Polarization studies of thermally grown chromium nitrides for possible applications in polymer electrolyte membrane fuel cells

Fuel cells can offer near-zero emission power generation at high efficiencies by using the chemical energy of a fuel to produce electrical energy. However, improvement on the materials used in the fabrication of these devices is necessary. In the polymer electrolyte membrane fuel cells (PEMFC) the particularly aggressive environment limits considerably the materials selection. The bipolar plates are responsible for electrical contact among the cells, and at the same time, they guide the reaction gases and products as they enter and leave the cells. Thick graphite plates (with low power densities) are the most commonly used material for this application. Research has been focused on metals, which offer higher electrical and thermal conductivities and better mechanical properties than graphite. However, several authors have reported decreases in the process efficiency due to the formation of resistive surface oxide layers on the plates; as well as, metal-ion contamination on the polymeric membrane5,6 The use of protective coatings, such as thermally grown metal nitrides (e.g. Cr2N), can help to overcome this problem2·9 Recent work has demonstrated that thermally grown Cr-nitrides on Ni-Cr base alloys show promise as a means to protect metallic bipolar plates7 In support of this premise, the corrosion protection obtained by different nitridation conditions of the Ni-50Cr (wt%) system was studied in more detail. Results suggest that the corrosion resistance increases with the nitridation temperature, and this change is related to the different phases and morphologies obtained. The nitrides seem to be fairly stable at voltages on the order of 750 mV(SHE). Even though a single and uniform phase after nitridation has not been obtained at this point, results seem to imply that a mixture of CrN+Cr2N gives better corrosion protection in aerated sulfuric acid solution (pH 3 at 80 °C) than that observed with a mixture of Cr2N and 1t phase. Polarization behavior observed in a hydrogen purged environment showed that both mixtures (CrN+Cr2N and Cr2N+1t phase) presented low corrosion currents (below lxl0-6 A/cm2) in this electrolyte. Preliminary XPS analysis suggests that the current densities measured in the aerated environment are at least partially due to oxygen incorporation into/at the nitride layer. Details of the nitridation conditions, corrosion behavior and microstructure are presented.