Graphene Nanosheets for Fuel Cell Application

Abstract

The unique chemical and physical properties of graphene enable it to be an alternative and better component in fuel cells as seen in recent studies. Fuel cells provide constant energy supply without any degradation of power over time with a continuous supply of fuel and show very little amount of emission of hazardous pollutants. Fuel cells can be a potent replacement for conventional petroleum energy sources. The limitations of producing efficient fuel cells are mainly high cost of materials, short lived membrane due to corrosion, cells being heavier in weight, efficient polymer electrolyte membrane (PEM), less efficient electrocatalysts, and slow reaction rates. To minimize all these limitations, graphene, a 2D allotrope of carbon, can be used due to its excellent properties; i.e., high surface area, high conductivity, high proton permeability, better electro catalytic performance, lower cost, greater corrosion resistivity and high bonding energy to hydrogen. In this chapter, the various components of fuel cells are discussed where graphene nanosheets and its derivatives (Graphene oxides and heteroatom doped graphene) are used to improve the fuel cells performance efficient.

1. Introduction

The use of global energy sources is extensively increasing due to the rapid growth and development of industrialization. The existing energy sources like coal, oil and natural gases are limited in nature, and energy production from these sources has a huge negative effect on the environment [1]. Energy production from renewable sources has much less impact on the environment but they also have some drawbacks such as low efficiency, higher cost [2]. The drawback led to the study of highly efficient energy sources, that is, fuel cell [3].

The significance of fuel cell is analyzed by the rapid rise of application worldwide in the past decade. A fuel cell generates electricity through an electrochemical reaction. It converts the chemical energy of a fuel and an oxidizing agent into electricity through redox reactions [2]. As the chemical energy of fuels converts into electricity by chemical reaction, it has a much higher efficiency than combustion cells along with low emission of pollutants; the by-products being only water [4]. Fuel cells need not to be charged, they can produce electricity under continuous fuel supply. If the waste heat can be utilized properly, up to 85% efficiency can be obtained from fuel cells [5].

According to the types of electrolytes used, fuel cells are classified into phosphoric acid fuel cell (PAFC), polymer electrolyte membrane fuel cell (PEMFC), alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC). Apart from these, there are different kinds of fuel cells, like direct methanol fuel cell (DMFC), and direct formic acid fuel cell (DFAFC) [1, 4]. Microbial fuel cells (MFC) can produce energy by microbial oxidation and can be used for wastewater treatment in a cost-effective and sustainable way [6].

A fuel cell mainly consists of bipolar plates (BPs), a cathode, an anode, and an electrolyte. The main reactions in a fuel cell include fuel oxidation on the anode and oxygen reduction on the cathode [1, 7]. The material selection for fuel cell components faces challenges including electrochemical performance, efficiency, economy, and durability [4]. The materials mostly used for bipolar plates include non-porous graphitic carbon and conducting polymers. This shows corrosion after a longer period of use and its mechanical strength is also poor. Metal and metal oxides (Pt, RuO₂, and IrO₂) are used as electrocatalysts. But the cost and availability of the materials limit their application. Also, RuO₂ oxidizes to RuO₄ under high anodic potential, which reduces the active surface area, therefore, decreasing efficiency. Self-poisoning created by the high adsorption rate of CO on the Pt surface reduces its efficiency as an electrocatalyst. Pt is also not a good electrocatalyst for organic fuels [3]. Electrodes like Pt/C after a certain period of use show corrosion of carbon and aggregation of Pt which decreases the efficiency of the fuel cell [8]. Sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion), dow membranes, per-fluorinated ionomers are mainly used as membrane or electrolyte in the fuel cell [2]. These polymers can work at an optimum rate under a certain temperature range. Changes in temperature can destabilize the system and decrease efficiency [9]. Their performance is also affected by hydrated conditions and fuel crossover issues [4].

A graphene nanosheet is a one-atom thick layer of sp² hybridized carbon arranged in a honeycomb structure. After its discovery in 2004, graphene is under research for different applications due to their excellent properties such as very high theoretical surface area (2600 m² g⁻¹), high mechanical strength (100–200 times stronger than steel), high thermal stability and excellent electrical conductivity (10⁶ S cm⁻¹), and excellent corrosion resistance [7]. Recent advancements in the use of graphene nanosheets for fuel cell applications have shown some promising results to overcome the problems discussed above related to the practical use of fuel cells [4]. As a result of the expansion of research, various derivatives of graphene are reported to be produced, like graphene oxide, multilayered graphene, porous graphene, porous graphene oxide, and impurity-doped graphene [10].

