Emerging Thin Films Electrochemical Applications: The Role of Interface

Abstract

Despite the natural cycle of climate change being unavoidable and the reality, history has been telling the living the previous civilizations that have overstressed themselves or pushed the consumption of natural resources to the limit, and the consequence soon shows the alert in the climate. It is a critical period of time to change the current in response to climate change by employing environmentally friendly and emission-free energy technologies. The applications of advanced functional thin films ranging from the quantum level to nano and microscale, from inorganic metal oxides to conductive polymers, have been pushing the rapid development of energy-saving technologies and clean and renewable energy production, storage, and conversion in the past decade. This specific chapter focuses on fundamental- and applied- science on various advanced thin films and their applications in reliable renewable energy devices and/or systems, including but not limited to electrochromics, supercapacitors, fuel cells, flow batteries, electrolysis, triboelectricity, etc. Given that much of the work is realized across interfaces, the spotlight is shielded onto the interface of thin films in electrochemistry with different emerging cutting-edge ongoing research examples.

1. Introduction

Nowadays, there are full of challenges and of course opportunities to respond to environmental change by replying on zero-emission clean energy technology developments. It is estimated about 40% of the primary global energy consumption lies in buildings, that is. lighting, heating, ventilation, and air conditioning (HVAC) sections [1, 2, 3, 4]. The understanding of taking the responsibility for reducing CO₂ emission is one of the main driving forces for the emerging and development of advanced electrochromic (EC) materials and devices today, particularly in terms of the utilization of smart windows or glazing for energy-saving houses, buildings, and skyscrapers [5] in cities. In the past decade, EC energy storage devices have become an emerging type of material in the rapid growing of miniaturized electronic devices, wearables, information encapsulation digital display, etc. [6, 7]. EC-based charging/discharging has gained elevated general public attention recently. Complementary to energy storage, they are able to adjust the relations between humans and sunlight utilizing their tunable and reversible optical response. EC glazing is estimated to reduce energy consumption by about 20% in the building sector [8, 9, 10, 11, 12, 13, 14, 15]. Charging capabilities and efficient energy derivation contribute to the energy recycling and have various functions from lighting and LEDs to wearable devices [16]. The visible charge level also provides much convenience to users. Most known renewable energy systems share concurrent electrochemical fundamentals, following thermodynamics but further restricted by kinetics. Enhancing the kinetics limited by nonideal conditions is dependent on the intrinsic understanding at as small a scale as we can.

Even though EC materials are being studied to obtain longer lifetime, the failure fundamentals are still unclear. A succession of chemical, physical, optical, electrical, and mechanical evolution in electrochemical energy devices arising from charge transfer (CT) kinetics has not been yet understood clearly. The potential future applications, with certainty, require a comprehensive deep digestion of the mechanisms beneath. The EC fundamentals and the durability issues should be mastered at a smaller scale. So far, the functional charging materials for small active ions (Li⁺)-based cells or systems have been found to be restricted by the instability events occurred at the interface across electrodes and electrolytes [17]. The across-interface activities, durability, and even the physical and nanoscale mechanical variations and related electrical changes remain poorly discovered, imposing a fundamental challenge for electrochemical CT in energy systems. Lots of inorganic transition metal-based oxide complex demonstrates a series of changes covering different aspects, from the physical and chemical, to optical, and even mechanical variations arising from electrochemistry. An EC case, WO₃, for example, changes its appearance from colorless to bright and deep blue colors upon reduction due to Li⁺/e⁻ dual incorporation. In addition to cathodic EC WO₃, other transition metal oxide, such as NiO is placed at the opposite position anode, switching between brownish and light status. In this electrochemical process, the mechanical stress/strain caused by the ion insertion/extraction brings non-neglectable volumetric expansion/shrinkage, which is termed “mechanical breathing.”

