Redox Flow Battery Stack Research Articles

Bonded Bipolar Plate-Graphite Felt Components for Redox Flow Batteries Manufactured via Thermal Fusion by Electro-Welding

Bipolar plates and graphite felt electrodes were thermally fused to one single component via a newly developed electro-welding process. A dropwise arrangement of the polymeric binder led to mechanically stable bonding and was documented by X-ray micro-computed tomography. Electrical conductivity of the fused components was achieved without any conductive filler and electro-welded samples show lower resistance values than unbonded ones at the same graphite felt electrode compression of 5% in ex situ electrical conductivity measurements. Applicability was demonstrated in situ in a 2-cell vanadium redox flow battery stack operated at a graphite felt electrode compression rate of 5% only. Obtained energy efficiencies of the battery are at least equal or even higher compared to state of the art vanadium redox flow battery with unbonded components at 20% graphite felt electrode compression. Composite components will simplify stack assembly and their increased electrode porosity during operation is expected to reduce pressure drop and to increase overall system efficiency.

Gas evolution induced vicious cycle between bubble trapping and flow choking in redox flow battery stacks

The uniform delivery of electrolyte to a parallelly fed redox flow battery stack might be impaired due to the maldistribution of evolved gas bubbles in each single electrode. Entrapped gas bubbles bring about flow choking, resulting in the decrease of liquid velocity, which in turn weakens bubble sweeping and aggravates bubble trapping. Thus, a vicious cycle between bubble trapping and flow choking occurs in porous electrodes. To quantitatively illustrate the development and hazard of this vicious cycle, we establish analytical models based on previous experimental data and present the correlation among gas saturation, liquid velocity, pressure drop and hydrogen evolution rate. When the liquid velocity drops from 10 mm s−1 to 1 mm s−1, the gas saturation exhibits more than fourfold increase at a given hydrogen evolution rate, showing that more bubbles tend to reside in the electrode and the flow path of liquid phase become narrower. With the decrease in the pressure drop, the liquid velocity becomes lower and more sensitive to the gas evolution rate and the electrode wettability. As the pressure drop further drops below a critical value, the liquid flow is very likely to be blocked, resulting in severe maldistribution of electrolyte feeding.

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 Flexible 7-in-1 Microsensor Embedded in a Hydrogen/Vanadium Redox Battery for Real-Time Microscopic Measurements.

The latest document indicates that the hydrogen/vanadium redox flow battery has better energy density and efficiency than the vanadium redox flow battery, as well as being low-cost and light-weight. In addition, the hydrogen, electrical conductivity, voltage, current, temperature, electrolyte flow, and runner pressure inside the hydrogen/vanadium redox flow battery can influence its performance and life. Therefore, this plan will try to step into the hydrogen/vanadium redox flow battery stack and improve the vanadium redox flow battery of this R&D team, whereof the electrolyte is likely to leak during assembling, and the strong acid corrosion environment is likely to age or fail the vanadium redox flow battery and microsensors. Micro-electro-mechanical systems (MEMS) are used, which are integrated with the flexible 7-in-1 microsensor, which is embedded in the hydrogen/vanadium redox flow battery for internal real-time microscopic sensing and diagnosis.

Current Distribution in the Discharge Unit of a 10-Cell Vanadium Redox Flow Battery: Comparison of the Computational Model with Experiment.

Shunting currents are among the main problems of all-vanadium redox flow battery stacks since, in addition to capacity losses, they cause negative effects associated with the local destruction of electrodes and bipolar plates. The values of both the shunting currents and their destructive effects on materials can be reduced at the battery development stage by adjusting the resistance of the electrolyte supply channels. The solution to this problem can be found using a calculation model for current distribution based on the current balance in the nodes as well as voltage drops and electromotive force in internal circuits according to Kirchhoff's laws. This paper presents the verification of the model of current distribution in an all-vanadium redox flow battery stack of an original design that allows for the determination of membrane-electrode assembly resistances and electrolyte supply channels via direct measurements. Based on a comparison of the calculated and experimental values of the coulombic efficiency of charge-discharge cycles, the capacity fade associated with the crossover of vanadium compounds through the membrane has been determined.

