Descriptions of major fuel cell types.
\r\n\t
\r\n\tRecently in 2019, International Council on Systems Engineering (INCOSE) has released the latest version of the “Guidelines for the Utilization of ISO/IEC/IEEE 15288 in the Context of System of Systems (SoS) Engineering” to industry for review and comments. The document was developed under the Partner Standards Development Organization cooperation agreement between ISO and IEEE, as it was approved by Council Resolution 49/2007. This document provides guidance for the utilization of ISO/IEC/IEEE 15288 in the context of SoS in many domains, including healthcare, transportation, energy, defense, corporations, cities, and governments. This document treats an SoS as a system whose elements are managerially and/or operationally independent systems, and which together usually produce results that cannot be achieved by the individual systems alone. This INCOSE guide book perceives that SoS engineering demands a balance between linear procedural procedures for systematic activity and holistic nonlinear procedures due to additional complexity from SoS perspectives.
\r\n\tThe objective of this book is to provide a comprehensive reference on Systems-of-Systems Engineering, Modeling, Simulation and Analysis (MS&A) for engineers and researchers in both system engineering and advanced mathematical modeling fields.
\r\n\tThe book is organized in two parts, namely Part I and Part II. Part I presents an overview of SOS, SOS Engineering, SOS Enterprise Architecture (SOSEA) and SOS Enterprise (SOSE) Concept of Operations (CONOPS). Part II discusses SOSE MS&A approaches for assessing SOS Enterprise CONOPS (SOSE-CONOPS) and characterizing SOSE performance behavior. Part II focuses on advanced mathematical application concepts to address future complex space SOS challenges that require interdisciplinary research involving game theory, probability and statistics, non-linear programming and mathematical modeling components.
\r\n\tPart I should include topics related to the following areas:
\r\n\t- SOS and SOS Engineering Introduction
\r\n\t- Taxonomy of SOS
\r\n\t- SOS Enterprise (SOSE), SOSE CONOPS, Architecture Frameworks and Decision Support Tools
\r\n\tPart II should address the following research areas:
\r\n\t- SOS Modeling, Simulation & Analysis (SOS M&SA) Methods
\r\n\t- SOS Enterprise Architecture Design Frameworks and Decision Support Tools
\r\n\t- SOS Enterprise CONOPS Assessment Frameworks and Decision Support Tools.
Thanks to the breakthroughs in microfabrication technologies, numerous concepts of microsystems including micro aerial vehicles, microbots, and nanosatellites have been proposed. Contrary to ordinary electronic devices, these microsystems perform mechanical work and require the extended operation. As their functions are getting complex and advanced, their energy consumption is also increasing exponentially. In order to activate these microsystems, a high density power source in a small scale is required. However, present portable devices still extract power from existing batteries. The energy density of the current batteries is too low to support these microsystems (Holladay et al., 2004). Therefore, a new micro power source is essential for the successful development of new microsystems.
\n\t\t\t\tVarious concepts for micro power generations have been introduced such as micro engines, micro gas turbines, thermoelectric generators combined with a micro combustor, and micro fuel cells. All of these concepts extract energy from a chemical fuel that have energy density much greater than that of the existing batteries. The first challenge to micro power source was the miniaturization of conventional heat engines. However, the development of micro heat engine reached a deadlock due to the difficulties of microfabrication and realization of miniaturized fast moving components and kinematics‘ mechanism to generate power in micro scale. Micro fuel cells have drawn attention as a primary candidate for a micro power source due to its distinctive merits that are absence of moving parts and high efficiencies. The fuel cell is an electrochemical device that directly converts chemical energy to electric energy. Due to its different energy conversion path, the fuel cell has high thermal efficiency compared to the heat engines. The energy density of the fuel cell is higher than that of the existing batteries because it uses a chemical fuel such as hydrogen (Nguyen & Chan, 2006).
\n\t\t\t\tThere are several types of fuel cell as summarized in Table 1 (O’Hayre et al., 2006), such as polymer electrolyte membrane fuel cell (PpngC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), molten carbon fuel cell (MCFC), and solid oxide fuel cell (SOFC). Of these fuel cells, PpngC is suitable to a micro power device due to its low operating temperature and solid phase of electrolyte. Direct methanol fuel cell (DMFC) is a kind of PpngC except that it directly uses methanol instead of hydrogen as a fuel. Formic acid, chemical hydrides, and other alcohols can be used as a direct fuel.
\n\t\t\t\t\n\t\t\t\t\t\t\t | PpngC | \n\t\t\t\t\t\t\tPAFC | \n\t\t\t\t\t\t\tAFC | \n\t\t\t\t\t\t\tMCFC | \n\t\t\t\t\t\t\tSOFC | \n\t\t\t\t\t\t
Electrolyte | \n\t\t\t\t\t\t\tPolymer | \n\t\t\t\t\t\t\tH 3 PO 4\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tKOH | \n\t\t\t\t\t\t\tMolten carbonate | \n\t\t\t\t\t\t\tCeramic | \n\t\t\t\t\t\t
Charge carrier | \n\t\t\t\t\t\t\tH +\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tH +\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tOH -\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tCO 3\n\t\t\t\t\t\t\t\t2-\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tO2\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Temperature | \n\t\t\t\t\t\t\t80 ˚C | \n\t\t\t\t\t\t\t200 ˚C | \n\t\t\t\t\t\t\t60-220 ˚C | \n\t\t\t\t\t\t\t650 ˚C | \n\t\t\t\t\t\t\t600-1000 ˚C | \n\t\t\t\t\t\t
Catalyst | \n\t\t\t\t\t\t\tPlatinum | \n\t\t\t\t\t\t\tPlatinum | \n\t\t\t\t\t\t\tPlatinum | \n\t\t\t\t\t\t\tNickel | \n\t\t\t\t\t\t\tPerovskite | \n\t\t\t\t\t\t
Cell components | \n\t\t\t\t\t\t\tCarbon | \n\t\t\t\t\t\t\tCarbon | \n\t\t\t\t\t\t\tCarbon | \n\t\t\t\t\t\t\tStainless | \n\t\t\t\t\t\t\tCeramic | \n\t\t\t\t\t\t
Fuel compatibility | \n\t\t\t\t\t\t\tH2, CH3OH | \n\t\t\t\t\t\t\tH2 | \n\t\t\t\t\t\t\tH2 | \n\t\t\t\t\t\t\tH 2 , CH 4\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\tH 2 , CH 4 , CO | \n\t\t\t\t\t\t
Descriptions of major fuel cell types.
In the beginning of research, DMFC has been widely investigated as a possible candidate for micro power generation due to the use of liquid fuel and its simple structure (Lua et al., 2004). However, the fuel crossover phenomena is an inherent problem of DMFC, which severely limits its power output. It is known that the power output of PpngC is much greater than that of DMFC, and there is no fuel crossover in PpngC. Major obstacle in the successful development of PpngC is the difficulties of the hydrogen storage with high density. Although possible to use hydrogen in either compressed gas or liquid form, it gives significant hazards due to its explosive nature. Metal hydride suffers from high weight per unit hydrogen storage and low response for a sudden increase in hydrogen demand. Chemical storage in the form of liquid fuel such as methanol has significantly higher energy density compared to the suggested technologies. It can be reformed to generate hydrogen gas when needed. The fuel reformer is a device that extract hydrogen from a chemical fuel including methanol, methane, propane, octane, gasoline, diesel, kerosene, and so on. The fuel choice is more flexible than the direct fuel cells. Although a fuel cell combined with the reformer is more attractive, it is complex and bulky compared to the DMFC due to the fuel reformer. Therefore, the miniaturization of the reformer has been a major research activity for the successful development of PpngC system in recent years (Pattekar & Kothare, 2004).
\n\t\t\t\tMEMS technology is a useful tool to reduce the size of reformer and fuel cell (Yamazaki, 2004). The use of MEMS technology in a thermo-chemical system is relatively new concept. It allows the miniaturization of conventional reactors while keeping its throughput and yield. The microreactor has a relatively large specific surface area, which provides the increased rate of heat and mass transport, and short response time. In addition, MEMS-compatible materials are suitable to various chemical reaction applications due to their high thermal and chemical resistances.
\n\t\t\tCatalytic steam reforming of methanol for hydrogen production using conventional reactors has been already carried out in the literature. However, the use of microreactors is a relatively new challenge and other approaches are required for the development of micro reformers using MEMS technologies. Nevertheless, the study on the methanol reforming reaction in the conventional reactors give a good background for the development of micro methanol reformer.
\n\t\t\t\tVarious research groups have successfully developed micro fuel reformers using MEMS technologies. Pattekar & Kothare, 2004 developed a micro-packed bed microreactor for hydrogen production, which is fabricated by deep reactive ion etching (DRIE). The width of microchannels was 1000 µm and the depth ranged from 200 to 400 µm. The microchannels were grooved on 1000 µm thick silicon substrate using photolithography followed by DRIE. A 10 µm thick photoresist (Shipley 1045, single/dual coat) was used as a etch mask for DRIE. Commercial Cu/ZnO/Al2O3 catalyst was load by passing the water-based suspension of catalyst particles ranging from 50 to 70 µm via microchannels. The microfilter was fabricated at the end of microchannels, and the catalyst particles larger than 20 µm were trapped in the microchannels. The platinum resistance temperature detector was used as a temperature sensor with a linear temperature versus resistance characteristic. The platinum microheater was deposited along the microchannels. The methanol conversion was 88% at the steam-to-carbon ratio (S/C) of 1.5 and the methanol feed rate of 5 ml/h. The hydrogen production rate was 0.1794 mol/h that is the sufficient flow to generate 9.48 W in a typical PpngC. Pattekar & Kothare, 2005 also developed a radial flow reactor that has less pressure drop compared to conventional one due to the increased flow cross section area along the reaction path.
\n\t\t\t\t\n\t\t\t\t\tKundu et al., 2006 fabricated a microchannel reformer on a silicon wafer using silicon DRIE process. The split type channels were made in the micro vaporizer region to reduce the back pressure at the inlet port and to get a more uniform flow of fluid. The dimensions of the micro reformer were 30 mm in length and 30 mm in width, and each channel was 28 mm in length. The width of each channel was 1 mm and the depth was 300 µm. The commercial CuO/ZnO/Al2O3 catalyst (Johnson Matthey) was packed inside the channels by injecting the water-based catalyst slurry. The catalyst particles were trapped in the microchannels by filters that were in the form of 90 µm thick parallel walls spaced 10 µm apart oriented parallel to the direction of the fluid flow. The catalyst deactivation was observed after operating continuously for 8 hours using catalyst characterization. It can be seen that the performance with the serpentine channel was higher than with the parallel channel due to the longer residence time. The hydrogen production rate was 0.0445 mol/h which can produce 2.4 W assuming an 80% fuel cell operation efficiency.
\n\t\t\t\tKazushi et al., 2006 developed a micro fuel reformer integrated with a combustor and a microchannel evaporator. Two fuel reforming reactors were placed on either side of a combustor to make the system compact and to use combustion heat efficiently. The silicon and Pyrex® glass wafer that are used as a substrate were stacked by anodic bonding. A commercially available reforming catalyst made of CuO/ZnO/Al2O3 (MDC-3, Süd-Chemie Catalysts Japan, Inc.) was filled into a microchamber fabricated on glass substrates after being powdered and hardened by polyvinylalcohol (PVA). The Pt loaded on TiO2 support made by sol-gel method was used as a catalyst of the combustor. Thin film resistive temperature sensors made of Pt/Ti (100 nm/50 nm) to measure temperature inside the fuel reformer was fabricated on the wall of the combustion chamber by the lift-off process. The six kinds of microchannel evaporators were fabricated on the silicon substrates; as a result, it was found that the design of the microchannel evaporator is critical to obtain larger hydrogen output. The 32.9 ml/min of hydrogen, which is equivalent to 5.9 W in lower heating value, was produced when input combustion power was 11 W. The maximum efficiency of 36.3% was obtained and the power density of the reformer was 2.1 W/cm3.
\n\t\t\t\tThough the work on the MEMS-based reformer has been continuously reported in the recent literature, there is no novel change and significant improvement. The literature could be classified into two standpoints. In terms of substrate materials, silicon wafers has been mostly used as a substrate of microreactors. Different materials have been also used such as glass wafer, polydimethylsiloxane (PDMS), and low temperature co-fired ceramic (LTCC). In terms of a method of catalyst loading in the reactor bed, either catalyst coating or packing has been used. In almost results, the heat to sustain the methanol steam reforming reaction was provided by an external heater, while some results presented the use of a catalytic combustor as a heat source.
\n\t\t\tFuel reforming is a chemical process that extracts hydrogen from a liquid fuel. Fuel reformer is a device that produces hydrogen from the reforming reaction. Liquid fuel is used as a feed of the reformer due to its higher density than gaseous fuels. Considering hydrogen content and ease of reforming, methanol was chosen as the primary fuel in hydrogen sources such as alcohols and hydrocarbons (Schuessler et al., 2003).
