Article ID: TETSU-2017-085
Separators for solid polymer fuel cells must have a low contact resistance with the carbon paper and stability in a corrosive environment of sulfuric acid in the cell. The titanium surface is highly resistant to corrosion thanks to a passive film but has high contact resistance.
In this study, titanium carbide or nitride as the electrical conductor was formed on the surface by annealing commercially pure titanium sheet. The contact resistances of these sheets were evaluated before and after a sulfuric acid aqueous solution exposure test, “pH4 at 80°C for 4 days”, briefly simulating the operating environment. In addition, the same evaluation test was conducted with a surface with TiC formed dipped in nitric acid to enhance the stability in a sulfuric acid solution.
The initial contact resistance falls below 10 mΩ·cm2 by formation of TiC and TiN, Ti2N on sheet surface. However, the contact resistance rises to 100 or above after the exposure test because a large amount of TiO2 precipitates. This is probably because TiC and TiN are dissolved by sulfuric acid, generating TiO2.
By contrast, dipping in nitric acid hardly raises the contact resistance from less than 10 even after the exposure test. It is considered from the results of surface analyses that Ti ion generated by partial dissolution of TiC is turned into TiO2 by the oxidizability of nitric acid, changing the surface structure covering TiC. It is considered that the newly formed TiO2 film enhanced stability in a sulfuric environment.