Two-layer hydrodynamic network model for redox flow battery stack with flow field design

A two-layer hydrodynamic network model is proposed to analyze the electrolyte distribution within a redox flow battery (RFB). The proposed model consists of a channel, a porous electrode, and an interconnection part through which the electrolyte flows through the channel and porous electrode. The flow rate and pressure drop of each network component are calculated using the Hardy-Cross method. The predicted pressure drop is consistent with experimental and three-dimensional computational fluid dynamics (CFD) results. The velocity and pressure distributions are also in good agreement with CFD results. Furthermore, the proposed model analyzes the flow distribution of cells in RFB stacks with different flow field designs. The results show that the flow rate difference between the first and last cells of the RFB stack with an interdigitated flow field is 20 times higher than that of a flow-through-type.

Connection of Bipolar Plate with Graphite Felt Electrode for Vanadium Flow Battery Using a Powder Thermal Unification Method

Carbon felt electrodes are usually compressed against the bipolar plates in order to decrease the contact resistance, but it leads to high pressure drop and consumption of pumping energy in conventional vanadium redox flow battery stacks. This paper describes the fabrication of an effective way to unify the bipolar plate and graphite felt electrode in order to improve the battery efficiency at lower felt compression pressure. The unification process involves a thermal fusion technique with a component design consisting of bipolar plate, graphite felt electrode and connecting powder. The powder comprises either polyvinylidene difluoride or a mixture of polyvinylidene difluoride and conductive carbon black. The interfacial bonding substance determines the electrical resistance of the resultant fabricated unified assembly. The electrical resistance results prove that the carbon black proportion in the bonding element substantially reduces the electrical resistance. The optimum quantity of carbon black in the powder is found to be 40 wt%. Micro-computed tomography analysis findings indicate that thermally unified assembly interfacial contact electrode porosity is slightly lower than that of an unbonded sample. A two-cell vanadium redox flow battery stack with integrated unified assembly achieves a rather constant energy efficiency of 76% during 100 cycles with 5% electrode compression at a current density of 80 mA cm−2.

Model for Rating a Vanadium Redox Flow Battery Stack through Constant Power Charge–Discharge Characterization

A method for estimating the stack rating of vanadium redox flow batteries (VRFBs) through constant power characterization was developed. A stack of 22 cells, each with 1500 cm2 of nominal electrode area, was constructed and tested using constant current and constant power protocols. Typical ratios of charging to discharging power that prevail in various applications (e.g., peak shaving, wind power/solar photovoltaic power integration) were employed in the test protocols. The results showed that fractional energy storage capacity utilization and round-trip energy efficiency varied linearly with the power at which the energy was charged or discharged. A zero-dimensional electrochemical model was proposed based on the area-specific resistance to account for the energy stored/extracted during constant power discharge in the state of charge (SoC) window of 20% to 80%. It was shown that this could be used to rate a given stack in terms of charging and discharging power from the point of view of its application as a power unit. The proposed method enables stack rating based on a single polarization test and can be extended to flow battery systems in general.

Shunt currents in vanadium redox flow batteries – a parametric and optimization study

Shunt currents can hinder the performance and reduce the lifespan of redox flow batteries. This work focuses on finding the best approach to mitigate the shunt currents in vanadium redox flow batteries. A 2-D dynamic phenomenological model was used to compute the shunt currents as a function of the design and operating conditions of a stack with 6 cells and 16 cm2 active area, operated at 80 mA cm−2. In parallel, an equivalent circuit model was used to compute the same shunt currents but with a far smaller computing time. Finally, an artificial intelligence-based software was used to obtain a simple mathematical correlation that best fit the simulated shunt currents, as a function of the external resistances (manifold and flow frame channels, from 0.08 to 2621 Ω), operating current (from 4 to 512 A) and number of cells (from 5 to 40) in a stack. The correlation was then used to perform a sensitivity analysis to the previous parameters.The results show that the shunt currents decrease with the internal resistances (activation, ohmic and mass transport). Nonetheless, the external resistances have the greatest impact in the shunt currents, particularly the manifold electrical resistance. Based on these findings, a novel redox flow battery stack design (343 cm2 active area), with a high manifold resistance and a low flow frame channel resistance, is proposed to mitigate the shunt currents; cells with no electrochemical activity (dumping cells), evenly inserted into the stack, were considered to increase the manifold resistance. This new stack displays 23.6% lower shunt current losses, 33% lower pressure drop and 19% higher nominal power output at 80 mA cm−2 when compared with a conventional stack.

