All-vanadium Redox Flow Battery Research Articles

Low-Index Facet Polyhedron-Shaped Binary Cerium Titanium Oxide for High-Voltage Aqueous Zinc-Vanadium Redox Flow Batteries.

Aqueous zinc-vanadium hybrid redox flow battery systems are an efficient strategy to address the problems of low voltage and high cost of conventional all-vanadium redox flow batteries. However, the low electrochemical activity of carbon-based electrodes toward a vanadium redox reaction limits the performance of redox flow batteries. In this study, polyhedral binary cerium titanium oxide (Ce2/3TiO3, CTO) is synthesized using molten salt synthesis. CTO is fabricated by adjusting the temperature and composition. Notably, the prepared CTO obtained at 1000 °C shows the highest catalytic activity for a VO2+/VO2+ redox reaction. Further, CTO is prepared as a composite electrocatalyst and applied to a high-voltage aqueous zinc-vanadium redox flow battery. The cell adopts an alkali zinc electrolyte containing a Zn/[Zn(OH)4]2- redox pair and exhibits a high operating voltage of 2.26 V. Remarkably, a zinc-vanadium redox flow battery using the composite electrocatalyst exhibits a high energy density of 42.68 Wh L-1 at 20 mA cm-2 and an initial voltage efficiency of 90.3%. The excellent cell performance is attributed to structural defects caused by A-site deficiency in the perovskite oxide structure as well as oxygen vacancies resulting from the low valence state of the metal ion, which enhance the catalytic activity of the vanadium ions.

Boosting performance of Ti3C2TX/Bi modified graphite felt electrode for all-vanadium redox flow battery

All-vanadium redox flow battery (VRFB) with high power density is urgent in energy storage area. This study investigated the impact of Ti3C2TX/Bi as catalyst on VRFB performance at high current density. The Ti3C2TX/Bi decorated electrode was prepared based on a facile dropping method. Owing to the synergistic effect between Bi and Ti3C2TX, the hydrogen evolution reaction (HER) is effectively inhibited and the electrochemical activity towards V2+/V3+ redox reaction is enhanced. With the current densities ranging from 100 mA·cm−2 to 220 mA·cm−2, VRFB equipped with Ti3C2TX/Bi modified graphite felt shows the best rate performance. Furthermore, at a current density of 200 mA·cm−2, the battery with Bi and Ti3C2TX modified electrode shows the highest capacity retention rate of 70.3 % after 100 charge-discharge cycles and the highest energy efficiency (EE) of 80.1 %.

A New Manganese Based-Metal Oxide Electrocatalyst Through Electrodeposition for Investigation of Effect on V(IV)/V(V) Side’s Reversibility of All-Vanadium Redox Flow Batteries

Within the objective of this work, it is aimed to research, examine and bring to the literature as a new electrocatalyst material for enhancing the positive side’s reaction of VRBs. To be able to achieve this goal, it has been synthesized a manganese-based electrocatalyst as MnO2 to enhance the reversibility reaction for the positive side of VRB and so, practical, feasible and reproducible pencil graphite electrode(PGE) has been chosen as a working electrode for the modification to be electrodeposited MnO2 onto it. It is characterized via X-ray methods to understand the structure and surface changings and electrochemical techniques to interpret electrochemical behavior and charge transfer resistance through cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS). Within the scope of synthesizing, potential range, sulfuric acid and precursor concentration, and cycle number are optimized via CV and EIS. 0.5 M KMnO4 and 3.0 M H2SO4 medium with 50 cycle number within 50 mV s−1 scan rate in a −0.5/1.5 V potential range was decided to produce Mn-based metal-oxide as MnO2 electrocatalyst as optimum conditions. The original contribution of this study is that electrodepositing of manganese-based electrocatalyst as MnO2 on graphite based electrodes for redox flow battery in the literature.

Exploring the Effectiveness of Carbon Cloth Electrodes for All-Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) have shown to be a promising technology for integrating intermittent renewable energy sources into the existing electrical grid. Incorporation of carbon cloth electrodes into VRFB is an area of interest for their enhanced electrochemical performance, however, issues with performance degradation throughout the duration of the experiment persist. This study investigates the performance evolution of carbon cloth electrodes during VRFB cycling to build a hypothesis on possible reasons for the declining performance. Electrochemical impedance spectroscopy and polarization curve measurements are used in conjunction to monitor the electrode degradation and shed light on the effectiveness of carbon cloth electrodes during extended cycling experiments. A detailed investigation into the structure of the carbon cloth electrodes before and after cycling, via several material characterization tests, provides insight needed to determine an explanation for the increasing resistance. The structural integrity and surface morphology of the carbon cloth electrodes are evaluated to compare the electrode before and after cycling, displaying any changes to the electrode due to cycling. Durability of hydrophilicity during RFB cycling is found to be a key feature for future carbon cloth electrode design efforts.