In recent years, elevated attention has been paid to renewable energy development, that is. environmentally friendly electrical vehicles for transportation [18]. Proton exchange membrane fuel cells (PEMFCs) demonstrate great potential for developing clean or green transportation technologies, based on oxygen and hydrogen electrochemical reactions by providing a renewable and zero-emission fuel source replacing the traditional combustion engine. However, one of the unaddressed performance and durability issues lies in the Nafion® membrane due to so-called harmful radical attack that brings many electro-mechanical negative effects in fuel cells [19]. These active radicals come from oxygen reduction reactions at overpotentials in this case the oxygen molecules associate with the catalyst, that is. platinum and travel to the polymer layer [20]. Such serious chemical attack is the main reason for polymer chain scission and physical and chemical irreversibility, causing the overall and regional mass loss of the membrane, which will cause producing fluorinated and sulfated species into reactant waste at the outlets. Synchronous fluorinated and sulfated degradation byproducts will accumulate at the exhaust. Because of chemical degradation, the fluoride emission quantity will significantly increase in a short period of time. Especially, the amount of the fluorinated products will significantly at certain accelerated operations, that is. higher temperatures and dry conditions. In one word, the fluoride and sulfate anions emission rates (FER; SER) can be defined as the signature of the membrane loss, and thus the fuel cell degradations status and a sign for predicting the remaining lifetime.

To precisely quantify the polymer breakdown, the fluoride anions of the wastewater coming from the fuel cell membrane are identified as degradation signatures. Detecting fluoride anions and converting the quantity as a sensor signal can be achieved based on specific fluoride-sensitive films. Fabricating highly selective sensors can be obtained by modifying the active sensing region; its integration in the electrical vehicle allows direct knowing ahead of the remaining life of the fuel cell polymers replacing the existing, wasteful, and costly methods of getting the fuel cell engine changed. Unfortunately, up to date, vehicle manufacturers advise users to change the fuel cell engine after covering a certain amount of miles or hours [21]. It may cause waste as potentially functional membranes in good condition can be changed or may be risky; in that case, a membrane in operation has had more degradation than what is theoretically projected according to the driving mileage or the time after experiencing intensive consumptions at extreme destructive real conditions in driving.

Benefiting from their lower cost, smaller size, robustness, and capabilities for continuous real-time monitoring, ion-sensitive field effect transistors have been promising microsensors. Leveraging the fast development of advanced sensor science and technologies, our team has designed and developed microsensors bringing opportunities for incorporating electrode outlets for showing the fuel cell state of health. The excellent leading signature for Nafion®-based fuel cells is the emission of fluoride anions (F⁻), coming along with the exhaust streams at the electrodes [22]. The F⁻ concentration is chosen as a signature, representing the membrane’s failure status in a precise way. Monitoring fluoride and converting its quantity into a voltage/current sensor readout is achieved via specific fluoride-sensitive nanofilms. It is a good idea to employ fluoride emission rate as a direct simplified but accurate diagnostic method. Specific fluoride-selective films (LaF₃) will be incorporated into a nanoscale layer of films for functionalization. The modification of the active area changes the selectivity/sensitivity of the developed sensors. It is different and complements to current fuel cell test and troubleshooting approaches. At present, some other methods for detecting fluoride exist; however, very few are compatible as a solution for real-time continuous monitoring of the fluoride emission rate (FER) in fuel cell test stands or electrical vehicles. Most of the existing tools are more delicate, costly, limited in detection, and not portable. The commercially available products fluoride ion selective electrode (FISE) is utilized to provide a reference and kind of ground truth to relatively compare the performance of the developed sensors. FISE is capable of detecting fluoride ions at ppb level and requires calibration operation, maintenance, and specific storage procedures, restricting its applications to fuel cells. It is of great significance to dedicate efforts to design and build microsensors for onsite real-time detection of the signature byproducts, and thus fuel cell membrane or polymer loss. The association of the F⁻ and fuel cell degradation has been established according to the ion quantity measurement to calculate the membrane degradation rate. The ions released from ionomer breakdown are targeted as fluorinated species, termed FER, which is being taken to tell fuel cell membrane failure degree [23].

