FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Polybenzimidazole

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Membranes: Polybenzimidazole

Membranes: Polybenzimidazole P Mustarelli, E Quartarone, and A Magistris, University of Pavia, Pavia, Italy & 2009 Elsevier B.V. All rights reserved. ...

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Membranes: Polybenzimidazole P Mustarelli, E Quartarone, and A Magistris, University of Pavia, Pavia, Italy & 2009 Elsevier B.V. All rights reserved.

Introduction The current research on proton-exchange membrane fuel cells (PEMFCs) is moving toward the optimization of a device working above 100 1C and at low humidity levels. Such operative conditions offer several advantages, such as a better catalyst stability toward the fuel impurities, a faster electrode kinetics, and chiefly a simplified water management design, which is particularly important in the case of stationary applications. The optimization of high-temperature PEMFCs does require the development of new membranes, as an alternative to Nafion, where the proton transport is not assisted by water molecules. Many systems are proposed in this regard, working on a wide range of ionomers such as modified perfluoropolymers, ormolytes, and blends of acid and base aromatic polymers. Among the wide variety of polymer membranes, aciddoped polybenzimidazoles (PBIs) are likely the most appealing ones because of high proton conductivity (s) with no or low humidification, promising fuel cell performances, and low methanol permeability. The low methanol permeability of PBIs makes it possible to use them also in direct methanol fuel cells (DMFCs). Hightemperature, PBI-based membrane–electrode assemblies (MEAs) (Celtecs) for PEMFCs have been recently developed by BASF-PEMEAS. At present, PBI-based systems show two relevant technological limitations that affect their use in PEMFC applications: (1) worsening of the mechanical strength of the polymer because of the high acid doping levels; and (2) possible leaching of the free acid in the presence of liquid water, which causes a serious drop in conductivity during fuel cell tests. Although the first problem may be overcome by stiffening the membrane with inorganic fillers or by means of crosslinking processes, the second one is still an open question. As a result, the use of PBI-based membranes in a fuel cell is now limited at temperatures higher than 150 1C (where the proton conductivity is high enough even after acid leaching) and no humidification, conditions that are actually far from the expected operative range of both automotive and stationary applications. Recently, new strategies for controlling acid leaching from PBI-based membranes in fuel cells were proposed. The basic idea is to increase the acid retention capability of the membrane by increasing the polymer basicity. Therefore, new PBI structures have been developed by (1) modulating the number of nitrogen atoms and their

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interspacing along the polymer backbone and (2) making new composites based on properly modified inorganic fillers.

Polymer Synthesis and Membrane Preparation Polybenzimidazoles are heterocyclic, basic, and thermoplastic polymers characterized by at least one aromatic unit, which confers high mechanical and thermal stability (Tg4400 1C; Tg: glass transition temperature). The basic units of this class of polymers are 2,5-polybenzimidazole and poly 2,20 -m-(phenylene)-5,50 -bibenzimidazole (in the following labeled as ABPBI and PBI_4N, respectively; see Scheme 1(a) and 1(b)), which are the most studied for applications in fuel cells. These macromolecules were synthesized for the first time in 1961 by Vogel and Marvel. The synthetic procedure is based on a thermal polycondensation reaction between amino groups and carboxylic units in polyphosphoric acid (PPA) as the polymerization agent. In the particular case of ABPBI, the process is an autocondensation of a diaminobenzoic acid, whereas the PBI_4N is formed through the reaction of a diaminobenzidine with isophtalic acid (see Scheme 1(a) and 1(b)), respectively). However, by changing the starting monomers, it is relatively easy to obtain many PBI-based structures with the possibility to modulate physico-chemical properties such as basicity, molecular weight, and solubility. Table 1 lists a series of new polymers developed recently and tested in proton exchange membranes for fuel cells. Generally speaking, the PBI syntheses are quite simple to perform. The average molecular weight of the PBIbased polymers cannot be determined by the standard gel permeation chromatography, because they are completely insoluble in a large number of solvents. It is commonly calculated by means of inherent viscosity (Z) measurements on polymer solutions in sulfuric acid through the Mark–Houwink–Sakurada equation: ½Z ¼ K 0 ðMWÞa

where MW is the polymer average molecular weight, which is related to the polymerization degree (DP) through the relationship DP ¼ MWpol/MWmon, where MWpol is the polymer molecular weight, and MWmon is the monomer molecular weight; K 0 and a are the Mark– Houwink constants whose values are 1.94  104 and

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

NH2 HOOC

NH2

PPA

H N

N2

N

735

n

(a)

COOH H2N

NH2

PPA

H N

H N

+ COOH H2N

NH2

N2

N

N

n

(b)

Scheme 1 Thermal polymerization of 2,5-polybenzimidazole (a) and poly 2,20 -m-(phenylene)-5,50 -bibenzimidazole (b). PPA, polyphosphoric acid.

