Vanadium Redox Flow Batteries Using meta-Polybenzimidazole

Oct 10, 2017 - Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses. Chanho Noh†⊥, Mina Jung†‡â...
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Vanadium Redox Flow Batteries Using meta-PolybenzimidazoleBased Membranes of Different Thicknesses Chanho Noh,†,⊥ Mina Jung,†,‡,⊥ Dirk Henkensmeier,*,‡,§,∥ Suk Woo Nam,‡,∥ and Yongchai Kwon*,† †

Graduate School of Energy and Environment, Seoul National University of Science and Technology, Nowon-gu, Seoul 01811, Republic of Korea ‡ Fuel Cell Research Center, Korea Institute of Science and Technology, Hwgrangno 14-gil 5, Seongbukgu, Seoul 02792, Republic of Korea § ET-GT Convergence, University of Science and Technology, Hwarangno 14-gil 5, Seongbukgu, Seoul 02792, Republic of Korea ∥ Green School, Korea University, Seoul 136-713, Republic of Korea S Supporting Information *

ABSTRACT: 15, 25, and 35 μm thick meta-polybenzimidazole (PBI) membranes are doped with H2SO4 and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H2SO4, PBI absorbs about 2 mol of H2SO4 per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, meta-PBI shows a negligible crossover of V3+ and V4+ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBI’s nanosized pores. It turns out that charge efficiency (CE) of VRFBs using meta-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick meta-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin meta-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H2SO4-doped meta-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm−2. KEYWORDS: meta-polybenzimidazole-based membranes, vanadium redox flow batteries, coulombic efficiency, vanadium crossover, electrostatic energy barrier

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INTRODUCTION

The VRFB is a device that is structurally similar to fuel cells. However, instead of a gaseous fuel and oxidant, water-based solutions including vanadium salts are passed through the two electrode chambers, which are separated by an ion-conducting membrane. The capacity of a VRFB system is determined by the volume of electrolyte solution, while the power is controlled by the stack dimensions. Another feature of the VRFB is that multioxidation states of vanadium salts are possible. During the charging process, under consumption of externally provided

With an increasing amount of highly fluctuating renewable energy fed into grids, there are demands to develop large-scale energy-storage solutions. For instance, the average contribution of renewable energy to the public electric power supply in Germany was 37.8% over the first 8 months of 2017. While the production of renewable energies was quite close to the average value on April 29 (35.1%), it shot up to 63.9% the next day.1 In line with this, ASDReports predicted that the global grid storage market will grow from 200 million USD in 2012 to 10.4 billion USD in 2017.2 One impetus inducing growth of the global grid storage market, especially for large-scale energy storage, is the vanadium redox flow battery (VRFB).3−5 © XXXX American Chemical Society

Received: July 24, 2017 Accepted: September 21, 2017

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DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Weight changes and acid doping level occurred during doping with H2SO4 solution.

energy, V3+ is reduced to V2+, while V4+ is oxidized to V5+ in the other electrode compartment. When the system is discharged, the opposite reactions occur.6,7 For maintenance of the electric neutrality of the electrolyte, protons and/or sulfate ions pass through the membrane. The performance of a VRFB is mainly evaluated by measuring the charge efficiency (CE, CE = 100% × discharging time/charging time, assuming that the same current density is used for charging and discharging) and voltage efficiency (VE, VE = 100% × average discharging voltage/charging voltage) and calculating the resulting energy efficiency (EE, EE = CE × VE).8 These efficiencies should be more improved, mainly by enhancing the membrane properties. Among many membrane candidates for VRFB, Nafion that consists of a perfluorinated polymer bearing sulfonated side chains, is the most well-known membrane due to its high oxidative stability and high proton conductivity.9 However, because it is a cation-exchange membrane, permeability of vanadium ions is high, and the ensuing crossover of vanadium ions reduces the CE of the VRFB. For minimization of the crossover of vanadium ions, membrane thickness should be increased. This reduces the VE, which is highly correlated with the ohmic resistance of the system. For a shift of the trade-off relation between CE and VE to a higher level, membranes with lower vanadium crossover and higher or similar conductivity are needed. One of the alternatives was the use of sulfonated poly(ether ether ketone) (SPEEK). According to the previously reported result, a 30 μm thick SPEEK membrane showed higher CE than nominally 25 μm thick Nafion 212.10 One explanation is that SPEEK has narrower channels than Nafion.11 In addition, if multiply charged vanadium ions interact with several sulfonate groups, they are immobilized, and the channel size may be further decreased in these points. Because of that, the positive charge on the immobilized vanadium ion can repel other vanadium ions and thus reduce crossover. Another way to repel vanadium ions and to minimize crossover of vanadium ions is the use of anion-exchange membranes (AEMs).8 AEMs are made of polymers with a fixed positive charge (typically quaternary ammonium groups) attached to the polymer chain and mobile counteranions.12 Theoretically, these materials should repel cations and only attract and conduct sulfate ions. However, because of the (i) small size of protons and (ii) possibly some excess sulfuric acid (H2SO4) in the membrane, the overall conductivity of AEM immersed in VRFB electrolyte is expected to be a mixed proton and anion conduction.

