Designing Advanced Catalysts for Urea Electrocatalytic Oxidation Reaction

Abstract

Urea is the primary end product of nitrogen metabolism in humans, excreted from the human body by kidney function through urine. Abnormal concentrations of urea in the human urine provide essential information on renal function and chronic kidney disease. Moreover, a recent investigation reveals that industrial urea factories produce wastewater, including two kiloton urea per day. Thus, treating wastewater with a high level of urea is also one of the difficulties confronted by different industrial plants. Developing catalysts toward urea oxidation reaction (UOR) is technologically essential for various research fields, including urea sensors, wastewater treatment, hydrogen production, and energy storage. Based on the extensive theoretical and experimental research effort, recent advancements in Ni-based catalysts toward UOR in alkaline media have proposed reaction mechanisms with high catalytic activity and cost-effectiveness. At the moment, UOR performances with Ni-based catalysts are generally restricted by the six-electron transfer process. Hence, various catalytic designs and synthesis strategies, including alloy formations, modification nanostructures, defect engineering, and surface treatment, propose to produce highly efficient UOR. This dissertation investigates the development of catalysts for the improvement of UOR performance, understanding of the mechanism of UOR, and exploration of flexible sweat-based urea sensor and direct urea fuel cell platform. Therefore, the nickel and nickel-based compounds undergo oxidation to nickel's active state (NiOOH) in an alkaline medium and then play the role of UOR catalyst. Adjusting the content ratio of Ni2+/Ni3+ could deliver the maximum catalytic ability for urea oxidation due to the efficient synergism for the high valence state of Ni species (Ni3+) formation, little electron transfer resistance, and effective catalytic kinetics. In addition, Ni-based catalysts inherently occupied Ni3+ ions (NiOOH as a catalyst) will promote the electrooxidation performance of urea more efficiently. The significant challenges and prospects for the future advancement of UOR and corresponding catalysts are also addressed.