Asymmetric Chemical Potential Activated Nanointerfacial Electric Field for Efficient Vanadium Redox Flow Batteries.

Constructing active sites with enhanced intrinsic activity and accessibility in a confined microenvironment is critical for simultaneously upgrading the round-trip efficiency and lifespan of all-vanadium redox flow battery (VRFB) yet remains under-explored. Here, we present nanointerfacial electric fields (E-fields) featuring outstanding intrinsic activity embodied by binary Mo2C-Mo2N sublattice. The asymmetric chemical potential on both sides of the reconstructed heterogeneous interface imposes the charge movement and accumulation near the atomic-scale N-Mo-C binding region, eliciting the configuration of an accelerator-like E-field from Mo2N to Mo2C sublattice. Supported with theoretical calculations and intrinsic activity tests, the improved vanadium ion adsorption behavior and charge-transfer process at the nanointerfacial sites were further substantiated, hence expediting the electrochemical kinetics. Accordingly, the pronounced promotion is achieved in the resultant flow battery, yielding an energy efficiency of 77.7% and an extended lifespan of 1000 cycles at 300 mA cm-2, outperforming flow cells with conventional single catalysts in most previous reports.

Vanadium Redox Flow Battery Stack Balancing to Increase Depth of Discharge Using Forced Flow Attenuation

Vanadium redox flow batteries are gaining great popularity in the world due to their long service life, simple (from a technological point of view) capacity increase and overload resistance, which hardly affects the service life. However, these batteries have technical problems, namely in balancing stacks with each other in terms of volumetric flow rate of electrolyte. Stack power depends on the speed of the electrolyte flow through the stack. Stacks are connected in parallel by electrolytes to increase battery power. If one of the stacks has a lower hydrodynamic resistance, the volume of electrolytes passing through it increases, which leads to a decrease in the efficiency of the remaining stacks in the system. This experimental study was conducted on a 10 kW uninterruptible power supply system based on two 5 kW stacks of all-vanadium redox flow batteries. It was demonstrated that forced flow attenuation in a circuit with low hydrodynamic resistance leads to an overall improvement in the system operation.

A Novel Biomimetic Lung-Shaped Flow Field for All-Vanadium Redox Flow Battery

The all-vanadium redox flow battery (VRFB) was regarded as one of the most potential technologies for large-scale energy storage due to its environmentally friendliness, safety and design flexibility. The flow field design and mass transfer performance in the porous electrodes were some of the main factors to influence the battery performance. A novel biomimetic lung-shaped flow field was designed, and the battery performance was compared with the serpentine flow field by numerical simulation analysis. The results showed that the charging voltage of the VRFB was reduced by about 5.34% when SOC = 0.9 compared with the serpentine flow field. On the other hand, the discharging voltage was promoted by about 9.77% when SOC = 0.1 compared with the serpentine flow field. The battery performance of the VRFB is obviously due to the enhancement of the mass transfer performance. The uniformity factor was promoted by 35.6% by the lung-shaped flow field when SOC = 0.1, which can reduce the polarization loss. The average concentration of the active ions was increased by about 18% by the lung-shaped biomimetic flow field, which was of significance to the electrochemical reaction. The design of the lung-shaped flow field can contribute to the application of the VRFB.

Physics-informed machine learning of redox flow battery based on a two-dimensional unit cell model

In this paper, we present a physics-informed neural network (PINN) approach for predicting the performance of an all-vanadium redox flow battery, with its physics constraints enforced by a two-dimensional (2D) mathematical model. The 2D model, which includes 6 governing equations and 24 boundary conditions, provides a detailed representation of the electrochemical reactions, mass transport and hydrodynamics occurring inside the redox flow battery. To solve the 2D model with the PINN approach, a composite neural network is employed to approximate species concentration and potentials; the input and output are normalized according to prior knowledge of the battery system; the governing equations and boundary conditions are first scaled to an order of magnitude around 1, and then further balanced with a self-weighting method. Our numerical results show that the PINN is able to predict cell voltage correctly, but the prediction of potentials shows a constant-like shift. To fix the shift, the PINN is enhanced by further constrains derived from the current collector boundary. Finally, we show that the enhanced PINN can be even further improved if a small number of labeled data is available.

A Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Aqueous Organic and Metalorganic Flow Batteries

The ion exchange membrane is a critical component of most aqueous flow batteries, where it provides a transport medium for charge carrying ions while suppressing undesired crossover of redox active species. The all-vanadium redox flow battery (VRFB), the most developed flow battery chemistry to date, is also the system for which most studies of flow battery membranes have been done. These studies have shown that the concentration and composition of the acidic vanadium electrolytes influence water content in the membrane phase, and therefore influence the transport phenomena through the membrane pores. In the present work, we build on this understanding by evaluating the effects of iron(II/III) hexacyanide electrolyte concentration, pH, and cations on cation exchange membrane performance, to inform design of aqueous organic and metalorganic flow batteries (AORFBs).The size and charge numbers of established redox active materials for AORFBs afford them lower crossover than metal ions like vanadyl or iron. However, many emerging AORFB chemistries with extreme stability use neutral or alkaline electrolytes where sodium or potassium, not protons, must carry ionic current through the membrane. In these flow batteries, the membrane resistance regularly contributes the most to polarization losses. The widely-used cation exchange membrane Nafion has a factor-of-ten reduction of conductivity in potassium form compared to when it is protonated in acidic environments. Ohmic resistance of the membrane is therefore a substantial limiting factor on practical current densities and power densities for battery operation.In order to investigate the membrane-electrolyte system, we first describe simple methods for conductivity and electrolyte uptake measurements that can be used to screen a variety of membrane materials and electrolytes. We show the influence of pre-treatment, cation (sodium or potassium), supporting electrolyte, and iron hexacyanide concentration on Nafion conductivity and electrolyte uptake, and compare these results with a non-fluorinated Fumatech membrane (E-620). We also assess pre-treatment, concentration, and cation effects on ferricyanide permeability. Considering the membrane and contacting flow battery electrolyte as one system, our results emphasize that the total ion concentration of the electrolyte affects membrane water content, and there are conditions where additional supporting electrolyte can have a harmful effect on membrane conductivity. We also show that maximizing the concentration of iron hexacyanide by using mixed cation electrolytes results in increased membrane resistance, signifying that conductivity and volumetric capacity become a tradeoff.

Novel Ion-Exchange (Blend)-Membranes Based on Free Amines for All-Vanadium Redox Flow Batteries

For reaching the aim of a climate-neutral EU within the European Green Deal until 2050 several new innovations in different fields, especially in the field of renewable energies, are necessary [1]. One challenge which has to be overcome is the fluctuating nature of wind and solar energy and the associated storage and release of excess energy for stabilisation of the power-grid [2]. One promising technology in this regard is the All-Vanadium Redox Flow Battery (VRFB) in which Nafion-based membranes are usually utilised [3, 4]. However, due to high costs and a low H+/V selectivity there are many attempts to find alternative materials with better performance, lower crossover and reduced costs [5, 6].In this work we present a new aromatic, ether-free and side-chain containing polymer with free non-quaternised amine groups prepared by polyhydroxyalkylation. A new monomer was designed to improve the conductivity and morphology of the resulting membranes. The membranes show sufficient mechanical properties in the wet state and improved conductivity compared to commercially available free-amine based VRFB membranes. Blend membranes with a PBI derivative were produced to improve the mechanical properties in the dry state and the crossover of vanadium species.The above-mentioned membranes were characterised ex-situ by using EIS, NMR spectroscopy, TGA, DMA and permeability tests. In addition, first promising in-situ characterisations were performed in a VRFB single-cell test station.Literature[1] European Commission: The European Green Deal, Brussels, 2019; URL: https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal_de#documents, accessed: 21.11.2022.[2] Reihani, E; Motalleb, M; Ghorbani, R; Saad Saoud, L.: Load peak shaving and power smoothing of a distribution grid with high renewable energy penetration. Renewable Energy 2016, 1372 – 1379.[3] Düerkop, D; Widdecke, H; Schilde, C; Kunz, U; Schmiemann, A.: Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review. Membranes 2021.[4] Guarnieri, M; Mattavelli, P; Petrone, G; Spagnuolo, G.: Vanadium Redox Flow Batteries: Potentials and Challenges of an Emerging Storage Technology. EEE Ind. Electron. Mag. 2016, 20 – 31.[5] Zeng, L; Zhao, T.S; Wei, L; Jiang, H.R; Wu, M.C.: Anion exchange membranes for aqueous acid-based redox flow batteries: Current status and challenges. Applied Energy 2019, 622 – 643.[6] Gubler, L.: Membranes and separators for redox flow batteries. Current Opinion in Electrochemistry 2019, 31 – 36.

Investigating Transport through Membranes in Non-Aqueous Redox Flow Batteries with Neutron Radiography

