Investigation of Pt supported on carbon, ZrO2, Ta2O5 and Nb2O5 as electrocatalysts for the electro–oxidation of SO2

Abstract

The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electro-thermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electro-oxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electro-catalysts were prepared using sodium borohydride as a reducing agent. These electro-catalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electro-chemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability.