Carboxysome-Inspired Electrocatalysis using Enzymes for the Reduction of CO2 at Low Concentrations.

The electrolysis of dilute CO2 streams suffers from low concentrations of dissolved substrate and its rapid depletion at the electrolyte-electrocatalyst interface. These limitations require first energy-intensive CO2 capture and concentration, before electrolyzers can achieve acceptable performances. For direct electrocatalytic CO2 reduction from low-concentration sources, we introduce a strategy that mimics the carboxysome in cyanobacteria by utilizing microcompartments with nanoconfined enzymes in a porous electrode. A carbonic anhydrase accelerates CO2 hydration kinetics and minimizes substrate depletion by making all dissolved carbon available for utilization, while a highly efficient formate dehydrogenase reduces CO2 cleanly to formate; down to even atmospheric concentrations of CO2 . This bio-inspired concept demonstrates that the carboxysome provides a viable blueprint for the reduction of low-concentration CO2 streams to chemicals by using all forms of dissolved carbon.

Fast CO2 hydration kinetics impair heterogeneous but improve enzymatic CO2 reduction catalysis.

The performance of heterogeneous catalysts for electrocatalytic CO2 reduction suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilized CO2 reduction enzymes-such as formate dehydrogenase-can operate with high turnover and selectivity at a minimal overpotential and are therefore 'ideal' model catalysts. Here, through the co-immobilization of carbonic anhydrase, we study the effect of CO2 hydration on the local environment and performance of a range of disparate CO2 reduction systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilization of carbonic anhydrase increases the kinetics of CO2 hydration at the electrode. This benefits enzymatic CO2 reduction-despite the decrease in CO2 concentration-due to a reduction in local pH change, whereas it is detrimental to heterogeneous catalysis (on Au) because the system is unable to suppress the H2 evolution side reaction. Understanding the role of CO2 hydration kinetics within the local environment on the performance of electrocatalyst systems provides important insights for the development of next-generation synthetic CO2 reduction catalysts.