Table 1

polybenzimidazole (PBI)-based polymers Polymer H N

Monomer A NH2

ABPBI

n*

N PBI_4N

H N

N

N H N

H N

* PBI_5N_2,6

*

PBI_5N_2,5

*

H N

N

N H N

N H N

N H N N

N PBI_6N_bipy

PBI_6N_pyraz

*

H N

N

N n*

N H N

*

* n

n*

H N N

H2N

n*

H N

H N

H N N

n

N

n*

n* N NH



NH2

COOH

NH2

H2N

N

N

* PBI_4N_b

HOOC

H N

PBI_4N_a

Monomer B

NH2 H2N

NH2

H2N

NH2

COOH COOH HOOC

H 2N

NH2

H2N

NH2

H2N

NH2

H2N

NH2

HOOC

H2N

NH2

HOOC

H2N

NH2

H2N

NH2

H2N

NH2

H 2N

NH2

COOH HOOC

N

COOH

COOH

N

COOH

HOOC N

N COOH

HOOC

N

NH2

N H ABPBI, poly [2,5-benzimidazole]; PBI_4N, poly [2,20 -(m-phenylene)-5,50 -bibenzimidazole; PBI_4N_a, poly{2,6-(2,6-naphtyliden)-1,7dihydrobenzo[1,2-d;4,5-d0 ]diimidazole}; PBI_4N_b, poly 2,20 -(2,6-naphtyliden)-5,50 -bibenzimidazole; PBI_5N_m, poly-2,20 -(2,6-pyridine)-5,50 bibenzimidazole; PBI_5N_p, poly-2,20 -(2,5-pyridine) 5,50 -bibenzimidazole; PBI_6N_bipy, poly-2,20 -(2,20 -bipyridine-5,50 )-5,50 -bibenzimidazole); PBI_6N_pyraz, poly-2,20 -(3,5-pyrazole)-5,50 -bibenzimidazole. N

N

0.791, respectively, in the case of PBI, and 8.7  103 and 1.10 for ABPBI. These values can be taken as rough references for other polymers whose constants are not reported in literature. Table 2 summarizes the inherent viscosity and the DP for a large class of PBIs obtained in our laboratory. The polymerization time and temperature were the same for all the polymers. From the analysis of the data reported in the table, the following points can be stressed: (1) an increase in the monomer

H2N

molecular weight reduces the condensation rate; and (2) the enhanced basicity provided by the presence of extra N atoms in the PBI backbone (see, for example, PBI_5N) determines an increase of the DP for equal condensation times, probably because of the improved solubility of the monomer in PPA. The inherent viscosity and, consequently, the DP are important parameters to be considered during the membrane preparation. In fact, optimal Z values should

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

Table 2 Inherent viscosity (Z), monomer molecular weight (MWmon), polymerization degree (DP), polymer molecular weight (MWpol), and doping level before (n) and after the elution tests (nr) of some polybenzimidazole (PBI)-based membranes Polymer

Z (dL g1)

