Electrode For Redox Flow Battery Research Articles

Morphological Properties and Electrochemical Performance for Compressed Carbon-fiber Electrodes in Redox Flow Batteries

Abstract Improving reactor performance of redox flow batteries is critical to reduce capital cost, and one of the main contributions to the internal resistance is generated by the electrodes, which also impact the pressure drop of the stack. Porous electrodes with optimized microstructure and physiochemical properties play a key role in enhancing electrochemical and fluid dynamic performance. Electrode compression significantly impacts morphology and battery behavior, but the relationship between microstructure and performance remains unclear. In the present study, three representative, commercially available, carbon-fiber electrodes (i.e., paper, felt, and cloth) with distinct microstructures were investigated, and a comprehensive study was conducted to compare morphology, hydraulic permeability, mechanical behavior, electrochemical performance in a lab-scale vanadium redox flow battery at compression ratios of 0-50%. The 3D electrode morphology was characterized through X-ray computed tomography and the extracted microstructure parameters (e.g., surface area and tortuosity) were compared with corresponding electrochemically determined parameters. The optimal trade-off between fluid dynamics and electrochemical performance occurred at the compression ratios of 30%, 20%, and 20% for the felt, paper, and cloth, respectively. Owing to the bi-modal porosity of the woven microstructure, the cloth showed a better trade-off between the electrochemical performance and pressure drop than the other electrodes.

In situ growth of CoO on MXene sheets for modification of all‑vanadium redox flow battery electrodes

In order to greatly improve the energy efficiency and electrolyte utilization efficiency of vanadium redox flow batteries (VRFBs) and to enable them to operate at high current densities, MXene/CoO composites were used for the first time in this research as electrocatalysts for the V3+/V2+ reaction. The catalytic effect of the materials was characterized by cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) tests. Then battery performance of MXene/CoO composite modified plasma-treated graphite felt (GF) negative electrodes (MXene/CoO/GF) were tested in a VRFB. The results show that MXene/CoO composites can improve the catalytic activity. There is a 22.93 % increase in energy efficiency and 124.94 % increase in electrolyte utilization efficiency when MXene/CoO/GF is applied as VRFB negative electrode at a current density of 200 mA cm−2 compared to GF. At a current density of 250 mA cm−2, the energy efficiency of MXene/CoO/GF is maintained at 75.55 %. The excellent performance is attributed to the addition of MXene/CoO, which increases the active sites for redox reactions and thus improves the kinetics of the V3+/V2+ reaction.

Recent advances and perspectives of practical modifications of vanadium redox flow battery electrodes

Electrode modification of VRFB with “3Es”.

Scaling up Electrode Manufacturing with Non-Solvent Induced Phase Separation Technology

