strontium cerate nanocomposites with enhanced proton conductivity for proton exchange membrane fuel cells operating at high temperature

Accepted Manuscript Title: Polybenzimidazole/strontium cerate nanocomposites with enhanced proton conductivity for proton exchange membrane fuel cells operating at high temperature Author: Akbar Shabanikiaa Mehran Javanbakht Hossein Salar Amoli Khadijeh Hooshyari Morteza Enhessari PII: DOI: Reference:

S0013-4686(14)02460-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.12.025 EA 23900

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

28-7-2014 3-10-2014 4-12-2014

Please cite this article as: Akbar Shabanikiaa, Mehran Javanbakht, Hossein Salar Amoli, Khadijeh Hooshyari, Morteza Enhessari, Polybenzimidazole/strontium cerate nanocomposites with enhanced proton conductivity for proton exchange membrane fuel cells operating at high temperature, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.12.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Polybenzimidazole/strontium cerate nanocomposites with enhanced proton conductivity for proton exchange membrane fuel cells operating at high temperature

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Department of Chemistry, Amirkabir University of Technology, Tehran, Iran

Solar Cell and Fuel Cell Lab, Renewable Energy Research Center, Amirkabir University of Technology,

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b

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Hooshyaria,b, Morteza Enhessarib,d

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Akbar Shabanikiaa, Mehran Javanbakhta,b*[email protected], Hossein Salar Amolic, Khadijeh

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c

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Tehran, Iran

Faculty of Chemical Industry, Iranian Research Organization for Science and Technology, IROST,

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Tehran, Iran

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Department of Chemistry, Naragh Branch, Islamic Azad University, Naragh, Iran

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Corresponding author at: Department of Chemistry, Amirkabir University of Technology, Tehran, Iran. Tel.: +98 21 64542764; fax: +98 21 64542762.

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Abastract

In this work, perovskite-type SrCeO3 nanoparticles were used for improving the properties of high temperature polybenzimodazole (PBI) based proton exchange membranes. Novel proton conducting membrane nanocomposites were prepared using different amounts of SrCeO3 nanopowders dispersed into polybenzimidazole by solution casting method.The nanocomposite membranes were studied by using

AC impedance spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray and thermo gravimetric analysis. The prepared nanocomposite membranes showed a higher acid uptake, proton conductivity and thermal stability compared with the pure PBI membranes. The highest acid uptake (190 %) and proton conductivity (0.105 S/cm at 180 oC and 0% RH) were observed for phosphoric acide-dopped PBI nanocomposite membranes containing 8 wt% of SrCeO3 nanoparticles (PSC8). The PSC8 nanocomposite membranes were tested in a fuel cell and the polarization and power curves were obtained at different temperatures. The PSC8 showed 0.44 W/cm2 power density and 0.88

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A/cm2 current density in 0.5 V at 180 ºC. The result obtained from our studies shows the enhanced potential of the PSC8 as proton exchange membranes for high temperature proton exchange membrane

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fuel cells.

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Keywords: Proton conductivity; Polybenzimidazole; Nanocomposite; Proton exchange membrane;

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Srantiom cerate

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1. Introduction

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Proton exchange membrane fuel cells (PEMFCs) are composed of an ion exchange membrane sandwiched between two electrode sheets and are being developed for transport applications as well as

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for stationary fuel cell applications and portable fuel cell applications. Polyperfluorosulfonic acid (PFSA)

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membranes, such as Nafion are the most widespread type of polymeric membranes used for PEMFCs [1]. But one great weakness of Nafion membranes is their dehydration at temperatures above 80 °C, causing

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a remarkable decrease in their proton conductivity and mechanical stability. Having high chemical as well as thermal resistance and proton conductivity after doping of polybenzimidazole (PBI) memberans by phosphoric acid are great options to be applied in the fuel cells which function at temperatures above 100 °C. The phosphoric acid doped polybenzimidazole membrane has been proposed for high temperature PEMFC [2-7]. Nanocomposite membranes are new groups of membranes which include nanoparticles

such as SiO2, TiO2 and ZrO2 and other compounds [8-10]. The introduction of nanoparticles especially inorganic oxides into the membranes is expected to improve the proton conductivity but in some cases leads to a decrease in proton conductivity. This improvement is associated with more efficient water management due to the hygroscopic properties of the oxide particles and with the high doping level achieved [11,12]. As mentioned before, the presence of some nanoparticles in PBI nanocomposite membranes results in high proton conductivity in ways such as increase in the water uptake, increase in the amount of doping level and increase in the number of proton conductive functional groups [13].

