Operando Scanning Electrochemical Microscopy of Electrocatalytic Nitrate Reduction Intermediates

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.