zirconium phosphate composite membranes for high temperature applications

zirconium phosphate composite membranes for high temperature applications

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Sulfonated polybenzimidazole/zirconium phosphate composite membranes for high temperature applications Wei Qian a, Yuming Shang a,b,*, Mou Fang a, Shubo Wang a,b, Xiaofeng Xie a,**, Jinhai Wang a, Wenxiao Wang a,c, Jinyan Du a,c, Yaowu Wang a,b, Zongqiang Mao a a

Institute of Nuclear and New Energy Technology, A316, INET, Tsinghua University, Beijing 100084, China Beijing Key Lab of Fine Ceramics, Tsinghua University, Beijing 100084, China c College of Chemical Engineering, China University of Petroleum, Beijing 102249, China b

article info

abstract

Article history:

Sulfonated polybenzimidazoles (SPBI) with different sulfonation degrees were prepared

Received 18 January 2012

from 3,30 ,4,40 -amino-diphenyl ether, 5-sulfonated isophthalate sodium and isophthalic

Received in revised form

acid by direct polycondensation in polyphosphoric acid. Considering the conductivity and

11 May 2012

oxidative stability of the SPBIs with different sulfonation degrees, the SPBI-10 membrane

Accepted 15 May 2012

was optimally selected as base membrane. In order to enhance the proton conductivity,

Available online 23 June 2012

Zirconium phosphate (ZrP) was physically mixed with SPBI-10 to get a serial of composite membranes. The water uptake rises slightly with the increase of ZrP and the phosphoric

Keywords:

acid swelling ratio of membranes greatly depends on the concentration of phosphoric acid,

Sulfonated Polybenzimidazole

dipping time and temperature. At the H3PO4 doping level of 5, the phosphoric acid swelling

Zirconium phosphate

ratio is up to 15%, and the membranes keep a good mechanical property. Due to the

Proton exchange membrane (PEM)

addition of ZrP, the conductivity of SPBI-10-Z3 composite membrane is up to 0.08S/cm at

High temperature

180  C, which increases about 20% compared with that of SPBI-10 membrane. This suggests

Proton exchange membrane fuel cell

that SPBI/ZrP composite membranes may be a promising polymer electrolyte for high

(PEMFC)

temperature PEMFCs. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Fuel cells are electrochemical devices with high energy conversion efficiency, minimized pollutant emission and other advanced features. Proton exchange membrane fuel cell (PEMFC), among the five types of fuel cells, is attractive both for automobile and stationary applications [1]. For most PEMFCs, the proton exchange membranes are currently based on perfluorosulphonic acid (PFSA) polymers, e.g. Nafion [2]. This membrane material has high conductivity, excellent chemical stability, mechanical strength and flexibility, and potentially

long-term durability. However, it functions only in a highly hydrated state and therefore it is limited to operation at relatively low temperatures of around 80  C under ambient pressure in order to maintain a high water content in the membrane. Several challenges for the PEMFC power technology are associated with low operating temperature [3]. Fuel processors, i.e. hydrogen storage tanks and hydrocarbon or alcohol reformers with subsequent CO removers are voluminous, heavy, costly and in most cases complex. Water management involves appropriate humidification of fuel and oxidant, airflow rate and power load regulation. Temperature

* Corresponding author. Institute of Nuclear and New Energy Technology, A316, INET, Tsinghua University, Beijing 100084, China. Tel.: þ86 10 8019 4009; fax: þ86 10 6278 4827. ** Corresponding author. E-mail addresses: [email protected] (Y. Shang), [email protected] (X. Xie). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.05.076