MWmon

DP

MWpol

n

nr

ABPBI PBI_4N PBI_4N_a PBI_4N_b PBI_5N_2,5 PBI_5N_2,6 PBI_6N-bipy PBI_6N_pyraz

3.36 0.54 0.41 0.32 1.7 0.5 1.0 1.6

116 308 282 358 309 309 386 298

225 43 34 27 300 64 123 289

2 6100 2 1887 1 5469 1 1320 9 2840 1 9864 4 7574 8 6012

1.1 4.3 6.0 10.4 6.0 9.5 5.7 8.0

– 2.0 – – 2.2 4.5 2.6 3.0

Doping Procedures and Acid Uptake Polybenzimidazole is a basic and hydrophilic polymer (pKaB4.5; Ka: acid dissociation constant) that may be doped by strong acids such as phosphoric acid. The doping level, N, which is generally expressed in terms of molecules of acid per monomer unit, is a key parameter for the proton transport. The higher the acid content of the membrane, the higher its proton conductivity. In the case of poly 2,20 -m-(phenylene)-5,50 -bibenzimidazole, a doping level up to 6 is easily obtained at room temperature, after at least 50 h of membrane immersion in acid aqueous solutions (e.g., 75 wt% in the case of phosphoric acid). The doping mechanism has been widely discussed by Bouchet and Li, by means of spectroscopic measurements and calculations of the dissociation/protonation constants. PBI_4N contains two nitrogen-based sites with different basicity (–N– and –NH–). The two more basic nitrogen sites of the monomer unit are firstly protonated by the acid and a doping level, n, of 2 is reached (the socalled bonded acid). In the case of doping levels higher than 2, the excess of acid is in the nondissociated form, in equilibrium with the ionic species. In terms of

conductivity and transport, the surplus of phosphoric acid shows a behavior similar to that of a pure concentrated acid solution. The monomer structure plays an important role in the acid doping and, consequently, in the proton transport. Table 2 reports the acid doping levels calculated for some PBI-based membranes. Two main points can be stressed: (1) both the increase in the monomer molecular weight and the introduction of additional basic nitrogen groups in the standard PBI backbone further improve the affinity with the acid, thereby allowing an acid retention higher than that observed in the case of PBI_4N; and (2) similar effects seem to take place by increasing the nitrogen group interspacing in the polymer backbone. The acid doping is affected even by the presence of properly functionalized fillers. Figure 1 shows the phosphoric acid uptake versus the content of silica functionalized by imidazole-based units. Free-standing, mechanically strong membranes could be prepared with up to 50 wt% of filler. This is likely possible because of the chemical affinity of the filler with the polymer. The role of the filler is twofold: On the one hand, it increases the retention capability of the membrane as prepared

100 As-doped 80 H3PO4 uptake (wt%)

be high enough to obtain films with satisfying handiness and processability, without compromising the polymer solubility. Good PBI membranes may be prepared starting from polymers with Z exceeding 0.5 dL g1. In general, PBI films are obtained by means of the standard casting of polymer solutions onto Petri dishes. Polybenzimidazoles may be dissolved in organic acids such as trifluoracetic and methanesulfonic acid, in bases such as dimethylacetamide, or in ethanol/NaOH mixtures. However, in some cases, the nature of the starting monomer may remarkably affect the solubility of the polymer in the solvents commonly used, for example, PBI_4N. An alternative film forming procedure was recently proposed by Benicewicz and colleagues, which reported the direct casting of the PPA polymerization solution with a consequent in situ doping process (see the next section).

60

40

20 After leaching 0

0

10

20

30

40

50

Functionalized SiO2 (wt%)

Figure 1 Acid uptake of the composite membranes vs the filler content, both for the as-prepared samples and after leaching. Proton conductivity at 80 1C and 50% RH versus n ¼ [H3PO4]/ [monomer unit].

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

and, on the other hand, it allows the membrane to retain a larger quantity of acid (up to 3 times more) in the presence of liquid water.

Proton Conductivity and Leaching The proton conductivity of acid-doped PBI membranes is strongly dependent on temperature, relative humidity (RH), and acid doping level, with the last parameter, in turn, dependent on the molar concentration of the doping solution as reported in the previous section. In contrast to what is generally observed for Nafion, which is highly conductive (s460 mS cm1) only at high RH (480%) and temperature below 100 1C, PBI membranes easily reach high conductivity values (s420 mS cm1) at low–medium humidification up to 200 1C. As an example, Figure 2 shows the behavior of conductivity versus acid concentration at 80 1C and 50% RH. This different behavior is related to the fact that in the case of Nafion, the proton migration occurs via vehicular mechanism through the water molecules. On the contrary, different mechanisms of proton transport have been proposed in the literature for PBI-based systems: 1. hopping along the nitrogen sites of the PBI chains. This term is relevant only for non-doped PBI; 2. hopping from the N–H sites to the phosphoric acid anions. This contribution is thought to be relevant for no2, and leads to conductivity values as high as 102 O1 cm1 at 200 1C; 3. hopping along the phosphoric acid anions (n42). This term is associated to the presence of free acid and can contribute to an increase in conductivity by several orders of magnitude (see the following);

100 10−1

80 °C (S cm−1)

10−2 10−3 10−4 10−5 10−6 10−7 10−8 10−9 10−10 0

2

4

6

8

10

n

Figure 2 Plot of proton conductivity, s, at 80 1C and 50% RH vs the doping level, n before the acid leaching. The dotted and dashed lines indicate the maximum degree of protonation of standard PBI (PBI_4N) and the conductivity of the H3PO4 solution, respectively.

