In-Situ Electrochemical X-Ray Diffraction of Pt Oxidation and Reduction in Hydrogen Fuel Cells

Abstract

The commercialization of proton exchange membrane (PEM) fuel cells has increasingly demanded lower precious metal catalyst loadings. A better understanding of platinum nanoparticle degradation mechanisms and electrochemistry will be essential in preserving fuel cell performance over increased device lifetimes. In-situ synchrotron x-ray diffraction/scattering is a powerful tool for characterizing commercial catalyst layers in an electrochemical cell1-2. Recent advances in hardware presently allow time-resolved scattering measurements and diffractive imaging3 without compromising electrochemical experiments. New cell designs4 greatly improve the ease and flexibility of measuring membrane electrode assemblies under environmental conditions and Pt loadings relevant for fuel cell operation and degradation. The structural effects and kinetics of platinum electrochemistry on nanoparticles can be directly probed using x-ray diffraction. The evolution of nanoparticle strain, atomic ordering, oxidation, and dissolution are monitored in the presence of adsorbing molecular species under potential control. We show how by monitoring Pt particle size, and Pt lattice strain, it is possible to deconvolute chemical and electrochemical steps in the mechanism of Pt oxidation and reduction. Measurements using conventional single crystal and half cell experiments are compared with x-ray diffraction on an operating PEM fuel cell cathode. Several differences in surface chemistry between these cell configurations are detected, which is directly relevant to the development of better accelerated aging and stress-testing protocols. Particular attention has been given towards developing these advanced synchrotron techniques as useful tools for the non-specialist. Recent advances improving the accessibility of in situ diffraction for the electrochemical community are discussed. Figure 1. X-ray transparent half-cell suitable for high-energy x-ray diffraction of membrane electrode assemblies (left). X-ray powder diffractogram obtained from fuel cell cathode during the oxygen reduction reaction (right). Imai, K. Izumi, M. Matsumoto, Y. Kubo, K. Kato and Y. Imai. J. Am. Chem. Soc., 2009, 131 (17), pp 6293–6300K. Sasaki, N. Marinkovic, H.S. Isaacs, and R.R. Adzic. ACS Catal., 2016, 6 (1), pp 69–76F. Reikowski, T. Wiegmann, J. Stettner, J. Drnec, V. Honkimaki, F. Maroun, P. Allongue, O.M. Magnussen. J. Phys. Chem. Lett., 2017, 8 (5), pp 1067–1071B. Pinaud, A. Bonakdarpour, L. Daniel, J. Sharman and D.P. Wilkinson. J. Electrochem. Soc. 2017, 164 (4), pp F321-F327 Figure 1