Chrome and cobalt-based novel electrolyte systems for redox flow batteries

In the present work, novel redox ion-pairs (as cobalt and chromium) have been used in aqueous medium for the first time in the literature as electrolyte component of redox flow battery system. The electrochemical performance of the Co(II) and Cr(III) redox species as anolyte and catholyte was investigated by cyclic charge-discharge tests, respectively. Electrochemical behaviors of Cr(III) solutions in sulfuric acid solution were determined by using differential pulse voltammetry, electrochemical impedance spectroscopy and cyclic voltammetry via a typical three-electrode system. Morphological analyses of surface of pencil graphite electrode, which was used as anode in differential pulse voltammetric analysis, were done by scanning electron microscopy. Discharge capacity of the battery system consisting of 1.0 M Cr(III) as anolyte (negative electrolyte) and 1.0 M of Co(II) as catholyte (positive electrolyte) in 4.0 M of sulfuric acid was determined as 682.5 mAh (1.4 Ah L⁻¹) with 4 mA cm⁻² charge current density and 0.4 mA cm⁻² discharge current density. Voltage efficiency, energy efficiency and coulombic efficiency of the battery were 70.1%, 53.8% and 57.2%, respectively. The discharge cell potential of the battery was also determined as 1.40 V.     

全钒液流电池用电极及双极板研究进展

全钒氧化还原液流储能电池是一种新型的储能装置,电极及双极板是其关键材料。介绍了全钒液流储能电池的两种电极（金属电极、碳素电极）和三种双极板（金属双极板、碳塑双极板和石墨双极板）以及一体化电极双极板的研究进展。     

Novel chlorine doped graphene electrodes for positive electrodes of a vanadium redox flow battery

One‐step electrochemical preparation of chlorine doped graphene coated electrodes was succeed by cyclic voltammetric method for the first time in the literature in this work. Because hetero atom and oxygen functional groups doped graphene‐based electrodes can improve the electrochemical performance of the battery with formed defects sides on the main structure of the electrodes, the modification of graphene‐based electrodes by chlorine and oxygen including functional groups such as ―Cl, ―ClO₄, ―ClO₃, ―OH, and ―C═O have been done by arranging of scanned potential in cyclic voltammetric treatment. The structural features of chlorine doped graphene electrodes were analyzed by scanning electron microscopic analysis. Chemical structures of the surface of the electrodes were elucidated by X‐ray photoelectron spectroscopy. Raman analyses were carried out to determine the optical properties of the electrodes. Chlorine doped graphene‐based electrodes were also used as positive electrode component of a vanadium redox battery for the first time in the literature. The electrodes showed great electrochemical performance as positive electrode materials of the battery.     

Characteristics of graphite based composite electrodes containing carbon nanotubes for vanadium redox flow batteries

以聚四氟乙烯(PTFE)为黏结剂制备了不同组分的石墨粉(GP)和多壁碳纳米管(MWCNT)复合电极,采用恒电位阶跃,循环伏安,电化学阻抗谱及恒电流充放电等方法系统考察了GP-MWCNT复合电极在全钒液流电池(VRFB)体系中的电化学性能.实验结果表明:复合电极中MWCNT含量的增加有利于VRFB正,负极反应的进行,纯MWCNT电极表现出最优的电化学性能;以纯MWCNT电极为正,负极构建的VRFB电池在30 mA/cm²的恒电流充放电条件下表现出了良好的稳定性和电化学性能,电流效率,电压效率和能量效率分别为96%,87%和84%.     

Developments of electrodes for vanadium redox flow battery

本文介绍了钒液流电池电极材料的研究现状。详细介绍了电极种类、电极材料的改性途径、改性效果,并对电极的老化机制进行了分析。全钒液流电池（VFB）电极材料改性的方法主要包括增加电极催化活性和增大电极电化学反应面积两种方式。通过对电极进行热处理、酸处理,可以改变电极表面结构,提高电极催化活性,从而提高电极反应可逆性。通过在电极表面生长碳纳米管或者负载石墨烯、氧化铱等而制备的复合电极材料,以及采用天然废弃物制备的多孔碳电极,可以达到同时提高电极表面催化活性和增大电极电化学反应面积的效果。还可以通过制备电极和双极板复合一体化电极,降低电池的接触电阻,减小电池极化。而电极的化学降解及电化学降解对于电极的寿命会产生影响,而且对电池负极的影响比正极更加明显。最后,总结了VFB电极材料的现状并展望了未来研究发展的方向。     