Recently polybenzimidazole-based (PBI-based) membranes were also investigated for VRFBs.13−15 It is known that PBI membranes can be doped with strong acids like phosphoric acid (H3PO4) or H2SO4.16 The pKa values of imidazolium, H3PO4, H2PO4−, H2SO4, and HSO4− are 6.95, 2.15, 7.20, −3, and 1.99, respectively. Therefore, when a PBI membrane is immersed in H2SO4 (the electrolyte used in VRFBs), it will absorb H2SO4 and water. Also, on the basis of the pKa values, it can be deduced that phosphoric acid can only protonate one imidazole group, while sulfuric acid could protonate two imidazole groups. It is expected that the narrow, nanosized channel structure hinders diffusion of large vanadium ions. In addition, the positive charge on the PBI polymer backbone effectively repels vanadium ions, while surplus of H2SO4 allows high conductivity. Although oxidative stability of the PBI polymer backbone is rather low as indicated by the Fenton test,17 aciddoped PBI bears two positive charges per repeat unit, which plays a role in increasing oxidative stability of the material. For instance, in the first reaction step of radical-induced degradation (as in the Fenton test), hydroxy radicals preferentially react with electron-rich aromatic systems.18 Similarly, degradation of polysulfones by V5+ ions may be hindered in the positively charged systems. Although more work on this is needed, Zhao et al. already demonstrated a high stability of PBI in 1 M V5+ solution.13 For cation-exchange membranes, the effect of the electric field on vanadium crossover has been discussed recently.19,20 While diffusion-based permeation of ions is inversely proportional to the membrane thickness, migration and electroosmosis only depend on the current. For membranes which repel vanadium ions, the contribution of diffusion to overall crossover should be negligible, and crossover should be dominated by active transport mechanisms. In addition, as shown in an extensive review that covers the use of PBI membranes in flow batteries,21 the thickness was varied only for porous, spongelike membranes, along with the porosity.14,22−24 However, the result is not clear about what the influences of the polymer and the pores are, because each pore adds two polymer/electrolyte interfaces to the pathway of crossing vanadium ions. In comparison to that, in this work we characterized dense meta-PBI membranes with a thickness of 15, 25, and 35 μm to evaluate the effect of thickness of the dense PBI layer on performance of VRFB single cells. The obtained data show that the CE is practically independent of thickness, with a slight indication that thinner PBI membranes may increase both CE and VE. If this is true, the reason can be that thicker membranes require higher charging potentials. B

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. SEM images of membranes after operation in the VRFB (red boxes indicate area used for elemental analysis by SEM−EDS).

increase. Furthermore, with increases in acid and water uptake, the diameter of the nanosized pores should increase, opening space for mobile ions.15 The water uptake is calculated by subtracting acid uptake from total weight gain. Therefore, errors are potentiated, and water uptake values are the least accurate values. However, all values are in a range 6−12%, meaning that water uptake is rather constant over the given concentration range and similar to the water uptake value reported for PBI-OO (11.6%),33 but still lower than that recorded for meta-PBI (15−19%).34 For evaluation of the method’s accuracy, blind samples that were “doped” in pure water were measured. The calculated ADL was 0.3 acid molecules per repeat unit, and total weight gain was 15.6%. This indicates that the real acid uptake values are a little lower, while the water uptake is a little higher. When membrane samples were immersed in a solution of 1.5 M VOSO4 + 2 M H2SO4, no large effect on the overall weight gain and acid uptake was observed in comparison to membranes doped in 2 M H2SO4. When PBI membranes absorb sulfuric acid, they are expected to change their dimensions. As long as the membranes are preswollen before they are assembled into the VRFB cell, the effect is of less importance than, for example, Nafion membranes in a fuel cell, where the membrane’s water content is highly affected by the relative humidity of the gas streams and the water produced in the electrochemical reaction. However, it is important to know the swelling behavior to cut the pristine membranes into the right shape before doping. It was found that swelling in the length doubles from 2.5% in water to 5.1% in 2 M sulfuric acid, but then hardly changes when the acid concentration is further increased to 8 M (6.4%, see Figure S12 in the Supporting Information). The length expansion is much larger than swelling in thickness. While it is 8% in water, it is 18% in 2 M sulfuric acid, and nearly doubles when the acid concentration is raised to 8 M (33%). Thus, the ratio of thickness to length swelling strongly increases to a maximum value of 5.2 in 8 M sulfuric acid. This is much higher than the

These higher potentials should promote side reactions and allow more vanadium ions to overcome the energy barrier, which hinders cations from entering the membrane.

