Phosphonated graphene oxide with high electrocatalytic performance for vanadium redox flow battery

Development of more efficient electrodes is essential to improve the competitiveness of vanadium redox flow battery (VRFB) systems. Covalent functionalization of carbon structure in graphene oxide with phosphonic acid groups was carried out to enhance the electrode wettability. The phosphonated graphene oxide (P-GO) was characterized and found displaying an improved electrocatalytic performance towards electrooxidation/electroreduction of vanadium ion pairs. The defect in P-GO structure increased the negative charge density on the surface leading to higher vanadium ions tendency for electrooxidation/electroreduction reactions. The battery performance was evaluated using electrodes made of carbon felt hosted GO and P-GO in a single cell VRFB and 180 charge-discharge cycles were recorded. The VRFB with P-GO displayed an improved performance and stable coulombic, voltage and energy efficiency compared to VRFB with GO.     

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.     

Shunt current loss of the vanadium redox flow battery

The shunt current loss is one of main factors to affect the performance of the vanadium redox flow battery, which will shorten the cycle life and decrease the energy transfer efficiency. In this paper, a stack-level model based on the circuit analog method is proposed to research the shunt current loss of the vanadium redox flow battery, in which the SOC (state of charge) of electrolyte is introduced. The distribution of shunt current is described in detail. The sensitive analysis of shunt current is reported. The shunt current loss in charge/discharge cycle is predicted with the given experimental data. The effect of charge/discharge pattern on the shunt current loss is studied. The result shows that the reduction of the number of single cells in series, the decrease of the resistances of manifold and channel and the increase of the power of single cell will be the further development for the VRFB stack.     

Development of three-dimensional model for the analysis of the mass transport in vanadium redox flow batteries

It is practical to equip the renewable energy system with the vanadium redox flow battery (VRFB) to improve energy utilization efficiency. A steady-stated, three-dimensional model is developed to study the flow and mass transfer in VRFB with serpentine flow field (SFF), and the corresponding experiments are also executed. The effect of the inlet flow rate on VRFB is analyzed by simulating the charge-discharge process, in which the uniformity factors, pressure drop, overpotential, protons concentration, and the battery efficiency are compared as indicators. Two split-SFFs are proposed to improve the pressure drop in the conventional SFF. The results show the efficiency of VRFB increase with the increase of the flow rate, but when the flow rate is higher than a critical flow rate, the performance on the VRFB is no longer sensitive to the flow rate. Compared to SFF, SFF(SY) performs well on pressure drop but poorly on discharge.     

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.     

Investigations on transfer of water and vanadium ions across Nafion membrane in an operating vanadium redox flow battery

Diffusion coefficients of the vanadium ions across Nafion 115 (Dupont) in a vanadium redox flow battery (VRFB) are measured and found to be in the order of V²⁺ > VO²⁺ > VO₂⁺ > V³⁺. It is found that both in self-discharge process and charge-discharge cycles, the concentration difference of vanadium ions between the positive electrolyte (+ve) and negative electrolyte (−ve) is the main reason causing the transfer of vanadium ions across the membrane. In self-discharge process, the transfer of water includes the transfer of vanadium ions with the bound water and the corresponding transfer of protons with the dragged water to balance the charges, and the transfer of water driven by osmosis. In this case, about 75% of the net transfer of water is caused by osmosis. In charge-discharge cycles, except those as mentioned in the case of self-discharge, the transfer of protons with the dragged water across the membrane during the electrode reaction for the formation of internal electric circuit plays the key role in the water transfer. But in the long-term cycles of charge-discharge, the net transfer of water towards +ve is caused by the transfer of vanadium ions with the bound water and the transfer of water driven by osmosis.     

Dynamic modelling of the effects of ion diffusion and side reactions on the capacity loss for vanadium redox flow battery

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.     

Novel branched sulfonated polyimide membrane with remarkable vanadium permeability resistance and proton selectivity for vanadium redox flow battery application

A series of novel branched sulfonated polyimide (bSPI-x) membranes with 8% branched degree are developed for application in vanadium redox flow battery (VRFB). The sulfonation degrees of bSPI-x membranes are precisely regulated for obtaining excellent comprehensive performance. Among all bSPI-x membranes, the bSPI-50 membrane shows strong vanadium permeability resistance, which is as 8 times as that of commercial Nafion 212 membrane. At the same time, the bSPI-50 membrane has remarkable proton selectivity, which is four times as high as that of Nafion 212 membrane. The bSPI-50 membrane possesses slower self-discharge speed than Nafion 212 membrane. Furthermore, the bSPI-50 membrane achieves stable VRFB efficiencies during 200-time charge-discharge cycles at 120–180 mA cm^?2. Simultaneously, the bSPI-50 membrane exhibits excellent capacity retention compared with Nafion 212 membrane. All results imply that the bSPI-50 membrane possesses good application prospect as a promising alternative separator of VRFB.     

