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Zirconium oxide nanotube–Nafion composite as high performance membrane for all vanadium redox flow battery
Journal of Power Sources xxx (2016) 1e9
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Zirconium oxide nanotubeeNafion composite as high performance membrane for all vanadium redox flow battery Md. Abdul Aziz, Sangaraju Shanmugam* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 333, Techno Jungang-daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu, 42988, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Nafion-ZrNT composite membrane was successfully fabricated. Nafion-ZrNT membrane shows low vanadium-ion crossover thus superior ion selectivity. VRB showed low self-discharge rate for Nafion-ZrNT composite membrane than Nafion-117. Nafion-ZrNT exhibits high discharge capacity and efficiency compared with Nafion-117.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2016 Received in revised form 25 October 2016 Accepted 31 October 2016 Available online xxx
A high-performance composite membrane for vanadium redox flow battery (VRB) consisting of ZrO2 nanotubes (ZrNT) and perfluorosulfonic acid (Nafion) was fabricated. The VRB operated with a composite (Nafion-ZrNT) membrane showed the improved ion-selectivity (ratio of proton conductivity to permeability), low self-discharge rate, high discharge capacity and high energy efficiency in comparison with a pristine commercial Nafion-117 membrane. The incorporation of zirconium oxide nanotubes in the Nafion matrix exhibits high proton conductivity (95.2 mS cm1) and high oxidative stability (99.9%). The Nafion-ZrNT composite membrane exhibited low vanadium ion permeability (3.2 109 cm2 min1) and superior ion selectivity (2.95 107 S min cm3). The VRB constructed with a Nafion-ZrNT composite membrane has lower self-discharge rate maintaining an open-circuit voltage of 1.3 V for 330 h relative to a pristine Nafion membrane (29 h). The discharge capacity of Nafion-ZrNT membrane (987 mAh) was 3.5-times higher than Nafion-117 membrane (280 mAh) after 100 charge-discharge cycles. These superior properties resulted in higher coulombic and voltage efficiencies with Nafion-ZrNT membranes compared to VRB with Nafion-117 membrane at a 40 mA cm2 current density. © 2016 Elsevier B.V. All rights reserved.
Keywords: Vanadium redox flow battery Nafion composite membrane ZrO2 nanotubes Electrospinning Vanadium crossover
1. Introduction Vanadium redox flow battery (VRB) has received great attention in energy storage technologies, such as smart grids, wind turbine
* Corresponding author. E-mail address: [email protected] (S. Shanmugam).
generators, emergency backup applications and remote area power source systems [1e5], due to its high energy efficiency, low cost, long cycle life, quick response and profound discharge capability [6e13]. VRB employs oxidation and reduction reactions to store energy, where two soluble redox couples act as electroactive materials [14]. In a typical setup, VRB consists of two electrolyte reservoir supplied with V2þ/V3þ and VO2þ/VOþ 2 redox species in sulphuric acid solution, a battery stack area for redox reaction and
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obtain ZrO2 nanotubes. The calcination process was carried under air atmosphere. Initially, the synthesized nonwoven mat was stabilized at 250 C for 1 h to remove the organic substance and heating was continued to 580 C for 1 h. The heating rate was fixed at 5 C per minute.
two pumps. The electrolyte solutions are pumped into the reservoirs parted by a membrane which allows diffusion of protons to fulfill the requirement of the circuit at the time movement of current. The ideal VRB membranes should possess good chemical stability, low vanadium ion permeability to achieve longer cycle life and better coulombic efficiency. Similarly, to result in higher voltage efficiency, a membrane with good proton conductivity should be highly recommended for VRB [14,15]. Perfluorosulfonic polymers, such as Dupont Nafion®, are widely used membranes for VRB, because of their good chemical stability and excellent proton conductivity [16]. Despite their several advantages, still, Nafion membranes exhibit high vanadium ion crossover that additionally contributes to the capacity loss and decreases energy efficiency of VRB. To overcome these challenges, certain modifications phenomena were offered to increase the ion-selectivity of Nafion membrane or minimize the crossover of vanadium ion [17e19]. For example, Nafion was modified with SiO2 [17], polypyrrole [19], polyelectrolyte [20], polyaniline [21], TiO2 [22], polyvinylidene fluoride [23], sulfonated poly (ether ether ketone) [24], diethoxydimethylsilane [25], organic silica modified TiO2 [26], polytetrafluoroethylene [27,28] All these composite membranes showed comparatively lower vanadium ion crossover and higher columbic efficiency compared with the pristine Nafion membrane. However, still a great challenge to maintain the proper ratio between proton conductivity over vanadium ion permeability to achieve superior vanadium ion selectivity. Based on this aspect, we synthesized ZrO2 nanotubes (ZrNT) with a diameter smaller than 50 nm. This was achieved by calcination of an electrospun polyacrylonitrile (PAN) containing a Zirconium precursor mat prepared using a single spinneret electrospinning method under an air atmosphere. The fabricated porous metal oxide nanotubes are incorporated in a Nafion ionomer and subsequently, a VRB single cell performance was evaluated using the composite membranes. The main reason for introducing ZrNT in Nafion is to enhance the ion selectivity of the membrane, because of ZrO2 partially block the polar structure (sulfonated ion) available in Nafion membrane by the electrostatic interaction, which acts as a permselective barrier for vanadium ions by tortuous pathway effect. Moreover, the proton can easily transfer through the modified membrane because the stoke radius of the proton is comparatively smaller than the vanadium ions [29]. The VRB results using the Nafion-ZrNT membranes were compared with results obtained using a commercial Nafion-117 membrane.
Field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800II) with a setup voltage of 3 kV was used to observe the morphology of the samples. Osmium coating was used before SEM observation. Field-emission transmission electron microscope (FETEM, Hitachi, HF-3300) with the voltage of 300 kV was used to determine the microstructures of samples. The crystal structure was determined using powder X-ray diffraction (Panalytical, Empyrean, XRD) with CuKa radiation at a setup current of 30 mA and voltage of 40 kV. The thermal stability of membrane sample was determined by using a thermal gravimetric analyzer (Thermo plus EVO, TG 8120). The small part of the membrane was placed in a crucible and thermally heated from 25 to 900 C with a heating rate of 10 C min1 under air atmosphere. Water uptake (WU) and swelling degree of membranes were obtained by comparison of weight and dimensions of dry and wet samples according to Eqs. (1) and (2). Membranes were heated at 90 C for 12 h using the oven and finally soaked in DI water (24 h) at room atmosphere.
2. Experimental
Water uptake ð%Þ ¼
2.3. Preparation of Nafion-ZrO2 nanotube composite membrane A required mass ratio of ZrNTs was incorporated in the Nafion ionomer using ethanol as a solvent. Nafion ionomer and ZrNTs were mixed together then the contents were ultra-sonicated for 1 h and finally stirred for 5 h by using a mechanical stirrer. The obtained mixture was poured into a glass petri dish and allowed to dry at 50 C for 2 h, 60 C for 2 h, 70 C for 2 h, and 80 C for 2 h using vacuum oven. Then, the composite membrane was peeled-off from the petri dish by immersing in DI water. The membrane thickness is optimized by adjusting the viscosity of Nafion ionomer solution. Finally, the composite membrane was pretreated by heating with 5% H2O2, H2O, 0.5 M H2SO4, and H2O in sequence for 1 h for each case. 2.4. Materials characterization
Wwet Wdry 100 Wdry
(1)
Lwet Ldry 100 Ldry
(2)
2.1. Materials Zirconium (IV) acetylacetonate (Zracac), polyacrylonitrile (PAN, Mw ¼ 150,000 g/mol) and Nafion-117 membrane were acquired from Sigma-Aldrich, Korea. Nafion ionomer (1100 EW, 15 wt%) was obtained from Ion Pow. Inc, USA. Ethanol, H2O2, H2SO4, and N, Ndimethylformamide (DMF) were purchased from Daejung Chemicals, Korea. Vanadium electrolyte 1.7 M V (±3%), V3þ/V4þ 1:1 (±3%), 4.5 M SO 4 (±5%) was purchased from FC International, Korea and were utilized as received. 2.2. Preparation of ZrO2 nanotubes Zracac (0.1 g) and PAN (1 g) were completely dissolved in DMF separately at 90 C. Then, both solutions were suspended together at 90 C and a clear homogenous solution was observed. The details of electrospinning process are given in our previous publications [30,39]. The prepared electrospun (e-spun) nonwoven mat was pyrolysed using a tubular furnace (Wisd Laboratory Instruments) to
Swelling degree ð%Þ ¼
where, Wwet is the membrane weight after 24 h and Wdry is the dry membrane weight. Similarly, Lwet is the membrane length after 24 h and Ldry is the dry membrane length. Ion exchange capacity (IEC) was obtained by neutralization reaction using phenolphthalein indicator. The membrane was dried in the vacuum oven at 90 C and then dipped in 3 M NaCl solution for 12 h to exchange Hþ of the membrane with Naþ. Finally, titrated using 0.01 M NaOH. The IEC value was determined according to Eq. (3).
