Two-dimensional materials for electrocatalytic water splitting

Abstract

Hydrogen is a kind of clean energy with high calorific value and non-pollution. There are many methods for hydrogen production. Fuel processing technologies transform a hydrogen containing material such as coal, petroleum, or natural gas into a hydrogen rich stream. However, these processes need an external heat source for the reactor and produce large amounts of carbon dioxide. Hydrogen production by electrolysis of water is regarded as an advanced technology to make effective use of renewable resources, such as wind power, solar power, etc., to achieve energy storage and conversion. Water electrolysis includes hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). These reactions are normally catalyzed by precious metals, such as platinum (Pt) and iridium (Ir)-based catalysts, which limits the large-scale application of electrolysis of water. Thus, it is necessary to develop alternative catalysts with low cost and high performance. Two-dimensional (2D) materials have considerable application prospect in electrocatalysis of H2O because of their unique structural and electronic properties. In addition, 2D materials with a reduced dimension compared with the bulk material exhibits several distinctive properties, such as high specifc surface area, high thermal and electric conductivity and more catalytic active sites. In this review, the key scientific issues and the latest advances in the two half-reactions (HER and OER) of electrocatalytic water splitting with 2D materials are systematically summarized. The mechanisms of HER and OER are discussed briefly. The involved 2D materials for HER in this work include graphene, graphene encapsulated transition-metal catalysts, g-C3N4 and 2D transition-metal dichalcogenides, while for OER contain layered double hydroxide (LDH) and graphene encapsulated transition-metal catalysts materials. For graphene, g-C3N4 and 2D transition-metal dichalcogenides, there are various techniques to enhance the catalytic activity of the materials, such as the introduction of defects, heteroatom-doped (N, B, P, S or metal atoms) and functional groups. For graphene encapsulating earth-abundant transition metal nanoparticles, the activity of electrocatalytic water splitting can be improved by the electron transfer from the metal core. Furthermore, the utilization of strong coupling between various 2D materials is another facile approach to optimize the catalytic activity. This review enumerates several typical 2D materials and recent applications for the two half-reactions of electrocatalytic water splitting respectively. The future challenges and opportunities in this field are also discussed. The strategy for designing novel HER electrocatalysts with high performance mainly focuses on the electronic structure engineering of 2D materials to modify electronic properties and optimize the adsorption and desorption behavior of H atoms. The design of high-performance and long-term durability OER electrocatalysts in acidic medium still remains a major challenge. Although the obtained electrocatalysts for water splitting still suffer from some serious problems when it comes to large-scale applications, the unique electronic structure of 2D materials and possibility of modifcations on the surface offer opportunities to synthesize novel electrocatalysts with low cost, high catalytic activity and high stability. Thus, it is possible to adopt 2D materials as catalysts in electrocatalytic water splitting reactions. It is expected to give guidance for the comprehension of 2D materials and their applications in electrocatalytic water splitting.