Vanadium Redox Flow Cell Research Articles

Inter-compartment concentration monitoring of key species in a vanadium redox flow cell operating with and without polarization: Experiments and construction of a predictive model

The usually encountered phenomenon of vanadium crossover in Vanadium Redox Flow Batteries (VRFB), leading to energy capacity losses, is specifically studied without (diffusion measurements) and with the application of an electric current (battery operating conditions). The experimental set-up is an electrochemical filter-press reactor integrating a NafionTM 424 membrane and two storage tanks. The electrolytes composition on both sides was determined using potentiometric titration (VO2+ and H+) and UV–Vis analysis (V2+, V3+ and VO2+). First, experiment is conducted without polarization and a model based on mass balance equation incorporating diffusion flux and electrolyte volumes change is derived to predict electrolytes composition on both sides. The effective diffusion coefficient of VO2+ in the membrane is obtained by fitting the model with experimental data. The experiment is then carried out under galvanostatic polarization (10 mA/cm2) and a model embedding, simultaneously, electrolyte volumes variation, diffusion and migration across the membrane and the electrochemical and self-discharge reactions, is proposed to predict electrolytes composition. The partitioning of the applied current between all possible electrochemical reactions is addressed using a specific algorithm based on logical tests on limiting currents. The resulting system of coupled ordinary differential equations is solved numerically using standard python functions. The results indicate a satisfactory agreement between predicted and experimental data and highlight the requirement of considering all phenomena for an accurate prediction of electrolytes composition in VRFB. The interest of applying the developed model to simulate and predict composition evolution in VRFB (State of Charge (SoC) and acidity) is discussed.

An all-extruded tubular vanadium redox flow cell - Characterization and model-based evaluation

The vanadium redox flow battery (VRFB) as one of the most promising electrochemical storage systems for stationary applications still needs further cost reductions. Tubular cell designs might reduce production costs by extrusion production of cell components and small sealing lengths. Based on a first study of the authors [1], this work demonstrates the feasibility of extruded tubular VRFB cells with high power density in the flow-by electrode configuration. Extruded cell components are the perfluorosulfonic acid cation exchange membrane with a diameter of 5.0 mm and carbon composite current collectors. The cell performance is experimentally characterized by polarization curve, ohmic resistance and galvanostatic cycling measurements. A maximum volumetric power density of 407 kW/m 3 and a maximum current density of 500 mA/cm 2 can be achieved. A non linear E cell / i -model is used to evaluate exchange and limiting current densities while in-situ half cell SoC monitoring is applied to evaluate the extruded membrane.

The first approach to dynamic modeling of a solar vanadium redox flow cell

The increasing offer of energy, especially electricity from renewable sources, has been fueling the investment in energy storage technologies. However, most of the energy harvesting technologies produce non-dispatchable electricity. The solar redox flow cell (SRFC) technology stands out by converting sunlight into storable energy. The latter is easily converted into electricity. However, there are challenges that need to be addressed before SRFCs become commercially available. Phenomenological models are a powerful tool that can be used to accelerate the development of any technology. This work reports the first multidimensional, dynamic and phenomenological model of a vanadium SRFC. The mass and momentum conservation equations are incorporated in the model to simulate the electrolyte flow. The charge transport is simulated by implementing the charge conservation, the Nernst-Planck and drift-diffusion equations. The photoelectrochemical and electrochemical reactions are simulated using the Butler-Volmer equation. The model is validated against experimental data of a vanadium SRFC using a cadmium sulfide semiconductor (n-type). The energy band shifting for different bias potentials and the accumulation and depletion of charges at the space charge region are simulated. The photopotential is determined using the minor carrier concentration and it is demonstrated to be equivalent to the potential difference between the quasi-Fermi energy levels of the electrons and holes. The impact of recombination mechanisms in the performance of the SRFC is analyzed; the surface recombination competes with surface reactions, affecting the onset potential, while non-surface recombination mechanisms affect the concentration of holes, decreasing the maximum current density.

