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Phosphoric acid doped poly(2,5-benzimidazole)-based proton exchange membrane for high temperature fuel cell application.

INTRODUCTION

Polymer electrolyte doped with sulfonic acid group commonly known as perfluorosulfonated solid electrolyte (PFSE) has been utilized for proton conduction. Nafion is one of such type of membrane available commercially, which is explored widely for polymer electrolyte fuel cell (PEFC) application [l]. Sulfonic acid present in the PEFC, provides [H.sup.+] ion only under hydrated condition and qualifies for the ion conduction. Therefore, the operating temperature of the sulfonated membrane based fuel cell is restricted below 100[degrees]C under atmospheric pressure. Furthermore, the fluorine backbone of this type of membrane is harmful. Nevertheless, it is a preferred candidate for low temperature fuel cell applications due to its excellent mechanical and chemical stability. Due to low temperature operation, ultra-pure hydrogen gas (CO level <25 ppm) is required to avoid the poisoning of the platinum catalyst used in PEFC application. However, the CO tolerance capacity might be improved up to 3%, when fuel cell was operated at elevated temperature [2]. High temperature operation further enhances the reaction kinetics on both the electrodes of the fuel cells. Moreover, during high temperature operation, the hydrogen desorption kinetics improves when metal hydride storage is used for hydrogen storage.

Above the boiling point of water, operation of PEFC involves only a single phase that is the water vapor, and therefore, avoids water flooding. The size of cooling system can be reduced substantially, which is otherwise very important for the transport application due to the increased temperature gradient. The heat can be recovered as steam, which in turn can be used either for direct heating or steam reforming or for pressurized operation. In this way, the overall system efficiency can be significantly increased. Several efforts have been made to develop proton-conducting membrane for operation at temperature above 100[degrees]C for the fuel cell application [3, 4].

Among various types of polymer suitable for high-temperature polymer electrolyte fuel cell (HT-PEFC) application such as, phosphonated perflurosulfonic acid membrane, sulfonated aromatic hydrocarbon polymer membrane, inorganic-organic composite membrane [3], and phosphoric acid doped polybenzimidazole (PBI) are reported as promising candidates due to its high performance, excellent oxidation and thermal stability, low fuel permeability, nearly zero water drag coefficient and high ionic conductivity at temperature up to 200[degrees]C.

A majority of the current work is focused towards developing polybenzimidazole (PBI) based membrane for high temperature fuel cell application. PBI could absorb up to 75% [H.sub.3]P[O.sub.4] and also less expensive as compared with Nafion membrane [5]. It is also impermeable to fuel gases and methanol and does not require humidification. Generally, para and meta-PBI are commercially available membranes in the benzimidazole family. Among the family of benzimidazole series; poly(2,5 benzimidazole) (ABPBI) (Fig. 1), is worth investigating for high temperature fuel cell application [6].

ABPBI exhibits simple structure as shown in the Fig. 1. PBI synthesis is a tedious process, where two monomers, tetra aminobiphenyl (TAB) and isophthalic acid (IPA) are needed. On the contrary, the synthesis process of ABPBI is simple and needs only single inexpensive commercially available monomer, namely: (3,4 diaminobenzoic acid) even without purification [7]. It is capable of absorbing higher amount acid as compared with PBI as it has higher concentration of benzimidazole group [8]. Moreover, the preparation method of ABPBI polymer is simple, safe, and cost-effective. Therefore, ABPBI is considered as a promising candidate for fuel cell application.

In present work, ABPBI membrane has been prepared through solvent casting method by considering the appropriate ABPBI-MSA solution concentration on the basis of relative viscosity, hardness, and membrane formation capacity mentioned in Table 1. Then the suitable membrane was considered for the further parametric study [9, 10]. Subsequently, the mechanical properties of the membrane were studied via Nano-indentation technique and proton conductivity measurement was carried out through impedance analysis at high temperature by varying the doping concentration of [H.sub.3]P[O.sub.4]. The electrochemical response of the doped membrane has been analyzed by an equivalent circuit model. In brief, structure-property relationship studies have been evaluated with commercially available ABPBI to develop proton conducting membrane for fuel cell application.

