The pH at the electrocatalyst surface plays a key role in defining the activity and selectivity of the CO 2 reduction reaction (CO 2 RR). We report here operando Raman measurements of the catalyst surface in a customized CO 2 RR flow cell that enable the measure of pH. Using this flow cell, we were able to measure surface pH as a function of time, current density, and proximity to the catalyst surface during the electrolysis of bicarbonate solutions. We observed that increasing the current density from 0 to 200 mA cm −2 increased the surface pH from 8.5 to 10.3. We also show here that operation at elevated temperatures (70 °C) results in an increased surface pH and serves to suppress the competing and undesirable hydrogen evolution reaction.

The deployment of electrolyzers that convert CO2 into chemicals and fuels requires appropriate integration with upstream carbon capture processes. To this end, the electrolytic conversion of aqueous (bi)­carbonate offers the opportunity to avoid the energy-intensive steps currently used to extract pressurized CO2 from carbon capture solutions. We demonstrate here that an optimized silver gas diffusion electrode (GDE) architecture enables conversion of model carbon capture solutions (i.e., 3 M KHCO3) into CO at partial current densities (J CO) greater than 100 mA cm–2 with CO2 utilization rates of ∼70%. These results exceed the performance of any previously reported liquid-fed CO2 electrolyzers and rival gas-fed devices. We were able to hit these metrics through the systematic design of gas diffusion layer (GDL) components (e.g., polytetrafluoroethylene) and catalyst layer constituents (i.e., Nafion, silver) on CO production. A key finding of this work is that hydrophobic GDE components (which are common to gas-fed CO2RR electrolyzers) decrease in situ CO2 generation and thus the formation of the final CO product. These findings show a clear path toward industrially relevant reactors that couple electrolytic CO2 conversion with carbon capture.

Bicarbonate electrolysers convert carbon capture solutions into chemicals and fuels and bypass the need for energy-intensive CO2 recovery. Porous metal electrodes are more effective than composite carbon electrodes for this type of electrolyser.

Electrochemical CO 2 reduction studies typically supply CO 2 to the cathode as a gas or dissolved in aqueous media. Both of these feedstocks present challenges when scaling a CO 2 electrolyzer: gaseous CO 2 feedstocks require significant energy to pressurize CO 2 , while the low solubility of CO 2 in water precludes high current densities. Using a liquid bicarbonate feedstock bypasses the need for a gaseous CO 2 feedstock while delivering higher concentrations of CO 2 to the cathode than currently possible with CO 2 dissolved in water. We show here that an electrochemical flow cell can be designed such that protons convert bicarbonate into CO 2 (at the catalyst interface), which is then reduced to generate formate. Electrolysis of 3.0 M KHCO 3(aq) solutions yield formate at partial current densities > 100 mA cm −2 , which is nearly commensurate with electrolyzers fed with gaseous CO 2 . The use of bicarbonate as a feedstock presents an opportunity to efficiently integrate carbon capture with CO 2 electrochemistry.

A novel aqueous rechargeable dual-ion battery system is demonstrated in this study, which consists of BiF3 as a fluoride ion electrochemical anode, NMO as a sodium ion electrochemical cathode, and aqueous NaF as the electrolyte.