Template Assisted Synthesis of SnO2 Supports for Pt Electrocatalysts for Polymer Eectrolyte Membrane Fuel Cells
Due to its physicochemical, optical and electric properties, SnO2 is widely used in fuel cells, lithium-ion batteries, gas sensors and sensitized solar cells . 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 make SnO2 suitable for use as an electrocatalyst support, properties such as high electrical conductivity, stability, and bimodal porous structure are required. In this work, the influence of synthesis parameters (aging time, mass ratio of polystyrene to precursor salt) on SnO2 electrical conductivity and electrochemical stability had been studied. Polystyrene (PS) microspheres with average diameter of ~ 250 nm were used as a hard template for the synthesis of macroporous SnO2. They were prepared using 4,4΄-azobis(4-cyanovaleric acid) as an initiator for polymerization in accordance with the following work . The first series of SnO2 was synthesized by aging of sols from SnCl4 in the presence of PS suspension in water-ethanol solution. The PS was removed from SnO2 samples by annealing at 450°C in O2. 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. Physicochemical characterization of SnO2 samples was carried out using low-temperature N2 adsorption (77 K), Hg porosimetry, the scanning electron microscopy, X-ray diffraction and CHNS elemental analysis. The electrochemical stability of SnO2 samples 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 curves of 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" protocol 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 a bimodal pore distribution and spherical macropores with an average diameter of 110-180 nm. The increase of sols aging time led to the increase in SnO2 stability and conductivity due to the larger size of crystallites. 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 salt led to the increase in SnO2 conductivity and stability. The stability of SnO2 samples was comparable with that of commercial carbon black Vulcan XC-72. 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"). References
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