Redox Flow Battery Applications Research Articles

Harnessing Interfacial Electron Transfer in Redox Flow Batteries

Summary Redox flow batteries (RFBs) have garnered increasing attention for their potential to enable the widespread adoption of renewable electricity. However, a critical need associated with the continued development of this technology involves designing electrode-electrolyte interfaces that exhibit rapid, stable electron transfer kinetics. This targeted review outlines key challenges associated with measuring and enhancing the electron transfer kinetics of established and emerging flow battery active materials. We discuss several promising opportunities for advancing flow battery science and technology using the tools of applied electroanalysis and catalysis science. These challenges and opportunities are broadly relevant for future research directed at advancing the commercial adoption of RFBs for grid-scale energy storage.

Diphenyl ditelluride as a low-potential and fast-kinetics anolyte for nonaqueous redox flow battery applications

Developing low-potential, fast-kinetics and highly concentrated anolyte materials is one of the major challenges for nonaqueous redox flow batteries (NRFBs). Organosulfides exhibit promising potentials for NRFBs owing to their high solubility and structure diversity. However, they suffer from poor reaction kinetics and intermediate reversible potential, leading to poor roundtrip efficiency and low full cell voltage. Herein, we report diphenyl ditelluride (PDTe) as a low-potential and fast-kinetics anolyte candidate. The reversible potential of PDTe (average 1.97 V vs. Li/Li+) is lower than that of its sulfur counterpart PDS (2.25 V), which offers higher full cell voltages. We demonstrated a Li/2.5 M PDTe half-cell with a high volumetric capacity (128 Ah L−1PDTe), and high coulombic and energy efficiency (>99.5% and 84.1%) over 100 cycles. A Li/0.1 M PDTe flow cell achieved a stable long cycling over 260 cycles (38 days) at 1.5 mA cm−2. Coupling with lithium iodide (LiI) catholyte, the PDTe/LiI full cells demonstrated stable cycling over 120 cycles. Density functional theory calculation reveals that the cleavage of ditelluride bond requires a much lower bond dissociation energy than that of disulfide bond, which significantly reduces battery voltage hysteresis (by 429 mV). This study demonstrates the promising potential of organotellurides as anolytes for energy storage applications.

Structures and Free Energies of Cerium Ions in Acidic Electrolytes and Structures of Electrolyte-Electrode Interfaces to Inform Kinetic Performance in Redox Flow Battery Applications

