Performance Of Vanadium Redox Flow Battery Research Articles

Coupled Experimental-Theoretical Characterization of a Carbon Electrode in Vanadium Redox Flow Batteries using X-ray Absorption Spectroscopy.

Vanadium redox flow batteries (VRFBs) have emerged as promising solutions for stationary grid energy storage due to their high efficiency, scalability, safety, near room-temperature operation conditions, and the ability to independently size power and energy capacities. The performance of VRFBs heavily relies on the redox couple reactions of V2+/V3+ and VO2+/VO2+ on carbon electrodes. Therefore, a thorough understanding of the surface functionality of carbon electrodes and their propensity for degradation during electrochemical cycles is crucial for designing VRFBs with extended lifespans. In this study, we present a coupled experimental-theoretical approach based on carbon K edge X-ray absorption spectroscopy (XAS) to characterize carbon electrodes prepared under different conditions and identify relevant functional groups that contribute to unique spectroscopic features. Atomic models were created to represent functional groups, such as hydroxyl, carboxyl, methyl, and aldehyde, bonded to carbon atoms in either sp2 or sp3 environments. The interactions between functionalized carbon and various solvated vanadium complexes were modeled using density functional theory. A library of carbon K-edge XAS spectra was generated for distinct carbon atoms in different functional groups, both before and after interacting with solvated vanadium complexes. We demonstrate how these simulated spectra can be used to deconvolve ex situ experimental spectra measured from carbon electrodes and to track changes in the electrode composition following immersion in different electrolytes or extended cycling within a functional VRFB. By doing so, we identify the active species present on the carbon electrodes, which play a crucial role in determining their electrochemical performance.

Ammonium Bifluoride‐Etched MXene Modified Electrode for the All−Vanadium Redox Flow Battery

AbstractThe development of electrodes with high performance and long‐term stability is crucial for commercial application of vanadium redox flow batteries (VRFBs). This study compared the performance of VRFB with thermal‐treated and MXene‐modified carbon paper. To prepare the MXene, a modified‐etching process with ammonium−bifluoride (NH4HF2) led to a mild and efficient conversion of the MAX‐phase to MXene compared to etching process with hydrofluoric‐acid (HF). Electron microscopy and X‐ray diffraction studies revealed that the etching process with NH4HF2 led to MXene nanostructures with a large interlayer spacing. The results show that at a current density of 60 mA cm−2, the energy efficiency increased by 25.5 % when using a NH4HF2 ‐etched MXene‐modified negative electrode, by 12.5 % with a thermal‐treated MXene‐modified electrode, and by 4 % with an HF‐etched MXene‐modified electrode, in comparison to the pristine electrode. The maximum power density of the battery was increased by more than 40 %. In long‐term cycling experiments the MXene modified electrode exhibited excellent stability over 1000 cycles of charge‐discharge, with 0.05 % discharge capacity decay per cycle, amongst the lowest values reported to date and four times lower than for thermally‐treated electrode. The superior performance was linked to the improved electrical conductivity and wettability, higher interlayer spacing, and lower charge transfer resistance for the V2+/V3+ redox reaction.

Carbon cloth modified by direct growth of nitrogen-doped carbon nanofibers and its utilization as electrode for zero gap flow batteries

The synthetic procedure and characterization of carbon nanofibers (CNFs) grown on carbon cloth (CC) are explored in this study, with a focus on their potential application as electrodes in vanadium redox flow batteries (VRFBs). CC offers an attractive platform for surface modification owing to its conductive properties and three-dimensional architecture, while the N-doped CNFs formed by nitrogen (N) rich composition of melamine precursor enhance wettability of electrolyte and redox reactivity of vanadium ions. Electrochemical assessments reveal that NCC electrodes significantly increase voltage efficiency (VE) and capacity retention in VRFBs compared to bare CC (BCC) electrodes. Notably, NCC demonstrates a VE of 65.9%, surpassing the 55.9% of BCC electrodes. Additionally, NCC maintains superior capacity retention under varying current densities, a crucial factor for VRFBs. Long-term stability tests over 1000 cycles highlight the NCC electrode's durability, with only a minimal decrease in VE. Post-experiment analysis confirms the structural integrity of the CNFs on the CC electrodes, validating their resilience in VRFB operations. In summary, the study introduces a novel approach for fabricating N-doped CNFs on CC, resulting in electrodes that significantly boost VRFB performance in terms of efficiency, capacity retention, and stability, marking a notable advancement in flow battery technology.

