Electrolyte For Vanadium Redox Flow Battery Research Articles

A Review of Electrolyte Additives in Vanadium Redox Flow Batteries.

Vanadium redox flow batteries (VRFBs) are promising candidates for large-scale energy storage, and the electrolyte plays a critical role in chemical-electrical energy conversion. However, the operating temperature of VRFBs is limited to 10-40 °C because of the stability of the electrolyte. To overcome this, various chemical species are added, but the progress and mechanism have not been summarized and discussed yet. This review summarizes research progress on electrolyte additives that are used for different purposes or systems in the operation of VRFBs, including stabilizing agents (SAs) and electrochemical mass transfer enhancers (EMTEs). Additives in vanadium electrolytes that exhibit microscopic stabilizing mechanisms and electrochemical enhancing mechanisms, including complexation, electrostatic repulsion, growth inhibition, and modifying electrodes, are also discussed, including inorganic, organic, and complex. In the end, the prospects and challenges associated with the side effects of additives in VRFBs are presented, aiming to provide a theoretical and comprehensive reference for researchers to design a higher-performance electrolyte for VRFBs.

Designing CNT-implanted graphite felt as a sustainable electron network for long-cycling of vanadium redox flow batteries

Carbon nanotubes/Carbon nanofibers (CNT/CNF), owing to their abundant active sites and high acidic resistance, have emerged as promising electrocatalysts for high-energy-density vanadium redox flow batteries (VRFB). However, the utilization of CNT/CNF has been obstructed owing to problems such as cost, and the detachment of CNT/CNF from the carbon electrode caused by the flowing electrolyte in VRFB. In this study, we proposed a novel strategy for uniformly implanted multiwall CNT in electrode materials such as graphite felt, for the development of efficient electron-networked electrodes. The weed root-like morphology of the electrode improved the charge transfer ability and minimized the increase in overpotential even after 1000 cycles of stability test. As a result, the electrode exhibited a high discharge capacity of 30.7 Ah·L−1 and an energy efficiency of 86.9%. Therefore, the proposed CNT implanted electrode can be used as an alternative design in practically feasible VRFB exhibiting sustainable electrochemical performance.

Clean preparation of mixed trivalent and quadrivalent vanadium electrolyte for vanadium redox flow batteries by catalytic reduction with hydrogen

The vanadium redox flow battery (VRFB) is a promising technology for large-scale stationary energy storage systems. However, the high preparation cost of mixed valent vanadium electrolyte hinders the large-scale commercial application of VRFB. In this work, a simple, green and low-cost method is proposed to prepare the mixed valent vanadium electrolyte for VRFB. The clean hydrogen is chosen as the reducing agent to obtain trivalent vanadium ions from quadrivalent vanadium ions. The Pt/C material is used as the catalyst to accelerate the reduction rate at atmospheric pressure. The impurity-free mixed valent vanadium electrolyte, with a mean valence number of 3.5, prepared by the catalytic reduction process exhibits excellent battery performance with CE of 93% and EE of 85%. Furthermore, a catalytic reactor using Pt/C decorated graphite felt is designed and used to continuously produce the mixed valent vanadium electrolyte. According to the result of simple cost analysis, the proposed catalytic hydrogen reduction process can reduce theoretically the manufacturing cost by approximately 22.6% compared with the present industrial electrolytic process. As a consequence, the simple, clean and low-cost manufacture method of the mixed valent vanadium electrolyte exhibits remarkable competitiveness and promising potential for the practical production application of VRFB.

State of Charge and Capacity Tracking in Vanadium Redox Flow Battery Systems

The vanadium redox flow battery electrolyte is prone to several capacity loss mechanisms, which must be mitigated to preserve electrolyte health and battery performance. This study investigates a simple and effective technique for the recovery of capacity loss arising from symmetrical mechanisms via automatic electrolyte rebalancing. However, chemical or electrochemical techniques must be used to mitigate capacity loss from asymmetrical mechanisms (e.g., air oxidation of V2+), which requires knowledge of the oxidation states present in the electrolytes. As such, this study assesses the suitability of SOC tracking via electrolyte absorption for independent monitoring of the anolyte and catholyte within an existing VRFB system. Testing is performed over cycling of a 40 cell, 2.5 kW with 40 L of electrolyte. Optical monitoring is performed using a custom-made flow cell with optical paths (interior cavity thicknesses) ranging from 1/4″ to 1/16″. Light transmitted through the cell by a 550 lumen white light source is monitored by a simple photodiode. The electrolyte rebalancing mechanism displayed success in recovering symmetrical capacity losses, while optical monitoring was unsuccessful due to the high absorbance of the electrolyte. Potential improvements to the monitoring system are presented to mitigate this issue.

