The Direct Electrochemistry of Fuel-Forming Enzymes on Semiconducting Electrodes: How Light-Harvesting Semiconductors Can Alter the Bias of Reversible Electrocatalysts in Favour of H₂ Production and CO₂ Reduction

Abstract

This chapter describes the direct electrochemistry of the fuel-forming enzymes [NiFeSe]-hydrogenase (Sect. 1.6.3) and carbon monoxide dehydrogenase (CODH, Sect. 1.6.4) immobilised on porous n-type semiconductor electrode materials. Being reversible electrocatalysts for CO₂/CO and H⁺/H₂ interconversion, these enzymes show high activities for both oxidation and reduction when attached to metallic-like graphite electrodes (PGE). Yet, whereas the most efficient catalysts for the formation of solar fuels should, in addition to operating close to reversible potentials, possess a catalytic bias for the fuel-forming direction, both [NiFeSe]-hydrogenase and CODH possess an inherent catalytic bias that favours the oxidation over the reduction half-reaction. When [NiFeSe]-hydrogenase and CODH are adsorbed on n-type semiconductor electrodes constructed from CdS and TiO₂ nanoparticles, a marked shift in bias is observed, now favouring CO₂ or H⁺ reduction. The fuel-forming reaction is efficiently catalysed at minimal overpotentials (both in the dark and under illumination), while the, in this case destructive oxidation reaction is greatly suppressed. This chapter describes how the electronic state of the electrode can strongly bias the direction of electrocatalysis of both CO₂ and H₂ cycling. Electrical impedance spectroscopy (EIS) experiments shed light on the binding mode of these enzymes on porous surfaces and indicate possible rate-limiting parameters in photocatalytic CO₂ or H⁺ reduction by metalloenzymes adsorbed on semiconductor nanoparticles.

Part of the work presented in this chapter has been published: Andreas Bachmeier, Vincent C.-C. Wang, Thomas W. Woolerton, Sophie Bell, Juan C. Fontecilla-Camps, Mehmet Can, Stephen W. Ragsdale, Yatendra S. Chaudhary, and Fraser A. Armstrong, J. Am. Chem. Soc. 2013, 135, 15026.