Solid polymer electrolytes

This invention provides a solid polymer electrolyte which is low in water absorption, from which no dopant runs out even in pressing, and which is excellent in stability in the presence of water or methanol, proton conductivity and methanol barrier properties, in which an imidazole ring-containing polymer such as a polybenzimidazole compound is doped with an acid in which at least one hydrogen atom of an inorganic acid such as phosphoric acid is substituted by a functional group having a phenyl group by blending the imidazole ring-containing polymer with the acid in a solution using a solvent such as trifluoroacetic acid, preferably at a rate of 1 to 10 molecules of the acid per repeating structure unit of a molecular chain of the imidazole ring-containing polymer, the solid polymer electrolyte.

FIELD OF THE INVENTION 
The present invention relates to a solid polymer electrolyte, and more 
specifically, to an acid-doped solid polymer electrolyte used in fuel 
cells. 
BACKGROUND OF THE INVENTION 
In recent years, fuel cells have occupied an important position as novel 
type clean energy sources. And solid polymer electrolytes comprising solid 
polymer electrolyte membranes having high proton conductivity have been 
developed for retaining the characteristics of their high output capacity 
and high energy density, and for capability in miniaturization and weight 
saving. As the solid polymer electrolyte membranes, hydrated membranes 
such as a sulfonated polyfluoroolefin (trade name: Nafion, manufactured by 
E. I. du Pont de Nemours and Company) and acid-doped polybenzimidazole 
(PBI) membranes are generally known. When methanol is used as a fuel for 
operating fuel cells, the solid polymer electrolytes are required to have 
barrier properties to fuel methanol (low methanol permeability) However, 
the hydrated membranes such as Nafion have a limitation to methanol 
barrier properties due to occurrence of hydrated proton hopping. On the 
other hand, the acid-doped PBI membranes are homogeneous membranes. And it 
is considered that, in the acid-doped PBI membranes, proton hopping occurs 
through acids forming complexes with basic N--H groups in a base polymer, 
PBI. Accordingly, in the acid-doped PBI membranes, the proton hopping does 
not occur by the movement of water. The acid-doped PBI membranes have 
therefore been expected as the solid polymer electrolytes excellent in 
methanol barrier properties. 
As acid-doped PBI membranes, for example, phosphoric acid-doped PBI 
membranes were prepared by immersing PBI membranes in phosphoric acid 
solutions [J. S. Wainright etal., J. Electrochem. Soc., Vol. 142, No. 7, 
p122, July (1995)]. Acid-doped PBI membranes were obtained by allowing 
acids to be adsorbed by PBI membranes in aqueous solutions of phosphoric 
acid or sulfuric acid (U.S. Pat. No. 5,525,436). And acid-impregnated PBI 
and acid-impregnated alkyl or arylsulfonated PBI membranes, or alkyl or 
arylsulfonated PBI membranes (Japanese Unexamined Patent Publication No. 
9-73908) are proposed, and obtained phosphoric acid-doped PBI membranes 
show superior characteristics. 
However, studies of these phosphoric acid-doped PBI membranes have revealed 
the following problems. 
PBI has slight water absorbing capability, however phosphoric acid has 
extremely high affinity for water. Therefore, a phosphoric acid-doped PBI 
is liable to cause wrinkles by water absorption. Accordingly, when an MEA 
(a membrane electrode assembly in which a membrane and electrodes are 
assembled) is fabricated using the phosphoric acid-doped (wrinkled) PBI 
membrane and a stack is assembled, followed by operation of it, the use of 
the phosphoric acid-doped PBI membrane causes the leakage of gas and 
liquid. Further, there is a limitation of thin film formation of the 
phosphoric acid-doped PBI membranes. 
In particular, when PBI doped with phosphoric acid at a rate of two or more 
molecules of per basic imidazole ring constituting PBI (one or more 
molecules of phosphoric acid per N--H group) is hot pressed in preparing 
the MEA, free phosphoric acid, not participating in bonding, seeps into an 
electrode layer or a diffusion layer. In a hydrogen fuel cell, therefore, 
phosphoric acid which has seeped out also acts as an ionomer. However, 
when the amount of the seeped phosphoric acid is excessive, diffusion of a 
reaction gas into a catalytic metal is inhibited. 
The phosphoric acid that has seeped into the electrode by the 
above-mentioned hot pressing is not fixed, so that it is apt to seep out 
of the electrode when water in a gas reaction cell is condensed by 
interruption of the operation. Further, when the PBI membrane is immersed 
in condensed water in the gas reaction cell, or when the PBI membrane is 
immersed in water and methanol in a liquid supply direct methanol fuel 
cell (DMFC), phosphoric acid fixed into the PBI membrane is also easily 
dedoped to run out, resulting in a reduction of ion conductivity of the 
PBI membrane. 
Inorganic phosphoric acid is a strong acid and has extremely high methanol 
solubility. Therefore, the PBI membranes are conventionally doped with 
phosphoric acid by immersing the PBI membranes having the basicity (N--H 
groups) in high concentrated methanol solutions of inorganic phosphoric 
acid. On the other hand, compounds other than inorganic phosphoric acid 
(for example, organic phosphoric acid compounds) are low in solubility. 
Accordingly, high concentrated solutions thereof can not be prepared, the 
acid dissociation degree of the prepared solutions is also low, and the 
molecular size of dopants is also large. It is therefore difficult to 
conduct doping by the above-mentioned immersing method. 
As described above, it has been difficult to use the phosphoric acid-doped 
PBI membranes as the solid polymer electrolyte membranes of the liquid 
supply DMFCs. 
SUMMARY OF THE INVENTION 
The present invention has been made against a background of the current 
problems of the phosphoric acid-doped PBI membranes as described above, 
and an object of the invention is to provide a solid polymer electrolyte 
having no wrinkle occurred by water absorption while film formation by 
virtue of its low water absorption. From the acid-doped PBI membranes of 
the present invention, no dopant runs out even when hot pressing is 
carried out. And the acid-doped PBI membranes of the present invention are 
excellent in stability in the presence of water or methanol, and excellent 
in proton conductivity and methanol barrier properties. 
