錳氧化物的制備、表征及電化學應用研究
發(fā)布時間：2019-04-20 11:00
【摘要】：開發(fā)高性能的空氣電極催化劑可提高電化學性能并降低電池成本,具有重要的研究價值。本論文主要從合成高比表面積的超細納米粒子以及特殊晶面暴露納米材料的可控合成著手以提高錳氧化物的電催化活性。采用改進的共沉淀法與水熱法制備錳氧化物催化劑,同時,為進一步拓展所制備Mn304八面體的應用領域,以其為前驅(qū)體合成了LiMn2O4鋰離子電池正極材料。論文主要研究結(jié)果如下： 首次將MnOx/CeO2固溶體用于電催化反應,采用改進的共沉淀法制備了系列MnOx/CeO2固溶體超細納米粒子,研究了Mn/Ce配比、煅燒溫度對電催化活性的影響。研究發(fā)現(xiàn)將MnO2與Ce02形成固溶體后作為氧還原催化劑,使得O2在催化活性位點(即Mn4+所處位置)的移動性得到顯著提高,導致更多吸附到空缺位的氧分子被活化,促進了大電流下氧還原過程的進行。另外Mn4+進入Ce02晶格能抑制Ce02晶體的生長,從而得到高比表面積的MnOx/CeO2固溶體,顯著增加了氧還原活性位點,進一步促進了大電流放電下對O2催化能力。 以KMnO4為原材料,N,N-二甲基甲酰胺(DMF)作還原劑和結(jié)構(gòu)導向劑,不使用任何表面活性劑或模板劑,采用水熱法合成了形貌規(guī)則,尺寸均一的Mn304八面體,研究了不同DMF加入量及水熱溫度對所得產(chǎn)物形貌的影響。研究發(fā)現(xiàn)在該水熱體系中,決定Mn304形貌的主要因素為溶劑組成和水熱溫度,通過采用Time-dependent形貌演化的研究方法發(fā)現(xiàn)八面體Mn304的形成是伴隨著自組裝機制和Ostwald成熟機制的晶體生長過程。電化學測量結(jié)果發(fā)現(xiàn)具有截頂八面體形貌的Mn304具有最優(yōu)的電催化性能。結(jié)合物相表征結(jié)果可推斷具有高能級晶面暴露可能導致截頂八面體Mn304上02越容易得到電子被還原,從而具有比普通Mn304納米粒子更高催化活性。 以Mn304八面體為前驅(qū)體,通過固相鋰化反應合成了LiMn2O4八面體作為鋰離子電池正極材料,與采用溶膠凝膠法合成的普通納米粒子進行了性能對比研究。研究發(fā)現(xiàn)采用化學鋰化氧化物前驅(qū)體的方法可以很好控制目標材料的八面體形貌,從而大大提高材料的電化學性能。在1C倍率下,LiMn2O4八面體材料首次放電容量為122.7mAhg-1,200次循環(huán)后的容量保持率為84.8%。與LiMn2O4內(nèi)米粒子對比八面體LiMn2O4電化學性能得到明顯提高的原因可能如下：(1)該材料在充放電過程中能形成更加穩(wěn)定的SEI膜,從而抑制充放電過程中Mn的溶解,提高循環(huán)性能；(2)合成的八面體具有良好的結(jié)晶度和單晶結(jié)構(gòu)。
[Abstract]:The development of high performance air electrode catalysts can improve the electrochemical performance and reduce the battery cost, which has important research value. In order to improve the electrocatalytic activity of manganese oxides, the synthesis of ultrafine nanoparticles with high specific surface area and the controllable synthesis of nano-materials exposed to special crystal planes were studied in this thesis. Manganese oxide catalysts were prepared by improved coprecipitation and hydrothermal method. In order to further expand the application field of Mn304 octahedron, LiMn2O4 cathode materials for lithium-ion batteries were synthesized by using them as precursors. The main results are as follows: for the first time, MnOx/CeO2 solid solution was used in electrocatalytic reaction. A series of MnOx/CeO2 solid solution ultrafine nanoparticles were prepared by improved coprecipitation method, and the ratio of Mn/Ce was studied. Effect of calcination temperature on electrocatalytic activity. It was found that when MnO2 and Ce02 were used as catalyst for oxygen reduction, the mobility of O2 at the catalytic active site (i.e., the position of Mn4) was significantly increased, resulting in the activation of more oxygen molecules adsorbed to the vacancy site. It promotes the process of oxygen reduction under high current. In addition, the addition of Mn4 into the Ce02 lattice can inhibit the growth of Ce02 crystal, and thus obtain MnOx/CeO2 solid solution with high specific surface area, which significantly increases the oxygen reduction activity site and further promotes the catalytic activity of O2 under high current discharge. Mn 304 octahedron with regular morphology and uniform size was synthesized by hydrothermal method using KMnO4 as raw material, N, N-dimethylformamide (DMF) as reducing agent and structure directing agent, without using any surfactant or template. The effects of different amount of DMF and hydrothermal temperature on the morphology of the products were studied. It is found that the main factors determining the morphology of Mn304 in this hydrothermal system are solvent composition and hydrothermal temperature. It is found that the formation of octahedral Mn304 is accompanied by self-assembly mechanism and Ostwald ripening mechanism by means of Time-dependent morphology evolution. Electrochemical measurements show that Mn304 with octahedral morphology has the best electrocatalytic performance. It can be concluded from the phase characterization that the high-level surface exposure may lead to the reduction of electrons on the octahedral Mn304 with a truncated octahedral surface, which leads to a higher catalytic activity than that of ordinary Mn304 nanoparticles. LiMn2O4 octahedron was synthesized as cathode material of lithium ion battery by lithium reaction using Mn304 octahedron as precursor. The properties of LiMn2O4 octahedron were compared with that of ordinary nano-particles synthesized by sol-gel method. It is found that the octahedral morphology of the target material can be well controlled by the method of chemical lithium oxide precursor, which greatly improves the electrochemical performance of the material. The first discharge capacity of limn _ 2O _ 4 octahedral material is 84.8% after the first cycle of 122.7mAhg-1200 at 1C rate, and the initial discharge capacity of limn _ 2O _ 4 octahedral material is 84.8%. The reasons why the electrochemical properties of octahedral LiMn2O4 can be obviously improved compared with the octahedral LiMn2O4 particles in LiMn2O4 may be as follows: (1) the material can form a more stable SEI film during charge-discharge process, thus inhibiting the dissolution of Mn during the charge-discharge process and improving the cycle performance; (2) the octahedron has good crystallinity and single crystal structure.
