pH-dependent Scaling Relations and Ion Insertion Promote Bifunctional Oxygen Electrocatalysis on MnO2

Designing electrocatalysts that efficiently mediate both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) remains a central challenge in electrochemical energy conversion and storage. Most catalysts exhibit activity for one reaction at the expense of the other due to adsorbate scaling relations. Here, we report that 𝛼-KxMnO2 defies this trade-off, exhibiting simultaneous improvements for both OER and ORR with increasing electrolyte pH. Using rotating ring disk electrochemistry (RRDE), high-resolution transmission electron microscopy (HRTEM), and operando X-ray and infrared spectromicroscopies, we rule out morphological changes and site blocking as causes of this unusual pH effect. Instead, grand-canonical DFT shows that a pH-dependent interfacial electric field strengthens the adsorption of key intermediates at high pH, leading to non-Nernstian shifts towards lower overpotentials. Simultaneously, electrochemical K+ (de)insertion promotes high bifunctional activity by stabilizing Mn3+ sites for ORR and Mn4+ sites for OER. These mechanistic findings establish a generalizable strategy for breaking conventional catalytic trade-offs by coupling electrolyte engineering and ion insertion to enhance electrocatalytic performance.