NOVELTY - Use of an ionic liquid on the basis of fluorinated or partially fluorinated alkyl phosphorus/phosphine, phosphonic- or alkyl sulfonic- acid, or on the basis of derivatives of di- or triacid as an electrolyte for preparing fuel cell membrane or fuel cell membrane electrode assemblies, is claimed.    USE - The ionic liquid is useful: as an electrolyte for preparing fuel cell membrane or fuel cell membrane electrode assemblies, and as binder in the electrode; for contacting the electrode into the membrane side or in a solution on the side pointing to a membrane or spreading in a solution, where the contacting takes place in vacuum and high temperature; for coating the catalyst side of the electrode; and for spraying or with a volatile solvent for coating the electrode, preferably in a hot press process. The fuel cell is useful in vehicles and for stationary uses. The compounds are useful for the preparation of fuel cell membrane electrode units for coating or immersing the electrodes (all claimed). The ionic liquid is useful in polymer-electrolyte-membrane fuel cell, and for small mobile power plant or domestic energy supply.    ADVANTAGE - The ionic liquid provides fuel cell with high efficiency, long-term stability at high temperature, high oxidation and reduction stability and low water solubility.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for the ionic liquid comprising a first composition comprising 1,1-dimethylpyrrolidinium bis(perfluoroalkyl)phosphinate with bis(perfluoroalkyl)phosphinic acid and 1,1-dimethylpyrrolidinium perfluoroalkylphosphonate with perfluoroalkylphosphonic acid in a weight ratio of 1:1; a second composition comprising 1,1-dimethylpyrrolidinium-perfluoroalkylfluorophosphate (VI) and triethyleneglycoldimethylether-perfluoroalkylfluorophosphoric acid complex (VII) in a weight ratio of 1:1, where the second composition optionally combines with phosphoric acid; a third composition comprising (VII) and polyethyleneglycol-200-trisperfluoroalkylfluorophosphoric acid complex (VIII), in a weight ratio of 1:1; guanidinium-perfluoroalkylfluorophosphate of formula (((R2N)3C)+((CnF2n+1)6-zPFz)) (IX); protonated polyethyleneimine (a proton per unit) of formula ((-NDiethyletherDiethylethern per Unit) of formula ((-NH-CH(R1a)-CH(R1a)-N(CH(R1a)-CH(R)-CR2)-CH(R1a)-CH(R1a)-)x.2xH+((CnF2n+1)6-zPFz-)) (XI) in combination with (I)-(V), phosphoric acid and its derivative; protonated polyethyleneglycol of formula (-(O-CH(R1a)-CH(R1a)-)5x.xH+((CnF2n+1)6-zPFz)-) (XII) in combination with compounds from perfluoroalkylphosphoric acid hydrate of formula (H+((CnF2n+1)6-zPFz)yH2O) (I), perfluoroalkylphosphoric acid in diethylether of formula (H+((CnF2n+1)6-zPFz).y(C2H5)2O) (II), bis(perfluoroalkyl)phosphinic acid of formula (CnF2n+1)2P(O)(OH) (III), perfluoroalkylphosphonic acid ((CnF2n+1)P(O)(OH)2) (IV) and fluorinated or partially fluorinated alkylsulfonic acid of formula ((CnF2n+1-tHt)SO3H) (V), where the compounds (I)-(V) is present in mixture or composition form, or contains further liquid electrolytes, phosphoric acid and its derivatives; alkylated poly(melamine-co-formaldehyde) of formula (XIII) in combination with (I)-(V), phosphoric acid and its derivative, preferably as di-(perfluoroalkylfluorophosphate); and poly(N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)- alpha , omega -diamine-CO-2,4-dichloro-6-morpholino-1,3,5-triazine) of formula (XIV) in combination with (I)-(V) phosphoric acid and its derivatives, preferably as pentaperfluoroalkylfluorophosphate.    n = 1-8;    z = 2-5;    R = H or 1-8C-alkyl;    R1a = H or 1-4C-alkyl; and    m = 0-10.    In formula (I):y = plus minus 3.    In formula (II):y = plus minus 2.    In formula (V):    t = 0-4, when n is 1 t is 0-2.