Electrochemical technologies offer a strong platform to decarbonize and electrify our energy economy and thus are poised to play an increasingly important role. Nevertheless, all electrochemical technologies suffer from thermodynamic, kinetic, and other operational losses that can drastically limit the performance of the device. Current macroscopic characterization techniques used to assess those losses (i.e., polarization, electrochemical impedance spectroscopy) fail to assess local inefficiencies challenging complete understanding of the system being investigated. Operando imaging of electrochemical systems, in tandem with electrochemical diagnostics offers a viable option to resolve local information and correlate it with macroscopic electrochemical performance. This dual characterization strategy has been successfully applied for the development of other electrochemical technologies (i.e., polymer electrolyte fuel cells[1,2], lithium-ion batteries[3,4]). In the past years, several groups developed and investigated imaging and spectroscopic techniques for operando characterization of redox flow batteries (RFBs) such as fluorescence microscopy[5], nuclear magnetic resonance spectroscopy[6,7] and infrared thermography[8]. Only recently, Clement et al. reported for the first time the use of neutron radiography to perform through-plane imaging of the gas evolution during operation of an all-vanadium RFB[9]. However, an approach enabling non-invasive operando concentration mapping of the species of interest within the reactor area has remained elusive.In this work, we develop two novel neutron radiography operando methodologies to quantify concentration distributions in RFBs. We first perform cuvette experiments to calibrate the neutron attenuation response of the redox active materials (i.e., TEMPO and TEMPO+) and supporting salts (i.e., BF4 -) used in this study. We find a linear response between the neutron absorption and the concentrations of the species of interest in the electrolyte in the studied range (0.0 to 0.5 M). Next, we performed experiments with redox flow cells under operation and use the ex-situ calibrations together with subtractive neutron imaging to quantify species concentration in the electrolyte. By employing an in-plane optical imaging setup[10], we can resolve species concentration across the porous electrodes (Figure 1). We observe strong variations in the concentration profiles within the reactor area upon changes in the operating conditions and link those differences to material properties (i.e., membrane electrostatic charge). Finally, I will discuss our latest results obtained using time-of-flight neutron imaging to deconvolute concentration maps for TEMPO/TEMPO+ and BF4 - species. Using this methodology, we study mass transport across a porous separator and a dense anion exchange membrane. We find that, while TEMPO/TEMPO+ and BF4 - show almost symmetrical concentration changes in the case of a porous separator, asymmetric transport occurs when a charge selective membrane is used where only BF4 - is allowed to crossover the anion exchange polymer. The developed methodology offers a non-invasive diagnostic tool to image concentration distributions in electrochemical cells and can be applied to support theoretical and experimental efforts aiming to understand the role of materials and reactor designs on complex mass transport phenomena.[1] P. Boillat, E. H. Lehmann, P. Trtik, M. Cochet, Current Opinion in Electrochemistry 2017, 5, 3.[2] J. Eller, T. Rosén, F. Marone, M. Stampanoni, A. Wokaun, F. N. Büchi, J. Electrochem. Soc. 2011, 158, B963.[3] B. Michalak, H. Sommer, D. Mannes, A. Kaestner, T. Brezesinski, J. Janek, Sci Rep 2015, 5, 15627.[4] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett, P. R. Shearing, Nat Commun 2015, 6, 6924.[5] A. A. Wong, M. J. Aziz, S. Rubinstein, ECS Trans. 2017, 77, 153.[6] E. W. Zhao, T. Liu, E. Jónsson, J. Lee, I. Temprano, R. B. Jethwa, A. Wang, H. Smith, J. Carretero-González, Q. Song, C. P. Grey, Nature 2020, 579, 224.[7] E. W. Zhao, E. Jónsson, R. B. Jethwa, D. Hey, D. Lyu, A. Brookfield, P. A. A. Klusener, D. Collison, C. P. Grey, J. Am. Chem. Soc. 2021, 143, 1885.[8] H. Tanaka, Y. Miyafuji, J. Fukushima, T. Tayama, T. Sugita, M. Takezawa, T. Muta, Journal of Energy Storage 2018, 19, 67.[9] J. T. Clement, Investigation of Localized Performance and Gas Evolution in All-Vanadium Redox Flow Batteries via in-Situ Distributed Diagnostic Techniques, Ph.D. thesis, University of Tennessee, 2016.[10] P. Boillat, G. Frei, E. H. Lehmann, G. G. Scherer, A. Wokaun, Electrochem. Solid-State Lett. 2010, 13, B25. Figure 1

All-Vanadium Redox Flow Battery Electrodes of Carbon Cloth Modified with Tungsten-Bismuth-Based Oxide Nanostructures for an Enhanced V(II)/(VIII) Reaction Kinetics and Inhibited H2 Evolution