IEC ¼
VNaOH CNaOH Wdry
(3)
where VNaOH was the NaOH volume (mL) used at the neutralized point, CNaOH was the NaOH concentration (M) and Wdry was the dry
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membrane weight. The membrane conductivity cell (Bekktech) was used to obtain proton conductivity of the membrane with the gas accessory. Membrane samples were assembled in the cell at a fixed position in contact with two platinum electrodes. Specific voltages were applied to Pt electrodes using potentiostat and the corresponding currents were obtained. The resistance (R) is calculated using the slope of the line, which joins the data points. During measurement, the temperature was fixed at 80 C with different relative humidity (RH %). The membrane conductivity was determined according to Eq. (4).
s¼
L RW T
(4)
where, L ¼ 0.425 cm is the constant length among Pt electrodes; R is the resistance of membrane in U; W is the sample width in cm and T is the membrane thickness in cm. For the oxidative stability test, a small part of the membrane was immersed in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 C. The oxidative stability was investigated using the remaining weight of the membrane after 1 h treatment with Fenton's reagent [31]. 2.5. Measurements of VO2þ permeability and ion selectivity The permeation of vanadium ion (VO2þ) between the membrane was evaluated via a membrane diffusion cell. Sample membrane which acts as a barrier with specific area 9 cm2 was clamped between two reservoirs. The left reservoir was loaded with 150 mL of 1.5 mol L1 VOSO4 in 3 mol L1 H2SO4, and the right reservoir was loaded with 150 mL of 1.5 mol L1 MgSO4 in 3 mol L1 H2SO4 solution to balance the ionic strengths as well as to decrease the effects of osmotic pressure. The solutions in two reservoirs were continuously stirred by magnetic stirrers throughout the entire measurement at normal temperature to avoid the concentration polarization. At regular time intervals, the aliquot sample was taken from the right reservoir and determined the VO2þ concentration using the UVeVis spectrometer. The permeability nature of vanadium ion was obtained by the following equation [32].
VR
dCR ðtÞ P ¼ A ½CL CR ðtÞ dt L
d P
between two graphite plates act as current collectors. Two copper sheets were attached in the outer positions of two graphite plates, which were worked for battery terminal connectors. This assembly was fastened by two polyvinyl chloride end plates. The carbon fleece (5 mm uncompressed thickness) was activated by thermal treatment at 500 C for 30 min. The cyclic chargeedischarge curves of the VRB cell constructed with the Nafion-ZrNT composite and the recast Nafion membranes were determined in normal temperature at 40 mA cm2 current density. Vanadium electrolyte with a composition of 1.7 M V (±3%), V3þ/ 4þ V 1:1 (±3%), 4.5 M SO 4 (±5%) was employed as electrolyte solution. The amount of electrolyte solution was 30 mL in each negative electrolyte vessel as well as in the positive electrolyte vessel. The electrolytes were recirculated with a peristaltic pump (Reglo ICC 2ch Pump) at a flow rate of 10 mL min1. Both electrolyte vessels were de-aerated using continuous N2 flow to avoid the chemical oxidation of the electrolyte solutions. An eight channel battery analyzer (BST8-3, MTI Corp.) was used to test the battery. For each membrane sample, such as Nafion-117 and Nafion-ZrNT same charge and discharge conditions were applied. To save the electrodes and graphite plates from corrosion, the maximum edge of the charge voltage was applied 1.6 V and the minimum edge of the discharge voltage was maintained to be 1 V. Finally, the self-discharge of the battery was evaluated at room temperature by time tracking the open circuit voltage decrease. All data points were collected until the open circuit voltage dropped below 0.8 V. The cyclic efficiencies including coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the VRB were determined by the following equations.
CE ¼
discharge capacity 100 charge capacity
(7)
VE ¼
middle point of discharge voltage 100 middle point of charge voltage
(8)
EE ¼ CE VE
(9)
3. Results and discussion
(5) 3.1. Membrane characterization
where, CR(t) is the concentration vanadium ion in the right reservoir with time measured by UVevis spectrometer, and CL is the concentration vanadium ion in the left reservoir. A is the active area and L is the thickness of the membrane. P is the vanadium ion permeability and VR is the volume of the right reservoir. It is considered that the concentration change of the vanadium ion in the left reservoir should be negligible as well as a pseudo-steadystate order is applied in between the membrane. The membrane ion selectivity (S) was measured via proton conductivity (d) with a crossover of vanadium ion (P), which was determined by Eq. (6),
S¼
3
(6)
2.6. Measurements of vanadium flow battery performance A single cell VRB was assembled by sandwiching the composite membrane between two carbon felt electrodes (9 cm2). The membrane-electrode assembly (MEA) was compressed and held
The membrane plays a significant role in the operation of VRB and the widely used membrane is based on the Nafion. The hydrophilic polar structure of Nafion membrane is beneficial for proton conductivity but it has a great impact on the vanadium ion permeability, which decreases the overall cell performance. Effective approaches in developing the ion selective Nafion membranes by filling of the polar cluster of Nafion with inorganic metal oxide fillers [33e36]. Incorporation of ZrO2 in Nafion membrane significantly reduces the vanadium ion permeability. The interaction between oxygen-functional groups on ZrO2 and the sulfonic acid group of Nafion matrix in the Nafion composite membrane partially block the polar structure available in Nafion membrane, which acts as a barrier for vanadium ions by tortuous pathway effect (Fig. 1). The FE-SEM image clearly shows the synthesized ZrO2 exhibits tubular structure and FE-TEM image shows the existence of porous nanotubes structure, where the tubular wall composed of nanoparticles as shown in Fig. 2. The composite membranes with filler amount of 0.5, 1.0 and 1.5 wt % were fabricated and denoted as NafioneZrNT (0.5%), NafioneZrNT (1%) and Nafion-ZrNT (1.5%), respectively. The presence of ZrNT filler in the composite membrane was confirmed by XRD. Fig. S1 displays the X-ray diffraction
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Fig. 1. Schematic illustration of VRB operation and formulation process of Nafion-ZrNT composite membrane.
