# 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

## TL;DR

This study investigates how high potentials affect carbon thin film electrodes in vanadium redox reactions, showing that sulfate anion insertion dominates and can be reversed with reductive potentials.

## Contribution

The study reveals the significant role of sulfate anion insertion in inhibiting vanadium redox reactions in carbon electrodes and demonstrates their reversibility.

## Key findings

- Sulfate anion insertion is the main process occurring alongside the vanadium(IV/V) redox reaction.
- Pyridine/pyrrole groups stabilize the insertion compound, strongly inhibiting the vanadium redox reaction.
- Electrochemical features and Raman spectra can be fully restored using reductive potentials after high polarization.

## Abstract

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 (EPP) 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.

This study examines how high potentials affect amorphous (nitrogen‐doped) carbon thin film electrodes in vanadium(IV/V) redox reactions, relevant for the all‐vanadium redox flow battery. Using electrochemical microscopy, the research highlights that sulfate anion insertion predominates and inhibits vanadium redox activity, especially with pyridine‐ and pyrrole‐like moieties. Despite high polarization, the electrodes’ properties could be fully restored by suitable reductive potentials.