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The acceptor-dopted perovskite- type oxides are known as high temperature proton conductors, which

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are promising materials for devices such as fuel cells [14]. It is known that the proton migrates in the interstitial sites around oxygen ions by hopping in crystals of these proton conductor [15, 16]. Most of

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the proton conductors currently under investigations are dependent on or related to water and water

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indeed plays a vital role in proton conduction processes. Perovskite structures from group II elements

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such as Sr, Ba and Ra can be used as hygroscopic materials to improve the humidity sensing

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performance. Due to proton conduction mechanism of these material upon interaction with water, they

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are well-known to the positive ionic conductors [17]. Doped perovskites have shown good proton

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conductor properties [18].

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Perovskite oxides based on SrCeO3 have been recognized to exhibit predominate proton conduction under hydrogen containing atmosphere at elevated temperatures [14]. The chemical composition of

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perovskite oxides could be written as AB1-xMxO3-δ, where A and B denote two different cations and M is

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some trivalent elements like rare earth elements and δ is the oxygen deficiency per perovskite unit cell. These perovskite-type ceramics exhibit p-type electronic (hole) conduction under oxidizing atmospheres

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while they become a proton conductor in the presence of water vapor and hydrogen [19]. The proton conduction in these oxides was directly verified by means of electrochemical transport of hydrogen across the oxides [14,20,21]. Recently, we introduced new proton conducting hybrid membranes based on phenyl or propyl sulfonic acid-functionalized nonporous silica [22,23] and poly(sulfonic acid)-grafted silica nanoparticles [24] for

PEM fuel cells. Nafion/Fe2TiO5 nanocomposite membranes were prepared by dispersion of Fe2TiO5 nanoparticles within the commercial Nafion membranes [25]. Incorporation of Fe2TiO5 nanoparticles in Nafion matrix improved the thermal stability of Nafion membranes which is essential for operation of PEMFCs at elevated temperatures. As mentioned before, it seems that in Fe2TiO5 nanoparticles when Fe3+ cations are placed near Ti4+ cations, the acidic character of these ions is increased. Fe2TiO5 singlephase nanoparticles, has better hydrophilic nature in comparison with both of TiO2 nanoparticles and Fe2O3 nanoparticles [25].

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In the present study, nanocomposite membranes based on polybenzimidazole/strontium cerate nanoparticles were prepared and characterized. The prepared nanocomposite PBI based membranes

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containing SrCeO3 nanoparticles showed higher acid uptake and proton conductivity.

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2. Experimental

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2.1. Materials

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Polybenzimidazole with chemical structure of ( poly(2,2'-m-(phenylene)-5,5'-bibenzimidazole)(PBI), with

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a glass transition temperature of 425-435 °C, molecular weight (62000- 59000 g mol-1) was obtained

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from FumaTech corporation. Phosphoric acid (PA) and N, N-dimethylacetamide (DMAc) were

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purchased from Merck. Distilled deionised water (DI) was used through all experiments. 2.2. Synthesis of SrCeO3 nanoparticles

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Strontium cerate (SrCeO3) nanoparticles were provided according to the literature procedure [26].

