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.