Fast CO 2 hydration kinetics impair heterogeneous but improve enzymatic CO 2 reduction catalysis.

The performance of heterogeneous catalysts for electrocatalytic CO 2 reduction suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilized CO 2 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 CO 2 hydration on the local environment and performance of a range of disparate CO 2 reduction systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilization of carbonic anhydrase increases the kinetics of CO 2 hydration at the electrode. This benefits enzymatic CO 2 reduction-despite the decrease in CO 2 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 H 2 evolution side reaction. Understanding the role of CO 2 hydration kinetics within the local environment on the performance of electrocatalyst systems provides important insights for the development of next-generation synthetic CO 2 reduction catalysts.