Surface Reconstruction of Polycrystalline Cu Electrodes in Aqueous KHCO_3 Electrolyte at Potentials in the Early Stages of CO_2 Reduction

The reconstruction of the Cu(pc) polycrystalline surface at potentials that correspond to the early stages of CO_2 reduction in 0.1 M KHCO_3 was investigated by electrochemical scanning tunneling microscopy (ECSTM) at −0.90 V (SHE). A kinetically hindered surface reconstruction of the topmost layers of Cu(pc) into the (100) face was observed, reminiscent of the transformation previously reported at the same electrode potential in 0.1 M KOH. Evidently, the same reconstructed surface, Cu(pc)-[Cu(100)], can be generated in either 0.1 M KHCO_3 (pH 8) or 0.1 M KOH (pH 13). In addition, only minimal structural disruption was observed when the reconstructed surface was transferred from KHCO_3 to KOH electrolyte, and vice versa, provided the solution exchange was executed potentiostatically at −0.90 V. The structural convergence toward the same (100) facet regardless of pH or supporting electrolyte strongly suggests that the Cu(pc) → Cu(pc)-[Cu(100)] surface reorganization is a general phenomenon driven primarily by the rather negative potential applied on the electrode.