The present inventors have conducted intensive investigation for attaining 
the above-mentioned object. As a result, the present inventors have 
discovered that a solid polymer electrolyte membrane obtained by using an 
acid enhanced in hydrophobicity by introducing an organic group, said acid 
comprising a functional group having a phenyl group substituted for a 
hydrogen atom of an inorganic acid, is used as a dopant instead of 
inorganic phosphoric acid, and mixing an imidazole ring-containing polymer 
with the above-mentioned dopant by a solution blend method, followed by 
film formation is excellent in dope stability, proton conductivity and 
methanol barrier properties, thus completing the present invention.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a solid polymer electrolyte in which an 
imidazole ring-containing polymer is doped with an acid in which at least 
one hydrogen atom of an inorganic acid is substituted by a functional 
group having a phenyl group. 
In this case, the imidazole ring-containing polymer is preferably a 
polybenzimidazole compound. 
Further, the above-mentioned polybenzimidazole compound is preferably 
poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole]. 
Furthermore, the above-mentioned polybenzimidazole compound is preferably 
poly[2,5-benzimidazole]. 
In addition, the above-mentioned inorganic acid is preferably phosphoric 
acid. 
The amount of the above-mentioned acid, with which the imidazole 
ring-containing polymer is doped, is preferably from 1 to 10 molecules per 
repeating structure unit of a molecular chain of the imidazole 
ring-containing polymer. 
The above-mentioned solid polymer electrolyte is preferably produced by a 
solution blend method. 
A solvent used in the above-mentioned solution blend method is preferably 
trifluoroacetic acid. 
The imidazole ring-containing polymers used in the present invention may be 
any polymers which have repeating structure units comprising imidazole 
rings which function as receptor groups for protons against acidic 
dopants. The imidazole ring-containing polymers show sufficient proton 
conductivity by means of acid doping. And acid-doped imidazole ring 
containing polymers are highly stable within the operating temperature 
range of fuel cells. Usually, the polymers having a molecular weight of 
1,000 to 100,000 are used. In the case of the molecular weight of the 
polymers less than 1,000, the physical properties of the resulting 
electrolytic base materials are deteriorated. On the other hand, in the 
case of the molecular weight of the polymers exceeding 100,000, solubility 
of the polymers in solvents decreases unfavorably, therefor, it becomes 
difficult to form membranes. 
Such imidazole ring-containing polymers include, for example, 
polybenzimidazole compounds and polybenzbisimidazole compounds. 
Usually, the polybenzimidazole compounds can be produced from aromatic 
dibasic acids and aromatic tetraamines. Examples thereof include 
poly[2,2'-(m-phenylene)-5,5'-bi-benzimidazole], 
poly[2,2'-(pyridylene-3",5")-5,5'-bibenz-imidazole], 
poly[2,2'-(furylene-2",5")-5,5'-bibenzimidazole], 
poly[2,2'-(naphthylene-1",6")-5,5'-bibenzimidazole], 
poly[2,2'-(biphenylene-4",4")-5,5'-bibenzimidazole], 
poly[2,2'-amylene-5,5'-bibenzimidazole], 
poly[2,2'-octamethylene-5,5'-bibenzimidazolel, 
poly[2,6'-(m-phenylene)-diimidazolebenzene], 
poly[2',2'-(m-phenylene)-5,5'-di(bibenzimidazole)ether], 
poly[2',2'-(m-phenylene)-5,5'-di(benzimidazole)sulfide], 
poly[2',2'-(m-phenylene)-5,5'-di(benzimidazole)sulfone], 
poly[2',2'-(m-phenylene)-5,5'-di(benzimidazole)methane], 
poly[2',2"-(m-phenylene)-5,5"-di(benzimidazole)-propane-2,2] and 
poly[2,2'-(m-phenylene)-5',5"-di(benzimidazole)-ethylene-1,2]. Preferred 
examples of the polymers include poly(2,2'-(m-phenylene) 
-5,5'-bibenzimidazole], the structure of which is represented by general 
formula (I). 
##STR1## 
These polybenzimidazole compounds can also be produced by self-condensation 
of at least one kind of aromatic compounds having a pair of amine 
substituent groups at the ortho-positions and a carboxylate ester group 
positioned in an aromatic nucleus. Such aromatic compounds include, for 
example, diamino-carboxylic acids or esters thereof, such as 
3,4-diamino-naphthalene-1-carboxylic acid, 
5,6-diaminonaphthalene-2-carboxylic acid, 
6,7-diaminonaphthalene-1-carboxylic acid, 
6,7-diaminonaphthalene-2-carboxylic acid and 3,4-diamino-benzoic acid. 
Preferred examples of the polybenzimidazole compounds include 
poly[5-(4-phenyleneoxy)benzimidazole] obtained from 
4-phenoxycarbonyl-3',4'-diaminophenyl ether, and 
poly[2,5(6)-benzimidazole] obtained from 3,4-diaminobenzoic acid. The 
structure of poly[2,5-benzimidazole] is represented by general formula 
(II). 
##STR2## 
Further, examples of the polybenzbisimidazole compounds include 
poly[2,6'-(m-phenylene)benzbisimidazolel, 
poly[2,6'-(pyridylene-2",6")benzbisimidazole], 
poly[2,6'-(pyridylene-3",5")benzbisimidazole], 
poly[2,6'-(naphthylene-1",6")benzbisimidazole] and 
poly[2,6'-(naphthylene-2",7")benzbisimidazole. The preferable polymer is 
poly[2,6'-(m-phenylene)benzbisimidazole]. 
In the present invention, the acids used in doping, namely the dopants, are 
acids obtained by substituting hydrogen atoms of inorganic acids by 
functional groups having phenyl groups. The dopants include organic acids 
obtained by substituting hydrogen atoms of sulfuric acid, phosphoric acid, 
phosphorous acid and the like, by functional groups having phenyl groups. 
In particular, organic phosphoric acids can be suitably used as a dopant 
in the present invention. 
Examples of the sulfuric acid compounds in each of which a hydrogen atom of 
sulfuric acid is substituted by a functional group having a phenyl group 
include phenylsulfuric acid. 
Further, the acids in each of which at least one hydrogen atom of 
phosphoric acid is substituted by a functional group having a phenyl 
group, namely the organic phosphoric acid compounds, include 
phenylphosphoric acid derivatives represented by general formula (III), 
and diphenylphosphoric acid derivatives represented by general formula 
(IV) 
##STR3## 
wherein R represents a hydrogen atom, an alkyl group having 1 to 5 carbon 
atoms, a halogen atom or a nitro group. 
