奈米結構錳氧化物之製備與超高電容儲能機制探討

Preparation of nanostructured manganese oxide and the supercapacitive charge storage mechanism

朱怡璇(Yi-Hsuan Chu)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/碩士(2012年)

摘要

本研究主要是針對奈米氧化物的製備以及超高電容機制的探討。論文結果主要分為三部分。第一部份結果探討錳氧化物在不同電為範圍下的充放電反應機制以及有添加劑在電解液中的影響。第二部分則為製備中孔洞奈米顆粒錳氧化物的方式以及其材料特性和電化學分析。第三部分則是研究在有無添加劑的電解液裡對製備出的錳氧化物活化以及多圈掃描後的效應，也探討了其與石墨烯組裝成的非對稱電容的效果。以下將簡要地討論各章所包含的內容。
第一部份藉由電化學石英天平(EQCM)和循環伏安掃描(CV)來偵測錳氧化物在硫酸鈉水溶液中進行電化學充放電反應時的伏安電流變化及重量變異，進而推知在不同電位下的充放電反應機制。由微石英天平震盪的研究中發現，錳氧化物於1.0~1.2V的電位之間因氧氣產生而大量剝離，相當不穩定。此外，由質荷比(MCR)可大略推估電位在-0.2~0.2V的區間主要為H+的嵌入與嵌出，而在電位範圍為0.2~0.8V之間主要進出錳氧化物晶格的應為Na+或H3O+。另外，當電解液中加入少量的NaHCO3或Na2HPO4時，HCO3-可與錳離子形成難溶鹽類，而HPO42-則可吸附於錳氧化物的表面，進而抑制錳氧化物的溶解，增加可利用電位範圍。
第二部分則是利用醋酸錳加入甲醇再經鍛燒的方式，簡單地製備出錳氧化物，並利用熱重分析儀(TGA)、X光繞射儀(XRD)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、比表面積測試儀(BET)、循環伏安掃描(CV)來檢驗。發現在分散有石墨烯氧化物的甲醇中能夠幫助形成孔徑分布窄，以及較大表面積和孔洞較多的錳氧化物，使其具有較大的比電容表現，以及在高掃速下因具有多孔洞而使得電解液裡的離子能夠快速補充而有較好的電容表現。
第三部分則是探討第二部分中製備出的錳氧化物，在有無碳酸氫鈉的電解液裡的活化及一千圈循環伏安掃描下的結果。發現在活化前即加入陰離子能夠在一千圈後還維持相當大的比電容及可逆的電化學行為，並且由感應耦合質譜儀(ICP)的結果也顯示有加添加劑的實驗中，剝落到電解液裡的錳氧化物濃度比沒加添加劑的低很多。並且活化完成後的錳氧化物也與石墨烯組裝成非對稱電容，其最大電位利用範圍可到達2.5V，且在比電流40A/g，電位範圍為2.4V情況下仍具有18.7Wh/kg的能量密度和約20kW/kg的功率密度。此外，在比電流為10A/g，電位範圍2.4V的情況下進行充放電一千圈後，其電化學行為仍然相當穩定。顯示此方法有效將可利用電位範圍擴大而增加可儲存電能空間。

關鍵字

錳氧化物 ； 超電容 ； 儲能材料 ； 石磨烯 ； 電化學石英微震盪天平 ； 充放電反應機制 ； 非對稱超電容

並列摘要

In this work, we study the preparation of nanostructured manganese oxide and the charge storage mechanism of supercapacitors. The results of this study are separated into three parts. The first part discusses different charge storage mechanisms between different potential regions and the effects of additive in the electrolyte on manganese oxide. The second part suggests a simple way to synthesize mesoporous MnOx and the textural characteristics and electrochemical properties are investigated. In the third part, the effects of additive on the activation and cycle-life test of manganese oxide are examined. Besides, we also assembled MnOx/graphene asymmetric supercapacitor in the electrolyte with or w/o additive.
In the first part, the electrochemical behavior and the corresponding mass variations of amorphous manganese oxide (denoted as a-MnOx) are examined simultaneously in neutral electrolyte containing 10 mM Na2SO4 without or with NaHCO3 or Na2HPO4 by cyclic voltammetry with a quartz crystal microbalance (QCM). From this EQCM study, a-MnOx is unstable between 1.0 and 1.2 V in 10 mM Na2SO4 because of significant dissolution of a-MnOx due to oxygen evolution. From the MCR (mass-to-charge ratio) value, the major ion involves is H+ when the potential window is between -0.2V and 0.2V. But when it is between 0.2V and 0.8V, H3O+ are considered to be the main ions intercalate/deintercalate within the manganese oxide. The dissolution phenomenon and oxygen evolution are successfully suppressed by the formation of insoluble manganese carbonate or the adsorption of phosphate by adding NaHCO3 or Na2HPO4 in the Na2SO4 electrolyte, enlarging the potential window for the charge/dichrage of a-MnOx.
In the second part, the Mn3O4 is synthesized through calcination of the mixture of methanol (or GO/methanol) and manganese acetate. The instrument X-ray diffractometer(XRD), scanning electron microscope(SEM), transmission electron microscope(TEM), Thermogravity analysis (TGA), surface area and pore size analyzer (BET), and cyclic voltammetry (CV) are employed to characterized the sample. It is discovered that manganese oxide synthesized with graphene oxide suspended in methanol possesses the properties of narrower pore size distribution, larger surface area and pore volume. Therefore, the higher capacitance is attributed to the larger pore volume which facilitates ion transportation.
In the third part, the effects of bicarbonate on the activation and cycle life of manganese oxide are studied. It is found that higher capacitance value can be gained when activating manganese oxide in the electrolyte with bicarbonate. From the results of ICP-MS, the concentration of manganese in the electrolyte with additive is much fewer than that in bare sodium sulfate. Besides, the assembly of asymmetric supercapacitor MnOx/graphene reaches 2.5V which is the largest maximum cell voltage in aqueous electrolyte to the best of our knowledge. In addition, the energy density reaches 18.66Wh/kg and power density of 20kW/kg at 2.4V cell voltage in 0.1M Na2SO4 with adding 3mM NaHCO3. Furthermore, it exhibits an excellent charge-discharge behavior after 1000 cycles at the cell voltage of 2.4V at current density of 10A/g.

並列關鍵字

manganese oxide ； supercapacitor ； graphene ； EQCM ； charge storage mechanism ； asymmetric supercapacitor

參考文獻

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