An optimal electrolyte addition strategy for improving performance of a vanadium redox flow battery

In this paper, the influences of multistep electrolyte addition strategy on discharge capacity decay of an all vanadium redox flow battery during long cycles were investigated by utilizing a 2-D, transient mathematical model involving diffusion, convection, and migration mechanisms across the membrane as well as the contact resistance in the battery. Results show that with various multistep electrolyte addition strategies, the discharge capacity decay of the battery can be diminished. An optimal multistep electrolyte addition strategy is presented, which is corresponding to adding 1.04 mol L⁻¹ V³⁺ electrolyte to a negative tank while adding 1.04 mol L⁻¹ VO₂⁺ electrolyte to a positive tank. Results show that capacity decay of the battery can be debased by 10.8%, which is due to increased vanadium ions in the negative side and the decreased state-of-charge (SOC) imbalance between two half-cells. This study will propose a practical method for mitigating the discharge capacity decay of the battery during operation.     

Vanadium redox flow batteries: a technology review

Flow batteries have unique characteristics that make them especially attractive when compared with conventional batteries, such as their ability to decouple rated maximum power from rated energy capacity, as well as their greater design flexibility. The vanadium redox flow batteries (VRFB) seem to have several advantages among the existing types of flow batteries as they use the same material (in liquid form) in both half‐cells, eliminating the risk of cross contamination and resulting in electrolytes with a potentially unlimited life. Given their low energy density (when compared with conventional batteries), VRFB are especially suited for large stationary energy storage, situations where volume and weight are not limiting factors. This includes applications such as electrical peak shaving, load levelling, UPS, and in conjunction with renewable energies (e.g. wind and solar). The present work thoroughly reviews the VRFB technology detailing their genesis, the basic operation of the various existing designs and the current and future prospects of their application. The main original contribution of the work was the addressing of a still missing in‐depth review and comparison of existing, but dispersed, peer reviewed publications on this technology, with several original and insightful comparison tables, as well as an economic analysis of an application for storing excess energy of a wind farm and sell it during peak demand. The authors have also benefited from their background in electric mobility to carry out original and insightful discussions on the present and future prospects of flow batteries in mobile (e.g. vehicle) and stationary (e.g. fast charging stations) applications related to this field, including a case study. Vanadium redox flow batteries are currently not suitable for most mobile applications, but they are among the technologies which may enable, when mature, the mass adoption of intermittent renewable energy sources which still struggle with stability of supply and lack of flexibility issues.Copyright © 2014 John Wiley & Sons, Ltd.     

Testing and analyzing power-energy response capability of the vanadium redox flow battery

全钒液流电池具有功率和能量分别独立设计的优点,因此在电力系统应用中具有显著的优势.本文基于5 kW/10 kW·h全钒液流电池系统,通过恒温,倍功率充放电实验模式,开展全钒液流电池的功率和能量特性的实验研究.功率型应用时,在全SOC范围内以备用SOC曲线为参考基准,其具备对称充放电能量不大于1.35倍额定功率的功率响应能力;能量型应用时,在恒功率充或放电能量基础上,通过不大于0.6倍额定功率充放电最大限度的存储或释放能量.     

A review of all‐vanadium redox flow battery durability: Degradation mechanisms and mitigation strategies

The all‐vanadium redox flow battery (VRFB) is emerging as a promising technology for large‐scale energy storage systems due to its scalability and flexibility, high round‐trip efficiency, long durability, and little environmental impact. As the degradation rate of the VRFB components is relatively low, less attention has been paid in terms of VRFB durability in comparison with studies on performance improvement and cost reduction. This paper reviews publications on performance degradation mechanisms and mitigation strategies for VRFBs in an attempt to achieve a systematic understanding of VRFB durability. Durability studies of individual VRFB components, including electrolyte, membrane, electrode, and bipolar plate, are introduced. Various degradation mechanisms at both cell and component levels are examined. Following these, applicable strategies for mitigating degradation of each component are compiled. In addition, this paper summarizes various diagnostic tools to evaluate component degradation, followed by accelerated stress tests and models for aging prediction that can help reduce the duration and cost associated with real lifetime tests. Finally, future research areas on the degradation and accelerated lifetime testing for VRFBs are proposed.     

