Vanadium Redox Flow Battery Cell Research Articles

A novel cell design of vanadium redox flow batteries for enhancing energy and power performance

The Vanadium Redox Flow Battery (VRFB) is one of the most promising electrochemical energy storage systems considered to be suitable for a wide range of renewable energy applications. In this work, a novel cell structure is designed for VRFB, which includes embedded serpentine flow channels in a non-porous and non-brittle case. This new design eliminates end plates and gaskets to improve safety, extend life, and promote ease of assembly with fewer components than traditional VRFB cells. In particular, a key challenge of conventional graphite flow field plates in which solution penetration can occur due to an easily broken and porous structure is solved by using rigid impermeable polyvinyl chloride (PVC) sheets with flow channels embedded in the sheets. Various tests, such as cycling and polarization measurements of a wide range of current and flow rates, and Electrochemical Impedance Spectroscopy (EIS) for internal resistance analysis, are performed to compare new and traditional designs. The results show that the new design keeps cycling performance stable, with better stability and energy efficiency, than conventional VRFB cells do. High power performance is also observed for the same external cell size. In addition, an abrupt potential drop caused by high contact resistance between the current collector and a porous electrode is solved by introducing a layer of the patterned graphite sheet to improve contact and electronic conductivity and, also, to function as a flow channel. Finally, an economic analysis shows that the new design not only improves battery performance, but also lowers assembly costs and facilitates assembly.

Enhanced vanadium redox flow battery performance using graphene nanoplatelets to decorate carbon electrodes

Rotating Disk Electrode (RDE) measurements on model glassy carbon (GC) substrates and Cyclic Voltammetry on more practical commercial carbon supports are used to demonstrate that the kinetics of the positive VO2+/VO2+ redox reaction can be substantially enhanced by using electrostatic layer-by-layer assembly (LbL) to decorate their surface with graphene nanoplatelets (GNPs). An exchange current density, i0, is obtained that is more than two orders of magnitude greater than that observed with standard carbon supported Pt nanocatalyst with the deposition of only 20 GNP layers. Tafel slope analysis is compared to electron microscopy imaging to conclude that while faster redox kinetics is associated with an increase in the available active area, the prevalence of smaller GNPs and associated edge sites the can attenuate activity gains with increasing number of layers. Practical implementation to existing Vanadium Redox Flow Battery (VRFB) configurations was demonstrated through the application of a 370 nm (20 layer) LbL GNP coating on carbon felt (CF). The GNP coating yielded a 5% increase relative in voltage and overall efficiency of charge discharge curves obtained under typical VRFB cell operating conditions at 40 mA cm−2. Furthermore, a substantial increase in the discharge time is observed with this GNP coating on CF.

Experimental study on the performance of a vanadium redox flow battery with non-uniformly compressed carbon felt electrode

Optimal electrode compression can efficiently reduce electrode contact resistance and enhance species mass transfer so that the performance of vanadium redox flow battery (VRFB) is consequently improved. New designs of VRFB with a serpentine flow field on the current collector and compressed thin electrodes are investigated to increase its power density. In this study, the intrusion ratio, porosity, strain–stress, area specific resistance, hydrodynamic characteristics, and charge/discharge performance of VRFBs are comprehensively characterized under different compression ratios (CRs) which can be adjusted by changing the assembly force. Then, VRFBs using carbon felts with different CRs are tested by experiments, and the influence of electrode compression on VRFB cell performance is quantitatively evaluated. The in-homogeneous compression of carbon felt electrode in a VRFB with a flow field leads to a non-uniform porosity distribution of the electrodes under the channel, intrusion, and rib regions. The intrusion ratio, local average porosity, and permeability at different CRs are obtained. The Kozney–Carman constant of carbon fiber felt is modified by measuring the flow pressure drop through the electrode. The charge/discharge curves are acquired and the corresponding energy efficiencies are calculated under different CRs. It is shown that the charge/discharge time increases with the CR, and the energy efficiency can be improved to a maximum of 19.4% when the CR varies from 0.3% to 41.8%.

High Oxidizing Stability and Ion Selectivity of Hybrid Polymer Electrolyte Membrane for Improving Electrochemical Performance in Vanadium Redox Flow Battery

The research work examines effects of organic-inorganic hybrid membrane made of low-cost sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) and graphene oxide (GO) to improve the membrane properties for application in vanadium redox flow battery (VRFB). SPPO membrane alone is economical but undesirable for VRFB operation because it is significantly vulnerable to degradation in vanadium electrolyte. For this reason, the inorganic GO is introduced into the SPPO polymer matrix, thus beneficial effects are acquired specifically that the hybrid SPPO-GO membrane shows enhanced chemical-physical properties, especially higher ion selectivity is obtained for 2 wt% GO incorporated into SPPO, the greater than pure SPPO and Nafion 212 membrane. Furthermore, vanadium permeability is lowered for composite membrane incorporated with GO. Importantly, the hybrid membrane shows increased oxidizing stability in comparison to the SPPO membrane alone. Accordingly, the hybrid SPPO-GO membrane obtains higher efficiencies than pure SPPO membrane in VRFB cell test. Therefore, the results of this work can reveal promising consideration on the use of economical polymer membrane for further development in VRFB application.

Investigation of Electrode Losses in All-Vanadium Redox Flow Batteries with an Interdigitated Flow Field

Redox flow batteries (RFBs) are one of the strong candidates for large-scale energy storage needed for effective use of solar and wind energy [1]. A redox couple in association with vanadium ions has been utilized for industrial applications and further improvement of cell performance that can lead to reduction of the system cost is of great importance for market penetration. Recently, cell performance has been drastically improved by replacing conventional thick carbon felt electrodes with thin carbon paper electrodes [2-4]. Heat and/or acid treatments applied to carbon electrode materials has been also demonstrated to improve reaction kinetics in all vanadium redox flow batteries (VRFBs) [2-4]. To develop carbon electrodes with higher catalytic activity for VRFB application, clarifying reaction kinetics [5], as well as performance limiting electrode under cell operation are strongly needed. Agar et al. utilized an asymmetric configuration to identify performance limiting electrode and showed the negative electrode limit the cell performance in VRFBs [3]. Aaron et al. applied a dynamic hydrogen reference electrode to an operating VRFB and found larger kinetic polarization at the negative electrode [6]. In these literature, cell polarization at high current density conditions was not fully examined. In this study, we examined cell polarization of a VRFB with an interdigitated flow filed that was applied to achieve high current density operation [7]. We investigated electrode losses in the VRFB cell operated up to 1A/cm2by using a dynamic hydrogen reference electrode inserted into the cell [6]. We used heat-treated and untreated (as-received) carbon porous materials. Heat treatment to the electrodes was performed in the air or nitrogen atmosphere. Figure 1 shows IR-free overpotentials assigned to the negative (Fig.1(a)) and the positive (Fig.1(b)) electrodes, respectively. As reported in the literature [3,6], the negative electrode showed slow kinetics, but was intensively improved by heat treatment in the air. This suggests that catalytic activity of the electrode was enhanced by oxygen-enriched functional groups that also causes better wettability of the electrolyte. This was also supported by X-ray photoelectron spectroscopy we performed to the electrodes. On the other hand, the positive electrode showed less performance gain especially in low current density condition, suggesting that reaction kinetics in the positive electrode is faster. In high current density operation, heat treatment both in the air and in the nitrogen improved cell performance and this can be attributed to concentration overpotential. In the positive electrode, vanadium-ion (VO2 +) concentration adjacent to the electrode surface should decrease due to vanadium consumption and water production in high current density operation during the discharge. This gave rise to concentration overpotentials in the untreated electrode. But, by applying heat to the electrodes, an electrochemical surface area increased due to shrinkage of polymer binders impregnated in the untreated electrode, effectively suppressing concentration overpotentials. Acknowledgements This research was supported by Japan Science and Technology Agency (JST), Precursory Research for Embryonic Science and Technology (PRESTO), Grant No. JPMJPR12C6.

