Efficient water management of composite membranes operated in polymer electrolyte membrane fuel cells under low relative humidity

Journal of Membrane Science 493 (2015) 285–298

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Efficient water management of composite membranes operated in polymer electrolyte membrane fuel cells under low relative humidity Kriangsak Ketpang a, Sangaraju Shanmugam a,n, Chonlada Suwanboon b, Noppavan Chanunpanich b,c, Dongha Lee d a Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1, Sang-Ri, Hyeonpung-Myeon, Dalseong-Gun , Daegu 711-873, Republic of Korea b Integrated Nanoscience Research Center, Science and Technology Research Institute, King Mongkut's University of Technology North Bangkok, 10800, Thailand c Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, 10800, Thailand d Robotics Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 3 March 2015 Received in revised form 22 June 2015 Accepted 29 June 2015 Available online 9 July 2015

High performance and durable electrolyte membrane operated in polymer electrolyte membrane fuel cells (PEMFCs) under low relative humidity (RH) has been achieved by incorporating various diameter sizes of mesoporous hygroscopic TiO2 nanotubes (TNT) in a perfluorosulfonic acid (Nafions) membrane. Porous TNTs with different tube diameters are synthesized by thermal annealing the electrospun polymer containing titanium precursor mat at 600 °C under an air atmosphere. The diameter of the TNT is significantly controlled by changing the concentration of the precursor solution. Compared to a commercial membrane (Nafion, NRE-212), the Nafion-TNT-10 composite membrane operated under 100% RH at 80 °C generates about 1.3 times higher current density at 0.6 V, and 3.4 times higher maximum power density operated under dry conditions (18% RH at 80 °C). In addition, the Nafion-TNT-10 composite membrane also exhibits stable and durable operation under dry conditions. The remarkably high performance of the Nafion-TNT-10 composite membrane is mainly attributed to the significant reduction of the ohmic resistance as well as the improvement of cathode catalyst utilization by incorporating TNTs, which greatly enhances the water retention and the water management capability through the membrane. Furthermore, Nafion–TNT membranes exhibit superior mechanical property. & 2015 Elsevier B.V. All rights reserved.

Keywords: Nafion composite membrane Mesoporous titanium dioxide nanotubes Water management PEM fuel cells Impedance spectroscopy

1. Introduction Global warming, a result of the emission of large quantity of CO2 gas in the earth's atmosphere, has seriously impacted on the climate change [1]. One of the biggest CO2 emitter is the transportation sector, primarily owing to the combustion of petroleum fuels [2]. The development of the zero-CO2 emission vehicle is thus an optimistic strategy to diminish the global warming problem [3– 5]. Polymer electrolyte membrane fuel cells (PEMFCs) have been the most promising power source for an electric vehicle due to zero-CO2 emission and high efficiency power generation [3,4]. In PEMFC, the polymer electrolyte membrane (PEM) is one of the crucial components which strongly determine the PEMFCs performance [6]. Perfluorosulfonic acid membrane (Nafions) has been commercially accepted as a state of the art PEM due to its n

Corresponding author. Fax: þ 82 53 785 6402. E-mail address: [email protected] (S. Shanmugam).

http://dx.doi.org/10.1016/j.memsci.2015.06.055 0376-7388/& 2015 Elsevier B.V. All rights reserved.

high proton conductivity under fully humid conditions as excellent as the oxidative and reductive environmental stability [6]. However, the PEMFCs performance employing Nafions membrane is poor under low relative humidity (RH) operation [7]. Developing PEMFCs with ability to operate under low RH have benefits in the removal of the external humidification system which beneficially decreases the cost of the system [7]. The serious degradation of PEMFCs performance under this condition is mainly due to the loss of proton conductivity of the Nafion membrane, which results in a drastic increase of the ohmic overpotential [7–9]. The proton conductivity of the Nafion membrane is highly affected by the amount of water absorbed in the membrane, and the maximum proton conductivity is attained when the membrane is fully saturated with water [9–11]. Under low RH, the absorbed water in the membrane vaporizes, which remarkably reduces the proton conductivity. It is thus highly important to enhance the proton conductivity of the Nafion electrolyte membrane under low RH in order to accomplish higher PEMFCs performance.

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Several effective approaches have been employed to improve the performance of the electrolyte membrane operated under low RH including introduction of anhydrous proton-conduction group into the membrane [12–14], discovery of new polyelectrolyte membrane which is less sensitive to RH [15,16], and incorporation of the hydrophilic materials to increase the water retention ability in the membrane [17–19]. Incorporation of the water retention fillers in the electrolyte membrane such as SiO2 [20–25], TiO2 [25– 38], ZrO2 [25, 39–44], and heteropolyacids [45–47] etc., which are both hygroscopic and proton conductors, is widely accepted strategy to improve the membrane performance operated under low RH. However, incorporating hygroscopic filler particularly nanoparticles in Nafion membrane does not enhance the fuel cell performance operated under fully humid conditions, owing to the serious aggregation of nanoparticles which yields insufficient both water electroosmotic drag and water back-diffusion through the membrane [23,25,27,35,36]. Generally, water supplied from an external humidification system carries the protons from anode to cathode electrodes through the membrane by electroosmotic diffusion. At the same time, water is produced at the cathode by oxygen reduction reaction (ORR). Thus, large amount of water accumulated at the cathode must be removed or diffuse back to the anode electrode, otherwise, the oxidant cannot reach the active sites of the cathode catalyst layer. As more oxidant reaches the cathode active catalyst layer, it results in suppressing the mass transport overpotential [23,30]. To overcome this obstacle, functionalized 1-dimensional carbon nanotubes (CNT) has been incorporated into the Nafion membrane so as to improve the Nafion membrane performance operated under both fully humid and dry conditions [48,49]. The acid functional group on the CNT filler plays a vital role in enhancing the proton conductivity of Nafion membrane under not only fully humid but dry conditions as well [49]. Furthermore, 2-dimensional graphene and its chemical modification have also been impregnated into Nafion membrane and the resulting composite membrane exhibits higher proton conductivity as well as better fuel cell performance under both fully humid and low RH operations [50–52]. Nevertheless, CNT and graphene are electron conductive material and hence incorporation of electron conductor filler into the membrane may result in short circuit of the cell. For this reason, incorporating the porous hygroscopic filler with tubular morphology, where the tube wall was composed of nanoparticle joined together, into electrolyte membrane could be an optimistic approach to enhance the performance of the electrolyte membranes operated under fully humid and low RH conditions [36]. This is because the water is capable of efficiently diffusing through both outer and inner surfaces of the tubular hygroscopic filler, resulting in effective enhancement of proton migration through the membrane. Additionally, because the nano-sized particles forming the tube wall are connected, they are highly stable without aggregation, and can provide more effective water absorption and retention capacities as compared to particle morphology filler. That incremental water retention and facile water diffusion could be capable of suppressing the ohmic resistance of the membrane as well as increasing the cathode catalyst utilization. Recently, we have reported the fabrication of porous titanium dioxide nanotubes (TNT), zirconium oxide nanotubes (ZrNT) and cerium oxide nanotubes (CeNT) using the simple and cost-effective conventional single spinneret electrospinning technique [36]. Additionally, fabricated porous metal oxide nanotubes were used as water retention fillers in a Nafion membrane to improve the PEMFC performance operated under low RH [36]. In the present study, we examined the effect of the tubular diameter size of TNT on the fuel cell performance of Nafion–TNT composite membranes operated under 18%, 50% and 100% RH at 80 °C. The diameter size of TNT was prepared in a range of 30–180 nm by controlling the