Leveraging Neural Networks and Genetic Algorithms to Refine Electrode Properties in Redox Flow Batteries

Redox flow batteries are a nascent, yet promising, energy storage technology for which widespread deployment is hampered by technical and economic challenges. A performance-determining component in the reactor, present-day electrodes are often borrowed from adjacent electrochemical technologies rather than specifically designed for use in flow batteries. A lack of structural diversity in commercial offerings, coupled with the time constraints of wet-lab experiments, render broad electrode screening infeasible without a modeling complement. Herein, an experimentally validated model of a vanadium redox flow cell is used to generate polarization data for electrodes with different macrohomogeneous properties (thickness, porosity, volumetric surface area, and kinetic rate constant). Using these data sets, we then build and train a neural network with minimal average root-mean squared testing error (17.9 ± 1.8 mA cm−2) to compute individual parameter sweeps along the cell polarization curve. Finally, we employ a genetic algorithm with the neural network to identify electrode property values for improving cell power density. While the developed framework does not supplant experimentation, it is generalizable to different redox chemistries and may inform future electrode design strategies.

A numerical study of electrode thickness and porosity effects in all vanadium redox flow batteries

Vanadium redox flow battery (VRFB) is one of the promising technologies suitable for large-scale energy storage in power grids due to high design flexibility, low maintenance cost and long-life cycle. Vanadium redox flow cell consists of two porous electrodes with serpentine flow channels and electrolyte solutions which is separated by an ion-exchange membrane. The temperature has been set to 298 K for the electrolytes which is composed of 1500 mol/m³ initial vanadium concentration with 4000 mol/m³ initial H2SO4 concentration. We developed a three-dimensional model to scrutinize the complexities of fluid dynamics and electrochemical reactions when considering different electrode thickness sizes, electrode porosity and electrolyte flow rates. In this study, a three-dimensional numerical simulation have been performed in order to investigate the effect of electrode thickness and electrode porosity on the performance of VRFB. The impact of electrolyte solution flow rate on the VRFB electrical characteristics and efficiencies are also numerically investigated. The results show that the cell voltage increases with increasing the electrolyte flow rate and electrode porosity during discharging process of VRFB. Increasing the initial vanadium concentration, the VRFB cell voltage is significantly increased due to reduced overpotential in the porous electrodes. The maximum power-based efficiency of 96.8% is calculated with the electrode thickness of 1 mm at 10 ml/min, while the power-based efficiency of 96.4% is calculated with the electrode thickness of 4 mm at 50 ml/min. This work gives comprehensive insights on electrode configurations for VRFBs.

Electrode Perforation Induced Performance Increase in Vanadium Redox Flow Batteries