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Initially, ammonium cerium (IV) nitrate ((NH4)2Ce(NO3)6) and n-butanol (C4H10O) are reacted to synthesize cerium-n-butoxide (Ce(OBu)4). Ammonium nitrate (NH4NO3) can be removed from the solution including the synthesized cerium-n-butoxide. Next, the cerium-n-butoxide, strontium acetate (Sr(CH3COO)2), and melted stearic acid (C18H36O2) were heated at 150° C for 12 h to form a

homogenous gel. The homogenous gel can be calcined at 1000 °C for 2 h. Finally, the strontium cerate nanoparticles with a particle size range of 21-32 nm were isolated. 2.3. Ion exchange capacity (IEC) The ion exchange capacity (IEC) of PA-doped membranes was determined by the titration method. PAdoped membranes were soaked in a 2 M sodium chloride solutions for 24 h at RT to ensure replacement of H+ groups with Na+ groups. Subsequently, the solution was titrated with 0.1 M sodium hydroxide.

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2.4. Preparation of PBI nanocomposite membranes

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The PBI nanocomposite membranes were prepared by a solution-casting method. In this method, the nanocomposite membranes were fabricated using DMAc as a casting solvent. At room temperature,

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different weight percentages of nanoparticles with respect to the PBI (2, 4, 8, 12 and 16%), were

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dispersed in DMAc using an ultrasonic bath. Then, appropriate amounts of PBI powder was added to

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this solution under stirring at 120 °C. The obtained brown solution was cast onto a glass plate and the

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solvent was evaporated slowly at 120 °C for 5 h. The glass plates were then soaked in a de-ionized water

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bath. The membranes were slowly removed from the glass plates. Finally, the prepared membranes

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immersed in PA. PSC membranes are easily doped by PA.

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PBI-SrCeO3 nanocomposite membranes were named PSC. The value of x in PSCx samples was assigned

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for the weight percent of the nanoparticles in the PBI membranes.

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2.5. Acid uptake and leaching test The resulting brown-colored PBI nanocomposite membranes were doped by immersion in aqueous PA

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(75 wt %.) for 5 days. In order to separate the water content from the doping acid, the acid-doped membranes were dried at 110 ºC under vacuum until the membrane weights were unchanged with time. The weight change in acid doping was measured and used for the calculation of acid doping level (acid molecule numbers per PBI repeating unit).

Leaching test is a method for the determination of PA retained by the PA-doped PSCx membranes after washing with hot water (90 ºC), which is considered one of the main degradation factors of this kind of membranes in the fuel cell. The PA-doped PSCx membranes were immersed in de-ionized hot water for 2 h and then the remaining acid was obtained by a similar manner as described above. 2.6. FT-IR ATR spectra The FT-IR ATR spectra (600-4000 cm-1, resolution 4 cm-1) were recorded with a Bruker Equinox 55 using an attenuated total reflectance (ATR, single reflection) accessory purged with ultra dry compressed

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air.

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2.7. Proton conductivity measurements

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The proton conductivity of membranes was calculated by the electrochemical impedance spectroscopy

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(EIS) with PGSTAT 303N potentiostat/galvanostat (Ecochemie). The sample PA-PSC membrane was

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sealed between two platinum plates electrodes. The measurements were carried out on the potentiostatic

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mode. The spectra were recorded with signal amplitude of 50 mV in the frequency range of 100 Hz – 1

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MHz with 100 points. The resistance of the membrane was obtained from the high-frequency intercept of

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the impedance. The conductivity values were calculated by using the equation (σ= L/RS), where, σ, L, R

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and S respectively refer to, proton conductivity, thickness, resistance from the impedance data and crosssectional area of the membranes.

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2.8. SEM and EDX measurements

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The morphology of nanocomposite membranes was investigated by using a scanning electron microscopy

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(SEM), (JSM-5600, Jeol Co.), coupled with energy dispersive X-ray (EDX) spectroscopy. The samples were freeze-fractured in liquid N2 and coated with gold plate before SEM observations were carried out. 2.9. Thermal properties

Thermogravimetry analysis (TGA) of the nanocomposite membranes was carried out by using a TGA TA instrument 2050 system , at the heating rate of 10 °C/min in nitrogen atmosphere from 25 to 600 °C. 2.10. XRD analysis The

XRD

(Equinox

3000)

analysis

was

carried

out

using

Cu

Kα

irradiation

((λ = 1.5418 A°). The detected diffraction angle (2) was scanned from 15° to 65° at the scanning speed of 3°/min. The compound was prepared as a KBr pellet and FT-IR spectra were collected using a

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Hyperion 3000 spectrometer. 2.11.Fuel cell tests

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The catalyst was Pt–C (E-TEK, 20 wt% Pt) and the Pt loadings of anode and cathode were 0.5 mg/cm2.