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control or cooling is more critical for larger stacks, and the heat is of low value. Therefore, modified Nafion and non-fluorinated polymer membranes have been intensively explored [3e6]. In recent years, research activities in the high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) have grown with a sign of stepping. PEMFC operated at high temperature, in particular, continues to generate interest in regards to the prevention of catalyst poisoning of CO, improved electrode reaction kinetics, and enhanced system efficiency [7,8]. Much research efforts have been devoted to the study of proton conducting membranes at temperatures above 100  C. Savinell et al. [9,10]first developed polybenzimidazole (PBI) that can be used as a polymer electrolyte at 200  C after doping with strong acids such as phosphoric acid (H3PO4) or sulphuric acid (H2SO4). He et al. [11]studied phosphoric acid doped polybenzimidazole (PBI) and PBI composite membranes for high temperature PEMFC. Gomez-Romero et al. [12] reported sulfonated poly(2,5-benzimidazole) (SABPBI) membrane impregnated with phosphoric acid for high temperature PEMFC. Ariza and Jennifer et al. [13,14] investigated sulfonated poly(benzimidazole) membranes (SPBI) for high temperature PEMFC. The PBI membranes have high thermal stability, but the conductivity is to be improved, besides, the solubility of PBI is poor that affects the formation of membrane [15]. In order to the solve the above problems, in this work, soluble sulfonated polybenzimidazoles (SPBI) with various sulfonation degrees were prepared from 3,30 ,4,40 -aminodiphenyl ether, 5-sulfonated isophthalate sodium and isophthalic acid by direct polycondensation in the polyphosphoric acid, which were then modified by zirconium phosphate (ZrP) to get serials SPBI/ZrP composite membranes. Experiments showed the proton conductivity of the obtained membranes was improved by the addition of ZrP which suggested the composite membranes might be potential polymer electrolyte for HT-PEMFCs.

2.2.

SPBI polymer was synthesized in PPA from 3, 30 , 4, 40 -tetra aminodiphenyl ether, 5-sulfonated sophthalate sodium and isophthalic acid at 200  C as presented in Scheme 1. A typical procedure for polycondensation was as follows: 30 g of polyphosphoric acid was placed in a 100 mL threenecked flask with a mechanical stirrer under a nitrogen atmosphere. 0.8024 g (3 mmol) of 5-sulfonated isophthalate sodium, 0.4972 g (3 mmol) of isophthalic acid and 1.3818 g (6 mmol) of 3, 30 , 4, 40 -tetraaminodiphenylether were then added to the flask. The mixture was heated at 100  C for 1 h and then 200  C for 5 h with continuous stirring. After the polymer cooled down, the polymer was carefully poured into deionized water and washed with deionized water several times to remove the residual acid. Finally, the product was vacuum-dried at 120  C for 24 h. The molar ratio of sulfonated monomer (5-sulfonated isophthalate sodium) used in these reactions respectively was 10%, 30%, 50%,70% and 100% of SPBI polymer. The names of these membranes were noted as SPB-10, SPBI-30, SPBI-50, SPBI-70 and SPBI-100.

2.3.

Materials and method

2.1.

Materials

3, 30 , 4, 40 -tetraaminodiphenylether, 5-sulfonated isophthalate sodium, isophthalic acid and Polyphosphoric acid (PPA) were purchased from Aldrich. Zirconium oxychloride was purchased from Tianjin Chemical Reagents Co. Other chemicals and solvents were obtained from Beijing Chemical Reagents Co. and employed without further purification.

Preparation of SPBI membranes

The above obtained SPBI polymers were dissolved in DMSO. Then a series of membranes were prepared by casting polymer solution onto dust-free glass plates and drying at 180  C for 24 h. After cooling to room temperature, the glass plates were immersed in deionized water and the membranes peeled off. After rinsing with deionized water several times, the membranes were dried in vacuum oven at 100  C for 24 h. The thickness of membranes was 0.05e0.06 mm.

2.4.

2.

