本研究主要在發展同步輻射X光吸收光譜技術於能源材料鑑定之應用，能源材料包括能源轉換與能源儲存材料。本論文以燃料電池與鋰電池材料為主要研究系統，利用X光吸收光譜之解析能力探討觸媒之奈米結構與電極材料之結構缺陷。
第一部分為雙金屬PtCo觸媒奈米結構之鑑定。研究室發展一限制空間合成法製備中孔碳承載之雙金屬觸媒，在700℃高溫還原氣氛下熱處理後，無序結構之PtCo雙金屬奈米粒子轉變為有序之面心正方結構(L10)而無顯著之粒子團聚，因其粒徑太小而不易由XRD分析，故利用X光吸收光譜鑑定其結構。Co K-edge 吸收近邊緣結構圖譜，看出其在白線區域(white line)分裂成兩根明顯之吸收峰，此為面心正方結構(fct)之特徵鋒；而Pt LIII-edge 吸收近邊緣結構圖譜白線區域強度下降，而d軌域電子較填滿。而由結構參數看出Pt-Co異相配位數明顯增加。此觸媒進行氧氣還原活性之量測，其質量活性為無序之PtCo/mc與商業化E-TEK PtCo/C觸媒之1.7倍。其優越之催化活性可歸因於有序結構之PtCo其Pt-Co合金化程度較高，Co的電子轉移造成Pt之電子結構有利於氧氣還原反應。亦進行加速老化測試，發現在電位掃描10000圈後，H2-PtCo/mc觸媒之活性衰退為26%，遠低於PtCo/mc的58%。其穩定性之提升推測為面心正方結構在酸下形成穩定核殼結構與限制空間效應的加成結果。接著利用X光吸收光譜鑑定商業化E-TEK PtCo/C TKK-Pt3Co/C觸媒結構及其氧氣還原催化特性。由Co K-edge 吸收近邊緣結構圖譜發現E-TEK為無序之結構，而TKK觸媒為有序之合金結構。將所得之結構參數套入研究室發展之結構模型中，量化數據指出此TKK具有高合金化程度，其JCo與JPt值皆約為120%，高於E-TEK PtCo/C觸媒(JCo與JPt值分別為63.8與43.9，為Ptrichcore-Corichshell之核殼型結構)。TKK觸媒氧氣還原反應活性與穩定性皆優於E-TEK觸媒。
第二部分為鋰離子電池LiCoO2陰極電極材料氧缺陷之鑑定。研究中利用X光吸收光譜瞭解過量鋰鈷氧(Li-overstoichiometric, Li1+tCo1-tO2-t)材料中，Li+離子取代結構中Co3+的位置後，整個結構的電荷平衡機制。由Co K-edge 之吸收近邊緣結構圖譜看出，過量鋰(Li1+tCo1-tO2-t)之吸收位置與LiCoO2相近，其價數為Co3+；由Co K-edge EXAFS曲線配適之結構參數得到Co-O配位數為4.7，代表氧缺陷之形成。觀察電子產率模式之Co L-edge吸收圖譜，表面之Co3+亦有氧化為Co4+現象；由 O K-edge之吸收圖譜看出吸收前緣(pre-edge)的特徵峰為氧O 2p 軌域與中間自旋(IS)態之Co3+ 3d電子能階形成混成軌域，電子由1s軌域躍遷到2p-3d混成軌域之吸收峰。由吸收光譜鑑定得知Li+離子取代Co3+後結構之電荷主要藉由形成氧缺陷而來平衡；第二個系統為摻雜Mg進入LiCoO2中形成之LiCo1-yMgyO2結構，利用X光吸收光譜釐清Mg在LiCoO2結構中的位置與其電荷平衡機制。由Co K-edge 與Co L-edge吸收圖譜發現不同Mg2+摻雜後，Co之氧化價數皆維持+3，而由Co K-edge EXAFS曲線配適之結構參數得到Co-O配位數隨著Mg摻雜量的增加而下降，表示氧缺陷隨著增加。Mg之摻雜取代Co的位置後，電荷亦由產生氧缺陷來平衡，O K-edge之吸收圖譜看出無Co3+中間自旋(IS)態之產生。

In this thesis, X-ray absorption spectroscopy (XAS) was developed to as a nano-characterization tool for energy-related materials. Here, electrocatalysts for fuel cells and active electrode materials for Li-ion batteries are taken as examples to illustrate the capability of XAS to correlate the observed electrochemical phenomena with their theoretical background. XAS has been utilized as a powerful tool to characterize the nano-structured catalysts and structural defects in electrode materials.