Despite the interest in renewable energy as a substitute for fossil fuels, renewables make up less than 20%1 of total electrical energy in the United States in part because their intermittency prevents them from being a dominant fraction without storage.2 The redox flow battery (RFB) is a promising energy storage technology because of its high-power density and long lifetime,2 which can be used to address intermittency by decoupling customer demand and renewable electricity generation. Currently, however, RFBs incur substantial energy losses during operation at high current densities and are therefore too costly for market deployment. A promising chemistry for the positive electrode of a RFB which would improve efficiency by increasing the total voltage is the cerium redox couple, which has a potential up to 0.74 V greater than the positive vanadium electrode.3,4 The Ce3+/Ce4+ electron transfer redox potential changes depending on the electrolyte,3 which we hypothesize is due to the anion complexation thermodynamics. It is not currently clear whether only one or both of the Ce ions are complexed by anions in solution. Knowledge of the structures and free energies of cerium ions in acidic electrolytes would enable control over the Ce3+/Ce4+ redox potential for RFB applications. Additionally, understanding whether the Ce3+/Ce4+ redox reaction involves a change in the inner sphere is important for discerning the charge transfer mechanism and kinetics.In addition to a lack of information on the structure of Ce3+ and Ce4+ anions in acids, previous work in zinc-cerium5,6 and hydrogen-cerium7 batteries show that the cerium electrode kinetics are a limiting factor on battery performance. Our preliminary kinetic studies of the redox couple in sulfuric acid indicate that the kinetics are influenced more by the electrolyte than by the electrode material, which is consistent with an outer-sphere electron transfer mechanism. If the Ce3+/Ce4+ electron transfer does proceed via an apparent outer-sphere mechanism, the reorganization energy,8 which is dependent on the electrolyte, will control the rate of reaction. To determine whether the reaction follows an outer-sphere mechanism, we must isolate the behavior of the cerium ions at the electrode interface. In this study, we therefore explore the structure of the electrode-electrolyte interface as well as study the structure of cerium anion complexes.To probe the electrode-electrolyte interface, we use in-situ Extended X-ray Absorption Fine Structure (EXAFS) of the platinum (Pt) L3-edge of a Pt electrode during Ce3+ oxidation in two of the most relevant electrolytes for battery applications, H2SO4 and CH3SO3H. At the high positive potentials applied, the Pt is oxidized, and therefore the relevant electrode is a Pt-oxide. To study the ionic structure in different aqueous acidic electrolytes (i.e., HCl, H2SO4, H3NSO3, CH3SO3H, HNO3, CF3SO3H, and HClO4) and extract free energies of Ce3+ and Ce4+ complexes, we use experimental UV-Vis spectroscopy and EXAFS, and Density Functional Theory (DFT) calculations. Based on the combination of spectroscopy, DFT calculations, and the direction of the redox potential shift from non-complexing media, we hypothesize that Ce4+ is complexed by anions in electrolyte, while Ce3+ is hydrated by water. We find from the UV-Vis spectra that the dominant Ce3+ structure is the same in all acids studied, and that the dominant Ce4+ complex changes with electrolyte. EXAFS of Ce3+ allows us to determine a coordination number of nine water molecules. Our DFT calculations enable us to extract bond lengths of the first coordination shell of the cerium ions. The DFT-predicted free energies of Ce4+ anion complexation agree with free energies we calculated from redox potentials reported in literature, supporting our hypothesis that the shift in redox potential can be attributed to the anion complexation of Ce4+. Additionally, this agreement shows that we can predict standard redox potentials for cerium in simple aqueous electrolytes. Ultimately, these characterization findings clarify the fundamental interactions of cerium ions with their electrolyte environment and a relevant electrode as well as the Ce3+/Ce4+ thermodynamics and will guide kinetics studies of the Ce3+/Ce4+ redox couple for use in redox flow battery applications. U.S. EIA, Electric Power Annual (2019).Weber, A. et al. J. Appl. Electrochem. 41, 1137–1164 (2011).Piro, N. et al. Coord. Chem. Rev. 260, 21–36 (2014).Smith, G. & Getz, C. Ind. Eng. Chem. Res. 10, 191–195 (1938).Walsh, F. et al. Chempluschem 80, 288–311 (2015).Nikiforidis, G. et al. Electrochim. Acta 140, 139–144 (2014).Tucker, M. et al. J. Power Sources 327, 591–598 (2016).Bard, A. & Faulkner, L. (John Wiley & Sons, Inc., 2001).

Investigation of Mass Transport Properties of Fibrous Electrodes in Vanadium Redox Flow Batteries By Lattice Boltzmann Simulation

Redox flow batteries (RFBs) are gathering much attention as one of the promising candidates for a large-scale electrical energy storage device that is vital to utilize intermittent renewable energy in the grid [1]. In RFBs, fibrous electrodes have been widely applied presumably due to their large specific surface area and better electric conductance. It has been well recognized that the electrode structure profoundly affects cell performance that is attributed to the reaction and transport properties of the electrode [2,3].In this study, we developed a numerical simulation method to analyze pore-scale inhomogeneous behaviors of fluid and electrochemical reaction in fibrous electrodes using Lattice Boltzmann method (LBM). We focused our attention on porous structure affecting mass transport properties and overpotential in the electrodes for the vanadium redox flow battery.We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1(a). As the calculation procedure first, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. All calculation was carried out by the Lattice Boltzmann code we developed. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 50mA/cm2 was performed with different fiber diameter and porosity. As the pressure loss was fixed, the change in total overvoltage was evaluated as the change in energy loss.Figure 1 shows the calculation results of the electrochemical reaction and transport in the fiber electrode with a diameter of 3.75 μm and a porosity of 0.70. A path through which the electrolyte selectively flows is formed in the sparse area of the fibrous electrode and the inhomogeneous vanadium ion concentration and local reaction current density are identical in the electrode. This indicates that the electrolyte flow is locally biased due to the non-uniformity of the electrode. The reactive species concentration decreases due to the electrochemical reaction in the dense fiber region and causes local reaction current density.Based on a series of simulation with different diameter and porosity of the fibrous electrode, we obtained an equation for the electrolyte velocity and pressure drop in the electrode. We also evaluated averaged mass transport coefficients. We confirmed that the mass transfer characteristics are improved when the fiber diameter is large and the porosity is high. This is attributed to the larger the voids, the higher the permeability and the transport by advection was promoted. However, it is noteworthy that the larger diameter and the higher porosity leads to less electrode surface area. In our previous study [3], an optimized electrode architecture with a diameter of 2.4 μm and a porosity of 0.89 was presented by macro-scale simulations.In this fiber-scale simulations, we evaluated overpotential depending on the fiber diameter and the electrode porosity, and demonstrated the LBM simulation applied for optimization of fiber diameter and the electrode porosity in the RFB applications. Acknowledgements This research was supported by a Precursory Research for Embryonic Science and Technology (PRESTO) grant (grant number JPMJPR12C6) from the Japan Science and Technology Agency(JST).

