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Introduction to Materials for PEMFC Electrodes Mardle, Peter; Du, Shangfeng
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Document Version Peer reviewed version Citation for published version (Harvard): Mardle, P & Du, S 2020, Introduction to Materials for PEMFC Electrodes. in A-G Olabi (ed.), Encyclopedia of Smart Materials. Elsevier.
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Download date: 27. Sep. 2021 Introduction to Materials for PEMFC Electrodes Peter Mardle and Shangfeng Du* School of Chemical Engineering, University of Birmingham, UK *Corresponding email: [email protected] Keywords: PEMFC, Polymer electrolyte membrane, Fuel cell, Electrode, Electrocatalyst, Catalyst support, Platinum, Nanoparticle, Nanostructure, Oxygen Reduction Reaction, Alloy.
Abstract
• As part of a sustainable energy future, much attention has been drawn to the use of hydrogen fuel as it can be formed renewably through the electrolysis of water. As part of this proposed hydrogen economy, polymer electrolyte membrane fuel cells (PEMFCs) play a key role as clean power generators in vehicles, building and portable devices. However, for extensive commercialization, high performance and durable devices need to be produced. This chapter introduces the material used for electrodes and how advanced development can help to address the challenge faced by PEMFCs.
1. Introduction Serious concerns about the sustainability and the environmental consequence of using fossil fuels as our primary energy source have led to several ingenious technologies, such as energy generation from hydrogen through the use of fuel cells. Such a technology provides a clean and sustainable way for producing electricity on-site such as portable power generators or in the transport sector. By avoiding the thermodynamic limitations of the Carnot cycle, fuel cells also have the additional benefits of providing a high energy efficiency while releasing extremely low levels of pollutants (Ramachandran & Stimming, 2015). Among various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) are favored for automotive and portable applications due to their low operating temperature, flexible scale, and quick start up/shut down. In recent decades, PEMFCs have therefore received the greatest efforts towards their development. However, despite significant progress has been achieved for this technology, PEMFCs still face some big challenges including poor reliability and robustness, and also high cost. In order to meet these challenges and move this clean technology for extensive commercialization, further understanding of the materials and structure of PEMFC components is essential.
1.1. PEMFC History
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In 1839, William R. Grove constructed what can be considered to be the first hydrogen fuel cell, and published the results as a mere postscript to his investigations of acidic salt solutions (Grove, 1839). The first PEMFC, however, was developed 120 years later by Thomas Grubb for General Electric (GE). After refinements by Leonard Niedrach in the mid of the 1960s, the PEMFC then saw active use by NASA as a power source in Project Gemini. The ‘Grubbs-Niedrach fuel cell’ employed for these missions was used in place of the batteries used in project Mercury as it could be used for longer durations. In over 50 years since the first PEMFC, the basic design and the materials used have barely changed. Initially phenolic membranes were used, but a series of developments led to a breakthrough in 1966 with the development of the NFI Nafion® membrane by DuPont (Zaidi & Matsuura, 2010). The use of the Nafion® membrane effectively tripled the durability of the electrolyte, and as such is still the conventionally used membrane today. It also so happens that the platinum (Pt) used in the Grove cell is inherently the most active single metal catalyst for use in PEMFCs (Nørskov et al., 2004). Interest dropped in PEMFCs until the 1990s where advances in nanotechnology and the understanding of electrode processes led to the common practice of dispersing Pt nanoparticles on high surface area carbon to act as the catalyst. Since then, heavy research and development has been carried out on PEMFCs, not least on the electrode materials leading to the release of commercial PEMFC powered vehicles in the form of the Hyundai iX35 (2013) and Toyota Mirai (2014) fuel cell electric vehicles (FCEVs) and more recently; the Honda Clarity (2017) and Hyundai Nexo (2018). However, for the mass adoption of such clean technology, the development of cheaper and more powerful PEMFC systems is required. In recent years, PEMFC development has been guided by the US Department of Energy (DoE) ultimate target of developing the PEMFC system at a 65% electricity efficiency with a durability of 8,000 hours, all at a cost of $30 kW-1 (Papageorgopoulos, 2019).
