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Title: Synthesis of ultra-low Pt loading catalysts for direct liquid fuel cells
Authors: Luk, Sin Yee
Degree: Ph.D.
Issue Date: 2018
Abstract: Due to the depleting and limited supply of fossil fuel and the related environmental impacts, fuel cell technology is widely adopted to enhance efficiency and reduce pollution. In direct liquid fuel cells, platinum (Pt) and Pt-based electrocatalysts are reported to be the most effective catalyst in the literature but the massive commercialization is limited owing to its high cost and limited reserve. The aim of this project is to develop ultra-low Pt loading catalysts with improved Pt utilization and enhanced catalytic activity. Platinum-gold core-shell (AuNP@Pt) nanostructures supported on multi-walled carbon nanotubes (MWCNTs) composites (AuNP@Pt/CNTs) were prepared by Pt halide ion adsorption onto gold nanoparticles (AuNPs) surface, followed by in-situ electrochemical reduction. Electrochemical studies show that different Pt halides show different adsorption affinity and different electrocatalytic activity towards the electrooxidations of formic acid and methanol. It is found that varying the concentrations of tetrahaloplantiante complex [PtX₄]²ˉ (X = Cl and Br) ion and tetraammineplatinum(II) nitrate (Pt(NH₃)₄(NO₃)₂) solutions from 0.02 mM to 5.12 mM could control the Pt coverage on gold nanoparticles supported on MWCNTs (AuNP/CNTs) composite while hexahaloplatinate complex ([PtX₆]²ˉ) solutions could not. Also, varying immersion time with the concentrations of [PtX₄]²ˉ ion and Pt(NH₃)₄(NO₃)₂ solutions could control the Pt coverage on AuNP/CNTs composite. The AuNP@Pt/CNTs composites prepared by different Pt halide ions with different Pt coverages show different electrocatalytic activity due to the different specific packing modes of the Pt atoms deposit on the AuNPs surface. The electrocatalytic studies show that the AuNP@Pt/CNTs composites with Pt coverage of lower than 30 % were active to direct oxidation of formic acid but inactive to methanol. The AuNP@Pt/CNTs composite prepared by tetrachloroplatinate complex ([PtCl₄]²ˉ) solution with Pt coverage of higher than 30 % and prepared by tetrabromoplatinate complex ([PtBr₄]²ˉ) solution with Pt coverage of higher than 80 % start becoming active to indirect oxidations of formic acid and methanol.
Pd@PdPt/CNTs alloy and Pt@Pd/CNTs phase-separate composites were synthesized from surfactant-free Pd/CNTs and Pt/CNTs composites respectively. Both clean palladium (Pd) and Pt metal surfaces are available for surface modification to form alloy and phase-separate PdPt surfaces. The PdPt shell of Pd@PdPt/CNTs alloy catalysts were synthesized by the galvanic replacement reaction between Pd and [PtCl₄]²ˉ ions. The Pt@Pd/CNTs phase-separate composites were prepared by adsorption of tetrachloropalladate complex ([PdCl₄]²ˉ) onto Pt surface followed by hydrogen reduction. Powder X-ray diffraction (XRD) suggests that both Pd@PdPt/CNTs alloy and Pt@Pd/CNTs phase-separate composites adopt a face-centred cubic (fcc) structure. X-ray photoelectron spectroscopy (XPS) analysis confirms the formation of PdPt alloy structure. Compared with monometallic Pd/CNTs and Pt/CNTs composites, the Pd@PdPt/CNTs alloy and Pt@Pd/CNTs phase-separate composites with particular Pd-to-Pt surface ratio exhibit enhanced catalytic activity towards the electrooxidations of ethanol and glycerol. Hollow PtAg nanowires (NWs) composites were synthesized in the galvanic replacement reaction using silver nanowires (AgNWs) as sacrificial template and potassium tetrachloroplatinate(II) (K₂PtCl₄) as metal precursor. The Pt-to-Ag atomic ratio from 1 : 3.0 to 3.5 : 1 and surface ratio from 1 : 8.0 to 1 : 3.5 of hollow PtAg NWs composites could be controlled by varying the amount of K₂PtCl₄ present. Both XRD and XPS analyses confirm the alloying of Ag and Pt in hollow PtAg NWs. Electrochemical investigations show that the hollow PtAg NWs composites with specific Pt-to-Ag surface ratio exhibit enhanced catalytic activity towards the oxidations of methanol, ethanol, glycerol and formic acid when compared with Pt black composite.
Subjects: Hong Kong Polytechnic University -- Dissertations
Fuel cells
Pages: xv, 334 pages : color illustrations
Appears in Collections:Thesis

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