Transition Metal Carbides for Electrochemical CO2 Reduction

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.

Carbon dioxide (CO2) is the most notorious greenhouse gas with steadily rising emissions. Both, CO2 production reduction and its conversion into chemical energy carriers are necessary for environmental protection. Electrochemical CO2 conversion can provide a sustainable low temperature redox cycle for energy conversion and storage. However, remaining challenges, such as the slow kinetics, the low energy efficiency, and the high energy consumption of CO2 electroreduction, prevent it to become a viable technology. These related issues still need to be resolved prior to applications of CO2 reduction to fuels. This can be achieved through the development of catalysts that are highly selective and efficient and that can substantially lower the overpotentials. For an effective development of such catalyst materials it is crucial to understand the reaction mechanisms. In the present project, Mo carbides (MoxC) and composites of Cu and Au with Ti carbides (TiC) will be characterized in terms of their structural, chemical and electronic properties, as well as be investigated in terms of their catalytic activity towards CO2 electroreduction. Specifically, this concerns their selectivity for products that can find use as fuels in energy conversion applications, such as hydrocarbons and alcohols. A concerted experiment-theory approach will be employed that combines electrochemical studies, ex-situ (emersion) surface science micro- and spectroscopy, in-operando interface analytics and first-principles calculations. The aim is a deep mechanistic understanding that reveals and allows to maximally exploit synergistic effects either on the level of the carbide alone or through metal-support interactions. Preceding theoretical work on Mo2C(001) has suggested a break of the scaling relation between CO and CHO intermediate binding. We will first assess if this break of scaling relations stands indeed behind the promising first reports on CO2 electroreduction at Mo2C(001). Simultaneously, detailed benchmarking against the obtained experimental reference data will allow for a careful gauging of the DFT-based first-principles approach - an endeavor that has rarely been accomplished in the electrochemical context and never for transition metal carbides. Studies of Cu (Au)/TiC composites aim to elucidate synergistic effects due to catalyst-support interactions. For such systems an enhanced activity towards gas-phase CO2 hydrogenation has been attributed to charge transfer from carbide to metal and to dispersion effects. Supported by calculations, the experimental activities will aim to disentangle corresponding sources of synergy, as well as to devise systems tailored to maximally exploit them for minimized overpotential at simultaneously maximized selectivity. This enables a systematic variation of materials properties towards high activity and selectivity for electrochemical production of usable fuels from CO2.