The electrocatalytic performance is governed by the immediate microenvironment surrounding the active site, particularly the hydrogen-bond network that stabilizes reaction intermediates. While cation effects in aqueous electrolytes allow tuning of this network, this powerful leveraging is absent in proton-exchange membrane fuel cells (PEMFCs), where proton is the sole cation. Here, we demonstrate a general strategy of "immobilized molecular perturbation" for single-atom catalysts, which moves the tuning function from the electrolyte to the catalyst's second coordination sphere. Using the oxygen reduction reaction (ORR) on Fe─N─C as a model, we demonstrate that proximal P─O groups act as steric and hydrogen-bonding perturbers. This engineered microenvironment selectively weakens the solvation shell of key *OH intermediates, as confirmed by spectroscopy and computations, thereby facilitating the rate-determining step of *OH desorption. This regulation endows the catalyst with exceptional performance, achieving a half-wave potential of 0.861 V in 0.5 m H2SO4 and a peak power density of 1024 mW cm-2 in a H2/O2 PEMFC. Furthermore, it exhibits outstanding stability with 72 % current retention after 253 h at 0.65 V, positioning it among the best-reported non-precious metal catalysts. This work shifts the paradigm from exclusive active-center optimization to deliberate local microenvironment engineering, enabling accelerated electrocatalysis in device-relevant environments.
Han et al. (Mon,) studied this question.