Novel Obstructed Flow Field Designs for Improved Perfomance in Vanadium Redox Flow Batteries

One of the major challenges that limits the widespread commercialization of vanadium redox flow batteries (VRFBs) is their relatively low power density, which often results in higher cost [1]. In recent years, significant effort has been put on understanding the effect of flow field design on the power density performance of VRFBs [2-3]. Flow fields play a crucial role in effective delivery and removal of active species from the electrode surface and hence they can significantly alter the power output of the VRFBs. Due to inherent similarities between fuel cells and redox flow batteries, flow field designs for VRFBs have been limited to previously studied conventional flow designs such as parallel, serpentine, and interdigitated. Though significant effort has been placed on analyzing which design performs better and how certain parameters can result in higher performance; investigation of novel flow designs outside the fuel cell research has remained much unexplored. Although conventionally utilized flow field designs have very different flow paths for delivery of the electrolyte in two dimensions; the channel depth seems to remain constant for the flow field studies reported so far. Considering the porous electrodes lie in parallel with the channel floors in most of the VRFB cell fixtures, it is believed that alterations of the channel depth throughout the length of the flow channels can help facilitate better transport of active species to the electrodes. Motivated by this, various forms of channel obstructions and ramped flow field designs have been investigated to understand the effect of channel depth and delivery of electrolytes on the power density of the VRFBs. Results have shown significant improvements in power density (up to 90%) with the addition of alterations to the channel depth with minor (10%) or no change in pumping losses. These findings suggests that engineering of the flow field designs in all three dimensions can help further improve the power density of VRFBs.

Activated Charcoal Modified Graphite Felts Using for Positive Electrodes of Vanadium Redox Flow Battery

In the present paper, a composite electrode material was developed for vanadium redox flow batteries (VRFBs). Activated charcoal particles were evenly immobilized on the graphite felt (GF) via a sucrose pyrolysis process for the first time. The in site formed pyrolytic carbon is used as the binder, because it is essentially carbon material as well as GF and activated charcoal, which has a natural tendency to realize good adhesion and low contact resistance. The activated charcoal decorated GF electrode (abbreviated as the composite electrode) possesses larger surface area (13.8 m2 g−1), more than two times as GF (6.3 m2 g−1). The oxygen content of composite electrode is also higher (7.0%) than that of GF (4.8%). The composite electrode was demonstrated to lower polarization and increase the reversibility toward the VO2+/VO2+ redox couple according to the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The charge–discharge cycling test was conducted with a single VRFB cell. The results indicate that the cell with composite electrode presents higher charge–discharge capacity, larger electrolyte utilization efficiency (EU), and higher energy conversion efficiency (79.1%) compared with that using GF electrode. The increasing electrochemical performances of composite electrodes are mainly ascribed to the high electrochemical activity of activated charcoal particles and increasing superficial area.

Design and experimental validation of a generalised electrical equivalent model of Vanadium Redox Flow Battery for interfacing with renewable energy sources

In this paper a MATLAB/Simulink based generalised electrical equivalent model of Vanadium Redox Flow Battery (VRFB) has been proposed with an objective of interfacing VRFB with renewable energy sources. The impact of practical parameters like flow rate, self discharge, pump power loss and charge-discharge current profile has been considered for analysing VRFB performance. The model involves online estimation of VRFB stack terminal voltage and state of charge (SOC) under different constant current charge-discharge profiles. The accurate estimation of electrical characteristics of VRFB leads to a benchmark for selection of the design parameters of an efficient charge controller taking into account the variability of renewable energy sources. An optimal range of variable electrolyte flow rates for dynamic SOC has been estimated to design suitable flow rate controller. This ensures minimum VRFB system power consumption during charging and maximum power output during discharging which leads to maximum overall VRFB system efficiency. A generic MATLAB library has also been created for optimal design of any generalised VRFB system. A practical 20 Cell 1kW 6h VRFB system has been used for experimental validation of simulation results of the proposed model which shows a very good agreement having maximum error within 0.5% and 0.21% for VRFB cell terminal voltage estimation during charging and discharging respectively. The proposed model of the VRFB system is then applied for interfacing and investigating the performance of a 1kW 6h VRFB with a microgrid system comprising of 10kW Solar PV, local loads and distribution grid.