##STR4## 
wherein R represents a hydrogen atom, an alkyl group having 1 to 5 carbon 
atoms, a halogen atom or a nitro group. 
Examples of the phenylphosphoric acid derivatives include alkyl-substituted 
phenylphosphoric acids such as phenylphosphoric acid, o-toluylphosphoric 
acid, p-toluyl-phosphoric acid, o-ethylphenylphosphoric acid, 
p-ethyl-phenylphosphoric acid and p-isopropylphenylphosphoric acid; 
halogen-substituted phenylphosphoric acids such as 
o-chlorophenylphosphoric acid, p-chlorophenylphosphoric acid and 
p-bromophenylphosphoric acid; and nitrophenylphosphoric acids such as 
m-nitrophenylphosphoric acid. 
Further, examples of the diphenylphosphoric acid derivatives include 
di(alkyl-substituted phenyl)phosphoric acids such as diphenylphosphoric 
acid, di(o-toluyl)phosphoric acid, di(o-ethylphenyl)phosphoric acid, 
di(p-ethylphenyl)-phosphoric acid and di(p-isopropylphenyl)phosphoric 
acid; di(halogen-substituted phenyl)phosphoric acids such as 
di(o-chlorophenyl)phosphoric acid, di(p-chlorophenyl)phos-phoric acid and 
di(p-bromophenyl)phosphoric acid; and di(nitrophenyl)phosphoric acids such 
as di(m-nitrophenyl)-phosphoric acid. 
Furthermore, the phosphorous acid compounds used in the present invention 
in each of which a hydrogen atom of phosphorous acid is substituted by a 
functional group having a phenyl group include phenylphosphorous acid 
derivatives represented by general formula (V), and examples thereof 
include di(alkyl-substituted phenyl)phosphorous acids such as 
phenylphosphorous acid, diphenylphosphorous acid, di(o-toluyl)phosphorous 
acid, di(p-toluyl)phosphorous acid, di(o-ethylphenyl)phosphorous acid, 
di(p-ethylphenyl)phos-phorous acid and di(p-isopropylphenyl)phosphorous 
acid; di(halogen-substituted phenyl)phosphorous acids such as 
di(o-chlorophenyl)phosphorous acid, di(p-chlorophenyl)phos-phorous acid 
and di(p-bromophenyl)phosphorous acid; and di(nitrophenyl)phosphorous 
acids such as di(m-nitrophenyl)-phosphorous acid. 
##STR5## 
wherein R represents a hydrogen atom, an alkyl group having 1 to 5 carbon 
atoms, a halogen atom or a nitro group. 
Usually, the doping is carried out by the following three methods: 
(1) A method of immersing an imidazole ring-containing polymer film in a 
dopant solution (immersing method); 
(2) A method of coagulating an imidazole ring-containing polymer at an 
interface between a solution of an imidazole ring-containing polymer and a 
dopant solution (interfacial coagulation method); and 
(3) A method of blending an imidazole ring-containing polymer with a dopant 
in a solution (solution blend method). 
For preparing the imidazole ring-containing polymer film used in the 
immersing method described in the above (1), a solution of the imidazole 
ring-containing polymer is first prepared. Various solvents can be used 
for preparing this polymer solution, and examples thereof include 
N,N-di-methylacetamide, N,N-dimethylformamide, N,N-dimethyl sulfoxide and 
N-methyl-2-pyrrolidone. 
In this case, the concentration of the polymer solution is preferably 5% to 
30% by weight. In the case of the concentration less than 5% by weight, it 
becomes difficult to obtain a film having a desired thickness. On the 
other hand, in the case of the concentration of the polymer solution 
exceeding 30% by weight, it results in difficulty in preparing the uniform 
polymer solution. 
Solvents for the dopant solutions include tetrahydrofuran (THF), methanol, 
ethanol, n-hexane and methylene chloride. In this case, the concentration 
of the dopant solution is preferably 50% to 90% by weight. In the case of 
the concentration less than 50% by weight, the amount of the dopant in the 
doped film (the dope amount) is decreased to cause reduced proton 
conductivity of the resulting doped film. On the other hand, in the case 
of the concentration of the dopant solution exceeding 90% by weight, it 
results in dissolution of the polymer film in the dopant solution. 
The solution is prepared at a temperature of room temperature to 
120.degree. C. Usually, in order to dissolve the polymer with a solvent 
homogeneously, the solution is heated below the boiling point of the 
solvent, and cooled to room temperature. Then, the solution is adjusted so 
as to give a solution viscosity of 50 to 4,000 poises, preferably 400 to 
600 poises (at 30.degree. C.). The above-mentioned solution viscosity 
depends on the temperature, the degree of polymerization, and the 
concentration of the polymer solution. In general, however, in the case of 
the solution viscosity less than 50 poises, it is difficult to form a 
film. Whereas, in the case of the solution viscosity exceeding 4,000 
poises, the solution viscosity becomes too high, resulting difficulty to 
prepare a homogeneous film. 
The polymer solution thus obtained can be cast on, for example, a glass 
plate, and the solvent is removed by an ordinary method to prepare a film. 
As solvents for the solutions of the imidazole ring-containing polymers and 
solvents for the dopant solutions used in the interfacial coagulation 
method described in the above (2), the solvents described in the 
above-mentioned immersing method (1) can be used. 
The solvent used in the solution blend method described in the above (3) is 
required to dissolve not only the imidazole ring-containing polymer and 
the dopant, but also the acid-doped polymer produced. For this reason, the 
solvent such as N,N-dimethylacetamide or N-methyl-2-pyrrolidone used as 
the solvent for the imidazole ring-containing polymer can not be used 
because of their very low solubility of the acid-doped polymer produced 
therein. The acid-doped polymer produced is dissolved only in strong acids 
such as concentrated sulfuric acid and methanesulfonic acid. However, it 
is difficult to treat these strong acids after film formation. The 
preferable solvent used in the solution blendmethod (3) is trifluoroacetic 
acid. 
