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Abstract In this paper, carbon paper (CP) was modified by three-dimensional reduced graphene oxide (3DrGO/CP) and binders. Based on physical (field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy) and electrochemical tests (cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charging/discharging curves), the physical and chemical properties of 3GrGO/CP electrode were investigated. The physical characterization and electrochemical performance analysis indicated that the different binders significantly influenced the hydrophilicity and the wet surface area of electrode. Nitrogen doping three-dimensional reduced graphene oxide facilitates the electron transfer on electrode/electrolyte interface for both oxidation and reduction processes of 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-HO-TEMPO), provides more reaction area and reduces concentration polarization. Therefore, the modified carbon paper electrode exhibits better electrochemical activity and reversibility compared with the pristine carbon paper electrode in a methyl viologen (MV)/4-HO-TEMPO redox flow battery (RFB). Graphical abstract Image 1 Highlights • N doping 3DrGO facilitates the electron transfer on electrode/electrolyte interface for 4-HO-TEMPO redox reactions. • The different binders significantly influenced the hydrophilicity and the wet surface area of electrode. • This simple modification method can effectively improve the performance of the RFB. [ABSTRACT FROM AUTHOR]

Spherical mesoporous carbon (SMC) was prepared by water in oil emulsification combined with sol-gel polymerization and activated with carbon dioxide. The SMCs had an average particle size of 44 μm, a specific surface area of 479 m2 g−1 and with a large mesopore volume of 1.12 cm3 g−1. The slit-shaped mesopores were embedded into the wall surface of the SMCs. As electrocatalysts, the electrochemical properties of SMCs toward the anthraquinone-2-sulfonic acid (AQS) and 1,2-benzoqinone-3,5-disulfonic acid (BQDS) redox couples were examined. The SMCs facilitated the fast reversible-electrochemical reaction with the kinetic rate constant of 9.73 × 10−4 cm s−1 for BQDS, and of 4.39 × 10−3 cm s−1 for AQS. Subsequently, the electrodes of an aqueous organic flow battery were fabricated onto carbon paper and carbon cloth using SMCs as the electrocatalysts. The flow battery performance was improved by around 1.7-fold, when the SMCs had a mass loading of 2 mg cm−2 with 20 wt% Nafion, fabricated onto carbon cloth. However, there was no significant performance improvement when the SMCs were fabricated onto carbon paper. Moreover, SMCs provided the most outstanding power density over other carbon electrocatalysts (i.e., graphene, Vulcan XC-72 and Vulcan XC-72R), from 2 to 3 fold, according to the superior physical and electrochemical properties. • SMCs prepared via sol-gel polymerization with water-oil emulsification system. • SMCs had superior electrochemical properties over carbon glass up to 100 times. • SMCs greatly improved an aqueous organic flow battery performance. • Power density from SMCs was about 2–3 time better over other carbon electrocatalysts. [ABSTRACT FROM AUTHOR]

Noble metal-free bifunctional catalysts are highly desirable for both oxygen reduction and evolution reactions in air-based flow batteries, but remain a big challenge. Herein, micropores dominated nitrogen and sulfur codoped carbon (NSC) film is directly growing on conductive carbon fiber paper (CFP) as self-standing air electrode through the electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole and followed pyrolysis process. NSC film provides abundant active sites, while conductive CFP with 3D macro-networks offers affluent channels for electron and mass transfer. Benefit from the strong interaction between NSC and CFP, the resultant CFP@NSC displays highly catalytic activity for both oxygen reduction (comparable half-wave potential to commercial Pt/C) and oxygen evolution (lower overpotential than commercial IrO 2), along with robust stability. As a proof of practical usage, zinc air flow battery with CFP@NSC displays lower charge voltage and higher discharge voltage, compared with commercial Pt/C and IrO 2 coated CFP electrode. Importantly, zinc air flow battery also exhibits excellent cycling stability including nearly 100% retention of coulombic efficiency and energy efficiency over 3400 min, representing the most efficient bifunctional oxygen electrode reported so far. A self-standing electrode was designed to overcome the incompatibility between catalysts and supports in traditional air electrodes. The unitary CFP@NSC oxygen electrode prepared by the in situ growing micropores dominated nitrogen and sulfur codoped carbon film on 3D carbon fiber paper endows superior activity and excellent stability for ORR and OER in practical zinc air flow batteries. Image 1 • A self-standing electrode was designed to overcome the incompatibility between catalysts and supports. • Micropores dominated N and S codoped carbon film was in situ grown on 3D carbon fiber paper. • Prepared CFP@NSC displays highly catalytic activity and robust stability for ORR and OER. • Zinc air flow battery with CFP@NSC-900 has a neglectable performance decay during 3300 min. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries are promising energy storage devices and are already ahead of lead–acid batteries in terms of installed capacity in energy systems due to their long service life and possibility of recycling. One of the crucial tasks today is the development of models for assessing battery performance and its residual resource based on the battery's present state. A promising method for estimating battery capacity is based on analyzing present voltage and current values under various load conditions. This paper analyzes the discharge characteristics of a 10 kW all-vanadium redox flow battery at fixed load powers from 6 to 12 kW. A linear dependence of operating voltage and initial discharge voltage on load power is established. It is also determined that the slope of the discharge curve linear section does not increase linearly in absolute value, and the Box–Lucas model can be used to describe it. Models for predicting current VRFB capacity based on different curve fitting functions are proposed. These models can be used to roughly estimate battery capacity at different load powers. [ABSTRACT FROM AUTHOR]

Hydrogen-bromine (H2-Br2) redox flow batteries (RFBs) have gained a lot of interest due to their advantages in mitigating the performance shortcomings of conventional zinc-bromine and vanadium flow batteries. Various carbon materials have been tested in H2-Br2 RFBs as bromine electrodes. However, a comparative study among the different carbon materials has not been reported in the literature. This work reports, for the first time, an evaluation of carbon papers, felt and cloth in a three-electrode half-cell setup as potential bromine electrodes, in pristine and thermally treated state. A systematic evaluation was performed by comparing the surface morphologies, kinetic parameters, polarisation curves and stability tests of different carbon electrodes. Thermally treated graphite felt electrode demonstrated the best electrochemical performance as bromine electrode owing to its improved surface area, hydrophilicity and intrinsic activity. Further in-depth studies will shed important insights, which will help understand the electrode characteristics for future bromine battery design. The current study will assist in evaluating the performance of upcoming novel electrode materials in a three-electrode assembly. [ABSTRACT FROM AUTHOR]

The hydrogen‐iron (HyFe) flow cell has great potential for long‐duration energy storage by capitalizing on the advantages of both electrolyzers and flow batteries. However, its operation at high current density (high power) and over continuous cycling testing has yet to be demonstrated. In this paper, we discuss our design and demonstration of a water management strategy that supports high current and long cycling performance of a HyFe flow cell. Water molecules associated with the movement of protons from the iron electrode to the hydrogen electrode are sufficient to hydrate the membrane and electrode at a low current density of 100 mA cm−2 during the charge process. At higher charge current density, more aggressive measures must be taken to counter back‐diffusion driven by the acid concentration gradient between the iron and hydrogen electrodes. Our water management approach is based on water vapor feeding in the hydrogen electrode, and water evaporation in the iron electrode, thus enabling the high current density operation of 300 mA cm−2. [ABSTRACT FROM AUTHOR]

The operation in energy arbitrage markets is an attractive possibility to energy storage systems developers and owners to justify an investment in this sector. The size and the point of connection to the grid can have significant impact on the net revenue in transmission and distribution systems. The decision to install an energy storage system cannot be based only on the cost of the equipment but also in its potential revenue, operation costs, and depreciation through its life cycle. This paper illustrates the potential revenue of a generic energy storage system with 70% round trip efficiency and 1–14 h energy/power ratio, considering a price-taking dispatch. The breakeven overnight installed cost is also calculated to provide the cost below which energy arbitrage would have been profitable for a flow battery. The analysis of the potential revenue was performed for 13 locations within the PJM Real-time market. We considered hourly data of day-ahead and real-time locational marginal prices over 7 years (2008–2014). Breakeven installed cost per MW ranged from $30 (1 MW, 14 MWh, 2009) to $340 (1 MW, 1 MWh, 2008). [ABSTRACT FROM AUTHOR]

Redox flow batteries (RFBs) have been widely recognized in the domain of large-scale energy storage due to their simple structure, long lifetime, quick response, decoupling of capacity and power, and structural simplicity. Because of the limited open circuit voltage (OCV) by hydrogen and oxygen evolution reactions, together with the relatively low solubility of active species, RFBs with aqueous electrolytes are challenging to reach high energy densities. Researchers have been trying to develop new solvent systems without water to remove the electrochemical window limitation of water and pursue higher cell potential. However, non-aqueous solvents are also hindered by some key problems, such as high viscosity and poor safety. Meeting these challenges require a comprehensive understanding of relevant structural design parameters and multi-variable operation in the non-aqueous flow battery (NAFB) system. Modeling and simulation are not only an effective way to understand the basic mechanism of flow batteries at different scales of size and time but also an ideal tool for optimizing the reaction process, battery assembly, and the whole flow battery installation. This review paper introduces the development of the non-aqueous flow battery, the challenges it faces, and the research progress of related modeling and simulation for verification or optimization. Finally, the future development prospects of the non-aqueous flow battery model are pointed out, especially for those systems and fields that have not yet been explored. [ABSTRACT FROM AUTHOR]

In this paper, we propose a sophisticated battery model for vanadium redox flow batteries (VRFBs), which are a promising energy storage technology due to their design flexibility, low manufacturing costs on a large scale, indefinite lifetime, and recyclable electrolytes. Primarily, fluid distribution is analysed using computational fluid dynamics (CFD) considering only half-cells. Based on the analysis results, a novel model is developed in the MATLAB Simulink environment which is capable of identifying both the steady-state and dynamic characteristics of VRFBs. Unlike the majority of published studies, the inherent characteristics of the flow battery, such as shunt current, ion diffusion, and pumping energy consumption, are considered. Furthermore, simplified charge transfer resistance (CTR) is taken into account based on electrochemical impedance spectroscopy (EIS) measurement results. The accuracy of the model was determined by comparing the simulation results generated by the equivalent circuit battery model developed in this study with real datasets. The obtained results indicate that the developed model has an accuracy of 3% under the sample operating conditions selected. This study can also be used to fill the gap left by the absence of the VRFB battery model in commonly used programs for renewable energy systems, such as TRNSYS. [ABSTRACT FROM AUTHOR]

Abstract Developing cost-effective and environmental-friend electrode material is critical to accelerate the large-scale commercialization of the all-vanadium redox flow batteries. Herein, we propose and develop a twin-cocoon-derived self-standing carbon material by a green and safe preparation method. This facile degumming and carbonization process allows the twin-cocoon-derived monolithic carbon material to be decorated with nitrogen defects and oxygen functional groups. It has been demonstrated that this nitrogen-oxygen-rich monolithic carbon electrode material yields a promoted electrochemical activity for both V2+/V3+ and VO2+/VO 2 + couples, causing a 50% decrease in redox potential difference and a 192% increase in diffusion slope as compared with the commercial carbon paper, thereby improving the electrochemical reversibility and enhancing the mass transfer kinetics. It is also found that the flow battery using nitrogen-oxygen-rich monolithic carbon electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than that with carbon paper at the current density of 100 mA cm−2, promising a high-performance and low-cost all-vanadium redox flow battery. This work opens a new way to developing monolithic carbon electrode material that possesses great potential applications in flow battery and other electrochemical energy conversion and storage systems. Graphical abstract Image 1 Highlights • Twin-cocoon-derived self-standing monolithic carbon catalyst material was proposed. • Nitrogen-doped monolithic carbon contains oxygen-rich functional groups. • NO-MC carbon catalyst material promises a high-performance and low-cost VRFB. • NO-MC carbon catalyst material yields a 50% decrease in redox potential difference. [ABSTRACT FROM AUTHOR]

