Development of platino-iridium/ruthenium telluride nanoalloy electrode systems for possible application in ammonia fuel cell
South Africa is undergoing a serious consideration of hydrogen economy in an effort to develop safe clean and reliable alternative energy sources for fossil fuels. Ammonia is one of the promising candidates due to its low production cost, ease in liquefaction at ambient temperatures, and high energy density as compared to methanol. Ammonia has a high content of hydrogen atoms per unit volume and can easily be cracked down into hydrogen and nitrogen. In the last four years carbon intensive coal dependent South Africa has become one of the leading global destinations for renewable energy investment. Another driving force behind the technology is the prevalence of platinum reserves found in South Africa. Platinum group metals are the key catalytic materials used in most fuel cells, and with more than 75 % of the world’s known platinum reserves found within South Africa. In this thesis, I have developed novel electrocatalysts that are highly specific and selective for production of hydrogen using ammonia as a fuel source. The electro-oxidation of ammonia on platinum electrode drop coated platinum nanoparticles (PtNP), platinum iridium nanoparticles (PtIrNP), platinum ruthenium nanoparticles (PtRuNP), platinum telluride nanoparticles (PtTeNP) and ternary nanoparticles (PtIrTeNP) finally (PtRuTeNP) was systematically studied in alkaline solution of potassium hydroxide (KOH) by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrocatalysts were synthesised using sodium borohydride as a reducing agent and polyvinylpyrrolidone (PVP) as a stabilising agent from aqueous solutions of H2PtCl6/IrCl3/RuCl3/NaHTe mixtures. XRD confirmed that the binary and ternary electro-catalyst displayed characteristic patterns which indicated that all catalysts have shown the Pt face-centred-cubic (fcc) crystal structure and that the nanoparticles were poly-orientated. The structural characterization was further confirmed with FTIR and UV-vis, FTIR showed the most striking evidence that the PVP stabilized Pt presented a broad peak between 1288 cm-1 and 1638 cm‐1 which corresponded to C‐N stretching motion and C=O stretching motion of monomer for PVP, respectively. The narrow absorption peak centered at 1420 cm‐1 and 2880 cm‐1 occurred in which was ascribed to the C–H bonding due to the presence of PVP. This was due to the formation of coordinate bond between the nitrogen atom of the PVP and the Pt2+, Ir3+, Ru3+ and Te2+ ions. UV-vis was able to show the oxidation state of the nanoparticles and obtained an exponential graph shape which indicated complete reduction because there was no peak observed. Morphological characterization in the form of high resolution scanning electron microscope (HRSEM) revealed the formation of poly-orientated nanoparticles with average particle size of 23- 46 nm with slightly aggregated crystalline materials. The elemental composition of the alloy nanoparticles measured using energy dispersive spectroscopy (EDS) showed the presence of the four elements; Pt, Ir, Ru and Te. High resolution transmission electron microscopy (HRTEM) revealed the formation of crystalline non-aggregated 0.6-5 nm sized nanoparticles. The elemental composition of the alloy nanoparticles measured using energy dispersive X-ray (EDX) showed the presence of the four elements; Pt, Ir, Ru and Te. Selected area electron diffraction pattern (SAED) nanoparticles showed characteristic electron diffraction rings of Pt, PtIr, PtRu, PtTe, PtIrTe and PtRuTe, confirmed the phase and crystallinity of the materials. The electrocatalytic behaviour of the PtIrTe and PtRuTe nanoparticles for ammonia oxidation in KOH solution showed reduced overpotential properties and an increased current density compared to the bare Pt nanoparticles electrode thus providing a promising alternative for development of low-cost and high-performance electrocatalyst for electro-oxidation of ammonia. In terms of minimising the ammonia oxidation overpotential, catalyst selection were ranked as follows PtTe > PtRuTe > PtIr > PtRu > PtIrTe > Pt, with regards to maximising the exchange current density, the ranking was PtTe > PtIrTe > Pt > PtRu > PtIr > PtRuTe. The results were further interrogated with EIS which revealed in terms of minimising charge transfer resistance (Rct) the nano catalysts selection were ranked as follows PtRuTe ˃ PtIrTe ˃ PtRu ˃ PtIr ˃ Pt ˃ Bare Pt electrode ˃ PtTe. That meant that the conductivity of the catalysts facilitated the flow of charge through the nanoalloys onto the surface of the electrode. The difference in charge transfer resistance revealed that PtRuTe and PtIrTe nanoalloys had an obvious advantage in reaction activity. The application of ternary metal nanoparticles had significantly enhanced the catalytic activity toward ammonia oxidation. The role of the third component (Te) had improved the catalysts in reducing Nads adsorption on Pt. The enhanced catalytic activity has been attributed by a number of factors including the change in Pt–Pt inter atomic distance, number of Pt nearest neighbours, Pt 5d band vacancy, and Pt metal content on particle surface.