Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries

Developing electrodes with high catalytic activity and cycling stability is essential to make vanadium redox flow batteries (VRFBs) viable on a commercial level. In this work, hollow Ti3C2Tx spheres, in the form of 3D nanostructured MXene, are investigated as electrocatalysts for V3+/V2+ redox reactions. The first-principles study demonstrates that Ti3C2 exhibits a metallic behavior with a high electrical conductivity. The catalytic effect of the material is then verified experimentally by cyclic voltammetry and electrochemical impedance spectra test. The hollow Ti3C2Tx spheres decorated graphite felt electrodes are finally prepared and tested in the VRFB. It is shown that the prepared electrode enables the electrolyte utilization efficiency and energy efficiency to be as high as 80.1% and 81.3% at 200 mA cm−2, which are 41.7% and 15.7% higher than that of the battery with the pristine electrode. At the current density of 300 mA cm−2, the electrolyte utilization efficiency and energy efficiency can be still maintained at 62.9% and 75.0%, which are 44.5% and 12.8% higher than the battery with the XC-72 carbon nanoparticle decorated electrode. More impressively, the battery exhibits excellent stability with a high capacity retention rate and no energy efficiency decay over 500 charge-discharge cycles at a current density of 200 mA cm−2. The superior performance is ascribed to significant improvement in the electrochemical kinetics and enlarged active sites towards V3+/V2+ redox reactions by the decoration of hollow MXene spheres.