Polymer Electrolyte Fuel Cell Durability

In this brief review of the dissolution and solubility of platinum under equilibrium conditions and the degradation of platinum nanoparticles at the cathode under various operating conditions, we discuss some mechanisms of degradation, and then offer recent possibilities for overcoming the problem. The data indicate that platinum nanoparticle electrocatalysts at the cathode are unstable under harsh operating conditions, and, as yet, often would be unsatisfactory for usage as the cathode material for fuel cells. Carbon corrosion, particularly under start/stop circumstances in automobiles, also entails electrical isolation and aggregation of platinum nanoparticles. We also discuss new approaches to alleviate the problem of stability of cathode electrocatalysts. One involves a class of platinum monolayer electrocatalysts that, with adequate support and surface segregation, demonstrated enhanced catalytic activity and good stability in a long-term durability test. The other approach rests on the stabilization effects of gold clusters. This effect is likely to be applicable to various platinum- and platinum-alloy-based electrocatalysts, causing their improved stability against platinum dissolution under potential cycling regimes.

Owing to its unique electrical and structural properties, high surface area carbon has found widespread use as a catalyst support material in proton exchange membrane fuel cell (PEMFC) electrodes. The highly dynamic operating conditions in automotive applications require robust and durable catalyst support materials. In this chapter, carbon corrosion kinetics of commercial conventional-carbon-supported membrane electrode assemblies (MEAs) are presented. Carbon corrosion was investigated under various automotive fuel cell operating conditions. Fuel cell system start/stop and anode local hydrogen starvation are two major contributors to carbon corrosion. Projections from these studies indicate that conventional-carbon-supported MEAs fall short of meeting automotive the durability targets of PEMFCs. MEAs made of different carbon support materials were evaluated for their resistance to carbon corrosion under accelerated test conditions. The results show that graphitized-carbon-supported MEAs are more resistant to carbon corrosion than nongraphitized carbon materials. Fundamental model analyses incorporating the measured carbon corrosion kinetics were developed for start/stop and local hydrogen starvation conditions. The combination of experiment and modeling suggests that MEAs with corrosion-resistant carbon supports are promising material approaches to mitigate carbon corrosion during automotive fuel cell operation.

Chemical degradation of perfluorinated sulfonic acid (PFSA) membranes during operation is a serious problem to be overcome before commercialization of electric vehicles and stationary cogeneration systems based on polymer electrolyte fuel cells. In this chapter, the mechanism for peroxide/radical formation, which works as an active species for membrane degradation, in the cell is comprehensively introduced, including recent findings. Accelerated testing methodology and the chemistry of PFSA membrane degradation are also outlined to improve the durability of PFSA membranes in polymer electrolyte fuel cells.

Membrane chemical degradation of polymer electrolyte membrane fuel cells (PEMFCs) is summarized in this paper. Effects of experimental parameters, such as external load, relative humidity, temperature, and reactant gas partial pressure, are reviewed. Other factors, including membrane thickness, catalyst type, and cation contamination, are summarized. Localized degradations, including anode versus cathode, ionomer inside the catalyst layer, degradation along the Pt precipitation line, gas inlets, and edges are discussed individually. Various characterization techniques employed for membrane chemical degradation, Fourier transform IR, Raman, energy-dispersive X-ray, NMR, and X-ray photoelectron spectroscopy are described and the characterization results are also briefly discussed. The detailed discussion on mechanisms of membrane degradation is divided into three categories: hydrocarbon, grafted polystyrene sulfonic acid, and perfluorinated sulfonic acid. Specific discussion on the radical generation pathway, and the relationship between Fenton's test and actual fuel cell testing is also presented. A comparison is made between PEMFCs and polymer electrolyte water electrolyzers, with the emphasis on fuel cells.

The world's first highly durable perfluorinated polymer-based membrane electrode assembly (MEA) for polymer electrolyte membrane fuel cells, under conditions of high temperature and low humidity, has been developed. The newly developed MEA, which is composed of a new perfluorinated polymer composite membrane, reduces the degradation rate to 1/100th to 1/1,000th of that of the conventional MEA. The new perfluorinated polymer composite MEA can be operated for more than 6,000 h at 120°C and 50% relative humidity.