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RESULTS AND DISCUSSION Sulfuric Acid Uptake of meta-PBI. meta-PBI membranes are not very conductive (conductivity of 10−9 mS cm−1).16 However, when they are immersed in concentrated solutions of strong acids like H3PO4, hydrochloric acid, or H2SO4, they absorb significant amounts of acid. In detailed explanation, the nitrogen atoms of the imidazole rings initially react with the strong acid in an acid−base reaction, leading to the protonated imidazolium form. In H3PO4, that strongly interacts with the polymer, its uptake does not stop after neutralization, and the H3PO4 molecules are linked to other H3PO4 molecules by hydrogen bonds. For example, if meta-PBI is immersed in 85% H3PO4 at room temperature, 5−6 H3PO4 molecules per imidazole ring are absorbed. After the H3PO4 doping, conductivity of meta-PBI membranes increases to the range 1−10 mS cm−1, depending on acid uptake and water contents. In the VRFB, the membranes are immersed in H2SO4-based electrolytes, and it is expected that they absorb both acid and water. Since the properties like acid uptake and water contents strongly influence membrane performance, the overall weight gain and acid uptake of meta-PBI membranes in H2SO4 solutions were measured (Figure 1). According to Figure 1, both total weight gain and acid uptake are proportional to concentration of the doping acid. For instance, in 2−4 M acid solutions, the polymers absorb around 2 acid molecules per repeat unit [acid doping level (ADL) = 2]. This indicates that all acid molecules formally exist as imidazolium hydrogen sulfate. When the acid concentration increases further, a surplus of the H2SO4 starts to enter the membrane. For example, if the membranes are doped in 8 M solution, formally 2 mol of hydrogen sulfate and 1 mol of H2SO4 per mol of repeat unit enter the membrane. With that, the local pH inside the membrane decreases, and its conductivity is expected to C

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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membranes will operate at higher potential, and more ions will be able to surpass the energy barrier to enter the membrane. Figure 4 schematically demonstrates the different size of sulfate, hydronium, and V4+ (actually hydrated VO2+) ions. Vanadium ions are relatively large and multiply charged ions [V2+, V3+, V4+ (which exists as VO2+), and V5+ (in the form of VO2+ under acidic conditions)],37 and thus are affected by electrostatic repulsion when they approach the positively charged PBI backbones. Possibly single-charged hydronium ions are also affected by electrostatic repulsion, but they are small enough to enter the water-filled domains easily. In contrast, sulfate ions are negatively charged and can enter the membranes more easily. According to reference data,38 the chain−chain distance in pure meta-PBI is just about 3.5−4.6 Å, which is compatible with the WAXS data represented in Figure S1. In Figure S1, all materials showed a broad signal with a d value of 3.6−3.7 Å. In addition, a small and sharp signal with a d value of 4.1−4.2 Å was observed at low acid concentrations (0−1 M H2SO4). At higher acid concentrations, this signal disappeared, indicating that H2SO4 interrupted the PBI−PBI interaction responsible for this signal (probably side-to-side packing),38 and that PBI doped at concentration of 2 M H2SO4 or higher is mainly amorphous. Membrane Permeability and Conductivity. For analysis of vanadium crossover, UV−vis spectra of magnesium salt solutions were measured, and absorbance peak intensities of the V4+ and V3+ ions were monitored. Figure 5a,b represents the peaks of the V4+ and V3+ ions in the magnesium sulfate solutions after 24 h, while Figure 5c,d displays the relationship between operating time and concentration of the V4+ and V3+ ions that are accumulated during the testing time. There were three noticeable findings from the tests. First, the concentration of permeated V4+ and V3+ ions has an almost linear relationship with the testing time. This justifies the use of eq 1 for determination of vanadium ion permeability. The crossover flux of vanadium ions was obtained as a product of the vanadium permeability and the concentration gradient. Second, permeability of meta-PBI membranes was much lower than that of Nafion membranes. In fact, the permeability of meta-PBI membranes was so low that no crossover was detected during the testing time of 24 h. In contrast, crossover of Nafion membranes was significant, with higher permeability of Nafion 212 (N212) than that of Nafion 117 (N117). Third, the absorbance spectra for V3+ ions showed two peaks because V3+ ions were easily oxidized to V4+ ions. Both ions found in reservoir B were considered permeated ones. Since V2+ and V5+ ions were even more unstable than V3+ ions in given ambient conditions, their permeability could not be measured. Regarding membrane resistance, it increased with the thickness of the membranes (Figure S7). The values for PBI15, PBI25, PBI35, N212, and N117 membranes were 1.62, 1.89, 2.03, 1.32, and 1.41 Ω cm2. When these area resistances are plotted against the membrane thickness, straight lines are obtained for both Nafion (obvious for 2 data points) and metaPBI. While the y-axis intercept represents the electrolyte resistance, the inverse slopes stand for the membrane conductivity, which was 4.9 mS cm−1 for meta-PBI while that for Nafion was 166.7 mS cm−1. The value for Nafion is just a rough approximation, because (i) of a lack of data points for obtaining a solid value and because (ii) extruded N117 and solution-cast N212 have slightly different properties. However,