patterns of Nafion-ZrNT (1%) membrane, ZrNT filler, and recast Nafion membrane XRD patterns. The broad peak at 17.0 (2q) and prominent peak at 17.8 (2q) are attributed to the Nafion crystallite structure [37]. XRD peaks observed at 28 , 30 and 50 (2q) for the NafioneZrNT (1%) composite membrane are attributed to the diffraction pattern of the ZrNT filler present in the composite membrane. The surface, as well as cross-section morphology of the Nafion-ZrNT composite membrane, was observed by SEM analysis (Fig. S2). It should be noticed that the composite membrane is opaque and invariable from cross-section to the surface. High magnification SEM image shows ZrNT filler randomly distributed in the composite membrane, where the Nafion ionomer coated the surface of the ZrNT fillers. Additionally, aggregation of ZrNT fillers was not detected by SEM images. The thickness of the membrane was investigated by the SEM cross-sectional image of the NafioneZrNT (1%) composite membrane. The obtained thickness was measured to be 150 ± 5 mm. Furthermore, the dispersion of fillers in NafionZrNT composites membranes was measured by capturing SEM images in back scattering mode and it was observed that the nanotubes were randomly distributed within the membrane as shown in Fig. S3. The physicochemical properties, like water uptake, swelling degree, IEC, proton conductivity, activation energy and oxidative stability performances were determined for all membranes and the
results are listed in Table 1. In the case of Nafion-ZrNT membrane, WU is higher than the Nafion-117 membrane. It was reported that the zirconium oxide surface contains the hydroxyl groups, which facilitates the water absorption and retention capacity [38]. The interaction between the ZrNT filler and the sulfonated species of the Nafion ionomer is expected to improve the swelling degree and IEC value of the composite membrane. This interaction has an improved effect on reduction of vanadium ion-permeability and thus the enhanced VRB performance is observed even in the fully oxidative operating environment. Therefore, the proton conductivity of various NafionZrNT composite membranes was evaluated and the corresponding results were compared with the Nafion-117 membrane at 80 C under different humidity levels. Fig. 3a displays the humidity dependent proton conductivity plots of Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes at 80 C. The proton conductivity values of the NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes were higher than Nafion-117 membrane under 100% RH (Table 1). The higher proton conductivity of all composite membranes compared to the Nafion-117 membrane was mainly due to the specific water retention properties of ZrO2 filler [39]. The ZrO2 filler with various morphologies (nanotubes, nanoparticles) showed different conductivity results (Table 1). In addition, the joined nanoparticles in a tubular structure of ZrO2 filler are distributed uniformly, which facilitates better surface area. The high surface area of ZrNTs is retained more water compared with only ZrO2 nanoparticles. Comparatively, the low surface area of nanoparticles can freely accumulate with each other and exhibits lower proton conductivity than ZrO2 nanotubes [39]. Generally, the mechanism of proton transport of Nafion-based membrane should be verified based on proton activation energy. This can be deduced using the Arrhenius equation (Eq. (10)) [40,41]. Ea
s ¼ so eRT
(10) 1
where, s is the proton conductivity (S cm ), so is the preexponential factor, Ea is the activation energy (kJ mol1), R is the universal gas constant (8.314 J mol1K1) and T is the absolute temperature (K). Fig. 3b presents the plot of ln s vs. 1000/T of the Nafion-117 membrane and Nafion-ZrNT composite membranes obtained under 100% relative humidity at different temperatures. It was obtained that with increasing temperature the proton conductivity values also increased for all membranes. The value of activation energy computed from the slope of ln s vs. 1000/T plot for Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are found to be 13.27, 13.18 and 13.22 kJ mol1, respectively, whereas the activation energy value of Nafion-117 membrane was 13.50 kJ mol1. The Nafion-ZrNT composite
Fig. 2. (a) FE-SEM image; (b) FE-TEM image of ZrO2 nanotube structures.
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Table 1 Physicochemical characteristics of Nafion-117 and Nafion-ZrNT membranes. Membrane
Nafion-117 Nafion-ZrNT Nafion-ZrNT Nafion-ZrNT Nafion-ZrNP
(0.5%) (1%) (1.5%) (1%)
Water uptake (%)
Swelling degree (%)
IEC (meq g1)
Proton conductivity (mS cm1) @100% RH; 80 C
Activation energy (kJ mol1)
Oxidative stability (%)
27.68 32.26 35.89 35.31 33.17
15.31 10.33 6.21 5.94 6.24
1.076 0.935 0.927 0.915 0.930
87.6 93.9 95.2 94.5 91.9
13.50 13.27 13.18 13.22 13.36
99.6 99.7 99.8 99.9 99.8
membranes activation energy were lower compared with the Nafion-117 membrane, indicating that the existence of ZrNTs in Nafion membrane enhanced the proton transport through the membrane. The values of activation energy of composite membranes were suggestive the migration of proton across the composite membranes via the vehicular mechanism [41]. The values of activation energy of composite membranes were identical to the activation energy measured with hybrid Nafion-silica, Nafionmesoporous titanium phosphate and Nafion-mesoporous zirconium phosphate membrane [40,42]. Thermal stability of Nafion-117, Nafion-ZrNT (0.5%), NafionZrNT (1%), Nafion-ZrNT (1.5%) membranes and ZrNT filler was determined under air atmosphere. Fig. 4 displays three distinctive
weight losses were observed for all membranes. The weight loss in the temperature range of 80e120 C (dehydration) was almost same for all membranes. On the other hand, the decomposition temperature (290e370 C) of the sulfonated groups in the NafioneZrNT composite membrane was lower compared with the Nafion-117 membrane. Nafion composite membrane exhibits drastic loss of the sulfonated group in a composite membrane compared with the Nafion-117 membrane at a temperature of 300 C [43]. Furthermore, the decomposition temperature (420e520 C) of the polymer backbone of the composite membrane was higher compared with the Nafion-117 membrane. The huge loss of the sulfonic acid groups is mainly attributed to the distinct molecular interaction between the sulfonic acid of the Nafion with a hydroxyl group of the metal oxide. The oxidative stability of Nafion-117 and Nafion-ZrNT composite membranes was investigated using Fenton's reagent (3 wt% H2O2 þ 2 ppm FeSO4) at 80 C. The specific weight loss of the membranes for 1 h are given in Table 1. The Nafion-ZrNT membranes exhibited good oxidative stability compared with Nafion117. This is mainly due to the oxidative affect generally happens in the hydrophilic regions of Nafion by radical groups (HO and HOO). The interaction between oxygen-functional groups of ZrO2 and the sulfonic acid group of Nafion matrix reduces the hydrophilic region in the composite membrane [44]. Moreover, it is a well-established that with the incorporation of a small portion of filler materials to proton exchange membrane is highly active to improve the oxidative stability of the membrane [45]. The addition of ZrNT filler in Nafion matrix further proved the high oxidative stability of Nafion-ZrNT membrane. 3.2. VO2þ permeability and ion selectivity The vanadium ion crossover through the membrane is
Fig. 3. (a) Relative humidity dependent proton conductivity of Nafion-117, NafioneZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and NafioneZrNP (1%) membranes obtained at 80 C; (b) Retrogression curves of proton conductivity versus 1000/T for Nafion-117, Nafione ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and NafioneZrNP (1%) membranes measured under 100% RH at different temperature.
Fig. 4. Thermograms of ZrNT filler, Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes measured in room atmosphere.
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considered a vital complication of VRB, due to rapid self-discharge of the VRB, resulting in low energy efficiency. Fig. S4 displays the schematic illustration of membrane separated diffusion cell. The presence of vanadium ion concentration in MgSO4 solution compartment was determined with the time for Nafion-117, Nafion-ZrNT membranes (Fig. 5a) and the corresponding vanadium ion permeability and ion selectivity was shown in Fig. 5b. At a given time, the vanadium ion concentration permeate across the Nafion-ZrNT composite membranes was lower than that of the Nafion-117 membrane. Nafion-117 membrane shows a high vanadium ion permeability (1.2 108 cm2 min1), while Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes show the permeability values of 3.6 109, 3.4 109 and 3.2 109 cm2min1, respectively. The lower permeability value of Nafion composite membrane benefits from the presence of ZrNT filler. As discussed above, the ZrO2 filler exhibits oxygen-functional groups and the sulfonic acid groups of Nafion matrix interact with each other and acts as a barrier for vanadium ion by tortuous pathway effect that has been partly filled the polar clusters of the Nafion membrane. The lower vanadium ion crossover further proved by lower IEC value of Nafion-ZrNT membranes. The vanadium ion selectivity value for Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) are found to be 2.60 107, 2.80 107 and 2.95 107 S min cm3, respectively. The values obtained for composite membranes are much higher when compared with the Nafion-117 membrane (7.5 106 S min cm3). The superior performance of Nafion-ZrNT membranes over Nafion-117 was mainly attributed from the barrier effect of vanadium ions. Compared with vanadium ions, proton transportation across the membrane was
much faster because the stoke radius of vanadium ions is comparatively bigger than the proton [29]. Moreover, it was observed that by incorporating ZrNT confines the swelling nature of the composite membrane that is closely interrelated to its ion selectivity. In a VRB system, a high degree of swelling would increase the vanadium ions crossover, which was largely affected the capacity loss of the cell. Incorporation of ZrNT, swelling is subsidized at a certain level, by which the composite membrane exhibits fairly high proton conductivity and lower crossover of vanadium ions.
Fig. 5. (a) Measurement of time dependent vanadium ion concentration; (b) Permeability of vanadium ions and ion selectivity through Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes.
Fig. 6. Charge-discharge curves of the VRB assembled with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes at 40 mA cm2.
