Negative Half-cell Research Articles

Rotating ring-disc electrode measurements for the quantitative electrokinetic investigation of the V3+-reduction at modified carbon electrodes

Thin film rotating-ring disc electrode (RRDE) technique is exploited to quantify the parasitic hydrogen evolution reaction (HER) competing with the desired V3+-reduction at surface modified carbon nanoparticles for application as electrocatalysts in the negative half-cell of vanadium redox-flow batteries (VRFB). Carbon based electrode materials are derived from standard Vulcan XC-72 carbon, treated by chemical surface etching techniques proposed for carbon felt-electrodes in the literature. Additional electrochemical characterization is performed using stationary cyclic voltammetry (CV) followed by fitting of CV data, Fourier-transform alternating-current cyclic voltammetry (FT-ACCV) and electrochemical impedance spectroscopy (EIS) followed by distribution of relaxation times (DRT) analysis. To our knowledge the present paper is the first study using the RRDE technique for separating HER and V3+-reduction reactions. It is demonstrated that the ratio of HER to V3+-reduction significantly depends on the chemical pretreatment of the carbon electrodes and that the V3+-reduction proceeds at an optimum rate at E − ERHE = −0.45 V. Separating the HER from the V3+-reduction also allows us to provide highly accurate values for the diffusion coefficient of the V3+-ion in sulfuric acid solutions.

Characterizing Degradation in Non-aqueous Vanadium(III) Acetylacetonate Redox Flow Batteries

A detailed study characterizing the degradation of non-aqueous vanadium flow batteries under moisture-free conditions is presented. Despite assembly and operation in an inert glovebox, rapid cell failure is still observed. The capacity loss is attributed to a combination of electrolyte degradation and increased cell impedance. Cyclic voltammetry verifies the formation of VO(acac)2 within the positive electrolyte, while Raman spectroscopy reveals the formation of additional degradation products in the negative electrolyte. Scanning electron micrographs and X-ray photoelectron spectroscopy reveal the growth of a surface film on cycled electrodes harvested post-mortem, particularly in the negative half-cell. The appearance of the surface film is correlated with electrochemical impedance measurements, which indicate a dramatic increase in charge-transfer resistance post-cycling. The individual electrode contributions to the total cell impedance are resolved post-mortem. It is determined that even when great care is taken to exclude water from the system, the stability of the non-aqueous vanadium flow battery is limited by degradation processes occurring at the negative electrode. Furthermore, the lifetime is not noticeably enhanced, compared to earlier studies where moisture was present.

Deconvolution of electrochemical impedance data for the monitoring of electrode degradation in VRFB

Understanding degradation phenomena occurring during the operation of vanadium redox-flow batteries (VRFB) requires a measurement technique which allows for differentiating the overall performance losses into individual performance losses of the cell components. For this purpose, electrochemical impedance spectroscopy (EIS) is a valuable and well established tool. However, the discrimination of processes taking place at similar time scales is challenging since they overlap in the commonly used Nyquist or Bode representation. Distribution of relaxation times (DRT) analysis tackles this issue by deconvoluting EIS data with respect to the time constants of the individual processes. It hence circumvents the necessity of finding a suitable equivalent circuit model and thus allows for data evaluation without any a-priori knowledge of the system under study. For the first time, we herein present the application of DRT transform to EIS data of a VRFB. By varying experimental conditions and employing full cell as well as double half cell operational modes, we are able to identify the faradaic process of the negative half cell. This enables us to visualize the negative half cell’s contribution to the overall impedance of a VRFB even in a full cell EIS measurement. By an accelerated degradation experiment we finally demonstrate the great potential of DRT analysis for future application in the monitoring of electrode degradation in VRFB.

