RU2364439C1 - Polymer composition for manufacturing proton-conducting membranes - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to technology of manufacturing propton-conducting membranes, in particular, membranes for high-temperature fuel elements. Polymer composition for manufacturing proton-conducting membranes consists of mixture of poly[2,2'-(l<-phenylene)-5,5'-bibenzimidazole] and benzimidazole-substituted polybenzazole with molecular mass 100000-200000 and structural formula of elementary link (1)

with fraction of total mass from 20% to 60%, or with structural formula of elementary link (2)

with fraction of total mass from 20% to 80%.

EFFECT: application of claimed composition allows to obtain proton-conducting membranes with improved complex of properties: higher proton-conductivity and mechanical strength at higher temperatures.

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Description

FIELD OF THE INVENTION

The invention relates to materials for proton-conducting membranes, in particular membranes for high-temperature fuel cells and, more particularly, relates to polymer compositions based on benzimidazole-substituted polybenzazoles as starting materials for the manufacture of the aforementioned membranes.

State of the art

Recently, most of the research in the field of proton-conducting membranes for high-temperature fuel cells has focused on the use of complexes of basic polymers with strong acids as materials for such membranes. The most popular polymers for doped membranes are polybenzimidazoles (PBI), and in particular poly [2,2 '- (m-phenylene) -5,5'-bibenzimidazole] of the formula (1a) (M. Rukikawa, K.Sanui . Prog. Polym. Sci., 25, 1463 (2000))

In order to obtain doped membranes with increased proton conductivity, a number of research groups studied PBIs with various chemical structures (2), (3) (JAAsensio, S. Borros, P. Gomez-Romero. J. Polym. Sci., A40, 3703 ( 2002)), (4) (L. Xiao, H. Zhang, E.-W. Chehoe, et. Al. Proc. Of the Conference on Advanced Materials for PEM Fuel Cell Systems. Asilomar, CA, USA, Poster abstract 34 (2003); J. Kiefer. Patent DE 10239701).

A common disadvantage of PBI (2a) and (3a) is the poor solubility in common solvents - dimethylacetamide (DMAA), dimethylformamide, dimethyl sulfoxide (DMSO) and other solvents that are most suitable for processing polymers into a film. PBI of the formula (4a) also become poorly soluble in ordinary solvents at a high content of n-phenylene fragments in the polymer unit, and at their low content, the polymer swells strongly and dissolves in phosphoric acid.

It is known (RF Patent No. 2276160) the use of benzimidazole-substituted polybenzimidazoles of the General formula

,

, R=-, O,Where:

,

, R = -, O,

as materials for high-temperature proton-conducting membranes.

The disadvantage of such polymers is their solubility in 85% phosphoric acid and, as a result, poor mechanical strength in doped form.

Disclosure of invention

The problem solved by the claimed invention is to obtain proton-conducting membranes with an improved set of properties.

The technical result consists in increasing the mechanical strength of proton-conducting membranes in doped form at elevated temperatures.

The technical result is achieved in that the polymer composition for the manufacture of proton-conducting membranes consists of a mixture of poly [2,2 '- (m-phenylene) -5,5'-bibenzimidazole] and a benzimidazole-substituted polybenzazole with a molecular weight of 100000-200000 and the structural formula of the elementary unit

with a mass fraction of from 20% to 60%, or of an elementary link with the structural formula

with a mass fraction of 20% to 80%.

The use of polymer compositions based on a mixture of benzimidazole-substituted polybenzazoles with poly [2,2 '- (m-phenylene) -5,5'-bibenzimidazole] in various ratios instead of individual benzimidazole-substituted polybenzazoles allows one to obtain membranes with higher mechanical strength at elevated temperatures. In addition, the use of such polymer compositions allows their doping with an acid of higher concentration than for individual benzimidazole-substituted polybenzazoles, which leads to membranes with higher proton conductivity. Thus, the invention can simultaneously increase the proton conductivity and mechanical strength of proton-conducting membranes.

As a solvent in the polymer composition can be used dimethylacetamide, as well as other amide solvents - dimethylformamide, N-methylpyrrolidone. The solvent content in the composition is determined by the polymer used and may be 90% -95% of the amount of polymer mixture.

The implementation of the invention

Mixtures of benzimidazole substituted polybenzazole of formula (1) and PBI of formula (1a) with a mass content of benzimidazole substituted polybenzazole PBI of formula (1) 20-60% or benzimidazole substituted polybenzazole PBO of formula (2) and PBI of formula (1a) are used as starting materials for the manufacture of membranes. the mass content of benzimidazole-substituted polybenzazole PBO of the formula (2) 20-80%.

