In Situ Identification of Dynamic Active Sites in the Electrocatalytic Nitrate Reduction Reaction

Deciphering the spatiotemporal evolution of catalytic active sites under operational conditions is pivotal for establishing precise design principles in electrocatalysis. However, conventional electrochemical characterization techniques, which are limited in correlating physical topographical features with chemical activity, fundamentally obstruct the linkage between nanoscale structural and macroscopic activity descriptors. To address this challenge, an integrated operando electrochemical atomic force microscopy-scanning electrochemical microscopy platform was implemented, enabling simultaneous nanoscale tracking of the topographic reorganization and local activity distribution during the nitrate reduction reaction. Morphological analysis revealed that the amorphous SnO₂ catalyst undergoes dynamic surface restructuring with an increase in roughness of up to 59.0% during catalysis, while operando current mapping confirmed that these roughened regions exhibited ∼100-fold enhancement in NH₃ production activity compared to basal planes. Height profiling demonstrated that amorphous catalysts undergo ∼60% greater thickness reduction than crystalline counterparts during activation, constructing a dual-functional architecture with surface-exposed highly active Sn(II) sites and subsurface Sn(0) enabling rapid electron transport. These findings provided direct experimental evidence of electrochemically driven defect proliferation in amorphous catalysts, which synergistically contributes to NH₃ yield several times higher than those of crystalline benchmarks, pointing to one of the best Sn-based catalysts. The developed correlative operando imaging strategy established a generalized paradigm for resolving transient catalytic dynamics across material systems with direct applicability to NO₃RR optimization and emergent electrocatalytic processes.