Transition Metal Carbides for Electrochemical CO2 Reduction

二氧化碳(CO2)是导致全球变暖的最臭名昭著的温室气体。因此，将产生的二氧化碳转化为合成燃料是减少排放和实现可持续能源经济的一条备受关注的途径。电化学二氧化碳转化似乎特别有利，因为它将从风能、太阳能或水电站的可再生电力中直接提取还原分子所需的能量。此外，它还可以在环境条件下运行，因此易于小规模分散使用。不幸的是，传统上用于电化学的金属电极只显示出不足的性能。反应速度太慢，需要的能量太多。这些限制归因于化学反应过程中形成的某些中间分子与金属电极结合强度的基本关系。修饰金属电极以优化用于反应的一种这种中间体的结合会自动恶化另一种中间体的结合。有初步迹象表明，如果使用更复杂的电极材料，这种困境就会被打破。因此，令人感兴趣的要么是将金属与其他元素混合的化合物材料，要么是由金属和另一种材料组成的复合体系。本项目的目标是评估金属碳化物在这方面的适宜性，即由金属和丰富的元素碳组成的化合物。具体研究了碳化钼及碳化钼与金、铜形成的复合体系。将实验和计算方法结合起来，雄心勃勃的目标是在原子尺度上分析电化学二氧化碳还原。相应的详细见解被认为提供了关于中间分子结合强度的基本关系在这些碳化物基材料上被打破的程度以及如果是这样的话为什么会被打破的总体理解。这反过来将为如何优化碳化物本身以用作二氧化碳电化学还原中的能效电极或设计替代材料提供思路。

Carbon dioxide (CO2) is the most notorious greenhouse gas that contributes to global warming. Converting produced CO2 into synthetic fuels represents thus a much sought route to reduce emissions and achieve a sustainable energy economy. Electrochemical CO2 conversion appears particularly advantageous, as it would draw the energy required to reduce the molecule directly from renewable electricity from wind, solar, or hydro power plants. It may furthermore be operated at ambient conditions and is therefore prone to small-scale decentralized utilization. Unfortunately, metal electrodes that are traditionally employed in electrochemistry show only an insufficient performance. The reaction runs too slow and too much energy is needed.These limitations are ascribed to a fundamental relation in the strength with which certain intermediate molecules that are formed in the course of the chemical reaction are bound to the metal electrodes. Modifying the metal electrode to optimize the binding of one such intermediate for the reaction automatically worsens the binding of another intermediate. There are first indications that this dilemma is broken when instead employing more complex electrode materials. Of interest are thereby either compound materials which mix metals with other elements or composite systems that comprise both a metal and another material. The objective of the present project is to assess the suitability of metal carbides in this respect, i.e. compounds formed of metals and the abundant element carbon. Specifically studied are molybdenum carbide and composite systems formed of molybdenum carbide and Au or Cu. Combining both experimental and computational approaches the ambitious goal is to analyze the electrochemical CO2 reduction at the atomic scale. Corresponding detailed insight is believed to provide a general understanding of how much and if so why the fundamental relation in the binding strength of the intermediate molecules is broken at these carbide based materials. This in turn will provide ideas of how to optimize carbides themselves for use as energy efficient electrodes in the electrochemical reduction of CO2, or design alternative materials for this purpose.