Design and Development of High-Performance Flow Field Architecture in a 300 cm2 Vanadium Redox Flow Battery

Vanadium Redox Flow Batteries (VFBs) occupy a critical position in the quest for sustainable energy solutions, offering an attractive option for large-scale energy storage due to their operational simplicity, longevity, and exceptional efficiency.[1] However, the broader deployment of VFBs is hindered by significant energy losses, primarily due to pressure drops across porous electrodes and suboptimal electrolyte utilization at high current densities, which limit their discharge capacity and overall energy efficiency.[2] This research introduces a comprehensive strategy that employs embroidered porous electrodes for substantial pressure drop reduction and corrugated bipolar plates to improve mass transport and electrolyte distribution, thus addressing concentration polarization and mass transport limitations that significantly impact performance at high current densities. Embroidered porous electrodes: Through an innovative embroidery technique, the study engineered carbon electrodes with distinct patterns—diamond, parallel, and perpendicular—to optimize flow dynamics and decrease resistance. This method, reinforced by hydraulic modeling, achieved remarkable pressure drop reductions of 36%, 39%, and 12% for diamond, parallel, and perpendicular patterns, respectively, when compared to unmodified electrodes. The diamond pattern, in particular, demonstrated a notable increase in energy efficiency of 5.5%, including pumping losses, across various current densities. The hydraulic model provides further insight into the ideal channel width and spacing for these patterns, leading to pressure drop reductions of up to 73.5%, marking a significant breakthrough in electrode design. This research not only highlights the potential of embroidered porous electrodes in addressing VFBs' pressure drop challenges but also broadens the application horizon for these technologies across different energy storage systems. Corrugated bipolar plates: The study also investigated the role of corrugated bipolar plates, manufactured via a hot-pressing technique. These plates, in comparison to flat counterparts, offer a fresh approach to refining the VRFBs' flow field. By adjusting the thickness of these corrugated plates (1.0, 1.5, and 2.0 mm), battery performances were methodically assessed across a range of current densities. The corrugation promotes electrolyte flow turbulence and distribution, effectively minimizing ohmic losses and facilitating higher ion transport rates—essential for high-current-density operations. The results demonstrate that the synergistic application of embroidered porous electrodes and corrugated bipolar plates significantly bolsters VRFB performance. Specifically, an uplift in energy efficiency from 62.3% to 72.3% and an enhancement in voltage efficiency from 64.1% to 74.5% were observed at a current density of 200 mA cm-2. Additionally, discharge capacity increased from 1.46 Wh to 1.89 Wh under these conditions, underscoring the efficacy of the design modifications in surmounting VRFBs' intrinsic high-current-density limitations.This holistic approach, merging innovations in both electrode and bipolar plate designs, signifies a shift in VRFB development and optimization. It addresses critical challenges of pressure drop and electrolyte utilization while laying groundwork for future enhancements in flow battery technologies. By improving pressure drop and discharge capacity, this study furthers the objective of integrating renewable energy into the grid, leading the way to more sustainable and reliable energy storage solutions. Keywords: Vanadium Redox Flow Batteries, Embroidered Porous Electrodes, Corrugated Bipolar Plate, Pressure Drop, Discharge Capacity.[1] H. Jiang, J. Sun, L. Wei, M. Wu, W. Shyy, T. Zhao, A high power density and long cycle life vanadium redox flow battery, Energy Stor. Mater. 24 (2020) 529-540.[2] T. Li, F. Xing, T. Liu, J. Sun, D. Shi, H. Zhang, X. Li, Cost, performance prediction and optimization of a vanadium flow battery by machine-learning, Energ. Environ. Sci. 13(11) (2020) 4353-4361. Figure 1

