Flow Battery Research Articles

A TOC- and deposition-free electrochromic window driven by redox flow battery

Under “green architecture” principles, electrochromic smart windows are employed to adjust optical transmittance and indoor temperature, yet their high costs limit the wide application. Here, an electrochromic window is driven by a redox flow battery (RFB), where TOC and deposition layers are no longer needed. The transmittance of the electrochromic window is modulated by the state of oxidation (SOC) of aqueous posolyte Fe(phen)3Cl2, which is coupled with BTMAP-Vi negolyte in RFB. Under optimized conditions, average CE, VE, and EE reach 93.25 %, 92.61 %, and 86.35 % for RFB with a capacity fading rate of 1.57 % per cycle. 88.66 % optical modulation and 9.36 cm2/C coloration efficiency are achieved in the electrochromic process, and 72.34 % optical modulation is maintained after 12000 s. Essentially, the indoor temperature declines 3 °C for posolyte with 100 % SOC when compared with the control experiment using circulating water for a model house. This means minimum electricity of 0.0185 kWh is saved when using an air conditioner to cool a 100 m3 house, which corresponds to declined CO2 emission (COE) of 0.0185 kg. This work provides a novel and cost-efficient strategy for modulating indoor comfort via electrochromic windows driven by RFB.

Mapping the flow: Knowledge development and diffusion in the global innovation system of flow battery technology

Redox flow batteries (RFB) are receiving increasing attention as promising stationary energy storage systems. However, while first innovation activities in this technological field date back to the 1950s, the commercialization and diffusion rates of RFB technology have remained limited. Apart from technological challenges, the lack of a functioning innovation system is one of the main barriers to mainstream this technology. Here, we carried out a comprehensive bibliometric analysis of the RFB research field to shed systematic light on a core aspect of innovation systems - the production of knowledge. Based on the analysis of 4,872 papers published in the years 1981–2021, we reveal developments over time, describe the geographical distribution of research activities, and explore collaboration networks as well as emerging lines of research. Overall, our study allows for strengthening the RFB innovation system by providing the foundation for aligning a scattered research field, identifying needs for technology development, and fostering collaboration and synergy creation.

Iron-vanadium redox flow batteries electrolytes: performance enhancement of aqueous deep eutectic solvent electrolytes via water-tuning

Deep eutectic solvents (DES) are being recognized as a highly promising electrolyte option for redox flow batteries. This study examines the impact of modifying the molar ratio of water to a DES consisting of urea and choline chloride on important measures of electrolyte performance, such as viscosity, cyclic voltammetry, and impedance spectroscopy. According to the findings, when the molar ratio of water to DES is 1:3, the viscosity of the electrolyte falls by 75 %, the peak current density increases by 5–9.5 times, the ionic conductivity increases by 5–10 times, and the ohmic resistance decreases by 60–90 %. This approach greatly enhances the conductivity and diffusion coefficient of the electrolyte, resulting in a novel, cost-effective, and highly efficient electrolyte for iron-vanadium redox flow battery applications. This study enhances the efficiency of DESs at the molecular level, establishing the foundation for future extensive implementations.

Covalent triazine frameworks crosslinked microporous polymer membranes with fast and selective ion transport for ultra-stable vanadium redox flow batteries