Grid-scale energy storage technologies must be deployed at a massive scale to balance the mismatch between energy generation using renewable technologies and demand. Redox flow batteries are an emerging technological option for large-scale and multi-hour electrochemical energy storage thanks to their ability to decouple energy and power1. Despite their intrinsic advantages, the current elevated costs of this technology hinders commercialization2. To improve cost competitiveness, research efforts have focused on developing novel electrolyte chemistries and optimized reactor concepts. To the latter, optimizing the porous electrode microstructure and surface chemistry offers a promising pathway to increase the performance of flow batteries. However, porous electrodes design is challenging as they must fulfil contradictory requirements such as providing high surface area for electrochemical reactions, facilitating mass transport of reactive species, and maintaining low operational pressure drop. Furthermore, state-of-the-art porous electrodes have been repurposed from other mature electrochemical technologies but have not been tailored for the unique requirements of redox flow batteries3.Non-solvent induced phase separation (NIPS) has been recently introduced as a versatile method to synthesize porous structures suitable for RFBs with unique properties, which are unattainable with current fibrous materials4,5. Drawing inspiration from membrane fabrication techniques, we reengineered the method for the synthesis of porous electrodes for use in redox flow batteries. The porous electrodes synthesized with NIPS were tested in a single-electrolyte cell configuration and then in an all-vanadium redox flow battery, showing promising electrochemical performance in comparison with existing commercial fibrous carbon materials. In this presentation, I will discuss our current efforts to scale up the NIPS method to manufacture electrodes at industrial scale. We believe that non-solvent induced phase separation is a cost-effective alternative to traditional carbon fibrous materials thanks to the few simpler steps required for the electrode preparation. Based on technoeconomic modeling, we estimate that the production cost of the NIPS electrodes can be less than half of the production costs of traditional carbon fibrous electrodes6. Moreover, the phase separated electrodes do not require post-manufacturing heat treatment to increase the surface area and wettability, which further translates into savings on energy costs. In this project, we aspire to scale-up the NIPS materials to a final size, after carbonization, of 30·30 cm2 while retaining similar microstructural and electrochemical performance properties as in laboratory scale. We perform a suite of microscopy, spectroscopic and flow cell diagnostics to correlate the electrode microstructure, surface chemistry and device performance. In this presentation, I will discuss the scalability of the method, reproducibility based on statistical analysis of sample properties and performance, and finally electrochemical performance on two flow battery chemistries.References Markard, J. (2018). The next phase of the energy transition and its implications for research and policy. Nature Energy, 3(8), 628-633.Darling, R. M. (2022). Techno-economic analyses of several redox flow batteries using levelized cost of energy storage. Current Opinion in Chemical Engineering, 37, 100855.Forner-Cuenca, A., and Brushett, F. R. (2019). Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities. Current Opinion in Electrochemistry, 18, 113-122.Wan, C. T. C., Jacquemond, R. R., Chiang, Y. M., Nijmeijer, K., Brushett, F. R., & Forner‐Cuenca, A. (2021). Non‐Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes. Advanced Materials, 33(16), 2006716.Jacquemond, R. R., Wan, C. T. C., Chiang, Y. M., Borneman, Z., Brushett, F. R., Nijmeijer, K., & Forner-Cuenca, A. (2022). Microstructural engineering of high-power redox flow battery electrodes via non-solvent induced phase separation. Cell Reports Physical Science, 3(7), 100943.Minke, Christine, Ulrich Kunz, and Thomas Turek. "Carbon felt and carbon fiber-A techno-economic assessment of felt electrodes for redox flow battery applications." Journal of Power Sources342 (2017): 116-124.

Stereolithography 3D Printing As a Versatile Tool to Manufacture Porous Electrodes for Redox Flow Batteries

Redox flow batteries show potential for large-scale energy storage; yet their current costs limit widespread implementation. As the porous electrode microstructure defines the available surface area for electrochemical reactions, electrolyte transport, and fluid pressure drop [1], enhancing the electrode performance is a promising strategy to increase power density and thus reduce system costs. Conventional fibrous porous electrodes are repurposed from fuel cell gas diffusion electrodes and have not been tailored to sustain the requirements of liquid-phase electrochemistry. Therefore, new manufacturing techniques offering high control over the electrode microstructure and resulting properties need to be developed [2]. In this context, 3D printing techniques are uniquely suited to manufacture controlled and deterministic architectures, enabling the tuning of electrochemical performance and hydraulic resistance [3].In this work, we employ stereolithography 3D printing using a commercial desktop printer, followed by carbonization, to manufacture model grid structures and investigate their potential as redox flow battery electrodes. We use microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics, to evaluate the thermal behavior, manufacturing fidelity, and fluid and mass transport performance of ordered lattice structures in non-aqueous redox flow cells. We find that the flow field geometry (flow through versus interdigitated), printing orientation with respect to the printing platform, and pillar geometry (Figure 1a) influence the flow cell performance. The printing orientation impacts the surface roughness and resin spreading, which determines the electrode morphology, influencing the internal surface area, shrinking direction after carbonization, and thus the charge transfer, mass transfer, and hydraulic resistances. Moreover, mass transfer rates within the electrode are increased by altering the pillar shape to a helical or triangular design, or by using an interdigitated flow field design, providing directions to improve electrolyte mixing through the electrode. Although the commercial electrodes feature a greater internal surface area and therefore a better electrochemical performance, the area-normalized mass transfer coefficients and pressure drop can be reduced by utilizing 3D printed electrodes (Figure 1b-c). Going forward, 3D printing technologies that enable finer features combined with carbonization at elevated temperatures can be used to manufacture multi-scale electrode structures simultaneously providing excellent electrochemical performance and low hydraulic resistance. We anticipate that combining 3D printing with emerging computational topology optimization approaches could enable the bottom-up design of advanced electrode materials for electrochemical devices [4]. Acknowledgments The authors gratefully acknowledge the Dutch Research Council (NWO) through the Talent Research Program Veni (17324) and the 4TU.HTM grant for financial support.