A successful transition from a vanadium redox flow battery stack to an energy efficient electrochemical desalination module

Vanadium Redox Flow Batteries (VRFB) are considered prime candidates for stationary energy storage due to their unique capability to decouple power and energy. With the aim of making electrochemical water desalination more energy efficient when scaling up the technology, potential benefits from repurposing a commercial VRFB stack as a Capacitive Deionization (CDI) module were studied. Accordingly, a 5-cell CDI Stack equipped with 1250 cm2 electrodes was assembled by revamping a 5kWh-VRFB. Electrosorption experiments showed a great versatility with a maximum salt adsorption capacity of 14.2 mg g−1 and an average salt adsorption rate of 0.321 mg g−1 min−1. Furthermore, a long-term test performed over 100 cycles validated the robustness of the system by achieving salt concentration reductions of 10–12 mM at a constant production rate of 4 L h−1 m−2. Moreover, adapting a VRFB stack specifically designed for energy optimization allowed a substantial reduction of energy consumption of 75% (from 0.84 kW h m−3 to 0.21 kW h m−3) by effectively recovering the energy employed in the desalination step. These results demonstrated the successful transition from VRFB to CDI showcasing the relevance of the Water-Energy Nexus and opening the path for exploring a second life of exhausted VRFB stacks.

Modelling of the treatment of wastewater by photovoltaic solar electrochemical oxidation (PSEO) assisted by redox-flow batteries

This work aims at the formulation of simple and pragmatic models to predict the behaviour of a photovoltaic solar electrochemical oxidation treatment assisted by an energy storage system. Those models will be later integrated in a software tool that allows the optimization and the management of the energy provided by solar panels to power electrochemical advanced oxidation processes coupled with a redox flow battery as energy storage system. Models for a PV panel, a redox flow battery stack and a conductive diamond electrochemical oxidation electrolyzer have been proposed and fitted to experimental data. Results showed a huge accuracy of the models to predict the electric and remediation parameters. Validation analyses reported high regression coefficients above 0.96 which confirm the precision of all the proposed models. Thus, once known the solar radiation in a located place, the level of remediation of a wastewater effluent treated by electrolysis may be estimated. Furthermore, it must be pointed out that the operational conditions of the treatment could be optimized and adjusted by means of an energy storage in the redox flow battery. Considering those facts, the sustainability and efficiency of electrochemically-assisted remediation processes could be highly increased.

Dataset of a vanadium redox flow battery 10 membrane-electrode assembly stack.

This paper contains a vanadium redox flow battery stack with an electrode surface area 40 cm2 test data. The aim of the study was to characterize the performance of the stack of the original design. The dataset include three series of galvanostatic charge-discharge cycling in the potential region 8–16 V with current densities 75, 150 and 200 mA/cm2 for 100 cycles. Coulomb, voltaic, energy efficiencies and capacity utilization coefficient are also provided for all three series.

Nine watt – Level aqueous organic redox flow battery stack using anthraquinone and vanadium as redox couple

In this study, nine watt (9 W) aqueous organic redox flow battery (AORFB) stack using vanadium (IV) oxide sulfate hydrate (VOSO4) and anthraquinone-2,7-disulfonic acid (2,7-AQDS) as redox couple is developed for the first time. This redox couple is dissolved in sulfuric acid with cell voltage of 0.9 V and this is higher than that of other acid based AORFBs. Although this couple has precipitation and crossover issues of vanadium ions, they are solved by the adoption of two additives, ammonium chloride (NH4Cl) and magnesium sulfate (MgSO4). NH4Cl plays a role in preventing the precipitation problem by enhancing their dispersion capability, while MgSO4 can reduce the crossover problem by keeping the balance of ionic strength between two electrolytes. With that, the effects of concentration of VOSO4, flow rate, and current density on the performances of AORFB single cells using these two additives are evaluated. When the effects are considered, capacity retention is improved and the precipitation and crossover of vanadium ions are suppressed, and these induce a better cycle stability of AORFB. Eventually, the five AORFB single cells are stacked and this AORFB stack shows the excellent maximum power of 9 W at 120 mA·cm−2, while its performance is well preserved for 200 cycles. From this study, we substantiate that the 9 W AORFB stack using organic and vanadium active materials is well achieved by using efficient additives and optimizing the operational factors.

