Enhancing Vanadium redox flow batteries performance through local compression ratio adjustment using stiffness gradient carbon felt electrodes

Improvement in catalytic activity of electrochemically treated carbon-based electrode (relative to that of the untreated carbon electrode) toward vanadium redox flow battery (VRFB) performance is widely reported in the literature [1-3]. In this work, performance characteristics of ex-situ treated carbon felt electrode is compared to the in-situ treated (charge-discharge cycles) carbon felt electrode. Also, the effect of porosity of electrode on the performance of VRFB is studied using different gasket thickness. The VRFBs constructed with the in-situ and ex-situ treated electrodes have comparable performance. Furthermore, the electrode compressed to 22 % delivers higher performance compared to that of the electrode compressed to 6 %. It is observed that porosity of electrode has a pivotal role in the cell performance. Thus, the thickness of the electrode must be considered along with the surface area (cm2) and it would be appropriate to report power density in terms of the volume of the electrode (cm3). Figure 1: Performance curves of VRFB using three different carbon felt electrodes of area 25 cm2 at a flow rate of 20 ml per minute.

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

In this study, the concentration of vanadium ions in a carbon felt electrode was examined quantitatively using UV-visible spectrophotometry with respect to the current density and flow rate to examine the effect of the concentration overpotential on the performance of a vanadium redox flow battery (VRB). Experimental results revealed that the concentration of vanadium ions in the positive electrode is different from that observed in the positive electrolyte tank, and the deviation increases with increasing current density and decreasing flow rate. The concentration of V(IV) is higher in the positive electrode compared to the positive electrolyte tank after the discharging process, which leads to a premature cut-off in the discharge of the VRB. Therefore, this result suggests that it is important to improve electrolyte transport through the electrode for better performance of VRBs, and the measurement of the concentration of vanadium ions in the electrode could be useful for studying the concentration overpotential.

Facile preparation of an oxygen-functionalized carbon felt electrode to improve VO2+/VO2+ redox chemistry in vanadium redox flow batteries

All-vanadium redox flow batteries (VRFBs) have been identified as a promising technology for bridging the gap to integrating renewables onto grids as well as improving grid-level power generation efficiency. VRFBs are prime candidates for this role on account of their modularity, scalability, and relatively high performance. [1] There are still, however, many design parameters that can be more highly optmized to improve and specialize the aspects of this technology. Capacity fade is a parasitic loss that occurs in all electrochemical enery storage systems and is one of the parameters that can be most improved in VRFBs. Fading capacity is due to a host of variables in the system, such as membrane permeability and electroactive species concentration.[2] Degradation of cell components (electrodes and membranes) can also contribute to capacity decay in the form of increased overpotentials throughout the systems operating range. The kinetics at the negative electrode (V(II) ↔ V(III)) are generally understood to be more sluggish than at the positive electrode [3] as well as being less stable (i.e. degrading more rapidly with respect to time). Several high performance materials have been identified for the negative electrode [4] [5], however extended cycling data on the order of years is uncommon in the literature. Mitigation of this degradation on this time scale is one such optimization that can improve voltage efficiency and mitigate capacity fade. In this work we present our most recent experimental work in which the Faradaic current that can be supported by the plates is quantified on different sets of VRFB hardware and isolated from the current on the combined plates with GFD 2.5 mm carbon felt electrodes. For this work symmetric cells were utilized, to isolate crossover effects, in conjunction with electrochemical impedance spectroscopy and polarization curves to quantify the contributions from the different processes at each overpotential. Likewise, experiments were also performed on a symmetric cell where the plates served as the electrodes to quantify the parallel contribution. The data were assessed via statistical tests in order to determine significance. The results of this study show that, graphitic bipolar plates can support 35-50% of the current density on the negative electrode at low overpotentials and 20 and 30% at higher overpotentials. The open circuit impedance data also show that the ASR of the cell with felt electrodes is increasing at a rate of around 25 mΩ cm2 min−1 at this stage in the electrode’s life. This degradation is nonlinear and asymptotes to a stable ASR, however, this implies that the percentage measured in this context is smaller than the steady state percentage. These combined results suggests a material oriented approach to improving the bipolar plate material in order to achieve a more stable negative electrode.

