Electrochemical Characterisitcs of Thin Carbon Coatings for Metal Bipolar Plates - Evaluation of the Relative Porosity Using EIS

The anticipation of a rapidly growing market demand for PEM fuel cell components, places the focus on the establishment of advanced, high-throughput processes to satisfy the material requests of the industry. Sandvik Materials Technology AB uses a roll-to-roll PVD process to produce carbon-coated steel for PEMFC bipolar plates that meet both technical requirements of corrosion resistance and electrical conductivity, as well as economic requirements. The main advantages of using pre-coated steel for bipolar plate production are increased productivity and decreased risks associated with parts handling. Furthermore, a consistently high material performance is achieved as the result of a very stable coating process combined with large batch sizes (several tons).

For improved corrosion resistance, low porosity coatings are generally desired. A method for electrochemical impedance spectroscopy (EIS) has been developed in which the relative porosity has been evaluated for carbon coated steel. The carbon coating is deposited on top of an intermediate metallic layer, which serves to enhance adhesion and corrosion resistance. Using this method, we have been able to verify that increasing the coating thickness and the number of coating layers resulted in a decrease in the porosity index. More interestingly, however, we have been able to show that the relative porosity index can be related to interfacial contact resistance measured for coated samples subjected to potentiostatic testing (100 h at 0.7 V vs. AgCl). The interfacial contact resistance of studied samples varied between 3-13 mΩcm² at a pressure of 140 N/cm².

Together these results indicate that the evaluation of relative porosity by means of EIS can be used as a relevant tool for assessing the corrosion resistance of coated metal intended for use as PEM bipolar plates.