1.2. PEMFC Principles The fuel cell is a device which can directly convert the chemical energy in fuels to electricity through electrochemical reactions. The scheme of a PEMFC is shown in Figure 1. At the anode, molecular hydrogen is split into two protons and two electrons by the hydrogen oxidation reaction (HOR), as shown by Equation 1. The protons then transfer though the polymer electrolyte membrane (PEM) to the cathode. As the PEM does not conduct electrons, they are forced to transfer to the cathode through the external load, allowing an electrical current to be drawn from the fuel cell. At the cathode, molecular oxygen then reacts with the transferred protons and electrons to form water (Equation 2). This process is called the oxygen reduction reaction (ORR). The overall chemical reaction (Equation 3) is simply therefore the recombination of oxygen and hydrogen to produce water.
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Under standard conditions the Gibbs free energy of the forwards reaction for the production of liquid water (Equation 3) is –237 kJ mol-1 (Dicks & Larminie, 2003), corresponding to a potential difference of 1.23 V between both sides of the fuel cell. The PEMFC can therefore be thought of as a method of splitting this simple redox reaction into its two half-cell processes. In doing so the change in Gibbs free energy is harnessed in the form of electrical energy. Figure 2 depicts the main components of a PEMFC, where in the center resides the membrane electrode assembly (MEA). In a conventional PEMFC the MEA consists of the cathode gas diffusion electrode (GDE) and the anode GDE hot pressed to the PEM. The GDE comprises of a polytetrafluoroethylene (PTFE) treated carbon paper/cloth-based gas diffusion layer (GDL) with the catalyst layer (CL) deposited on top. Gases are distributed to the GDL through the flow field plate (FFP). In this chapter, we only discuss the electrodes, while leaving the PEM, GDLs and FFPs to next chapter.
Figure 2: The components of a single PEMFC. The power performance of PEMFCs is usually presented by a polarization curve, or voltage-current density curve, as shown in Figure 3a. The polarization curve is usually determined by holding the cell potential at a series of set values for 15-30 minutes prior to recording the current (Gasteiger, Kocha, Sompalli, & Wagner, 2005; Multi-Year Research, Development, and Demonstration Plan: 3.4 Fuel Cells, 2017). It can also be replotted as power-current density curve, representing the power output from the PEMFC to the external load. The durability of the PEMFC is usually tested either by monitoring the power performance over a long-term operation at a specified condition, e.g. holding the cell potential at 0.6 V for a few thousand hours, or by running dynamic load cycles which can accelerate the fuel cell degradation rate, thus reducing the time required for the test (Tsotridis, Pilenga, Marco, & Malkow, 2015). A typical dynamic load cycle for fuel cells in automotive applications is shown in Figure 3b.
1.3. Electrode requirements and fabrication From Figure 1, it can be seen that in order to get a larger power output from a PEMFC, highly efficient fuel cell reactions (Equations 1-3) are necessary. To achieve this, both reactants and products need to be quickly supplied to and removed from the reaction sites, respectively. Therefore, the electrodes need to be highly electrically conductive to allow for rapid transfer of electrons between the reaction sites and external circuit. While the PEM has a high proton conductivity requirement to facilitate the transport
3 of protons from the reaction sites of the anode to the cathode, proton conducting ionomer (typically a dispersion of the electrolyte material) is required throughout the CL to provide the final proton transfer pathways to the catalytic sites and increase overall catalyst utilization (Figure 4). Additionally, the electrodes should possess a large degree of porosity for both hydrogen and oxygen to be delivered to the reaction sites in time. The electrode structure and composition must also allow for the efficient removal of the produced water from the reaction sites. Without efficient water transport, the water will cover the reaction sites or even block the pores, resulting in water flooding, hindering the transfer of gaseous fuels to the reaction sites.
