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.     

Research progress of vanadium redox flow battery for energy storage in China

Principle and characteristics of vanadium redox flow battery (VRB), a novel energy storage system, was introduced. A research and development united laboratory of VRB was founded in Central South University in 2002 with the financial support of Panzhihua Steel Corporation. The laboratory focused their research mainly on the selection and preparation of electrode materials, membrane material and modification, stable concentrated electrolyte producing approach, test cell configuration design and optimization. Some relevant foundation problems, such as state of vanadium in sulfurous acid with various additives, the difference of electrochemical reaction rate in anode and in cathode, the crossover of vanadium ions and so on, have been emphasized. The details of these studies have been given and discussed. A 5 kW VRB stack was fabricated in the laboratory and its performances, especially electrochemical performance such as voltage efficiencies, energy efficiencies, and durability, were fully tested. The results will be shown in the talk.The key technologies of developing VRB, such as to improve the activity of its electrode materials, the stability of electrolyte and selectivity of separator, were also discussed. In addition, the research progresses in other laboratories in China were briefly introduced.     

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.     

A novel vanadium/cobalt redox couple in aqueous acidic solution for redox flow batteries

In this work, a novel aqueous electrolyte system consisting of cobalt and vanadium for redox flow battery was prepared to increase the cell voltage of the system for the first time in the literature. Electrolyte systems were characterized by using of cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy to determine the effects of sulfuric acid and active ion concentration on the performance of the battery. Optimum sulfuric acid concentration was determined as 4.0 M for anolyte and catholyte. The effect of diffusion on mass transfer mechanism was higher than that of adsorption in each electrolyte of the flow battery system. Cyclic charge-discharge tests were carried out for the prepared novel electrolyte system. Discharge capacities of the electrolyte were determined as 430.1, 417.4, and 428.7 mAh for first cycle, second cycle, and third cycle, respectively. The cell potential of the redox flow battery in the electrolyte system during the charging process increased to 2.35 V which was also relatively higher than those of aqueous vanadium redox flow battery and aqueous iron flow battery. Obtained redox flow battery composition with its high cell potential can bring a new approach for the different application areas in the electrochemical energy storage.     

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。     

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.     

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。     

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.     

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%.     

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.     

液流电池储能技术研究现状与展望

液流电池技术利用流动的电解液作为电化学储能介质,适合于进行大容量电能与化学能的转化与储存。液流电池通常具有寿命长、效率高等技术特征,在平滑风能、太阳能等可再生能源发电出力以及微型电网、智能电网建设等方面有着广阔的应用前景。本文论述了液流电池的研究与开发现状,概述了目前逐渐具备工程实施能力的全钒液流电池体系,分析了液流电池新体系的研究开发状况,指明了它们各自需要进行技术突破的重要问题,最后展望了金属/ 空气液流电池的技术优势与未来发展前景。     

Development of the all‐vanadium redox flow battery for energy storage: a review of technological,financial and policy aspects

The commercial development and current economic incentives associated with energy storage using redox flow batteries (RFBs) are summarised. The analysis is focused on the all‐vanadium system, which is the most studied and widely commercialised RFB. The recent expiry of key patents relating to the electrochemistry of this battery has contributed to significant levels of commercialisation in, for example, Austria, China and Thailand, as well as pilot‐scale developments in many countries. The potential benefits of increasing battery‐based energy storage for electricity grid load levelling and MW‐scale wind/solar photovoltaic‐based power generation are now being realised at an increasing level. Commercial systems are being applied to distributed systems utilising kW‐scale renewable energy flows. Factors limiting the uptake of all‐vanadium (and other) redox flow batteries include a comparatively high overall internal costs of $217 kW^?1 h^?1 and the high cost of stored electricity of ≈ The commercial development and current economic incentives associated with energy storage using redox flow batteries (RFBs) are summarised. The analysis is focused on the all‐vanadium system, which is the most studied and widely commercialised RFB. The recent expiry of key patents relating to the electrochemistry of this battery has contributed to significant levels of commercialisation in, for example, Austria, China and Thailand, as well as pilot‐scale developments in many countries. The potential benefits of increasing battery‐based energy storage for electricity grid load levelling and MW‐scale wind/solar photovoltaic‐based power generation are now being realised at an increasing level. Commercial systems are being applied to distributed systems utilising kW‐scale renewable energy flows. Factors limiting the uptake of all‐vanadium (and other) redox flow batteries include a comparatively high overall internal costs of $217 kW^?1 h^?1 and the high cost of stored electricity of ≈ $0.10 kW^?1 h^?1. There is also a low‐level utility scale acceptance of energy storage solutions and a general lack of battery‐specific policy‐led incentives, even though the environmental impact of RFBs coupled to renewable energy sources is favourable, especially in comparison to natural gas‐ and diesel‐fuelled spinning reserves. Together with the technological and policy aspects associated with flow batteries, recent attempts to model redox flow batteries are considered. The issues that have been addressed using modelling together with the current and future requirements of modelling are outlined. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrolyte flow optimization and performance metrics analysis of vanadium redox flow battery for large-scale stationary energy storage