Modeling the Impact of Sequential Two-Electron Transfer Compounds on Redox Flow Battery Performance

Redox flow batteries (RFBs) are promising candidates for grid-scale energy storage, but further cost reductions are needed for widespread use.1 As such, redox-active organic molecules and metal-coordination complexes are emerging as charge storage materials considering their redox potential, solubility, and stability can be tailored through molecular functionalization.2,3 Moreover, a number of these compounds can support multiple electron transfers, affording dramatic increases in energy storage capacity.4,5 However, the nature of the electron transfer events, concerted or sequential, plays a key role in determining whether the added capacity can be accessed efficiently. Specifically, depending on molecular features and cell operation, voltaic inefficiencies may be incurred which, in turn, offset any benefits of increased capacity. Therefore, we sought to quantify these losses and their relationship to molecular- and cell-level parameters to better understand the feasibility of multi-electron transfer in RFBs.In this presentation, we use a zero-dimensional electrochemical model, accounting for thermodynamic, kinetic, and mass transfer, to simulate the performance of two-electron compounds in redox flow cells. First, using a single half-cell, we show that voltaic efficiency is significantly influenced by the average redox potential and potential difference between the redox events. We also discuss the effects of different mass transfer coefficients and comproportionation reaction rates. Second, using a full cell, we consider the effects of utilizing two-electron compounds in both half-cells and subsequent effects of charge imbalance, both resulting in further inefficiencies. By understanding the operational tradeoffs of multi-electron compounds, we can develop design criterion to guide molecular engineering efforts towards more efficient high energy density electrolytes for RFBs.AcknowledgementsThis work was funded by the National Science Foundation (NSF) under Award Number 1805566. B.J.N gratefully acknowledges the NSF Graduate Research Fellowship Program under Grant Number 1122374. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.References(1) Chalamala, B. R.; Soundappan, T.; Fisher, G. R.; Anstey, M. R.; Viswanathan, V. V.; Perry, M. L. Redox Flow Batteries: An Engineering Perspective. Proceedings of the IEEE 2014, 102 (6), 976–999.(2) Hogue, R. W.; Toghill, K. E. Metal Coordination Complexes in Nonaqueous Redox Flow Batteries. Current Opinion in Electrochemistry 2019.(3) Ding, Y.; Zhang, C.; Zhang, L.; Zhou, Y.; Yu, G. Molecular Engineering of Organic Electroactive Materials for Redox Flow Batteries. Chem. Soc. Rev. 2018, 47 (1), 69–103.(4) Kowalski, J. A.; Casselman, M. D.; Kaur, A. P.; Milshtein, J. D.; Elliott, C. F.; Modekrutti, S.; Attanayake, N. H.; Zhang, N.; Parkin, S. R.; Risko, C.; Brushett, F. R.; Odom, S. A. A Stable Two-Electron-Donating Phenothiazine for Application in Nonaqueous Redox Flow Batteries. J. Mater. Chem. A 2017, 5 (46), 24371–24379.(5) Sevov, C. S.; Fisher, S. L.; Thompson, L. T.; Sanford, M. S. Mechanism-Based Development of a Low-Potential, Soluble, and Cyclable Multielectron Anolyte for Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc. 2016, 138 (47), 15378–15384.

Modifying Carbon Felt Electrodes Using High-Temperature Ammonia Treatment to Improve the Performance of All-Vanadium Redox Flow Batteries

High temperature ammonia treatments were performed on carbon felt electrodes in this study. Their physical and electrochemical properties were investigated.Carbon felt electrodes are often used in vanadium redox flow batteries (VRFBs). Without intervention, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to make it more effective, which has been historically accomplished with thermal treatments to promote oxygen functional groups. [1] Many attempts have been made to improve performance of carbon felt in VRFBs, but some of the more successful modifications have resulted in an increase in the specific surface area. [2-4] Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. [5]For our experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at 900°C for varying lengths of time. The samples were tested in a single-cell flow battery using cyclic voltammetry (CV), polarization curves, and electrochemical impedance spectroscopy (EIS). This treatment results in a significant increase in electrochemical surface area and performance.The physical properties were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and Raman spectroscopy. A pitting effect was observed in SEM images, preceded by surface blistering at shorter lengths of exposure.Greatly increased active surface area, along with a substantial increase in cell performance, was achieved with the ammonia-modified carbon felts described in this work. Sun, B. and M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment. Electrochimica Acta, 1992. 37(7): p. 1253-1260.Lu, W., et al., High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces. Energy & Environmental Science, 2016. 9(7): p. 2319-2325.Zhou, X.L., et al., A high-performance dual-scale porous electrode for vanadium redox flow batteries. Journal of Power Sources, 2016. 325: p. 329-336.Wei, L., et al., Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries. Energy Storage Materials, 2020. 25: p. 885-892.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016. Figure 1

