Au Dendrite Electrocatalysts for CO₂ Electrolysis

Electrochemical CO₂ reduction can convert CO₂ into fuels and valuable chemicals using renewable electricity, which provides a prospective path toward large-scale energy storage. Au nanostructured electrodes have demonstrated the best catalytic performance for CO₂ conversion: high catalytic selectivity for CO formation at low overpotentials, high current density, and long-term durability. Here, we report selective electrocatalytic CO₂ reduction to CO on nanostructured Au with various morphologies, prepared via electrocrystallization with a megahertz potential oscillation. X-ray diffraction showed that the proportion of {100} and {110} to {111} surfaces increased at more negative deposition potentials. Cyclic voltammetry showed the potential of zero charge on an Au film was approximately 0.35 V vs standard hydrogen electrode (SHE) and that the surface energy decreased by ∼1 eV/nm² at -0.5 V vs SHE, tending to 0 within several volts in either direction. Scanning electron micrograms showed that the Au crystals grow primarily in the 〈110〉 directions. From these data, a model for crystallization from melts was adapted to calculate the roughening temperature of the {111}, {100}, and {110} Miller indices as 7000, 4000, and 1000 K, decreasing for more negative deposition potentials. This offers a framework for exposed facet control in electrocrystallization. In CO₂ electrocatalysis, -0.35 V vs reversible hydrogen electrode was observed to be a turn-on potential for improved CO₂ reduction activity; dendrites showed 50% Faradaic efficiency for CO production at more cathodic potentials. The Tafel slope was measured to be 40 mV/decade for {100} and {110}-rich Au dendrites and 110 mV/decade for {111}-dominated Au plates, suggesting the higher surface energy crystal facets may stabilize all of the CO₂ reduction reaction intermediates.