In the solution blend method described in the above (3), the solution is 
prepared at a temperature of room temperature to 200.degree. C., 
preferably from 40.degree. C. to 120.degree. C. 
With respect to the doping of the imidazole ring-containing polymer with 
the organic substituting acids, the present inventors have compared the 
above-described three methods (1) to (3), and studied which method is 
industrially applicable, as described below. 
With respect to the immersing method (1), films of the imidazole 
ring-containing polymer are immersed in dopant solutions each of which has 
a specified concentration, at room temperature for 48 hours, and the 
immersed films are dried under vacuum. The amounts of the dopant in the 
doped film are calculated from changes in weight before and after the 
doping operation. 
With respect to the interfacial coagulation method (2), a solution of the 
imidazole ring-containing polymer in N,N-dimethylacetamide is cast on PTFE 
films, and sunk in solutions of the dopant in tetrahydrofuran, thereby 
coagulating the imidazole ring-containing polymer at an interface of both 
solutions, followed by drying under vacuum. Then, the amounts of the 
dopant in the doped films are calculated from changes in weight before and 
after the doping operation. 
With respect to the solution blend method (3), a powder of the imidazole 
ring-containing polymer is dissolved in a strong acid such as 
trifluoroacetic acid, and then, specified amounts of the dopant are added 
thereto, followed by stirring overnight at room temperature. After the 
preparation, the homogenized solutions are cast on polytrifluoroethylene 
(PTFE) sheets, and the solvent is removed at 40.degree. C. The amounts of 
the dopant in the doped films are calculated from the charged amount 
ratios of the imidazole ring-containing polymer to the dopant. 
Then, for examining the stability of the doped films, the doped films 
obtained by the above-mentioned respective methods (1) to (3) are dried 
under vacuum. Each film is set in a glass filter, and extracted by the 
Soxhlet's extraction method with period of time. The film is taken out 
together with the glass filter, and dried under vacuum. The amount of the 
eliminated dopant is measured by a decrease in weight. 
Of these, results of the study of the immersing method described in the 
above (1) are shown in Table 1, and results of the study of the 
interfacial coagulation method described in the above (2) are shown in 
Table 2. 
That is to say, PBI (poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole]) films 
having a thickness of 30 .mu.m were immersed in methylphosphoric acid/THF 
solutions, methylphosphoric acid/MeOH solutions, phenylphosphonic acid/THF 
solutions, and di (2-ethylhexyl) phosphoric acid/THF solutions, and 
results thereof are shown in Table 1. In the interfacial coagulation 
method (2), a 10 wt. % solution of PBI 
(poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole]) in DMAc was cast on PTFE 
films, which were rapidly sunk in methylphosphoric acid/THF solutions, 
thereby carrying out doping and coagulation at the same time. 
TABLE 1 
______________________________________ 
Immersing Method 
After Film Immersion Treatment 
Amount of 
Amount of 
Organic Organic 
Before Treatment Phos- Phos- 
Dopant Weight Weight 
Change 
phoric phoric 
Concen- 
of Unit Of In Acid Acid 
tration 
Sample Number Sample 
Weight 
(.times. 10.sup.-4) 
(mol/PBI 
(%) (g) of PBI (g) (g) (mol) unit) 
______________________________________ 
Methylphosphoric Acid/THF 
100 0.0440 0.0002 0.1146 
0.0706 
6.30 4.11 
90 0.0457 0.0002 0.1087 
0.0630 
5.62 3.53 
80 0.0570 0.0002 0.1178 
0.0608 
5.43 2.73 
70 0.0450 0.0002 0.0987 
0.0537 
4.79 3.05 
60 0.0488 0.0002 0.0893 
0.0405 
3.62 2.12 
Methylphosphoric Acid/MeOH 
100 0.0448 0.0002 0.1193 
0.0705 
6.29 3.70 
90 0.0443 0.0002 0.1180 
0.0737 
6.58 4.26 
80 0.0486 0.0002 0.1156 
0.0670 
5.98 3.53 
70 0.0484 0.0002 0.1084 
0.0600 
5.36 3.17 
Phenylphosphonic Acid/THF 
60 0.0465 0.0002 0.0580 
0.0115 
0.727 0.45 
50 0.0475 0.0002 0.0555 
0.0080 
0.506 0.31 
40 0.0494 0.0002 0.0548 
0.0054 
0.342 0.20 
Di(2-ethylhexyl)phosphonic Acid/THF 
100 0.0433 0.0002 0.0448 
0.0015 
0.0465 0.03 
90 0.0427 0.0001 0.0439 
0.0012 
0.0372 0.02 
80 0.0405 0.0001 0.0415 
0.0010 
0.0310 0.02 
70 0.0457 0.0002 0.0469 
0.0012 
0.0372 0.02 
______________________________________ 
TABLE 2 
______________________________________ 
Interfacial Coagulation Method 
Amount of 
Amount Of 
Amount Molec- Organic 
Organic 
Dopant 
of Dope ular Change 
Phos- phosphoric 
Concen- 
Solu- Number Amount 
in phoric Acid 
tration 
tion of PBI of PBI 
Weight 
Acid (mol/PBI 
(%) (g) (mol) (g) (g) (mol) unit) 
______________________________________ 
Methylphosphoric Acid/THF 
100 2.53 0.0082 0.45 0.16 0.0015 0.18 
90 4.55 0.0148 0.80 0.71 0.0063 0.43 
80 5.04 0.0163 0.89 1.94 0.0173 1.06 
______________________________________ 
In the immersing method described in (1), methylphosphoric acid is liquid 
at room temperature, and dissolves in MeOH at high concentration, like 
inorganic phosphoric acid. However, when methylphosphoric acid is used as 
a dopant, the doping rate at a concentration of 70% or more is inferior to 
phosphoric acid (see Table 1). Therefore, a methylphosphoric acid doped 
product on the level of 5 molecules per PBI unit can not be obtained, 
whereas a phosphoric acid doped product on the same level is easily 
obtained. Phenylphosphonic acid is solid, and dissolved in THF in an 
amount of about 60%. However, when phenylphosphonic acid is used, the 
doping rate becomes lower than that of methylphosphoric acid (see Table 
1). Di(2-ethylhexyl)phosphoric acid is liquid at ordinary temperature and 
entirely insoluble in water and Methanol. Therefore, using 
di(2-ethylhexyl)phosphoric acid, the doping can be hardly performed in 
spite of high concentrated solutions (see Table 1). These results of the 
doping of the PBI membranes with the organic phosphoric dopant solutions 
have revealed that it is difficult to obtain a doped membrane containing a 
dopant at a high concentration such as that obtained in the case of 
inorganic phosphoric acid. Because, the high concentrated solutions of the 
organic phosphoric dopants show reduced doping ability, and it is 
difficult to prepare of a high concentrated solution owing to a reduction 
in solubility of the dopant. 
