Carbon felt coated with tungsten–bismuth-based oxides as highly active and selective negative electrodes for high power density all-vanadium redox flow batteries

Abstract

All-vanadium redox flow batteries (VRFBs) are one of the future strategic energy storage technologies for large-scale applications. For developing the VRFB negative electrode, tungsten/bismuth-based oxides (W–Bi) were solvothermally synthesized and grown on carbon felt (W_x–Bi_y@CF) using different initial W/Bi molar ratios. X-ray diffraction (XRD) and Raman spectroscopy results revealed the presence of BiOCl, WO_3−x, and Bi₂WO₆ at various ratios, according to the W to Bi initial molar ratio. The XPS results indicated the oxygen-defective nature of WO_3−x and the significant electronic effect between Bi and WO_3−x that increases on increasing the Bi contents. Even though all the W_x–Bi_y@CF electrodes exhibited high catalytic activity for the V²⁺/V³⁺ redox reaction, the W–Bi₂@CF electrode showed superior catalytic activity and selectivity, indicated by the suppressed hydrogen evolution reaction (HER) and high reversibility. The CV results showed an anodic peak current (I_pa) of 95.8 mA cm⁻², a peak potential separation (ΔE) of 83.5 mV, and an anodic to cathodic peak current ratio (I_pa/I_pc) of ≈ 1 at a scan rate of 5 mV s⁻¹. The EIS results, with the distribution of relaxation time (DRT) calculations, showed that the W–Bi₂@CF electrode has minimal charge transfer resistance (95.6 mΩ) among the different electrodes. The assembled devices of the W–Bi@CF electrodes showed outstanding stability during 1000 cycles for the negative electrode, with a VE of 89.8 and EE of 83.8 at a current density as high as 250 mA cm⁻², indicating the high potential for employing the W–Bi₂@CF electrode as a negative electrode in VRFBs.