Preparation and properties of sulfonated poly(fluorenyl ether ketone) membrane for vanadium redox flow battery application

In order to develop novel membranes for vanadium redox flow battery (VRB) with low self-discharge rate and low cost, sulfonated poly(fluorenyl ether ketone) (SPFEK) was synthesized directly via aromatic nucleophilic polycondensation of bisphenol fluorene with 60% sulfonated difluorobenzophenone and 40% difluorobenzophenone. The SPFEK membrane shows the lower permeability of vanadium ions. The open circuit voltage evaluation demonstrates that the SPFEK membrane is superior to Nafion 117 membrane in self-discharge test. Both energy efficiencies (EE) and power densities of the VRB single cell based on the SPFEK membrane are higher than those of the VRB with Nafion 117 membrane at the same current densities. The highest coulombic efficiency (CE) of VRB with SPFEK membrane is 80.3% while the highest CE of the VRB with Nafion 117 membrane is 77.0%. The SPFEK membrane shows the comparative stability to Nafion 117 membrane in VO₂⁺ electrolyte. The experimental results suggest that SPFEK membrane is a promising ion exchange membrane for VRB.     

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.     

Numerical simulation of the power-based efficiency in vanadium redox flow battery with different serpentine channel size

We present numerical investigations on the power-based efficiency of vanadium redox flow battery (VRFB). A three-dimensional numerical model is developed to capture the complexities of electrochemical reactions and fluid dynamics when considering different serpentine channel sizes and electrolyte flow rates. It is shown that the reduced channel size and increased electrolyte flow rate improve the electrochemical performance of the VRFB due to the enhanced distribution of molar centration at the electrodes. Nonetheless, the channel size reduction and increased electrolyte flow rate also increases pressure drop between inlet and outlet of the serpentine channels for negative and positive sides. In this, we calculate the power-based efficiency by considering the generated power of VRFB and power loss due to overpotentials, ohmic loss, and required pump power. The maximum power-based efficiency of 96.6% is calculated with the channel size of 1.9 mm at 60 mL min⁻¹, while it is 95.5% with 9.6 mm in channel size at 100 mL min⁻¹. The proposed numerical approach can be useful to determine the channel size with optimized electrolyte flow rate and maximum VRFB efficiency.     

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.     

Sulfonated poly (fluorenyl ether ketone) membrane with embedded silica rich layer and enhanced proton selectivity for vanadium redox flow battery

A series of novel organic-inorganic hybrid membranes with special microstructure, based on sulfonated poly (fluorenyl ether ketone) ionomer (SFPEK, IEC = 1.92 mequiv. g⁻¹) and SiO₂ or sulfonic acid group containing SiO₂ (SiO₂-SO₃H), has been successfully designed and prepared for vanadium redox flow battery (VRB) application. The SiO₂-SO₃H is synthesized by co-condensation of tetraethoxysilane and γ-propyl mercaptotrimethoxysilane via sol-gel process to control the same IEC with neat SPFEK. The hybrid membranes are prepared by simply adding the inorganic particles into the SPFEK solution in N,N′-dimethylacetamide, followed by ultrasonic dispersion, casting and profiled temperature drying process. The morphology is examined by SEM-EDX which is applied to the top surface, bottom surface and cross-section of the hybrid membranes. The water uptake, oxidative stability, thermal property, mechanical property, proton conductivity, VO²⁺ permeability and single cell performance are investigated in detail in order to understand the relationship between morphology and property of the membranes. All the hybrid membranes show dramatically improved proton selectivity at 20 °C and 40 °C when compared with Nafion117. The VRB assembled with the SPFEK/3%SiO₂ and SPFEK/9%SiO₂ membranes exhibit higher coulombic efficiency and average discharge voltage than the VRB assembled with the SPFEK membrane at all the tested current densities.     

