MoO2 nanocrystals interconnected on mesocellular carbon foam as a powerful catalyst for vanadium redox flow battery

Abstract

To increase the performance of vanadium redox flow battery (VRFB), the rate of the reaction of the VO²⁺/VO₂⁺ redox couple, known as the rate determining reaction, should be increased. To increase the rate of this reaction, molybdenum dioxide nanocrystals interconnected on mesocellular carbon foam (MoO₂/MSU-F-C) are suggested as a new catalyst. Initially, the optimal amount of MoO₂ embedded on MSU-F-C is determined, whereas its activity, reversibility and charge–discharge behavior are investigated. The specific surface area, crystal structure, surface morphology and component analysis of the composite are also measured using BET, XRD, TEM, TGA, EELS and XPS. As a result, the MoO₂/MSU-F-C results in a high peak current, small peak potential difference and high electron transfer rate constant, confirming that the composite is an excellent catalyst for the VO²⁺/VO₂⁺ redox reaction. In terms of multiple charge–discharge tests, a VRFB single cell, including MoO₂/MSU-F-C, induces high voltage and energy efficiencies with high specific capacity and a low capacity loss rate. These results are attributed to the intercalation of MoO₂ by metal cations such as VO²⁺ and VO₂⁺ and the existence of hydrophilic functional groups on the surface of MoO₂/MSU-F-C. The intercalated MoO₂ plays an excellent conductor role in promoting fast ionic and electron transfer and reducing overpotential, whereas the hydrophilic functional groups improve the VO²⁺/VO₂⁺ redox reaction by lowering its activation energy.