Activity gradient carbon felt electrodes for vanadium redox flow batteries

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

With dwindling fossil resources and growing environmental concerns, clean and efficient energy conversion and storage technologies are getting more and more attention from the public. Lithium ion batteries, redox flow batteries as well as fuel cells hold the promise and burden to sustain our energy-demanding society. In recent years, several efforts have been addressed to vanadium redox flow batteries (VRFB) which show great promise for the economical storage of electrical energy intermittently available from wind and solar. The excess energy is stored in liquid electrolytes using the four oxidation states of Vanadium, V2+/3+ at the negative, V4+/5+at the positive side. The liquid electrolytes flow through porous carbon felt electrodes, which provide a heterogeneous reaction surface. The main reasons to use carbon felts are their corrosion resistance in acidic electrolyte, high overpotential towards the unwanted hydrogen evolution reaction (HER) and low material costs. In addition, their activity towards the positive and negative reaction, respectively, can be modified using thermal oxidation in air atmosphere, acid treatment, heteroatom doping, and others. With respect to stationary applications with projected system runtimes of 10˙000 hours and more, not only activity, but also durability of the electrode materials moves into the focus of current research. It has been reported [1] that the carbon felts undergo significant degradation when cycled for a certain number of times. However, already the electro-less contact with the highly acidic electrolytes seems to affect the felts properties. In this study, static electro-less ageing and dynamic degradation of heat-treated carbon felt electrodes were investigated by combining different electrochemical and analytical methods. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to monitor the performance loss of the carbon-based electrodes, whereas scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to study cause and effects of material’s degradation. In this respect, oxygen functional groups, electron conductivity, wettability, accessible surface area and other characteristics play an important role. In general, the HER is a parasitic reaction, which should be hindered in favor of the V3+ reduction. It has been reported that bismuth additions to the electrolyte enhance the activity of the felt for the negative reaction without enhancing the HER [2]. However, the competition between HER and V3+reduction can also be used as an indicator of the felt’s state of health when probed at different temperatures and in different electrolyte concentrations. We will discuss this for differently degraded felts. Furthermore, we use our battery tester with implemented reference electrodes to compare the individual half-cell potentials with full cell potentials and to study the effect of different cut-off voltages on the projected electrode degradation. Higher cut-off potentials might prove effective for the system’s operation strategy, if they do not lead to the expected increased degradation phenomena. [1] I. Derr, M. Bruns, J. Langner, A. Fetyan, J. Melke, C. Roth „Degradation of all-vanadium redox flow batteries (VRFB) investigated by electrochemical impedance and X-ray photoelectron spectroscopy: Part 2 electrochemical degradation, Journal of Power Sources 325 (2016) 351-359. [2] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang Nano Letters. 13 (2013) 1330-1335.

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.

Effects of atmospheric air plasma treatment of graphite and carbon felt electrodes on the anodic current from Shewanella attached cells

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.

Regulating flow field design on carbon felt electrode towards high power density operation of vanadium flow batteries

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.

Carbon felt electrode modified by lotus seed shells for high-performance vanadium redox flow battery

The effects of surface treatment combining corona discharge and hydrogen peroxide (H2O2) on the electrochemical performance of carbon felt electrodes for vanadium redox flow batteries (VRFBs) have been thoroughly investigated. A high concentration of oxygen functional groups has been successfully introduced onto the surface of the carbon felt electrodes by a specially designed surface treatment, which is mainly responsible for improving the energy efficiency of VRFBs. In addition, the wettability of the carbon felt electrodes also can be significantly improved. The energy efficiency of the VRFB cell employing the surface modified carbon felt electrodes is improved by 7% at high current density (148 mA cm−2). Such improvement is attributed to the faster charge transfer and better wettability allowed by surface-active oxygen functional groups. Moreover, this method is much more competitive than other surface treatments in terms of processing time, production costs, and electrochemical performance.

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

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.

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.

In this study, carbon felt (CF) electrodes were chemically treated using alkali metal salt (potassium nitrate) in the molten state as a mild oxidizing agent. Various temperatures in the range of 380–500 °C and oxidation duration (up to 12 h) were analyzed in order to find the most appropriate oxidation conditions. As-prepared carbon felts were electrochemically investigated in a three-electrode set-up as well as in a lab-scale vanadium redox flow battery (VRFB). The battery set-up included additional in-situ potential measurements allowing to measure the polarization of each electrode separately and compare the performance of heat- and chemically-treated electrodes. The physico-chemical properties of CFs have been analyzed using X-ray photoelectron spectroscopy (XPS) and other advanced techniques. Combination of extended oxidation time with temperature increase causes satisfactory surface functionalization and enhanced vanadium redox reaction reversibility. The chemically treated samples showed a better performance with lower overpotentials and higher efficiencies. Improved reversibility of V2+/V3+ reaction is obtained for the carbon felt treated with salt presence. What is more, a 4 times reduced CF electrode oxidation time as usually presented in literature (12 h for heat-treated electrodes vs. 3 h for salt- and heat-treated ones) can be achieved with nitrate salt oxidation.

Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application

Redox flow batteries (RFBs) are a promising technology for grid scale stationary energy storage to complement renewable energy systems. Although RFBs have a relatively low energy density, they offer important benefits such as long lifetime, decoupled energy and power, high round-trip efficiency, scalability and design flexibility, fast response and low environmental impacts. These benefits make them superior to many other stationary energy storage technologies [1-3]. The vanadium RFB (VRFB) is the most widely used RFB in industry for grid scale energy storage and for integration of renewable energy generation systems [4]. However, due to their slow redox reaction rates, their performance and stability are limited and VRFBs would thus benefit from enhancement of the electrode kinetics [5]. Various catalysts have been employed to improve the redox reaction rates [5-6]. Bismuth is considered a suitable candidate due to its good reactivity with oxygen, low toxicity, relatively low cost and comparable catalytic properties to other metal competitors [5, 7-8].In this study, we present a novel modified VRFB electrode composed of carbonized bismuth-polyaniline (Bi-PANI) on a thermally-treated carbon paper substrate. The battery performance was evaluated in a 5 cm2 flow cell using a ‘zero-gap’ design with a 1.6 M VOSO4 in 3 M H2SO4 electrolyte. Charge-discharge of the VRFB was performed at a constant current density (ranging from 10 to 80 mA cm−2) with upper and lower voltage cut-offs of 1.65 and 0.8 V, respectively. Thermally treated carbon paper electrodes were tested as a comparison, and it was found that the VRFB efficiency was significantly enhanced, from 51% to 68% at a current density of 80 mA cm− 2, due to the modification of the electrodes with Bi-PANI. The charge-discharge capacity was also improved by around 17.2%. In addition, the stability of the battery using the modified electrodes was evaluated for over 200 cycles. The improved battery performance was attributed to the catalytic effect of the Bi particles on the VO2+/VO2 + and V2+/V3+ redox reactions leading to a significant increase in capacity, voltage, and energy efficiency.