NOVELTY - Proton conducting composite polymer electrolyte comprises a proton conducting ionomer and a metal-ligand complex additive, where the metal is a metal, metal alloy, metal oxide and/or metal salt, and the ligand in the complex comprises a heterocyclic molecule (A) or its polymer. The heterocyclic molecule (A) is substituted (2,2')bipyridinyl compounds (I), substituted (1,10)phenanthroline compounds (II)-(XI), or substituted (2,2';6',2'')terpyridine compounds (XII).    USE - The proton conducting composite polymer electrolyte is useful in a membrane electrode assembly of a solid polymer electrolyte fuel cell (all claimed).    ADVANTAGE - The proton conducting composite polymer electrolyte improves durability or performance of a solid polymer electrolyte fuel cell.    DETAILED DESCRIPTION - Proton conducting composite polymer electrolyte comprises a proton conducting ionomer and a metal-ligand complex additive, where the metal is a metal, metal alloy, metal oxide and/or metal salt, and the ligand in the complex comprises a heterocyclic molecule (A) or its polymer. The heterocyclic molecule (A) is substituted (2,2')bipyridinyl compounds of formula (I), substituted (1,10)phenanthroline compounds of formula (II)-(XI), or substituted (2,2';6',2'')terpyridine compounds of formula (XII).    R1, R3, R4 = H, CH3(CH2)n-, CH3(CH2)n-O-, CF3(CF2)n-, CF3(CF2)n-O-, -COOH, PO(OH)2, SO3H, NH2, OH, or substituted poly-phenyl moiety of formula (a1);    X = H, COOH, PO(OH)2 or SO3H;    n = 0-10; and    R2 = CH3-(CH2)n, CH3(CH2)n-O-, CF3(CF2)n, CF3(CF2)n-O-, COOH, PO(OH)2, SO3H, NH2, OH, or (a1).    INDEPENDENT CLAIMS are also included for:    (1) a membrane electrode assembly for a solid polymer electrolyte fuel cell comprising an anode catalyst layer, a membrane electrolyte, a cathode catalyst layer and the composite polymer electrolyte;    (2) a solid polymer electrolyte fuel cell comprising the membrane electrode assembly;    (3) a method of making the composite polymer electrolyte comprising: preparing a metal from a precursor for the metal; preparing a ligand; mixing the metal and the ligand in a solution or dispersion comprising the proton conducting ionomer thus preparing the composite polymer electrolyte in the solution or dispersion; and separating out the composite polymer electrolyte from the solution or dispersion;    (4) a method of making the composite polymer electrolyte comprising: preparing a metal from a precursor for the metal; preparing a ligand; synthesizing the metal-ligand complex additive from the metal and the ligand; dissolving the metal-ligand complex additive in a solution or dispersion comprising the proton conducting ionomer thus preparing the composite polymer electrolyte in the solution or dispersion; and separating out the composite polymer electrolyte from the solution or dispersion; and    (5) a method of improving durability or performance of a solid polymer electrolyte fuel cell comprising a membrane electrode assembly comprising an anode catalyst layer, a membrane electrolyte, and a cathode catalyst layer, the method comprising using the composite polymer electrolyte in at least one of the anode catalyst layer, the membrane electrolyte, or the cathode catalyst layer.