Integration of energy storage functionalities into fiber reinforced spacecraft structures

One of the common challenges of spaceflight and e-mobility is the energy storage. The operational ranges of spacecraft or electric cars as well as operational usability is strongly dependent on the capacity of the energy storage, which is usually constrained by the mass and volume of the vehicle. For example, the battery mass of large communication satellites ranges from 6% to 9% of the dry mass. This amount increases for smaller satellites like CubeSats up to 13% without accounting wiring harnesses and subsystem volume. Thus, a system can benefit from a reduction in mass and volume by combining multifunctional use of the components and materials. In the presented research, energy storage is integrated into lightweight carbon fiber materials. Carbon fibers have a distinct mass advantage compared to metal structures. In addition, they have very low thermal expansions that can reduce thermal stresses during the operation of a satellite. Fiber composites or laminates consist of two components, the fibers and the matrix material. In laminates, the typically resin based matrix content is 30–40% of the component volume and could be substituted with novel solid state battery materials. Latter have the advantage that they have nearly similar physical properties as usually used resins and are also able to store energy electrochemically. Thus, about 25% of the volume can be used for electrochemical energy storage without compromising structure integrity. To reach such electrochemical functions the host structures have to be infiltrated with anode, cathode, and separator materials. Using the developed recipes and a component thickness of 5 mm, an energy amount per component area of 1130 Wh/m2 can be reached depending on the applied battery active material.This article will give an overview of how fiber composite materials, which are increasingly being used in lightweight construction processes, can be combined with energy storage materials to be used in spacecraft structures. The goal of the research on this topic at the Technische Universtät Braunschweig is a structure battery for a spacecraft. However the article addresses as a first step the characterization and manufacturing of structural battery composite negative half-cells from carbon fiber reinforced PEO/LiTFSI. An estimation of the expected performance is carried out. The initial material evaluation and the manufacturing process as well as mechanical and electrical result will be discussed.

Improved Electrolyte for Negative Side of Zinc-Cerium Redox Flow Batteries

Redox flow batteries (RFBs) are one of the most recent and promising technologies for large-scale energy storage applications. In an RFB, the active materials for the two redox reactions are dissolved in separate electrolytes stored in external tanks. During charge/discharge, these electrolytes are pumped through the cell where the redox reactions occur. Compared to conventional energy storage systems, RFBs are advantageous due to their portability, low maintenance cost, modularity and flexibility [1]. RFBs based on zinc-cerium chemistry are very attractive since they provide the highest cell voltage among flow batteries and thus the highest theoretical energy density [2]. Nevertheless, this system has a number of challenges, the most important of which is the occurrence of side reactions. During charge, hydrogen and oxygen evolution side reactions occur at the negative and positive electrodes, respectively, leading to a lower charge efficiency, wasted energy and eventual premature battery failure [3,4]. During discharge, self-discharge of the zinc deposit due to acid attack also contributes to reduction in the efficiency. Thus, the suppression of these side reactions is necessary for the improvement and ultimate success of zinc-cerium RFBs. Toward this goal, our work focuses on the enhancement of the Zn/Zn(II) redox reaction through the use of a mixed-acid electrolyte. From half-cell experiments on a glassy carbon electrode, we have shown that the introduction of 0.2 – 0.3 mol dm−3 of chloride anions to the conventional methanesulfonic acid electrolyte significantly increases the transport properties of Zn(II), reduces the zinc nucleation overpotential and enhances the exchange current density of the Zn/Zn(II) redox reaction [5]. The same effect is observed when glassy carbon is replaced by polyvinyl ester (PVE), which is a commonly-used negative electrode in Zn-Ce RFBs. The use of this improved electrolyte on the charge, voltage and energy efficiency of a bench-scale zinc-cerium RFB has also been investigated. Similar to results obtained in the half-cell studies, the full-cell experiments show that the mixed methanesulfonate-chloride electrolyte facilitates the zinc redox reaction and thus increases the voltage efficiency of the battery as high as 10%. Additionally, the charge efficiency increases significantly under the conditions where negative half-cell is the limiting reaction. The improvement is more than 20% at an operating current density of 25 mA cm-2. The performance and life-cycle analysis of the battery with this new electrolyte under different operating conditions will also be discussed in this presentation. [1] De Leon, C. P., Frías-Ferrer, A., González-García, J., Szánto, D. A., & Walsh, F. C. (2006). Redox flow cells for energy conversion. Journal of power sources, 160(1), 716-732. [2] Walsh, F. C., Ponce de Léon, C., Berlouis, L., Nikiforidis, G., Arenas‐Martínez, L. F., Hodgson, D., & Hall, D. (2015). The development of Zn–Ce hybrid redox flow batteries for energy storage and their continuing challenges. ChemPlusChem, 80(2), 288-311. [3] Leung, P. K., Ponce-de-León, C., Low, C. T. J., & Walsh, F. C. (2011). Zinc deposition and dissolution in methanesulfonic acid onto a carbon composite electrode as the negative electrode reactions in a hybrid redox flow battery. Electrochimica Acta, 56(18), 6536-6546. [4] Nikiforidis, G., Berlouis, L., Hall, D., & Hodgson, D. (2014). An electrochemical study on the positive electrode side of the zinc–cerium hybrid redox flow battery. Electrochimica Acta, 115, 621-629. [5] Amini, K., & Pritzker, M. D. (2018). Electrodeposition and electrodissolution of zinc in mixed methanesulfonate-based electrolytes. Electrochimica Acta, 268, 448-461.