1,2-Dimethylimidazole based bromine complexing agents for vanadium bromine redox flow batteries

To stabilize bromine produced during a vanadium-bromine redox flow batteries (VBr RFBs) charging, a bromine complexing agent (BCA) should be effectively used as a supporting material in VBr electrolyte. However, there remains a problem of improving the unstable reversibility between V²⁺ and V³⁺ in electrolyte including halogen elements (Br and Cl). This paper describes two imidazole-based BCAs, which are 1,2-dimethyl-3-ethylimidazolium bromide (DMEIm: C₇H₁₃BrN₂) and 1,2-dimethyl-3-propylimidazolium bromide (DMPIm: C₈H₁₅BrN₂), for not only confirming the capture of bromine but also improving the redox reaction of vanadium ions in VBr electrolyte. The effectiveness of the proposed two imidazole-based BCAs is demonstrated through the following experiments: cyclic voltammetry (CV), nuclear magnetic resonance analysis (NMR), scanning electron microscopy (SEM) analysis and cyclic cell operation test. Experimental results show that both the diffusion coefficient and the peak currents of each electrolyte using the proposed imidazole-based BCAs increases linearly with the rise of scan rate on the recorded CV curves, providing improved reversible reaction of V²⁺/V³⁺ in negative electrolyte. It also exhibits that the electrolytes using the DMEIm and DMPIm provide significantly improved charge (discharge) capacities which are 9.38 (31.01) % and 11.8 (35.66) % higher than the pristine one, respectively, resulting in 13.27% and 14.36% higher current efficiencies. In addition, corrosion cracks on the separator surface due to bromine attack are not observed after the cyclic cell operation. Consequently, these results indicate that the proposed two imidazole-based BCAs can not only sequester bromine during the VBr RFB charging, but also enhance electrochemical reversibility caused by improving diffusion coefficient of vanadium.     

Nitrogen-doped mesoporous carbon for energy storage in vanadium redox flow batteries

We demonstrate an excellent performance of nitrogen-doped mesoporous carbon (N-MPC) for energy storage in vanadium redox flow batteries. Mesoporous carbon (MPC) is prepared using a soft-template method and doped with nitrogen by heat-treating MPC in NH₃. N-MPC is characterized with X-ray photoelectron spectroscopy and transmission electron microscopy. The redox reaction of [VO]²⁺/[VO₂]⁺ is characterized with cyclic voltammetry and electrochemical impedance spectroscopy. The electrocatalytic kinetics of the redox couple [VO]²⁺/[VO₂]⁺ is significantly enhanced on N-MPC electrode compared with MPC and graphite electrodes. The reversibility of the redox couple [VO]²⁺/[VO₂]⁺ is greatly improved on N-MPC (0.61 for N-MPC vs. 0.34 for graphite), which is expected to increase the energy storage efficiency of redox flow batteries. Nitrogen doping facilitates the electron transfer on electrode/electrolyte interface for both oxidation and reduction processes. N-MPC is a promising material for redox flow batteries. This also opens up new and wider applications of nitrogen-doped carbon.     

Technical and economic comparison of grid supportive vanadium redox flow batteries for primary control reserve and community electricity storage in Germany

