Ammonium Bifluoride-Etched MXene Modified Electrode for theAll-Vanadium Redox Flow Battery

The development of electrodes with high performance and long-term stability is crucial for commercial application of vanadium redox flow batteries (VRFBs). This study compared the performance of VRFB with thermal-treated and MXene-modified carbon paper. To prepare the MXene, a modified-etching process with ammonium-bifluoride (NH4HF2) led to a mild and efficient conversion of the MAX-phase to MXene compared to etching process with hydrofluoric-acid (HF). Electron microscopy and X-ray diffraction studies revealed that the etching process with NH4HF2 led to MXene nanostructures with a large interlayer spacing. The results show that at a current density of 60 mA cm(-2), the energy efficiency increased by 25.5 % when using a NH4HF2 -etched MXene-modified negative electrode, by 12.5 % with a thermal-treated MXene-modified electrode, and by 4 % with an HF-etched MXene-modified electrode, in comparison to the pristine electrode. The maximum power density of the battery was increased by more than 40 %. In long-term cycling experiments the MXene modified electrode exhibited excellent stability over 1000 cycles of charge-discharge, with 0.05 % discharge capacity decay per cycle, amongst the lowest values reported to date and four times lower than for thermally-treated electrode. The superior performance was linked to the improved electrical conductivity and wettability, higher interlayer spacing, and lower charge transfer resistance for the V2+/V3+ redox reaction.