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

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