In the pursuit of providing reliable energy to an increasingly energy dependent society, the development of electrochemical devices that can meet various demand output conditions is not only important, but necessary. The Redox Flow Battery (RFB) is one option that can provide for this demand by offering the ability to modularize energy and power at a high standard of safety over a long product lifetime. While the RFB has already been successfully implemented at the commercial scale, there exists a need to research further methods towards cost/kWh reduction and power density increase. A conventional cell stack for a RFB consists of end plates that sandwich a set of current collectors, bipolar plates, electrodes, and an exchange membrane arranged in a symmetrical cell.[1] In this study we explore the potential of electrode perforation towards increases in battery performance using cell reactive resistivity as a performance indicator. The perforation method in this study has been chosen for its commercial feasibility and ease of implementation; it has been achieved through the use of a scalable punch press to bore a matrix of holes with variable diameter dimensions of 1-2 mm and a range of hole densities from ~3 holes/cm2 to ~6 holes/cm2. Three sets of serpentine bipolar plates were also prepared with varying turn densities of 4, 7, and 11. These bipolar plates were created keeping effective channel diameter consistent while varying depth and width. The channel to ridge ratios in all three plates were kept at 1:1 and the effect of electrode perforation was explored. The test cells were assembled with the aforementioned bipolar plates and SGL 39AA carbon paper electrodes activated at 600°C for 30 min. Nafion NR211 was used as the membrane material. The cell was assembled with 0.5 mm silicon spacers and 3 sheets of activated SGL 39AA for both the positive and negative electrodes. The electrolyte for the experiment was a 1.5M Vanadium solution prepared with VOSO4-nH2O, 20%w/w H2SO4, and DI water. An averaged improvement in reactive resistivity of 20.8% from 0.262 Ohms*cm2 to 0.207 Ohms*cm2 was demonstrated for a set of three serpentine flow fields via the mechanical perforation of 1 mm diameter holes at a density of ~3 holes/cm2. It is noted that reactive resistivity was separated out of ASR resistivity using a LCR meter to measure frequency independent bulk effects which is referred to as conductive resistance.[2] Reactive resistivity was further broken down into mass transport, charge transfer, and diffusion resistivities through the analysis of relaxation times during cell charge and discharge. Through this, the effect of perforated electrodes for charge transfer resistivity and mass transfer resistivity are shown. (Figure 1) It is realized that electrode perforation via 1 mm bores improves all subsections of reactive resistivity, and it is hypothesized that electrode perforation allows for increased reactant mass transport to the membrane side electrode partition, which has been shown to exhibit lower electrode volumetric utilization than the bipolar plate side electrode partition.[3] Is it also observed that the difference in performances of the 4, 7, 11 turn bipolar plates, which were significant and proportional to channel depth when evaluated with non-perforated electrodes showed a significant decrease in variance with the introduction of perforated electrodes. It can thus be extrapolated that the electrode modification method yields a higher apparent consequence to cell performance than the variation of channel density for the same active electrode area. In conclusion, it is shown that the introduction of 1 mm diameter perforations at ~3 holes/cm2 in carbon paper electrodes is effective in improving performance in a vanadium redox flow cell with serpentine flow channels. The over perforation of electrodes at ~6 holes/cm2 was noted to have the decreased performance associated with an extreme reduction in useable electrode surface area. Pressure drop analysis also reveals that the introduction of perforated electrodes reduces the pump pressure needed to operate at the same nominal flow rate. It is thus realized that the introduction of perforations in electrode material can improve both system efficiency and cell efficiency in the VRFB. [1] N. Tokuda et al.: “Development of a Redox Flow Battery System”, SEI Technical Review, No. 151 (2000) [2] J. Ross Madonald.: “Simplified impedance/frequency response results for intrinsically conducting solids and liquids”, The Journal of Chemical Physics 61, 3977 (1974); doi: 10.1063/1.1681691 [3] Q. Liu et al.: “In Situ potential disctribution measurement in an all-vanadium flow battery”, Chem. Commun. 49, 6292-6294 (2013); doi: 10.1039/C3CC42092B Figure 1

Current Density Improvement for Vanadium Redox Flow Cell Having Interdigitated Flow-Field

Tuning of operational conditions and by using pre-treatment of electrodes can lead to the increase in performance of Vanadium redox flow cell (VRFC) as evident through our published article [1]. This work reports the tuning of operational conditions of a VRFC having impregnated graphite bipolar plate with interdigitated flow-field as compared to conventional flow-field. The schematic is shown in figure 1. The as-received Fumatech FAP-450 anion exchange membrane (AEMs) were used without any treatment. All membranes were cleaned in 1M H2SO4 solution at 80 oC until all vanadium stains were removed and stored in deionized water before being used again. The electrolyte consisted of a commercially available 4 M sulfuric acid-based vanadium solution (1.6 M V3+/V4+ 50:50 M ratio, GfE Metalle und Materialien GmbH, Germany) and it was used without further treatment or purification. Graphite felt electrode (GFE) of thickness 4.6 mm was used with compression ratio of 25%. GFE were boiled in 1M H2SO4 for 12 hours and then left for another 12 hours inside the same solution. Then, GFE are washed with DI water and dried at 100 oC in oven. Afterwards, GFE are thermally treated at 390 oC for 12 hrs inside the furnace injected by air and then cooled down in the furnace upto room temperature. These arrangements allowed to reach higher current density at lower flowrate than used in published article [1]. Charge discharge cycling was done at 40ml/min using current densities of 120mA/cm2, 150mA/cm2, 170mA/cm2, 190 mA/cm2 and 210 mA/cm2. Results of efficiencies (%), energy density (Wh/L) and capacity (mAh) are shown in attached figure 2,3 and 4 respectively. Electrochemical impedance spectroscopy (EIS) is carried out to measure the contribution of Area Specific Resistance at 40 ml/min which is 1.36 Ωcm2 for interdigitated flow-field. EIS results are further fitted using equivalent circuits to determine the contribution of each component. As per the previous article [1], in the absence of flowfield, it was observed that increase in flowrate is feasible to increase the energy efficiency. But, current work highlighted the significance of using flowfields as an efficient way is to decrease the pressure drop to avoid continous increase in flowrate. This approach helps to reach higher current densities at much lower requirement of pumping flowrate. Energy efficiency start decreasing significantly with increment in current density. Only voltaic efficiency is effected whereas columbic efficiency remain around 98% at all current densities. More research efforts are required to optimise the treatment of electrodes and also flowrate distribution to get higher efficiency at higher current density. Reference: [1] R. Monteiro, J. Leirós, M. Boaventura, and A. Mendes, "Insights into all-vanadium redox flow battery: A case study on components and operational conditions," Electrochimica Acta, vol. 267, pp. 80-93, 2018. Figure 1

