Roles of Metal-Free Materials in Photoelectrodes for Water Splitting

ConspectusPhotoelectrochemical (PEC) water splitting is an appealing approach to the hydrogen evolution reaction since it converts sunlight in the form of hydrogen fuel, which has the potential to revolutionize the fossil fuel-based energy systems of the modern society. In the last half century, progress has been made with respect to the material, synthesis, and system. Recent developments of multilayered photoelectrodes have made a breakthrough to improve the sunlight conversion efficiency and strengthen the physiochemical stability. The exploration of new materials for the functional layers of photoelectrodes offers a new opportunity for practical application. Among the emerging materials, metal-free species have shown superior properties, such as high stability, sustainability, and renewability. With respect to inorganic materials, their physiochemical properties can be readily regulated, including thermodynamics and kinetics, and thus increasing attention has been devoted.In this Account, the functional components of a photoelectrodes are specified, namely conductive layers for charge separation and transfer, photocatalytic layers for light absorption, passivation layers for protection of the electrode, and cocatalysts for the acceleration of the surface reaction. We have focused metal-free materials for the functional layers, including the (1) carbon cloth and carbon quantum dots (CQDs) for the conductive layer, 2) polymeric carbon nitrides (PCNs), carbon doped boron nitride, and covalent organic frameworks for photocatalytic layers, and (3) CQDs and black phosphorene for cocatalysts. We aim to identify the relationship between the function of the metal-free materials and the performance of the water splitting reaction. With respect to the outlook, discussions have been presented for the further development of metal-free materials to improve the performance of each functional layer. Despite the fact that the use of metal-free materials for PEC water splitting is still in its infancy, this Account not only provides new thoughts and opportunities to pursue an improved performance for the water-splitting reaction but also stimulates advanced applications by using metal-free PEC systems.