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The mechanical condition in stack assembly can impose a significant influence in performance and reliability of the vanadium flow battery. In order to analyze the influence and optimize the overall performance, the effect of clamping force on electrode morphology, mass transfer and polarizations is investigated in this study by measurement of contact resistance and development of a coupled mechanical-electrochemical model. The experimental results indicate that the contact resistance and ohmic polarization decreases with an increase in clamping force, while the simulation results demonstrate that the clamping force can significantly impact on the electrode morphology and mass transport phenomena, and subsequently affect the distributions of activation and concentration polarizations. Despite the concentration polarization presenting to rise at a large clamping force, the overall polarization is proved to be decreased demonstrating that an increased clamping force can offer high voltage efficiency and power density. By further considering the mechanical failure probability and trading off the cell performance and reliability, an optimal clamping force is successfully determined. Such an analysis focusing on assembling condition is vital for understanding the effect of mechanical condition on the battery performance, while also highlighting the significance of rational mechanical design and assembly in flow battery manufacture.

A multi-stack module consisting of a number of stacks connected in series and parallel serves as a basis for installation of MW-scale vanadium flow battery system in grid storage applications. Due to the existence of stack-to-stack variation in resistance, the module performance can be notably limited by an inappropriate module layout that magnifies the impact of stack resistance variation through series and parallel connections. To understand the layout effect on performance, an in-depth investigation is conducted for an eight-stack 250 kW module in this study. Based on experimental measurements, the correlation of module layout to performance is firstly revealed on both the 250 kW module and a laboratory mini-module. Subsequently, 35 different layouts are specified for the 250 kW module and their performance is fully evaluated by means of development of dynamic models for the module. Simulation results prove that the module charging capacity can be effectively improved by grouping stacks with similar resistances into the same branch and be further promoted by optimizing the flow rate for the stack with the largest resistance. The present study offers not only mechanistic insights into the importance of module layout but a cost-effective way to evaluate the module performance as well.

Stack reliability is of great importance in commercialization of vanadium redox flow battery (VFB) since practical VFB stacks are prone to undergo material failure and electrolyte leakage caused by unreliable stack design and improper assembling conditions. A comprehensive evaluation of mechanical behavior and analysis of stack failure is thus highly valued for material fabrication, stack design and assembly. In this study, mechanical behavior and Weibull statistics based failure analysis of the VFB stacks are investigated. The Weibull parameters of two key components are firstly determined from tensile strength tests, which, in combination with finite element analysis of the stack mechanical behavior, are subsequently used to calculate the stack failure probability at specified clamping forces for two different stack designs that both contains 20 individual cells. The results demonstrate that the stack failure probability can be significantly reduced by properly decreasing the clamping forces for both designs, while adding a thick plate to the middle of the stack can effectively lower the probability of failure thus offering a superior stack mechanical performance and a prolonged stack life cycle. Such an approach to analyze stack failure can be readily accessed by flow battery engineers for design and assembly of commercial VFB stacks.

Zinc-manganese flow batteries have drawn considerable attentions owing to its advantages of low cost, high energy density and environmental friendliness. On the positive carbon electrode, however, unstable MnO2 depositions can be formed during oxidation through disproportionation reaction of Mn3+, which result in poor reversibility of Mn2+/MnO2 and bring instability to Zn–Mn flow battery limiting its performance and further development. To tackle this issue, in this study, we reported a highly reversible Zn–Mn flow battery by employing EDTA-Mn as the positive electrolyte. In aqueous solutions, EDTA exhibits a strong ligand field for Mn2+ from ab initio calculations, which proves to form bonds with Mn2+ through carboxyl/amino groups and replace bonded waters in the solvation structure of Mn2+. Benefiting from the coordination effect of EDTA, both electrochemical and material characterizations demonstrate a highly reversible Mn2+/Mn3+ redox reaction in EDTA-Mn, which effectively inhibits the disproportionation reaction of Mn3+ without forming any deposited MnO2 on carbon electrode. The constructed Zn–Mn flow cell adopting EDTA-Mn not only demonstrates excellent rate performance with a high CE over 95 % operated at 10–50 mA cm−2, but also realizes a superior cycling stability over 300 cycles at 20 mA cm−2 affording 98 % CE and 75 % EE.