Electrocatalytic Coating Electrodes in the All-Vanadium Redox Flow Battery

Redox flow batteries (RFB) are used to reversibly transform electrical energy into chemical energy that can be stored for long periods. They are adaptable and are used for a wide range of storage capacities depending on the electrode areas and the amount of electrolyte used. RFB system’s flexibility and modularity allows them to be easily transported to different environments (at any charge/discharge level) and increase or decrease their capacity to the desired level [1]. One of the most developed RFB systems is the all-vanadium redox flow battery (VRFB) which uses carbon electrodes. However, carbon degrades when used as the positive electrode during charging, which significantly decreases the lifetime of the battery and increases maintenance costs [2]. This is the main reason to test different electrode materials that have been successfully used in other electrochemical applications with similar extreme conditions of operation such as electrowinning and the chlor-alkali industry. In this sense, the so-called Dimensional Stable Anodes (DSA), introduced by Beer in the 1970’s [3], consists in a metallic substrate coated with different oxides which provide improved electrocatalytic properties and high resistance to corrosion. The Pechini and ionic liquid [4] methods of preparation of DSA-type electrodes were chosen to manufacture the electrocatalytic coating electrodes with a composition of on a titanium plate substrate and tested in the positive electrolyte of the VRFB (0.01 mol dm-3 V2O5 in 5 mol dm-3 H2SO4). These methods have proven good morphology and electrochemical performance in the final coating in previous studies [5]. In this particular coating composition, iridium dioxide behaves as the electrocatalytic oxide for the V(IV)/V(V) redox reactions, tin oxide provides stability to the coating oxides solution and the antimony oxide helps to increase the electrical conductivity of the coating. The morphology and physical characteristics of the coating are related to the lifetime of the electrode due to deactivation by detachment of the coating. This failure of the electrode could happen when oxygen passes through the coating to the titanium substrate and leads to the formation of titanium oxides/hydroxides between the substrate and the coating. In this study, the effect of the different preparation methods of electrodes on their morphology and composition were studied through physical characterization (SEM, EDS, XRD). Figure 1 a) and b) show the characteristic mud-cracked morphology obtained by the Pechini method [5] where the width of the cracks are between 1 and 2 µm. Moreover, a semi-homogeneous distribution of the oxides is also seen. In Figure 1 c) and d), the surface of the electrocatalytic coating electrode prepared by the ionic liquid method is more homogeneous with the presence of cracks with a width below 1 µm. The presence of more cracks in the electrode prepared by the Pechini method could lead to a shorter lifetime due to the easier passage of oxygen from the surface of the coating to the titanium substrate. Cyclic voltammetry measurements were used to compare the electrocatalytic properties of the electrodes prepared by both methods towards the V(IV)/V(V) redox reaction. The presentation includes the influence of the preparation method to manufacture the electrocatalytic layer in the electrochemical performance of the electrodes in the positive electrolyte of the VRFB. [1] C. Ponce de León, A. Frías-Ferrer, J. González-García, D. A. Szánto, and F. C. Walsh, "Redox flow cells for energy conversion," Journal of Power Sources, vol. 160, pp. 716-732, 9/29/ 2006. [2] M. Skyllas-Kazacos and F. Grossmith, "Efficient Vanadium Redox Flow Cell," Journal of The Electrochemical Society, vol. 134, pp. 2950-2953, December 1987 1987. [3] H. B. Beer, "Electrode and coating therefor," United States of America Patent 3,632,498, 1972. [4] D. T. Araújo, M. de A. Gomes, R. S. Silva, C. C. de Almeida, C. A. Martínez-Huitle, K. I. B. Eguiluz, et al., "Ternary dimensionally stable anodes composed of RuO2 and IrO2 with CeO2, SnO2, or Sb2O3 for efficient naphthalene and benzene electrochemical removal," Journal of Applied Electrochemistry, vol. 47, pp. 547-561, 2017. [5] T. É. S. Santos, R. S. Silva, C. Carlesi Jara, K. I. B. Eguiluz, and G. R. Salazar-Banda, "The influence of the synthesis method of Ti/RuO2 electrodes on their stability and catalytic activity for electrochemical oxidation of the pesticide carbaryl," Materials Chemistry and Physics, vol. 148, pp. 39-47, 2014. Figure 1