Novel 2D porous carbon nanosheet derived from biomass: Ultrahigh porosity and excellent performances toward V2+/V3+ redox reaction for vanadium redox flow battery

In this paper, biomass-derived carbon-based nanosheet from a kind of domestic waste (fish scales) has been successfully prepared as catalysts with high electrocatalytic activity for V²⁺/V³⁺ couple of vanadium redox flow battery (VRFB) using hydrothermal, lyophilization, and carbonization treatments. KOH is employed as template and activator, which results in nanosheet structure, ultrahigh porosity, and introduction of oxygen-containing groups. The obtained catalyst (FSC-K) exhibits the advantages of large surface area, many active sites and excellent wettability. FSC-K can effectively reduce electrochemical polarization of electrode reaction and accelerate mass transfer of active species. The cell using FSC-K demonstrates a high energy storage efficiency. Compared with pristine cell, the discharge capacity of FSC-K modified cell is 67.2 mA h at 150 mA cm⁻², increased by 18 mA h, and the corresponding energy efficiency reaches 63.8%, enhanced by 7.9%. The capacity retention of FSC-K modified cell keeps 83.1% after 50 cycles at 75 mA cm⁻², 16.7% larger than that for pristine cell. This work reveals that the electrocatalyst for V²⁺/V³⁺ couple obtained from biomass is significant for improving the comprehensive energy storage performance of VRFB.     

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.     

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.     

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.     

Preparation and characterization of a novel layer-by-layer porous composite membrane for vanadium redox flow battery (VRB) applications

By the solution casting method, a novel porous membrane has been prepared for VRB by doping sulfonated poly(fluorenyl ether ketone) (SPFEK) with imidazole, and then imidazole was washed out by extraction with solution. The proton conductivity of porous membrane increased with increasing the content of imidazole, but proton/vanadium ion (H/V) selectivity decreased. Layer-by-layer (LbL) technique was used to improve the porous membrane with high selectivity. Moreover, the performance of VRB using SPFEK-20.7imidazole-(PDDA/PSS)₈ membrane which is doped with 20.7 wt.% content of imidazole and then removed imidazole, and then deposited with eight LbL bilayers exhibits the highest columbic efficiency (CE) of 92.5% at 30 mA cm⁻².     

Investigations on physicochemical properties and electrochemical performance of graphite felt and carbon felt for iron-chromium redox flow battery

In this paper, polyacrylonitrile-based graphite felt (GF), carbon felt (CF) and the effect of thermal activation on them with or without the catalyst (BiCl₃) are comprehensively investigated for iron-chromium redox flow battery (ICRFB) application. The physical-chemical parameters of GF and CF after the thermal activation is affected significantly by their graphitization degree, oxygen functional groups, and surface area. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results manifest that GF and CF before and after the thermal activation have different electrocatalytic activities owing to oxygen functional groups number increase and the graphitization degree decrease. In terms of the capacity decay rate, as oxygen functional groups provide shorter electrocatalytic pathways than bismuth ions, the performance of GF and CF after the thermal activation is more ideal. As a result, GF before and after the thermal activation exhibits higher efficiency (EE: 86%) and better stability at a charge-discharge current density of 60 mA·cm⁻² than those of CF during charge-discharge cycling, as the dominant limitation in an ICRFB is ohmic and activation polarization. Therefore, GF after thermal activation together with the addition of BiCl₃ in the electrolyte is a more promising electrode material for ICRFBs application than CF.     

Simulation of the effects of electrode parameters on all-vanadium redox flow battery performance

以全钒液流电池为研究对象,利用电池内部传递与反应相耦合的机理模型,模拟电池二维,等温,稳态条件下,电池充电过程电极参数对电池内部极化的影响规律,包括碳毡电极的几何结构参数(厚度Lt和压缩比CR),电学特性参数(比表面积a和电导率)和操作参数(充电电流密度i)的影响.数值模拟结果给出Lt从1.5 mm增至3.5 mm,端电压仅降低3 mV;CR从0.1增至0.5,端电压降低16 mV;a从3.5×10⁴ m²/m³增至3.5×106 m²/m³,端电压降低30 mV;从18.9 S/m增至164.4 S/m,端电压降低87 mV,并给出多孔电极内部过电势在不同条件下不同的二维分布特点;i从100 mA/cm²增至150 mA/cm²,端电压增大57 mV,若同比增大比表面积a,则端电压只增大46 mV.将数值模拟与宏观实验相对比,取得良好的一致性,表明了数值模拟与分析的可靠性.通过增大CR,a,可以明显提高电池性能,为进一步提高电极材料的性能,设计电极结构参数,选择操作参数提供了重要依据.