Polymer synthesis

Preparation of zirconium phosphate(ZrP)

Zirconium Phosphate was prepared by reflux method [16]. The procedure followed was: 2.1 g of ZrOCl2  8H2O and 2M HCl solution (50 ml) were added to the beaker, and then the dropwise added to the mixture solution of 4M phosphoric acid (3.2 ml) and 2M hydrochloric acid (2 ml), and it formed sol. The sol was added to 12M phosphoric acid, and keep for 48 h at a temperature of 90  C. The product was obtained by filtered and washed with deionized water several times, and were dried in vacuum oven at 80  C for 48 h. The grain size of Zirconium Phosphate (ZrP) was 50e60 nm.

Scheme 1 e Synthesis of SPBIs.

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2.5. Preparation of SPBI/ZrP membranes and H3PO4 doping ZrP nanoparticles composition membranes were prepared by adding ZrP nanoparticles to the DMSO solution. This suspension was stirred at 30  C for 1 h and then the SPBI was added while stirring for 3 h at 150  C. And SPBI/ZrP composition membranes were prepared by the process similar to that of SPBI membrane. The amount of ZrP nanoparticles used in these reactions respectively was 0.5%, 1%, 2% and 3% of SPBI polymer. The names of these composites were noted as SPBIZ0.5, SPBIZ-1, SPBIZ-2 and SPBIZ-3. The SPBI/ZrP nanoparticles composition membranes were immersed in 70% (w/w) H3PO4 solution for 24 h at 70  C.

2.6.

Measurements

1H NMR was recorded on a Brucker 600 MHz NMR Spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) as solvent. FT-IR spectra was obtained from a Shimadzu FTIR-8400. Thermal gravimetric analysis (TGA) was done using Thermal Analyzer PerkinElmer7 at nitrogen. TGA curves were recorded in the range of 20e700  C at a heating rate of 20  C/min. The membrane morphology was studied with a Scanning Electron Microscope (SEM, Hitachi S-5500). The water uptake and the phosphoric acid swelling were determined according to the following steps. The membrane sample (4  4  50e60 cm2mm) was dried in vacuum oven at 100  C for 24 h. The weight (Wdry) and area (Sdry) of the dry membrane were measured. Thereafter, the membrane was immersed in deionized water at 70  C for 24 h, taken out and wiped dry, and then the weight (Wwet) of the membranes in wet state were measured immediately. Similarly, the membrane was immersed in 70%(w/w) H3PO4 at 70  C for 24 h, taken out and wiped dry, and the area (Swet) of the membranes in wet were measured. The water uptake and the phosphoric acid swelling were calculated by the following equations: water uptakeð%Þ ¼

Wwet  Wdry  100 Wdry

phosphoric acid swellingð%Þ ¼

Swet  Sdry  100 Sdry

Similar to the previous report [17], the oxidative stability of SPBI membranes were investigated by observing the degradation behavior of membranes (2  2  50e60 cm2mm)when immersed in 25 mL Fenton’s reagent (2 ppm FeSO4 in 3% H2O2) at 80  C. AC conductivity measurements as a function of temperature were made by the in-plane conductivity testing in air using a ZAHNER IM6 apparatus with AC voltage amplitude of 10 mV over frequency ranging from 3 MHz to 100 Hz. The area of membrane was 1  4 cm2. The membrane was put on the surface of the two platinum electrodes that is a distance of 2 cm. The membrane and electrodes were sandwiched with PTFE plates, which ensured the membrane good contact with the electrode and two wires were used to connect the

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electrodes with electrochemical workstation. Components are placed in a closed container, the temperature in the container could be heated by the heating apparatus. The proton conductivity of membrane was calculated using the relation s ¼ L/RWd, where R is the resistance of the sample, L represents the distance between two electrodes, W and d denote the width and thickness of the membrane, respectively.

3.

Results and discussion

3.1.