The molecular weight of benzimidazole-substituted polybenzazoles is 100000-200000. The use of polymers with molecular weights in excess of 200,000 results in membranes with even greater mechanical strength.

The preparation of benzimidazole-substituted polybenzazoles is carried out by polycondensation of bis-benzoylenbenzimidazole with 3,3'-diaminobenzidine or 3,3'-dihydroxybenzidine in 85% polyphosphoric acid. Polymers are isolated from the reaction mass by precipitation in water.

A polymer composition is prepared by mixing the PBI of formula (1a) and a benzimidazole-substituted polybenzazole of formula (1) or (2) in the required ratio and solvent (dimethylacetamide, dimethylformamide, N-methylpyrrolidone). The solvent content in the composition is determined by the polymer used and may be 90% -95% by weight of the polymer mixture. The resulting mixture is mixed with heating at 80-90 ° C until a solution forms.

The drawing shows the thermomechanical curves for the considered polymers.

The invention is illustrated by the following examples:

Example 1. Synthesis of bis-benzoylenbenzimidazole.

Bis-benzoylenbenzimidazole is prepared by the following reaction:

7.35 g of O-phenylenediamine is dissolved in 200 ml of nitrobenzene and 10 g of dianhydride are added portionwise with vigorous stirring to the resulting solution. The reaction mass is stirred at room temperature for 1 hour, after which it is boiled under reflux and a water separator for 7 hours. The resulting solution was kept overnight, after which the precipitate formed was filtered off and dried to constant weight at 200 ° C and 0.1 mm Hg. The yield of crude product is 13 g (87%). Purification of the substance is carried out by recrystallization from dry DMSO. The purified product is dried at 200 ° C and 0.1 mm Hg. within 3 hours. The purified product melts in the range of 359-368 ° C. The yield of pure product is 5.2 g (35%).

Example 2. Obtaining benzimidazole-substituted polybenzazole of the formula (1).

Weighed portions of bis-benzoylenbenzimidazole (5 g) obtained according to the procedure described in Example 1, 3,3'-diaminobenzidine (2.4435 g) and 85% polyphosphoric acid (100 g) are charged into a two-necked flask equipped with a mechanical stirrer. The flask was purged with argon for 30 minutes, after which the temperature of the reaction mixture was brought to 210 ° C, and the reaction was carried out in a stream of argon for 6 hours. The hot reaction mixture was poured into water, the polymer obtained was washed with water and kept in aqueous ammonia solution (pH 10 ) for 5 hours to neutralize residual phosphoric acid. Then the polymer is washed three times with water and dried at 150 ° C to constant weight. The polymer yield is 6.95 g. The reduced viscosity is 1.24 dl / g (H ₂ SO ₄ , 25 ° C).

The polymer is a brown fiber.

Found,%: C = 77.70, H = 4.01, N = 18.26.

Calculated,%: C = 77.92, H = 3.89, N = 18.19.

The molecular weight determined by the study of the free diffusion of macromolecules in a sulfuric acid solution (Lavrenko P.N., Okatova O.V. Vysokomolek. Soed., A19, 2640 (1977)) was 130,000 (n = 211).

In the IR spectrum there are vibration bands C = C and C = N (1520-1587 cm ^-1 ) of benzimidazole cycles. In the range of 2000-3800 cm ^-1 , a wide intense absorption region of hydrogen-bound and free stretching vibrations of NH and aromatic CH is manifested. In the region of 700-800 cm ^-1, bands characteristic of the disubstituted benzene ring appear.

Example 3. Obtaining benzimidazole-substituted polybenzazole of the formula (2).

The synthesis of the polymer of formula (2) is carried out according to the procedure described in Example 2, from bis-benzoylenbenzimidazole (5 g) and 3,3'-dihydroxybenzidine (2,4663 g). The polymer yield is 7 g. The reduced viscosity is 0.89 dl / g (H ₂ SO ₄ , 25 ° C).

The polymer is a brown fiber.

Found,%: C = 77.51, H = 3.90, N = 13.41, O = 5.42.

Calculated,%: C = 77.66, H = 3.57, N = 13.59, O = 5.18.

The molecular weight determined by the study of the free diffusion of macromolecules in a sulfuric acid solution (Lavrenko P.N., Okatova O.V. High-molecular compounds., A19, 2640 (1977)) 100,000 (n = 161).

In the IR spectrum there are vibration bands C = C and C = N (1533-1615 cm ^-1 ) of benzimidazole cycles. A peak of 103 6 cm ^{−1 is} characteristic of the COC bond of the benzoxazole ring. In the range of 2000-3800 cm ^-1 , a wide intense absorption region of hydrogen-bound and free stretching vibrations of NH and aromatic CH is manifested. In the region of 700-800 cm ^-1, bands characteristic of the disubstituted benzene ring appear.