Graphite is the most used material for bipolar plates but its mechanical strength is poor and it comes with a higher fabrication cost. Metallic plates have much higher electrical conductivity but show higher corrosion rates in an acidic environment. Graphene with an anti-corrosive nature has been used as a protective layer for bipolar plates [11]. Graphene-coated copper plates showed 3 times more charge transfer resistance than uncoated copper after 720 hours of immersion in 0.5 mol L⁻¹ H₂SO₄ solution [12]. A sharp increase in electrical conduction was observed using graphene-coated Ni foam due to numerous micropores in the three-dimensional Ni foam. By this coating, the corrosion resistance of the bipolar plate is increased by almost 2 times [13]. Graphene-based materials can also be used as fillers to build up highly conductive and durable polymeric bipolar plates. A highly filled graphene/poly benzoxazine composite with 60 wt% graphene content, exhibiting high flexural modulus (18 GPa), flexural strength (42 MPa), thermal conductivity (8.0 W m⁻¹ K⁻¹), electrical conductivity (357 S cm⁻¹), and low water absorption (0.06% at 24 hours immersion) [4, 14].

One of the graphene derivatives, graphene oxide (GO), contains oxygen functional groups with very high surface area, which results in a higher transfer rate of protons through channels and holds the water for better water uptake. Various properties of graphene nanosheets such as electrical insulation, gas permeability, and hydrophilicity make it a promising material for composite membranes in PEMFCs [2]. Graphene composite membranes can work in a much higher temperature range than currently available membranes used. F-GO/Nafion shows almost 4 times higher proton conductivity than unmodified Nafion at 30% relative humidity and 120°C [15]. By incorporating 0.5 wt% ionic liquid polymer-modified graphene sheets, the ionic conductivity (7.5 × 10⁻³ S cm⁻¹ at 160°C) of sulfonated polyimide membrane was improved by about 2.6 times [16]. Graphene is flexible for working with proton-conducting groups like sulfonic acid to further facilitate proton transport [17]. A critical drawback of the Nafion membrane is the fuel crossover, especially when using methanol. Graphene can act as a barrier layer to stop fuel gas permeability without compromising the proton exchange action which increases overall cell performance [18].

Graphene and graphene-based materials like graphene oxides show structural properties which help them to possess carbonyl, hydroxy, and epoxy on their basal plane and the carboxylic groups at the edges. These oxygen-containing groups enhance graphene to make a desired structure to exhibit high conductivity and stability [8]. The PEMFC with Pt-Co/N-doped graphene cathode showed a four times higher maximum power density (805 mW cm⁻² at 60°C) than that of a commercial Pt/C cathode. Graphene-supported single-atom catalysts (SACs) show high oxygen reduction reaction activity and long-term stability in alkaline or acidic media [4]. Pt/NrEGO₂-CB₃ maintains 86.35% of the initial electrochemical surface area after 30,000 cycles compared to 2% of the electrochemical surface area of Pt/C [19]. atomically dispersed Ru on N-doped graphene exhibited higher ORR activity, better durability, and tolerance toward methanol and CO poisoning than commercial Pt/C catalyst in 0.1 molL⁻¹ HClO₄ [20]. The MFCs suffer from low power density and poor energy conversion efficiency due to the slow transfer rate of an electron from electron donors to electrodes producing electricity. Graphene having high conductivity and high biocompatibility can be used in MFCs to enhance electron transfer rate by increasing surface area and reducing the internal resistance [21].

In this chapter, the use of graphene nanosheets and its derivatives are described below for the various fuel cell components, such as, electrocatalyst, membrane, and bipolar plate.