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4. hopping via water molecules. This term, concurrent with the previous one, is more significant at high temperature. Increases from 0.03 to 0.07 O1 cm1 have been reported at 200 1C from 0.15% to 0.5% RH. The prevalent hopping-like nature of the transport mechanism is demonstrated by the Arrhenius behavior of conductivity. The activation energies (Ea) for n42 are in the range 15–25 kJ mol1, i.e., very near to that of concentrated phosphoric acid aqueous solutions. As already stressed, the major drawback of PBI membranes is related to the leaching of free phosphoric acid during the fuel cell operation. By means of elution tests, it has been demonstrated that, in the case of the standard PBI_4N, the amount of acid retained gives a residual doping nrD2, leading to a 4 orders of magnitude decrease of conductivity with respect to the as-prepared samples. Both n and nr are a complex function of the number of nitrogen atoms, N, in the monomer, and of their interspacing on the polymer backbone. Table 2 reports the values of n and nr for several membranes obtained from different monomers with 4pNp6. The highest acid affinity of the as-prepared membrane is provided by the PBI_5N_2,5 membrane, where n ¼ 9.5 is reached. Following the elution test, promising results are obtained in the case of PBI_5N_2,6, which shows a residual doping level of 4.5. In this case, a conductivity of 2  104 O1 cm1 has been obtained at 120 1C and 50% RH.

Blend and Composite Membranes Blends of PBI with acidic polymers such as sulfonated polysulfone (SPSU), polyethersulfone (SPSF), sulfonated polyphenylene oxide (SPPO), and sulfonated polyetheretherketone (SPEEK) have been reported in the literature. Here, the aims were to improve the mechanical strength and flexibility of pure PBI chiefly at high temperature, and to obtain a lower gas permeability without losing in proton conductivity. As a matter of fact, the tensile strength of 50–50 wt% PBI–SPSF blends at 150 1C is up to a factor 2–3 better than that of pure PBI in the doping range 5ono15. Composite membranes may be prepared with both organic and inorganic fillers, which, in turn, can be considered as active or passive from the point of view of their effects on proton conductivity. Again, the rationale for preparing composite systems is manifold: from improving mechanical properties such as tensile strength, stiffness, or filmability, to reducing fuel crossover, to increasing thermal, chemical, and electrochemical stability of the MEAs. The use of a perfluorocarbon sulfonic acid ionomer as the coupling agent allowed PBI–polytetrafluoroethylene (PTFE) composite membranes as thin as B20 mm being prepared, with better electrical and mechanical properties compared to B100-mm-thick PBI

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

membranes; H3PO4/Nafion/PBI composite membranes showed a better cell durability without humidification than the corresponding H3PO4/PBI composite membranes at 150 1C. As far as PBI composites with inorganic materials are concerned, high surface area silicon dioxide and layered silicates such as montmorillonite (MTM) have been used. MTM/PBI membranes showed a reduced thermal expansion coefficient, better thermo-oxidative stability, and lower methanol permeability. Generally speaking, the proton conductivity slightly decreases by increasing the amount of the filler. In the case of nanoscale silicon dioxide, in contrast, a nonlinear behavior is found (see Figure 3), with a maximum at near 5–10 wt% of filler. Although a conclusive physico-chemical explanation is still lacking, this behavior is often observed in lithium polymer-based composite electrolytes. The group of active fillers includes inorganic or organic– inorganic proton conductors. The former includes hydrated zirconium phosphate (Zr(HPO4)2  nH2O) and superacids such as phosphotungstic acid (H3PW12O40  nH2O (PWA)) and silicotungstic acid (H4SiW12O40  nH2O (SiWA)). The active nature of these fillers leads to the possibility of adding up to 30 wt% of filler to PBI without lowering (or even increasing) the proton conductivity. This allows

80 °C (S cm−1)

10−1

101

10−2

10−3

100

0

10 20 30 HiSilTM (wt%)

As-doped

120 °C (S cm−1)