High Energy Density Non-Aqueous Organic Redox Flow Batteries

Renewable energy sources such as wind and solar are replacing fossil fuels for electricity generation. However, intermittency of wind and solar limits their wide-spread adoptions. The energy fed into the power grid must be matched with the consumer energy demand to prevent blackouts and destabilization of the grid [1, 2]. Recently, redox flow batteries (RFBs) have gained practical interest among the other energy storage technologies in light of their long lifetime, independent sizing of power and energy, high round-trip efficiency, scalability and design flexibility, fast response, and low environmental impact [3-5]. Organic redox-active materials have recently received attention as they provide competitive electrochemical characteristics, flexible design, and they are abundant in nature [6]. Aqueous designs face commercial difficulty because RFBs have low energy and power density due to the limited cell voltage of 1.23 V. The limited voltage is due to the evolutions of hydrogen and oxygen in the water electrolysis [7]. Solvent substitution is one solution to enable higher energy densities in RFBs, using non-aqueous solution also provides a large design space for enhancement of material solubility, cell potential and the number of electrons stored in the redox species [7-8].In this study, a new organic redox molecule, tetra amino anthraquinone (Disperse Blue 1: DB-1), is evaluated and compared with other organic systems reported in the literature [5, 7] such as benzoquinone (BQ), naphthoquinone (NQ), anthraquinone (AQ), tetramethyl piperidinyloxyl (Tempo), and phenylenediamine (PD) in non-aqueous solvent by means of cyclic voltammetry. A three-electrode system was utilized to conduct cyclic voltammetry (CV) experiments using glassy carbon working electrodes. The battery performance was evaluated using a flow cell with an electrode area of 2.5 cm2. The electrolytic solution, containing 40 mM DB-1 solution in dimethyl sulfoxide solvent (DMSO) and 1 M Bis (trifluoromethane) sulfonimide lithium salt, was circulated through the cell at a flow rate of 10 cm3 min-1. Graphite felt and Nafion 115 were used as the electrode and membrane, respectively.In addition, density functional theory (DFT) calculations were used to better understand the electrochemical behavior of the active quinone molecules at different oxidation states. Figure 1 shows the molecular orbital energy levels (HOMO and LUMO) of the DB-1 organic dye and other similar organic molecules obtained by DFT calculations in DMSO. A relatively small HOMO-LUMO gap means a lower overpotential required for the oxidation and reduction processes [8]. The DB-1 had narrower bandgaps (<3 eV) than other quinone molecules (> 3.9 eV), suggesting that the selected molecule has better kinetics than other organic molecules. The result describes a systematic evaluation of DB-1 as a redox active material for energy storage applications by means of electronic structures, electrochemical properties, and flow cell studies. Voltammetric studies and DFT calculations (in DMSO solvent) demonstrated that this was capable of forming both anions and cations at electrode potentials ranging from 1.7 to 4.4 V vs. Li, involving up to 6 electron-transfers and exhibiting one of the highest electrode potentials (up to 4.4 V vs. Li) in the literature. Since these quinone derivatives can be extracted from biomass materials, the proposed organic RFB system provides a greener and more sustainable option for grid-scale energy storage applications than metal based redox systems. The proposed chemistry in this work showed high energy efficiency (ca. 68 %) at a current density (20 mA cm-2) with close to 100% capacity retention. These results reveal that the molecular tuning of quinone structures is promising for RFBs applications. Figure 1

Employing Neural Network Regression Models to Screen the Electrode Design Space for Redox Flow Batteries

