Polymer electrolyte membrane for solid polymer fuel cell, membrane-electrode assembly and fuel cell

1. A polymer electrolyte membrane for polymer electrolyte fuel cells, comprising a block copolymer which comprises

a polymer block (A) having an aromatic vinyl compound unit whose α-carbon is a quaternary carbon, and

a flexible polymer block (B),

wherein said block copolymer has ion-conducting groups on the polymer block (A), and

wherein the percentage of the aromatic vinyl compound unit whose α-carbon is a quaternary carbon in the polymer block (A) is 50% by mass or more of the polymer block (A).

2. The electrolyte membrane according to claim 1 , wherein the ion-conducting groups are monovalent cation-conducting groups or anion-conducting groups, and when the ion-conducting groups are anion-conduction groups, the polymer block (A) has monovalent anion-conducting groups, or polyvalent anion-conducting groups which are bonded so as to crosslink the polymer blocks (A) and/or so as to crosslink the aromatic vinyl compound units inside the polymer block (A).

3. The electrolyte membrane according to claim 1 , wherein the mass ratio of the polymer block (A) to the polymer block (B) is 95:5 to 5:95.

4. The electrolyte membrane according to claim 1 , wherein the aromatic vinyl compound unit whose α-carbon is quaternary carbon is an aromatic vinyl compound unit wherein the hydrogen atom bonded to the α-carbon is replaced with an alkyl group having 1 to 4 carbon atoms, a halogenated alkyl group having 1 to 4 carbon atoms or a phenyl group.

5. The electrolyte membrane according to claim 1 , wherein the polymer block (B) is a polymer block comprising at least one unit selected from the group consisting of an alkene unit having 2 to 8 carbon atoms, a cycloalkene unit having 5 to 8 carbon atoms, a vinylcycloalkene unit having 7 to 10 carbon atoms, a conjugated diene unit having 4 to 8 carbon atoms and a conjugated cycloalkadiene unit having 5 to 8 carbon atoms; and a vinylcycloalkene unit having 7 to 10 carbon atoms, a conjugated diene unit having 4 to 8 carbon atoms and a conjugated cycloalkadiene unit having 5 to 8 carbon atoms in which units, part or all of carbon-carbon double bonds are hydrogenated.

6. The electrolyte membrane according to claim 5 , wherein the polymer block (B) is a polymer block comprising at least one unit selected from the group consisting of an alkene unit having 2 to 8 carbon atoms, a conjugated diene unit having 4 to 8 carbon atoms, and a conjugated diene unit having 4 to 8 carbon atoms wherein part or all of carbon-carbon double bonds are hydrogenated.

7. The electrolyte membrane according to claim 1 , wherein the aromatic vinyl compound unit whose α-carbon is quaternary carbon is an α-methylstyrene unit, and the polymer block (B) is a polymer block comprising at least one unit selected from the group consisting of a conjugated diene unit having 4 to 8 carbon atoms and a conjugated diene unit having 4 to 8 carbon atoms wherein part or all of carbon-carbon double bonds are hydrogenated.

8. The electrolyte membrane according to claim 1 , wherein the ion-conducting group is a cation-conducting group, and the cation-conducting group is a group represented by −SO 3 M or −PO 3 HM wherein M represents a hydrogen atom, an ammonium ion or an alkali metal ion.

9. The electrolyte membrane according to claim 8 , wherein in the radical stability test at 60° C. which is conducted by adding the block copolymer to a radical reaction reagent obtained by dissolving D-glucose and iron (II) chloride tetrahydrate in an aqueous 3% by mass hydrogen peroxide solution, the ion exchange capacity retention proportion of the block copolymer after 8 hours is 50% or more.

10. The electrolyte membrane according to claim 1 , wherein the ion-conducting group is an anion-conducting group, and the anion-conducting group is at least one selected from the following groups of (1) to (13)

wherein R 1 to R 3 represent respectively independently hydrogen atoms or alkyl groups having 1 to 8 carbon atoms, R 4 to R 9 and R 11 represent respectively independently hydrogen atoms, methyl groups or ethyl groups, R 10 represents a methyl group or an ethyl group, X − represents a hydroxide ion or an acid anion, m represents an integer of 2 to 6, and n represents 2 or 3.

11. The electrolyte membrane according to claim 10 , wherein R 1 to R 9 and R 11 are methyl groups or ethyl groups, and X − is a hydroxide ion.

12. The electrolyte membrane according to claim 11 , wherein R 1 to R 11 are methyl groups.

13. The electrolyte membrane according to claim 10 , wherein in the oxidation stability test at 60° C. which is carried out by adding the block copolymer to an aqueous 3% by mass hydrogen peroxide solution, the mass retention proportion of the block copolymer after 8 hours is 85% or more.

14. The electrolyte membrane according to claim 1 , wherein the ion exchange capacity of the block copolymer is 0.30 meq/g or more.

15. A membrane electrode assembly comprising the electrolyte membrane according to claim 1 .

16. A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 15 .