High power output and efficient charge-discharge operation are deemed to be the pivotal for technological development of vanadium flow batteries. Due to large polarizations however, cycling efficiency is still to a great extent limited in high-power commercialized stacks adopting the conventional flow-through design. To facilitate an in-depth understanding of polarizations and realize a high-power and efficient operation, polarization analyses are performed in this study on a flow-through design based vanadium flow cell. Combined experimental and calculated results show that under the investigated testing conditions, the ohmic polarization dominates the cell polarization across 20–250 mA cm−2, while the electrolyte resistance can account for 70% of the cell resistance. To reduce the electrolyte resistance, further experimental results uncover that enhancing the ionic conductivity through adopting a dilute vanadium electrolyte at a high temperature can readily yield an energy efficiency of 80% at 250 mA cm−2. Such an efficiency enhanced operation can aid to reduce the stack cost, and further discussions concerning other electrolyte properties (e.g., viscosity and surface tension) and their associated effects on battery design and operation also manifest that the electrolyte optimization strategy is practically viable and can be considered for certain flow battery application scenarios.

Full commercialization of vanadium flow batteries requires a high current density operation. However, this can be only realized when associated large polarizations of the cell are properly reduced. Of all the cell components, the porous electrode plays a critical role in determining cell polarizations since it directly relates to each of the polarizations. Despite that, an in-depth understanding of electrode compression impact on polarizations of a flow cell is still limited in literature. In this work, a quantitatively experimental study to unveil the electrode compression impact on each of the polarizations as well as the performance of a vanadium flow cell is conducted by employing a symmetric cell configuration. Four different compression ratios are investigated by both ex-situ characterizations and in-situ symmetric cell tests, which successfully reveal its influence on activation, ohmic and concentration polarizations at varied operating current densities. Charge-discharge cycling tests further prove the significance of electrode compression to both efficiency and discharge capacity, while also delivering an optimal compression ratio for the investigated flow cell. Such a quantitative analysis not only promotes a deep understanding of the importance of electrode compression to cell performance, but is also of vital importance for stack design and optimization in practice.

Organic redox flow batteries with many technological advantages over conventional metal-based flow batteries represent a new generation flow battery technology for electric grid storage applications. Herein, we report a dopamine-based catholyte with a high redox potential of 0.77 V for aqueous organic redox flow batteries. In sulfuric acid, the dopamine exhibits an excellent combination of good electrochemical reversibility and high solubility up to 0.7 M, and undergoes a fast two-electron transfer redox reaction with a rate constant of 1.82 × 10−3 cm s−1. Paired with V3+/V2+, the dopamine-vanadium full cell delivers an average round-trip efficiency of 65% at 20 mA cm−2 and achieves a capacity retention of 50% for 100 consecutive charge-discharge cycles. Moreover, an energy efficiency of 51.3% can be steadily realized at an increased current density of 60 mA cm−2, while an enhanced capacity retention over extended 200 charge-discharge cycles is also observed by introducing saturated ammonium chloride into sulfuric acid. The proposed dopamine-based catholyte not only successfully offers a new alternative for aqueous organic redox flow batteries, but its performance can be also further promoted by rational molecule design and optimizations in mass transfer and electrode kinetics.

Flow batteries are being increasingly deployed in grid-scale energy storage applications. However, long-term operation of flow batteries still suffers from a different extent of capacity decay. While the effects of ion diffusion and side reactions on capacity degradation have been identified and further minimized by improvement in materials, the mechanism of volumetric transfer and its influence in capacity still receive insufficient attentions that impedes further capacity optimizations for flow batteries. In order to gain an in-depth understanding of volumetric transfer mechanism in flow batteries, six different types of flow batteries are adopted in this study and further classified in accordance with electrolyte viscosities for investigations. Experimental results show that a net volumetric transfer in a conventional flow battery highly depends on viscosity values of the two half-cell electrolytes and is virtually towards the half-cell possessing a smaller electrolyte viscosity. For flow batteries with a mixed electrolyte in both half-cells, moreover, cycling tests further demonstrate a zero net transfer under similar viscosity measurements of both half-cell electrolytes. Unraveling the viscosity impact on volumetric transfer is greatly beneficial to facilitate deeper understandings of transport phenomena in flow batteries, which can contribute to realize long-term flow battery operation with a superior capacity retention.

In vanadium flow batteries, electrolyte transfer across the membrane can lead to a volumetric imbalance between the two half-cell electrolytes and a subsequent loss of available capacity. However, the transfer mechanism has not been comprehensively understood and this lack of knowledge has significantly limited long-term discharge capacity and stability of the vanadium flow battery. To overcome this issue, the electrolyte transfer mechanism is systematically developed in this study by analyzing the pressure drop across the membrane in accordance with Darcy's law and further validated by experiments. The experimental results show that the viscosity difference between the two half-cell electrolytes contributes greatly to the electrolyte transfer from negative half-cell to positive half-cell, while a large flow rate applied to both half-cells may also exacerbate the electrolyte transfer. Moreover, further experiments also demonstrate that the electrolyte transfer in continuous charge-discharge operation can be effectively suppressed by optimizing the flow rates based on viscosity measurements, which subsequently yields a notable improvement in discharge capacity. Revealing the electrolyte transfer mechanism is not only beneficial to enhancing long-term performance and stability of the vanadium flow battery, but also highly valued for understanding the transport phenomena in other flow battery systems.