10−1 10−2

a better modulation of the mechanical properties of the membranes. The most interesting application of active fillers is likely to increase the amount of the bonded acid in the PBI membrane. Organic–inorganic fillers based on silica functionalized with imidazole groups (see Scheme 2) have been prepared by sol–gel. Figure 3 shows the proton conductivity behavior of the samples as prepared and after the elution test (washing in water at 80 1C till the complete removal of the free acid, as determined by inductively coupled plasma (ICP) analysis). In the case of the as-prepared membranes, the conductivity is nearly independent of the filler amount. In the case of the washed membranes, in contrast, the addition of the filler results in dramatic increases in conductivity by more than a factor of 1000 compared to the pure membrane. Moreover, an increase of more than 100 times is already obtained with 2 wt% of filler, which means that it is very efficient in promoting the acid retention. As a matter of fact, the amount of acid retained increases from B18 to B37 wt%.

Membrane Electrode Assembly and Functional Tests In the case of PBI-based fuel cells, the electrodes are normally prepared by spraying on the gas diffusion layer (GDL) a mixture containing the catalyst, PBI as the binder, and N-methyl pyrrolidone as the solvent. The platinum loading is in the range of 0.1–2 mg cm2. To date, the attempts to spray the catalyst directly onto the PBI membrane have not given satisfactory results. The MEAs are then prepared by hot pressing or even by simple mechanical torque. Because of the high proton conductivity at low RH, PBI-based MEAs can be operated without gas humidification, which greatly simplifies the construction of the fuel cell when compared with those based on perfluorosulfonic ionomer-based membranes. Moreover, because of the high thermal stability of the polymer, the

10−3 After leaching

10−4 10−5

O

−6

10





O

O

Si O

Si O

n

O 0

10

20 30 40 Functionalized SiO2 (wt%)

50

Figure 3 Proton conductivity, s, at 120 1C and 50% RH of a composite PBI_4N–silica/imidazole filler membrane vs the filler content. Filled circles: samples as prepared; open circles: samples after the elution test. Inset: proton conductivity at 80 1C and 50% RH vs the content of nanoscale silica (HiSilTM, Degussa) of PBI_4N membranes as prepared.

N ∅ = OH; Si−O N

Scheme 2 Chemical structure of the imidazole-containing silica.

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

PBI-based fuel cells have been tested up to 200 1C, where power densities as high as 1 W cm2 have been reported with H2/O2 feeding under a pressure of 3/3 bar and no gas humidification. The membrane performance is quite comparable to that observed in the case of Nafion-based single cells at lower temperatures and higher RHs (power density 40.6 W cm2 depending on the system pressure). The major problems affecting the lifetime of PBIbased fuel cells are probably (1) the loss of phosphoric acid, (2) polymer oxidative degradation, and (3) the agglomeration of the catalyst during operation. Lifetimes of B1000 and B5000 h have been reported at 200 and 150 1C, respectively. Concerning the poisoning of catalysts, it has been reported that 3% carbon monoxide, in hydrogen can be tolerated at 200 1C up to a current density of 0.8 A cm2, whereas at 125 1C 0.1% carbon monoxide, can be still tolerated below 0.3 A cm2. These results are very encouraging when compared with the 25 ppm tolerated by Nafion at 80 1C and current densities not exceeding 0.2 A cm2. The poisoning caused by carbon dioxide, is of less extent. As carbon dioxide, is inert toward the catalyst, the poisoning is due to the formation of carbon monoxide, caused by the water–gas shift reaction CO2 þ H2 $CO þ H2 O

As PBI-based fuel cells work under low humidity, the equilibrium is shifted to the right. However, even for a fuel containing 25% carbon dioxide, the carbon monoxide formation is estimated to be below the tolerance threshold, at least above 125 1C. For these reasons, PBIbased fuel cells can also work with reformate hydrogen. Because of the low permeability of the PBI membrane to methanol, which is typically 1 order of magnitude lower than Nafion, PBI-based cells can also be fuelled with direct methanol. The performance reduction is B50% with respect to pure hydrogen.

Conclusions Fuel cells based on PBI membranes are very promising when compared with both Nafion cells and phosphoric acid fuel cells. Their main advantage is the possibility to operate at up to 200 1C practically negligible RH, thereby reducing the complexity of the stack, increasing the carbon monoxide tolerance, allowing the integration of a methanol reformer, and giving a high efficiency of heat utilization. The challenge for the next future is to lower the working temperature to 120 1C in order to allow the use of fuel cells in electric cars. This chiefly requires an improvement in the membrane, where the major issues to

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be solved are (1) the improvement of the mechanical properties in order to fabricate cells with larger active areas, (2) the improvement of thermal and chemical stability, and (3) the reduction of acid leaching during operation.

