Doping Effects of Metal-Doped Tin Oxide Nanoparticles Used As Carbon-Free ORR Catalysts for Polymer Electrolyte Fuel Cells
Authors: Fukuda Takeshi , Tsuji Ryuki , Tanabe Maito , Ito Seigo 
Published: December 2024

Polymer electrolyte fuel cells (PEFCs) are an important energy device for the realization of a hydrogen society. However, a longer service life is essential for the further spread of PEFCs. Currently, catalyst ( platinum supported on carbon, Pt/C) is the most mainstream cathode one. But, it has durability issues due to carbon degradation during high potential sweeps. As an alternative material to carbon, tin oxide (SnO2) is attracting attention due to its electrical conductivity and chemical stability. It has already been reported that Pt/SnO2 catalysts using SnO2 doped with antimony (Sb), niobium (Nb), tantalum (Ta), etc., and they exhibit high durability. However, they have a lower Pt electrochemical surface area (ECSA) and oxygen reduction mass activity (MA) compared to Pt/C. Therefore, further improvement of ECSA and MA is required for the practical use of Pt/SnO2. Other dopants may improve catalyst activity, so it’s worth checking the effects for doping other elements. In this study, a total of 20 metal-doped tin oxides (M-doped SnO2) were synthesized. Pt metal-doped tin oxide (Pt/ M-SnO2) was prepared by loading platinum onto the synthesized M-doped SnO2, and its catalytic performance was evaluated.

M-doped SnO2 nanoparticles were synthesized by the solvothermal method. Tin chloride pentahydrate, metal chlorides as doping materials, methanol, and tetramethylammonium hydroxide solution were mixed and stirred to form a precursor. The solution was placed in a Teflon-coated stainless steel autoclave and heat treated. After 12 hours, the reactants were washed and dried to obtain M-doped SnO2.

Pt/M-SnO2 was prepared using the polyol method. The prepared M-doped SnO2 powder was mixed and dispersed in a mixed solvent of ethylene glycol and water, then platinum chloride hexahydrate was added and stirred overnight. The solution was then heated at 120°C for 2 hours to produce the platinum nanoparticles on the SnO2 surface.

The activity evaluation of the prepared catalysts was carried out at a rotating disk electrode in a three-electrodes cell system. The electrolyte was 0.1 M HClO4, the counter electrode was a platinum wire electrode, the reference electrode was a reversible hydrogen electrode, and the working electrode was a glassy carbon disk (GCD) electrode. Catalyst ink was dripped onto the working electrode and dry by spinning at 300 rpm for 1 hour. Measurements were performed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). From the obtained peak, ECSA and MA values were calculated.

Among the Pt/M-doped SnO2, Pt/Mo-SnO2 and Pt/Sb-SnO2 showed specific peaks. In the CV, the peak area of the lower left part corresponding to the H2 adsorption was greatly enlarged in Pt/Mo-SnO2. As a result, Pt/Mo-SnO2 recorded the highest ECSA value (74.4 m2g-1
-Pt), exceeding that of Pt/pure-SnO2 (24.1 m2g-1
-Pt). In the LSV, the onset potential of the Pt/Sb-SnO2 peak shifted to the higher side than Pt/pure-SnO2. As a result, Pt/Sb-SnO2 recorded the highest value of Mass Activity (93.5 A g-1
-Pt), which is much higher than that of Pt/pure-SnO2 (17.1 A g-1
-Pt). The catalytic performance is significantly changed by doping metallic elements into tin oxide crystal.