Flow Battery Research Articles

Optimal Selection for Redox Couples and Enhanced Performance through Magnetic Nanofluid Electrolyte in Solar Flow Batteries

The limited photoelectric conversion efficiency poses one of the critical constraints on commercializing solar flow batteries (SFBs). This study compares the chemical and photoelectrochemical properties of three commonly used redox couples. Additionally, magnetic Fe3O4 nanoparticles, for the first time, are introduced to optimize the electrolyte, and they are compared with the original electrolyte. Across different redox couples, the variations in semiconductor flat-band potentials and carrier concentrations result in changes in photoelectric current density. Notably, FeCl2/FeCl3 redox coupled with TiO2 photoelectrodes exhibits the highest photoelectric current density, reaching 75.7 µA cm−2. However, the trade-off of this electrolyte, i.e., providing high photocurrent while being unable to supply sufficient open-circuit voltage, imposes limitations on the practical application of SFBs. Alternatively, for TEMPO and 4-OH-TEMPO electrolytes, which can provide a higher open-circuit voltage, the electrochemical activity is enhanced, and the solution ohmic resistance is reduced by introducing magnetic nanoparticles to form a magnetic nanofluid. As a result, the photoanode’s photocurrent density increases by 36.6% and 17.0%, respectively, in the two electrolytes. The work reported here effectively addresses the current issue of low photocurrent density in SFBs and presents new optimization strategies for SFBs.

Surface Electroactive Sites of Tungstated Zirconia Catalysts for Vanadium Redox Flow Batteries.

Surface electroactive sites for tungstate zirconia (WZ) were created by utilizing tungstate-immobilized UiO-66 as precursors via a double-solvent impregnation method under a mild calcination temperature. The WZ-22-650 catalyst, containing a moderate W content (22%), demonstrated a high density of surface electroactive sites. Proper heat treatment facilitated the binding of oligomeric tungsten clusters to stabilized tetragonal ZrO2, resulting in improved catalytic performance toward the VO2+/VO2+ redox couples compared to other tested samples. The substantial surface area, mesoporous structure, and establishment of new W-O-Zr bonds affirm the firm anchoring of WOx to ZrO2. This robust attachment enhances surface electroactive sites, elevating the electrochemical performance of vanadium redox flow batteries (VRFBs). Charge-discharge tests further demonstrate that the superior voltage efficiency (VE) and energy efficiency (EE) for VRFBs using the WZ-22-650 catalyst are 87.76 and 83.94% at 80 mA cm-2, which are 13.42% VE and 10.88% EE better than heat-treated graphite felt, respectively. Even at a higher current density of 160 mA cm-2, VRFBs utilizing the WZ-22-650 catalyst maintained considerable efficiency, recording VE and EE values of 76.76 and 74.86%, respectively. This facile synthesis method resulted in WZ catalysts displaying superior catalytic activity and excellent cyclability, offering a promising avenue for the development of metal-oxide-based catalysts.

ZSM‐5 zeolite incorporated flow battery membranes with regulated pore channels for high proton conduction

AbstractFast proton conduction plays an essential role in ion conductive membranes (ICMs) for energy conversion and storage technologies, but the demand for low reactive species permeation always limits further enhancement in proton transport rate. Herein, an ICM incorporating ZSM‐5 zeolite with optimized physicochemical property of pore channels was developed to achieve highly conductive ion transport via regulating the silicon‐aluminum atomic (Si/Al) ratio of ZSM‐5. Benefiting from the regulation of pore channels of zeolites, the proton conductivity of the pristine membrane was remarkably improved to 1.7‐fold. As expected, the optimized membrane exhibited outstanding voltage efficiency (VE) of 85.7% and energy efficiency (EE) of 85.1% at 120 mA/cm2, which is considerably better than pure polymer membrane (VE of 80.5% and EE of 79.4%) and commercial Nafion 212 (VE of 81.3% and EE of 77.4%) membrane, respectively.

Charge-Dependent Crossover in Aqueous Organic Redox Flow Batteries Revealed Using Online NMR Spectroscopy.

