Interfacial processes involving strong electronic interactions in solar energy conversion and storage

Abstract

It has been known for a long time that strong chemical bonding of intermediate species is necessary for efficient electrocatalysis, particularly of electron transfer mechanisms. This important aspect has been neglected in previous investigations of materials for fuel-producing and energy-storing photoelectrochemical reactions. In order to emphasize this statement the progress of experiments with two groups of photoelectrodes which undergo strong interaction with reactants in the electrolyte is discussed. The first group consists of d band semiconductors ( e.g. PtS 2 and RuS 2) in which the photogeneration of holes in the valence band derived from transition metal d states leads to the formation of interfacial coordination complexes with electron donors ( e.g OH −). The second group consists of electrodes exhibiting combined electronic and ionic conduction which are able to photo-intercalate guest species ( e.g. Cu + into Cu 6− x PS 5I or Cu 3PS 4). The behaviour of anodically polarized RuS 2 (Δ E G = 1.2 eV) as a stable photoelectrode which is able to liberate oxygen from water with a high quantum efficiency during illumination with visible and near-IR light confirms that complicated energy conversion reactions can take place in the presence of strong interactions even with thermodynamically unstable materials. By comparing the performances of various ruthenium dichalcogenides it is shown that the photoelectrochemical reaction path depends strongly on the d state density in the upper region of the valence band. Energy losses due to kinetic inhibition (unfavourable energetic position of the intermediate states) can be reduced by using semiconductors containing transition metal pairs or clusters in their crystal structure. Semiconductor materials which are able to photo-intercalate or photo-insert cations as a result of strong electron transfer interactions could be developed for light-powered ion pumps, intercalation batteries which can be charged by solar energy and systems capable of storing optical information. Photoelectrode systems which undergo strong interactions with redox systems open up new perspectives for photoelectrochemical energy conversion and storage but also demand major research efforts. New materials with specific electronic and interfacial properties for which simplified electron transfer models are not adequate need to be developed. It will also be necessary to deal with high concentrations of surface states.