The polymer electrolyte fuel cell (PEFC) gas-diffusion layer (GDL) is the critical bridging component between the bipolar plate flow-field and elec-trocatalyst layer. It must participate in all mass-transport processes of a PEFC. These consist primarily of reactant transport and liquid-water handling – either excess water removal to prevent catalyst-layer flooding under humidified conditions or suppression of water removal to prevent membrane dehydration under subsaturated conditions. Other requirements of a GDL include electron collection and transport, and sharing stack compression load with the cell gaskets. To achieve this broad range of functions, state-of-the-art GDLs consist of a complex, porous composite network of graphite fibers, carbon particles, and hydrophobic fluoropolymer. They are manufactured via a series of intricate processing steps, all of which can affect the final properties of the GDL, and may contain several discrete layers in the final form. The most popular configuration is a bilayer structure with the macroporous substrate facing the flow field and a microporous layer (MPL) facing the catalyst layer. All properties of the GDL must be preserved within the PEFC operating environment to ensure required stack lifetimes and power densities. This chapter discusses GDL substrate processing variables, hydrophobic posttreatments, MPL addition, and material selection in the context of their affects on long-term PEFC performance, i.e., loss of hydrophobicity, loss of MPL material, carbon corrosion, increase in mass-transport resistance, etc. Advanced physical property characterization methods are shown and are related to durability data. Finally, considerations for improving GDL durability and extending membrane lifetime under dry operating conditions through novel GDL designs are discussed.

BASF Fuel Cell (formerly PEMEAS) produces polybenzimidazole-based high-temperature membrane electrode assemblies (MEAs). These Celtec®-P MEAs operate at temperatures between 120 and 180°C, and, therefore, are especially suitable for use in reformed-hydrogen-based polymer electrolyte fuel cells. Owing to these high operating temperatures, CO tolerances up to 3% can be achieved. Additional fuel gas impurities (inorganic or organic) can be tolerated to a much higher concentration than in low-temperature fuel cells. From a fuel cell system perspective, waste heat can be effectively used which increases the overall system efficiency. However, besides the distinct advantages over low-temperature polymer electrolyte fuel cells, some challenges have to be overcome. Especially on the catalyst level, there are several requirements which have to be met. In detail these are (1) anode catalyst activity for the oxidation of CO in the presence of hydrogen, (2) cathode catalyst activity in the presence of an adsorbing electrolyte such as phosphoric acid, and (3) high corrosion stability of the catalyst metal and catalyst support, especially under transient operation conditions such as start/stop or local fuel starvation. Especially the last point is important since for successful commercialization of MEAs, durability, reliability, and robustness are critical factors. That is, all materials used in MEAs have to be highly durable even under nonideal daily life conditions outside the laboratory. This contribution gives insight into the degradation mechanism during start/stop operation. Several tests are presented giving a better understanding of corrosion effects in high-temperature MEAs.

This chapter provides an overview of performance durability issues typically occurring in the direct methanol fuel cell (DMFC), in both single cells and short DMFC stacks. The focus of this chapter is on those sources of performance degradation that have been recognized as impacting DMFC operation in a major way (1) the loss of cathode activity due to surface oxide (hydroxide) formation, (2) ruthenium crossover from the anode to the cathode through the proton-conducting membrane, and (3) membrane–electrode interface degradation. Much attention is devoted to the interpretation of performance losses observed during extended operation of DMFCs under “realistic” DMFC operating conditions, including high-voltage cell operation. A separation of the anode and cathode performance losses is attempted whenever possible. Also addressed in this chapter are various methods of mitigating DMFC performance losses, either at the stage of membrane–electrode assembly design and fabrication or in an operating fuel cell.

This chapter describes the behavior and stability of metallic bipolar plates in polymer electrolyte fuel cell application. Fundamental aspects of metallic bipolar plate materials in relation to suitability, performance and cell degradation in polymer electrolyte fuel cells are presented. Comparing their intrinsic functional properties with those of carbon composite bipolar plates, we discuss different degradation modes and causes. Furthermore, the influence and possible improvement of the materials used in bipolar plate manufacturing are described.