Expanding the Scanning Electrochemical Microscopy Toolset for Evaluating Electron Transfer Kinetics of Concentrated and Specialized Redox Electrolytes on Carbon Surfaces

Performing quantitative measurements of heterogeneous electron transfer reactions at electrodes (e.g., determining k0 and a) under conditions of high redox active molecule concentration in solution, such as those encountered in electrolytes for redox flow batteries (RFBs), presents significant technical challenges. For example, attempting to use transient voltammetry methodology quickly leads to limitations imposed by iR drop, as currents can be large and resistance, especially in non-aqueous systems, can be high. Additionally, using steady-state techniques based on convective principles, such as the rotating disk electrode (RDE), involve the use of relatively large equipment, with moving parts, and usually with requirements of tens to hundreds of mL of solution, which may be limiting for exploring new compounds at high concentration.To address these issues, here we introduce the use of scanning electrochemical microscopy (SECM) for the determining the rates of electron transfer of redox active species at high concentration in non-aqueous [1] and other unconventional systems, such as deep eutectic solvents. SECM enables the resolution of higher rates of electron transfer compared to transient voltammetry and RDE, and it also enables experiments with spatial resolution, enabling the exploration of reactivity on various sites of a heterogeneous electrode. In this presentation, we will describe theoretical and practical aspects of these measurements, and we will demonstrate their utility using two model systems. The first one consists on the observation of inherent rate asymmetries observed for ferrocene derivatives, ubiquitous redox probes, at the graphite/carbonate solvent interface. These asymmetries are not present on aqueous systems or metallic electrodes, demonstrating the ability of the SECM technique to capture unique behaviors. In the second case, we will discuss how SECM measurements can be used on deep eutectic solvents such as ethaline to address how interactions between redox actives and the electrode or the solvent systems modulate the rate of electron transfer. By systematically modifying the strength of interactions by means of molecule choice, solvent composition, and electrode type and structure, we can gain insight into the processes that determine reactivity at redox flow battery electrodes. This insight is critical for exploring the interfacial aspects such as adsorption, passivation, and double-layer effects on kinetics of electron transfer.[1] Gaddam, R.; Sarbapalli, D.; Howard, J.; Curtiss, L.A.; Assary, R.S.; Rodríguez-López, J. An SECM-Based Spot Analysis for Redoxmer-Electrode Kinetics: Identifying Redox Asymmetries on Model Graphitic Carbon Interfaces Chem: Asian J., 2023, 18, 2, e202201120. Figure 1

Revealing the Multifaceted Impacts of Electrode Modifications for Vanadium Redox Flow Battery Electrodes.

Carbon electrodes are one of the key components of vanadium redox flow batteries (VRFBs), and their wetting behavior, electrochemical performance, and tendency to side reactions are crucial for cell efficiency. Herein, we demonstrate three different types of electrode modifications: poly(o-toluidine) (POT), Vulcan XC 72R, and an iron-doped carbon-nitrogen base material (Fe-N-C + carbon nanotube (CNT)). By combining synchrotron X-ray imaging with traditional characterization approaches, we give thorough insights into changes caused by each modification in terms of the electrochemical performance in both half-cell reactions, wettability and permeability, and tendency toward the hydrogen evolution side reaction. The limiting performance of POT and Vulcan XC 72R could mainly be ascribed to hindered electrolyte transport through the electrode. Fe-N-C + CNT displayed promising potential in the positive half-cell with improved electrochemical performance and wetting behavior but catalyzed the hydrogen evolution side reaction in the negative half-cell.