The vanadium redox flow batteries (VRFBs) are an essential pathway to long-duration energy storage with the merits of power and energy decoupling, long lifetime and safety. However, low coulombic efficiency stemming from vanadium crossover across the membrane together with notable ohmic loss originated from low proton conductivity of the membrane still limits high performance operation of the VRFBs. Herein, we proposed 2,2′-bipyridine-based covalent triazine frameworks (bipCTF) crosslinked sulfonated-poly (4,4′-diphenylether-5,5′-bibenzimidazole) (SOPBI) (bipCTF/SP-100) for use as the membrane in VRFBs, which synergistically prevents vanadium crossover and enhances proton conductivity. By delicately tuning the sulfonation degree and properly incorporate bipCTF, the bipCTF/SP-100 membrane is successfully synthesized based on OPBI polymers, which delivers a reduced area resistance of 0.3 Ω cm2 and a low VO2+ permeability of 19.05 × 10−9 cm2 s−1, affording an excellent trade-off between proton conductivity and ion selectivity. Theoretical calculation further corroborates that the high proton conductivity of bipCTF/SP-100 is attributed to the ionic channels formed by bipCTF, while the superior ion selectivity is assigned to protonated imidazole and pyridine. Benefiting from microporous bipCTF/SP-100 membrane, the VRFB exhibits a high CE of 99.8 % and an excellent EE of 78.3 % at 250 mA cm−2 and meanwhile yields a high capacity retention of 94.8 % after 200 cycles at 200 mA cm−2, which shows enormous potential for high power density operation of the VRFBs.

Balancing current density and electrolyte flow for improved zinc-air battery cyclability

Zinc-air batteries (ZABs), known for their high energy density and environmental friendliness, are emerging as promising solutions for sustainable energy storage. However, the irregular deposition of zinc on electrodes hinders the widespread utilization of rechargeable ZABs due to limited durability and stability. This study investigates the role of electrolyte flow in enhancing zinc electrodeposition and overall performance in zinc-air flow batteries (ZAFBs) at high current densities. We explore the interplay between current density, flow rate, and their influence on electrode surface morphology and the removal of the passivating zinc oxide layer to improve battery efficiency and lifespan. Using advanced in-situ synchrotron radiation x-ray tomography, we found that incorporating flowing electrolyte at specific current densities results in rounded dendrite tips, thinner and more uniform deposition layers, and improved zinc adhesion. Imaging and electrochemical analyses further reveal that flowing electrolyte enhances zinc morphology, reduces charge transfer resistance, diminishes passivation, and lowers galvanostatic charge/discharge polarization across various current densities, thereby improving battery cycling performance. Notably, the impact of flowing electrolyte is more pronounced at a moderate current density of 50 mA/cm2 compared to lower or higher currents. Our findings underscore the importance of electrolyte flow and current density management in developing high-performance ZAFBs, providing valuable insights for their future commercialization.

A Cu-Zn bimetallic organic framework as protective interlayer for dendrite-free Zn deposition in zinc-based flow batteries

Zinc-based flow batteries hold great promise for grid-scale energy storage. However, the formation of zinc dendrites challenges their lifespan and safety. In this work, we devise a bimetallic organic framework material on carbon cloth (CuZn@MOF-CC) using a one-step dip coating method to address the dendrite issue. The prepared CuZn@MOF-CC promotes the creation of zincophilic sites on the electrode surface, facilitating smooth and uniform zinc deposition. Electrochemical csharacterizations and density functional theory (DFT) calculations reveal much stronger electronic interactions between CuZn@MOF and the zinc atoms compared to the commonly used anode material through an obvious electron interaction between the Zn atom and the Cu sites linked in the CuZn@MOF chain. By mitigating the zinc dendrite problem, this approach effectively improves the coulombic efficiency, cycle life, and safety of zinc-based flow batteries. The battery incorporating the prepared material demonstrates impressive stability, exceeding 450 cycles without dendrites at a high current density of 320 mA cm−2, paving the way for efficient and reliable energy storage solutions and a sustainable future.

Construction of Fe2O3-CuO Heterojunction Photoelectrode for Enhanced Efficiency of Solar Redox Flow Batteries

Construction of Fe2O3-CuO Heterojunction Photoelectrode for Enhanced Efficiency of Solar Redox Flow Batteries

Sulfonate and Ammonium-Grafted Poly(isatin triphenyl) Membranes for the Vanadium Redox Flow Battery

Sulfonate and Ammonium-Grafted Poly(isatin triphenyl) Membranes for the Vanadium Redox Flow Battery

Deep Entropy-Learning based virtual inertia control for VRFB regulation considering Phase-Locked loop dynamics