In complementary to the interfacial studies for electrochemical performance and durability improvements, a unique and fascinating application is to implement the thin films and cantilever platform to envision the activities at the interface at extraordinary spatial and temporal resolution. The research activities regarding this topic are creative and original due to the synergy between the quantitative diagnostic sensor developments and energy-focused fundamental questions related to nanoscale electrochemistry and energy storage, highly demanding to date. When spectroscopy has opportunities to meet nanomechanics, it is hypothesized that the thermal-mechanical cantilever resonator (TMCR) in situ/Operando platform will be capable of decoding modern electrochemistry nanoverse based on its exceptional specificity/sensitivity [24]. This multimodal single utility TMCR is conceptually novel and is of great interest to the chemistry and electrochemistry community for nanometer and ultimate atomic scale studies. Its spatial and temporal resolution is up to sub-nm and μs, respectively; the mass sensitivity is six orders of magnitude higher than commercial quartz crystal microbalance (QCM) [25]; illuminating the infrared (IR) light can obtain compositional identification based on the photo-thermal nanomechanical effects, superior to Fourier transform infrared spectroscopy (FTIR) bounded by its opto-output loss [25]. Three focused characteristics of the versatile TMCR are (1) cantilever static bending and resonant dynamics, (2) nanomechanical holography enabled by multifrequency excitation/detection, and (3) IR spectroscopy through photo-thermal nanomechanical response. Uniquely, this single entity enables real-time simultaneous multi-signal output realized by orthogonal vectors. This real-time data acquisition not only surpasses the limitations of current ex situ state-of-the-art equipment but also consolidates multiple measurements into a single platform, promoting measurement efficiency and disseminating research accessibility.

2. Interfacial deformation- electrochromic monolithic ultra-thin-film device ITO/WO₃/Ta₂O₅:Li/NiO/ITO

Sustainability development of the society, clean energy demand, performance the cost are driving modern energy science and technologies fast growing. Much progress in these realms is made including electrochromic (EC) materials and devices by improving their performance and durability. Yet, ion insertion/de-insertion induced charge transfer (CT) nanomechanics, that is. repetitive electrode size change and generated stress/strain during electrochemical cycling, termed the “nanomechanical breathing” effect, has remained unexplored. Electro-chemo mechanics is the focus here due to its intimate correlation to the elastic and plastic deformation at the interface. EC transitional metal oxides with intervalence electrons and excellent electrochemical kinetics show dramatic color changes at various valence states upon redox reactions, which enable them as an emerging category of energy storage supercapacitors. WO₃ and NiO films are configurated as the cathode and anode, respectively. The void generation and delamination at the interface account for the device degradation after extended cycles. Nanoindentation mechanical test and electrical kelvin probe force microscopy are employed to investigate the interface. WO₃ with a charge density of ~40mC/cm² shows a ~45% increase in the elastic Young’s modulus compared to the pristine transparent state. The redox-induced surface stress originating from ion adsorption and CT is revealed. Noteworthy, upon charging (coloration; lithiation) with a fair smooth surface status giving about 3.4 times more electrostatic surface potential, the electrical work function of the films surprisingly becomes lower because of the dominant effect of the dipole layer potential against the chemical potential. From a physics point of view, it is suggested that the interatomic cohesive energy and equilibrium distance increase lead to the mechanical deformation in the long-term charging/discharging. The dependence of surface potential, stress, work function, and cohesive energy on electrochemical kinetics is interpreted. It provides fundamental insights into electro-chemo mechanics and interdisciplinary concerted interfacial effects at the nano/atomic level (Figure 1).

Figure 1.

Author’s work on electrochromic film interfacial nanomechanical deformation [26].

3. Interfacial manipulation- sensing purpose: Diagnostics tool for PFSA-fuel cells

Fuel cell electrical vehicles have drawn much attention due to the elevated environmental concern and governmental initiatives on hydrogen infrastructures recently. Perfluorosulfonic acid (PFSA) fuel cells have great potential to be employed for vehicles based on oxygen and hydrogen electrochemical reactions. Unfortunately, one of the limiting factors to the performance is the PFSA membrane. The radical attack and associated irreparable electro-mechanical damage in degradation result in the global and local thinning of the ionomer, causing fluorinated and sulfated degradation chemicals into reactant outlet streams; the options in the market lack specific diagnostics and a legitimate indication of the exact time that the membrane should be changed. To address the need, this work emphasizes developing an onsite sensor for quantifying the degradation by detecting fluoride in effluent water on the basis of surface functionalization and interface manipulation. The sensor’s specificity/sensitivity has been achieved in real-time at a sub 10 ppb level, relying on the spin-coating deposition of ~50 nm sensing membrane LaF₃ and post-annealing procedures. The multimodal data collection is obtained, including the characterizations of open circuit potential, cyclic voltammetry, chronoamperometry, and differential pulse voltammetry, demonstrating a consistent linear decrease of Faraday current from the established redox marker, while favoring the capacitive behaviors at the interface by absorbing F- ions (Figure 2).