concentration of the precursor solution. The main purpose of incorporating TNT is to provide the water retention capability as well as the water diffusion ability in the electrolyte membrane. The excellent water retention and the facile water diffusion would result in depressing the ohmic resistance of the composite membrane as well as improving catalyst utilization at the cathode electrode. In addition, incorporation of TNT fillers could enhance thermal stability as well as mechanical property of the composite membrane. It was found that the smaller diameter of TNT provided the greater water retention as well as the more facile water diffusion in the composite membrane, yielding remarkable enhancement of the fuel cell performance operated under fully humid and dry conditions. To understand the outstanding PEMFC performance of the composite membrane in comparison with a commercial membrane (NRE-212), electrochemical impedance spectroscopy was conducted to analyze the ohmic resistance as well as the mass transport resistance. It was observed that the extremely lower both ohmic resistance and mass transport overpotential were responsible for the markedly improving PEMFCs performance of the composite membrane than the NRE-212 membrane. Additionally, the Nafion–TNT composite membrane exhibited excellent stability and durability operated under dry conditions.

2. Experimental 2.1. Materials Polyacrylonitrile (PAN, Mw ¼ 150,000 g/mol) and titanium (IV) oxyacetylacetonate (TiOacac) were purchased from Aldrich, Korea. Nafion ionomer (1100 EW, 15 wt%) was purchased from Ion Pow. Inc, USA. Nafion 212 (NRE-212) membrane was purchased from Aldrich, Korea. N,N-dimethylformamide (DMF), ethanol, H2O2, H2SO4, HNO3 and isopropyl alcohol (IPA) were purchased from Daejung Chemicals, Korea and were used as received. 2.2. Preparation of electrospun (e-spun) PAN/TiOacac composites nonwoven web Required amount of TiOacac with a mass ratio of 10%, 20% and 50% based on polymer weight was completely dissolved in 3.00 g of DMF at 90 °C. PAN with amount of 1.00 g was separately dissolved in DMF 6.00 g at 90 °C. The TiOacac solution and PAN solution were then mixed and stirred at 90 °C until a clear homogenous solution was observed. The detail of electrospinning process is described elsewhere [36,53]. The solution was electrospun using electrospinning set up (NanoNC.Ltd) under conditions: traveling distance between spinneret to collector of 10 cm, high voltage power supply of 15 kV, volume feed rate of 1.0 mL h  1 and rotating speed of 300 rpm, respectively under humidity o30% RH at 20–25 °C. 2.3. Preparation of mesoporous TiO2 nanotubes TNT was prepared by calcining the as-spun PAN/TiOacac composite nonwoven web in a tubular furnace (Wisd Laboratory Instruments) at 600 °C for 3 h. The calcination process was carried out under an air atmosphere in which the composite nonwoven web was thermally stabilized at 250 °C for 1 h to decompose an organic materials and then heating to 600 °C for 3 h. The heating rate was 5 °C per minute [36]. The calcined mat with TiOacac content of 10%, 20%, and 50% were denoted as TNT-10, TNT-20, and TNT-50, respectively.

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2.4. Preparation of Nafion–TNT composite membrane Various concentrations of TNT-10, TNT-20 and TNT-50 were impregnated in Nafion ionomer using ethanol as a solvent and the resultant admixtures were then ultra-sonicated for 60 min followed by mechanical stirring for 6 h. The composite membranes were prepared by casting these solutions on 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. For comparison, Nafion ionomer was casted in a similar manner without any filler materials. The membranes were peeled off by adding DI water and the dry membrane thicknesses of all membranes were measured at 5 random points over the surface using a digital micrometer and the average thickness was found to be 5075 μm. Finally, the membranes were pre-treated by boiling in 5% H2O2, H2O, 0.5 M H2SO4, and H2O in sequence for 1 h in each case. 2.5. TiO2 nanotubes characterizations The morphology of the samples was observed using a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4800II) with an accelerating voltage of 3 kV. Before the observation, the samples were coated with osmium. The microstructures and lattice fringe of samples were determined by field-emission transmission electron microscope (FE-TEM, Hitachi HF-3300) with an acceleration voltage of 300 kV. For TEM analysis, samples were ultrasonically dispersed in ethanol, and then a drop of dispersion was deposited on carbon coated copper grid and dried under UV lamp. The crystal structure of calcined samples was investigated by powder X-ray diffraction (XRD, Panalytical Empyrean) using Cu Kα radiation at a generator voltage of 40 kV and a tube current of 30 mA. The pore structure and the specific surface area of samples were investigated by N2 adsorption–desorption isotherm analysis using a Micromeritics ASAP 2020 surface area analyzer. Pore size distribution calculation was based on the Kelvin equation and the BJH method. Before measurement, the samples were degassed at 150 °C under vaccum overnight.

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The water retention capability of membranes was analyzed by investigating the –OH functional group using Fourier transform infrared spectroscopy (FT-IR) equipped with attenuated total reflectance. Before measurement, the membranes were dried at 110 °C overnight and the membrane samples were scan in the range of 4000–600 cm  1 under room temperature. Small angle X-ray scattering (SAXS, Panalytical Empyrean) was utilized to measure the ionic cluster size of membranes. The hydrated membranes were tightly sealed in plastic zip bag and they were subsequently irradiated by X-ray source with 40 kV. The SAXS experiment set up was reported elsewhere [15]. The thermal stability of membranes was studied using the thermal gravimetric analyzer (Thermo plus EVO, TG 8120). The sample was loaded in a crucible and then it was thermal heat treatment from room temperature to 900 °C under an air atmosphere. The heating rate was fixed at 20 °C per min. The mechanical property of membranes was analyzed using the tensile test instrument (Cometech, B2-type) with a load cell of 50 N. The membrane samples were inserted in the tensile machine whose gauge length was 40 mm. The tensile measurement was conducted at a speed of 10 mm min  1. Tensile test was carried out at least 7 specimens per membrane sample. 2.7. Proton conductivity measurement The membrane proton conductivity was investigated in the longitudinal direction with a four-probe method using membrane conductivity cell (Bekktech) with gas flowing options. Membrane sample with area of 0.4 cm  3 cm was assembled in the cell, in contact with two platinum electrodes placed at a fixed position. The potentiostat is set to apply specific voltages between two Pt electrodes and resulting currents were measured. The resistance (R) is derived from the slope of the line that connects the data points. The membrane conductivity as a function of relative humidity percentages (RH %) at 80 °C was determined according to the Eq. (3)

σ=

L R×W×T

(3)

2.6. Membrane characterizations

where, L ¼0.425 cm is the fixed distance between two Pt electrodes; R is the membrane resistance in Ω; W is the width of the sample in cm and T is the thickness of the membrane in cm.

