Mechanistic insights into Fe-M dual-metal-site catalysts for the oxygen reduction reaction

Introducing transition metals adjacent to the Fe site in Fe-N-C single-atom catalysts to construct double-atom catalysts (DACs) presents a promising strategy for enhancing the performance of the oxygen reduction reaction (ORR). However, the understanding of the catalytic mechanisms of DACs remains controversial, thereby hindering the rational design of ideal DACs. In this work, we constructed seven iron-metal-nitrogen-carbon catalysts (denoted as Fe-M@NC, M = Cr, Mn, Fe, Co, Ni, Cu, and Zn) to gain in-depth insights into the catalytic mechanisms. We found that Fe-Fe@NC and Fe-Co@NC exhibit superior ORR activity compared to the other constructed catalysts, featuring higher limiting potentials of 1.081 V and 0.888 V, respectively. Detailed reaction mechanism analysis revealed that in the absence of potential effects, the ORR on these two catalysts follows the dissociative pathway. By contrast, when the potential was considered, they catalyze the ORR via the associative mechanism. This discrepancy arises because O₂ adsorption energy weakens under uniform potential conditions, which is unfavorable for direct cleavage of the OO bond. Therefore, it manifests as the associate mechanism. Under this mechanism, the limiting potentials of these DACs are both around 0.9 V, consistent with the experimental results. Additionally, the adsorption energies of other ORR intermediates also exhibit nonlinear dependence on the electrode potential. This study demonstrates the advantages of Fe-Fe@NC and Fe-Co@NC catalysts in the ORR and emphasizes the critical role of potential effects in understanding catalyst reaction mechanisms.