Effective charge separation in photoelectrochemical water splitting: a review from advanced evaluation methods to materials design

Abstract

Photoelectrochemical (PEC) water splitting has garnered significant interest in recent years for alleviating and addressing concerns regarding energy security and environmental pollution. The total efficiency of PEC water splitting is typically divided into three components, i.e., light absorption efficiency, charge separation efficiency, and surface injection efficiency. The charge separation process, including charge recombination and transportation, occurs within the time range of ns–μs, which is much slower than the generation of electron–hole pairs activated by light absorption but faster than that of surface reactions. This step is valuable to modulate but it is challenging to clarify its working mechanism. Nowadays, there is a lack of reviews focusing on both the charge separation process and its advanced evaluation methods. Thus, the present review emphasizes the strategies for enhancing the charge separation efficiency within the semiconductor bulk, across the heterojunction interface, and at the semiconductor/electrolyte surface. Firstly, we introduce the fundamentals of PEC water splitting cells, covering key physical concepts, basic characteristics for water splitting, and evaluation methods. Subsequently, the strategies to facilitate charge separation in metal-based (metal oxides, metal nitrides, and metal sulfides) semiconductor photoelectrodes are mainly reviewed, together with the related qualitative and quantitative methods for analyzing the carrier separation efficiency. Finally, a summary and outlook toward enhanced charge separation efficiency on photoelectrodes and in PEC cells are discussed. The theoretical and experimental insights summarized herein are not only beneficial for understanding the working mechanism of the transportation and recombination of carriers but also for designing and modifying photoelectrodes to improve the solar to hydrogen efficiency of PEC cells.