ratio of 1.2−1.5 reported for PBI membranes which are immersed in 85% phosphoric acid.17 However, it should be noted that the acid uptake in 85% phosphoric acid is around 350−440 wt %, while it is between 60 and 100 wt % in the tested sulfuric acids. This could indicate that the polymer morphology (e.g., a lamellar structure parallel to the casting plane) is less interrupted at low sulfuric acid concentrations, but is already strongly disintegrated at 85% phosphoric acid at room temperature, until PBI dissolves in phosphoric acid at 190 °C and in concentrated sulfuric acid at room temperature. Analysis of Membranes after Operation in the VRFB. After disassembly, no membrane degradation was visible to the naked eye. SEM images showed a smooth, homogeneous crosssectional surface for Nafion membranes, but a more structured fracture surface for meta-PBI membranes (Figure 2), which is common.35 More information is gained by EDS spectra taken from the whole cross-sectional membrane area (Figure 3). According to

Figure 3. Ratio of sulfur to vanadium atoms obtained from SEM−EDS data of the cross-sectional area of membranes.

Figure 3, the ratio of sulfur to vanadium ions (S/V ratio) was 20.6, 26.8, and 10.1 for 15, 25, and 35 μm thick meta-PBI membranes, respectively, but only 3.6 for Nafion 212 membrane (N212). Considering the large measurement error, it can be concluded (qualitatively) that PBI membranes have high S/V ratios, while that of N212 is low, suggesting a higher vanadium uptake. Zawodzinski et al. reported that the molality of H2SO4 in Nafion membranes at 22 °C was about 40% of the molality of the doping solution.36 Thus, a Nafion membrane with an ionexchange capacity (IEC) of 0.9 mmol g−1 that is immersed in 2 M H2SO4 (ca. 1.75 m solution) is expected to absorb 0.7 mol of H2SO4 per kg of doped membrane, which is equivalent to 0.8 mol of H2SO4 per mole of sulfonic acid (SO3H) group. If vanadium does not interfere with this equilibrium, Figure 3 shows that Nafion absorbs about 0.5 vanadium ions and 0.8 H2SO4 molecules per SO3H group. It is anticipated that the S/V ratio in an equilibrium state is independent of thickness. However, there seems to be a qualitative difference, as shown in Figure 3. One explanation is that the applied potentials affect ion composition in the membrane. Namely, large cations cannot enter PBI membranes easily because of the narrow diameter of the ionic pathways and the positive charge on the protonated PBI backbone. Therefore, vanadium ions should surpass an energy barrier to enter the membrane. For operation at a given current, the required potential increases with the membrane resistance. Thus, thicker D

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Chemical structure of meta-PBI, and schematic illustration explaining why protons (repulsive electrostatic interactions with polymer chains, but very small) and sulfate ions (attractive electrostatic interaction with polymer chains) are easily transported across PBI membranes and vanadium ions are rejected (large ion size and strong repulsive electrostatic interaction with polymer chains).

spectroscopy) of VRFB full cells filled with 2 M H2SO4. The resistance of VRFB P15, VRFB P25, and VRFB P35 was 1.62, 1.89, and 2.03 Ω cm2, respectively. The ohmic resistance of VRFB P15 is lowest, and thus it should induce higher VE due to the reciprocal relationship between VE and the resistance. In terms of the charging and discharging time (Figure 6b), the VRFB N series has a similar charging time, but significantly shorter discharging time than the VRFB P series, resulting in a lower CE. For examination of the durability of the five VRFB single cells, 20 charge−discharge cycles were run (Figure S2). According to the cycle test, while the apparent capacity of the VRFB P series decreased linearly, capacity loss of the VRFB N series followed a logarithmic trend. Fitting and extrapolation of the curves revealed that VRFB P15 would reach a similarly reduced capacity as VRFB N212 only after 119 cycles (at about 44% of the initial capacity), and VRFB P35 would even perform better than VRFB N212 until the 142nd cycle (also at about 44% of the initial apparent capacity). The loss ratio of the charge−discharge capacity after the first 10 cycles is shown in Figure S1. Although the capacity loss during charge−discharge steps was observed for all VRFBs, the loss rate of the VRFB P series was lower than that of the VRFB N series. The average capacity loss rate for VRFB P15, P25, and P35 was 0.14, 0.11, and 0.10 A h L−1 cycle−1, respectively. This is slightly higher than those observed for some hydrocarbon-based cationexchange membranes (0.036 A h L−1 cycle−1 at 1 M vanadium concentration),10 but is much lower than that reported for an N115 membrane (0.9 A h L−1 cycle−1 at 1 M vanadium concentration).26 For further investigation of the impact of membranes on the VRFB performance, efficiencies of the VRFB single cells were calculated from the cycling data (Figure 7). In terms of CE,

