Efficient electrochemical conversion of wastewater nitrate to sustainable ammonia requires comprehensive mechanistic understanding and stringent control over competing pathways among multiple aqueous nitrogen intermediates─a challenge that underscores the need for in situ and operando surface analyses of electrocatalysts. In this study, we describe scanning electrochemical microscopy (SECM)-based approaches to characterize the nitrate reduction reaction (NO3RR) on Ti and Cu. These methods facilitate the assessment of catalytic activity, product selectivity, and surface-adsorbed intermediates. By combining tailored chronoamperometric protocols, we achieved operando visualization of NO3RR onset potentials (−0.35 V vs RHE for Ti and −0.16 V vs RHE for Cu) and verified potential-dependent nitrogen product distributions through stepwise quantification of ammonia, hydroxylamine, and nitrite. Furthermore, in situ surface interrogation SECM directly titrates surface-adsorbed H* and O*, revealing their coverage shifts under varying potentials and reaction times during the NO3RR and providing evidence for a hydrogen-mediated pathway on Ti. The methodology establishes a robust SECM-based platform for systematically benchmarking electrocatalyst performance and is readily applicable to the NO3RR on other catalysts and even other reactions that produce a complex array of intermediates.
Lee et al. (Fri,) studied this question.