Electrochemical analysis of nano-electro fuels based on multi-walled carbon nanotubes and graphene nanoplatelets for vanadium redox flow batteries

The limited energy performance of vanadium redox flow batteries (VRFBs) prevents their commercialization on a large scale. Active research is conducted aiming enhancement of electrochemical properties of VRFBs, nanotechnology is constantly penetrating the energy sector with its superior electrochemical properties. Multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) are the two highly sought out nanomaterials in VRFBs performance enhancement research due to their exceptional chemical stability in acid environments. In the current research, electrochemical performance analysis of commercially available MWCNTs and GNPs are performed by their dispersion in base vanadium electrolyte of VRFBs, forming nano-electro fuels. It is concluded from electrochemical characterizations that nano-electro fuels based on MWCNTs and GNPs perform better when compared to conventional electrolytes.

Development of economical and highly efficient electrolyte using vanadium pentoxide for vanadium redox flow battery.

Vanadium pentoxide can be an inexpensive replacement to vanadium sulfate in synthesizing vanadium redox flow battery (VRFB) electrolytes. In this study, VRFB electrolyte is synthesized from vanadium pentoxide using an indigenously developed process and setup. In order to have the same performance as that of vanadium sulfate, the supporting electrolyte environment constituted by H+ and sulfate ion is replicated based on the calculations from standard synthesis mechanisms. The calculations reveal that 4M H2SO4 is required while preparing 1.5M Vn+ electrolyte from V2O5 to replicate 1.5M Vn+ in 2.5M H2SO4 from VOSO4. The effect of variation of sulfuric acid concentration is explored using thermal stability testing, which shows stable V(V) electrolytes for more than 30days for 4M H2SO4 concentration. Furthermore, the electrochemical performance of developed electrolyte from vanadium pentoxide shows similar charge-discharge profile, yet 1/5 the cost as compared to vanadium sulfate.

Acid/base flow battery environmental and economic performance based on its potential service to renewables support

An innovative technology, called Acid Base Flow Battery (AB-FB), has been developed to overcome the intermittent supply of wind and solar electricity generation. It stores electrical energy using pH and salinity differences in the water and compared with other battery technologies, such as Vanadium Redox Flow Battery (VRFB), the new system is expected to be safer, more sustainable and to become a cost competitive option. To provide a deeper knowledge of the new system potentials, in this research, Life Cycle studies under cradle to grave approach have been carried out to assess the environmental and economic performance of 1 MW/6 MWh AB-FB system. Furthermore, 1 MW/6 MWh VRFB has been considered as the reference case. According to the comparative analysis, the AB-FB system exhibited the best environmental and economic performance, placing the AB-FB system as the most sustainable technology. In terms of environmental impacts related to the three process stages, the AB-FB system operation stage yielded the most relevant environmental burden, mostly attributed to energy losses due to the system efficiency. Manufacturing of the AB-FB system was the second stage with the more significant quote to the total environmental burden. Particularly, impacts were related to the power subsystem components being steel, copper, polyethylene and polyvinylchloride identified as the key materials responsible of this tendency. In contrast, the VRFB manufacturing was the most relevant process stage in terms of environmental implications. The energy subsystem was responsible for this tendency due to the vanadium-based electrolyte production related impacts. This component of the VRFB system was also its main constrains in terms of costs. The VRFB investment cost (339 €/kWh) was almost twice the AB-FB one (184 €/kWh), mostly affected by the VRFB electrolyte cost production.

The Effect of Metal Impurities on the All-Vanadium Redox Flow Battery Performance

