Dataset for "<b>Closed-Loop Discovery of H Spillover-Driven CO</b>_{<strong>2</strong>}<b> Reduction in Acidic Electrolyte by Phosphine-Modified Cu</b>"

Electrochemical CO₂ reduction (CO₂RR) in acidic media provides high carbon efficiency but suffers from strong competition with hydrogen evolution. Using a closed-loop discovery platform that integrates autonomous experimentation with active learning, we systematically explored phosphine-modified Cu catalysts and identified 1,1,1-tris(diphenylphosphinomethyl)ethane–Cu with 79% C₂₊ Faradaic efficiency and a highest partial current density of 319 mA cm⁻². Beyond catalyst optimization, isotope-labeling and depth-profiling studies revealed a striking mechanistic feature: C₂ products originate predominantly from gas-phase CO₂ reacting inside the dry porous Cu layer, where *H species, generated at the water/Cu interface, spill over to hydrogenate accumulated *CO. This contrasts with the prevailing view that multi-carbon products mainly form at the wetted water–Cu–gas triphasic interface. By decoupling *H generation from CO₂ supply, the optimized catalyst suppresses hydrogen evolution and promotes C–C coupling under acidic conditions. Our study thus demonstrates how closed-loop catalyst discovery not only delivers high-performing systems but also leads to uncovering overlooked mechanistic pathways in electrochemical CO₂ conversion.