silica composite membrane with high performance for vanadium redox flow battery

Journal of Power Sources 272 (2014) 113e120

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Ultra-thin polytetrafluoroethene/Nafion/silica composite membrane with high performance for vanadium redox flow battery Xiangguo Teng a, Jicui Dai a, Fangyuan Bi a, Geping Yin b, * a b

School of Marine Science and Technology, Harbin Institute of Technology at Weihai, Weihai 264209, PR China School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China

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


Ultra-thin polytetrafluoroethylene/ Nafion/silica composite membrane was prepared.
The membrane was prepared by solution casting and solegel method.
SEM results show that the pores of PTFE membrane were well filled.
The vanadium battery with the composite membrane showed good performance.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2014 Received in revised form 12 August 2014 Accepted 14 August 2014 Available online 27 August 2014

Ultra-thin and high performance polytetrafluoroethene (PTFE)/Nafion/silica composite membrane has been successfully prepared by solution casting and solegel method for all vanadium redox flow battery (VRB). Thickness of ~25 mm polytetrafluoroethene/Nafion (P/N) membrane is first prepared by impregnating porous PTFE membrane with Nafion solution, and then the P/N membrane is immersed in tetraethoxysilane (TEOS) solution to prepare PTFE/Nafion/silica (P/N/S) composite membranes. The chemical structures of membranes are investigated by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR), which prove that the Nafion resin and silica are well impregnated in PTFE membrane. The water uptake, proton conductivity, vanadium permeability and VRB single cell tests of the composite membrane are also investigated in detail. At 80 mA cm2, coulombic efficiency, voltage efficiency and energy efficiency of the VRB with P/N/S-7 (7 wt.% SiO2 in P/N/S) membrane are 93.9%, 87.2% and 81.9%, respectively. Furthermore, the self-discharge rate of the VRB with P/N/S membrane is much slower than that of the VRB with P/N membrane, which indicates that the membrane has good vanadium block ability. Fifty cycles chargeedischarge test proves that the P/N/S membrane is very stable and possesses high chemical stability under the strong acid solutions. © 2014 Elsevier B.V. All rights reserved.

Keywords: Polytetrafluoroethene Nafion Silica Vanadium redox flow battery

1. Introduction In order to solve environmental pollution and decrease the usage of traditional fossil fuels, renewable energy such as solar and

* Corresponding author. Tel./fax: þ86 631 5687232. E-mail addresses: [email protected] (X. Teng), [email protected] (G. Yin). http://dx.doi.org/10.1016/j.jpowsour.2014.08.060 0378-7753/© 2014 Elsevier B.V. All rights reserved.

wind energy have been developed rapidly in recent years. However, the renewable energy sources are intermittent in nature and thus require a safe and effective large scale energy storage system (EES) to improve the reliability, power quality and economy of these renewable energies [1]. Compared with the other EESs, all vanadium redox flow battery (VRB) invented by M. Skyllas-Kzazcos and co-workers is the one of the most promising EESs due to its long cycle life, low cost, flexible design and high energy efficiency [2e4].

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As a critical component, proton exchange membrane is employed to separate the negative and positive electrolytes while still allowing transfer of the protons from anode to cathode. The ideal membrane in VRB should possess low vanadium ion permeability, high proton conductivity, good chemical stability and low cost to satisfy the large scale commercialization for VRB. Despite many new developed membranes [5e10] used in VRB, perflurosulfonic acid membranes such as Nafion (DuPont) is still one of the most popular membranes used in VRB due to its high chemical stability and proton conductivity [11,12]. However, Nafion membrane suffers from high vanadium permeability and the extremely high cost which have limited their further commercialization. To solve these problems, various research progresses of modified Nafion membranes with improving VRB performances have been made. Recently, our group has reported a low Nafion content with 45 mm P/N (polytetrafluoroethene/Nafion) composite membrane in VRB application [13]. The membrane shows much higher ion selectivity, low cost and good chemical strength and etc. [14]. Inspired by the good performances of P/N composite membrane and thinner membrane means low Nafion consumption and low ionic resistance, in this work, ultra-thin (~25 mm) P/N/silica (P/N/S) membranes were prepared by modifying P/N membrane using tetraethoxysilane (TEOS) via in-situ solegel method. The properties of the P/N/S composite membrane such as water uptake, proton conductivity, vanadium permeability, Fourier transform infrared spectra (FT-IR), morphology and cell performance were investigated and discussed in detail. 2. Experimental 2.1. Materials Nafion 117 (Du Pont, U. S. A.), PTFE membrane (Xinxiang Xinxing Fenghua Film Factory, Henan Province, China), TEOS and N,N-dimethylformamide (DMF) (Tianjin Bodi Chemical Co., Ltd., China), Nafion solution of 5 wt.% was made according to literature [15]. All the other chemical reagents were purchased from local chemical corporations and used as received. All water was deionized.

