Improved All-Vanadium Redox Flow Batteries using Catholyte Additive

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Improved All-Vanadium Redox Flow Batteries using Catholyte Additive and a Crosslinked Methylated PBI Membrane Ruiyong Chen, Dirk Henkensmeier, Sangwon Kim, Sang Jun Yoon, Tatiana Zinkevich, and Sylvio Indris ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01116 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Energy Materials

Improved All-Vanadium Redox Flow Batteries using Catholyte Additive and a Crosslinked Methylated PBI Membrane Ruiyong Chen,*,† Dirk Henkensmeier,‡,§ Sangwon Kim,† Sang Jun Yoon,†,# Tatiana Zinkevich,‖,⊥ Sylvio Indris‖ †

Transfercenter Sustainable Electrochemistry, Saarland University and KIST Europe, 66123 Saarbrücken, Germany

‡

Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea §

Division of Energy and Environment Technology, KIST School, University of Science and Technology, Seoul 02792, Republic of Korea

# Center

for Membranes, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea

‖Institute

for Applied Materials – Energy Storage Systems, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany ⊥

Helmholtz Institute Ulm, Helmholtzstraße 11, 89081 Ulm, Germany

Keywords: Energy storage, redox flow batteries, vanadium electrolyte, imidazolium chloride, temperature stability

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Abstract: Through controlling the crosslinking and thickness of low-cost methylated polybenzimidazole anion exchange membranes and by using imidazolium chloride as a catholyte additive, we improved the energy efficiency at a current density of 100 mA cm-2 for allvanadium redox flow batteries (RFBs) to 82%, compared to 76% with a Nafion 212 membrane and 67-78% for previously reported polybenzimidazole membranes with standard electrolyte. Moreover, thermal stability analysis of the charged catholytes, by direct observations of vanadium precipitation and variable temperature 51V nuclear magnetic resonance, confirmed an optimized content of 0.16 M of the electrochemically inert additive in 1.6 M vanadium solution, consistent with the electrochemical data. The imidazolium chloride additive with such a low concentration is sufficient to induce interaction with vanadium species in catholyte. This explains the improved electrolyte stability and enhanced cycling efficiency.

1. Introduction Utilization of renewable energy such as solar and wind to address the environmental challenges needs suitable energy storage devices with technological and economical merits. With good system scalability and operational flexibility, rechargeable RFBs are steadily receiving more attention for grid-scale energy storage by using redox active chemical components dissolved in liquids contained in the external tanks of the systems.1 It is considered that flow batteries using aqueous electrolytes are inherently safer and less expensive over lifetime than lithium-ion batteries.2 During operation, redox reactions of active species in different compartments of the cell, separated by an ion exchange membrane, are charge-balanced through the transport of counter ions across the membrane. The operation efficiencies, such Coulombic, voltage and energy efficiencies, are largely determined not only by the selectivity and ion conductivity of the membranes, but by the feature of the electrolytes.


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Currently, vanadium RFBs using sulfuric acid as supporting electrolytes and perfluorinated Nafion cation exchange membranes are benchmark systems on the market.3,4 Despite various advantages of the system, drawbacks such as high cost of Nafion membranes and crossover of vanadium ions through the membranes remain to be solved.5,6 Large swelling of Nafion membranes in aqueous electrolytes leads to interconnected channels facilitating transport of vanadium ions and consequently poor proton/vanadium selectivity. Alternative membranes with low cost, low vanadium ion permeability and durable performance are crucial to promote the application of vanadium RFBs. Various strategies have been employed to use low cost non-perfluorinated membranes,7-10 to design membranes with controlled hydrophilic/hydrophobic domains,11,12 to control the dimension of ion channel for sieving hydrated proton and vanadium ions with different size.13,14 With electrostatic repulsion between the positively charged backbone and the vanadium cations, and size sieving effect, anion exchange membranes (AEM) with narrow ion channel can reduce effectively the vanadium permeability, which however usually sacrifices the voltage efficiency of the battery due to their low ion conductivity.15,16 Polybenzimidazoles (PBI) are known with excellent chemical stability to oxidation.17 With the capability to adsorb acid, PBI membranes form a positively charged protonated backbone with acidic supporting electrolytes.18,19 Thus, it is considered that PBI has a mixed proton/anion conduction mechanism when used in typical vanadium electrolytes. Unfortunately, due to low ion conductivity PBI membranes showed higher energy efficiency than Nafion membranes only at low current densities (< 50 mA cm2).16,19

PBI membranes with modified backbone structure and enhanced acid adsorption

capability have been reported by Hong et al.20 In addition to variation in the chemical structure, porosity21,22 and the backbones,18,23 strategies to crosslink the membranes have been applied to


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tailor the microstructure of the polymer matrix and accordingly the ion conductivity.24-27 However, previous work often focused exclusively on the membrane properties. For RFBs, there is a high demand for the compatibility between electrolytes and ion exchange membrane. Apart from the membranes, modification of the electrolyte formulation is the key to the deployment of high performance RFBs. Local solvation structures of vanadium cations,28,29 electrolyte stability and vanadium crossover behavior depend largely on the electrolyte composition. It has been well known that the vanadium stability depends on the concentration, temperature, oxidation states, state of the charge and acid type.4,30-32 Significant improvement in the temperature stability has been achieved by using mixed acid electrolytes30 and additives.33,34 Temperature adaptability of employed electrolytes and membranes plays important role for durable and safe operation. Catholytes tend to be instable at high temperature above 40 °C.35 Various stabilizing agents have been applied to stabilize vanadium ions.36-38 In addition, the presence of appropriate electrolyte promoters can enhance the reaction kinetics.39 Moreover, additives should be chemical/electrochemically stable in strongly oxidizing electrolytes.34,37 Based on the finding that the 1-butyl-3-methylimidazolium cation (BMIm+) and the Cl- anion have lower reduction potential than V3+/V2+ couple and higher oxidation potential than the VO2+/VO2+ couple, respectively,40,41 BMIm+Cl- is suitable as an electrochemically stable additive. In addition, surfactant cationic additive such as BMIm+ may inhibit the aggregation of vanadium ions into V2O5 through a Coulombic repulsion and steric hindrance,42 while Cl- ions may form soluble VO2Cl species with VO2+.30 Herein, through adding BMImCl into catholyte as complexing agent and meanwhile using a crosslinked methylated meta-polybenzimidazole (cmPBI) membrane, we demonstrate improved cycling efficiency at high current density, and fast VO2+/VO2+ reaction kinetics. The additive-vanadium interaction and the positive effect of using