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2. Catalyst Materials When comparing the kinetic rates of HOR and ORR in the PEMFC, ORR at the cathode is comparatively a much more sluggish reaction, which is about 6 orders slower than HOR (Shao, Chang, Dodelet, & Chenitz, 2016). Therefore, in PEMFC development, most efforts have been attracted to developing advanced cathodic catalysts towards a fast ORR. The amount of effective reaction sites on the catalyst surface, with respect to the total amount of catalyst material, is defined by the electrochemical effective surface area (ECSA) given in m2/mg. For Pt catalysts, this can be determined electrochemically by measuring the number of hydrogen molecular adsorbed on the effective reaction sites on the catalyst surface. A large ECSA indicates a high degree of catalyst utilization whereby there are more surface sites available to catalyze the reaction. The catalytic activity of the catalysts is defined as the current obtained at a specific voltage, i.e. 0.9 V divided by the mass of catalyst material present. This is often referred to as the mass activity, the most commonly used figure to compare different catalysts. The ratio of the mass activity to the ECSA then provides the specific area activity, indicating how active the catalyst surface is towards a fuel cell reaction. Electrochemical testing of the fuel cell catalysts can be conducted both ex-situ half- cell measurement (outside of the fuel cell), or in-situ (testing in the MEA). The most popular ex-situ test is that of the rotating disk electrode (RDE) in aqueous electrolytes (e.g. H2SO4 or HClO4) where a three electrode setup allows the intrinsic catalytic activity of the catalysts to be quickly measured (Garsany, Baturina, Swider-Lyons, & Kocha, 2010; Gasteiger et al., 2005). Although it is a useful screening technique, the recorded activities often do not correlate well with the performance in fuel cells for novel non-Pt/C catalysts and is a key challenge in effective use of novel catalyst materials in operating PEMFCs (Stephens, Rossmeisl, & Chorkendorff, 2016). Asides from measuring catalytic activity, accelerated degradation testing (ADT) is frequently used to determine the stability of the catalysts. ADT is usually conducted by cycling the fuel cell operation for a fixed time, e.g. 30,000 cycles between low current densities (low load) and high current densities in set conditions (Multi-Year Research, Development, and Demonstration Plan: 3.4 Fuel Cells, 2017). Such cycling accelerates the dissolution and aggregation of the catalyst particles and so the susceptibility of the catalyst to processes such as Ostwald ripening can be examined. The dynamic load cycle test is one type of ADT and better simulates the voltage changes in an operating PEMFC in practical driving of vehicles.
2.1. Platinum Nanoparticles For both HOR and ORR, Pt is recognized as the most efficient catalyst where Pt nanoparticles supported on carbon nanospheres (Pt/C) are still the commonly used catalysts today. As such, little has changed with regards to the materials used in PEMFC electrodes since 1839 and the reasons were cleared by the work conducted by Nørskov et al. (Nørskov et al., 2004) and Trasatti et al (Trasatti, 1972). Utilizing density functional theory (DFT) it was found that the ORR activity is directly related to the oxygen and hydroxyl binding energies on the catalyst surface. If the binding energy is too strong, then the rate of ORR would be limited by desorption of the
5 reaction products from the surface. If the binding energy is too weak, then the adsorption of oxygen becomes less favorable and so the reaction becomes limited by the proton and electron transfer to dissociate molecular oxygen. Because of this trade- off, the volcano plot in Figure 6 forms (Nørskov et al., 2004). Closest to the peak is Pt, demonstrating that it is inherently the most effective single metal catalyst towards ORR. A similar plot was constructed by Wendt et al (Wendt, Spinacé, Oliveira Neto, & Linardi, 2005) who made the analogous argument for HOR. With lower bond strengths there would be a reduced activation of the hydrogen-hydrogen bond. Conversely, if the metal-hydrogen bond strength is too high, then the rate of the heterogeneous reaction would be limiting.
