Carbon Based Thin Film Electrodes as Model Systems for the Electrocatalytic Hydrogenation of Unsaturated Organics

Abstract

On the transition away from fossil fuels towards a more sustainable economy electrocatalysis can play a crucial role. One example of this is the electrocatalytic hydrogenation of unsaturated organic molecules, which provides a route to store energy from renewable sources as fuels and chemicals. Additionally, the electrocatalytic hydrogenation has the potential to enable the use of biomass as a sustainable carbon feedstock, further reducing the need for fossil resources. For the ECH to become a real alternative, the development of new electrocatalysts is essential. The electrocatalytic hydrogenation generates the required hydrogen in-situ from the electrolyte, which makes catalysts in the hydrogen evolution reaction materials of interest in the electrocatalytic hydrogenation. Precious metals deliver a high activity, but their scarcity, high cost and tendency to catalyst poisoning makes them less desired in the electrocatalytic hydrogenation. As a result, transition metals and materials derived from them, like transition metal dichalkogenides are receiving increased attention. However, these materials come with different challenges, like corrosion in the conditions used for the ECH or low electric conductivity. Carbon and carbon based materials can be used to overcome some of these drawbacks and thereby allowing the effective use of these materials in the ECH. Carbon is chemically stable in a wide range of conditions, abundantly available and its chemistry can be tailored for specific applications. In electrocatalysis carbon can be used as a metal-free electrode material, as a catalyst support or for encapsulation. This thesis will explore all three of these applications. The opening section will give the motivation behind this work by looking at the current reliance of fossil resources today. From there, the electrocatalytic hydrogenation will be introduced followed by a discussion of potential electrocatalytic materials based on non-precious metals. The role of carbon in electrochemistry particularly in electrocatalytic application will be highlighted. The three use cases of carbon - metal-free electrodes, catalyst support and encapsulation - will be looked at, along with the use of model electrode systems. Finally, the aims of the thesis will be outlined. The second chapter looks at the materials and methods used and introduces the theory behind the main techniques of this work. The first result chapter, Chapter 3, focuses on the synthesis and characterisation of metal-free nitrogenated carbon films. For this, the synthesis procedure present in the groupwas updated to ensure reliable synthesis of nitrogenated carbon films. The two step synthesis allowed control over the distribution of nitrogen sites and the graphitisation. The electrochemical performance of the nitrogenated films was investigated in the HER and the ECH. While only a poor activity was found, the results helped a better understanding of the ECH. In Chapter 4 the nitrogenated carbon films synthesised in the previous chapter were used as support for molybdenum disulfide, a material with promising activity in the HER. The heterostructures were synthesised via a CVD process using a close proximity “microreactor” setup. After the characterisation of the materials, the activity in the HER and ECH of the heterostructures was investigated. The heterostructures showed promising activity in the ECH, which can be the basis of future studies. Carbon encapsulation of copper was investigated in Chapter 5. Varying mass loadings of carbon were deposited on sputtered copper films. After a comprehensive characterisation the electrochemical activity of the films was investigated. And a negligible effect of the carbon was found for the HER, which was attributed to the microroughness of the copper film. While the electrocatalytic activity did not seem affected by the carbon, a positive effect on the selectivity towards benzyl alcohol was found. The cause for which could not be finally explained. In Chapter 6 the use of the carbon thin films as substrates in nano-electrochemistry measurements using scanning electrochemical cell microscopy (SECCM), which will be briefly introduced, will be shown. An extended characterisation of the carbon films will be shown. Two projects within the group will be used to show the potential of the carbon films in correlative electrochemistry of nanomaterials. To conclude the work, design principles derived from the results will be discussed together with general insights in the ECH and the role that carbon materials can play. After, that, promising future research directions will be discussed.