Identifying non-proton-coupled electron transfer as a potential limiting step for the electrosynthesis of ammonia

Abstract

Comprehensively determining the potential limiting step (PLS) and understanding the sophisticated mechanisms of a certain electrochemical reaction become increasingly crucial. Up to now, there is no exception that previous reports default to consider the proton-coupled electron transfer (PCET) process only when identifying the limiting potential, which is dictated by the compromise between the kinetic and thermal equilibrium during proton migration. Unlike before, we theoretically proposed an unusual realization that the non-PCET (*NH₃ → * + NH₃) can also be regarded as PLS during nitric oxide reduction reaction (NORR) on single atom alloy (SAA) catalysts, which is well coincided with the experimentally observable anomaly. More importantly, the further hydrogenation process of *NH₃ to NH₄⁺ was confirmed to be the rate-determining step during the whole reaction, instead of the common hydrogenation steps of NORR. In this regard, as a non-hydrogenation step, NH₃ desorption is expected to serve as an independent evaluation criterion that predicts catalytic performance toward the NH₃ electrosynthesis, getting rid of conventional linear rules caused by the Sabatier principle. This discovery not only reasonably introduces the long-ignored factors to study electrocatalysis but also tackles the sharp contradiction between theory and experiment.