PROTON EXCHANGE MEMBRANE WITH ENHANCED CHEMICAL STABILITY AND METHOD OF PREPARING THEREOF

Abstract

polymeric ion-conducting membrane with an enhanced stability against attacks of free radicals for exteding its service time, which comprises (a) a polymer matrix, and (b) a redox stabilizer, where the redox stabilizer is attached to the polymer matrix by chemical or ligand bonding, or the redox stabilizer is physically mixed with the polymer matrix.

Claims

1. A polymeric ion-conducting membrane with an enhanced stability against attacks of free radicals, comprising: (a) a polymer matrix, and (b) a redox stabilizer wherein said redox stabilizer is attached to said polymer matrix by chemical or ligand bonding, or said redox stabilizer is physically mixed with said polymer matrix.

2. The polymeric ion-conducting membrane of claim 1, wherein said redox stabilizer is one or more molecules each independently comprising a ferrocyanide or a ferricyanide group.

3. The polymeric ion-conducting membrane of claim 2, wherein said molecule comprising a ferrocyanide or a ferricyanide group is selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, ammonium ferricyanide, hexacyanoferrous acid, hexacyanoferric acid, potassium nitroprusside, sodium nitroprusside, sodium pentacyanoammineferroate, and ammonium disodium pentacy anoammineferroate.

4. The polymeric ion-conducting membrane of claim 3, wherein said molecule comprises a ferrocyanide or a ferricyanide group is potassium ferricyanide or sodium pentacy anoammineferrate.

5. The polymeric ion-conducting membrane of claim 1, wherein said redox stabilizer is a hydroquinone-based molecule that undergoes a redox cycle.

6. The polymeric ion-conducting membrane of claim 5, wherein said is selected from the group consisting of hydroquinone, benzoquinone, naphthoquinone, phenanthraquinone, anthraquinone and all their related derivatives.

7. The polymeric ion-conducting membrane of claim 1, wherein said polymer matrix has a polymer chain architecture selected from the group consisting of homopolymer, random or block copolymer, random or block terpolymer, crosslinked polymer, interpenetrating network, and a polymer containing side chains.

8. A method of making a proton exchange membrane, comprising: (a) preparing a polymer matrix; (b) adding an amount of a redox stabilizer to said polymer matrix in a predetermined mass ratio to form a membrane formulation or, alternatively, attaching an amount of a redox stabilizer directly to said polymer matrix in a predetermined mass ratio by ligand or chemical bonding to from a modified polymer matrix; (c) dissolving said membrane formulation or modified polymer matrix in a solvent to afford a membrane casting solution; (d) casting said membrane casting solution and allowing the solvent evaporating therefrom to form a membrane; and (e) conducting acidification of said membrane to obtain a proton exchange membrane.

9. The method of claim 7, wherein said redox stabilizer is a molecule comprising a ferricyanide or a ferricyanide group.

10. The method of claim 8, wherein said molecule comprising a ferrocyanide or a ferricyanide group is selected from the group consisting of potassium ferrocyanide, sodium ferrocyanide, ammonium ferrocyanide, potassium ferricyanide, sodium ferricyanide, ammonium ferricyanide, hexacyanoferrous acid, hexacyanoferric acid, potassium nitroprusside, sodium nitroprusside, sodium pentacyanoammineferroate, and ammonium disodium pentacyanoammineferroate.

11. The method of claim 7, wherein said redox stabilizer is a hydroquinone-based molecule that undergoes a redox cycle.

12. The method of claim 7, where said polymer matrix is prepared from one or more ingredients selected from the group consisting of Nafion, sulfonated poly(ether ether ketone), sulfonated polysulfone, sulfonated poly(ether sulfone), sulfonated polyimide, sulfonated polybenzimidazoles, sulfonated polystyrene, sulfonated polynitrile, sulfonated polyphenylenes, sulfonated poly(phenylene oxide)s, sulfonated polyphenylene sulfide, sulfonated polyphosphazene, poly(vinyl pyridine), poly(vinyl chloride), polytetrafluoroethylene, poly(vinylidene fluoride) and copolymers of vinylidene fluoride and hexafluoropropylene.

13. The method of claim 7, wherein in step(b) said predetermined mass ratio of polymer matrix to redox stabilizer is (99-85):(1-15)

14. The method of claim 13, wherein in step(c) said solvent is selected from the group consisting of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, diphenyl ether, hexamethylphosphoramide, hexaethylphosphoramide, ethylene glycol monophenyl ether, triethylene glycol, diethylene glycol, dimethylbenzene, dimethylphenol, tetrahydrofuran, methyltetrahydrofuran and dioxane.

15. The method of claim 7, wherein said evaporation in step(d) is conducted at a temperature between 20 and 160 C. and a pressure between 0 and 1 atm.

16. The method of claim 7, wherein said acidification in step(e) is conducted in an acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and acetic acid.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0025] FIG. 1 shows the change of the open circuit voltage value of a proton exchange membrane (Nafion-Redox) prepared in the embodiment 1 and a comparative proton exchange membrane (recast Nafion) prepared similarly by using only a commercial Nafion solute, tested over time in the absence of operating current of the fuel cell. It is comparative OCV curves of the PEMFCs based on recast commercial Nafion membrane and Nafion membrane with redox stabilizer. The Nafion membrane with redox stabilizer shows a large improvement in stability under conditions of 90 C. and 30% RH.

[0026] FIG. 2 shows the change of the open circuit voltage value of a proton exchange membrane (SPEEK-Redox) prepared in the embodiment 2 and a proton exchange membrane (SPEEK) prepared by using only a sulfonated poly(ether ether ketone) with a 70% degree of sulfonation, tested over time in the absence of operating current of the fuel cell. It is comparative OCV curves of the PEMFCs based on SPEEK membrane with 70% degree of sulfonation and SPEEK membrane with redox stabilizer. The SPEEK membrane with redox stabilizer shows a large improvement in stability under conditions of 90 C. and 30% RH.

[0027] FIG. 3 shows the change of the open circuit voltage value of a proton exchange membrane (SPSf-Redox) prepared in the embodiment 3 and a proton exchange membrane (SPSf) prepared by using only a sulfonated polysulfone, tested over time in the absence of operating current of the fuel cell. It is comparative OCV curves of the PEMFCs based on commercial SPSf membrane with 40% degree of sulfonation and SPSf membrane with redox stabilizer. The SPSf membrane with redox stabilizer shows a large improvement in stability under conditions of 90 C. and 30% RH.

[0028] FIG. 4 shows Comparative OCV curves of the PEMFCs based on FC2178 membrane and FC2178 membrane with redox stabilizer. The FC2178 membrane with redox stabilizer shows a large improvement in stability under conditions of 90 TC and 30% RH.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The present invention is further described in detail below, through specific embodiments. One skilled in the art can understand the present invention more comprehensively through the following embodiments, which, however, do not limit the present invention in any way.

Embodiment 1

[0030] (1) A solvent of commercial Nafion D521 dispersion is evaporated, to obtain Nafion polymer.

[0031] (2) The Nafion polymer and potassium ferrocyanide are physically mixed at a mass ratio of 95:5, to obtain a membrane formulation.

[0032] (3) The membrane formulation is dissolved in dimethylformamide to prepare a membrane casting solution with a total solute concentration of 100 g/L, and the solution is left to stand for defoaming.

[0033] (4) The membrane casting solution is decanted into a casting dish and evaporated for 20 h at the temperature of 80 C. and 1 atm pressure (ambient conditions) to form a membrane.

[0034] (5) After solvent evaporation and membrane formation process are complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with much improved chemical stability.

[0035] FIG. 1 shows the change of the open circuit voltage (OCV) value of a proton exchange membrane (Nafion-Redox) prepared in the embodiment 1 and a comparative proton exchange membrane (recast Nafion) prepared similarly by using only commercial Nafion solute, tested over time in the absence of operating current of the fuel cell. Before the OCV test, membranes were processed into membrane electrode assembly (MEA). Commercial Pt/C (60 wt % Pt, Johnson Matthey, England) was used as the catalyst for both anode and cathode. The catalysts were dispersed in Nafion binder (Nafion D521 dispersion, Alfa Aesar, China), where the mass ratio of Nafion to the catalyst was 20 wt %. The resulting dispersion was sprayed by an air gun (Iwata, Japan) onto carbon paper (Toray 250, Japan) to achieve 0.4 mg cm.sup.2 catalyst loadings (0.24 mg Pt cm.sup.2 ) on both anode and cathode with an effective area of 4 cm.sup.2. MEA was fabricated from the anode-membrane-cathode sandwich by hot press under a pressure of 4.0 MPa at 120 C. for 3 min. The OCV testing environment is as follows: the anode hydrogen flow rate is 120 sccm, the cathode oxygen flow rate is 160 sccm, test temperature is 90 C., test humidity is 30% RH, and test back pressure is 1 atm. In conditions of elevated temperature, low humidity, and no operating current, a large number of free radicals are generated in the fuel cell, resulting in rapid chemical degradation of the proton exchange membrane. FIG. 1 shows that the open circuit voltage value of Nafion-Redox shows slight decline of 7.3% over a period of 300 h, while the open circuit voltage value of comparative recast Nafion decreases by 40% within 300 h. Test results of the open circuit voltage durability of the fuel cells prove that the addition of the redox reagent composed of ferricyanide or ferricyanide with a strong negative charge greatly improves the chemical stability of the recast commercial Nafion proton exchange membrane.

Embodiment 2

[0036] (1) 10.0g of poly(ether ether ketone) is dissolved in 300 mL of concentrated sulfuric acid for a reaction for 60 h at room temperature. The obtained solution is poured into ice water, and a precipitate is washed with pure ice water until the pH value reaches 7.0. The recovered polymer is then dried for 12 h at room temperature, to obtain sulfonated poly(ether ether ketone) with a 70% degree of sulfonation.

[0037] (2) The sulfonated poly(ether ether ketone) and potassium ferricyanide are physically mixed at a mass ratio of 90:10 to obtain a membrane formulation.

[0038] (3) The membrane formulation is dissolved in dimethylacetamide to prepare a membrane casting solution with a total concentration of 50 g L.sup.1, and the solution is left to stand for defoaming and degassing.

[0039] (4) The membrane casting solution is decanted into a casting dish and evaporated for 12 h at the temperature of 120 C. under ambient 1 atm pressure conditions to form a membrane.

[0040] (5) After membrane formation is complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with much improved chemical stability.

[0041] The proton exchange membrane (SPEEK-Redox) prepared by physically mixing the sulfonated poly(ether ether ketone) with redox stabilizer potassium ferricyanide in embodiment 2 and a comparative proton exchange membrane (SPEEK) prepared by using only the sulfonated poly(ether ether ketone) without redox stabilizer are assembled into fuel cells. Changes in the open circuit voltage value over time are tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. FIG. 2 demonstrates that the open circuit voltage value of SPEEK-Redox decreases by about 15% within 300 h, while the open circuit voltage value of comparative SPEEK without redox stabilizer comes to a catastrophic damage within 55 h. Test results of the open circuit voltage durability of the fuel cells prove that redox stabilizer ferricyanide or ferricyanide compounds greatly improve the chemical stability of the SPEEK proton exchange membrane.

Embodiment 3

[0042] (1) Commercial sulfonated polysulfone (SPSf) with 40% degree of sulfonation (Shandong Jinlan special polymer Co. Ltd, China) is dissolved in dimethylformamide, and the polymer solution is poured into water to precipitate purified sulfonated polysulfone.

[0043] (2) The sulfonated polysulfone and sodium pentacyanoferrate are physically mixed at a mass ratio of 99:1 to obtain a membrane formulation.

[0044] (3) The membrane formulation is dissolved in N-methylpyrrolidone to prepare a membrane casting solution with a total concentration of 500 g L.sup.1, and the solution is left to stand for defoaming.

[0045] (4) The membrane casting solution is decanted into a casting dish and evaporated for 48 h at a temperature of 20 C. under ambient conditions of 1 atm pressure to form a membrane.

[0046] (5) After membrane formation is complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with high chemical stability.

[0047] The proton exchange membrane (SPSf-Redox) prepared by physically mixing the sulfonated polysulfone and the sodium pentacyanoferrate in embodiment 3 and a comparative proton exchange membrane (SPSf) prepared by using only the sulfonated polysulfone are assembled into fuel cells. Changes in the open circuit voltage value over time are tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. FIG. 3 shows that the open circuit voltage value of SPSf-Redox shows no decrease within 32 h, while the open circuit voltage value of comparative SPSf without redox stabilizer decreases by more than 30% within 27 h. Test results of the open circuit voltage durability of the fuel cells prove that the ferricyanide or ferricyanide redox stabilizers greatly improve the chemical stability of the SPSf proton exchange membrane.

Embodiment 4

[0048] (1) Commercial sulfonated poly(ether sulfone) with 30% degree of sulfonation (YANJIN Technology Co. Ltd, China) is dissolved in dimethylformamide, and the polymer solution is poured into water to precipitate purified sulfonated poly(ether sulfone).

[0049] (2) The sulfonated poly(ether sulfone) and sodium pentacyanoferrate are physically mixed at a mass ratio of 97:3 to obtain a membrane formulation.

[0050] (3) The membrane formulation is dissolved in dimethyl sulfoxide to prepare a membrane casting solution with a total concentration of 300 g and the solution is left to stand for defoaming.

[0051] (4) The membrane casting solution is decanted into a casting dish and evaporated for 40 h at a temperature of 40 C. under ambient conditions of 1 atm pressure to form a membrane.

[0052] (5) After membrane formation process is complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with much improved chemical stability.

[0053] The proton exchange membrane prepared by physically mixing the sulfonated poly(ether sulfone) and the sodium pentacyanoferrate in embodiment 4 and a comparative proton exchange membrane prepared by using only sulfonated poly(ether sulfone) without redox stabilizer are assembled into fuel cells. Changes in the open circuit voltage value over time are tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. The open circuit voltage value of the redox stabilized membrane decreases by about 3% within 300 h, while the open circuit voltage value of the membrane without redox stabilizer decreases by more than 40% within 120 h. Test results of open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide redox stabilizer greatly improves the chemical stability of the poly(ether sulfone) proton exchange membrane.

Example 5

[0054] (1) Following a reported synthetic procedure (Polymer 44 (2003) 4509-4518), 2.55 g of 3-(2,4-diaminophenoxy)propane sulfonic acid is dissolved in 21 mL of m-cresol and 2.76 mL of triethylamine, with stirring under nitrogen flow. Then 2.412 g of 1,4,5,8-naphthalenetetracarboxylic dianhydride and 1.56 g of benzoic acid are added. The mixture is heated to 80 C. for 6 h and then 180 C. for 30 h. After cooling to room temperature, additional 30 mL of m-cresol is added to dilute the highly viscous solution. The solution mixture is poured into acetone. The resulting precipitate is collected by filtration, washed with acetone, and dried for 12 h at a temperature of 30 C., to obtain sulfonated polyimide with a 100% degree of sulfonation.

[0055] (2) The sulfonated polyimide and potassium ferricyanide are physically mixed at a mass ratio of 98:2 to obtain a membrane formulation.

[0056] (3) The membrane formulation is dissolved in m-cresol to prepare a membrane casting solution with a total concentration of 200 g L.sup.1, and the solution is left to stand for defoaming.

[0057] (4) The membrane casting solution is decanted into a casting dish and evaporated for 48 h at a temperature of 20 C. under ambient conditions of 1 atm pressure to form a membrane.

[0058] (5) After the membrane formation is complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with much improved chemical stability.

[0059] The proton exchange membrane prepared by physically mixing the sulfonated polyimide and the potassium ferricyanide in embodiment 5 and a comparative proton exchange membrane prepared by using only the sulfonated polyimide are assembled into fuel cells. Changes in the open circuit voltage value over time are tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. The open circuit voltage value of the redox stabilize sulfonate polyimide membrane decreases by about 8% within 500 h, while the open circuit voltage value of the non-redox-stabilized membrane decreases by more than 30% within 180 h. Test results of open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide redox stabilizer greatly improves the chemical stability of the sulfonated polyimide proton exchange membrane.

Embodiment 6

[0060] (1) 5.0 g of vinyl benzene and 5.0 g of a sodium vinylbenzenesulfonate monomer are dissolved in benzene, and 0.7 g of azobisisobutyronitrile is added as an initiator for free radical polymerization. The polymerization reaction is performed for 18 h at a temperature of 120 C. under the protection of nitrogen. The reaction solution is decanted into water to precipitate the resulting sulfonated polystyrene with a 35% degree of sulfonation.

[0061] (2) The sulfonated polystyrene and potassium ferrocyanide are physically mixed at a mass ratio of 91:9 to obtain a membrane formulation.

[0062] (3) The membrane formulation is dissolved in dimethylformamide to prepare a membrane preparation solution with a total concentration of 350 g L.sup.1, and the solution is left to stand for defoaming.

[0063] (4) The membrane casting solution is decanted into a casting dish and evaporated for 30 h at a temperature of 50 C. under ambient conditions of 1 atm pressure to form a membrane.

[0064] (5) After membrane formation is complete, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a sulfonated polystyrene proton exchange membrane with much improved chemical stability.

[0065] The proton exchange membrane prepared by physically mixing the sulfonated polystyrene and potassium ferrocyanide redox stabilizer in embodiment 6 and a comparative proton exchange membrane prepared by using only the sulfonated polystyrene without redox stabilizer are assembled into fuel cells. Changes in the open circuit voltage value over time are tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. The open circuit voltage value of the redox-stabilized membrane decreases by about 5% within 200 h, while an open circuit voltage value of the non-redox-stabilized membrane decreases by more than 50% within 90 h. Test results of the open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide redox stabilizer greatly improves the chemical stability of the sulfonated polystyrene proton exchange membrane.

Embodiment 7

[0066] (1) 10.0 g of vinyl pyridine monomer is dissolved in benzene, and 0.5 g of azobisisobutyronitrile is added as an initiator for free radical polymerization. The polymerization reaction is performed for 12 h at 100 C. under the protection of nitrogen. The reaction solution is decanted into water for precipitate the resulting poly(vinyl pyridine).

[0067] (2) 1.6 g of sodium pentacyanoferrate and 3.8 g of 15-crown-5 are dissolved in 10 mL of water. 0.4 g of the poly(vinyl pyridine) is dissolved in 10 mL of methanol. The two solutions are mixed for a reaction time of 1 h at a temperature of 40 C. The reaction solution is poured into water in an ice bath environment, and the resulting precipitate is washed with 1 M sulfuric acid, washed with isopropanol three times and then dried for 12 h at room temperature to obtain a product with a formula of

##STR00001##

which is used as a proton-conducting membrane formulation, wherein the proportion x of modified chain segments is 70%. Here, the redox stabilizer is physically attached to the polymer chain, rather than simply being mixed.

[0068] (3) The membrane formulation is dissolved in methanol to prepare a membrane casting solution with a total concentration of 10 g and the solution is left to stand for defoaming.

[0069] (4) The membrane casting solution is decanted into a casting dish and evaporated for 42 h at a temperature of 30 C. under ambient conditions of 1 atm pressure to form a membrane.

[0070] (5) After a membrane formation process completed, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with much improved chemical stability.

[0071] The proton exchange membrane prepared by using the poly(vinyl pyridine) modified by the sodium pentacyanoferrate in embodiment 7 and a proton exchange membrane prepared by using unmodified poly(vinyl pyridine) are assembled into membrane electrode assemblies for fuel cell tests. Changes in the open circuit voltage value over time is tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. The open circuit voltage value of the redox-stabilized membrane decreases by about 9% within 360 h, while the open circuit voltage value of the non-redox-stabilized membrane decreases by more than 55% within 60 h. Test results of the open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide group greatly improves the chemical stability of the proton exchange membrane.

Embodiment 8

[0072] (1) Commercial poly(vinyl chloride) is dissolved in tetrahydrofuran, and the polymer solution precipitation is precipitated into water to obtain purified poly(vinyl chloride).

[0073] (2) 5 g of purified poly(vinyl chloride) is reacted with 300 mL of a dimethylformamide solution of 0.5 g of sodium hydride and 5 g of p-hydroxypyridine for 2 h at a temperature of 0 C. The resulting reaction solution is decanted into water and dried for 12 h at a temperature of 30 C. to obtain a precursor polymer. 9.6 g of sodium pentacyanoferrate and 24.0 g of 15-crown-5 are dissolved in 50 mL of water. Separately, 1.0 g of the precursor polymer is dissolved in 50 mL of dimethylformamide. The two solutions are then mixed and reacted at a temperature of 40 C. for 8 h. The resulting reaction solution is poured into water, and the precipitate is washed with 1 M sulfuric acid three times, then washed with pure water until the pH value is 7. After recovering the polymer, it is then dried for 12 h at the temperature of 80 C. to obtain a product with the structural formula of

##STR00002##

which is used as a proton-conducting membrane formulation, wherein the proportion x of modified chain segments is 35%.

[0074] (3) The membrane formulation is dissolved in tetrahydrofuran to prepare a membrane casting solution with a total concentration of 250 g L.sup.1 and the solution is left to stand for defoaming.

[0075] (4) The membrane casting solution is decanted into a casting dish and evaporated for 16 h at the temperature of 90 C. under ambient conditions of 1 atm pressure to form a membrane.

[0076] (5) After the membrane formation process is completed, the membrane is removed from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification, to obtain a proton exchange membrane with high chemical stability.

[0077] The proton exchange membrane prepared by using the poly(vinyl chloride) modified by the sodium pentacyanoferrate in embodiment 8 and a comparative proton exchange membrane prepared by using unmodified poly(vinyl chloride) are assembled into fuel cells. Changes in the open circuit voltage value over time is tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. The open circuit voltage value of the former membrane modified by the pentacyanoferrate decreases by about 5% within 400 h, while the open circuit voltage value of the latter membrane decreases by more than 32% within 150 h. Test results of open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide group greatly improves the chemical stability of the proton exchange membrane.

Embodiment 9

[0078] (1) 4.0 g of vinylidene fluoride and 6.0 g of hexafluoropropylene are dissolved in 100 mL of dimethylformamide, and 0.4 g of benzoyl peroxide is added as an initiator for free radical polymerization. The polymerization reaction is performed for 18 h at a temperature of 120 C. under the protection of nitrogen. The reaction solution is then decanted into water for precipitation to obtain a copolymer of vinylidene fluoride and hexafluoropropylene (FC2178).

[0079] (2) 3 g of FC2178 are reacted with 300 mL of a dimethylformamide solution of 0.1 g of sodium hydride and 1 g of p-hydroxypyridine at a temperature of 0 C. for 1 h. The resulting reaction solution is slowly poured into water and the resulting precipitate is dried at a temperature of 30 C. for 12 h to obtain a precursor polymer. 1.2 g of sodium pentacyanoferrate and 3.0 g of 15-crown-5 are dissolved in 10 mL of water. Separately, 1.0 g of the precursor polymer is dissolved in 10 mL of dimethylformamide. The two solutions are then mixed to allow reaction at the temperature of 50 C. for 6 h. The reaction solution is then slowly decanted into water, and the precipitate is washed with 1 M sulfuric acid three times, then washed with pure water until pH value is 7. After polymer recovery, it is then dried at the temperature of 80 C. for 12 h to obtain a product with the formula of

##STR00003##

which is used as a proton-conducting membrane formulation, wherein the proportion x of modified chain segments is 1%.

[0080] (3) The membrane formulation is dissolved in dimethyl sulfoxide to prepare a membrane casting solution with a total concentration of 200 g and the solution is left to stand for defoaming.

[0081] (4) The membrane casting solution is decanted into a casting dish and evaporated at the temperature of 100 C. for 15 h under ambient conditions of 1 atm pressure to form a membrane.

[0082] (5) After the membrane formation process is completed, the membrane is taken out from the casting dish and immersed in 1 M sulfuric acid in an ice bath environment for acidification treatment, to obtain a proton exchange membrane with high chemical stability.

[0083] The proton exchange membrane prepared by using FC2178 which is modified by the sodium pentacyanoferrate pentacyanoammineferrate in embodiment 9 and a comparative proton exchange membrane prepared by using the unmodified FC2178 are assembled into fuel cells. Changes in the open-circuit voltage value over time is tested in the absence of operating current of the fuel cell, with test conditions being the same as those in embodiment 1. FIG. 4 shows that the open circuit voltage value of the former redox-stabilized membrane decreases by about 3% within 33 h, while the open circuit voltage value of the latter non-redox-stabilized membrane decreases by more than 12% within 30 h. Test results of open circuit voltage durability of the fuel cells prove that the negatively charged ferricyanide or ferricyanide group greatly improves the chemical stability of the proton exchange membrane.

[0084] Although the preferred embodiments of the present invention are described above in combination with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely for illustration rather than limitation. Inspired by the present invention, one skilled in the art may make many specific transformations without departing from the essence of the present invention and the protection scope of the claims, which, however, all fall into the protection scope of the present invention.