Redox flow batteries (RFBs) are a promising electrochemical technology for integrating renewable energies into the current grid infrastructure, but further cost reductions are needed for widespread deployment.1 Porous electrodes are an integral component of the RFB reactor providing heterogeneous active sites for electrochemical reactions, a solid-void matrix that facilitates electrolyte dispersion, and a conductive solid for transporting electrons throughout the reaction zones. Many electrodes employed in RFBs use gas diffusion layers (GDLs)—such as carbon papers, cloths, and felts—that are repurposed from fuel cell applications. While GDLs possess some favorable properties, they are not broadly tailored to support electrochemical reactions and forced liquid-phase flow. As such, these materials are suboptimal for RFB applications, and, consequently, research has focused on post-process modifications to improve electrochemical and fluid dynamic performance.2,3 In contrast, the ideal electrode would be a bottom-up design with a property profile specific to the particular redox chemistry and operating conditions. Unfortunately, manufacturing limitations and time-intensive experimentations stymie broad screening of electrode designs. This motivates the use of computational tools.The advances of computational fluid dynamics and multi-physics solvers enable a platform to construct novel electrode designs and investigate resulting structure-performance relationships. By incorporating physical properties and operating conditions, the electrode can be modeled at a variety of length scales.4 Simulation domains can be used to span a wide variety of electrode and system parameter values allowing for the generation of large data sets for statistical and sensitivity analysis. Further, the current epoch of machine learning algorithms engender opportunities to build regressive models for predicting RFB electrochemical and fluid dynamic performance that can be subsequently used for optimization protocols.In this presentation, we use an experimentally-validated RFB model to generate >8,000 electrochemical simulations across a range of electrode properties. We then use feature engineering to reduce data complexity for building a robust and accurate neural network to predict output current density. Subsequently, we use this trained and validated neural network in conjunction with metaheuristics to search the electrode design space and report optimal morphological properties. This approach—as shown in the figure below—represents a first step to exploring the electrode design space for advanced RFBs and may ultimately support performance optimization of existing battery installation, materials selection for prototypes, or new approaches to electrode design.AcknowledgmentsThis work was funded by the Joint Center for Energy Storage Research, an Energy Innovation Hub of the U.S. Department of Energy, Office of Science, Basic Energy (De-AC02-06CH11357). K.M.T. recognizes additional support from the U.S. NSF Graduate Research Fellowship (1122374).References A. Z. Weber et al., J Appl Electrochem, 41, 1137 (2011).K. J. Kim, Y.-J. Kim, J.-H. Kim, and M.-S. Park, Materials Chemistry and Physics, 131, 547–553 (2011).K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Appl. Mater. Interfaces, 10, 44430–44442 (2018).A. Gayon Lombardo, B. A. Simon, O. Taiwo, S. J. Neethling, and N. P. Brandon, Journal of Energy Storage, 24, 100736 (2019). Figure 1

Electrospinning as a route to advanced carbon fibre materials for selected low-temperature electrochemical devices: A review

Electrospinning is a versatile fabrication method for synthesising mesostructured nanofibrous mats and has been explored in this review for the following electrochemical energy storage devices: redox flow batteries, fuel cells, metal air batteries and supercapacitors. Electrospinning has been proven as a highly versatile fabrication method for producing nano-structured fibres with controllable morphology, of both the fibres themselves and the void structure of the mats. Additionally, it is possible to use heteroatom doped polymers or to include catalytic precursors in the electrospinning solution to control the surface properties of the fibres. These factors make it an ideal method for the production of electrodes and flow media for a variety of electrochemical devices, enabling reduction in mass transport and activation overpotentials and therefore increasing efficiency. Moreover, the use of biomass as a polymer source has recently gained attention for the ability to embed sustainable principles in the materials of electrochemical devices, complementing their ability to allow an increase in the use of renewable electricity via their application. In this review, the historical and recent developments of electrospun materials for application in redox flow batteries, fuel cells, metal air batteries and supercapacitors are thoroughly reviewed, including an overview of the electrospinning process and a guide to best practice. Finally, we provide an outlook for the emerging use of this process in the field of electrochemical energy devices with the hope that the combination of tailored microstructure, surface functionality and computer modelling will herald a new era of bespoke functional materials that can significantly improve the performance of the devices in which they are used.

Electrochemical Reactions and Failure Mechanism Study of Sodium‐Aqueous Polysulfide Conversion Reactions in Redox Flow Batteries

Low‐cost aqueous polysulfide (APS) chemistry has recently attracted intensive research as it is a promising candidate for redox flow batteries. However, the intrinsic properties, electrochemical stability, and compatibility with sodium super ionic conductor (NASICON) membrane have been rarely demonstrated to clarify the challenges on the road to practical application. Herein, a detailed study on the electrochemical reactions and failure mechanism of aqueous polysulfides is presented. Starting from investigating the intrinsic stability, the optimized concentration of NaOH is determined. Through constructing a hybrid Na‐APS cell to avoid the effect from the counterelectrode, the cycling behavior is found to be stable under the capacity cut‐off condition, while severe decay occurs after ≈80 cycles. Comprehensive analysis of the decay mechanism shows the decomposition and loss of active materials during the long‐term cycles. Meanwhile, the NASICON separator surface is found to be corrosive after electrochemical exposure with APS solution under a high‐pH environment. It is anticipated that the systematic study of the APSs’ electrochemical reactions and decay mechanism could be beneficial for their further application in redox flow batteries.