A poly(2-ethylaniline) blend membrane for vanadium redox flow batteries

Processable E-PANI polymer blend SPES membrane for enhancement of vanadium redox flow battery performance.

Investigation of Polarization in Vanadium Redox Flow Batteries with a Fluctuating Supply of Electrolyte Solution

The rapid development of renewable energy has led to increasing demand for energy storage equipment. The vanadium redox flow battery (VRFB) has attracted more attention due to its advantages such as good safety, long service life, and flexible design [1].In the VRFB, the electrolyte flow characteristics are important for the concentration overpotential as it significantly affects the supply of vanadium ions [2]. Therefore, many studies are focusing on the influence of flow rates and electrolyte distribution on VRFB performance [3-4]. However, few studies have focused on the effects of fluctuations in electrolyte flow rates on battery polarization and overpotential, which often occurs in practical situations and affects the mass transfer of ions. In this study, we focused on the effect of fluctuations in electrolyte supply induced by the peristaltic pump on the polarization behavior of the VRFB.The electrodes of the experimental VRFB are Sigracet® GDL 38AA, the membrane is Nafion® 211, and the vanadium ion concentration of the electrolyte is 1.7 mol/L. Before the formal experiment, the cell was charged to 50% state of charge. To determine the fluctuation effect on battery polarization, a peristaltic pump was used on one electrode side and a smooth pump on the other.Fig. 1 (a) and (b) show the charging and discharging voltage variation with a current of 200 mA/cm2 at different flow rates. The average voltages during charging decrease with increasing flow rates, while the average voltages during discharging increase with increasing flow rates, and all voltages fluctuate up and down.It can be known that the voltage variation is attributed to the fluctuating supply of electrolyte affecting the mass transfer. At higher flow rates, the vanadium ion transport speed is faster, and the concentration gradient of the reaction surface is smaller, resulting in lower overpotential and polarization. Under fluctuating conditions, the changing electrolyte flow rates also lead to changes in vanadium ion supply speed, resulting in the voltage fluctuation periodically.AcknowledgementThis work was supported by JSPS KAKENHI Grant Number 21H04540. The first author was supported by the China Scholarship Council.

Adaptation of a Vanadium Redox Flow Battery for Thermal Applications Using a Solid Capacity Booster

In this work a novel approach for the use of the Vanadium Redox Flow Batteries (VRFB) is made by viewing it as a dual system with the ability to store both electrical and thermal energy. Electrical energy is thanks to the electrochemical reactions of the vanadium ions dissolved in the electrolyte, while thermal energy will use the sensible heat of the electrolyte mass.The performance of VRFB has been widely studied over the last 40 years and there is an extensive bibliography on the subject regarding the electrochemical performance attending only to the conventional energy storage requirements, but the hybrid electrical-thermal function is a novel subject that has been scarcely explored. One of the most critical aspects is the narrow operating temperature range of these batteries (15-40°C) that strongly limits their thermal energy storage capacity.Practical vanadium electrolytes operate with electrolytes containing a vanadium concentration between 1.6 and 2.0 M, and usually also employ cooling and heating systems to maintain the temperature between 15 and 40ºC. In particular, at temperatures below 15ºC the slower reaction kinetics, high viscosity and reduced conductivity of the concentrated electrolytes introduce remarkable energy losses in the battery. On the other hand, at temperatures over 35ºC the positive electrolyte is not stable due to the limited solubility of VO2 + species. High temperatures also boost the kinetics of parasitic reactions such as hydrogen and oxygen evolution, thus increasing energy losses.In this study, we address these limitations aiming to open up new possibilities of a successful integration of vanadium redox flow batteries into thermal applications. For this, the functionality and stability limits of a diluted vanadium electrolyte are analyzed for an extended operational temperature window from 5ºC to 50ºC and compared to commercial electrolyte formulations. In a second step, the use of a solid capacity booster is investigated to compensate the loss of energy density due to electrolyte dilution.[1] S. Berling, S. Mavrikis, N. Patil, E. García – Quismondo, J. Palma, C. Ponce de León “Lignin as redox-targeted catalyst for the positive vanadium electrolyte”, Electrochemistry Communications 142 (2022) 107339.[2] S. Berling, J. M. Hidalgo, N. Patil, E. García - Quismondo, J. Palma, C.Ponce de León, “A mediated vanadium flow battery: Lignin as redox-targeting active material in the vanadium catholyte”. Submitted