Highly graphite filled polymer composites were developed for use as bipolar plates in polymer electrolyte fuel cells (PEFCs). For use in PEFCs, composites should possess excellent durability in a hot and humid environment in addition to high electrical conductivity and good mechanical properties. Therefore, the stability of different composites in hot water was estimated by comparison with the initial properties and target values. On the basis of this comprehensive estimation, we obtained thermosetting composites for compression molding and thermoplastic composites for injection molding that enabled the production of precise bipolar plates. A PEFC stack assembly using the composite bipolar plates showed good, stable performance comparable to that of conventional machined graphite plates.

In the past, the construction and optimization of single fuel cell components was often considered most important and relatively little attention has been paid to the sealing of the cells. Enduring sealing solutions though are a prerequisite for functionality, continuous operation and achievement of high efficiencies. The requirements for the applied sealing materials are multifarious; some of them are common to all types of polymer electrolyte fuel cells (PEFCs), while others depend on the type of fuel cell in question. Besides the usual requirements for sealing materials such as optimized relaxation behavior and a good processability allowing for inexpensive mass production, all suitable sealing materials must have a general fuel cell compatibility. First, the materials must not contain potential catalyst poisons which might migrate and deactivate the catalyst layer of the PEFC; second the materials must not contain any substances which might reduce the performance of the PEFC; and finally the materials must not contain any components which might be eluted and thus have the potential to block pores of the gas-diffusion layer, coat other active surfaces, or interfere in whatever way with the electrochemistry of the cell. The differences among the three main types of polymer-electrolyte-based fuel cells (PEFC, direct methanol fuel cell, high-temperature PEFC) for the sealing material are, on the one hand, the different temperatures at which the cells are operated and on the other hand, the different media against which the materials need to be resistant (water, fuel: H₂, O₂, reformate, methanol, formic acid, phosphoric acid, coolants). The resulting catalogue of requirements necessitates an in-depth understanding of the material behavior within the cell; therefore fundamental investigations need to emphasize a profound understanding of the deterioration mechanisms (e.g., oxidative and thermal processes, hydrolysis, chemical nature of the neighboring parts, influence of surrounding media, etc.). Many times the existing and commonly employed methods for evaluating the sealing performance of a gasket are found not to be sufficient, so either known methods have to be adapted or completely new methods have to be set up. With the resulting knowledge base optimized sealing solutions can be developed, including new materials and composites as well as innovative gasket designs.

Commercialization of the proton exchange membrane fuel cell, an efficient energy-conversion device, requires additional gains in system lifetime. Contamination represents a key degradation mode. Its status is summarized and analyzed to identify research needs. Contaminant sources include ambient air, system components located upstream of the fuel cell stack, and fuel and coolant loops. The number of reported contaminants was conservatively estimated at 97, but many contaminant compositions are still unclear and many gaps remain to be explored, including airstream system components and coolant and fuel streams. For the latter cases, contaminants may reach the cathode compartment by diffusion through the membrane or as a result of seal or bipolar plate failure, thus representing potential interaction sources. In view of this large potential inventory of contaminants, recommendations were made to accelerate studies, including the addition of identification tests performed by material developers, development of standard tests, and definition of an exposure scale for ranking purposes. Because anions are excluded from the membrane in contact with weak solutions (Donnan exclusion), mechanisms involving anions need to be reevaluated. Contaminant mechanisms were synthesized, resulting in only eight separate cases. This situation favors the development of two key simple mathematical models addressing kinetic and ohmic performance losses that are expected to positively impact the development of test plans, data analysis, model parameter extraction, contaminant classification (use of apparent rate constants), and hypothetical scenario evaluation. Many mitigation strategies were recorded (41) and were downselected by elimination of untimely material-based solutions. The remaining strategies were grouped into three generic approaches requiring further quantitative evaluation and optimization: cathode compartment wash, cathode potential variations, and manufacturing material and processing specifications.