Vanadium redox flow battery (VRFB) is the best choice for large-scale stationary energy storage, but its low energy density affects its overall performance and restricts its development. In order to improve the performance of VRFB, a new type of spiral flow field is proposed, and a multi-physics coupling model and performance metrics evaluation system are established to explore the electrolyte distribution characteristics. The results show that the new spiral flow field can effectively improve the uniformity of electrolyte flow and alleviate the phenomenon of local concentration polarization as compared with the traditional serpentine flow field and parallel flow field. Due to the long flow channel and large pressure drop, the system efficiency is low. However, coulombic efficiency, voltage efficiency and energy efficiency are significantly better than the traditional flow fields. Therefore, the novel flow field has obvious advantages in the application of small stacks.     

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.     

Effects of nitrogen doping on the electrochemical performance of graphite felts for vanadium redox flow batteries

Vanadium redox flow batteries (VRFB), originally proposed by Skyllas‐Kazacos et al., have been considered as one of the most promising energy storage systems for intermittently renewable energy. However, the poor electrochemical activity and hydrophobicity of graphite felt electrode greatly limit energy storage efficiency of VRFB system. In this paper, two nitrogen‐doped (N‐doped) graphite felts, obtained by heat‐treating in an NH₃ atmosphere at 600 °C and 900 °C, have been investigated as electrodes with high electrochemical performance for vanadium redox flow batteries. In particular, the one obtained at 900 °C exhibits an excellent electrochemical activity for both V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples. The cells with different graphite felt electrodes were assembled, and the charge–discharge performance was evaluated. The cell with the N‐doped graphite felts has larger discharge capacity, discharge capacity retention, and energy efficiency, especially with the sample treated at 900 °C. The average energy efficiency of the cell with the 900 °C treated N‐doped graphite felts is 86.47%, 5.44% higher than that of the cell with the pristine graphite felt electrodes. These enhanced electrochemical properties of the N‐doped graphite felt electrodes are attributed to the increased electrical conductivity, more active sites, and better wettability provided by the introduction of the nitrogenous groups on the surface of graphite felts. It indicates that N‐doped graphite felts have promising application prospect in VRFB. Copyright © 2015 John Wiley & Sons, Ltd.     

Redox flow batteries—Concepts and chemistries for cost-effective energy storage

Electrochemical energy storage is one of the few options to store the energy from intermittent renewable energy sources like wind and solar. Redox flow batteries (RFBs) are such an energy storage system, which has favorable features over other battery technologies, e.g. solid state batteries, due to their inherent safety and the independent scaling of energy and power content. However, because of their low energy-density, low power-density, and the cost of components such as redox species and membranes, commercialised RFB systems like the all-vanadium chemistry cannot make full use of the inherent advantages over other systems. In principle, there are three pathways to improve RFBs and to make them viable for large scale application: First, to employ electrolytes with higher energy density. This goal can be achieved by increasing the concentration of redox species, employing redox species that store more than one electron or by increasing the cell voltage. Second, to enhance the power output of the battery cells by using high kinetic redox species, increasing the cell voltage, implementing novel cell designs or membranes with lower resistance. The first two means reduce the electrode surface area needed to supply a certain power output, thereby bringing down costs for expensive components such as membranes. Third, to reduce the costs of single or multiple components such as redox species or membranes. To achieve these objectives it is necessary to develop new battery chemistries and cell configurations. In this review, a comparison of promising cell chemistries is focused on, be they all-liquid, slurries or hybrids combining liquid, gas and solid phases. The aim is to elucidate which redox-system is most favorable in terms of energy-density, power-density and capital cost. Besides, the choice of solvent and the selection of an inorganic or organic redox couples with the entailing consequences are discussed.     