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Pt–C/PBI/LiCl/DMAc (3.6/1/0.2/38 by wt.%) catalyst solution was prepared by ultrasonic disturbing for

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1 h. The Pt-C inks were loaded on to the carbon paper (Toray TGP-H-090) by a painting method and

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dried at 190 ºC in a conventional oven to calculate catalyst loading. The catalyst coated carbon papers

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were then doped with phosphoric acids by dipping in 10 % PA solution. The acid doped membrane was

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sandwiched between two pieces of gas diffusion electrodes on each side and hot-pressed under a pressure

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of about 50 kg/cm2 at 130 ºC for 5 min. Polarization curves were obtained using a fuel cell evaluation

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system (FCT-150s). The cell temperatures were at 100, 150 and 180 oC with ambient pressure. Each MEA with an active area of 2.3×2.3 cm2 was performed the fuel cell test with the H2/O2 flow rates at

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300/500 mL/min under dry condition.

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3. Results and Discussion 3.1. FT-IR ATR spectra The PA-doped PBI membranes were characterized by FT-IR ATR spectroscopy. In order to clarify the interaction between PA and PBI, the characteristic absorption of PA molecules in PA-PBI membranes

was investigated. The bands at 500-1300 cm-1 were attributed to the vibration of HPO42- and PA groups. Three characteristic absorptions of the HPO42-, P-OH, and H2PO4- groups for PA-PBI membranes appear at 1090, 1008, 970 cm-1, respectively [27-29]. The peak around 1445 cm-1 suggests the deformation of benzimidazole “Breathing” mode of imidazole rings [30]. The peak at 1600 cm-1 was assigned to the C=C and C=N stretching groups. The bands at 2250-2500 cm-1 and 2500-3000 cm-1 were attributed to the O– H stretching and N+–H stretching in present of PA, receptivity. The peak at 2900 cm-1 was assigned to the stretching vibration of aromatic C–H groups. The bands at 3195 cm-1 and 3390 cm-1 were attributed

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to the hydrogen bonded N–H groups and non-hydrogen bonded N–H stretching groups, receptivity. The

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peak around 3615 cm-1 suggests the O–H stretching due to absorbed water [2].

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3.2. Acid uptake and proton conductivity of PSCx nanocomposite membranes

The reported proton conductivity value for pure PBI was 10-12 S/cm [31]. Thus, pure PBI is not a proton

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conducting polymer, and can not be used as solid electrolyte. PBI has both donor and acceptor bonding

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sites and is thus capable of specific ion interaction. PBI is one of the most famous membranes doped with

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PA, in which PA acts as protons conducting carrier and no water is needed for protons conduction in the

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membranes.

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In PA-doped PBI type membranes, a PA molecule is immobilized protonating a benzimidazole ring. The proton conductivity mechanism of acid-doped polybenzimidazoles is mainly by a Grotthus mechanism

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[32]. In this Grotthus mechanism, proton transfer hope between two molecules (acid–acid, acid–water,

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or acid–benzimidazole ring). Only about 2 molecules of PA interacts nearly quantitatively with a PBI unit containing two imidazole groups and excess PA to imidazole groups is required to give sufficient

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conductivity [33]. The existence of HPO42- and H2PO4- anions implies that the proton conduction may happen according to the Grotthus mechanism, which involves an exchange of protons between H3PO4 and HPO42- or H2PO4- and PBI. In addition, strong acids could form polymer complexes due to the acid– base interaction or hydrogen bonding interactions between imidazole group of PBI and acid molecules.

