Electrical and corrosion properties of metal nitride films prepared by evaporation system

Abstract

Fuel cell technologies have received much attention owing to their high efficiencies and low emission. The polymer electrolyte membrane (PEM) fuel cells are electrochemical device that convert the chemical energy directly to electrical energy. They have been considered variety of applications due to their high power density, rapid start up, work at low temperature. Especially, the PEM fuel cells a promising candidate as the next generation power sources for transportation applications. The PEM fuel cells have been consisted of many parts such as membrane, catalyst, bipolar plate, flow field, end plate and gas diffusion layer. The bipolar plates perform multiple functions, such as conducting electrons between adjacent cells, separating and distributing fuel and oxidant, and carrying away reaction products and heat from each cell. Therefore, high interfacial electrical conductivities and corrosion resistance are important properties for practical application.

Evaporation methods were used to deposit a film on AISI 316L stainless steel to satisfy the requirements for bipolar plate of PEM fuel cells. The chemical and electrical characteristics of films were studied for satisfied united states department of energy (US DoE) standards. Titanium nitride (TiN), Chromium nitride (CrN), and ternary (Ti,Cr)N thin films were produced by electron beam evaporation and thermal evaporation with inductively coupled plasma processes. With two different deposition methods, effects of high density plasma chemical and electrical properties were investigated. Davies method was used for measurement of the interfacial contact resistance (ICR) between samples and carbon paper. Potentiodynamic and potentiostatic test were performed at in 0.1 N H2SO4 + 2 ppm HF solution at 80℃ to simulate PEM fuel cells operation conditions.

First of all, the TiN coatings deposited by e-beam evaporation, ICR and corrosion values were 2.48 μAcm-2 (at 0.6 V) and 3.96 mΩcm2 (150 Ncm-2), respectively. While, TiN coatings deposited by thermal evaporation with ICP, ICR and corrosion values were 5.30 μAcm-2 (at 0.6 V) and 15.12 mΩcm2 (150 Ncm-2) each. Secondly, the CrN coatings ICR and corrosion values were 2.91 μAcm-2 (at 0.6 V) and 11.7 mΩcm2 (150 Ncm-2) for e-beam evaporation, 3.20 μAcm-2 (at 0.6 V) and 9.9 mΩcm2 (150 Ncm-2) for thermal evaporation with ICP. Finally, the (Ti,Cr)N coatings deposited by e-beam evaporation ICR and corrosion values was 6.37 μAcm-2 (at 0.6 V) and 4.5 mΩcm2 (150 Ncm-2). The (Ti,Cr)N coatings deposited by thermal evaporation with ICP showed the lowest corrosion current 0.93 μAcm-2 (at 0.6 V). Interfacial contact Resistance (ICR) values of (Ti,Cr)N coatings was lower than AISI 316L stainless steel used as substrate and ICR value was 10.62 mΩcm2 at the compaction force of 150 Ncm-2. Following measurements, only (Ti,Cr)N coating of corrosion and ICR were satisfied the US DoE targets of < 1 μAcm-2 and < 20 mΩcm2.