Improved Vanadium Redox Flow Battery Performance through a Pulsating Electrolyte Flow Regime

The increase in emissions of greenhouse gases and their environmental impact lead to a transition from primarily fossil fuel-based energy sources to renewable and eco-friendly ones, such as solar panels and wind turbines. However, these sources are intermittent, resulting in fluctuating energy production levels that require buffering, which poses significant challenges in matching production-consumption profiles. [1,2] In order to stabilise the grid and reduce dependency on fossil fuels, integrating electrical energy carriers into a smart grid to store excess energy produced during peak periods and employ it during periods of low production is a potential solution. Redox flow batteries hold promise in this regard, as the storage capacity can be decoupled from the battery size, enabling capacity to be scaled independently of power output by increasing the volume of the electrolyte. [3] In contrast to conventional secondary battery types, such as lead-acid or lithium-ion batteries, the electrolyte in a redox flow battery is non-stationary, inducing convective mass transport effects that are critical for achieving a high power output.In this work we introduced pulsations to the electrolyte flow of an all-vanadium redox flow battery, leading to a reduced stagnant boundary layer attributed to shear forces. Previously, Pérez-Gallent et al. demonstrated that the conversion can be increased by a factor of two while selectivity gains of 15-20% can be made in a electrolyser by introducing a pulsating flow. [4] Furthermore, Vranckaert et al. studied a pulsating flow combined with pillar field electrodes, using the well-known ferri-ferrocyanide redox couple, and concluded that the average limiting current density can be increased fourfold compared to working with a conventional steady state flow regime. [5] Overall, a pulsating flow behaviour yields an enhanced mass transfer of electroactive species by increasing the local velocity through pulsations. The net residence time of the reactants remains unchanged since the pulsating flow has no influence on the overall flow velocity. However, the resulting improvement of mass transfer leads to an increase in conversion. As it is clear that mass transfer enhancement strategies open the door for more efficient systems, our work will for the first time focus on the influence of a pulsating electrolyte flow on the redox flow battery performance. The results show a significant improvement: the discharge capacity at 100 mA/cm2 increased by 38.7% when applying a pulse with pulse amplitude (PA) 0.4ml and pulse frequency (PF) 2.4 Hz at a flow rate of 5 ml/min (Figure 1 A). Furthermore, this 38.7% increase in discharge capacity surpassed the performance achieved at a constant steady-state flow of 25 ml/min. Therefore, a fivefold reduction of the net flow rate is possible without performance losses, decreasing necessary pumping costs. When regarding the stability of an all-vanadium redox flow battery operated with a pulsating electrolyte flow initial fluctuations of the current efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) were recorded. However, the efficiencies became stable at 98.4%, 70.5% and 69.4% respectively, showing no decline over time (Figure 1 B). This novel and improved redox flow battery system allows to take the next step towards integrating electrical energy carriers into a smart grid suitable for renewable and green energy sources.[1] P. D. Lund, J. Lindgren, J. Mikkola, J. Salpakari, Renew. Sustain. Energy Rev. 2015, 45, 785–807.[2] H. Kondziella, T. Bruckner, Renew. Sustain. Energy Rev. 2016, 53, 10–22.[3] E. Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, J. Power Sources 2021, 481, 228804.[4] E. Pérez-Gallent, C. Sánchez-Martínez, L. F. G. Geers, S. Turk, R. Latsuzbaia, E. L. V. Goetheer, Ind. Eng. Chem. Res. 2020, 59, 5648–5656.[5] M. Vranckaert, H. P. L. Gemoets, R. Dangreau, K. Van Aken, T. Breugelmans, J. Hereijgers, Electrochim. Acta 2022, 436, 141435. Figure 1

Proton conducting zeolite composite membrane boosts the performance of vanadium redox flow battery

