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

Abstract

The performance of heterogeneous catalysts for electrocatalytic CO2 reduction (CO2R) suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilised CO2R 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-immobilisation of carbonic anhydrase, we study the effect of CO2 hydration on the local environment and performance of a range of disparate CO2R systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilisation 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 CO2R catalysts.