All-vanadium redox flow batteries (VRFBs) are the most promising and highly efficient storage systems for renewable energy power plants. VRFBs can store high amounts of energy and are designed to work for long periods and their solutions can be used indefinitely.1 VRFBs are mainly carbon-based electrodes as they have high conductivity, high surface area, and high chemical stability, are relatively cheap and available,2 and can be tailored as desired for specific properties. However, these electrodes' performance is limited in their pristine form due to their resistance, hydrophobicity, and surface structure among other factors, resulting in VRFBs with a low power density. The VRFBs' low power density is usually related to the electrodes’ poor performance. Modifying the carbon electrodes using nanomaterials was proven to increase the power density of the battery. Metal oxides like tungsten oxide3 are known to enhance vanadium reactions when used as composites with carbon nanomaterials, while bismuth is known to inhabit the parasitic hydrogen evolution reaction.4 The material of the electrode and the metal molar ratio and the types of metal oxide modifiers highly affect the final performance of the carbon-based electrodes in VRFBs.In this work, different tungsten-bismuth-based oxide nanostructures were synthesized using the hydrothermal method. It is suggested that using different metal molar ratios, pH, and metal precursors significantly affects the electrode's physical and chemical structure and hence its electrochemical performance. The synthesized BiWOx nanostructures are evaluated using XRD, SEM, FT-IR, EDS, XPS, and water contact angle measurements. The effect of using BiWOx nanostructures as a modifier for carbon cloth electrodes on the kinetics of the VRFBs negative half-cell reaction (V2+/V3+) was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Results showed that using BiWOx as an electrode modifier led to an increase in electrode activity and stability, with BiWOx showing the best stability and BiW2Ox showing the best activity. The enhancement effect of BiWOx nanostructures is related to the interaction between the two metal oxides and the electrode surface, the enhanced hydrophilicity of the modified electrode, and the increase of the catalytic active sites.

Efficient Ion-Selective Transport in Sulfonated Diels-Alder Poly(phenylene) Membrane Enables Aqueous Organic Flow Batteries with Undetectable Crossover

In most aqueous redox flow batteries (RFBs), ion exchange membranes are used to suppress crossover of redox active species while enabling conductivity of charge carrying ions. Many aqueous RFB chemistries, such as the all-vanadium RFB, involve metal cations dissolved in acid. In such systems, high proton conductivity affords low ohmic resistance from the membrane, but crossover rates are also high. Organic and metalorganic reactants with extreme chemical stability are an emerging direction for the development of RFB electrolytes, and may enable batteries with long calendar life composed of earth-abundant elements. Moreover, the size and charge numbers of these reactants enable much lower crossover than observed for systems like vanadium RFBs. However, the most stable chemistries use neutral or alkaline electrolytes where protons are not abundant enough to carry the ionic current. Currently, this presents a drawback: the incumbent cation exchange membrane, Nafion, exhibits a factor-of-ten lower conductivity of potassium compared to protons. The present work introduces a new membrane cast from sulfonated Diels-Alder poly(phenylene) to enable improved area-specific resistance while suppressing crossover.Nafion contains a perflourinated hydrophobic backbone with intervals of a flexible side chain that terminates in hydrophilic sulfonic acid moieties. This structure of a hydrophobic backbone with hydrophilic side chains is common to most ion exchange membranes. When a membrane with this polymer structure is hydrated and ion exchanged to an alkali metal cation form, the cations promote aggregation of the ionic groups via rearrangement of the flexible side chains, giving a configuration that is not optimal for ion conductivity. In contrast, the new membrane has a sulfonated hydrophilic backbone and external hydrophobic moieties, which we hypothesize prevents the aggregation effects seen in Nafion to enable efficient ion-selective transport. In the present work, we show that the new membrane affords negligible ferricyanide permeability (< 2 × 10−13 cm2/s) and promising area-specific resistances (< 1 Ω cm2) for a variety of cations. The conductivity of the new membrane in 1 M KCl electrolyte is 3.6 mS cm-1, more than double that of Nafion NR212 measured in the same flow battery cell. We demonstrate the use of this membrane in an alkaline organic RFB, with a ferri-/ferrocyanide posolyte and a 4,4’-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ) negolyte. Three-electrode voltametric methods for post-cycling detection of crossed-over species will be discussed and their sensitivities compared, in an effort to quantify crossover fluxes during cycling and their possible contributions to capacity fade. We conclude that in this flow battery, the new membrane suppresses crossover of both iron hexacyanide and 2,6-DBEAQ to the extent that any crossed over compounds cannot be detected by our methods, and we assign an upper limit of 2 × 10−7 μmol cm−2 s−1 on the possible crossover flux, amounting to a projected < 1 %/year capacity fade from crossover in the lab-scale cell.

Microstructural Engineering of Flow Battery Electrodes with Non-Solvent Induced Phase Separation