High surface area bio-waste based carbon as a superior electrode for vanadium redox flow battery

Activated carbon (AC) with high surface area (1901 m2 g−1) is synthesized from low cost bio-waste orange (Citrus sinensis) peel for vanadium redox flow battery (VRB). The composition, structure and electrochemical properties of orange peel derived AC (OP-AC) are characterized by elemental analyzer, field emission-scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. CV results show that OP-AC coated bipolar plate demonstrates improved electro-catalytic activity in both positive and negative side redox couples than the pristine bipolar plate electrode and this is ascribed to the high surface area of OP-AC which provides effective electrode area and better contact between the porous electrode and bipolar plate. Consequently, the performance of VRB in a static cell shows higher energy efficiency for OP-AC electrode than the pristine electrode at all current densities tested. The results suggest the OP-AC to be a promising electrode for VRB applications and can be incorporated into making conducting plastics electrode to lower the VRB cell stack weight and cost.

Enhanced VRB electrochemical performance using tungsten as an electrolyte additive

In this study, we investigated the addition of a tungsten catalyst into the electrolyte of a vanadium redox flow battery (VRBs) and measured the cell performance with respect to the concentration of the W6+ precursor. The VRBs showed a clear improvement in electrochemical performance because of the deposition of tungsten on the electrode. The presence of tungsten allowed the ionization of tungsten in the electrolyte during battery cycling. The optimized concentration of the tungsten additive was examined using electrochemical methods, which showed that the efficiency of the VRBs improved by 2–3% after the addition of 3mM tungsten to the electrolyte. In particular, the performance of VRB cells in an anolyte containing a W6+ catalyst was significantly enhanced at high current rates.

Improvement of Performance By Using Catalytically Etched Carbon Paper As Electrode for Vanadium Redox Flow Batteries

The environmental aspects of fossil fuels and depletion of their reservoirs are main reasons for the growing use of renewable energy sources and environmentally sustainable storage technologies in recent years. Solar, wind, biomass and tidal are among climate-friendly energy sources. But the integration of these sources into large scale grid application is limited by their intermittent nature. Electrical energy storage (EES) technology solves this issue by storing energy on large scales. Redox flow batteries (RFBs) due to unique advantages of design flexibility, long cycle life, decoupled scaling of power and energy, are amongst suitable candidates of EES. All Vanadium Redox Flow Battery (VRFB) introduced in 1980s by Maria Skyllas-Kzacos et al has advantage over conventional RFBs as it employs four different oxidation states of vanadium; V4+/V5+ to cathode side and V2+/V3+ to anode side hence minimizes crossover effect through membrane. A typical VRFB contains two electrolyte reservoirs connected to cell sandwiching an ion exchange membrane between electrodes. V4+ is oxidized to V5+ on cathode while V3+ is reduced to V2+ on anode during charging and these charged species are reversed back during discharging. Carbon paper or felt due to its large reactive surface area, chemical stability in highly acidic solution has been widely used as electrode in VRFB, however poor electrochemical activity, low kinetics toward oxidation and reduction of vanadium ions, and less wettability arising from its hydrophobic nature limit its wide-spread application. Several modification techniques have been used to improve the catalytic activity of carbon electrode toward vanadium redox couples mainly including thermal, chemical and electrochemical treatments. Most of the modification techniques result increased activity of carbon electrode attributed to its enhanced available surface area and surface functional groups. In this work carbon paper is etched by using cobalt oxide (Co3O4) as catalyst and is used as electrode for vanadium redox flow battery cell. The etched carbon paper is found better than pristine one in terms of charging overpotential, IR drop, charge/discharge capacities and energy efficiency at all working current densities. Etched carbon paper showed almost 31.0 Ah/L discharge capacity at initial working current density of 50 mA/cm2 compare to 16.0 Ah/L of pristine one, While 16 Ah/L at 150 mA/cm2 where pristine and thermally treated (TT) carbon paper have no performance. Similar effect was noticed in terms of energy efficiency, the etched carbon paper showed 70% energy efficiency at 150 mA/cm2 which is dramatically higher than pristine and thermally treated carbon paper electrodes. The improved performance is attributed to increased surface area, wettability and presence of microscopic pores on the surface of etched carbon paper.