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‘polyol process.’ This is a process where a polyol such as ethylene-glycol is used as both the solvent and reducing agent for the metallic salt (Fievet, Lagier, & Figlarz, 1989). Microwave assistance can be employed to increase the reaction rate to mere minutes while producing consistently small particle sizes (Figure 7) (Sharma et al. 2019; H. Li, Zhang, Yan, Lin, & Ren, 2013). Modifications of this process such as the use of capping agents, e.g. oleic acid have also been used to achieve size control of the precipitated product (Y. Chen, Liang, Yang, Liu, & Chen, 2011; Santiago, Varanda, & Villullas, 2007).
2.2. Platinum alloys and hybrids Alloying platinum with transition metals has been proven to vastly improve the sluggish ORR kinetics of the cathode catalyst because of geometric and electronic effects. By extending the aforementioned work, Nørskov et al (Nørskov et al., 2004), Stamenkovic et al , Greeley et el (Greeley et al., 2009) and Chorkendorff et al (Stephens et al., 2012) extended the volcano plot to include platinum-transition metal alloys (Figure 8). By reducing the d-band center for the platinum-oxygen bond, the bond strength is weakened and the activities for the ORR therefore increased. The ORR activity of these alloys is also enhanced through lattice strain effects. As a result of the strained lattice structure, surface Pt-Pt bonds contract, weakening the Pt-OH bond strength through an increased downwards d-band shift (Mukerjee, Srinivasan, Soriaga, & Mcbreen, 1995; Xiaoming Wang et al., 2013). By simply adding a second precursor alongside the chloroplatinic acid in the salt reduction process, other metal ions can be easily incorporated into the reaction mixture to form alloyed nanoparticles, e.g. Pt-Cr/C (E. Antolini et al., 2006), Pt-Ni/C (E. Antolini et al., 2005; K. W. Park et al., 2002), Pt-Co/C (Salgado et al., 2004), Pt-Cu/C (Zhou & Zhang, 2015) alloyed nanoparticles have all been successfully prepared for fuel cell catalyst application.
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Although vastly improved ORR activities can be obtained by this alloying approach, popular alloy materials such as Co, Fe and Ni are prone to leaching out of the catalyst nanoparticles in the highly acidic environment in PEMFC operation. Such leaching not only leads to a drop of the catalytic activity of the catalysts, but also results in the degradation of both ionomer in CL and membrane due to the contamination of the leaching metal ions to the ionomer molecular chain (Yu, Pemberton, & Plasse, 2005). As a result of these durability concerns, core-shell structures have attracted much interest in recent years as the shell of Pt can provide a degree of protection for the transition metals in the core. The vast majority of alloy structures investigated have been those with a Pt3M lattice structure due to superior performances over its other stoichiometric counterparts (Fernandes, Paganin, & Ticianelli, 2010; Ma & Balbuena, 2008; Mukerjee et al., 1995). It is also well known that for the majority of alloys, Pt readily segregates to the surface (Ma & Balbuena, 2008) to form a Pt-rich shell layer after annealing. The second layer for Pt3Fe, Pt3Co and Pt3Ni however contains the alloyed M element. In theory then the electrochemically active sites consist of surface Pt, with the ORR activity enhancement provided by the alloyed metal directly underneath the surface. In addition to the simple annealing, a prior de-alloying process is usually employed to assist the formation of the Pt-rich skin (Snyder, McCue, Livi, & Erlebacher, 2012; Strasser et al., 2010). The favored method is of electrochemical de-alloying where potential cycling of the catalyst in acidic electrolyte dissolves the exposed non-noble metal particles. De-alloying is highly effective for smaller particles with a large Pt:M ratio, but for larger particles (> 15 nm diameter) the de-alloying process results in a nanoporous structure (Snyder, Livi, & Erlebacher, 2013; Snyder et al., 2012) rather than prompting the surface re-arrangement to core-shell nanoparticles (Figure 9). The porosity of such structures can also be advantageous for heterogeneous catalysis as higher attempt frequencies of the reactants can be achieved, although the leaching is still a problem.