Aqueous organic redox-flow batteries (AORFBs) are promising candidates for low-cost grid-level energy storage. However, their wide-scale deployment is limited by crossover of redox-active material through the separator membrane, which causes capacity decay. Traditional membrane permeability measurements do not capture all contributions to crossover in working batteries, including migration and changes in ion size and charge. Here we present a new method for characterizing crossover in operating AORFBs using online 1H NMR spectroscopy. By the introduction of a separate pump to decouple NMR and battery flow rates, this method opens a route to quantitative time-resolved monitoring of redox-flow batteries under real operating conditions. In this proof-of-concept study of a 2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide model system, we observed a doubling of the 2,6-DHAQ crossover during battery charging, which we attribute to migration effects. This new membrane testing methodology will advance our understanding of crossover and accelerate the development of improved redox-flow batteries.

Upscaling of Reactive Mass Transport through Porous Electrodes in Aqueous Flow Batteries

Porous electrodes (PEs) are an important component of modern energy storage devices, such as lithium-ion batteries, flow batteries or fuel cells. Their complicated multiphase structure presents a considerable challenge to modeling and simulation. In this paper, we apply the volume-averaging method (VAM) as an efficient approach for the evaluation of effective macroscopic transport parameters in PEs. We consider the transport of electro-active species coupled to heterogeneous Butler-Volmer type reactions at the electrode surface. We identify the characteristic scales and dimensionless groups for the application to aqueous flow batteries. We validate the VAM-based model with direct numerical simulation results and literature data showing excellent agreement. Subsequently, we characterize several simplified periodic PE structures in 2D and 3D in terms of hydraulic permeability, effective dispersion and the effective kinetic number. We apply the up-scaled transport parameters to a simple macroscopic porous electrode to compare the overall efficiency of different pore-scale structures and material porosity values over a wide range of energy dissipation values. This study also reveals that the Bruggeman correction, commonly used in macroscopic porous electrode models, becomes inaccurate for realistic kinetic numbers in flow battery applications and should be used with care.

Aggregation and Capacity Limiting Effects in Anthraquinone-Based Flow Battery Negolytes

Anthraquinone-based molecules are promising electroactive materials for use in aqueous organic flow batteries. At high concentrations in aqueous solutions, the well-known negolyte molecule anthraquinone disulfonic acid (AQDS) molecule has been observed to aggregate under near-neutral and acidic conditions. Aggregation has been hypothesized to be directly linked to observed concurrent capacity reduction. In this study, we investigated three different water-soluble anthraquinones in electrolytes of varying compositions and pH to gain further insight into the possible causes of capacity loss. We used low-field benchtop 1H-NMR and diffusion NMR measurements directly in non-deuterated aqueous flow-battery electrolytes to investigate molecular aggregation. Single-cell testing was performed under exhaustive electrolysis conditions to determine the number of electrons exchanged per molecule. We observed a decrease in the number of electrons exchanged per molecule in the presence of carbonate ions due to CO2 adduct formation. The aggregation constants were determined from both concentration-dependent chemical shifts and self-diffusion coefficients. We show that aggregation of the oxidized form of all three molecules studied here occurs under near-neutral and alkaline conditions and does not affect the number of accessible e−.

Assessing the Feasibility of Implementing Porous Carbon Foam Electrodes Derived from Coal in Redox Flow Batteries

The feasibility of carbon foam electrodes derived from coal for the vanadium redox flow battery (VFB) is assessed as a pathway to repurpose mining waste for use in renewable energy storage technologies. Three-electrode, half-cell, and full-cell measurements provide proof-of-concept for coal foam as an electrode material for VFBs. Similarities in physical and chemical properties between the coal foam used here and other VFB electrode materials is characterised via SEM, micro-CT, XPS, MRI, and Raman spectroscopy. We show that significant improvement in electrochemical performance of the coal foam electrodes can be achieved via simple techniques to improve material wetting and remove impurities. The overall characteristics and electrochemical behaviour indicate that coal-derived foam can be feasibly utilised as an electrode material, and with further electrode activation, may provide a competitive solution to both cost-efficient VFBs and waste reduction.

