Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr23C6 forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr23C6 as electro-conductive channels for stainless steel.