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additive were further studied through variable temperature experiments to observe the presence of precipitation and the change in the local vanadium coordination environment. 2. Experimental Dry cm-PBI membranes with dark brown color in iodide form were synthesized following a route described elsewhere,26 with additional control in the amounts of dibromoxylene crosslinker and thickness. The cross-section morphology of the cm-PBI membranes was characterized by scanning electron microscopy (SEM). By soaking the membranes in 1 M NaCl for 2 days, the iodide form was exchanged to chloride form. Water uptake and chloride through-plane conductivity were then measured. A weight loss was generally observed. Afterwards, the membranes were soaked in 2 M H2SO4 for 2 days. Sulfuric acid uptake and sulfate through-plane conductivity were recorded. Such membranes in sulfate form are then readily to be assembled into a flow battery. Alternatively, the membranes in chloride form were soaked in 1 M HCl for comparative purpose. Vanadium permeability of the membranes were measured at room temperature with a diffusion cell. The left reservoir was filled with 1 M VOSO4 in 2 M H2SO4 (55 mL) while the right one was filled with 1 M MgSO4 in 2 M H2SO4 (55 mL) to minimize the osmotic pressure and to equalize the ionic strengths. The solutions were stirred with magnetic bars during the experiments to avoid concentration polarization. The effective area of the membranes was 9.616 cm-2. Solutions in the right cell were analyzed at regular time intervals. The concentration of VO2+ was determined using a UV-vis spectrometer. Then, the VO2+ permeability was calculated according to Fick’s diffusion law.3 For comparison, the VO2+ permeability of Nafion 212 membrane was also measured.


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Commercial vanadium electrolyte with a total vanadium concentration of 1.6 M (50% VO2+ and 50% V3+) in 2 M H2SO4 (GfE Metalle und Materialien GmbH) was used. BMImCl (≥98.0%, Sigma Aldrich) as additive in catholyte was dissolved with a BMImCl : V ratio between 0:1 to 0.2:1. For flow battery tests at room temperature, a home-made flow cell with an active area of 4 cm2 was used. Graphite felts with uncompressed thickness of 5 mm (GFD4.6 EA, SGL) were pretreated in 3 M H2SO4 solution for 24 h and then thermally activated at 500°C for 12 h in static air. An electrode compression of 20% was used. Commercial Nafion 212 and cm-PBI membranes were soaked in 2 M H2SO4 overnight prior to use. Catholyte (12 mL) and anolyte (12 mL) with flow rates of 50 mL min-1 were delivered to each compartment using a peristaltic pump (Ismatec, Germany). Charge/discharge cycles and electrochemical impedance spectra (EIS) were measured with a Bio-Logic VMP3 multichannel potentiostat (BioLogic, France). Cyclic voltammetry measurements were performed using a three-electrode cell, consisting of a glassy carbon working electrode, a Pt foil counter electrode and a Ag/AgCl reference electrode. Catholytes were collected after the cell was charged to 1.7 V for thermal stability tests. For cyclic voltammetry measurements, the collected catholytes were further diluted to 0.16 M vanadium with 3 M H2SO4 to minimize electrolyte mass transport effects. 51V

nuclear magnetic resonance (NMR) spectra were measured using a Bruker NMR

spectrometer at a magnetic field of 4.7 T, corresponding to a 51V Larmor frequency of 52.1 MHz. A single-pulse sequence was used for data acquisition with a /2 pulse length of 5.6 μs and a recycle delay of 3 s. The 51V chemical shifts are referenced to VOCl3. 256 scans were acquired for each spectrum. Temperature dependent measurements were performed after 30 min stabilization time prior to data acquisition.


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Figure 1. (a) Chemical structure of a cm-PBI membrane with positively charged backbone, which repulses the vanadium cations, but allows the free transport of anions, (b) Cross section SEM image of the cm-PBI membrane.

3. Results and discussion A series of methylated meta-PBI membranes in initial iodide form with different amounts of dibromoxylene as crosslinker (x/%, weight ratio of crosslinker to cm-PBI, x = 0, 5 and 10) and thickness (y/μm, y = 15 and 50), denoted as cm-PBI-x or cm-PBI-x-y, were studied with respect to their chemical stability, ion conductivity and electrochemical properties for RFBs. We found that the non-crosslinked membranes showed rather poor mechanical/chemical stability after ex situ immersion test using highly oxidizing VO2+ solution. Crosslinking of the PBI membranes was then applied to reinforce the chemical stability, mechanical strength and reduce deformation, as schematically shown in Figure 1a. Cross-section SEM image of the cm-PBI membrane shows a typical dense matrix (Figure 1b).


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Table 1. Physical properties of the cm-PBI membranes Membrane cm-PBI-5-50 amount of crosslinker 5% degree of methylation 83% ion exchange capability 3.3 / mmol Cl-1 g-1 thickness of dry membrane 46.2 / μm conductivity in 16.2 1 M NaCl / mS cm-1 conductivity in 38.4 2 M H2SO4 / mS cm-1 weight change after soaking in +31% H2SO4 for 20 h conductivity in 1 M HCl / mS cm-1

cm-PBI-10-15 10% 68% 1.9

cm-PBI-10-50 10% 68% 1.9

14.9

47.6

1.5

3.5

19.3

22.8

+33%

+32%

17.7

-

During ion exchange from iodide form to chloride form, cm-PBI-5 membranes with less crosslinker (5%) swell excessively in spite of high ion conductivity (Table 1). In contrast, cmPBI-10 showed negligible water swelling in NaCl solution. The through-plane ion conductivity of 10% crosslinked cm-PBI membranes is rather low at neutral pH electrolytes (Table 1). However, after soaking the cm-PBI-10 membranes in 2 M H2SO4 or in 1 M HCl, the ion conductivity increased to about 20 mS cm-1, which is in line with their capability to absorb acid molecules. In comparison with a recent study of an unmodified meta-PBI membrane with a conductivity of about only 4.9 mS cm-1,19 a fast anion transport channel was created under acidic environment for the present membranes through methylation and crosslinking of the PBI polymer networks. A dimensional change of swelling in length is about 13% after soaking in 2 M H2SO4, which is close to the observed values of a meta-PBI membrane.19 Higher quantity of crosslinker was not studied, which may lead to a membrane with more compact structure with reduced reactant permeability and ion conductivity.22,43 In addition, VO2+ permeability of the cmPBI-10 membrane was measured using a diffusion cell. The change of the VO2+ concentration


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and solution color in the right reservoir of the diffusion cell was recorded as a function of diffusion time, as shown in Figure 2. Compared to a Nafion 212 membrane (thickness: 50 μm) with a measured VO2+ permeability of 1.38×10-6 cm2 min-1, the cm-PBI-10-15 membrane shows a significantly reduced VO2+ permeability of 8.58×10-8 cm2 min-1. Compared with a cation exchange Nafion 212 membrane and previous work on other PBI membranes,16,19 higher operation current density and competitive cycling efficiency have been achieved for the cm-PBI membranes with a mixed proton/anion conduction mechanism in combination with a control in the catholytes, as will be discussed below.