\n\t\t\t\tThere are a number of fuel reforming techniques available, including steam reforming (Lindström & Pettersson, 2001), partial oxidation (Wang et al., 2003), and autothermal reforming (Lindström et al., 2003). Of all considered techniques, the steam reforming process provides the highest attainable hydrogen concentration in the reformate gas. This reaction takes place at relatively low temperature in the range of 200-300 ˚C. The chemical reaction of the methanol steam reforming process is expressed below:
\n\t\t\t\t\n\t\t\t\t\tEquation 1 is a primary reforming process that is the stoichiometric conversion of methanol to hydrogen. It can be regarded as the overall reaction of the methanol decomposition and the water-gas shift reaction. First, the methanol decomposes to generate carbon monoxide.
\n\t\t\t\tThe presence of water can convert carbon monoxide to carbon dioxide through the water-gas shift reaction.
\n\t\t\t\tThe formation of carbon monoxide lowers the hydrogen production rate and the carbon monoxide also acts as a poison for the fuel cell catalyst. Typically, carbon monoxide is converted to carbon dioxide either in a separate water-gas shift reactor or a preferential oxidation called PROX (Delsman et al., 2004). Palladium/silver alloy membrane is also used to separate selectively the carbon monoxide. Other byproducts such as carbon dioxide and excess water vapor can be safely discharged to atmosphere.
\n\t\t\t\tCu-based catalysts are used for the steam reforming of methanol, and the well-known one is Cu/ZnO/Al2O3. Generally, it has been claimed that Cu0 provides catalytic activity and ZnO acts as a stabilizer of Cu surface area. Addition of Al2O3 to the binary mixture enhances Cu dispersion and catalyst stability (Agrell et al., 2003).
\n\t\t\t\tThe steam reforming of methanol is endothermic reaction. An external electric heaters or catalytic combustors can be used as a heat sources to sustain the reforming reaction. The amount of the endothermic heat per a mole of methanol is 48.96 kJ/mol at 298 K. The electric microheater is the simplest method to supply heat to the reformer because its control is relatively easy and the fabrication can be simply integrated into MEMS process. However, the electric heater is usually used for startup period only due to its low thermal efficiency. The catalytic combustors are an ideal alternative heat source to the electric heater because its high thermal efficiency. Methanol can be directly used in the combustor to facilitate methanol reforming reaction. Part of the hydrogen produced out of the reformer can be fed to the combustor. While it is possible that the catalytic hydrogen combustion with Pt as the catalyst even at room temperature, the methanol combustion requires preheaters to initiate the reaction. In the present study, the catalytic combustion of hydrogen and the catalytic decomposition of hydrogen peroxide were used as heat sources of the methanol steam reformer. Hydrogen peroxide as a heat source is the first attempt in the world.
\n\t\t\t\t\n\t\t\t\t\tFigure 1 shows the schematic of a typical reformer-combined fuel cell system, which consists of a fuel reformer and a fuel cell. The fuel reformer is classified into four units; fuel vaporizer/preheater, steam reformer, combustor/heat-exchanger, and PROX reactor. First, methanol is fed with water and is heated by the vaporizer. The methanol is reformed by the reforming catalyst to generate hydrogen in the steam reformer. To supply heat to the steam reformer, part of hydrogen from the anode off-gas of fuel cell can be fed to the combustor. The combustor generates the sufficient amount of heat to sustain the methanol reforming reaction. As mentioned before, the extremely small amount of carbon monoxide deactivates the fuel cell catalyst, which should be reduced to below 10 ppm by PROX.
\n\t\t\t\tSchematic of the fuel cell system combined with the fuel reformer.
This chapter presents design, fabrication and evaluation of MEMS methanol reformer. First, a methanol reformer was fabricated and integrated with a catalytic combustor. Cu/ZnO was selected as a catalyst for the methanol steam reforming reaction and Pt for the hydrogen catalytic combustion. Wet impregnation method was used to load the catalysts on a porous support. The catalyst-loaded supports were inserted in the cavity made on the glass wafer. The performance of the micro methanol reformer was measured at various test conditions and the optimum operation condition was sought. Next, new concept of micro methanol reformer was proposed in the present study. The micro reformer consists of the methanol reforming reactor, the catalytic decomposition reactor of hydrogen peroxide, and a heat-exchanger between the two reactors. In this system, the catalytic decomposition of hydrogen peroxide is used as a process to supply heat to the reforming reactor. The decomposition process of hydrogen peroxide produces water vapor and oxygen as a product, which can be used efficiently to operate the reformer/PpngC system. Microreactor was fabricated for preferential oxidation of carbon monoxide using a photosensitive glass process integrated with a catalyst coating process. A γ-Al2O3 layer was coated as a catalyst support on the surface of microchannels using sol-gel method. The wet impregnation method was used to load Pt/Ru in the support. The conversion of carbon monoxide was measured with varying the ratio of oxygen to carbon (O2/C) and the catalyst loading amount. Micro fuel cell was fabricated and the integrated test with the MEMS methanol reformer was performed to validate the micro power generation from the micro fuel cell system.
\n\t\t\t\n\t\t\t\t\tFigure 2 depicts the construction of the integrated micro methanol reformer. The mixture of methanol and water enters the steam reformer at the top and the reformate gas leaves the reactor. The mixture of hydrogen and air flows into the catalytic combustor at the bottom with counter flow stream against the reforming stream. The heat generated from the catalytic combustor is transferred to the steam reformer through the heat-exchanger layer that has micro-fins to increase the surface area and the suspended membrane to enhance the heat transfer rate. The porous catalyst supports were inserted in the cavity made on the glass wafer as shown in Fig. 2. The micro reformer structure was made of five glass wafers; two for top and bottom, one for the steam reformer, one for the catalytic combustor, and the reminder for the heat-exchanger in-between.
\n\t\t\t\tConstruction of the integrated micro methanol reformer.
The porous ceramic material (ISOLITE®) was used as a catalyst support due to its large surface area and thermal stability (Kim et al., 2007). The typical ceramic support is composed of 40% Al2O3 and 55% SiO2 with traces of the other metal oxides, and the porosity is approximately 71%. Figure 3 shows SEM images of the support material. The scale of the bulk pores was between 100 and 300 μm, while smaller scale pores were a few microns. This structure of the porous support can enhance the heat and mass transport between catalyst active sites and reactants.
\n\t\t\t\tSEM images of the porous ceramic material used as a catalyst support.
The overall fabrication process was integrated with a catalyst loading step as shown in Fig. 4. The fabrication process for an individual glass wafer is as follows: (1) exposure to ultraviolet (UV) light under a mask at the intensity of 2 J/cm2; (2) heat treatment at 585 ˚C for 1 hour to crystallize portion of the glass that was exposed to UV; and (3) etching the crystallized portion of the glass in the 10% hydrofluoric (HF) solution to result in the desired shape. The etching rate was 1 mm per hour. With step 1-3 in Fig. 4, two covers, a reformer layer, and a combustor layer were fabricated. To obtain the membrane heat-exchanger, the glass wafer was exposed by UV light on both sides of the wafer. After the heat treatment, the wafer was etched standing in the etching bath. The tooth shape cross-section of the membrane heat-exchanger layer was fabricated by controlling etching time as shown in the step 4-6 of Fig. 4. The complete micro methanol reformer was constructed by fusion-bonding the fabricated glass layers, where the porous catalyst supports were inserted in the reformer layer and the combustor layer, respectively. The best fusion-bonding between glass wafers was obtained by pressing the wafers against each other at 1000 N/m2 in a furnace held at 500 ˚C (Kim & Kwon, 2006a).
\n\t\t\t\tAs a final step, the catalysts were loaded on the porous catalyst supports. The Cu/ZnO was selected as a catalyst for methanol reforming reaction, considering its proven reactivity and selectivity (Kim & Kwon, 2006b). The Pt was chosen as a catalyst for the hydrogen catalytic combustion. The wet impregnation method was used to load both catalysts on the porous supports. A mixture of a 0.7 M aqueous solution of Cu(NO3)2 and a 0.3 M aqueous solution of Zn(NO3)2 was prepared. The mixture was injected in the catalyst support inserted in the reformer layer using a syringe pump. The moisture was removed by drying the catalyst-loaded support in a convection oven at 70 ˚C for 12 hours. Calcination procedure followed in a furnace at 350 ˚C for 3 hours. The similar procedures were used for Pt coating with 1 M aqueous solution of H2PtCl6. The amount of the loaded Cu/ZnO was 7.0 wt % while Pt was 5.0 wt % of the total weight of the catalyst support. The catalysts were reduced for 4 hours in an environment of mixture of 4% H2 in N2, which is steadily flowing into the reformer at a rate of 10 ml/min in a furnace of 280 ˚C.
\n\t\t\t\t\n\t\t\t\t\tFigure 5 shows the fabrication results, including etched glass wafers, a complete micro methanol reformer, a cross-section view of the reformer and SEM image of the membrane heat-exchanger. The total volume of the reformer was 3.6 cm3 (20 mm30 mm6mm) and the weight was approximately 13.4 g.
\n\t\t\t\tOverall fabrication procedure of the micro methanol reformer.
Fabricated results of the micro methanol reformer.
Experimental setup was equipped to measure the performance of the micro methanol reformer. A syringe pump (KDS200, KD Scientific) supplied a mixture of methanol and water to the reformer at a controlled rate. The flow rate of hydrogen and air was controlled by mass flow controllers (EL-FLOW, Bronkhorst). After mixed them in a mixing chamber, the mixture gas was supplied to the combustor. The temperature of each reactor was recorded by thermocouples. The product gas of the reformer was cooled and the condensable portion was removed in a cold trap. The non-condensable product gas was analyzed by a gas chromatography (Agilent HP6890). The flow rate of dry gas was measured by a bubble meter. The column in the gas chromatography was Carboxen-1000 (60/80 mesh, 1/8”, 18 ft) that can separate H2, N2, CO, CO2, CH4 and others. Nitrogen carrier gas at known flow rate was mixed with the product gases before entering the gas chromatography. The exact hydrogen production rate can be calculated by comparing the ratio of hydrogen to nitrogen because the flow rate of the carrier gas is known. The gas composition was detected by a TCD (thermal conductivity detector) with Ar as a reference gas. The product gas of the catalytic combustor was analyzed, after moisture was removed in a cold trap.
\n\t\t\t\tThe energy balance between the methanol reformer and the catalytic combustor was calculated as shown in Table 2. The total heating energy consists of the energy to raise the reformer temperature and the heat of reaction. The heat of reaction is the sum of the reforming heat, the evaporation heat and the heat to raise mixture to reforming temperature (sensible heating). The energy to reform 1 mole methanol with 1 mole water is 158.3 kJ, which can be provided by burning 0.66 mole hydrogen by the catalytic combustor. The hydrogen can be provided by recycling the off-gas of the fuel cell. The reformer produces 2.7 moles hydrogen from 1 mole methanol when methanol conversion is 95% and hydrogen selectivity is 95%. Assuming that hydrogen utilization of the fuel cell is 72%, the amount of the hydrogen off-gas is 0.756 mole, which is greater than the hydrogen requiremnt for the combustor to sustain the reformer. Based on this calculation, the expected production of hydrogen is 54.5 ml/min when the methanol feed rate is 2 ml/h. The fuel cell consumes 72% portion (39.2 ml/min) in the reformed hydrogen and the remainder (15.3 ml/min) can be used to operate catalytic combustor.
\n\t\t\t\t\n\t\t\t\t\t\t\t | Calculation | \n\t\t\t\t\t\t\tFlow rate | \n\t\t\t\t\t\t
Methanol input | \n\t\t\t\t\t\t\t1 mol | \n\t\t\t\t\t\t\t2 ml/h | \n\t\t\t\t\t\t
Energy requirement for the reformer * | \n\t\t\t\t\t\t\t153.8 kJ | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Evaporation and sensible heating of methanol | \n\t\t\t\t\t\t\t48.4 kJ | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Evaporation and sensible heating of water | \n\t\t\t\t\t\t\t51.5 kJ | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Heat of reaction | \n\t\t\t\t\t\t\t58.4 kJ | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Expected production of hydrogen ** | \n\t\t\t\t\t\t\t2.7 mol | \n\t\t\t\t\t\t\t54.5 ml/min | \n\t\t\t\t\t\t
Hydrogen requirement for the combustor | \n\t\t\t\t\t\t\t0.66 mol | \n\t\t\t\t\t\t\t13.3 ml/min | \n\t\t\t\t\t\t
Anode off-gas of fuel cell *** | \n\t\t\t\t\t\t\t0.756 mol | \n\t\t\t\t\t\t\t15.3 ml/min | \n\t\t\t\t\t\t
* Reforming temperature: 250 ˚C , ** 95% methanol conversion, 95% hydrogen selectivity , *** Fuel cell utilization: 72% | \n\t\t\t\t\t\t
Energy balance calculation between the methanol reformer and the combustor.
