Optimizing Cu⁺-Cu⁰ synergy by operando tracking of Cu₂O nanocatalysts during the electrochemical CO₂ reduction reaction

Tracking the evolution of electrocatalysts over oxide-derived Cu materials during the electrochemical CO₂ reduction reaction (eCO₂RR) is pivotal for optimizing the product selectivity toward desired multi-carbon (C₂₊) products. However, the identification of the true intermediate active catalyst is still unclear. Here, we adopted a multi-modal characterization approach, primarily based on operando powder X-ray diffraction and operando micro-Raman spectroscopy, to study three Cu₂O precursors with different morphologies, namely, octahedral (O-), cubic (C-), and nanowire (N-Cu₂O). This multi-modal approach allows us to investigate the Cu₂O nano-crystallites from the interface to the bulk structure. The results suggested notably different electrochemical reduction kinetics. 26.1% O-Cu₂O and 90.6% C-Cu₂O were reduced to much smaller Cu(0) domains after two hours of time-on-stream; N-Cu₂O, with notably higher surface-to-volume ratio, was completely reduced within 45 min of time-on-stream. We accordingly observed a structure-reactivity correlation where a more intricate Cu₂O/Cu grain network (and hence Cu⁺-Cu⁰ junctions) as observed in O-Cu₂O, can lead to stable and quantitative production of ethylene at the Faradic efficiency of around 40% (in stark contrast to those of C- and N-Cu₂O). The synergy between the Cu₂O and Cu phases was also verified by density functional theory calculations. The upshifted D-band center of Cu₂O/Cu in O-Cu₂O is the most conducive toward the production of ethylene, whereas the downshifted D-band center of Cu₂O/Cu in C-Cu₂O leads to a decreased production of ethylene in the expense of unwanted production of hydrogen. We envisage that system optimization and design of new catalysts will become more facile and efficient using a related multi-modal operando characterization philosophy.