Vanadium redox flow batteries (VRFB) are a rapidly emerging technology for grid scale energy storage and the integration of the renewable energy generation. However, the cost of the electrolyte is a major barrier for implementation of VRFBs [1, 2]. The quality of the electrolyte has a significant impact on the cell performance and cost. The presence of impurities even with low concentrations in the vanadium electrolyte solution can alter the stability of the electrolyte and influence cell performance, energy density, operating temperature range, electrochemical kinetics and production costs [1-4]. Because a universal standard for the electrolyte specifications has not been defined in the market, a high purity electrolyte is always favored by researchers and customers to avoid potential detrimental impacts of impurities on the system performance. High purity greatly contributes to the high cost of electrolyte for VRFBs. Thus, understanding the impact of the impurities present in electrolyte on the performance of VRFBs is vital for commercialization of VRFBs [5]. This paper aims to conduct a systematic study on the effect of iron, aluminum and manganese ions (Mn2+, Fe2+ and Al3+) on the performance of VRFBs.A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments. Carbon paper (thermally treated at temperature of 500o C for 1 h in an air atmosphere), a platinum wire, and a saturated calomel electrode (SCE), were used as the working, counter and reference electrodes, respectively. The battery performance was evaluated in a flow cell using a ‘zero-gap’ cell design with an electrode area of 5 cm2. The electrolytic solution (1 M VOSO4 solution in 3 M H2SO4) was circulated through the cell. Thermally treated carbon papers were used as cathode and anode electrodes. For charge-discharge experiments, constant current density (10, 20, 30, 40 and 60 mAcm−2) was applied with 1.65 and 0.8 V as upper and lower voltage limits. The effects of each impurity were studied at five different concentrations (0.02, 0.04, 0.06, 0.08 & 0.1 M) through CV and charge-discharge experiments.The CV results shown in Figure 1 indicate that side reactions of gas evolution from water electrolysis will increase as the concentrations of the metal impurities increase. Comparison of the effect of three transition metal ions (Mn2+, Fe2+ and Al3+) showed that the highest peak separation was obtained in the presence of Fe2+. The peak separations for Mn2+and Al3+ were almost the same. Figure 2 shows the battery performance obtained in the zero-gap cell for a range of current densities versus cycle number, for a pure electrolyte and in the presence of aluminum and iron. The coulombic efficiency improved as the current density increased. Generally, the presence of impurities affected the coulombic efficiency and could result in side reactions and capacity fading, which will have a negative effect on the battery performance. The voltage and energy efficiencies obtained for a range of current densities are shown in Figures 2 (b) and (c), respectively. Similar to the coulombic efficiency results, the highest voltage efficiency was observed with the pure electrolyte. Based on the cyclic voltammetry results, the kinetics of the vanadium reaction falls, and the peak separation increased with increasing concentration of the contaminant metal ions Al3+ and Fe2+. Thus, the voltage efficiency decreased in the presence of these metal ions in the electrolyte. The energy efficiencies have the same trend and illustrates that the contaminant metal ions are competitive with vanadium ions for adsorption on the electrode surface and thus affect the vanadium redox reaction kinetics [6]. The side reactions caused by the metal ions have a negative influence on the performance of VRFBs.[1] A. Parasuraman, T.M. Lim, Ch, Menictas, M. Skyllas-Kazacos, A review of electrolyte additives and impurities in vanadium redox flow batteries, Electrochimica Acta, 27-40, 2013.[2] Cao, L., Skyllas-Kazacos, M., Menictas, Ch., Noack, A review of electrolyte additives and impurities in vanadium redox flow batteries, Energy Chemistry, pp.1269-1291, 2018.[3] A.K. Singh., N. Yasri., K. Karan, E.P. L. Roberts, Electrocatalytic Activity of Functionalized Carbon Paper Electrodes, ACS Appl. Energy Mater. 2019, 2324−2336.[4] John, J. St., Imergy uses recycled vanadium to cut materials costs for flow batteries, Greentech media, 2014.[5] J.H. Park, J.J. Park, H.J Lee, B.S. Min, J.H. Yang, Influence of Metal Impurities or Additives in the Electrolyte of a Vanadium Redox Flow Battery, The Electrochemical Society, page 1263-1268, 2018.[6] Ding, M., Liu, T., Zhang, Y., Cai, Z., Yang, Y., Yuan, Y., Effect of Fe(III) on positive electrolyte for vanadium redox flow battery, R. SOC. open sci. 181309, 2019. Figure 1

Laboratory XANES to study vanadium species in vanadium redox flow batteries

The speciation of vanadium in the electrolyte of vanadium redox flow batteries (VRFBs) is important to determine the state of charge of the battery. To obtain a better understanding of the transport of the different vanadium species through the separator polymer electrolyte membranes, it is necessary to be able to determine concentration and species of the vanadium ions inside the nanoscopic water body of the membranes. The speciation of V in the electrolyte of VRFBs has been performed by others at the synchrotron by X-ray absorption near-edge structure analysis (XANES). However, the concentrations are quite high and not necessarily justify the use of a large-scale facility. Here, we show that vanadium species in the electrolyte and inside the ionomeric membranes can be determined by laboratory XANES. We were able to determine V species in the 1.6 M electrolyte with a measurement time of 2.3 h and V species having a concentration of 9.8 g kg−1 inside the membranes (178 µm thick) with a measurement time of 5 h. Our results show that laboratory XANES is an appropriate tool to study these kind of samples.