Premature voltage cut-off in the operation of the vanadium redox flow battery is largely associated with the rise in concentration overpotential at high state-of-charge (SOC) or state-of-discharge (SOD). The use of high constant volumetric flow rate will reduce concentration overpotential, although potentially at the cost of consuming excessive pumping energy which in turn lowers system efficiency. On the other hand, any improper reduction in flow rate will also limit the operating SOC and lead to deterioration in battery efficiency. Pressure drop losses are further exacerbated by the need to reduce shunt currents in flow battery stacks that requires the use of long, narrow channels and manifolds. In this paper, the concentration overpotential is modelled as a function of flow rate in an effort to determine an appropriate variable flow rate that can yield high system efficiency, along with the analysis of pressure losses and total pumping energy. Simulation results for a 40-cell stack under pre-set voltage cut-off limits have shown that variable flow rates are superior to constant flow rates for the given system design and the use of a flow factor of 7.5 with respect to the theoretical flow rate can reach overall high system efficiencies for different charge–discharge operations.

In vanadium redox flow batteries (VFB), the power of the battery is determined by the number of cells in the stack. Serial and parallel layouts are commonly adopted interactively to suit the designed power demand. The bipolar stack design inevitably introduces shunt currents bypassing into the common manifolds in the stack and thereby resulting in a parasitic loss of power and energy. During standby, shunt current and its associated internal discharge reactions can generate heat and increase stack temperature, potentially leading to thermal precipitation in the positive half-cell. This study aims to investigate the effect of shunt current on stack efficiency and temperature variation during standby periods for a 40-cell stack. Dynamic models based on mass balance, energy balance and electrical circuit are developed for simulations and the results provide an insight into stack performance that will aid in optimising stack design and suitable cooling strategies for the VFB.

During the operation of vanadium redox flow battery, the vanadium ions diffuse across the membrane as a result of concentration gradients between the two half-cells in the stack, leading to self-discharge reactions in both half-cells that will release heat to the electrolyte and subsequently increase the electrolyte temperature. In order to avoid possible thermal precipitation in the electrolyte solution and prevent possible overheating of the cell components, the electrolyte temperature needs to be known. In this study, the effect of the self-discharge reactions was incorporated into a thermal model based on energy and mass balances, developed for the purpose of electrolyte temperature control. Simulations results have shown that the proposed model can be used to investigate the thermal effect of the self-discharge reactions on both continuous charge–discharge cycling and during standby periods, and can help optimize battery designs and fabrication for different applications.

Studies have shown that the temperature of the electrolyte solutions in the vanadium redox flow battery (VFB) has a significant impact on the battery performance. In this paper, a thermal model for the VFB has been developed on the basis of the conservation of energy to predict the battery temperature as a function of time under different operating conditions and structure designs. Simulations of battery and electrolyte temperature at both constant and varying environmental temperatures show that the presenting model is able to effectively forecast the fluctuation of the battery temperature in the presence of different charge and discharge currents. As expected, increasing current or reduced flow rate will increase the stack and electrolyte temperature. Thermal properties of the tank material and its surface area can however be adjusted to optimize heat transfer to the atmosphere to reduce overheating. This model can be employed to develop a model-based control system which will manage the electrolyte temperature in the optimal range. Further possible improvements to the model are also discussed.

The diffusion of vanadium ions across the membrane along with side reactions can have a significant impact on the capacity of the vanadium redox flow battery (VFB) over long-term charge–discharge cycling. Differential rates of diffusion of the vanadium ions from one half-cell into the other will facilitate self-discharge reactions, leading to an imbalance between the state-of-charge of the two half-cell electrolytes and a subsequent drop in capacity. Meanwhile side reactions as a result of evolution of hydrogen or air oxidation of V2+ can further affect the capacity of the VFB. In this paper, a dynamic model is developed based on mass balances for each of the four vanadium ions in the VFB electrolytes in conjunction with the Nernst Equation. This model can predict the capacity as a function of time and thus can be used to determine when periodic electrolyte remixing or rebalancing should take place to restore cell capacity. Furthermore, the dynamic model can be potentially incorporated in the control system of the VFB to achieve long term optimal operation. The performance of three different types of membranes is studied on the basis of the above model and the simulation results together with potential operational issues are analysed and discussed.