A benzophenone-based anolyte for high energy density all-organic redox flow battery

A new electroactive species benzophenone (BP) is identified for the anolyte of an all-organic redox flow battery (AORFB) due to its low redox potential, high electrochemical reversibility and stability, and high solubility in non-aqueous electrolyte. BP also shows high stability in a wide temperature range, and the elevated temperature enhances the redox reactions and the mass transfer rates, leading to an exponential increase of the diffusion coefficient. An AORFB based on BP anolyte is achieved with a cell voltage of 2.41 V and a theoretical energy density of 139 Wh L^?1. The cycling performance shows a stable coulombic efficiency of 81%. The results presented indicate that organic molecule BP is a promising anolyte active species for high-energy density AORFBs.     

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.     

Gradient-microstructural porous graphene gelatum/flexible graphite plate integrated electrode for vanadium redox flow batteries

A novel 3D electrochemically reduced graphene oxide (ERGO) porous gelatum material was self-assembled on graphite plate (GP) by the simple and controllable electrodeposition method to construct a new-type integrated electrode (ERGO-GP) for all vanadium redox flow batteries (VRFBs). Benefited from the ideal preparation method, a porous and gradient microstructure of ERGO was formed easily and facilitated the electrolyte, ions and electron transport simultaneously. Besides, the direct growth of ERGO on GP could reduce the contact resistance between the reaction layer and the conduction layer effectively. As a result, the electrochemistry and battery performance for the ERGO-GP as both negative and positive electrode of VRFBs were better than the common carbon felts (CF) and GP electrode system (CF-GP) due to the lower polarization. The obtained results also indicated the great promise of this material for application in other flow redox batteries and helped us to improve the design of carbon-based porous electrodes.     

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.     

Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose

High ion selectivity and mechanical strength are critical properties for proton exchange membranes in vanadium redox flow batteries. In this work, a novel sulfonated poly(ether sulfone) hybrid membrane reinforced by core-shell structured nanocellulose (CNC-SPES) is prepared to obtain a robust and high-performance proton exchange membrane for vanadium redox flow batteries. Membrane morphology, proton conductivity, vanadium permeability and tensile strength are investigated. Single cell tests at a range of 40–140 mA cm⁻² are carried out. The performance of the sulfonated poly(ether sulfone) membrane reinforced by pristine nanocellulose (NC-SPES) and Nafion® 212 membranes are also studied for comparison. The results show that, with the incorporation of silica-encapsulated nanocellulose, the membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm⁻², which is stable during 200 charge-discharge cycles.     

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.     

Branched sulfonated polyimide/s-MWCNTs composite membranes for vanadium redox flow battery application

A novel sulfonated multi-wall carbon nanotubes (s-MWCNTs) filler is synthesized by ring-opening reaction. And then, a series of branched sulfonated polyimide (bSPI)/s-MWCNTs composite membranes are also prepared for application in vanadium redox flow batteries (VRFBs). The optimized bSPI/s-MWCNTs-2% composite membrane has lower vanadium ion permeability (2.01 × 10⁻⁷ cm² min⁻¹) and higher proton selectivity (1.06 × 10⁵ S min cm⁻³) compared to those of commercial Nafion 212 membrane. Moreover, the VRFB with bSPI/s-MWCNTs-2% composite membrane exhibits higher coulombic efficiencies (CEs: 96.0–98.2%) and energy efficiencies (EEs: 79.7–69.5%) than that with Nafion 212 membrane (CEs: 86.5–92.5% and EEs: 78.5–67.6%) at 80–160 mA cm⁻². The VRFB with bSPI/s-MWCNTs-2% composite membrane has stable battery performance over 400 cycles at 100 mA cm⁻², whose EE value is in the top level among previously reported SPI-based composite membranes. The results show that the bSPI/s-MWCNTs-2% composite membrane has a great prospect in VRFB application.     

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.     

Balancing the specific surface area and mass diffusion property of electrospun carbon fibers to enhance the cell performance of vanadium redox flow battery

Electrospun carbon fibers are featured with abundant electroactive sites but large mass transport resistances as the electrodes for vanadium redox flow battery. To lower mass transport resistances while maintaining large specific areas, electrospun carbon fibers with different structural properties, including pore size and pore distribution, are prepared by varying precursor concentrations. Increasing the polyacrylonitrile concentration from 9 wt% to 18 wt% results in carbonized fibers with an average fiber diameter ranging from 0.28 μm to 1.82 μm. The median pore diameter, in the meantime, almost linearly increases from 1.32 μm to 9.05 μm while maintaining the porosity of higher than 82%. The subsequent electroactivity evaluation and full battery testing demonstrate that the mass transport of vanadium ions through the electrode with larger fiber diameters are significantly improved but not scarifying the electrochemical activity. It is shown that the flow battery with these electrodes obtains an energy efficiency of 79% and electrolyte utilization of 74% at 300 mA cm⁻². Hence, all these results eliminate the concern of mass transport when applying electrospun carbon fibers as the electrodes for redox flow batteries and guide the future development of electrospun carbon fibers.     