In a vanadium redox flow battery (VRFB), the selectivity of protons over vanadium ions for an ion-conducting separator is crucial to achieving capacity retention in charge/discharge cycles. Conventionally used perfluorinated or hydrocarbon-based membranes are not able to resolve cross-contamination of vanadium during VRFB operation. Engineered inorganic materials like zeolites have inherent selectivity towards specific ions based on the size and charge of the ions. In this work, ZSM-5 zeolites with different Si/Al ratios were modified by a facile ion-exchange process to obtain proton rich zeolites, followed by successful infiltration into the SPEEK matrix. The composite membranes demonstrated excellent oxidative stability in 2 M H2SO4 containing 1.5 M VO2+ ions as well as favourable electrochemical properties. In a single-cell VRFB, the membrane with the highest proton conductivity i.e., SPEEK-ZSM 5 (25) 2H+ was able to achieve an average Coulombic efficiency of 97.0% and average energy efficiency of 78.0% over 300 charge/discharge cycles at 80 mA cm−2 current density. The polarization curve and self-discharge experiment results illustrated 21.7% higher peak power density and 2.5 times slower self-discharge compared to state-of-the-art Nafion-117 in identical operating conditions indicating zeolite-composite membranes as a potential candidate to enhance the durability and performance of VRFBs.

Large-scale all-climate vanadium batteries

The vanadium redox flow battery (VRFB) is a highly promising technology for large-scale energy storage applications due to its exceptional longevity and virtually unlimited capacity. However, for this technology to be widely applicable across different geographical locations, a thorough understanding of its all-climate properties is essential. In recent years, VRFBs have been improved by optimising vanadium electrolytes, exchange membranes, carbon fabric electrodes, and casings. However, most research activities have been limited to batteries operating in moderate temperature conditions, and all-climate vanadium batteries are rarely considered. Therefore, this research work focuses on the effect of temperature on the performance of VRFBs. Existing models that deal with the thermal-hydraulic-chemical modelling of VRFBs either ignore the coupling between the governing physical processes or the variation of key material properties with temperature. In this study, we consider both the coupling of the multi-physics processes that influence the behaviour of VRFBs and the variation of their underpinning properties with temperature.

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

Vanadium redox flow batteries (VRFBs) are assembled by compressing the electrodes to reduce the contact resistance between electrodes and bipolar plates and prevent liquid electrolyte leakage. However, an excessive compression ratio can impede the mass transfer and cause high-pressure drops in the electrolyte, ultimately reducing energy and system efficiencies. Accordingly, conventional stacks require a compromised compression ratio of 20 to 30%, resulting in substantial contact resistance and lower cell performance. To address this issue, we proposed a new approach to adjust the local compression ratio of carbon felt electrodes by creating a gradient in compressive stiffness along the through-thickness direction. We achieved this by selectively treating the carbon felt electrode to increase the interaction of carbon fibers in a treated region. This approach deforms the electrode non-uniformly, increasing the electrical contact point between the electrode and bipolar plate in regions with higher deformation and improving the convection and diffusion of ions at the interface between the electrode and membrane in regions with lower deformation, where porosity is relatively high. In a VRFB single-cell test, the electrode with the stiffness gradient demonstrated the highest energy efficiency (86.5%) under a current density of 100 mA∙cm−2, 10% higher than that of neat CF.

Review on the Applications of Biomass-Derived Carbon Materials in Vanadium Redox Flow Batteries.

The development of vanadium redox flow batteries (VRFBs) requires the exploration of effective and affordable electrodes. In order to increase the electrochemical activity of these electrodes and decrease the polarizations, they are doped with an electrocatalyst. In this context, the use of biomass-derived materials as electrocatalysts in VRFBs has received much attention recently due to their widespread availability, renewable nature, low cost, and high energy efficiency. This paper aims to review the synthesis methods of biomass-derived carbon materials and their applications in VRFBs. In line with this aim, recent developments in carbon-based electrode modification methods and their electrochemical performance in VRFBs are summarized. The studies show that porous carbon electrocatalysts increase energy efficiency by reducing overpotentials and improving electrocatalytic activation. In addition, it is thought that biomass carbon doped electrocatalysts can improve the hydrophilicity of the electrodes, the transfer of vanadium ions, and the reaction kinetics. The highest charge voltage decrease rate of 8.61% was obtained in the Scaphium scaphigerum, whereas the highest discharge voltage increase rate of 14.29% was observed in the twin cocoon, as in all reviewed studies. Furthermore, the maximum energy efficiency (75%) was achieved in a VRFB equipped with an electrode doped with carbon derived from Scaphium scaphigerum and cuttlefish. It can be concluded from the reviewed studies that the electrochemical performances of electrodes doped with biomass-derived carbons in VRFBs are more effective than those of the bare electrodes.