2. Graphene nanosheets: synthesis, characterization, and properties

2.1 Synthesis of graphene nanosheets

The first graphene nanosheets were synthesized by extracting mono-layer sheets from three-dimensional graphite with Scotch tape using a technique called micromechanical cleavage [3]. Since then, a wide range of synthesis processes has been developed and have been broadly divided into two types; the bottom-up method and top-down method. The bottom-up method includes mainly chemical vapor deposition (CVD) on catalytically active metals, epitaxial growth on single crystal SiC, and arc discharge approach [3, 22]. The top-down method includes mechanical exfoliation, chemical exfoliation, ball milling, ultrasonic treatment, electrochemical exfoliation, liquid-phase exfoliation, and high-shear mixing [4, 23]. In chemical vapor deposition method, graphene nanosheets are prepared on transition metal substrates such as Ni, Pd, Ru, Ir, and Cu and the thickness is dependent on the carbon solubility in these metals and their cooling rate [22]. The transfer process of the CVD is time-consuming and contaminated whereas electrochemical-assisted de-lamination of the CVD provides a fast, environmentally friendly, and more controllable approach to graphene production [4]. Different substrates are used in CVD for graphene film growth; they include Ni, Fe, Cu, and stainless steel [24]. High-quality graphene can be produced on SiC wafers under high temperature [22]. This process is limited by the high cost of the single crystal SiC and, due to the wafer size, the graphene nanosheets are produced at a limited size [4]. In the arc discharge approach, carbon nanosheets are prepared by evaporating carbon sources at high temperatures [3]. In the top-down synthesis process, a single layer of graphene is obtained by mechanical exfoliation [4]. In chemical exfoliation, graphene nanosheets are obtained with fewer defects by a stable alkali metal salt intercalation compound which is industrially applicable [22]. In liquid phase exfoliation, graphene is derived from organic solvent with certain surface tension provided by external forces like ball milling, ultrasonic treatment, and shear mixing [3]. It can be noted that graphene can be synthesized by using various methods as reported in the literature. Moreover, its characterization is to be supported for the successful synthesis. The various characterization methods of graphene nanosheets are described in the subsequent section.

2.2 Characterization of graphene nanosheets

Characterization of graphene is an important aspect to understand its physical and chemical properties. The development of graphene and its derivatives were found through different physical and chemical methods. Characterization includes study of graphene morphology, properties, defects, and layered structure. The nanomaterials are dispersed in the matrix or substrate material through exfoliation and then studied [24, 25].

There are different characterization methods includes Fourier Transform Infrared Spectroscopy (FTIR), Solid-state nuclear magnetic resonance (SS-NMR), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, Thermo-Gravimetric Analysis (TGA), and different kind of microscopic technique including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) [26].

FTIR can be used to detect the functional groups of the specimen sample. In case of graphene oxides presence of different oxygen containing functional groups can be identified by the vibrational peaks obtained. The chemical changes in transition from edges to the inner region of GO flakes can be seen in the FTIR images [25, 26]. The electrical and mechanical properties can be varied with the presence of functional group. The SS-NMR is used to determine the properties of functional group doped graphene nanosheets. XPS provides some advantages over FTIR and SS-NMR in giving both basic information and quantification about the sample. It also shows the percentage of different oxygen containing functional group on the graphene surface [26]. Raman spectroscopy has an advantage in studying graphene with polar impurities. Three peaks (D, G, and 2D) obtained in Raman spectroscopy explains disorder in sp² hybridization, lattice vibration, and stacking degree of graphene respectively. TGA is used to study the graphene characteristics and its stability under different temperatures [24, 26]. SEM, TEM, and AFM are mainly used to analysis the surface morphology of the graphene nanosheets and its derivatives. It helps in the study of size of the nano particles and layered structure of graphene [24, 25, 26].

It can be seen that there are many methods to characterize the graphene nanosheet for its wide range of applications. Based on requirements, some of the methods are to be used to characterize graphene, which helps to choose the perfect graphene nanoparticles which provide expected results. Below the sections will discuss the various properties of graphene and its derivatives.

2.3 Properties of graphene nanosheets and derivatives of graphene

2.3.1 Properties of graphene nanosheets

Graphene and graphene derivatives exhibit exceptional physical and chemical properties, such as, large specific area, high electrical and thermal conductivity, greater durability, and chemical strength. These excellent properties make graphene one of the most appropriate materials for fuel cell components [4].