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.     

Electrocatalytic activity of nitrogen-doped CNT graphite felt hybrid for all-vanadium redox flow batteries

Novel nitrogen-doped CNT-containing graphite felt (N-CNT/GF) hybrid electrodes with high electrocatalytic activity were developed for all-vanadium redox flow batteries (VRFBs). A simple, effective preparation method for N-CNT/GFs using metal (Fe, Co, and Ni) phthalocyanines as the carbon and nitrogen precursor is presented. We found that different metal precursors generated different densities of N-CNTs on the surface of the GFs due to the various interactions of the metals (Fe, Co, Ni) with the carbon precursor during carbonization. Higher density and longer N-CNTs were obtained for N-CNT/GF (Fe), which gave rise to a higher N-doping concentration, enhanced wettability and conductivity, and improved electrochemical reactivity. When used as an electrode in a VRFB single cell, this material showed outstanding performance with an increase in energy efficiency of more than 20% compared to pure GF at a high current density (150 mA/cm²).     

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.     

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.     

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.     

Highly stable graphene oxide composite proton exchange membrane for electro-chemical energy application

Proton exchange membrane is a basic element for any redox flow battery. Nafion is the only commercial available proton exchange membrane used in different electro-chemical energy systems. High cost restrict it's used for energy generation devices. In present work, we synthesised styrene divinylbenzene based composite proton exchange membranes (PEMs) with varying sulfonated graphene oxide (sGO) content for redox flow battery (RFB). Synthesized copolymer PEMs were analyzed in terms of their chemical structure with the help of FT-IR spectroscopy to confirm desired functional groups at appropriate position. Electrochemical characterization was performed in terms proton-exchange capacity, protonic conductivity and water uptake. Membrane shows adequate proton exchange capacity with good proton conductivity. Vanadium ion permeability was also tested for the prepared membrane to assess capability for vanadium redox flow battery (VRFB) in contrast with commercially available Nafion 117 PEM. Higher VO⁺² ion cross-over resistance was found for CEM-4 with 7.17 × 10⁻⁷ cm² min⁻¹ permeability, which is about half of the CEM-1. Further CEM-4 was also evaluated for charging-discharging phenomenon for single cell VRFB. The values of columbic, voltage and energy efficiency for VRFB confirms prepared membrane as a good candidate for redox flow battery. Composite PEM also shows better mechanical and thermal stability. Results indicates that synthesized composite membrane can be used in vanadium redox flow battery.     

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.     

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.     

Development of electrolyte thermodynamic studies for vanadium flow battery

钒液流电池作为可以规模化储能的一种二次电池越来越受到人们的重视,和其它类型的化学电池不同,其电解液不仅是离子导体,也是实现能量存储的电活性物质,是钒电池的核心.为了提高钒电池储能系统的性能并使之稳定运行,必须深刻认识钒电解液的热力学性质及变化规律.溶液热力学的研究可以提供如离子的存在形式,离子的活度及活度系数,不同形式离子对的解离常数等信息.这些热力学基础数据在设计电池和提升其性能方面都有着重要意义.本文对钒液流电池电解液热力学性质的研究进行了综述,重点介绍了量热法,密度法,电导法等在钒溶液研究中的应用,还介绍了Pitzer电解质溶液理论应用于钒溶液的研究情况,最后对热力学在钒液流电池电解液研究中的应用前景进行展望.     

Preparation of N‐doped graphene‐based electrode via electrochemical method and its application in vanadium redox flow battery

An N‐doped graphene electrode has been prepared by cyclic voltammetric method in 5.0 M of HNO₃ solution on a graphite‐based electrode at room temperature. The modification of the electrode surface with different types of N‐containing groups, such as nitro groups, pyrrolic N, and pyridinic N, has been controlled by changing the scanned potential ranges. The formation of an N‐doped graphene electrode has been confirmed by scanning electron microscopic, atomic force microscopic, X‐ray photoelectron, and Raman spectroscopic methods. The prepared N‐doped graphene‐modified electrodes have been used in positive electrolyte of a vanadium‐based redox flow battery. As positive electrodes, the electrochemically modified electrodes prepared in 5.0 M of HNO₃ solution ?1.0 to (+1.9) and ?0.7 to (+1.9) V had more than 140 and 120 mA/cm² anodic and cathodic peak currents, respectively, in vanadium redox battery. This fast, low‐cost, and environmentally friendly method can be used in many application areas, such as optical devices, (bio)sensors, energy storage materials, and electronic devices.     

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.