The oxygen reduction reaction (ORR) on platinum-based catalysts in the cathode catalyst layer is affected by several kinds of impurities, such as impurity cations or organic impurities in membrane electrode assemblies and in polymer electrolyte membranes. These impurities may come from outside the cathode chamber, or may be generated inside as decomposition products on the catalyst surface or as crossed-over fuels from the anode. In this chapter the effect of inorganic and organic impurities of 0.1–10 mmol dm^-3 on the kinetics of the ORR investigated by electrochemical measurements is discussed. Cationic species, aldehydes, and alcohols are found to degrade strongly the ORR current. A method to cope with such impurity problems is proposed where small amounts of additives in the membrane electrode assembly or in the membrane suppress the degradation and affect positively the ORR performances at the catalyst surface.

The performance and durability of a polymer electrolyte fuel cell (PEFC) operating with reformate is discussed. Brief overviews are given on how dilution affects the thermodynamic driving force and how diffusion of N₂ and CO₂, two major components in a typical reformate mix, affects the overall voltage. The primary focus is on the impact of CO on the voltage performance of the PEFC, i.e., the anode overpotential at different CO levels. Specifically, the effects of CO concentration and the impact of various CO mitigation methods on durability and degradation are presented. CO/air bleed interactions are discussed in connection with peroxide-induced membrane/ionomer degradation rates. Furthermore, the possibility of in situ anode CO formation from CO₂ via the reverse water-gas-shift reaction is assessed for realistic PEFC operating conditions. The discussion includes results obtained at high CO levels and the stability of Pt–Ru catalysts. The impact of trace impurities such as NH₃, H₂S, and small organic molecules is also described.

One of the most critical aspects of proper fuel cell design is water management: too little water, and the membrane will dry out; too much water, and the catalyst layer will flood and block access of the reactant gases to the electro-catalyst surface. Developers have put considerable effort into the optimization of three-dimensional structures that can accommodate the simultaneous demands of membrane hydration and gas access to the catalyst layer under normal operating conditions, as well as of the power-plant-level designs that can ensure water is properly distributed and managed. In fuel cell power plants that operate intermittently and are exposed to atmospheric conditions, such as in automotive applications, the challenges of water management are complicated by the fact that the system will frequently have to be started from subfreezing conditions. Given the volume change associated with freezing water, one expects that any water retained in the pores of the catalyst layer or at the catalyst layer interface with the gas-diffusion layers will expand and can therefore create considerable stresses on the porous materials, possibly deforming them from their initial state. To design a cell that can accommodate these changes, a thorough understanding of the physics of water movement and freezing is necessary. Researchers have seen degradation associated with freeze/ thaw cycles and, more specifically, with drawing current from the fuel cell when the temperature of the cell itself is below freezing, a procedure that will be frequently experienced for fuel cell power plants deployed in automotive applications. Experiments demonstrate a marked increase in the mean pore size and width of the pore size distribution of the catalyst layer after thermal cycling. Modeling of the phase transition associated with thawing and freezing, however, as well as the coupled phenomena of water management and thermal management under partially frozen conditions is rather limited in the open literature. In this chapter, we examine the limits of current understanding, as well as the data that suggest freezing-point depression in polymer electrolyte fuel cell materials and the implications of lowered temperatures on fundamental kinetic processes.

Accelerated testing and statistical lifetime modeling are important tools in the development of durable membrane electrode assemblies (MEAs). There are several reasons for using accelerated tests, such as demonstrated durability improvement, marketing a product 's competitive advantage, and reduced product development time. Three types of accelerated testing are often used; screening tests, mechanistic tests, and lifetime tests. Accelerated lifetime tests are particularly useful when combined with statistical analysis to provide predictive capability for MEA lifetimes in “real life” conditions. This contribution outlines the main techniques for accelerated testing and important rules to follow for accurate results, such as observing the MEA failure modes for consistency.

Successful developers of fuel cells have learned that the keys to achieving excellent durability are controlling potential and temperature, as well as proper management of the electrolyte. While a polymer-electrolyte fuel cell (PEFC) has inherent advantages relative to other types of fuel cells, including low operating temperatures and an immobilized electrolyte, PEFC stacks also have unique durability challenges owing to the intended applications. These challenges include cyclic operation that can degrade materials owing to significant changes in potential, temperature, and relative humidity. The need for hydration of the membrane as well as the presence of water as both liquid and vapor within the cells also present complications. Therefore, the development of durable PEFC stacks requires careful attention to the operating conditions and effective water management.

