Chiajen Hsu

Graduation Semester and Year

2013

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Fuqiang Liu

Abstract

Unique Au/Pd core-shell nanoparticles were synthesized via galvanic replacement of Cu by Pd on hollow Au nanoparticles. Au/PtCu core-shell nanoparticles were also synthesized using this method. These core-shell nanoparticles exhibited unique electrochemical properties - the Au/Pd nanoparticles demonstrated superior electrochemical activity for formic acid oxidation and the Au/PtCu nanoparticles improved the kinetics in oxygen reduction reaction, when compared to their corresponding pure metal particles. In this dissertation, we aim at investigating catalytic abilities of these core-shell nanoparticles and understanding electrochemical reactions in direct formic acid fuel cells.Different Pd thicknesses on hollow Au nanoparticles in the core-shell configuration have been investigated. The hollow Au nanoparticles served as substrates. First, a sacrificial Cu layer was coated on the Au nanoparticles and then replaced by Pd in a PdCl2 solution via a galvanic replacement reaction. The thickness of the Pd layer was determined by the Cu layer thickness which was tailored by the coating time. It has been found that the Au/Pd nanoparticles exhibited superior formic acid oxidation performance, lower CO-stripping peak potential, and long-term durability and stability compared to commercial Pd black due to enhanced electronic coupling between Au core and Pd shell.To optimize the cathodic reaction (so-called oxygen reduction reaction) in direct formic acid fuel cells, it is necessary to investigate the electro-catalytic abilities of cathodic materials. Herein, we adopted the Au/PtCu core-shell nanoparticles to study their performance and electro-catalytic mechanism in oxygen reduction reaction. The Au/PtCu core-shell nanoparticles were fabricated via galvanic replacement of Cu by Pt on hollow Au nanoparticles. The Pt thickness was also controlled by the Cu coating time. We found that up to 83 wt. % of Cu can be replaced in 5 mM K2PtCl4 solution, forming an alloyed PtCu phase which was confirmed by X-ray diffraction data. Results showed 2-2.5 times higher in area specific activity and mass specific activity of the Au/PtCu catalysts than the commercial Pt black and Pt/C in oxygen reduction reaction, measured by a rotating disk electrode system. The kinetic electrochemical studies showed a four-electron transfer process, suggesting an efficient pathway of O2 directly reduced to H2O. Besides, Au nanoparticles with a thinner PtCu shell (25 nm thickness) demonstrated a significant CO oxidation peak shift (by 0.13 V) and improved long-term durability probably due to the core-shell structure and the electronic coupling effect. Further improvement in both activity and durability of the Au-based core-shell nanoparticles relies on the electronic coupling between core and shell which can be manipulated by tuning the Au core surface roughness. Two methods have been attempted: varying Au particle size by changing of the Au solution concentration and controlling surface roughness by adding a Na2SO3 solution. Smaller and more porous Au nanoparticles were formed when dilute Au solutions were used. The Au/Pd nanoparticles synthesized using a dilute Au concentration (7.775 g L-1) showed the highest formic acid oxidation activity (0.93 mA cm-2 at 0.3 V). In addition, the roughness of the hollow Au cores was also tailored by adding a Na2SO3 solution. It was found that the higher concentration of Na2SO3 was used, the rougher Au nanospheres became. However, the rough Au surface may reduce the electronic coupling with the Pd layer and decrease the catalytic abilities. The Au/Pd nanoparticles synthesized without the Na2SO3 solution yielded the smoothest Pd surface and demonstrated the highest formic acid oxidation activity (0.71 mA cm-2 at 0.3 V).Raman spectroscopy was adopted to study formic oxidation mechanism on the Au/Pd core-shell nanoparticles thanks to the enhanced surface plasmon resonance from the hollow Au cores. By combining with electrochemical stripping analysis, the in situ Raman studies helped reveal the electrochemical intermediate species at differently applied potentials. In this study, CO stripping test in conjunction with the in situ Raman studies showed a lower oxidation potential (0.6 V vs. Ag/AgCl) of CO on the Au/Pd nanoparticles than the Pd black (> 0.6 V), suggesting a strong electronic coupling between Au and Pd in the core-shell nanoparticles which contributes to the enhanced electro-oxidation of formic acid. To further explore the formic acid oxidation mechanism, oxidation of formate-based solutions were studied on the Au/Pd nanoparticles. Compared to the Pd black, the Au/Pd nanoparticles showed superior catalytic activities and stabilities especially when using concentrated formate solutions, indicating formate as the main electro-oxidation intermediate species in the reactions. Furthermore, oxidation of the formate-based solutions was numerically simulated. The results indicated a lower formate and hydroxyl coverage at the same applied potential for the Au/Pd nanoparticles than the Pd black, suggesting an efficient removal of these intermediates probably due to the strong electronic coupling within these nanoparticles.

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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