Boosting the performance of positive electrolyte for VRFB by employing zwitterion molecule containing sulfonic and pyridine groups as the additive

The electrolyte, one of the key components for the energy storage in the vanadium redox flow battery (VRFB), hinders the further commercial applications of the VRFB due to the poor thermal stability of the positive electrolyte at the elevated temperature and the inferior activity on the interface of electrode. To enhance the performance of the positive electrolyte, a zwitterion-type molecule pyridinium propyl sulphobetaine (PPS) with negatively charged sulfonic group and positively charged pyridine group is adopted as the additive for the positive electrolyte. In this study, we have proved that PPS can be adsorbed on the electrode through sulfonic group and pyridine group under both positive potential and negative potential, which can effectively modify the interface of electrode. Compared to other additives containing –NH2/–SO3H groups, PPS containing pyridine group may effectively eliminate the side effects of protonation (–NH3+) and intermolecular hydrogen bonding (-SO3−---+H3N-) in acidic electrolyte. Moreover, the PPS-containing V(V) electrolyte presents a better stability at 50 °C compared to the blank V(V) electrolyte. As a consequence, with the addition of 1% PPS in positive electrolyte, the VRFB exhibits higher efficiencies (80.78% VE and 78.17% EE compared to 78.95% VE and 76.87% EE of the blank VRFB at 100 mA cm−2) and 76.28% capacity retention after 100 cycles which is 5.22% higher than that of the pristine VRFB. Herein, the study of zwitterion molecules provides a meaningful reference for the further research on organic additives for VRFB.

Effect of nanofluidic electrolyte on the electrochemically enhanced long‐term efficiency of vanadium redox flow battery

AbstractThe low‐energy density of flow batteries using aqueous electrolytes such as vanadium redox flow batteries is a limitation of commercialization. Our ground‐breaking research has increased the limited energy density by 21.5% and the reversibility to oxidation/reduction by 8.5% through a nanofluidic electrolyte in which commercial multiwalled carbon nanotubes (MWCNTs) are dispersed in an electrolyte. Three nanofluidic electrolytes (0.05, 0.1, and 0.2 wt%) are prepared according to the electrolyte weight ratio and electrochemical performance is verified experimentally through half‐cell and single‐cell test. The increased active area due to the graphitic edge sites of MWCNT is an effective nanofluidic electrolyte to increase electrochemical performance. Our research provides the concept of nanofluidic electrolytes for increasing the electrochemical performance of VRFB, and the weight ratio of the optimal nanofluidic electrolyte of the vanadium redox flow battery electrolyte through single‐cell test is 0.1 wt% MWCNTs.

Seawater as an alternative to deionized water for electrolyte preparations in vanadium redox flow batteries

The vanadium redox flow battery (VRFB) has been identified as one of the most promising candidates for large-scale energy storage systems. Nevertheless, the high capital cost, particularly the cost of electrolytes, has hindered its broad market penetration. In this work, we use abundant seawater to replace deionized water for VRFB electrolyte preparation. Testing results show that both the coulombic efficiency and capacity retention rate in the case of using seawater electrolytes are higher than using the deionized-water electrolytes, although the battery voltage efficiency slightly decreases due to the lower mass transport diffusivity of V3+ in seawater. Moreover, the solubility of vanadium sulfate in seawater increases by ∼12%. The overall cost of seawater-prepared electrolytes is cheaper by 3.6% than that of the deionized water prepared electrolytes. The results indicate that seawater offers the potential to replace deionized water as a new avenue for VRFB electrolyte preparations.