Operando studies on through-plane cell voltage losses in vanadium redox flow battery

Through-plane potential and overpotential distributions in a vanadium redox flow battery (VRFB) is investigated in-situ using saturated Ag/AgCl reference electrodes attached to each side of the membrane with edge-type configuration. Combined with electrochemical impedance spectroscopy (EIS), this operando approach enables detailed quantification of the voltage losses contributed from the individual VRFB cell components with the magnitudes of ohmic and charge transfer losses at practical charge-discharge conditions. It is found that the overpotentials for vanadium redox reactions on graphite felt electrodes at the positive and negative half-cells are one order magnitude larger than the voltage loss through the typical cation-exchange membrane (Nafion®117) for charging-discharging at current densities from 50 to 120 mA cm−2. Within the overall half-cell overpotentials, the ohmic voltage loss accounts for about 40%. The analysis reveals that the voltage loss by the anode charge transfer reaction is about three folds larger than that by the cathode reaction, suggesting that sluggish V2+/V3+ redox kinetics in the negative half-cell is the major factor limiting the overall VRFB cell performance. This operando methodology is effective for the in-depth analysis of VRFB performance in practical conditions as well as the development of high-performance cell components and efficient total cell design by a collective and quantitative approach.

Degradation Phenomena of Bismuth-Modified Felt Electrodes in VRFB Studied by Electrochemical Impedance Spectroscopy

The performance of all-V redox flow batteries (VRFB) will decrease when they are exposed to dynamic electrochemical cycling, but also when they are in prolonged contact with the acidic electrolyte. These phenomena are especially severe at the negative side, where the parasitic hydrogen evolution reaction (HER) will be increasingly favored over the reduction of V(III) with ongoing degradation of the carbon felt electrode. Bismuth, either added to the electrolyte or deposited onto the felt, has been reported to suppress the HER and therefore to enhance the kinetics of the V(II)/V(III) redox reaction. This study is the first to investigate degradation effects on bismuth-modified electrodes in the negative half-cell of a VRFB. By means of a simple impregnation method, a commercially available carbon felt was decorated with Bi 2 O 3 , which is supposedly present as Bi(0) under the working conditions at the negative side. Modified and unmodified felts were characterized electrochemically using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode setup. Surface morphology of the electrodes and composition of the negative half-cell electrolyte were probed using scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (TXRF), respectively. This was done before and after the electrodes were subjected to 50 charge-discharge cycles in a battery test bench. Our results suggest that not only the bismuth catalyst is dissolved from the electrode during battery operation, but also that the presence of bismuth in the system has a strong accelerating effect on electrode degradation.

