Synthesis and Characterization of Platinum on Carbon Nanoparticles Selectively Coated with Titanium Nitride (TiN)
Authors: Ortiz Richard , Ilhom Saidjafarzoda , Kattan Mor , Bayansal Fatih , Maramo Matthew , Ein-Eli Yair , Biyikli Necmi , Jankovic Jasna 
Published: August 2024

The growing demand for clean and efficient energy sources has been a direct result of the serious effect on the climate and the environment associated with high emissions of pollutants that are mainly produced by fossil fuel-based energy generators. Currently, green technology like proton exchange membrane fuel cells (PEMFC) can produce zero-emissions energy power by primarily using the electrochemical reactions of hydrogen and oxygen to generate electrical energy. Additionally, due to their high activity, Platinum (Pt) nanoparticles (NPs) distributed on a carbon catalyst support are considered the most efficient catalyst materials used in this technology. However, the main drawbacks of this technology are (i) the scarcity of Pt, making it very expensive for widespread fuel cell application and commercialization, and (ii) the susceptibility of the carbon support to corrosion and degradation when operating even at the most typical PEMFC conditions. Carbon corrosion can induce the loss of Pt NPs' active surface area when Pt detaches from the carbon support or agglomerates into larger Pt NPs, leading to premature performance losses in a PEMFC. In this work, we propose a novel approach to prevent carbon corrosion by selectively depositing a corrosion-resistant thin layer (~2 nm) of titanium nitride (TiN) only on the surface of the carbon support while the catalyst centers are left uncoated and accessible for reagent reactions (oxygen O and H+ protons). TiN was used as the coating material because of its attractive properties, such as electronic conductivity, enough to enable the free movement of electrons from the current collector toward the catalytic centers. The TiN layer would also act as an anchoring mechanism of Pt NPs, preventing their mobility on the carbon support and subsequent agglomeration. The deposition of TiN layers was carried out at 150°C with a hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) reactor and using tetrakis(dimethylamino)titanium (IV) as the metal precursor in Ar/N2 plasma, from which nitrogen was used as the co-reactant. Before HCPA-ALD, to prevent TiN from depositing on top of the Pt NPs, the surface of Pt NPs was selectively coated with a thin film of oleylamine (OAm), a polymer that would only absorb onto Pt NPs, which is later removed from the catalyst system (after the HCPA-ALD process is finished) by heat treatment at 185°C. 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. Electrochemical characterization, including cyclic voltammetry (CV) and oxygen reduction reaction (ORR), was performed in a rotating disc electrode (RDE) to further confirm the activity and effectiveness of our designed catalyst. Our preliminary results have shown that highly conformal TiN can be successfully grown onto Pt/C nanoparticles (as powders) by using HCPA-ALD with a custom-made agitator mechanism. The second phase of this research will be focused on inspecting the degradation mechanisms this novel catalyst system experiences after potential cycling in a Membrane Electrode Assembly (MEA).