Amongst the accessible electrochemical energy storage technologies, vanadium redox flow batteries (VRFBs) are the important candidate for large-scale energy storage due to their elongated life cycle, quick response, ability to go for deep discharge, low cost, and independent energy-power outputs. To date, most of the current research has been focused on new membranes, electrodes, electrolytes and redox chemistries for better VRFB performance in terms of energy efficiencies, however, flow fields and their effects are sparsely explored. Flow fields are a crucial component in VRFB that plays vital role to distribute the electrolytes and hence significantly impacts the mass transport performance. Few flow field geometries have been studied experimentally and computationally for mass transport and pressure drop in VRFB.1–3 Nevertheless, optimal flow fields yet to be defined independently of operating conditions.4 In this research, therefore, we have studied three different flow fields: serpentine, interdigitated and flow through under the identical operating conditions. Effects of flow field geometry on operating current densities and charge/discharge capacities were evaluated. In addition, flow field effect on polarization curves have been investigated. Results show that the performance of VRFBs strongly depends on flow fields and peak power density above 500 mW/cm2 could be achieved even with thick graphite felt electrodes. The outcome of this study should be useful for researcher to further optimize VRFB system for high-performance. Acknowledgement This work is supported by Laboratory Directed Research & Development, Los Alamos National Laboratory.

In vanadium redox flow batteries, the flow field geometry plays a dramatic role on the distribution of the electrolyte and its design results from the trade-off between high battery performance and low pressure drops. In the literature, it was demonstrated that electrolyte permeation through the porous electrode is mainly regulated by pressure difference between adjacent channels, leading to the presence of under-the-rib fluxes. With the support of a 3D computational fluid dynamic model, this work presents two novel flow field geometries that are designed to tune the direction of the pressure gradients between channels in order to promote the under-the-rib fluxes mechanism. The first geometry is named Two Outlets and exploits the splitting of the electrolyte flow into two adjacent interdigitated layouts with the aim to give to the pressure gradient a more transverse direction with respect to the channels, raising the intensity of under-the-rib fluxes and making their distribution more uniform throughout the electrode area. The second geometry is named Four Inlets and presents four inlets located at the corners of the distributor, with an interdigitated-like layout radially oriented from each inlet to one single central outlet, with the concept of reducing the heterogeneity of the flow velocity within the electrode. Subsequently, flow fields performance is verified experimentally adopting a segmented hardware in symmetric cell configuration with positive electrolyte, which permits the measurement of local current distribution and local electrochemical impedance spectroscopy. Compared to a conventional interdigitated geometry, both the developed configurations permit a significant decrease in the pressure drops without any reduction in battery performance. In the Four Inlets flow field the pressure drop reduction is more evident (up to 50%) due to the lower electrolyte velocities in the feeding channels, while the Two Outlets configuration guarantees a more homogeneous current density distribution.

Definition of multi-objective operation optimization of vanadium redox flow and lithium-ion batteries considering levelized cost of energy, fast charging, and energy efficiency based on current density

Numerous researches have been conducted to look for better design of cell architecture of redox flow battery. This effort is to improve the performance of the battery with respect to further improves of mass transport and flow distribution of electroactive electrolytes within the cell. This paper evaluates pressure drop and flow distribution of the electroactive electrolyte in three different electrode configurations of vanadium redox flow battery (V-RFB) cell, namely square-, rhombus- and circular-cell designs. The fluid flow of the above-mentioned three electrode design configurations are evaluated under three different cases i.e. no flow (plain) field, parallel flow field and serpentine flow field using numerically designed three-dimensional model in Computational Fluid Dynamics (CFD) software. The cell exhibits different characteristics under different cases, which the circular cell design shows promising results for test-rig development with low pressure drop and better flow distribution of electroactive electrolytes within the cell. Suggestion for further work is highlighted.

A pluggable current collector for in-operando current measurements in all-vanadium redox flow batteries with flow field

