In Situ Structural Evolution and Activity Descriptor of Atomically Dispersed Catalysts During Nitrate Electroreduction

Single-Atom Catalysts (SAC) have emerged as a promising class of materials for various catalytic applications, including the electrochemical nitrate reduction reaction (eNO₃RR) and consequently ammonia production. While the efficiency and selectivity of these materials have been extensively highlighted for the eNO₃RR, the in situ evolution to their structure and composition during electrocatalysis is largely unexplored and lacks catalyst design principles. To solve this, we investigated a series of high utilization metal-nitrogen-carbon (MNC) SACs (M = Cr, Fe, Co, Ni, and Cu) for eNO₃RR. Except for CuNC, which selectively produced nitrite, all catalysts exhibited Faradaic efficiencies (FE) for ammonia exceeding 50%. NiNC demonstrated the highest performance (FE of 78.0 ± 2.9% at -0.4 V versus reversible hydrogen electrode (RHE) at pH 13 and maximum ammonia production rate of 615.7 ± 176.5 µmol·h^-1· <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>cm</mml:mi> <mml:mi>geo</mml:mi> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>2</mml:mn></mml:mrow> </mml:msubsup> <mml:annotation>${\mathrm{cm}}_{{\mathrm{geo}}}^{ - {\mathrm{2}}}$</mml:annotation></mml:semantics> </mml:math> , corresponding to an energy efficiency of 15.1 ± 1.4% at -0.6 V_RHE), followed by CoNC. In situ Synchrotron X-ray fluorescence (SXRF) mapping at various cathodic potentials (from open circuit potential to 0.0 V_RHE and then -0.6 V_RHE at 100 mV steps) revealed significant mobility of Ni within the carbon matrix, leading to the formation of metallic clusters from 0.0 V_RHE. Similar in situ metal clustering is observed for CoNC. Structure-activity plots are generated from both MNC literature and results obtained here, finding a clear trend between OH binding energy and turnover frequency, with the high activity of NiNC and CoNC in this work explained by their stronger OH binding in the metallic structure compared to their SAC coordination. This work therefore, reveals the structure-activity-stability of MNCs for eNO₃RR and provides a simple descriptor for identifying highly active eNO₃RR catalysts and their in situ structural evolution.