The first part focuses on the characterization of PtCo bimetallic nanocatalysts. Here we present a novel synthetic route to prepare carbon supported PtCo catalysts with structurally ordered face-centered tetragonal (L10) phase. After thermal annealing at 700℃, the disordered PtCo was converted to structurally ordered PtCo L10 structure without severe particle aggregation/sintering. The structure features were studied by X-ray absorption spectroscopy. The XANES spectra of Co K-edge showed characteristic features indicating the formation of chemically ordered PtCo structure. However, the Pt LIII-edge XANES spectra showed successive decrease in white line intensity, caused by increasing charge transfer from Co to Pt. These charge transfer indicates the progression of PtCo alloying caused by reductive annealing process. The extracted structural parameters also confirm that the formation of chemically ordered fct structure. Structurally ordered PtCo catalyst exhibits superior catalytic activity and stability for oxygen reduction reaction. The kinetic mass current density of H2-PtCo/mc-700 catalyst at the potential 0.9 V is 1.7 fold improvement over those of disordered PtCo/mc and commercial E-TEK PtCo/C. The XAS characterization proves that the catalyst with fct structure has higher degree of alloying between Co and Pt atoms compared with fcc structure. The electron-rich Pt in fct structure, resulting from electron transfer from Co to Pt, is the reason for the improvement in its catalytic activity towards oxygen reduction reaction. This work demonstrated the space confinement effect plays an important role in the preparation and stabilization of fct-PtCo nanoparticles against particle sintering/aggregation which is driven by high temperature and potential cycling. For comparison, E-TEK PtCo/C and TKK Pt3Co/C electrocatalysts were chosen to observe the changes in the structure effects towards the catalytic activity of these nanocatalysts. X-ray absorption spectroscopy was employed to characterize the properties such as alloying extent and electronic structure. For E-TEK catalyst, the PtCo NPs possess a Pt-rich in core and Co-rich in shell structure (Ptcore-Coshell). For TKK Pt3Co/C catalyst, the total coordination number of ΣNPt-i is very similar with ΣNCo-i , indicating the well-alloyed structure of Pt3Co nanoparticles.
The second part emphases the characterization of defect structure of electrode materials in Li-ion battery. For Li-overstoichiometric LiCoO2, the crystal field splitting of Co d orbitals was changed due to the missing of Co-O bonding and local structural distortion. The molecular re-hybridization between Co 3d and O 2p orbitals due to formation of square-based pyramids results in the appearance of pre-edge peaks in the oxygen K-edge spectra. From our previous proposal, intermediate state Co3+ was proposed to exist in square-based pyramids. The appearance of pre-edge peaks in oxygen K-edge spectra were proposed due to the electronic transition from 1s orbital to unoccupied 3d level in which the hybridization of 3dxy and 3dz2 takes places with oxygen 2p orbitals. The excitation energy for transition of 1s electron to those hybridized orbitals is lower than transition to molecular orbitals with eg-2p hybridization. In Mg-doped LiCoO2, the Co K-edge and L-edge XAS spectra showed that the oxidation state of Co is 3+ and remains the same for all Mg-doped samples. Formation of oxygen vacancies is mainly to compensate the charge balance during doping of lower valance Mg.

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