Roles of Metal-Free Materials in Photoelectrodes for Water Splitting

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Photoelectrochemical (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.

Tailor‐Engineered 2D Cocatalysts: Harnessing Electron–Hole Redox Center of 2D g‐C3N4 Photocatalysts toward Solar‐to‐Chemical Conversion and Environmental Purification

“…[84] Moreover, the well-known hybridization technique with other semiconductors or cocatalyst materials to form diversity heterostructure has also been discussed in the work of Li et al, [85] Hasija et al, [86] Ong et al, [87] Zhang et al, [88] Bie et al, [29a] and Ng et al [89] In addition, Wang and co-workers discussed the intriguing role of metal-free materials (e.g., carbon nitride) as the photocatalytic layer in multilayered photoelectrode system. [90] These few works have discussed the fascinating interactions between g-C 3 N 4 and other materials to form different types of heterojunctions such as Type-I, Type-II, Schottky, Z-scheme, and S-scheme. Distinctively, several review papers focused on the cocatalyst loading modification in photocatalytic energy and environmental applications as reported by Zhu et al, [91] Biswal et al, [92] Liang et al, [93] Tong et al, [94] Yuan et al, [95] You et al, [96] Li et al, [97] Bie et al, [29a] and Xu et al [98] These works have implied the significant contribution and breakthrough of cocatalyst engineering technique to the advancement of photocatalysis.…”

“…[ 89 ] In addition, Wang and co‐workers discussed the intriguing role of metal‐free materials (e.g., carbon nitride) as the photocatalytic layer in multilayered photoelectrode system. [ 90 ] These few works have discussed the fascinating interactions between g‐C 3 N 4 and other materials to form different types of heterojunctions such as Type‐I, Type‐II, Schottky, Z‐scheme, and S‐scheme. Distinctively, several review papers focused on the cocatalyst loading modification in photocatalytic energy and environmental applications as reported by Zhu et al., [ 91 ] Biswal et al., [ 92 ] Liang et al., [ 93 ] Tong et al., [ 94 ] Yuan et al., [ 95 ] You et al., [ 96 ] Li et al., [ 97 ] Bie et al., [ 29a ] and Xu et al.…”

One‐Pot Synthesis of CoS2Merged in Polymeric Carbon Nitride Films for Photoelectrochemical Water Splitting

“…[8] However, it is still a challenge to form photoelectrodes with the crystallized PCN materials. [9] To facilitate photoexcitation charge transfer, attempts have been made to develop conductive scaffolds at the bottom of the PCN films, and the performance of PEC water splitting could therefore be significantly improved. [10] Typically, yttrium-doped zinc oxide nanorods were acquired under PCN films to improve the photoexcitation charge separation and transfer, [11] effectively improving the performance.…”

“…Crystallized PCN has presented an improved performance for water splitting in a particle suspension system owing to the promoted charge transfer [8] . However, it is still a challenge to form photoelectrodes with the crystallized PCN materials [9] …”

High-performance potassium poly(heptazine imide) films for photoelectrochemical water splitting

“…Solar-driven water splitting for hydrogen fuel production using a photoelectrochemical (PEC) platform is an appealing strategy by which to sustain energy demand. [1][2][3][4] In the PEC system, semiconducting photoelectrodes with the properties of the efficient absorption of visible light and high quantum yield have been researched for almost half a century, 5 and a variety of materials, including metal oxides, [6][7][8][9] metal oxynitrides, 10,11 perovskites, 12 polymers, 13 metal-organic frameworks [14][15][16] and other materials, [17][18][19][20][21] have been reported. Among them, conjugated polymers have received broad attention recently.…”