Water uptake (WU) of samples was determined by drying the membranes in the oven at 90 °C overnight and the membranes were then immersed in DI water for 24 h at room temperature. WU was calculated by the following Eq. (1)

2.8. Fabrication of membrane electrode assembly and fuel cell performance evaluation

WU (%) =

Wswollen − Wdry Wdry

× 100

(1)

where, Wswollen is the weight of the membrane that was soaked in water for 24 h and Wdry is the weight of dry membrane. Ion exchange capacity (IEC) was investigated by an acid–base titration using phenolphthalein as an indicator. The samples were dried in the oven at 90 °C overnight and the dry membranes were then immersed in 3 M NaCl solution for 12 h so that the H þ of membrane could be exchanged with Na þ . The solution was finally titrated with 0.01 M NaOH. IEC value was calculated according to Eq. (2)

IEC =

VNaOH × CNaOH Wdry

(2) 1

where, IEC was the ion exchange capacity (mmol g ), VNaOH was the added volume of NaOH at the equivalent point (mL), CNaOH was the concentration of NaOH (M) and Wdry is the weight of dry membrane.

The PEMFC performance of membranes was evaluated by making membrane electrode assemblies (MEAs). Diffusion-layer coated carbon papers (SGL, thickness¼0.27 mm) were used as the backing layers. For the catalyst layer, 40 wt% Pt/C catalyst (Johnson Matthey) was firstly mixed with DI water and 30 wt% Nafion solution (5% Nafion ionomer) followed by isopropyl alcohol. The resultant slurry was ultrasonicated for 60 min and it was represented as cathode catalyst. For anode catalyst layer, 40 wt% Pt/C was prepared a similar manner to cathode reaction layer but the concentration of Nafion ionomer was 7 wt%. Catalyst was coated on the backing layers with loading of 1.0 mg cm  2 for both the anode and cathode. The active area for the PEMFC was 5 cm2. A thin layer of Nafion ionomer was applied to the catalyst surface of both the electrodes. MEAs were obtained by sandwiching the membrane between the cathode and anode followed by its hotcompaction under a pressure of 20 kg cm  2 at 130 °C for 2 min. MEAs were coupled with teflon gas-sealing gaskets and placed in single-cell test fixtures with parallel serpentine flow-field machined on graphite plates. Before conducting the polarization plots, the MEAs were stabilized at 0.6 V for 6 h under each RH

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conditions (18%, 50% and 100% RH) by passing hydrogen and oxygen gases which were heated at a required dew point temperature, to anode and cathode sides, respectively. The hydrogen and oxygen gas flow rates were fixed at 300 sccm and the cell temperature was fixed at 80 °C. The polarization data are collected point by point and 1 min. is given to the system to come to steady state. Electrochemical impedance spectroscope (EIS) was utilize to evaluate the resistance for NRE-212, recast Nafion, and NafionTNT-10 membranes operated at 80 °C under 100% RH and 18% RH. The EIS experiment under operating conditions of 18% and 100% RH were obtained by applying the current density of 400 and 1800 mA cm  2, respectively. The reproducibility of the data was ascertained by repeating the experiments at least twice. All the MEAs were evaluated in PEMFCs under atmospheric pressure without back pressure.

3. Results and discussion 3.1. Characterization of TiO2 nanotubes The porous TNT with different diameter sizes was synthesized by a two-step process. Electrospun titanium precursor embedded polymer fibers were first fabricated using a single spinneret electrospinning technique under an ambient atmosphere. An as-spun mat of light yellow color was obtained on a piece of Al foil. The composite mats with different concentrations of titanium precursors (10, 20, and 50 wt%) were fabricated and these samples are denoted as Ti-10, Ti-20, and Ti-50, respectively. The morphology of as-spun samples was evaluated using FE-SEM. It was found that all mats were perfect fibers without beaded (Fig. S1(a), (c), and (e)). The average fibers diameter of Ti-10, Ti-20, and Ti-50 was found to be 210, 280, and 460 nm (Fig. S1(b), (d), and (f)), respectively. The fiber diameter increased with increasing the precursor content due to an increase of the solution viscosity [54]. Additionally, elemental C, N, Ti and O were clearly observed in the fibers mat using SEM elemental mapping analysis (Fig. S2), indicating uniform dispersion of precursor in the fibers. The as-spun mats were thermally stabilized at 250 °C for 1 h and then the stabilized mats were pyrolysed at 600 °C for 3 h under an air atmosphere. The calcined mats Ti-10, Ti-20, and Ti-50 are, hereafter, denoted as TNT-10, TNT-20, and TNT-50, respectively. Fig. 1 displays the X-ray diffraction patterns of TNT-10, TNT-20, and TNT-50 samples. The

Fig. 1. Power X-ray diffraction pattern of samples TNT-10 (a), TNT-20 (b), and TNT50 (c).