De-alloying processes have also been used to form intricate Pt3M frameworks. By the subsequent chemical de-alloying of PtNi3 polyhedra, Pt3Ni nanoframes were developed by Chen et al (C. Chen et al., 2014). Thermal annealing of the obtained nanoframes formed a highly active Pt skin on the surface of the frame. Remarkably this catalyst exhibited a 22-fold increase in specific activity with respect to commercial Pt/C. This enhancement was in part attributed to the electronic and geometric effects of the alloy, but also due to the accessibility provided by the open structure for the efficient transport of reactants and products to/from the catalytic sites.
8 best catalytic performance and durability for the catalyst. With the sequential reduction method, the ratio of Pt:M is commonly controlled simply by the quantity of the precursors used in the synthesis (Y. Chen et al., 2011; G. Wang, Wu, Wexler, Liu, & Savadogo, 2010). Furthermore, individual monolayers of platinum can also be added by using a sacrificial metal such as Cu. Zhang et al (J. Zhang et al., 2005) synthesized a variety of alloy nanoparticles by reducing an “almost dry slurry of carbon and metal salts” using hydrogen gas at high temperatures. The temperatures involved help to induce surface segregation of Pt to the surface of the alloy particle. A copper monolayer was then adsorbed on the surface of the particle, to be replaced by platinum using galvanic displacement (J. Zhang et al., 2004).
2.3. 1D Platinum Nanostructures Following on from the nanoframe structures and with mass transport limitations in mind, 1D structures have gained significant interest in recent years. 1D Pt nanostructures can provide durability enhancements for PEMFC catalysts as due to their inherent structures, the effects of Ostwald ripening can be significantly reduced (Sun, Jaouen, & Dodelet, 2008; Sun et al., 2011). In addition, 1D nano-structures usually possess a much higher electrical conductivity which does not suffer from the size effect as much as their 0D nanoparticle analogues. Both advantages mean that no carbon support is required as that in Pt/C. Thus, the carbon corrosion that has been considered as one of the largest challenges in PEMFC development can be fully avoided. The preparation methods of 1D nanostructures for PEMFC applications was recently reviewed by Du (Du, 2012). One of the simple ways is by using template methods. For example, silica, polymer and anodic aluminum oxide (AAO) templates can be used to form highly regular arrays of nanowires (S. M. Choi, Kim, Jung, Yoon, & Kim, 2008; W. C. Choi & Woo, 2003; Zhao, Xu, Guo, Li, & Li, 2006). By a further kinetic control of the chemical reduction of the metal precursor salts, single crystal nanowires can be achieved with preferred exposure of highly active crystal facets so that catalytic activity can be maximized. For the polyol process this can be achieved by adding a trace amount of Fe2+ or Fe3+ ions (J. Chen, Herricks, Geissler, & Xia, 2004; Xia et al., 2003) to reduce the saturation of Pt atoms. Alternatively, in the wet chemical reduction method in aqueous solution using formic acid, temperature control can be used to control the growth of nanowires (Du, 2010; Du, Lin, et al., 2014; Lu, Du, & Steinberger-Wilckens, 2015; Sun et al., 2011) (Figure 10). At room temperature, crystal growth favors the lower energy crystallographic planes and so single crystal nanowires can form. However, even though the 1D nanostructures provide better stability and very high specific area activities, such performance enhancements must overcome the reduced ECSA caused by the large bulky volume, i.e. to synthesize ultrathin catalyst nanowires.
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A class of ultrathin Pt and Pt alloys nanowires have therefore been developed in -1 recent years resulting in the highest recorded ORR mass activity of 13.6 A mgPt (31- -1 fold the DoE 2020 target of 0.44 A mgPt ). A variety of Pt alloy and jagged Pt nanowires have been synthesized by refining oleylamine based methodologies whereby highly strained crystalline surfaces are complemented by extremely high ECSAs (Bu et al., 2015; M. Li et al., 2016). Improving scalability and more in-situ tests are required for these materials to find real use in PEMFC applications.
