CO₂ Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu₂O Films

TLDR

The study aims to use modified Cu electrodes to probe Cu structural features affecting CO₂/H₂O reduction selectivity and to develop practical electrolytic fuel synthesis catalysts. Cu foils were annealed in air to form Cu₂O layers, then electrochemically reduced to create modified Cu electrodes. Electrodes made from thick Cu₂O layers (≥3 µm) annealed at 500 °C show higher roughness, reduce CO₂ at 0.5 V lower overpotential, achieve >1 mA cm⁻² at <0.4 V, outperform polycrystalline Cu, and remain stable for hours, whereas thin layers behave like polycrystalline Cu.

Abstract

Modified Cu electrodes were prepared by annealing Cu foil in air and electrochemically reducing the resulting Cu(2)O layers. The CO(2) reduction activities of these electrodes exhibited a strong dependence on the initial thickness of the Cu(2)O layer. Thin Cu(2)O layers formed by annealing at 130 °C resulted in electrodes whose activities were indistinguishable from those of polycrystalline Cu. In contrast, Cu(2)O layers formed at 500 °C that were ≥~3 μm thick resulted in electrodes that exhibited large roughness factors and required 0.5 V less overpotential than polycrystalline Cu to reduce CO(2) at a higher rate than H(2)O. The combination of these features resulted in CO(2) reduction geometric current densities >1 mA/cm(2) at overpotentials <0.4 V, a higher level of activity than all previously reported metal electrodes evaluated under comparable conditions. Moreover, the activity of the modified electrodes was stable over the course of several hours, whereas a polycrystalline Cu electrode exhibited deactivation within 1 h under identical conditions. The electrodes described here may be particularly useful for elucidating the structural properties of Cu that determine the distribution between CO(2) and H(2)O reduction and provide a promising lead for the development of practical catalysts for electrolytic fuel synthesis.