Figure 2. Change of concentration of VO2+ and color with diffusion time in the right reservoir of the diffusion cell for the Nafion 212, and the cm-PBI-10-15 membranes. Electrochemical performance of a single cell using commercial vanadium electrolytes (1.6 M vanadium in 2 M H2SO4) with different amounts of BMImCl additive in the catholytes, and cmPBI-10 membranes with different thickness was investigated, as summarized in Table 2. For flow battery with cation exchange membrane Nafion 212, when BMImCl was added into the catholyte, high cell resistance (3.6 Ω) was observed, indicating that large BMIm+ cations may block the ion transport channel of the membrane. Thus, BMImCl is only added into the catholyte when cm-PBI membranes were used. For thinner cm-PBI-10 membrane, the cell resistance


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values (0.21~0.25 Ω for the studied vanadium electrolytes) measured from the impedance Nyquist plot at high frequency are comparable to that (0.36 Ω) for the Nafion membrane, implying excellent ion conductivity. With an increase in the thickness of the cm-PBI membrane, the cell resistance increases accordingly (0.49 Ω). This will lead to high ohmic polarization at high current densities. Note that the cm-PBI membranes conduct mixed sulfate/chloride anions in case of using BMImCl in catholytes. It has been found that the Cl- anions mostly help stabilize30 and affect the first solvation shell of vanadium species28 in catholytes (V4+, V5+), rather than in anolyte (V2+, V3+). Therefore, it is considered that diffusion of partial Cl- anions into anolyte during cycling will not alter significantly the battery performance. For practical operation, it is necessary to restore the capacity by periodically remixing the catholyte and anolyte. The effect of additive on the anolyte will be examined in the future.

Table 2. Properties of initial catholytes and membranes used Membrane Nafion 212 cm-PBI-10-15

cm-PBI-10-50

Ratio between BMImCl : V (1.6 M) in the Cell resistance / Ω initial catholyte 0:1 0.36 0.1 : 1 3.60 0:1 0.23 0.05 : 1 0.21 0.1 : 1 0.25 0.2 : 1 0.20 0.1 : 1 0.49

Single cell galvanostatic charge/discharge cycles were performed at room temperature between cutoff voltages of 1.7 and 0.8 V, at current densities ranging from 50 to 100 mA cm-2 (Figure 3). The difference in the cycling efficiencies including Coulombic, voltage and energy efficiency for Nafion 212, cm-PBI-10-15 and cm-PBI-10-50 is compared at first (Figure 3a-3c). The Coulombic efficiency remains nearly constant from 50 to 100 mA cm-2 over about 40 cycles for


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each individual test, demonstrating excellent chemical/mechanical stability of the cm-PBI membranes. The cell with Nafion 212 has the lowest Coulombic efficiency (96.7%, Figure 3a). It is a common observation that Nafion membranes have relatively low ion selectivity and accordingly high vanadium crossover.5 In contrast, enhanced Coulombic efficiencies of 99.6% and 99.1% were observed for cm-PBI-10-50 and cm-PBI-10-15, respectively (Figure 3b,3c). This indicates that the positively charged backbone of the cm-PBI membranes functions well to the Donnan exclusion of vanadium cations (Figure 1). The increase in Coulombic efficiency with increasing thickness is related to the increased length of the diffusion pathway, which affects the diffusion-based permeation of vanadium ions.5,44

Figure 3. (a-c) Cycling efficiencies for different membranes at varied current densities at room temperature, (a) Nafion 212 without catholyte additive, (b,c) cm-PBI-10 AEM with a BMImCl : V ratio of 0.1:1, (d) Voltage profiles at 75 mA cm-2 and room temperature for different membranes with varied amounts of catholyte additive, (e) Dependence of cycling efficiencies of cm-PBI-10-15 membrane at 50 mA cm-2 and room temperature, and the conductivity of the studied electrolytes on the ratio of BMImCl : V, (f) Cyclic voltammetry curves of the catholytes with a BMImCl : V ratio of 0:1 and 0.1:1, measured at a scan rate of 5 mV s-1.


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From 50 to 100 mA cm-2, cm-PBI-10-15 membrane has the highest voltage and energy efficiencies. At 50 mA cm-2, cm-PBI-10-15 has an energy efficiency of about 92%, compared to 85% for Nafion 212. Even at 100 mA cm-2, cm-PBI-10-15 has a steady energy efficiency of about 82% over cycling, compared to 76% for Nafion 212 and 58% for cm-PBI-10-50, respectively (Figure 3a,3b). Interestingly, this is higher than that of a previously reported metaPBI membrane (about 67%, 15 μm in thickness),19 a dense acid-doped PBI membrane (about 78%, 25 μm in thickness),20 and a recent work for poly(p-phenylene)-based ionomers (80%, 40– 50 μm in thickness).45 Interestingly, this is a striking contrast to the previous electrochemical data that PBI membranes (with a thickness between 15 and 30 μm) show higher energy efficiency than the Nafion membranes only at low current densities (typically below 50 mA cm2).16,19

Figure 3d shows the voltage profiles of a single cell measured at 75 mA cm-2 with

different membranes and catholytes. In comparison with those of the thinner cm-PBI-10-15 and the Nafion 212, thicker cm-PBI-10-50 membrane with higher ohmic resistance leads to higher overpotential and accordingly lower voltage efficiency (70%), and also lower reversible capacity. Furthermore, we examined the influence of the amounts of the catholyte additive on the electrochemical performance (Figure 3d,3e). For the cm-PBI-10-15 membrane, depending on the ratio of BMImCl : V in the catholytes ranging from 0:1 to 0.2:1, difference in the voltage profiles such as the voltage efficiency and the accessible capacity were observed (Figure 3d). The dependence of cycling efficiencies on the amounts of BMImCl in catholytes become more significant at 50 mA cm-2 (Figure 3e). The Coulombic efficiency increases continuously with the amount of BMImCl and reaches a maximum at a ratio of 0.1:1. Vanadium species may form


