Prediction Method for PEMFC Cathode Kinetic Losses Induced By Contaminants

Proton exchange membrane fuel cells (PEMFCs) are attractive energy conversion devices that are expected to be durable but are exposed to a variety of contaminants either present in air and hydrogen or released by fuel cell system materials, which may be deleterious to the cell performance [1-3]. In most if not all cases, the catalyst is affected by the contaminant. Only a small fraction of the large number of likely contaminants has been investigated. For instance, only 21 ambient air species from a list of more than 200 candidates were recently studied [4]. Furthermore, fuel cell contamination tests require significant resources and may last several ten or a few hundred of hours even if accelerated conditions achieved with higher contaminant concentrations are used. As a result, there is a need to develop a simple method to predict the impact of contaminants on fuel cell performance.

A prediction method for the performance loss associated with contamination has not been suggested for PEMFCs. However, the use of a molecule dipole moment was proposed to correlate the contaminant effect on the half wave potential of the oxygen reduction reaction [5]. Only 5 contaminants were considered and data were obtained with a thin film catalyst layer electrode mounted in a conventional three-electrode cell. Tests were completed with an aqueous electrolyte solution maintained at room temperature. These operating conditions are quite different than those used to operate a PEMFC with a solid electrolyte and the presence of a gas diffusion layer covering the catalyst layer. The adsorption energy has also been used as a predictor of catalytic performance [6].

The PEMFC performance database resulting from separate tests performed with 21 airborne contaminants [4] is re-examined with the purpose of exploring the usefulness of two correlations between the kinetic loss at steady state and a contaminant physico-chemical parameter. The contaminant dipole moment is first considered thus significantly extending the range of the previous study [5] from 5 to 20 species. Data are also correlated with the contaminant adsorption energy on Pt. The database contains impedance spectra acquired before, during and after a temporary contamination period. The use of impedance spectroscopy during the galvanostatic experiments was more economical because only a single experiment was needed to assess the different cell performance losses. The analysis focuses on kinetic cell performance losses because Pt is a common catalyst used for many processes [7]. Therefore, it was expected that a significant amount of information would be available for physico-chemical parameters characterizing interactions between Pt and selected contaminants including the adsorption energy.

The extraction of a surrogate parameter from impedance data representing the kinetic losses and resulting correlations with the dipole moment and the contaminant adsorption energy on Pt will be discussed. The adsorption energy correlation is also related to the O₂ adsorption energy on Pt. The impact of contaminant chemical and electrochemical reactions on the Pt surface adsorbate coverage is also considered.

ACKNOWLEDGMENTS

Authors are indebted to the United States Department of Energy for funding (award DE-EE0000467). The authors are grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.

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