Polymer characterization

The 1H NMR spectrum of SPBI-100 was shown in Fig. 1. The signal peaks of SPBI-100 are assigned. The broad shape peak around 13e14 ppm is due to the exchanged proton in eNH group with small amount of water in solvent. Other chemical shift signals attributed to different proton of SPBI molecular can also be found and with good agreement with the molecular structure. In addition, the FT-IR spectra of polybenzimidazoles were extensively investigated [18]. As shown in Fig. 2, there is no residual absorption band between 1780 and 1650 cm1, suggesting that the cyclization of benzimidazole moieties is complete [18e20]. The characteristic bands of benzimidazole rings are observed at 1630 cm1 (the C]C/C]N stretching), 1530 cm1 (the ring vibration of conjugation between benzene and imidazole rings), and 1465 cm1 (the in-plane vibration of imidazole rings). The absorption bands at 1048, 988, and 628 cm1 are found in the IR spectra of SPBI whereas they are absent in that of PBI. The absorptions centered at 1048 and 988 cm1 are assigned to the asymmetric and symmetric stretching vibrations of eSO3Na, respectively, and the band around 628 cm1 is ascribed to the stretching vibration of the SeO in sulfonate groups. The above 1H NMR and IR characterization of the polymers suggested the successful synthesis of SPBI.

3.2.

Thermal analysis

The thermal property of SPBI was investigated by TGA which was displayed in Fig. 3. As can be seen from the TGA curves, there is small amount of water inside all the membranes which is lost up to a temperature of 250  C. The first weight loss is at about 450  C, due to desulfonation of the polymer. Besides, with the increasing degree of sulfonation, the mass loss of polymer gradually increases. At 700  C the residual weight is greater than of 65%, which further demonstrates the good thermal stability of SPBIs.

3.3.

Conductivity of SPBIs

Proton conductivity is an important indicator of membrane performance. Before adding ZrP, it is of great importance to choose appropriate polymer with good proton conductivity as the base membrane. Fig. 4 illustrates the proton conductivity of different SPBI membranes doped with H3PO4 as a function of temperature at 5%RH. The proton conductivity of SPBI/ H3PO4 is increased with the increase of polymer sulfonation degree in some humidity condition and a maximum

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Fig. 1 e 1H NMR spectrum of SPBI-100.

conductivity is founded to be 0.076 S cm1 of the SPBI-100 membrane at 180  C. Also, the proton conductivity of SPBI rises with the increasing of temperature. As presented in Fig. 4, the order of maximum conductivity is SPBI-100 >SPBI70 > SPBI-50 > PBI-30 >SPBI-10 membrane. Although H3PO4 is considered a proton donor for conductance at high temperature, the eSO3H group still plays a vital role in conductivity. When the membrane keeps in a hydrate state, the eSO3H group can play a proton donor to enhance the conductivity. That supports the fact that some humidity condition at high temperature can increase the conductivity [21].

3.4.

Oxidative stability

PEMs exhibit inferior resistance to oxidation due to the benzene ring possesses a high electron density [1]. However, there are already a few aromatic polymer based PEMs with excellent durability for PEMFC applications, such as sulfonated polyphenylquinoxalines, sulfonated poly(2,6-diphenyl-4phenylene oxide), sulfonated polyarylethersulfone, polybenzimidazoles [3,25]. The oxidative stability of aromatic PEMs is critical for its application. The oxidative stability of SPBI samples was evaluated by the mass loss in Fenton’s reagent (2 ppm FeSO4 in 3% H2O2) at 80  C in the same time. As shown in Fig. 5, the mass loss of SPBI membranes doped with H3PO4 is increased with the increase of sulfonation degree which is due to the fact that the side chains with eSO3N groups posses poor resistance to the attack of free radicals.

During the operation of fuel cells, the radicals such as HO and HOO will form and attack proton exchange membranes leading to the degradation of membranes [22e24]. In comparison with the perfluorinated PEM, the aromatic polymer based

Fig. 2 e FT-IR spectra of PBI and SPBIs.

Fig. 3 e TGA curves of PBI and SPBIs.

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Fig. 6 e SEM images of SPBI/ZrP composite membrane. (a: surface image; b: cross-section image).

