Engineering advanced noble-metal-free electrocatalysts for energy-saving hydrogen production from alkaline water via urea electrolysis.

When compared to OER at the anode of the water electrolyzer, UOR for urea electrolysis can effectively fulfill energy-saving hydrogen production. Various noble-metal-free materials have been regarded as promising electrocatalysts to accelerate the UOR process. The progress, design trends, challenges, and prospects of such electrocatalysts toward UOR are summarized here, with a focus on the activity-enhancing strategies by representing typical examples. [Display omitted] • The energy-saving hydrogen production can be fulfilled by replacing OER with UOR at anode. • Various noble-metal-free catalytic materials for efficiently catalyzing UOR are summarized. • The underlying activity-structure relationships are discussed in detail. • The current challenges and further prospects in this field are also presented. When the anodic oxygen evolution reaction (OER) of water splitting is replaced by the urea oxidation reaction (UOR), the electrolyzer can fulfill hydrogen generation in an energy-economic manner for urea electrolysis as well as sewage purification. However, owing to the sluggish kinetics from a six-electron process for UOR, it is in great demand to design and fabricate high-performance and affordable electrocatalysts. Over the past years, numerous non-precious materials (especially nickel-involved samples) have offered huge potential as catalysts for urea electrolysis under alkaline conditions, even in comparison with frequently used noble-metal ones. In this review, recent efforts and progress in these high-efficiency noble-metal-free electrocatalysts are comprehensively summarized. The fundamentals and principles of UOR are first described, followed by highlighting UOR mechanism progress, and then some discussion about density functional theory (DFT) calculations and operando investigations is given to disclose the real reaction mechanism. Afterward, aiming to improve or optimize UOR electrocatalytic properties, various noble-metal-free catalytic materials are introduced in detail and classified into different classes, highlighting the underlying activity-structure relationships. Furthermore, new design trends are also discussed, including targetedly designing nanostructured materials, manipulating anodic products, combining theory and in situ experiments, and constructing bifunctional catalysts. Ultimately, we point out the outlook and explore the possible future opportunities by analyzing the remaining challenges in this booming field. [ABSTRACT FROM AUTHOR]