Primary control reserve and maximising self‐consumption are currently two of the main applications for large‐scale battery storage systems. Although being currently the most profitable application for large‐scale batteries in Germany, storage systems applying primary control reserve have not been implemented in a grid supportive manner in distribution grids yet. Despite a current unfavourable regulatory framework and reimbursement scheme for community electricity storages in Germany, they are potentially more profitable than residential storages, which is mainly due to their economy of scale, and thus they may become the major large scale battery application in the future. The two applications: primary control reserve and maximising self‐consumption, are combined with a grid supportive behaviour by providing reactive power control and/or peak shaving and are fitted to a vanadium redox flow battery prototype, which is installed in a distribution grid in southern Germany. Based on measured data from the prototype, two battery models for two different time resolutions (1s, 1min) are presented in detail along with their respective operation models. The operation strategy model for primary control reserve comprises the so‐called degrees of freedom used to reduce the energy needed to recharge the battery. The operation strategy to maximise self‐consumption is based on a persistence forecast. The model for the operation strategy for a grid supportive primary control reserve was validated in a field test revealing a relative error of 2.5 % between the simulated and measured state of charge of the battery for a multi‐week time period. The technical assessment of both applications shows that the use of the degrees of freedom can reduce the energy to recharge the battery by 20 %; and in the case of self‐consumption, the curtailment losses can be kept under 1 %. The economic assessment, however, indicates that even for the most promising primary control reserve case, the investment costs of vanadium redox flow batteries must be reduced by at least 30 % in order to break even. Finally, the encouraging key finding is that the negative impact of a grid supportive behaviour, additionally to its primary purpose, is less than 1 % of the revenues. This may encourage distribution grid and battery operators to consider the integration of large scale batteries in distribution grids as part of the solution of a rising share of a decentralised renewable energy generation.     

A novel electrode-bipolar plate assembly for vanadium redox flow battery applications

A novel electrode-bipolar plate assembly has been developed and evaluated for application in the vanadium redox flow battery (VRB). It is composed of three parts: a graphite felt (electrode), an adhesive conducting layer (ACL) and a flexible graphite plate (bipolar plate). The ACL connects the electrode with the bipolar plate to an assembly. By the evaluations of cost, resistivity, surface morphology, electrolyte permeation and single cell performance, this novel assembly demonstrates its applicability in VRB as evident in the following outcomes: (1) lowers the cost and area resistivity to about 10% and 40% of the conventional setups, respectively; (2) improves electrical conductivity to 4.97 mΩ cm as compared to over 100 mΩ cm of the carbon-plastic composite bipolar plate; (3) attains zero electrolyte permeation; and (4) achieves a higher energy efficiency of 81% at a charge/discharge current density of 40 mA cm⁻² when employed in a VRB single cell, which is 73% for the conventional setup. All these indicate that the novel electrode-bipolar plate assembly is a promising candidate for VRB applications.     

Vanadium redox flow batteries for the storage of electricity produced in wind turbines

In this work, accelerated degradation charge‐discharge tests have been applied to compare the performance of a bench‐scale vanadium redox flow battery (VRFB), when charged under galvanostatic conditions and under the highly variable conditions of current produced by wind turbines. Wind speed patterns applied for the VRFB charge were obtained during 3 representative days in winter, in Ciudad Real (Spain). The accumulated and delivered charge capacities and the different efficiencies (coulombic, voltage, and energy) were analyzed during 3 charge and discharge cycles. The conversion of the different vanadium species during the charge‐discharge cycles indicated that the operation mode had a strong influence on the performance of the VRFB and helped to explain the charge profiles obtained. Although, similar efficiencies and charge/discharge capacities were found, the VRFB operated in wind‐charging mode performs slightly worse than the VFRB operated in galvanostatic mode. Increased crossover of vanadium species in the negative electrolyte compartment explains the differences found. Nevertheless, it can be concluded that this type of technology seems to be promising for the storage of electricity produced by wind turbines.     

High-temperature conductive binder for an integrated electrode bipolar plate and its application in vanadium redox flow battery

A high-temperature conductive binder for preparing an integrated electrode bipolar plate (IEBP) was proposed. The electrical resistance and stability of IEBP samples with different component proportions were tested after treated at different temperatures. The results showed that the mass ratio of phenolic resin, graphite powder, B4C, and SiO2 in the conductive binder was 1:0.5:0.5:0.1, and the IEBP prepared by it had the lowest electrical resistance and the highest stability in vanadium solution after treated at 800°C. The characterization results of thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Fourier transformed infrared radiation (FTIR), and scanning electron microscope (SEM) indicated that the bonding strength was closely related to the formation of borosilicate glass and the volume compensation from B₄C's oxidation. The battery with IEBP had better performance than those with no binder, and it operated well at a current density up to 150 mA cm⁻². Furthermore, the battery with IEBP had a stable cycling performance and the IEBP remained integrated after up to 100 charge-discharge cycles.     