The water uptake and phosphoric acid swelling of membranes represent their water retention and dimensional stability, respectively. Water content could affect the proton

conductivity and dimensional stability which affects the mechanical property of membranes. The absorbed water in the membrane associates with protons and promotes the transport of hydrated proton (H3Oþ) from anode to cathode, leading to the increase of proton conductivity. On the other hand, the mechanical properties of the membrane would deteriorate due to the excessive water uptake [6]. In order to balance the two properties of membranes, the investigation of the water uptake and phosphoric acid swelling of membranes is important [26]. As displayed in Fig. 7,the water uptake of SPBI/ZrP composite membranes rises slightly with the increase of ZrP, because ZrP is a group of inorganic polymers where the O3POH groups of the a-type Zr(O3POH)2 nH2O and the O2P(OH)2 groups of g-type ZrPO4O3P(OH)2 nH2O, and it owns water retention property [3,27]. The phosphoric acid swelling of membrane has relationship with the concentration of phosphoric acid, dipping time and temperature which also determines the H3PO4 doping level of the membranes, and finally affects the conductivity. The SPBI/ZrP composition membranes were immersed in 70% (w/w) H3PO4 for 24 h at 70  C. At the H3PO4 doping level of about 5, the phosphoric acid swelling is up to 15%, and the membrane can keep acceptable mechanical property. At the same time, the conductivity of SPBI-10-Z3 composite membranes is up to 0.08S/cm (at 180  C and RH 5%), which is increased by 20% compared to SPBI-10 membrane (0.069 S/cm) at same testing conditions.

Fig. 5 e Mass loss ratio of SPBI membranes with different sulfonation.

Fig. 7 e Water uptake of different composite membranes.

Fig. 4 e Proton conductivities of different SPBI membranes doped with H3PO4 at 5%RH.

3.5.

Microscopic morphology of composite membranes

In order to improve the proton conductivity, ZrP was introduced into SPBIs to get composite membranes. Considering the conductivity and oxidative stability of the SPBIs, SPBI-10 is optimally chosen as basement membrane. Fig. 6 shows the SEM morphology of surface and a cross-section of the SPBI-10Z2 composite membrane. The grain size of ZrP is about 100 nm which is well distributed in the membrane. Fig. 6 a shows that the surface of SPBI-10-Z2 composite membrane which is not smooth, suggesting that the membrane-making processes need to be controlled. Fig. 6 b depicts the distribution of ZrP inside the membrane which looks very even.

3.6. Water uptake, phosphoric acid swelling and conductivity of composite membranes

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Conclusions

Sulfonated polybenzimidazoles with various sulfonation degrees were synthesized from 3,30 ,4,40 -amino-diphenyl ether, 5-sulfonated isophthalate sodium and isophthalic acid by direct polycondensation in the polyphosphoric acid. The conductivity and oxidative stability of the SPBI polymers were studied, SPBI-10 membrane was optimally chosen as basement membrane. The SPBI/ZrP composite membranes were investigated, and the following conclusions can be obtained: (1) The water uptake rises slightly with the increase of ZrP due to the water retention property of ZrP. The phosphoric acid swelling ratio of membranes depends on the concentration of phosphoric acid, dipping time and temperature. At the H3PO4 doping level of about 5, the phosphoric acid swelling is up to 15%, and the membranes keep a good mechanical property. (2) The SPBI-10-Z3 composite membrane shows a considerably increased conductivity up to 0.08S/cm at 180  C which increased by 20% compared to SPBI-10. (3) Current work is to show the properties of composite membrane, and test the membrane in an actual fuel cell is carried out and will be reported later.

Acknowledgements This research was financial supported by the National Natural Science Foundation of China (51003053, 50973055), Tsinghua University Initiative Scientific Research Program (2011Z23152) and the State Key Basic Science Research Project of China (2012CB215500).

references

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