The worldwide effort to decarbonize the electricity grid has made battery energy storage system (BESS) an essential technology. A promising technology for long duration storage on the grid is Vanadium Redox Flow Batteries (VRFBs), which can support the grid by regulating peak load, reducing intermittency of large-scale renewables, and as emergency power supply. There are two primary methods of integrating a BESS to the grid. Frist, the BESS is directly connected to the DC-link of a DC-AC converter that interfaces with the grid. Second, the BESS is connected to a DC-DC converter that in turn interfaces with the DC-link of a DC-AC converter. These converters all have a characteristic ripple voltage and current during operation. The magnitude and frequency of the ripples depend on topology and switching frequency of a converter, controller design and protocol, type of connection to the grid, voltage magnitude and load characteristics. Currently, studies on the impact of ripples have been limited to Lithium-ion and Lead-Acid batteries. The impact of ripple current and voltage has not been studied in VRFBs. In a VRFB, potential impact includes excessive gassing of hydrogen and carbon corrosion of the bipolar plate and porous electrode and cell heating. To better integrate and operate VRFBs to the electricity grid, the impact of ripples needs to be studied in detail.Using a novel experimental platform, where both magnitude and frequency of ripples will be incorporated, we will emulate the performance of different power electronic converter configurations. It will comprise standard 5cm2 VRFB cells with a carbon felt electrode and Nafion ion exchange membrane. The various converter ripples in an electricity grid setting will be emulated by superimposing the signals from a potentiostat (constant current) and a high frequency transformer (modulated ripples), which will be connected to the cathode of the cells.The experimental testbed will be used to cycle the VRFB cells at various states of charge with ripples that emulate the performance of power electronic converters during grid operations and disturbances. Varied parameters will include ripple amplitude, ripple frequency, electrolyte flow rate, applied current density and state of charge. Measurement data from monitoring voltage of each electrode, cell voltage and current, gas generation, electrolyte crossover and electrolyte loss will be analyzed to determine the impact of ripples on VRFB cells. Finally, the cells will be monitored for signs of gas generation and postmortem signs of carbon corrosion.Our analysis will determine the impact of various ripple magnitude and frequencies on VRFBs in a simulated grid integration environment. This work will facilitate the development of design standards for power electronic converters that are intended for battery applications, while also informing future efforts to optimize VRFB cell and stack performance.

Electrode-integrated bipolar plate structure for multi-cells in vanadium redox flow batteries

Activity gradient carbon felt electrodes for vanadium redox flow batteries

Modification and application of spiral flow fields in vanadium redox flow batteries

Highly porous zeolitic-imidazole frameworks (ZIFs) are synthesized to produce N-doped mesoporous carbon electrocatalysts via calcination. The N-doped carbon (m-NC) and carbon nanotubes (m-NCNT) are obtained from ZIF-8 and ZIF-67, while the core-shell structure of ZIF-8@ZIF-67 produced with ZIF-8 seeds (m-NC@NCNT) is prepared by hydrothermal method. Chemical and optical evaluations of the catalysts are characterized using BET, FT-IR, XPS, XRD, Raman spectroscopy and SEM/STEM and they are used as the catalysts for redox reactions of vanadium ions and redox flow battery (VRFB) performance. In the utilization, m-NC@NCNT and m-NCNT are effective for improving VO2+/VO2+ redox reaction, although m-NC does not influence that. Even in VRFB tests using the catalysts, charge/discharge potential and energy efficiency (EE) of m-NC@NCNT and m-NCNT are highest, not to mention excellent EE resilience after undergoing tougher cycling condition. These results are due to the large graphitic-N portion of the two catalysts. Namely, electrons produced by the graphitic-N are delocalized, forming pi-conjugated system and vanadium–nitrogen transition state. This state then promotes electron transfer during VO2+/VO2+ redox reaction and VRFB performance.