Recent development of membrane for vanadium redox flow battery applications: A review

Abstract Responding to rapid growth of the renewable energy applications, it is crucial to develop low cost and high efficient large-scale energy storage systems in order to smooth out the intermittency of the renewable energy resources. As one of the most promising large-scale energy storage systems, vanadium redox flow battery (VRFB) has attracted great attention in recent times. Membrane is one of the key components of VRFB which not only affects the whole cyclability performance but also determines the economic viability of the system. The membrane separates the positive and negative half-cells and prevents the cross-mixing of vanadium ions while providing required ionic conductivity. The ideal membrane should have good ionic exchange capacity; high ionic conductivity, low water uptake, swelling ratio, area electrical resistance and vanadium and other poly-halide ions permeability; and good chemical stability, as well as low cost. Numerous efforts have been spent on the development of different types of membranes, including different functional groups ion exchange membrane and non-ionic porous membrane. This paper reviews the research on membranes in VRFB system, including the properties, development of traditional commercial membranes as well as recently developed membranes. It explores various methods of fabrication of the membrane products which have received relatively little attention. A detailed summary table of the new membranes with their properties, fabrication and costs is provided to serve as a reference guide for researchers and industrialists interested in VRFB system building and dynamic modelling set. Subsequently, the challenges and future directions of membrane research are examined.

A complex four-point method for the evaluation of ohmic and faradaic losses within a redox flow battery single-cell

We propose a complex 4-point method for characterization of flow batteries. The distribution of ohmic and faradaic losses within a single-cell is evaluated from electrochemical impedance spectra and load curves of positive and negative half-cells measured with platinum wire pseudo-reference electrodes positioned in respective electrode compartment. The developed method can be used e.g., for the component screening and in-situ durability studies on single-cell scale. The method was validated on a vanadium redox flow battery single-cell; however, it can be analogically employed for various chemistries of flow battery.•Complex 4-point method for characterization of flow battery single-cell was developed.•Method is based on electrochemical impedance spectra and load curve measurements.•Direct evaluation of ohmic and faradaic losses distribution within battery single-cell by the method.

Detrimental role of hydrogen evolution and its temperature-dependent impact on the performance of vanadium redox flow batteries

Abstract This paper addresses the damaging role of the parasitic hydrogen evolution reaction (HER) in the negative half-cell of a vanadium redox flow battery (VRFB) on state-of-the-art carbon felt electrodes at different temperatures. It was found that increasing the temperature resulted in a better catalytic performance for both the positive and negative half-cell reactions. In addition, increasing the temperature significantly enhanced the undesired HER at the negative side. Operating the VRFB cell at higher temperature led to a decrease in the coulombic efficiency attributed to the higher hydrogen production. More pronounced hydrogen production caused an oxidation on the surface of the carbon fibers and a degradation of the electrode as indicated from scanning electron microscopy and X-ray photoelectron spectroscopy measurements. This observed degradation results in fading of the overall performance of the vanadium redox flow battery over time.

Performance evaluation of thermally treated graphite felt electrodes for vanadium redox flow battery and their four-point single cell characterization

In our contribution we study the electrocatalytic effect of oxygen functionalization of thermally treated graphite felt on kinetics of electrode reactions of vanadium redox flow battery. Chemical and morphological changes of the felts are analysed by standard physico-chemical characterization techniques. A complex method four-point method is developed and employed for characterization of the felts in a laboratory single-cell. The method is based on electrochemical impedance spectroscopy and load curves measurements of positive and negative half-cells using platinum wire pseudo-reference electrodes. The distribution of ohmic and faradaic losses within a single-cell is evaluated for both symmetric and asymmetric electrode set-up with respect to the treatment conditions. Positive effect of oxygen functionalization is observed only for negative electrode, whereas kinetics of positive electrode reaction is almost unaffected by the treatment. This is in a contradiction to the results of typically employed cyclovoltammetric characterization which indicate that both electrodes are enhanced by the treatment to a similar extent. The developed four-point characterization method can be further used e.g., for the component screening and in-situ durability studies on single-cell scale redox flow batteries of various chemistries.

