COMPOSITE HIGH-TEMPERATURE PROTON EXCHANGE MEMBRANE FOR FUEL CELL, PREPARATION METHOD THEREFOR AND USE THEREOF

Abstract

A composite high-temperature proton exchange membrane for a fuel cell is prepared using materials include PBI and composite A@B and phosphoric acid. A is nanoparticles with a free radical quenching function and B is C.sub.3N.sub.4 having a nanosheet structure. The mass fraction of composite A@B is 0.05-2 wt. % and the mass ratio of A to B in A@B is 1:1-1:20. Composite A@B is firstly prepared, and A@B is then ultrasonically dispersed with a strong polar aprotic solvent to obtain a dispersion S1. PBI solution S2 is obtained from PBI and a strong polar aprotic solvent. S1 and S2 are uniformly mixed and stirred to obtain a casting solution S3, which is cast on plate glass with a groove. The membrane is then soaked in phosphoric acid after dying to obtain a composite membrane for a high-temperature proton fuel cell.

Claims

1. A composite high-temperature proton exchange membrane for fuel cell, comprising raw materials of polybenzimidazole, composite A@B and phosphoric acid, wherein A is nanoparticles with free radical quenching function, B is C.sub.3N.sub.4 with a nanosheet structure, a mass fraction of the composite A@B is 0.05-2 wt. %, and amass ratio of A to B in the composite A@B is 1:1-1:20.

2. The composite high-temperature proton exchange membrane for fuel cell according to claim 1, wherein the composite A@B is that A loads on B, a diameter of the nanoparticles A is 2-10 nm, and a thickness of the nanosheet B is 4-10 nm.

3. The composite high-temperature proton exchange membrane for fuel cell according to claim 1, wherein the polybenzimidazole is at least one of mPBI (poly 2,2-(m-phenyl)-5,5-bibenzimidazole), ABPBI (poly(2,5-benzimidazole)), OPBI (poly 2,2-(p-diphenyl ether)-5,5-bibenzimidazole), PBI with sulfonic acid group side chain, PBI with phosphonic acid group side chain, and hyperbranched PBI; the A is at least one of MnO.sub.2, Mn.sub.2O.sub.3, Fe.sub.3O.sub.4, TiO.sub.2 and CeO.sub.2.

4. The composite high-temperature proton exchange membrane for fuel cell according to claim 1, wherein the composite A@B is that nanoparticles CeO.sub.2 load on nanosheet C.sub.3N.sub.4.

5. The composite high-temperature proton exchange membrane for fuel cell according to claim 1, wherein a preparation method of the composite A@B is as follows: (1) calcining dicyandiamide after grinding, grinding the calcined dicyandiamide into powder, washing the powder with 0.25-1.5M hydrochloric acid solution for 0.5-3 hours and with deionized water for 0.5-2 hours respectively, and drying the obtained solid for standby; and (2) mixing the solid obtained in step (1) with a precursor of nanoparticles with free radical quenching function to prepare a suspension, adding 0.5-2.5M KOH solution into the suspension so that the pH value of the suspension is 12-14, stirring and centrifuging the suspension to obtain a solid precipitate, washing the precipitate with water to neutral, and calcining the precipitate after drying to obtain the composite A@B.

6. The composite high-temperature proton exchange membrane for fuel cell according to claim 5, wherein in step (1), a mass ratio of dicyandiamide to the precursor of the nanoparticles with free radical quenching function is 15:1 to 5:1; calcining conditions are of heating from room temperature to 500-600 C. with a heating rate of 3-8 C. min.sup.1 in an air atmosphere, and maintaining the temperature for 3-6 hours after heating to a set temperature; a drying temperature is 60 C.; and calcining conditions in step (2) are of calcining for 2 hours in an air atmosphere at 250 C.

7. A preparation method of the composite high-temperature proton exchange membrane for fuel cell according to claim 1, comprising the following steps of: (a) ultrasonically dispersing the composite A@B with a strongly polar aprotic solvent, and preparing a dispersion liquid S1 after ultrasonically dispersing for a period of time; (b) dissolving polybenzimidazole (PBI) in the strongly polar aprotic solvent, and obtaining a PBI solution S2 after stirring and heating; and (c) obtaining a casting solution S3 after mixing S1 and S2, casting S3 onto a grooved plate glass to obtain a membrane, and soaking the membrane in phosphoric acid after drying.

8. The preparation method of the composite high-temperature proton exchange membrane for fuel cell according to claim 7, wherein a mass concentration of the composite A@B in the dispersion liquid S1 in step (a) is 0.05-2 mg/10 ml; and a mass fraction of polybenzimidazole in the solution S2 in step (b) is 0.8-5 wt. %.

9. The preparation method of the composite high-temperature proton exchange membrane for fuel cell according to claim 8, wherein the aprotic solvents in steps (a) and (b) are independently at least one of N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMAc) and N-methyl-2 pyrrolidone (NMP); in step (1), an ultrasonic power is 50-300 W and an ultrasonic time is 0.5-6 hours; in step (3), a mixing mode is magnetic stirring with a stirring power of 50-100 W and a stirring time of 1-12 hours; and a concentration of phosphoric acid for soaking the polybenzimidazole membrane is 50-85%, an soaking temperature is 50-150 C., and an soaking time is 6-24 hours.

10. A use of the composite high-temperature proton exchange membrane for fuel cell according to claim 1 in fuel cells.

Description

DETAILED DESCRIPTION OF DRAWINGS

[0027] FIG. 1 shows scanning electron microscope diagrams of the nano-composite CeO.sub.2@C.sub.3N.sub.4 involved in Embodiment 1, Comparative examples 1, 2, and 4 of the present invention. Panel a-Embodiment 1, panel b-Comparative example 1, panel c-Comparative example 2, and panel d-Comparative example 4.

[0028] FIG. 2 is a scanning electron microscope diagram of CeO.sub.2 in Comparative example 3.

[0029] FIG. 3 shows diagrams of interaction model of g-C.sub.3N.sub.4 and phosphoric acid molecules. panel abefore adsorption, and panel bafter adsorption.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0030] The present invention is further described below with reference to specific embodiments, but is not limited in any way. The synthesis methods of the three hyperbranched PBI in the present application can refer to Journal of Membrane Science 593 (2020) 117435.

EMBODIMENT 1

[0031] Taking polybenzimidazole and CeO.sub.2@C.sub.3N.sub.4 as the raw materials, the composite high-temperature proton exchange membrane was prepared according to the following steps:

[0032] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the powder was washed with 1 M hydrochloric acid solution for 1.5 hours and with deionized water for 2 hours, respectively, and the obtained solid was dried in an oven at 60 C. Then the dried solid was mixed with 0.2 g of cerium nitrate, and then 50 g of deionized water was added to the mixture and magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH value of the suspension to 13, and then a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that the composite CeO.sub.2@C.sub.3N.sub.4 was obtained.

[0033] Step 2: 0.12 mg of the composite CeO.sub.2@C.sub.3N.sub.4 was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at an ultrasonic power of 100 W for 4 hours to obtain a dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0034] Step 3: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

EMBODIMENT 2

[0035] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the powder was washed with 1 M hydrochloric acid solution for 1.5 hours and with deionized water for 2 hours, respectively, and the obtained solid was dried in an oven at 60 C. Then the dried solid was mixed with 0.25 g of cerium nitrate, and then 50 g of deionized water was added to the mixture and magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH value of the suspension to 13, and then a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that the composite CeO.sub.2@C.sub.3N.sub.4 was obtained.

[0036] Step 2: 2.5 mg of the composite CeO.sub.2@C.sub.3N.sub.4 was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at an ultrasonic power of 100 W for 4 hours to obtain a dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0037] Step 3: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

COMPARATIVE EXAMPLE 1

[0038] A composite high-temperature proton exchange membrane was prepared according to the method of Embodiment 1, and the calcined product of the dicyandiamide was not washed with hydrochloric acid.

[0039] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the power was mixed with 0.2 g of cerium nitrate, and 50 g of deionized water was added to the mixture and magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH value of the suspension to 13, and then a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that composite the CeO.sub.2@C.sub.3N.sub.4 was obtained.

[0040] Step 2: 0.12 mg of the composite CeO.sub.2@C.sub.3N.sub.4 was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at a ultrasonic power of 100 W for 4 hours to obtain a dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0041] Step 3: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

COMPARATIVE EXAMPLE 2

[0042] A composite high-temperature proton exchange membrane was prepared by using polybenzimidazole and nanosheet C.sub.3N.sub.4 as raw materials.

[0043] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the powder was washed with 1 M hydrochloric acid solution for 1.5 hours and with deionized water for 2 hours, respectively; the obtained solid was dried in an oven at 60 C., so that C.sub.3N.sub.4 nanosheets were obtained.

[0044] Step 2: 0.12 mg of the C.sub.3N.sub.4 nanosheets was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at a ultrasonic power of 100 W for 4 hours to obtain a dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0045] Step 3: the casting solution was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

COMPARATIVE EXAMPLE 3

[0046] A composite high-temperature proton exchange membrane was prepared by using polybenzimidazole and the nanosheet CeO.sub.2 as raw materials.

[0047] Step 1: 0.2 g of cerous nitrate was weighed followed by adding 10 g of deionized water, and the mixture was magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH of the suspension to 13, and a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that the nanoparticles CeO.sub.2 was obtained.

[0048] Step 2: 0.12 mg of the nanoparticles CeO.sub.2 was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at a ultrasonic power of 100 W for 4 hours to obtain a dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0049] Step 3: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

COMPARATIVE EXAMPLE 4

[0050] A composite high-temperature proton exchange membrane was prepared according to the method of Embodiment 1, with excessive doping of CeO.sub.2.

[0051] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the powder was washed with 1 M hydrochloric acid solution for 1.5 hours and with deionized water for 2 hours, respectively; the obtained solid was dried in an oven at 60 C. Then the dried solid was mixed with 0.2 g of cerium nitrate, and then 50 g of deionized water was added to the mixture and magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH value of the suspension to 13, and a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that the composite CeO.sub.2@C.sub.3N.sub.4 was obtained.

[0052] Step 2: 0.12 mg of the composite CeO.sub.2@C.sub.3N.sub.4 was weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at a ultrasonic power of 100 W for 4 hours to obtain the dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0053] Step 3: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

COMPARATIVE EXAMPLE 5

[0054] Step 1: 15 g of dicyandiamide was weighed and was placed in a crucible after sufficient grinding, then the crucible was put in a tubular furnace for calcination, and the tubular furnace was heated from room temperature to 550 C. with a heating rate of 5 C. min.sup.1 under an air atmosphere; the temperature was maintained for 4 hours after the temperature is increased to the set temperature, then the temperature was decreased along with the furnace; the calcined dicyandiamide was transferred into a mortar and was carefully ground into powder, and then the powder was washed with 1 M hydrochloric acid solution for 1.5 hours and with deionized water for 2 hours, respectively; and the obtained solid was dried in an oven at 60 C.

[0055] Step 2: 0.2 g of cerium nitrate was weighed and was added in 10 g of deionized water, the mixture was magnetically stirred for 1 hour to obtain a suspension. Subsequently, 1 M KOH solution was added to the suspension to adjust the pH value of the suspension to 13, and a solid precipitate was obtained by centrifugation after magnetic stirring for 2 hours. The solid precipitate was washed with large amounts of deionized water until the solution pH was neutral. Finally, the materials were completely dried in an oven and calcined at 250 C. for 2 hours under an air atmosphere, so that the nanoparticles CeO.sub.2 was obtained.

[0056] Step 3: 0.10 mg of C.sub.3N.sub.4 and 0.02 mg of CeO.sub.2 were weighed and 10 ml of NMP was measured to mix, and the mixture was ultrasonically dispersed at an ultrasonic power of 100 W for 4 hours to obtain the dispersion liquid S1. 0.27 g of mPBI (poly-2,2-(m-phenyl)-5, 5-bibendazole) and 30 g of NMP were weighed, mixed and magnetically stirred, so that a PBI solution S2 was obtained after sufficient dissolution. S1 and S2 were uniformly mixed with stirring at a stirring power of 50 W for 6 hours to obtain a casting solution S3.

[0057] Step 4: the casting solution S3 was poured onto a grooved plate glass, and the plate glass was vacuum-dried at 80 C. for 24 hours, followed by vacuum-drying at 120 C. for 10 hours to obtain a base membrane. Finally, the base membrane was soaked in phosphoric acid with a concentration of 85% at 80 C. for 20 hours, so that a composite membrane was obtained.

[0058] The composite membranes prepared in the embodiments and the comparative examples were tested by the scanning electron microscopy (SEM), and the results are shown in FIG. 1. It can be seen from FIG. 1 that, in the composite CeO.sub.2@C.sub.3N.sub.4 of Embodiment 1, the amount of CeO.sub.2 is moderate and CeO.sub.2 is uniformly distributed on the surface of C.sub.3N.sub.4, so that the tolerance to the free radicals can be improved without reducing the conductivity and tensile strength.

[0059] The composite membranes prepared in embodiments and comparative examples were tested for conductivity and tensile strength, and the results are shown in Table 1. It can be seen from Table 1 that the conductivity and tensile strength of the composite membranes of the present invention are both improved, and the effects thereof is better than those of the Comparative examples 1 to 5.

TABLE-US-00001 TABLE 1 Case Conductivity/S .Math. cm.sup.1 Tensile strength/MPa Embodiment 1 0.048 17.3 Embodiment 2 0.054 17.8 Comparative example 1 0.036 16.8 Comparative example 2 0.043 16.5 Comparative example 3 0.034 14.2 Comparative example 4 0.038 15.9 Comparative example 5 0.044 16.7

[0060] The composite membrane un-soaked in phosphoric acid in Embodiments 1 and PBI/CeO.sub.2@C.sub.3N.sub.4 composite membranes un-soaked in phosphoric acid in Comparative examples 1 to 5 were immersed in Fenton reagent for durability test, and the results are shown in Table 2.

TABLE-US-00002 TABLE 2 Mass residual rate of composite Case membrane after 100 hours/% Embodiment 1 93 Embodiment 2 95 Comparative example 1 85 Comparative example 2 81 Comparative example 3 87 Comparative example 4 90 Comparative example 5 89

[0061] As can be seen from the table, due to C.sub.3N.sub.4 in Comparative example 1 was not treated with hydrochloric acid, resulting in more agglomeration of C.sub.3N.sub.4 and more agglomeration of CeO.sub.2, so that the CeO.sub.2@C.sub.3N.sub.4 composite does not significantly improve the conductivity and tensile strength, and the effect on durability is also limited. In the present application, C.sub.3N.sub.4 is treated with hydrochloric acid, so that the C.sub.3N.sub.4 is protonated, which is helpful for the dispersion of C.sub.3N.sub.4 and improves the proton conductivity. Therefore, the operation of treating C.sub.3N.sub.4 with hydrochloric acid is necessary. C.sub.3N.sub.4 treated with hydrochloric acid has more layered structures; Comparative example 2 is only doped with the treated C.sub.3N.sub.4, which can significantly improve the conductivity and tensile strength of the composite membrane. However, the durability of the composite membrane is poor due to the absence of free radical quencher CeO.sub.2. Comparative example 3 is only doped with CeO.sub.2, but CeO.sub.2 itself does not conduct protons, nor can it transfer stress, so that the proton conductivity and tensile strength of the composite membrane are decreased. In Comparative example 4, the amount of CeO.sub.2 is excessive, which covers part of the active sites where C.sub.3N.sub.4 react with phosphoric acid, thereby reducing the contact area between C.sub.3N.sub.4 and phosphoric acid as well as between PBI resin and phosphoric acid, resulting in limited effects on improving proton conductivity and tensile strength. In Comparative example 5, CeO.sub.2 and C.sub.3N.sub.4 were added, so that the two substances do not interact well with each other, and quenching of free radicals and adsorption anchoring of phosphoric acid cannot be achieved at the same site at the same time. In summary, Embodiment 1 has the best implementation effect.