There has been an increase in wind power penetration into the utility grid. The wind speed variations exhibit undesirable oscillations that influence power quality and system reliability. Therefore, the grid operation is challenging in this scenario. Battery energy storage system (BESS) smooths out wind power production and reduces fluctuations. However BESSs low power density and high capacity requirements to compensate for fluctuation limit its implementation for high-power applications. This paper introduces a wind energy conversion system (WECS) based on a doubly fed induction generator (DFIG) incorporating a vanadium redox flow battery (VRFB) in the DC-link, which supports grid power smoothing and power balance even under varying wind speed. Real-site wind data was used to develop a technique for designing the VRFB. The rotor side converter (RSC) employs an off-maximum power point tracking to keep the VRFB operation within the limit and deliver consistent power to the grid. An improved phase lock loop control is designed for DFIG that suppresses DC offset at the input and allows RSC and grid side converter (GSC) to perform appropriately even under grid disturbances. The GSC control is designed with an improved DC-link voltage control by adopting an anti-windup fractional-order proportional-integral (AWFOPI) controller, which can improve transient response. A sparrow search algorithm is adopted to tune the AWFOPI controller. Matlab Simulink is used to simulate the system. Test results show that the system performs well under various operating conditions and is satisfactory. It has also been tested on the OPAL-RT real-time simulator test bench. [ABSTRACT FROM AUTHOR]

As a crucial component of redox flow batteries (RFBs), porous electrode provides active sites for redox reaction to realize electrochemical energy conversion. The coupling reaction kinetics and mass transfer process of active materials in porous electrode have great impact on the battery energy efficiency. As pore structure of porous electrode and its internal electrolyte flow regime jointly determine the convective mass transfer properties and electrochemical reaction efficiency in RFBs, based on the existing studies about the gradient porous electrode and the developed flow channel structure in recent years, this paper aims to investigate the coupling effects of gradient porous electrode and flow channel pattern on the mass transfer process in electrodes and the electrochemical performance of RFBs. A macroscopic three-dimensional model on non-aqueous iron-vanadium flow battery is established according to the actual operation characteristics of RFBs by finite element method. The numerical results reveal the operation process properties of iron-vanadium flow battery affected by the different flow field and gradient structural porous electrode, thus obtaining the optimal flow channel pattern and gradient morphology of porous electrode to improve the cell performance. [ABSTRACT FROM AUTHOR]

In this work, we present a simple and cost-effective method to form a dual-scale porous electrode by KOH activation of the fibers of carbon papers. The large pores (∼10 μm), formed between carbon fibers, serve as the macroscopic pathways for high electrolyte flow rates, while the small pores (∼5 nm), formed on carbon fiber surfaces, act as active sites for rapid electrochemical reactions. It is shown that the Brunauer-Emmett-Teller specific surface area of the carbon paper is increased by a factor of 16 while maintaining the same hydraulic permeability as that of the original carbon paper electrode. We then apply the dual-scale electrode to a vanadium redox flow battery (VRFB) and demonstrate an energy efficiency ranging from 82% to 88% at current densities of 200–400 mA cm −2 , which is record breaking as the highest performance of VRFB in the open literature. [ABSTRACT FROM AUTHOR]

The Br/H redox flow cell shows promise as a high-power, low-cost energy storage device. The effect of various aspects of material selection, processing, and assembly of electrodes on the operation, performance, and efficiency of the system is determined. In particular, (+) electrode thickness, cell compression, hydrogen pressure, and (−) electrode architecture are investigated. Increasing hydrogen pressure and depositing the (−) catalyst layer on the membrane instead of on the carbon paper backing layers have a large positive impact on performance, enabling a limiting current density above 2 A cm and a peak power density of 1.4 W cm. Maximum energy efficiency of 79 % is achieved. In addition, the root cause of limiting-current behavior in this system is elucidated, where it is found that Br reversibly adsorbs at the Pt (−) electrode for potentials exceeding a critical value, and the extent of Br coverage is potential-dependent. This phenomenon limits maximum cell current density and must be addressed in system modeling and design. These findings are expected to lower system cost and enable higher efficiency. [ABSTRACT FROM AUTHOR]

Abstract: The soluble lead flow battery (SLFB) is conventionally configured with an undivided cell chamber. This is possible, unlike other flow batteries, because both electrode active materials are electroplated as solids from a common species, Pb2+, on the electrode surfaces during charging. Physically separating the active materials has the advantage that a single electrolyte and pump circuit can be used; however, failure mechanisms such as electrical shorting may be observed. In addition, a common electrolyte requires that any electrolyte additives are compatible with both half-cell reactions. This paper introduces two new configurations; semi- and fully divided for the SLFB. Cationic, anionic, and microporous separators are assessed for ionic conductivity in SLFB electrolytes, showing that their incorporation adds as little as a 20 mV to the cell voltage. Voltammetry shows the effect of additives on the equilibrium potential and stripping overpotential of PbO2. It is then demonstrated that the incorporation of a separator into the SLFB can reduce failure due to electrical shorting and permit electrode-specific additives to be used. A unit flow cell with electrode area of 100 cm2 is shown to operate for over 300 Ah in the semi-divided configuration, more than doubling the previously reported cycle life for cells of similar size.Graphical Abstract: [ABSTRACT FROM AUTHOR]

Acid‐base flow batteries (ABFB) are a promising and environmentally benign class of flow batteries that utilize neutralization energy. Among the other flow batteries, ABFBs stand out with low cost and high solubility of the electrolytes and the possibility to harvest neutralization energy of acidic and alkaline wastewaters. However, the main ABFB issues, such as low power caused by discharge current limitation and low energy density, are limiting the possibility of their implementation. In this work, a novel two‐membrane ABFB with two hydrogen electrodes was developed to overcome main ABFB issues. The proposed concept demonstrated high power density up to 6.1 mW cm−2 at 13 mA cm−2. It was shown that battery performance was greatly limited by negative electrode overvoltage. Analysis of the voltage losses allowed to estimate main power losses and highlight the possible ways to its minimization. [ABSTRACT FROM AUTHOR]

