Bulk pH contribution to CO/HCOO− production from CO2 on oxygen-evacuated Cu2O electrocatalyst

- Electrocatalytic reduction of CO2 on oxygen-evacuated Cu2O.
- Controlling production of CO and HCOO−via different bulk pH contribution.
- Prioritizing bulk and local pH effect on product selectivity.

Controlling product selectivity is regarded as one of the main barriers in the application of Cu-based electrodes to electrochemical CO2 reduction and hence significant studies on experimental factors to improve the selectivity towards desired products. In particular, two pH parameters appear to be important in controlling product selectivity of the CO2 reduction process. We study how both bulk and local pH conditions affect product distributions during CO2 reduction on oxygen-evacuated Cu2O electrode. In contrast with previous studies of local pH effect on CO2 reduction using metallic Cu, our results suggest that not always is the preferential production of C2 compounds (C2H4 + C2H6) achieved by high local pH condition irrespective of catalyst materials, but rather bulk pH condition could contribute to ability to control product selectivity. It is finally demonstrated that the bulk pH effect, often underestimated but related to carbonate speciation, is associated with the direct reduction of HCO3− to HCOO−.

While the electrochemical reduction of CO2 into C2 compounds on Cu metal was selectively achieved by higher local pH condition, the local pH effect became less important in the CO2 reduction on the oxygen-evacuated Cu2O. Instead, on the oxygen-evacuated Cu2O, we demonstrate that changing the bulk pH condition could influence on product selectivity and the bulk pH effect is derived from the molar fraction of HCO3− with its direct reduction to HCOO−.121