Method of making an electrically conductive element for use in a fuel cell
The present invention provides an electrically conductive element for a proton exchange membrane fuel cell having low electrical contact resistance and high corrosion resistance. The conductive element comprises a corrosion susceptible metal substrate with a surface, which is preferably treated to activate the surface (i.e., to remove a passivation layer of oxides from the surface) with an acidic treatment solution. The treated surface is then overlaid with an electrically conductive, corrosion-resistant, protective coating to protect the substrate re-forming a passivation layer while exposed to the corrosive environment of the fuel cell. The present invention also provides methods of preparing an electrically conductive element to have low electrical contact resistance and high corrosion resistance.
1. A method of making an electrically conductive element, comprising:
providing a conductive metal substrate comprising stainless steel for use in a fuel cell having a surface susceptible to passivation in the presence of oxygen by forming oxides, wherein oxides are formed thereon;
applying an acidic treatment solution to said surface to remove said oxides and reduce contact resistance of said surface to less than about 10 mOhm-cm 2 under a compressive force of greater than or equal to about 200 psi (about 1400 kPa), wherein said acidic treatment solution comprises an anionic species selected from the group: fluorine (F − ), chlorine (Cl − ), bromine (Br − ), iodine (I − ), sulfate (SO 4 2− ), nitrate (NO 3 − ), and mixtures thereof, wherein said anionic species is present at about 1×10 −1 to about 1×10 −6 molarity;
rinsing said surface to remove substantially all of said treatment solution; and
coating said surface with a corrosion resistant electrically conductive coating.
2. The method of claim 1 , wherein said applying is performed by submerging said surface in said treatment solution.
3. The method of claim 1 , wherein said applying is performed by spraying said surface with said treatment solution.
4. The method of claim 1 , wherein said rinsing is performed by first rinsing with a first acidic rinse solution and then by rinsing with a second rinse solution comprising de-ionized water.
5. The method of claim 1 , wherein prior to said applying, a cleaning of said surface is conducted to remove loosely adhered contaminants therefrom.
6. The method of claim 5 , wherein said cleaning comprises applying an alkaline cleaning solution effective for cleaning at least one contaminant selected from the group consisting of: oils, grease, waxy solids, metallic particles, carbon particles, dust, dirt, silica, scale, and combinations thereof.
7. The method of claim 6 , wherein said cleaning comprises mechanically cleaning said surface.
8. The method of claim 1 , wherein said applying is conducted for a time and at a temperature sufficient to reduce a contact resistance of said surface to less than about 10 mΩ-cm 2 measured at a pressure of 2700 kPa.
9. The method of claim 1 , wherein said coating has a thickness less than or equal to about 30 μm.
10. The method of claim 1 , wherein said coating comprises a polymeric matrix comprising a corrosion-resistant polymer and a plurality of electrically conductive particles.
11. The method of claim 1 , wherein said applying is conducted for a duration of between about 3 seconds to about 600 seconds.
12. The method of claim 1 , wherein said applying is conducted at a temperature of from about 250° C. to about 100° C.
13. The method of claim 1 , wherein said acidic treatment solution has a pH of from greater than 0 to about 4.
14. The method of claim 1 , wherein said applying is conducted at a temperature from about 25° C. to about 100° C. for a time of between about 3 to about 600 seconds, and wherein said acidic treatment solution has a pH from greater than 0 to about 4.
15. The method of claim 1 , wherein said acidic treatment solution comprises a halogen anion selected from the group consisting of: F − , Cl − , Br − , I − , and mixtures thereof.
16. The method of claim 15 , wherein said acidic treatment solution comprises hydrofluoric acid, wherein said anion is fluoride.
17. The method of claim 1 , wherein said acidic treatment solution comprises a solvent comprising water.
18. The method of claim 1 , wherein said anionic species comprises a halogen ion and is present at about 1×10 −3 to about 1×10 −5 molarity.
19. A method of making an electrically conductive element, comprising:
providing a conductive metal substrate comprising stainless steel for use in a fuel cell, having a surface susceptible to passivation in the presence of oxygen by forming oxides, wherein oxides are formed thereon;
applying an acidic treatment solution to said surface to remove said oxides for a time and temperature sufficient to reduce a contact resistance of said surface to less than about 10 mOhm-cm 2 under a compressive force of greater than or equal to about 200 psi (about 1400 kPa), wherein said acidic treatment solution comprises an anionic species present at about 1×10 −1 to about 1×10 −6 molarity, wherein said anionic species is a halogen ion;
rinsing said surface to remove substantially all of said treatment solution; and
coating said surface with a corrosion resistant electrically conductive coating.