The Role of the Anion Insertion-Extraction Reaction in Amorphous Carbon Thin Film Electrodes on the Vanadium(IV/V) Reaction Probed by Scanning Electrochemical Cell Microscopy

The influence of high potentials on amorphous nitrogen-free and nitrogen-doped hydrogenated carbon thin film electrodes with thicknesses of 9 to 30 nm is probed toward the vanadium(IV/V) redox reaction by scanning electrochemical cell microscopy (SECCM), which mimics the reaction of the positive side of the all-vanadium redox flow battery (VRFB). Besides the evaluation of the peak separation (E_PP) from cyclic voltammograms (CV), the localized probing is adapted in a way that the influence of high overpotentials on the stability of the carbon materials, as well as competitive electrochemical processes, can be analyzed. The sulfate anion insertion process is found to be the predominant process in all samples, with its onset appearing in parallel to the vanadium(IV/V) reaction. The presence of pyridine/pyrrole groups can stabilize the insertion compound, which inhibits the vanadium(IV/V) reaction much more strongly. In all cases, the electrochemical redox features of the vanadium(IV/V) reaction, as well as the initial Raman spectra of the carbon thin films, are fully reconstructed by applying reductive potentials in a suitable time frame, even after polarizing to drastically high potentials (2.5 V vs. RHE). Overall, this competing insertion reaction must be given greater consideration when discussing electrochemical data of the vanadium(IV/V) redox reaction.