Influence of surface roughness and hydrophobicity of bipolar plates on PEM performance

The effect of surface roughness on wettability of different coating materials on bipolar plates employed in proton exchange membrane fuel cells (PEMFC) was investigated in this study. Aluminum substrate was coated with three different materials (NiCr, NiCrBSi, and Co(Ni)CrAlY) using high velocity oxy fuel (HVOF) process. Surface topology and roughness were then evaluated using atomic force microscopy and roughness profilometer. Moreover, the contact angle (CA) of different coatings was measured both in as-sprayed condition and after grinding in order to investigate the influence of roughness on the surface hydrophobicity parameter. It was found that among the three coatings with hydrophobic nature, NiCrBSi and Co(Ni)CrAlY showed the highest (128 degrees) and the lowest (106 degrees) CAs, respectively, whereas the CA of the uncoated Aluminum was determined to be 84 degrees showing the hydrophilic nature of the substrate. The surface roughness of NiCrBSi, Co(Ni)CrAlY and NiCr were Ra = 7.632 mu m, Ra = 5.510 mu m and Ra =5.974, respectively. It shows that NiCrBSi and Co(Ni)CrAlY exhibited the highest and lowest surface roughness. It was also found that for all coating materials, power-current curves of the single cells illustrated a rising trend in power with increasing relative humidity from 30% to 100%. Such behavior was found to be due to the hydrophobic property of the coated surfaces through the removal of the produced water. This in turn prevented water blocking and flooding known as fuel cell performance and durability weakness points.. Among the sprayed coating materials used in this study, NiCrBSi exhibited the highest hydrophobic property, and hence, the greatest cell performance.