Recent advances in perovskites for electrocatalytic and photocatalytic water splitting processes: materials, characterizations, synthesis and applications

Abstract

Perovskites have attracted attention for hydrogen production due to their structural flexibility, high efficiency, high stability, low cost, tunable bandgap, good thermal stability, and high electron transfer. This review describes the recent progress made in the field of perovskites, which includes their characterizations, synthesis and applications in the electrocatalytic and photocatalytic water splitting processes. The development of stable, low cost, efficient, earth abundant and active electrocatalysts is crucial for hydrogen production. Compared to other oxides, the key differences that make perovskites advantageous for electrocatalytic and photocatalytic water splitting come from their tunable structure and electronic properties. Especially, perovskite oxides are preferable for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to their perfect catalytic ability, diversity in the formation of abundant oxygen vacancies and good electronic conductivity. Perovskite oxides can accelerate charge transfer, ease the formation of oxygen vacancies, tend to change the oxidation state and increase electrocatalytic activity. Some methods such as doping and coupling with metals, non-metals and cocatalysts are applied to improve their performances. This review underlines the effect of doping on perovskite oxides in the OER and HER mechanisms and the improvement of bifunctional perovskite oxides. The effect of anion and cation doping is investigated because doping enhances the surface properties and electronic structures of catalysts by improving their intrinsic catalytic activity. A-site doping influences both OER and HER performances by activating the B-site. B-site doping is a more effective method for designing effective perovskite oxides in terms of stability, activity and performance. Especially, Fe doping is preferable at the B-site due to its high valence to improve OER performance. O-site doping affects the crystal structure, electronic structure, ionic conductivity, chemical stability, band structure, charge balance, concentration of oxygen vacancies and oxygen ion mobility. Dual doping (A, B-site and B, O-site) also results in the formation of multi active sites, especially for the HER. Generally, high entropy perovskite oxides possess a low overpotential and Tafel slope with faster reaction kinetics due to their high covalency. There is a growing interest in developing bifunctional electrocatalysts that can catalyze both the HER and OER. The development of an ideal bifunctional electrocatalysts is also crucial because they have high stability and high activities to provide long-term stability for both HER and OER mechanisms. This review also highlights the photocatalytic water splitting performance of perovskite oxides. Perovskite oxides are preferable as photocatalysts in the photocatalytic water splitting process for hydrogen production because of their facilely tuned band gap, significant chemical and optical properties, low cost, high structural/chemical/thermal stability as well as perfect structural and compositional flexibilities. The selection of proper cations and anions is crucial for the photocatalytic process. Functional doping is a useful method to enhance the photocatalytic performance of perovskite oxides for the hydrogen evolution reaction. Cation doping at the A- and B-sites increases the light absorption capability of perovskite oxides with the formation of intra-band energy levels. However, this may cause some disadvantages such as inferior stability. Instead of cation doping, anion doping into the O-site in perovskite oxides is a more effective method for the regulation of photocatalytic activity, and the most preferred anion dopants are fluorine, chlorine, sulfur and nitrogen for doping into the crystal lattice of perovskite oxides. This review demonstrates that doping improves both electrocatalytic and photocatalytic performances of perovskite oxides. The novelty of the work lies in the fact that the doping effect has importance in the rational design of perovskite oxides with high electrocatalytic and photocatalytic performances. Future studies may focus on the doping effect on perovskite oxides, especially for HER mechanisms and the development of high-entropy perovskite oxides with excellent bifunctional electrocatalytic activities.