Spin State versus Potential of Zero Charge as Predictors of Density-Dependent Oxygen Reduction in M-N-C Electrocatalysts

TL;DR

This study demonstrates that the potential of zero charge (PZC), rather than spin state, better predicts density-dependent oxygen reduction activity and selectivity in M-N-C electrocatalysts, supported by simulations and experiments.

Contribution

It introduces PZC as a superior descriptor over spin state for predicting oxygen reduction trends in M-N-C catalysts, integrating simulations and experimental validation.

Findings

PZC shifts correlate with density-dependent activity changes.

Spin state varies weakly across different densities.

PZC-based models accurately reproduce experimental activity trends.

Abstract

Metal-site density strongly influences oxygen reduction activity and selectivity in M-N-C electrocatalysts, but the descriptors that predict these trends remain under debate. Here, we compare spin state and the potential of zero charge as predictors of density-dependent oxygen reduction behavior in Fe-N-C and Co-N-C catalysts. Using constrained-magnetization calculations combined with Landau analysis, we find that the ground-state magnetic moments vary only weakly across a broad range of metal-site densities, suggesting that magnetic descriptors alone cannot account for the pronounced performance changes. In contrast, explicit-solvent simulations reveal systematic density-dependent shifts in PZC, which alter the interfacial electric field and thereby modulate field-sensitive adsorption energetics of ORR intermediates. Incorporating these PZC shifts into a pH-field-coupled microkinetic model captures the density-dependent activity trends and reproduces the experimentally observed increase in two-electron selectivity at lower site densities under acidic conditions. Experimental PZC measurements further support the predicted trend. Together, these results show that PZC is a more effective predictor than spin state for density-dependent oxygen reduction activity and selectivity in M-N-C electrocatalysts.