To integrate large-scale renewable energy into energy systems, an effective participation from private investors and active customer engagement are essential. Virtual power plants (VPPs) are a very promising approach. To realize this engagement, an efficient monitoring and control system needs to be implemented for the VPP to be flexible, scalable, secure, and cost-effective. In this paper, a realistic VPP in Western Australia is studied, comprising 67 dwellings, including a 810 kW rooftop solar photovoltaic (PV) system, a 700 kWh vanadium redox flow battery (VRFB), a heat pump hot water system (HWS), an electric vehicle (EV) charging station, and demand management mechanisms. The practical and detailed concept design of the monitoring and control system for EEBUS-enabled appliances, and also for the PV and VRFB system, with smart inverters, is proposed. In addition, a practical fog-based storage and computing system is developed to enable the VPP owner to manage the PV, VRFB, and EV charging station for maximizing the benefit to the customers and the VPP owner. Further, the proposed cloud-based applications enable customers to participate in gamified demand response programs for increasing the level of their engagement while satisfying their comfort level. All proposed systems and architecture in this paper have the capability of being implemented fully and relevant references for practical devices are given where necessary. [ABSTRACT FROM AUTHOR]

There are many advantages to utilize storages in electric power system. Peak shaving, load leveling, load frequency control, integration of renewable, energy trading and spinning reserve are the most important of them. Batteries, especially redox flow batteries, are one of the appropriate storages for utilization in distribution network. This paper presents a novel, heuristic and practical method for optimal scheduling in distribution network with flow battery storage. This heuristic method is more suitable for scheduling and operation of distribution networks which require installation of storages. Peak shaving and load leveling is considered as the main objective in this paper. Several indices are presented in this paper for determine the place of storages and also scheduling for optimal use of energy in them. Simulations of this paper are based on real information of distribution network substation that located in Semnan, Iran. [ABSTRACT FROM AUTHOR]

This paper develops a general model with which to evaluate flow uniformity and pressure drop within interdigitated-channel structures, especially in the context of redox flow batteries. The governing equations are cast in dimensionless variables, leading to a set of characteristic dimensionless parameter groups. The systems of governing equations are solved computationally, with the results presented graphically. Because the results are general, the underlying model itself is not needed to apply the quantitative design guidelines. However, the paper presents and discusses all the information required to recreate the model as needed. [ABSTRACT FROM AUTHOR]

This paper presents a review of methods for assessing the performance of all-Vanadium Flow Batteries (VFBs), derived from the experience deduced on laboratory kW-class VFB facilities. The main factors affecting the performance of VFBs, namely thermal, hydraulic, electric, and energetic, are identified and the main strategies for assessing them are discussed. An analysis of the different loss terms involved in large VFBs is presented. Thereafter criteria and methods for the unambiguous evaluation and comparison of the performance of these devices are proposed. Capacity fades are also considered, their causes are analyzed and measures to counteract them are presented. The state of art of flow batteries international standard and regulation is also reported. The final aim of the paper is contributing to the development of metrics for the industrialization of VFBs. • An overview of methods for evaluating industrial Vanadium Flow Batteries is given. • Thermal, hydraulic, electric, and efficiency tests are described. • The differences between testing flow batteries single cells and stacks are explained. • Regulation and standards are also presented. • These topics play a role in the industrializations and marketing of flow batteries. [ABSTRACT FROM AUTHOR]

We describe a ZnBr2 single cell which has a highly modular symmetrical structure. With designed polyethylene shell frames, membrane frame and composite titanium-carbon felt electrodes, it has a higher energy density and is more flexible compared with traditional flow batteries. We repeatedly tested its performance, which showed good tightness, high reliability and a high energy efficiency of 75%. Due to the special symmetrical structure and modular design, it is easy to assemble and disassemble, which makes it suitable as a test platform for electrodes, membranes and electrolyte performance testing. The designed modular flow cell has low cost and high energy density, and can provide good guidance for flow battery research. [ABSTRACT FROM AUTHOR]

Hydrogen-bromate flow battery is a promising hybrid current source for air-deficient environment that functions by electrocatalyzed reactions of hydrogen oxidation and aqueous LiBrO3 reduction. The flow cell consists of porous carbonaceous cathode, platinum catalyzed hydrogen oxidation gas diffusion anode, and separating proton exchange membrane. Performance of the hydrogen-bromate flow battery single cell has been optimized by varying the catholyte feed rate, LiBrO3 concentration in catholyte, hydrogen pressure, membrane thickness, amount of porous carbon at cathode, and Pt loading at anode. Shape of the I-V curve is characterized by a sharp maximum of current, which indicates passivation of one of the electrodes. Combination of conventional reference electrode and home-made thin-film Luggin capillary has been used to monitor separately the polarizations of both flow cell electrodes. Poisoning of platinum hydrogen oxidation electrocatalyst by bromine species, which permeated the membrane, is shown as a major source of performance losses of hydrogen-bromate flow battery at high power density. Hypothesis supported by experiments claims that the degree of the platinum electrocatalyst poisoning is determined by the balance between the rates of the bromine species supply to anode and their removal by liquid water that permeates the membrane. Use of thinner proton exchange membrane and thinner carbonaceous cathode is a prerequisite to achieving high power density of the cell at high current efficiency of the cathode process. At 40 °C, area-specific power reaches 0.74 W cm−2 at the level of catholyte utilization equal to 0.93. [ABSTRACT FROM AUTHOR]

Aluminum-air batteries (AAB) are regarded as one of the most promising beyond-lithium high-energy-density storage candidates. This paper introduces a three-dimensional (3D) Al 7075 anode enabled by femtosecond laser and friction-stir process which, along with a special double-face anode architecture provides world-class performance. Electrochemical characterizations prove that the corrosion resistance of the modified 3D Al 7075 FSP anode was enhanced, and electrochemically active surface area (ECSA) was increased compared with that of normal Al 7075 anode. Friction-stir processing reduced the mean grain size from 30 μm to 3 μm. The discharge performance of 3D Al 7075 FSP anode is shown to be quite stable, and the average values of energy density are significantly increased from 2256 mWh g−1 to 2941 mWh g−1 at 100 mA cm−2. In a double-face flowing Al-air battery system, the 3D Al 7075 FSP anode exhibited significantly better electrocatalytic performance (discharge voltage of 0.76 V at 400 mA cm−2, and power density of 337.8 mW cm−2) than that of a commercial Al 7075 anode. • Al-air batteries based on three-dimensional (3D) structured Al anodes. • Periodic grooving for enhanced electrochemically active surface area (ECSA). • Friction stirring for grain refinement and improved corrosion resistance. [ABSTRACT FROM AUTHOR]