3.3. Vanadium flow battery performance The VRB charge-discharge tests constructed with composite and
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Fig. 7. (a) Discharge capacity retention of the VRB constructed with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes; (b) Self-discharge curves of the VRB constructed with Nafion, Nafion-ZrNT (0.5%), and Nafion-ZrNT (1%) membranes.
Nafion-117 membranes are carried out at room temperature using a current density 40 mA cm2. Fig. 6 displays the charge-discharge plots of the VRB constructed with Nafion-ZrNT and Nafion-117 membranes. The first cycle charge-discharge time for NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) composite membranes observed to be 7.40, 8.46 and 7.77 h, respectively. Whereas, the charge-discharge time for Nafion-117 membrane was 4.04 h. The discharge capacity at 40 mA cm2 of Nafion-117, NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes was 694.3, 1306.2, 1469.8 and 1369.8 mAh, respectively. Moreover, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes showed 1.8, 2.1 and 2-times higher capacity compared with Nafion-117 membrane. At 100 charge-discharge cycles, the Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-117 membranes generated the discharge capacity of 767, 987, 881 and 280 mAh, respectively. The capacity retention is also higher for Nafion-ZrNT membrane compared with Nafion-117 membrane. After 100th cycle, the capacity retention for NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are 59, 66 and 64%, respectively shown in Fig. 7a. On the other hand, Nafion-117 membrane is 40%. Moreover, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are 1.5, 1.7 and 1.6 times higher, respectively, than that of the Nafion-117 membrane. To further understand the excellent VRB results, the selfdischarge analysis was performed to determine the vanadium ion crossover through the composite Nafion-ZrNT and Nafion-117 membranes. The self-discharge degree of VRB is assessed by monitoring the OCV change for Nafion-117 and Nafion composite membranes. Fig. 7b displays, the OCV value steadily decline with time before 1.3 V and then drop sharply to 0.8 V for all membranes. In contrast, the VRB assembled with Nafion-ZrNT composite membrane exhibited higher performance, retaining OCV above 1.3 V for Nafion-ZrNT (0.5%) and Nafion-ZrNT (1%) membranes are 315 and 330 h, respectively, which are much longer than that obtained with same voltage for Nafion membrane (29 h). This performance is in-line with the vanadium ion-permeation test, with a much lower crossover of V (IV) ions occur through the Nafion-ZrNT membranes compared with the Nafion membrane. Similarly, low vanadium ion crossover is another reason for observing longer the self-discharge time for Nafion composite membrane. The high performance of composite membrane should be further seen from the coulombic efficiency, voltage efficiency, and energy efficiency of the VRB with all Nafion composite membranes compared with the Nafion-117 membrane at 40 mA cm2 (Fig. 8).
The performance of all efficiencies for Nafion-ZrNT composite membranes obtained at higher compared with the Nafion-117 membrane. In case of coulombic efficiency for the VRB with Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes was 97.2, 97.5 and 97.3%, respectively. These values are higher compared with VRB constructed with the Nafion-117 membrane (96.1%). While for voltage efficiency, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes performed 79.29, 80.48 and 78.85%, respectively. The improved performance of Nafion-ZrNT (1%) membrane is due to its highest proton conductivity compared with Nafion-ZrNT (0.5%) and Nafion-ZrNT (1.5%) membranes but obviously, NafionZrNT membranes exhibited higher voltage efficiency compared with the Nafion-117 membrane (77.45%). Similarly, as a result of higher coulombic efficiency and voltage efficiency, an improvement of the resulting energy efficiency for VRB operated with NafionZrNT membranes (<4%) compared with the Nafion-117 membrane. Generally, VRB constructed with the Nafion membrane suffered vanadium ion permeation during charge and discharge operation, where vanadium ions interact with sulfonated species present in a polar cluster of the Nafion membrane. Different valance states of vanadium ions result in low coulombic efficiency and energy efficiency for VRB [17]. Furthermore, the improved performance is attributed to the preferable proton conductivity, low vanadium ion crossover of the composite membranes. In order to evaluate the strong chemical stability of Nafion-ZrNT composite membrane in VRB, FE-SEM analysis was conducted for the composite membrane after 100 cycles of charge-discharge VRB operation. The results displayed in Fig. S5 indicate the Nafion-ZrNT composite membrane morphology retains its initial structure without much change to the overall initial morphology. The surface and cross-section images show the ZrNT fillers are still uniformly distributed among the surface of the membrane correspond to the good chemical stability of Nafion-ZrNT composite membranes. To study the impact of ZrO2 filler morphology in Nafion composite membrane, certain amount of ZrO2 nanoparticles (ZrNP) filler, 0.5, 1 and 1.5% was added in Nafion monomer. The optimized Nafion-ZrNP (1%) membrane was chosen for VRB cycling performance (Fig. S6). The discharge capacity for Nafion-ZrNP (1%) membrane delivered 1279.2 mAh at the first cycle. Under the same condition, the Nafion-ZrNT (1%) membrane exhibited 1.15-times higher discharge capacity (1469.8 mAh) compared with the NafionZrNP (1%) composite membrane. The capacity retention of NafionZrNP (1%) membrane was also lower compared with Nafion-ZrNT membranes. Fig. 8 shows all efficiencies (coulombic, voltage and
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assembled with Nafion-ZrNT (1%) composite membrane exhibits a lower vanadium ion permeability (3.6 109 cm2 min1) than those with membrane of Nafion/SiO2 (3.0 107) [17], Nafion with polypyrrole (PHB12) (5.4 107) [19], Nafion/TiO2 (6.7 106) [22], Nafion/Si/Ti (4.3 107) [26], and Nafion-117 (3.6 106) [46]. The VRB operated with Nafion-ZrNT (1%) composite membrane showed higher coulombic efficiency (97.5%) than other reported membranes, such as Nafion-polyvinylidene fluoride (N0.8P0.2) (91.8%) [23], and Nafion-117 (93.8%) [46], at a current density of 40 mA cm2. The high performance of proton conductivity value and self-discharge time (OCV) of Nafion-ZrNT (1%) composite membrane also obtained compared with all comparable membranes. 4. Conclusion The Nafion-ZrNT membrane was successfully fabricated by using porous ZrNT which was prepared by the electrospinning process of a polymer mat consisting of Zracac with PAN at 580 C under air atmosphere. Structural investigation reveals that ZrNT filler was tubular like the format, where the tube wall composed of small particles joined together. Compared with the pristine Nafion-117 membrane, the Nafion-ZrNT composite membranes displayed higher water uptake, proton conductivity, and oxidative stability as well as the lower value of swelling degree, IEC, and activation energy. The VRB constructed with Nafion-ZrNT membranes showed excellent capacity retention, low self-discharge rate (open circuit voltage was maintained above 1.3 V after a period of 330 h), higher performance of coulombic efficiency (97.5%), voltage efficiency (80.5%) and energy efficiency (78.5%) compared with Nafion-117 membrane at current density 40 mA cm2. Permeability nature of vanadium ions through Nafion-ZrNT membranes are much lower than the Nafion-117 membrane, which corresponds to superior ion selectivity. Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) by the Ministry of Education, Science, and Technology (2014R1A1A2057056). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.113. Fig. 8. Cycle efficiencies of the VRB constructed with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes as a function of cycling numbers.