Graphene has a theoretical surface area of 2630 m² g⁻¹ which is very much higher than graphite powders. It has a mechanical strength of young modulus of 1100GPa. It shows stable properties even at a temperature higher than 1000°C [27]. It shows high electrical conductivity at an order of 106 S cm⁻¹ and electrical mobility of 200,000 cm² V⁻¹ s⁻¹at a carrier density of ~1012 cm². It has a breaking stress of 42 N m⁻¹ and excellent thermal conductivity (~5000 W m⁻¹ K⁻¹) [28]. The approximate thickness of a single layer of graphene is 0.345 nm. It is a transparent monolayer structure which shows properties of light absorption. In addition, the atomic C-C bonds at 284 eV and the wavenumber at 1550 cm⁻¹ have been recorded with the narrow scan of X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared (FTIR) spectra, respectively [10]. Good thermal conductivity helps in an application that requires strict heat management and the reactions exhibiting a strong exothermicity or endothermicity [29]. Other than graphene nanosheets, there are other derivatives, namely, graphene oxide and heteroatom doped graphene, which are greatly applicable in fuel cell research.

2.3.2 Properties of graphene oxide

Graphene oxide (GO) nanosheet is a single-layered structure usually produced by chemical oxidation of graphite. Oxygen functional groups like hydroxyl groups and epoxy groups are found on the basal planes, other groups like carboxy, carbonyl, phenol etc. were mostly formed at the sheet edges. These oxygen containing groups and defects change its electrical and thermal properties. GO is strongly hydrophilic and disperses uniformly in water, which makes it an excellent electrode material [30]. GO has a high proton conductivity (1.1 × 10⁻⁵ - 2.8 × 10⁻³ S cm⁻¹) making it an excellent choice for polymer electrolyte membrane materials in PEMFC [4]. GO is unlikely to work at high temperatures and hence, limiting its use at low temperature applications [31]. However, the electrical conductivity of GO is lower than graphene due to present of functional group on the surface.

2.3.3 Properties of heteroatom doped graphene

The properties of graphene can be tuned and modified as per the requirements by doping heteroatoms into the basal planes and reactive edges. It is reported that the doping increases the active site and enhances electrocatalytic activity of graphene significantly [32]. Nitrogen is the most popular dopant used owing to its chemical stability against chemical oxidation [32]. However, carrier mobility and conductivity of N-doped graphene are slightly lower than pristine graphene due to hinder effect. On the other hand, the Boron-doped and thiophene rich S-doped graphene show higher conductivity and charge transfer ability. Hydrogenation of graphene can control its semiconducting properties [33].

From the above it can be understand that graphene nanosheet has the several properties which can be used to enhance the fuel cell electrochemical activity as compared to commercial Pt/CB electrocatalyst. Subsequent section will discuss the application of graphene nanosheet for the various component in PEM fuel cell and microbial fuel cell.

3. Application of graphene nanosheets in fuel cells

3.1 Polymer electrolyte membrane fuel cell (PEMFC)

3.1.1 Electrocatalyst support

The effectiveness of the fuel cell depends on the Hydrogen evolution reaction (HER), Oxygen evolution reaction (OER), and Oxygen reduction reaction (ORR) at the electrode. These reactions proceed at a very slower rate, decreasing the efficiency of the overall fuel cell. Metals and metal oxides, such as, Pt, RuO₂, and IrO₂ are the most popular electrocatalysts used to increase the rate of the reactions [4, 8]. Although these electrocatalysts show promising results, various reasons limit their wide implementation, including very high cost, oxidation of RuO₂, CO poisoning of Pt, limited abundance etc. [34].

Graphene or its derivatives are an excellent alternative to these conventional electrocatalysts due to its comparatively lower cost and unique properties. Graphene is a zero-band gap semiconductor which decreases its catalytic activity, but doping of graphene with different heteroatoms varies its electronic properties and increases active sites and enhances its electrocatalytic activity significantly [32]. The morphology of a graphene-based electrocatalyst can significantly affect the different electrocatalytic reactions in PEMFC. The different morphological structures of graphene nano sheets can be obtained by distorting graphene nanosheets either by topological defects, heteroatom doping, or through creation of active edges [34]. Conductive graphene derivatives are used to facilitate electron transfer rates. Many graphene composites show positive results solving many critical problems associated with PEMFC [4]. By topological defect engineering, the number of active sites can be increased in graphene nanosheets. Interfacial effect engineering in graphene-based electrocatalysts can be used to produce low-cost, high-performance OWS (Overall water splitting) electrocatalyst [34]. Heteroatom doped graphene can be used as a support for single metal atoms, which increases active site area largely and as a result it shows high ORR activity and selectivity for the 4-electron route. It also provides long term stability and a greater charge distribution to the system [4, 20].