Techno-economic analysis of off-grid hybrid wind-photovoltaic-battery power system by analyzing different batteries for the industrial plant in Shiraz Industrial Town, Iran.

The world has moved toward renewable energy resources for three major reasons: (1) to mitigate climate change arising from the excessive emission of greenhouse gases, (2) to protect health by lowering greenhouse gas emissions, and (3) to meet ever-increasing demands for energy. Shiraz is a major city in Iran and struggles with pollution challenges due to the presence of highly polluting industries. The increased energy demand and the lack of a demand-supply trade-off have led to frequent power outages in Shiraz in recent years. Batteries have been of great interest to researchers as they have a wide range of compounds and variety in the market and strongly influence the function and initial costs of hybrid energy systems. This study models a hybrid renewable energy system using four different batteries, that is, lead-acid, Li-ion, vanadium redox, and zinc-bromine batteries. These four scenarios were subjected to techno-economic analysis in HOMER. The system was assumed to generate 3000 kW of industrial power and 300 kWh of office/domestic power. It was demonstrated that the hybrid system with the lead-acid battery was the most optimal system to supply power to the case-study industrial plant for both industrial and domestic load, with a levelized cost of energy of 0.47 USD/kWh and an initial cost of 6.02 million USD. However, the hybrid system with the Li-ion battery will become more optimal than the system with the lead-acid battery if Li-ion batteries continue to become more affordable in < 5 years. This system would decrease CO2 emissions by 1,060,133 kg every year as compared to the diesel system.

Voltage prediction of vanadium redox flow batteries from first principles*

Global energy demand has been increasing for decades, which has created a necessity for large scale energy storage solutions for renewable energy sources. We studied the voltage of vanadium redox flow batteries (VRFBs) with density functional theory (DFT) and a newly developed technique using ab initio molecular dynamics (AIMD). DFT was used to create cluster models to calculate the voltage of VRFBs. However, DFT is not suited for capturing the dynamics and interactions in a liquid electrolyte, leading to the need for AIMD, which is capable of accurately modeling such things. The molarities and densities of all systems were carefully considered to match experimental conditions. With the use of AIMD, we calculated a voltage of 1.23 V, which compares well with the experimental value of 1.26 V. The techniques developed using AIMD for voltage calculations will be useful for the investigation of potential future battery technologies or as a screening process for additives to make improvements to currently available batteries.

Spatial Distribution of Pressure Using Fluid Physics for the Vanadium Redox Flow Battery and Minimizing Fluid Crossover Between the Battery Electrodes

Our focus in this treatment is a relatively novel approach to minimizing the fluid transfer imbalance between the negative and positive electrodes of a vanadium redox flow battery (VRFB) through the membrane, and determination of the horizontal/lateral change in pressure across each electrode, as well as the vertical pressure distributions. Underpinnings of the fluid transfer are the Darcy continuum equation. Here we develop analytical equations from the field equations affecting the fluid flow in the VRFB, which are very useful for controlling settings in battery stacks consisting of several cells.

Ionic Diffusion in Slurry Electrolytes for Redox Flow Batteries

Slurry electrodes have been proposed as a method to decouple the storage and power capacities of hybrid redox flow batteries by allowing the reduced metal to adhere to a flowing dispersion of electrically conductive particles rather than accumulate in the flow cell. In this work, the ionic mass transport through these slurry electrolytes is investigated via voltammetry at a rotating disc electrode and in a parallel channel flow cell. A modified Levich equation, il=0.62nFCω1/2ν−1/6(D+r2Φωm)2/3, is developed to describe limiting currents to a rotating disc electrode when using a particle filled electrolyte. Mass transfer is demonstrated to be enhanced by greater particle loading so long as there are sufficient particles such that their respective flow velocity boundary layers interact. In the flow cell, faradaic current in a carbon black slurry electrolyte was observed to decrease over time. This suggests that the carbon particles adhere and accumulate on the stationary electrode, impeding mass transport by imposing diffusion path tortuosity. This accumulation is undesirable in hybrid redox flow batteries as it encourages metal deposition in the cell, limiting the scalability of the technology.