energy) were much lower for Nafion-ZrNP (1%) membrane than Nafion-ZrNT membranes. The high performance of Nafion-ZrNT over Nafion-ZrNP membranes was mainly due to the porous tubular morphology of ZnNT present in Nafion-ZrNT membrane, which exhibited lower ohmic resistance. The higher ohmic resistance of Nafion-ZrNP (1%) membrane resulted in higher activation energy and lower proton conductivity than Nafion-ZrNT (1%) composite membrane (Table 1). Moreover, Nafion-ZrNP (1%) membrane showed low water uptake, high swelling degree, and IEC compared with the Nafion-ZrNT (1%) membrane. This is because of the nanoparticles can freely accumulated with each other that conclusion poor distribution in the Nafion composite membrane. The performance of Nafion-ZrNT composite membrane is compared with the other composite membrane (Table S1). The VRB
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Zirconium oxide nanotubeeNafion composite as high performance membrane for all vanadium redox flow battery Md. Abdul Aziz, Sangaraju Shanmugam* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 333, Techno Jungang-daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu, 42988, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The Nafion-ZrNT composite membrane was successfully fabricated. Nafion-ZrNT membrane shows low vanadium-ion crossover thus superior ion selectivity. VRB showed low self-discharge rate for Nafion-ZrNT composite membrane than Nafion-117. Nafion-ZrNT exhibits high discharge capacity and efficiency compared with Nafion-117.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2016 Received in revised form 25 October 2016 Accepted 31 October 2016 Available online xxx
A high-performance composite membrane for vanadium redox flow battery (VRB) consisting of ZrO2 nanotubes (ZrNT) and perfluorosulfonic acid (Nafion) was fabricated. The VRB operated with a composite (Nafion-ZrNT) membrane showed the improved ion-selectivity (ratio of proton conductivity to permeability), low self-discharge rate, high discharge capacity and high energy efficiency in comparison with a pristine commercial Nafion-117 membrane. The incorporation of zirconium oxide nanotubes in the Nafion matrix exhibits high proton conductivity (95.2 mS cm1) and high oxidative stability (99.9%). The Nafion-ZrNT composite membrane exhibited low vanadium ion permeability (3.2 109 cm2 min1) and superior ion selectivity (2.95 107 S min cm3). The VRB constructed with a Nafion-ZrNT composite membrane has lower self-discharge rate maintaining an open-circuit voltage of 1.3 V for 330 h relative to a pristine Nafion membrane (29 h). The discharge capacity of Nafion-ZrNT membrane (987 mAh) was 3.5-times higher than Nafion-117 membrane (280 mAh) after 100 charge-discharge cycles. These superior properties resulted in higher coulombic and voltage efficiencies with Nafion-ZrNT membranes compared to VRB with Nafion-117 membrane at a 40 mA cm2 current density. © 2016 Elsevier B.V. All rights reserved.
Keywords: Vanadium redox flow battery Nafion composite membrane ZrO2 nanotubes Electrospinning Vanadium crossover
1. Introduction Vanadium redox flow battery (VRB) has received great attention in energy storage technologies, such as smart grids, wind turbine
* Corresponding author. E-mail address: [email protected] (S. Shanmugam).
generators, emergency backup applications and remote area power source systems [1e5], due to its high energy efficiency, low cost, long cycle life, quick response and profound discharge capability [6e13]. VRB employs oxidation and reduction reactions to store energy, where two soluble redox couples act as electroactive materials [14]. In a typical setup, VRB consists of two electrolyte reservoir supplied with V2þ/V3þ and VO2þ/VOþ 2 redox species in sulphuric acid solution, a battery stack area for redox reaction and
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obtain ZrO2 nanotubes. The calcination process was carried under air atmosphere. Initially, the synthesized nonwoven mat was stabilized at 250 C for 1 h to remove the organic substance and heating was continued to 580 C for 1 h. The heating rate was fixed at 5 C per minute.
two pumps. The electrolyte solutions are pumped into the reservoirs parted by a membrane which allows diffusion of protons to fulfill the requirement of the circuit at the time movement of current. The ideal VRB membranes should possess good chemical stability, low vanadium ion permeability to achieve longer cycle life and better coulombic efficiency. Similarly, to result in higher voltage efficiency, a membrane with good proton conductivity should be highly recommended for VRB [14,15]. Perfluorosulfonic polymers, such as Dupont Nafion®, are widely used membranes for VRB, because of their good chemical stability and excellent proton conductivity [16]. Despite their several advantages, still, Nafion membranes exhibit high vanadium ion crossover that additionally contributes to the capacity loss and decreases energy efficiency of VRB. To overcome these challenges, certain modifications phenomena were offered to increase the ion-selectivity of Nafion membrane or minimize the crossover of vanadium ion [17e19]. For example, Nafion was modified with SiO2 [17], polypyrrole [19], polyelectrolyte [20], polyaniline [21], TiO2 [22], polyvinylidene fluoride [23], sulfonated poly (ether ether ketone) [24], diethoxydimethylsilane [25], organic silica modified TiO2 [26], polytetrafluoroethylene [27,28] All these composite membranes showed comparatively lower vanadium ion crossover and higher columbic efficiency compared with the pristine Nafion membrane. However, still a great challenge to maintain the proper ratio between proton conductivity over vanadium ion permeability to achieve superior vanadium ion selectivity. Based on this aspect, we synthesized ZrO2 nanotubes (ZrNT) with a diameter smaller than 50 nm. This was achieved by calcination of an electrospun polyacrylonitrile (PAN) containing a Zirconium precursor mat prepared using a single spinneret electrospinning method under an air atmosphere. The fabricated porous metal oxide nanotubes are incorporated in a Nafion ionomer and subsequently, a VRB single cell performance was evaluated using the composite membranes. The main reason for introducing ZrNT in Nafion is to enhance the ion selectivity of the membrane, because of ZrO2 partially block the polar structure (sulfonated ion) available in Nafion membrane by the electrostatic interaction, which acts as a permselective barrier for vanadium ions by tortuous pathway effect. Moreover, the proton can easily transfer through the modified membrane because the stoke radius of the proton is comparatively smaller than the vanadium ions [29]. The VRB results using the Nafion-ZrNT membranes were compared with results obtained using a commercial Nafion-117 membrane.
Field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800II) with a setup voltage of 3 kV was used to observe the morphology of the samples. Osmium coating was used before SEM observation. Field-emission transmission electron microscope (FETEM, Hitachi, HF-3300) with the voltage of 300 kV was used to determine the microstructures of samples. The crystal structure was determined using powder X-ray diffraction (Panalytical, Empyrean, XRD) with CuKa radiation at a setup current of 30 mA and voltage of 40 kV. The thermal stability of membrane sample was determined by using a thermal gravimetric analyzer (Thermo plus EVO, TG 8120). The small part of the membrane was placed in a crucible and thermally heated from 25 to 900 C with a heating rate of 10 C min1 under air atmosphere. Water uptake (WU) and swelling degree of membranes were obtained by comparison of weight and dimensions of dry and wet samples according to Eqs. (1) and (2). Membranes were heated at 90 C for 12 h using the oven and finally soaked in DI water (24 h) at room atmosphere.
2. Experimental
Water uptake ð%Þ ¼
2.3. Preparation of Nafion-ZrO2 nanotube composite membrane A required mass ratio of ZrNTs was incorporated in the Nafion ionomer using ethanol as a solvent. Nafion ionomer and ZrNTs were mixed together then the contents were ultra-sonicated for 1 h and finally stirred for 5 h by using a mechanical stirrer. The obtained mixture was poured into a glass petri dish and allowed to dry at 50 C for 2 h, 60 C for 2 h, 70 C for 2 h, and 80 C for 2 h using vacuum oven. Then, the composite membrane was peeled-off from the petri dish by immersing in DI water. The membrane thickness is optimized by adjusting the viscosity of Nafion ionomer solution. Finally, the composite membrane was pretreated by heating with 5% H2O2, H2O, 0.5 M H2SO4, and H2O in sequence for 1 h for each case. 2.4. Materials characterization
Wwet Wdry 100 Wdry
(1)
Lwet Ldry 100 Ldry
(2)
2.1. Materials Zirconium (IV) acetylacetonate (Zracac), polyacrylonitrile (PAN, Mw ¼ 150,000 g/mol) and Nafion-117 membrane were acquired from Sigma-Aldrich, Korea. Nafion ionomer (1100 EW, 15 wt%) was obtained from Ion Pow. Inc, USA. Ethanol, H2O2, H2SO4, and N, Ndimethylformamide (DMF) were purchased from Daejung Chemicals, Korea. Vanadium electrolyte 1.7 M V (±3%), V3þ/V4þ 1:1 (±3%), 4.5 M SO 4 (±5%) was purchased from FC International, Korea and were utilized as received. 2.2. Preparation of ZrO2 nanotubes Zracac (0.1 g) and PAN (1 g) were completely dissolved in DMF separately at 90 C. Then, both solutions were suspended together at 90 C and a clear homogenous solution was observed. The details of electrospinning process are given in our previous publications [30,39]. The prepared electrospun (e-spun) nonwoven mat was pyrolysed using a tubular furnace (Wisd Laboratory Instruments) to
Swelling degree ð%Þ ¼
where, Wwet is the membrane weight after 24 h and Wdry is the dry membrane weight. Similarly, Lwet is the membrane length after 24 h and Ldry is the dry membrane length. Ion exchange capacity (IEC) was obtained by neutralization reaction using phenolphthalein indicator. The membrane was dried in the vacuum oven at 90 C and then dipped in 3 M NaCl solution for 12 h to exchange Hþ of the membrane with Naþ. Finally, titrated using 0.01 M NaOH. The IEC value was determined according to Eq. (3).