EXPERIMENTAL

Materials and Experimental Methods

ABPBI (poly 2,5 benzimidazole) polymer was obtained from Gharda Chemicals, Mumbai, India (G-5020 with a trade name GAZOLE, synthesized from 3,4 dibenzoic acid with intrinsic viscosity of 2.0-2.5 dL/g). ABPBI membrane was prepared by solvent casting method using methane sulfonic acid (MSA, MERCK, Germany) as a solvent. The concentration of ABPBI has been varied from 22 to 78 vol% and the corresponding membranes were prepared by pouring the solution on a petridish with gradually increasing the temperature up to 200[degrees]C for 90 min.

Characterizations

Relative viscosity of ABPBI solution in methane sulfonic acid was determined with the help of Cannon-Fenske viscometer by measuring the efflux time of the solvent as well as solution at different concentration at 30[degrees]C. The appropriate solution concentration was determined by considering the relative viscosity of the solution, the membrane forming ability(which yields a membrane of thickness of ~60-100 [micro]m) of the solution and the mechanical properties (modulus and the hardness) of the membrane. Hardness and the Young's modulus of the corresponding membrane were determined from nano-indentation technique described below. Surface morphology of the membrane was performed using an Alicona infinite focus optical three-dimensional (3D) surface measurement device (Austria). Wide angle X-ray Diffraction (WAXD) studies were carried out on a Phillips X-Pert Pro. The incident X-rays ([lambda] = 1.54 [Angstrom]) were monochromatized using a Ni filter. WAXD patterns were recorded with a step scan and step size of 0.02[degrees] between 5[degrees] and 60[degrees] (2[theta]). The elemental analysis of carbon, hydrogen, nitrogen, and sulfur was calculated from FLASH EA 1112 SERIES (Thermofinnigan instrument, Italy). Fourier transform infrared spectroscopic (FTIR) analysis was carried out in the range of 400-4000 [cm.sup.-1] with Vertex 80 FTIR instrument (BRUKER, The United States). Thermal stability of the various membranes was investigated in the temperature range between room temperature to 1000[degrees]C using a thermogravimetric analyzer (TGA; Mettler Toledo STA 851, The United States); with nitrogen flow rate of 60 mL/min. Membrane electrode assembly consists of an ion-exchange membrane sandwiched between a pair of catalyst supported electrodes under a pressure of 19.6 MPa. Hence, investigations of the mechanical properties of the doped membranes were essential to qualify for fuel cell application. Mechanical properties such as hardness (H) and Young's modulus (E) of the [H.sub.3]P[O.sub.4] doped and undoped ABPBI membrane were determined using a nano-indenter (Tribolndenter TI-900, Hysitron Inc., Minneapolis) with a Berkovich diamond indenter (tip radius of 150 nm and semi apex angle of 65[degrees]). The Nano-indenter had a load and depth resolution along z-axis of 1 nN and 0.04 nm, respectively. Dielectric impedance spectrometer (Concept 80, Novocontrol Technologies, Germany) was used to measure the proton conductivity of doped and undoped membrane in the frequency range of 1 Hz-1 MHz from room temperature to 200CC. The experiment was carried out by placing the ABPBI membrane in between two bronze electrodes by through plane assembly. The experiment was carried out by Novocontrol Alpha-A Analyzer, Germany in the frequency range of 1 Hz-10 MHz and Z-view software was used for designing the model equivalent circuit.