Insights into all-vanadium redox flow battery: A case study on components and operational conditions

This works reports the effect of operation conditions on the performance of a 25 cm2 vanadium redox flow cell (VRFC) featuring commercial carbon felt (CF) as electrode and different commercial membranes, namely electrolyte flow rate and charge-discharge current density. The effect of electrode compression and thermal treatment was also assessed.The battery performance was evaluated based on the charge-discharge curves and average efficiencies during 20 charge-discharge cycles; the capacity fading was evaluated after 20 cycles. The limiting current was obtained through polarization curves and the ohmic resistance of the battery was obtained by electrochemical impedance spectroscopy (EIS).The electrodes yielding the highest energy efficiency (EE), ca. 85%, at 40 mA cm−2 current density were thermally treated at 390 °C or 415 °C during 12 h under air. This result represents an improvement of more than 50% compared with the as-received electrodes. Additionally, energy efficiency increased ca. 10% when electrode compression rose from 8% to 25%. Increasing the flow rate from 37.5 cm3 min−1 to 150 cm3 min−1 EE was also enhanced from 76% to 84% at 40 mA cm−2. The type of membrane affects the performance: a comparison between a commercial anion exchange membrane (AEM) and Nafion® series cation exchange membranes (CEM) showed that AEM, generally, yields higher EE than all Nafion® membranes tested; however, the capacity fading of AEM is much higher than CEM.The information provided in this work shows that, by tuning operational conditions and using suitable components, it is possible to increase the performance of a VRFC without additional costs.

Redox flow batteries as the means for energy storage

As the deployment of solar and wind electrical energy increases, the intermittency of these power plants necessitates some means of energy storage for rebalancing the load and the supply. Storage of excess produced energy in electrochemical cells (batteries) is an obvious choice, and perhaps even the most practical method. However, the challenge is to find batteries that would be reliable, widely available, effective and cost-attractive. Within electrochemical storage the redox flow batteries constitute an important subgroup of storage and both the concepts and a number of redox systems is discussed here. The possible chemistries cover the more traditional vanadium redox flow cell. The modern trends are covered in the bromine-hydrogen, flow-through lead, zinc-bromine, cells with organic electrolytes and chemically regenerated redox fuel cells. A table comparing these systems is included.

Membranes based on carboxyl-containing polyacrylonitrile for applications in vanadium redox-flow batteries

A novel-type ion-exchange membrane has been fabricated by the controlled hydrolysis of a polyacrylonitrile film. The efficiency of this membrane in a model vanadium redox-flow cell has been found comparable to that of a commercial Nafion ® membrane.

Modification Based on MoO3 as Electrocatalysts for High Power Density Vanadium Redox Flow Batteries

AbstractMolybdenum oxide MoO3 has been widely used in catalysts and electrochemical energy devices, but, to date, has not been investigated as an electrocatalyst in redox flow cells. In this study, MoO3 in the form of micro‐flakes was introduced onto carbon paper as an electrocatalyst and MoO42− as electrolyte additive for the vanadium redox couple reactions in the vanadium redox flow battery (VRFB). When the carbon paper was modified with MoO3 or MoO42− added to the electrolyte, the peak potential separations were measured as 171 and 203 mV for V2+/V3+ and 130 and 111 mV VO2+/VO2+ redox reactions, respectively, as compared with 306 and 144 mV for the bare pre‐treated carbon paper. A vanadium redox flow cell with MoO3‐CP electrodes exhibited a voltage efficiency of 85.4 % at 100 mA cm−2. The power density reached 200 mW cm−2 at a current density of 150 mA cm−2 when cycled within the voltage range of 0.6–1.7 V. These results indicate that MoO3‐based modification is a promising method for high power density VRFB applications.

