Electrocatalytic urea oxidation: advances in mechanistic insights, nanocatalyst design, and applications

Abstract

The urea oxidation reaction (UOR) has emerged as one of the promising half-reactions for energy conversion and storage devices due to its low thermodynamic potential (−0.46 V vs. SHE). However, the complex 6-electron transfer process in the UOR causes intrinsic sluggish kinetics, still requiring a significant enhancement in the catalytic activity and energy utilization of electrocatalysts. Hence, this review aims to provide comprehensive insights into how to design high-performance electrocatalysts for the UOR and develop highly efficient UOR-based energy devices. We first highlight the recent progress in the proposed mechanisms (e.g., adsorbate evolution, lattice oxygen involvement, and chemical–electrochemical processes) and general electrochemical evaluation standard parameters for the UOR. Subsequently, five effective strategies of nanocatalyst design are summarized and discussed for improving electrocatalytic urea oxidation. Rationally engineering nanocatalysts' crystal and geometric structure is available for increasing the densities of catalytic active sites. The improvement of intrinsic activity can depend on heteroatom doping and defect engineering to adjust the electronic and coordination environment of active sites. The heterojunction construction strategy can also be adopted to regulate the Janus charge distribution for the adsorption of urea. In addition, relevant practical applications are introduced in detail, such as photoelectrochemical urea splitting for hydrogen production, urea-assisted reversible zinc–air batteries (ZABs), and direct urea fuel cells (DUFCs). The remaining concerns and challenges in the urea electrocatalysis field are also discussed.