The virtual inertia control (VIC) technique is often adopted as a prominent mechanism to address the inertia deficiency of modern integrated power systems with high penetration of sustainable energy units. In such control mechanisms, a phase-locked loop (PLL) is required to estimate the grid frequency. However, utilization of such frequency measurement platforms is accompanied by large frequency fluctuations due to its inherent dynamics. In particular, this paper proposes an adaptive integral recursive backstepping (I-RBSC) based on virtual inertia for the regulation of vanadium redox flow battery (VRFB) in stand-alone Micro-Grids (SMGs). Unlike previous works that neglected the effect of frequency measurement devices, this work investigates the effect of PLL dynamics in virtual inertia control. The I-RBSC controller is designed by an entropy-driven actor-critic neural networks (NNs) to adaptively adjust the tunable coefficients during its interaction with the SMG. By training the ability of deep NNs, the entropy is maximized to regulate the system output. The VRFB unit can be discharged up to 90% which can quickly respond to the randomness of loads and sustainable energy units. The transient outcomes of SMG reveal the feasibility of I-RBSC (designed by entropy learning) to address the challenges of low inertia and PLL dynamics. The real-data of wind turbine and solar radiation are utilized to simulate the SMG in a more realistic framework from a systematic point of view. The comprehensive analysis under typical scenarios confirms the supremacy of the suggested VIC controller over prevalent state-of-the-art virtual inertia controllers including conventional VIC, fuzzy logic, backstepping, and model-predictive control (MPC).

Performance enhancement of vanadium redox flow battery with novel streamlined design: Simulation and experimental validation

Electrolyte utilization and the consequent concentration polarization significantly limit the potential increase in power density and contribute to electrode degradation in vanadium redox flow batteries during cycling. This study investigates a novel curvature streamlined design, drawing inspiration from natural forms, aiming to enhance the performance of vanadium redox flow battery cells compared to conventional square and rectangular flow-through cell designs. The simulated 3D single-cell model shows a notably superior uniformity in both current and species' concentration distribution within the streamlined design compared to the square and rectangular shapes. Experimental analysis conducted on 3D-printed flow frames demonstrated 2 % enhanced energy efficiency and 47 % improved capacity when compared to rectangular design at 150 mA cm−2, all achieved with minimal increase in pressure drop.

A comprehensive study in experiments combined with simulations for vanadium redox flow batteries at different temperatures

Ensuring the appropriate operation of Vanadium Redox Flow Batteries (VRFB) within a specific temperature range can enhance their efficiency, fully exploiting the advantages of renewable energy. This study employs a comprehensive approach combining experimentation and simulation to systematically investigate the impact of temperature on VRFB performance. Experimental evaluations of VRFB performance across various temperatures were conducted through charge and discharge experiments, electrochemical impedance spectroscopy, hydraulic pressure drop testing, and cycling charge and discharge trials. Additionally, a three-dimensional multi-field coupled model was developed to provide a theoretical analysis and elucidate VRFB performance under varying temperature conditions, incorporating temperature-dependent variations in the physical parameters of different components. The results suggest that in VRFB systems, an increase in temperature leads to a reduction in overpotential and pressure drop, resulting in improved system efficiency, albeit accompanied by a slight decrease in Coulombic efficiency. Furthermore, compared to ohmic overpotential and electrochemical overpotential, the impact of temperature on concentration overpotential appears to be relatively insignificant. Increasing the flow rate or temperature could contribute to a more stable degradation rate of capacity and Coulombic efficiency during the battery cycling process. Higher flow rates enhance the voltage efficiency of the battery, with the degree of improvement decreasing as temperature increases.

Performance of an All-Aqueous Thermally Regenerative Flow Battery Using CNT/Carbon Cloth Composite Electrodes

Performance of an All-Aqueous Thermally Regenerative Flow Battery Using CNT/Carbon Cloth Composite Electrodes

Improving the Volumetric Capacity of Gallocyanine Flow Battery by Adding a Molecular Spectator.

