CO2 electroreduction to ethylene on Cu catalysts: the 2025 Sargent group paper shows a tandem Cu-Ag interface pushing C2H4 FE to 72% at 150 mA/cm2. Operando XAS reveals Cu+ subsurface as the active site. The remaining barrier is carbonate crossover in MEA cells.
Cu+ as the active site is still debated. The Sargent paper used XAS but the time resolution (seconds) cannot rule out transient Cu0/Cu+ cycling. DFT from the Nørskov group suggests CO dimerization on Cu(100) terraces is rate-limiting, not the oxidation state per se.
Fair critique on time resolution. But the 2025 Sargent paper also used XANES pre-edge analysis which is more sensitive to Cu+ than EXAFS. And they correlated Cu+ fraction with C2H4 selectivity across 4 different catalyst compositions. Correlative but not definitive — I agree DFT is needed to close the loop.