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

Reliability studies of vanadium redox flow batteries: upper limit voltage effect

All-vanadium redox flow batteries (VRFBs) show promise as a long-duration energy storage (LDES) technology in grid applications. However, the continual performance fading over time poses a significant obstacle for VRFBs. This study systematically investigates the impact of increased upper limit voltage (1.6 V, 1.7 V, and 1.8 V) in the reliability and degradation of a scaled VRFB cell (49 cm2) over long-term testing (500+ cycles). The findings indicate that higher upper voltages significantly decrease capacity and voltage efficiencies. Although electrolyte remixing can restore the majority of the capacity, it only partially recovers voltage efficiency at 1.7 V and 1.8 V, suggesting substantial cell degradation. Analysis reveals that the overpotential increase induced degradation is mainly contributed by the anode during charging and the cathode during discharging. Increased upper voltage amplifies degradation, with the anode being more affected. As confirmed by electrochemical impedance spectroscopy (EIS) and polarization curves, elevated voltages lead to significant resistance increases, driven by charge transfer resistance (mostly from the anode). Moreover, the morphological, surficial, and electrochemical characterization results of cycled electrodes suggest that the degree and mode of degradation were contingent upon the cutoff voltage. For instance, the cathode experienced severe surface degradation at the maximal upper voltage of 1.8 V. This work highlights the importance of optimizing voltage limits to improve the lifetime of VRFBs and offers valuable insights into the development of predictive models through using accelerated stressor lifetime testing (ASLT) protocols for VRFBs.

Unveiling the Role of Electrografted Carbon-Based Electrodes for Vanadium Redox Flow Batteries

Carbon-based electrodes are used in flow batteries to provide active centers for vanadium redox reactions. However, strong controversy exists about the exact origin of these centers. This study systematically explores the influence of structural and functional groups on the vanadium redox reactions at carbon surfaces. Pyridine, phenol and butyl containing groups are attached to carbon felt electrodes. To establish a unique comparison between the model and real-world behavior, both non-activated and commercially used thermally activated felts serve as a substrate. Results reveal enhanced half-cell performance in non-activated felt with introduced hydrophilic functionalities. However, this cannot be transferred to the thermally activated felt. Beyond a decrease in electrochemical activity, a reduced long-term stability can be observed. This work indicates that thermal treatment generates active sites that surpass the effect of functional groups and are even impeded by their introduction.

Extending the lifetime of vanadium redox flow batteries by reactivation of carbon electrode materials

The degradation and aging of carbon felt electrodes is a main reason for the performance loss of Vanadium Redox Flow Batteries over extended operation time. In this study, the chemical mechanisms for carbon electrode degradation are investigated and distinct differences in the degradation mechanisms on positive and negative electrodes have been revealed. A combination of surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Electrochemical Impedance Spectroscopy (EIS) was applied for this purpose. In addition to understanding the chemical and physical alterations of the aged electrodes, a thermal method for reactivating aged electrodes was developed. The reactivation process was successfully applied on artificially aged electrodes as well as on electrodes from a real-world industrial vanadium redox flow battery system. The aforementioned analysis methods provided insight and understanding into the chemical mechanisms of the reactivation procedure. By applying the reactivation method, the lifetime of vanadium redox flow batteries can be significantly extended.

Systematic Characterization of Degraded Anion Exchange Membranes Retrieved from Vanadium Redox Flow Battery Field Tests

Commercially available anion exchange membranes were retrieved from VRFB field tests and their degradation due to the various operation conditions is analyzed by in-situ and ex-situ measurements. Ion exchange capacity, permeability and swelling power are used as direct criteria for irreversible changes. Small-angle X-ray scattering (SAXS) and Differential scanning calorimetry (DSC) analyses are used as fingerprint methods and provide information about the morphology and change of the structural properties. A decrease in crystallinity can be detected due to membrane degradation, and, in addition, an indication of reduced polymer chain length is found. While the proton diffusion either increase or decline significantly, the ion exchange capacity and swelling power both are reduced. The observed extent of changes was in good agreement with in-situ measurements in a test cell, where the coulombic and voltage efficiencies are reduced compared to a pristine reference material due to the degradation process.

Graphene quantum dot-decorated carbon electrodes for energy storage in vanadium redox flow batteries

Nitrogen-doped graphene quantum dots (GQDs) and graphitic carbon nitride (g-C₃N₄) quantum dots are synthesized via a solid-phase microwave-assisted (SPMA) technique. The resulting GQDs are deposited on graphite felt (GF) and are employed as high-performance electrodes for all-vanadium redox flow batteries (VRFBs). The SPMA method is capable of synthesizing highly oxidized and amidized GQDs using citric acid and urea as the precursor. The as-prepared GQDs contain an ultrahigh O/C (56-61%) and N/C (34-66%) atomic ratio, much higher than the values reported for other carbon-based nano-materials (e.g. oxidized activated carbon, carbon nanotubes, and graphene oxide). Three types of quantum dots, having an average particle size of 2.8-4.2 nm, are homogeneously dispersed onto GF electrodes, forming GQD/GF composite electrodes. Through deposition of GQDs onto the electrode structure, the catalytic activity, equivalent series resistance, durability, and voltage efficiency are improved. The capacity utilization using GQD/GF electrode is substantially enhanced (∼69% increase within 40 cycles). The improved performance is attributed to the synergistic effect of GQDs containing oxygen functionalities (epoxy, phenolic and carboxylic groups) and lattice N atoms (quaternary, pyrrolic and pyridinic N) which result in enhanced wettability and increased electrochemical surface area providing increased reaction sites.