Fig. 1. Preparation scheme of P/N and P/N/S composite membranes.

2.3. Characterization of the composite membranes 2.3.1. Water uptake Water uptake of the membrane was defined as mass ratio of the absorbed water to that of the dry membrane. The weighed dry membranes were immersed into water at room temperature for 24 h at first. Subsequently, the membranes were taken out and wiped off the residual water adhered to the surface quickly and weighed. The water uptake was determined according to the following Equation (1).


Wwet  Wdry Water uptake % ¼  100% Wdry

(1)

where Wwet and Wdry are the weights of wet and dry state, respectively.

2.2. P/N and P/N/S composite membranes preparation The methods for preparation of P/N and P/N/S composite membranes are referred to literature [14e17] and the preparation scheme is shown in Fig. 1. Porous PTFE membranes (~15 mm), with pore size distribution in the range of 0.3e0.5 mm and porosity of 85%, were treated by ethanol at 55  C for 2.5 h to improve its compatibility with Nafion resin. The pretreated PTFE membranes were impregnated with 7 mL of 5 wt.% Nafion/DMF solution for 2 h and then annealed at 140  C for 10 h to obtain P/N composite membranes. After thermal treatment, P/N membranes were then treated according to the standard treatment procedures for Nafion membranes [18]. The process for preparation of P/N/S composite membranes is similar to that of the method first invented by Deng et al. [19]. Briefly, Mixed TEOS/water/HCl (molar ratio ¼ 1:4:0.5) solution was obtained by continuous stirring at room temperature for 5 h. Subsequently, the MeOH/H2O (molar ratio ¼ 2:1) swollen P/ N membranes were immersed in the above mixed TEOS/water/HCl solution for certain time to obtain a series of P/N/S membranes. Silica composition in the prepared dry P/N/S composite membranes were controlled to be 1%, 3%, 5%, 7% and 9%. Accordingly, they are identified as P/N/S-1, P/N/S-3, P/N/S-5, P/N/S-7 and P/N/S-9, respectively. And the thickness of all the P/N/S membrane was ~25 mm.

2.3.2. Proton conductivity Proton conductivity of the sample membranes was carried out on a electrochemical impedance spectroscopy (EIS) with a IM6e electrochemical station (Zahner, Germany) according to literature [20] in the frequency range of 1 Hze105 Hz at room temperature and 100% relative humidity. The proton conductivity (s) of the membranes was calculated by Equation (2).

  s mS cm1 ¼

L RS

(2)

where L (cm) is the distance between two electrodes, R (U) is the sample membranes resistance, and S (cm2) is the cross-sectional area of the membranes. 2.3.3. VO2þ permeability The permeability of VO2þ was determined using a self-made osmotic cell according to the literature [21]. The membranes were exposed into a solution of 1.5 mol L1 VOSO4 in 2.5 mol L1 H2SO4 (left reservoir) and 1.5 mol L1 MgSO4 in 2.5 mol L1 H2SO4 (right reservoir), respectively. The effective membrane area is 1.77 cm2 and the volume of each cell was 18 mL. Solution in MgSO4

X. Teng et al. / Journal of Power Sources 272 (2014) 113e120

side was taken and measured by a UVevis spectrometer (Purkinje General, China) at regular time intervals. The absorbance of the samples was measured at a wavelength of 762 nm. The permeability of different membranes can be calculated by Equation (3) according to literature [22].

VR

 
dCR ðtÞ P ¼ A CL  CR t dt L

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resolution set at 4 cm1 and 32 scans were accumulated over the range from 4000 to 675 cm1. The germanium crystal has a circular configuration with diameter of 2 mm and a 45 incident angle. All the membranes were dried under 80  C for 12 h before measurements. 2.4. VRB single cell performance

