Improvements of Electrical and Electrochemical Activities at Nb-SnO2 Supports by Aggregation and Pt Loading as Fuel Cell Cathode Catalysts
Authors: Kakinuma Katsuyoshi , Senoo Yuichi , Chino Yuji , Taniguchi Koji , Uchida Makoto , Uchida Hiroyuki , Deki Shigehito , Watanabe Masahiro 
Published: February 2020

Polymer electrolyte fuel cells (PEFCs) are highly attractive power generation systems for fuel cell vehicles and residential co-generation systems. Platinum-based catalysts supported on carbon are important cathode catalysts for PEFCs, but the degradation of the carbon support in the high potential range is a key issue to be solved. Tin oxide (SnO2) is one of the good candidates for alternative non-carbon support with high electrical conductivity and stability in the high potential range.1 We have succeeded the synthesis of a Nb-doped tin oxide (Nb-SnO2) support with an aggregated network structure, like carbon black (CB), by a flame oxide-forming method.2-4 In this research, we evaluate the electrical conductivity and electrochemical activity of the Pt supported Nb-SnO2 with such unique morphology (Pt/ Nb-SnO2) by rotating disk electrode (RDE) and membrane electrode assembly (MEA). The obtained particles (crystallite size, 10-20 nm; specific surface area, 40-100 m2/g) were partially sintered with nearest neighbors and constructed an aggregated network structure with primary pore (diameter < 30 nm) and secondary pore (diameter > 30 nm). We evaluated the level of necking between nanoparticles with “Necking Index” (NI=SBET/SXRD) defined by the fraction of a specific surface area of particles measured by BET method (SBET) to that of the particles estimated from the mean crystallite size determined by the XRD method (SXRD). The apparent electrical conductivity (σapp.) of the support increased with decreasing NI, (Fig. 1, bold line), which rely on both the decrease of contact resistance by development of the necking among the particles and the change of microstructure from a closed-packed aggregate type to a chain-like aggregate type. Some particles exhibited a deviation from the simple correlation curve to higher conduction values even though the NI was nearly the same (Fig. 1: dotted line). The electrical conductivities of corresponding Nb-SnO2 increased with increasing their primary pore volumes due to a development of ramification of the chain-like aggregated structure with improvement of the electron conducting paths (Fig. 2). Moreover, we found that σapp. of Pt/ Nb-SnO2 catalysts were more than two orders of magnitude higher than those of the corresponding supports.
The kinetically controlled current density (j
k,Pt) for the oxygen reduction reaction (ORR) at 0.80 V of Pt/Nb-SnO2 (17 wt%, Pt particle size, 3.0 nm) increased with increasing σapp. and exceeded that of commercial Pt supported on carbon black catalyst (Pt/CB) (Fig. 3). The single serpentine pattern cell (Japan Automobile Research Institute (JARI) standard cell) with Pt/Nb-SnO2 also showed superior performance to that with Pt/CB.
This work was supported by funds for the “HiPer-FC Project” of NEDO, Japan.

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