📄 Our Lab Paper

Enhanced electrochemical performance of copper-doped cobalt oxide nanowire-modified graphite felt as positive electrode material for vanadium redox flow batteries

Chemical Engineering Journal, 505 (2025) 159170 | DOI: 10.1016/j.cej.2024.159170

Authors：Aknachew Mebreku Demeku, Chun-Hong Guo, Daniel Manaye Kabtamu, Zih-Jhong Huang, Guan-Cheng Chen, Anteneh Wodaje Bayeh, Chen-Hao Wang*

📄 Abstract

This research investigates copper (Cu) doped cobalt oxide (Co₃O₄) as a catalyst for vanadium redox flow batteries (VRFBs). Cu-doped Co₃O₄ is prepared on a heat-treated graphite felt (Cu-Co₃O₄-HGF) electrode through hydrothermal and calcination, enhancing the electrochemical performance of the VO₂⁺/VO₂⁺ redox couple. Electrochemical analysis confirms that the Cu-Co₃O₄-HGF electrode demonstrates superior catalytic activity for vanadium redox reaction, evidenced by a substantial reduction in charge transfer resistance compared to the undoped Co₃O₄-HGF. At a higher current density of 200 mA cm⁻², the VRFB using the Cu-Co₃O₄-HGF electrode achieves an energy efficiency (EE) of 76.04 %. This marks an improvement of 20.82 %, 12.67 %, and 4.93 % compared to the pristine graphite felt (PGF, 55.22 %), heat-treated graphite felt (HGF, 63.37 %), and Co₃O₄-HGF (71.11 %) electrode, respectively. Moreover, no significant efficiency decay is observed even after 500 cycles of VRFB operation, indicating the excellent stability of the Cu-Co₃O₄-HGF electrode during prolonged cycling. The superior performance of the Cu-Co₃O₄-HGF electrode is mainly due to the synergistic effects of Cu and Co, abundant oxygen vacancies, enhanced hydrophilicity, relatively high surface area, and increased Co₃⁺ concentration in the structure of Cu-Co₃O₄. These properties collectively enhance electron transfer kinetics and provide abundant active sites for redox reactions. This research contributes valuable insights to developing advanced electrode materials for next-generation energy storage technologies.

🔬 Five Core Findings

Cu doping dramatically reduces charge transfer resistance: Cu-Co₃O₄-HGF's Rct value is 9.24 Ω, far lower than Co₃O₄-HGF (10.78 Ω), HGF (28.93 Ω), and PGF (35.55 Ω). Electron transfer kinetics significantly improved, with conductivity increased 50-fold (Cu-Co₃O₄: 9.5×10⁻⁵ S/m vs. Co₃O₄: 1.9×10⁻⁶ S/m).

Significant energy efficiency improvement: At 200 mA cm⁻², Cu-Co₃O₄-HGF achieves 76.04% EE, representing improvements of 20.82% over PGF (55.22%), 12.67% over HGF (63.37%), and 4.93% over Co₃O₄-HGF (71.11%)—one of the best GF-based catalysts in literature.

Exceptional cycling stability: At 200 mA cm⁻² for 300 cycles, Cu-Co₃O₄-HGF shows only 1.17% EE decay, far superior to HGF's 12.27% decay; even after 500 cycles, no significant decay observed, demonstrating excellent long-term durability.

Increased oxygen vacancies and Co₃⁺ concentration: XPS and EPR analysis confirm Cu-doped Cu-Co₃O₄ has richer oxygen vacancies (OV ratio improved) and higher Co₃⁺/Co²⁺ ratio (main active sites increased)—key factors for enhanced VO₂⁺/VO₂⁺ reaction kinetics.

Enhanced hydrophilicity and surface area: Water contact angle shows Cu-Co₃O₄-HGF at 0° (super-hydrophilic), dramatically improved from PGF's 110.5°; N₂ adsorption-desorption analysis shows Type IV isotherm with mesoporous structure and relatively high specific surface area, beneficial for electrolyte wetting and active site accessibility.

📊 Key Figures

Key Figure 1: Material structure and electrochemical analysis, caption embedded in image.

Key Figure 2: Cycling stability test, caption embedded in image.