(3)

where P is permeability of the vanadium ions, CR(t) and CL are the vanadium ion concentration in the right and left cell, respectively. A and L are the area and thickness of the membrane, and VR is the volume of right cell. 2.3.4. Scanning electron microscopy (SEM) The surface morphology of composite membranes were investigated by a scanning electron microscope (SEM, Quanta 200F, Czech Republic) equipped with energy dispersive X-ray (EDX) spectroscope. The cross-section morphology of the membrane was studied by a Hitachi (Su8010, Japan) scanning electron microscope system equipped with an energy dispersive X-ray (EDX) spectrometer. The sample for cross-section view was manually fractured in liquid nitrogen. All the samples surfaces were coated with gold under vacuum before SEM observations were conducted. 2.3.5. Fourier transform infrared spectroscopy (FT-IR) The structure of the composite membranes was examined by FT-IR ATR (attenuated total reflection) method. Samples were placed on a smart OMNI-Sampler germanium crystal and recorded using an iS10 FT-IR spectrometer (Themo Scientific Nicolet, U. S. A.) equipped with a DTGS KBr detector. Spectra were collected with

A VRB single cell was fabricated similar to our previous work [13]. Membranes with effective area of 9 cm2 were sandwiched between two pieces of 5 mm graphite felt electrodes. Two graphite polar plates were used as current collectors. In the beginning of chargeedischarge cycles, 40 mL of 1.5 mol L1 V(IV) in 2.5 mol L1 H2SO4 solution and 1.5 mol L1 V(III) in 2.5 mol L1 H2SO4 solution were pumped into the negative and positive electrode, respectively. The cell was controlled by a land CT2001C (Wuhan Land Electronics Co., Ltd., China) battery analyzer. Chargeedischarge of the cell was conducted at current densities from 50 to 80 mA cm2. The cutoff voltages for the test were set at 1.7 V and 0.8 V respectively to avoid the corrosion of graphite felts and graphite polar plates. For selfdischarge test, the cell was firstly charged to 50% state of charge (SOC) at the current density of 60 mA cm2, and then the cell was immediately laid with peristaltic pumps still working until the single cell open circuit voltage (OCV) reached to 0.8 V. The cycle life measurement of the cell was conducted at the constant current density of 80 mA cm2, and the cut-off voltages were also set as 1.7 V and 0.8 V, respectively. The coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the cell were calculated by Equations (4)e(6).

CE ¼

discharg capacity  100% charge capacity

Fig. 2. SEM micrographs of different composite membranes: (a) PTFE, (b) Nafion 212, (c) P/N, (d) P/N/S-7.

(4)

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3. Results and discussions

membranes is investigated by SEM and the images are illustrated in Fig. 2. Apparently, the pure PTFE membrane displays a very rough appearance with lots of pinholes and fibers, which is in accordance with the literature [15,17]. For pristine Nafion 212 (Fig. 2(b)) and P/ N membrane (Fig. 2(c)), they present planar structure and they are dense with no visible holes. These results indicate that porous PTFE membrane is well impregnated with Nafion resin. As expected, the P/N/S-7 composite membrane exhibits different morphology compared with P/N membrane, PTFE membrane and Nafion 212. It is clear that a white thin film is embedded in the surface of P/N/S-7 membrane indicating that the SiO2 is uniformly dispersed into the P/N matrix. Si element distribution on the surface of the P/N/S-7 membrane was further tested by EDX method and the results were shown in Fig. 3. It can be seen that the SiO2 are dense and uniformly distributed throughout the composite membrane without any visible aggregation, indicating the Si element has been successfully introduced into the P/N/S membrane. Cross-section morphology of P/N/S-7 membrane are further conducted and presented in Fig. 4(a) and (b). It can be seen that the cross-section of P/ N/S-7 is also dense and homogenous with no obvious pores. Corresponding to Fig. 4(b), the EDX mapping of Fig. 4(c) shows that the element Si distribution is uniform throughout the membrane crosssection, which proves that the pores of PTFE in cross-section are also well filled by SiO2.

3.1. Consumption of Nafion

3.3. FT-IR analysis

For the 45 mm P/N membrane as we have previously prepared, the weight of Nafion resin consumed is 0.0075 g per square centimeter [13]; while for the 25 mm P/N membrane prepared in this work, the Nafion resin consumed is only about 0.0035 g per square centimeter. Compared with that of 45 mm P/N membrane, the Nafion consumption of 25 mm P/N membrane has decreased by 53.3%, and it is only about 34.7% compared to that of Nafion 212 membrane. Consequently, the cost of the membrane will be greatly decreased if this kind of membrane can be used in VRB application.

