纳米二氧化锰三维结构的可控合成与氧还原催化性能的研究

At present, as the cathode catalysts of metal-air batteries, noble-metal catalysts (Pt and Pt) are known to be the best active catalysts for oxygen reduction reaction, but their application is limited because of the scarce resource and high price. Therefore, it is extremely essential for us to develop alternative catalysts based on non-noble elements. Among them, manganese dioxides (MnO2) are particularly attractive due to their high abundance, benign environment and considerable electrocatalytic activity, and thus they are really potential as cathode catalysts of metal-air batteries. However, compared with noble-metal catalysts, manganese dioxides have generally weak electrical conductivity, low overpotentials and poor capability to catalyze 4-electron reduction. In this project, we aim to develop a one-step liquid in-situ synthesis method and synthesis the different crystal phases of nano-manganese dioxides with three-dimensional structure, and produce in-situ oxygen vacancies on the surface of manganese dioxide. Then the electrocatalytic activity of manganese dioxide will be evaluated by the oneset potensial, reduction current and transferred electron number. Based on the above study, the relationship between the crystal structure, morphology, oxygen vacancy and the electrocatalytic activity for oxygen reduction reaction will be built. Moreover, the mechanism of oxygen reduction reaction on nano-manganese dioxide with three-dimensional structure will be illuminated. The project will finally provide an efficient controlled synthesis new route for preparing nano-manganese dioxides oxygen reduction catalysts with high activity and satisfied stability.

目前，作为金属-空气电池阴极催化剂，Pt、Pd等贵金属纳米催化剂在氧还原反应中显示出优异的催化活性。然而，贵金属资源稀缺、价格昂贵制约了其在氧还原反应的应用，因此开展非贵金属氧还原催化剂的研究尤为重要。二氧化锰资源丰富、环境友好且具有良好的氧还原催化活性，是一种极具发展潜力的阴极催化剂。但与贵金属催化剂相比，二氧化锰催化剂的氧还原催化活性偏低，主要由于二氧化锰的弱导电性、较高的过电位和较低的催化氧还原四电子能力。本项目拟通过发展一步液相原位合成法，可控合成不同晶型的纳米二氧化锰三维结构催化剂，并在其表面原位产生氧空位以及调控其浓度，从而提高其氧还原催化活性和稳定性。通过研究，建立纳米二氧化锰晶型、三维结构、表面氧空位浓度与氧还原催化性能间的关系，阐明氧还原催化反应机理。本项目的开展为高活性和高稳定性金属-空气电池阴极纳米二氧化锰氧还原催化剂的制备开辟了一条高效、原位可控的合成新路线。

MnO2在碱性电解液中表现出对氧还原反应良好的催化活性，是一种极具发展潜力的金属-空气电池阴极催化剂。但是，由于MnO2的弱导电性、较高的过电位和较低的催化氧还原四电子能力等性质，MnO2的氧还原催化活性还有待进一步提高。为此本课题工作的重点是发展三维MnO2电催化剂的一步液相合成法，实现三维MnO2的晶型与形貌的可控合成，并进一步调控其表面氧空位的浓度，改善其导电性，从而提高其电催化活性。研究结果表明：（1）通过温和的水热法制备了晶型可控的三维MnO2多级结构，通过比较发现，α-MnO2具有较优异的氧还原催化性能，这是因为α-MnO2具有更强的导电性与O2吸附能力；（2）利用不同的氧化剂（K2S2O4和KClO3）合成了两种三维辐射状的α-MnO2，蒲公英状α-MnO2具有更大的比表面积和电化学活性面积、更多的表面缺陷和更小的电荷传递电阻，因此该样品具有更好的电催化活性。在电流密度为-3 mA cm-2时，蒲公英状α-MnO2的电位是0.73 V，正于海胆状α-MnO2的电位（0.71 V）；（3）利用水热合成法，通过调节反应温度（60, 80和140 ℃），合成了一系列具有相同形貌的三维α-MnO2微球。结果表明，80 ℃下合成的α-MnO2微球达到了导电性和活性位点的平衡，因此其具有最优异的氧还原催化性能。在电流密度为-3 mA/cm2时，80 ℃制备的α-MnO2微球的电位是0.79 V，正于140 ℃合成的α-MnO2微球（0.76 V）和60 ℃合成的α-MnO2微球（0.75 V）这两个样品。此外，我们还采用其它制备方法合成了一系列MnO2催化剂，并建立其晶型、形貌、氧空位浓度与催化活性之间的关系。本项目的开展为合成晶型、形貌以及氧空位可控的MnO2提供了一种简单的化学合成途径，此类MnO2材料在氧还原反应中存在潜在的应用价值。