In Table 1, the performances of graphene based electrocatalyst are presented. A heteroatom doped graphene molecule Ru-N/G catalyst was synthesized for the application in PEMFC as an electrocatalyst. The ORR electrocatalytic performance of the Ru-N/G catalystwas calculated by rotating disk electrode (RDE) measurements which was performed in acidic electrolyte (0.1 M HClO₄) at a rotation speed of 1600 rpm. Based on different annealing temperatures in the presence of NH3, these products are sub-categorized into Ru-N/G-550, Ru-N/G-650, Ru-N/G-750, and Ru-N/G-800. The products obtained at lower annealing temperature show lower current density and at higher temp (~800°C) Ru shows agglomeration. So, the optimum electrocatalytic behavior is shown by Ru-N/G-750. Mass activity of the ORR for Ru-N/G-750 is 7.5 times higher than that of the commercial Pt/C, thus increasing metal utilization and decreasing the cost for electrocatalyst. Durability of the electrocatalyst is notably increased. After 10,000 cycles, the catalyst holds up to 93% of initial saturated current [20]. Reduced graphene oxide supported platinum (Pt/NrEGO₂-CB₃) is also a graphene derivative which showed promising results under considerable testing. It shows very high charge retention, a higher energy density than commercial Pt/C, and lower mass transfer resistance [19]. In a recent study, it was reported that Co₃O₄ nanocrystals on reduced modified graphene oxide (rmGO) can be used as a catalyst for Oxygen reduction reaction (ORR). The composite material showed similar properties to commercially available Pt/C composites but its durability is very high in adverse conditions and manufacturing cost is much lower [35]. By highly dispersing ultrafine (~2.3 nm) Ni nanoparticles on reduced graphene oxide (rGO), the composite showed high mass activity of 1600 mA m g⁻¹ and high stability [4]. The nitrogen doped Fe-Co/rGO electrocatalyst improved electrocatalytic performance significantly (for HER and OER) in alkaline solution. The over potentials is reported of 0.215 V for HER and 0.308 V for ORR at 10 mA cm^−2. The developed catalyst shows a high durability (after 45 h) [36]. Using Pt/functionalized graphene (Pt/FG) in comparison to Pt/C gives a higher amount of electrochemical active surface area (ECSA) and forward peak current density (mA cm⁻²) [37].

Graphene based material	Properties & Performance	References
Ru-N/G-750	ORR activity is 7.5 times higher than commercial Pt/C catalyst system. Lower manufacturing cost. Exceptional high durability. Exhibits onset potential at 0.89 V and half wave potential of 0.75 V.	[20]
Pt/NrEGO2-CB3	Maintains high effective surface area value after 30,000 cycles (63.83 cm^2 g^−1pt) Offers smaller amount of resistance in compared to commercial electrocatalysts. Shows high power density of 0.411 W cm^−1. Lower level of Pt agglomeration under stressed condition.	[19]
Co3O4/N-rmGO	Low cost of the catalyst High durability High ORR activity	[35]
Ni/rGO	High mass activity (1600 mA mg^−1) High stability (maintains 1012 mA mg^−1 after 1000 cycles)	[4]
Fe-Co-N/rGO-700	Shows Over potential of 0.215 V for HER at the current density of 10 mA cm^−2. Shows over potential of 0.308 V for OER at current density of 10 mA cm^−2. Shows Cell potential of 1.5 V	[36]
Pt/FG	ESA after 1000th scan is 53.33 m^2 g^−1Pt Forward peak current density after 1000th scan is 42.06 mA cm^−2	[37]

Table 1.

Summary of properties and performances of recent-past graphene based Electrocatalyst.

3.1.2 Reinforcement in the polymer electrolyte membrane

Polymer electrolyte membranes (PEM) play a vital role in the PEMFC fuel cell system [1]. Commercially used PEMs include Nafion, Flemion, and Aciplex, which are mainly based on Perflurosulfonic acid (PFSA). Though they all show promising results, different conditions restrict its widespread application [17]. The most common problems related to PEMs are low rate of performance in highly humid conditions and high working temperature; fuel crossover issues; high cost; critical synthesis methods; and water balancing issues [2, 17, 38]. The development of a PEM with adequate conductivity operating under high temperature range and low humid conditions has become a primary objective of the fuel cell technology [9]. Impregnation of graphene in PEM enhances reaction rate, thus, improving the efficiency and the durability of fuel cell devices [1, 4]. The application of graphene and its 2D derivatives has proved to be profitable due to its properties like large electrical conductivity, high charge carrier rates, vast surface area, and a lower production cost.