as-prepared samples were a mixture of anatase and rutile phases TiO2. The amount of anatase and rutile phases detected from XRD was approximately 75% and 25%, respectively. Fig. 2(a), (c), and (e) show the morphology of samples TNT-10, TNT-20, and TNT-50. SEM images present a tubular morphology with various lengths for all samples. The average outer diameter of samples TNT-10, TNT-20, and TNT-50 was found to be 50, 90, and 150 nm, respectively (Fig. S3). The inner diameter of TNT-10, TNT20, and TNT-50 was in the range of 10–50 nm, 20–80 nm, and 30– 110 nm, respectively. The TNT outer diameter increased with increasing the precursor (TiOacac) content in the polymer fibers. A transmission electron microscope (TEM) was used to observe the microstructure of the TNT samples. The TEM image obviously indicated the presence of porous nanotubes, where the tube wall comprises of small particles joined together, (Fig. 2(b), (d), (f)). In addition, the particle size increased as the concentration of Ti precursor increased (Fig. 2(b), (d), (f)). The high resolution TEM images show lattice fringes with an inter-planar distance of 0.35 nm (inset of Fig. 2(b), (d), and (f)), corresponding to the (011) plane of anatase TiO2. Additionally, elements Ti, O, and N were clearly observed in STEM elemental mapping analysis of the sample TNT-10 (Fig. S4), suggesting that N may be incorporated into the TiO2 framework [55]. In order to determine the pore structure and the specific surface area, the TNT-10, TNT-20, and TNT-50 samples were analyzed by N2 adsorption–desorption isotherms and the result is depicted in Fig. 3(a) and (b). According to IUPAC classification, the sorption isotherm of TNT-10, TNT-20, and TNT-50 was type IV and the hysteresis loop was of type H3 (Fig. 3(a)) [56]. The sorption isotherm results pointed out that the synthesized TNT-10, TNT-20, and TNT-50 were a mesoporous material with mainly cylindrical pore shape [56]. In addition, the type H3 hysteresis loop was indicative of an aggregation of joined particles giving rise to slitshaped pores [56]. The sorption isotherm and pore size distribution results highly supported the porous morphology observed by TEM analysis. The BET surface area of TNT-10, TNT-20, and TNT-50 was 35.4, 28.8 and 15.2 m2 g  1, respectively. The smaller TNT diameter yielded higher surface area. Furthermore, all samples exhibited a narrow pore size distribution (10–18 nm) as shown in Fig. 3(b). The BET surface area and BJH desorption cumulative pore volume are listed in Table 1. In order to understand the mechanism underlying the formation of TNT by this method, the as-spun Ti-20 mat was thermally stabilized at 250 °C for 1 h and the stabilized mat was subsequently pyrolysed at a temperature of 400, 500, and 600 °C under an air atmosphere. The sample heat treated at 400 °C was physically observed to be dark brown color. On the other hand, samples calcined at 500 and 600 °C turned out to be light yellow and white colors, respectively. X-ray diffraction experiment was performed to analyze the phase of samples calcined at 400, 500, and 600 °C and the result is shown in Fig. S5. The sample prepared at 400 °C was a single phase of anatase TiO2. The observed broad peak at the 2θ of 25° was indicative of the presence of carbon in the sample. On the other hand, the samples synthesized at 500 and 600 °C were the mixture of anatase and rutile phase TiO2 without carbon. In addition, the XRD peak intensity increased with increasing the annealing temperature, exhibiting more crystallize TiO2. The amount of rutile phase was found to be higher when the sample was synthesized at high temperature. Fig. 4 depicts the morphology of calcined Ti-20 samples at different temperature using FE-SEM. The morphology of sample annealed at 400 °C was rod with an average diameter size of 155 nm (Fig. 4(a) and (b)), whereas, the samples pyrolysed at 500 and 600 °C exhibited tubular morphology (Fig. 4(c) and (e)). The average outer diameter sizes of samples calcined at 500 and 600 °C was 130 nm (Fig. 4(d)) and 90 nm (Fig. 4(f)), respectively. The tube inner diameter

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Fig. 2. FE-SEM images of samples TNT-10 (a), TNT-20 (c), TNT-50 (e), TEM images and its corresponding lattice fringes (inset) of sample TNT-10 (b), TNT-20 (d), and TNT-50 (f).

prepared at 500 and 600 °C was in the range of 10–50 nm and 20– 80 nm, respectively. Base on the XRD and SEM results, it could be concluded that the TiOacac precursor was uniformly dispersed in the electrospun fibers during the electrospinning process. In the calcinations process, titanium ions on the fiber surface interacted with air, forming TiO2 clusters on the fiber surface at temperature 400 °C. These TiO2 clusters aggregated together along the fiber template forming a rigid shell, however, the polymeric carbon did not completely decompose yet at this temperature. Upon reaching a temperature of 500 and 600 °C, the carbon would be decomposed, leaving a porous hollow structure [36,57]. 3.2. Characterization of Nafion–TNT membranes 3.2.1. The existence of TNT in the membranes Various diameters of synthesized TNTs were employed as the water retention and water diffusion filler in the Nafion membrane

and were then incorporated in a Nafion ionomer. The composite membranes were fabricated by ultrasonically dispersing various concentrations of TNT-10, TNT-20, and TNT-50 in Nafion ionomer using ethanol as a solvent and subsequently the admixture was casted in a form of membrane. The composite membranes utilized TNT-10, TNT-20, and TNT-50 were assigned to be Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50, respectively. Fig. 5 presents XRD patterns of TNT-20 sample, recast Nafion, and Nafion–TNT composite membranes with TNT concentration of 1.5% based on Nafion ionomer. A small peak was observed at 25 (2θ), which corresponding to 011 plane of anatase TiO2, in the Nafion–TNT composite membranes. This observation indicated the successful incorporation of TNTs fillers in the Nafion membranes. It should be noted that the low XRD intensity of TNT observed in the composite membranes would be due to low amount of the TNT fillers existing in the membrane (1.5% based Nafion ionomer). In addition, FESEM elemental mapping analysis was utilized to verify the

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existence of TNT and the result is shown in Fig. S6. It was found that elemental Ti was uniformly observed in the mapping result, suggesting the uniform dispersion of TNT fillers in Nafion membrane.

Nafion ionomer (Fig. S7). The amount of water absorbed in the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes was 33.7, 31.3, and 29.6 wt%, respectively. The water uptake of the NRE-212 and recast Nafion membranes was 23.7 and 21.8 wt%, respectively. The Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT50 membranes exhibited 1.4, 1.3, and 1.2 times higher water uptake than that of NRE-212 membrane and 1.5, 1.4, and 1.4 times higher water uptake than that of recast Nafion membrane. The higher water uptake of the Nafion–TNT composite membranes than the NRE-212 membrane indicates the facile migration of proton through the membrane. In order to visualize the facile migration of proton in the Nafion–TNT composite membrane, the water self-diffusion coefficients of the Nafion-TNT-10 and NRE-212 membranes was determined using the pulsed field gradient Nuclear Magnetic Resonance (PFG NMR) technique on the swelled membranes at a temperature of 25 °C [36]. The water self-diffusion coefficient of the Nafion-TNT-10 and NRE-212 membranes was found to be 3.527  10  9 and 2.003  10  9 m2 s  1, respectively [36]. The remarkably higher water self-diffusion coefficient of the Nafion–TNT membrane than NRE-212 membrane indicates that the mesoporous tubular TiO2 filler could effectively enhance the water electroosmotic diffusion though the membrane. The TiO2 surface composes of negatively charged hydroxyl groups which are profitable in water adsorption and retention ability [58]. Additionally, the adsorbed water is not evaporated under dry conditions due to the electrostatic attraction within electrical double layers (EDL) [56]. In order to verify the water retention in the composite membrane, therefore, the swollen membranes were dried at 110 °C overnight and subsequently the dried membranes were analyzed for the presence of –OH group using FT-IR spectroscopy. It should be noted that the physically adsorbed water in the membrane is easily removed by heating the membrane at a temperature 100 °C. Fig. 6 presents the FT-IR spectra of dried Nafion–TNT composite membranes in comparison with IR spectrum of recast Nafion membrane. Peaks at wavenumber of 3455 and 1625 cm  1, are corresponding to the –OH stretching vibration and –HOH bending vibration, respectively, and were clearly observed in IR-spectra of Nafion–TNT composite membranes [31]. However, these peaks were not present in the IR spectrum of recast Nafion membrane. The observed chemical –OH peaks in FT-IR indicated the excellent water retention of Nafion composite membranes under dry condition. Additionally, the intensity of the –OH peak at 3455 cm  1 was in the order of NafionTNT-10 4Nafion-TNT-20 4 Nafion-TNT-50 4recast Nafion membranes. The highest water retention capability of Nafion-TNT-10 composite membrane was mainly attributed to the highest surface area of TNT-10 filler.

