Hierachical design of Fe-N-C catalysts for the electroreduction of oxygen

质子交换膜燃料电池(PEM-FC)是一种很有前途的车辆可持续推进技术。整个反应被描述为在水的形成下氢与氧的反应。这两种半电池反应都由铂基催化剂加速。由于铂/碳对氧还原反应(ORR)的动力学有限，有几种科学方法可以用廉价的替代品来取代阴极上的铂。在这方面，Fe-N-C催化剂是当今最有希望的ORR替代品。标记为Fe-N-C与以下事实有关：为了形成活性物质，必须在前体成分中或在热处理过程中利用铁和氮，并且活性中心嵌入在碳中。在活性中心的分子性质方面，给出了与铂/碳催化剂相比的主要区别。因此，一些被发现对描述铂/碳催化剂相当有用的技术不能被应用，或者对Fe-N-C有很大的限制。在这一方案中，制备了分级设计的Fe-N-C催化剂，以考察特定结构单元对催化活性和选择性的影响。两种不同的基于聚合物的合成和加工策略被用于制造。在这种方法中，聚合物的组成和分子结构是不同的，所获得的催化剂的性质可以被调节(孔隙率，FeN4位的密度)。因此，可以考察不同的孔类(由IUPAC定义)对这些非贵金属催化剂的催化活性和选择性的影响。此外，人们可以估计不同的结构单元或Fe-N-C催化剂中铁的浓度在多大程度上影响还原机理。用扫描和隧道电子显微镜、拉曼光谱和Möber Bauer光谱以及光电子和X射线吸收光谱对聚合物前驱体、模型催化剂和参比催化剂进行了结构表征。为了将催化剂与催化性能联系起来，催化剂的电化学表征尤其是选择性和活性起着至关重要的作用。在此基础上，从催化剂的活性中心的分子结构和宏观的碳结构两个方面提出了一个完整的概念。

The proton exchange membrane fuel cell (PEM-FC) is a promising technology for sustainable propulsion in vehicles. The overall reaction is described by the reaction of hydrogen with oxygen under the formation of water. Both half-cell reactions are accelerated by platinum-based catalysts. Caused by the limited kinetics of Pt/C for the oxygen reduction reaction (ORR) there are several scientific affords to replace platinum on the cathode by cheap alternatives. In this respect, Fe-N-C catalysts are todays most promising alternative for the ORR. The label as Fe-N-C is related to the fact that for the formation of active materials iron and nitrogen have to be utilized in the precursor composition or during the heat treatment and that the active sites are embedded in carbon. A mayor difference in comparison to Pt/C catalysts is given in the molecular nature of active sites. Therefore, several techniques that were found as rather useful for the description of Pt/C catalysts cannot be applied or a strongly limited for Fe-N-C. Within this proposal hierarchically designed Fe-N-C catalysts are fabricated in order to investigate the role of specific structure units on the catalytic activity and selectivity. Two different polymer-based synthesis and processing strategies are applied for the fabrication. In this approach the composition and molecular structure of the polymers is varied in such a way that the properties of the obtained catalysts can be modulated (porosity, density of FeN4 sites). Hence, the influence of different pore classes (as defined by IUPAC) on the catalytic activity of these non-precious metal catalysts and its selectivity can be investigated. In addition, one can estimate to what extend the reduction mechanism is affected by distinct structure units or e.g. the concentration of iron in Fe-N-C catalysts. Structural characterization of the polymer precursors, the model and reference catalysts is made by scanning and tunneling electron microscopy (SEM/TEM), Raman and Mößbauer spectroscopy, as well as photoelectron and x-ray absorption spectroscopy. For the electrochemical characterization of the catalysts especially the selectivity and activity play a crucial role in order to correlate them with the catalytic properties. Based on this an integral concept of the catalyst from its molecular structure of active sites and of the carbon towards the macroscopic dimension is visualized.