The performance of the reformer was measured at various test conditions and an optimum operation condition was sought. The measured performance of the reformer was expressed in terms of the methanol conversion, which is defined as follows:
\n\t\t\t\t\n\t\t\t\t\tFigure 6 shows the methanol conversion as a function of the reformer temperature at each methanol feed rate with the steam-to-carbon ratio of 1.1. The methanol conversion decreased as the methanol feed rate increased, while the methanol conversion increased as the reformer temperature increased. The maximum methanol feed rate was 2 ml/h to obtain the methanol conversion higher than 90% at temperature lower than 250 ˚C. At the feed rate of 2 ml/h and the reformer temperature of 250 ˚C, the hydrogen production rate was 53.9 ml/min and the composition of carbon monoxide in the reformate gas was 0.49%.
\n\t\t\t\tMethanol conversion as a function of the reformer temperature.
The performance of the catalytic combustor was measured at various conditions. Figure 7 shows the temperature variation of the catalytic combustor as a function of the reaction time at an equivalence ratio of 1.0. This plot includes the change of reformer temperature, which has to reach 250 ˚C to obtain the optimal methanol conversion. The temperatures of reformer and catalytic combustor were measured as varying the hydrogen feed rate. The air was mixed with hydrogen in the mixing chamber at the equivalent ratio of 1.0 and the gas mixture was fed into the combustor. In the energy balance calculation, the hydrogen requirement of the combustor was 15.3 ml/min to sustain the methanol reforming reaction at the methanol feed rate of 2 ml/h. At the feed rate of 15.3 ml/min, the temperature of the catalytic combustor reached 148.7 ˚C when 18 min elapsed after the initiation of the reaction. The hydrogen feed rate increased to reduce the time for the startup of the reformer. At the hydrogen feed rate of 41.3 ml/min, the combustor temperature reached 271 ˚C within 8.6 min after the start of operation and the reformer temperature was 250 ˚C. As the hydrogen feed rate increased, the combustion heat increased and the time for startup decreased. However, the hydrogen conversion decreased at the increase of the hydrogen feed rate due to the short residence time that is proportional to the inverse of the feed rate. Furthermore, the hot-spot appeared in the fore part of the combustor, which can damage the catalyst and the reactor substrate. The temperature difference between the reformer and the combustor increased with the hydrogen feed rate. At the feed rate of 41.3 ml/min, the temperature difference was 21 ˚C when the reformer temperature reached 250 ˚C.
\n\t\t\t\tTemperature variation of the catalytic combustor as a function of the reaction time.
\n\t\t\t\t\tFigure 8 represents the result of simultaneous operation of the methanol steam reformer and the catalytic combustor. The reformer was heated up to 250 ˚C by an external preheater with the increasing rate of temperature of 11.4 ˚C/min. The combustor was operated when the reformer temperature reached 250 ˚C. The hydrogen feed rate was 15.3 ml/min, which can be supplied from the anode off-gas of fuel cell when the methanol feed rate is 2 ml/h. The air was mixed with hydrogen to fix the equivalent ratio at 1.0. The methanol was fed into the reformer with the feed rate of 2 ml/h. The water feed rate was 0.98 ml/h to satisfy the steam-to-carbon ratio of 1.1. The reformer temperature was maintained constantly after the methanol reforming reaction was initiated. After 8 minutes into the simultaneous operation, steady reforming reaction was attained and the methanol conversion was higher than 90%. The maximum conversion of methanol was 95.7%. The temperature difference between the reformer and the combustor was approximately 4 ˚C.
\n\t\t\t\tSimultaneous operation of the methanol steam reformer and the catalytic combustor.
The composition of reformate gas and the production rate of hydrogen.
\n\t\t\t\t\tFigure 9 shows the composition of reformate gas and the hydrogen production rate after the start of complete operation. As the steady reforming reaction lasted, the composition of reformate gas remained constant. The reformate gas composition was 74.4% H2, 24.36% CO2, and 1.24% CO, and its flow rate was 67.2 ml/min. The hydrogen production rate was approximately 50 ml/min, which can generate 4.5 W electric power on a typical PpngC. The concentration of carbon monoxide at the integrated test was higher than that at the separate test of the reformer. Although the catalytic combustor gave the sufficient amount of heat to operate the reformer, it could not form uniform temperature distribution within the reformer. As a result, the high temperature gradient occurred in the reformer, increasing the selectivity of carbon monoxide. The thermal efficiency of the conventional reformer combined with the combustor is defined by:
\n\t\t\t\twhere the LHV means the lower heating value. The thermal efficiency of the integrated micro methanol reformer was 76.6%. The operating conditions and the performance of the micro methanol reformer is summarized in Table 3.
\n\t\t\t\tOperating condition | \n\t\t\t\t\t\t\tReformer | \n\t\t\t\t\t\t\tCombustor | \n\t\t\t\t\t\t
Feed flow rate | \n\t\t\t\t\t\t\t2 ml/h CH 3 OH | \n\t\t\t\t\t\t\t15.3 ml/min H 2 | \n\t\t\t\t\t\t
S/C (steam - to - carbon ratio) | \n\t\t\t\t\t\t\t1.1 | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Equivalence ratio | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 1.0 | \n\t\t\t\t\t\t
Temperature | \n\t\t\t\t\t\t\t250 ˚C | \n\t\t\t\t\t\t\t251 ˚C | \n\t\t\t\t\t\t
Performance | \n\t\t\t\t\t\t\tReformer only | \n\t\t\t\t\t\t\tIntegrated operation | \n\t\t\t\t\t\t
Temperature | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 247 ˚C (reformer) | \n\t\t\t\t\t\t
Conversion | \n\t\t\t\t\t\t\t96.2% | \n\t\t\t\t\t\t\t95.7% | \n\t\t\t\t\t\t
H 2 production rate | \n\t\t\t\t\t\t\t53.9 ml/min | \n\t\t\t\t\t\t\t50 ml/min | \n\t\t\t\t\t\t
CO composition | \n\t\t\t\t\t\t\t0.49% | \n\t\t\t\t\t\t\t1.24% | \n\t\t\t\t\t\t
Thermal efficiency | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 76.6% | \n\t\t\t\t\t\t
The operating conditions and the performance of the micro methanol reformer.
In the previous section, the catalytic combustor is used as a heat source of the methanol steam reformer. However, it is still problematic that non-uniform distribution of reaction and hot spot formations in the fore region of the combustor. In the present study, the catalytic decomposition of hydrogen peroxide is used as a process to supply heat to the reformer. The decomposition reaction of hydrogen peroxide is expressed below:
\n\t\t\t\tThe construction of the micro methanol reformer complete with a heat source is presented in Fig. 10, in which the catalytic reactor for the hydrogen peroxide decomposition is included. The hydrogen peroxide decomposition is a highly exothermic reaction and generates the sufficient amount of heat to sustain the methanol steam reforming reaction. The catalytic decomposition of hydrogen peroxide has great reactivity and selectivity on various metal elements, such as Fe, Cu, Ni, Cr, Pt, Pd, Ir, and Mn (Teshima et al., 2004). The hydrogen peroxide decomposition generates steam and oxygen as products. The steam can be recycled into the reformer for the steam reforming reaction. The oxygen can be used as an oxidizer at the fuel cell cathode and to remove carbon monoxide in the preferential oxidation. The present concept renders the system far more compact than the existing reformer/combustor model because hydrogen peroxide is stored and used in condensed phase and oxygen enrichment enhances the system efficiency.
\n\t\t\t\tIn the present study, the performance evaluation of the methanol steam reformer with hydrogen peroxide heat source was carried out at various test conditions and an optimum operation condition was sought.
\n\t\t\t\tConcept of methanol steam reformer integrated with hydrogen peroxide heat source.
Experimental apparatus for the performance measurement of the reformer system is similar with the combustor experiment. Two syringe pumps supplied reactants to the reactor at a controlled rate; one for the mixture of methanol and water, and the other for hydrogen peroxide. The temperature of each reactor was recorded by thermocouples. The analysis of the product gas composition was the same with the section 2.3. The concentration of hydrogen peroxide was measured using a refractometer (PR-50HO, ATAGO) with a small quantity of sample. The product gas of hydrogen peroxide decomposition was analyzed, after moisture removed in a cold trap.
\n\t\t\t\tThe measured performance of the reformer was expressed in terms of the methanol conversion, hydrogen selectivity and hydrogen peroxide conversion, which are defined as follows:
\n\t\t\t\tThe chemical equation of methanol steam reforming reaction is expressed below:
\n\t\t\t\twhere symbol s is the molal ratio of water to methanol (H2O/CH3OH), which is the same with the steam-to-carbon ratio. Decomposition reaction of hydrogen peroxide is expressed below:
\n\t\t\t\twhere symbol a and x are the molal ratio of hydrogen peroxide to methanol (H2O2/CH3OH) and the molal concentration of hydrogen peroxide, respectively. The performance of the reformer system depends on these parameters. In order to determine the reaction condition, the concentration of hydrogen peroxide and the weight hourly space velocity (WHSV) were used as control parameters. The weight hourly space velocity indicates the ratio of the reactant flow rate to the catalyst mass as follows:
\n\t\t\t\tOverall heat output of the integrated reformer system was calculated as shown in Fig. 11. Figure 11 (a) shows the variation in the decomposition reaction heat of hydrogen peroxide as a function of the weight concentration of hydrogen peroxide. It can be seen that the hydrogen peroxide concentration has to be higher than 73.9 wt % to generate the sufficient heat to complete the reforming reaction of methanol at s = 1.0 and a = 9.0, respectively. Hydrogen peroxide with even higher concentration is needed when the steam-to-carbon ratio is higher or the hydrogen peroxide-to-methanol ratio is lower.
\n\t\t\t\t\n\t\t\t\t\tFigure 11 (b) illustrates the net heat output that amounts to the difference between the decomposition heat of hydrogen peroxide and the heat required to maintain the reformer at the optimum operation condition. The decomposition heat of 5.3 moles hydrogen peroxide at 81.5 wt % concentration releases the sufficient amount of heat to reform the mixture of 1 mole methanol and 1 mole water. The required amount of hydrogen peroxide will decrease when the hydrogen peroxide concentration increases or the steam-to-carbon ratio decreases. In the calculation that leaded to Fig. 11, the heat loss to the surrounding was ignored. Considering the heat loss of the reformer, higher concentration of hydrogen peroxide or higher hydrogen peroxide-to-methanol ratio is required. In the present study, hydrogen peroxide of 82 wt % concentration was used and the steam-to-carbon ratio was fixed at 1.1 for convenience in the experiment. The performance characteristics of the reformer was investigated with three control parameters; methanol space velocity, hydrogen peroxide space velocity, and hydrogen peroxiode-to-methanol ratio.
\n\t\t\t\tOverall heat output of the integrated reformer system.
The temperature of the hydrogen peroxide decomposition reactor was measured as varying the hydrogen peroxide space velocity. Figure 12 (a) shows the temperature of the hydrogen peroxide decomposition reactor as a function of reaction time at each space velocity, in which the hydrogen peroxide conversion is included. At the space velocity of 6.32 mol/g-h, the hydrogen peroxide conversion was 98.2% and the reactor temperature reached 150 ˚C when 200 seconds elapsed after the initiation of reaction. At the space velocity of 37.3 mol/g-h, the reactor temperature reached 250 ˚C, which is the optimal temperature for the methanol reforming reaction, within a minute after the start of operation. The amount of reaction heat increases with the feed rate of hydrogen peroxide, reducing the time to obtain the optimal reformer temperature. At high space velocity, however, reactants does not take the residence time enough to react on the catalyst, resulting in the decrease of hydrogen peroxide conversion. At the low space velocity, the temperature difference between the reformer and the decomposition reactor was within 5 ˚C. At the space velocity of 37.3 mol/g-h, however, the temperature difference increased with the time after the start-up as shown in Fig. 12 (b). When the temperature of decomposition reactor reached 250 ˚C, the reformer temperature was less than 200 ˚C.
\n\t\t\t\t\n\t\t\t\t\tFigure 13 represents the simultaneous operation result of the methanol steam reformer and the hydrogen peroxide decomposition reactor. The reformer was heated up to 250 ˚C by the decomposition reactor with 82 wt% hydrogen peroxide at the space velocity of 9.48 mol/g-h. The mixture of methanol and water was fed into the reformer with the steam-to-carbon ratio at 1.1. The space velocity of methanol was 0.68 mol/g-h. The temperature increased steadily after the methanol reforming reaction was initiated. It implies that the hydrogen peroxide feed rate exceeds the minimum to sustain the methanol reforming reaction. By reducing the feed rate down to the space velocity of 6.32 mol/g-h after 5 minutes into the operation, an ideal reaction condition was obtained as shown in Fig. 13. After 8 minutes into the operation, steady methanol reforming reaction was obtained and the methanol conversion was higher than 91.2%. The temperature inside the reformer and the decomposition reactor were 253 ˚C and 278 ˚C, respectively.
\n\t\t\t\tThe performance of hydrogen peroxide decomposition reactor.
Simultaneous operation of the micro reformer with hydrogen peroxide heat source.