3.2.2. Water absorption and retention analysis Water content plays an important role in proton migration through the membrane. Higher amount of water absorption in the membrane leads to facile proton diffusion through the membrane. Table 2 illustrates water uptake of Nafion–TNT composite membrane in comparison with that of recast Nafion and commercial NRE-212 membranes. It should be noted that the optimum filler concentration for Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT50 membranes was found to be 1.5, 1.0, and 1.5 wt% based on

3.2.3. Proton conductivity Proton conductivity is an essential property which directly affects the Nafion membrane performance. The higher proton conductivity of the membrane results in better the PEMFC performance. Fig. 7(a) shows the proton conductivity of NRE-212, recast Nafion, and Nafion–TNT membranes measured at 80 °C under different humidity levels. The proton conductivity under 100% RH at 80 °C of the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes was 155, 142, and 121 mS cm  1, respectively. The

Fig. 3. (a) N2 adsorption–desorption isotherm and (b) pore-size distribution for TNT-10, TNT-20, and TNT-50.

Table 1 Surface area and pore volume of synthesized TNTs. Samples Surface area/m2 g  1 BJH desorption cumulative pore volume/cm3 g  1 TNT-10 TNT-20 TNT-50

35.4 28.8 15.2

0.107 0.091 0.068

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Fig. 4. FE-SEM images of sample TNT-20 calcined at 400 °C (a), 500 °C (c), 600 °C (e) and its corresponding outer diameter size distribution of sample annealing at 400 °C (b), 500 °C (d) and 600 °C (f).

proton conductivity of recast Nafion and NRE-212 membranes was 97 and 103 mS cm  1, respectively. This means that the proton conductivity of Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes was 1.6, 1.5, and 1.2 times higher than the recast Nafion membrane, respectively, and 1.5, 1.4, and 1.2 times higher than that of the NRE-212 membrane. In addition, the proton conductivity of the Nafion–TNT composite membranes was also higher than the recast Nafion and NRE-212 membranes in all humidity regions. In particular, the proton conductivity of the NafionTNT-10, Nafion-TNT-20, and Nafion-TNT-50 composite membranes was ultimately 2.1, 1.5, and 1.5 folds higher than that of the NRE212 under 20% RH. The remarkably higher proton conductivity of Nafion–TNT composite membranes over NRE-212 and recast Nafion membranes was strongly influenced by the incorporation the TNTs fillers which effectively improved not only the water retention but water diffusion capability in Nafion membrane as well. Generally, the proton transport mechanism in Nafion-based membrane can be either vehicular or Grotthus depending strongly

on the amount of water in the membrane [9,36]. Both vehicular and Grotthus mechanisms can be identified by the evaluation of the activation energy of the membrane, which can be determined according to the Arrhenius equation [36]. Fig. 7(b) presents the plot of ln s vs. T  1 of the NRE-212, recast Nafion and Nafion–TNT composite membranes measured under 100% RH at different temperature. It was found that the proton conductivity value of all membranes increased with increasing temperature. The activation energy calculated from the slope of ln s vs. T  1 plot of the NafionTNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes was found to be 7.10, 7.81, and 7.98 kJ mol  1, respectively. The activation energy of the NRE-212 and recast Nafion membranes was 8.04 and 8.31 kJ mol  1, respectively. The activation energy of the Nafion– TNT composite membranes under 100% RH was lower than the NRE-212 and recast Nafion membranes, indicating that the incorporation of the TNT fillers in the Nafion membrane facilitates the proton migration through the membrane. In addition, incorporation of the higher TNT surface area in Nafion membrane

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Fig. 5. Power X-ray diffraction pattern of samples TNT-20, recast Nafion, and Nafion–TNT composite membranes. The TNT filler content was 1.5% based on Nafion ionomer.

Table 2 The water uptake, IEC, tensile strength and Young modulus of various Nafion composite membranes. Samples

Water uptake (%)

IEC/meq g  1 Tensile strength Young modulus (MPa) (MPa)

NRE-212 Recast Nafion Nafion-TNT10 Nafion-TNT20 Nafion-TNT50

23.7 21.8

0.937 0.938

– 11

– 159

33.7

0.918

14

236

31.3

0.921

13

222

29.6

0.915

–

–

Fig. 7. (a) Proton conductivity of NRE-212, recast Nafion, Nafion-TNT-10, NafionTNT-20, and Nafion-TNT-50 membranes measured at 80 °C and (b) Regression curves of proton conductivity versus 1000/T for NRE-212, recast Nafion, NafionTNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes measured under 100% RH at different temperature.

Nafion–mesoporous titanium phosphate membranes [59,60].

Fig. 6. FT-IR spectra of recast Nafion, Nafion-TNT-10, Nafion-TNT-20 and NafionTNT-50 membranes measured under ambient condition.

resulted in the more facile proton diffusion. These activation energy values of the composite membranes are indicative of the proton migration through membranes via vehicular mechanism [36]. The activation energy values of the composite membranes were similar to the activation energy obtained with that of hybrid Nafion–silica, Nafion–mesoporous zirconium phosphate and

3.2.4. Fuel cell performance Fig. 8(a) compares the PEMFCs performance of the Nafion-TNT10, Nafion-TNT-20, and Nafion-TNT-50 membranes with optimum filler contents, with that of recast Nafion and NRE-212 membranes operated at 80 °C under 100% RH and under ambient pressure. The catalyst loading on gas diffusion layers was kept identical for all MEAs studies. The open circuit voltage (OCV) was roughly 0.97– 1.03 V for all membranes, indicating very low amount of H2 gas permeability from the anode to the cathode through the membrane. To evaluate the effect of TNT filler on the ohmic resistance of the composite membranes, the current density of the membranes at 0.6 V was compared [36]. The Nafion-TNT-10, NafionTNT-20, and Nafion-TNT-50 membranes provided current density of 1777, 1609, and 1498 mA cm  2, respectively. The NRE-212 and recast Nafion membranes generated current density of 1374 and 1357 mA cm  2, respectively, at the same potential. At 0.6 V, the current density of Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT50 membranes was 1.3, 1.2, and 1.1 times higher, respectively, than the NRE-212 membrane, and 1.3, 1.2, and 1.1 times higher than the recast Nafion membrane. In other words, Nafion-TNT-10 membrane exhibited the highest current density at 0.6 V. The highest

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Fig. 8. Polarization and high frequency resistance (HFR) plots of NRE-212, recast Nafion, Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes operated at 80 °C under 100% RH (a), under 50% RH (b), and under 18% RH (c). The H2 and O2 gases flow rate were fixed at 300 SCCM. The catalyst loading on anode and cathode electrodes was 1.0 mg cm  2 of 40 wt% Pt/C.

