Catalyst layer, catalyst layer forming liquid and membrane electrode assembly

A catalyst layer comprising

• a supported catalyst having a carrier containing a metal oxide, and a catalyst supported on the carrier, and
• a polymer having at least one type of units containing a cyclic ether structure, selected from the group consisting of units represented by the formula (u12) and units represented by the formula (u22), and having ion exchange groups,
• wherein the total of the content of the units containing a cyclic ether structure is at least 30 mol % to all units which the polymer contains:
• in the formula (u12), R21 is a C1-6 perfluoroalkylene group, or a C2-6 perfluoroalkylene group having an ether-bonding oxygen atom between carbon-carbon bonds, and R22 is a fluorine atom, a C1-6 perfluoroalkyl group, a C2-6 perfluoroalkyl group having an ether-bonding oxygen atom between carbon-carbon bonds, or a group represented by —R21(SO2X(SO2Rf)a)−M+, where M+is H+, a monovalent metal cation, or an ammonium ion in which at least one hydrogen atom may be substituted by a hydrocarbon group, and Rf is a linear or branched perfluoroalkyl group which may have an ether-bonding oxygen atom, X is an oxygen atom, a nitrogen atom, or a carbon atom, and when X is an oxygen atom, a=0, when X is a nitrogen atom, a=1, and when X is a carbon atom, a=2;
• in the formula (u22), s is 0 or 1, R51 and R52 are each independently a fluorine atom, a C1-5 perfluoroalkyl group, or a spiro ring formed by linking with each other (provided in the case where s is 0), R53 and R54 are each independently a fluorine atom, or a C1-5 perfluoroalkyl group, and R55 is a fluorine atom, a C1-5 perfluoroalkyl group, or a C1-5 perfluoroalkoxy group.