Ta-Based Catalyst Support for Proton Exchange Membrane Fuel Cell Applications

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising energy conversion devices. However, corrosion of the carbon catalyst support at the cathode is one of the main problems that is impeding commercialization of PEMFCs and, therefore, the development of a durable catalyst support material is a major technical objective. PEMFCs typically use carbon black as the support material for Pt nanoparticles.  However, carbon is susceptible to electrochemical corrosion, especially under conditions of fuel starvation during startup/shutdown cycles. Therefore, alternative catalyst supports, with many based on metal oxides (e.g., Ta₂O₅, TiO_x, WO_x, SnO₂, IrO₂), carbides (e.g., WC_x), metal nitrides (e.g., Ti_xN_y), etc., have been investigated to replace carbon. These metal oxides/carbides/nitrides/oxynitride have been reported to have good corrosion resistance under PEMFC operating conditions, but their conductivity needs to be improved, especially compared to carbon. Therefore, carbon is often added, forming composite materials. Furthermore, these alternative materials may offer strong metal/support interactions (SMSI), potentially enhancing catalyst activity and durability. Although Ta₂O₅/C composites have been investigated as a support material previously, no work has yet been reported on the use of TaO_xN_y/C for this purpose. TaO_xN_yis a material under study in our group, normally in the form of nanotubes.

The authors examine the properties of TaO_xN_y 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 TaO_xN_y 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 TaO_xN_y 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 TaO_xN_yNPs/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 TaO_xN_y NPs were successfully hydrothermally synthesized on the surface of the CICs.  For the 80 nm pore size CIC, the size of the TaO_xN_y NPs is 30-70 nm, presumed to be depositing both outside and inside the pores of the CIC nanostructure. The TaO_xN_y NPs also exhibit regular voids in their structure, assumed to arise from the higher density of TaO_xN_y (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, TaO_xN_y is electrochemically active negative of 0.6 V vs. RHE, giving a reversible redox feature proposed to be due to the Ta^5+/4+ redox reaction. Accelerated durability tests carried out by potential stepping between 0.8 and 1.4 V in acidic solutions showed that the TaO_xN_y NPs/CIC materials are more corrosion resistant than the CIC alone.  Overall, the use of hydrothermally synthesized TaO_xN_y NPs, combined with nitrided CIC, appears to be a promising route towards achieving enhanced corrosion resistance support materials for PEMFC cathode applications.