A PEM must offer high proton permeability, greater electrochemical and thermal stability, high water uptake rates, low permeability to reactant species, high mechanical stability, and lower cost [2]. Table 2 shows the comparison in the properties of graphene/polymer nanocomposites for fuel cell applications.

Polymers	Graphene material	Graphene content	Advancements	References
Sulfonated poly (ether etherketone)	Sulfonated reduced graphene oxide	1.0 wt%	Very less amount of gas crossover. Increase in open circuit voltage. High proton conductivity.	[17]
Sulfonated; polyamide	Ionic liquid polymer modified graphene sheets	10 wt%	Ionic conductivity, tensile strength and mechanical properties were improved.	[16]
		0.5 wt%	2.6 times better proton conductivity than unaltered membrane.
Nafion	Polyoxometalate coupled graphene oxide	1 wt%	Increase in proton conductivity. Reduction in ohmic resistance. Water retention. Maximum power density of 841 mW cm^−2	[39]
Polybenzimidazole (PBI)	3-amino propyl-triethoxysilane ionic liquid with functionalized Graphite oxide	5 wt%	Improves proton conductivity and ionic conductivity significantly.	[40]

Table 2.

Graphene nanosheet/polymer composites for PEMFCs.

Sulfonated reduced graphene oxide (SRGO) as a conductive layer incorporated with sulfonated poly (ether ether ketone) (SPEEK). Among many tryouts, SPEEK/SRGO-1.0 membrane shows excellent proton conductivity also under low relative humidity (RH). It shows high proton conductivity of 8.6 mScm-1 at 80°C/50% RH, which is 3 times greater than commercially available SPEEK membranes. Also, a higher power output of 705 mW cm⁻² is generated compared to 636 mW cm⁻² for non-modified membrane [17]. Another study shows graphene modified protic ionic liquid-based membrane which improves ionic conductivity, proton conductivity, tensile strength and mechanical properties of fuel cell remarkably at a lower production cost [16]. Modification of Nafions with Polyoxometalate coupled with graphene oxide show higher amounts of proton conductivity and power density compared to nafion alone. It also increases its efficiency at working for a longer period [39].

It is also seen that modified graphene oxide material and incorporated it with the polybenzimidazole membrane material and improved its protonic and ionic conductivity notably [40].

3.1.3 Graphene nanosheet reinforced carbon-polymer composite bipolar plate

Bipolar plate (BP) is one of the important parts in PEMFCs. Its function includes provide support to the cell, distribute fuel and oxidant to the electrode surface connecting, collect current from the cells. Therefore, it should have high electrical conductivity, good mechanical strength and low gas permeability [7, 41]. Both metals (Ti, Ni etc.) and non-metals (graphite) are used as the material for bipolar plates in PEMFCs. Graphite is one of the important carbon materials now a days attracted as the reinforcement in the carbon-polymer composite for bipolar plates in PEMFCs. Because, graphene has high corrosion resistance, low specific gravity, and high electrical conductivity (10⁶ Scm⁻¹) [4, 11]. However, carbon-polymer bipolar plate suffers lack of mechanical strength. This can be overcome using metallic bipolar plate. Metallic plate has high electrical conductivity and high mechanical strength, but it suffers due to corrosion problem [27]. By incorporating graphene as a supporting material for bipolar plates in both metallic and non-metallic bipolar plates, researchers found some significant improvement in the results. 1% graphene reinforcement increased by around 6% and 35% in cases of in-plane and through plane electrical conductivities of graphitic bipolar plate composites with phenyl-formaldehyde resin (RPFR) as binder [7, 11].

It has been presented in Table 3, Polyphenylene sulfide (PPS) as a matrix, compositing it with graphene nanoplatelets (G-NPs), making a new bipolar plate material which showed excellent properties under different GNP loading according to different working conditions [13]. The second material presented is by using graphene 0.2 parts per hundred parts of resin (phr) in graphitic bipolar plate composite with vinyl ester resin (VER) as binder shows promising thermal conductivity and flexure strength values are found [7]. It has been proposed graphene-Poly benzoxazine composites as bipolar plate material based on the promising experimental results. 60 wt% of graphene loading showed optimum results in electrical conductivity and flexure strength [14]. Graphene nano particles (7.5 wt%) and polypropylene composite showed electrical conductivity of 5.28 × 10⁻¹¹ S cm⁻¹. Graphene oxide polypyrole (PPY) composite showed 0.059 μA cm-2 of corrosion current, which is very low compared to commercial metallic BPs [27]. Reduced modified graphene oxide (RMGO), titanium composite bipolar plate showed corrosion current of <10–6 A cm⁻², and interfacial contact resistance (ICR) is as low as 4 mΩ cm² [41].

Bipolar plate material	Electrical; Conductivity; (S cm^−1)	Flexure strength; (MPa)	Corrosion current; (μA cm^−2)	References
Graphene/Carbon Fiber/Carbon Black/Natural Graphite/RPFR 1/5/5/64/25 vol%	435.31^i; 130.17^t	57.28	NA	[7, 11]
G-NP/PPS (60 wt% G-NP)	1.7	66	NA	[13]
Graphene/VER/Natural Graphite 0.2phr/70/30 wt%	286.4^i	49.2	NA	[7]
G-NP/ Poly benzoxazine (60 wt% G-NP)	357	41.7	NA	[14]
G-NP/Poly Propylene (7.5 wt% G-NP)	5.28 × 10^−11	NA	NA	[27]
GO/PPY/SS304; (1 mg mL^−1 GO)	NA	NA	0.059	[27]
RMGO/Ti	NA	NA	<1	[41]

Table 3.

Graphene nanosheet reinforced bipolar plates.

t = through-plane; i = in-plane; NA = not available.

3.2 Microbial fuel cell

Microbial fuel cells are cells which convert chemical energy into electrical energy. The cell performs critical fuel oxidation reaction on the cathode while oxygen reduction reactions on the anode. Among many metals used, graphene nanosheets exhibit better electrolytic performance with increases in power density as well as current density [42]. In Table 4, the recent studies that apply material doped graphene nanosheets to increase the electrocatalytic activities reduction reaction (ORR) catalytic activities in the cathode as well as in the anode are tabulated.

Fuel cell component	Doping compound with graphene	Maximum power density; (mW m^−2)	Open circuit potential; (mV)	Electro; chemical active surface area; (m^2 g^−1)	Reference
Cathode	Cobalt nanoparticles embedded in N doped carbon shell with graphene	611 ± 9	NA	NA	[43]
	B-doped graphene	703.55	NA	NA	[44]
	Iron (III) oxide and boron nitride-doped reduced graphene oxide nanosheets	2066 ± 15	663 ± 4	73.1638	[42]
	Polyaniline-graphene nanosheets	Improved from 0.85 to 99	580	NA	[45]
Anode	Nitrogen doped graphene sheet	1008	NA	NA	[46]
	Titanium suboxides with graphene	2073	980	26.5	[47]

Table 4.

Applications of graphene composites in MFCs.

To enhance the cathode performance, studies show cobalt nanoparticles embedded in nitrogen doped carbon nanosheets coupled with graphene show excellent ORR activity. The compound pyrolyzed at 800°C optimizes the ORR activity. Moreover, it also provides long term stability [43]. The B-doped graphene quantum dots also serve as a durable ORR cathode catalyst which gives a power density 1.5 times the Pt/C cathode [44]. Iron (III) oxide and boron nitride-doped reduced graphene oxide nanosheets enhance the cathode performance determined by impedance analysis which is conducted over the range of 1 MHz to 100 MHz at 10 mV [42]. Polyaniline graphene nanosheets improve electricity generation capacity with a maximum power density of 99 mW m⁻², promoting electrolytic performance [45].

To enhance the anode performance, nitrogen doped graphene is used to exhibit maximum electrooxidation current [46]. Titanium suboxide with graphene as anode provides a steady voltage of 980 mV, highly enhancing charge transfer efficiency [47].

4. Conclusions

This chapter summarized the progress currently done by implementing graphene-based material on fuel cell components, such as, electrode, electrocatalysts, and bipolar plates. It is observed that the graphene, owing to its excellent properties including thermal, chemical, and electro-chemical properties showed significant improvement in each of the application.