Further, the results of the interfacial coagulation method described in (2) 
have indicated that not only the dopant solutions are significantly 
coagulated and gelled on the film interfaces preventing an increase in the 
doping rate, but also the films easily to wrinkle by the doping, resulting 
in the difficulty of putting them to practical use. 
In the solution blend method described in (3) preferably used in the 
present invention, the dope amounts (mol) of the acids per unit of the 
polymers are calculated from the above-mentioned dope amounts (increases 
in weight), and the weight of the polymers before treatment, and the 
amounts of eliminated dopants (dope acid stability) are compared and 
studied. The results thereof have revealed that the amounts of dopants 
eliminated from the doped films obtained by the solution blend method are 
small. Further, the dope amount (mol) of the above-mentioned acid ranges 
from 1 to 10 molecules based on a repeating structure unit of a molecular 
chain of the polymer of the doped film obtained by the solution blend 
method (from 0.5 to 5 molecules per N--H group in a repeating structure 
unit) 
This dope amount is 10 molecules or less, and preferably from 1 to 6 
molecules, per a repeating structure unit of a molecular chain of the 
imidazole ring-containing polymer. In the case of exceeding 10 molecules, 
it results in failure to form a solid membrane shape. 
Thus, the solid polymer electrolyte membranes obtained by the solution 
blend method described in (3) exhibit decreased water absorption achieved 
by the doping, so that no wrinkles are developed by water absorption. 
Further, the dope stability in aqueous solutions of methanol, which is 
required in using the membranes in liquid supply type direct methanol fuel 
cells (DMFCs), is also substantially improved, compared with inorganic 
phosphoric acid. Furthermore, the solid polymer electrolyte membranes 
obtained by the solution blend method are excellent in proton conductivity 
and methanol barrier properties, and useful as solid polymer electrolyte 
membranes for fuel cells. 
The present invention will be illustrated with reference to examples in 
more details below, but these examples are not intended to limit the scope 
of the present invention. Parts and percentages in the examples and 
comparative examples are on a weight basis, unless otherwise specified. 
Main materials used in the examples and comparative examples are as 
follows: 
(1) Polybenzimidazole (PBI) Resin 
Poly[2,2'-(m-phenylene) -5,5'-bibenzimidazole] was used as a 
polybenzimidazole (PBI) resin, an imidazole ring-containing polymer. PBI 
powder (trade name: CELAZOLE) manufactured by Aldrich Co. was dissolved in 
dimethylacetamide at a concentration of 10%, and the resulting solution 
was filtered under pressure. Then, the solution was coagulated in 
distilled water, and the resulting purified product was used after vacuum 
drying. The term "PBI" as used in the examples hereinafter means 
poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole]. 
(2) Preparation of Polybenzimidazole (PBI) film 
The above-prepared PBI powder was dissolved in DMAc at a concentration of 
15%, and cast with a doctor blade. Then, the solvent was removed at 40C. 
The residue was boiled in distilled water, and dried under vacuum to 
obtain 30 .mu.m and 33 .mu.m thick films (PBI membranes). Using the 30 
.mu.m thick PBI membrane, the doping was conducted by the immersing method 
described in the above (1). The 33 .mu.m thick PBI membrane was used as a 
non-doped PBI membrane in Reference Example 2. 
(3) Poly[2,5-benzimidazole] (2,5-PBI) Resin 
A 2,5-PBI resin was synthesized according to the description of Y. Imai, 
Macromol. Chem., Vol.85, p179, (1965). One gram of 3,4-diaminobenzoic acid 
was heated in 35 g-of 116% polyphosphoric acid at 160.degree. C. for 1.5 
hours to obtain a polymer. The resulting polymer was neutralized in a 4% 
aqueous solution of NaHCO.sub.3 overnight, and washed with water and 
methanol, followed by vacuum drying at 120.degree. C., thus obtaining 
2,5-PBI. The inherent viscosity .eta..sub.inh of 2,5-PBI thus obtained was 
0.36. 2,5-PBI heated for 6 hours and 12 hours showed inherent viscosities 
of 0.71 and 0.86, respectively. The inherent viscosity .eta..sub.inh was 
calculated from the viscosity of a 5 g/liter solution of 2,5-PBI in 
concentrated sulfuric acid measured by means of a capillary viscometer. In 
the present invention, the above-prepared 2,5-PBI having a inherent 
viscosity of 0.86 was used. 
(4) Phosphoric Acid and Organic Phosphoric Acid Compounds 
As phosphoric acid and organic phosphoric acid compounds, commercially 
available ones were used as such. That is to say, phenylphosphoric acid 
(R=H in general formula (I)) and diphenylphosphoric acid (R=H in general 
formula (II)) were used in the examples, and phosphoric acid [HO-P(O)(OH) 
.sub.2 ], methylphosphoric acid [MeO-P(O)(OH) .sub.2 ], 
di(2-ethylhexyl)phosphoric acid [(C.sub.8 H.sub.18 O).sub.2 P(O)OH], 
di(n-butyl)phosphoric acid [(n-BuO).sub.2 P(O)OH] and phenylphosphonic 
acid [general formula (VI)] were used in the comparative examples. 
##STR6## 
EXAMPLES 1 AND 2 AND COMATIVE EXAMPLES 1 TO 5 (Preparation of Doped PBI 
Membranes by Solution Blend Method) 
In 20 ml (concentration: 10%) of trifluoroacetic acid, 2.000 g of the PBI 
powder was dissolved, and each dopant shown in Table 3 was added thereto 
in an amount shown in Table 3, followed by stirring at room temperature 
overnight. Each homogenized solution was cast on a PTFE sheet, and the 
solvent was removed at 40.degree. C., followed by vacuum drying at 
80.degree. C., thus obtaining a doped PBI membrane. The molecular number 
of the dopant per PBI unit (dope molecular number/PBI unit) was calculated 
from the amounts of the PBI powder and the dopant charged. The PBI 
membrane doped with di(2-ethylhexyl)phos-phoric acid in Comparative 
Example 3 was insoluble in the solvent, resulting in failure to form a 
film. Further, the doped PBI membrane having 5 molecules of 
phenylphosphonic acid per PBI unit obtained in Comparative Example 5 was 
also insoluble in the solvent, so that it could not form a film. 