A New Enclosed Design for Vanadium Redox Flow Battery Cells

Vanadium Redox Flow Batteries (VRFBs) represent one class of electrochemical energy storage devices, which are principally nominated for stationary applications. In this work, a new enclosed-type single-cell design of VRFB that includes serpentine flow channels is fabricated and tested experimentally. The introduced design includes desirable features such as safety, ease of assembly, and fewer components than traditional VRFB cells, meaning that it does not require gaskets, insulators, graphite plates, and end plates, making it a very cost-effective design. It also has more mechanical strength due to the replacement of the graphite flow field plate (brittle and porous) with a hard and impermeable thermostatic material, PVC (Poly Vinyl Chloride) sheet, which ensures a longer lifespan, especially in harsh environments exposed to shocks and vibrations. Many tests have been conducted in order to compare this new design with the traditional one, including cycling, polarization over a range of currents along with a range of flow rates, and electrochemical impedance spectroscopy (EIS) for internal resistance analysis. The results show that the new design has very stable cycling performance with better columbic and energy efficiencies compared to the traditional one, also it reflects the normal behavior of single cycles at different loading conditions, and it can provide more output current through the polarization test and, subsequently, higher output power. Finally, an economic analysis was conducted that compares the costs associated with the two designs and it is found that the new one is cheaper, less weight, and easy assembly.

Water-Activated Graphite Felt As a High-Performance Electrode for Vanadium Redox Flow Batteries

A simple, green, novel, time-efficient, and potentially cost-effective water activation method was employed to enhance the electrochemical activity of graphite felt (GF) electrodes for vanadium redox flow batteries (VRFBs). The GF electrode prepared with a water vapor injection time of 5 min at 700 °C exhibits the highest electrochemical activity for the VO2+/VO2 + couple among all the tested electrodes. This is attributed to the small, controlled amount of water vapor that was introduced producing high contents of oxygen-containing functional groups, such as –OH groups, on the surface of the GF fibers, which are known to be electrochemically active sites for vanadium redox reactions. Charge–discharge tests further confirm that only 5 min of GF water activation is required to improve the efficiency of the VRFB cell. The average coulombic efficiency, voltage efficiency, and energy efficiency are 95.06%, 87.42%, and 83.10%, respectively, at a current density of 50 mA cm−2. These voltage and energy efficiencies are determined to be considerably higher than those of VRFB cells assembled using heat-treated GF electrodes without water activation and pristine GF electrodes. Figure 1

Low Permeable Hydrocarbon Polymer Electrolyte Membrane for Vanadium Redox Flow Battery.

Polymer electrolyte membrane (PEM) confirms the life span of vanadium redox flow battery (VRFB). Products from Dupont, Nafion membrane, is mainly used for PEM in VRFB. However, permeation of vanadium ion occurs because of Nafion’s high permeability. Therefore, the efficiency of VRFB decreases and the prices becomes higher, which hinders VRFB’s commercialization. In order to solve this problem, poly(phenylene oxide) (PPO) is sulfonated for the preparation of low-priced hydrocarbon polymer electrolyte membrane. sPPO membrane is characterized by fundamental properties and VRFB cell test.

Effects of sulfonated side chains used in polysulfone based PEMs for VRFB separator

Polysulfone (PSU) –based Proton Exchange Membranes (PEMs) were synthesized from different pre-sulfonated monomers and evaluated for use in a vanadium redox flow battery (VRFB). Two types of sulfonated monomers, containing either an allyl sulfonate or styrene sulfonate group, were synthesized by a Mizoroki-Heck reaction from a 4,4′-difluorodiphenyl sulfone derivative. These monomers were polymerized with 4,4′-(hexafluoroisopropylidene)diphenol to give partially fluorinated PSUs with controlled placement and content of sulfuric acid moieties. PEMs were then fabricated by solution casting and characterized against the membranes made from traditional backbone sulfonated PSU. The new PEMs revealed desirable physical properties and performance in a VRFB cell competitive with the Nafion membrane. Furthermore, each PEM with a different acid side-chain structure exhibited varying properties and performance depending on its acid content, providing a path to optimization.

Water-activated graphite felt as a high-performance electrode for vanadium redox flow batteries

Abstract A simple, green, novel, time-efficient, and potentially cost-effective water activation method was employed to enhance the electrochemical activity of graphite felt (GF) electrodes for vanadium redox flow batteries (VRFBs). The GF electrode prepared with a water vapor injection time of 5 min at 700 °C exhibits the highest electrochemical activity for the VO 2+ /VO 2 + couple among all the tested electrodes. This is attributed to the small, controlled amount of water vapor that was introduced producing high contents of oxygen-containing functional groups, such as OH groups, on the surface of the GF fibers, which are known to be electrochemically active sites for vanadium redox reactions. Charge–discharge tests further confirm that only 5 min of GF water activation is required to improve the efficiency of the VRFB cell. The average coulombic efficiency, voltage efficiency, and energy efficiency are 95.06%, 87.42%, and 83.10%, respectively, at a current density of 50 mA cm -2 . These voltage and energy efficiencies are determined to be considerably higher than those of VRFB cells assembled using heat-treated GF electrodes without water activation and pristine GF electrodes.

Outstanding electrochemical performance of a graphene-modified graphite felt for vanadium redox flow battery application

Abstract The development of more efficient electrode materials is essential to obtain vanadium redox flow batteries (VRFBs) with enhanced energy densities and to make these electrochemical energy storage devices more competitive. A graphene-modified graphite felt synthesized from a raw graphite felt and a graphene oxide water suspension by means of electrophoretic deposition (EPD) is investigated as a suitable electrode material in the positive side of a VRFB cell by means of cyclic voltammetry, impedance spectroscopy and charge/discharge experiments. The remarkably enhanced performance of the resultant hybrid material, in terms of electrochemical activity and kinetic reversibility towards the VO 2+ /VO 2 + , and mainly the markedly high energy efficiency of the VRFB cell (c.a. 95.8% at 25 mA cm −2 ) can be ascribed to the exceptional morphological and chemical characteristics of this tailored material. The 3D-architecture consisting of fibers interconnected by graphene-like sheets positively contributes to the proper development of the vanadium redox reactions and so represents a significant advance in the design of effective electrode materials.

Block Copolymer Sdapp Membranes for Vanadium Redox Flow Batteries-Strategy to Address Transport and Durability

