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In an effort to reduce the platinum group metal (PGM) content in proton exchange membrane fuel cells (PEMFCs) to a value comparable with conventional vehicles, a concerted global effort over the past decade has been made by the fuel cell community to develop high activity oxygen reduction reaction (ORR) catalysts. However, as PGM loadings in membrane electrode assemblies (MEAs) decrease below 0.125 mg/cm2, it has become clear that ORR mass activity is just one of the challenges which must be overcome. At the anode, it appears that a kinetic limitation may have been reached and new research may now be required to develop hydrogen oxidation reaction (HOR) catalysts with higher turnover frequencies vs. conventional Pt. At the cathode, transport limitations have been uncovered which are related to ionomer/catalyst interactions. Recently, the critical role of the carbon support (used to disperse the PGM catalyst) on these ionomer/catalyst interactions has been revealed. Thus, rational design of carbon structures either through modification of commercial carbons or design of completely new structures, is now of utmost importance. The challenges and discoveries related to these concepts are critically analyzed in this review, along with suggestions for future research directions to help overcome the remaining hurdles. • In recent years the PGM content of PEMFCs has been greatly reduced. • As this occurs, new performance losses are being experienced at both electrodes. • This review discusses the observations and proposed mechanisms for these losses. • The role of carbon support microstructure on cathode losses is also highlighted. [ABSTRACT FROM AUTHOR]

Anode reversal occurs in proton exchange membrane fuel cells (PEMFCs) when one or several membrane electrode assemblies (MEAs) in a stack experience global hydrogen starvation, causing the anode potential to reach values > 1.5 V leading to rapid carbon corrosion and eventually catastrophic failure of the MEA. To mitigate this, a common solution has been to add an anti-reversal catalyst (ARC) into the anode which can promote water oxidation in favor of carbon corrosion. The most common accelerated stress test (AST) for evaluating new ARCs in single-cell MEAs has been a galvanostatic test whereby a constant current is applied to the MEA while air/N 2 are used for the cathode/anode gas streams. Less commonly, another AST that has been used is a pulsing experiment whereby the anode is cycled between normal operating conditions and reversal conditions. These two types of ASTs have both been used in the literature for determining the durability of ARCs and novel anode design concepts. Here, by directly comparing two well known ARCs, one with high stability (IrO x) and one with low stability (RuO x), it is shown that the less commonly used pulsing AST provides a more comprehensive gauge of anode durability during reversal events. [Display omitted] • Anti-reversal catalysts (ARCs) in anodes can mitigate reversal of PEMFC. • Accelerated stress testing (AST) are effective methods for evaluating ARCs. • Galvanostatic AST is commonly used but unreliable. • Pulsing AST can be used to accurately gauge the anti-reversal performance of ARCs. [ABSTRACT FROM AUTHOR]

Membrane electrode assemblies (MEAs) for proton exchange membrane fuel cell (PEMFC) applications must survive fuel starvation events which occur when the flow field becomes blocked by dust/debris/or ice (during freeze events) leading to high (>2 V) potentials at the anode. While most research has focused on optimizing these 'reversal' catalysts, recent work has also demonstrated that anode layer thickness/structure has an enormous impact on anode durability. In the present work, the impact of catalyst layer structure is studied in more detail, with a focus on understanding the mechanism for this improvement. The effect of both anode layer thickness as well as the spatial distribution of Ir within the anode layer is evaluated. The longest reversal time is found to result from thin layers, or placing the Ir adjacent to gas diffusion layer (GDL) interface. While both approaches lead to improved reversal times, reducing anode layer thickness is found to be a more promising approach as it results in less degradation during reversal, and is easier to manufacture vs. selectively placing the Ir adjacent to the GDL. [Display omitted] • Help to understand the role of anode thickness and IrO x distribution in anode layer. • Analyze the key factors affecting anode reversal time. • Recommendations for designs of anode layer are provided. [ABSTRACT FROM AUTHOR]

Fuel starvation events occur in proton exchange membrane fuel cells (PEMFCs) when the flow fields become blocked by ice or dust/debris from the environment. When this occurs, the electrons at the anode catalyst layer (ACL) must be generated by either water oxidation or carbon corrosion which forces the cell into a 'reversal' event. Such events rapidly degrade the anode catalyst layer, but can be mitigated through the use of reversal tolerant catalysts (RTCs) such as IrO x. While widely used in industry, this topic has not received significant attention in the literature. At the membrane electrode assembly (MEA)-level, there are two main approaches to evaluate reversal tolerance: 1) Galvanostatic and 2) Pulsing. In galvanostatic accelerated stress tests (ASTs), the current is held at a constant value until the cell voltage reaches a predetermined value. For pulsing ASTs, the MEA is put through short term 'reversal' conditions before going back to normal operating mode. These ASTs are in fact quite different in terms of the stress they put on the ACL. Here, we directly compare these two ASTs, showing that, when normalized for the same amount of total reversal time, the pulsing experiments result in more severe anode damage than galvanostatic testing. • Blockage of H 2 supply to the anode of a PEMFC results in severe anode damage. • This can be overcome with the use of a 'reversal tolerance catalyst' (RTC). • Accelerated stress testing (ASTs) are not widely standardized for RTCs. • A direct comparison of the two most common ASTs (Galvanostatic and Pulsing) is provided. • Recommendations for interpreting and choosing between these ASTs are provided. [ABSTRACT FROM AUTHOR]