The reduction of carbon dioxide to renewable fuels and feedstocks offers opportunities for large-scale, long-term energy storage. The synthesis of efficient CO2 reduction electrocatalysts with high C2:C1 selectivity remains a field of intense interest. Here we present electro-redeposition, the dissolution and redeposition of copper from a sol-gel, to enhance copper catalysts in terms of their morphology, oxidation state and consequent performance. We utilized in situ soft X-ray absorption spectroscopy to track the oxidation state of copper under CO2 reduction conditions with time resolution. The sol-gel material slows the electrochemical reduction of copper, enabling control over nanoscale morphology and the stabilization of Cu+ at negative potentials. CO2 reduction experiments, in situ X-ray spectroscopy and density functional theory simulations revealed the beneficial interplay between sharp morphologies and Cu+ oxidation state. The catalyst exhibits a partial ethylene current density of 160 mA cm-2 (-1.0 V versus reversible hydrogen electrode) and an ethylene/methane ratio of 200.

Efficient wide-bandgap perovskite solar cells (PSCs) enable high-efficiency tandem photovoltaics when combined with crystalline silicon and other low-bandgap absorbers. However, wide-bandgap PSCs today exhibit performance far inferior to that of sub-1.6-eV bandgap PSCs due to their tendency to form a high density of deep traps. Here, we show that healing the deep traps in wide-bandgap perovskites-in effect, increasing the defect tolerance via cation engineering-enables further performance improvements in PSCs. We achieve a stabilized power conversion efficiency of 20.7% for 1.65-eV bandgap PSCs by incorporating dipolar cations, with a high open-circuit voltage of 1.22 V and a fill factor exceeding 80%. We also obtain a stabilized efficiency of 19.1% for 1.74-eV bandgap PSCs with a high open-circuit voltage of 1.25 V. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed cation-halide perovskites heals the defects that introduce deep trap states.