Reduction of Contact Resistance on Titanium Sheet Surfaces by Formation of Titanium Carbide and Nitride, and its Stability in Sulfuric Acid Aqueous Solution

摘要

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, "with a pH of 4 at 80℃ 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-cm~2 by formation of TiC and TiN, Ti_2N on sheet surface. However, the contact resistance rises to 100 or above after the exposure test because a large amount of TiO_2 precipitates. This is probably because TiC and TiN are dissolved by sulfuric acid, generating TiO_2. 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 TiO_2 by the oxidizability of nitric acid, changing the surface structure covering TiC. It is considered that the newly formed TiO_2 film enhanced stability in a sulfuric environment.