ReviewPEM fuel cell electrodes
Introduction
The first application of a proton exchange membrane (PEM), also referred to as a polymer electrolyte membrane, in a fuel cell was in the 1960s as an auxiliary power source in the Gemini space flights. Subsequently, advances in this technology were stagnant until the late 1980s when the fundamental design underwent significant reconfiguration. New fabrication methods, which have now become conventional, were adopted and optimized to a high degree. Possibly, the most significant barrier that PEM fuel cells had to overcome was the costly amount of platinum required as a catalyst. The large amount of platinum in original PEM fuel cells is one of the reasons why fuel cells were excluded from commercialization. Thus, the reconfiguration of the PEM fuel cell was targeted rather directly on the electrodes employed and, more specifically, on reducing the amount of platinum in the electrodes. This continues to be a driving force for further research on PEM fuel cell electrodes.
A PEM fuel cell is an electrochemical cell that is fed hydrogen, which is oxidized at the anode, and oxygen that is reduced at the cathode. The protons released during the oxidation of hydrogen are conducted through the proton exchange membrane to the cathode. Since the membrane is not electrically conductive, the electrons released from the hydrogen travel along the electrical detour provided and an electrical current is generated. These reactions and pathways are shown schematically in Fig. 1.
At the heart of the PEM fuel cell is the membrane electrode assembly (MEA). The MEA is pictured in the schematic of a single PEM fuel cell shown in Fig. 1. The MEA is typically sandwiched by two flow field plates that are often mirrored to make a bipolar plate when cells are stacked in series for greater voltages. The MEA consists of a proton exchange membrane, catalyst layers, and gas diffusion layers (GDL). Typically, these components are fabricated individually and then pressed to together at high temperatures and pressures.
As shown in Fig. 1, the electrode is considered herein as the components that span from the surface of the membrane to gas channel and current collector. A schematic of an electrode is illustrated in Fig. 2. Though the membrane is an integral part of the MEA, a review of the design and fabrication of polymer electrolyte membranes is beyond the scope of this paper. However, the interface between the membrane and the electrode is critical and will be given its due attention. Current collectors and gas channels, typically in the form of bipolar plates, will not be reviewed herein.
An effective electrode is one that correctly balances the transport processes required for an operational fuel cell, as shown in Fig. 2. The three transport processes required are the transport of:
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protons from the membrane to the catalyst;
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electrons from the current collector to the catalyst through the gas diffusion layer; and
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the reactant and product gases to and from the catalyst layer and the gas channels.
Protons, electrons, and gases are often referred to as the three phases found in a catalyst layer. Part of the optimization of an electrode design is the attempt to correctly distribute the amount of volume in the catalyst layer between the transport media for each of the three phases to reduce transport losses. In addition, an intimate intersection of these transport processes at the catalyst particles is vital for effective operation of a PEM fuel cell. Each portion of the electrode will now be introduced.
The catalyst layer is in direct contact with the membrane and the gas diffusion layer. It is also referred to as the active layer. In both the anode and cathode, the catalyst layer is the location of the half-cell reaction in a PEM fuel cell. The catalyst layer is either applied to the membrane or to the gas diffusion layer. In either case, the objective is to place the catalyst particles, platinum or platinum alloys (shown as black ellipses in Fig. 2), within close proximity of the membrane.
The first generation of polymer electrolyte membrane fuel cells (PEMFC) used PTFE-bound Pt black electrocatalysts that exhibited excellent long-term performance at a prohibitively high cost. [1]. These conventional catalyst layers generally featured expensive platinum loadings of 4 mg/cm2. A generous amount of research has been directed at reducing Pt loading below 0.4 mg/cm2 [2], [3]. This is commonly achieved by developing methods to increase the utilization of the platinum that is deposited. Recently, platinum loadings as low as 0.014 mg/cm2 have been reported using novel sputtering methods [4], [5]. As a consequence of this focused effort, the cost of the catalyst is no longer the major barrier to the commercialization of PEM fuel cells.
In addition to catalyst loading, there are a number of catalyst layer properties that have to be carefully optimized to achieve high utilization of the catalyst material: reactant diffusivity, ionic and electrical conductivity, and the level of hydrophobicity all have to be carefully balanced. In addition, the resiliency of the catalyst is an important design constraint [1].
The porous gas diffusion layer in PEM fuel cells ensures that reactants effectively diffuse to the catalyst layer. In addition, the gas diffusion layer is the electrical conductor that transports electrons to and from the catalyst layer. Typically, gas diffusion layers are constructed from porous carbon paper, or carbon cloth, with a thickness in the range of 100–300 μm. The gas diffusion layer also assists in water management by allowing an appropriate amount of water to reach, and be held at, the membrane for hydration. In addition, gas diffusion layers are typically wet-proofed with a PTFE (Teflon) coating to ensure that the pores of the gas diffusion layer do not become congested with liquid water.
Proven and emerging methods that are used to construct integrated membrane–electrodes are illuminated in this review. Two widely employed electrode designs are the PTFE-bound and thin-film electrodes. Emerging methods include those featuring catalyst layers formed with electrodeposition and vacuum deposition (sputtering). In general, electrode designs are differentiated by the structure and fabrication of the catalyst layer. As well, we highlight recent accomplishments in the development of gas diffusion layers. However, most commercial PEM fuel cells and the majority of those reported herein still employ conventional carbon cloth or paper. There has been a significant amount of research conducted on producing composite gas diffusion layers with graded porosity and wet-proofing, as well as the optimization of carbon and PTFE loading in the gas diffusion layer. This report also includes a section describing some recent advances in increasing the surface area of the catalyst by optimization of catalyst supports.