RESULTS AND DISCUSSION

The Selection of Appropriate ABPBI Concentration for Membrane Formation

Acid-doped ABPBI membrane with good proton conductivity value at higher temperature is considered as the best suitable membrane for HTPEFC application. The crucial part of the membrane preparation depends on the specific concentration of the polymer in a good solvent. It primarily decides the film forming capability of the solution and also the strength of the well-prepared membrane. The addition of varying amount of the polymer to a solvent (viz., methane sulfonic acid), affects the relative viscosity of the solution. The development of the membrane was carried out with a suitable solution on the basis of the hardness and film forming capability supported by the relative viscosity value and the topography associated with the membrane. Figure 2 shows the variation of hardness and relative viscosity as a function of molar concentration of ABPBI in methane sulfonic acid. It is observed that the ABPBI solution of molar concentration between 8.5 and 6.8 mol/L could not form the membrane continuously. However, the ABPBI solution of molar concentration between 5.6 and 4.2 mol/L yielded continuous membrane. Molar concentration of 5.6 mol/L with relative viscosity of 125 of the solution having hardness of 0.074 GPa associated with the membrane was optimized for HTPEFC application. Table 1 shows the hardness and the relative viscosity values for the various ABPBI membranes and their corresponding solution, respectively.

Doping of the ABPBI Membrane

The membrane samples (2 cm x 2 cm) obtained from solvent casting, were immersed in the varying volume concentration (20, 40, 60, and 80 vol %) of the phosphoric acid ([H.sub.3]P[O.sub.4]) solution. Although, it is reported in the literature, that the complete doping could be achieved within 12 h, the membrane was doped for 15 h to ensure complete doping in the present study [11, 12]. It has been observed that the membrane was completely dissolved in 80 vol% [H.sub.3]P[O.sub.4] solution; however, membrane survived up to 60 vol% [H.sub.3]P[O.sub.4] solution. Extent of acid doping was estimated based on the following correlation (Eq. 1) and it is summarized in Table 2.

Acid dopoing level = Weight difference/Initial weight x [M.sub.w] of ABPBI repeat unit/[M.sub.w] of [H.sub.3]P[O.sub.4] (1)

Membrane Topography

The surface topography of undoped ABPBI membrane (~110 [micro]m thickness) has been investigated via Optical 3D surface measurement. The surface of the ABPBI membrane was investigated as it was prepared through solvent casting method. The average roughness and root mean square roughness of the ABPBI membrane show 832 and 1000 nm, respectively (Fig. 3), which is in good agreement for the membrane prepared via solvent casting method. The maximum peak to valley height of roughness and minimum peak to valley height of roughness found to be 5.9 and 4.5 [micro]m, respectively.

In general, the top surface of a membrane would come in contact with air, whereas the bottom surface stayed in contact with the plane glass surface of the petri-dish during the solvent casting process; however, the SEM images exhibited almost similar features for both the surfaces [8, 11]. The roughness varies more towards the end corner of the membrane with [10.sup.-14] [micro]m height; maximum peak valley height was found to be 5.9 [micro]m from Fig. 3 with a mean value to peak height of 4.5 [micro]m. The low peak height value does not exhibit any major impact on the membrane properties.

Wide Angle X-ray Diffraction Analysis

ABPBI is semicrystalline in nature, but the doping process destroys significantly the crystalline nature associated with the ABPBI membrane. The variation in the crystalline structure has been studied through WAXD analysis. Figure 4 shows the WAXD pattern of doped and undoped ABPBI membrane. A single peak is observed at 2[theta] = 27[degrees], which corresponds to parallel benzimidazole ring forming a stacked structure of ABPBI [13, 14]. The sharpness of the peak shows the highly crystalline nature of the membrane. The peak is broadened with increasing extent of doping level, referring to an increase in the amorphous nature, which may lead to reduced mechanical strength. The decrease in the extent of crystallinity may be due to the discontinuity in the molecular chain arising from the free [H.sub.3]P[O.sub.4] molecules trapped in the intermolecular free volume space.

