Fig. 6.

Band energy diagrams and device architectures of proposed heterogeneous photoredox methanation systems. a A single semiconductor photocatalyst that drives both the water oxidation and CO₂ reduction reactions at its valence (VB) and conduction band (CB) sites. b A semiconductor photocatalyst with co-catalysts added to facilitate reduction and oxidation half-reactions. Electrons and holes can be transferred to the co-catalysts to initiate the associated half-reactions. c A Z-scheme consisting of two semiconductor photocatalysts, in which the water oxidation reaction is occurring at VB 1, and the CO₂ reduction reaction occurs at CB 2. Electron transfer between the two semiconductors can be facilitated through the appropriate choice of semiconductors. Photoelectrochemical device architectures of (d) a monolithic device, in which protons and electrons are transferred from the anode to the cathode through an electrolyte or via conduction, respectively; (e) a wired device, in which protons and electrons are transferred from anode to cathode via a membrane and an external circuit, respectively; and (f) a photoelectrochemical cell, in which the anode and cathode are separated by a proton-conducting membrane with integrated electron-conducting material⁹⁹. Adapted from ref. ⁹⁹—Published by the Royal Society of Chemistry