The effect of counter-ion substitution on poly(phthalazinone ether ketone) amphoteric ion exchange membranes for vanadium redox flow battery

The study proposed a novel method to prepare poly(phthalazinone ether ketone) amphoteric ion exchange membranes (Q/S-M) with both increased efficiency and chemical stability for vanadium redox flow battery (VRB) applications. Q/S-M membranes were obtained after the successive amination and acidification process of blend base membranes prepared from brominated poly(phthalazinone ether ketone) (BPPEK) and sulfonated poly(phthalazinone ether ketone) substituted by counter-ions (SPPEK-M), where M was defined as counter-ions: Li+, Na+ and K+. The study analyzed the effect of counter-ion size on properties of Q/S-M membranes, which were compared with those for Q/S membrane obtained from BPPEK and SPPEK in acid form. Q/S and Q/S-M membranes maintained the same composition of ionic groups. The Q/S-M membranes induced by bigger counter-ion size showed the increasing of water content and ion diffusion rate, which was higher than that of Q/S membrane. Related to Q/S membrane, Q/S–K membrane possessed almost half area resistance (0.67 Ω cm2 vs 1.43 Ω cm2), and the value was close to that of Nafion115. Q/S-M membranes exhibited a 96.0–98.8% decrease in VO2+ permeability over Nafion115. Q/S–K membrane displayed a 6.5% increase in VE and correspondingly a 6.2% increase in EE over Q/S membrane, maintaining superior EE than Nafion115 (89.7% vs 86.5%). The ex-situ degradation test in VO2+ solutions indicated that despite close EE, Q/S–K membrane showed superior chemical stability over amphoteric ion exchange membranes (AIEMs) in our previous works. There was no obvious efficiency decay of Q/S–K membrane during 100-cycles test, indicating the good duration in operating VRB.

Nitrogen and sulfur co-doped graphene composite electrode with high electrocatalytic activity for vanadium redox flow battery application

Nitrogen and sulfur co-doped graphene has been decorated on graphite felt electrode via a hydrothermal treatment using thiourea as sulfur and nitrogen sources for vanadium redox flow battery application. Our results demonstrate that the modification of sulfur and nitrogen co-doped graphene on graphite felt provides an effective way to improve surface area and introduce functional groups for vanadium redox reaction, resulting in a significant improvement in energy efficiency (85.37% at a current density of 80 mA/cm2), power density (0.4925 W/cm2), discharge capacity and capacity retention. These results reveal that the modification of nitrogen and sulfur co-doped graphene on graphite felts is a promising method for the enhancement of vanadium redox flow battery performance.

Novel sulfonated polyimide membrane blended with flexible poly[bis(4-methylphenoxy) phosphazene] chains for all vanadium redox flow battery

To improve SPI stability, poly[bis(4-methylphenoxy) phosphazene] (PMPP) was designed and synthesized using polyphosphazene's unique and strong flexibility. A novel sulfonated polyimide (SPI) composite membrane embedded with flexible PMPP chains was prepared for all vanadium redox flow battery applications. SPI/PMPP composite membrane characterizations, such as proton conductivity, vanadium ion permeability, stability, and single cell performance, were thoroughly investigated. PMPP's unique properties not only allow the membrane to exhibit excellent anti-hydrolysis and anti-oxidation stability, but also maintain high proton conductivity and low vanadium ion permeability (as low as 4.81 × 10−7 cm2 min−1). It can also form a rigidity-flexibility network with SPI to improve membrane mechanical properties. It can be deduced that introducing PMPP has achieved many things in one stroke. Results show that the hydrolysis and oxidation stabilities of SPI/PMPP membrane were significantly improved. The SPI/2% PMPP membrane exhibits higher columbic efficiency (CE: 97.52%), voltage efficiency (VE: 84.88%), and energy efficiency (EE: 82.21%) than that of the N212 membrane (CE: 94.97%, VE: 82.04%, EE: 76.99%) at a high current density of 100 mA cm−2. In addition, the charging capacity loss of a battery with a SPI/2% PMPP membrane has also significantly decreased by about 206.70 mAh after 110 cycles at 100 mA cm−2 with the same electrolyte volume compared to a pristine SPI. Meanwhile, the high temperature resistance of SPI/PMPP membrane also shows the possibility of being applied with fuel cells.

