Comparison of photoelectrochemical water oxidation activity of a synthetic photocatalyst system with photosystem II

Abstract

This discussion describes a direct comparison of photoelectrochemical (PEC) water oxidation activity between a photosystem II (PSII)-functionalised photoanode and a synthetic nanocomposite photoanode. The semi-biological photoanode is composed of PSII from the thermophilic cyanobacterium Thermosynechococcus elongatus on a mesoporous indium tin oxide electrode (mesoITO|PSII). PSII embeds all of the required functionalities for light absorption, charge separation and water oxidation and ITO serves solely as the electron collector. The synthetic photoanode consists of a TiO₂ and NiO_x coated nanosheet-structured WO₃ electrode (nanoWO₃|TiO₂|NiO_x). The composite structure of the synthetic electrode allows mimicry of the functional key features in PSII: visible light is absorbed by WO₃, TiO₂ serves as a protection and charge separation layer and NiO_x serves as the water oxidation electrocatalyst. MesoITO|PSII uses low energy red light, whereas nanoWO₃|TiO₂|NiO_x requires high energy photons of blue-end visible and UV regions to oxidise water. The electrodes have a comparable onset potential at approximately 0.6 V vs. reversible hydrogen electrode (RHE). MesoITO|PSII reaches its saturation photocurrent at 0.84 V vs. RHE, whereas nanoWO₃|TiO₂|NiO_x requires more than 1.34 V vs. RHE. This suggests that mesoITO|PSII suffers from fewer limitations from charge recombination and slow water oxidation catalysis than the synthetic electrode. MesoITO|PSII displays a higher ‘per active’ site activity, but is less photostable and displays a much lower photocurrent per geometrical surface area and incident photon to current conversion efficiency (IPCE) than nanoWO₃|TiO₂|NiO_x.