Engineering Morphology and Electron Redistribution of a Ni/WO3 Mott–Schottky Bifunctional Electrocatalyst for Efficient Alkaline Urea Splitting
Kai Zhang,
Fengchen Guo,
Nigel Graham
et al.
Abstract:Construction of the desired morphology and nanointerface to expose the active sites and modulate the electronic structure offers an effective approach to boosting urea splitting for energy-saving hydrogen generation. Herein, we fabricate a Ni/WO 3 Mott−Schottky heterojunction electrocatalyst with a hedgehog-like structure supported on Ni foam toward alkaline urea splitting. Different Ni/WO 3 morphologies, such as microspheres, hedgehog-like structures, octahedrons, and cubes, were obtained when various ratios … Show more
In contrast to the thermodynamically unfavorable anodic oxygen evolution reaction, the electrocatalytic urea oxidation reaction (UOR) presents a more favorable thermodynamic potential. However, the practical application of UOR has been hindered by sluggish kinetics. In this study, hierarchical porous nanosheet arrays featuring abundant Ni‐WO3 heterointerfaces on nickel foam (Ni‐WO3/NF) is introduced as a monolith electrode, demonstrating exceptional activity and stability toward UOR. The Ni‐WO3/NF catalyst exhibits unprecedentedly rapid UOR kinetics (200 mA cm−2 at 1.384 V vs. RHE) and a high turnover frequency (0.456 s−1), surpassing most previously reported Ni‐based catalysts, with negligible activity decay observed during a durability test lasting 150 h. Ex situ X‐ray photoelectron spectroscopy and density functional theory calculations elucidate that the WO3 interface significantly modulates the local charge distribution of Ni species, facilitating the generation of Ni3+ with optimal affinity for interacting with urea molecules and CO2 intermediates at heterointerfaces during UOR. This mechanism accelerates the interfacial electrocatalytic kinetics. Additionally, in situ Fourier transform infrared spectroscopy provides deep insights into the substantial contribution of interfacial Ni‐WO3 sites to UOR electrocatalysis, unraveling the underlying molecular‐level mechanisms. Finally, the study explores the application of a direct urea fuel cell to inspire future practical implementations.
In contrast to the thermodynamically unfavorable anodic oxygen evolution reaction, the electrocatalytic urea oxidation reaction (UOR) presents a more favorable thermodynamic potential. However, the practical application of UOR has been hindered by sluggish kinetics. In this study, hierarchical porous nanosheet arrays featuring abundant Ni‐WO3 heterointerfaces on nickel foam (Ni‐WO3/NF) is introduced as a monolith electrode, demonstrating exceptional activity and stability toward UOR. The Ni‐WO3/NF catalyst exhibits unprecedentedly rapid UOR kinetics (200 mA cm−2 at 1.384 V vs. RHE) and a high turnover frequency (0.456 s−1), surpassing most previously reported Ni‐based catalysts, with negligible activity decay observed during a durability test lasting 150 h. Ex situ X‐ray photoelectron spectroscopy and density functional theory calculations elucidate that the WO3 interface significantly modulates the local charge distribution of Ni species, facilitating the generation of Ni3+ with optimal affinity for interacting with urea molecules and CO2 intermediates at heterointerfaces during UOR. This mechanism accelerates the interfacial electrocatalytic kinetics. Additionally, in situ Fourier transform infrared spectroscopy provides deep insights into the substantial contribution of interfacial Ni‐WO3 sites to UOR electrocatalysis, unraveling the underlying molecular‐level mechanisms. Finally, the study explores the application of a direct urea fuel cell to inspire future practical implementations.
Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni3+ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni3+ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH−, CO32−, SiO32−, MoO42−, and WO42− were used to bond with Ni2+ to fabricate the pre-catalysts β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni3+ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm–2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts.
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