| Summary: | 博士 === 國立中央大學 === 材料科學與工程研究所 === 100 === In this study, the highly effective PdCo/C catalysts toward oxygen reduction reaction (ORR) are prepared. The preparation and modification approaches, including the control over pH values, addition of ceria (CeO2), oxidation heat treatment and incorporation of Au are used to synthesize Pd-Co/C with tunable structure and surface compositions and high catalytic activity. The PdCo/C catalysts with 20 wt. % metal loading are synthesized through the deposition-precipitation (DP) method and reduced under H2 atmosphere. The fundamental understanding of relationship between structures, morphologies, surface species and electrochemical properties is studied by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and various electrochemical measurements.
The study is divided into four parts. In the 1st part, the PdCo/C alloy catalysts with a Pd/Co atomic ratio of 3/1 are prepared at various pH values (pH = 9, 11 and 13) and reduced at 390 or 620 K under H2 atmosphere. The pH value affects the deposition and reduction rates of Pd and Co ions significantly and thus the structures, surface species and ORR activity of the PdCo/C catalysts are also influenced. Due to the enhancement of Co surface segregation and the formation of Co oxides on the alloy surface, the deterioration of ORR activity for PdCo/C catalysts prepared under the high pH values (pH = 11 and 13) and high temperature (620 K) is observed. On the other hand, the alloy catalysts deposited at pH = 9 and reduced at 390 K have well-formed Pd-Co alloy structure, Pd-rich surface and excellent ORR activity.
In the 2nd part, the ORR activity enhancement and surface species alteration of PdCo/C alloy catalysts with various Ce additions (10 ~ 20 wt. %) is studied. The electrocatalysts are prepared and subsequently reduced at 390 or 620 K. Although Co ions can be fully reduced only at 620 K, the reduction process conducted at 390 K can prevent the Pd-Co alloys from grain growth and undesired surface Co segregation. The addition of 15 wt. % CeO2 (Ce15) along with low reduction temperature (390 K) into the Pd-Co system promotes the enrichment of Pd surface species, resulting in the formation of a Pd skin / alloy core structure. As a consequence, Ce15 can present the superior ORR activity with a 2.3-fold improvement as compared to the un-modified, and CeO2 acting as a textural promoter and surface modifier for the PdCo/C alloy catalysts, can significantly improve their ORR activity.
In the 3rd part, the PdCo/C catalysts prepared in the 1st part are further oxidation treated at various temperatures (To = 320 ~ 620 K) and the oxidation induced improvement of the ORR activity for PdCo/C is investigated. Based on the XPS characterization, a new term, degree of surface oxidation (DSO), is proposed to illustrate the relationship between ORR activity enhancement and surface oxidation extent of the PdCo/C. The optimal temperature for the promotion of ORR activity on various oxidized samples is 520 K where its surface is majorly comprised of PdO. Besides, various heat treatment atmospheres (H2 and CO) are applied on Pd-Co system without changing their particle size. It is clearly evident that the PdCo/C catalysts oxidized at 520 K exhibit a better performance relative to that of the non-oxidized ones, confirming that the improved ORR activity is solely ascribed to the formation of surface PdO species with 100 % DSO value rather than the large particle size effect.
In the 4th part, ternary catalysts of Pd75Co25-xAux/C with varying x content (x = 5 ~ 25) are synthesized and the role of Au in altering structures, surface conditions and electrochemical properties of PdCo/C catalysts is systematically investigated. XRD results reveal that the incorporation of Au into Pd-Co system contributes to the formation of inhomogeneous alloy structure. Meanwhile, the fine structural details determined by XAS indicates that Au improves the heteroatomic intermix extent of alloy nanocatalysts significantly, especially for the Pd75Co10Au15/C (Au15) catalysts. TPR results suggest that the Pd-rich surface gradually changes to Pd, Au and alloy mixed surface species when Au content is larger than 15 at. %. All of the Au-modified PdCo/C catalysts have the particles size of ca. 4.9 ~ 5.4 nm and Au15 has the narrowest size distribution among all catalysts. For the electrochemical properties, the Pd mass activity of Au15 is ca. 4-fold and 2.5-fold higher than that of Pd/C and Au0 (Pd75Co25/C). Thus, it is demonstrated that Au15 displays the excellent ORR performance due to the improved extent of heteroatomic intermix in bulk structure, narrow size distribution, large electrochemical surface area (ECSA) and the concomitant multiple coexisting species predominated on the outermost surface.
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