Chemically stable anion exchange membranes based on C2-Protected imidazolium cations for vanadium flow battery

Imidazolium-based anion exchange membranes (AEMs) are attractive as the separator for vanadium redox flow battery (VFB) application. However, the lack of fundamental understanding of the correlations between imidazolium chemical structure and their physical properties as well as cell performance constraints the design of advanced AEMs in VFBs. In this work, by designing the “clickable” imidazolium compounds from C2-methyl or C2-phenyl substituted imidazolium to C2-phenyl substituted benzimidazolium, a series of PSf-based AEMs having pendant C2-protected imidazolium derivatives were prepared by efficient CuAAC reaction to explore how the nature of C2 substitution affected the electrochemical property of AEMs and the resulting VFB performance. Interestingly, PSf-MIm with C2-methyl protected imidazolium group showed lowest area resistance (0.3 Ω cm2, IEC = 1.66 meq./g) in 3 M H2SO4 aqueous solution but unfavorable vanadium permeability due to its highest swelling ratio. The introduction of bulky C2-phenyl substituted benzimidazolium led to the distinct microphase separation in PSf-PhBIm membrane, and thus reduced water uptake, high vanadium selectivity, and comparable conductivity were observed. As a result, the single VFB with PSf-MIm-1.2 membrane exhibited better electrochemical performance with a coulombic efficiency (CE) of 97.0%, and an energy efficiency (EE) of 82.4% at a current density of 120 mA/cm2, higher than those of Nafion N115 membrane (EE = 75.5%) and unsubstituted imidazolium-based AEMs (EE = 80.6%). More importantly, both ex situ stability testing in 1.5 M (VO2)2SO4/3 M H2SO4 solution for 90 days and in situ cycling performance at a current density of 120 mA/cm2 demonstrated that the chemical structure of C2-substituted imidazolium based AEMs remained intact after stability tests as confirmed by NMR analysis, while significant degradation was found for unsubstituted PSf-Im membrane via possible nucleophilic addition mechanism. Therefore, the VFB with PSf-MIm membrane showed the best long-term durability with only 34.1% loss in EE for 3638 h of operating (4800 cycles). This work not only fills the knowledge gap on the structure-property relationship of imidazolium-based AEMs for VFB application, but also gives us new directions to design stable AEMs for durable VFBs.

Use of Carbon Additives towards Rechargeable Zinc Slurry Air Flow Batteries

The performance of redox flow batteries is notably influenced by the electrolyte, especially in slurry-based flow batteries, as it serves as both an ionic conductive electrolyte and a flowing electrode. In this study, carbon additives were introduced to achieve a rechargeable zinc slurry flow battery by minimizing the zinc plating on the bipolar plate that occurs during charging. When no carbon additive was present in the zinc slurry, the discharge current density was 24 mA∙cm−2 at 0.6 V, while the use of carbon additives increased it to up to 38 mA∙cm−2. The maximum power density was also increased from 16 mW∙cm−2 to 23 mW∙cm−2. Moreover, the amount of zinc plated on the bipolar plate during charging decreased with increasing carbon content in the slurry. Rheological investigation revealed that the elastic modulus and yield stress are directly proportional to the carbon content in the slurry, which is beneficial for redox flow battery applications, but comes at the expense of an increase in viscosity (two-fold increase at 100 s−1). These results show how the use of conductive additives can enhance the energy density of slurry-based flow batteries.

Sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) nanocomposite membranes with high selectivity, stability, and vanadium‐ion barrier for vanadium redox flow batteries

Polyethyleneimine‐functionalized reduced graphene oxide (PEI‐RGO)‐embedded sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) (SPVDF‐co‐HFP) acid‐base composite membranes are fabricated by solution casting method for vanadium redox flow battery (VRFB) applications. The denser and crumbled morphology revealed by FESEM images and EDX results indicates the successful synthesis of PEI‐RGO nanosheets. The interfacial acid‐base pairs formed between PEI‐RGO and SPVDF‐co‐HFP matrix of composite membranes result in improvement of ion exchange capacity, water uptake, proton conductivity, as well as effective control in swelling ratio and vanadium‐ion permeability. In addition, the dispersing ability of PEI‐RGO in the SPVDF‐co‐HFP matrix improved the thermal and mechanical stability of the composite membranes. Homogeneous dispersion of PEI‐RGO nanosheets at the SPVDF‐co‐HFP surface is revealed by the FESEM images of the composite membranes, and their presence is confirmed by EDX results. The SPVDF‐co‐HFP membrane with 0.75 wt% PEI‐RGO exhibited the highest proton conductivity of 5.69 × 10−3Scm−1at 25°C, membrane selectivity of 25.52 × 10−4Scm−3min, and lowest vanadium‐ion permeability of 2.23 × 10−8cm2min−1. Overall results suggested that the SPVDF‐co‐HFP‐0.75 nanocomposite membranes are found to be a suitable alternative for commercially costly Nafion in VRFB applications.

