PEMFC Contamination - Fundamentals and Outlook

As the commercialization of proton exchange membrane fuel cells (PEMFCs) comes closer to reality, systems are taken out of the controlled laboratory environment with purified air and hydrogen reagents. The impact of foreign species is coming to the forefront because air quality is currently beyond our ability to control and the cost of hydrogen is partly tied to its composition. A multitude of contaminants, impurities, poisons and inhibitors have already been characterized (1,2). However, many species remain to be identified including their deactivation mechanisms, preventive mitigation approaches and recovery procedures. Foreign species have the capability to harmfully disrupt the functions of the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) catalysts, and the ion exchange polymer. Both of these membrane/electrode assembly materials are essential for PEMFC operation. The incumbent Pt ORR and HOR catalyst is poorly selective and thus has multiple uses (3) rendering it susceptible to deactivation (4,5). The present Nafion ion exchange polymer composed of perfluorovinyl ether groups terminated with sulfonate groups attached to a tetrafluoroethylene backbone, is also poorly selective and permeable to almost any species including gases, solvents and cations (6-11). The ionic conductivity of the ionomer is smaller in the presence of foreign species (12,13). 

In this tutorial, fundamentals aspects of PEMFC contamination will be discussed. Foreign species originate from the ambient air, H₂ production processes such as methane reforming, cooling fluids including ethylene glycol, fuel cell stack and balance of plant materials (either released or washed away (14)), corrosion processes, manufacturing operations (intrusion during fabrication, assembly and storage), and cleaning agents. A down selection procedure is essential in view of the significant number of foreign species left to study and to focus resources on the most relevant and deleterious ones while avoiding duplication of efforts (15,16). The characterization of the multiple effects of foreign species on PEMFC performance under different operating conditions constitutes the next step. The foreign species concentration is particularly important as high values are an option to accelerate degradation processes that would otherwise be slow at practical concentrations (15). The fuel flow configuration is also crucial. Dead end, recirculation and other operating modes (17) that maximize fuel utilization promote foreign species accumulation. The use of in situ as well as ex situ diagnostic methods is essential to establish contamination mechanisms, lay foundations to facilitate the development of preventive and recovery strategies, and encourage the derivation of predictive performance loss correlations. These methods include impedance spectroscopy, segmented cell for current or voltage distributions (18), gas chromatography/mass spectroscopy for product and intermediate species identification, liquid water content in cells for the scavenging impact on soluble foreign species, rotating ring/disc electrode and membrane conductivity cell. Mathematical modeling takes both diagnostic and predictive roles such as the extraction of meaningful parameters from experimental data (13) and the creation of tolerance levels based on accelerated tests (19). In turn, tolerance levels are helpful to set air filter and H₂fuel specifications, and, select more appropriate construction materials, cooling fluids and cleaning agents (prevention). Maintenance procedures are strategically important (performance recovery) if preventive measures are judged insufficient to achieve the desired system robustness.   

Several commercially pertinent topics have hardly been explored: foreign species mixtures, alternatives to Pt catalysts and the scale up of maintenance procedures to fuel cell stacks. A more detailed outlook will be provided.

ACKNOWLEDGMENTS

Authors are grateful to the Office of Naval Research (award N00014-11-1-0391), the Department of Energy (award DE-EE0000467 and National Renewable Energy Laboratory sub-contract ZFT-0-40588-01) and the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility. The authors are also indebted to William Collins formerly from UTC Power and collaborators at the Naval Research Laboratory, the University of Connecticut Center for Clean Energy Engineering, Ballard Power Systems, the National Renewable Energy Laboratory, General Motors, the Los Alamos National Laboratory and the University of South Carolina.

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