The results of acid uptake of PA-PSCx nanocomposite membranes are shown in Fig. 1. The added nanoparticles in the nanocomposite membranes enhanced the ability to trap PA. The content of the nanoparticles and the properties of the nanocomposite membranes are key parameters in manufacturing which improve the proton conductivity of the nanocomposite membranes. Hence, PA-PSC nanocomposite membranes displayed a high acid uptake (190% for PSC8). At high content of SrCeO3 nanoparticles (>8 wt.%) the acid uptake of PSC samples was decreased. These results were attributed to the self-aggregate of SrCeO3 nanoparticles in PSC samples. The proton conductivity of new

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nanocomposite membranes at room temprature (RT) is shown in Fig. 2. As it is clear, the proton conductivity of the nanocomposite membranes increases with addition of nanoparticles up to 8 wt%

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(0.075 S/cm at RT). This improvement in the proton conductivity of PSC8 samples were attributed to

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hygroscopic nature of the SrCeO3 nanoparticles in the membrane. In the Grotthus mechanism for proton

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transport in SrCeO3 nanoparticles, proton diffuses by an arrangement between a molecular reorientation

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around the oxygen and jump of the proton from oxygen to a nearest neighbor ion. The low activation

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enthalpy of proton mobility in perovskites structure can attributed to the large A-site cations [34].

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Consequently, at higher content of SrCeO3 nanoparticles (>8 wt.%) due to the self-aggregate of SrCeO3

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nanoparticles in PSC samples the proton conductivity was decreased. These results show that the amount

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of nanoparticles of membranes is very important parameter in increasing proton conductivity. In the case of PA doped PSC16 nanocomposite membranes, the nanoparticles tend toward agglomeration in the

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polymer solution. The self-aggregate of SrCeO3 reduces the active surface of acid adsorption in PSC16.

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Fig. 3. displays a Nyquistand bode modulus plots of PA doped PSCx nanocomposite membranes (2-16 wt %) at fully acid uptake. Fig. 3(a) shows that PA- PSC8 nanocomposite membranes have the lowest

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resistance (highest proton conductivity) compared with other nanocomposite membranes. the impedance values represented in Bode Modulus plots and Nyquist plots (Fig. 3(b)) confirm these results. To estimate the proton conductivity of the PSCX membranes at high temperature, the PSC4 and the PSC8 membranes were used to evaluation of proton conductivity in different temperature up to 180 oC. Fig. 4

displays the conductivity of PBI, PSC4 and PSC8 nanocomposite membranes at room temperature up to 180 ºC. As seen in Fig. 4 these membranes (PSC4 and PSC8) due to hygroscopic nature of the SrCeO3 nanoparticles, still exhibited higher proton conductivities than the pristine PA-PBI membrane. This finding could be due to the higher acid doping levels achieved in these membranes. Table.1 shows a comparison between the proton conductivity of new nanocomposite membranes (PSC4 and PSC8) and the other works [35-40].

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3.3.Acid leaching test of PA- PSCx nanocomposite membranes

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The acid leaching tests for all the membranes were carried out in order to determine the acid retention capability of the membranes, which is considered one of the main degradation factors of this sort of

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membranes in the fuel cell [41]. Fig. 5 shows the results obtained from the acid leaching tests. It can be

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observed from Fig. 4 that the membranes with 8 wt. % of the SrCeO3 retain higher amounts of the acid

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than the other PA- doped PSCx nanocomposite membranes. This result shows that the presence of

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SrCeO3 in the composite improves the properties of the membrane; in this case, the improved property is

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the capability of the membrane to retain acid after being washed with hot water. The formed

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agglomerates for composite membranes with higher amount of SrCeO3 than 8 wt % reduces the active

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surface area of the nanoparticles and increases the acid leaching from the membrane.