• A variable cross-section electrode structure was proposed. • Discharge voltage was improved by 2.35% by the variable coss-section electrode. • The uniformity factor was promoted 14.24% by the variable coss-section design. • The variable coss-section electrode#1 was the best design. The global warming can be relieved by the application of the renewable energy, which should be supported by the large-scale energy storage such as the vanadium redox flow battery (VRFB). The charge-discharge reactions mainly take place in the porous electrodes, which can influence the battery performance. In the paper, a variable cross-section electrode with interdigital flow field was proposed to enhance the mass transfer in the porous electrode. The charge-discharge voltage, overpotential, concentration distribution and uniformity factors are predicted by a 3-D numerical model to evaluate the battery performance of different electrodes (the Conventional electrode; Variable coss-section electrode #1: the variable coss-section electrode with the interdigital flow field distributed on one side; Variable coss-section electrode #2: the variable coss-section electrode with the interdigital flow field distributed on both sides). The discharging voltage of the variable coss-section electrode #2 was 1.25% higher than that of the variable coss-section electrode #1. The uniformity factor of the variable coss-section electrode #1 and the variable coss-section electrode #2 was 14.24% and 19.56% higher than that of the conventional electrode, respectively. Considering the machining process and promotion of the battery performance, the variable coss-section electrode#1 was the best design of the three battery models mentioned in this article. [ABSTRACT FROM AUTHOR]

To integrate large-scale renewable energy into energy systems, an effective participation from private investors and active customer engagement are essential. Virtual power plants (VPPs) are a very promising approach. To realize this engagement, an efficient monitoring and control system needs to be implemented for the VPP to be flexible, scalable, secure, and cost-effective. In this paper, a realistic VPP in Western Australia is studied, comprising 67 dwellings, including a 810 kW rooftop solar photovoltaic (PV) system, a 700 kWh vanadium redox flow battery (VRFB), a heat pump hot water system (HWS), an electric vehicle (EV) charging station, and demand management mechanisms. The practical and detailed concept design of the monitoring and control system for EEBUS-enabled appliances, and also for the PV and VRFB system, with smart inverters, is proposed. In addition, a practical fog-based storage and computing system is developed to enable the VPP owner to manage the PV, VRFB, and EV charging station for maximizing the benefit to the customers and the VPP owner. Further, the proposed cloud-based applications enable customers to participate in gamified demand response programs for increasing the level of their engagement while satisfying their comfort level. All proposed systems and architecture in this paper have the capability of being implemented fully and relevant references for practical devices are given where necessary.

Vanadium redox flow batteries are promising energy storage devices and are already ahead of lead–acid batteries in terms of installed capacity in energy systems due to their long service life and possibility of recycling. One of the crucial tasks today is the development of models for assessing battery performance and its residual resource based on the battery’s present state. A promising method for estimating battery capacity is based on analyzing present voltage and current values under various load conditions. This paper analyzes the discharge characteristics of a 10 kW all-vanadium redox flow battery at fixed load powers from 6 to 12 kW. A linear dependence of operating voltage and initial discharge voltage on load power is established. It is also determined that the slope of the discharge curve linear section does not increase linearly in absolute value, and the Box–Lucas model can be used to describe it. Models for predicting current VRFB capacity based on different curve fitting functions are proposed. These models can be used to roughly estimate battery capacity at different load powers.

Abstract Solvent treatment has been proved to be a very simple and effective method to prepare high-performance porous membranes for flow batteries. However, the initial morphology and the properties of membrane materials will exert pivotal effects on the resultant performance of porous membranes. In this paper, according to comparing the differences in the morphology and performance of untreated and treated porous poly(ether sulfone) (PES) membranes with different porous structures, the influence of the initial morphology of membranes was clarified in detail. Moreover, the differences in the morphology and performance of untreated and treated porous poly(vinylidene fluoride) (PVDF), polusulfone (PSF) and poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes were also investigated. By comparing various properties of PES, PVDF, PSF and PVDF-HFP polymers, the importance and criteria for selecting proper membrane materials when using the solvent treatment method were clarified. Furthermore, the VFB assembled with the resultant membrane could achieve an energy efficiency of 82.93% at 80 mA cm−2, which could be comparable to that of the commercial Nafion 115. Therefore, this article is beneficial for designing high-performance porous membranes for flow batteries. Graphical abstract fx1 Highlights • The membrane formation mechanism was investigated for flow battery application. • The membrane morphology was tuned by solvent treatment. • The factors of initial morphology and polymer properties were studied in detail. • A controlled performance can be achieved by changing preparation parameters. [ABSTRACT FROM AUTHOR]

Vanadium redox flow batteries are gaining great popularity in the world due to their long service life, simple (from a technological point of view) capacity increase and overload resistance, which hardly affects the service life. However, these batteries have technical problems, namely in balancing stacks with each other in terms of volumetric flow rate of electrolyte. Stack power depends on the speed of the electrolyte flow through the stack. Stacks are connected in parallel by electrolytes to increase battery power. If one of the stacks has a lower hydrodynamic resistance, the volume of electrolytes passing through it increases, which leads to a decrease in the efficiency of the remaining stacks in the system. This experimental study was conducted on a 10 kW uninterruptible power supply system based on two 5 kW stacks of all-vanadium redox flow batteries. It was demonstrated that forced flow attenuation in a circuit with low hydrodynamic resistance leads to an overall improvement in the system operation. [ABSTRACT FROM AUTHOR]

