Copper Carbonate Hydroxide as Precursor of Interfacial CO in CO₂ Electroreduction

Copper electrodes are especially effective in catalysis of C₂ and further multi-carbon products in the CO₂ reduction reaction (CO₂ RR) and therefore of major technological interest. The reasons for the unparalleled Cu performance in CO₂ RR are insufficiently understood. Here, the electrode-electrolyte interface was highlighted as a dynamic physical-chemical system and determinant of catalytic events. Exploiting the intrinsic surface-enhanced Raman effect of previously characterized Cu foam electrodes, operando Raman experiments were used to interrogate structures and molecular interactions at the electrode-electrolyte interface at subcatalytic and catalytic potentials. Formation of a copper carbonate hydroxide (CuCarHyd) was detected, which resembles the mineral malachite. Its carbonate ions could be directly converted to CO at low overpotential. These and further experiments suggested a basic mode of CO₂ /carbonate reduction at Cu electrodes interfaces that contrasted previous mechanistic models: the starting point in carbon reduction was not CO₂ but carbonate ions bound to the metallic Cu electrode in form of CuCarHyd structures. It was hypothesized that Cu oxides residues could enhance CO₂ RR indirectly by supporting formation of CuCarHyd motifs. The presence of CuCarHyd patches at catalytic potentials might result from alkalization in conjunction with local electrical potential gradients, enabling the formation of metastable CuCarHyd motifs over a large range of potentials.