It is evident throughout the report that the most common electrode design currently employed is the thin-film design. The thin-film design is characterized by the thin Nafion film that binds carbon supported catalyst particles. The thin Nafion layer provides the necessary proton transport in the catalyst layer. This is a significant improvement over its predecessor, the PTFE-bound catalyst layer, which requires the less effective impregnation of Nafion. However, one fault of the Nafion thin-film method is its reduced resiliency. Methods of increasing this resiliency, such as using a thermoplastic form of the ionomer, have been found and are reported herein. Sputter deposited catalyst layers have been shown to provide some of the lowest catalyst loadings, as well as the thinnest layers. The short conduction distance of the thin sputtered layer dissipates the requirement of a proton-conducting medium, which can simplify production. The performance of the state of the art sputtered layer is only slightly lower than that of the present thin-film convention.
The performances of many of the electrodes reviewed are reported to accommodate comparison between designs. The performances are provided in the form of power densities at 200 mA/cm2 and 0.6 V. These power densities are benchmarked because they typically represent two characteristics of the electrode. At 200 mA/cm2, the losses can be associated to activation overpotential (the losses associated with the irreversibilities of the chemical reaction). The 0.6 V benchmark depicts the resistive components of the cell and its ability to provide adequate transport of gases, electrons, and protons to the catalyst sites. Together, these two benchmarks provide an overall picture of a PEM fuel cell’s electrode performance. However, when comparing electrode designs it is important to weigh the operating characteristics such temperatures, pressures, and the purity of the gases as they can have an overriding effect on the fuel cell performance.
Section snippets
PTFE-bound methods
Before the development of the thin-film catalyst layer [3], PTFE-bound catalyst layers were the convention [6], [7], [8], [9]. In these catalyst layers, the catalyst particles were bound by a hydrophobic PTFE structure commonly cast to the diffusion layer. This method was able to reduce the platinum loading of prior PEM fuel cells by a factor of 10; from 4 to 0.4 mg/cm2 [9]. In order to provide ionic transport to the catalyst site, the PTFE-bound catalyst layers are typically impregnated with
Thin-film methods
The present convention in fabricating catalyst layers for PEM fuel cells is to employ thin-film methods. In his 1993 patent, Wilson [3] described the thin-film technique for fabricating catalyst layers for PEM fuel cells with catalyst loadings less than 0.35 mg/cm2. In this method the hydrophobic PTFE traditionally employed to bind the catalyst layer is replaced with hydrophilic perfluorosulfonate ionomer (Nafion). Thus, the binding material in the catalyst layer is composed of the same material
Vacuum deposition methods
Common vacuum deposition methods include chemical vapor deposition, physical or thermal vapor deposition, and sputtering. Sputtering is commonly employed to form catalyst layers and is known for providing denser layers than the alternative evaporation methods [25]. The sputtering of catalyst layers consists of a vacuum evaporation process that removes portions of a coating material (the target) and deposits a thin and resilient film of the target material onto an adjacent substrate. A schematic
Electrodeposition methods
The first disclosure of electrodeposition of the catalytic layer in PEM fuel cells was in the form of Vilambi Reddy et al.’s 1992 US patent [30]. This patent detailed the fabrication of electrodes featuring low platinum loading in which the platinum was electrodeposited into their uncatalyzed carbon substrate in a commercial plating bath. The uncatalyzed carbon substrate consisted of a hydrophobic porous carbon paper that was impregnated with dispersed carbon particles and PTFE. Nafion was also
Impregnated catalyst layer
The ability to use fabrication techniques that require meltable materials, such as molding and extruding, would be extremely valuable in the production of membrane electrode assemblies. The conventional perfluorosulfonate acid membranes are not melt-processable because of side chain entanglement and the ionic interactions between the functional groups [37]. Kim et al. [37] have been working on a melt-processable membrane and the encapsulating MEA, which is formed out of perfluorosulfonyl
Catalyst supports
The most common supported catalyst is platinum supported by high surface area carbon and is used in both the cathode and anode. When CO is present in the fuel stream because of reforming, the platinum is alloyed with other materials such as Ruthenium to reduce poisoning of the fuel cell and retain the performance. Electrocatalysts are commonly prepared by solution precipitation, which is followed by reduction of platinum salt in either the gas or liquid phase [13]. Though platinum and platinum
Gas diffusion layer development
The gas diffusion layer has many roles to fulfill. Firstly, it is the electronic conductor between the current collecting bipolar plates and the catalyst layers. Thus, thin gas diffusion layers with a high conductivity is desired for electrical efficiency. Secondly, the gas diffusion layer is fabricated in the form of porous media to allow the passage of the reactant and product flows. To improve mass transport, gas diffusion layers can be made more porous at the cost of increased electrical
Conclusion
This report outlined major advances made in the fabrication of electrodes for PEM fuel cells from the PTFE-bound catalyst layers of almost twenty years ago to the present investigation of membrane-impregnated catalyst layers. It was found that the most common form of electrode is that featuring a thin-film catalyst layer. This is a common selection because of the ample proton conductivity provided by the binding Nafion film. This method is shown to significantly increase performance and reduce
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Present address: Institute for Integrated Energy Systems, University of Victoria, Vic., Canada V8W 3P6.