Vanadium Redox Flow Batteries Fabricated by 3D Printing and Employing Recycled Vanadium Collected from Ammonia Slag

In order to realize sustainable renewable energy supply, large-scale energy storage system is needed to overcome the problem of intermittency of power generation. Vanadium redox flow battery (VRFB) presents the most viable solution but faces the problem of high material cost. In this study, we have established a cost-effective process to prepare vanadium electrolyte for VRFB from an untouched industrial waste, ammonia slag, by pH control under atmospheric condition (< 95°C). The extracted solution changed color during electrolytic reduction as yellow, blue, dark green and purple, matched with the colors of V5+, V4+, V3+, and V2+, respectively, indicating accurate change of the valences without forming precipitates. Electrolyte prepared from the recycled vanadium showed almost the same charging/discharging performances as the one prepared from commercial V2O5 reagent battery tests during the first several cycles, but degraded rapidly after 16 cycles, caused by impurities that deactivate the negative electrode for the reduction of V3+ to V2+. The miniature VRFB prototype built by employing 3D printing technique showed a much higher performance than the H-cell, indicating the flow cell configuration could help to push up the diffusion limit of vanadium redox by flowing the electrolyte solution through the electrodes, as well as reducing IR loss and water splitting to increase the efficiency.

Vanadium Redox Flow Batteries Fabricated by 3D Printing and Employing Recycled Vanadium Collected from Ammonia Slag

In order to realize sustainable renewable energy supply, large-scale energy storage system is needed to overcome the problem of intermittency of power generation. Vanadium redox flow battery (VRFB) presents the most viable solution but faces the problem of high material cost. In this study, we have established a cost-effective process to prepare vanadium electrolyte for VRFB from an untouched industrial waste, ammonia slag, by pH control under atmospheric condition (< 95°C). The extracted solution changed color during electrolytic reduction as yellow, blue, dark green and purple, matched with the color of V5+, V4+, V3+, and V2+, respectively, indicating accurate change of the valences without forming precipitates. Electrolyte prepared from the recycled vanadium showed almost the same charging/discharging performances as the one prepared from commercial V2O5 reagent in in battery tests. The miniature VRFB prototype built by employing 3D printing technique showed a much higher performance than the H-cell, indicating the flow cell configuration could help to push up the diffusion limit of vanadium redox by flowing the electrolyte solution through the electrodes, as well as reducing IR loss and water splitting to increase the efficiency.

Insights into the synergistic effect of ammonium and phosphate-containing additives for a thermally stable vanadium redox flow battery electrolyte

In the field of large-scale electrochemical energy storage, the vanadium redox flow battery possesses many advantageous features. However, its performance is hampered by the low stability of the positive electrolyte at high temperatures, raising the necessity to use stabilizing agents. Phosphoric acid is commonly used to thermally stabilize the positive vanadium electrolyte, in place of effective hydrohalic acids additives, e.g. HCl, which have the risk of toxic halogen gas formation. However, the effectivity of phosphoric acid is restricted by its self-precipitation with V(V) ions in the sulfuric acid supported vanadium electrolyte to form VOPO4. In this study, we explore the synergistic effect of ammonium and phosphate ions and show that ammonium ions can effectively prevent the formation of VOPO4, hence leading to a significant improvement of the thermal stabilizing effectivity of ammonium phosphates over phosphoric acid. This study provides an insight into the thermal stabilizing effectivity of ammonium- and phosphate-containing additives for the positive vanadium electrolyte, which is useful for the understanding in the design of new additive formula.

Insights into the solvation of vanadium ions in the vanadium redox flow battery electrolyte using molecular dynamics and metadynamics

The interaction of vanadium ions (V2+, V3+, VO2+ and VO2+) with counter ions in the condensed-phase vanadium redox flow battery (VRFB) system is investigated using force-field based molecular dynamics (MD), coupled with well-tempered metadynamics (WT-MetaD). In the conventional VRFB electrolyte, (i) bisulphate ions are found relatively less stable in the first solvation shell of vanadium ions than in the bulk, and (ii) the free energy of sulphate ions in the first solvation shell of vanadium ions is marginally lower than that in the bulk. Thus, significant proportion of vanadium ions in the conventional VRFB will have an undisturbed water coordination sphere. The presence of additives like hydrochloric acid and phosphate salts introduces chloride and dihydrogen phosphate ions into the electrolyte. These counter ions are found to be significantly more stable in the first solvation shell of vanadium ions, thereby modifying their local solvation structure by replacing water molecules. The activation free energy barriers for the diffusion of all counter ions into the first solvation shell of vanadium ions can be overcome at room temperature. We believe that the MD + WT-MetaD tool, as presented here, can be used to screen and select potential additives for enhancing the solubility of vanadium ions in VRFB.