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complexes with the additive, which will then suppress their cross-mixing. Accordingly, the Coulombic efficiency can be enhanced and reaches a maximum at an optimal amount of additive. The voltage efficiency increases from 90% to 92% with the presence of additive, but keeps constant from the ratio of 0.05:1 to 0.2:1. Accordingly, a maximal energy efficiency was then obtained at a ratio of 0.1:1. In the presence of Cl- in the catholyte, it is considered that soluble VO2Cl is formed as oxidized neutral species.30 In addition, additive with only a low fraction is sufficient to stabilize vanadium ions due to a possible rapid and dynamic interaction mechanism.46 The conductivity of the studied electrolytes reduces continuously from 248 to 210 mS cm-1 from a BMImCl : V ratio of 0:1 to 0.2:1 (Figure 3e), indicating that the additive can deteriorate the ion transport properties. The positive effect of BMImCl is also reflected in the reaction kinetics. Cyclic voltammetry curves of the charged VO2+ catholytes using glassy carbon working electrode obtained at a scanning rate of 5 mV s-1 are shown in Figure 3f. The oxidation and reduction reactions of the VO2+/VO2+ couple located at about 1.05 V and 0.7 V were observed.47 A large difference was observed in the peak potential separation (ΔE) between the oxidation and reduction for the catholyte with (0.30 V) and without (0.45 V) BMImCl. This implies considerably improved reaction kinetics with the presence of BMImCl, which may be related to a change in the coordination environment of vanadium species and possible adsorption of additive on the electrode surface.38,39 This explains the enhanced voltage efficiency in the presence of additive, as observed in Figure 3e. It has been reported that the catalytic behavior of an organic additive for the VO2+/VO2+ couple is independent of its concentration (from 0.05 to 0.3 M).39 This is consistent with our observation that the voltage efficiency remains nearly unchanged with the BMImCl : V ratio from 0.05:1 to 0.2 :1 (Figure 3e).


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Figure 4. (a) Normalized capacity retention of a Nafion 212 membrane, and cm-PBI-10 membranes at varied current densities, (b) Normalized discharge capacity retention of a cm-PBI10-15 membrane at 150 mA cm-2 over 100 cycles, with a BMImCl : V ratio of 0.1:1, and (c) Long-term cycling performance of a cm-PBI-10-50 membrane at different current densities from 125 to 250 mA cm-2 over 600 cycles between 1.74 and 0.76 V (fresh electrolyte and electrode were used after 75 cycles at 125 mA cm-2 due to leakage of the cell), with a BMImCl : V ratio of 0.1:1. Inset in (c) shows negligible change in the electrolyte volume after 600 cycles.

Ion crossover and solvent transport through the membranes can cause changes in electrolyte volume and concentration of vanadium ions in the half cells. Such imbalance will then lead to reduced utilization ratio of vanadium and concentration overpotential. Interestingly, negligible change in electrolyte volume has been observed over 60 cycles for about 150 h for the thin cmPBI-10-15 membranes, suggesting low net electroosmotic transfer of water, unlike the one


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reported previously for a Nafion membrane48 or a dense acid-doped PBI membrane.20 Compared to Nafion 212, cm-PBI membranes showed slightly improved capacity retention (Figure 4a). Typically, depending on the thickness, a trade-off between voltage efficiency and capacity retention of membranes has been reported.49 It can be clearly seen that thicker cm-PBI-10-50 membrane can retain the capacity well with a capacity decay of 0.28% per cycle at 75 mA cm-2, which is 1.12% and 0.70% for Nafion 212 and cm-PBI-10-15, respectively (Figure 4a). At 150 mA cm-2, the cm-PBI-10-15 membrane showed a capacity retention of 67.4% after 100 cycles (i.e., a capacity decay of 0.33% per cycle, Figure 4b). Figure 4c shows the long-term cyclability of the cm-PBI-10-50 membrane over 600 charge/discharge cycles at different current densities from 125 to 250 mA cm-2. Over cycling, a steady Coulombic efficiency of about 99.4% was observed. After the initial 75 cycles at 125 mA cm-2, the electrolyte and electrode were replaced and the cell was reassembled due to leakage of the cell. The membrane was washed in 2 M H2SO4 and reused. Interestingly, after changing the electrolyte and setting the current density to 175 mA cm-2, the utilization ratio of the vanadium electrolyte (theoretical capacity: 514.5 mAh) increases from about 22 to 58%, and the voltage efficiency of the cell increases from about 52 to 65%, respectively. This indicates an activation of the fresh cm-PBI membrane over the first 75 cycles. Such phenomenon has not yet been reported. At highly applied current densities of 200 and 250 mA cm-2, a high starting utilization ratio of the electrolyte of 47 and 22% was observed, respectively. There is a decrease in the utilization ratio of electrolyte over cycling, as also found in Figure 4a,4b. In addition, a high voltage efficiency of 60% at 200 mA cm-2, and about 50% at 250 mA cm-2 has been observed, respectively. After 470 cycles at 250 mA cm-2, the voltage efficiency reduces slightly to 45%. These results confirm a stable chemical and electrochemical performance of the studied cm-PBI-10 membrane.


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Table 3. Electrolyte stability test at elevated temperatures. Catholytes with different amounts of additive were collected after charging to 1.7 V. BMImCl : V ratio before heating

0:1

0.1 : 1

0.2 : 1

after 50°C for 2h after 60°C for 0.5h

precipitation no precipitation

precipitation

In order to understand the additive-vanadium interaction and to examine the effect of the amounts of additive, the catholytes were then analyzed at high temperature. It has been reported that the vanadium electrolyte forms precipitates at above 40°C due to the endothermic nature of the precipitate reaction from VO2+ to V2O5.31,38 Thus, charged vanadium catholytes with varied amounts of BMImCl additive were collected and subjected to elevated temperatures (40°C for 0.5 h, 50°C for 2 h and 60°C for 0.5 h) to observe intermittently the presence of precipitates (Table 3). No obvious changes were found from room temperature up to 50°C. However, after keeping the catholytes at 60°C for 0.5 h, catholytes with BMImCl : V ratio of 0:1 and 0.2:1 turn into turbid and large amounts of precipitates were observed at the bottom of the glass containers. In contrast, this sample with 0.1:1 ratio is still clear, suggesting enhanced thermal stability. Without additive, it was reported that the V5+ species exist as hydrated penta-coordinated vanadate ion, namely [VO2(H2O)3]+,50 which will evolve into neutral VO(OH)3 via a deprotonation process, and then into insoluble V2O5 through a hydrolysis reaction at high temperature.14,50 In the presence of certain amounts of Cl-, improved thermal stability of VO2+ can be obtained through the formation of water soluble species VO2Cl(H2O)2.30 It has been


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observed that VO2+ and VO2+ can be stabilized with Cl- in 6 M HCl.30 However, the use of highly concentrated HCl may cause increased risk of metal corrosion and unwanted chlorine evolution side reaction.51 In this work, we demonstrate that an optimized concentration of only 0.16 M BMImCl can stabilize 1.6 M VO2+ solutions, without using concentrated hazardous hydrochloric acid. To examine the local coordination environment of vanadium species with and without BMImCl additive in the catholytes, three 1.7 V charged catholytes with a BMImCl : V ratio of 0:1, 0.1:1 and 0.2:1 were studied with 51V NMR, as shown in Figure 5. The collected catholytes may still contain certain amounts of VO2+, as observed from the red brown color. Note that only V5+ ions are active in the

51V

NMR owing to its diamagnetism. With the presence of VO2+,

paramagnetic dipolar broadening of the VO2+ signal has been reported.32 Figure 5a shows a comparison among samples with different BMImCl contents at room temperature. All of the spectra are characterized by a single broad line centered at approximately -552 ppm, in a good agreement with the value of -551 ppm for 1.6 M vanadium solution.52 Catholytes 0.1:1 and 0.2:1 have almost equal linewidth, indicating similar local environments around vanadium ions. On the other hand, catholyte 0:1 is characterized by a larger linewidth, suggesting possibly a different structure, viscosity and diamagnetic/paramagnetic vanadium ratio.


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Figure 5. 51V NMR spectra of vanadium electrolytes with different additive content. (a) Spectra at room temperature, (b-d) Temperature-dependent 51V NMR spectra with a BMImCl : V ratio of 0:1, 0.1:1 and 0.2:1, respectively, (e,f) Linewidth and chemical shift as a function of temperature, respectively.

Variable temperature

51V

NMR spectra were collected to examine the change in the local

coordination environment of vanadium species. Upon heating, all samples also exhibit significant line broadening (Figure 5b-5d), reflecting a variation of electron density in the molecular orbitals and the mobility of atoms.33 At 53°C, the signal is hardly visible for the 0:1 vanadium catholyte (Figure 5b), but is still resolvable for the 0.1:1 and 0.2:1 catholytes (Figure 5c,5d). Moreover, the initial and the terminal line widths of the 0.1:1 and 0.2:1 catholytes are essentially narrower compared to those of 0:1 sample (Figure 5e). The broadening in the linewidth at high temperatures can be attributed to the formation of the paramagnetic VO2+.32 Lu53 ascribed such line broadening to the polymerization of vanadium species in the solution, which leads to an increase in viscosity of the electrolyte. The BMIm+ cations may inhibit the


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polymerization of vanadium species, and suppress irreversible precipitation through forming vanadium-additive complex.39 To address the chemical stability of different catholytes from about 20 to 58°C, the temperature dependence of the chemical shift is shown in Figure 5f. With an increase in temperature, the chemical shift of

51V

moves towards higher frequency due to

nuclear deshielding was observed for all samples.30,50 However, a clear difference can be seen among different samples. The 51V chemical shift of 0:1 catholyte stays roughly constant until a temperature of 43°C is reached. Afterwards, an abrupt increase of the chemical shift was observed. Similar behavior is found for 0.2:1 catholyte, but at a higher onset temperature. In contrast, the chemical shift for the 0.1:1 catholyte remains nearly invariable upon heating up to 58°C. Thus, it is concluded that the 0:1 vanadium catholyte has poor stability against temperature above 50°C, in line with previous work,30 and the visual observation in Table 3. The addition of BMImCl stabilizes the vanadium species at elevated temperatures and an optimal BMImCl : V ratio of 0.1:1, as found from electrochemical tests, has been confirmed. Preliminary tests of the chemical stability of the cm-PBI-10-15 membrane were performed by recording the Fourier transform infrared spectra (FTIR), as shown in Figure 6. After disassembling the flow cells (with 68 h cycling), membrane with areas that unexposed and exposed to the vanadium electrolytes was examined. The bands in the 1630, 1521 and 1465 cm-1 of the spectra overlapped well and can be assigned to different vibration and stretching modes of the aromatic backbone.54 In addition, the characteristic peak at 810 cm-1 from the C-H bending of the benzene ring of the membrane maintained well after flow battery tests.55 Changes were observed for the weak bands at 2927 and 2857cm-1 that are related to the methyl asymmetric and symmetric stretching.56 These bands became even weaker after exposing to the vanadium electrolytes, indicating possible membrane degradation due to the attack from the electrolyte.


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The peak at 1016 cm-1 assigned to a benzene ring vibration of PBI decreases after exposing the membrane to electrolyte (Figure 6a, 6b).57 Additionally, a strong band at 1164 cm-1 was observed for the membrane with more additive in the catholyte (Figure 6a). This band was observed for pure BMImCl.58 The changes in the chemical structure of the membrane may probably have impact on the capacity retention properties, as observed in Figure 4a. Further optimization of the membrane and electrolyte formulation is needed in the future.

Figure 6. FTIR spectra of the cm-PBI-10-15 membrane collected from the disassembled flow cells with areas unexposed and exposed to vanadium electrolytes.

4. Conclusions We report that low-cost PBI-based mixed anion exchange/ion solvating membranes with high ion conductivity under acidic conditions can be obtained through methylation and crosslinking and then investigate their electrochemical performance in vanadium RFBs. Earlier reported PBI membranes showed higher energy efficiency than Nafion membranes only at low current density


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about below 50 mA cm-2. Interestingly, in this work, enhanced cycling efficiency at high current density up to 100 mA cm-2 has been achieved by simultaneously tailoring the catholyte formulation and using a thin cm-PBI-10 membrane. A maximal energy efficiency has been observed when BMImCl as catholyte additive with an optimized content of 0.16 M in 1.6 M vanadium solution was used. Up to 600 charge/discharge cycles have been carried out for a cmPBI-10-50 membrane. Such a membrane undergoes an activation process during the initial 75 cycles. Enhanced utilization ratio of the vanadium electrolyte, and improved voltage efficiency have been observed after the activation cycling. Furthermore, high temperature stability analysis of the charged catholytes by direct visual observations and

51V

NMR experiments help to

understand the positive effect of adding BMImCl in vanadium catholytes. Again, these results confirm the optimized ratio of BMImCl : V of 0.1:1, consistent with the electrochemical data. This work is suggestive for developing high performance RFBs by using compatible membrane and electrolyte.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT We acknowledge the KIST Europe basic research funding: “New electrolytes for redox flow batteries”. D.H. thanks the German-Korean joint SME R&D project of ZIM-AIF and MOTIE/KIAT. R.C. thanks Prof. R. Hempelmann for his continuing support, and J.Y. Bae for his assistance to determine the membrane conductivity.