To further investigate the structure of the prepared membranes, the ATR- FTIR spectra of Nafion 212, P/N and P/N/S-7 membranes are measured and shown in Fig. 5. In comparison with P/N and commercial Nafion 212, there is an additional absorption peaks around 806 cm-1 in the FTIR of P/N/S-7, which can be ascribed to the SieOeSi symmetric stretching vibration [23,24]. The absorption peaks at 1204 cm1 and 1148 cm1 are corresponding to the two bands which are anti-symmetric nas mode of CF2 and SO-3 groups in Nafion [25]. This result further proves the successful condensation reaction of TEOS in P/N/S membrane.

3.2. SEM analysis

3.4. Water uptake and conductivity

In order to explain the microstructure change of the composite membrane, the morphology of PTFE, Nafion 212, P/N and P/N/S-7

The water uptake and proton conductivity of P/N membrane and P/N/S membrane with different contents of SiO2 are presented in

Fig. 3. SEM and corresponding EDX image of P/N/S-7 composite membrane: (a) SEM photograph, (b) EDX image of the element Si.

VE ¼

middle point of discharg voltage  100% middle point of charge voltage

EE ¼ CE  VE

(5)

(6)

Fig. 4. SEM and corresponding EDX image of cross-section of P/N/S-7 membrane: (a) and (b), SEM photographs, (c) EDX image of the element Si corresponding to (b).

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Fig. 7. Dependency of VO2þ concentration as a function of time for P/N and P/N/S membranes. Fig. 5. FT-IR spectra of Nafion 212, P/N and P/N/S-7 membranes.

Fig. 6. Water uptake is a crucial parameter for ion exchange membrane and it has great influence on the proton conductivity. Generally, high water content can improve the proton conductivity. As shown in Fig. 6, both the water uptake and proton conductivity of all P/N/S composite membranes are slightly higher than that of the P/N composite membrane and increase with the increase of silica content. Studies by Miyake N. et al. [26] and Ke C. et al. [27] show that the water content in solegel prepared Nafion/silica hybrid membranes also increases with the increase of silica content. The reason should be due to the excellent capability of water maintenance caused by hydrophilic effect of silica in composite membranes [27]. It also can be seen from Fig. 6, the conductivity of all the P/N/S composite membranes are all slightly higher than that of P/N membranes and increase with the increase of silica content. This result is different from that of Nafion/silica hybrid membranes reported by some groups, but it is in accordance with studies by Zeng R. et al. [28]. Dimitrova P. et al. suggest that the increased water may change the transport properties of proton by bringing about a size growth of the ionic clusters and pores inside membranes. [29]. On the other hand, Zeng R. et al. [28] think that the condensed silica in the P/N/S composite membranes might decrease the distance between two sulfonated groups and make

Fig. 6. Variation of water uptake and conductivity with the silica content.

the conductivity of the membranes increase. It can be expected that the P/N/S membranes should possess good cell performance due to the higher conductivity. 3.5. VO2þ permeability Permeability of VO2þ plays an important role of VRB which demonstrates the ability to prevent the crossover of electrolyte. Fig. 7 shows the concentration change of VO2þ with time for different membranes. The VO2þ ion concentration increased with the permeation time increased for all the membranes. The VO2þ permeability values of all P/N/S membranes are lower than that of P/N membrane. The slopes of the diffusion curves reflect the permeability (P) of different membranes. By calculating according to the Eq. (3), the P values of different membranes are presented in Fig. 8. As can be seen that the P values of all the silica modified P/N/S membranes are much lower than that of the P/N membranes. The difference between P/N and P/N/S membranes is determined by their microstructure. A series of pioneered studies by Mauritz K.A. and co-workers have shown that cyclic/linear networks of SieOeSi groups can be formed in

Fig. 8. VO2þ permeability of P/N and different P/N/S membranes.