current density at 0.6 V of the Nafion-TNT-10 composite membrane compared to other membranes implies that the ohmic resistance of the Nafion-TNT-10 membrane would be the lowest than the other membranes. To verify the ohmic resistance of the membranes, high frequency resistance (HFR) was then estimated for the Nafion–TNT composite membranes in comparison with that of NRE-212 and recast Nafion membranes. Fig. 8(a) illustrates the HFR of recast Nafion, NRE-212, and Nafion–TNT composite membranes at 80 °C under  100% RH. Apparently, the NafionTNT-10, Nafion-TNT-20, and Nafion-TNT-50 composite membranes exhibited markedly lower ohmic resistance than the NRE-212 and recast Nafion membranes. On the other hand, the ohmic resistance of Nafion–TNT composite membrane was in the order of NafionTNT-10 oNafion-TNT-20 oNafion-TNT-50 membranes. The remarkably lower ohmic resistance under 100% RH of the Nafion– TNT composite membranes compared to the NRE-212 membrane was possibly attributed to the porous tubular hygroscopic TiO2, which facilitate the proton diffusion through the membranes. In addition, the smallest ohmic resistance value of Nafion-TNT-10 composite membrane mainly affected by the highest surface area of TNT-10 fillers (Table 1) which provided the largest water retention capability in the composite membrane (Table 2). Another important observation in Fig. 8(a) is that the Nafion-TNT-10 composite membrane generated apparently higher current density at low potential (0.3 V) than the other membranes under 100% RH. The significant higher current density at 0.3 V of the Nafion-TNT10 membrane implies the efficient water removal from the cathode electrode by back diffusing to the anode electrode through the composite membrane, which beneficially resulted in suppressing the mass transport overpotential. Fig. 8(b) compares the polarization and HFR plots of the NafionTNT-10, Nafion-TNT-20, and Nafion-TNT-50 composite membranes with that of recast Nafion and NRE-212 membranes under  50% RH at 80 °C and ambient atmospheric pressure. The current density of the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 composite membranes at 0.6 V was 1021, 935, and 915 mA cm  2. The current density of NRE-212 and recast Nafion membranes at 0.6 V was 293 and 348 mA cm  2, respectively. This means that the current density of Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-

50 composite membranes under 50% RH was ultimately 3.5, 3.2, and 3.1 times higher than the NRE-212 membrane. Additionally, the current density at low potential (0.3 V) of the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes ( 2686, 2056 and 1778) was higher than the NRE-212 and recast Nafion membranes ( 1419 and 1208 mA cm  2). Under 50% RH, the fuel cell performance of the Nafion–TNT composite membranes was substantially higher than that of the recast Nafion and NRE-212 membranes (Fig. S8). The outstanding PEMFC performance of Nafion–TNT composite membranes operated at 80 °C under 50% RH was apparently due to the lower ohmic resistance (Fig. 8(b)) of the composite membranes than NRE-212 and recast Nafion membranes. In addition to operation of the composite membranes at 50%, 100% RH at 80 °C, we also evaluated the fuel cell performance of Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 composite membranes under 18% RH at 80 °C in comparison with that of recast Nafion and NRE-212 membranes (Fig. 8(c)). The current density of the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT-50 membranes at 0.6 V was 390, 378, and 361 mA cm  2, respectively. The current density of NRE-212 and recast Nafion membranes was 134 and 164 mA cm  2, respectively, at the same potential. The maximum power density of the Nafion-TNT-10, Nafion-TNT-20, Nafion-TNT-50, NRE-212, and recast Nafion membranes corresponded to 641, 521, 432, 186, and 175 mW cm  2 (Fig. S9). A 3.4, 2.8, and 2.3 folds enhancement in maximum power density was observed for the Nafion-TNT-10, Nafion-TNT-20, and Nafion-TNT50 composite membranes compared with the NRE-212 membrane. The superior performance of the Nafion–TNT composite membranes under low RH (18%) was mainly attributed to the extremely low ohmic resistance (Fig. 8(c)), due to the excellent water retention ability and the tubular structure of TiO2. Once again, the current density at 0.3 V of the Nafion–TNT composite membranes was in the order of Nafion-TNT-10 4Nafion-TNT-20 4Nafion-TNT50 composite membranes. This could be mainly resulted from the highest surface area of TNT-10 (Table 1) which effectively facilitated the water back-diffusion from cathode to anode electrodes. It could be highlight that the incorporation of mesoporous TNT fillers can greatly enhance water retention (free water and

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bound water in ionic groups of polymer matrix) in the membrane as well as efficiently facilitate the proton transport through the Nafion membrane. Additionally, the higher TNT surface area significantly provided the greater water retention capability in the composite membrane. The facile proton migration resulted in the extremely low ohmic resistance leading to an enhancement of PEMFC performance of the composite membrane operated under both fully humid and dry conditions. 3.2.5. Electrochemical impedance analysis As observed in polarization plots (Fig. 8), the Nafion-TNT-10 composite membrane exhibited significantly higher current density at low potential (0.3 V) than the NRE-212 membrane and the recast Nafion membrane under both fully humid and dry conditions. The remarkably higher current density at 0.3 V of the Nafion–TNT composite membranes than the NRE-212 and recast Nafion membranes was assumed to be due to the efficient water removal from the cathode catalyst layer by diffusing back to the anode. As more water diffuses back from cathode to anode electrodes, more oxidant will reach the active sites of the cathode catalyst layer, leading to suppressing the mass transport overpotential. Thus, the electrochemical impedance spectroscopy was employed to prove the assumption. Fig. 9(a) illustrates the impedance plots of the Nafion-TNT-10, NRE-212, and recast Nafion membranes operated at 80 °C under 100% RH. All impedance plots were obtained by applying the current density of 1800 mA cm  2 and the impedance data were fitted with the equivalent circuit (Fig. 9(c)) to extract the resistance value. It was found that the resistance value at high frequency region of the Nafion-TNT-10, NRE-212, and recast Nafion membranes operated under 100% RH at 80 °C was 0.018, 0.032, and 0.033 Ω, respectively (Fig. 9(a)). On the other hand, the resistance value at low frequency region of the Nafion-TNT-10, NRE-212, and recast Nafion membranes operated under 100% RH at 80 °C was 0.018, 0.032, and 0.035 Ω, respectively (Fig. 9(a)). Under 100% RH, the Nafion-TNT-10 composite membrane exhibited 1.8 and 1.8 times lower resistance value at high frequency region, and 1.8 and 1.9 times lower resistance at low frequency region than the NRE-212 and recast Nafion membranes, respectively. The lower resistance at high frequency region implies the higher ionic conductivity of the electrolyte membrane. On the other hand, the smaller resistance value at low frequency region indicates the facile ORR, exhibiting better cathode catalyst utilization. Fig. 9(b) depicts the impedance plot at a fixed current density of 400 mA cm  2 of the Nafion-TNT-10, NRE-212, and recast Nafion membranes operated under 18% RH at 80 °C. The resistance value at high frequency region of the Nafion-TNT-10, NRE212, and recast Nafion membranes operated under 18% RH was 0.037, 0.061, and 0.059 Ω, respectively. The resistance at low frequency region of the Nafion-TNT-10, NRE-212, and recast Nafion membranes operated under 18% RH was 0.014, 0.093, and 0.098 Ω, respectively. Under 18% RH, the Nafion-TNT-10 composite membrane exhibited 1.6 time lower resistance at high frequency region, and 6.6 times lower resistance at low frequency region than the NRE-212 membrane. The extremely low resistance value under 18% RH at 80 °C of the Nafion-TNT-10 was mainly attributed to the TNT filler which possessed the excellent water retention capability as well as very efficient water back-diffusion from the cathode to the anode through the membrane. Thus, it could be stressed that the incorporation of mesoporous hygroscopic TNT in the Nafion membrane efficiently improved not only water retention capability but the water back-diffusion from the cathode to the anode electrodes through the membrane as well. The facile water backdiffusion from the cathode to anode electrodes had benefits in not only keeping the membrane wet (facile proton migration), but also suppressing the mass transport overpotential (more oxidant reaching to cathode catalyst layer, yielding better cathode catalyst