IEC ¼
VNaOH CNaOH Wdry
(3)
where VNaOH was the NaOH volume (mL) used at the neutralized point, CNaOH was the NaOH concentration (M) and Wdry was the dry
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membrane weight. The membrane conductivity cell (Bekktech) was used to obtain proton conductivity of the membrane with the gas accessory. Membrane samples were assembled in the cell at a fixed position in contact with two platinum electrodes. Specific voltages were applied to Pt electrodes using potentiostat and the corresponding currents were obtained. The resistance (R) is calculated using the slope of the line, which joins the data points. During measurement, the temperature was fixed at 80 C with different relative humidity (RH %). The membrane conductivity was determined according to Eq. (4).
s¼
L RW T
(4)
where, L ¼ 0.425 cm is the constant length among Pt electrodes; R is the resistance of membrane in U; W is the sample width in cm and T is the membrane thickness in cm. For the oxidative stability test, a small part of the membrane was immersed in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 C. The oxidative stability was investigated using the remaining weight of the membrane after 1 h treatment with Fenton's reagent [31]. 2.5. Measurements of VO2þ permeability and ion selectivity The permeation of vanadium ion (VO2þ) between the membrane was evaluated via a membrane diffusion cell. Sample membrane which acts as a barrier with specific area 9 cm2 was clamped between two reservoirs. The left reservoir was loaded with 150 mL of 1.5 mol L1 VOSO4 in 3 mol L1 H2SO4, and the right reservoir was loaded with 150 mL of 1.5 mol L1 MgSO4 in 3 mol L1 H2SO4 solution to balance the ionic strengths as well as to decrease the effects of osmotic pressure. The solutions in two reservoirs were continuously stirred by magnetic stirrers throughout the entire measurement at normal temperature to avoid the concentration polarization. At regular time intervals, the aliquot sample was taken from the right reservoir and determined the VO2þ concentration using the UVeVis spectrometer. The permeability nature of vanadium ion was obtained by the following equation [32].
VR
dCR ðtÞ P ¼ A ½CL CR ðtÞ dt L
d P
between two graphite plates act as current collectors. Two copper sheets were attached in the outer positions of two graphite plates, which were worked for battery terminal connectors. This assembly was fastened by two polyvinyl chloride end plates. The carbon fleece (5 mm uncompressed thickness) was activated by thermal treatment at 500 C for 30 min. The cyclic chargeedischarge curves of the VRB cell constructed with the Nafion-ZrNT composite and the recast Nafion membranes were determined in normal temperature at 40 mA cm2 current density. Vanadium electrolyte with a composition of 1.7 M V (±3%), V3þ/ 4þ V 1:1 (±3%), 4.5 M SO 4 (±5%) was employed as electrolyte solution. The amount of electrolyte solution was 30 mL in each negative electrolyte vessel as well as in the positive electrolyte vessel. The electrolytes were recirculated with a peristaltic pump (Reglo ICC 2ch Pump) at a flow rate of 10 mL min1. Both electrolyte vessels were de-aerated using continuous N2 flow to avoid the chemical oxidation of the electrolyte solutions. An eight channel battery analyzer (BST8-3, MTI Corp.) was used to test the battery. For each membrane sample, such as Nafion-117 and Nafion-ZrNT same charge and discharge conditions were applied. To save the electrodes and graphite plates from corrosion, the maximum edge of the charge voltage was applied 1.6 V and the minimum edge of the discharge voltage was maintained to be 1 V. Finally, the self-discharge of the battery was evaluated at room temperature by time tracking the open circuit voltage decrease. All data points were collected until the open circuit voltage dropped below 0.8 V. The cyclic efficiencies including coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the VRB were determined by the following equations.
CE ¼
discharge capacity 100 charge capacity
(7)
VE ¼
middle point of discharge voltage 100 middle point of charge voltage
(8)
EE ¼ CE VE
(9)
3. Results and discussion
(5) 3.1. Membrane characterization
where, CR(t) is the concentration vanadium ion in the right reservoir with time measured by UVevis spectrometer, and CL is the concentration vanadium ion in the left reservoir. A is the active area and L is the thickness of the membrane. P is the vanadium ion permeability and VR is the volume of the right reservoir. It is considered that the concentration change of the vanadium ion in the left reservoir should be negligible as well as a pseudo-steadystate order is applied in between the membrane. The membrane ion selectivity (S) was measured via proton conductivity (d) with a crossover of vanadium ion (P), which was determined by Eq. (6),
S¼
3
(6)
2.6. Measurements of vanadium flow battery performance A single cell VRB was assembled by sandwiching the composite membrane between two carbon felt electrodes (9 cm2). The membrane-electrode assembly (MEA) was compressed and held
The membrane plays a significant role in the operation of VRB and the widely used membrane is based on the Nafion. The hydrophilic polar structure of Nafion membrane is beneficial for proton conductivity but it has a great impact on the vanadium ion permeability, which decreases the overall cell performance. Effective approaches in developing the ion selective Nafion membranes by filling of the polar cluster of Nafion with inorganic metal oxide fillers [33e36]. Incorporation of ZrO2 in Nafion membrane significantly reduces the vanadium ion permeability. The interaction between oxygen-functional groups on ZrO2 and the sulfonic acid group of Nafion matrix in the Nafion composite membrane partially block the polar structure available in Nafion membrane, which acts as a barrier for vanadium ions by tortuous pathway effect (Fig. 1). The FE-SEM image clearly shows the synthesized ZrO2 exhibits tubular structure and FE-TEM image shows the existence of porous nanotubes structure, where the tubular wall composed of nanoparticles as shown in Fig. 2. The composite membranes with filler amount of 0.5, 1.0 and 1.5 wt % were fabricated and denoted as NafioneZrNT (0.5%), NafioneZrNT (1%) and Nafion-ZrNT (1.5%), respectively. The presence of ZrNT filler in the composite membrane was confirmed by XRD. Fig. S1 displays the X-ray diffraction
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Fig. 1. Schematic illustration of VRB operation and formulation process of Nafion-ZrNT composite membrane.