9 Kw VRFB Experimental Facility: High Current Polarization Tests and Energy Analyses

A Vanadium redox flow battery test facility of industrial size (IS-VRFB) has been recently built at the Energy Storage and Conversion Laboratory of University of Padua in order to investigate the performance potential of scaling-up from small-scale single-cell bench experiments. The facility is built around a 40-cell 600-cm­ 2 active-area stack and two 550-litre tanks. The facility is provided with advanced electronic bidirectional power supply, multi-physics instrumentation and data acquisition and control system. This paper reports on the experimental results obtained so far. Two working modes were tested, i.e. fast and steady-state discharge, representative of real operative conditions, such as primary frequency regulation and peak shaving, respectively. During discharge tests at SOC=0.9 on a passive load, currents as high as 400 A (665 mA cm­– 2) where measured at a specific flow rate of 20.5×10–3 cm s–1, with a steady state value in excess of 350 A (of 583 mA cm–2). Maximum power exceeded 9 kW (370 mW cm–2 per cell), with a steady state value approaching 8 kW (330 mW cm–2 per cell). Energy losses measurement showed a round trip efficiency (RTE) in excess of 70%. This performance required a proper programming of the control strategies provided by the system supervisor. References Alotto, M. Guarnieri and F. Moro, “Redox flow batteries for the storage of renewable energy: A review,” Renew Sus. Energ Rev. 29, (2014): 325–335.Guarnieri, A. Trovò, A. D’Anzi, P. Alotto, “Developing vanadium redox flow technology on a 9-kW 26-kWh industrial scale test facility: design review and early experiments”, Appl Energy, 230 (2018) 1425-1434. DOI: 10.1016/j.apenergy.2018.09.021.Trovò, G. Marini, A. Sutto, P. Alotto, M. Giomo, F. Moro, M. Guarnieri, “Standby thermal model of a vanadium redox flow battery stack with crossover and shunt-current effects”, Appl Energy, 240 (2019) 893-906.Trovò, A. Saccardo, M. Giomo, F. Moro, M. Guarnieri, “Thermal modeling of industrial-scale vanadium redox flow batteries in high-current operations”, J Power Sources, in print. Figure 1

9 Kw VRFB Experimental Facility: Early Results from a Multichannel Electrochemical Impedance Spectroscopy

A Vanadium redox flow battery test facility of industrial size (IS-VRFB) has been recently built at the Energy Storage and Conversion Laboratory of University of Padua in order to investigate the performance potential of scaling-up from small-scale single-cell bench experiments. The facility is built around a 40-cell 600-cm­ 2 active-area stack and two 550-litre tanks. The facility is provided with advanced electronic bidirectional power supply, multiphysics instrumentation and data acquisition and control system. This paper reports on the commissioning of a multichannel Electrochemical Impedance Spectroscopy (EIS) system, tailored for such large stack, capable of operating in the range 0-30 kHz, with bias currents up to 400 A. Results from early experimental campaign will be also reported, which show the lack of uniformity among cells and how differences enlarge with increasing battery load conditions. Results can be instrumental in addressing the design of flow patterns inside a large stack. References Alotto, M. Guarnieri and F. Moro, “Redox flow batteries for the storage of renewable energy: A review,” Renew Sus. Energ Rev. 29, (2014): 325–335.Guarnieri, A. Trovò, A. D’Anzi, P. Alotto, “Developing vanadium redox flow technology on a 9-kW 26-kWh industrial scale test facility: design review and early experiments”, Appl Energy, 230 (2018) 1425-1434. DOI: 10.1016/j.apenergy.2018.09.021.Trovò, G. Marini, A. Sutto, P. Alotto, M. Giomo, F. Moro, M. Guarnieri, “Standby thermal model of a vanadium redox flow battery stack with crossover and shunt-current effects”, Appl Energy, 240 (2019) 893-906.Trovò, A. Saccardo, M. Giomo, F. Moro, M. Guarnieri, “Thermal modeling of industrial-scale vanadium redox flow batteries in high-current operations”, J Power Sources, in print. Figure 1

Research on performance of vanadium redox flow battery stack

The vanadium redox flow battery is a power storage technology suitable for large-scale energy storage. The stack is the core component of the vanadium redox flow battery, and its performance directly determines the battery performance. The paper explored the engineering application route of the vanadium redox flow battery and the way to improve its energy efficiency, and studied high-power vanadium redox flow battery stack. 10 single cells, half-stack and full stack were assembled and tested for constant current and constant power charge and discharge respectively. The results showed that bipolar plate’s conductive filler was increased, besides, reasonable control of the thickness of the graphite felt could reduce the contact resistance between the materials inside the stack. The internal seal was added in the liquid flow frame to reduce the electrolyte permeability and improve the coulombic efficiency; Increasing the exchange capacity of the ion exchange membrane enhanced the choice of permeability and conductivity. All of the above factors could improve the energy efficiency of the battery. The energy efficiency of the 25kW stack could reach 78.6%, and the 31.5kW stack could reach 76.7%.

Computational study of effects of contact resistance on a large-scale vanadium redox flow battery stack

Computational models are developed to allow for a deeper understanding of design factors that affect the lifetime of a vanadium redox flow battery (VRFB) stack, particularly related with the contact-resistance issue of end cells in a large-scale stack. A simplified microcontact-resistance model and a physics-based macrocontact-resistance model are constructed to investigate the effect of contact resistance on the performance and longevity of VRFB stacks. A microcontact-resistance model predicts significant heat accumulation in the current-collector plate that can result in irreversible damage of plastic materials and an electrical-voltage loss if the contact resistance is not properly engineered in the stack design. Furthermore, the physics-based macrocontact-resistance model investigates abrupt voltage and current distortion in the bipolar plate that is in imperfect contact with the current collector; this results in the local corrosion of the bipolar plate. To ensure a long lifetime of VRFBs, a stack design with minimal contact resistance (less than 0.1 Ω cm2) is required. The structural design of the endplate as well as the selection of a high-stiffness material is critical to mitigate the bending issue and reduce contact resistance.

Standby thermal model of a vanadium redox flow battery stack with crossover and shunt-current effects

This paper presents an original model capable of simulating the thermal behavior of a vanadium redox flow battery stack in standby condition, i.e. without power and reactant flow, where the temperature distribution in the cells evolves because of ions crossover through the membrane, Joule losses due to shunt currents and inherent self-discharge effects. For the first time, a model is presented that is capable of simulating the cell temperature distribution in the stack and its time evolution considering all above effects. The model is applied to a 9 kW/40-cell stack and validated against measurements from a thermal imager. Numerical results show that shunt currents affect the temperature in the stack and can be responsible for local increases of cell temperatures up to 10 °C if the solutions are initially at high state of charge. This effect can be critical if standby occurs after a period of operation, with the electrolyte stack temperature markedly higher than air temperature. In addition, results show that shunt currents can play a major role in the thermal behavior of compact stacks, based on new materials capable of high power density and low ion crossover. The model presented here can constitute the basis for advanced cooling strategies.

A two-dimensional mathematical model for vanadium redox flow battery stacks incorporating nonuniform electrolyte distribution in the flow frame

Lumped models have been widely adopted to predict the performance of the vanadium redox flow battery (VFRB) stack, which mainly due to its simplicity in modeling transient behaviors during operation cycles. However, average transport and electrochemical properties in previous lumped models make it impossible to obtain the information of electrolyte distributions in stacks. To address this issue, in this work, we report a two-dimensional mathematical model for a VRFB stack considering the effect of nonuniform electrolyte distributions in the flow frame via a flow network equivalence method. With this new model, battery performance and its temperature at different operating conditions are determined accurately. It is demonstrated that (i) temperature fluctuations of the stack reach up to 10 K at different current densities and flow rates; (ii) 25% blockage in the middle cell can lead to the capacity reduction by up to 80%.

Vanadium redox flow battery with slotted porous electrodes and automatic rebalancing demonstrated on a 1 kW system level

Capacity loss due to electrolyte crossover through the membrane and pump losses due to pressure drop at the porous electrodes are widely known issues in vanadium redox flow batteries during operation. In commercial systems, these losses account for a significant reduction in the overall efficiency. Previous studies have been focused on the development of new membranes to solve the capacity loss, and design modification to reduce the pressure drop. In this work, we propose unique solutions to solve both problems and are demonstrated in a multi-cell stack for the first time. A 20-cell, 1 kW vanadium redox flow battery stack was assembled using thin bipolar plates and porous electrodes featuring interdigitated flow channels. Such a stack design is novel of its kind and can mitigate various problems associated with flow distribution and pump power in flow batteries. In addition, the electrolyte tanks were shunted together to rebalance the electrolyte automatically. The stack showed a very good and stable performance with an energy efficiency of 80.5% at a current density of 80 mA cm−2. The use of hydraulic shunt resulted in a constant capacity over 250 cycles while the use of flow channels on the porous electrodes resulted in ∼40% reduction in pressure drop, compared to a stack with standard felts. The reduction in pressure drop by employing flow channels reduced the pump power proportionally. Overall, capacity retention and utilization of active materials have been improved substantially. These methods are simple and applicable to any size of vanadium redox flow battery.