Acid Stability and Demetalation of PGM-Free ORR Electrocatalyst Structures from Density Functional Theory: A Model for "Single-Atom Catalyst" Dissolution

Platinum group metal-free (PGM-free) materials based on pyrolyzed M-N-C precursors offer a promising approach to replacing rare and expensive platinum group metal-based oxygen reduction reaction (ORR) electrocatalysts in proton exchange fuel cells (PEFCs). A major issue, however, is the stability of these materials in acidic environments and at potentials experienced in situ in PEFC cathodes and rotating disk electrode (RDE) experiments. Density functional theory (DFT)-based approaches have been valuable to understand how atomic scale structures couple to ORR activity. Little has been reported, however, on quantification of active site structure stability. This work proposes a set of DFT-accessible descriptors for M dissolution (demetalation) that directly address this need. Through the application of this approach to a specific Fe-N-4 bilayer graphene-hosted active site structure, the roles of the environment (pH and potential), ORR intermediates, and graphene underlayers are explored. Ranges of stability are reported and hypotheses explaining previously reported experimental behavior based on these findings are proposed. In particular, proposed are model implications for experimental trends in stability with respect to alkaline and acidic conditions; experimental trends for dissolution to occur below a given potential; and observed discrepancies in stability for materials in O-2-bearing vs O-2-purged environments. Based on these findings, suggestions for improving active site resistance to metal dissolution are provided.