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3.4.Ion exchange capacity (IEC)

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Fig. 6 displays ion exchange capacity (IEC) of PA-doped PSCx membranes. When nanocomposite membranes were doped with PA, dissociable H+ ions are increased and consequently IEC is dramatically

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increased. Fig. 6 demonstrates that the IEC of PA-PSCx nanocomposite membranes were increased with increasing nanoparticle (acid uptake) percent. 3.5. Morphological stuidies of nanocomposite membranes

SEM images of PSCx nanocomposit membranes were shown in Fig. 7. Significant agglomerations of SrCeO3 nanoparticles were clearly observable in the PSC16 samples (Fig. 7 (d)). Energy dispersive X-ray spectroscopy confirmed the presence of SrCeO3 in the composite membrane. EDX distribution of nanoparticles in the cross-section of PSC8 nanocomposite membranes were displayed in Fig. 8. A homogenous distribution of Sr and Ce nanoparticles in the cross-section of PSC8 nanocomposite membranes was also observed. Three fold proportion of oxygen in respect to the two other elements causes denser mapping. On the other hand a rare earth metal (Ce(IV)) with stronger

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binding energy and smaller radius indicates less dense mapping in respect to a main metal with greater

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radius (Sr2+).

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3.6. Thermogravimetric analysis (TGA)

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The thermal stability results for the PA-doped PSCx membranes were studied. Fig. 9 reveals that all the

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samples show two well-defined weight decays. The first goes from room temperature to 130 oC that is

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due to the desorption of adsorbed or absorbed water from polymer. The second one, appearing at around

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160 oC, is due to the thermal changes in phosphoric acid, forming the pyrophosphoric and triphosphoric

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2 H3PO4→ H4P2O7 + H2O

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acid, as shown by the following equations [42]:

(2)

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3 H4P2O7→ 2 H5P3O10 + H2O

(1)

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At temperatures over 500 oC a significant weight loss occurs, which is accompanied by the formation of carbon dioxide. This loss in weight, observed at the temperature range of 500–530 ºC, is also attributed

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to the thermal degradation of polymeric backbone of the nanocomposite membranes [13]. As it can be seen in Fig. 9, thermogravimetric analysis shows that the PBI membrane was not subject to a serious damage by the presence of SrCeO3 until 500 ºC. The presence of SrCeO3 in the PBI nanocomposite membranes reduce the weight loss rate of the PA-PBI nanocomposites in high

temperature. Incorporation of SrCeO3 nanoparticles in PBI polymer matrix leads to an increase in the decomposition temperature of nanocomposite membranes compared with PA-PBI membranes. Thus, the result obtained from TGA shows the enhanced potential of the PSCx nanocomposite membrane for higher temperature operations for PEMFC applications. 3.7. X-Ray diffraction analysis of PSCx membranes The X-ray patterns of the different membranes prepared in this work are shown in Fig. 10. It can be

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observed from Fig. 10 that the PBI membranes are amorphous [2] and show a broad peak at around 2θ = 25°. All the PSCx samples exhibited the main peak that is the characteristic of SrCeO3 (JCPDS card no.

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47-1689). The higher the SrCeO3 content, the more intense the main peak appeared. This fact confirms

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the presence of the SrCeO3 nanoparticles in the polymer matrix and the structure of it did not change when was introduced into the film. Thus, it can be concluded that the SrCeO3 is located in the spaces

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between the chains of polymer without modifying the structure of the polymer.

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3.8. Fuel cell performance tests

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To evaluate the fuel cell performance of the PSCx membranes, the PSC8 membrane was used to prepare

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MEAs and the fuel cell performance test was carried out. The membrane thickness was around 55 µm.

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The PEMFC singel- cell performances of these MEAs were tested at 100, 150 and 180 ºC under ambient pressure with non-humidified H2/O2 flows. The flow rates for both hydrogen and oxygen gases were kept

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as 300 ml/min and 500 ml/min, respectively. Fig. 11 shows the temperature dependence of the single cell

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performance for PSC8 nanocomposite membranes at 100, 150 and 180 °C. Significant increase in performance by increasing the temperature from 100 to 180 ºC was observed. Increasing the temperature

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can enhance the electrochemical properties of fuel cell. This fact can generally be explained by the increased membrane conductivity and enhanced reaction kinetics at elevated temperatures. Table 2 summarizes the PEMFC open circuit voltages (OCVs), power density in 0.5 V and the current density in 0.5 V. The PSC8 nanocomposite membrane showed 0.44 W/cm2 power density and 0.88 A/cm2 current

density in 0.5 V at 180 ºC. The OCV value for PSC8 was 0.86 at 180 °C (with the membrane thickness of 55 µm) that more or less is in agreement with the other pervious studies [43]. However it is known that the membrane thickness, the poor seal of the cell and the fuel penetration across the PEMs caused low OCV in the fuel cell performance tests [43].