[0062] The mechanism of proton conduction of the high-temperature composite membrane in the present application is to use g-C.sub.3N.sub.4 to adsorb and anchor phosphoric acid, increase the adsorption capacity and bonding effect of the composite membrane to phosphoric acid, and reduce the loss rate of phosphoric acid (see Table 4). The present invention uses a density-functional calculation to describe in detail the mechanism that g-C.sub.3N.sub.4 adsorbs phosphoric acid to promote the improvement of the composite membrane conductivity (see FIG. 3 and Table 3).

TABLE-US-00003 TABLE 3 Changes of bond length of phosphoric acid molecules before and after adsorption to g-C.sub.3N.sub.4 Before After Before After adsorp- adsorp- adsorp- adsorp- H.sub.3PO.sub.4.sup.1 tion/ tion/ H.sub.3PO.sub.4.sup.2 tion/ tion/ (O1H1) 1.03 0.99 (O5H4) 1.03 0.99 (O3H2) 0.99 1.15 (O7H5) 0.99 1.15 (O4H3) 1.03 0.99 (O8H6) 1.03 0.99 (P1O1) 1.57 1.57 (P2O5) 1.57 1.57 (P1O2) 1.52 1.51 (P2O6) 1.52 1.51 (P1O3) 1.57 1.62 (P2O7) 1.57 1.62 (P1O4) 1.56 1.57 (P2O8) 1.56 1.57

[0063] As shown in FIG. 3 and Table 3, according to results of the density-functional calculation, the bond length of PO bond (P1-O3, P2-O7) in the phosphoric acid molecule changes from 1.57 to 1.62 , and the bond length of HO bond (H2-O3, H5-O7) changes from 0.99 to 1.15 , which means that phosphoric acid can be further adsorbed on the surface of g-C.sub.3N.sub.4, so that is easier to dissociate protons and improves the proton conductivity of the composite membrane. This is the mechanism that g-C.sub.3N.sub.4 improves the conductivity of the composite membrane described in the present invention.

TABLE-US-00004 TABLE 4 Mass of remaining phosphoric acid in composite membrane after 72 hours under 40% relative humidity at 80 C. Retention rate of phosphoric acid in Case composite membrane after 72 hour/% Embodiment 1 81.4 Embodiment 2 80.9 Comparative example 1 78.2 Comparative example 2 75.5 Comparative example 3 68.7 Comparative example 4 73.2 Comparative example 5 76.3

[0064] As shown in Table 4, Embodiment 1 has the optimal phosphoric acid retention rate, indicating that the method of the present invention can improve the retention rate of phosphoric acid in the composite membrane, and significantly reduce the loss rate of phosphoric acid.

[0065] For any skilled in the art, without departing from the scope of the technical solution of the present invention, many possible changes and modifications can be made to the technical solution of the present invention by using the technical contents disclosed above, or modified into equivalent embodiments with equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the contents of the technical solution of the present invention shall still belong to the protection scope of the technical solution of the present invention.