ABSTRACT

Titanium oxynitride (TiON) coatings by reactive magnetron sputtering were studied to functionalize metallic bipolar plates of PEMFC (Polymer Electrolyte Membrane Fuel Cell). Stoichiometric TiN is a ceramic often used as a coating for its conductivity, hardness, and golden color. Under PEMFC's environment (acidic and 80°C), it displays a low interfacial contact resistance (ICR), however, its chemical stability, notably at high potential, is not enough to avoid oxidation and increase of ICR. Here, oxynitride titanium coatings are studied to offer enhanced stability under high-potential PEMFC conditions while a low ICR is maintained.

Ex-situ potentiodynamic tests, Electrochemical Impedance Spectrometry (EIS), and ICR measurements have been carried out to identify the optimum content of oxygen that enables maintaining a low ICR for the application while improving corrosion properties compared to TiN. The microstructure and chemical composition of the coating have been investigated.

It was found adding oxygen to TiN improves the chemical stability of the material, in particular at high potential. This is coming from the higher stability of ionocovalent bonds than the metallic bonds in the mixture. A composition was found which maximizes chemical stability while maintaining low ICR. Oxygen also modifies the microstructure which is more refined with larger columns and less active surface.

INTRODUCTION

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is attracting interest as a generator of electricity that does not emit greenhouse gases, provided that hydrogen is produced via clean processes. However, their large-scale commercialization is still limited by high cost and limited durability. As essential elements of the PEMFC, the bipolar plates (BPPs) enable the transport of electrons to the external circuit, guarantee the mechanical assembly of the cell, distribute the reactive gases, and evacuate the water. Thus, Bipolar Plates should provide gas tightness and high electrical conductivity to minimize electrical losses, and mechanical strength to reduce cell volume for mobility-related applications. It should be added to these specifications that the BPPs must resist corrosion in the acidic and humid environment of the PEMFC, to prevent chemical poisoning of the stack, and maintain low electrical losses.

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