AbstractCopper (Cu)-based catalysts generally exhibit high C₂₊selectivity during the electrochemical CO₂reduction reaction (CO₂RR). However, the origin of this selectivity and the influence of catalyst precursors on it are not fully understood. We combineoperandoX-ray diffraction andoperandoRaman spectroscopy to monitor the structural and compositional evolution of three Cu precursors during the CO₂RR. The results indicate that despite different kinetics, all three precursors are completely reduced to Cu(0) with similar grain sizes (~11 nm), and that oxidized Cu species are not involved in the CO₂RR. Furthermore, Cu(OH)₂- and Cu₂(OH)₂CO₃-derived Cu exhibit considerable tensile strain (0.43%~0.55%), whereas CuO-derived Cu does not. Theoretical calculations suggest that the tensile strain in Cu lattice is conducive to promoting CO₂RR, which is consistent with experimental observations. The high CO₂RR performance of some derived Cu catalysts is attributed to the combined effect of the small grain size and lattice strain, both originating from the in situ electroreduction of precursors. These findings establish correlations between Cu precursors, lattice strains, and catalytic behaviors, demonstrating the unique ability ofoperandocharacterization in studying electrochemical processes.