Vanadium redox flow battery (VRFB) is a promising technology for energy storage because of its independent energy to power ratio and long cycle life. However, VRFB commercialization is still hindered by some technological issues, among which the mass transport of the electrolyte over the porous electrode is one of the most important, since it leads to increased overpotential at high current and limits the power density of the system. Electrolyte mass transport is regulated by different physical phenomena, occurring at different scale (channel, electrode, pore) with different intensity throughout cell active area. Therefore, the quantification of the corresponding performance loss is crucial in order to improve VRFB competitiveness.Electrochemical impedance spectroscopy (EIS) is a powerful in-situ measurement technique that permits to separate physical phenomena characterized by different relaxation frequencies. However, despite its potentialities, the interpretation of experiments is not trivial and physics-based modelling plays a key role to derive quantitative information [1].In this work, local impedance spectra were measured in 10 different regions of a 25 cm2 segmented cell [2] at different electrolyte flow rates and current densities, adopting symmetric cell configuration with both positive and negative electrolyte. Impedance spectra were recorded in a frequency range included between 10 kHz to 10 mHz. The cell hardware was composed by single serpentine graphite distributors, manufactured in-house with the 10 electrically insulated regions held together by means of a 0.8 mm layer of insulating inert epoxy resin. Both positive and negative electrodes were Sigracet® 39 AA.The local impedance spectra were subsequently analyzed with the aid of a 1D+1D physics-based analytical model, able to compute system EIS with a reduced computational time. The developed model coupled the electrochemistry of red-ox reactions with different electrolyte mass transport mechanisms, each associated to an impedance (i.e., voltage loss). Electrolyte is firstly transported along the channel, which is responsible for an impedance related to vanadium consumption and where a convective mass transport resistance at the channel/electrode interface accounts the voltage loss due to the concentration gradients between the channel and the electrode. Then vanadium ions are transported through the porous electrode and from the bulk of electrode pores to the active surface of the carbon fibers, where red-ox reactions occur.The model was validated with respect to local impedance spectra: Figure 1 illustrates an example in the case of symmetric cell with negative electrolyte at 0.1 A cm2 and 20 ml min-1. It can be noticed that the developed model reproduces with an acceptable accuracy the local EIS, permitting to quantify the contribution of different mass transport losses throughout cell active area. In order to correctly reproduce the evolution of impedance along channel length it was fundamental to consider the propagation of the state of charge oscillation along the channel, which is induced by the local oscillating consumption of reactants. This induces the reactions occurring in the downstream portion of the electrode to experience an oscillating reactants concentration. Figure 1 – Comparison between simulated and measured impedance spectra along channel length in symmetric cell with negative electrolyte at 0.1 A cm-2 and 20 ml min-1. For the negative electrode, in most of the investigated conditions the voltage loss associated to the electrolyte transport from the bulk of the pores to the surface of carbon fibres appears predominant compared to the other transport mechanisms, while the greatest contribution is given by kinetic losses. Moreover, all the contributions are substantially uniform along the channel length, except for the increasing effect of state of charge oscillation propagation.For the positive electrode, the greatest contribution turns out to be the transport through the electrode thickness at high flow rate, while at low flow rate the most relevant effect is related to mass transport at channel-electrode interface. Along the channel length, all the mass transport contributions show a slight increase, but the greatest variation is observed once again for the state of charge oscillation propagation.