Rising levels of greenhouse gases urge the decarbonization of human activities. Electrification of strategic sectors such as transport, industry, and heating necessitates affordable, safe, and dispatchable large-scale energy storage systems. Redox flow batteries (RFBs) stand as a promising long-duration and large-scale energy storage technology due to their design features[1]. Ongoing research focuses on engineering key cell components (e.g. membranes, electrodes, active materials) to improve cost competitiveness. This work focuses on the fabrication of novel porous carbon electrodes which strongly influence the performance and cost of the technology. The requirements for porous electrodes liquid-phase electrochemical reactors are contradictory: a large surface area is needed to overall exchange current density, but also minimal pressure losses to decrease pumping costs. Additionally, surface properties of the electrode dictate the reaction kinetics whereas its microstructure controls the mass transport of the reactants within the reactor area[2]. However, the current state-of-the-art porous electrodes for RFBs consists of carbon-fiber materials repurposed from other electrochemical technologies (e.g. polymer electrolyte fuel cells) which do not offer a favorable tradeoff between transport properties and surface area. Additionally, their production involves complex manufacturing processes resulting in high cost and limited morphological versatility. Therefore, there is a need to develop alternative synthetic platforms which offer tailorable microstructures and surface properties to match the requirements of emerging electrochemical applications, such as redox flow batteries. In this work, we draw inspiration from the membrane technology and employ non-solvent induced phase separation (NIPS) as a method to synthesize porous carbon electrodes for RFBs[3,4].Here, we present our latest efforts to develop a manufacturing route enabling the control of microstructural features of carbonaceous porous electrodes for RFBs. The microstructure of the carbon scaffold is determined by the nature and composition of the polymer blend used to cast the electrode precursor. We use polyacrylonitrile as scaffold precursor and polyvinylpyrrolidone as a pore forming agent, dissolved in an organic solvent (i.e., dimethylformamide, dimethylsulfoxide or N-methylpyrrolidone). The polymer blends were cast into a mold and immersed in a water coagulation bath prior to being stabilized (270 °C) and carbonized (1050 °C). Various process parameters (e.g. polymer concentration, bath temperature, solvent type) governing the final electrode properties were leveraged to control structural features, opening new opportunities for the fabrication of diverse microstructures (i.e., porosity gradient, isoporosity and with macrovoids). Electrochemical double layer capacitance and pressure drop measurements show an increased design space compared to carbon-fiber electrodes, confirming the versatility of the synthetic route. Microstructural analysis of the prepared NIPS electrodes reveals two porosity domains: large finger-like macrovoids (throat diameter >50 μm) and smaller interconnected microvoids (pore diameter 2-15 μm). Flow cell polarization and electrochemical impedance spectroscopy measurements in two common flow battery electrolytes, an Fe2+/Fe3+ single electrolyte and vanadium full cell, show improvements in performance compared to traditional electrodes. An increase in the polymer concentration of the polymer blend solution produced NIPS electrodes with smaller pores and resulted in better mass transport within the electrode. When the temperature of the coagulation bath was increased to 40 ºC, the electrode has higher electrochemically active surface area, producing a significant power increase in the sluggish vanadium electrolyte. Overall, the NIPS electrodes operate at significantly lower activation and mass transport overpotentials in all-vanadium RFBs, generating higher electrical power at the expense of a slight pressure drop increase. Although preliminary, we demonstrate the potential of NIPS to manufacture electrodes with tailorable microstructures for convection-enhanced electrochemical technologies.[1] T. M. Gür, Energy Environ. Sci. 2018, 11, 2696.[2] A. Forner-Cuenca, E. E. Penn, A. M. Oliveira, F. R. Brushett, J. Electrochem. Soc. 2019, 166, A2230.[3] R. R. Jacquemond, C. T.-C. Wan, Y.-M. Chiang, Z. Borneman, F. R. Brushett, K. Nijmeijer, A. Forner-Cuenca, Cell Reports Physical Science 2022, 3, 100943.[4] C. T.-C. Wan, R. R. Jacquemond, Y.-M. Chiang, K. Nijmeijer, F. R. Brushett, A. Forner-Cuenca, Adv. Mater. 2021, 10. Figure 1

Measuring Electrochemical Potentials for Health Monitoring of Redox Flow Batteries

Long-duration energy-storage (LDES) systems have the potential to transform the electric grid. Redox Flow Batteries (RFBs) have many unique attributes that make this battery architecture ideally suited for grid-scale LDES, namely: independent power and energy; long cycle life, even with deep cycles; inherently superior safety [1]; and easy recyclability. However, despite a recent renaissance in RFB technologies, assessing the durability of RFB cells is a topic that has not received much attention in the open literature, and further research is needed to better understand the performance and degradation mechanisms [2]. In-situ health monitoring of RFBs is critical for maximizing their lifetime and efficiency. Measurements of the electrochemical potentials of the electrodes and the electrolyte can provide insights into whether operating parameters are being adhered to and can potentially detect failure mechanisms occurring real-time within the battery. Since the electrochemical potential of both the positive and negative electrolytes exhibits Nernstian behavior, they are convertible to state-of-charge (SOC) measurements. Ensuring that the electrolyte stays within a desired SOC window during operation is essential for the mitigation of undesirable side reactions, such as the hydrogen-evolution reaction (HER) [3]. In addition, measurements of the potential difference between the electrode and the electrolyte, commonly known as overpotential, during charging and discharging processes allow for independent quantification of the reaction efficiency at each electrode. Factors affecting this reaction efficiency may include reaction kinetics, the mass transport of active species to the electrodes, the plating of impurities onto the electrode surface, and the mobility of ions across the ion-exchange membrane. Analysis of overpotential can also be useful in detecting failure mechanisms relating to these various factors [4]. Largo Clean Energy has developed multiple methods to measure electrochemical potentials in all-vanadium RFBs (VRFBs). The focus of this talk will be on how one can use these different measurements to assess the performance of VRFB cells. References R. Wittman, M. L. Perry, T. N. Lambert, B. R. Chalamala, and Y. Preger, “Perspective – On the Need for Reliability and Safety of Grid-Scale Aqueous Batteries,” J. Electrochemical Society, 167 (2020).Y. Yao, J. Lei; Y. Shi; F. Ai; T. C. Lu, “Assessment methods and performance metrics for redox flow batteries,” Nature Energy, 6 (2021).C. Tai-Chieh Wan, K. E. Rodby, M. L. Perry, Y.-M. Chiang, F. R. Brushett, “Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte,” J. of Power Sources, 554 (2023).M. L. Perry and R. M. Darling, “Development of a Simple and Rapid Diagnostic for Redox Flow-Battery Cells,” Symposium I02: Frontiers of Chemical/Molecular Engineering in Electrochemical Energy Technologies, 242nd ECS Meeting, (2022).