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Nafion/silica composite membranes prepared by solegel method [30e33]. It is believed that those SieOeSi groups are the result of in situ solegel co-polymerization between the hydrolyzed silanol and SieO groups of TEOS [34,35]. Due to the formation of SiO2 nanoparticles in Nafion clusters by in situ solegel method, Nafion/SiO2 and Nafion/ORMOSIL hybrid membranes with lower vanadium ions permeability have been successfully used in VRB [36,37]. In this work, the lower permeability of P/N/S membranes could be due to the fact that the polar clusters of P/N membrane have been partly filled by SiO2 particles, which is the same as SiO2 and ORMOSIL modified Nafion membranes [36,37]. Furthermore, due to the lowest P for P/N/S-7 membrane, it is thus chosen to conduct the VRB single cell tests accompanied with P/N membrane. 3.6. VRB single cell performance As discussed in the above sections, the P/N/S membranes possess higher conductivity and water uptake as well as lower VO2þ permeability than that of P/N membrane. In order to further study the effect of SiO2 on the comprehensive performance of membrane for P/N/S applications, the chargeedischarge characteristics of the VRB single cell equipped with P/N and P/N/S-7 membrane at current density of 50e80 mA cm2 are demonstrated in Fig. 9. As can be seen, chargeedischarge capacities of the VRB with P/N/S-7 membrane are all much higher than that of the P/ N membrane. The average charge voltage of the VRB with all P/N membrane are higher than that of the VRB with P/N/S-7 membrane, while the average discharge voltage of the VRB with all P/N membrane are lower than that of the VRB with P/N/S-7 membrane at the tested current densities. Based on Fig. 9 and by calculating according to Eqs. (4)e(6), the coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the VRBs with P/N and P/N/S membranes are illustrated in Fig. 10. As shown, the CEs of the VRBs with P/N and P/N/S-7 membranes increase with current density due to the lower vanadium ions permeability at higher current density. The VEs of two VRBs all decrease with current density ascribed to the higher ohmic polarization at higher current

Fig. 10. Efficiency of VRBs with P/N and P/N/S-7 membranes at current density of 50e80 mA cm2 in the chargeedischarge test.

density. Furthermore, the CE, VE and EE of the VRB with P/N/S-7 membrane are all higher than that of the VRB with P/N membrane at all current density. The higher CE and VE can be attributed to the lower vanadium ions permeability and higher proton conductivity of P/N/S membrane, respectively. The EE of the VRB with P/N/S-7 membrane is about 82% at current density of 80 mA cm2, which is 11.4% higher than that of the P/N membrane. The open circuit voltage (OCV) curves of the VRBs assembled with P/N and P/N/S membranes are conducted and illustrated in Fig. 11. As can be seen, the OCV gradually decreased with time at first and then sharply dropped to 0.8 V. The time to 0.8 V were 21 h and 11 h for P/N/S-7 membrane and P/N membrane, respectively. The OCV loss is mainly ascribed to the permeation of V2þ/V3þ and V4þ/V5þ across membrane in anolyte and catholyte, respectively. It is obvious that P/N/S-7 membrane possesses longer retention time than P/N membrane due to lower VO2þ permeation than the latter. And the result is in agreement with the vanadium permeability experiment.

Fig. 9. Chargeedischarge curves of VRBs with P/N and P/N/S-7 membranes at current density of 50e80 mA cm2.

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and silica particles. And the surface of the P/N/S membrane is dense and homogeneous with no visible holes. In single cell test, the CE, VE and EE of the VRB with P/N/S-7 membrane are all higher than that of P/N membrane at all tested current densities due to its lower vanadium ion permeability and higher proton conductivity. Selfdischarge rate of the VRB with P/N/S membrane was much slower than that of the VRB with P/N membrane, which further proves the good vanadium ion block ability of the membrane. The cycle performance of VRB employed with P/N/S composite membrane is very stable during 50 cycles test. Considering that the thickness of P/N/S-7 membrane is only ~25 mm and the VRB with this membrane possesses excellent performance, the cost of the membrane will be greatly decreased if it can be used in VRB. Thus, this composite membrane is a promising substitute of Nafion membrane for further VRB commercial application. Acknowledgments Fig. 11. OCV curves of the VRBs with P/N and P/N/S-7 membranes.

In order to further investigate the stability of the P/N/S membrane, the VRB with P/N/S-7 membrane is charge-discharged for 50 times at current density of 80 mA cm2 and the image is presented in Fig. 12. It can be seen that the CE of the cell remains about 94% without obvious decay suggesting that the membrane was chemically stable. In case of the slight decrease of VE and EE, it is due to either the polarization or internal resistance of the cell.

4. Conclusions Ultra-thin (~25 mm) P/N and P/N/S composite membranes are successfully prepared by solution casting and solegel method in order to decrease the vanadium permeation and enhance the performance of the VRB system. The influence of the silica content on the water uptake, proton conductivity and vanadium permeability of the membranes are investigated. Higher water uptake and proton conductivity of P/N/S composite membrane are found as compared with P/N membrane. SEM and EDX analysis show that the porous PTFE membrane is well impregnated by Nafion resin

Fig. 12. Efficiency of VRBs assembled with P/N/S-7 membrane at 80 mA cm2 in 50 cycles' chargeedischarge test.