Fig. 9. Impedance plots of NRE-212, recast Nafion, and Nafion-TNT-10 membranes operated at 80 °C under (a) 100% RH, (b) 18% RH, and (c) the equivalent circuit used to model the impedance spectra. The impedance plots under 18 and 100% RH were obtained by applying the current density of 400 and 1800 mA cm  2, respectively.

utilization). The result of the resistance value at high frequency region under 18% and 100% RH was consistency to the proton conductivity result. The impedance result was the obvious evidence demonstrating the remarkably high PEMFC performance of the Nafion–TNT composite membrane operated under 18% and 100% RH at 80 °C. 3.2.6. Mechanism of TNTs–Nafion interactions It is believed that the negatively charged hydroxyl groups on the surface of TiO2 is able to become the positively charged by surface protonation under pH lower than the isoelectric point [56]. Under fuel cell operation conditions (pH  1–2), thus, the charge on the surface of TiO2 is positively charge. During fuel cell operation, the proton attached to sulfonic acid group diffuses to the cathode, leaving negatively charged sulfonic acid group. Thus, the positively charged ion on the surface of TiO2 can counterbalance the negative charge of the sulfonic acid group of Nafion membrane, resulting in improving the fuel cell performance under low humid and at elevated temperature conditions [58]. Nevertheless,

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our experimental results were not in agreement with this assumption. Because Nafion ionomer is very strong acid and it prefers to donate proton rather than gain proton. Moreover, if the surface charge of TiO2 filler is the positively charged proton, the ion exchange capacity (IEC) value of the electrolyte membrane should be increased. However, the observed experimental IEC value was found to be lower than recast Nafion membrane (Table 2), which means that there is some interaction between Nafion and TNT filler. To clarify this hypothesis, wide-angle XRD was utilized to observe the change in the microstructure of the membranes and the result is depicts in Fig. 5. Morphologically, Nafion membrane is composed of the crystalline hydrophobic Teflon backbone and the amorphous hydrophilic cluster [6]. The difference in electron density between hydrophobic backbone and hydrophilic cluster results in the appearance of the XRD peak at 2θ of 17° [50]. It was demonstrated that incorporation of sulfonated graphene oxide (SGO) into Nafion membrane leads to the reduction of the XRD peak intensity at 17° (2θ) as compared to recast Nafion membrane [50]. The lower XRD intensity of the composite membrane versus recast Nafion membrane indicates the reorganization of the Nafion microstructure, due to the interaction between SGO filler and Nafion membrane. In addition, the stronger interaction between SGO filler and Nafion matrix results in larger the ionic cluster size, yielding effective improvement of the proton conductivity [50]. Fig. 5 illustrates the XRD pattern of Nafion–TNT composite membranes in comparison with recast Nafion membrane. It was found that the XRD peat intensity at 17° (2θ) was in the order of Nafion-TNT-10 oNafion-TNT-20 o Nafion-TNT50 ¼recast Nafion membranes (Fig. 5). The lowest XRD peak intensity of Nafion-TNT-10 composite membrane implied the strongest interaction between TNT filler and Nafion matrix, leading to the reorganization of the phase-separated structure by incorporation of TNT filler into Nafion membrane. Additionally, the interaction between TNT with Nafion matrix was stronger as the surface area of TNT filler was increased (Fig. 5). In order to specifically determine the interaction between TNT and Nafion ionomer, small-angle X-ray scattering (SAXS) was conducted to visualize the ionic cluster size of Nafion-TNT-10 composite membrane compared to recast Nafion and NRE-212 membranes and the result is shown in Fig. 10. It was assumed that the TNT filler should coordinately interact with the sulfonic acid group of Nafion, and hence the size of ionic cluster should be increased [25,50]. The peak position (qMAX) in Fig. 10 is correlated to the Bragg spacing (dMAX) which indicates the ionic cluster size of the Nafion

Fig. 10. Small angle X-ray scattering of hydrated NRE-212, recast Nafion and Nafion-TNT-10 membranes.

295

Fig. 11. TGA thermograms of TNT-10 filler, recast Nafion, Nafion-TNT-10, NafionTNT-20, and Nafion-TNT-50 membranes measured under an air atmosphere.

membrane. The dMAX can be calculated using following the equation, dMAX ¼2π/qMAX [15, 50]. It was found that the dMAX of the hydrated NRE-212 and recast Nafion was found to be 5.7 nm which was in good agreement with the reported value [15,30,50]. On the other hand, Nafion-TNT-10 composite membrane exhibited a peak at qMAX of 0.12 (Å  1), corresponding to 5.1 nm in dMAX. It has been reported that the smaller dMAX value suggests the larger ionic cluster size [25]. As expected, the ionic cluster size of Nafion–TNT composite membrane was larger than recast Nafion and NRE-212 membranes. The larger ionic cluster size was highly a result of the interaction between the TNT filler and sulfonic acid group, that has profitable in facile proton migration through the membrane [25,30,50,61]. Furthermore, to confirm the interaction between sulfonic acid group of Nafion and TNT filler, we employed TGA to analyze the weight loss of Nafion–TNT composite membrane compared to recast Nafion membrane. It is expected that this interaction should induce the rapid loss of sulfonic acid group at around 300 °C. Fig. 11 shows the TGA thermograms of the TNT-10 filler, NafionTNT-10, Nafion-TNT-20, Nafion-TNT-50, and recast Nafion membranes measured from room temperature to 900 °C under an air atmosphere. It was found that all membranes showed three stages of weight loss, i.e. the dehydration at the first region (80–120 °C), the desulfonation (290–370 °C), and decomposition of polymer backbone (420–520 °C). As expected, the sulfonic acid group of the Nafion–TNT membranes was decomposed at a temperature lower than that of recast Nafion membrane. Moreover, the weight loss at 300 °C of Nafion–TNT composite membranes was observed to be higher than the recast Nafion membrane. In particular, the NafionTNT-10 membrane exhibited the highest weight loss value at 300 °C. It has been demonstrated that the incorporation of TiO2 into the Nafion membrane results in more drastic loss of the sulfonic acid group of the composite membrane than the recast Nafion membrane at a temperature of 300 °C [25]. The rapid loss of the sulfonic acid group is due to the coordination bonding between the sulfonate group and a coordinately unsaturated Ti (IV) surface site [25]. This interaction has beneficially affected to the enhancement of fuel cell performance of Nafion–TiO2 composite membrane operated under low RH and at elevated temperature [25]. Therefore, TGA results confirmed the interaction between sulfonic acid group of Nafion and TiO2 nanotube, which is consistency to SAXS analysis. Based on the weight loss at 300 °C, the interaction between the TNT and the sulfonic acid group was in the order of Nafion-TNT-10 4Nafion-TNT-20 4Nafion-TNT-50

