MXene quantum dots induced Ni site d-band center self-optimization for advanced photovoltaic-driven urea-assisted water splitting

doi:10.1016/S1872-2067(26)65152-2

Abstract

Abstract:

Urea-assisted electrolytic water splitting for hydrogen production represents an effective contemporary strategy for generating green hydrogen. The sluggish kinetics of the anodic urea oxidation reaction (UOR) and cathodic hydrogen evolution reaction (HER) limit their practical application. The MXene quantum dots (MQDs) promote pronounced d-d orbital hybridization between Ni and Mo sites. The constructed heterostructures facilitate electron transfer and modulate charge redistribution. The extensive formation of heterostructures between numerous quantum dots and nickel molybdate results in the emergence of flower-like clusters at the tips of rod-like NiMoO₄ particles. The augmentation of the specific surface area results in the exposure of a larger quantity of active sites, which facilitates the redistribution of charge across these active sites, consequently enhancing the hydrolytic activity. The electrocatalyst exhibits remarkable performance in both the UOR and HER, achieving 1.31 V and 55 mV at 10 mA cm^-2, respectively. The catalyst for urea-assisted water electrolysis requires only 1.38 V at 10 mA cm^-2. The prepared catalyst can be employed for seawater splitting and photovoltaic-driven hydrogen production. In-situ Raman spectrum confirms that Ni sites on the NiMoO₄/MQDs surface transform into Ni-OOH under hydrolysis and subsequently govern the hydrolytic reaction. This work indicates that d-d orbital hybridization can provide pathways for electronic transitions, thereby enhancing charge-transfer efficiency and effectively lowering reaction energy barriers.

Key words: Urea oxidation reaction, Hydrogen evolution reaction, Heterostructure, MXene Quantum Dots, Charge redistribution

Yufeng Jiang, Shaobin Li, Li Zhang, Yang Yang, Kun Cheng, Fengbo Li, Xiaoqing Lv. MXene quantum dots induced Ni site d-band center self-optimization for advanced photovoltaic-driven urea-assisted water splitting[J]. Chinese Journal of Catalysis, 2026, 89: 207-217.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65152-2

https://www.cjcatal.com/EN/Y2026/V89/I10/207

Figures/Tables 6

Fig. 1. (a) Schematic diagram of the synthesis of NiMoO4/MQDs. (b) SEM image of NiMoO4. SEM image (c), TEM image (d), HRTEM image (e), and SAED pattern (f) of NiMoO4/MQDs. Contact angle photographs (g) and EDS mapping plot (h) of NiMoO4/MQDs.

Fig. 2. (a-c) The XRD patterns of NiMoO4 and NiMoO4/MQDs. XPS survey (d) spectra and O 1s (e) of NiMoO4/MQDs. Ni 2p (f) and Mo 3d (g) XPS spectra of NiMoO4/MQDs.

Fig. 3. (a) Polarization curves of NiMoO4/MQDs in UOR and OER. (b) LSV curves for various catalysts. (c) Tafel slopes. (d) Bode-phase plots of NiMoO4/MQDs measured at different anodic polarization potentials. (e) Calculated Cdl values for the electrode. (f) EIS spectra obtained at a fixed overpotential of 10 mA cm-2. (g) Comparative analysis of overpotentials and Tafel slopes. (h) Property comparison between NiMoO4/MQDs and NiMoO4. (i) Chronopotentiometry curve recorded at 10 mA cm-2 (without iR compensation); inset: LSV curves before and after 1000 CV cycles.

Fig. 4. LSV curves (a) and Tafel plots (b) of NiMoO4/MQDs, NiMoO4, Pt/C and NF in 1 mol L-1 KOH. (c) Comparison of overpotentials of different catalysts at 10 mA cm-2 and Tafel plots. (d) The calculated value of Cdl for the relevant electrode. (e) Nyquist plot. (f) The TOF value of catalysts of NiMoO4, NiMoO4/MQDs-2, NiMoO4 /MQDs, NiMoO4 /MQDs-6 and NiMoO4/MQDs-8. (g) The time-potential curve of the HER NiMoO4/MQDs at 10 mA cm-2 (inset): LSV curves before and after 1000 CVs stability test. (h) Comparison of different electrochemical properties of NiMoO4/MQDs and NiMoO4. (i) Some recent reports on the comparison of electrocatalysts with NiMoO4/MQDs 10 mA cm-2 overpotential and Tafel electrocatalysts for HER.

Fig. 5. (a) Schematic illustration of the urea electrolysis cell employing NiMoO4/MQDs as both the anode and cathode catalysts. (b) LSV curves comparing the UOR and OER. (c) Time-dependent potential profiles for both processes at a current density of 10 mA cm-2. (d) The Faradaic efficiency for hydrogen generation as a function of operation time in the urea electrolysis system. (e) LSV curves of NiMoO4/MQDs and the noble-metal benchmark (Pt/C||RuO2) in 1.0 mol L-1 KOH with 0.33 mol L-1 urea. (f) Schematic of urea-assisted water splitting powered by a solar cell.

Fig. 6. (a) In-situ Raman spectrum of NiMoO4/MQDs in 1 mol L-1 KOH + 0.33 mol L-1 urea. NiMoO4 (b) and NiMoO4/MQDs (c) of differential charge density at the NiMoO4/MQDs heterostructure. The side view and top view models of urea adsorption at different sites: Ni site (d) and Mo site (e). (f) The DOS of the pristine and NiMoO4/MQDs unit cell. (g) PDOS of Ni 3d. The corresponding d-orbital occupancy of NiMoO4 (h) and NiMoO4/MQDs (i). PDOS of Ni 3d in NiMoO4 (j) and NiMoO4/MQDs (k).

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