Elemental Analysis

The amount of absorbed [H.sub.3]P[O.sub.4] by the ABPBI membrane can be calculated via elemental analysis. The membrane is completely dissolved in 80 vol% [H.sub.3]P[O.sub.4] solution after 12 h, as it absorbed 87 wt% of [H.sub.3]P[O.sub.4] (Table 3) whereas, membrane swelled up moderately in 60 vol% [H.sub.3]P[O.sub.4] solution. The residue is different from C, H, N, and S, which attributes to phosphate ion (P[O.sup.3-.sub.4]) content in the membrane. Hydrogen is associated with both [H.sub.3]P[O.sub.4] and the polymer, so the phosphate ion concentration only can provide the idea of amount of [H.sub.3]P[O.sub.4] present in the doped membrane [15]. During the casting of ABPBI in MSA, the sulfur present in MSA protonates the benzimidazole group of the polymer and being displaced by [H.sub.3]P[O.sub.4] with increasing doping level. Therefore, only sulfur is present in the ABPBI membrane, which is mentioned in the Table 3; however, it is completely disappeared after doping. The residue for undoped ABPBI is 35.82%, which confirms only about the presence of oxygen due to MSA but the residue of remaining doped sample may explain the amount of [H.sub.3]P[O.sub.4] present in the membrane.

Fourier Transform Infrared (FTIR) Analysis

The molecular signature of phosphoric acid in the doped ABPBI membrane can also be determined by the FTIR spectroscopic analysis. FTIR analysis (Fig. 5) shows three major absorption peaks, which provide the molecular signature for the undoped and the doped ABPBI membrane. The absorption band at 3,300-3,600 [cm.sup.-1] indicates N-H stretching [11]. The broadening of the band may be due to doped sample with the presence of [N.sup.+]-H stretching [7]. It implies that proton can jump from one imidazole group to another non protonated imidazole group or any phosphoric acid molecule, which is not observed in the case of undoped membrane [16]. Moreover, FTIR analysis exhibits the O-H stretching vibration (2500-3000 [cm.sup.-1]) as both are hygroscopic in nature. The most important two peaks in the range of 1,100-1,500 [cm.sup.-1], may indicate the main absorption peak for [H.sub.3]P[O.sub.4].

Thermo-Gravimetric Analysis

The physical and chemical changes of the membrane as a function of temperature are essential to study to evaluate the stability of the membrane for high temperature fuel cell application, which is studied through TGA. Figure 6 explains the mass loss of ABPBI polymer as a function of temperature of both doped and undoped state. The initial weight loss starts at 50[degrees]C-60[degrees]C due to the absorbed water as [H.sub.3]P[O.sub.4] doped ABPBI membrane is very much hygroscopic in nature [17]. During preparation and handling, it might have absorbed 8% of water. The second weight loss starts at 150[degrees]C up to 210[degrees]C due to free phosphoric acid elimination from intermolecular free volume spaces. Phosphoric acid changes to pyrophosphoric acid above 180[degrees]C and then detached from the main chain. The DTG curve (inset in Fig. 6) also clearly depicts the larger peak above 600[degrees]C and smaller peak at 50[degrees]C-60[degrees]C of the ABPBI membrane. The molecular chain vibrates violently causing weaken of the backbone at above 600[degrees]C and starts to degrade at 800[degrees]C [18].

