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

Abstract

The performance of heterogeneous catalysts for electrocatalytic CO 2 reduction (CO 2 R) suffers from unwanted side reactions and kinetic inefficiencies at the required large overpotential. However, immobilised CO 2 R 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 CO 2 hydration on the local environment and performance of a range of disparate CO 2 R systems from enzymatic (formate dehydrogenase) to heterogeneous systems. We show that the co-immobilisation 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 R catalysts.

Carboxysome-inspired electrocatalysis using enzymes for the reduction of CO2 at low concentrations

“…While we have previously shown that FDh produces formate with nearly 100% FE under pure CO2 on ITO, 28,38 the reduced FE under low CO2 concentrations can be tentatively attributed to the residual background reduction of the metal oxide electrode. [39][40][41] This can be observed in the absence of enzymes with no liquid or gaseous products being detected by Ion or Gas Chromatography, respectively (Figure S5) at current densities that match well with the unattributed FE under low CO2 concentrations.…”

“…The formate partial current density of CO2R by immobilized FDh (jformate, points, Figure 2a) in 1% and 10% CO2 purged solutions with N2 as the balance gas, unlike when purged with concentrated CO2 28 , was not affected by CA co-immobilization. Formate was the quantitative product as no other liquid or gas phase products (such as H2, CO or CH4) were detected by ion chromatography or headspace gas chromatography, respectively (for total and partial current densities from all experiments including controls see Table S2).…”

“…A Finite Element Model (FEM) using a previously reported approach 28,38 was used to model the local environment that was experimentally inaccessible (Figure S6). 42 This uses well understood physical and analytical equations, with enzyme properties determined from solution assays, 37 solved over a mesh that represents the geometry of the system to model the response and was in good agreement with experimentally observed currents (lines, Figure 2a), allowing the concentrations of species within the porous electrode to be determined (Figure 2b-c).…”

“…The fuel-forming enzyme formate dehydrogenase (FDh) has previously been combined with CA for enzymatic CO2R to demonstrate that CO2 (and not HCO3 − ) is the substrate of FDh. 27 In addition, CA has been used to catalyze the conversion of CO2 to HCO3 − , decreasing local CO2 concentrations and increasing HCO3 − and H + (directly contrasting its role in increasing CO2 concentrations in the carboxysome) either to mitigate local pH changes in saturated CO2 28 or for the use of HCO3 − as a substrate. 29 Enzymatic CO2R at low CO2 concentrations with CA has also relied on an energy inefficient NADH cofactor recycling system to transfer electrons from an electrode to enzymes in bulk solution giving a low FE and offering no capability to mimic the nanoconfined environment of a carboxysome on an electrode [29][30][31] , a property that may be highly beneficial for CO2R at reduced rates as it has been shown to improve enzyme's affinity for substrate and kinetic rates.…”

Infrared Spectroscopy Reveals Metal independent Carbonic Anhydrase Activity in Crotonyl CoA Carboxylase/Reductase

“…Taken together, in situ ATR FTIR difference spectroscopy allowed investigating the interaction of different enzymes with CO2, providing a simple, yet powerful approach to directly identify CA activity for a given biological sample. Moreover, our method is also suited to identify and characterize the source of water and will serve as an important tool to analyze CO2 hydration in biocatalysis [28][29][30][31] , homogenous or heterogeneous catalysts [32][33][34] , and (de-)hydration reactions in general. [35,36]…”