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

Abstract

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–N4 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 O2-bearing vs O2-purged environments. Based on these findings, suggestions for improving active site resistance to metal dissolution are provided.

Dynamically Unveiling Metal–Nitrogen Coordination during Thermal Activation to Design High‐Efficient Atomically Dispersed CoN4 Active Sites

“…Even worse, the Fenton reactions between Fe 2+ and H 2 O 2 are significant concerns due to the generation of radical oxygen species (ROS) [19–21] . Both ROS and H 2 O 2 could attack the FeN 4 active sites, carbon support, organic ionomers, and polymer membranes, thereby accelerating performance degradation [22–24] . An alternative Co‐N‐C catalyst, which does not significantly promote Fenton reactions, has recently garnered attention due to prominent improvements in ORR activity and stability in acidic electrolytes (Table S1), [25–30] and, most importantly, encouraging fuel cell power density and performance durability [14, 28, 29, 31–34] …”

“…[19][20][21] Both ROS and H 2 O 2 could attack the FeN 4 active sites,c arbon support, organic ionomers,a nd polymer membranes,t hereby accelerating performance degradation. [22][23][24] An alternative Co-N-Cc atalyst, which does not significantly promote Fenton reactions,h as recently garnered attention due to prominent improvements in ORR activity and stability in acidic electrolytes (Table S1), [25][26][27][28][29][30] and, most importantly,encouraging fuel cell power density and performance durability. [14,28,29,[31][32][33][34] Despite the significant progress in improving Co-N-C catalysts activity and stability via empirical engineering of the composition and structure, [25] the fundamental understanding of the CoN 4 site formation mechanism remains unknown for decades.T his is primarily due to the complexity of the processes occurring during the critical high-temperature pyrolysis step,involving the simultaneous carbonization of carbon structure,n itrogen doping,a nd the CoN 4 site generation.…”

Single-Atom Catalysts: A Perspective toward Application in Electrochemical Energy Conversion

“…(c) Fe–N x stability versus dissolution diagram as a function of pH and potential. Reproduced with permission from ref ( 86 ). Copyright 2020 American Chemical Society.…”

“…Based on density functional theory (DFT) calculations, Holby et al proposed a model and descriptor for leaching of single-atom metal species. 86 By applying various experimental and reaction variables into their calculations (e.g., reaction environments, intermediates, etc. ), they drew a stability Pourbaix diagram of FeNC catalysts ( Figure 3 c) that can explain the previous experimental findings of the instability of FeNC catalysts.…”

Dynamically Unveiling Metal–Nitrogen Coordination during Thermal Activation to Design High‐Efficient Atomically Dispersed CoN4 Active Sites

“…Even worse, the Fenton reactions between Fe 2+ and H 2 O 2 are significant concerns due to the generation of radical oxygen species (ROS) [19–21] . Both ROS and H 2 O 2 could attack the FeN 4 active sites, carbon support, organic ionomers, and polymer membranes, thereby accelerating performance degradation [22–24] . An alternative Co‐N‐C catalyst, which does not significantly promote Fenton reactions, has recently garnered attention due to prominent improvements in ORR activity and stability in acidic electrolytes (Table S1), [25–30] and, most importantly, encouraging fuel cell power density and performance durability [14, 28, 29, 31–34] …”