Ruthenium (Ru) is a commercially used electrocatalysts in PEMFCs. Its ORR activity is improved by 7.5 times when graphene was used to make Ru-N/G-750 composite. It also showed higher durability and lower synthesis cost. On the other hand, Pt/NrEGO₂-CB₃ composite electrocatalyst shows higher value of ECSA of 63.83 cm² g⁻¹_pt even after 30,000 cycles of use and provides lower resistance and higher power density. Several modified electrocatalyst discussed in this chapter which includes Co₃O₄/N-rmGO, Ni/rGO, Fe-Co-N/rGO-700, and Pt/FG. All these catalysts show enhanced electrochemical activities in PEMFCs. The HAD and ORR activity of the graphene nanosheet supported electrocatalyst improved significantly.

It is also added that the GO reinforced SPEEK membrane nanocomposite improved the properties significantly. This nanocomposite membrane reduces the gas crossover, increased open circuit voltage, and enhanced proton conductivity. Moreover, it can be observed that the sulfonated polyamide, Nafion, and PBI all this membrane modified by using graphene showed promising results also under adverse conditions.

The Graphene reinforced carbon-polymer bipolar plate showed excellent electrical conductivity of 435.32 S cm⁻¹, 130.17 S cm⁻¹ respectively for in-plane and through-plane conditions and showed flexure strength of 57.28 Mpa. The use of graphene nanosheet in the various component of PEMFC is shows the improved in the overall performance significantly including power density of a single cell and/or fuel cell stack. Therefore, the use of graphene nanosheet in other fuel cell also attracted to the researchers and scientist.

In MFCs both cathode and anode materials are enhanced by using graphene nanosheets and its derivative. Using Cobalt nanoparticles embedded in N-doped carbon shell with graphene as cathode in MFC gives a high-power density of 611 mW m⁻². Iron (III) oxide and boron nitride doped rGO cathode in MFC showed higher power density of ~2066 mW m⁻², open circuit potential of ~663 mV and ECSA of 73.1638 m² g⁻¹. Other different electrode compound such as B-doped graphene, Polyaniline graphene nanosheets, N-doped graphene, and Titanium suboxides with graphene shows greater performances in MFCs. From this study, it can be attributed that the graphene nanosheets and its derivatives provide a new approach to fabricating high performance fuel cell components including excellent electrocatalyst in the simulated environment.

Acknowledgments

The authors gratefully acknowledge the contribution of Indian Institute of Chemical Engineers for guidance and support in writing this book chapter.

Appendices and nomenclature

AFC	Alkaline fuel cell
AFM	Atomic force microscopy
BPs	Bipolar Plates
CVD	Chemical vapor deposition
DFAFC	Direct formic acid fuel cell
DMFC	Direct methanol fuel cell
ECSA	Electro chemical active surface area
FTIR	Fourier Transform Infrared Spectroscopy
F-GO	Functionalized graphene
G-NPs	Graphene nano platelets
GO	Graphene Oxide
HER	Hydrogen evolution reaction
MFC	Microbial fuel cell
MCFC	Molten carbonate fuel cell
OWS	Overall water splitting
OER	Oxygen evolution reaction
ORR	Oxygen reduction reaction
Phr	Parts per hundred parts of resin
PFSA	Perflurosulfonic acid
PAFC	Phosphoric acid fuel cell
PBI	Polybenzimidazole
PEM	Polymer electrolyte membrane
PEMFC	Polymer electrolyte membrane fuel cell
PPS	Polyphenylene sulfide
PPY	Polypyrole
rEGO	reduced Exfoliated Graphene Oxide
rGO	reduced Graphene Oxide
rmGO	reduced modified Graphene Oxide
RH	Relative humidity
RDE	Rotating Disk electrode
RPFR	phenyl-formaldehyde resin
SEM	Scanning Electron Microscopy
SACs	Single atom catalysts
SOFC	Solid oxide fuel cell
SS-NMR	Solid State Nuclear Magnetic Resonance
SPEEK	Sulfonated poly (ether ether ketone)
SRGO	Sulfonated reduced Graphene Oxide
TGA	Thermo Gravimetric Analysis
TEM	Transmission Electron Microscopy
VER	Vinyl ester resin
XPS	X-ray Photoelectron Spectroscopy