Microwave-assisted synthesis of palladium-based core-shell nanocatalysts and iron phthalocyanines and their applications in direct alkaline alcohol fuel cells

Abstract

Palladium-based nano-alloys are well-known for their unique electrocatalytic properties for direct alcohol alkaline fuel cells (DAAFCs). This dissertation describes, for the first time, the synthesis of a novel ternary palladium-based core-shell nanocatalyst containing iron, cobalt and palladium (i.e., FeCo@Fe@Pd) using a microwave-assisted solvothermal technique. This microwave-induced fast and efficient synthesis of sub-10 nm sized palladium-decorated FeCo@Fe core-shell nanoparticles (ca. 3 – 7 nm) from a large-sized FeCo@Fe (0.21 – 1.5 μm) precursor, clearly suggests a ‘top-down’ nanosizing. I have termed this technique the “microwave-induced top-down nanostructuring and decoration (MITNAD)”. The nanocatalysts were successfully supported on commercial carbon (C, Vulcan®) and carboxy- and sulphonate-functionalised multi-walled carbon nanotubes (CNT-OH and CNT-SO3). They were thoroughly characterised using HRTEM, HAADF-STEM, EELS, SAED, FESEM, and EDX. The characterisation clearly proved the elemental composition, nanoparticulate nature, particle size ranges, core-shell nature, and morphology of the nanocatalysts. The nanocatalysts were subjected to electrochemical characterisation to establish their possible application as viable catalysts for DAAFCs. Half-cell anodic oxidation studies were carried out in alkaline medium using both monohydric alcohols (i.e., methanol and ethanol) and polyhydric alcohols (i.e., ethylene glycol and glycerol). Oxygen reduction reaction (ORR) was done in alkaline medium. Electrocatalytic parameters like oxidative onset potential, current response, electron transfer capabilities, Tafel values, ion exchange current densities, catalytic rate constants, and cycling stability as well as alcohol cross-over tests were all determined using various electrochemical techniques. Interestingly, the nanocatalysts exhibited excellent resistance to possible alcohol cross-over during ORR, which gives excellent promise for application in direct alkaline alcohol fuel cells (DAAFC). In general, the electrochemical performance of the nanocatalysts follows this trend: FeCo@Fe@Pd/CNT-OH >> FeCo@Fe@Pd/CNT-SO3 ≈ FeCo@Fe@Pd/C. Based on the findings in the half-cell studies, a preliminary screening of the FeCo@Fe@Pd/CNT-OH and FeCo@Fe@Pd/C were carried out in the complete single fuel cell membrane electrode assembly (MEA) test. The catalysts were tested in DAAFCs using the above stated four alcohols. Faradaic and Energy efficiency performances were obtained in the monohydric fuel cells, while the fuel cell exhaust components were analysed in the polyhydric fuel cells to determine their capability for complete oxidation of these fuels. Using NMR, the exhaust products were carefully determined and, surprisingly, the FeCo@Fe@Pd/CNT-OH showed excellent selectivity towards complete oxidation of the polyhydric alcohols. The second aspect of this research deals with the study of the electrocatalytic properties of a novel N4-macrocyclic complex (i.e., metallophthalocyanine, MPc) catalysts for ORR in alkaline fuel cells. A new hydrophobic form of Fe (II) tetrasulfophthalocyanine was synthesized. As a cathode catalyst for fuel cells, its exceptional resistance to high concentrations of methanol during the cathodic reaction has been quite surprising. It was able to selectively continue the ORR even in the presence of high quantities of alcohol in the electrolyte. This clearly proves that a cheap MPc can serve as an alternative to the costly noble metal catalyst for ORR. Further studies using MEA in real fuel cells will be necessary to ascertain this assumption.