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Fig. 12. Tensile test of recast Nafion, NafionTNT-10 and Nafion-TNT-20 membranes.

membranes, which strongly affected by the surface area of TNT fillers. The higher surface area of the TNT filler resulted in the stronger interaction with sulfonic acid group of the composite membrane, yielding the greater PEMFC performance operated under fully humid and dry conditions. TGA result is highly supported the enhancement of the fuel cell performance of the Nafion–TNT composite membranes. 3.2.7. Mechanical property Mechanical property is one of the important properties which can estimate the strength of the fabricated membrane. The most common technique to investigate mechanical property is tensile test [31]. Fig. 12 presents the tensile test of the dry Nafion-TNT-10, Nafion-TNT-20 composite membranes compared to that of dry recast Nafion membrane and the values of the tensile strength, and Young modulus are listed in Table 2. It was found that the Nafion-TNT-10 and Nafion-TNT-20 composite membranes exhibited higher not only the stress but elongation as well. Obviously, the mechanical properties (tensile strength and Young modulus, Table 2) of Nafion–TNT composite membrane were higher than that of recast Nafion membrane. The better mechanical properties of composite membranes towards recast Nafion membrane was possibly due to the interaction between the sulfonic acid group of Nafion and TNTs fillers. In particular, incorporation of smaller TNT size provided the higher mechanical property, which was mainly owing to the stronger interaction as observed in TGA analysis. On the basis of tensile test, it could be pointed out that TNT could enhance the mechanical property of Nafion membrane. 3.2.8. Stability test The stability of the TNT in maintaining the PEMFC performance operated under dry conditions is one of the essential parameter needs to investigate. Fig. 13 presents the stability of Nafion-TNT-10 composite membrane operated at 0.5 V under 18% RH at 80 °C for 100 h and the obtained result was compared with result of NRE212 membrane. The initial power density of the Nafion-TNT-10 membrane at 0.5 V was 470 mW cm  2 and it declined to 442 mW cm  2 after 100 h operated under 18% RH at 80 °C. The power density of NRE-212 membrane at the beginning was 55 mW cm  2 and it decreased to 21 mW cm  2 after 100 h operation under 18% RH at 80 °C. After 100 h operation, Nafion-TNT10 composite membrane exhibited 21 times higher PEMFC performance under 18% RH at 80 °C. The main reason for obtaining such high and stable performance of the composite membrane

Fig. 13. Time dependent on the power density at 0.5 V of Nafion-TNT-10 and NRE212 membranes operated under 18% RH at 80 °C. The H2 and O2 gases flow rate were fixed at 300 SCCM.

over NRE-212 under dry conditions was due to the tubular hygroscopic TNT which not only keeps the membrane wet (water retention) but also very efficient water back-diffusion from cathode to the anode. As water located at the cathode electrode was able to sufficiently migrate back to the anode electrode through the membrane, it will keep the anode catalyst layer wet. As water supplied to the anode catalyst layer, the produced protons at the anode electrode are capable of diffusing to cathode easily, resulting high and stable performance PEMFC operated under dry conditions. 3.2.9. Durability test Membrane degradation is typically due to the presence of H2O2 which is formed by the permeation of O2 gas from the cathode through the membrane to the anode [62]. The gas permeability can be directly detected by the reduction of the OCV [62]. Fig. 14 represents the durability studies of the Nafion-TNT-10 compared to NRE-212 membranes for 300 h under 18% RH at 80 °C. The OCV of the Nafion-TNT-10 membrane decreased from 1.01 to 0.99 V after 300 h operation. On the other hand, the OCV of NRE-212 membrane at the beginning was 1.01 V and it declined to 0.52 V after 120 h. The OCV reduction rate of the Nafion-TNT-10 and NRE-

Fig. 14. Time dependent on the open circuit voltage (OCV) of Nafion-TNT-10 and NRE-212 membranes operated under 18% RH at 80 °C. The H2 and O2 gases flow rate were fixed at 300 SCCM.

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212 membranes was 0.06 and 4.08 mV h  1, respectively. The smaller OCV reduction rate of the composite membrane compared to NRE-212 membrane highly stress that the TNTs were capable of preventing O2 gas diffusion from the cathode to the anode, which hinders the generation of H2O2 at the anode. The OCV test result indicated TNT filler was able to prevent the Nafion-TNT-10 composite membrane was extremely durable to operate under 18% RH at 80 °C.

4. Conclusions We successfully incorporated various diameter sizes of porous hygroscopic TNTs, which served as the water retention fillers under low RH and as the water diffusion route, in a Nafion membrane. TNTs were synthesized by thermal annealing of an electrospun PAN/TiOAc non-woven web, which was fabricated by a conventional single spinneret electrospinning technique, at 600 °C under an air atmosphere. The morphology of TNTs composed of small particles joined together forming tubular structure. The diameter size of synthesized TNTs was significantly controlled by the concentration of the precursor solution. The smaller the diameter of TNTs provided higher surface area, which expands the ionic cluster size of Nafion ionomer. The larger the ionic cluster size results in the greater water retention capability as efficient as water diffusion. The outstanding water retention capability and the facile water diffusion led to the effective enhancement of the proton conductivity of composite membrane under both fully humid and dry conditions. Compared to NRE-212, the Nafion–TNT membrane exhibited remarkably better PEMFC performance with operation under fully and low RH. The improved fuel cell performance of composite membranes can be attributed to the extremely low not only ohmic resistance but as well as the mass transport resistance, due to the excellent water retention ability and efficient water diffusion through the membrane. On the other hand, Nafion–TNT exhibited stable and durable to operate under dry condition. Also, incorporation of TNT improved the mechanical property of Nafion membrane.

Acknowledgment This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2014R1A1A2057056). This work was also partly supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (15-BD-01).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at 10.1016/j.memsci.2015.06.055.

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