In Situ Quantification of Hydrogen Radicals Disentangles Direct and Hydrogen-Radical-Mediated Pathways in Green...

The electrochemical reduction of nitrate (ERN) to ammonia (NH₃) has attracted increasing attention as a sustainable route for nitrogen recovery and green ammonia production, enabled by major advances in electrocatalyst design over the past decade. Two mechanistic pathways are generally well-recognized: direct electron transfer and a hydrogen radical (H*)-mediated mechanism. However, the latter remains difficult to quantify under practical electrochemical conditions, limiting mechanistic comparison across catalyst configurations. Herein, Ni/Co, Ni/Pt, and Ni/Pt/Co electrocatalysts were investigated to elucidate the interplay between direct and indirect ERN pathways. Quantitative electron spin resonance (ESR) measurements of H* under ERN-relevant conditions, combined with bulk electrolysis in the absence and presence of an H* scavenger, enabled direct correlation between H* availability and NH₃ production. Ni/Co predominantly follows direct electron transfer, whereas Ni/Pt transitions to an H*-mediated regime above a threshold current density. In contrast, Ni/Pt/Co exhibits synergistic behavior in which both pathways coexist. Moreover, the H* role varies with electrocatalyst chemical composition, facilitating either NO₃ ^- activation or NO₂ ^- hydrogenation. These findings establish a quantitative framework for resolving H*-mediated contribution in ERN and provide mechanistic design principles applicable to other electrocatalytic hydrogenation reactions.