The Relationship between the Structure and the Electrocatalytic Activity of Nanocatalysts for the Oxygen Reduction Reaction

• Main Authors: Shih-Hong Chang, 張士浤
• Other Authors: Bing-Joe Hwang
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/43997984971319007115

博士 === 國立臺灣科技大學 === 化學工程系 === 99 === The oxygen reduction reaction (ORR) is the one of the most significant electrochemical reactions in proton exchange membrane fuel cells (PEMFCs). The main strategies for enhancing durability and reducing the cost of membrane electrode assembly (MEAs) are to improve the ORR kinetics and the utilization of catalyst. The main focus of this work is the manipulation of the size of Pt clusters and to investigate the relationship between the structure of bimetallic heterostructured nanoparticles and their electrocatalytic activities towards the oxygen reduction and methanol oxidation reactions. The following are the topics of research addressed in this dissertation: 1) CO assisted synthesis of finely size-controlled platinum nanoparticles - carbon monoxide (CO) retards the catalytic reactions of Pt because it behaves as a surface poison. Taking advantage of this phenomenon, we propose a new concept, namely the CO-assisted synthesis of Pt NPs, which is a wet-chemical method for achieving size-controllable clusters with high dispersion in surfactant-free solutions. To relieve the excess free energy, the existing nuclei coalesce thermodynamically to form large clusters, upon which deposition of subsequently formed nuclei occurs. In this protocol, the CO coverage on the Pt nuclei can be regulated by adjusting the CO/Ar ratio in the gas stream, this being a predominant parameter that controls the uniformity and dispersion on carbon black (Vulcan XC-72). EXAFS, XANES, XPS, TEM, CVs of anodic CO stripping supports the notion that CO coverage on the cluster surface plays a key role during the CO-assisted synthesis of Pt NPs. A sufficient level of CO (CO/Ar ≧ 75 %) not only hinders the coalescence (d < 2 nm) and deposition of nuclei but also facilitates further dispersion. We confirmed the validity of this CO-assisted synthesis for controlling the cluster size in a surfactant free solution. We believe that the concept of a “gaseous stabilizer” might be useful for preparing supported size-controllable clusters in the absence of contamination of surfactants on large scales. 2) Structural and electronic effects of carbon-supported PtxPd1-x nanoparticles on the electrocatalytic activity of the oxygen reduction reaction and on methanol tolerance: Among the PtxPd1−x/C nanocatalysts with various Pt: Pd atomic ratios (x= 0.25, 0.5 and 0.75) studied, rotating disk electrode measurements revealed that the Pt3Pd1/C nanocatalyst shows a synergistic effect on 50 % enhancement towards ORR and an antagonistic effect on 90 % reduction towards MOR compared to JM 20 Pt/C on a mass basis. The alloying extent and Pt d band vacancies of the PtxPd1−x/C nanocatalysts were explored by extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure spectroscopy (XANES), respectively. The structure-activity relationship indicates that ORR activity and methanol tolerance of the nanocatalysts strongly depend on their alloying extent and d band vacancies. The Pt3Pd1/C nanocatalyst with high alloying extent and low Pt d band vacancy represents an optimal composition for the enhanced ORR activity owing to the favourable O-O scission and the inhibited formation of oxygenated intermediates. Furthermore, MOR activity also shows the structure-dependence, such as: Pt1Pd3/C with the Ptrich-corePdrich-shell structure possesses lower MOR activity than Pt3Pd1/C nanocatalyst with the random alloy structure. In this paper, the alloying extent and d band vacancies shed new insight onto the synergistic and antagonistic effects with respect to the surface reactivity of the PtxPd1−x/C nanocatalysts. 3) Based on the knowledge gained from above-mentioned, the electrocatalytic activity toward ORR was enhanced by tailoring Pt with Pd due to the changed geometrical and electronic structures. In this study, the electrocatalytic activity toward ORR for bimetallic shell layer of Pt-M (M: Au, Ir) depositied on Pd core were investigated. The cyclic voltammogram suggested that the addition of Au and Ir inhibit the OH-formation, which may improve the ORR kinetics.The electrocatalytic activity toward ORR for Pd@Pt/C and Pd@Pt3Ir1/C was enhanced with a factor of 1.4 as compared with JM20 due to poor kinetics toward ORR for Au. We have demonstrated the decrease to ~ 45% of initial H2-ECSA for Pt/C and Pd@Pt/C after 3,000 cycles, indicating that the addition of Pd does not show the significant improvement in stability due to electrochemical dissolution of Pd.