The performance characteristics of the micro reformer with hydrogen peroxide heat source was investigated at various conditions. Figure 14 (a) shows the effect of the methanol space velocity on the methanol conversion and the reformer temperature with the conditions of the decomposition reactor fixed (S/C = 1.1, 82 wt% H2O2, H2O2 WHSV 6.32 mol/g-h). As the methanol space velocity increased, the reformer temperature decreased gradually because the hydrogen peroxide decomposition heat was consumed to vaporize the methanol supplied in liquid phase. As a result, the reformer decreased in temperature and did not sustain the methanol reforming reaction. Figure 14 (b) shows the effect of the reformer temperature on the methanol conversion. The feed rate of the methanol was fixed while the reformer temperature was determined by varying the feed rate of hydrogen peroxide (CH3OH WHSV 0.68 mol/g-h, S/C = 1.1, 82 wt% H2O2). The reformer temperature increased with the space velocity of hydrogen peroxide because the decomposition heat of hydrogen peroxide increased. The methanol conversion increased with the reformer temperature, when the temperature was below 250 ˚C. For the reformer temperature higher than 250 ˚C, the methanol conversion maintained its value at 250 ˚C.
\n\t\t\t\tPerformance characteristics of micro reformer with hydrogen peroxide heat source.
Hydrogen selectivity and thermal efficiency as a function of reformer temperature.
\n\t\t\t\t\tFigure 15 shows the hydrogen selectivity and the thermal efficiency of the system as a function of reformer temperature with the conditions of the reformer fixed. The thermal efficiency of the conventional reformer/combustor model is defined by Eq 5.
\n\t\t\t\tThis formula could not be applied to the methanol reformer integrated with the hydrogen peroxide decomposition reactor, because the LHV of hydrogen peroxide is not defined. In the present study, the thermal efficiency for the reformer system is defined as follows:
\n\t\t\t\tThe LHV was replaced with the heat of reaction. The LHV of hydrogen provided to the combustor in Eq. 5 was replaced with the decomposition heat of hydrogen peroxide. The hydrogen selectivity increased with the thermal efficiency as the reformer temperature increased. At the reformer temperature higher than 250 ˚C, however, the hydrogen selectivity decreased as the reformer temperature increased, because the production of carbon monoxide increased. The maximum hydrogen selectivity and the thermal efficiency were 86.4% and 44.8%, respectively. The product gas included 74.1% H2, 24.5% CO2 and 1.4% CO, and the total volume production rate was 23.5 ml/min. The hydrogen production rate is the sufficient amount to generate 1.5 W electrical power on a typical PpngC. The optimum condition and the performance of the methanol reformer with hydrogen peroxide heat source are shown in Table 4.
\n\t\t\t\tThe overall efficiency of typical PpngC system using a methanol reformer is approximately 40% (Ishihara et al., 2004). In present study, the exergy loss can be reduced by the use of hydrogen peroxide decomposition reaction. The use of oxygen generated by the decomposition reaction raises the cell voltage, resulting in the increase of the fuel cell efficiency. It is understood that the overall efficiency of fuel cell system presented in present study is higher than that of the existing fuel cell model.
\n\t\t\t\t\n\t\t\t\t\t\t\t | H 2 O 2 reactor | \n\t\t\t\t\t\t\tReformer | \n\t\t\t\t\t\t
Temperature | \n\t\t\t\t\t\t\t278 ˚C | \n\t\t\t\t\t\t\t253 ˚C | \n\t\t\t\t\t\t
S/C (steam - to - carbon ratio) | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 1.1 | \n\t\t\t\t\t\t
H 2 O 2 concentration | \n\t\t\t\t\t\t\t82 wt% | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t |
Feed flow rate | \n\t\t\t\t\t\t\t2 ml/h | \n\t\t\t\t\t\t\t10 ml/h | \n\t\t\t\t\t\t
WHSV | \n\t\t\t\t\t\t\t0.68 mol/g-h | \n\t\t\t\t\t\t\t6.32 mol/g-h | \n\t\t\t\t\t\t
Conversion | \n\t\t\t\t\t\t\t98.4 % | \n\t\t\t\t\t\t\t91.2 % | \n\t\t\t\t\t\t
H 2 production rate | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 23.5 ml/min | \n\t\t\t\t\t\t
CO composition | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 1.4 % | \n\t\t\t\t\t\t
Hydrogen selectivity | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 86.4% | \n\t\t\t\t\t\t
Thermal efficiency | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t | 44.8% | \n\t\t\t\t\t\t
The optimum operation conditions and the performance of the integrated reformer.
Removal of carbon monoxide from the reformate gas mixture is of paramount importance for development of a reformer in fuel cell applications because carbon monoxide deactivates the anode catalyst of PpngC. There are several processes for the carbon monoxide removal including pressure/temperature swing adsorption (PSA/TSA), methanation, membrane separation, and preferential oxidation. PSA/TSA are energy-intensive and expensive. Methanation consumes three moles of hydrogen to convert 1 mole CO into 1 mole methane as given below:
\n\t\t\t\tIt is therefore not recommended. The membrane separation is attractive method because high purity hydrogen can be obtained. PROX also is the preferred method because the small amount of oxygen is required to oxidize CO into CO2 as expressed below:
\n\t\t\t\tMicroreactor for PROX was prepared as shown in Fig. 16. Pt/Ru was selected as a catalyst of PROX. Microchannels were fabricated on a photosensitive glass.
\n\t\t\t\tMicroreactor for preferential oxidation.
As a washcoat layer, γ-Al2O3 was coated on the microchannels using sol-gel method and the catalyst was loaded by wet impregnation method. First, aluminum isopropoxide was hydrolyzed in deionized water with vigorous stirring for 1 hour at 80 ˚C. The sol was peptized by adding nitric acid (HNO3) with adjusting the pH. Polyvinyl alcohol (PVA) solution was prepared by dissolving the PAV in deionized water at 75 ˚C. The presence of PVA can reduce crack formations of the washcoat layer at the drying time. The peptized sol and the PVA solution were mixed with adding the γ-Al2O3 powder to increase the concentration of γ-Al2O3 in the slurry. The mixture slurry was ball-milled for 72 hours. The glass substrate was then dipped into the prepared γ-Al2O3 slurry and dried for 2 hours at 120 ˚C after blowing off the excess slurry. This procedure was repeated to obtain the desired weight of the γ-Al2O3 washcoat layer. The washcoated microchannels were then calcined at 350 ˚C for 4 hours. A mixture of a 0.5 M aqueous solution of H2PtCl6 and a 0.5 M aqueous solution of RuCl3 were prepared. The substrate was immersed in the mixture for 12 hours. The moisture was removed by drying the catalyst-loaded substrate in a convection oven at 70 ˚C for 12 hours. The calcination followed in a furnace at 350 ˚C for 3 hours. The catalyst was activated by reduction in a steady flowing hydrogen environment at 350 ˚C for 5 hours.
\n\t\t\t\tThe carbon monoxide conversion of PROX reactor as a function of the reaction temperature with varying the ratio of oxygen to carbon is shown in Fig. 17. Mixture gas including 69.91% H2, 3.06% CO, 2.03% CH4, and 25% CO2 was used in the test of PROX reactor. The carbon monoxide conversion increased with the oxygen-to-carbon ratio and the reactor temperature. In the case of 5 wt% Pt/Ru/γ-Al2O3 catalyst, the carbon monoxide was removed completely with oxygen-to-carbon ratio of 4 at 200 ˚C.
\n\t\t\t\tConversion of carbon monoxide of PROX microreactor.
MEMS fuel cell was fabricated for integrated tests with the micro reformer. The structure of the micro fuel cell is shown in Fig. 18. Membrane electrode assembly (MEA) was prepared by coating 0.3 mg/cm2 Pt-Ru/C for an anode catalyst and 0.3 mg/cm2 Pt/C for a cathode catalyst on a Nafion-112 membrane. The reason to select Pt-Ru/C as an anode catalyst is because Pt/C is poisoned by carbon monoxide in the reformate gas even if removed via PROX reaction. Carbon paper (TGP-H-090, 260 μm) was used as a gas diffusion layer (GDL). Flow channels were fabricated by etching the photosensitive glass wafer, on which the current collectors, Ag/Ti layer, were sputtered. Overall fabrication process is presented in Fig. 18 and the fabricated micro fuel cell is shown in Fig. 19.
\n\t\t\t\tStructure and fabrication process of MEMS fuel cell.
Experimental layout for integrated tests of the reformer with the micro fuel cell is shown in Fig. 20. The micro fuel cell was tested with pure hydrogen to compare with the result with the reformate gas. Simultaneous operation of the micro reformer, PROX reactor, and micro fuel cell was carried out.
\n\t\t\t\tFabricated results of the micro fuel cell.
Schematic of the integrated test of micro reformer-PROX reactor-micro fuel cell.
Performance of MEMS fuel cell system with pure hydrogen and the reformate gas is shown in Fig. 21. Pure hydrogen gas feed rate was set in 50 ml/min. When methanol feed rate was 2 ml/h, the flow rate of reformate gas was 71.96 ml/min. The reformate gas included 74.4% hydrogen, thus the hydrogen flow was 53.5 ml/min. The power density was 184 mW/cm2 when the potential was 0.64 V. The performance was low compared with the result for pure hydrogen due to the feed at the fuel cell that included undesired CO, CO2, and N2.
\n\t\t\t\tPerformance curve of MEMS fuel cell system.
Specific energy density of the micro fuel cell system was calculated to compare with the state-of-art batteries. First, the overall energy budget for operation of the fuel cell system was calculated. Figure 22 presents the energy specification of each reaction step.
\n\t\t\t\tEnergy budget for a fuel cell system.
The 20 W fuel cell system requires the hydrogen of 0.42 mol/hr. Thus, methanol feed rate of 0.219 mol/hr is required, assuming 95% methanol conversion and 95% hydrogen selectivity of the reformer. The energy requirement of the reformer consists of sensible heat, vaporization heat, and endothermic reforming reaction heat as given below:
\n\t\t\t\tThe total energy input for the methanol reformer is 9.956 W. The catalytic combustor generates 10.658 W heat energy with the fuel cell off-gas of 0.163 mol/hr, which is greater than the reformer energy requirement. It means that the fuel cell system can be operated without the additional heat supply to sustain the methanol reforming reaction.
\n\t\t\t\tThe methanol storage of 4.386 moles is required for the duration of 20 hours (0.219 mol/hr × 20 hr). The water feed requirement is 0.241 mol/hr at the steam-to-carbon ratio of 1.1, thus the water storage is 4.825 moles (0.241 mol/hr × 20 hr). These translate into 140.49 g (178.97 cc) methanol, and 87.093 g (87.25 cc) water, respectively. Therefore, the net fuel mixture storage requirement would be 227.58 g or 266.22 cc.
\n\t\t\t\tThe specifications of the fabricated fuel cell are: mass of 0.5 g, volume of 2.7 cc, active area of 4 cm2, and power density of 180 mW/cm2. Thus 20 W fuel cell would have a mass of 13.89 g (0.5 g × 20 W / (0.18 W/cm2 × 4 cm2)) and a volume of 75 cc. The specific power density of the micro reformer was 0.34 W/g or 1.25 W/cc. The reformer would have a mass of 59.62 g and a volume of 16 cc for 20 W fuel cell to be operated in the sufficient hydrogen supply. Therefore, the mass and volume of the total system were 301 g and 357 cc, respectively.
\n\t\t\t\tThe energy storage capacity was 400 W∙hr (20 W × 20 hr). So, the fuel cell system would have a weight specific energy density of 1329 W∙hr/kg and a volume specific energy density of 1120 W∙hr/L, which are values 10 times higher than the state-of-art of rechargeable batteries. The system energy density as the duration is shown in Fig. 23. The water production rate in the fuel cell was 0.42 mol/hr, which is greater than the water supply of the reformer (0.241 mol/hr) as shown in Fig. 22. Thus, the water from the fuel cell can be recycled into the reformer, improving the system energy densities. The specific energy densities for 10 days duration would be 2728 W∙hr/kg and 2144 W∙hr/L, respectively. It means that the micro fuel cell system can be an ideal alternative solution for portable micro power sources in the future.
\n\t\t\t\tSystem energy density as a function of the duration.
The design, fabrication and performance evaluation of micro methanol reformer integrated with a heat source were described in this chapter. The micro methanol reformer consists of the steam reformer, the catalytic combustor, and the heat exchanger in-between. The two heat sources for the reformer were used; one is the hydrogen catalytic combustion and the other is the hydrogen peroxide decomposition.
\n\t\t\t\tAll reactions, the methanol reforming reaction, the hydrogen combustion, and the hydrogen peroxide decomposition, are the catalytic process. Cu/ZnO was used for the reformer and Pt for the catalytic combustor. The porous ceramic material was used as the catalyst support to enhance the catalytic surface area. The catalytic microreactor was fabricated on five photosensitive glass wafers; top and bottom covers, a reformer layer with Cu/ZnO/support insert, a combustor layer with Pt/support insert, and a heat exchanger layer in-between.