patterns of Nafion-ZrNT (1%) membrane, ZrNT filler, and recast Nafion membrane XRD patterns. The broad peak at 17.0 (2q) and prominent peak at 17.8 (2q) are attributed to the Nafion crystallite structure [37]. XRD peaks observed at 28 , 30 and 50 (2q) for the NafioneZrNT (1%) composite membrane are attributed to the diffraction pattern of the ZrNT filler present in the composite membrane. The surface, as well as cross-section morphology of the Nafion-ZrNT composite membrane, was observed by SEM analysis (Fig. S2). It should be noticed that the composite membrane is opaque and invariable from cross-section to the surface. High magnification SEM image shows ZrNT filler randomly distributed in the composite membrane, where the Nafion ionomer coated the surface of the ZrNT fillers. Additionally, aggregation of ZrNT fillers was not detected by SEM images. The thickness of the membrane was investigated by the SEM cross-sectional image of the NafioneZrNT (1%) composite membrane. The obtained thickness was measured to be 150 ± 5 mm. Furthermore, the dispersion of fillers in NafionZrNT composites membranes was measured by capturing SEM images in back scattering mode and it was observed that the nanotubes were randomly distributed within the membrane as shown in Fig. S3. The physicochemical properties, like water uptake, swelling degree, IEC, proton conductivity, activation energy and oxidative stability performances were determined for all membranes and the
results are listed in Table 1. In the case of Nafion-ZrNT membrane, WU is higher than the Nafion-117 membrane. It was reported that the zirconium oxide surface contains the hydroxyl groups, which facilitates the water absorption and retention capacity [38]. The interaction between the ZrNT filler and the sulfonated species of the Nafion ionomer is expected to improve the swelling degree and IEC value of the composite membrane. This interaction has an improved effect on reduction of vanadium ion-permeability and thus the enhanced VRB performance is observed even in the fully oxidative operating environment. Therefore, the proton conductivity of various NafionZrNT composite membranes was evaluated and the corresponding results were compared with the Nafion-117 membrane at 80 C under different humidity levels. Fig. 3a displays the humidity dependent proton conductivity plots of Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes at 80 C. The proton conductivity values of the NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes were higher than Nafion-117 membrane under 100% RH (Table 1). The higher proton conductivity of all composite membranes compared to the Nafion-117 membrane was mainly due to the specific water retention properties of ZrO2 filler [39]. The ZrO2 filler with various morphologies (nanotubes, nanoparticles) showed different conductivity results (Table 1). In addition, the joined nanoparticles in a tubular structure of ZrO2 filler are distributed uniformly, which facilitates better surface area. The high surface area of ZrNTs is retained more water compared with only ZrO2 nanoparticles. Comparatively, the low surface area of nanoparticles can freely accumulate with each other and exhibits lower proton conductivity than ZrO2 nanotubes [39]. Generally, the mechanism of proton transport of Nafion-based membrane should be verified based on proton activation energy. This can be deduced using the Arrhenius equation (Eq. (10)) [40,41]. Ea
s ¼ so eRT
(10) 1
where, s is the proton conductivity (S cm ), so is the preexponential factor, Ea is the activation energy (kJ mol1), R is the universal gas constant (8.314 J mol1K1) and T is the absolute temperature (K). Fig. 3b presents the plot of ln s vs. 1000/T of the Nafion-117 membrane and Nafion-ZrNT composite membranes obtained under 100% relative humidity at different temperatures. It was obtained that with increasing temperature the proton conductivity values also increased for all membranes. The value of activation energy computed from the slope of ln s vs. 1000/T plot for Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are found to be 13.27, 13.18 and 13.22 kJ mol1, respectively, whereas the activation energy value of Nafion-117 membrane was 13.50 kJ mol1. The Nafion-ZrNT composite
Fig. 2. (a) FE-SEM image; (b) FE-TEM image of ZrO2 nanotube structures.
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Table 1 Physicochemical characteristics of Nafion-117 and Nafion-ZrNT membranes. Membrane
Nafion-117 Nafion-ZrNT Nafion-ZrNT Nafion-ZrNT Nafion-ZrNP
(0.5%) (1%) (1.5%) (1%)
Water uptake (%)
Swelling degree (%)
IEC (meq g1)
Proton conductivity (mS cm1) @100% RH; 80 C
Activation energy (kJ mol1)
Oxidative stability (%)
27.68 32.26 35.89 35.31 33.17
15.31 10.33 6.21 5.94 6.24
1.076 0.935 0.927 0.915 0.930
87.6 93.9 95.2 94.5 91.9
13.50 13.27 13.18 13.22 13.36
99.6 99.7 99.8 99.9 99.8
membranes activation energy were lower compared with the Nafion-117 membrane, indicating that the existence of ZrNTs in Nafion membrane enhanced the proton transport through the membrane. The values of activation energy of composite membranes were suggestive the migration of proton across the composite membranes via the vehicular mechanism [41]. The values of activation energy of composite membranes were identical to the activation energy measured with hybrid Nafion-silica, Nafionmesoporous titanium phosphate and Nafion-mesoporous zirconium phosphate membrane [40,42]. Thermal stability of Nafion-117, Nafion-ZrNT (0.5%), NafionZrNT (1%), Nafion-ZrNT (1.5%) membranes and ZrNT filler was determined under air atmosphere. Fig. 4 displays three distinctive
weight losses were observed for all membranes. The weight loss in the temperature range of 80e120 C (dehydration) was almost same for all membranes. On the other hand, the decomposition temperature (290e370 C) of the sulfonated groups in the NafioneZrNT composite membrane was lower compared with the Nafion-117 membrane. Nafion composite membrane exhibits drastic loss of the sulfonated group in a composite membrane compared with the Nafion-117 membrane at a temperature of 300 C [43]. Furthermore, the decomposition temperature (420e520 C) of the polymer backbone of the composite membrane was higher compared with the Nafion-117 membrane. The huge loss of the sulfonic acid groups is mainly attributed to the distinct molecular interaction between the sulfonic acid of the Nafion with a hydroxyl group of the metal oxide. The oxidative stability of Nafion-117 and Nafion-ZrNT composite membranes was investigated using Fenton's reagent (3 wt% H2O2 þ 2 ppm FeSO4) at 80 C. The specific weight loss of the membranes for 1 h are given in Table 1. The Nafion-ZrNT membranes exhibited good oxidative stability compared with Nafion117. This is mainly due to the oxidative affect generally happens in the hydrophilic regions of Nafion by radical groups (HO and HOO). The interaction between oxygen-functional groups of ZrO2 and the sulfonic acid group of Nafion matrix reduces the hydrophilic region in the composite membrane [44]. Moreover, it is a well-established that with the incorporation of a small portion of filler materials to proton exchange membrane is highly active to improve the oxidative stability of the membrane [45]. The addition of ZrNT filler in Nafion matrix further proved the high oxidative stability of Nafion-ZrNT membrane. 3.2. VO2þ permeability and ion selectivity The vanadium ion crossover through the membrane is
Fig. 3. (a) Relative humidity dependent proton conductivity of Nafion-117, NafioneZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and NafioneZrNP (1%) membranes obtained at 80 C; (b) Retrogression curves of proton conductivity versus 1000/T for Nafion-117, Nafione ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and NafioneZrNP (1%) membranes measured under 100% RH at different temperature.
Fig. 4. Thermograms of ZrNT filler, Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes measured in room atmosphere.
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considered a vital complication of VRB, due to rapid self-discharge of the VRB, resulting in low energy efficiency. Fig. S4 displays the schematic illustration of membrane separated diffusion cell. The presence of vanadium ion concentration in MgSO4 solution compartment was determined with the time for Nafion-117, Nafion-ZrNT membranes (Fig. 5a) and the corresponding vanadium ion permeability and ion selectivity was shown in Fig. 5b. At a given time, the vanadium ion concentration permeate across the Nafion-ZrNT composite membranes was lower than that of the Nafion-117 membrane. Nafion-117 membrane shows a high vanadium ion permeability (1.2 108 cm2 min1), while Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes show the permeability values of 3.6 109, 3.4 109 and 3.2 109 cm2min1, respectively. The lower permeability value of Nafion composite membrane benefits from the presence of ZrNT filler. As discussed above, the ZrO2 filler exhibits oxygen-functional groups and the sulfonic acid groups of Nafion matrix interact with each other and acts as a barrier for vanadium ion by tortuous pathway effect that has been partly filled the polar clusters of the Nafion membrane. The lower vanadium ion crossover further proved by lower IEC value of Nafion-ZrNT membranes. The vanadium ion selectivity value for Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) are found to be 2.60 107, 2.80 107 and 2.95 107 S min cm3, respectively. The values obtained for composite membranes are much higher when compared with the Nafion-117 membrane (7.5 106 S min cm3). The superior performance of Nafion-ZrNT membranes over Nafion-117 was mainly attributed from the barrier effect of vanadium ions. Compared with vanadium ions, proton transportation across the membrane was
much faster because the stoke radius of vanadium ions is comparatively bigger than the proton [29]. Moreover, it was observed that by incorporating ZrNT confines the swelling nature of the composite membrane that is closely interrelated to its ion selectivity. In a VRB system, a high degree of swelling would increase the vanadium ions crossover, which was largely affected the capacity loss of the cell. Incorporation of ZrNT, swelling is subsidized at a certain level, by which the composite membrane exhibits fairly high proton conductivity and lower crossover of vanadium ions.
Fig. 5. (a) Measurement of time dependent vanadium ion concentration; (b) Permeability of vanadium ions and ion selectivity through Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes.
Fig. 6. Charge-discharge curves of the VRB assembled with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes at 40 mA cm2.
3.3. Vanadium flow battery performance The VRB charge-discharge tests constructed with composite and
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Fig. 7. (a) Discharge capacity retention of the VRB constructed with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes; (b) Self-discharge curves of the VRB constructed with Nafion, Nafion-ZrNT (0.5%), and Nafion-ZrNT (1%) membranes.