Revealing the role of phosphoric acid in all-vanadium redox flow batteries with DFT calculations and in situ analysis.

The present work suggests the use of a mixed water-based electrolyte containing sulfuric and phosphoric acid for both negative and positive electrolytes of a vanadium redox flow battery. Computational and experimental investigations reveal insights on the possible interactions between the vanadium ions in all oxidation states and sulphate, bisulphate, dihydrogen phosphate ions and phosphoric acid. In situ cycling experiments and ion-specific electrochemical impedance measurements confirmed a significant lowering of the charge-transfer resistance for the reduction of V(iii) ions and, consequently, an increase of the voltaic efficiency associated with the negative side of the battery. This increase of performance is attributable to the complexation of this oxidation state by phosphoric acid. So far, mixed acids have mainly been discussed with the focus on V(v) solubility. In this work we rationalize the impact of the mixed acids on the electrochemical efficiency opening new strategies on how to improve the cycling performance with ionic additives.

Effect of quinone additives on the performance of electrolytes for vanadium redox flow Batteries

Anthraquinone-2, 6-disulfonic acid (AQDS) was added to the electrolyte of a vanadium redox flow battery (VRFB) with the goal of enhancing its performance. This study utilized carbon paper electrodes and a Nafion 117 ion exchange membrane for the membrane-electrode assembly. The cycling performance was investigated over multiple charge and discharge cycles. The addition of AQDS was found to increase capacity efficiency by an average of 7.6% over the control VRFB containing no AQDS, while decreasing the overall cycle duration by ~18%. It is thus reported that the addition of AQDS to a VRFB electrolyte has the potential to inexpensively increase the activity and capacity.

A high-temperature tolerance solution for positive electrolyte of vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is attractive for energy storage applications, but there still remains a problem of preventing V-precipitation reaction (i.e., V2O5) to provide the thermal stability of electrolyte employed in VRFB. The V2O5 precipitation is accelerated in the positive electrolyte under high temperature, resulting in decreasing the charge capacity and energy efficiency of VRFB. So far, previous supporting materials, which are used to solve such precipitation problem, provide a disadvantage of decreasing voltage efficiency of VRFBs by increasing electrolyte resistance. This paper describes an advanced vanadium-positive electrolyte with high-temperature tolerance for high-efficiency VRFBs, which uses a sodium formate as a supporting material. The sodium formate plays a role as an agent capable of preventing precipitation reaction in the positive electrolyte and it also provides an effect of decreasing a viscosity of the positive electrolyte. The effectiveness of the proposed electrolyte solution is demonstrated through the following experiments: UV–vis spectrometry, viscosity measurement, cyclic voltammetry (CV), VRFB operation and scanning electron microscopy (SEM) analysis. Then, for the performance comparison of high temperature stability, all experiments are carried out at 60°C. Experimental results show that using the sodium formate leads to near 4.5 times increase of diffusion coefficient as compared to conventional electrolyte, and also provides 20.2% higher charge capacity (50th cycle) and 2.27% higher average energy efficiency (50cycles) at the current density of 80mAcm−2. In addition, it appears that the precipitation of vanadium species is not observed in the electrolyte during VRFB operation. Therefore, this paper provides that new direction about effect of the additive in the positive electrolyte and the sodium formate can be considered as a promising additive for high-performance electrolyte of VRFBs.

Towards an all-vanadium redox-flow battery electrolyte: electrooxidation of V(III) in V(IV)/V(III) redox couple

The electrochemistry of the V(IV)/V(III) redox couple influences the initial pre-charging of the electrolyte in vanadium redox-flow batteries to produce a catholyte and anolyte containing V(V)/V(IV) and V(III)/V(II) couples respectively. The electrochemical conversion of V(III) to V(IV) at a glassy carbon electrode has been studied in detail by cyclic voltammetry in sulfuric acid solutions containing 1.6M and 0.25M of V(III) species. A sigmoid-shaped voltammetric response with a half-wave potential at 0.1V vs. Hg/Hg2SO4 was observed at low scan rates and was attributed to the oxidation of V(III) at an oxidatively pretreated glassy carbon electrode. The attribution of voltammetric responses was consistent with results from a spectroelectrochemical study in 0.25M vanadium electrolyte solution at a reticulated glassy carbon electrode. Initial pre-charging of V3.5+ electrolyte in vanadium redox-flow batteries is assumed to be accompanied by the generation of V(V) in the catholyte and its chemical reaction with V(III) to form V(IV).