DFG-RSF: Doped-graphene for electrochemical energy storage and conversion: Impact of the electronic structure on electrocatalytic activity in oxygen redox reactions

在化学键中储存能量使电力供应从亚瓦到几十兆瓦。将化学键的能量直接转化为电能的电化学系统可以储存GWh的能量，并利用大量的化学系统。分子氧在环境大气中很容易接近，几乎取之不尽，是电化学能量转换和存储系统的优秀组成部分，因此氧氧化还原反应对燃料电池或金属-空气电池等许多电化学设备具有决定性的重要性。在这种体系中，氧的异相电子转移发生在电极表面。碳电极重量轻、价格便宜、导电性好，在大多数情况下是首选材料。碳电极表面由类石墨烯结构域组成，但不同的碳对氧氧化还原过程的效率不同。最近的研究表明，在碳中掺杂N、B或S等轻元素可以显著提高氧中电子转移的动力学，使其具有与贵金属相当的电催化活性。然而，其背后的物理机制还没有完全被理解。针对实际碳电极的高度复杂性，即电子结构、杂质、微结构等的个体效应难以区分的问题，提出以外延石墨烯为模型体系，阐明掺杂碳中的电子结构和杂质在氧的异相电子转移中的作用。我们将从外延石墨烯作为纯化学模型体系开始，并计划利用转移的石墨烯作为电极来开发日益复杂的功能电化学电池。为了实现化学模型体系，我们将通过化学气相沉积的方法在Ni(111)和Co(0001)表面原位生长掺杂石墨烯的单分子膜。利用角度分辨光电子能谱和密度泛函理论计算，对N-、B-和S-石墨烯的电子性质进行了理论和实验研究。氢和碱金属在掺杂石墨烯薄膜上的化学吸附将被用来控制超高真空条件下的费米能级位置。然后将使用ARPES、XPS和近环境压力XPS(NAP XPS)来跟踪系统在氧气暴露期间的演变。类似的方法将被应用于使用掺杂石墨烯作为电极的电化学电池的操作。石墨烯将生长在金属箔上，然后转移到固体电解质上，或者在背面提供液体电解质的情况下转移到Si3N4栅上。利用光电子能谱和X-射线吸收光谱(NEXAFS)研究了石墨烯与氧之间的电子转移和新的含氧物种的形成。

Storing energy in chemical bonds enables power supply from sub-W to tens of MW. Electrochemical systems that convert energy of chemical bonds directly to electric power can store GWh of energy and utilize a huge variety of chemical systems. Being easily accessible in ambient atmosphere and nearly inexhausable, molecular oxygen is an excellent component for electrochemical energy conversion and storage systems, so that oxygen redox reactions are of decisive importance for many electrochemical devices such as fuel cells or metal-air batteries. In such systems, heterogeneous electron transfer to/from oxygen occurs at the surface of an electrode. Carbon electrodes, being light-weight, cheap and well conductive, are the materials of choice in the most cases. The surface of carbon electrodes consists of a graphene-like domains, various carbons, however, reveal different efficiency for oxygen redox processes. It was recently demonstrated that doping of carbons with light elements like N, B or S enhances the kinetics of electron transfer to/from oxygen remarkably and leads to an electrocatalytic activity comparable to that of noble metals. However, the underlying physics has yet not been fully understood. A number of studies devoted to this topic suffer from high complexity of real carbon electrodes, i.e. individual effects of electronic structure, impurities, microstructure, etc. can be hardly distinguished.We propose to use epitaxial graphene as a model system to elucidate the role of electronic structure and impurities in doped carbons in heterogeneous electron transfer to and from oxygen. We will start from epitaxial graphene as purely chemical model system and are planning to develop functional electrochemical cells with increasing complexity using transferred graphene as electrode.To realize the chemical model systems monolayers of doped graphene will be grown in situ by chemical vapor deposition on Ni(111) and Co(0001) surfaces. The electronic properties of N-, B- and S-graphene will be explored experimentally and theoretically employing angle resolved photoemission spectroscopy (ARPES) and DFT-based calculations. Chemisorption of hydrogen and alkali metals on doped graphene layers will be used to control the Fermi level position under ultra-high vacuum conditions. The evolution of the system during oxygen exposure will then be traced using ARPES, XPS and near ambient pressure XPS (NAP XPS).A similar methodology will be applied to operating electrochemical cells utilizing doped graphene as an electrode. Graphene will be grown on metallic foils and then transferred onto solid electrolytes or onto Si3N4 grids with liquid electrolyte supplied from the back side. The electron transfer between graphene and oxygen as well as the formation of new oxygen-containing species will be followed using photoelectron spectroscopy and X-ray absorption spectroscopy (NEXAFS) under operando conditions.