Review on ion exchange membranes for vanadium redox flow battery applications

全钒氧化还原液流电池(VRB)作为一种新兴的电化学储能系统在解决可再生能源利用方面具有良好的应用前景.离子交换膜作为全钒液流电池的关键功能材料之一,应具有钒离子透过率低,电导率高,化学稳定性好等性能.本文论述了VRB的工作原理和特点,综述了近年来国内外相关的研究进展,对商品化离子膜,新型阳离子膜,新型阴离子膜,两性离子膜在VRB中的研究应用进行了对比与分析,并指出它们各自需要改进的地方;最后提出应大力开发低成本的国产全氟磺酸离子膜,为实现VRB大规模的产业化奠定基础.     

Electrode materials for all vanadium flow batteries:A review

电极材料在钒电池发展及应用中起到了至关重要的作用,对于钒电池电极的研究既包括不同基体电极材料的筛选,制备,也包括电极材料的结构优化,表面改性及模拟等.本文首先简要介绍了钒电池用电极材料的分类及其相关代表性结果,明确了电极材料的发展方向,其中具有耐腐蚀性,高电导率,高比表面积,高活性及价格低廉的电极材料成为研究的重点.随后以最具有实用价值的炭素电极材料为中心,详细介绍了该类材料的研究进展以及当前研究过程中存在的问题及可能的解决途径.最后,对电极材料的下一步研究工作进行了展望,认为不仅在电极材料产业化方面需要有进一步的突破,在与电极相关的基础研究如电极过程动力学等方面也需要有更为深入的探讨.     

A critical review on progress of the electrode materials of vanadium redox flow battery

Nowadays, renewable energy sources are taken great attention by the researchers and the investors around the world due to increasing energy demand of today's knowledge societies. Since these sources are non-continuous, the effective storage and re-use of the energy produced from renewable energy sources have great importance. Although classical energy storage systems such as lead acid batteries and Li-ion batteries can be used for this goal, the new generation energy storage system is needed for large-scale energy storage applications. In this point, vanadium redox flow batteries (VRFBs) are shinning like a star for this area. VRFBs consist of electrode, electrolyte, and membrane component. The battery electrodes as positive and negative electrodes play a key role on the performance and cyclic life of the system. In this work, electrode materials used as positive electrode, negative electrode, and both of electrodes in the latest literature were complained and presented. From graphene-coated and heteroatom-doped carbon-based electrodes to metal oxides decorated carbon-based electrodes, a large scale on the modification of carbon-based electrodes is available on the electrode materials of the VRFBs. By the discovering of novel electrode components for the battery system, the using of the VRFBs probably increase in a short time for many industrial and residential applications.     

Numerical analysis of cycling performance of vanadium redox flow battery

The mass transport system in vanadium redox flow batteries (VRFBs) is very complex, which makes it difficult to predict the cycling performance and analyze the characteristics of VRFBs. In particular, ions and water move through the membrane by various transport mechanisms such as diffusion, convection, electro-osmosis drag, and osmosis accompanied by side reactions. This complex transport system causes an imbalance in the electrolyte volume and concentration difference between the anolyte and catholyte tanks during charge/discharge cycling. As the performance of a VRFB is strongly affected by the electrolyte concentration, predicting the volume and concentration of the electrolyte is crucial to predict the performance of a VRFB and plan a rebalancing strategy for it. This study aims to accurately predict the cycling performance and efficiencies (coulomb, voltaic, and energy efficiency) of a VRFB by conducting a computational simulation that considers the electrolyte volume change, owing to the complex mass-transport system in a VRFB, for the first time. The simulation result shows that the diffusion of water and electro-osmosis of proton for an internal circuit have a dominant influence on the electrolyte volume change during the cycling process. It is observed that the electrolyte volume change is mainly caused by water diffusion in the initial cycles. Thereafter, it is found that osmosis predominantly influences the electrolyte volume change in a VRFB. The cycling performance and efficiency are calculated and validated with experimental data, which confirms the high fidelity of the model.     

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.