基于Mn-Ce-O/N-C复合纳米结构铝空气电池氧还原催化剂的构筑及协同催化机制研究

On the basis of the cheap manganese oxides and carbon substrates, in this project, CeO2, a unique material with high oxygen storage/releasing ability will be employed to form composite or solid solution with manganese oxides, both which are then expected to anchor in the nitrogen doped carbon materials to build low-cost hierarchical nanostructures with high electrocatalytic performance towards oxygen reduction reaction (ORR). MnOx with certain exposed facets are firstly designed by structure regulation to achieve high electrocatalytic performance. The crystal growth mechanism of MnOx in hydrothermal reaction will be elaborated. Different kinds of carbon substrates will be investigated to give the insight into the effect of the structure and morphology of carbon to ORR. The effect of the molar ratio of Mn to Ce and the various mixed methods for MnOx and CeO2 to the electrocatalytic performance will be investigated in details. In particular, we will study the difference of electrocatalytic performance towards ORR and their functional mechanisms by comparing the mixed way with that of forming solid solution. At the same time, the catalytic performance of carbon materials will be further improved by nitrogen doping with binary nitrogen sources. Theoretical calculation of Mn-Ce based oxide(s) will be carried out by the first principles method. In combination with the experiments, the formation, distribution of oxygen vacancies in Mn-Ce based oxide and the effect of oxygen molecules absorption will be investigated. The mechanism of oxygen reduction reactions and their kinetic processes will be studied. In order to elaborate the reason why the composites show superior electrocatalytic performance, the synergistic electrocatalytic mechanism towards ORR of as-constructed Mn-Ce-O/N-C composite catalysts will be discussed.

本项目从廉价的锰氧材料与碳基材料入手，拟将具有高储放氧能力的氧化铈与氧化锰复合负载在氮掺杂的碳基材料上以构筑低成本高催化活性的多级纳米复合催化剂。通过结构调控实现MnOx高能晶面的选择性生长，获得具有高催化活性的特定裸露晶面，研究水热过程中的晶体生长机制；考察不同碳基载体，研究碳材料结构和形态对氧还原过程的影响；深入研究铈锰配比、氧化铈与氧化锰的复合方式对产物电催化性能的影响规律，重点考察氧化铈与氧化锰的两相复合与形成固溶体两种方式对氧还原催化的差异及作用机理；通过双氮源氮掺杂技术进一步提升碳材料的催化活性，研究氮掺杂的作用机制。采用第一性原理法对锰铈氧化物进行模拟计算，结合实验设计，考察材料中氧空位形成与分布及氧分子吸附的影响规律。深入研究复合催化剂的氧还原反应机理及动力学过程，计算动力学参数，深入剖析Mn-Ce-O/N-C的协同催化作用，揭示复合催化剂具有高效催化活性的内在机制.

铝空气电池是用铝代替氢而构建的一种金属燃料电池，具有能量密度高、安全、成本低、无污染、放电电压平稳等优点，且铝资源丰富，能再生利用，可更换铝电极进行机械充电，是很有发展前景的电池技术。空气电极是铝空气电池最昂贵、最关键的部件，它决定了铝空气电池能否进入实用化阶段。而催化剂是空气电极的关键材料，其电催化性能的优劣决定了铝空气电池的能量输出。因此开发设计出一种具有高电催化活性，低成本的空气电极催化剂是目前关于铝空气电池的研究热点。本项目从廉价的锰氧材料与碳基材料入手，拟将具有高储放氧能力的氧化铈与氧化锰复合负载在碳基材料上以构筑低成本高催化活性的多级纳米复合催化剂。主要研究内容及结果如下：（1）通过两步法将CeO2与MnOx纳米颗粒负载在商业化导电炭黑KB上，结果发现，CeO2-MnOx/KB的半波电位达到0.81 V（vs. RHE），显著优于单纯的MnOx和CeO2都有所提高，并且其稳定性要好于商业化铂碳；（2）通过一步溶剂热法合成CeO2修饰MnOOH纳米线（CeO2/MnOOH）复合催化剂，CeO2/MnOOH材料的半波电位为0.80 V，稳定性优于商业化Pt/C，全电池放电最大功率达到209.9 mW cm−2；（3）采用声化学法合成CeOx-MnO2@CNTs催化剂，碳基复合材料具备较高的电导率，且高能超声能在常温下驱动KMnO4和碳发生氧化还原反应，避免了传统反应的高温高压条件，提高了材料合成的安全性。CeOx-MnO2@CNTs材料的ORR催化性能优异，半波电位0.82 V，稳定性优于商业化Pt/C，全电池放电最大功率达到204.1 mW cm−2。.尽管本项目合成的Mn-Ce-O复合物具有较高的氧还原催化活性，但是要实现商业化应用，仍然需要开展进一步的工作，如：深入研究CeO2与MnOx的协同作用机理，解析CeO2的助催化作用机制；碱性铝-空气电池放电会产生Al(OH)3沉淀。在静态电池中，随着放电时间增加，放电产物Al(OH)3会沉积在催化层表面，会堵塞孔道，导致电极失效。因此，可以设计液流型铝-空气电池，通过电解液循环将放电产物及时带走，从而延长空气电极的使用寿命。