【學位授予單位】：中南大學
【學位級別】：碩士
【學位授予年份】：2014
【分類號】：O643.36;TM912
本文編號：2461545
[Abstract]:The development of high performance air electrode catalysts can improve the electrochemical performance and reduce the battery cost, which has important research value. In order to improve the electrocatalytic activity of manganese oxides, the synthesis of ultrafine nanoparticles with high specific surface area and the controllable synthesis of nano-materials exposed to special crystal planes were studied in this thesis. Manganese oxide catalysts were prepared by improved coprecipitation and hydrothermal method. In order to further expand the application field of Mn304 octahedron, LiMn2O4 cathode materials for lithium-ion batteries were synthesized by using them as precursors. The main results are as follows: for the first time, MnOx/CeO2 solid solution was used in electrocatalytic reaction. A series of MnOx/CeO2 solid solution ultrafine nanoparticles were prepared by improved coprecipitation method, and the ratio of Mn/Ce was studied. Effect of calcination temperature on electrocatalytic activity. It was found that when MnO2 and Ce02 were used as catalyst for oxygen reduction, the mobility of O2 at the catalytic active site (i.e., the position of Mn4) was significantly increased, resulting in the activation of more oxygen molecules adsorbed to the vacancy site. It promotes the process of oxygen reduction under high current. In addition, the addition of Mn4 into the Ce02 lattice can inhibit the growth of Ce02 crystal, and thus obtain MnOx/CeO2 solid solution with high specific surface area, which significantly increases the oxygen reduction activity site and further promotes the catalytic activity of O2 under high current discharge. Mn 304 octahedron with regular morphology and uniform size was synthesized by hydrothermal method using KMnO4 as raw material, N, N-dimethylformamide (DMF) as reducing agent and structure directing agent, without using any surfactant or template. The effects of different amount of DMF and hydrothermal temperature on the morphology of the products were studied. It is found that the main factors determining the morphology of Mn304 in this hydrothermal system are solvent composition and hydrothermal temperature. It is found that the formation of octahedral Mn304 is accompanied by self-assembly mechanism and Ostwald ripening mechanism by means of Time-dependent morphology evolution. Electrochemical measurements show that Mn304 with octahedral morphology has the best electrocatalytic performance. It can be concluded from the phase characterization that the high-level surface exposure may lead to the reduction of electrons on the octahedral Mn304 with a truncated octahedral surface, which leads to a higher catalytic activity than that of ordinary Mn304 nanoparticles. LiMn2O4 octahedron was synthesized as cathode material of lithium ion battery by lithium reaction using Mn304 octahedron as precursor. The properties of LiMn2O4 octahedron were compared with that of ordinary nano-particles synthesized by sol-gel method. It is found that the octahedral morphology of the target material can be well controlled by the method of chemical lithium oxide precursor, which greatly improves the electrochemical performance of the material. The first discharge capacity of limn _ 2O _ 4 octahedral material is 84.8% after the first cycle of 122.7mAhg-1200 at 1C rate, and the initial discharge capacity of limn _ 2O _ 4 octahedral material is 84.8%. The reasons why the electrochemical properties of octahedral LiMn2O4 can be obviously improved compared with the octahedral LiMn2O4 particles in LiMn2O4 may be as follows: (1) the material can form a more stable SEI film during charge-discharge process, thus inhibiting the dissolution of Mn during the charge-discharge process and improving the cycle performance; (2) the octahedron has good crystallinity and single crystal structure.
【學位授予單位】：中南大學
【學位級別】：碩士
【學位授予年份】：2014
【分類號】：O643.36;TM912
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