the values are still in a good agreement with previously reported ones. For example, the reported value for PBI was 3 mS cm−1,25 while that of Nafion was 110 mS cm−1.36 Both values were measured in 2 M H2SO4 at 25 °C. The in-plane conductivity of membranes doped in 1−8 M H2SO4 was also determined ex situ in air at 22 °C and a low relative humidity of 13−15% (Figure S8). A noteworthy result is that the conductivity more than triples when the bath concentration increased from 2 to 3 M H2SO4. Higher acid concentrations reduce electrolyte stability and render it more corrosive. However, considering the exponential increase of conductivity with the acid doping level, a higher acid concentration in the electrolyte will significantly reduce the cell resistance and therefore increase the VE, as discussed later and shown in Figure S10. Effect of Membrane Thickness on Charge/Discharge Behavior of VRFB under Galvanostatic Control. Charge− discharge curves of a VRFB full cell were examined at 40 mA cm−2 for 20 cycles in a bid to evaluate the meta-PBI membrane thickness effect on VRFB performance. The results were compared with those of Nafion membranes (Nafion 117 and 212). In terms of the meta-PBI membrane thickness, 15, 25, and 35 μm were considered. We denote VRFB full cells using the three different PBI membranes as VRFB P15, VRFB P25, and VRFB P35, while those using Nafion 117 and 212 are denoted as VRFB N117 and VRFB N212. Figure 6 represents charge−discharge curves of the five VRFB full cells (VRFB P15, VRFB P25, VRFB P35, VRFB N117, and VRFB N212). As the meta-PBI thickness decreased, the voltage gap between the charge and discharge curves decreased, proving that the membrane resistance is proportional to the membrane thickness. This was also confirmed by measuring Nyquist plots via EIS (electrochemical impedance E

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Vanadium permeation through membranes separating a vanadium ion solution and a magnesium sulfate solution; UV−vis spectra of the magnesium sulfate solution after 24 h for (a) V4+ ions and (b) V3+ ions. Relationship between vanadium concentration in the magnesium sulfate solution and operating time for (c) V4+ ions and (d) V3+ ions.

Figure 6. Charge−discharge curves measured at 40 mA cm−2 for the purpose of comparison for (a) three VRFB P series (VRFB P15, VRFB P25, and VRFB P35), and (b) two VRFB N series (VRFB 117 and VRFB N212) and VRFB P15.

there were three main observations. First, the CE of the VRFB P series was larger than that of the VRFB N series. Namely, after their final cycle, the VRFB P series showed a CE of 97− 99%, while that of the VRFB N series was 87% (VRFB N212) and 92% (VRFB N117). Second, although the value difference was not significant, amid the VRFB P series, the CE was reciprocal to membrane thickness. Third, the CE of all VRFB single cells generally increased during cycling, while the VE decreased. Because of the much higher CE and just slightly lower VE of VRFB P15 and VRFB P25 in comparison to those

of the VRFB N series, these two VRFB single cells showed a better EE than the VRFB N series over all cycles. The CE depends mainly on the crossover flux of vanadium salts penetrating the membrane by diffusion and convection.27 A minor contribution can stem from hydrogen evolution at the negative electrode. Therefore, the main parameters influencing the CE are membrane thickness (if diffusion is important) and chemical structure of the corresponding membranes. In terms of the membrane thickness, thinner membranes usually lead to higher concentration gradients across the F

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) Charge efficiency, (b) voltage efficiency, and (c) energy efficiency of the five VRFB single cells using P15, P25, P35, N117, and N212 membranes. (d) Initial efficiencies of VRFB single cells using meta-PBI membranes of different thicknesses. Inset compares the efficiencies of VRFB P15 and VRFB N212. The current density was 40 mA cm−2.

