Electrocatalytic activity of sputtered Pt and Pt3Pd2 thin films for SO2(aq) electro-oxidation
The ever growing demand for energy and worldwide focus on global warming have dictated the course of research in energy production for the past couple of decades, with much emphasis placed on the need for clean and renewable energy sources as an alternative to the traditional fossil fuels. Possible methods for a clean, renewable and environmentally friendly energy source are based on the production of hydrogen through electrolysis. These methods have drawn considerable attention, as hydrogen can be used in a fuel cell where it combines with oxygen to produce H2O and electrical energy. The electro-oxidation of aqueous SO2 to H2SO4 represents one of the most promising avenues for large scale hydrogen production. In this reaction, H2O is split indirectly to produce hydrogen gas as a product. The attractiveness of this reaction lies in the fact that it occurs at a much lower standard potential (E0 = 0.158 V) than direct water electrolysis (E0 = 1.230 V). Although the electro-oxidation of SO2 appears to be a highly efficient method for splitting H2O, experimental results show that a large overpotential is required to drive this reaction at acceptable rates. This extra energy input is mostly due to the sluggish electrode kinetics of the SO2 electro-oxidation reaction. Therefore, the optimisation of this reaction through electrocatalyst development is of great importance. Most studies have been done using Pt as catalyst material, and relatively little research has been conducted on binary catalysts. In this study, binary PtxPdy and Pt/Pd thin film catalysts were produced on glassy carbon supports with magnetron sputtering and compared to Pt to determine the viability of these binary electrocatalyst compositions as a replacement for pure Pt. The most promising PtxPdy catalyst, as well as Pt, was also subjected to rapid thermal annealing to study the effect that annealing has on the performance of the electrocatalysts. The electrocatalytic activity and stability of the various catalysts were determined with multiple electrochemical techniques, which included cyclic voltammetry, linear polarisation and chronoamperometry. Rotating disc electrode experiments were also conducted in order to determine the kinetic properties of the catalysts through the use of various analysis methods. Other techniques such as scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray diffraction were also employed to characterise various physical properties of the thin film electrocatalysts. It was found that Pd was not as active towards the electro-oxidation of SO2 when compared to Pt, and that Pd and Pd-rich binary catalyst compositions suffered from stability issues due to the dissolution of Pd in acidic media. However, it was determined that some of the PtxPdy catalysts exhibited superior activity when compared to Pt as well as acceptable stability. Pt3Pd2 was found to be the most active binary catalyst and was thus used in further experiments alongside Pt. Physical characterisation of the catalyst thin films revealed that annealing caused various changes in the structure and properties of the catalysts. Annealing was also shown to negatively impact the activity of the electrocatalysts; however, it did improve stability in some cases. Tafel analysis of the data obtained from the rotating disc electrode experiments confirmed that annealing does indeed have a negative effect on catalyst activity. Kinetic parameters determined with Tafel analysis showed that while non-annealed Pt had the highest exchange current density, non-annealed Pt3P2 exhibited the lowest Tafel slope, and that for the SO2 electro-oxidation reaction the Tafel slope appeared to be the most important kinetic parameter. It was concluded that Pt3Pd2 is a promising replacement for Pt as catalyst material and that annealing has a positive effect on the stability of Pt3Pd2. Although annealing had a negative effect on the activity of the electrocatalyst thin films, this effect was much less pronounced on Pt3Pd2, and annealing could possibly be employed to improve the stability while retaining the activity of the binary catalyst, if the correct annealing temperature can be determined.