Ta-Based Catalyst Support for Proton Exchange Membrane Fuel Cell Applications
The authors examine the properties of TaOxNy nanoparticle (NP)/CIC (colloidal imprinted carbon) composites as supports for Pt nanoparticles, specifically for use in PEMFC cathodes, and the influence of the synthesis method of the TaOxNy NPs/ CIC/Pt NPs on the durability of the catalyst support. This presentation has a specific focus on the effect of the method of preparation of TaOxNy NPs/CIC/Pt NPs on catalyst durability. It has been reported earlier that, when metal oxides are synthesized in the presence of a carbon support, they deposit preferentially on the defects or oxygenated sites, similar to Pt NPs, thus blocking some of the COR initiation sites (oxygenated functional groups on the carbon) by utilizing them to anchor the NPs. Thus, we have used hydrothermal synthesis to first deposit a Ta oxide precursor on the CIC surface, where Ta alkoxide is hydrolysed to Ta oxide at high temperatures (250 oC) in an autoclave. In the second approach, pre-prepared Ta oxide NPs were physically mixed with the CIC powder, assuming that the preferential deposition of the Ta oxide NPs on the oxygenated sites of the CIC would not occur, hence predicting a lower durability of this composite material. After formation of Ta oxide NPs/CIC by these two methods, the samples were subjected to ammonolysis to obtain TaOxNyNPs/CIC, which were then functionalized with various wt % of Pt NPs by the incipient wet impregnation method. Scanning and Transmission electron microscopies were used to determine the structure, morphology, and composition of these composite support materials, while cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to determine electrochemical characteristics and durability as a catalyst support.
The results have shown that the TaOxNy NPs were successfully hydrothermally synthesized on the surface of the CICs. For the 80 nm pore size CIC, the size of the TaOxNy NPs is 30-70 nm, presumed to be depositing both outside and inside the pores of the CIC nanostructure. The TaOxNy NPs also exhibit regular voids in their structure, assumed to arise from the higher density of TaOxNy (14 g·cm-3) relative to that of the Ta oxide precursor (8.2 g·cm-3). A systematic study of the electrochemical properties of both of the composites, as well as of the CIC component, was performed at each stage of the synthesis. It was found that the conductivity of the CIC improves somewhat after exposure to the ammonia environment, consistent with its significantly enhanced N content. Also, TaOxNy is electrochemically active negative of 0.6 V vs. RHE, giving a reversible redox feature proposed to be due to the Ta5+/4+ redox reaction. Accelerated durability tests carried out by potential stepping between 0.8 and 1.4 V in acidic solutions showed that the TaOxNy NPs/CIC materials are more corrosion resistant than the CIC alone. Overall, the use of hydrothermally synthesized TaOxNy NPs, combined with nitrided CIC, appears to be a promising route towards achieving enhanced corrosion resistance support materials for PEMFC cathode applications.