🤝 Collaborative Paper

High entropy alloy oxide coating of VNbMoTaWOx as a novel electrode modification of vanadium redox flow batteries

Journal of Energy Storage 94 (2024) 112344 (Elsevier) | DOI: 10.1016/j.est.2024.112344

Authors：Krishnakant Tiwari, Chen-Hao Wang, Bih-Show Lou, Chaur-Jeng Wang, Igamcha Moirangthem, Ismail Rahmadtulloh, and Jyh-Wei Lee*

📄 Abstract

The vanadium redox flow battery (VRFB) is the leading candidate for a large-scale renewable energy storage solution. A pivotal element of the VRFB, the electrode, plays a significant role in influencing both the energy efficiency and the cost of the system. This research introduced a groundbreaking single-step technique employing high power impulse magnetron sputtering (HiPIMS) to enhance graphite felt (GF). Through this method, a thin layer of VNbMoTaW high entropy alloy oxide (HEAO) was uniformly coated on the GF surface by adjusting the oxygen gas flow, markedly boosting the VRFB electrode's reaction kinetics. The VRFB equipped with the HEAO-modified graphite electrode, specifically the HEAO50 electrode, demonstrated a superior energy efficiency of 80.50% at a current density of 100 mA cm⁻², outperforming the VRFB with the unmodified graphite electrode by 9.49%. These results are attributed to the plenty of oxygen vacancies and enhanced hydrophilicity of the GF, both of which contribute to improved electrode performance. Additionally, the amorphous structure reduces ion diffusion distances, further enhancing mass transportation and reaction kinetics within the GF electrode. This inventive method presents a viable and cost-effective strategy for electrode enhancement in VRFB, promising significant advancements in renewable energy storage technologies.

🔬 Five Key Findings

HiPIMS single-step coating: First application of high power impulse magnetron sputtering (HiPIMS) to deposit VNbMoTaW high-entropy alloy oxide (HEAO) coating on graphite felt, enabling rapid and uniform electrode modification without conventional heat treatment or chemical processes.

80.50% EE at 100 mA cm⁻²: The HEAO50-modified electrode achieves 80.50% energy efficiency, a 9.49% improvement over unmodified graphite felt, demonstrating the exceptional effectiveness of HEAO coating for VRFB electrode modification.

Stable over 250 cycles: Strong adhesion between the HEAO coating and graphite felt ensures stable Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) over 250 charge-discharge cycles, confirming long-term operational reliability.

Amorphous structure + oxygen vacancies + hydrophilicity: The amorphous HEAO structure shortens ion and electron diffusion paths, abundant oxygen vacancies provide more active sites, and enhanced hydrophilicity promotes electrolyte wetting—all synergistically improving mass transport and reaction kinetics.

>2.5× current density improvement over conventional methods: The HiPIMS HEAO coating method delivers more than 2.5× improvement in current density compared to conventional graphite felt modification methods, setting a new benchmark in electrode modification technology.

📊 Key Figures

Figure 10: Electrochemical performance of bare graphite felt (GF) and HEAO50-coated GF. (a) Charge-discharge curves; (b-d) CE, EE, and VE comparisons at current densities of 80, 100, and 120 mA cm⁻².