Figure 8. Average efficiencies gained by the five VRFB single cells (VRFB P15, VRVB P25, VRFB P35, VRFB N117, and VRFB N212) at different current densities. The current density was varied from 40 to 80, 120, and 40 mA cm−2, and at each current density, five cycles were run: (a) charge efficiency, (b) voltage efficiency, and (c) energy efficiency.

membrane and thereby increased vanadium crossover flux, as experimentally seen by comparison of VRFB N212 and thicker VRFB N117 (Figure 7a). However, the CE of VRFBs using acid-doped PBI membranes decreased with increasing thickness. As explained before, this may be because vanadium ions need to surpass an energy barrier to enter the protonated PBI membranes. Namely, as shown in Figure 6a, an about 100 mV higher potential was needed for the charging step of VRFB P35 compared to that of VRFB P15. This indicates that in VRFB P35 a larger portion of vanadium ions can overcome the energy barrier and enter the membrane. Therefore, although PBI 35 is thicker, the thinner PBI 15 membrane leads to higher CE. Another explanation would be hydrogen evolution, which

should increase with the potential. Regardless of the exact reason, the important message here is that thinner PBI membranes lead to increased VE without compromising the CE. For further investigation of these findings, another set of VRFB single cells was assembled and tested for 5 cycles at 40, then 80, 120, and again 40 mA cm−2. The result is represented in Figure 8 and Figures S3−S5. Because higher current densities generally reduce the charging time, the CE of the VRFB single cells is proportional to the applied current density. According to the test result, at all tested current densities, VRFB P15 showed the highest CE. This strongly supports that G

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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unchanged over 5 cycles at the given current density and then decreased with increasing current density. When the current density was again brought back to 40 mA cm−2, the capacity was almost restored to its initial value. Effect of Membrane Thickness on Charge/Discharge Behavior of VRFB under Potentiostatic Control. For further verification of the effect of thickness on the CE, three VRFB single cells (VRFB N212, VRFB P15, and VRFB P35) were cycled at a fixed potential of 1.6 V. The purpose of these cycling tests is to investigate whether the CE of VRFBs using both meta-PBI membranes (VRFB P15 and VRFB P35) is similar. According to the measurements, their CE values were very close, above 99.8% (Figure 10 and Figure S9). This is because (i) crossover occurring by diffusion of vanadium ions (that depends on time and thickness) is negligible, and (ii) crossover occurring by migration only depends on potential and possibly the totally transported charges (assuming that the ratio of protons to metal ions is constant, e.g., every xth ion entering the membrane is not a proton but a metal ion at a given potential). Essentially, this supports the finding that thickness of the meta-PBI membrane does not affect the CE of the VRFB, as would be the case for cation-exchange membranes.10,28

the observed thickness effect is not a single coincidence, and independent of the current density. Also at current densities of 80 and 120 mA cm−2, the VRFB P series demonstrated much higher CE than the VRFB N series. This positively affects the lifetime of the VRFBs. Over 20 cycles, the apparent charge capacity of VRFB N117 was degraded to 74% of its initial value (from 31.0 in the first cycle to 23.0 A h L−1 in the 20th cycle), and that of VRFB N212 was degraded to 69% (from 29.2 in the first cycle to 20.1 A h L−1 in the 20th cycle), whereas that of VRFB P15 was still at 98% of its initial value (from 30.2 in the first cycle to 29.7 A h L−1 in the 20th cycle). Assuming that the state of charge is 0% for the fully discharged cells, which seems to be justified because of the observed voltage drop at the end of the discharge curves, and knowing that the vanadium concentration in the anolyte refers to a maximum capacity of 40.2 A h L−1, an initial capacity of 30.2 A h L−1 refers to a state of charge of 75%. When it comes to VE, it dominantly relies on potential losses by (i) overpotentials and (ii) membrane resistance. Figures 8 presents that the VE of the VRFB N series is higher than that of the VRFB P series at the given current density range. This implies that the conductivity of PBI membranes is much lower than that of Nafion membranes. Within each series, the VE increased with decreasing membrane thickness as expected. Taken together, the VRFB P series showed higher VE over the entire tested current density range than the VRFB N series. Furthermore, at 40 mA cm−2, CE and EE of VRFB P15 and VRFB P25 were highest. However, as the current density increased, although the CE of the VRFB P series was still high, the VE significantly decreased. Thus, from 80 mA cm−2, VRFB N212 had higher EE than other VRFB single cells. An interesting feature is the trend of charge and discharge capacity over 20 cycles (five at 40, 80, 120, and again at 40 mA cm−2). As observed in Figure 9 and Figures S5 and S6, the