Green functionalization for chelating ligand-like and π-conjugated active sites on carbon nanotubes and its use for vanadium redox flow batteries

An imide bond-based functional group is proposed for the catalyst's active site in vanadium redox flow batteries (VRFBs). Based on the functional group's inherent property produced by the optimized process, the catalytic activity toward vanadium ion redox reaction (VIRR) on both sides was increased sixfold without adversely affecting VIRR reversibility in comparison to the raw material, carboxylic acid-functionalized carbon nanotubes. These advantages were attributed to the imide bond-based functional groups acting as a chelating ligand to form strong interactions with vanadium redox couples while also inducing facile electron transfer by the π-conjugated bonds, resulting in increased catalytic activity and reversibility. Moreover, the treatment used an eco-friendly process comprising low-cost precursors (citric acid and urea) and mild conditions (80 °C, 2 h), thereby improving the commercial availability of VRFBs. In VRFB driving tests, the fabricated electrode outperformed the electrode with a polymer-like structure in terms of catalytic activity and long-term stability due to leaching of the active sites from the latter. Overall, the proposed catalyst could be a practical option for enhancing VRFB performance based on a low-cost and eco-friendly treatment method.

Hydrodynamic and Electrochemical Analysis of Compression and Flow Field Designs in Vanadium Redox Flow Batteries

This numerical study investigates compression and flow field design effects on electrode behaviour in vanadium redox flow batteries (VRFBs). Through 3D simulations and analysis of various flow field designs, including conventional, serpentine, interdigitated, and parallel configurations, this study investigates three compression scenarios: uncompressed, non-homogeneously compressed, and homogeneously compressed electrodes. Hydrodynamic and electrochemical analyses reveal the impact on velocity, pressure, current density, overpotential, and charge–discharge performance. Interdigitated flow field is found to display the lowest charging potential and highest discharging potential among all flow fields under all three compression scenarios. Moreover, uncompressed electrode condition shows the conservative estimates of an average charging potential of 1.3647 V and average discharging potential of 1.3231 V in the case of interdigitated flow field, while compressed electrode condition and the non-homogeneously compressed electrode condition show an average charging potential of 1.3922 V and 1.3777 V, and an average discharging potential of 1.3019 V and 1.3224 V, respectively. Results highlight the significance of non-uniform compression while modelling and analysing the performance of VRFBs as it is a more realistic representation compared to the no-compression or homogeneous compression of the electrodes. The findings of this work provide insights for optimising VRFB performance by considering compression and flow field design.

A 3D modelling study on all vanadium redox flow battery at various operating temperatures

To understand whether the optimization of the operating/electrode structural parameters are temperature dependent, a 3D numerical model is developed and validated to gain insight into the impact of practical operating temperature (273.15 K–323.15 K) on vanadium redox flow battery (VRFB) performance, in which the property parameters are from published experimental data. The operating temperature is found significantly influence the optimal design of VRFBs. Increasing the inlet flow rate and state of charge (SOC), decreasing the electrode porosity and fibre diameter can all improve the battery performance with interdigitated flow channels, and the improvement increases with increasing temperature. In contrast, decreasing the fibre diameter or porosity increases the flow resistance and costs higher pump consumption, which is more pronounced at a lower temperature due to higher electrolyte viscosity. The effect of electrode thickness is also different at various temperatures. The gradient porosity electrode is applied in VRFB with interdigitated flow channels. The electrochemical performance of VRFB with gradient electrode (porosity increases from 0.8 at channel side to 0.93 at membrane side) performs similarly with the VRFB with 0.8 porosity electrode, while the pressure drop is reduced by 40% at all temperature. This model provides a deep understanding of effects of a wide range of working temperature on the optimization of operating/electrode parameters and on the VRFBs’ performance.