Improved chemical stability and proton selectivity of semi‐interpenetrating polymer network amphoteric membrane for vanadium redox flow battery application

AbstractIn this work, semiinterpenetrating polymer network (semi‐IPN), consisting of sulfonated poly (arylene ether sulfone) (SPAES) and crosslinked vinyl imidazole grafted polysulfone (VMPSU), is prepared and characterized. FTIR, EDS, and solubility test indicate the successful preparation of amphoteric membranes. The semi‐IPN amphoteric membranes exhibit better stability than pure SPAES membrane, as demonstrated by thermogravimetric analysis and ex situ immersion testing results. More importantly, it is shown that the amphoteric membrane can effectively hinder vanadium ion crossover through the membrane, which is attributed to the semi‐IPN structure and Donnan exclusion. As expected, the amphoteric membrane containing 20% VMPSU exhibits the highest proton selectivity (6.86 × 104 S min cm−3), comparing to pristine SPAES (1.90 × 104 S min cm−3) as well as Nafion117 (1.31 × 104 S min cm−3).

RETRACTED ARTICLE: Automatic generation control with fuzzy logic controller incorporating tandem and cross compound turbine

This dissertation characterize the Automatic Generation Control (AGC) knit together with tandem reheat and cross reheat thermal system wielding Proportional Integral (PI) and Fuzzy Logic Controller (FLC). Multi area thermal system is designed with non-linearities such as governor dead band in addition with generation rate constraint. This study is come up with a knowledgeable application of Redox flow batteries (RFB) correlative along Static Synchronous Series Compensator. In order to enhance dynamic reaction of the load frequency control, redox flow battery has been appended in couple of the units as a result of being active rejoinder and nether time persistent. Ziegler Nichols method is wielded to regulate the PI controllers. During enormous load distraction within areas, conventional control methods alone would be ineffective in diminishing frequency deflection and power of tie-line swinging as a result of medium reaction by speed governor process. In order prevail this hindrance; fuzzy logic controller has been executed within the plant. Simulation studies affirm the efficacy of the fuzzy logic controller, depends upon the parameters like overshoots, under shoots and settling time, hence AGC functioning during abnormality in power system has been ameliorated. System is also studied for hydro thermal power system with simulation outcome.

Quaternized poly(arylene ether benzonitrile) membranes for vanadium redox flow batteries

A series of quaternized poly (arylene ether benzonitrile)s (QAxCN; x = percent quaternization) were synthesized for the vanadium redox flow battery (VRFB) application. The presence of highly polar nitrile groups incorporated in the quaternized polymer chain provides desirable polymer properties (low water uptake, high dimensional stability and low VO2+ permeability) without compromising ion conductivity. VRFB single cells using the aforementioned membranes showed excellent performance. The cell using a 15 μm-thick QA50CN membrane exhibited superior coulombic efficiency (98.2%) and comparable energy efficiency (86.5%) at 40 mA cm-2 compared to the single cell using Nafion-212 with a coulombic efficiency of 95.5% and an energy efficiency of 86.2%. The cell cycling stability and membrane oxidative stability data confirmed that the QAxCN membranes have the potential to replace Nafion membranes for energy storage in VRFB applications.

Preparation of sulfonated polyimide/polyvinyl alcohol composite membrane for vanadium redox flow battery applications

A novel series of sulfonated polyimide (SPI)/polyvinyl alcohol (PVA) composite membranes applied in vanadium redox flow battery (VRFB) is prepared and characterized in this work. With the hydrophilicity and good barrier of PVA, the composite membranes exhibit better hydrolysis stability and vanadium ion resistance than normal SPIs with five-membered rings. In VRFB single-cell tests, because of the highest proton selectivity, the SPI5/PVA5 composite membrane shows lower self-discharge rate, higher coulombic efficiency, and energy efficiency (EE) (79.5% vs 77.8% at 30 mA cm−2) than that of Nafion117. Besides, the composite membranes present stable performances up to 100 cycles without significant decline in EE.