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CONCLUSIONS meta-PBI membranes of 15, 25, and 35 μm thickness were prepared, and their properties and VRFB performances were compared with those of Nafion-based membranes (Nafion 117 and 212). In 2 M H2SO4, the ADL was just around 2 mol of H2SO4 per mole of repeat unit of PBI, resulting in a proton conductivity of 4.9 mS cm−1, and relatively low VE. While the VE increased with decreasing membrane thickness because of the decreasing ohmic resistance, VRFBs using meta-PBI-based membranes showed lower VE than those using thicker Nafion membranes. Meanwhile, the EE of VRFBs using 15 and 25 μm thick meta-PBI membranes was higher than that of VRFBs using 50 μm thick N212 and 175 μm thick N117 membranes. This is attributed to the excellent CE of VRFBs using meta-PBIbased membranes, on the basis of the negligible vanadium crossover observed in ex-situ permeability tests. The crossover of vanadium ions through PBI membranes was strongly reduced because the vanadium ions (positive charges) were repelled by electrostatic repulsion from the protonated PBI backbone (positive charges). Regarding the CE of VRFB single cells, the CE of VRFBs using meta-PBI membranes was hardly affected by the membrane thickness. Furthermore, it was observed that the CE actually increased when the thickness of acid-doped PBI membranes decreased. Since permeability due to diffusion was not observed in the ex-situ permeation experiment, the observed inverse correlation between the CE of the VRFB and the thickness of the meta-PBI membrane probably depends on the electric potential. For instance, a 35 μm thick PBI membrane needs ∼100 mV higher potential than a 15 μm thick PBI membrane to achieve the same current. This additional potential apparently either led to increased hydrogen evolution, and/or allowed a larger number of vanadium ions to surpass the membrane/electrolyte interfacial barrier and to enter the membrane. In any case, this means that thinner PBI membranes not only increase the VE but also the CE. Also, since thinner membranes can be produced for a cheaper price than thick membranes, the thin PBI membranes can be used for future VRFB systems, and even thinner membranes (e.g., 8 μm, or 1

Figure 9. Charge capacity of VRFB P15 and VRFB N212 over the 20 cycles shown in Figure 8.

capacity decreased monotonically for VRFB N212 irrespective of the applied current density. Only when the current density decreased again from 120 to 40 mA cm−2, a very slight increase of the capacity was observed. This implies that the capacity loss is not recoverable. In a striking contrast to this, the capacity of VRFB P15 strongly depended on the current density (an effect which scales with the membrane resistance), but did not show much degradation over time. The capacity remained nearly H

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 10. (a) Charge efficiency under potentiostatic control at 1.6 V. (b) Development of current over the duration for the 2nd charge/discharge cycle.

μm on a cheap support material) can be considered. On the basis of the observed resistance values, it can be estimated that a 4 μm thick PBI membrane would lead to a similar cell resistance as N117. A further improvement could be brought by changing the H2SO4 concentration in the electrolyte from 2 to 3 M, as verified by in-situ and ex-situ measurements. According to the measurements, the increase in the H2SO4 concentration resulted in a 3.1 times larger conductivity. In the VRFB full-cell tests, the change in H2SO4 concentration from 2 to 3 M dramatically increased the VE, leading to an EE of 92% at 40 mA cm−2 (see Figures S10 and S11).

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SEM and SEM−EDS data were measured with an Inspect F50 SEM (FEI) instrument. Samples were prepared by freeze-breaking membranes in liquid nitrogen and sputtering the samples with platinum. WAXS curves of membranes were recorded at ambient conditions with a Rigaku MiniFlex2 instrument at a wavelength of 1.54 Å (Cu Kα). d values were calculated according to the Bragg equation, nλ = 2d sin(θ). Membrane resistances were obtained by using the electrochemical impedance spectroscopy (EIS) mode of a potentiostat (Bio-Logic sp240). For Nyquist plot measurements of the EIS data, the corresponding membranes with an active area of 4 cm2 were fixed on an equally sized carbon-felt working electrode and immersed into 2 M H2SO4 electrolyte. In fixed distances, platinum counter and reference electrodes were immersed in the same electrolyte. The impedance spectra were determined in the frequency range from 7 MHz to 10 Hz with 10 steps per decade, and modulating potential was 100 mV. Vanadium crossover flux through membranes was obtained by measuring the permeability of V3+ and V4+. For this, two reservoirs that were separated by the tested membrane were prepared (reservoirs A and B). Regarding the permeability test for V4+, reservoir A was filled with a solution containing 0.5 M VOSO4 and 1 M H2SO4. For the test for V3+, reservoir A was filled with a solution containing 0.5 M V3+ and 1 M H2SO4. In all cases, reservoir B was filled with a solution containing 0.5 MgSO4 and 1 M H2SO4. The volume of each solution was 32 mL. As previously reported, the MgSO4 was here used to reduce osmotic pressure and to make the ionic strength of two mixtures equivalent.24 During the test, the MgSO4 solution was taken from reservoir B, stored, and then analyzed for its vanadium ion content by UV−vis spectroscopy. The characteristic absorption peak for V4+ appears at 764 nm, and its intensity was calibrated against reference solutions.29 For the permeability of V3+, a similar procedure was followed, but because reservoir A was filled with a solution containing 0.5 M V3+ and 1 M H2SO4, the absorption peak for V3+ was monitored at 621 nm. With that, vanadium permeability was calculated by eq 1, and the vanadium crossover flux was evaluated as a product of the vanadium permeability and the concentration gradient.30−32