This work was funded by Shandong Provincial Natural Science Foundation, China (ZR2013BQ002) and Co-establishment project of Harbin Institute of Technology and Weihai City (2013DXGJ15). The authors thank Dr. Jianpeng Zuo for language improvements on the original manuscript. References [1] L. Li, S. Kim, W. Wang, M. Vijayakumar, Z. Nie, B. Chen, J. Zhang, G. Xia, J. Hu, G. Graff, J. Liu, Z. Yang, Adv. Energy Mater. 1 (2011) 394e400. [2] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, J. Electrochem. Soc. 133 (1986) 1057e1058. [3] W. Wang, Q. Luo, B. Li, X. Wei, L. Li, Z. Yang, Adv. Funct. Mater. 23 (2013) 970e986. [4] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chem. Rev. 111 (2011) 3577e3613. [5] R. Yang, Z. Xu, S. Yang, I. Michos, L. Li, A.P. Angelopoulos, J. Dong, J. Membr. Sci. 450 (2014) 12e17. [6] Z. Li, W. Dai, L. Yu, J. Xi, X. Qiu, L. Chen, J. Power Sources 257 (2014) 221e229. [7] S. Liu, L. Wang, Y. Ding, B. Liu, X. Han, Y. Song, Electrochim. Acta 130 (2014) 90e96. [8] S. Maurya, S. Shin, K. Sung, S. Moon, J. Power Sources 255 (2014) 325e334. [9] Y. Li, H. Zhang, H. Zhang, J. Cao, W. Xu, X. Li, J. Membr. Sci. 454 (2014) 478e487. [10] H. Zhang, C. Ding, J. Cao, W. Xu, X. Li, H. Zhang, J. Mater. Chem. A 2 (2014) 9524e95431. [11] B. Schwenzer, J. Zhang, S. Kim, L. Li, J. Liu, Z. Yang, ChemSusChem 4 (2011) 1388e1406. [12] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Energy Environ. Sci. 4 (2011) 1147e1160. [13] X. Teng, J. Dai, J. Su, Y. Zhu, H. Liu, Z. Song, J. Power Sources 240 (2013) 131e139. [14] L. Huang, L. Chen, T.L. Yu, H. Lin, J. Power Sources 161 (2006) 1096e1105. [15] F. Liu, B. Yi, D. Xing, J. Yu, H. Zhang, J. Membr. Sci. 212 (2003) 213e223. [16] G. Jung, F. Weng, A. Su, J. Wang, T. Leon Yu, H. Lin, T. Yang, S. Chan, Int. J. Hydrogen Energy 33 (2008) 2413e2417. [17] T.L. Yu, H.L. Lin, K.S. Shen, L.N. Huang, Y.C. Chang, G.B. Jung, J.C. Huang, J. Polym. Res. 11 (2004) 217e224. [18] F. Damay, L.C. Klein, Solid State Ionics 162e163 (2003) 261e267. [19] Q. Deng, R.B. Moore, K.A. Mauritz, Chem. Mater. 7 (1995) 2259e2268. [20] T.A. Zawodzinski, M. Neeman, L.O. Sillerud, S. Gottesfeld, J. Phys. Chem. 95 (1991) 6040e6044. [21] F. Grossmith, P. Liewellyn, A.G. Fane, M. Skyllas-Kazacos, in: Proc. Electrochem. Soc. Symp., Honolulu, 1988, p. 363. [22] X.L. Luo, Z.Z. Lu, J.Y. Xi, Z.H. Wu, W.T. Zhu, L.Q. Chen, X.P. Qiu, J. Phys. Chem. B 109 (2005) 20310e20314. [23] A.A.R.D. Oliveira, V.S. Gomide, M.D.F. Leite, H.S. Mansur, M.D.M.E. Pereira, Mater. Res. 12 (2009) 239e244. [24] A. Kalampounias, Bull. Mater. Sci. 34 (2011) 299e303. gis, T. Schmatko, P. Colomban, Vib. Spectrosc. 26 (2001) [25] A. Gruger, A. Re 215e225. [26] N. Miyake, J.S. Wainright, R.F. Savinell, J. Electrochem. Soc. 148 (2001) A905eA909. [27] C. Ke, X. Li, Q. Shen, S. Qu, Z. Shao, B. Yi, Int. J. Hydrogen Energy 36 (2011) 3606e3613. [28] R. Zeng, Y. Wang, S. Wang, P.K. Shen, Electrochim. Acta 52 (2007) 3895e3900. [29] P. Dimitrova, K.A. Friedrich, B. Vogt, U. Stimming, J. Electroanal. Chem. 532 (2002) 75e83.

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