Electrochemistry of V4+/V5+ reaction on ionic liquid-derived catalytic carbon electrode materials for vanadium redox flow battery

Abstract

Carbon-based materials for various electrochemical applications can be produced through the pyrolysis of ionic liquids (ILs). However, at the high temperatures used for pyrolysis, the decomposition products of ILs like tris(dibutylamino)cyclopropenium bis(2-ethylhexyl)phosphate ([TDBaCp][BEHB]) can have significant volatility, leading to very low yields (< 0.5%) of the final carbon material. Mixing pre-made carbon materials (e.g., diamond nanopowder, graphene, or carbon black) into the IL prior to carbonization can dramatically increase the yield of IL-derived carbon.

In this work, IL-derived carbons were prepared with and without addition of carbon supports and characterized using X-ray photoelectron spectroscopy, N₂ adsorption/desorption, Raman spectroscopy, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry techniques. The electrochemistry of the V⁴⁺/V⁵⁺ redox reaction was examined at the IL-derived carbon by drop-casting the synthesized carbon materials onto a glassy carbon disc electrode. The results demonstrated improved activity and kinetics for the V⁴⁺/V⁵⁺ redox couple at the IL-derived carbon electrode compared to both the carbon support electrode and the glassy carbon electrode alone. The activity, assessed by peak potential separation (ΔE_p), was further confirmed by EIS. A correlation was found between the presence of heteroatoms such as N, O and P in the IL-derived carbon, which increased structural disorder (as indicated by Raman Spectroscopy) and enhanced activities. The oxygen-functionalized groups on the carbon improved surface wettability, making them viable for vanadium redox flow battery (VRFB) applications. The effectiveness of IL-derived carbon modified graphite felt (GF) electrode was shown in VRFB single cell assembly, which achieved higher energy efficiency compared to bare GF.