📄 Our Lab Paper

Molybdenum-doped NiCo₂O₄ nanowires with enriched oxygen vacancies for wide-current-density VRFBs

Surface & Coatings Technology, 528 (2026) 133385 (Elsevier) | DOI: 10.1016/j.surfcoat.2026.133385 | Published: March 18, 2026

Authors：Zih-Jhong Huang, Hailegnaw Gizaw Workie, Aknachew Mebreku Demeku, Johan Nabiel Raihan, Sun-Tang Chang, Tai-Chin Chiang, Chen-Hao Wang*

📄 Abstract

Molybdenum-doped nickel–cobalt oxide (NCO–Mo) nanowires were directly grown on graphite felt to construct high-performance electrodes for vanadium redox flow batteries (VRFBs). Morphological and structural characterizations (XRD, HAADF-STEM, XPS, XANES, and EXAFS) confirm a well-integrated nanowire coating with uniform elemental distribution and Mo incorporation into the NiCo₂O₄ host without detectable crystalline impurity phases. Mo incorporation induces oxygen vacancies (+4.75 percentage points) and modulates near-surface electronic states, which are expected to benefit interfacial charge transfer and provide abundant active sites for vanadium redox reactions. Electrochemical tests demonstrate that the optimized NCO–Mo₃ electrode delivers an energy efficiency of 86.96% at 80 mA cm⁻², 12.3 percentage points higher than pristine graphite felt (74.68%). Notably, it maintains strong performance over a wide current-density range (80–260 mA cm⁻²), achieving 63.46% efficiency at the maximum current density. The electrode also exhibits excellent durability over 250 charge–discharge cycles with coulombic efficiency above 97% and negligible performance decay (<0.2% per cycle). Mechanistically, oxygen-vacancy-mediated defect engineering reduces charge-transfer resistance, suppresses hydrogen evolution, and enhances intrinsic catalytic activity toward the VO₂⁺/VO²⁺ redox reactions.

🔬 Five Key Findings

Mo⁶⁺ incorporation induces oxygen vacancies (+4.75 pp) and modulates Co²⁺/Co³⁺ ratio and near-surface electronic states.

Uniform 1D nanowire arrays without impurity phases: Mo integrates into the spinel lattice without crystalline impurity, forming uniform nanowire arrays (~40 nm diameter).

Significantly improved energy efficiency: NCO–Mo₃ achieves 86.96% EE at 80 mA cm⁻² (+12.3 pp), operates stably over 80–260 mA cm⁻², maintaining 63.46% at max current density.

Excellent long-term cycling stability: 250 cycles, CE > 97%, decay < 0.2%/cycle; bare GF degrades after ~150 cycles.

Oxygen-vacancy-mediated defect engineering: (1) reduces Rct, (2) suppresses HER, (3) enhances intrinsic catalytic activity toward VO₂⁺/VO²⁺.

📊 Key Figures

Figure 1: XRD patterns and SEM images of NCO-Mo nanowires, confirming crystal phase and surface morphology.

Figure 2: TEM images and elemental mapping of NCO-Mo nanowires, showing uniform Mo distribution in the spinel structure.