2.4. Non-platinum catalysts The most efficient way to reduce the amount of expensive Pt is to simply use none of it. Pt group metal (PGM) free catalysts have shown much promise in recent years. Carbon based materials that are commonly used as catalyst support materials (see Section 3) have demonstrated catalytic activity towards the ORR. Wang et al (Xiqing Wang et al., 2010) showed that nitrogen doped mesoporous carbon can be used as ORR catalysts where a computational study by Ikeda et al (Ikeda et al., 2008) suggested that a graphitized nitrogen atom could promote catalytic activity on a nearby carbon site. Hetero-atom dopants have also shown to greatly increase the activity of graphene (Molina-garcía & Rees, 2018) and carbon nanotube (CNT) catalysts (Shui, Wang, Du, & Dai, 2015) toward the ORR, however, performance is not comparable to Pt in acidic media. Metal oxides, nitrides, carbide and chalcogenides are just a few of the other materials that have been investigated as PGM-free catalysts (Shao et al., 2016; Wang et al., 2019). However, a most promising class of materials are M-N-C based where the metal is typically a transition metal such as Fe. Development of these materials has intensified where Ballard in conjunction with Nisshinbo have developed an MEA which has a peak power density of 570 mW cm-2 under practical (air) operating conditions (Banham et al., 2018). Current research challenges are in overcoming the poor stability of these catalysts as well as circumventing high mass transport limitations imposed by the relatively thick CL and high catalyst loadings required (Banham, Choi, Kishimoto, & Ye, 2019).
3. Catalyst Support Materials To achieve a uniform size distribution in the synthesis and reduce Oswald ripening in PEMFC operation, catalyst nanoparticles are usually supported on high surface area materials, in particular carbon black. The excellent electrical conductivity of the support materials can also facilitate the electron transfer through the active sites of the poorly conductive catalyst nanoparticles.
3.1. Carbonaceous catalyst supports To meet the requirements of the high electrical conductivity, large specific surface area (SSA) and high resistance to corrosion, carbon blacks are commonly used at the minimal cost. Carbon blacks are composed primarily of graphite agglomerates/nanospheres with size of up to 50 nm (Sharma & Pollet, 2012). The
10 agglomerates of carbon black provide a porous network, which allows the maximization of surface area without sacrificing electrical conductivity. Perhaps most importantly is the fact that carbon blacks can be formed easily by the incomplete thermal decomposition of hydrocarbons. The catalyst is then deposited onto the support by means of sputter deposition (Zacharia, Rather, Hwang, & Nahm, 2007), electrochemical deposition (Day, Unwin, & Macpherson, 2007) or simple impregnation processes (Matsumoto et al., 2004). Despite these benefits, the corrosion of carbon black in the acidic PEMFC, in particular the high potential during start-up is still a big challenge prompting the investigation of other carbonaceous support materials. More stable carbonaceous materials including CNTs and graphene nanosheets have been studied for this application as both of them possess a larger SSA than carbon black (Sharma & Pollet, 2012). Also with the development of materials synthesis technologies, even though the cost of CNTs and graphene is still high (Dai, 2002), when compared with the expensive Pt catalyst this is not so much of an obstacle for PEMFC application. A more significant drawback of these supports is the difficulty for the direct deposition of Pt nanoparticles on the pristine inert surface, and prior functionalization is usually required (Wildgoose, Banks, & Compton, 2006). The biggest challenge with both materials is caused by their anisotropic particle morphology compared to the carbon black, i.e. 1D CNTs and 2D graphene nano- sheets. The PEMFC catalyst layer made from CNT-based catalysts shows a very loose structure and it is not easy for the electrolyte ionomer coated on surface to form a continuous network through the whole catalyst layer to membrane, leading to low robustness and performance in comparison to other supports (Du, Lu, Malladi, Xu, & Steinberger-Wilckens, 2014; Mardle et al., 2020). Graphene nano-sheets also tend to stack on top each other due to their 2D morphology forming large aggregates (Si & Samulski, 2008). This dramatically reduces the catalyst utilization where reaction sites become embedded between the stacked nanosheets, consequently leading to a poor electrode performance (Ermete Antolini, 2012; Mardle, Fernihough, Du, 2018). Fully erect graphene nanosheets for the whole catalyst layer or a scaffolding architecture (Du, Lu, & Steinberger-Wilckens, 2014; S. Park et al., 2011; Si & Samulski, 2008) (Du, Lu and Steinberger-Wilckens, 2014; Park et al, 2011; Si and Samulski, 2008) can be potential solutions (Figure 11(a)-(b), but their real performance in PEMFCs is still unclear.