Example 4. Obtaining a polymer composition.

A polymer composition is prepared by mixing the PBI of the formula (1a) and the benzimidazole-substituted polybenzazole of the formula (1) in such a ratio that the mass fraction of the benzimidazole-substituted polybenzazole is 20-60%.

Example 5. Obtaining a polymer composition.

The polymer composition is prepared by mixing the PBI of the formula (1a) and the benzimidazole-substituted polybenzazole of the formula (2) in such a ratio that the mass fraction of the benzimidazole-substituted polybenzazole is 20-80%.

Example 6. Obtaining films based on mixtures of polymers.

2 g of a mixture of PBI of formula (1a) and benzimidazole-substituted polybenzazole of formula (1) or benzimidazole-substituted polybenzazole of formula (2), taken in the required ratio, are dissolved in 20 ml of DMAA with the addition of 0.2 g of LiCl. The resulting solution is filtered through a No. 2 glass filter, distributed evenly on a glass substrate and dried with a gradual increase in temperature from 50 to 160 ° C for 1 h. The film is removed from the substrate in a stream of water, washed with hot water from lithium chloride (3 times 1 h) and dried at 180 ° C to constant weight.

Example 6. Obtaining a membrane by doping.

The polymer film is placed for 48 hours in an aqueous solution of phosphoric acid of the desired concentration. The extracted film is dried with filter paper until there is a drop of moisture on the surface, and then over P ₂ O ₅ for 24 hours.

Table 1 presents a comparative assessment of the resistance of composite membranes and pure polybenzazoles to phosphoric acid of various concentrations.

Table 1. Resistance of pure polybenzazoles and their mixtures to phosphoric acid. Polymer The maximum concentration of phosphoric acid,% 1a > 85 one 60 2 65 1 + 1a (60% ()) 70 2 + 1a (80% ()) 80 1 + 1a (20% ()) 75 2 + 1a (20% ()) 85

Table 2 presents a comparative assessment of the amount of phosphoric acid absorbed by an individual PBI of formula (1a) and its mixtures with polybenzazoles (1) and (2).

Table 2. The amount of acid absorbed by various polybenzazoles. Polymer The concentration of the dopant H ₃ PO ₄ ,% The content of H ₃ PO ₄ in the membrane,% one 85 40 1 + 1a (60% (6)) 70 78 2 + 1a (80% (7)) 80 79 1 + 1a (20% (6)) 75 83 2 + 1a (20% (7)) 85 85

From table 2 it follows that mixtures of polybenzazoles sorb a greater amount of acid compared with pure polybenzimidazole (1A)

The Figure 1 shows the thermomechanical curves for the considered polymers. Comparative assessment of the heat resistance of pure polybenzazoles (1) and (2) (curves 1 and 2) and their mixtures with PBI of the formula (1a (curve 3 - (1 + 1a (50% (1)); curve 4 - (2 + 1a ( 70% (2)) shows that at temperatures above 100 ° С polymers (1) and (2) become viscous, while their mixtures with PBI of formula (1a) only deform to a limited extent even at higher temperatures.

Table 3 presents a comparative analysis of the values of proton conductivity for the doped PBI of formula (1a) and its mixtures with polybenzazoles (1) and (2). It follows from the table that membranes based on polymer mixtures have higher proton conductivity under the same conditions.

Table 3. Some characteristics of membranes based on doped polybenzazoles. Polymer Proton steam conductivity, S / cm (150 ° С) The current density of the test cell, mA / cm ² (0.5 V, 170 ° C) 1a 1.42 × 10 ^-2 40 1 + 1a (50% (6)) 2.73 × 10 ^-2 133 2 + 1a (70% (7)) 3.47 × 10 ^-2 one hundred

The study of the current-voltage characteristics of the membranes in question in a test hydrogen-air fuel cell with an active surface of 2 cm ² at atmospheric gas pressure, a temperature of 170 ° C and a voltage of 0.5 V showed that membranes based on mixtures of benzimidazole-substituted polybenzazoles and PBI have better performance characteristics compared to with pure PBI of the formula (1a) (table 3).

Industrial applicability

The invention can be used in the manufacture of proton-conducting membranes, in particular membranes for high-temperature fuel cells.

Claims (1)

A polymer composition for the manufacture of proton-conducting membranes, consisting of a mixture of poly [2,2 '- (m-phenylene) -5,5'-bibenzimidazole] and a benzimidazole-substituted polybenzazole with a molecular weight of 100000-200000 and the structural formula of the elementary unit (1)

with a mass fraction of from 20 to 60%, or with the structural formula of an elementary link (2)

with a mass fraction of from 20 to 80%.