State of charge monitoring of vanadium redox flow batteries using half cell potentials and electrolyte density

The operation of vanadium redox flow batteries requires reliable in situ state of charge (SOC) monitoring. In this study, two SOC estimation approaches for the negative half cell are investigated. First, in situ open circuit potential measurements are combined with Coulomb counting in a one-step calibration of SOC and Nernst potential which doesn't need additional reference SOCs. In-sample and out-of-sample SOCs are estimated and analyzed, estimation errors ≤0.04 are obtained. In the second approach, temperature corrected in situ electrolyte density measurements are used for the first time in vanadium redox flow batteries for SOC estimation. In-sample and out-of-sample SOC estimation errors ≤0.04 demonstrate the feasibility of this approach. Both methods allow recalibration during battery operation. The actual capacity obtained from SOC calibration can be used in a state of health model.

Validating and improving a zero-dimensional stack voltage model of the Vanadium Redox Flow Battery

Simple, computationally efficient battery models can contribute significantly to the development of flow batteries. However, validation studies for these models on an industrial-scale stack level are rarely published.We first extensively present a simple stack voltage model for the Vanadium Redox Flow Battery. For modeling the concentration overpotential, we derive mass transfer coefficients from experimental results presented in the 1990s. The calculated mass transfer coefficient of the positive half-cell is 63% larger than of the negative half-cell, which is not considered in models published to date.Further, we advance the concentration overpotential model by introducing an apparent electrochemically active electrode surface which differs from the geometric electrode area. We use the apparent surface as fitting parameter for adapting the model to experimental results of a flow battery manufacturer. For adapting the model, we propose a method for determining the agreement between model and reality quantitatively. To protect the manufacturer's intellectual property, we introduce a normalization method for presenting the results.For the studied stack, the apparent electrochemically active surface of the electrode is 41% larger than its geometrical area. Hence, the current density in the diffusion layer is 29% smaller than previously reported for a zero-dimensional model.

Hydrogen Diffusion Transport in an All-Iron Flow Battery Rebalance System

All-iron flow batteries (IFBs) are being developed for low-cost large-scale energy storage applications. IFBs use abundantly available, non-toxic and low cost electrolyte materials[1]. The all-iron flow battery involves plating and stripping of metallic iron in the negative half-cell and utilizes the Fe II/III (ferrous/ferric) redox couple reaction at its positive electrode[2-3]. Hydrogen evolution occurs as the iron deposits at the negative electrode during the flow battery charging and even if the amount might be small, there will be an impact on the electrolyte balance over many cycles. Recently, Selverston et al.[3] demonstrated a concept of hydrogen rebalance in an all-iron flow battery through designing a capillary-action galvanic reactor (CGR) to consume hydrogen and reduce excess ferric ions in the positive electrolyte reservoir. This design can maintain the balance of protons and reduce undesired precipitation reactions. A tube connects the negative tank headspace where hydrogen collects to the positive tank headspace where the CGR reactor is placed. An understanding of design factors on hydrogen transport through the tube between headspaces is desired and mathematical modeling[4-7] is the approach we took to achieve this. In this poster, we report a mathematical model of hydrogen diffusion between the headspaces of the negative electrolyte tank and the positive electrolyte tank. The effects of dimensional parameters, including diameter of tank, height of tank, diameter of tube, length of tube and diffusion coefficient on hydrogen diffusion rates and partial pressure are investigated during both battery charge and battery standby cases. In Figure 1, we demonstrate that the mathematical model is reasonably validated by the previous published experimental data for a specific all-iron flow battery rebalance device. Acknowledgements This work is supported by the all-iron flow battery project (grant number: DE-AR0000352) funded by Department of Energy (DOE) of the United States.