Nafion-117 membranes are carried out at room temperature using a current density 40 mA cm2. Fig. 6 displays the charge-discharge plots of the VRB constructed with Nafion-ZrNT and Nafion-117 membranes. The first cycle charge-discharge time for NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) composite membranes observed to be 7.40, 8.46 and 7.77 h, respectively. Whereas, the charge-discharge time for Nafion-117 membrane was 4.04 h. The discharge capacity at 40 mA cm2 of Nafion-117, NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes was 694.3, 1306.2, 1469.8 and 1369.8 mAh, respectively. Moreover, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes showed 1.8, 2.1 and 2-times higher capacity compared with Nafion-117 membrane. At 100 charge-discharge cycles, the Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-117 membranes generated the discharge capacity of 767, 987, 881 and 280 mAh, respectively. The capacity retention is also higher for Nafion-ZrNT membrane compared with Nafion-117 membrane. After 100th cycle, the capacity retention for NafionZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are 59, 66 and 64%, respectively shown in Fig. 7a. On the other hand, Nafion-117 membrane is 40%. Moreover, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes are 1.5, 1.7 and 1.6 times higher, respectively, than that of the Nafion-117 membrane. To further understand the excellent VRB results, the selfdischarge analysis was performed to determine the vanadium ion crossover through the composite Nafion-ZrNT and Nafion-117 membranes. The self-discharge degree of VRB is assessed by monitoring the OCV change for Nafion-117 and Nafion composite membranes. Fig. 7b displays, the OCV value steadily decline with time before 1.3 V and then drop sharply to 0.8 V for all membranes. In contrast, the VRB assembled with Nafion-ZrNT composite membrane exhibited higher performance, retaining OCV above 1.3 V for Nafion-ZrNT (0.5%) and Nafion-ZrNT (1%) membranes are 315 and 330 h, respectively, which are much longer than that obtained with same voltage for Nafion membrane (29 h). This performance is in-line with the vanadium ion-permeation test, with a much lower crossover of V (IV) ions occur through the Nafion-ZrNT membranes compared with the Nafion membrane. Similarly, low vanadium ion crossover is another reason for observing longer the self-discharge time for Nafion composite membrane. The high performance of composite membrane should be further seen from the coulombic efficiency, voltage efficiency, and energy efficiency of the VRB with all Nafion composite membranes compared with the Nafion-117 membrane at 40 mA cm2 (Fig. 8).
The performance of all efficiencies for Nafion-ZrNT composite membranes obtained at higher compared with the Nafion-117 membrane. In case of coulombic efficiency for the VRB with Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes was 97.2, 97.5 and 97.3%, respectively. These values are higher compared with VRB constructed with the Nafion-117 membrane (96.1%). While for voltage efficiency, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%) and Nafion-ZrNT (1.5%) membranes performed 79.29, 80.48 and 78.85%, respectively. The improved performance of Nafion-ZrNT (1%) membrane is due to its highest proton conductivity compared with Nafion-ZrNT (0.5%) and Nafion-ZrNT (1.5%) membranes but obviously, NafionZrNT membranes exhibited higher voltage efficiency compared with the Nafion-117 membrane (77.45%). Similarly, as a result of higher coulombic efficiency and voltage efficiency, an improvement of the resulting energy efficiency for VRB operated with NafionZrNT membranes (<4%) compared with the Nafion-117 membrane. Generally, VRB constructed with the Nafion membrane suffered vanadium ion permeation during charge and discharge operation, where vanadium ions interact with sulfonated species present in a polar cluster of the Nafion membrane. Different valance states of vanadium ions result in low coulombic efficiency and energy efficiency for VRB [17]. Furthermore, the improved performance is attributed to the preferable proton conductivity, low vanadium ion crossover of the composite membranes. In order to evaluate the strong chemical stability of Nafion-ZrNT composite membrane in VRB, FE-SEM analysis was conducted for the composite membrane after 100 cycles of charge-discharge VRB operation. The results displayed in Fig. S5 indicate the Nafion-ZrNT composite membrane morphology retains its initial structure without much change to the overall initial morphology. The surface and cross-section images show the ZrNT fillers are still uniformly distributed among the surface of the membrane correspond to the good chemical stability of Nafion-ZrNT composite membranes. To study the impact of ZrO2 filler morphology in Nafion composite membrane, certain amount of ZrO2 nanoparticles (ZrNP) filler, 0.5, 1 and 1.5% was added in Nafion monomer. The optimized Nafion-ZrNP (1%) membrane was chosen for VRB cycling performance (Fig. S6). The discharge capacity for Nafion-ZrNP (1%) membrane delivered 1279.2 mAh at the first cycle. Under the same condition, the Nafion-ZrNT (1%) membrane exhibited 1.15-times higher discharge capacity (1469.8 mAh) compared with the NafionZrNP (1%) composite membrane. The capacity retention of NafionZrNP (1%) membrane was also lower compared with Nafion-ZrNT membranes. Fig. 8 shows all efficiencies (coulombic, voltage and
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assembled with Nafion-ZrNT (1%) composite membrane exhibits a lower vanadium ion permeability (3.6 109 cm2 min1) than those with membrane of Nafion/SiO2 (3.0 107) [17], Nafion with polypyrrole (PHB12) (5.4 107) [19], Nafion/TiO2 (6.7 106) [22], Nafion/Si/Ti (4.3 107) [26], and Nafion-117 (3.6 106) [46]. The VRB operated with Nafion-ZrNT (1%) composite membrane showed higher coulombic efficiency (97.5%) than other reported membranes, such as Nafion-polyvinylidene fluoride (N0.8P0.2) (91.8%) [23], and Nafion-117 (93.8%) [46], at a current density of 40 mA cm2. The high performance of proton conductivity value and self-discharge time (OCV) of Nafion-ZrNT (1%) composite membrane also obtained compared with all comparable membranes. 4. Conclusion The Nafion-ZrNT membrane was successfully fabricated by using porous ZrNT which was prepared by the electrospinning process of a polymer mat consisting of Zracac with PAN at 580 C under air atmosphere. Structural investigation reveals that ZrNT filler was tubular like the format, where the tube wall composed of small particles joined together. Compared with the pristine Nafion-117 membrane, the Nafion-ZrNT composite membranes displayed higher water uptake, proton conductivity, and oxidative stability as well as the lower value of swelling degree, IEC, and activation energy. The VRB constructed with Nafion-ZrNT membranes showed excellent capacity retention, low self-discharge rate (open circuit voltage was maintained above 1.3 V after a period of 330 h), higher performance of coulombic efficiency (97.5%), voltage efficiency (80.5%) and energy efficiency (78.5%) compared with Nafion-117 membrane at current density 40 mA cm2. Permeability nature of vanadium ions through Nafion-ZrNT membranes are much lower than the Nafion-117 membrane, which corresponds to superior ion selectivity. Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) by the Ministry of Education, Science, and Technology (2014R1A1A2057056). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.10.113. Fig. 8. Cycle efficiencies of the VRB constructed with Nafion-117, Nafion-ZrNT (0.5%), Nafion-ZrNT (1%), Nafion-ZrNT (1.5%) and Nafion-ZrNP (1%) membranes as a function of cycling numbers.
energy) were much lower for Nafion-ZrNP (1%) membrane than Nafion-ZrNT membranes. The high performance of Nafion-ZrNT over Nafion-ZrNP membranes was mainly due to the porous tubular morphology of ZnNT present in Nafion-ZrNT membrane, which exhibited lower ohmic resistance. The higher ohmic resistance of Nafion-ZrNP (1%) membrane resulted in higher activation energy and lower proton conductivity than Nafion-ZrNT (1%) composite membrane (Table 1). Moreover, Nafion-ZrNP (1%) membrane showed low water uptake, high swelling degree, and IEC compared with the Nafion-ZrNT (1%) membrane. This is because of the nanoparticles can freely accumulated with each other that conclusion poor distribution in the Nafion composite membrane. The performance of Nafion-ZrNT composite membrane is compared with the other composite membrane (Table S1). The VRB
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Please cite this article in press as: M.A. Aziz, S. Shanmugam, Journal of Power Sources (2016), http://dx.doi.org/10.1016/j.jpowsour.2016.10.113