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REFERENCES (1) Skyllas-Kazacos, M.; Chakrabarti, M. H.; Hajimolana, S. A.; Mjalli, F. S.; Saleem, M. Progress in Flow Battery Research and Development. J. Electrochem. Soc. 2011, 158, R55-R79. (2) Park, M.; Ryu, J.; Wang, W.; Cho, J. Material Design and Engineering of Next-Generation Flow-Battery Technologies. Nat. Rev. Mater. 2016, 2, 16080. (3) Li, X.; Zhang, H.; Mai, Z.; Zhang, H.; Vankelecom, I. Ion Exchange Membranes for Vanadium Redox Flow Battery (VRB) Applications. Energy Environ. Sci. 2011, 4, 1147-1160. (4) Ding, C.; Zhang, H.; Li, X.; Liu, T.; Xing, F. Vanadium Flow Battery for Energy Storage: Prospects and Challenges. J. Phys. Chem. Lett. 2013, 4, 1281-1294. (5) Jiang, B.; Wu, L.; Yu, L.; Qiu, X.; Xi, J. A Comparative Study of Nafion Series Membranes for Vanadium Redox Flow Batteries. J. Membr. Sci. 2016, 510, 18-26. (6) Shin, D. W.; Guiver, M. D.; Lee, Y. M. Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability. Chem. Rev. 2017, 117, 4759-4805. (7) Xi, J.; Li, Z.; Yu, L.; Yin, B.; Wang, L.; Liu, L.; Qiu, X.; Chen, L. Effect of Degree of Sulfonation and Casting Solvent on Sulfonated Poly(ether ether ketone) Membrane for Vanadium Redox Flow Battery. J. Power Sources 2015, 285, 195-204. (8) Liao, J. B.; Lu, M. Z.; Chu, Y. Q.; Wang, J. L. Ultra-Low Vanadium Ion Diffusion Amphoteric Ion-Exchange Membranes for All-Vanadium Redox Flow Batteries. J. Power Sources 2015, 282, 241-247.


22

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Lu, W.; Yuan, Z.; Zhao, Y.; Li, X.; Zhang, H.; Vankelecom, I. F. J. High-Performance Porous Uncharged Membranes for Vanadium Flow Battery Applications Created by Tuning Cohesive and Swelling Forces. Energy Environ. Sci. 2016, 9, 2319-2325. (10) Winardi, S.; Raghu, S. C.; Oo, M. O.; Yan, Q.; Wai, N.; Lim, T. M.; Skyllas-Kazacos, M. Sulfonated Poly (ether ether ketone)-based Proton Exchange Membranes for Vanadium Redox Battery Applications. J. Membr. Sci. 2014, 450, 313-322. (11) Chae, I. S.; Luo, T.; Moon, G. H.; Ogieglo, W.; Kang, Y. S.; Wessling, M. Ultra-High Proton/Vanadium Selectivity for Hydrophobic Polymer Membranes with Intrinsic Nanopores for Redox Flow Battery. Adv. Energy Mater. 2016, 6, 1600517. (12) Teng, X.; Zhao, Y.; Xi, J.; Wu, Z.; Qiu, X.; Chen, L. Nafion/Organic Silica Modified TiO2 Composite Membrane for Vanadium Redox Flow Battery via in Situ Sol-Gel Reactions. J. Membr. Sci. 2009, 341, 149-154. (13) Kim, S.; Choi, J.; Choi, C.; Heo, J.; Kim, D. W.; Lee, J. Y.; Hong, Y. T.; Jung, H.-T.; Kim, H.-T. Pore-Size-Tuned Graphene Oxide Frameworks as Ion-Selective and Protective Layers on Hydrocarbon Membranes for Vanadium Redox-Flow Batteries. Nano Lett. 2018, 18, 3962-3968. (14) Zhang, H.; Zhang, H.; Li, X.; Mai, Z.; Zhang, J. Nanofiltration (NF) Membranes: The Next Generation Separators for All Vanadium Redox Flow Batteries (VRBs)? Energy Environ. Sci. 2011, 4, 1676-1679. (15) Schwenzer, B.; Zhang, J.; Kim, S.; Li, L.; Liu, J.; Yang, Z. Membrane Development for Vanadium Redox Flow Batteries. ChemSusChem 2011, 4, 1388-1406.


23


Page 24 of 36

(16) Zhou, X. L.; Zhao, T. S.; An, L.; Wei, L.; Zhang, C. The Use of Polybenzimidazole Membranes in Vanadium Redox Flow Batteries Leading to Increased Coulombic Efficiency and Cycling Performance. Electrochim. Acta 2015, 153, 492-498. (17) Luo, T.; David, O.; Gendel, Y.; Wessling, M. Porous Poly(benzimidazole) Membrane for All Vanadium Redox Flow Battery. J. Power Sources 2016, 312, 45-54. (18) Ye, R.; Henkensmeier, D.; Yoon, S. J.; Huang, Z.; Kim, D. K.; Chang, Z.; Kim, S.; Chen, R. Redox Flow Batteries for Energy Storage: A Technology Review. J. Electrochem. En. Conv. Stor. 2017, 15, 010801. (19) Noh, C.; Jung, M.; Henkensmeier, D.; Nam, S. W.; Kwon, Y. Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses. ACS Appl. Mater. Interfaces 2017, 9, 36799-36809. (20) Jang, J.-K.; Kim, T.-H.; Yoon, S. J.; Lee, J. Y.; Lee, J.-C.; Hong, Y. T. Highly Proton Conductive, Dense Polybenzimidazole Membranes with Low Permeability to Vanadium and Enhanced H2SO4 Absorption Capability for Use in Vanadium Redox Flow Batteries. J. Mater. Chem. A 2016, 4, 14342-14355. (21) Luo, T.; Dreusicke, B.; Wessling, M. Tuning the Ion Selectivity of Porous Poly(2,5benzimidazole) Membranes by Phase Separation for All Vanadium Redox Flow Batteries. J. Membr. Sci. 2018, 556, 164-177. (22) Qiao, L.; Zhang, H.; Li, M.; Yuan, Z.; Zhao, Y.; Li, X. A Venus-Flytrap-Inspired pHResponsive Porous Membrane with Internal Crosslinking Networks. J. Mater. Chem. A 2017, 5, 25555-25561.