Bismuth nanoparticles anchored on N-doped graphite felts to give stable and efficient iron-chromium redox flow batteries

Iron-chromium redox flow batteries (ICRFBs) use abundant and inexpensive chromium and iron as the active substances in the electrolyte and have great potential as a cost-effective and large-scale energy storage system. However, they are still plagued by several issues, such as the low electrochemical activity of Cr3+/Cr2+ and the occurrence of the undesired hydrogen evolution reaction (HER). We report the synthesis of amorphous bismuth (Bi) nanoparticles (NPs) immobilized on N-doped graphite felts (GFs) by a combined self-polymerization and wet-chemistry reduction strategy followed by annealing, which are used as the negative electrodes for ICRFBs. The resulting Bi NPs react with H+ to form intermediates and greatly inhibit the parasitic HER. In addition, the combined effect of Bi and N dopants on the surface of GF dramatically increases the electrochemical activity of Fe2+/Fe3+ and Cr3+/Cr2+, reduces the charge transfer resistance, and increases the mass transfer rate compared to plain GF. At the optimum Bi/N ratio of 2, a high coulombic efficiency of up to 97.7% is maintained even for 25 cycles at different current densities, the energy efficiency reaches 85.8% at 60.0 mA cm−2, exceeding many other reported materials, and the capacity reaches 862.7 mAh L−1 after 100 cycles, which is about 5.3 times that of bare GF.

Determination of the Ion Concentrations in VRFB by Non-Invasive Optical Techniques Due to Chemical Reactions, Complexes, and Side Reactions

The Vanadium redox flow battery has been intensively examined since the 1970s, with researchers looking at its electrochemical time varying electrolyte concentration time variation (both tank and cells, for negative and positive half cells), its thermal time variation, and fluid flow behavior. Chemistry behavior of the electrolyte ions have also been intensively examined too. Our focus in this paper is to examine the disturbance and effect of other chemical reactions and constituents to those typically found in the positive electrode electrolyte, while handling the less problematic negative electrode. Appropriate formulas are developed which allow assessment of the size of the effect, and their evaluation indicates something on the order of up to 14%. Therefore, for unambiguous non-invasive optical measurements, knowledge of the disturbance effect, and taking it into account will allow proper concentration determination. This treatment includes a comprehensive examination of identified side reactions which occur due to crossover ions between the electrodes.

Multiple‐dimensioned defect engineering for graphite felt electrode of vanadium redox flow battery

AbstractThe scarcity of wettability, insufficient active sites, and low surface area of graphite felt (GF) have long been suppressing the performance of vanadium redox flow batteries (VRFBs). Herein, an ultra‐homogeneous multiple‐dimensioned defect, including nano‐scale etching and atomic‐scale N, O co‐doping, was used to modify GF by the molten salt system. NH4Cl and KClO3 were added simultaneously to the system to obtain porous N/O co‐doped electrode (GF/ON), where KClO3 was used to ultra‐homogeneously etch, and O‐functionalize electrode, and NH4Cl was used as N dopant, respectively. GF/ON presents better electrochemical catalysis for VO2+/VO2+ and V3+/V2+ reactions than only O‐functionalized electrodes (GF/O) and GF. The enhanced electrochemical properties are attributed to an increase in active sites, surface area, and wettability, as well as the synergistic effect of N and O, which is also supported by the density functional theory calculations. Further, the cell using GF/ON shows higher discharge capacity, energy efficiency, and stability for cycling performance than the pristine cell at 140 mA cm−2 for 200 cycles. Moreover, the energy efficiency of the modified cell is increased by 9.7% from 55.2% for the pristine cell at 260 mA cm−2. Such an ultra‐homogeneous etching with N and O co‐doping through “boiling” molten salt medium provides an effective and practical application potential way to prepare superior electrodes for VRFB.