Evaluation of the Electrolyte Charge Process and the Operation Process of a Vanadium Redox Flow Battery with Potentiostatic and Galvanostatic Perturbations in Different Volume Schemes

The energy storage technologies are primordial for the working of several applications at high power density and low power density. Particularly, for intermediate and big electric applications and grid administration and support, which need high power density such as auxiliary power units, the vanadium redox flow batteries are an interesting option for the energy storage because of their low cost compared with more complex systems, durability, independence between energy density and power, large discharge time and their relatively simplicity in components considering their material constituents and well behaved - chemical internal processes [1-2].In this work, a simple laboratory prototype of a vanadium redox flow battery with spectroscopy carbon electrodes is utilized to test two classic electrochemical perturbations, a potentiostatic perturbation and a galvanostatic perturbation in the charge of electrolytes process and in the operation of the battery, also using three different volume schemes, aiming to establish the best way to transfer charge in the preliminary electrolyte charge process and in the operation of the battery.As main results, it is found that the battery has a better electrolyte charge process and a better operation process with the potentiostatic perturbation with a volume ratio of 2:1 between the anodic compartment and cathodic compartment. Also a comparison of the state of charge of this battery and the charge/discharge curves with others in literature is achieved, showing a good behavior of the tested conditions [3-7].1. P. Alotto, M. Guarnieri, and F. Moro, “Redox flow batteries for the storage of renewable energy: A review,” Renew. Sustain. Energy Rev., vol. 29, pp. 325–335, Jan. 2014.2. X. Wu, H. Xu, Y. Shen, P. Xu, L. Lu, J. Fu, and H. Zhao, “Treatment of graphite felt by modified Hummers method for the positive electrode of vanadium redox flow battery,” Electrochim. Acta, vol. 138, pp. 264–269, Aug. 2014.3. M. R. Mohamed, H. Ahmad, M. N. A. Seman, S. Razali, and M. S. Najib, “Electrical circuit model of a vanadium redox flow battery using extended Kalman filter,” J. Power Sources, vol. 239, pp. 284–293, 2013.4. A. Z. Weber, M. M. Mench, J. P. Meyers, P. N. Ross, J. T. Gostick, and Q. Liu, “Redox flow batteries: a review,” J. Appl. Electrochem., vol. 41, no. 10, pp. 1137–1164, Oct. 2011.5. M. Skyllas-Kazacos and F. Grossmith, “Efficient Vanadium Redox Flow Cell,” J. Electrochem. Soc., vol. 134, no. 12, p. 2950, 1987.6. M. Skyllas-Kazacos, G. Kazacos, G. Poon, and H. Verseema, “Recent advances with UNSW vanadium-based redox flow batteries,” Int. J. Energy Res., vol. 34, no. 2, pp. 182–189, Feb. 2010.7. G. Colt, “Evaluación de una celda redox de vanadio,” no. 1, pp. 183–188, 2010. Figure 1

In Situ Enhancement of Flow-through Porous Electrodes with Carbon Nanotubes via Flowing Deposition

This study describes a novel flowing deposition method which can be used in situ on a fully assembled electrochemical cell to enhance the electrochemical active surface area of flow-through porous electrodes. As a test case, vanadium redox reaction rates are enhanced by flowing deposition of carbon nanotubes on carbon paper electrodes. The deposition is characterized using a recently developed analytical flow cell technique and shown to increase the electrochemical active surface area and therefore the exchange current for the limiting V2+/V3+ reaction by over an order of magnitude. When applied to the anode of an assembled vanadium redox flow cell, the method nearly eliminates the overpotential associated with this reaction, thereby significantly improving the power output of the cell by up to 70% in the high voltage efficiency regime. As a surface area treatment, this flowing deposition method could be used to similarly enhance the energy conversion efficiency of many other electrochemical flow cells such as fuel cells, flow batteries and capacitive deionization cells.