TABLE 3 
______________________________________ 
Dope Amount 
Amount (Dope 
of Molecular 
Dopant Number/ 
Kind of Dopant (g) PBI unit) 
______________________________________ 
Example 
1a Phenylphosphoric Acid 
1.130 1 
1b " 2.258 2 
1c " 5.650 5 
2a Diphenylphosphoric Acid 
1.620 1 
2b " 3.240 2 
2c " 8.110 5 
Comparative 
Example 
1a Phosphoric Acid 0.635 1 
1b " 1.270 2 
1c " 3.180 5 
2a Methylphosphoric Acid 
0.727 1 
2b " 1.453 2 
2c " 3.633 5 
3a Di(2-ethylhexyl)phosphoric Acid 
2.090 1 
3b " 4.180 2 
3c " 10.460 5 
4a Di(n-butyl)phosphoric Acid 
1.365 1 
4b " 2.730 2 
4c " 6.820 5 
5a Phenylphosphonic Acid 
1.025 1 
5b " 2.050 2 
______________________________________ 
EXAMPLES 3 and 4 (Preparation of Doped 2,5-PBI Membranes by Solution Blend 
Method) 
With 10 ml (concentration: 10%) of trifluoroacetic acid, 2.000 g of 2,5-PBI 
having a inherent viscosity .eta..sub.inh of 0.86 was mixed, and each 
dopant shown in Table 4 was added thereto in an amount shown in Table 4, 
followed by stirring at room temperature overnight. 
2,5-PBI itself was insoluble in trifluoroacetic acid at a concentration of 
10%, and also insoluble even when inorganic phosphoric acid was added as a 
dopant. It was therefore impossible to prepare 2,5-PBI doped with 
phosphoric acid as a comparative example. However, when phenylphosphoric 
acid or diphenylphosphoric acid was added at a rate of two or more 
molecules per N--H group in 2,5-PBI, 2,5-PBI could be dissolved in the 
solution. When diphenylphosphoric acid was added at a rate of one molecule 
per N--H group, heating at 80.degree. C. was required for dissolving at a 
2,5-PBI concentration of 10%. In the case of the concentration of 
diphenylphosphoric acid higher than 10%, it was dissolved at room 
temperature. Each dissolved (homogenized) solution was cast on a PTFE 
sheet, and the solvent was removed at 40.degree. C., followed by vacuum 
drying at 80.degree. C., thus obtaining a doped 2,5-PBI membrane. Using a 
diphenylphosphoric acid solution of a concentration of not more than 4 
molecules/N--H group, a film could be formed. However, from a 
diphenylphosphoric acid solution of a concentration of higher than 4 
molecules/N--H group, no solidification occurs, even when the solvent was 
totally removed. The dope amount was calculated from the amounts of 
2,5-PBI and the dopant charged. 
TABLE 4 
______________________________________ 
Dope Amount 
(Dope Molecular 
Example Kind of Dopant Number/N-H group) 
______________________________________ 
3a Diphenylphosphoric Acid 
1.0 
3b " 1.5 
3c " 2.0 
3d " 3.0 
3e " 4.0 
4a Penylphosphoric Acid 
2.0 
______________________________________ 
Evaluation Methods 
Dope Stability (Dopant Elimination Rate) 
The dope stability was evaluated by the Soxhlet's extraction method. Each 
film was set in a glass filter, and extracted with 1 M aqueous solution of 
methanol for a specified period of time at a temperature of 85.degree. C. 
to 90.degree. C. After extraction, the film was taken out together with 
the glass filter, and dried under vacuum. The amount of the eliminated 
dopant is measured from a decrease in weight. 
For the doped PBI membranes and the doped 2,5-PBI membranes of Examples 1 
to 4 and Comparative Examples 1, 2 and 5, the film stability was 
evaluated. The amounts of the dopants eliminated from the PBI membranes 
are shown in Table 5 as the dopant molecular number per PBI unit, and the 
amounts of the dopants eliminated from the 2,5-PBI membranes are shown in 
Table 6 as the dope molecular number per N--H group. Two N--H groups are 
contained in one PBI unit. The relationship between the amounts of the 
eliminated dopants (molecular number per N--H group) and the amounts of 
the blended dopants (molecular number per N--H group) of the doped PBI 
membranes of Examples 2 to 4 is shown in FIG. 1. 
TABLE 5 
______________________________________ 
Dope 
Amount Amount of 
(dope Eliminated 
Elimina- 
molecular Dopant tion 
number/PBI 
(molecule/ 
Rate 
Kind of Dopant unit) PBI) (%) 
______________________________________ 
Example 
1a Phenylphosphoric 
1 0.48 48.40 
Acid 
1b Phenylphosphoric 
2 0.73 36.42 
Acid 
1c Phenylphosphoric 
5 3.10 62.01 
Acid 
2a Diphenylphosphoric 
1 0.00 0.00 
Acid 
2b Diphenylphosphoric 
2 0.00 0.00 
Acid 
2c Diphenylphosphoric 
5 2.02 40.44 
Acid 
Com- 
parative 
Example 
1a Phosphoric 1 0.69 69.01 
Acid 
1b Phosphoric 2 1.35 67.72 
Acid 
1c Phosphoric 5 3.38 67.57 
Acid 
2a Methylphosphoric 
1 0.89 88.67 
Acid 
2b Methylphosphoric 
2 1.90 94.80 
Acid 
2c Methylphosphoric 
5 4.99 99.98 
Acid 
5a Phenylphosphonic 
1 0.76 75.95 
Acid 
5b Phenylphosphonic 
2 1.47 73.28 
Acid 
______________________________________ 
TABLE 6 
______________________________________ 
Dope Amount of 
Amount Eliminated 
Elimina- 
(molecular 
Dopant tion 
number/ (molecule/ 
Rate 
Example 
Kind of Dopant 
N-H unit) N-H group) 
(%) 
______________________________________ 
3a Diphenylphosphoric 
1.0 0.0 0.0 
Acid 
3b Diphenylphosphoric 
1.5 0.6 40.0 
Acid 
3c Diphenylphosphoric 
2.0 0.9 45.0 
Acid 
3d Diphenylphosphoric 
3.0 1.7 56.7 
Acid 
3e Diphenylphosphoric 
4.0 2.8 70.0 
Acid 
4a Phenylphosphoric 
2.0 0.8 40.0 
Acid 
______________________________________ 
From the results of Table 5, with respect to the PBI membranes doped with 
almost dopants, it is apparent that the amounts of the eliminated dopants 
from the membranes doped with a dopant at a rate of 5-molecule per PBI 
unit are larger than that of the membranes doped with a dopant at a rate 
of two molecules or less per PBI unit. The PBI molecule has two sites 
(N--H groups) per unit that can interact with phosphoric acid groups, and 
a dopant introduced in an amount exceeding two molecules per PBI unit is 
presumed to be in a relatively free state. There is therefore no 
contradiction in this fact. 