Accelerated Stress Test Method for the Assessment of Membrane Lifetime in Vanadium Redox Flow Batteries

An accelerated stress test (AST) method was developed to estimate the lifetime of ion-exchange membranes in a vanadium redox flow battery. The oxidative VO₂⁺ ions present in the charged positive electrolyte are the predominant stressor causing loss of functional groups and membrane conductivity. Membrane aging was accelerated in ex situ tests by exploiting elevated temperatures and the increased oxidative strength of Ce⁴⁺. Acceleration factors were determined on the basis of the analysis of aged radiation grafted g(S-AN) membranes. The degradation in a Ce⁴⁺ solution was found to be ∼4 times faster than in VO₂⁺. The highest acceleration factor of ∼200 was found for the degradation with Ce⁴⁺ at 80 °C. The degradation reaction present in the accelerated stress test showed a similar activation energy of ∼50 kJ/mol in VO₂⁺ and Ce⁴⁺ solutions, suggesting a similar reaction pathway. The applicability of the test was further evaluated with the second membrane, g(AMS-MAN). Its lifetime was estimated based on the accelerated stress test and acceleration factors previously determined for the g(S-AN) membrane and compared to the lifetime projected from an extended cycling experiment in the cell. The two values were in the same range of ∼4000 h. The proposed AST can serve as a basis for predictive modeling of membrane lifetime in vanadium redox flow batteries. The potential of the method and the limitations are discussed.

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.

Elucidating the Nuanced Effects of Thermal Pretreatment on Carbon Paper Electrodes for Vanadium Redox Flow Batteries

Sluggish vanadium reaction rates on the porous carbon electrodes typically used in redox flow batteries have prompted research into pretreatment strategies, most notably thermal oxidation, to improve performance. While effective, these approaches have nuanced and complex effects on electrode characteristics hampering the development of explicit structure-function relations that enable quantitative correlation between specific properties and overall electrochemical performance. Here, we seek to resolve these relationships through rigorous analysis of thermally pretreated SGL 29AA carbon paper electrodes using a suite of electrochemical, microscopic, and spectroscopic techniques and culminating in full cell testing. We systematically vary pretreatment temperature, from 400 to 500 °C, while holding pretreatment time constant at 30 h, and evaluate changes in the physical, chemical, and electrochemical properties of the electrodes. We find that several different parameters contribute to observed performance, including hydrophilicity, microstructure, electrochemical surface area, and surface chemistry, and it is important to note that not all of these properties improve with increasing pretreatment temperature. Consequently, while the best overall performance is achieved with a 475 °C pretreatment, this enhancement is achieved from a balance, rather than a maximization, of critical properties. A deeper understanding of the role each property plays in battery performance is the first step toward developing targeted pretreatment strategies that may enable transformative performance improvements.

Asymmetric vanadium flow batteries: long lifespan via an anolyte overhang strategy

Fast capacity decay is a serious problem in vanadium flow batteries (VFBs). How to eliminate or slow down capacity fading has become a critical issue for the practical application of VFBs. Herein, the concept of an asymmetric vanadium flow battery (aVFB) is introduced, in which the asymmetric design of a catholyte and an anolyte is used to suppress the capacity decay of the VFB. Based on the comprehensive analysis of the capacity decay and electrolyte imbalance process of the traditional symmetric VFB, it was found that the capacity fading is mainly owing to the loss of the anolyte in the long-term cycling test. Therefore, this work attempts to use excess anolyte (i.e. 10%, 20% and 30%) to mitigate the capacity decay during the long-term operation of the VFB. To gain deeper insights into the capacity retention mechanism of these novel anolyte overhang aVFBs, long-term cycle performance of the corresponding symmetric overhang VFBs and catholyte overhang aVFBs is investigated for comparison. The optimal excess ratio of anolyte and how to add the excess anolyte are also suggested for future study.

Influence of Membrane Equivalent Weight and Reinforcement on Ionic Species Crossover in All-Vanadium Redox Flow Batteries

One of the major sources of lost capacity in all-vanadium redox flow batteries (VRFBs) is the undesired transport (usually called crossover) of water and vanadium ions through the ion-exchange membrane. In this work, an experimental assessment of the impact of ion-exchange membrane properties on vanadium ion crossover and capacity decay of VRFBs has been performed. Two types of cationic membranes (non-reinforced and reinforced) with three equivalent weights of 800, 950 and 1100 g·mol⁻¹ were investigated via a series of in situ performance and capacity decay tests along with ex situ vanadium crossover measurement and membrane characterization. For non-reinforced membranes, increasing the equivalent weight (EW) from 950 to 1100 g·mol⁻¹ decreases the V(IV) permeability by ~30%, but increases the area-specific resistance (ASR) by ~16%. This increase in ASR and decrease in V(IV) permeability was accompanied by increased through-plane membrane swelling. Comparing the non-reinforced with reinforced membranes, membrane reinforcement increases ASR, but V(IV) permeability decreases. It was also shown that there exists a monotonic correlation between the discharge capacity decay over long-term cycling and V(IV) permeability values. Thus, V(IV) permeability is considered a representative diagnostic for assessing the overall performance of a particular ion-exchange membrane with respect to capacity fade in a VRFB.

Electrochemical evaluation methods of vanadium flow battery electrodes

A reliable device as well as parameters is important for the electrochemical evaluation of a VFB electrode to achieve more convincing results.