Highlights • A transient model of the VRB system is developed. • The model is used for dynamic control for electrolyte flow rate of VRB. • Flow rate is optimized under varying (dis-)charge power and SoC conditions. • Concentration discrepancy of active species in the stack and tank is considered. • Relatively low electrolyte flow rates are preferred with high pressure drops. Abstract The vanadium redox flow battery (VRB) is considered to be one of the most promising technologies for large-scale energy storage, with the electrolyte flow rate capable of significantly affecting the mass transfer, temperature rise, and pump power losses of the VRB system. Although the flow-rate optimization under constant current has been addressed in the literature, few studies have investigated the control strategy for the electrolyte flow rate under varying (dis-)charge power that is common in practical applications. Moreover, fewer studies have considered the concentration discrepancy of the active species in the tank and stack in the flow-rate optimization. In this paper, the electrolyte flow-rate optimization is investigated by incorporating the influences of the flow rate on the mass transfer, temperature rise, and required pump power. A transient model of the VRB system is developed to derive the total power losses (by which the overall system energy efficiency is determined; include losses resulting from overpotentials, ohmic drops, and required pump power) as a function of the applied current, concentration of the active species in the stack, and flow rate of the electrolyte. Based on this model, a dynamic flow-rate control strategy is proposed for determining the optimal flow rate under varying (dis-)charge power and state-of-charge conditions. The simulation results show that the proposed control strategy can deliver a high VRB system efficiency of 87.7%, and manage the electrolyte temperature to the safe range during mild summer days. [ABSTRACT FROM AUTHOR]

Air-based flow batteries have triggered huge interests as promising large-scale energy storage devices due to their high energy density and extremely low cost. However, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) lower their energy conversion efficiencies. Herein, we in situ grow amorphous NiS x -FeO y particles on sulfur doped carbon fiber paper (termed as NiS x -FeO y /SCFP) as practical self-supported oxygen electrode through a simple and economic synthesis route. Benefiting from the synergetic effects of amorphous NiS x -FeO y and SCFP, NiS x -FeO y /SCFP possesses massively accessible active sites, enhanced electrons and reactants transfer, and stable integrated structure, exhibiting a low overpotential (0.37 V) at 10 mA cm −2 for OER, a positive half-wave potential for ORR and a small potential gap (0.82 V). Zinc air flow battery assembled with newly developed NiS x -FeO y /SCFP shows a coulombic efficiency of 100% and a voltage efficiency of 50% at the charge-discharge current of 10 mA cm −2 , and vigorously operates 110 h without performance decay, showing highly catalytic activity and super cycling stability, which are among the best performance ever reported. We believe that this work will open up new ways for fabricating practical oxygen electrode for metal air based batteries applications. [ABSTRACT FROM AUTHOR]

The intervention of renewable energy for curbing the supply demand mismatch in power grids has projected the added advantage of having lower greenhouse gas (GHG) emissions. Non-depleting sources are characterised by variability and unpredictability. This necessitates the adequate design and sizing of Energy Storage Devices (ESD). This study focusses on life cycle study of three different types of storage devices, Valve Regulated Lead Acid Battery (LAB), Lithium Iron Phosphate (LFP-G) Battery and Polysulphide Bromine Flow Battery (PSB). It has been concluded that the PV-VRLA system has an Energy Pay Back Time (EPBT) of 4.3 years, PV-LFP-G system having 4.56 years and PV-PSB system with a value of 8.2 years. The environmental impact of the systems is measured by the GHG emission factor expressed in kgCO2eq/kWh of energy generated. The PV-PSB system has the highest value owing to the material production and operating energy component, the values being 0.321, 0.343 and 0.70 kgCO2eq/kWh for the LAB, LFP-G and PSB, respectively. The impact of the generation mix for the present, Business as Usual (BAU) and a future Renewable Energy Intensive has also been studied. It has been concluded that emission metrics of the PSB system is more sensitive to generation mix characteristics. [ABSTRACT FROM AUTHOR]

The soluble lead flow battery utilises the Pb/Pb 2+ and Pb 2+ /Pb 4+ redox couples. The electrolyte is methanesulfonic acid, in which Pb 2+ species are soluble, up to 2.6 mol dm −3 . Previous publications have presented data demonstrating differing performances for the electrode and cell reactions. In this paper, electrolyte properties including density, viscosity, ionic conductivity and species concentration are systematically investigated to identify their impact on the efficiency and cycle life of a soluble lead cell under static conditions. The relationship between ionic conductivity and species concentration (Pb 2+ and methanesulfonic acid) in the starting electrolyte is shown to be key to cell performance. An electrolyte initially containing 0.7 mol dm −3 Pb(CH 3 SO 3 ) 2 & 1.0 mol dm −3 CH 3 SO 3 H is shown to provide optimal electrochemical performance for the soluble lead cell, achieving charge and voltage efficiencies of greater than 80% and 70% respectively along with Pb 2+ utilisation of over 80%. [ABSTRACT FROM AUTHOR]

Energy storage has become an absolute necessity for the growth of renewable power systems today. Vanadium Redox Battery is rapidly gaining popularity in integrated hybrid renewable power systems due to its high life cycle count, modularity and flexible capacity. This paper puts forth an electrical model of a vanadium battery to study its operation while integrated with a standalone photovoltaic power source. The model includes evaluation of cell stack voltages and the state of charge of the storage capacity. A simple energy management strategy is included by modeling a charge controller to avoid any instances of overcharging/discharging. The solar panel is modeled using solar cells which act as the primary source to supply a purely resistive domestic load. The entire work is simulated in a Matlab/Simulink environment and the results investigated for constant and varying irradiation cases. The results obtained prove efficient operation of the battery as per requirements. [ABSTRACT FROM AUTHOR]

The capital cost of conventional redox flow batteries is relatively high (>USD$ 200/kWh) due to the use of expensive active materials and ion-exchange membranes. This paper presents a membrane-less hybrid organic-inorganic flow battery based on the low-cost elements zinc (92.7% with the use of carbon felt electrodes. In the presence of a fully oxidized active species close to its solubility limit, dissolution of the deposited anode is relatively slow (<2.37 g h −1 cm −2 ) with an equivalent corrosion current density of <1.9 mA cm −2 . In a parallel plate flow configuration, the resulting battery was charge-discharge cycled at 30 mA cm −2 with average coulombic and energy efficiencies of c.a. 71.8 and c.a. 42.0% over 20 cycles, respectively. [ABSTRACT FROM AUTHOR]

