A Comparative Study of Functionalized High-Purity Carbon Nanotubes towards the V(IV)/V(V) Redox Reaction Using Cyclic Voltammetry and Scanning Electrochemical Microscopy

Functionalized high purity carbon nanotubes (CNTs) with various amounts of oxygen containing surface groups were investigated towards the relevant redox reactions of the all-vanadium redox flow battery. The quinone/hydroquinone redox peaks between 0.0 and 0.7V vs. Ag|AgCl|KClsat. were used to quantifying the degree of functionalization and correlated to XPS results. Cyclic voltammetry in vanadyl sulfate-containing 3M H2SO4 as a common supporting electrolyte showed no influence of the amount of surface groups on the V(IV)/V(V) redox system. In contrast, the reactions occurring at the negative electrode (V(II)/V(III) and V(III)/V(IV)) are strongly affected by oxygen surface groups. However, under modified experimental conditions, SECM experiments detecting the consumption of VO2+ molecules by CNT thin films in pH=2 solution show improved onset potentials with increased surface oxygen content up to approximate to 3 at%. Further increase in surface oxygen up to 8 at% led to minor improvement. These dissimilar results under different experimental conditions are rationalized by suggesting that oxygen functional groups do not form the active site for the V(IV)/V(V) reaction but wetting of the catalyst layer is of high importance.