Electrochemical impedance spectroscopy and cyclic voltammetry were used to investigate the electrode kinetics of V II -V III and V IV -V V in H 2 SO 4 on glassy carbon, carbon paper, carbon xerogel, and carbon fibers. It was shown that, for all carbon materials investigated, the kinetics of V II -V III is enhanced by anodic, and inhibited by cathodic, treatment of the electrode; in contrast, the kinetics of V IV -V V is inhibited by anodic, and enhanced by cathodic, treatment. The potential region for each of these effects varied only slightly with carbon material. Rate constants were always greater for V IV -V V than for V II -V III except when anodized electrodes were compared, which may explain discrepancies in the literature. The observed effects are attributed to oxygen-containing functional-groups on the electrode surface. The considerable differences between the potentials at which enhancement of V II -V III and inhibition of V IV -V V occur indicates that they do not correspond to a common oxidized state of the electrode. Likewise inhibition of V II -V III and enhancement of V IV -V V do not correspond to a common reduced state of the electrode. It is possible that enhancement of both V II -V III and V IV -V V is due to the same (active) state of the electrode. There is considerable interest in vanadium flow batteries (VFBs), also known as vanadium redox flow batteries (VRFBs or VRBs), for storage of electrical energy particularly in conjunction with renewable energy sources such as wind and solar. [1][2][3][4][5][6] Active areas of research include cell design and modelling, [7][8][9] performance and state-of-charge monitoring, 10-16 coulombic and energy efficiencies, 5,17,18 electrolytes, [11][12][13][14][15][16]19,20 membranes, 4,21 and electrodes. Cells typically have porous carbon electrodes and electrode performance can depend strongly on electrode treatment. Various electrochemical, 22-27,36-41 chemical, 36,40,43,44 and thermal [45][46][47][48][49] treatments have been reported. These treatments often have the effect of oxidizing or reducing the surface, and the influence of surface oxygen species on electrochemical kinetics at carbon electrodes is recognized, 22,57-60 although often not well understood. Thermal 45-49 and chemical 36,40,43,44 treatments of electrodes for VFBs have been tested on a range of carbon-based electrodes and, in general, these treatments result in higher activities of the electrode toward the vanadium redox reactions. There are also a number of reports of the effect of electrochemical treatment of electrodes. Anodic treatment of carbon felt was reported 22,36 to cause a decrease in the kinetic rates of the V IV -V V redox couple. In contrast, there are also reports of enhancement of V IV -V V kinetics after electrochemical oxidation [38][39][40][41] (of graphite and carbon felt electrodes) and of V II -V III kinetics after potential cycling 61 (of highly-oriented-pyrolytic-graphite and glassy carbon electrodes). However, in considering the effects of anodization on a c...

Novel carbon fiber microelectrode (CFME) and flow cell experiments were used to investigate electrode treatments for vanadium flow batteries (VFBs). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) on CFMEs showed that electrode treatments at positive potentials enhance the kinetics of V 2+ /V 3+ and inhibit the kinetics of VO 2+ /VO 2 + , while electrode treatments at negative potentials inhibit the kinetics of V 2+ /V 3+ and enhance the kinetics of VO 2+ /VO 2 + . XPS analysis showed changes in oxygen-containing species on electrode surfaces after treatment, supporting the suggestion that such species are responsible for the observed effects. The kinetics of VO 2+ /VO 2 + are significantly faster than that of V 2+ /V 3+ . Based on the CFME results, the range of potential experienced by a negative electrode in a flow cell during operation corresponds to a region where it is being deactivated by reduction, and the redox potential of the positive half-cell falls in a region where the electrode is activated for the V 2+ /V 3+ reaction. This is supported by flow cell experiments which showed that the overpotential at the negative electrode increases with charge-discharge cycling but decreases significantly when the positive and negative electrolytes are interchanged. The all-vanadium flow battery (VFB) has received attention as a load leveling technology for large-scale energy storage.1-4 This technology is capable of interfacing with renewable energy sources and provides an alternative solution to balancing power consumption and generation. Despite the advantages, VFBs have not yet been widely commercialized. Significant improvements are needed to enhance flow battery systems. Limitations include ion transport through the membrane, mass transport resistances within the electrodes, and electrode reaction kinetics. 5 Recently, attention has been directed toward improvement of electrochemical properties of carbon based electrode materials. Modifications via thermal treatments, chemical oxidation, or electrochemical oxidation are thought to enhance electrochemical activity.6-12 The presence of oxygen containing functional groups has been shown to directly affect the kinetics; surface oxides resulting from the aforementioned treatments are thought to act as active sites, catalyzing the vanadium reactions.13 Some researchers have reported an increase [6][7][8][9][10][11][12] while others reported a decrease 14 in activity upon functionalization of carbon electrodes. Many of these studies have been conducted using glassy carbon, 15,16 graphite, 17 carbon paper, 18 multi-walled carbon nanotubes, 19 or carbon composites. 20 One group concluded that the kinetics for the VO 2+ /VO 2 + reaction are faster than the V 2+ /V 3+ reaction at a plastic formed carbon electrode, but found the opposite when using pyrolytic graphite. 21 Previously, we reported that electrochemical oxidation treatments enhanced the V 2+ /V 3+ reaction kinetics, whereas electrochemical reduction treatments enhanced the VO 2...