The Influence of Additives on the Polybenzimidazole Membranes in Vanadium Redox Flow Batteries

In recent years, membrane development has been a focus to replace Nafion as the only viable choice for Vanadium redox flow batteries (VRFBs). Efforts were made to develop polymeric membranes with a lower cost and especially with lower vanadium ions crossover to minimize the costs and hassles associated with frequent electrolyte rebalancing. Among researched VRFB membranes, polybenzimidazole (PBI) membrane stands out as one of the most promising candidates thanks to its incredibly low vanadium permeability and lower cost than Nafion. However, its lower ionic conductivity when compared to Nafion which results in low voltage and energy efficiency – is a main barrier for its widespread acceptance in VRFBs.In order to improve the conductivity of PBI membranes, efforts have been made, such as chemical alteration of PBI backbone, and addition of ionic and non-ionic side groups. However, these approaches add complexity in the synthetic procedure resulting in a higher membrane cost. Therefore, simple approaches such as blending PBI polymer with additives (e.g. Nafion, polypentafluorostyrene, etc.) provide composite membranes with suitable conductivities for VRFBs. However, these additives leach into the vanadium electrolyte solution depending on their solubility. They can affect the kinetics of the anode and cathode reactions which is largely omitted in the literature. Therefore, in this study, we have investigated the effects of different additives, used for the fabrication of composite PBI membranes, on the electrode and VRFB performance. The kinetic performance loss is studied in-situ and ex-situ by electrochemical impedance spectroscopy. We have also established the Structure–Performance Relationship of additives with VRFB performance.

LTO/TiO2 Nanowire-Decorated on Graphite Felt As Electrode for High-Performance Vanadium Redox Flow Battery

Vanadium redox flow battery(VRFB) is one of the most promising large-scale energy storage systems. However, the graphite electrode materials employed for VRFB often have shortcomings such as poor electrochemical activity and insufficient conductivity, which will lead to poor performance of VRFB. To modify the graphite electrode, in this study, lithium titanate/titanium dioxide(LTO/TiO2) composite catalyst with the nanowire morphology was directly grown on the surface of the heat treatment graphite felt(HGF). The graphite felt electrode through the nanowire morphology, the reaction area between the electrode and the electrolyte and active sites are increased, thereby improving the performance of VRFB. In order to further prove the effect of lithium titanate/titanium dioxide composite catalyst modified graphite felt(LTO/TiO2@HGF), the modified graphite felt electrode was applied to the positive and negative electrodes of VRFB, and the charge and discharge tests at different current densities were carried out. In terms of efficiency, the LTO/TiO2-modified graphite felt electrode has obviously better results. When the current density is increased to 200 mA/cm2, its energy efficiency can reach 60.57%, which is 8.63% higher than that of HGF. In the stability test with a current density of 160 mA/cm2, the VRFB was charged and discharged for 60 cycles, and the overall performance did not change significantly, which means that the composite material of lithium titanate and titanium dioxide not only effectively improve the performance of the VRFB, but also show the stability that can be used for a long time.

Electrode Treatments for Redox Flow Batteries: An Industrial Perspective

Renewables sources of energy like solar and wind are expected to supply ~35% of the total electricity demand by 2050.1 The intermittent nature of solar and wind necessitates the development of energy storage technologies. Redox flow batteries (RFBs) are a promising technology for storing energy over several hours to days; however, RFBs suffer from inefficiencies in part due to the overvoltages at the electrode surface. Vanadium redox flow batteries (VRFBs) are the most commercialized RFB chemistry till date due to the use of same element both sides of battery, eliminating the issue of cross-contamination. Carbon felts (CFs) are the most commonly used electrodes in RFBs due to their high conductivity and porous nature. More than 70 electrode treatments for VRFBs alone have been previously shown to reduce the overvoltages and improve performance of VRFBs. Some of these CF electrode treatments include thermal, acid/base, elemental doping, and loading with electrocatalysts.2–5 However, the differences in the operating conditions between treatment studies in literature makes the comparison among different treatments very difficult. Additionally, the complexity associated with each treatment makes identification of industrially implementable treatments extremely challenging.In this talk, I will discuss our efforts to tackle the above problem by comparing treatments under similar operating conditions and accounting for the treatment complexity. We compare the VRFB performance, stability, and economic feasibility with various CF treatments, and treatment complexity to identify the most promising industrially implementable treatments. We extract the round-trip energy efficiencies (EE) of VRFBs using CFs with (and without) various treatments from literature at laboratory and industrial scale. The performance is evaluated using the EE at various operating current densities and stability is identified based on the change in EE per cycle. Economic feasibility of a treatment is measured based on the affordable capital cost (ACC) obtained from an in-house techno-economic model. ACC is defined as the maximum capital cost that can be invested in a treatment without an increase in the overall capital cost of VRFB without treated electrodes. The treatment complexity is measured using the number of steps and the process units required in a treatment. Based on carefully chosen metrics for satisfying the performance, stability, economic feasibility, and treatment complexity criteria, we identify the high performing, stable, and economic feasible treatments that are easy to implement industrially. We conduct a cost analysis of these promising treatments by utilizing the similarities in production of carbon fibers and CFs. We encourage the RFB community to standardize of RFB testing procedures for fast industrial implementation of CF treatments as we move towards a society powered by renewable energy.

