Preparation And Properties Of Hydrophobicity Coatings On 304 Stainless Steel Surfaces As Bipolar Plates For PEMFC

Bipolar plate is one of key component in Proton Exchange membrane fuel cell(PEMFC), which accounts for 60~80% weight of cell and 40% of the cost. Bipolar plate performs a critical function of collecting and transmitting electric current, separating the anode and cathode, supporting the MEA and conducting thermal energy. Stainless steel is the most promising candidate materials for bipolar plate for PEMFC due to its high electric and thermal conductivity, low gas permeability, low material and processing costs. Channel is one of important design and element of bipolar plate, which will directly influence the performance and efficiency of PEMFC. Channel performs a critical function of transporting fuel and gas, collecting water; water management is the urgency problem affecting the actual application of PEMFC, therefore surface modification is needed. One of direct and effective way to improve its surface hydrophobicity is preparing excellent properties of hydrophobicity film on metal bipolar plate. In this paper, self-assemble excellent hydrophobicity film on stainless steel surface is investigated.In our previous work, it is found that multy-step treatments with acid treatment-heat treatment-electrochemical treatment on the 304 SS surface show advantages over others. So further hydrophobic coating for the surface is carried out in this paper. The properites of hydrophobic and corrosion resistance with different coatings is analyzed in simulate PEMFC environments. The optimal DB film is obtained by immersing the SS304 in 6-N,N-(dibutyl)-1,3,5-triazine-2,4-dithiol(DB) solution for self-assemble 15 min. The 304SS/DB sample with optimazed property shows average contact angle is 99.08°, self-corrosion potential of 22.7mV and corrosion current density of 3.33×10-8A·cm-2, respectively. After 10 h polarization in simulated PEMFC environment its property of hydrophobic and corrosion resistance reach stable, average contact angle is 89.85°, self-corrosion potential and corrosion density current density is-135.1mV and 1.81×10-8A·cm-2, respectively. The composition and elements analysis of 304SS/DB sample by FT-IR and XPS indicated that 304 SS sample was covered by DB film basically. And DB film have good associativity with the SS substrate through mercaptan formed-S-Fe/Ni bond. FE-SEM and EDS analysis showed that after DB film coated on 304 SS surfaces micro roughness structure is formed on surface. After long time polarization, it is found that the roughness of surface decreased. And with polarization time increased, the surface elements content of C, N, S decreased, but the content of Fe and Cr increased.In order to further improve its hydrophobicity, octadecyl trichlorosilane(OTS) film was covered on 304SS/DB surface. The optimal film is obtained by immersing the SS304/DB in OTS solution for 6h self-assemble. The 304SS/DB/OTS sample show excellent hydrophobic of 148.72°contact angle, self-corrosion potential of-5.3mV, self-corrosion current density is 1.03×10-8A·cm-2. After 10 h polarization in simulated PEMFC environment, its property of hydrophobic and corrosion resistance still keep good, average contact angle is 138.18 °, self-corrosion potential is-196.4mV, and self-corrosion density current density is 0.73×10-8A·cm-2. The composition and morphology analysis of 304 SS sample by FT-IR and FE-SEM indicated that after dealing with OTS, the DB/OTS composite film have good associativity with the SS substrate. The surface is observed with macro-nano hierarchical structure roughness which is in favor of hydrophobicity. After long time polarization, the surface roughness decrease, but still have excellent hydrophobic and corrosion resistance properties.