24

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Yuan, Z.; Duan, Y.; Zhang, H.; Li, X.; Zhang, H.; Vankelecom, I. Advanced Porous Membranes with Ultra-High Selectivity and Stability for Vanadium Flow Batteries. Energy Environ. Sci. 2016, 9, 441-447. (24) Xia, Z.; Ying, L.; Fan, J.; Du, Y.-Y.; Zhang, W.-M.; Guo, X.; Yin, J. Preparation of Covalently Cross-Linked Sulfonated Polybenzimidazole Membranes for Vanadium Redox Flow Battery Applications. J. Membr. Sci. 2017, 525, 229-239. (25) Weissbach, T.; Wright, A. G.; Peckham, T. J.; Alavijeh, A. S.; Pan, V.; Kjeang, E.; Holdcroft, S. Simultaneous, Synergistic Control of Ion Exchange Capacity and Cross-Linking of Sterically-Protected Poly(benzimidazolium)s. Chem. Mater. 2016, 28, 8060-8070. (26) Chang, Z.; Henkensmeier, D.; Chen, R. One-Step Cationic Grafting of 4-HydroxyTEMPO and its Application in a Hybrid Redox Flow Battery with a Crosslinked PBI Membrane. ChemSusChem 2017, 10, 3193-3197. (27) Chen, W.-F.; Lin, H.-Y.; Dai, S. A. Generation and Synthetic Uses of Stable 4-[2Isopropylidene]-phenol Carbocation from Bisphenol A. Org. Lett. 2004, 6, 2341-2343. (28) Bon, M.; Laino, T.; Curioni, A.; Parrinello, M. Characterization of Vanadium Species in Mixed Chloride–Sulfate Solutions: An Ab Initio Metadynamics Study. J. Phys. Chem. C 2016, 120, 10791-10798. (29) Sepehr, F.; Paddison, S. J. Effect of Sulfuric and Triflic Acids on the Hydration of Vanadium Cations: An ab Initio Study. J. Phys. Chem. A 2015, 119, 5749-5761.


25


Page 26 of 36

(30) Li, L.; Kim, S.; Wang, W.; Vijayakumar, M.; Nie, Z.; Chen, B.; Zhang, J.; Xia, G.; Hu, J.; Graff, G.; Liu, J.; Yang, Z. A Stable Vanadium Redox‐Flow Battery with High Energy Density for Large-Scale Energy Storage. Adv. Energy Mater. 2011, 1, 394-400. (31) Rahman, F.; Skyllas-Kazacos, M. Vanadium Redox Battery: Positive Half-Cell Electrolyte Studies. J. Power Sources 2009, 189, 1212-1219. (32) Kim, S.; Choi, C.; Kim, R.; Kim, H. G.; Kim, H.-T. Temperature-Dependent 51V Nuclear Magnetic Resonance Spectroscopy for the Positive Electrolyte of Vanadium Redox Flow Batteries. RSC Adv. 2016, 6, 96847-96852. (33) Choi, C.; Kim, S.; Kim, R.; Choi, Y.; Kim, S.; Jung, H.-y.; Yang, J. H.; Kim, H.-T. A Review of Vanadium Electrolytes for Vanadium Redox Flow Batteries. Renew. Sust. Energ. Rev. 2017, 69, 263-274. (34) Lu, W.; Li, X.; Zhang, H. The Next Generation Vanadium Flow Batteries with High Power Density – A Perspective. Phys. Chem. Chem. Phys. 2018, 20, 23-25. (35) Xiao, S.; Yu, L.; Wu, L.; Liu, L.; Qiu, X.; Xi, J. Broad Temperature Adaptability of Vanadium Redox Flow Battery–Part 1: Electrolyte Research. Electrochim. Acta 2016, 187, 525534. (36) Zhang, J.; Li, L.; Nie, Z.; Chen, B.; Vijayakumar, M.; Kim, S.; Wang, W.; Schwenzer, B.; Liu, J.; Yang, Z. Effects of Additives on the Stability of Electrolytes for All-Vanadium Redox Flow Batteries. J. Appl. Electrochem. 2011, 41, 1215-1221. (37) Roe, S.; Menictas, C.; Skyllas-Kazacos, M. A High Energy Density Vanadium Redox Flow Battery with 3 M Vanadium Electrolyte. J. Electrochem. Soc. 2016, 163, A5023-A5028.


26

Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Li, S.; Huang, K.; Liu, S.; Fang, D.; Wu, X.; Lu, D.; Wu, T. Effect of Organic Additives on Positive Electrolyte for Vanadium Redox Battery. Electrochim. Acta 2011, 56, 5483-5487. (39) Hwang, J.; Kim, B.-m.; Moon, J.; Mehmood, A.; Ha, H. Y. A Highly Efficient and Stable Organic Additive for the Positive Electrolyte in Vanadium Redox Flow Batteries: Taurine Biomolecules Containing –NH2 and –SO3H Functional Groups. J. Mater. Chem. A 2018, 6, 4695. (40) Chen, R.; Hempelmann, R. Ionic Liquid-Mediated Aqueous Redox Flow Batteries for High Voltage Applications. Electrochem. Commun. 2016, 70, 56-59. (41) Zhang, Y.; Ye, R.; Henkensmeier, D.; Hempelmann, R.; Chen, R. “Water-in-Ionic Liquid” Solutions Towards Wide Electrochemical Stability Windows for Aqueous Rechargeable Batteries. Electrochim. Acta 2018, 263, 47-52. (42) Chang, F.; Hu, C.; Liu, X.; Liu, L.; Zhang, J. Coulter Dispersant as Positive Electrolyte Additive for the Vanadium Redox Flow Battery. Electrochim. Acta 2012, 60, 334-338. (43) Hu, M.; Ni, J.; Zhang, B.; Neelakandan, S.; Wang, L. Crosslinked Polybenzimidazoles Containing Branching Structure as Membrane Materials with Excellent Cell Performance and Durability for Fuel Cell Applications. J. Power Sources 2018, 389, 222-229. (44) Darling, R. M.; Weber, A. Z.; Tucker, M. C.; Perry, M. L. The Influence of Electric Field on Crossover in Redox-Flow Batteries. J. Electrochem. Soc. 2016, 163, A5014-A5022. (45) Shin, H. Y.; Cha, M. S.; Hong, S. H.; Kim, T.-H.; Yang, D.-S.; Oh, S.-G.; Lee, J. Y.; Hong, Y. T. Poly(p-phenylene)-based Membrane Materials with Excellent Cell Efficiencies and Durability for Use in Vanadium Redox Flow Batteries. J. Mater. Chem. A 2017, 5, 12285-12296.