The main requirements for the bipolar plate are electrical contacting of electrodes, current conduction, supply of gases and cooling media, removal of products, and the separation of reactant gases and cooling media. Another important factor for efficient operation and high durability is a homogeneous gas distribution. The homogeneity of the local reactant partial pressures is influenced by the depletion of oxygen and the accumulation of water and is locally affected by covered contact areas (landings) of the bipolar plate. Degradation effects in connection with hydrogen or oxygen undersupply and starvation are strongly influenced by bipolar plate design. Bipolar plate design may be influenced by the material, the production process, the gas diffusion layer properties, the membrane properties, the temperature profile requirements, the temperature level requirements, the interaction of the anode and the cathode, and especially by the liquid water transport through the membrane electrode assembly. As a result, different cell types have been established, e.g., gases can be conducted by a porous structure or in channels. Nonoptimized bipolar plate structures reduce the performance or involve the danger of increased degradation of the membrane electrode assembly and the bipolar plate. Requirements for an efficient and durable stack operation are a homogeneous distribution of fluids and a homogeneous and defined cell compression. The manifold of a fuel cell stack has the function to carry reactants and cooling media to the stack or to different cell rows and to carry off products and cooling media; therefore, the design of the manifold has an impact on the distribution of gas and cooling media. The stack-compression hardware has the function to lock the stack components into position with a defined and homogeneous pressure. Too high, too low, or inhomogeneous compressions have negative effects on the performance and durability of the stack.

Polymer electrolyte fuel cell stacks, in the commonly used bipolar arrangement, consist of multiple stacked single cells in a filter-press-type arrangement. The bipolar arrangement connects the cells in series electrically and in parallel for the reactant and coolant flows; therefore, all cells have to carry the same current but they can receive different reactant mass flows. The reactant and the coolant supply may be different owing to statistically varying percolation resistances of the fluids and owing to the position of the cells in the stack. Therefore, the commonly made assumption that individual cells perform equally is valid neither for normal operation nor for the degradation of individual cells. Differences between cells can be of systematic or stochastic nature and translate into differences in the degradation rate under operation or start/stop conditions. The four main cases are discussed.

Characteristics of operation conditions of polymer electrolyte fuel cells (PEFCs) for stationary use compared with automobile use are reviewed in terms of durability, and the degradation factors found in PEFCs for residential cogeneration systems are described on the basis of long-term operation data. It was observed that degradation of the membrane, loss of electrochemical surface area mainly due to sintering of noble metals, decrease in carbon monoxide tolerance, and decrease in gas diffusivity due to loss of hydrophobicity of the catalyst layer are the main degradation factors. It was demonstrated that the degradation of the membrane is greatly suppressed by saturated humidification conditions and that the sintering is limited under the operation conditions of cogeneration systems. Although the decreases in CO tolerance and gas diffusivity are the most important factors for long-term durability, the potential of the durability of the existing PEFCs has also been demonstrated by long-term operation of single cells for more than 50,000 h as well as that of actual cogeneration systems for 18,000 h.

In recent years, the importance of fuel cell vehicles has been increasing in the North American, European, and Japanese markets amid desires to reduce CO₂ emissions and resolve energy problems. Therefore, improving stack durability has become an increasingly important issue. However, many membrane electrode assembly degradation phenomena occur in the stack under various vehicle operating conditions. This chapter presents an analysis of membrane electrode assembly degradation phenomena and the results obtained with several durability improvement measures.

The New Energy and Industrial Technology Development Organization (NEDO) has been promoting the national development of polymer electrolyte fuel cells under the direction of the Ministry of Economy, Trade, and Industry. NEDO proposed an R&D target road map for the technical development of stationary and vehicle systems in 2005 and made some revisions in 2008. This road map shows the technical development themes and target values to be achieved in each stage of development. This chapter describes the polymer electrolyte fuel cell R&D targets of NEDO.