Improved Performance of Ti/Mn Redox Flow Battery by Thermally Treated Carbon Paper Electrodes

In this paper we investigate the effect of heat treatment of carbon papers on Ti-Mn redox system. The heat treatment improves the redox reaction kinetics of carbon paper electrode for both positive and negative half-cell. It is considered that the enhanced kinetic performance is attributed to the increased wettability of carbon surface, because a significant increase in the surface area was not observed. In the full-cell evaluation, mass transport loss increased significantly with increasing the state of charge. It is considered that the precipitation of MnO2 caused decrease of reactants and prevented reactants diffusion. The low kinetics of negative half-cell limited the cell performance. The cell performance has been greatly improved by using a high capacitance carbon felt as negative electrode. A high energy efficiency of 83.2% at current density of 100 mA cm-2 was obtained in a single cell test.

Evaluation of Redox Chemistries for Single-Use Biodegradable Capillary Flow Batteries

The rate of battery waste generation is rising dramatically worldwide due to increased use and consumption of electronic devices. A new class of portable and biodegradable capillary flow batteries was recently introduced as a solution for single-use disposable applications. The concept utilizes stored organic redox species and supporting electrolytes inside a dormant capillary flow cell which is activated by the dropwise addition of aqueous liquid. Herein, various organic redox species are systematically evaluated for prospective use in disposable capillary flow cells with regards to their electrochemical characteristics, solubility, storability and biodegradability. Qualitative ex-situ techniques are first applied to assess half-cell solubility, redox potential and kinetics, followed by quantitative in-situ measurements of discharge performance of selected redox chemistries in a microfluidic cell with flow-through porous electrodes. Para-benzoquinone in oxalic acid and either hydroquinone sulfonic acid or ascorbic acid in potassium hydroxide are identified for the positive and negative half-cells, respectively, yielding a maximum discharge power density of 50 mW/cm2. A prototype capillary flow battery using the same redox chemistries demonstrates robust cell voltages above 1.0 V and maximum discharge power of 1.9 mW. These results show that practical primary battery performance can be achieved with biodegradable chemistries in a disposable device.

The porous membrane with tunable performance for vanadium flow battery: The effect of charge

Abstract Porous membranes with different charge on the surface and internal pore walls are prepared via the solvent-responsive layer-by-layer (SR-LBL) method. The effect of charge on the transport properties of different ions through the membranes is investigated in detail. The charge property of prepared membranes is tuned by assembling different charged polyelectrolytes (PEs) on the pore walls and the surface of the porous membranes. The results show that in a vanadium flow battery (VFB), the PE layers assembled on the surfaces (including pore walls) are capable to construct excellent ion transport channels to increase proton conductivity and to tune the ion selectivity via Donnan exclusion effect. Compared with the porous membrane with negative charges (7 bilayers), a VFB single cell assembled with a positively charged membrane (7.5 bilayers) yields a higher coulombic efficiency (98%). The water and ion transfer behavior exhibits a similar tendency. In the negative half-cell, the amount of V 3+ gradually increases as cycles proceed and the amount of V 2+ stays at a low and stable level. In the positive half-cell, the amount of VO 2+ decreases; while VO 2 + is accumulated. The imbalance of vanadium ions at both sides induces the discharge capacity fade.

Balancing Osmotic Pressure of Electrolytes for Nanoporous Membrane Vanadium Redox Flow Battery with a Draw Solute.