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...

The all-Iron flow battery utilizes the iron II/III redox couple at the positive electrode and the iron II/0 reaction at the negative electrode. The standard reduction potential of the iron II/0 reaction is at −0.44 V vs. NHE, suggesting that hydrogen evolution could be a significant factor in coulombic losses on the negative electrode. Methods of increasing the coulombic efficiency of iron plating are considered, such as anion concentration and electrolyte additives. The use of a chloride anion containing electrolyte showed less hydrogen evolution rates and faster plating kinetics than an electrolyte containing the same concentration of sulfate anions. Increasing the chloride concentration significantly reduced the hydrogen evolution observed on an iron electrode, and plating efficiencies of 97% were demonstrated on a rotating rod electrode. The effect of complexing ligands on plating and hydrogen evolution was also investigated.As renewable energy sources increasingly become a part of the grid, the need for large scale energy storage grows alongside. 1,2 One such storage technology is the redox flow battery (RFB), typically involving redox couples dissolved in an externally stored electrolyte. [3][4][5] Traditional redox flow batteries, such as the all-vanadium 6 and ironchromium 7 chemistries use redox couples as both the anode and cathode, and the electrolyte is pumped through a battery stack, where the electrochemical reactions occur, decoupling the energy and power of the battery. The amount of energy storage is solely based on the reservoir of electrolyte, and the power delivery is determined by the size of the battery stack, making scale up of redox flow batteries convenient. The all-iron flow battery utilizes the Fe II/III redox reaction at its positive electrode, and the negative reaction involves the plating and stripping of iron. 8,9 The plating reaction occurs in the stack, so the energy and power of the battery are no longer decoupled: the energy storage density is dependent on the plated iron.The standard reduction potential of the iron II/0 reaction is −0.44 V vs. NHE, well negative of the thermodynamic potential for hydrogen evolution. As such, the coulombic efficiency of iron plating will depend on the pH of the electrolyte, as well as the pH at the electrode surface. 10,11 Increasing the pH of the electrolyte will shift the equilibrium potential for hydrogen evolution more negative and decrease the diffusion limited current. 10 However, Fe 2+ will precipitate as a hydroxide species before the pH is sufficiently high to completely avoid hydrogen evolution in the potential range for iron plating. If the hydrogen evolution current is large enough, bubbles can cling to the electrode surface and cause the deposit to form around the bubble. 12 Corrosion researchers have investigated the effect of anions on the kinetics of iron corrosion. 13,14 The presence of specifically adsorbed halide ions was found to alter the mechanism by which iron corrodes. 13 The concentration of halide ions in electrolyte wa...

Iron chloride in a deep eutectic solvent containing high concentrations of iron with choline chloride and ethylene glycol have been synthesized. It was found that physical properties of the electrolytes, as well as the nature of the electroplated iron are greatly influenced by electrolyte composition. This is not surprising in that electrochemical reactivity of the solute ions as well as the physical properties of the electrolyte are controlled by speciation of the metals in solution. When the chloride to iron ratio is ≥4:1, complexes such as [FeCl 4 ] − and [FeCl 4 ] 2− were shown to be the dominant species using X-ray absorption near edge structure measurements coupled with Raman spectroscopy. However, when the chloride to iron ratio falls below 4:1, the ethylene glycol was found to complex the iron; the presence of this complex hinders fluid properties as shown by an order of magnitude decrease in solution conductivity as well as alter the iron deposition mechanism. © The Author(s) 2017. Flow batteries present a potentially low cost energy storage solution that is flexible in design due to external storage of the electrolyte. They offer reversible energy storage along with the load-leveling capabilities needed to facilitating implementation of renewable energy sources. Unlike conventional batteries, dissolved reactive species are stored in tanks and flowed through an electrolytic stack, where the electrochemical reactions occur, allowing for scalability. Redox flow batteries (RFBs) employ a redox chemistry at both electrodes; examples include the all-vanadium, 1 iron-chrome, 2,3 or a metal-free system utilizing quinones. 4 There are also several hybrid configurations, such as the all-iron, 5 all-copper, 6 and zinc halide 7 system which involve metal deposition/dissolution at one electrode. With the hybrid configurations, the storage capacity is related to the amount of metal that can be stored within the stack. The reactions for the all-iron chemistry are shown by Equations 1 and 2 where iron metal is deposited at the negative electrode upon charge. The all-iron chemistry represents a cost effective and environmentally friendly RFB chemistry and was thus chosen for this study.Positive Electrode: FeAqueous based flow battery systems have been widely studied, [8][9][10] however there is a growing interest in non-aqueous electrolytes with larger electrochemical windows (1.5−5.0 V 11 ) as a means to increase power density. Unfortunately, organic solvents have safety hazards associated with them due to their volatile and flammable nature and contamination by trace amounts of moisture or oxygen can have detrimental effects on battery performance. 12 The solubility of active species in these types of electrolytes are often very low, limiting the energy storage capacity. 13,14 Ionic liquids (IL) offer a different approach to increase energy and power densities of current non-aqueous redox flow battery (RFB) technologies. They possess wide electrochemical windows similar to organic solvents but with the advantages o...