Electrocatalysis of fuel cell molecules on carbon nanotube platinum-ruthenium based electrodes

Abstract

The investigation of the kinetics of fuel cell (FC) molecules such as methanol (MeOH), ethylene glycol (EG) and formic acid (FA) on platinum (Pt), platinum/ruthenium (PtRu) and platinum based metal complexes (ruthenium tetrakis(diaquaplatinum)octacarboxy-phthalocyanine (RuOcPcPt) modified basal plane pyrolytic graphite electrode (BPPGE) was carried out. One of the major limitations of FC molecules is that Pt undergoes surface poisoning by strongly adsorbed reaction intermediates, carbon monoxide (CO) that eventually decreases the fuel cell efficiency. Thus, the integration of Pt and or Pt/Ru with functionalized multi-walled carbon nanotubes (fMWCNTs) and some N4-macrocycles such as ruthenium phthalocyanine complexes on the BPPGE towards these FC molecules have been studied in this work. However, this study focused mainly on Pt, Pt/Ru, ruthenium octacarboxy-phthalocyanine (RuOcPc) and RuOcPcPt nanoparticles. The MWCNTs, metal and N4-macrocycles provided the needed platform for the efficient electrooxidation of FC molecules with minimum or no poisoning. The first part of the thesis deals with electrocatalytic oxidation of the FC molecules using electrodes prepared by electrodeposition techniques. The section describes the comparative electrocatalytic behaviour of MeOH, EG and FA at MWCNT-Pt/Ru immobilized on BPPGE. The Pt/Ru nanoparticles were deposited on the substrate using the electrodeposition technique. The second part of this work deals with electrocatalysis of the FC molecules using BPPG electrode modified with chemically synthesized Pt nanoparticles integrated with RuOcPc. In both cases, successful modification of the electrodes with the metal nanoparticle/carbon nanotube or metal nanocomplex nanocomposite was established using the field emission /high resolution scanning electron microscopy (FESEM/HRSEM), high resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD) spectroscopy and electron dispersive x-ray spectroscopy (EDS). The average particle size for the synthesised Pt nanoparticles is 1.4 nm. The electrocatalytic behaviour of the modified electrodes was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results of the electrodeposition study showed that the presence of Pt nanoparticles, together with Ru nanoparticles, gave better performance with the FA showing the least tolerance to electrode poisoning. The impedance spectra of the MWCNT-Pt/Ru hybrids in all the FC materials studied showed some dependence on the oxidation potential. These spectra were somewhat complicated but generally followed electrical equivalent circuit models characteristic of adsorption-controlled charge transfer kinetics. EG and MeOH showed conventional positive Faradaic impedance spectra, irrespective of the applied oxidation potential. FA impedance spectra exhibited an inductive loop only at the extreme forward anodic peak potential, characteristic of Faradaic current being governed by the occupation of an intermediate state. On the other hand, the presence of phthalocyanine with the synthesized Pt-Ru nanocatalysts showed an improvement on the tolerance to CO poisoning during MeOH oxidation and therefore its application in the direct fuel cell oxidation is encouraged. The synthesized Pt-based nanoparticles gave better performance compared to the electrodeposited Pt-based nanoparticles. The comparative electrocatalytic behaviour of the chemically synthesized nanocatalysts indicated that the BPPGE-fMWCNT/RuOcPcPt electrode gives the best performance towards MeOH oxidation compared to other electrodes studied, while FA oxidation was favoured on the BPPGE-RuOcPcPt electrode without CNTs support. However, EG oxidation was not successful at the electrodes at all. The oxidation of these FC molecules are characterized by both diffusion (forward) and adsorption-controlled (reverse) processes. The two electrodes (BPPGE-fMWCNT/RuOcPcPt and BPPGE-RuOcPcPt) gave better tolerance to oxidation poison with the ratio of the current density of the forward anodic peak to the reverse anodic peak (Jfa/Jra) and (Jfa1/Jfa2) of 4.0 and 1.0 respectively. The electrodeposited and chemical synthesized nanocatalysts results shown in this work have for the first time provided some useful insights into the electrocatalytic response of FC molecules (MeOH, FA, EG) for potential application in fuel cell technology. The third part of the thesis describes the electrocatalytic reduction of molecular oxygen in alkaline solution using a novel ruthenium tetrakis (diaquaplatinum)octacarboxyphthalocyanine (RuOcPcPt) electrocatalyst supported on MWCNTs. The results revealed that the MWCNT-RuOcPcPt electrode is electro-catalytically active than MWCNT, MWCNT-RuOcPc, RuOcPc and RuOcPcPt electrodes towards oxygen reduction reaction. The study shows that the oxygen reduction activity follows a direct 4-electron transfer process with high kinetic rate constant, 3.57 x 10-2 cm s-1. The results obtained imply that more energy has been achieved and therefore the electrode is a promising candidate as a catalyst in the cathodic reaction of fuel cell.