A process for producing proton-conducting membrane, the process comprising: mixing (i) 5% to 60% by volume of an electrically nonconductive inorganic powder having a good acid absorption capacity, the powder comprising essentially nanosize particles; (ii) 5% to 50% by volume of a polymeric binder that is chemically compatible with acid, oxygen and the fuel; and (iii) 10 to 90% by volume of an acid or aqueous acid solution, wherein the mixing is conducted at various rate steps, thereby producing a proton-conducting mixture; continuously casting the proton-conducting mixture on rolled paper, non-woven matrix or the like at ambient temperature; drying the casted proton-conducting mixture at a temperature of greater than 100° C. for approximately 5 to 30 minutes, thereby forming a dry film; and laminating a plurality of the dry films together under pressure, and thereafter extracting pore-former out of pores of the dry films, thereby forming the proton-conducting membrane having an average pore size of less than 30 nanometers.