Unbiased solar energy storage: Photoelectrochemical redox flow battery

Electricity from renewable energy sources is craving for efficient storage technologies, in particular solar industry, to enable practical small-scale solutions for residential and offices use. The best stationary technology is probably the redox flow batteries. This article proposes the direct conversion of sunlight into electrochemical energy stored in a redox flow battery. A photoelectrochemical cell is used to charge a vanadium redox flow cell (CdS(s)|V3+, VO2+||V3+, V2+|Carbon Felt(s), E0=0.6 VNHE). A CdS thin film photoelectrode is prepared to directly charge the cell, pairs V3+/VO2+. CdS photoanode exhibits competitive photocurrents, when compared to other photoelectrochemical devices, and yields enough photovoltage to charge the vanadium battery up to 75% with no external bias. An overlayer of CdSe improves the performance of CdS with current densities up to ca. 1.4mAcm−2. Finally, a tandem configuration is tested using a dye-sensitized solar cell and a CdS photoanoade creating ~1.3V of photovoltage. This tandem arrangement proves to charge conventional all vanadium redox cell (DSC/CdS(s)|VO2+, VO2+||V3+, V2+|Carbon Felt(s), E0=1.2 VNHE) without external bias.

The Effect of Additives on the High‐Temperature Stability of the Vanadium Redox Flow Battery Positive Electrolytes

AbstractSeveral inorganic additives are investigated as potential precipitation inhibitors for the positive half‐cell electrolyte of the vanadium redox flow battery (VRB) at elevated temperatures. Electrolyte stability tests on the more concentrated 2 m vanadium electrolyte show that of the additives tested, the best results are obtained with the addition of 1 % phosphoric acid that give induction times of 40, 22 and 18 days for 80, 90 and 95 % state of charge (SOC) solutions at 50 °C, compared with 5, 2 and 1 days respectively for the corresponding blank solution. Several additives are also evaluated at 45 °C in a vanadium redox flow cell operating with a 2 m vanadium electrolyte. In the case of the cell employing the blank solution, a sudden drop in capacity, followed by pump failure is observed after around 70 cycles, while the cells with the additives continue to cycle without failure for 120 and 150 cycles for ammonium sulphate and ammonium phosphate additives respectively. A similar stabilising effect has previously been observed with these additives at low temperatures in the negative half‐cell electrolyte of the VRB, confirming that these stabilising agents have the potential to extend the upper and lower operating temperature limits of a 2 m vanadium electrolyte in the VRB to 45 and 5 °C respectively, while also significantly enhancing its energy density without the need for potentially hazardous chloride additions.

A High Energy Density Vanadium Redox Flow Battery with 3 M Vanadium Electrolyte

In this paper, a high energy density vanadium redox battery employing a 3 M vanadium electrolyte is reported. To stabilise the highly supersaturated vanadium solutions, several additives were evaluated as possible stabilizing agents for the thermal precipitation of supersaturated V(V) solutions at elevated temperatures. The Blank 3 M V(V) solution in a sulfuric acid supporting electrolyte containing 5 M total sulfates, showed thermal precipitation after 3 days, while the solution containing 1 wt% H3PO4 additive increased the induction time for precipitation to over 47 days at 30°C. After 32 days, 3 M V(V) solutions containing 1 wt% sodium pentapoIyphosphate, 1 wt% K3PO4 and 2 wt% (NH4)2SO4 + 1 wt% H3PO4 showed final V(V) concentrations of 2.7, 2.7 and 2.6 M respectively, compared to 2.4 M V(V) in the Blank solution. From the screening tests, selected additives were used in vanadium redox flow cell cycling studies employing a 3 M vanadium electrolyte. The cell was subjected to 90 charge-discharge cycles and no precipitation or capacity loss was observed in the presence of 1wt% H3PO4 + 2 wt% ammonium sulfate. These results demonstrate that a significant enhancement in the energy density of the VRB can be achieved in the presence of additives that act as precipitation inhibitors for the vanadium ions.