EXPERIMENTAL SECTION

Materials. meta-PBI was obtained from Danish Power Systems. DMAc (dimethylacetamide) was obtained from Sigma. Vanadium(IV) oxide sulfate hydrate (97% pure) was obtained from Zhejiang Eazy Pharmchem. Membrane Fabrication. A 28.11 g portion of DMAc was transferred into a large vial with a magnetic stir bar. The amount of PBI given in Table 1 was added to the stirred solution. After

Table 1. Casting Conditions for Membrane Fabrication thickness of dry membrane [μm]

amount of PBI in casting solution [g]

gap thickness for casting [μm]

15 25 35

5.52 4.30 4.18

160 230 320

completing dissolution, the solutions were cast on a glass plate. The plate with the wet film was kept in a closed oven at 40 °C for 24 h; then, vacuum was applied for another 24 h. The membrane was delaminated from the glass plate by wetting with water, and residual DMAc was removed by immersing the membrane in DI (deionized) water for 24 h. After this, the membranes were again dried in the vacuum. Membrane Characterizations. For measuring the sulfuric acid uptake of membranes, PBI membranes were cut to 1 cm × 4 cm size, washed in DI water at 75 °C for 24 h, and then dried in an oven to remove all water at 60 °C for 48 h under vacuum. Completely dried membranes were weighed and then immersed in water, 2, 4, 6, and 8 M H2SO4, or 1.5 M VOSO4 in 2 M H2SO4 at room temperature (RT) for 48 h. Samples were wiped by tissue paper to remove excess solution from the surface; the weight of wet membranes was measured. Samples were immersed in 20 mL of 0.05 M KOH solution to remove H2SO4 from the membrane at RT for 24 h. Each solution was titrated with 0.1 M HCl solution to know how much sulfuric acid was absorbed in meta-PBI.

VR

dC R P = A [C L − C R (t )] dt L

(1)

Here, CL is the vanadium ion concentration of reservoir A, and CR(t) is the vanadium ion concentration of reservoir B as a function of time. A is the membrane area, and L is the membrane thickness, while P is the permeability of vanadium ions, and VR is the volume of reservoir B. VRFB Single-Cell Tests. For VRFB single-cell tests, the single cell consisting of graphite bipolar plates and gold-coated copper current collectors was used. Membranes with an active area of 4 cm2 were assembled between carbon felt, which was either JNT GF-040 (JNT; 4.0 mm thick) or XF30A (Toyobo; 3.5 mm thick) and gaskets I

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ACS Applied Materials & Interfaces (silicone, 0.3 mm thick). The electrolyte was prepared by dissolving vanadium(IV) oxide sulfate hydrate in 2 M H2SO4 to give a 1.5 M solution. Anolyte (negative electrode) and catholyte (positive electrode) volumes were 15 and 17 mL to avoid overcharging. The amount of 15 mL refers to a maximum possible capacity of 40.2 A h L−1. The system was preactivated by a first charging step up to a potential of 1.75 V using a WBCS3000K8 potentiostat from Won-A Tech. After that, the catholyte (now V5+) was quickly exchanged with a fresh solution, so that the anolyte and catholyte were in the V3+ and V4+ state. Then, at a flow rate of 11.5 mL min−1, the cells were charged and discharged over several cycles between 1.7 and 0.8 V.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10598. WAXS curves, data on capacity loss, efficiencies for each cycle, charge−discharge curves, and conductivity data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +82 29585298. E-mail: [email protected]. *Phone: +82 29706805. Fax: +82 29706800. E-mail: kwony@ seoultech.ac.kr. ORCID

Dirk Henkensmeier: 0000-0003-2330-953X Yongchai Kwon: 0000-0003-3118-401X Author Contributions ⊥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the German−Korean joint SME R&D projects program of MOTIE/KIAT and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (20164030201060). We thank Robert Jan Sedelmayer for preparation of some of the membranes.

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REFERENCES

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K

DOI: 10.1021/acsami.7b10598 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX