Enhanced Oxygen Reduction Reactions and Stable Long-term Activity on TiO2-supported Dealloyed PtCu Nanoparticles in Acidic Aqueous Solutions

In the research and development of polymer electrolyte fuel cells (PEFCs), one of the challenges is to design better alternatives to the state-of-art Pt catalyst as anode and cathode catalysts in PEFCs, for which high power density has been obtained at room temperature. In particular, the oxygen reduction reaction (ORR) kinetics in the cathode is very slow, even at the surface of the Pt catalyst. Therefore, a large overpotential is required for the ORR to proceed at any practical speed under the operating conditions of PEFCs. To accelerate the ORR kinetics to reach a practical usable level in fuel cells, there has been a strong demand for the development of cathode ORR catalysts that can solve significant cost and durability issues as well as sluggish ORR kinetics. The partial [1] or complete [2] replacement of Pt metal with other metals has attracted considerable interest due to its potential to reduce the high costs of market batteries and to enhance electrocatalytic activity. Recently, we reported that PtPb/TiO₂ showed substantial electrocatalytic activity for ORR [3]. The nature of the support, the composition of catalytic sites as well as their interaction with the support, and the electronic structure of catalytic sites all most likely influenced the observed electrochemical behavior. Such enhancement of PtPb NPs in the ORR activity was mainly explained as follows: (i) change in the orbital structure of Pt atoms caused by coexisting with Pb atoms, (ii) change in Pt-Pt interatomic distance by insertion of Pb atoms into the Pt crystal structure. In addition, the enhancement of the catalytic activity, due to the presence of metal oxide support, is often called strong metal support interactions (SMSI) and significant effort has been devoted to understand this phenomenon [4]. SMSI has been reported also in the papers on the enhancement of ORR [5]. SMSI is usually explained in terms of partial charge transfer [6] or substrate-induced change in the lattice parameter of the metal deposited [7]. Particularly the change of the electronic properties of the NPs was attributed to overlapping of d orbitals (occupied) from deposited metal and the unoccupied d orbitals of the support. This PtPb NPs/TiO₂ system can be expected to exhibit the synergistic effect of the inherent electrocatalytic activity of PtPb ordered intermetallic surfaces and the electronic interaction between PtPb NPs and TiO₂ in the enhancement of ORR. However, in our previous study, the voltammograms obtained using a PtPb NPs/TiO₂-fixed glassy carbon (GC) electrode for ORR exhibited a broad shape caused by high electron resistance (IR resistance) because the PtPb nanoparticles (NPs, particle size 3.0 nm) were deposited on high resistivity TiO₂ particles (particle size < 25 nm) and because the Pt NPs/TiO₂ was fixed on a GC electrode with carbon black (CB) and Nafion. In this study, PtPb NPs were chemically deposited on small, thin TiO₂ particles that were prepared on CB, to obtain ORR voltammograms that did not show IR resistance. The step-by-step deposition of Pt and Pb intentionally designed for this study achieved the restrictive fixation of PtPb NPs on the small, thin TiO₂particles (Fig.1). Among the Pt NPs/CB, PtPb NPs/CB, Pt NPs/TiO₂/CB and PtPb NPs/TiO₂/CB samples, The PtPb NPs/TiO₂/CB showed the highest ORR activity (Fig.2).

References

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