Coulombic Efficiency of Negative and Positive Half-Cells in a Vanadium Redox Flow Cell

Flow batteries offer a lot of promise for large scale energy storage applications.1 In particular, Vanadium Redox Flow Batteries (VRFB) have received a lot of attention and significant commercialisation of the system has already begun.2,3 VRFB cells can operate at coulombic efficiencies of over 90%4,5 and their carbon electrodes have very good stability as long as the positive half-cell is not overcharged.6,7 Since active species of the anolyte and catholyte are just different forms of vanadium in H2SO4, cross-contamination problems are effectively eliminated.8 In addition, because the active species are all in liquid form in solution, these batteries have long lifetimes.9 Furthermore the state of charge of the system may be obtained by monitoring the anolyte and catholyte using physical techniques.10 It has previously been reported that the coulombic efficiency of a VRFB increases with increasing current density, and this has been attributed mainly to the transfer of vanadium ions across the proton exchange membrane,11 leading to an imbalance in the state of charge of the two half-cells. However, hydrogen evolution at the negative electrode cannot be neglected and may also be a significant issue for VRFBs.12 Charging and discharging experiments were carried out using 1.6 mol dm-3 VOSO4 (VIV) in 3 mol dm-3 H2SO4 as starting catholyte and 1.6 mol dm-3 V2(SO4)3 (VIII) in 3 mol dm-3 H2SO4 as starting anolyte in a laboratory scale flow cell. The electrolytes were circulated by means of a peristaltic pump, typically at a flow rate of 0.3 cm3 s-1. Carbon felt electrodes, 6 mm in thickness (compressed to 5 mm in the cell), were used in both the positive and negative half cell. The two half cells were separated by a 180-µm Nafion® 117 membrane. The electrodes and the membrane were 50 mm × 50 mm in area and the electrodes were contacted by means of carbon-filled polymer contacts. The negative half cell potential, negative open circuit potential, positive half cell potential and the positive open circuit potential were measured relative to Hg/Hg2SO4 reference electrodes. We accurately determined the coulombic efficiency of each half cell by measuring its open circuit potential as the battery was charged and discharged. Experiments were performed in which cells were charged and discharged between states of charge of approximately 15% and 75%. The state of charge was determined from open circuit potentials in each half cell using the Nernst equation. The efficiency was then determined from the ratio of charge passed during charging and discharging between these endpoints. It was found that, in general, the positive half cell was more efficient than the negative half cell. Possible reasons for this imbalance in efficiency will be discussed. We will also present results of a more detailed study of coulombic efficiency of the full VRFB cell. Experiments were performed in which the charge rate was varied but the discharge rate has been kept constant. Similarly, experiments were performed in which the discharge rate was varied and the charge rate was held constant. It will be shown that while the coulombic efficiency increased at a higher rate of charging, the coulombic efficiency was not as significantly affected by the variation in the rate of discharging (see Fig. 1). This is attributed to the effect of H2 formation at the negative electrode. Results will be presented for various cell and charging conditions and will be discussed in detail.

Investigation of several graphite-based electrodes for vanadium redox flow cell

Several graphite-based electrodes are investigated for vanadium flow battery applications. These materials are characterized both as-received and after chemical or electrochemical treatments in order to vary the content of oxygen functional groups on the electrode surface. The surface properties of the samples are investigated by X-ray photoelectron spectroscopy. Electrochemical performance is evaluated by cyclic voltammetry and electrochemical impedance spectroscopy measurements in a three electrode half-cell. The chemical treatment with HNO3 causes a cleaning of the electrode surface from adsorbed oxygen species or labile bonded functional groups in highly graphitic samples. Whereas, carbonaceous materials characterized by smaller graphitic domains or a higher degree of amorphous carbon show an increase of oxygen functional groups upon chemical and electrochemical pre-treatments. In both cases, an increase of oxygen species content on the surface above 5% causes a decrease of electrochemical performance for the redox battery determined by an increase of ohmic and charge transfer resistance.

Investigations on V(IV)/V(V) and V(II)/V(III) redox reactions by various electrochemical methods

The positive pole V(IV)/V(V) and negative pole V(II)/V(III) reactions in a vanadium redox flow cell have been investigated on a glassy carbon electrode by various techniques. Several parameters for these electrode reactions were obtained at high concentrations of both sulfuric acid and V species in addition to the mobility (diffusion coefficient and Stokes radius) of V(V), V(IV), V(III) and V(II) species.