Young's Modulus and Hardness of the Doped ABPBI Membrane

Nano-hardness (H) and Young's modulus (E) of the ABPBI membrane of varying level of [H.sub.3]P[O.sub.4] doping along with undoped ABPBI membrane were determined using nano-indentation technique. H and E values were evaluated according to DIN 50359-1 standard from the load (P) versus depth of penetration (h) plot using the well-established Oliver and Pharr method with a fixed constant load of 100 nN. The variations in depth of penetration with applied load for different undoped and doped membranes are shown in Fig. 7. The total area under the P-H plot is much reduced in un-doped membrane as compared with that of the doped membrane. The hardness and Young's moduli data of the doped and un-doped membrane derived from Fig. 7 are summarized in Table 4. The results clearly demonstrate that increasing doping concentration results in a decreased value of H and E. The final depth of penetration in the undoped membrane is approximately 25% lower than the 60 vol% [H.sub.3]P[O.sub.4] doped ABPBI membrane. For pure ABPBI membrane, inter molecular H-bonding may act as a dominant force, which influences the mechanical strength of the ABPBI membrane [19]. When phosphoric acid is introduced, the molecular cohesion of ABPBI is decreased; however, strong hydrogen bond persists between--N=-and--NH--group [20].

It is also observed that the doped membrane shows less elastic recovery than the corresponding undoped membrane. It may be also due to the fact that the excess free acid present between the intermolecular chains, enhances the chain flexibility and acts like a plasticizer in ABPBI membrane. For the same reason, the open space volume of polymer increases [21] and the membrane swells more and more with increasing doping concentration, which causes the reduction of E and H value [22]. Undoped ABPBI membrane shows (Table 4) the E and H value of 2.46 [19] and 0.92 GPa, respectively, and the corresponding E and H values for 1.65 doping level of 60 vol% [H.sub.3]P[O.sub.4] doped membrane exhibits the value of 0.14 and 0.067 GPa, respectively.

Proton Conductivity and Activation Energy

Proton conductivity is increased with increasing temperature for all the membranes expect for the undoped ABPBI membrane. Figure 8a shows that proton conductivity remains constant up to 100[degrees]C for the undoped and doped ABPBI samples until 40% [H.sub.3]P[O.sub.4] concentration or there is a marginal increase in proton conductivity for all the membranes, which indicate Grotthus hopping mechanism of proton transfer, where the hopping between two molecules (acid-acid, acid-water, acid-benzimidazole ring) may take place in these membranes. This mechanism exhibits less significant role below 100[degrees]C. Even the undoped membrane exhibits very less proton conductivity, which matches well with the reported value of [10.sup.-12] S/cm [12]. However, the proton conductivity exponentially increases with increase in temperature for all the doped membranes. 20% [H.sub.3]P[O.sub.4] doped ABPBI membrane with 0.25 doping level shows a proton conductivity of [10.sup.-8] S/cm and increases to [10.sup.-7] S/cm with an increasing doping level of 0.49. The 60% [H.sub.3]P[O.sub.4] doped membrane with doping level of 1.65 shows highest proton conductivity value of 2.2 x [10.sup.-2] S/cm (Fig. 8b). This suggests that the excess [H.sub.3]P[O.sub.4], which leads to higher doping level in ABPBI membrane, may help in proton conduction. At higher temperature, proton transfer occurs primarily through one N--H site to [H.sub.3]P[O.sub.4] anion and [H.sub.3]P[O.sub.4] to [H.sub.3]P[O.sub.4] for contributing conductivity. The increase in proton conductivity in ABPBI membrane with increasing doping level may be due to the presence of excess acid around the molecular chain. It is to be noted that 80% [H.sub.3]P[O.sub.4] doped ABPBI membrane swelled up significantly and dissolved when it was impossible to measure proton conductivity.

The activation energy (Ea) can be calculated from the Arrhenius plot of various ABPBI membranes, which is shown in Fig. 9. Undoped ABPBI membrane registers activation energy of 147.58 KJ/mol; whereas it is decreased gradually to 20.85 KJ/ mol for 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane. Activation energy of ABPBI 40% [H.sub.3]P[O.sub.4] doped ABPBI membrane shows a lower value (31.6 KJ/mol) as compared with ABPBI 20% [H.sub.3]P[O.sub.4] doped ABPBI membrane (33.7 KJ/mol). In general, undoped membrane shows higher activation energy than doped ABPBI membrane. It implies that energy needed to transport proton requires less energy for doped ABPBI membrane as compared with undoped membrane. The lowest activation energy of 20.85 KJ/mol corresponding to 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane suggests that the proton conduction would be faster than other corresponding membrane [14].

Impedance Spectroscopic Analysis

The impedance spectra (Fig. 10) exhibit a suppressed semicircle at high frequency domain, which may be due to the contribution of the membrane resistance coupled with constant phase element (CPE), which may be in parallel combination. Further, a straight line in low frequency region corresponds to the linear diffusion process of charged particle and described by Warburg element [23]. The model circuit (Fig. 10a) is considered based on the experimental data obtained from the sample at 119.95[degrees]C. The trend of slowly increasing conductivity from undoped ABPBI to 40% [H.sub.3]P[O.sub.4] doped ABPBI membrane and then sudden increase to 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane matches well with the fitting value, which is seen from Fig. 10.

An electron cannot cross from the electrode to the electrolyte membrane, so the electrode/electrolyte interface is likely to be presented as double layer capacitance [24]. However, an ideal capacitance is generally expressed by a vertical line in Z' versus Z" plot. Here, it deviates for solid electrolyte membrane due to irregularities on the surface of the electrode. Therefore, it is represented as CPE mentioned in (Eq. 2), the CPE behavior is contributed by surface inhomogeneity, reactivity, porosity, current, and potential distribution associated with electrolyte geometry, which is known as fractal geometry [25].

[Z.sub.CPE] = 1/Q(j[omega])[empty set] (2)

Where, [Z.sub.CPE] is the impedance of CPE, [omega] is the rotational frequency, and [PHI] is the CPE component When [PHI] = 1, ZCPE behaves like a complete capacitor and when [PHI] = 0, it is completely independent of frequency and the value of [PHI] should always be less than 1. The resistance value at 119.95[degrees]C is considered for modeling analysis. The output of modeling result is compared with the experimental value. The %error in resistance (R) of undoped ABPBI, 20% [H.sub.3]P[O.sub.4] doped ABPBI membrane, 40% and 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane show 3.60%, 3.25%, 7.21%, 5.87% of error for constant phase element (CPE) shows 1.55%, 4.26%, 3.22%, 2.92%, respectively. It implies modeling results are appreciably fitting with the experimental results.

The suitability of [H.sub.3]P[O.sub.4] doped ABPBI membrane as a proton conducting membrane for fuel cell application is elucidated via various characterization techniques employed in the present work and is compared in Table 5 considering some of the representative reported literature. It is observed from Table 5 that extensive characterization techniques have been utilized to analyze the [H.sub.3]P[O.sub.4] doped ABPBI membrane in the present investigation, which shows high proton conductivity. Moreover, nanoindentation technique has been utilized for evaluating mechanical properties of the [H.sub.3]P[O.sub.4] doped ABPBI membrane. Further, an extensive impedance analysis has been used to understand the various parameters, which affect the proton conduction in the [H.sub.3]P[O.sub.4] doped ABPBI membrane.