Investigating Mass Transfer Relationships in Stereolithography-Based 3D Printed Electrodes for Redox Flow Batteries

Redox flow batteries are a promising option for large-scale energy storage but remain too costly for widespread deployment. One approach to increase cost competitiveness is to improve the power density and efficiency of the electrochemical cell. Porous electrodes determine the performance of the cell, and their three-dimensional structure determines the electrolyte transport, fluid pressure drop, and the available surface area for electrochemical reactions [1]. Conventional porous electrodes are fibrous mats assembled in coherent structures [2]; however, they are repurposed from fuel cell gas diffusion electrodes and have not been tailored for redox flow batteries. Hence, an effective means to improve the performance of redox flow batteries is by engineering porous electrode structures to sustain the requirements of liquid-phase electrochemistry. To this purpose, there is a need to develop new manufacturing techniques affording a higher degree of control over the electrode microstructure and resulting properties. Additive manufacturing, or 3D printing, can be employed to manufacture customized, controlled, and deterministic architectures, enabling the fine-tuning of the electrical and hydraulic performance of porous electrodes [3].In this study, we manufacture model grid structures using stereolithography 3D printing followed by carbonization (Figure 1a) to tune the physiochemical properties of electrodes to be used in redox flow batteries. We employ a suite of microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics to understand the impact of the electrode structure on the mass transport and hydraulic performance of ordered lattice structures in flow cells. Here, the influence of the printing direction, pillar geometry (Figure 1b), and flow field type on mass transport is investigated using an organic electrolyte. We elucidate correlations between the electrode structure and performance metrics, namely surface area, pressure drop, and mass transfer correlations. We find that the printing orientation impacts the electrode performance through a change in electrode morphology caused by resin spreading and surface roughness, affecting the shrinking direction upon carbonization, internal surface area, and thus the charge transfer, mass transfer, and hydraulic resistances. Furthermore, we find that mass transfer rates within the electrode are enhanced by using an interdigitated flow field or by altering the pillar shape to a helical or triangular design, which we hypothesize improves mixing. Compared to commercial carbon-fiber electrodes, the pressure drop is significantly reduced (Figure 1c) as expected due to larger pore sizes (~500 μm for the grids vs. 2-100 μm for the Freudenberg H23 paper electrode and 2-300 μm for the ELAT Cloth electrode). Even though the commercial electrodes feature a superior internal surface area compared to the 3D printed electrodes, their area normalized mass transfer coefficients are lower (Figure 1d). Going forward, the use of printing technologies enabling finer features in combination with carbonization at elevated temperatures can be used to manufacture multiscale electrodes simultaneously providing low hydraulic resistance and excellent electrochemical performance. Additive manufacturing in combination with emerging computational approaches in topology optimization [4] might enable the bottom-up design and manufacturing of advanced electrode materials.

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.

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.

Investigating Mass Transfer Relationships in Stereolithography 3D Printed Electrodes for Redox Flow Batteries

AbstractPorous electrodes govern the electrochemical performance and pumping requirements in redox flow batteries, yet conventional carbon‐fiber‐based porous electrodes have not been tailored to sustain the requirements of liquid‐phase electrochemistry. 3D printing is an effective approach to manufacturing deterministic architectures, enabling the tuning of electrochemical performance and pressure drop. In this work, model grid structures are manufactured with stereolithography 3D printing followed by carbonization and tested as flow battery electrode materials. Microscopy, tomography, spectroscopy, fluid dynamics, and electrochemical diagnostics are employed to investigate the resulting electrode properties, mass transport, and pressure drop of ordered lattice structures. The influence of the printing direction, pillar geometry, and flow field type on the cell performance is investigated and mass transfer vs. electrode structure correlations are elucidated. It is found that the printing direction impacts the electrode performance through a change in morphology, resulting in enhanced performance for diagonally printed electrodes. Furthermore, mass transfer rates within the electrode are improved by helical or triangular pillar shapes or by using interdigitated flow field designs. This study shows the potential of stereolithography 3D printing to manufacture customized electrode scaffolds, which could enable multiscale structures with superior electrochemical performance and low pumping losses.

Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non‐Solvent‐Induced Phase Separation

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction‐limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non‐solvent‐induced phase‐separation (NIPS) technique that enables the reproducible synthesis of macrovoid‐free electrodes with well‐defined across‐thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity‐gradient electrodes are compared with macrovoid‐containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity‐gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid‐containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity‐gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.

Gradient-distributed NiCo2O4 nanorod electrode for redox flow batteries: Establishing the ordered reaction interface to meet the anisotropic mass transport

In this work, a gradient-distributed NiCo2O4 nanorod-composed graphite felt electrode (G-NiCo2O4-GF) was proposed and developed to achieve an ordered reaction toward the nonuniform mass transport in flow batteries. To this end, the ordered-nucleation hydrothermal method was designed, for which the NiCo2O4 nanorod was grown concentrated in one side of electrode, and gradually decreased along the through-plane direction until hardly any in the other side. Theoretical calculating results indicated that the G-NiCo2O4-GF electrode yielded an obviously lower difference of local current density along through-plane direction, nearly one third of the uniformly-enhanced electrode. Flow battery assembled with the as-prepared electrode exhibited high energy efficiency (88.81 % at 100 mA cm−2), superior operation current density (500 mA cm−2), and long cycling stability (162 h, 200 cycles). This work provides an effective approach to develop the high-performance electrode for flow batteries.

Why electrode orientation and carbon felt heterogeneity can influence the performance of flow batteries

The orientation of carbon felt electrodes in redox flow batteries is found to influence battery performance due to the differences in electrode activity between each face of the carbon felt. Polarisation curves were measured between 1 and 150 mA cm−2 using the positive vanadium reaction in a flowing half-cell configuration. Galvanostatic discharge measurements revealed that one face of the carbon felt sample resulted in significantly lower overpotentials indicating that this face had a greater redox activity. Thermal treatment of the carbon felt in air at 400 °C decreased the activity differences between each face of the felt, suggesting that one side of the carbon felt may have been previously exposed to heat during manufacturing. XPS analysis revealed a correlation between both the sp2/sp3 ratio and pyridinic nitrogen groups and the electrode activity, suggesting that these are descriptors for electrode activity. These findings highlight that commercial carbon felts can exhibit surprisingly large face-to-face variability which should be considered when assembling a flow battery system. Furthermore, it is shown that refining the thermal treatment of the felt during manufacturing can not only introduce relevant active sites, but also improve the overall uniformity of the electrode.

Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes.

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Impedance analysis of electrodes made of continuous carbon filaments in a 20 cm2 redox flow cell

• Continuous filament electrodes permit studies on electrode filament arrangements. • Continuous filament electrodes allow for side electrical contact without compression. • A common charge-transfer rate constant, k 0 of 2.2·10 -4 cm s -1 , was estimated. • Mass-transfer resistance improved with non-uniform filament spatial density. The most commonly used electrodes in redox flow batteries are carbon fiber felts, which are assemblies of randomly oriented short fibers. The avenues to improve cell performance are usually pre-treatment to enhance their catalytic activity, modifications of their porosity, and reducing resistivity by increasing compression. Our investigations focus on carbon electrodes made of continuous carbon filaments, where filaments were arranged in specific geometric patterns in order to investigate the effect of the fiber density and spatial distribution across direction of flow on the cell performance. To identify the source of cell voltage losses, electrochemical evaluations were performed with electrochemical impedance spectroscopy (EIS), and compared with commonly used felt electrodes. The electrochemically reversible ferri/ferrocyanide system at 50% state-of-charge (SOC) was used as redox probe for both anolyte and catholyte to avoid varying electrode activities of anode/cathode due to differences in redox species. The findings generate electrode design considerations to reduce voltage losses in redox flow batteries.