High Performance Metal Oxide–Reduced Graphene Oxide Electrocatalyst for All Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) is one of the most favorable technologies for large-scale energy storage applications owing to its numerous gorgeous features, such as flexible design, high safety, and long cycle life. In this study, metal oxide (MOx)–reduced graphene oxide (rGO) nanohybrid was successfully synthesized via a facile hydrothermal route followed by annealing and employed as an electrocatalyst to modify the graphite felt (GF) electrode material for all-vanadium redox flow batteries (VRFBs) application. The as-synthesized MOx–rGO nanohybrid reveals good electrocatalytic reversibility and higher anodic and cathodic peak current density for V3+/V2+ and VO2 +/VO2+ redox couples compared with those of rGO and MOx samples. The voltage efficiency of the VRFB using MOx–rGO nanohybrid reaches 84% at 80 mA cm−2, which is 4% and 13% higher than the VRFBs using the rGO-loaded GF electrode and the pristine GF electrode, respectively. It still shows the voltage efficiencies of 76% and 70% at relatively high current densities of 120 mA cm−2 and 140 mA cm−2, respectively, but other samples have no effective discharge. This improvement is accredited to the uniform distribution of MOx nanoparticles on the rGO surfaces, avoiding the restacking of the rGO sheets and conquering nanoparticles aggregation, which might boost the effective active surface area and improve mass transport at the electrode-electrolyte interfaces. Besides, the existence of oxygen vacancies on MOx, the high electrical conductivity of rGO, and the high content of oxygen functional groups play the prodigious roles to facilitate the reversibility of vanadium ions redox reactions.

Densification, Precipitation, and Dissolution of Vanadium Electrolyte for Vanadium Redox Flow Battery Systems

The all-vanadium redox flow battery system (VRFB) is the most mature RFB technology since it uses a single active species, which does not degrade. Rebalancing electrolytes to maintain maximum energy capacity in an RFB system that uses a single active species, or symmetrical electrolytes, is simple and enables the use of relatively low-selectivity separators, which have higher ionic conductivities.1 Conversely, finite-lifetime RFB chemistries, especially asymmetric chemistries, result in significant negative impacts on the levelized cost of storage (LCOS).2 Currently, the vanadium electrolyte used in a MWh-scale VRFB system is typically shipped to the installation site in liquid form in multiple totes and the VRFB-system tanks are filled onsite. A new approach explored in this study is to densify and precipitate the electrolyte in a solid form, which can then be quickly dissolved to its intended concentration onsite by the addition of de-ionized water.3 This approach can enable multiple benefits. First, shipping the electrolyte in solid form provides more stability and higher density during shipping, which significantly reduces shipping costs. Second, the solid electrolyte can be shipped in the actual containers that will be used onsite, which thus simplifies the required onsite work. This presentation will discuss a method used to densify the vanadium electrolyte, as well as nucleation and precipitation processes that generate the solid electrolyte form, which will quickly dissolve when deionized water is added onsite. The nucleation and precipitation process leverages recent work on the solid-liquid high-energy storage density method for RFB systems.4 References M. L. Perry, J. D. Saraidaridis, and R. Darling, “Crossover Mitigation Strategies for Redox-Flow Batteries,” Current Opinion in Electrochemistry, 21 (2020). 10.1016/j.coelec.2020.03.024 K. E. Rodby, M. L. Perry, and F. R. Brushett, “Assessing Capacity Loss Remediation Methods for Asymmetric Redox Flow Battery Chemistries Using Levelized Cost of Storage,” J. Power Sources, 506 (2021). 10.1016/j.jpowsour.2021.230085 M. L. Perry, Provisional U.S. Patent Application 63/312,999 filed on Feb. 2, 2022.Yuanchao Li and Trung Van Nguyen, “A Solid/Liquid High-Energy-Density Storage Concept for Redox Flow Batteries and Its Demonstration in an H2-V System,” J. Electrochem. Soc., 169 (2022). 10.1149/1945-7111/ac97c6