\n\t\t\t\tThe performance of the reformer complete with the catalytic combustor was measured. The methanol conversion was 95.7%, and the thermal efficiency was 76.6%. The reformate gas flow including three major elements, 74.4% H2, 24.36% CO2, and 1.24% CO was 67.2 ml/min. The hydrogen flow in the reformate gas was the sufficient amount to run 4.5 W PpngC.
\n\t\t\t\tThe performance characteristics of the methanol reformer with the hydrogen peroxide heat source was investigated. The methanol conversion over 91.2% and the hydrogen selectivity over 86.4% were obtained. A modified thermal efficiency using the reaction heat of hydrogen peroxide instead of the LHV was defined and the thermal efficiency of the system was 44.8%. The reformate gas flow including 74.1% H2, 24.5% CO2 and 1.4% CO was 23.5 ml/min. This hydrogen was the sufficient amount to run 1.5 W PpngC. The performance of the present methanol reformer can be further enhanced by using hydrogen peroxide with higher concentration.
\n\t\t\t\tThe microreactor for the PROX reaction was fabricated using the photosensitive glass process integrated with the Pt/Ru/γ-Al2O3 sol-gel coating process. The carbon monoxide in the reformate gas was removed to use directly in the micro fuel cell.
\n\t\t\t\tThe micro fuel cell was fabricated and connected with the micro reformer and PROX reactor.
\n\t\t\t\tThe power density of the micro fuel cell system was 184 mW/cm2 at the potential of 0.64 V and is lower than that in the case of pure hydrogen test, because the reformate gas included the undesired CO, CO2, and N2.
\n\t\t\t\tThe system energy density of the micro fuel cell system integrated with the methanol reformer was calculated. The overall energy budget was calculated to operate the reformer-combined fuel cell system. The system energy storage density of the micro fuel cell system was obtained in the range of 1329 W∙hr/kg to 2728 W∙hr/kg. It is estimated that the micro fuel cell combined with the micro reformer has the energy density of up to 10 times higher than existing batteries, thus expecting to appear in the mobile energy market of the future.
\n\t\t\tAlthough the integrated methanol reformer developed in the present study can be used directly to operate the micro fuel cell, several works may be continued such as a fully integrated microfabrication, thermal packing, and optimization.
\n\t\t\t\tThe micro reformer should be insulted thermally to obtain the high thermal efficiency and the low package temperature of the micro fuel cell system. The excess heat loss of the reformer makes the catalytic combustor difficult to sustain the methanol reforming reaction. The thermal insulation of the reformer facilitates the integration of the reformer with the micro fuel cell at the low package temperature. The heat loss through conduction and convention can be prevented by the vacuum packaging technology using an anodic bonding process. The thermal design of the micro reformer through the extensive modeling of the heat transfer will be preceded to improve the overall thermal efficiency of the micro fuel cell system.
\n\t\t\t\tThe fully integrated microfabrication of the micro fuel cell system is the next challenge to improve the system packaging efficiency. The batch fabrication of all elements including the micro reformer, PROX reactor, and micro fuel cell can reduce the fabrication cost. The overall integrated design of the micro fuel cell system should be optimized in consideration of the thermal balance and fluidic interconnections between the reactors. The micropump, microvalve, and control circuitry will be integrated with the micro reformer and micro fuel cell in the future.
\n\t\t\ta - Molal ratio of hydrogen peroxide to methanol
\n\t\t\tCp- Constant pressure specific heat, kJ/mol-K
\n\t\t\tLHV - Lower heating value, kJ/mol
\n\t\t\tO2/C - Oxygen-to-carbon ratio
\n\t\t\tS/C - Steam-to-carbon ratio
\n\t\t\ts - Molal ratio of water to methanol
\n\t\t\tWHSV - Weight hourly space velocity, mol/g-h
\n\t\t\tx - Molal concentration of hydrogen peroxide
\n\t\t\tηT- Thermal efficiency
\n\t\t\t∆HR- Heat of reaction, kJ/mol
\n\t\t\t∆HV- Vaporization heat, kJ/mol
\n\t\tThe field of plasmonics has advanced immensely over the years and is spreading more and more into the area of sensor technology [1, 2, 3, 4]. This is due to the unique interaction of light with noble metals [5]. The excitation of conduction electrons to perform collective vibrations, both in volume as well as at the surface, shows in brilliant colors, which have fascinated people since the medieval times. Countless prominent examples of art objects still exist today, such as the Lycurgus Cup (4th century AD) and stained-glass windows of cathedrals.
As early as 1857, Michael Faraday published his groundbreaking findings (The Bakerian Lecture of the Royal Society of London [6]) on the experimental interactions of gold and other metals with light [7]. “Light has a relation to the matter which it meets with in its course, and is affected by it, being reflected, deflected, transmitted, refracted, absorbed, etc. by particles very minute in their dimensions [6].” He studied the emergence of different colors for fluids containing gold reduced to diffused particles and described the metallic character of the divided gold: “Hitherto it may seem that I have assumed the various preparations of gold, whether ruby, green, violet, or blue in color, to consist of that substance in a metallic divided state [6].”
Between 1900 and 1920, the famous contributions of James Clerk Maxwell Garnett [8], Gustav Mie [9], Richard Gans [10, 11], and Richard Adolf Zsigmondy [12], just to name a few, proved that plasmonic colors were based on optical resonances that occur for particles smaller than the wavelength of light and that can be theoretically described and precisely predicted [13, 14, 15]. This birthed the field of colloidal plasmonic nanoscience [16]. During the last few decades, a variety of different nanostructures, both in the form of individual NPs and particle assemblies [17], have been developed with a special focus on their use as colorimetric or SERS spectroscopy sensors [18, 19].
Today’s pronounced diversity of available nanostructures demonstrates the high level of interest in these optically functional materials. Within the scope of this book chapter, this diversity can, of course, only be covered to a limited extent. For this reason, the focus here is on NPs as defined building blocks for discrete nanostructures by guided self-assembly [20, 21, 22]. Figure 1 provides a rough overview of functional structures which have been found to be particularly suitable for sensor applications. The first area is represented by individual nanocrystals, for example, in spherical, rod-shaped, triangular, or star-shaped morphologies with strong intrinsic electromagnetic hot spots. This can be extended by coupling these particles to a metal surface or thin-film, the formation of branched structures by overgrowth, or hollow, core/shell, and nested structures with nanoscale interior gaps. Nanoparticles at short distances form extrinsic hot spots by strong electromagnetic interactions [23], referred to as plasmonic coupling [24]. Even in the disordered state, particle aggregates produce strong field enhancements. However, it is the ordered assembly of particles that allows plasmonic hybridization to emerge [25]. Hybridization can result in highly sensitive modes for spectral shift sensing and intense near-field enhancement for chemical sensing [14]. Again, the diversity ranges from discrete single nanoclusters with uniform coordination numbers to complex superstructures, supercrystals, and patterned structures both in 2D and 3D, which can be fabricated by colloidal engineering.
Schematic overview of the diversity in plasmonic nanoparticles and -structures: (top, left) nanocrystals, film-coupled particles, and hollow/nested morphologies on single particle level; (top, right) disordered assemblies of aggregated NPs; (bottom) ordered assemblies of discrete NP oligomers, clusters, and defined superstructures in 2D and 3D.
In this chapter, we will first address the field of plasmonic colorimetric sensing. Here, the fundamental concepts for sensing of surface plasmon resonances of metallic thin films are reviewed and then extended to localized surface plasmon resonances [26] of NPs and ordered NP assemblies by guided self-assembly [20, 21, 22]. Second, we will review SERS analytics divided into several steps: the first principles of SERS [27] and off-resonance excitation, SERS analytics of dispersed particles with a focus of the tasks of functional shells, SERS analytics using disordered aggregates under controlled conditions, and finally ordered assemblies designed for high SERS activity [28, 29].
To begin, we will first briefly review SPR spectroscopy of thin metallic films as a foundation for LSPR spectroscopy. Colorimetric sensing describes the optical determination of chemical or physicochemical properties of a sample. Subsequently, we will discuss concepts in which the plasmonic response is used to obtain information at interfaces or at the near-field environment of metal structures, which would otherwise be inaccessible.
Surface plasmons (SPs) are electromagnetic waves, emerging from surface plasmon resonances (SPRs), that propagate along the surface or interface of a conductor, usually a metal/dielectric interface [30]. Essentially, surface plasmons are light waves trapped at interfaces because of their strong interaction with the free electrons of the conductor. Surface structuring can guide this interaction [31]. The response of the free electrons takes place collectively in the form of oscillations in resonance with the light wave. The consequent charge density oscillation at the surface leads to a concentration of light and thus an enhancement of the local electric nearfield. The high sensitivity of this light-matter interaction renders it attractive for sensing applications. SP-based sensing builds on a simple resonance condition:
The resonance condition requires the SP mode (with frequency-dependent wave-vector ksp) to be greater than that of a free-space photon of the same frequency (free-space wave-vector k0). In addition, for SPRs, the frequency-dependent permittivity of metal (εm) and dielectric (εd) need to be of opposite signs. As a consequence, the SP resonance phenomenon has been employed for biochemical sensing [32] and clinical diagnosis [33]. Under appropriate conditions, the reflectivity of a thin metal film is extremely sensitive to changes in the local refractive index environment. Figure 2A (left) shows an exemplary SPR sensor in a fluidic channel in Kretschmann configuration using a prism for coupling p-polarized light into the metallic film interface [34]. The resulting evanescently decaying field reaches beyond the metallic interface into the sensing medium. When the SPR condition is satisfied, the reflection spectrum for monochromic light shows a characteristic resonance dip (Figure 2A, right). Here, the resonance condition is
where n denotes the refractive indices of the dielectric prism (p) and the sensing medium (s), λ is the wavelength in free space, and θ is the incident angle of light [34]. Thus, changes of the refractive index (Δn) of the sensing medium will shift the resonance dip by altering the resonance angle (Δθ) and/or the resonance wavelength (Δλ). The sensing of this resonant spectral response can be realized in different micro- and nanostructured sensor configurations (e.g., prism-, waveguide-, channel-, grating-based setups) [35]. Oates et al. demonstrated that the established methods of SPR spectroscopy for chemical and biological sensing can be enhanced by using the ellipsometric phase information [31]. Next, we focus on colorimetric sensing using plasmonic NPs.
Fundamental concepts of colorimetric plasmonic sensing. (A) SPR sensor in a fluidic channel. Copyright 2011 MDPI, adapted with permission [34]. (B) Pregnancy test using AuNPs as inert dye. (C) Refractive index sensitivity of an AgNP-based optical sensor. Copyright 2003 ACS, adapted with permission [44]. (D) LSPR sensor concept for enzyme-guided inverse sensitivity [45]. (E) Dual-responsive hydrogel/AuNP hybrid particles. Copyright 2016 NPG, adapted with permission [47]. (F) Protease-triggered dispersion of AuNP assemblies. Copyright 2007 ACS, adapted with permission [48].
The transition from SPR to localized surface plasmon resonance (LSPR) sensing is accompanied by the step from sensors using metallic thin-films to nanosensors in the form of particulate matter [36, 37, 38]. The plasmon generated on a small nanoparticle, for example, a sphere, experiences strong spatial confinement because of its hindered and limited propagation [39]. This confinement, also known as localization, results in discrete charge density oscillations [9, 40], which manifest themselves by intensive colors [41]. The excitation frequency of localized plasmons (absorbance band) is highly sensitive for the size, shape, composition, and refractive index environment of the NP. Though LSPRs were capitalized for various nanophotonic applications covering many fields [16, 26, 42], this chapter is limited to their use as sensor elements. For this purpose, we briefly survey the most commonly used and prominent concepts for colorimetric sensing.
Figure 2 summarizes the fundamental colorimetric sensing concepts using single NPs and disordered NP assemblies. The first example shows the working principle of a pregnancy test for which AuNPs (conjugated to anti-hCG antibodies, blue) serve as an inert dye to detect the presence of hCG antigens (green, Figure 2B) [43]. The test is basically a lateral flow sandwich immunoassay consisting of a test line with anti-hCG antibodies (violet), a control line with immunoglobulin G (IgG, red) antibodies, and a mixing with immobilized anti hCG-conjugated AuNPs. By application of a urine sample, the NPs bind to available hCG antigens (which are indicative for a pregnancy). This is followed by the selective binding of AuNPs to the control and test line, while the latter only happens in the presence of hCG antibodies. In this example, the AuNPs serve as an inert dye which does not interfere with the antibody–antigen binding by biorecognition and possesses high chemical stability.