The Redox Flow Battery (RFB) systems are unique chemically, mechanically, and electrically compared to other kinds of batteries. Among RFBs, the Vanadium Redox Flow Batteries (VRFBs) are the most commercialized type. VRFBs are a suitable option for large-scale energy storage with exceptional advantages like the decoupled power and energy design (scalability), long life-time, safe battery chemistry (non-toxic, and non-flammable), etc. The power and energy designs in VRFBs are decoupled, known as the scalability benefit of the VRFBs. This implies more output energy from the battery is possible only by adding more electrolytes to the reservoir tanks.There are only a few research studies in the literature about the optimized operation of the Vanadium redox flow batteries. Most of these papers are not applicable to develop in real practice. The introduced optimized operation algorithms of VRFBs in this lecture can help the participants learn how to improve the performance of the battery and operate the battery more efficiently. Different objective functions are considered in this lecture to be optimized, e.g. minimizing the time duration of battery charging (for fast charging), energy loss, voltage loss, and capacity loss of the battery. The participants can find the results of this useful in optimized operation and implementation of the VRFBs.High charging current density results in faster charging and reduces the capacity fading in Vanadium Redox Flow Batteries (VRFB). On the other hand, it leads to the reduced energy efficiency of the battery. Also, the lower electrolyte flow rate in VRFBs results in less energy consumption by the pumps leading to the higher energy efficiency of the VRFBs. However, higher flow rates have the benefit of reducing voltage loss of VRFBs. To address these trade-offs, closed-loop charge control and flow management in VRFBs are necessary. In this lecture, a multi-objective optimization is proposed in first section to optimize the charging duration and flow management of the VRFB simultaneously during its charging. An innovative method is proposed for modeling pump consumption based on affinity laws for centrifugal pumps, which leads to new electrolyte flow management. Further, three case studies are defined in charging mode on a nine-cell VRFB unit laboratory prototype to validate the proposed optimization's performance, involving the duration of charging, flow management, and energy efficiency of the VRFB. The method is compared with previously published research studies on the optimal operation of VRFBs, which shows the uniqueness and consistency of the proposed optimization method for simultaneous controlling VRFB's charging duration and flow management.Capacity fade in Vanadium Redox Flow Batteries (VRFB) relies on the loss of electrolyte volume in each of charge and discharge cycles. The loss of volume in each cycle, also known as the bulk electrolyte osmosis, is due to Vanadium ions' diffusion from the membrane. The lower electrolyte flow rate in VRFB can reduce capacity fade as the electrolyte's velocity across the membrane decreases. However, the lower electrolyte flow increases the battery’s voltage loss. A new electrolyte flow management is introduced in second section of the lecture to address this trade-off, which considers the decrease of both capacity fade and voltage loss in VRFBs simultaneously. The proposed multi-objective flow management shows a significant reduction of both capacity and voltage losses in VRFBs.Moreover, typically complex electrochemical models and equations are needed to model capacity fade in VRFBs, which are not straightforward to model. The capacity fade modeling can lead to the estimation of available capacity and the battery’s State of Health (SoH). Therefore, a new mathematical model is proposed for the VRFB’s available capacity based on the electrochemical-based capacity fade model results. The model further is developed to estimate the State of Charge (SoC) and the SoH of VRFBs per cycle of charge and discharge.

Effect of flow field geometry on operating current density, capacity and performance of vanadium redox flow battery

Numerical investigations of flow field designs for vanadium redox flow batteries

Synchrotron X-Ray radiography of vanadium redox flow batteries – Time and spatial resolved electrolyte flow in porous carbon electrodes

Application of flexible integrated microsensor to internal real-time measurement of vanadium redox flow battery

Structural modification of vanadium redox flow battery with high electrochemical corrosion resistance

The vanadium redox flow battery (VRFB) is one of the strong candidates for massive electrical energy storage [1]. For the further implementation of the VRFBs, improvement of cell performance is important to achieving cost reduction. Recently, cell performance has been drastically improved by using thin carbon paper electrodes instead of conventional thick carbon felt electrodes [2-4]. Heat and acid treatments to carbon electrode materials has been also demonstrated to show better reaction kinetics in VRFBs [2-4]. To develop carbon electrodes with higher catalytic activity for VRFB application, clarifying reaction kinetics [5], as well as performance limiting electrode under cell operation are strongly needed. To identify performance-limiting electrode, asymmetric cell configuration was used in a VRFB [3] and showed the negative electrode limit the cell performance in VRFBs. A dynamic hydrogen electrode was applied to an operating VRFB and larger kinetic polarization was found at the negative electrode compared to the positive electrode [6]. In these literature, cell polarization at low current density operation where the reaction kinetics determine overall cell performance was intensively examined. It is also noteworthy that the reaction distribution in the carbon electrode has not been fully explored yet. In this study, we performed polarization experiments by using the symmetric cell geometry in a VRFB. The VRFB in which an interdigitated flow field was embedded [7] was used to achieve high current density operation. To examine the performance limiting electrode, we used heat-treated and raw (as-received) carbon porous materials as the better- and poor- kinetics electrode as reported [3]. Furthermore, we applied two sheets of electrode material to each the negative and positive electrode in order to investigate reaction distribution in the electrodes. In the experiments, one of the two sheets of the heat-treated electrodes was replaced by the raw electrode to determine the performance-limiting side of the electrode. Accordingly, we performed polarization experiments in the following five cases: Base case: (−) HT|HT|PEM|HT|HT (+) Case 1: (−) Raw|HT|PEM|HT|HT (+) Case 2: (−) HT|Raw|PEM|HT|HT (+) Case 3: (−) HT|HT|PEM|HT|Raw (+) Case 4: (−) HT|HT|PEM|Raw|HT (+) Figure 1 shows polarization curves obtained in case 1 and 2 where the negative electrode was partially replaced by the raw electrode. Both polarization curves corresponding to case 1 and 2 shows larger overpotentials than the base case in entire operational condition. This indicates that both the current collector side and the membrane side of the negative electrode contributed to the V(II)/V(III) reaction. However, polarization curves at low current density condition (<0.5A/cm2) indicate that cell performance was more deteriorated in case 2. Case 2 corresponds to the raw carbon material placed in the membrane side of the negative electrode. This suggests that V(II)/V(III) reaction was slightly concentrated in the membrane side of the negative electrode during the discharging process of the VRFB at low current density condition. On the other hand, case 1 showed less cell performance at high current density condition (>0.5A/cm2), indicating more reaction in the current collector side of the negative electrode at high current density condition. These observations can be attributed to the transport of proton (H+) and V(II) ion in the negative electrode. At low current density condition (<0.5A/cm2), sufficient amount of V(II) ion was supplied to the electrode and thus proton transport resistance in the electrode can affect the cell performance, resulting in the more reaction in the membrane side of the electrode. However, at high current density condition (>0.5A/cm2), reaction distribution was spatially shifted to the collector side possibly due to an insufficient supply of V(II) ion to the negative electrode. This further causes the concentration overpotential in the negative electrode at high current density operation. Acknowledgements This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO). The authors acknowledge Prof. Mench for his fruitful communication on high performance VFRBs. The authors appreciate Dr. Kumbur for his useful suggestion on VFRB experiments. Prof. Nguyen is greatly acknowledged for his valuable comments on improving cell performance.

