Macroporous SnO2 as an Electrocatalyst Support
SnO2 is wide bandgap semiconductor which has been investigated in many application fields such as lithium-ion batteries, gas sensors, sensitized solar cells and electrocatalysis . One of the most perspective SnO2 application is to use it as a support for Pt electrocatalysts in fuel cells because they have a higher stability compared to that of conventional Pt catalysts based on carbon black . To use SnO2 as a support for a Pt electrocatalyst, it is necessary to obtain a SnO2 material with high electrical conductivity, stability and a bimodal porous structure that provides the active mass transport of reagents and products. Thus, the aim of this work is to obtain SnO2 with high conductivity, stability and bimodal porous structure using template technique and to study the influence of synthesis parameters on SnO2 electric conductivity and electrochemical stability.
Polystyrene (PS) microspheres with average diameter of ~ 250 nm were used as a template for the synthesis of macroporous SnO2. They were prepared using 4,4΄-azobis(4-cyanovaleric acid) as an initiator in accordance with the work . The first series of SnO2 was synthesized by aging of sols from SnCl4 in the presence of PS suspension in H2O-C2H5OH solution. The PS was removed from SnO2 samples by annealing at 450°C. The final powders were yellow. The second series of SnO2 was synthesized from SnC2O4. At first stage, SnCl2 was precipitated by (NH4)2C2O4 in the presence of PS suspension while stirring. Then PS was extracted from SnC2O4 by toluene at 110°C and then the material was heat treated at 400°C to provide thermal decomposition. The final SnO2 powders were brown.
The SnO2 samples were characterized by using low-temperature N2 adsorption (77 K), Hg porosimetry, the scanning electron microscopy (SEM), X-ray diffraction and CHNS analysis. The SnO2 stability was studied by using accelerated “Start/Stop cycling” protocol in 1–1.5 V RHE of the potential range in 0.1 M HClO4. The cyclic voltammograms were recorded in the range of 0.05-1.2 V RHE with a sweep rate of 0.05 V/s before the "Start/Stop cycling" and every 2000 cycles. Electrical conductivity was studied by the impedance spectroscopy in 10-1-105 Hz of the frequency range using home-made cell.
The final SnO2 samples had the BET surface areas of 31-45 m2/g. The SnO2 samples had macropores (110-180 nm) and mesopores (6-14 nm) according to SEM and BET, respectively. The increase of sols aging time led to the increase in SnO2 stability and conductivity due to the larger crystallite size. Using thermal decomposition of SnC2O4, SnO2 with high conductivity up to 0.275 S/cm were synthesized. The increase of the mass ratio of PS to the precursor led to the increase in SnO2 conductivity and stability. The stability of SnO2 samples was comparable with that of carbon black Vulcan XC-72.
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The work was financially supported by the Ministry of Education and Science of the Russian Federation (RFMEFI60417X0159, title of the agreement: "Development of methods for hydrotreating of vacuum residue into high-quality marine fuels on macroporous catalysts").