Figure 2.

Author’s work on sensor developments based on the capacitive process at the interface [27].

4. Interfacial in situ/operando envision- spectroscopy meets nanomechanics: To decode interfacial nanoverse on a thermal-mechanical cantilever resonator (TMCR)

To decode the interface across electrode and electrolyte, an in situ/Operando envision tool is highly on-demand. It provides the potential to enable real-time simultaneous multi-signal output based on a conceptually novel single entity, a “thermal-mechanical cantilever resonator (TMCR).” TMCR in situ/Operando envisioned tool is creative and original due to the synergy between the quantitative diagnostic sensor developments and energy-focused fundamental questions related to interface, nanoscale electrochemistry, and energy storage, which is critically demanding to date. It is expected that thin-film-based TMCR will be capable of decoding interfacial charge transfer nanoverse based on exceptional specificity/sensitivity. Three targeted characteristics of the versatile TMCR are (1) cantilever static bending and resonant dynamics, (2) nanomechanical holography by multifrequency excitation/detection, and (3) infrared (IR) spectroscopy via photo-thermal nanomechanical response. It is complementary to conventional macroscopic characterizations (Figure 3).

Figure 3.

Thermo-mechanical cantilever resonator (TMCR): Multimodal single entity at micro-dimension as electrode/electrolyte interface envision tool.

Based on its nano/micro dimension, geometric feature, and thermomechanical effects, TMCR provides extraordinary sensitivity and orthogonal vector readouts. The spatial and temporal resolution is up to sub-nm and μs, respectively; the mass sensitivity is six orders of magnitude higher than commercial costly quartz crystal microbalance (QCM); illuminating the infrared (IR) light can obtain compositional identification through photo-thermal nanomechanics, superior to Fourier transform IR spectroscopy (FTIR) bounded by the opto-output loss. The ultrasensitive and informative TMCR is expected to reveal the electrochemical dynamics and unclear fundamental mechanistics at the nanoscale, generally uncaptured with classic macroscopic measurements. It will assist in decoding (i) real-time charge transfer nanomechanics, that is. repetitive electrode size change during electrochemical cycling, termed “nanomechanical breathing,” (ii) inhomogeneities, local ion concentration, and other locality properties based on nanomechanical holography, and (iii) quantifying minute changes in metastable intermediates, competing species, and impurities. TMCR real-time data acquisition not only surpasses the limitations of current ex situ state-of-the-art equipment but also consolidates multiple measurements into a single platform, promoting measurement efficiency and disseminating research accessibility, not limiting to the thin films and electrochemical energy device research.

5. Conclusions

This chapter focuses on the interface in thin films in electrochemistry and renewable energy applications, which involves fundamental science and challenges from the delamination issue at the interface, and thus the full device degradation, the electrochemical sensing applications based on the interactions and communications across the interface to an advanced envision measurement tool for capturing the minute changes occurred in the electrode/electrolyte. Bridging the gap between the electro-chemo nanomechanics and sensing capabilities at the interface with renewable energy applications would require interdisciplinary research with surface physics/chemistry, materials science, nanotechnologies, and engineering. Therefore, the research area can bring scientists and researchers from diverse backgrounds, including chemistry, physics, materials science and engineering, electrical engineering, and mechanical engineering.

Acknowledgments

This work is partially supported by the Million Mile Fuel Cell Truck (M2FCT) consortium, funded by the Department of Energy (DoE), US, under contract No. DE-AC02-05CH11231. The author would like to specially thank Dr. Thomas Thundat for the insightful discussions on thermal-mechanical cantilevers.

Conflict of interest

The authors declare no conflict of interest.