Figure 2C highlights the refractive index sensitivity of an AgNP-based optical sensor in various solvent environments (left) [44]. Van Duyne et al. found a linear relationship between the refractive index environment and the LSPR position. This enabled to detect the adsorption of fewer than 60,000 1-hexadecanethiol molecules on single AgNPs which corresponded to a 41 nm shift followed by dark-field spectroscopy. However, for ultralow concentrations, the variations in the physical property (e.g., LSPR shift) become increasingly smaller, and thus, harder to detect with confidence. Contrary to conventional transducers which generate a signal that is directly proportional to the concentration of the target molecule, Stevens and coworkers proposed an LSPR sensor with inverse sensitivity (Figure 2D) [45]. The key for this inverse sensitivity is the enzymatic control over the rate of nucleation of Ag on Au nanostars (top: overgrowth; bottom: nucleation), accompanied by a blueshift of the LSPR. Different biosensing strategies have been proposed building on enzymatic reactions and NPs [46].
Apart from dispersed NPs, the plasmonic coupling between NPs, which is dependent on their spatial interspacings, can be utilized for colorimetry. Song and Cho reported dual-responsive architectures by mixing hydrogel and AuNP-decorated hydrogel particles (Figure 2E) [47]. This hybrid ensemble responds to both temperature and ions by means of a volume and color change in aqueous systems. Both stimuli can be used to reversibly trigger the transition of uncoupled well-separated AuNPs (red tint) to a state that allows for plasmonic coupling (blue tint), mediated by the hydrogel matrix. Ulijn and Stevens et al. demonstrated the bioresponsive transition from aggregated to dispersed state [48]. Figure 2F shows the protease-triggered dispersion of AuNP assemblies using thermolysin for the removal of attractive self-assembly groups and revelation of repulsive charged groups. The consequent blueshift of the LSPR allowed for simple and highly sensitive detection of the presence of thermolysin, which could be tailored for different proteases.
Another approach builds on measuring the orientation of a sample. For this, it is necessary to align the ensemble of NPs macroscopically. This was achieved for anisotropic NPs homogenously dispersed in an elastic polymer matrix [49, 50]. By uniaxial stretching of the material, the NPs are oriented along the direction of elongation. As a result, the material exhibits uniform plasmonic response, which enables for optical detection of the orientation of the material. Continuing on, we will examine assemblies containing ordered NPs and patterns.
Figure 3 highlights colorimetric sensing examples of ordered NP assemblies and defined patterned superstructures. Because the LSPR depends on the local dielectric environment at the NP surface, the LSPR shift can be evaluated to detect changes in effective refractive index. The first example is a macroscopic plasmonic library consisting of well-separated non-coupling AuNPs with a gradient in size, induced by Au overgrowth [51]. Along the array, the size increase goes along with a color change from colorless to pink (Figure 3A, left). Aided by electromagnetic simulations, it was possible to evaluate the effective refractive index and thus changes in the local density of the hydrogel shell around the substrate-supported particles (right). The initial increase of the refractive index indicated a densification of the hydrogel network upon particle growth from 10 to 30 nm. The subsequent decrease above 30 nm might result from internal breakup/rupture of the network.
Colorimetric sensing concepts of ordered NP assemblies and defined patterned superstructures: (A) Effective refractive index of a substrate-supported array of particles with a gradient in sizes. Copyright 2014 ACS, adapted with permission [51]. (B) Core/satellite assemblies for highly sensitive refractive index sensing. Copyright 2015 ACS, adapted with permission [52]. (C) Biomolecular detection by disassembly of core/satellite assemblies. Copyright 2011 ACS, adapted with permission [54]. (D) Stress memory sensor based on disassembly of AuNP chains. Copyright 2014 ACS, adapted with permission [56]. (E) Mechanochromic strain sensor based on AuNP-decorated microparticles dispersed in a polymer matrix. Copyright 2017 Wiley, adapted with permission [59]. (F) Reversible strain-induced fragmentation of quasi-infinite linear assemblies to defined plasmonic oligomers. Copyright 2017 ACS, adapted with permission [62].
Sönnichsen et al. proposed the use of core/satellite assemblies for highly sensitive refractive index sensing [52]. Figure 3B (left) shows 60 nm AuNPs as cores linked to 20 nm AuNPs as satellites. The average number of satellites allowed tuning the LSPR from 543 to 575 nm. The core/satellite nanostructures showed about twofold higher colorimetric sensitivity (Δλ/Δn) than similar sized gold NPs (right). Lee et al. developed a theory-based design of such core/satellite assemblies to optimize the spectral shift due to satellite attachment or release. They provided clear strategies for improving the sensitivity and signal-to-noise ratio for molecular detection, enabling simple colorimetric assays [53]. Figure 3C depicts the disassembly of substrate-supported core/satellite assemblies for biomolecular detection [54]. By addition of trypsin, the cysteine/biotin-streptavidin peptide tethers were proteolytically cleaved to release the satellites into solution enabling colorimetric detection of the protease.
Different concepts have been developed for opto-mechanic sensitivity and control [55]. Yin et al. reported a stress-responsive colorimetric film that can memorize the stress it has experienced (Figure 3D, left) [56]. This stress memory sensor is based on the LSPR shift associated with the disassembly of chains of AuNPs embedded in a polymer matrix (middle). By plastic deformation, the LSPR experiences a blueshift by irreversible breaking of the linear AuNP assemblies, initially formed in colloidal suspension [57]. The sensitivity of the optical change to stress could be tuned by doping with different amounts of PEG as plasticizer (right) [56]. Instead of mixing NPs with an elastic matrix, AuNPs can also be grown at the surface of a flexible substrate [58]. This enables mechanical control of the plasmonic coupling and electromagnetic fields at the surface. Dreyfus and coworkers designed mechanochromic AuNP-decorated microparticles as strain sensor (Figure 3E) [59]. After dispersion in an elastic polymer matrix of PVA, the capsules change in color upon elongation. When the film is stretched, the capsules are deformed into elongated ellipsoidal shapes and the distance between the AuNPs, embedded in their shells, concomitantly increases. Another mechanoplasmonic approach has been proposed for substrate-supported chains of AuNPs. Figure 3F (left) shows oriented linear assemblies, above the so-called infinite chain limit [60, 61], in a periodic pattern over cm2 areas on an elastic support [62]. Upon external strain, the assemblies experience a transition from long to short chains by reversible strain-induced fragmentation. The transition from plasmonic polymers to oligomers was accompanied by a pronounced spectral shift (right). A similar strain sensing approach was reported by Minati and coworkers using 1D arrays of broader line widths [63]. These multiparticle arrays showed a blueshift of the reflectance, lineally scaling with the external strain. Here, we will leave the field of colorimetric sensing and turn our attention to the enhancement of the electric field and the concomitant SERS activity.
Since its discovery by Martin Fleischmann, Patrick J. Hendra, and A. James McQuillan in 1974, surface-enhanced Raman scattering has become an indispensable tool for analytical chemistry [64]. In this study, two types of pyridine adsorptions have been identified at Ag electrodes [65]. At the same time, the detected signal strengths were far beyond what could be expected to arise from local concentration of absorbed molecules. In 1977, two enhancement theories were proposed independently, namely chemical enhancement [66] by Albrecht and Creighton and electromagnetic enhancement [67] by Jeanmaire and Van Duyne. Here, we will limit our discussion on the electromagnetic theory, because it is most closely linked to the optical and structural properties of nanocrystals and their assemblies. After an initial review of the origin of SERS enhancement [68, 69], we will go over prominent examples for the application of SERS for chemical spectroscopy.
The Raman process describes the inelastic scattering of light by atoms or molecules discovered by C. V. Raman in 1928 [70]. This process had already been proposed theoretically by Adolf Smekal in 1923 [71]. In principle, Raman scattering builds on the interaction of a photon and a molecule (or crystal) for which an energy transfer can occur (Figure 4A) [72]. However, the Raman process has an inherently low cross-section because the probability of this transfer is very low and only 1 in 107 photons are scattered inelastically. For that reason, long measurement times are necessary to compensate for the low photon yield. Therefore, the low time resolution and the limitation of the spatial resolution demand on excellent optics. The SERS effect dramatically improves the photon yield, overcoming these limitations. To clarify this, it is necessary to look at the origin of the enhancement [68].
First principles of surface-enhanced Raman scattering: (A) schematic of the two-photon non-resonant stokes scattering between two vibrational states of a molecule (n = 0 - > n = 1) mediated by a virtual state v. A harmonic potential (solid line) approximates the energy landscape of the ground electronic level (dashed line). Copyright 2016 ACS, adapted with permission [72]. (B) Schematic of the twofold amplification process. (C) Single-molecule spectra of rhodamine Rh123 of AgNP aggregates (solid lines) reflecting the approximate shape of their corresponding Rayleigh scattering spectra (dotted lines). Copyright 2007 APS, adapted with permission [79]. (D) Comparison of individual single-molecule events to average signal (7500 spectra). Copyright 2010 ACS, adapted with permission [75]. (E) Electric field confinement of nanosphere, nanorod, nanotriangle, and core/satellite nanocluster; scaled to maximum visibility. Copyright 2016 ACS, adapted with permission [77]. (F) Selective excitation and spatial transfer of hot spots in dimer gap, in both junctions, or only in the particle−film junctions. Copyright 2016 ACS, adapted with permission [78].
Raman scattering is, in its most basic and phenomenological form, the emission of a molecular Raman dipole (λsca = λexc ± Δλ) [27]. Induced by the electric field Eexc of the exciting laser light λexc, a Raman dipole p0 oscillates at a Raman-shifted frequency (ωsca = 2πc/λsca).
For a randomly oriented Raman scatterer, the radiated power is proportional to |p0|2 [68]. Thus, in order to enhance the Raman signal either the polarizability α or the electric field E need to be magnified. The former refers to chemical enhancement, this is the modification of the local polarizability of the molecule (α0 -> αloc), whereas the latter is called electromagnetic enhancement EM and describes the augmentation and confinement of the local near-field (E0 -> Eloc). The enhanced dipole p can be written as
The dipole enhancement upon excitation can be expressed by
Likewise, the dipole enhancement upon emission, that is, the Raman scattering, is given by
Although it is convenient from a theoretical point of view to separate the excitation and scattering into two steps, the Raman scattering process is instantaneous and both occur simultaneously [27]. The combined enhancement MSERS contains both contributions of the twofold amplification process (Figure 4B) [73]: the augmented excitation of a molecular dipole by an incident photon (λexc) and the stimulated emission of a scattered photon (λsca) of higher (anti-Stokes, λsca < λexc) or lower energy (Stokes, λsca > λexc).
Often, the total enhancement is simplified by the so-called |E|4-approximation, which neglects the Raman shift. However, this approximation needs to be treated with caution as many cases have shown [74].
The questions of why and how the shapes of SERS spectra change has been addressed by Yamamoto and Itoh [73]. The transformation of the Raman spectrum, in principle the Raman cross section σRS, by the SERS process can be described as follows:
If Eq. 9 holds true, this would enable the reconstruction of a SERS spectrum from the unenhanced Raman spectrum and an enhancement function MSERS(λ). Yamamoto and Itoh proposed that this reconstruction might be given by the product of the Raman spectrum times the Rayleigh scattering spectrum, based on observations using AgNP nanoaggregates (Figure 4C) [73]. They found that single-molecule spectra of rhodamine Rh123 (solid lines) were well reflecting the approximate shape of their corresponding Rayleigh scattering spectra (dotted lines) both for Stokes as well as Anti-Stokes scattering [43, 47]. Figure 4D shows a comparison of individual single-molecule events to average signal (7500 spectra) for Nile blue measured with Ag aggregates at high spectral resolution [75]. The ensemble averaging of many events revealed the characteristic broadening of the average spectrum (top) and fitted to Lorentzian line shapes (bottom).
All the reviewed examples, up to now, have capitalized from aggregated Ag nanocolloids as, for example, introduced by Lee and Meisel [76]. Such disordered aggregates enabled breakthrough results in regard to pushing the limit for ultrasensitive detection, down to single-molecule resolution, as well as nurtured the fundamental understanding of the origin of SERS enhancement. In the following section, we will turn our attention from disordered aggregated systems to individual nanocrystals and their ordered assemblies. The main aim is to find appropriate criteria for the design of nanostructures with both high and robust enhancement.
As a first step, we address the formation of hot spots, so-called confined locations with the highest electric field strengths. For individual nanoparticles, hot spots can be found at the poles of nanospheres (e.g., in respect to an excited dipolar mode, see Figure 4E, left) or near areas of high surface curvatures, as at the tips of nanorods and nanotriangles (middle). In multiparticle systems, hot spots form inside interparticle gaps, cavities, or crevices, as depicted for an exemplary core/satellite assembly (right) [77]. Depending on the nature of the excited LSPR modes and considering plasmonic hybridization in multiparticle systems, the distribution of hot spots can be quite diverse. For instance, the lighting of special hot spots has been demonstrated for film-coupled multiparticle configurations [78]. Figure 4F shows the basic example for selective excitation and spatial transfer of hot spots in a AuNP dimer. Depending on the excitation wavelength, the hot spots can be formed either in the dimer gap, in the particle-film junctions, or in both junctions at the same time [78]. Electromagnetic simulations can give valuable guidance for the design of targeted hot spots in plasmonic nanostructures, especially for complex systems [15, 23].
