Effect of Cathodic and Anodic Treatments of Carbon on the Electrode Kinetics of V^IV/V^VOxidation-Reduction

Abstract

Cyclic voltammetry and electrochemical impedance spectroscopy were used to examine several types of carbon electrodes in V IV /V V in H 2 SO 4 . The materials investigated included glassy carbon, graphite, carbon paper, reticulated vitreous carbon and carbon fibers. In all cases the electrode kinetics of the V IV /V V oxidation-reduction reactions are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. Pronounced activation typically occurs at potentials more negative than +0.1 V (vs. Hg/Hg 2 SO 4 ); the effect begins to saturate at about -0.6 V. Pronounced deactivation typically occurs at potentials more positive than +0.7 V. Both activation and deactivation occur rapidly during the first ∼10 s at the most negative and most positive potentials, respectively. The activation effect saturates quickly at the most negative potentials but the deactivation effect does not saturate on the time scales investigated. There is a considerable shift (∼1.1 V) between the potentials for activation and deactivation. Activated electrodes showed no significant loss of activity after standing in the electrolyte for 3 weeks; deactivated electrodes regained about 50% of their activity. The activation and deactivation effects were observed regardless of whether vanadium was present in the electrolyte and are attributed to oxygen-containing functional groups on the electrode surface. All-Vanadium Flow Batteries (VFBs) are a promising technology to meet energy storage requirements for large scale and remote area applications.1-6 Like other flow battery systems the VFB is an electrochemical device that converts electrical energy to chemical energy which is stored in the electrolyte. Typical cells have carbon felt electrodes separated by a proton exchange membrane. The catholyte and the anolyte are circulated through the electrodes from reservoirs. Both electrolytes are highly acidic, typically 3 mol dm A variety of electrode treatments have been reported, including electrochemical, 17-28 chemical, 22,23,27,29,30 and thermal 31-34 treatments. These treatments often have the effect of oxidizing or reducing the surface and the influence of surface oxygen species on the performance of carbon electrodes is recognized, 50-53 although often not well understood.In order to better understand and improve the performance of VFB electrodes, it is important to investigate the electrode kinetics. The kinetics of both the V II /V III and V IV /V V redox couples have been studied for a range of different carbon materials using a variety of techniques and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface. 31,[35][36][37][38] Generally, the kinetics are found to be faster for V IV [17][18][19]21 and inhibition of V IV /V V electrode kinetics 17-21 by anodic treatment. In this paper we report detailed results on the effects of both anodic and cathodic treatment on the kinetics of the V IV /V V couple at carbon electrodes (glassy carbon, gra...

Kinetic Study of Electrochemical Treatment of Carbon Fiber Microelectrodes Leading to In Situ Enhancement of Vanadium Flow Battery Efficiency

“…21 Previously, we reported that electrochemical oxidation treatments enhanced the V 2+ /V 3+ reaction kinetics, whereas electrochemical reduction treatments enhanced the VO 2+ /VO 2 + reaction kinetics. 22,23 This was demonstrated using a range of carbon materials including glassy carbon, carbon paper, carbon xerogel, and carbon fiber electrodes.It is apparent that kinetics are dependent on the nature of the electrode surface; thus, it is important to investigate factors affecting ki- * Electrochemical Society Student Member.* * Electrochemical Society Member. * * * Electrochemical Society Fellow.…”

“…Before use, electrodes were preconditioned as discussed later. In addition, to minimize any effects of electrode history, electrodes were also subjected to a normalization treatment 22,23 between experiments, consisting of three cycles, each of 60 s at −2.0 V and 60 s at +1.5 V.Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) experiments on CFMEs and electrochemical treatments of felt for surface analysis by X-ray photoelectron spectroscopy (XPS) were performed in a standard three-electrode cell with a potentiostat/galvanostat (Solartron SI 1287) and an impedance phase analyzer (Solartron SI 1260). The cell had a platinum counter electrode and saturated Hg/Hg 2 SO 4 reference (mercury/mercurous sulfate …”

“…21 Previously, we reported that electrochemical oxidation treatments enhanced the V 2+ /V 3+ reaction kinetics, whereas electrochemical reduction treatments enhanced the VO 2+ /VO 2 + reaction kinetics. 22,23 This was demonstrated using a range of carbon materials including glassy carbon, carbon paper, carbon xerogel, and carbon fiber electrodes.…”

“…Effects of treatment on kinetics.-We have previously reported 22,23,37 for a range of carbon materials that the kinetics of V 2+ /V 3+ are enhanced by electrode treatments at positive potentials while the kinetics of VO 2+ /VO 2 + are inhibited. Conversely, we have shown that the kinetics of VO 2+ /VO 2 + are enhanced by electrode treatments at negative potentials while the kinetics of V 2+ /V 3+ are inhibited.…”

Study of Loss Mechanisms Using Half-Cell Measurements in a Regenerative Hydrogen Vanadium Fuel Cell

“…18 From VRB research, information has been gathered on the whole cell as well as on the cathode using the V(IV)/V(V) redox couple which is of interest in this work. Currently whole cell EIS analysis fails to decouple contributions from the anode and cathode, while half-cell measurements are generally not gathered in-situ 19,20 However Sun et al have presented in their work an in-situ EIS study using a dynamic hydrogen reference electrode positioned using the "sandwich" method in the cell. 21 Their results demonstrated that the anode is the limiting electrode in VRBs, contributing most of the losses.…”