Synthesis and Characterization of Platinum on Carbon Nanoparticles Selectively Coated with Titanium Nitride (TiN)

Abstract

The serious impact that high emissions of pollutants, derived from fossil fuel-based energy power generation, have on climate change resulted in the growing demand for cleaner and more efficient power sources. From the currently available green technologies, Proton Exchange Membrane Fuel Cell (PEMFC) offers the prospect of zero-emissions power production by using the electrochemical reactions of hydrogen and oxygen to generate electrical energy. Platinum (Pt)/Platinum alloy nanoparticles distributed on a carbon (C) catalyst support are the most efficient catalyst materials due to their high activity, even though the scarcity of Pt makes it expensive for widespread fuel cell application and commercialization. Furthermore, the carbon support is prone to corrosion and subsequent degradation when operating in typical PEMFC environment. This phenomenon can induce the loss of Pt nanoparticles (NPs) active surface area due to its detaching from the support, Pt NPs growth and Pt NPs agglomeration, which leads to premature performance losses in a PEMFC. Here we propose a novel way to prevent the carbon corrosion by depositing a corrosion-resistant layer of titanium nitride (TiN) on platinum on carbon (Pt/C) catalyst (in powder state). The goal of fabricating such designed catalysts is to selectively coat the carbon support with TiN, while the Pt catalyst centers are left uncoated and accessible for reagents reactions (oxygen O and H+ protons). Another attractive property of this TiN coating is its electronic conductivity, which is enough to enable the free movement of electrons from the current collector toward the catalytic centers. The deposition of the approximately 1.5 nm thick layer of TiN was carried out employing hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) while using tetrakis(dimethylamino)titanium (IV) (TDMAT) and Ar/N2 plasma as the metal precursor and nitrogen co-reactant, respectively, at 150°C. To maintain the Pt NPs protected from TiN deposition on its surfaces during HCPA-ALD, its surface was selectively coated with a thin film of oleylamine, a polymer that would only absorb onto Pt NPs, which is later removed from the catalyst and TiN system (after the ALD process is over), by heat treatment. Here we will mainly focus on the TiN deposition process and the subsequent Transmission Electron Microscopy (TEM) characterization performed on these nanoparticles, which allows us to prove the successful formation of homogenous coatings and architecture characteristics of these nanoparticles after oleylamine application and TiN deposition. Our preliminary results have shown that highly conformal TiN can be successfully grown onto Pt/C powder nanoparticles by using HCPA-ALD with a custom-made agitator mechanism. The second phase of this research will be focused on the electrochemical responses of the TiN coated catalysts and the effect TiN coating has on its catalytic response, as well as inspecting the degradation mechanisms this novel catalyst system experiences after potential cycling.