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Page 28 of 36

(46) Yamamura, T.; Watanabe, N.; Yano, T.; Shiokawa, Y. Electron-Transfer Kinetics of Np3 + ∕ Np4 +, NpO2 + ∕ NpO22 +, V2 + ∕ V3 +, and VO2 + ∕ VO2 + at Carbon Electrodes. J. Electrochem. Soc. 2005, 152, A830-A836. (47) Li, W.; Zhang, Z.; Tang, Y.; Bian, H.; Ng, T.-W.; Zhang, W.; Lee, C. S. Graphene‐Nanowall‐Decorated Carbon Felt with Excellent Electrochemical Activity Toward VO2+/VO2+ Couple for All Vanadium Redox Flow Battery. Adv. Sci. 2016, 3, 1500276. (48) Nibel, O.; Rojek, T.; Schmidt, T. J.; Gubler, L. Amphoteric Ion-Exchange Membranes with Significantly Improved Vanadium Barrier Properties for All-Vanadium Redox Flow Batteries. ChemSusChem 2017, 10, 2767-2777. (49) Luo, Q.; Li, L.; Wang, W.; Nie, Z.; Wei, X.; Li, B.; Chen, B.; Yang, Z.; Sprenkle, V. Capacity Decay and Remediation of Nafion-based All-Vanadium Redox Flow Batteries. ChemSusChem 2013, 6, 268-274. (50) Vijayakumar, M.; Li, L.; Graff, G.; Liu, J.; Zhang, H.; Yang, Z.; Hu, J. Z. Towards Understanding the Poor Thermal Stability of V5+ Electrolyte Solution in Vanadium Redox Flow Batteries. J. Power Sources 2011, 196, 3669-3672. (51) Chen, R.; Trieu, V.; Natter, H.; Kintrup, J.; Bulan, A.; Hempelmann, R. Wavelet Analysis of Chlorine Bubble Evolution on Electrodes with Different Surface Morphologies. Electrochem. Commun. 2012, 22, 16-20. (52) Ding, C.; Ni, X.; Li, X.; Xi, X.; Han, X.; Bao, X.; Zhang, H. Effects of Phosphate Additives on the Stability of Positive Electrolytes for Vanadium Flow Batteries. Electrochim. Acta 2015, 164, 307-314.


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(53) Lu, X. Spectroscopic Study of Vanadium(V) Precipitation in the Vanadium Redox Cell Electrolyte. Electrochim. Acta 2001, 46, 4281-4287. (54) Aili, D.; Cleemann, L. N.; Li, Q.; Jensen, J. O.; Christensen, E.; Bjerrum, N. J. Thermal Curing of PBI Membranes for High Temperature PEM Fuel Cells. J. Mater. Chem. 2012, 22, 5444-5453. (55) Aili, D.; Hansen, M. K.; Pan, C.; Li, Q.; Christensen, E.; Jensen, J. O.; Bjerrum, N. J. Phosphoric Acid Doped Membranes Based on Nafion, PBI and Their Blends – Membrane Preparation, Characterization and Steam Electrolysis Testing. Int. J. Hydrogen Energy 2011, 36, 6985-6993. (56) Eren, E.; Sarihan, A.; Eren, B.; Gumus, H.; Kocak, F. O. Preparation, Characterization and Performance Enhancement of Polysulfone Ultrafiltration Membrane Using PBI as Hydrophilic Modifier. J. Membr. Sci. 2015, 475, 1-8. (57) Deimede, V.; Voyiatzis, G. A.; Kallitsis, J. K.; Li, Q.; Bjerrum, N. J. Miscibility Behavior of Polybenzimidazole/Sulfonated Polysulfone Blends for Use in Fuel Cell Applications. Macromolecules 2000, 33, 7609-7617. (58) Shi, F.; Peng, J.; Deng, Y. Highly Efficient Ionic Liquid-Mediated Palladium Complex Catalyst System for the Oxidative Carbonylation of Amines. J. Catal. 2003, 219, 372-375.


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Table of Contents Graphic


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Figure 1. (a) Chemical structure of a cm-PBI membrane with positively charged backbone, which repulses the vanadium cations, but allows the free transport of anions, (b) Cross section SEM image of the cm-PBI membrane. 64x70mm (300 x 300 DPI)



Figure 2. Change of concentration of VO2+ and color with diffusion time in the right reservoir of the diffusion cell for the Nafion 212, and the cm-PBI-10-15 membranes. 99x69mm (300 x 300 DPI)


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Figure 3. (a-c) Cycling efficiencies for different membranes at varied current densities at room temperature, (a) Nafion 212 without catholyte additive, (b,c) cm-PBI-10 AEM with a BMImCl : V ratio of 0.1:1, (d) Voltage profiles at 75 mA cm-2 and room temperature for different membranes with varied amounts of catholyte additive, (e) Dependence of cycling efficiencies of cm-PBI-10-15 membrane at 50 mA cm-2 and room temperature, and the conductivity of the studied electrolytes on the ratio of BMImCl : V, (f) Cyclic voltammetry curves of the catholytes with a BMImCl : V ratio of 0:1 and 0.1:1, measured at a scan rate of 5 mV s-1. 159x79mm (300 x 300 DPI)



Figure 4. (a) Normalized capacity retention of a Nafion 212 membrane, and cm-PBI-10 membranes at varied current densities, (b) Normalized discharge capacity retention of a cm-PBI-10-15 membrane at 150 mA cm2 over 100 cycles, with a BMImCl : V ratio of 0.1:1, and (c) Long-term cycling performance of a cm-PBI-1050 membrane at different current densities from 125 to 250 mA cm-2 over 600 cycles between 1.74 and 0.76 V (fresh electrolyte and electrode were used after 75 cycles at 125 mA cm-2 due to leakage of the cell), with a BMImCl : V ratio of 0.1:1. Inset in (c) shows negligible change in the electrolyte volume after 600 cycles. 159x111mm (300 x 300 DPI)


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Figure 5. 51V NMR spectra of vanadium electrolytes with different additive content. (a) Spectra at room temperature, (b-d) Temperature-dependent 51V NMR spectra with a BMImCl : V ratio of 0:1, 0.1:1 and 0.2:1, respectively, (e,f) Linewidth and chemical shift as a function of temperature, respectively. 169x89mm (300 x 300 DPI)



Figure 6. FTIR spectra of the cm-PBI-10-15 membrane collected from the disassembled flow cells with areas unexposed and exposed to vanadium electrolytes. 74x96mm (300 x 300 DPI)


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Improved All-Vanadium Redox Flow Batteries using Catholyte Additive

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