CONCLUSIONS

The highest doping level of 1.65 of phosphoric acid was achieved for the investigated ABPBI membrane. Presence of [H.sub.3]P[O.sub.4] molecule is detected by FTIR peak at 1300 and 1400 [cm.sup.-1] also it is supported by the elimination of same molecule at 180[degrees]C and 200[degrees]C through TGA. The nano indentation analysis clearly demonstrated that an increasing doping concentration might result in a decreased value of H and E. The final depth of penetration in the undoped membrane was approximately 25% lower than the 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane. For pure ABPBI membrane, inter molecular H-bonding might act as a dominant force, which influenced the mechanical strength of the ABPBI membrane. Undoped ABPBI registered E and H value of 2.46 and 0.92 GPa, which are much higher than 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane. 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane achieved only 0.14 and 0.067 GPa value for E and H because of its highest doping level of 1.65. The undoped ABPBI membrane exhibited very low proton conductivity; however, the proton conductivity exponentially increased with increase in temperature for all the [H.sub.3]P[O.sub.4] doped ABPBI membranes. The 20% [H.sub.3]P[O.sub.4] doped ABPBI membrane with 0.25 doping level showed a proton conductivity of [10.sup.-8] S/cm and increased to [10.sup.-7] S/ cm, with an increasing doping level of 0.49. About 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane with doping level of 1.65 exhibited highest proton conductivity value of 2.2 X [10.sup.-2] S/ cm. Undoped ABPBI membrane registered an activation energy of 147.58 KJ/mol; whereas it was decreased gradually to 20.85 KJ/mol for 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane. The lowest activation energy of 20.85 KJ/mol corresponding to 60% [H.sub.3]P[O.sub.4] doped ABPBI membrane might suggest that the proton conduction would be faster than other corresponding membranes. Equivalent circuit model was employed in order to understand the important circuit elements (viz., electrolyte resistance, constant phase element, and Warburg element) influencing the proton conductivity of [H.sub.3]P[O.sub.4] doped ABPBI membrane. It was proposed that the circuit involved resistance, which was parallel to the constant phase element, and was in series with Warburg element that matched perfectly with the experimental results.

In brief, [H.sub.3]P[O.sub.4] doped ABPBI membrane was developed as proton conducting membrane for fuel cell application, which showed the proton conductivity of 2.2 X [10.sup.-2] S/cm at 1.65 doping level and showed E and H values of 0.14 and 0.067 GPa, respectively.

ACKNOWLEDGMENTS

The authors would like to acknowledge Nano-indenter and Broad-band Dielectric Spectroscopy Central Facilities at MEMS, IIT Bombay. The authors would like to thank Mr. Jayant Kolte (MEMS, IIT Bombay) for his valuable suggestions on impedance analysis. The authors would also acknowledge Gharda Chemicals, Mumbai for supplying ABPBI and methane-sulfonic acid for carrying out this research work.

NOMENCLATURE

ABPBI                 Poly(2,5-benzimidazole)
DAB 3,4               Diaminobenzoic acid
[H.sub.3]P[O.sub.4]   Phosphoric acid
HT-PEFC               High-temperature polymer electrolyte fuel cell
IPA                   Isophthalic acid
PEFC                  Polymer electrolyte fuel cell
PFSE                  Perfluorosulfonated solid electrolyte
TAB                   Tetra aminobiphenyl
WAXD                  Wide angle X-ray diffraction
CPE                   Constant phase element


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Ratikanta Nayak, (1) Tapobrata Dey, (1) Prakash C. Ghosh, (1) Arup R. Bhattacharyya (2)

(1) Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India

(2) Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India

Correspondence to: P.C. Ghosh; e-mail: pcghosh@iitb.ac.in or A.R. Bhattacharyya; e-mail: arupranjan@iitb.ac.in

DOI 10.1002/pen.24370

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Hardness and relative viscosity for
different concentration of ABPBI membrane.

Molar               Relative
concentration       viscosity      Hardness
(mol/L)          ([[eta].sub.r])     GPa)         Membrane

8.5                    705          0.044     Partially formed
6.8                    136          0.061     Partially formed
5.6                    125          0.074          Formed
4.8                    60           0.069          formed
4.2                    33           0.051          formed

TABLE 2. Doping level for various doped membranes.

Sample                              Doping (volume of      Doping
                                 [H.sub.3]P[O.sub.4], %)   level

ABPBI 20% [H.sub.3]P[O.sub.4]              20               0.25
ABPBI 40% [H.sub.3]P[O.sub.4]              40               0.49
ABPBI 60% [H.sub.3]P[O.sub.4]              60               1.65
ABPBI 80% [H.sub.3]P[O.sub.4]              80               4.18

TABLE 3. Summary of the elemental analysis performed for
[H.sub.3]P[O.sub.4]-doped ABPBI membranes.