Decoupling Activation and Transport by Electron‐Regulated Atomic‐Bi Harnessed Surface‐to‐Pore Interface for Vanadium Redox Flow Battery (Adv. Mater. 6/2024)

Decoupling Activation and Transport by Electron‐Regulated Atomic‐Bi Harnessed Surface‐to‐Pore Interface for Vanadium Redox Flow Battery (Adv. Mater. 6/2024)

A neutral pH all-polymer redox flow battery employing organic and organometallic redox active polymers

Aqueous all-polymer redox flow batteries (APRFBs) working with size exclusion membranes are safe, low-cost, scalable solutions for energy storage applications. However, their development is still limited owing to challenges in optimizing the redox potential and solution viscosity of the polymers to deliver optimal energy density. Herein, we demonstrate an APRFB composed of ferrocene and viologen-based polymers with tunable redox potential. The theoretical calculations and electrochemical measurements reveal the contribution of HOMO energy change and polyelectrolyte dynamics in the charge transfer process with an increase in the molecular weight of the Catholyte-P. The electrochemical experiments yielded reasonable activity of the Catholyte-P24 and Anolyte-P12 with an oxidation rate constant of 1.26 × 10−4 cm s−1 and a reduction rate constant of 5.38 × 10−3 cm s−1, respectively. The application of synthesized polymers with size exclusion membrane yielded a 0.85 V APRFB with energy efficiency up to 72%, capacity retention of 99.93% per cycle at 10 mA cm−2, demonstrated energy density of 2.6 W h L−1, and capacity retention of >82% over continuous 250 cycles.

Double-Doped Carbon-Based Electrodes with Nitrogen and Oxygen to Boost the Areal Capacity of Zinc–Bromine Flow Batteries

Double-Doped Carbon-Based Electrodes with Nitrogen and Oxygen to Boost the Areal Capacity of Zinc–Bromine Flow Batteries

Can Flow Batteries Finally Beat Lithium?: Nanoparticles may boost energy density enough for EVs

Can Flow Batteries Finally Beat Lithium?: Nanoparticles may boost energy density enough for EVs

Static Three-Chamber ZnI2 Redox Batteries and Analyzed Electrochemically

Static three-chamber ZnI2 redox batteries are designed and analyzed electrochemically. The reversibility of the zinc plating/stripping process is evaluated by monitoring the cathodic/anodic peaks in the cyclic voltammograms. The electrochemical redox process of iodide on the cathode is analyzed based on the impedance spectra. The reversibility of zinc plating/stripping is related to I2 adsorption and improving performance of three-chamber ZnI2 redox battery could be attributed to the isolation of redox ions. The multi-electrolyte battery limits the influence of I2 adsorption and delivers better performance including of the high discharging voltage and power density.

Vanadium Redox Flow Battery Using an N-Doped Porous Carbon-Coated Positive Electrode Derived from Zeolitic Imidazolate Framework-8-Coated Graphite Felt

The poor electrochemical activity and low wettability of graphite felt (GF) electrodes significantly limit the energy efficiency and power of vanadium redox flow batteries (VRFBs). To solve these problems, we developed an N-doped porous carbon-coated electrode by the carbonization of zeolitic imidazolate framework-8- (ZIF-8-) coated GF. By controlling the carbonization temperature, the effects of temperature on the structural morphology, pore structure, and amount and type of functional groups of the electrode were confirmed. In addition, the effect of the change in the pore structure and the amount and type of functional groups on the electrocatalytic activity for the vanadium redox reaction was demonstrated. The synthesized N-doped porous carbon-coated electrode, which showed the best electrochemical activity, had a sufficient amount of nitrogen functional groups with a high ratio of graphitic N and the highest surface area and pore volume among all the samples, providing abundant active sites for the VO2+/VO2+ redox reaction. The VRFB fabricated using prepared electrode exhibited energy and voltage efficiencies of 70.44% and 73.44%, respectively, at 200 mA cm-2, which were 11.77% and 11.08% higher than those of the bare GF electrode. Furthermore, the modified battery operated effectively at a high current density of 350 mA cm-2. The energy efficiency exhibited excellent stability over 500 charge/discharge cycles at 250 mA cm-2. Hence, this study presents a promising method for the development of high-performance VRFB electrodes.