The electrolyte separator (membrane) is a key component in vanadium redox flow batteries (VRFB) for large scale electrical energy storage. In the VRFB cell, the separator serves to physically separate positive and negative electrolyte solutions while establishing ionic conduction to pass current between two half cells. High ionic conductivity, low vanadium permeability, chemical stability and low cost are desired properties of RFB separators. To serve as a separator, the membrane should deter unwanted redox species transport while maintaining excellent ionic transport properties to minimize ohmic resistance. In the harsh electrolyte environment, the membrane must be stable enough to withstand chemical attacks in the electrolyte environment for a long lifespan. Moreover, the cost of the separator must be kept low enough to be integrated into large-scale battery system. Generally, ion exchange membranes are used as electrolyte separator in VRFBs. Sulfonated Diels Alder poly(phenylene) (SDAPP) cation exchange membranes have emerged as a good membrane candidate for both proton exchange membrane fuel cell and VRFB. The first generation SDAPP is a partially sulfonated poly(phenylene) random polymer. The random SDAPP membrane has shown comparable performance to Nafion in transport study and battery testing.1 From the perspective transport properties, SDAPP is satisfactory as anelectrolyte separator in VRFBs. However, random SDAPP is not chemically stable enough in the electrolyte environment to serve for a targeted lifespan because the unfunctionalized benzene ring in SDAPP is vulnerable to attack from V5+ (VO2 +) ion, a strong oxidant. Sulfonated polyphenylene/polysulfone block copolymer SDAPP membranes show significantly improved resistance to V5+ attack and excellent cell performance.2 The SDAPP block copolymer consists of a hydrophilic segment of fully sulfonated polyphenylene and hydrophobic segment of polysulfone. The block-SDAPP membrane provides an extended lifespan within VRFB environment in comparison to random SDAPP. Chemical stability of block-SDAPP is enhanced by full sulfonation of poly(phenylene) in the hydrophilic segment. Ionic conductivity is not sacrificed by block copolymerization. In the battery, the performance of block-SDAPP can be better than state of art Nafion 212. As is shown in the figure, at 500 mA/cm2cycling current density, cells equipped with three different block-SDAPP membranes can reach 76% energy efficiency, compared to 70% with Nafion 212, with the same electrode and electrolyte conditions. About 80% voltage efficiency was achieved with block-SDAPP membranes, because of the lowered internal resistance. Moreover, the coulombic efficiency reached by the three block SDAPPs is respectively 97%, 95% and 93%, compared to 95% of Nafion. Vanadium crossover across block SDAPP membranes is similar to that in Nafion 212. Considering that block-SDAPPs have lower ASR in the battery, the ionic selectivity of block SDAPP is essentially better than Nafion. To further optimize the structure of block-SDAPP for VRFB application, we are investigating the correlation between structure and performance of block-SDAPP. Under battery electrolyte conditions, ionic equilibrium, ionic transport and membrane morphology will be studied via the protocol established in previous studies.3Sulfuric acid, water uptake and vanadium ion partitioning in membrane will be determined after being equilibrated in electrolyte of various acid and vanadium composition to simulate battery environment. Conductivity and vanadium permeability of block SDAPP will also be measured after the same equilibration. Since water is a critical mediator for proton and cation transport in aqueous media, pulsed field gradient nuclear magnetic resonance (PFG-NMR) will be performed to investigate proton and vanadium transport mechanisms in equilibrated membranes. Membrane morphology will be studied by transmission electron microscopy and small angle X-ray scattering will be used to establish the geometry of ionic cluster channel. Acknowledgement This work is supported by Dr. Imre Gyuk, Office of Electricity Delivery and Energy Reliability, US Department of Energy.

Quaternized adamantane-containing poly(aryl ether ketone) anion exchange membranes for vanadium redox flow battery applications

Quaternized adamantane-containing poly(aryl ether ketone) anion exchange membranes (QADMPEK) are prepared and investigated for vanadium redox flow batteries (VRFB) application. The bulky, rigid and highly hydrophobic adamantane segment incorporated into the backbone of membrane material makes QADMPEK membranes have low water uptake and swelling ratio, and the as-prepared membranes display significantly lower permeability of vanadium ions than that of Nafion117 membrane. As a consequence, the VRFB cell with QADMPEK-3 membrane shows higher coulombic efficiency (99.4%) and energy efficiency (84.0%) than those for Nafion117 membrane (95.2% and 80.5%, respectively) at the current density of 80 mA cm−2. Furthermore, at a much higher current density of 140 mA cm−2, QADMPEK membrane still exhibits better coulombic efficiency and energy efficiency than Nafion117 membrane (coulombic efficiency 99.2% vs 96.5% and energy efficiency 76.0% vs 74.0%). Moreover, QADMPEK membranes show high stability in in-situ VRFB cycle test and ex-situ oxidation stability test. These results indicate that QADMPEK membranes are good candidates for VRFB applications.