                              0%          20%          40%
                           H.sub.3]     H.sub.3]     H.sub.3]
Component (wt%)           P[O.sub.4]   P[O.sub.4]   P[O.sub.4]

Nitrogen                    12.05        11.15         7.35
Carbon                      40.15        33.53        23.23
Hydrogen                     3.35         3.09         3.20
Sulfur                       8.63          0            0
Total                       64.20        47.78        33.79
Residue                     35.82          0            0
  C[H.sub.3S[O.sub.3]H
Residue                       0          52.22        66.20
  [H.sub.3]P[O.sub.4]

                             60%          80%
                           H.sub.3]     H.sub.3]
Component (wt%)           P[O.sub.4]   P[O.sub.4]

Nitrogen                     5.56         1.52
Carbon                      17.26         8.13
Hydrogen                     3.52         3.72
Sulfur                        0            0
Total                       26.34        13.37
Residue                       0            0
  C[H.sub.3S[O.sub.3]H
Residue                     73.65        86.63
  [H.sub.3]P[O.sub.4]

TABLE 4. Young's modulus and hardness of undoped and
doped ABPBI membrane.

Sample           Young's modulus (GPa)      Hardness (GPa)

ABPBI 0%          2.46 [+ or -] 0.34     0.147 [+ or -] 0.03
ABPBI 20%         1.85 [+ or -] 0.35     0.089 [+ or -] 0.028
ABPBI 40%         1.31 [+ or -] 0.16     0.096 [+ or -] 0.011
ABPBI 60%         0.92 [+ or -] 0.13     0.067 [+ or -] 0.006

TABLE 5. A comparison of the present work with the existing literature.

Parameters        Present work

Viscosity         0.52 Pa s
Doping            ABPBI. 4.18
                  [H.sub.3]P[O.sub.4]
Topography        Average roughness = 832.2 nm
Mechanical        Hardness:0.067 [+ or -] 0.006 GPa
  properties        Modulus:0.92 [+ or -] 0.13 GPa
XRD peak          27[degrees]
Model             Parallel combination of resistance
  equivalent        and constant phase element, then
  circuit           series with Warburg element
Proton            2.2 X [10.sup.-2] at 200[degrees]C
  conductivity
  (S/cm)

                                     Previous work

                                            J.A. Asensio
Parameters           J.A. Asensio           et al. [14]
                      et al. [7]        Direct acid casting

Viscosity              2.4 dL/g             2.3-2.4 dL/g
Doping                ABPBI. 3.0             ABPBI. 3.0
                  [H.sub.3]P[O.sub.4]   [H.sub.3]P[O.sub.4]
Topography                --                     --
Mechanical                --                     --
  properties
XRD peak              26[degrees]           25[degrees]
Model                     --                     --
  equivalent
  circuit
Proton             6.2 X [10.sup.-2]    1.5 X [10.sup.-2] at
  conductivity     at 150[degrees]C        180[degrees]C
  (S/cm)              and 30% RH

                                  Previous work

                      J.A. Asensio           Haitao Zheng
Parameters             et al. [18]           et al. [19]
                    Sulfonated ABPBI         Porous ABPBI

Viscosity               2.4 dL/g                  --
Doping                 SABPBL 4.6                 --
                   [H.sub.3]P[O.sub.4]
Topography                 --                     --
Mechanical                 --                 Stress at
  properties                                break 0.6 MPa
XRD peak                   --                     --
Model                      --                     --
  equivalent
  circuit
Proton              3.5 X [10.sup.-2]     2.23 X [10.sup.-2]
  conductivity    S/cm at 185[degrees]C     180[degrees]C
  (S/cm)


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Author:Nayak, Ratikanta; Dey, Tapobrata; Ghosh, Prakash C.; Bhattacharyya, Arup R.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Dec 1, 2016
Words:5652
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