Method for preparing membrane electrode assemblies from fluoropolymer dispersions

Abstract

Described are methods for producing membrane electrode assemblies containing crosslinked membranes and electrodes, which membranes and electrodes are prepared from fluoropolymer organic-liquid dispersions containing a homogeneous mixture of reacted and unreacted sulfonyl halide groups.

1 . A method to prepare a membrane electrode assembly comprising the steps of: 
 a) providing a first solution comprising a polymer solvent and a first polymer containing pendant SO 2 X groups, wherein the first polymer comprises a fluorinated backbone containing pendant groups described by the formula —(O—CF 2 CFR f ) a —(O—CF 2 ) b —(CFR′ f ) c SO 2 X, where X is a halogen, R f  and R′ f  are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6;    b) combining the first solution of step a) with a nucleophilic compound Y and a first polar liquid, to form a first reaction mixture;    c) removing by distillation substantially all of the polymer solvent from the first reaction mixture of step b) to form a first dispersion wherein about 5% to about 95% of the pendant SO 2 X groups of the first polymer have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO 2 X groups remain unreacted;    d) forming a layer of the first dispersion of step c), and removing the first polar liquid from the layer of the first dispersion to form a polymer membrane;    e) providing a second solution comprising a polymer solvent and a second polymer containing pendant SO 2 X groups, wherein the second polymer comprises a fluorinated backbone containing pendant groups described by the formula —(O—CF 2 CFR f ) a —(O—CF 2 ) b —(CFR′ f ) c SO 2 X, where X is a halogen, R f  and R′ f  are independently selected from F, Cl or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0 to 2, b=0 to 1, and c=0 to 6;    f) combining the second solution of step e) with a nucleophilic compound Y and a second polar liquid, to form a second reaction mixture;    g) removing by distillation substantially all of the polymer solvent from the second reaction mixture of step f) to form a second dispersion wherein about 5% to about 95% of the pendant SO 2 X groups of the second polymer have reacted with the nucleophilic compound Y and about 95% to about 5% of the pendant SO 2 X groups remain unreacted;    h) combining the second dispersion of step g) and an electrocatalyst to form an electrode ink;    i) forming a layer of the electrode ink and removing the second polar liquid to form an electrode, and positioning said electrode against the membrane of step d); and    j) forming crosslinks within said membrane and between said membrane and said electrode.    
   
   
       2 . The method of  claim 1  wherein in the first dispersion of step c) about 25% to about 75% of the pendant SO 2 X groups in the first polymer of the first dispersion have reacted with the nucleophilic compound Y and about 75% to about 25% of the pendant SO 2 X groups in the first polymer of the first dispersion remain unreacted.  
   
   
       3 . The method of  claim 1  wherein in the second dispersion of step g) about 25% to about 75% of the pendant SO 2 X groups in the second polymer of the second dispersion have reacted with the nucleophilic compound Y and about 75% to about 25% of the pendant SO 2 X groups in the second polymer of the second dispersion remain unreacted.  
   
   
       4 . The method of  claim 1  additionally comprising the step of mixing the first reaction mixture of step b) or the first dispersion of step c) with a crosslinkable compound.  
   
   
       5 . The method of  claim 4  wherein the crosslinkable compound is of the formula HNR 1 , R 2 , and about 1% to about 100% of the remaining pendant SO 2 X groups in step c) are converted to pendant SO 2 NR 1 R 2  groups, wherein R 1  and R 2  are independently hydrogen or optionally substituted alkyl groups.  
   
   
       6 . The method of  claim 4  additionally comprising the step of contacting the membrane of step d) and the electrode of step i) with a crosslinking promoter so that crosslinks are formed between pendant groups of the first polymer in the membrane and pendant groups of the second polymer in the electrode.  
   
   
       7 . The method of  claim 6  wherein the crosslinks comprise one or more sulfonimide moieties.  
   
   
       8 . The method of  claim 7  wherein the sulfonimide moieties comprise SO 2 NR 7 SO 2 R 8 SO 2 NR 9 SO 2 , wherein R 7  and R 9  are independently hydrogen or optionally substituted alkyl groups, and R 8  is a substituted or unsubstituted alkyl, a substituted or unsubstituted aryl, a substituted sulfonimide polymer, an ionene polymer, or a substituted or unsubstituted heteroatomic function.  
   
   
       9 . The method of  claim 1  additionally comprising the step of mixing the second reaction mixture of step f) or the second dispersion of step g) with a second crosslinkable compound.  
   
   
       10 . The method of  claim 9  wherein the second crosslinkable compound is of the formula HNR 1 R 2 , and about 1% to about 100% of the remaining pendant SO 2 X groups in step g) are converted to pendant SO 2 NR 1 R 2  groups, wherein R 1  and R 2  are independently hydrogen or optionally substituted alkyl groups.  
   
   
       11 . The method of  claim 9  additionally comprising the step of contacting the membrane of step d) and the electrode of step i) with a crosslinking promoter so that crosslinks are formed between pendant groups within the membrane, between pendant groups within the electrode, and between pendant groups in the membrane and pendant groups is the electrode.  
   
   
       12 . The method of  claim 11  wherein the crosslinks comprise one or more sulfonimide moieties.  
   
   
       13 . The method of  claim 1  wherein step d) includes the step of incorporating a reinforcement material into the dispersion of step c) as the membrane of step d) is prepared, and additionally comprising the step of contacting the membrane of step d) with a crosslinking promoter so that crosslinks are formed between pendant groups of the first polymer.  
   
   
       14 . The method of  claim 1  wherein the first polar liquid is selected from DMF, DMAC, NMP, DMSO, acetonitrile, propylene carbonate, methanol, ethanol, water, or a combination thereof.  
   
   
       15 . The method of  claim 1  wherein X in the first polymer is F and wherein X in the second polymer is F.  
   
   
       16 . The method of  claim 1  wherein the polymer solvent of step a) is fluorinated and wherein the polymer solvent of step e) is fluorinated.  
   
   
       17 . The method of  claim 1  wherein the nucleophilic compound Y of step b) is water admixed with a non-nucleophilic base.  
   
   
       18 . The method of  claim 17  wherein the non-nucleophilic base is selected from LiH, NaH, and NR 4 R 5 R 6 , wherein R 4 , R 5  and R 6  are optionally substituted alkyl groups.  
   
   
       19 . The method of  claim 1  wherein the nucleophilic compound Y of step b) is selected from LiOH, NaOH, KOH, CsOH, and any combination thereof.  
   
   
       20 . The method of  claim 1  wherein in step c) the pendant SO 2 X groups have reacted to SO 3 M, wherein M is a univalent cation.  
   
   
       21 . The method of  claim 1  wherein the first polymer of step a) comprises pendant groups of the formula —O—CF 2 CF(CF 3 )—O—CF 2 CF 2 SO 2 F or —OCF 2 CF 2 SO 2 F, or any combination thereof.  
   
   
       22 . The method of  claim 1  wherein the first polymer of step a) is highly fluorinated and wherein the second polymer of step e) is highly fluorinated.  
   
   
       23 . The method of  claim 22  wherein the first polymer of step a) and the second polymer of step e) are the same polymer.  
   
   
       24 . The method of  claim 1  wherein the first polymer of step a) is perfluorinated and wherein the second polymer of step e) is perfluorinated.  
   
   
       25 . A membrane electrode assembly prepared by the method of  claim 1 .  
   
   
       26 . An electrochemical cell comprising the membrane electrode assembly of  claim 25 .  
   
   
       27 . The electrochemical cell of  claim 26  that is a fuel cell.