Gallocyanine (GAL) was recently introduced as a promising aqueous-soluble electroactive molecule for preparing two-electron storage alkaline flow battery (FB) negolytes. The development of a cost-effective GAL FB electrolyte is limited by the unexpectedly low solubility of GAL. In this work, the compound 7-amino-4-hydroxy-2-naphthalenesulfonic acid was introduced as a molecular spectator to modulate the solubility of GAL in KOH; this formulation allowed the preparation of a negolyte with a theoretical volumetric capacity of 32.00 Ah L-1. The cycling stability of an improved GAL electrolyte was demonstrated by operating a FB cell outside of the glovebox (bubbling N2 in the tanks). The cell exhibited a Coulombic efficiency close to 98% and began operating with 72.1% of its theoretical capacity (16.06 Ah L-1), retaining 88% of it after 110 cell cycles. This work demonstrates that significant improvement in electrolyte performance can be obtained with suitable additives and electrolyte engineering.

Insights into the interaction of nitrobenzene and the Ag(111) surface: A DFT study

This study explores the potential of nitrobenzene as an anolyte material for nonaqueous redox flow batteries (RFBs) by theoretically examining its low-coverage adsorption behavior on neutral and charged Ag(111) model electrode surfaces. At the low coverage limit, DFT calculations show a preference for nitrobenzene to adsorb parallel to the surface, with the benzene ring and nitro group centered over HCP sites. Interactions between nitrobenzene and the surface were analyzed using induced charge density analysis, Bader charge analysis, and projected density of states (PDOS). It was found that nitrobenzene adsorbs primarily through van der Waals interactions with the surface. As nitrobenzene accumulates negative charge, the strength of adsorption diminishes. Understanding the electrode-electrolyte interface is crucial for enhancing RFB electrochemical performance, and this study sheds light on nitrobenzene's interaction with a model Ag electrode.

Boosting the kinetics of bromine cathode in Zn–Br flow battery by enhancing the electrode adsorption of the droplet of bromine sequestration agent/polybromides complex

Zinc-bromine (Zn–Br) flow battery is a promising option for large scale energy storage due to its scalability and cost-effectiveness. However, the sluggish reaction kinetics of Br2/Br− have hindered further advances. In this study, we report that a nitrogen-doped carbon felt electrode derived from a metal-organic framework can facilitate the adsorption of N-methyl N-ethyl pyrrolidinium polybromide (MEP-pBr) complex droplet dispersed in aqueous electrolyte. Density functional theory (DFT) calculation and Zeta potential measurement suggest that the adsorption is primarily due to the positively charged surface induced by graphitic nitrogen defects. Electrochemical analysis indicates that the enhanced adsorption of MEP-pBr droplet significantly boosts the redox kinetics and decreases the crossover of bromine-bearing species. Consequently, the cell performance improves, achieving an energy efficiency of over 79 % during 900 cycles at a current density of 100 mA cm−2. Our work underscores the importance of providing access of MEP-pBr droplet to the electrode surface in augmenting the energy efficiency of Zn–Br flow battery.

Stability and Performance of Commercial Membranes in High-Temperature Organic Flow Batteries.

Redox flow batteries (RFB) often operate at extreme pH conditions and may require cooling to prevent high temperatures. The stability of the battery membranes at these extreme pH-values at high temperatures is still largely unknown. In this paper, a systematic screening of the performance and stability of nine commercial membranes at pH 14 and pH ≤ 0 with temperatures up to 80 °C is conducted in an organic aqueous RFB. Swelling, area resistance, diffusion crossover, battery performance and membrane stability after 40-80 °C temperature treatment are shown, after which a recommendation is made for different user scenarios. The Aquivion E98-05 membrane performed best for both the Tiron/2,7-AQDS battery and the DHPS/Fe(CN)6 battery at 40 mA/cm2, with stable results after 1 week of storage at 80 °C. At 80 mA/cm2, E-620-PE performed best in the DHPS/Fe(CN)6 battery, while Sx-050DK performed best in the Tiron/2,7-AQDS battery.