To transition toward sustainable energy systems, next generation power sources must provide high power density at minimum cost. Using inexpensive and environmentally friendly fabrication methods, this work describes a room temperature electrochemical flow cell with a maximum power density of 2.01 W cm −2 or 13.4 W cm −3 . In part, this is achieved by minimizing ohmic resistance through decreased electrode spacing, implementation of current collectors and improvement of electrolyte conductivity. The majority of the performance gain is provided by a novel in operando dynamic flowing deposition method for which the cell design has been optimized. Carbon nanotubes (CNTs) are deposited dynamically at the entrance of and within the carbon paper electrodes during operation of the cell. A natural equilibrium is reached between deposition and detachment of CNTs at which the electrochemical surface area and pore size distribution of the flow-through porous electrodes are greatly enhanced. In this way, the novel deposition method more than doubles the power density of the cell and sets a new performance benchmark for what is practically attainable with aqueous electrochemical flow cells. Overall, it is expected that the design and operation methods illustrated here will enable a wide range of electrochemical flow cell technologies to achieve optimal performance. [ABSTRACT FROM AUTHOR]

A network of conveniently located fast charging stations is one of the possibilities to facilitate the adoption of Electric Vehicles (EVs). This paper assesses the use of fast charging stations for EVs in conjunction with VRFBs (Vanadium Redox Flow Batteries). These batteries are charged during low electricity demand periods and then supply electricity for the fast charging of EVs during day, thus implementing a power peak shaving process. Flow batteries have unique characteristics which make them especially attractive when compared with conventional batteries, such as their ability to decouple rated power from rated capacity, as well as their greater design flexibility and nearly unlimited life. Moreover, their liquid nature allows their installation inside deactivated underground gas tanks located at gas stations, enabling a smooth transition of gas stations' business model towards the emerging electric mobility paradigm. A project of a VRFB system to fast charge EVs taking advantage of existing gas stations infrastructures is presented. An energy and cost analysis of this concept is performed, which shows that, for the conditions tested, the project is technologically and economically viable, although being highly sensitive to the investment costs and to the electricity market conditions. [ABSTRACT FROM AUTHOR]

Redox flow batteries are one of the most relevant emerging large-scale energy storage technologies. Developing control methods for them is an open research topic; optimizing their operation is the main objective to be achieved. In this paper, a strategy that is based on regulating the output voltage is proposed. The proposed architecture reduces the number of required sensors. A rigorous design methodology that is based on linear H ∞ synthesis is introduced. Finally, some simulations are presented in order to analyse the performance of the proposed control system. The results show that the obtained controller guaranties robust stability and performance, thus allowing the battery to operate over a wide range of operating conditions. Attending to the design specifications, the controlled voltage follows the reference with great accuracy and it quickly rejects the effect of sudden current changes. [ABSTRACT FROM AUTHOR]

This paper presents a literature review about the concept of redox flow batteries and its automation and monitoring. Specifically, it is focused on the presentation of all-vanadium redox flow batteries which have several benefits, compared with other existing technologies and methods for energy stored purposes. The main aspects that are reviewed in this work correspond to the characterization, modeling, supervision and control of the vanadium redox flow batteries. A research is presented where redox flow batteries are contextualized in the current energy situation, compared with other types of energy storage systems. Furthermore, a presentation about the current challenges on research, and the main existing installations is view. A discussion about the main dynamic models that have been proposed during last years, as well as the different control strategies and observers, is presented. [ABSTRACT FROM AUTHOR]

Achieving the renewable energy integration target will require the extensive engagement of consumers and the private sector in investment and operation of renewable-based energy systems. Virtual power plants are an efficient way to implement this engagement. In this paper, the detailed costs and benefits of implementing a realistic virtual power plant (VPP) in Western Australia, comprising 67 dwellings, are calculated. The VPP is designed to integrate and coordinate rooftop solar photovoltaic panels (PV), vanadium redox flow batteries (VRFB), heat pump hot water systems (HWSs), and demand management mechanisms. An 810-kW rooftop solar PV system is designed and located using the HelioScope software. The charging and the discharging of a 700-kWh VRFB are scheduled for everyday use over a year using an optimization algorithm, to maximize the benefit of it for the VPP owners and for the residents. The use of heat pump HWSs provides a unique opportunity for the residents to save energy and reduce the total cost of electricity along with demand management on some appliances. The cost-and-benefit analysis shows that the cost of energy will be reduced by 24% per dwelling in the context of the VPP. Moreover, the internal rate of return for the VPP owner is at least 11% with a payback period of about 8.5 years, which is a promising financial outcome. [ABSTRACT FROM AUTHOR]

In the present work, a flexible carbon sponge is experimentally characterized and proposed as an alternative electrode for advanced vanadium redox flow batteries. Such an electrode is prepared via directly carbonizing the commercially‐available and inexpensive melamine formaldehyde resin sponge in argon, to inherit the well‐defined and three‐dimensional bi‐continuous architecture of the melamine sponge with 99.6% porosity and 40 μm average pore size. By applying the carbon sponges as the electrodes, it is demonstrated that the vanadium flow battery at 200 mA cm−2 can yield an energy efficiency of 77.9%, significantly higher than that with commonly‐used graphite felt electrodes (72.9%). After a thermal treatment in air, the energy efficiency of carbon sponge can further be improved to 81.2% at mA cm−2 due to introduction of favorable oxygen containing functionalities. The operating stability with the carbon sponge is proven by a 200 cycling test with minor efficiency decay. [ABSTRACT FROM AUTHOR]

In this work, a high-performance porous electrode, made of KOH-activated carbon-cloth, is developed for vanadium redox flow batteries (VRFBs). The macro-scale porous structure in the carbon cloth formed by weaving the carbon fibers in an ordered manner offers a low tortuosity (∼1.1) and a broad pore distribution from 5 μm to 100 μm, rendering the electrode a high hydraulic permeability and high effective ionic conductivity, which are beneficial for the electrolyte flow and ion transport through the porous electrode. The use of KOH activation method to create nano-scale pores on the carbon-fiber surfaces leads to a significant increase in the surface area for redox reactions from 2.39 m 2 g −1 to 15.4 m 2 g −1 . The battery assembled with the present electrode delivers an energy efficiency of 80.1% and an electrolyte utilization of 74.6% at a current density of 400 mA cm −2 , as opposed to an electrolyte utilization of 61.1% achieved by using a conventional carbon-paper electrode. Such a high performance is mainly attributed to the combination of the excellent mass/ion transport properties and the high surface area rendered by the present electrode. It is suggested that the KOH-activated carbon-cloth electrode is a promising candidate in redox flow batteries. [ABSTRACT FROM AUTHOR]