Vanadium redox flow batteries with nanoporous membranes (VRFBNM) have been demonstrated to be good energy storage devices. Yet the capacity decay due to permeation of vanadium and water makes their commercialization very difficult. Inspired by the forward osmosis (FO) mechanism, the VRFBNM battery capacity decrease was alleviated by adding a soluble draw solute (e.g., 2-methylimidazole) into the catholyte, which can counterbalance the osmotic pressure between the positive and negative half-cell. No change of the electrolyte volume has been observed after VRFBNM being operated for 55 h, revealing that the permeation of water and vanadium ions was effectively limited. Consequently, the Coulombic efficiency (CE) of nanoporous TiO2 vanadium redox flow battery (VRFB) was enhanced from 93.5% to 95.3%, meanwhile, its capacity decay was significantly suppressed from 60.7% to 27.5% upon the addition of soluble draw solute. Moreover, the energy capacity of the VRFBNM was noticeably improved from 297.0 to 406.4 mAh remarkably. These results indicate balancing the osmotic pressure via the addition of draw solute can restrict pressure-dependent vanadium permeation and it can be established as a promising method for up-scaling VRFBNM application.

Electrochemical analysis of the performance loss in all vanadium redox flow batteries using different cut-off voltages

The performance loss observed in vanadium redox flow batteries over time strongly depends on the material and the cycling conditions. We have chosen 1.65V and 1.8V as the cut-off voltages in order to compare the performance loss and the risk of evolving hydrogen on the negative side and CO2 on the positive side. The 1.8V experiment was expected to show stronger degradation because of the higher risk of hydrogen and CO2 evolution due to the higher maximum half-cell potentials. To address this issue we implemented a reference electrode at the inlet of the negative half-cell of a 10cm2 test cell. The discharge process on the negative half-cell was assumed to be the process with the highest overpotential after running 50 charge and discharge cycles. After 50 cycles the reduction of V(II) remained the favored reaction compared to the HER in the negative side, even though the overpotentials for both reactions increased significantly. As for the positive half-cell, the risk of CO2 evolution increased with increasing cut-off voltage. The higher cut-off voltage led to a higher rate of degradation, but it also showed a better overall performance. The potentials in the half-cells were changing dynamically with each cycle depending on the degradation rate of the electrode and the state of the electrolyte. An imbalance of the electrolyte could be observed by measuring the half-cell open circuit potential after each charge and discharge process. The obtained results can be used for the diagnostics and optimization of VRFB systems in order to achieve maximum system output at reasonable risks of parasitic reactions and degradation.

Countering Degradation of Carbon Felt Electrodes in All-Vanadium Redox Flow Batteries (VRFB)

Carbon felt electrodes are used as electrode material in VRFB. The main reasons to use carbon felt electrodes are their corrosion resistance in acidic electrolyte, high overpotential towards hydrogen evolution reaction and low material costs. Different origins of degradation can be related to dynamic cycling conditions or to possible side reactions. Chemical degradation or aging on the other hand can be caused by simple contact with the electrolyte. The static aging and the dynamic degradation of heat-treated carbon felt electrodes in all-Vanadium redox flow battery systems were investigated by different electrochemical and analytical methods. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to determine the performance loss of the carbon-based electrodes. Different characterization methods, such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), were applied to study cause and effects of material’s degradation. Chemical aging was found to take place during the first 6 days of mere contact with the electrolyte (1.6 M V3+/4+, 2 M H2SO4, 0.05 M H3PO4). Both, the charge transfer resistance and the diffusion resistance increased with time. Electrochemical degradation was observed for 50 charge and discharge cycles of a 10 cm2 cell with flow-through design and 100 mL of electrolyte. The half-cell potentials shifted towards more negative potentials with an increasing amount of cycles. Chemical aging and electrochemical degradation have a strong impact on the negative half-cell. XPS and SEM reveal that the main reason for the observed performance loss is a decrease of electrochemically active surface area (ECSA) due to the oxidation of the carbon felt to CO and CO2. The durability of the electrodes was increased by introducing bismuth coated electrodes in the negative half-cell. Bismuth was deposited on the carbon felt with a pulse deposition protocol out of an aqueous solution. Charge and discharge experiments exhibited a more stable performance compared to pure carbon felt electrodes. The half-cell potentials remained constant during cycling but the overall cell resistance was higher, because the electrical conductivity of bismuth is lower compared to graphitic carbon.