Three kinds of sulfonated poly(ether ether ketone) (SPEEK)/nano oxide (Al2O3, SiO2, and TiO2) composite membranes are fabricated for vanadium redox flow battery (VRFB) application. The composite membranes with 5 wt% of Al2O3, SiO2, and TiO2 (S/A-5 %, S/S-5 %, and S/T-5 %) exhibit excellent cell performance in VRFB. Incorporation of nano oxides (Al2O3, SiO2, and TiO2) in SPEEK membrane improves in aspect of thermal, mechanical, and chemical stabilities due to the hydrogen bonds’ interaction between SPEEK matrix and nano oxides. The energy efficiencies (EEs) of composite membranes are higher than that of Nafion 117 membrane, owing to the good balance between proton conductivity and vanadium ion permeability. The discharge–capacity retentions of composite membranes also overwhelm that of Nafion 117 membrane after 200 cycles, indicating their good stability in VRFB system. These low-cost SPEEK/nano oxide composite membranes exhibit great potential for the application in VRFB.

It has become increasing clear that renewable electricity can be generated on a large scale through a variety of different methods. While this progress has begun to decarbonize baseload electricity generation, the intermittent nature of the required resources has proven difficult to incorporate into regional electrical grids. Grid-scale electricity storage methods are a promising solution to the inefficiencies associated with unpredictable electricity generation. Redox flow batteries have demonstrated the ability to be used as a reasonably low-cost, long-term electrical storage method. Specifically, vanadium redox flow batteries (VRBs) have been of special interest due to their chemical stability, long life cyclability, and potential for high capacity electrical storage. One of the main factors limiting market penetration of redox flow batteries in energy storage portfolios is the high capital cost associated with this comparatively low energy density technology. To this end, many current research efforts focus on improving the energy density of VRBs to warrant this cost in the long term. This work focuses on improving the energy density of VRBs by probing the effects of a variety of different metal electrocatalysts. This process uses metal salts to form catalytically active nanoparticles on the surface of the anode to improve the kinetics of the anodic reaction in the VRB. The systems of interest in this study involved using tin, copper, and lead metals deposited on the surface of the anode using a variety of techniques. The direct reduction of metal salts results in the formation of nano-scale and micron-scale electroplated particles on the surface of the anode, which can be used to improve the kinetics of the sluggish anodic reaction in VRBs. This work shows that when as little as 0.02 M concentrations of tin chloride are added to 1.0 M vanadium electrolyte, electrochemically reduced tin nanoparticles have excellent catalytic properties toward the anodic reaction, improving current densities by over 30% and improving the overall capacity of the battery. There were also some synergistic effects observed when the tin salts were coupled with copper salts, further improving the capacity of the battery. A series of methods were used to probe the effects of synthesized lead particles as well. We will present how these processes can have a dramatic effect on increasing both capacity and energy density of aqueous vanadium redox flow batteries.

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.

In this study, a SnO2-carbon nanotube (SnO2-CNT) composite as a catalyst for vanadium redox flow battery (VRFB) was prepared using a sol-gel method. The effects of this composite on the electrochemical performance of /VO2+, and on the V2+/V3+ redox reactions and VRFB performance were investigated. The SnO2-CNT composite has better catalytic activity than pure SnO2 and CNT due to the synergistic catalysis of SnO2 and the CNT. SnO2 mainly provides the catalytic active sites and the CNTs mainly provide the three-dimensional structure and high electrical conductivity. Therefore, the SnO2-CNT composite has a larger specific surface area and an excellent synergistic catalytic performance. For cell performance, it was found that the SnO2-CNT cell shows a greater discharge capacity and energy efficiency. In particular, at 150 mA cm−2, the discharge capacity of the SnO2-CNT cell is 28.6 mAh higher than that of the pristine cell. The energy efficiency of the modified cell (7%) is 7.2% higher than that of the pristine cell (62.8%). This study shows that the SnO2-CNT is an efficient and promising catalyst for VRFB.

Amino-silica modified Nafion membrane for vanadium redox flow battery