NH₄HF₂-etched MXene as an electrocatalyst for the V²⁺/V³⁺ redox reaction in a vanadium redox flow battery increased the power density by ~40 % due to high interlayer spacing, allowing for better ion transport and higher capacity retention, which is beneficial for electrochemical energy storage.

Abstract

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.