Preparation and Characterization of Highly Durable and Effective TiO₂-Based Pt Catalyst Support for Polymer Electrolyte Membrane Fuel Cell

Polymer electrolyte membrane fuel cells (PEMFCs) attracted attention as next-generation energy conversion devices since they use hydrogen as fuel and produce only water. PEMFC electrodes are mainly composed of a precious Pt metal and a carbon support which has large surface areas and high electrical conductivities. However, carbon materials such as carbon black, graphene, and carbon nanotubes can affect the electron transport environment and Pt catalyst stability. During the operation of PEMFC, carbon support corrosion can be caused by the high potential above 1.2 V at the cathode during start-up/shut-down and it will lead to the loss of Pt nanoparticles which results in the severe performance degradation. Metal oxide was prospectively suggested because it is stable in oxidizing atmospheres. Among them, TiO₂ has excellent chemical, electrochemical, and thermal stabilities. Furthermore, strong interaction between Pt and TiO2 can bring a higher electron density on Pt which can result in higher stability and enhanced catalytic activity for oxygen reduction reaction (ORR). However, it has two main drawbacks as a Pt catalyst support, low electron transfer and small surface area.<br/>To overcome low electrical properties, TiO₂ was firstly doped with nitrogen. N-doped TiO₂ (TiON) produces Ti³⁺-enriched TiO_2-x phases and the characterization with TEM, HR-PXRD, and XPS suggested that it affect the electrical structure and charge transport properties. Pt/ N-doped TiO₂cathode enhanced electrochemical activity and improved durability. Secondly, Pt/Nb-doped TiO2 have a smaller Pt 4f binding energy than that of Pt/C and the H_UPD desorption and PtO reduction peaks in half-cell CV measurements supported higher stability. Nevertheless, modified TiO₂ with N or Nb are still lower electrically conductive than carbon materials. Hybridization of doped TiO2 with highly conductive materials can be a candidate for Pt support and pristine graphene was selected because of its high electrical conductivity, large surface area, and mechanical strength. Graphene was also doped with nitrogen. N-doped graphene (NG) exhibited higher electrical conductivity and catalytic activity and strengthened the chemical bonding between the support and catalyst, leading to the prevention of catalyst aggregation and the uniform distribution of platinum nanoparticles on the graphene support. NG-TiON hybrid support was analyzed with TEM, XRD, XPS and TG, and the investigation of electrochemical ORR activity and durability of Pt-supported with NG-TiON showed high stability and corrosion resistance with a performance degradation rate of only 9 % after 5,000 accelerated durability test cycles while the performance of Pt/C decreased 83 %. However, low surface area is an intrinsic drawback of TiO₂, compared to carbon.<br/>In this study, the electrical conductivity of TiO₂was controlled with Nb-doping, and SiO₂ precursor was added with a nonionic surfactant in the hydrothermal preparation of TiO₂particles from precursors. It was expected that SiO₂ contributes to increase the surface area with the maintenance of TiO₂ structure and electrical properties by suppressing the agglomeration of TiO₂ during heat treatment. The effect of SiO₂ on the surface area of Nb-doped TiO₂-SiO₂ support was investigated by the addition of different amount of SiO₂ presursors. Pt supported Nb-doped TiO₂-SiO₂ were analyzed with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and their electrochemical properties were investigated with potentiostat/galvanostat and electrochemical impedance spectroscopy (EIS). Performance of Pt/ Nb-doped TiO₂-SiO₂ catalyst as a cathode in a single-cell system will be reported.