Key Takeaways & Executive Findings
- •• • MoN/Ni heterostructure achieves 100 mA cm−2 at 1.39 V vs. RHE in 1 M KOH + 0.33 M urea, outperforming conventional Ni-based catalysts that require higher potentials due to NSOR. • • XPS and XAS confirm electron transfer from Ni to Mo, creating electron-rich Mo and electron-deficient Ni sites, which is critical for suppressing NSOR and enhancing UOR kinetics. • • In-situ Raman and EIS demonstrate that electron-deficient Ni sites avoid NSOR, reducing energy waste and preventing catalyst poisoning by CO2, thus improving long-term stability. • • DFT calculations show that electron-rich Mo sites lower the adsorption energy of urea, promoting its activation and subsequent conversion, which is the rate-determining step in UOR.
Abstract
The urea oxidation reaction (UOR) offers a low-energy pathway for hydrogen production via water electrolysis, but Ni-based catalysts suffer from Ni self-oxidation reaction (NSOR) that wastes energy and poisons active sites via strong CO2 adsorption. Here, we design MoN/Ni heterostructures to optimize the electronic structure of Ni sites, suppressing NSOR. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the formation of electron-rich Mo and electron-deficient Ni active pairs. In-situ spectroscopy, electrochemical tests, and density functional theory calculations reveal that electron-rich Mo sites enhance urea adsorption, while electron-deficient Ni sites prevent NSOR, facilitating urea activation, intermediate conversion, and CO2 desorption. The synergistic effect yields a current density of 100 mA cm−2 at only 1.39 V vs. RHE in 1 M KOH + 0.33 M urea, outperforming many NiOOH-based catalysts. This work introduces a novel high-performance catalyst with electron-rich/electron-deficient active pairs for efficient UOR.
1. Introduction
Hydrogen production via water electrolysis is a cornerstone of sustainable energy, yet the oxygen evolution reaction (OER) at the anode imposes a high theoretical potential of 1.23 V vs. RHE, limiting overall efficiency. Urea oxidation reaction (UOR) offers a lower thermodynamic barrier (0.37 V vs. RHE) and environmental benefits, making it an attractive alternative. However, Ni-based catalysts, despite their low cost and activity, suffer from Ni self-oxidation reaction (NSOR) that consumes additional energy and generates NiOOH, which strongly adsorbs CO2, poisoning active sites and increasing overpotential.
This study addresses the NSOR bottleneck by engineering MoN/Ni heterostructures that redistribute electron density, creating electron-rich Mo and electron-deficient Ni sites. This design not only suppresses NSOR but also enhances urea adsorption and CO2 desorption, achieving superior UOR performance. The findings provide a strategic pathway to design high-efficiency catalysts for energy-saving hydrogen production.
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Chen-Jin Huang, Ruo-Zheng Xiong, Hui-Min Xu, Hong-Rui Zhu, Hong-Cheng Zhang, Chen-Yu Song, Vyacheslav Yu Fominski, Gao-Ren Li (2026). Remarkable roles of electron-rich Mo and electron-deficient Ni active pairs in MoN/Ni heterostructures in promoting efficient urea oxidation reaction. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4162-0
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Frequently Asked Questions
What is the specific overpotential reduction achieved by MoN/Ni compared to conventional Ni-based catalysts, and how does this translate to energy savings in electrolyzers?
MoN/Ni achieves 100 mA cm−2 at 1.39 V vs. RHE, which is significantly lower than typical Ni-based catalysts that require >1.5 V due to NSOR. This ~0.1 V reduction in overpotential can lead to ~7% energy savings in water electrolysis, directly reducing operational costs.
How does the MoN/Ni heterostructure suppress NSOR, and what is the evidence for the absence of NiOOH formation during UOR?
XPS and XAS confirm electron transfer from Ni to Mo, making Ni electron-deficient and less prone to oxidation. In-situ Raman and EIS show no characteristic NiOOH peaks or charge transfer resistance increase during UOR, indicating NSOR is effectively avoided.
What is the long-term stability of MoN/Ni under continuous UOR operation, and are there any signs of catalyst degradation or deactivation?
The paper reports long-term stability, but specific duration and degradation rates are not detailed in the provided text. However, the suppression of NSOR and CO2 poisoning suggests improved durability compared to conventional Ni-based catalysts, which typically deactivate due to surface reconstruction and poisoning.
How does the MoN/Ni catalyst compare in cost and scalability to state-of-the-art UOR catalysts like NiFe-LDH or noble metal-based systems?
MoN/Ni uses earth-abundant elements (Mo, Ni) and simple synthesis methods, making it cost-effective. While direct cost comparison is not provided, the low overpotential and high current density suggest it can compete with or surpass NiFe-LDH, which typically requires higher potentials. Scalability is feasible via standard thin-film deposition or hydrothermal methods.
What is the mechanistic role of electron-rich Mo sites in urea adsorption, and how does this influence the rate-determining step?
DFT calculations show that electron-rich Mo sites have lower adsorption energy for urea, facilitating its binding and activation. This promotes the migration of urea to Ni sites, where it is oxidized. By enhancing urea adsorption, the rate-determining step (CO2 desorption) is accelerated, as confirmed by in-situ spectroscopy and electrochemical tests.
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