NOVELTY - Polymer electrolyte membrane fuel cells includes an acid-doped solid electrolyte e.g., polymer blend comprising polyimidazole and a thermoplastic polymer sandwiched between gas diffusion anode and gas diffusion cathode.    USE - Fuel cell is used in automobiles.    ADVANTAGE - The fuel cell has good performance and high tolerance to fuel impurities e.g., carbon monoxide. The polymer blend electrolyte exhibits high ionic conductivity, enhanced mechanical properties at operational temperatures and is easy to handle during its preparation.    DETAILED DESCRIPTION - Method for preparing a polymer electrolyte membrane for fuel cells comprises:    (a) providing an acid-doped solid electrolyte (5);    (b) providing a gas diffusion cathode by:    (i) providing a first hydrophobic carbon support substrate (10) by treatment of a carbon substrate with a hydrophobic polymer solution;    (ii) providing a first supporting layer (9) on the first support substrate by casting a slurry comprising carbon black and a hydrophobic polymer onto the substrate;    (iii) providing a first catalyst layer (8) on the first supporting layer by casting a slurry comprising carbon-supported noble metal catalysts and a polymer binder on the supporting layer; and    (iv) doping the first catalyst layer with an acid or a mixture of acids, preferably a non-volatile acid and a volatile acid; and    (c) providing a gas diffusion anode which comprises a second hydrophobic carbon support substrate (4), a second supporting layer (3) and a second catalyst layer (2) by carrying out steps (i) to (iv) above; and    (d) assembling the polymer electrolyte membrane (PEM) by sandwiching the gas diffusion anode, the solid electrolyte and the gas diffusion cathode so that the first and second catalyst layers both face the solid electrolyte.    INDEPENDENT CLAIMS are also included for the following:    (i) a polymer electrolyte membrane (PEM) for use in fuel cells;    (ii) a polymer electrolyte membrane fuel cell;    (iii) a fuel cell;    (iv) a method for operating a polymer electrolyte membrane fuel cell; and    (v) a solid electrolyte for polymer electrolyte membrane fuel cells.    DESCRIPTION OF DRAWING(S) - The diagram shows the structure of a fuel cell.    Gas channel (1)    Anodic catalyst layer (2)    Anodic supporting layer (3)    Anodic back (4)    Solid electrolyte (polymer membrane electrolyte) (5)    Anodic current collector (6)    Cathode current collector (7)    Cathodic catalyst layer (8)    Cathodic supporting layer (9)    Cathodic back (10)    Oxidant gas (11)    Fuel gas (12)