Hydrophilic fuel cell bipolar plate having a plasma induced polymerization coating
View Patent ↗A fuel cell electrically conductive element is provided with a polymeric surface comprising a flow field, at least a part of the flow field having a grafted, permanently hydrophilic, polymer coating. The element is made by applying a layer of a monomer mixture of a hydrophilic, ethylenically-unsaturated monomer and a crosslinking monomer to the electrically conductive element surface and polymerizing the applied monomer mixture layer with a plasma whereby the polymerized layer is grafted to the polymeric surface.
1. A fuel cell electrically conductive element, comprising a thermoplastic polymeric surface comprising a flow field, at least a part of the flow field having a permanently hydrophilic, crosslinked polymer coating that is formed from a mixture of a first ethylenically-unsaturated monomer having a hydrophilic group and a second distinct crosslinking monomer having at least one ethylenically unsaturated group for crosslinking, wherein said mixture comprises about 50 to about 99 parts by weight of said first hydrophilic monomer and about 1 to about 50 parts by weight of said second crosslinking monomer, and said mixture forms said polymer coating grafted to the polymeric surface after cross-linking.
2. A fuel cell electrically conductive element according to claim 1 , wherein the element is a bipolar plate comprising two surfaces, each comprising a flow field.
3. A fuel cell electrically conductive element according to claim 2 , wherein the coating is grafted to the surface during plasma polymerization of the coating.
4. A fuel cell electrically conductive element according to claim 2 , wherein the bipolar plate comprises a polymer/graphite composite.
5. A fuel cell element according to claim 1 , wherein said first hydrophilic monomer of the polymer coating comprises at least one hydrophilic group selected from hydroxyl groups, acid groups, and amine groups.
6. A fuel cell element according to claim 1 , wherein flow field has lands and channels, and further wherein the lands are free of the coating.
7. A fuel cell element according to claim 1 , wherein flow field has lands and channels, and further wherein electric current flows through coating on the lands.
8. A fuel cell element according to claim 1 , wherein the permanently hydrophilic polymer coating has a dynamic contact angle for water of 30° or less for both advancing and receding contact angles.
9. A fuel cell electrically conductive element according to claim 1 , wherein said first ethylenically-unsaturated monomer comprising said hydrophilic group comprises at least one monomer selected from the group consisting of: hydroxy-functional monomers, acid-functional monomers, amine-functional monomers, and combinations thereof.
10. A fuel cell electrically conductive element according to claim 1 , wherein said first ethylenically-unsaturated monomer having said hydrophilic group is selected from the group consisting of: vinyl esters, vinyl acetate; hydroxyalkyl (meth)acrylates, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate; α,β-ethylenically unsaturated monocarboxylic acids containing 3 to 5 carbon atoms, α,β-ethylenically unsaturated dicarboxylic acids containing 4 to 6 carbon atoms, anhydrides and monoesters of α,β-ethylenically unsaturated monocarboxylic and dicarboxylic acids, acrylic acid, β-carboxyethyl acrylate, methacrylic acid, crotonic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride; sulfonic acid-functional monomers, styrene p-sulfonic acid, ethylmethacrylate-2-sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid; acrylamide, methacrylamide, and N-alkoxy acrylamides; 2-vinyl pyrrolidone; and combinations thereof.
11. A fuel cell electrically conductive element according to claim 1 , wherein said second crosslinking monomer comprises at least one of alkylene glycol di(meth)acrylates and polyalkylene glycol di(meth)acrylates.
12. A fuel cell electrically conductive element according to claim 1 , wherein said second crosslinking monomer having at least one ethylenically unsaturated group for crosslinking is selected from the group consisting of: (meth)acrylate esters of polyols, alkylene glycol di(meth)acrylates, polyalkylene glycol di(meth)acrylates, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate; trimethylolpropane tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate, pentaerythritol tetra(meth )acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, allyl methacrylate, diallyl phthalate, diallyl terephthalate, divinylbenzene, and combinations thereof.
13. A fuel cell electrically conductive element according to claim 1 , wherein said mixture forming said permanently hydrophilic, crosslinked polymer coating comprises about 70 to about 95 parts by weight of said first hydrophilic monomer and about 5 to about 30 parts by weight of said second crosslinking monomer.
14. A fuel cell electrically conductive element according to claim 1 , wherein said first ethylenically-unsaturated monomer having said hydrophilic group comprises polyethylene glycol (meth)acrylate and said second crosslinking monomer having at least one ethylenically unsaturated group for crosslinking comprises polyalkylene glycol di(meth)acrylates.