A three-dimensional pore-scale model for redox flow battery electrode design analysis

A three-dimensional (3-D) pore-scale model has been developed to construct multiscale fibrous electrodes for redox flow batteries (RFB). New designs, such as biporous electrodes modify electrode structures by creating secondary pores on single carbon fiber to reduce internal battery resistance. Existing pore-scale models only resolve electrodes to the single carbon fiber scale and cannot incorporate recent multiscale electrode designs into numerical models. Our new model aims to bridge this gap and provide a tool to rapidly screen new electrode configurations. Two multiscale electrodes, laser-perforated and biporous electrodes, were investigated at varying operation conditions with the proposed 3-D pore-scale models. The laser-perforated electrode exhibits a reduced pressure drop, but follows the same permeability correlation compared to the corresponding pristine electrode. For the biporous electrode, the added specific surface area and faster reaction kinetics from the secondary pores are the most influential factors leading to improved battery efficiency. However, operating the biporous electrode in limiting current density conditions should be avoided due to the decreased mass transfer efficiency and a more significant voltage loss. We believe that our 3-D pore-scale model can accelerate the flow battery electrode design process and provide new insights into electrode geometry optimizations.

Bottom-up design of porous electrodes by combining a genetic algorithm and a pore network model

The microstructure of porous electrodes determines multiple performance-defining properties, such as the available reactive surface area, mass transfer rates, and hydraulic resistance. Thus, optimizing the electrode architecture is a powerful approach to enhance the performance and cost-competitiveness of electrochemical technologies. To expand our current arsenal of electrode materials, we need to build predictive frameworks that can screen a large geometrical design space while being physically representative. Here, we present a novel approach for the optimization of porous electrode microstructures from the bottom-up that couples a genetic algorithm with a previously validated electrochemical pore network model. In this first demonstration, we focus on optimizing redox flow battery electrodes. The genetic algorithm manipulates the pore and throat size distributions of an artificially generated microstructure with fixed pore positions by selecting the best-performing networks, based on the hydraulic and electrochemical performance computed by the model. For the studied VO2+/VO2+ electrolyte, we find an increase in the fitness of 75 % compared to the initial configuration by minimizing the pumping power and maximizing the electrochemical power of the system. The algorithm generates structures with improved fluid distribution through the formation of a bimodal pore size distribution containing preferential longitudinal flow pathways, resulting in a decrease of 73 % for the required pumping power. Furthermore, the optimization yielded an 47 % increase in surface area resulting in an electrochemical performance improvement of 42 %. Our results show the potential of using genetic algorithms combined with pore network models to optimize porous electrode microstructures for a wide range of electrolyte composition and operation conditions.

Turning Trash to Treasure: Reusable Glucose Kit as a Cell Using ZnO Derived from Metal Organic Framework (MOF) Electrode for Redox Flow Battery

Redox flow batteries (RFBs) are a promising candidate that are capable of meeting the energy storage applications to fulfill the needs of renewable resources. Herein, we prepare an electrochemical device that holds higher energy density. In this work, a reusable glucose kit used as a flow cell which in turn helps to minimize the cost and also balance the pump losses in electrochemical systems. For fabricating RFB, ZnO, from the metal organic framework (Zn-MOF/ZnO), uses an electrode material: ZnCl2 in aqueous KOH used as both anolyte and catholyte solution. Upon the new cell fabricating in this investigation, we demonstrated the voltage efficiency of 92% at 5 mA cm−2, which reduces the cost of the cell upon being implemented in the flow battery application.

Effects of Structured 3D Electrodes on the Performance of Redox Flow Batteries

AbstractIn recent years, the interest in redox flow batteries as stationary energy storage devices is growing, for which disordered three‐dimensional electrodes such as felts have been the state‐of‐the‐art due to their large surface area and improved mass transfer properties. However, such electrodes present a high pressure drop and operating cost. In this work, the performance of structured three‐dimensional electrodes in a redox flow battery was studied. Compared to the carbon felt electrode similar electrochemical behaviour was demonstrated with these structured electrodes, which were based on the most common static mixer designs, but at a power consumption that was two to three orders of magnitude lower. Moreover, due to its ordered electrode design unwanted side reactions were suppressed. Consequently, these results demonstrate the influence of structured static mixer based three‐dimensional electrodes in redox flow batteries and the opportunities they hold to surpass the current state‐of‐the‐art.