Electronic States and Transport Phenomena of Pt Nanoparticle Catalysts Supported on Nb-Doped SnO₂ for Polymer Electrolyte Fuel Cells

Abstract

Semiconducting oxide nanoparticles are strongly influenced by surface-adsorbed molecules and tend to generate an insulating depletion layer. The interface between a noble metal and a semiconducting oxide constructs a Schottky barrier, interrupting the electron transport. In the case of a Pt catalyst supported on the semiconducting oxide Nb-doped SnO2 with a fused-aggregate network structure (Pt/Nb-SnO2) for polymer electrolyte fuel cells, the electronic conductivity increased abruptly with increasing Pt loading, going from 10–4 to 10–2 S cm–1. The Pt X-ray photoemission spectroscopy (XPS) spectra at low Pt loading amount exhibited higher binding energy than that of pristine Pt metal. The peak shift for the Pt XPS spectra was larger than that of the Pt hard X-ray photoemission spectroscopy (HAXPES) spectra. For all of the spectra, the peaks approached the binding energy of pristine Pt metal with increasing Pt loading. The Sn XPS spectral peak proved the existence of Sn metal with increasing Pt loading, and the peak intensity was larger than that for HAXPES. These spectroscopic results, together with the scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX) spectra, proved that a PtSn alloy was deposited at the interface between Pt and Nb-SnO2 as a result of the sintering procedure under dilute hydrogen atmosphere. Both Nb spectra indicated that the oxidation state of Nb was +5 and thus that the Nb cation acts as an n-type dopant of SnO2. We conclude that the PtSn alloy at the interface between Pt and Nb-SnO2 relieved the effect of the Schottky barrier, enhanced the carrier donation from Pt to Nb-SnO2, and improved the electronic transport phenomena of Pt/Nb-SnO2.

Engineering Catalyst Layers for Next‐Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

“…Kakinuma et al. [ 402 ] reported that the Pt deposited onto Nb‐doped SnO 2 formed a PtSn alloy between the catalyst and the surface, which enhanced the electron transfer to the catalyst by reducing the Schottky‐type barrier. They also reported that the conductivity had a significant dependence on the Pt loading.…”

Enhancement of the Catalytic Activity and Load Cycle Durability of a PtCo Alloy Cathode Catalyst Supported on Ta-Doped SnO2 with a Unique Fused Aggregated Network Microstructure for Polymer Electrolyte Fuel Cells

“…We reported that the PtSn alloy was inserted in the heterointerface between Pt and SnO 2 , and then, the Pt catalyst became well-orientated along with the relaxation of the lattice distortion and formation of the intermediate phase of PtSn alloy. The latter would diminish the effect of a possible Schottky barrier, enhance the high electronic conductivity, and impede the movement of Pt catalyst particles, as reported in previous papers. − , The loading amounts of Pt 100– x Co x alloy were 15.4 wt % ( x = 19), 16.4 wt % ( x = 23), 18.6 wt % ( x = 24-HR), 17.4 wt % ( x = 25), and 17.9 wt % ( x = 33). The chemical composition ratios (Pt/Co) of all of the catalysts also corresponded well to the chemical ratios measured by ICP-MS.…”

“…The Pt 75 Co 25 alloy was also well-oriented with respect to the Ta–SnO 2 support (inset of Figure ). In our previous paper, we evaluated the interface between the Pt catalyst and the SnO 2 support by XPS, hard X-ray photoelectron spectroscopy (HAXPES), and STEM–EDX and confirmed that the PtSn metal alloy layer was inserted at the interface . The PtSn metal alloy was formed during the heat treatment procedure after the Pt catalyst was deposited on the SnO 2 support.…”

“…In our previous paper, we evaluated the interface between the Pt catalyst and the SnO 2 support by XPS, hard X-ray photoelectron spectroscopy (HAXPES), and STEM−EDX and confirmed that the PtSn metal alloy layer was inserted at the interface. 50 The PtSn metal alloy was formed during the heat treatment procedure after the Pt catalyst was deposited on the SnO 2 support. We reported that the PtSn alloy was inserted in the heterointerface between Pt and SnO 2 , and then, the Pt catalyst became wellorientated along with the relaxation of the lattice distortion and formation of the intermediate phase of PtSn alloy.…”

Composition, thickness, and homogeneity of the coating of core–shell nanoparticles—possibilities, limits, and challenges of X-ray photoelectron spectroscopy

“…From these observations, a pronounced core–shell structure with an Er-rich shell is indicated for sample A, whereas in sample B, intermixing between Y, Yb, and Er with a slight enrichment of Yb in the near-surface region is consistent with the data. Such spatial distribution of the sensitizer and activator ions enables control of the donor–acceptor interaction and dynamics thereby eliminating or reducing deleterious cross-relaxation between lanthanide dopants and thus fine tuning of the optical properties [ 68 , 69 ]. Additionally, the location of emitting Er 3+ centres in the shell region enables very efficient luminescence resonance energy transfer (LRET) to organic molecules bound to the surface of the UCNPs due to the minimum distance between LRET donors and acceptors.…”