Enhancement in vanadium redox flow battery performance using reduced graphene oxide nanofluid electrolyte

Low energy storage performance of aqueous flow batteries is the main limitation in commercialization and worldwide implementation. The effect of nanofluids on the electrochemical behaviour of electrolytes to alleviate this problem has been rarely studied in contrast with the extensive heat-transfer-related literature on nanofluids from the field of thermal science. Therefore, this study aims to experimentally investigate the long-term effect of using reduced graphene oxide (rGO) nanofluidic electrolyte on vanadium redox flow battery performance, placing it amongst the very few studies on the effectiveness of nanofluids in flow batteries. Scanning Electron Microscopy (SEM), EDS elemental mapping, X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) characterizations reveal changes in morphology, structural and elemental properties of the carbon felt electrodes due to the presence of suspended nano catalysts in the electrolyte. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) exhibited significant improvements of 36 % and 26 % in oxidation/reduction peaks, ~ 95 % of reduction in charge transfer resistance, and enhanced redox reactions reversibility of the optimum concentration of nanofluidic electrolyte (0.1 wt%). Moreover, active surface area of the electrodes were determined to be increased by ~130 % and ~ 135 % for positive and negative 1.0 wt% nanofluid-affected electrode samples compared to pure electrolyte-affected electrode. In addition, considering the optimum nanoparticle concentration of 0.1 wt%, charge, and discharge capacities of the flow battery outperformed where pure electrolyte was applied in different current densities. These enhancements show considerable increase in energy storage of the VRFB. Furthermore, energy efficiency and voltage efficiency of the flow cell were observed to be increased respectively by 15 % to 24 % and 8 % to ~21 % for the current densities from 25 to 150 mAcm−2 when the optimum concentration of nanoparticles was applied. This study gives new insight into the rarely studied applicability of nanofluids in vanadium flow batteries.

Enhancement of vanadium redox flow battery performance with nitrogen-functionalized graphite felt electrodes etched by K2FeO4

Enhancement of vanadium redox flow battery performance with nitrogen-functionalized graphite felt electrodes etched by K2FeO4

Physics-Based Electrochemical Model of Vanadium Redox Flow Battery for Low-Temperature Applications

Vanadium redox flow batteries (VRFBs) operate effectively over the temperature range of 10 °C to 40 °C. However, their performance is significantly compromised at low operating temperatures, which may happen in cold climatic conditions. The loss of performance can be attributed to reduced kinetics and decreased diffusivity of ions in the electrolyte. In this paper, we present a physics-based electrochemical model of a vanadium redox flow battery that allows temperature-related corrections to be incorporated at a fundamental level, thereby extending its prediction capability to low temperatures. The model follows the conventional evaluation of the cell overpotential as the sum of contributions from overpotentials associated with activation, ionic conduction and mass transfer polarization. New data-driven models have been proposed to make these sub-models temperature sensitive. The overall model has been validated with a wide range of data from VRFB cells of sizes up to 900 cm2 and operating temperatures down to −10 °C. The model results indicate that enhancement of electrochemical performance of VRFB below subzero temperatures requires electrode and membrane activation and improvement in ionic conductivity of the electrolyte.