Nitrogen-Functionalized Metal Carbide-Modified Graphite Felt As a High Performance Electrocatalyst for an All-Vanadium Redox Flow Battery

To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs), we synthesize nitrogen-doped tungsten carbide nanowires (N-WC NWs), which are grown on the surface of GF through the two-step approach. When applied to VRFBs, the prepared electrode exhibits an apparent electrocatalytic effect toward the VO2+/VO2 + and V3+/V2+ redox reaction compared with pristine GF. The presence of negatively charged density on nitrogen promotes the absorption of vanadium ions. The nanowire structure exposes more active sites for the redox reaction of vanadium ions. The prepared N-WC NWs-GF electrode displays the best electrochemical performance with energy and voltage efficiencies of 74.8% and 78.9%, respectively, which is much higher than those of 59.8% and 63.2% obtained from the pristine GF at 100 mA cm−2. The N-WC NWs-GF exhibited durability and stability in an acidic electrolyte during long-term operation for 100 cycles at the higher current density of 100 mA cm−2.

Monitoring the State of Charge Imbalance of Vanadium Redox Flow Batteries Via Dual Online UV/Visible Spectroscopy

Electrolyte imbalance in Redox Flow Batteries (RFBs) is known to result in significant capacity decay over cycling [1]. This drawback is inherent to the flow battery technology and is caused by unwanted physico-chemical phenomena, such as species and water crossover, hydrogen evolution, air oxidation and precipitation of vanadium ions. The time evolution of electrolyte imbalance is often disregarded or underestimated when measuring the open-circuit voltage (OCV) for keeping track of the battery State of Charge (SOC). In order to correct such asymmetry, in operando real-time monitoring of the SOC of the positive and negative electrolytes would be highly desirable.In this work, we present an experimental test bench for the continuous online monitoring of the anolyte and catholyte SOCs via UV/visible spectroscopy during cycling of an all-vanadium redox flow battery (VRFB). The system notably includes 3D printed microfluidic optical flow cells, an optical switch, and an in-house microfluidic electrochemical cell. Key steps for the calibration of UV/visible spectroscopy with vanadium are presented and applied independently to both electrolytes to accurately estimate the SOC while taking into account the total vanadium concentration [2]. An experimental campaign is then carried out under varying conditions (initial average oxidation state (AOS), air oxidation, crossover, etc.) to illustrate the real-time evolution of electrolyte imbalance during VRFB battery cycling. The results suggest that the imbalance may arise from asymmetric initial Average Oxidation State (AOS ≠ 3.5) [3], vanadium crossover and also hydrogen evolution, which notably reduces the charging speed of the negative electrolyte [4]. The results highlight the benefits of UV/Visible spectroscopy to obtain SOC data for any of the three stable vanadium electrolytes: i) negative, VII/VIII ii) positive VIV/VV and even the iii) VIII/VIV pair. Moreover, the measurements are online, non-invasive and could provide real-time data to a battery management system either in conventional or microfluidic VRFBs to correct the SOC imbalance and maintain the battery capacity and efficiency [1]. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Project PID2019-106740RB-I00, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid. A. A. Maurice Energy acknowledges the support of an MCSF-Cofund “Energy for Future” (E4F) postdoctoral research fellowship by the Spanish Iberdrola Foundation (GA-101034297).

Levelized cost of electricity and greenhouse gas emissions of Ce- and V-based redox flow batteries

Inexpensive energy storage technologies are critical to meeting rising renewable electricity demand and maintaining grid stability, but there is insufficient storage capacity to meet this demand. Redox flow batteries are a promising long duration (>10 h) storage method because they scale efficiently with delivery time compared to non-flow batteries. Because of the connection between energy storage and intermittent renewables, the greenhouse gas emissions associated with using flow batteries are important to consider. Here, we investigate the all-vanadium redox flow battery and a Ce–V battery, where the V4+/V5+ couple is replaced by Ce3+/Ce4+. We use performance metrics from the literature and our previous work to model the batteries' operation. We develop technoeconomic assessment and life cycle inventory models and determine that the Ce–V RFB minimum levelized cost of electricity is lower and the two RFBs’ levelized GHG emissions are similar, suggesting Ce should be considered further in RFB applications. Ultimately, the production of the RFBs results in minor emissions compared to use, except in scenarios where 100% renewables are used to generate electricity. The redox potential and exchange current density are highly influential to the Ce–V RFB cost and emissions, motivating further work into the phenomena that control thermodynamics and kinetics.