Transport Properties Study on Carbon Electrodes for Vanadium Redox Flow Batteries

The electrode is a key component to optimize in the vanadium redox flow battery (VRFB) to achieve high battery performance and market competitiveness. Generally, carbon based materials are the most prevalent type of electrode utilized in commercial and experimental VRFBs.1 Fibrous carbon materials and their derivatives are widely used as electrodes in VRFB system because they are porous to allow electrolyte penetration, resistant to the electrolyte chemical environment and relatively low-cost. Recently, significant performance improvement of VRFB has been achieved by utilization of carbon paper materials with enhanced surface properties.2 Mass transport is a major factor in the efficiency loss in VRFB, particularly at low states of charge. Ohmic, activation and mass transport overpotential losses cause efficiency loss in an electrochemical system.3The electrode loss is mainly comprised of activation and mass transport losses, while the ohmic loss is mainly due to separator resistance. The activation overpotential is caused by limited reaction rate of redox couple and limited surface area. The mass transport overpotential is caused by reaction active species depletion due to insufficient active species supply on electrode surface. Because of the slow motion of cation in solution, it is critical to understand the relationship between the electrode properties and mass transport in electrode. Future material development and system optimization can benefit from this work. The properties of the carbon electrode can have a strong influence on the active species mass transport inside the cell. In Figure 1, the IR corrected polarization curves of positive and negative redox reactions on SGL 10AA and X-2 in VRFB are shown at varying electrolyte flow rates. The electrode materials were housed in a 5 cm2 battery hardware (Fuel Cell Technologies Inc.) with serpentine flow field. The electrolyte was composed of 1.7 M vanadium ions with 5 M total sulfate/bisulfate. The state of charge was maintained at 55% during the entire polarization curve measurement. In both reactions, the polarization curves show a typical mass transport limiting current density (iMT ) at high overpotential. In both reactions, higher iMT on 10AA indicates the better mass transport towards the surface of 10AA in the experimental scenario. X-2 is advantageous in reaction kinetics, since the activation overpotential is always lower on X-2 than 10AA in both reactions. However, the current density on 10AA generally surpasses that of X-2 at elevated overpotential. This trend should be attributed to better mass transport property of 10AA, though the detailed mechanism is not clear yet. We will present a thorough study of the mass transport properties of carbon paper electrode in VRFB cell. A systematic study of the physical properties and their influence on the convective and diffusive transport of redox active species within the electrode will be discussed. The transport study will be correlated to electrochemical test to develop a protocol to evaluate the mass transport properties’ influence on battery’s performance. This work is supported by Dr. Imre Gyuk, Office of Electricity Delivery and Energy Reliability, US Department of Energy. Reference: 1. M. H. Chakrabarti, N. P. Brandon, S. A. Hajimolana, F. Tariq, V. Yufit, M. A. Hashim, M. A. Hussain, C. T. J. Low, and P. V. Aravind, J. Power Sources, 253, 150–166 (2014) 2. A. M. Pezeshki, J. T. Clement, G. M. Veith, T. A. Zawodzinski, and M. M. Mench, J. Power Sources, 294, 333–338 (2015) 3. D. S. Aaron, Z. Tang, A. B. Papandrew, and T. A. Zawodzinski, J. Appl. Electrochem., 41, 1175–1182 (2011). Figure 1. IR corrected discharging polarization curves of negative and positive redox couple on SGL 10AA and X-2 in 5 cm2 FCT flow battery cell of varying flow rates with 1.7 M V2/3+/5 M bisulfate/sulfate electrolyte at 55% SoC. Figure 1