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Research Progress of Urea Splitting Catalysts for Hydrogen Generation
WANG Liuliu, REN Jie, LU Xingyu, ZOU Li, XIE Jiale
Materials Reports
2023, 37 (12):
21070195-15.
DOI: 10.11896/cldb.21070195
Hydrogen energy is both a zero-carbon fuel and a bridge for the transition and conversion between fossil fuels and renewable energy. Compared to water splitting for hydrogen production, urea splitting is an energy-saving strategy, which can also solve the problem of urea pollution. Urea has the merits of vast reserves, safety, and low cost, and its theoretical potential for urea splitting is far lower than water splitting, which is one of the important sources of hydrogen in the future. Urea oxidation reaction (UOR)is an important half-reaction of the urea splitting for hydrogen production, which determines the efficiency of the urea electrolysis cells or the urea fuel cells. Catalyst plays a key role in UOR reaction, and its physicochemical and surface properties will significantly affect the kinetics of UOR reaction. At present, nickel-based catalysts can be classified into three subsidiary sets:unsupported nickel-based catalysts, supported nickel-based catalysts, and non-nickel-based catalysts. However, the overpotential of the UOR reaction is high, which is about 1 000 mV higher than the theoretical value. The UOR reaction is a complex reaction process involving several proton-coupled electron transfer steps, whose deep mechanism is much more complex than the simple C-N bond cleavage. Precious metal-based catalysts are frequently applied as benchmarks for UOR catalysis. However, the drawbacks of expensiveness and scarcities are the major reasons for their limiting roles in current scientific research and future commercialization of urea-based technologies. Nonprecious transition metal-based catalysts have become the focus of research, whose catalytic performance may match or even surpass those of the precious metal catalysts. Nickel-based catalyst has become a class of star materials to catalyze UOR reaction, and many nickel-containing catalysts such as nickel metal, nickel hydroxide, nickel phosphate, and nickel metal organic framework materials. Further, nickel-containing composite catalysts such as Ni-Zn-Co, MoO2/Ni2P are also developed recently. The newly controllable methods of UOR catalysts such as the defect engineering, structural engineering and interface engineering are also developed. The UOR catalysts with nickel as the main active component are the focus of electrocatalytic or photoelectrocatalytic UOR reaction. According to the growth substrate of catalysts, it can be divided into two types of unsupported nickel-based catalysts and supported nickel-based catalysts. In recent years, non-precious metal and high-efficiency nickel-based electrocatalysts in the form of metals, oxides, hydrogen oxides, phosphorides, and metal-organic framework materials have been attracting wide attention for their cost reduction and accelerated reaction kinetics. On the other hand, a bimetal-catalyst or trimetal-catalyst formed by the hybridization of transition metals and nickel elements, including cobalt, copper, molybdenum, manganese, and iron, has also been extensively studied for UOR reactions. To solve the common problem of low conductivity of nickel-based catalyst, continuous attempts have been made to use nickel foam, carbon material, titanium mesh, and other high conductivity substrates with nickel-containing catalysts. In addition, some new non-nickel-based materials are also used as catalysts for the UOR reaction. This review offers a retrospection of the research efforts with respect to the urea splitting catalysts, and provides elaborate descriptions about the principle of urea splitting, the electrocatalysts for urea splitting, the photoelectrocatalysts for urea splitting, and the rational design/synthesis of UOR catalysts. We then pay attention to the problems confronting the current state-of-the-art UOR catalysts. This review can be used as a refe-rence for the preparation of high-efficient catalysts for hydrogen production from urea splitting.
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