No abstract

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.

Electrocatalytic palladium membrane reactors (ePMRs) use electricity and water to drive hydrogenation reactions without forming H2 gas.

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO 2 reduction reaction (CO 2 RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO 2 RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst�cobalt octaethoxyphthalocyanine (EtO 8 −CoPc)�that exhibited high selectivity (FE CO ≥ 95%), high partial current density (J CO ≥ 300 mA/cm 2 ), and high durability (ΔFE CO < 0.1%/h at 150 mA/cm 2 ) in a flow cell. This work demonstrates how to accurately identify CO 2 RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO 2 RR catalysis in a flow cell is the metal center.

The reduction of CO 2 to CO from a bicarbonate feedstock offers an opportunity to directly use aqueous carbon capture solutions, while bypassing ex-situ energy-intensive gaseous CO 2 regeneration. In this study, we resolved how the electrolyte cation identity (Li + , Na + , K + , Cs + ) affects the two reactions that make bicarbonate electrolysis possible: (i) the production of insitu CO 2 formed through reaction of HCO 3 À (from the catholyte) with H + (sourced from the membrane); and (ii) the electroreduction of CO 2 into CO. Our results show that cation identity does not change the rate of in-situ CO 2 formation, but it does enhance the rate of the CO 2 reduction reaction (CO2RR).Electrolysis experiments performed with a constant [HCO 3 À ] showed that CO selectivities progressively increased for the series Li + , Na + , K + , and Cs + , respectively. Optimization of the electrolyte composition yielded a CO selectivity of~80 % during electrolysis of 1.5 M CsHCO 3 solutions at 100 mA cm À 2 , while saturated LiHCO 3 solutions (0.84 M) yielded CO selectivities values of merely 30 % at the same current density. This study demonstrates a quantitative relationship between CO product selectivity and the cation radius, which provides a pathway to integrate bicarbonate electrolysis to carbon capture.