Aqueous colloid flow batteries with nano Prussian blue

Flow battery is a safe and scalable energy storage technology in effectively utilizing clean power and mitigating carbon emissions from fossil fuel consumption. In the present work, we demonstrate an aqueous colloid flow battery (ACFB) with well-dispersed colloids based on nano-sized Prussian blue (PB) cubes, aiming at expanding the chosen area of various nano redox materials and lowering the cost of chemicals. Taking advantage of the two redox pairs of PB, the developed all-PB cell employing a low-cost dialysis membrane with the synthesized PB on both sides displays an open-circuit voltage (OCV) of 0.74 V. Moreover, when paired with an organic tetra pyridine macrocycle the cell with PB as positive electrolyte exhibits an OCV of 1.33 V and a capacity fade rate of 0.039 %/cycle (0.8 %/day). Redox-active colloids exhibit enduring physicochemical stability, with no evident structural or morphological changes after extensive cycling, highlighting their potential for cost-effective and reliable ACFB energy storage.

Advanced Vanadium Redox Flow Battery Facilitated by Synergistic Effects of the Co2P-Modified Electrode

Advanced Vanadium Redox Flow Battery Facilitated by Synergistic Effects of the Co₂P-Modified Electrode

Optimized Sulfonated Poly(Ether Ether Ketone) Membranes for In-House Produced Small-Sized Vanadium Redox Flow Battery Set-Up.

The ionic exchange membranes represent a core component of redox flow batteries. Their features strongly affect the performance, durability, cost, and efficiency of these energy systems. Herein, the operating conditions of a lab-scale single-cell vanadium flow battery (VRFB) were optimized in terms of membrane physicochemical features and electrolyte composition, as a way to translate such conditions into a large-scale five-cell VRFB stack system. The effects of the sulfonation degree (SD) and the presence of a filler on the performances of sulfonated poly(ether ether ketone) (SPEEK) ion-selective membranes were investigated, using the commercial perfluorosulfonic-acid Nafion 115 membrane as a reference. Furthermore, the effect of a chloride-based electrolyte was evaluated by comparing it to the commonly used standard sulfuric acid electrolyte. Among the investigated membranes, the readily available SPEEK50-0 (SD = 50%; filler = 0%) resulted in it being permeable and selective to vanadium. Improved coulombic efficiency (93.4%) compared to that of Nafion 115 (88.9%) was achieved when SPEEK50-0, in combination with an optimized chloride-based electrolyte, was employed in a single-cell VRFB at a current density of 20 mA·cm-2. The optimized conditions were successfully applied for the construction of a five-cell VRFB stack system, exhibiting a satisfactory coulombic efficiency of 94.5%.

High-performance SPEEK membrane with polydopamine-bridged PTFE nanoparticles for vanadium redox flow batteries

With the growing demand of energy storage techniques in carbon-neutral environments, vanadium redox flow batteries (VRFBs) have emerged as outstanding systems for long-duration energy storage. Developing high-performance ion exchange membrane is essential for broad deployment of RFBs. In this work, a SPEEK/PTFE membrane is designed by incorporating polytetrafluoroethylene (PTFE) nanoparticles into sulfonated poly (ether ether ketone) (SPEEK) membrane by bio-inspired polydopamine (PDA) bridge. PDA combines SPEEK membrane and hydrophobic PTFE nanoparticle tightly by its abundant functional groups. The ion selectivity of optimal SPEEK/PTFE-0.5 % membrane is 8.9 times greater than that of Nafion 115. Remarkably, VRFB with optimal SPEEK/PTFE-0.5 % membrane exhibits excellent battery performances including impressive columbic efficiency (CE, 99.5 %) and energy efficiency (EE, 86.3 %) at 100 mA cm−2 surpassing Nafion 115 (CE, 95.8 %; EE, 83.4 %). Besides, VRFB employing SPEEK/PTFE-0.5 % membrane demonstrates excellent cycling stability, with higher capacity retention (71.4 % after 300 cycles) compared to Nafion 115, indicating its capability of extended operation. This work illustrates that the design of PDA bridge in composite membrane is a useful approach for developing high-performance membranes in RFBs.