Figure 11 (a): Schematic of the use of spacers between graphene sheets. Reproduced (adapted) from (S. Park et al., 2011) with permission from Elsevier. Copyright (2011) (b): Reduced graphene oxide nanosheets with erect PtPd nanowires. Reproduced (adapted) from (Du, Lu, & Steinberger-Wilckens, 2014) with permission from Elsevier. Copyright (2014).
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3.2. Non-Carbonaceous Catalyst Supports The very nature of carbonaceous materials means it’s impossible to completely mitigate the issue of carbon corrosion in PEMFC operation. Metal oxides have thus been studied for this application because they can avoid this corrosion problem, albeit at the price of reduced electrical conductivities (E. Antolini & Gonzalez, 2009). Therefore, a large degree of effort has focused on the doping of metal oxides in order to improve their electrical conductivities. For example, the conductivity of titanium dioxide is known to increase by introducing dopants, e.g. niobium (K.-W. Park & Seol, 2007) or by using sub-stoichiometric technology. In fact it has been reported that Ti4O7 exhibits a higher electrical conductivity compared to carbon (Ioroi, Siroma, Fujiwara, Yamazaki, & Yasuda, 2005). However, this did not entirely translate into an improvement in catalytic activity due to the comparatively low surface area. Other categories of non-carbonaceous supports include tin dioxide (Okanishi, Matsui, Takeguchi, Kikuchi, & Eguchi, 2006) which can also promote the electro-oxidation of poisonous carbon monoxide on Pt catalyst, and tungsten trioxide (E. Antolini & Gonzalez, 2009) which also adds a benefit in that the support itself can demonstrate some degree of proton transfer (Nakajima & Honma, 2002). Although a wide variety of catalyst support materials have been investigated, significant drawbacks are present with each of them. Although imperfect, the simplicity of carbon black has made this catalyst support very difficult to be replaced on an industrial scale.
4. Proton Conducting Ionomers in Catalyst Layers One of the main goals of PEMFC development is to reduce the PGM content and so -2 lower the overall cost. The biggest reduction in loading from around 4 mgPt cm to -2 0.4 mgPt cm was made in the 1990’s where alongside use of the carbon support, the incorporation of proton conducting ionomer into the catalyst layer much improved catalyst utilization (Wilson & Gottesfeld, 1992). Despite huge advancements in catalyst material in recent three decades, the common PGM loadings in the catalyst layer have not much decreased since then. Recent studies have shown that one major factor is that at low Pt loadings, the diffusion resistance of oxygen through the ionomer thin film coated on the catalyst surface becomes a significant contributor to the overall mass transport resistance (Jinnouchi, Kudo, Kitano, & Morimoto, 2016). Paradoxically to the PEM, high gas permeability is therefore required of the ionomer material. For example, highly oxygen soluble ionic liquids have been shown to improve the ORR activity of Pt based catalysts Zhang, Munoz, & Etzold, 2015, Wang et al., 2019, Zhang et al., 2019). Due to environmental concerns with the production of Nafion®, efforts are made to replace current PEM materials with non-fluorinated alternatives, typically consisting of a poly-aromatic backbone. These alternative ionomer materials exhibit reduced gas permeability, a desirable feature for PEM materials in order to reduce crossover and hence degradation, but undesirable for an ionomer. Differences in ionic exchange capacity (IEC), water uptake and catalyst utilization are also found with these materials and are to be much considered when incorporating in a PEMFC (Peron, Shi, & Holdcroft, 2011). This is also the case with short side chain ionomer materials such
12 as AquivionTM (Hyflon® in Figure 12) which through the removal of the ether group demonstrate increased chemical stability (J. Li, Pan, & Tang, 2014).
