LaMnO3作為擬電容應用之電荷存儲特性

鈣鈦礦氧化物為常見的擬電容材料(pseudocapacitor)，在室溫下可展現高電容量，在充放電過程中，電極發生氧化還原反應，透過改變材料金屬價態儲存電荷。然而其儲存之電荷量遠超過材料表面可儲存之最大電容量，表明其電荷儲存不僅來自表面金屬價態改變，還需要涉及體相(bulk)金屬價態改變，即充放電過成中存在電荷嵌入機制(intercalation)。傳統上認為鈣鈦礦氧化物將氧離子作為電荷載子(charge carrier) [1]，氧離子嵌入並透過晶格中的氧空缺位(oxygen vacancy)儲存電荷，因此提升電容的策略大多集中於提高氧空缺濃度。然而，理論上氧化物中的氧離子在室溫下不具備移動能力，因此嵌入機制可能是由其他電荷載子主導。本研究旨在探討嵌入機制的電荷載子，以及缺陷濃度對電容的影響。以微米級LaMnO3作為研究模型，探討鈣鈦礦氧化物作為擬電容的儲能機制。微米級粉末有助於區別材料的表面以及體相區域的訊號差異，並且微米級粉末的低表面積有助於探討體相儲能的現象。電化學實驗結果顯示，電容值隨著充放電次數增加而增強，並且超越表面儲存量一個數量級，由縱深分析(depth profile)發現隨著充放電次數增加電荷有嵌入的現象，因其涉及體相的金屬價態改變，如此便說明了該類材料的高電容來源。X光光電子能譜(X-ray photoelectron spectroscopy, XPS)結果顯示充放電過程中表面生成氫氧鍵(M-OH)，且X光吸收光譜(X-ray absorption spectroscopy, XAS)的結果說明氫氧鍵的生成伴隨錳價態的降低(Mn3+Mn2+)。綜觀以上結果，電荷載子並非如同文獻提及的氧離子，而是將氫離子作為電荷載子，在充放電過程中利用氫化反應與材料氧形成金屬-氫氧鍵儲存電荷。不同於文獻中透過提高氧空缺濃度提升電容量，本研究發現Mn-OH濃度才是顯著影響電容表現的關鍵因素，對提升儲能能力提供一個新的方向。希望可以透過缺陷影響鈣鈦礦氧化物的儲電能力，獲得製備高性能材料的策略。

關鍵字

鈣鈦礦氧化物；擬電容；電荷載子；嵌入機制

並列摘要

Perovskite oxides exhibit high capacitance as pseudocapacitors at room temperature. During charging and discharging, redox reactions occur at the electrodes, and charges are stored by changing the metal valence state of the material. However, the amount of stored charge exceeds the capacitance of the material surface, indicating that its charge storage not only comes from the change of the valence state of the surface metal, but also needs to involve the change of the valence state of the bulk metal., that is, there is a charge intercalation mechanism in the charge-discharge process. Perovskite oxides have traditionally been thought to use oxygen ions as charge carriers [1] , which intercalate and store charges through oxygen vacancy in the lattice, so most strategies to improve capacitance focus on increasing the oxygen vacancy concentration. Theoretically, oxygen atoms in oxides are not mobile at room temperature, as the result, the intercalation mechanism may be dominated by other charge carriers. This study aims to investigate the charge intercalation mechanism, and the effect of defect concentration on capacitance. Using micron-sized LaMnO3 as model material to explore the energy storage mechanism of perovskite oxides as pseudocapacitors. The micron-sized powder helps to distinguish the signal difference between the surface of the material and the bulk region, and the low surface base of the micron-sized powder helps to explore the phenomenon of energy storage in the bulk. Electrochemical experimental results show that the capacitance value increases with the number of charge and discharge, and exceeds the surface storage by an order of magnitude. From the depth profile, it was found that with the increase of the number of charge and discharge, there is a phenomenon of charge intercalation, because it involves the change of the metal valence state of the bulk, which explains the high capacitance of this type of material. The results of X-ray photoelectron spectroscopy (XPS) show that the hydrogen-oxygen bond (M-OH) is generated on the surface during the charging and discharging process, and X-ray absorption spectroscopy (XAS) indicates that the formation of hydrogen-oxygen bond is accompanied by the decrease of manganese valence (Mn3+Mn2+). Looking at the above results, the charge carriers are not oxygen as mentioned in the literature, but hydrogen ions are used as charge carriers, and the hydrogenation reaction is used to form a metal-hydrogen bond with the material during the charge and discharge process to store the charge. Unlike the literature in which the capacitance is improved by increasing the oxygen vacancy concentration, this study found that the Mn-OH concentration is the key factor that significantly affects the capacitance performance, providing a new direction for improving the energy storage capacity. It is hoped to adjust defects concentration to improve storage capacity of perovskite oxides, and obtained a strategy for preparing high-performance materials.

並列關鍵字

perovskite oxides ； pseudocapacitor ； charge carrier ； intercalation mechanism

參考文獻

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