However, the design of NPs for SERS has shown to be more involved than simply maximizing the local electric field [80]. Murphy et al. initially reported the surprising finding of the highest SERS signals for LSPRs, blue-shifted from the wavelength of laser excitation. They studied an ensemble of Au nanorods of different aspect ratios (but uniform tip curvature) for which the longitudinal LSPRs shifted from 650 nm to 800 nm (Figure 5A, left). Contrary to expectations, the batches with matching of LSPR and laser excitation performed worse than off-resonant batches (right). This has been explained by a competition between SERS enhancement and extinction. The initial assumption that the LSPR is an adequate prediction for the best SERS performance has been shown to be incorrect.
Off-resonance SERS enhancement: Non-linear dependency of (left) optical properties and (right) SERS signal intensities for variably sized (A) nanorods and (B) nanotriangles. (A) Copyright 2013 ACS, adapted with permission [80]. (B) Copyright 2018 ACS, adapted with permission [81].
Further validation was provided by a similar approach, screening the correlation of Au nanotriangle size and SERS performance [81]. Upon overgrowth, the LSPRs showed a continuous shift toward near-infrared (Figure 5B, left). Again, the highest SERS enhancement did not coincide with matching of the LSPR to the laser wavelength. Instead, the highest SERS activity was identified in off-resonance conditions (right). To rule out the influence of possible differences in tip/edge sharpness, the nanotriangle morphology was characterized by a close correlation of electron microscopy (TEM, field-emission SEM) and small-angle X-ray scattering (SAXS) analysis [50, 77, 81] combined with 3D atomistic modeling. This revealed a uniform tip curvature for all sizes.
The assumption that aggregation might be the cause for the off-resonant SERS activity was thoroughly investigated. Optical spectroscopy (UV-vis-NIR) before and after addition of the Raman marker did not show any indication of aggregation. Reversible aggregation/association of particles could be ruled out because of the presence of a sufficient amount of stabilizing agent (CTAB) in the solution. The amount of Raman marker (4MBA) added was much lower than that of the available stabilizing agent (>50-fold molar excess). The marker was not expected to act as a molecular linker. Also, in the case of irreversible aggregation, one would expect a loss of particles over time, which was not observed. The video feedback of the Raman microscopy did not give any indication of possible formation of aggregates even after the extended laser exposure. The colloidal dispersions were found to exhibit high colloidal stability for the reported surfactant concentrations, and the SERS signals could be detected reproducibly. Thus, even if aggregation (below the detection limit) was present, it would be of negligible contribution and should not have affected the registered SERS spectra [81].
Consequently, in many situations, the hot spot intensity is not directly correlated to the optical properties; thus, the extinction maximum is often not the best excitation wavelength for SERS [81]. As a rule, an increase in SERS activity can be expected for situations in which the Stokes-shifted Raman signals lies in the low-energy shoulder of the LSPR band; in other words, when the laser line is shifted to the red, compared to the LSPR extinction maximum.
At this point, we move to the application of SERS for analytical purposes. The term “single or individual particles” indicates that the NPs are present in a dispersed state in colloidally stable dispersions and that aggregation is avoided. First, examples will be discussed that highlight the retrieval of chemical information at the NP surface. Then, we will address the specific tasks of the different shells and its versatile functions for SERS applications.
SERS analytics have proven to be a valuable tool for the detection, studies of the exchange and competitive adsorption, as well as localized chemical reactions of ligands at NP surfaces in dispersion.
Hafner et al. applied SERS analytics to study the structural transition in the surfactant layer that surrounds Au nanorods (Figure 6A, left) [82]. This was achieved by following the displacement of surfactant by thiolated poly(ethylene glycol). At the same time, they characterized the surfactant bilayer, revealing the absorbed layer of counterions at the gold surface Au+X−, the ammonium head group CN+, and the skeletal vibrations of the alkane chain of the surfactant (right). Building on these findings, Chanana et al. investigated the quantitative exchange of surfactant against protein (bovine serum albumin) on Au nanorods (Figure 6B, left) [83]. In the context of biotoxicity, the cationic surfactant (CTA+X−) needs to be removed from the solution and NP surface. This required analyzing the broad spectral range from 100 to 3100 cm−1, covering all the characteristic signals associated with the ligand exchange (right). This evidenced the complete surfactant removal, which is a key step toward safe bioapplication of protein-coated NPs.
Detection, exchange, competition, and chemical reaction of ligands by SERS analytics in dispersion: (A) surfactant bilayer characterization on au nanorods (counter ion au+X−, ammonium head group CN+). Copyright 2011 ACS, adapted with permission [82]. (B) Quantitative exchange of surfactant against protein on au nanorods. Copyright 2015 ACS, adapted with permission [83]. (C) Competition of aromatic surfactants (BDA+), non-aromatic surfactants (CTA+), and thiol ligands (4MBA) on au nanotriangles. Copyright 2018 ACS, adapted with permission [81]. (D) Hot electron-induced reduction of small ligands (NTP to ATP) on Ag core/satellite assemblies. Copyright 2015 NPG, adapted with permission [85].
The competition of aromatic surfactants (BDA+), non-aromatic surfactants (CTA+), and thiol ligands (4MBA) was studied on Au nanotriangles (Figure 6C) [81]. Contrary to expectations, first evidence for the nonquantitative nature of the ligand exchange of aromatic versus non-aromatic surfactants was found. Differences in binding affinity toward the NP surface are attributed to additional π-interactions of the electron-rich benzyl headgroup [84]. It was possible to detect a trace amount of BDA+ of 1–10 nM, which originated from the seed synthesis and could not be removed even after excessive washing with CTAB surfactant [81].
Schlücker and Xie applied SERS analytics to follow nano-localized chemistry on Ag core/satellite assemblies (Figure 6D, left) [85]. They reported the hot electron-induced reduction of small ligands (NTP to ATP) in the absence of chemical reduction agents. The reduction was shown to be dependent on the available halide counterions X− (middle, right).
The shell can provide for different functionalities, such as inertness, hosting of Raman markers, molecular trapping, harvesting/accumulation of analyte molecules, but also by means of improving the SERS activity.
The first examples represent chemically inert shells to protect SERS-active nanostructures from contact with whatever is being probed. This concept has been introduced by Tian et al. and entitled as shell-isolated NP-enhanced Raman spectroscopy (SHINERS) [86]. The silica shell acts as a nanoscale dielectric spacer. Such particles have been applied for the in-situ detection of pesticide contaminations on food/fruit [87]. Silica encapsulation can also be used to protect a molecular codification, a layer of Raman markers at the NP surface (Figure 7A) [88]. A silica shell enables further functionalization, chemical post-modification, and assembly of superstructures [89]. At the same time, silica overgrowth can stabilize superstructures as well as increase their colloidal stability and robustness (Figure 7A).
Functional shells for augmented SERS applications of single nanoparticles: (A) silica coating of dispersed AuNPs and overcoating of core/satellite superstructures. Copyright 2011 Wiley, adapted with permission [88]. (B) SERS-encoded NPs enclosed in silica. Copyright 2015 ACS, adapted with permission [90]. (C) Hydrogel-encapsulation for thermo-responsive molecular trapping. Copyright 2008 Wiley, adapted with permission [93]. (D) Branched morphologies grown inside radial mesoporous silica shells. Copyright 2015 ACS, adapted with permission [94]. (E) Schematic core/shell particle with nanoscale interior gaps and nanobridges [95]. (F) Interior nanogap within double-shelled au/Ag nanoboxes. Copyright 2015 NPG, adapted with permission [96]. (G) Protective au overcoating of AgNPs to increase their chemical stability. Copyright 2018 Wiley, adapted with permission [99].
Pazos-Pérez and Alvarez-Puebla et al. demonstrated the SERS encoding of particles. For this, the NP surface is first partially functionalized by a stabilizing ligand, followed by a codification with a SERS marker (Figure 7B), and the final overcoating with silica to create an inert outer surface layer [90]. Also, hyrogel shells have shown the potential for molecular trapping of analyte molecules [91, 92]. The partial hydrophobic/hydrophilic nature of poly(N-isopropylacrylamide) during the reversible volume-phase transition can harvest analyte molecules from solution. After accumulation, the induced collapse of the shell can serve to increase the local concentration of analytes at the NP surface (Figure 7C, right) [93].
The shell can serve as a means to improve the SERS activity of NPs. For example, as a template for the growth of complex NP morphologies. Liz-Marzán et al. developed an approach for the templated growth of branched Au structures inside of mesoporous silica shells (Figure 7D, left) [94]. The formed tips branching out from the cores (spheres, rods, triangles) improve the plasmonic performance (right) while favoring the localization of analyte molecules at highly SERS-active regions. Another example is the formation of nanoscale DNA-tailorable interior gaps in spherical core/shell particles (Figure 7F) [95]. Interestingly, it has been found that the presence of nanobridges between shell and core does not perturb the hot spot formation. In Figure 7G, this concept is adopted for cubic NPs [96]. For such shelled Au/Ag nanoboxes and nanorattles, a pronounced electric field confinement was found in the interior nanocavity [50].
Metal overgrowth can increase surface roughness by forming bumpy structures [97] or undulated surfaces [98], which both showed increased SERS activity. At the same time, metal overgrowth can increase the chemical stability of NPs. A sub-skin depth Au layer has been shown to enable oxidant stability and functionality without altering the optical properties of Ag nanocubes (Figure 7G) [99].
The field of particle assemblies is sophisticated and versatile. Here, we start with disordered aggregates and then progressively move from small discrete oligomers to large organized multiparticle assemblies in dispersion or supported on substrates. The examples shown are by no means to be considered universally valid but should serve as guidelines for a rational design of assemblies for SERS.
In this section, we briefly delve back into the field of disordered aggregates to explore to what extent these can be formed under controlled conditions for SERS analytics. As introduced in Section 3.1, aggregates can exhibit high SERS activities but suffer from problems concerning reproducibility and robustness. Several approaches addressed these issues and proposed conditions under which aggregation can be induced in a controllable manner.
Gucciardi et al. reported the aggregation of Au nanorods mediated by optical forces and plasmonic heating (Figure 8A) for SERS detection of biomolecules at physiological pH [100]. The formed disordered NP clusters can embed molecules from solution, even in buffered media at neutral pH. The dynamics of the signal increase during aggregate formation have been studied revealing different states (Figure 8B: onset, stabilization, size increase, and saturation). The laser scattering and bright field images showed that such aggregates can reach micron-scale dimensions after extended laser irradiation (over several tens of minutes). Aggregation can also be induced by specific binding, for example, glyco-conjugation of galectin-9 and glycan-decorated AuNPs. The real-time dynamic SERS sensing of galectin-9 in binding buffer, mimicking neutral conditions, revealed three different aggregation ranges, namely an early fast cluster growth, followed by a slower aggregation, before ultimately reaching the final equilibrium state [101]. Similarly, Mahajan et al. employed the selective guest sequestration of cucurbit[n]uril (CB[n]) molecules for molecular-recognition-based SERS assays (Figure 8C) [102]. These macrocyclic host molecules feature sub-nanometer dimensions capable of binding AuNPs. The aggregation produces clusters with a defined interparticle spacing of 0.9 nm. The entitled “host-guest SERS” builds on the capturing of analyte guest molecules inside the barrel-shaped geometry of CB[n] host molecules [102, 103].
SERS sensing based on the formation of disordered aggregates under controlled conditions: (A) laser-induced aggregation by plasmonic heating and (B) aggregation dynamics. Copyright 2016 NPG, adapted with permission [100]. (C) Molecular-recognition-based SERS assay by selective guest sequestration. Copyright 2011 ACS, adapted with permission [102]. (D) Droplet-based microfluidic setup for lab-on-a-chip SERS measurements. Copyright 2016 ACS, adapted with permission [104].
Lab-on-a-chip (LOC) SERS is an approach that is particularly well-suited for diagnostic applications. Figure 8D depicts a droplet-based microfluidic setup for LOC SERS measurements [104]. For measurements, equal amounts of Ag colloids and buffer/analyte solutions were mixed and, finally, 1 M KCl was added for inducing the aggregation in a controlled and reproducible manner [105]. Such microfluidic devices with a liquid/liquid segmented flow, where analyte-containing droplets are formed in a carrier liquid e.g. oil, show great potential for bioanalytics because minimal sample amounts are required and only short analysis times are needed, leading to significant cost reduction [106, 107, 108].
The ultimate goal is to obtain full control over the organization of NPs into discrete clusters. In other words, finding appropriate conditions under which the local association of particles proceeds in a controlled way. This would give access to uniform and discrete assemblies of defined coordination numbers both in 2D and 3D.
The first step toward this goal was achieved by efficient fractionation of 3D clusters by density gradient separation. After purification, the mixed ensemble of clusters was separated yielding pure cluster populations (Figure 9A) [109]. The obtained fractions of monomers (single NPs), dimers, trimers, tetramers, pentamers, and higher species were characterized for their optical properties as well as their capabilities for SERS. The SERS activity per assembly was found to depend on the coordination number, which determines the number of available gaps in a cluster, but also the respective contribution of each gap.
