Influence of Carbon Material Properties on Activity and Stability of the Negative Electrode in Vanadium Redox Flow Batteries: A Model Electrode Study

Abstract

The V 2+ /V 3+ reaction occurring at the negative electrode in the all-vanadium redox flow battery has been identified as performance limiting in the system. Given the complexity of the commercial carbon electrodes typically used for this application, a model electrode approach is adopted in this work to study electrode activity and stability. This was done using edge and basal plane pyrolytic graphite electrodes modified by electrochemical oxidation. Differential electrochemical mass spectrometry was used for the first time in this work to investigate the parasitic hydrogen evolution reaction on the oxidized carbon surfaces. The basal plane surface showed a higher faradaic efficiency for V 3+ reduction compared to that for the edge plane surface. The oxidized surfaces were subject to extended cycling, after which the basal electrode showed a dramatic loss in activity compared to the edge surface which was relatively stable. This activity loss was related to the poor mechanical stability of the basal plane surface. The electrodes were analyzed before and after cycling by different techniques including X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy. The influences of the type of carbon and its related properties on electrode activity and stability are discussed.

Influence and Electrochemical Stability of Oxygen Groups and Edge Sites in Vanadium Redox Reactions

“…The authors conclude that the activity towards the vanadium redox couples is dependent on OFGs on edges and a well‐preserved basal plane. Similar results were obtained by another group, which found active OFGs that are only stable on edge sites, to catalyze the V III /V II redox reaction [20] …”

“…The stability of edge sites is important, since OFGs on edge plane graphite are far more stable than on basal planes. [20] In Figure 7(h), the I D /I G ratios are given for each sample before and after cycling in the corresponding half-cell. After cycling in the positive half-cell, all samples lose a severe number of edge sites, reflected by a decrease of the I D /I G ratio.…”

“…Similar results were obtained by another group, which found active OFGs that are only stable on edge sites, to catalyze the V III /V II redox reaction. [20] However, even more questions towards surface functional groups seem to arise when carbon-based nanomaterials such as graphene or multi-walled carbon nanotubes (MWCNTs) are investigated. The activity of MWCNTs has been examined and it was concluded that they have no intrinsic catalytic effect on the V V O 2 + /V IV O 2 + redox pair.…”

Conversion of Spent Coffee Beans to Electrode Material for Vanadium Redox Flow Batteries

“…Moreover, for the BC-modified electrode, the peak potential separation of the V 4+ /V 5+ redox couple was also significantly (0.40 V vs. SCE) decreased when compared to the unmodified GP electrode (0.55 V vs. SCE), suggesting the facilitated electrochemical reversibility of the reactions. However, no change in peak separation was observed for the V 2+ /V 3+ redox couple which is usually the limiting part of the VRB system [22].…”

“…Moreover, for the BC-modified electrode, the peak potential separation of the V 4+ /V 5+ redox couple was also significantly (0.40 V vs. SCE) decreased when compared to the unmodified GP electrode (0.55 V vs. SCE), suggesting the facilitated electrochemical reversibility of the reactions. However, no change in peak separation was observed for the V 2+ /V 3+ redox couple which is usually the limiting part of the VRB system [22]. The AC3-modified GP electrode (biochar activated with steam for 3 h) also showed higher anodic and cathodic peak currents for both redox couples than for the pure GP electrode, though the redox process facilitation was less pronounced than for the BC-modified electrode.…”