In this work a variety of modifications of porous glass membranes are investigated considering their application in vanadium redox-flow batteries. Through 3D-printing technology a cell assembly was established that made replicable long-term testing of single glass membranes with a predefined geometry possible. Native as well as modified porous glass membranes with pore diameters in the range of 2 nm to 50 nm and thicknesses varying from 300 μm to 500 μm are electrochemically characterized (cf. Chapter IV). Pore surface modifications included the chemical bonding of sulfonic acid groups, propyl-N, N, N-trimethylene-ammonia-groups and propyl-pyridinium groups (cf. Chapter III). Membranes with large pore diameters and thus high porosity showed the highest performance concerning achievable current densities. However, self-discharge and selectivity are affected negatively in these membranes (cf. Chapter V). A newly developed point system identified the best performance of membranes with a thickness of 300 μm, a mean pore diameter between 5 nm and 10 nm and finely dispersed silica particles throughout the pore system. This corresponds with the membrane type 503FD. With this type a mean charging power density of 54.1 mA cm-2 at 1.8 V and a mean discharging power density of 71.7 mA cm-2 at 0.8 V was achieved during cycling. The maximum power density reached 75.3 mW cm-2 at a current density of 100.1 mA cm-2 and a voltage of 752 mV during discharge starting from a SoC of 100 %. Furthermore very high Coulomb Efficiencies (CE) of 98.1 % are possible with such membranes. Further surface modification of this membrane type does not lead to improvements of the performance. Although surface modifications contribute in reducing the self-discharge of the battery through cation crossover, other electrochemical characteristics suffer considerably. Given the additional work and expense of the process, modification of this membrane type is negligible (cf. Chapter VI). The overall performance of the investigated porous glass membranes and their modifications are still well below those of the referenced system of Nafion™ 117. This is explained by the relatively low porosity of the porous glasses. If it was possible to establish large-size, thin and stable glass membranes with high porosities and mean pore diameters in the range well below of 10 nm at reasonable prices, their longevity and aging resistance as well as the large variety of possible chemical surface modifications might lead to drastic performance and efficiency gains. Therefore porous glass membranes might become a real competition for the widespread use of polymer membranes in vanadium redox-flow batteries. The great potential of porous glasses in electrochemical applications is still to be developed in further studies. Their biggest advantage being that their chemical properties can be adopted to their very purpose through chemical modification. In summary the chemical stability and electrochemical performance of porous glass membranes provide very good preconditions for various long-term applications under the harsh conditions of redox flow-battery systems.