Benchmarking Nanoscale Electrochemistry in <i>Operando</i> Transmission Electron Microscopy with a Standard Reference Electrode

<i>Operando</i>/<i>in situ</i> methods have significantly advanced understanding of the nanoscale dynamics at solid-liquid interfaces under real-time reaction conditions. In particular, <i>operando</i> electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) enables real-time tracking of structural evolution, yet precise potential control remains challenging due to the incompatibility of standard reference electrodes in confined liquid cells. Here, we report real-time reference electrode (RE) calibration via a metal-bridge strategy connecting an on-chip Pt pseudo-RE with an external Ag/AgCl RE. Electrochemical benchmarking using three redox couples confirms reliable potential referencing and Nernstian hydrogen kinetics on Pt across a wide pH range (1-12). <i>Operando</i> EC-STEM reveals accelerated Cu nucleation and growth at higher overpotentials and shows that Cu²⁺ depletion drives the transition from mossy to dendritic growth, which can be suppressed by external electrolyte flow, providing rational guidance for energy or other industrial applications that demand minimal dendrite formation.