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.     

Graphite felt electrode modified by square wave potential pulse for vanadium redox flow battery

In this paper, graphite felts treated by square wave potential pulse were modified as electrode for vanadium redox flow battery (VRFB). X‐ray photoelectron spectroscopy measurements indicated that the treatment for graphite felt can introduce oxygen‐containing groups on the surface. Moreover, graphite felt can also be etched to form nano‐scale pores, without damage of mechanical property by treatment. The formed nano‐scale pores and introduced oxygen‐containing groups can enhance the wettability of electrolyte on graphite felt and electrochemical kinetics of V(II)/V(III) and V(IV)/V(V) redox reactions. The treated graphite felt was employed as electrode to assemble the static cell, and its electrochemical performance was evaluated. The cell using modified graphite felt exhibits larger discharge capacity and energy efficiency compared with the pristine graphite felt. The average energy efficiency for graphite felt treated for 1600 s can reach 87.0% at 30 mA cm^?2, 4.2% larger than that for the pristine graphite felt. This study demonstrates that the square wave potential pulse treatment is an efficient way to enhance the electrochemical properties of graphite felt for VRFB system. Copyright © 2016 John Wiley & Sons, Ltd.     

Electrocatalytic performance of TiO2 with different phase state towards V2+/V3+ reaction for vanadium redox flow battery

In this paper, titanium dioxide (TiO₂) nanoparticles were employed as catalysts towards V²⁺/V³⁺ redox couple of vanadium redox flow battery (VRFB). The effect of TiO₂ phase on the electrocatalytic performance for negative couple was systematically investigated. The electrochemical properties of TiO₂ with different phase were assessed via cyclic voltammetry and electrochemical impedance spectroscopy by using AB as conductive agent. Obtained from the results, anatase TiO₂ (α‐TiO₂) exhibits superior electrocatalytic activity to rutile TiO₂ (γ‐TiO₂). The VRFB cell performs well at discharge capacity, voltage efficiency, and energy efficiency by employing α‐TiO₂‐modified negative electrode with current density varying between 50 and 100 mA cm^?2. The discharge capacity of α‐TiO₂‐modified cell with vanadium ion concentration of 1.6 M comes up to 113.5 mA h at 100 mA cm^?2 current density, which is increased by 39.1 mA h after modification for negative electrode. Moreover, the corresponding energy efficiency increases by 7.5% after modification of α‐TiO₂. Experimental results show that TiO₂ is an ideal catalyst for VRFB. Moreover, α‐TiO₂ demonstrates superior electrocatalytic performance to γ‐TiO₂ towards V²⁺/V³⁺ reaction.     

Carbon paper decorated with tin dioxide particle via in situ electrodeposition as bifunctional electrode for vanadium redox flow battery

This work describes a two-step method of in situ electrodeposition and following oxidation to prepare carbon paper (CP) modified with tin dioxide (SnO₂) particle as bifunctional electrode for vanadium redox flow battery. Electrodeposition technique was employed to load metal tin particle on CP, which was subsequently oxidized to tin dioxide at high temperature. CPs modified by SnO₂ with different content (CP/SnO₂-1, CP/SnO₂-2, and CP/SnO₂-3) were obtained by controlling electrodeposition time. CP/SnO₂ presents an increase in electrochemical performance including faster charge and mass transfer compared with pristine CP. Among all samples, CP/SnO₂-2 with proper decorated SnO₂ exhibits superior electrocatalytic performances for V³⁺/V²⁺ and VO₂⁺/VO²⁺ reactions. Raised performances can be attributed to that SnO₂ nanoparticles provide more active sites leading to rapid electrochemical kinetic of vanadium redox reactions. In addition, mass transfer can be accelerated due to the excellent hydrophilicity of SnO₂. The cell using CP/SnO₂-2 as bifunctional electrode exhibits better stability and higher capacity retention during 50-cycle charge-discharge test. The cell for CP/SnO₂-2 shows higher energy and voltage efficiency, suggesting that introduction of SnO₂ can decrease electrochemical polarization. At 150 mA cm⁻², energy efficiency of the cell increases by 7.8% through using CP/SnO₂-2.