With respect to the mono-substituted acids, methylphosphoric acid of 
Comparative Example 2 shows an elimination rate equal to or more than that 
of inorganic phosphoric acid of Comparative Example 1. On the other hand, 
in the case of phenylphosphoric acid of Example 1, the elimination is 
depressed to less than half those of the above-mentioned acids. Further, 
for diphenylphosphoric acid that is a di-substituted acid, no elimination 
is observed at all at a dope rate of 1 to 2 molecules per PBI unit. This 
apparently indicates that the stability of the dopant at basic sites (N--H 
groups) of the PBI molecule. 
From the results of FIG. 1, the doped PBI membranes and the doped 2,5-PBI 
membranes showed similar dope stability. That is to say, in the doped PBI 
membranes and the doped 2,5-PBI membranes, one molecule of dopant per N--H 
group remained even after methanol extraction, and the membranes were 
stabilized. 
From the results of Tables 5and 6, the diphenylphosphoric acid-doped PBI 
membrane of Example 2b and diphenylphosphoric acid-doped 2,5-PBI membrane 
of Example 3a, each having one molecule of diphenylphosphoric acid per 
N--H group, showed good stability. 
Proton Conductivity 
The proton conductivity of each PBI film was measured for the vacuum dried 
membrane in a dried state by the four-terminal method. For the 
measurement, the complex impedance was measured at 750 mV with an 
impedance analyzer "YHP 4192A" manufactured by YOKOGAWA-HEWLETT KARD, 
LTD., and the direct current component R was obtained by Cole-Cole plots. 
Then, the proton conductivity was calculated therefrom. Results thereof 
are shown in Table 7 and FIG. 2. 
The proton conductivity of each 2,5-PBI film was measured and calculated in 
the same manner as described above with the exception that the 
two-terminal method was used in place of the four-terminal method. 
Further, as Reference Example 1, the proton conductivity of Nafion 112 
(manufactured by E. I. du Pont de Nemours and Company) was measured and 
calculated. Results thereof are shown in Table 7 and FIG. 3. 
TABLE 7 
______________________________________ 
Proton Conductivity (.times. 10.sup.-3) (S/cm) 
Example 2b 
Example 3a Example 3B 
Doped PBI Doped 2,5-PBI 
Doped 2,5-PBI 
Condi- (1 molecule 
(1 molecule 
(1.5 molecules 
tions of diphenyl- 
of diphenyl- 
of diphenyl- 
Reference 
Tem- phosphoric 
phosphoric phosphoric 
Example 1 
perature 
acid/ acid/ acid/ Nafion 
(.degree. C.) 
N-H group) 
N-H group) N-H group) 
112 
______________________________________ 
65 -- -- -- 45 
75 0.51 7.9 11 38 
80 -- -- -- 31 
85 -- -- -- 21 
100 0.84 9.4 18 -- 
125 1.7 8.8 26 -- 
______________________________________ 
As shown in FIG. 2, by use of phenylphosphoric acid or diphenylphosphoric 
acid as a dopant, doped membranes at a rate of 5 molecules per PBI unit 
(2.5 molecules per N--H group) showed conductivity similar to that of the 
membranes doped with phosphoric acid at a rate of 2 molecules per PBI unit 
(1 molecule per N--H group). In the case that PBI is doped with inorganic 
phosphoric acid (liquid state at room temperature) at a rate of two or 
more molecules, excessive free phosphoric acid not participating in 
bonding with the N--H groups seeps out from the doped membranes, owing to 
the pressure of hot pressing or stack assembling in preparing an MEA 
(membrane electrode assembly). However, from these solid acids (doped 
membranes) doped with phenylphosphoric acid and the like, the dopants 
hardly seep out by pressing, so that the dopants can be introduced in 
somewhat larger amounts by the solution blend method. Therefore, the 
solution blend method can overcome the defect that the acid dissociation 
degree thereof is lower than that of inorganic phosphoric acid. 
As shown in FIG. 2, methylphosphoric acid of Comparative Example 2, having 
the structure that hydrogen of a hydroxyl group of phosphoric acid is 
substituted by methyl, was used as a dopant. Conductivity measured in 
Comparative Example 2 was similar to that of phosphoric acid in 
Comparative Example 1. Further, in Comparative Example 5 in which 
phenylphosphonic acid having the structure that a hydroxyl group of 
phosphoric acid is substituted by phenyl, the conductivity was extremely 
low even in a high temperature region. 
As shown in FIG. 3, the diphenylphosphoric acid-doped 2,5-PBI membranes of 
Examples 3a and 3b showed higher conductivity than the diphenylphosphoric 
acid-doped PBI membrane of Example 2a. This is because N--H density of 
2,5-PBI is higher than that of PBI. However, the conductivity of the doped 
2,5-PBI membranes was lower than that of Nafion 112 of Reference Example 1 
which has been generally used at present. This is because the conductivity 
of phosphoric acid-doped PBI is originally considerably lower than that of 
Nafion, and moreover, because the acid dissociation degree of diphenyl 
phosphoric acid, in which two hydrogen atoms of phosphoric acid are 
substituted by phenyl groups, is lower than that of phosphoric acid. 