4. Conclusion

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In this work advanced nanocomposite membranes based of PBI/SrCeO3 were prepared by solution casting method. The results showed that the acid uptake and proton conductivity of the nanocomposite

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membranes were higher than that of pure PBI based membrane. This fact is related to the high

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hygroscopic character and perovskite structure of SrCeO3 nanoparticles. The proton conductivitiy of

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0.105 S/cm, power density of 0.440 W/cm2 and current density of 0.88 A/cm2 in 0.5 V at 180 ºC were

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seen for the nanocomposite membranes containing 8 wt.% of the SrCeO3 nanoparticles. Based on the

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SEM micrographs, the SrCeO3 nanoparticles were well distributed into the polymer matrix for the

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composite membranes cotaining upto 8 wt %. In the case of the nanocomposite membranes with higher

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amounts of SrCeO3 than 8 wt. %, the nanopowders could not be well distributed inside the membrane

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and aggregate. The PSC8 nanocomposite membrane showed a high thermal stability and is suitable for

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use in high temperature PEMFCs.

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Acknowledgement

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The authors are grateful to the Renewable Energy Research Center (RERC), Amirkabir University of Technology (Tehran, Iran) for the financial support of this work.

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Figure Captions

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Fig. 1. Acid uptake plots of PA- PSCx nanocomposite membranes.

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Fig. 2. Proton conductivity plots of PA- PSCx nanocomposite membranes at room temperature.

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Fig. 3. Nyquist (a) and Bode Modulus (b) plots for PSCX nanocomposite membranes.

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Fig. 4. Proton conductivity of PBI, PSC4 and PSC8 composite membranes at different temperatures.

Fig. 5. Remaining acid plots of PA- PSCx nanocomposite membranes.

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Fig. 6. Ion exchange capasity (IEC) of PA- PSCx nanocomposite membranes.

Fig. 7. SEM micrographs of the PSCx nanocomposite membranes surface: (a) PSC2 (b) PSC4 (c) PSC8(d)

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D

PSC16.

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Fig. 8. SEM-EDX of the PSC8 nanocomposite membrane surface.

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Fig. 9.Thermogravimetric analysis of the PA- PSCx nanocomposite membranes.

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Fig. 10. X-Ray diffraction patterns of SrCeO3, PBI and PSCx nanocomposite membranes.

Fig. 11. Electrochemical fuel cell performance for single-cell based on the PSC8 nanocomposite

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membranes.

Ref.

0.105

This work

DC

0.079

This work

10

0.080

[32]

180

5

0.065

[32]

180

DC

0.032

[33]

180

5

0 .080

[34]

160

DC

0.067

[35]

Sulfonated silica (10 %)

160

DC

0.040

[36]

Zirconium tricarboxybutylphosphonat e (50 %)

200

100

0.052-0.081

[37]

T (°C)

SrCeO3 (4 %)

180

Clay (15 %)

180

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Clay (15 %) ZrO2 (10 %)

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Zirconium phosphate (3 %)

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Silica (10 %)

DCa = Dry condition

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180

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SrCeO3 (8 %)

% RH/ DC

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Modifier (wt %)

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Proton conductivity (S/cm)

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Table 1- Comparison of proton conductivity of several PA doped PBI nanocomposite membranes.

a

Table 2- Fuel cell performance for single-cell based on PSC8 nanocomposite membranes.

Temperature (°C)

Current

Power density

(A/cm2) in 0.5 V

(W/cm2) in 0.5 V

OCV (V)

0.84

0.42

0.21

150

0.84

0.54

0.27

180

0.86

0.88

PT

100

A

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TE

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A

N

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0.44