Flowable slurry electrodes have received increased interest recently for use in electrochemical energy storage and water treatment systems. In an electrochemical cell these flowing electrodes can simultaneously support both faradaic currents (through reactions occurring at the slurry/solution interface) and nonfaradaic currents (through charging of the double layer capacitance of each slurry particle as it passes through the cell). In this paper, a model based on a three-dimensional set of macrohomogeneous equations that combines the effects of both types of current is developed. The model equations are applicable to flow battery, electrochemical flow capacitor, and flow-electrode capacitive deionization systems. Using this modeling approach, the performance of slurry electrodes in symmetric electrochemical cells is examined. Scaling techniques are applied to the equations in order to permit the prediction of the steady-state current that can be achieved from a slurry electrode as a function of cell dimensions, slurry properties, flow rate, reaction kinetics, and the potential applied across the cell. Depending on the values of four dimensionless combinations of the pertinent variables (the capacitive Graetz number, the nondimensional exchange current density, the conductivity ratio, and the flow behavior index) slurry electrodes are then shown to operate in one of four distinct operational regimes. Lastly, the variation of the physical extent of the reaction zone (i.e. electroactive zone extension) with respect to the relative magnitude of advection is characterized. [ABSTRACT FROM AUTHOR]

The increased number of renewable power plants pose threat to power system balance. Their intermittent nature makes it very difficult to predict power output, thus either additional reserve power plants or new storage and control technologies are required. Traditional spinning reserve cannot fully compensate sudden changes in renewable energy power generation. Using new storage technologies such as flow batteries, it is feasible to balance the variations in power and voltage within very short period of time. This paper summarises the controlled use of hybrid flow battery, thermal and hydro power plant system, to support wind power plants to reach near perfect balance, i.e. make the total power output as close as possible to the predicted value. It also investigates the possibility of such technology to take part in the balance of the Lithuanian power system. A dynamic model of flow battery is demonstrated where it evaluates the main parameters such as power, energy, reaction time and efficiency. The required battery size is tested based on range of thermal and hydro power plant reaction times. This work suggests that power and energy of a reasonable size flow battery is sufficient to correct the load and wind power imbalance. [ABSTRACT FROM AUTHOR]

This paper presents the performance of a vanadium–cerium redox flow battery using conventional and zero-gap serpentine architectures. Mixed-acid solutions based on methanesulfonate-sulfate anions (molar ratio 3:1) are used to enhance the solubilities of the vanadium (>2.0 mol dm −3 ) and cerium species (>0.8 mol dm −3 ), thus achieving an energy density ( c.a. 28 Wh dm −3 ) comparable to that of conventional all-vanadium redox flow batteries (20–30 Wh dm −3 ). Electrochemical studies, including cyclic voltammetry and galvanostatic cycling, show that both vanadium and cerium active species are suitable for energy storage applications in these electrolytes. To take advantage of the high open-circuit voltage (1.78 V), improved mass transport and reduced internal resistance are facilitated by the use of zero-gap flow field architecture, which yields a power density output of the battery of up to 370 mW cm −2 at a state-of-charge of 50%. In a charge–discharge cycle at 200 mA cm −2 , the vanadium–cerium redox flow battery with the zero-gap architecture is observed to discharge at a cell voltage of c.a. 1.35 V with a coulombic efficiency of up to 78%. [ABSTRACT FROM AUTHOR]

The vanadium redox flow battery (VRFB) is a promising grid-scale energy storage technology, but future widespread commercialization requires a considerable reduction in capital costs. Determining the appropriate battery size for the intended power range can help minimize the amount of materials needed, thereby reducing capital costs. A physics-based model is an essential tool for predicting the power range of large scale VRFB systems to aid in the design optimization process. This paper presents a modeling framework that accounts for the effects of flow rate on the pumping losses, local mass transfer rate, and nonuniform vanadium concentration in the cell. The resulting low-order model captures battery performance accurately even at high power densities and remains computationally practical for stack-level optimization and control purposes. We first use the model to devise an optimal control strategy that maximizes battery life during discharge. Assuming optimal control is implemented, we then determine the upper efficiency limits of a given VRFB system and compare the net power and associated overpotential and pumping losses at different operating points. We also investigate the effects of varying the electrode porosity, stack temperature, and total vanadium concentration on the peak power. [ABSTRACT FROM AUTHOR]

Nanofluid electrodes with high loading of active solid materials have significant potential as high energy density flow battery electrolytes; however, two key criteria need to be met: they must have a manageable viscosity for pumping and simultaneously exhibit good electrochemical activity. A typical dispersion of nickel hydroxide nanoparticles (~100 nm) is limited to 5–10 wt.% of solids, above which it has a paste-like consistency, incompatible with flow applications. We report on the successful formulation of stable dispersions of a nano-scale nickel hydroxide cathode (β-Ni(OH)2) with up to 60 wt.% of solids and low viscosity (32 cP at 25 °C), utilizing a surface graft of small organic molecules. The fraction of grafting moiety is less than 3 wt.% of the nanoparticle weight, and its presence is crucial for the colloidal stability and low viscosity of suspensions. Electrochemical testing of the pristine and modified β-Ni(OH)2 nanoparticles in the form of solid casted electrodes were found to be comparable with the latter exhibiting a maximum discharge capacity of ~237 mAh/g over 50 consecutive charge–discharge cycles, close to the theoretical capacity of 289 mAh/g. [ABSTRACT FROM AUTHOR]

Stationary energy storage methods such as flow batteries are one of the best options to integrate with smart power grids. Though electrochemical energy storage using flow battery technologies has been successfully demonstrated since the 1970s, the introduction of ionic liquids into the field of energy storage introduces new dimensions in this field. This reliable energy storage technology can provide significantly more flexibility when incorporated with the synergic effects of ionic liquids. This mini-review enumerates the present trends in redox flow battery designs and the use of ionic liquids as electrolytes, membranes, redox couples, etc. explored in these designs. This review specifically intends to provide an overview of the research prospects of ionic liquids for redox flow batteries (RFB). [ABSTRACT FROM AUTHOR]