Water Absorption of Film 
Each dried doped film was allowed to stand in an atmosphere of saturated 
water vapor of 80 C for 6 hours, and changes in weight thereof were 
measured. 
Seepage of Dopant in Pressing 
Each dried doped film was placed between two PTFE sheets and pressed at 224 
kgf/cm.sup.2 for 10 minutes, and changes in weight thereof were measured. 
The ratios (%) of the amounts of water absorption (changes in weight after 
6 hours under saturated water vapor at 80.degree. C.) and the seeping 
amounts of dopants (changes in weight after pressing at 224 kgf/cm.sup.2) 
to the initial amounts of the phenylphosphoric acid-doped PBI membranes 
and the diphenylphosphoric acid-doped PBI membranes of Examples 1 and 2, 
the phosphoric acid-doped PBI membranes of Comparative Examples 1b and 1c, 
and a non-doped PBI membrane of Reference Example 2 are shown in Table 8. 
TABLE 8 
______________________________________ 
Dope Change in 
Amount Weight Change in 
(Dope after Weight 
Molecular 
Water after 
Number Absorption 
Pressing 
Dopant PBI unit) 
(%) (%) 
______________________________________ 
Example 
1b Phenylphosphoric 
2 3.77 -- 
Acid 
1c Phenylphosphoric 
5 5.63 -0.43 
Acid 
2a Diphenylphosphoric 
2 2.40 -- 
Acid 
2c Diphenylphosphoric 
5 4.89 -0.65 
Acid 
Comparative 
Example 
1b Phosphoric Acid 
2 6.51 -- 
1c " 5 10.82 -19.01 
Reference 
Not used -- 6.27 -- 
Example 2 
(Intact PBI) 
______________________________________ 
When the dopants were phenylphosphoric acid or diphenylphosphoric acid in 
Examples 1 and 2, the amounts of water absorption were small, and the 
decreases of weight by pressing were extremely small, compared with that 
of Comparative Example 1 in which phosphoric acid is used. Like this, the 
organic phosphoric acid-doped PBI membranes are low in water absorption, 
and the dopants are suppressed to run out of them by pressing. The dopants 
can therefore be introduced in membranes in somewhat larger amounts. 
Methanol Barrier Properties 
A 1 M aqueous solution of methanol pressurized to 0.5 kgf/cm.sup.2 was 
supplied to an anode chamber at a rate of 0.5 ml/minute, and a permeated 
material exhausted from a cathode chamber through a 3 cm square cell 
heated to a temperature of 70.degree. C. was collected with a cold trap. 
The permeation amount of methanol (g/cm.sup.2) was calculated from the 
weight of the permeated material and the composition determined by gas 
chromatographic analysis. Thus obtained permeation amounts of methanol 
were employed as evaluation results of methanol permeability. 
The permeation amounts of methanol were measured for the diphenylphosphoric 
acid-doped 2,5-PBI membrane (having a film thickness of 70 .mu.m) of 
Example 3a, the non-doped PBI membrane of Reference Example 2 (having a 
film thickness of 33 .mu.m) and a Nafion 117 membrane (manufactured by E. 
I. du Pont de Nemours and Company) (having a film thickness of 201 .mu.m) 
of Reference Example 3. Results thereof are shown in FIG. 4 and Table 9. 
TABLE 9 
______________________________________ 
Total Permeation 
Permeation 
Amount Amount of Speed of 
Test of Permeated 
Methanol Methanol 
Time Material (.times. 10.sup.-3) 
(.times. 10.sup.-6) 
(min.) 
(g) (g) [g/(cm.sup.2 .multidot. min.)] 
______________________________________ 
Example 3a 
30 0.08 0.491 1.82 
60 0.16 0.982 1.82 
120 0.24 1.47 1.36 
Reference 
Example 2 
(Non-doped PBI) 
15 0.21 0.908 6.73 
60 0.41 1.77 3.28 
125 0.57 2.47 2.19 
Reference 
Example 3 
(Nafion 117) 
5 0.75 49.8 1110 
15 1.93 128 949 
30 4.66 309 1150 
______________________________________ 
From the results of FIG. 4 and Table 9, the diphenylphosphoric acid-doped 
2,5-PBI membrane showed methanol barrier properties similar to those of 
the non-doped PBI membrane. From this fact, it is apparent that the 
methanol barrier properties are hardly affected even if the higher-order 
structure of 2,5-PBI is changed by doping. The permeation amount of 
methanol of the diphenylphosphoric acid-doped 2,5-PBI membrane was about 
one-tenth that of the Nafion 117 membrane having a thickness about 3 times 
that of the 2,5-PBI membrane. 
In general, the methanol permeation of a solid polymer electrolyte membrane 
for direct methanol fuel cells (DMFCs) is considered to occur according to 
two kinds of mechanisms. One kind of mechanism is due to the structure of 
a membrane material. And the other one is caused in conjunction with 
proton hopping owing to electric power generation. However, the latter 
proton hopping is considered to be substantially about 0 in a PBI membrane 
in which proton transmission occurs in a non-hydrated state [D. Weng, J. 
S. Wainright, U. Landau, and R. F. Savinell, J. Electrochem. Soc., 
Vol.143, No.4, p1260, April (1996)]. Accordingly, when the above-mentioned 
membranes are incorporated in fuel cells to carry out electric power 
generation, the difference in the permeation amount of methanol between 
the PBI membrane and the Nafion membrane is likely to be further widened. 
The PBI membrane of the present invention is therefore excellent in 
methanol barrier properties, compared with the Nafion membrane. 
The solid polymer electrolytes of the present invention in which the 
imidazole ring-containing polymers are doped with the acids in which 
hydrogen atoms of inorganic acids are substituted by the functional groups 
having phenyl groups are superior to the conventional inorganic phosphoric 
acid-doped PBI membranes in stability, and can be used at substantially 
high doping rates. In particular, the 2,5-PBI membranes are low in water 
absorption, and excellent in durability, proton conductivity and methanol 
barrier properties, so that they are useful as the solid polymer 
electrolytes for fuel cells, particularly as the solid polymer electrolyte 
membranes for DMFCs.