CO2-triggered break-in and formation of accessible high surface area nanoporous Cu cathode for CO2RR from CuO-MgO nanocomposites

Accessible high surface area copper nanoporous electrode is created in a zero-gap CO₂ reduction reaction (CO₂RR) electrolyzer in-situ from a nanocomposite of copper oxide and magnesium oxide (CuO-MgO). Upon the removal of MgO, a nanoporous CuO exhibiting a very compelling surface area of 201 m²/g with regular 30 nm pores is obtained. After electrochemical reduction, it transformed into a Cu electrode with an unprecedented high roughness factor of 36 cm²_Cu/cm²_geo based on the monolayer surface oxidation charge measured under aqueous half-cell conditions. Under MEA testing conditions, the on-site dissolution of MgO under the presence of water and CO₂ leads to formation of nanoporous copper that exposes large amounts of triple-phase boundaries accessible for mass and charge carriers. The uniformity of the obtained Cu electrode, spanning from macro to nanoscale, is a key essence towards good CO₂RR performance and the suppression of hydrogen evolution reaction. The nanoporous high surface area Cu delivers a maximum CO₂RR current density (j_CO2RR) of 150.8 ± 4.8 mA/cm² at a cell voltage of 3.4 V. At 3.2 V, it delivered a maximum C₂ Faradaic efficiency of 49.6 ± 6.7 % at 103.4 ± 9.9 mA/cm² that was dominated by ethylene production, exhibiting a Faradaic efficiency of 31.0 ± 0.5 %.