Nomenclature Symbols and Units Ea K0 and a Ka MWmon MWpol nr N Tg g r

activation energy, kJ mol  1 Mark-Houwink constants acid dissociation constant monomer molecular weight polymer molecular weight doping level after elution tests number of nitrogen atoms glass transition temperature, 1C inherent viscosity, dL g  1 proton conductivity, ohm  1 cm  1

Abbreviations and Acronyms DMFC DP GDL ICP MEA MTM PBI PEMFC PPA PTFE PWA RH SiWA SPEEK SPSF SPPO SPSU

direct methanol fuel cell polymerization degree gas diffusion layer inductively coupled plasma membrane electrode assembly montmorillonite polybenzimidazole proton-exchange membrane fuel cell polyphosphoric acid polytetrafluoroethylene phosphotungstic acid relative humidity silicotungstic acid sulfonated polyetheretherketone polyethersulfone sulfonated polyphenylene oxide sulfonated polysulfone

See also: Fuel Cells – Proton-Exchange Membrane Fuel Cells: Membranes: Elevated Temperature; Cells; Membranes; Overview Performance and Operational Conditions; Membrane–Electrode Assemblies.

Further Reading Asensio JA and Gomez-Romero PG (2005) Recent developments on proton conducting poly(2,5-benzimidazole) (ABPBI) membranes for high temperature polymer electrolyte membrane fuel cells. Fuel Cells 5: 336--343. Bouchet R and Siebert E (1999) Proton conduction in acid doped polybenzimidazole. Solid State Ionics 118: 287. Carollo A, Quartarone E, Tomasi C, et al. (2006) Developments of new proton conducting membranes based on different

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Membranes: Polybenzimidazole

polybenzimidazole structures for fuel cells applications. Journal of Power Sources 160: 175--180. He R, Li Q, Jensen JO, and Bjerrum NJ (2007) Doping phosphoric acid in polybenzimidazole membranes for high temperature proton exchange membrane fuel cells. Journal of Polymer Science. Part A: Polymer Chemistry 45: 2989--2997. Herring AM (2006) Inorganic-polymer composite membranes for proton exchange membrane fuel cells. Journal of Macromolecular Science. Part C: Polymer Reviews 46: 245--296. Kreuer KD, Paddison SJ, Spohr E, and Schuster M (2004) Transport in proton conductors for fuel-cell applications: Simulations, elementary reactions, and phenomenology. Chemical Reviews 104: 4637--4678 and references cited therein. Li Q, He R, Jensen JO, and Bjerrum NJ (2004) PBI-based polymer membranes for high temperature fuel cells – preparation, characterization and fuel cell demonstration. Fuel Cells 4: 147--159. Lin H-L, Yu TL, Chang W-K, Cheng C-P, Hu C-R, and Jung G-B (2007) Preparation of a low proton resistance PBI/PTFE composite membrane. Journal of Power Sources 164: 481--487.

Ma Y-L, Wainright JS, Litt MH, and Savinell RF (2004) Conductivity of PBI membranes for high-temperature polymer electrolyte fuel cells. Journal of the Electrochemical Society 151: A8--A16. Mustarelli P, Carollo A, Grandi S, et al. (2007) Composite protonconducting membranes for PEMFCs. Fuel Cells 7: 441--446. Mustarelli P, Quartarone E, Grandi S, Carollo A, and Magistris A (2008) Polybenzimidazole-based membranes as a real alternative to Nafion for fuel cells operating at low temperature. Advanced Materials 20: 1339--1343. Quartarone E, Carollo A, Mustarelli P, et al. (2007) New polybenzimidazole-based membranes for fuel cells. Material Research Society Symposia Proceedings 972: 125--130. Quartarone E, Mustarelli P, and Magistris A (1998) PEO-based composite polymer electrolytes. Solid State Ionics 110: 1--14. Wang JT, Savinell RF, Wainright J, Litt M, and Yu H (1996) A H-2/O-2 fuel cell using acid doped polybenzimidazole as polymer electrolyte. Electrochimica Acta 41: 193--197. Zhai YF, Zhang HM, Zhang Y, and Xing DM (2007) A novel H3PO4/ Nafion/PBI composite membrane for enhanced durability of high temperature PEM fuel cells. Journal of Power Sources 169: 259--264.