5. Extended surface area catalysts Extended surface area (ESA) catalysts have gained a lot of interest in recent years. They comprise large area surfaces extended in two dimensions such as thin films, possessing larger radii of curvature and having greater resistance to surface area loss (Debe, 2012). The specific area activities towards ORR are usually higher than Pt/C, owing to electronic structure properties versus nanoparticles. In ESA catalysts, thin film catalyst layer technology on high-aspect-ratio nanoparticles shows great potential for PEMFC applications. The most famous ESA catalyst is nanostructured thin-film (NSTF) catalysts developed by 3M. This advanced technique was deliberate from 1995 and has been reviewed recently (Debe, 2013). The thin catalyst layer is firstly introduced by using a monolayer array of perylene whiskers (1 µm tall, 30 nm×55 nm in cross-section) with surface coated 20 nm polycrystalline PtCoMn film. This extremely small thickness of the catalyst layer enables a much higher catalyst utilization ratio thus significantly enhancing the power performance. However, the approach is intrinsically limited by challenges facing with water management issues due to the hydrophilic Pt-transitional 2 metal alloy catalyst surface and the very low ECSA (only 10-15 m /gPt). Recently, based on this concept, Du’s group developed an approach by directly growing ultrathin Pt nanowire arrays on GDL surface in aqueous solution using a very simple method (Du, 2010). Pt nanowires have a diameter about 3-4 nm a length between 10- 2 200 nm. This Pt nanowire electrode shows an enhanced ECSA of 34.4 m /gPt in MEA, together with the advantage of high catalyst utilization ratio of the thin film catalyst nanostructure, a better power performance is achieved in PEFCs than the conventional one from Pt/C nanoparticles, especially at a high catalyst loading (Lu et al., 2015). This work was reviewed in detail by the group very recently (Lu, Du, & Steinberger-Wilckens, 2016). However, there is a challenge with this approach to directly grow Pt alloy nanowires to further improve the electrode performance.
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6. Conclusions and Perspectives Even though the core materials in PEMFC technology haven’t changed much over two centuries, vast improvements in electrode performance are clearly obtainable by creating novel nanoscale materials. In this chapter it has been shown that for the catalyst in particular, remarkable size, morphology, structure and compositional control of precious group and transition metals can be achieved. Pt-based transitional metal alloy nanoparticles deposited on carbon are still the most preferred commercialized catalysts in the conventional PEMFC electrodes, and electrodes from extended surface area catalysts also show promising potential, including both NSTF and Pt-nanowire array electrodes. These progresses have led to much improved understanding of the first principles underlying PEMFC technology. With the expansion of such knowledge, focused design can begin to supersede the development of new preparation methods. It is of importance that the research focus shifts to considering electrodes as a single entity rather than just its materials. The interaction and behavior of catalysts, support and electrolyte ionomer with the whole electrode, their impact on the catalytic performance, water and gas distribution, electrical and proton conductivity, and their contact with polymer electrolyte membrane must all be cleared. It is only through considering all of the variables that we will successfully pass the demonstration stage today and the extensive commercialization will become possible, allowing the potential role of PEMFCs in a sustainable energy future to be fully realized.
7. References
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