Ordered assemblies designed for high SERS activity. (A) Nanoparticle clusters of discrete coordination numbers. Copyright 2012 Wiley, adapted with permission [109]. (B) DNA-directed 2D core/satellite structures. Copyright 2012 Wiley, adapted with permission [110]. (C) Monolayer of close-packed nanotriangles. Copyright 2017 ACS, adapted with permission [118]. (D) AgNP-decorated silica microparticles. Copyright 2015 ACS, adapted with permission [119]. (E) Patterned supercrystal array of close-packed vertically aligned nanorods. Copyright 2012 Wiley, adapted with permission [124]. (F) Supercrystals of NPs ranging from the nano- to the microscale. Copyright 2012 ACS, adapted with permission [121]. (G) Pattern of pyramidal supercrystal architectures. Copyright 2017 ACS, adapted with permission [126]. (H) Macroscale patterns of quasi-infinite linear NP arrangements. Copyright 2014 ACS, adapted with permission [61].
For the case of substrate-supported “flat” clusters, Bach and coworkers fabricated core/satellite structures using DNA ligands to direct the self-assembly (Figure 9B) [110]. Such “nanoflower”-like 2D structures have shown improved SERS activity as compared to commercial SERS substrates. However, the critical parameter is the size ratio of core to satellite as it controls the coupling strength and plasmonic hybridization. For a high mismatch is sizes, this is for satellites of much smaller size than the core NP, the orbit of NPs is effectively uncoupled from the central NP. But the possible sensing applications of such structures go beyond the formation of hot spots for SERS. For matching building block sizes, Fano-like resonances have been predicted and experimentally observed for structures prepared by e-beam lithography [111, 112, 113]. Such resonances show the characteristic signature in the form of two LSPR peaks separated by a transparency window, a dip in extinction.
Building on this, one might ask what happens if such core/satellite patterns are formed on a curved surface, such as a microparticle, or if the substrate consists of different metallic layers. The latter results in a complex architecture of multilayered core/satellite assemblies surrounding Au/Ag rattles as cores [114]. The rattles exhibit intrinsic hot spots inside the interior cavity between the central rod-like Au core and the frame of Au/Ag alloy [50, 115, 116]. By adsorption of small NPs at the exterior, the LSPR band broadened and shifted further toward red and additional extrinsic hot spots were formed. This broadening was found to be controlled by the density of the satellite particles.
In general, the density of assemblies is a critical parameter for SERS activity. The interparticle distances determine the coupling strength. Here, the coupling threshold of the smallest, in other words, weakest coupling partner limits the interaction strength [117]. For a nanofilm, that is, a dense layer of close-packed particles, this limitation is overcome aided by collective long-range modes. For instance, a layer of nanotriangles prepared by film-casting and solvent evaporation shows nicely arranged edge-to-edge and tip-to-tip ordered arrangements in the monolayer (Figure 9C) [118]. The resulting extrinsic hot spots have been shown allowance for hot electron-driven catalytic reactions such as the dimerization of 4-NTP to 4,4′-dimercaptoazobenzene (DMAB), followed by SERS spectroscopy in a dry state. Radziuk and Möhwald reported close-packed nanofilms of 30-nm sized AgNPs on silica microparticles for intracellular SERS (Figure 9D) [119]. The primary (of individual microparticles) and secondary hot spots (between adjacent microparticles) were utilized for chemical imaging of live fibroblasts [120]. These AgNP-decorated microparticles have been compared to AgNPs with a 5 nm silica shell and silica NPs with a 5 nm thin Ag coating.
The nanofilm concept can be extended to thicker layers ultimately yielding supercrystals of close-packed NPs. Figure 9F presents a study in which supercrystals of NPs ranging from nano- to the microscale dimension have been realized [121]. During this transition, the LSPR band of the building blocks becomes increasingly broad because of higher polar modes. The formed superlattices exhibited pronounced interparticle coupling, whereas only the top layer is available for SERS sensing because of a limited penetration depth. However, patterning can also be utilized to discretize the electromagnetic response of the films/crystals. By comparison of different geometries of discrete supercrystals, it was found that patterning can have a deep impact on the surface electric field generation [122].
In addition to size, the shape of the used building block NPs plays an important role. For anisotropic particles such as nanorods, the local organization becomes essential. Organized nanorod supercrystals in lying-down [123] and standing configurations have shown significant contributions of side-to-side interactions between the close-packed NPs [124]. Bach et al. developed a method for directed nanorod assembly to prepare micropatterned substrates of vertically oriented nanorods (Figure 9F).
Finally, we turn our attention toward complex superstructures of organized NPs such as periodical arrays of pyramidal supercrystals. These superstructures accumulate the electric near-field at the apex of the pyramid [125]. The focalization can be expected to strongly depend on the “sharpness” of the pyramid but also on the internal order. Liz-Marzán and coworkers further developed the templated self-assembly to obtain “high quality” pyramids with highly ordered face-centered cubic lattices with one {100} facet oriented parallel to the pyramidal base and the {111} facets corresponding to the four lateral sides (Figure 9G) [126].
Further examples, highlighting the importance of structural order, are linear 1D assemblies with uniform interspaces. Here, lateral offsetting of NPs and inconsistent spacing can perturb the coupling along the chain [62, 127]. Figure 9H presents macroscale patterns of quasi-infinite linear NP arrangements [61]. The strong plasmonic coupling builds on the regular nanoscale spacing. Both single-line and double-line arrays have been shown to provide for uniform enhancement, with higher values for the single-line arrays [128, 129]. Likewise, the use of anisotropic NPs, which is more challenging, causes the LSPRs to shift further toward the red [130].
Ultimately, the challenge in ordered systems is the control of the interparticle spacings because the gap sizes correlate with the achievable field enhancements. Here, the coating of the NPs plays an essential role, as it may act as a guide for the self-assembly while at the same time act as a spacer in the final nanostructure [20].
This chapter gave an overview of plasmonics in the field of sensing. Starting with nanoparticles with specific morphological and optical properties up to the complex arrangements of these into ordered and even hierarchical superstructures, there is much diversity of accessible nanostructures. Despite this diversity, it must be considered that the structure-property relations need to be well understood to design functional materials with tailored properties.
For colorimetric detection, this represents the fabrication of uniform systems with high spectral sensitivity. Changes in particle size, shape, composition, and arrangement can be used for this purpose. This gives access to many possible applications in the field of biosensing, ion/temperature sensing, mechanosensing, to name a few. In each case, the plasmonic response of a system to a specific internal or external stimulus represents the central design criterion. For colorimetry, the task of plasmonics is very direct, meaning that the plasmonic response of a material is evaluated directly. For this reason, the plasmonic material/substrate is often closely linked to the analytical question.
Building on this, SERS spectroscopy demonstrates the power of surface-enhanced analytics. Here, plasmonics serve to provide the enhancement as a means to an end. The central task is the detection of chemical information—often independent of the actual plasmonic material. In fact, the analysis is, in many cases, blind to the plasmonic material. This opens the door for optimizing the nanostructure for higher signal yields. In this overview, we have found various structural approaches to increase the sensitivity. The confined and localized enhancement in these structures gives access to unprecedented details about the chemistry of and at material interfaces and nanoparticle surfaces [131].
As a final thought, one needs to bear in mind that some questions persist regarding the enhancement mechanism in action. For multiparticle systems and complex nanostructures, the prediction of hot spot localization and intensity becomes increasingly challenging. Several examples have shown the surprising circumstance of highest SERS response at unexpected conditions. In retrospect, it is likely that a fair share of SERS studies in the literature might be affected by particle aggregation. For that reason, targeted investigations are necessary to explain such phenomena. Also, the accurate prediction of SERS activity is still challenging both on a single-particle level as well as in multiparticle assemblies. In view of these points, besides increasing the sensitivity, the sensing robustness of the system must be a central design criterion in the development.
C.K. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 799393 (NANOBIOME). Honest gratitude is addressed to Andreas Fery for his fruitful suggestions, his critical thinking, and his continued support.
The author declares no competing financial interest.
IntechOpen aims to ensure that original material is published while at the same time giving significant freedom to our Authors. To that end we maintain a flexible Copyright Policy guaranteeing that there is no transfer of copyright to the publisher and Authors retain exclusive copyright to their Work.
',metaTitle:"Publication Agreement - Chapters",metaDescription:"IN TECH aims to guarantee that original material is published while at the same time giving significant freedom to our authors. For that matter, we uphold a flexible copyright policy meaning that there is no transfer of copyright to the publisher and authors retain exclusive copyright to their work.\n\nWhen submitting a manuscript the Corresponding Author is required to accept the terms and conditions set forth in our Publication Agreement as follows:",metaKeywords:null,canonicalURL:"/page/publication-agreement-chapters",contentRaw:'[{"type":"htmlEditorComponent","content":"The Corresponding Author (acting on behalf of all Authors) and INTECHOPEN LIMITED, incorporated and registered in England and Wales with company number 11086078 and a registered office at 5 Princes Gate Court, London, United Kingdom, SW7 2QJ conclude the following Agreement regarding the publication of a Book Chapter:
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The Corresponding Author (acting on behalf of all Authors) and INTECHOPEN LIMITED, incorporated and registered in England and Wales with company number 11086078 and a registered office at 5 Princes Gate Court, London, United Kingdom, SW7 2QJ conclude the following Agreement regarding the publication of a Book Chapter:
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\n\n6.1 Unless prevented from doing so by events outside its reasonable control, IntechOpen, in its discretion, agrees to publish the Chapter attributing it to the Corresponding Author and any Co-Author.
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\n\n6.3 IntechOpen is granted the authority to enforce the rights from this Publication Agreement, on behalf of the Corresponding Author and any Co-Author, against third parties (for example in cases of plagiarism or copyright infringements). In respect of any such infringement or suspected infringement of the copyright in the Chapter, IntechOpen shall have absolute discretion in addressing any such infringement which is likely to affect IntechOpen's rights under this Publication Agreement, including issuing and conducting proceedings against the suspected infringer.
\n\n7. MISCELLANEOUS
\n\n7.1 Further Assurance: The Corresponding Author shall and will ensure that any relevant third party (including any Co-Author) shall, execute and deliver whatever further documents or deeds and perform such acts as IntechOpen reasonably requires from time to time for the purpose of giving IntechOpen the full benefit of the provisions of this Publication Agreement.
\n\n7.2 Third Party Rights: A person who is not a party to this Publication Agreement may not enforce any of its provisions under the Contracts (Rights of Third Parties) Act 1999.
\n\n7.3 Entire Agreement: This Publication Agreement constitutes the entire agreement between the parties in relation to its subject matter. It replaces and extinguishes all prior agreements, draft agreements, arrangements, collateral warranties, collateral contracts, statements, assurances, representations and undertakings of any nature made by or on behalf of the parties, whether oral or written, in relation to that subject matter. Each party acknowledges that in entering into this Publication Agreement it has not relied upon any oral or written statements, collateral or other warranties, assurances, representations or undertakings which were made by or on behalf of the other party in relation to the subject matter of this Publication Agreement at any time before its signature (together "Pre-Contractual Statements"), other than those which are set out in this Publication Agreement. Each party hereby waives all rights and remedies which might otherwise be available to it in relation to such Pre-Contractual Statements. Nothing in this clause shall exclude or restrict the liability of either party arising out of its pre-contract fraudulent misrepresentation or fraudulent concealment.
\n\n7.4 Waiver: No failure or delay by a party to exercise any right or remedy provided under this Publication Agreement or by law shall constitute a waiver of that or any other right or remedy, nor shall it preclude or restrict the further exercise of that or any other right or remedy. No single or partial exercise of such right or remedy shall preclude or restrict the further exercise of that or any other right or remedy.
\n\n7.5 Variation: No variation of this Publication Agreement shall be effective unless it is in writing and signed by the parties (or their duly authorized representatives).
\n\n7.6 Severance: If any provision or part-provision of this Publication Agreement is or becomes invalid, illegal or unenforceable, it shall be deemed modified to the minimum extent necessary to make it valid, legal and enforceable. If such modification is not possible, the relevant provision or part-provision shall be deemed deleted.
\n\nAny modification to or deletion of a provision or part-provision under this clause shall not affect the validity and enforceability of the rest of this Publication Agreement.
\n\n7.7 No partnership: Nothing in this Publication Agreement is intended to, or shall be deemed to, establish or create any partnership or joint venture or the relationship of principal and agent or employer and employee between IntechOpen and the Corresponding Author or any Co-Author, nor authorize any party to make or enter into any commitments for or on behalf of any other party.
\n\n7.8 Governing law: This Publication Agreement and any dispute or claim (including non-contractual disputes or claims) arising out of or in connection with it or its subject matter or formation shall be governed by and construed in accordance with the law of England and Wales. The parties submit to the exclusive jurisdiction of the English courts to settle any dispute or claim arising out of or in connection with this Publication Agreement (including any non-contractual disputes or claims).
\n\nLast updated: 2020-11-27
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