Designing materials for the production of tailored and sustainable fuels from waste CO2 and water

Brewis, Ian (2024) Designing materials for the production of tailored and sustainable fuels from waste CO2 and water. Doctoral thesis, Northumbria University.

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Abstract

The selective and efficient electrochemical reduction of carbon dioxide (CO2) to carbonaceous fuels poses a promising method of energy storage, as well as providing a crucial method of recycling otherwise waste products following the burning of hydrocarbon fuels. The use of atmospherically abundant CO2 as a chemical feed-stock in the formation of fuels aids not only in reducing the build up of harmful greenhouse gasses, but also in producing fuels compatible with existing infrastructure such as within the transport and aviation sectors.

To make this dream of green hydrocarbon fuels a reality however, the field of electrochemical CO2 reduction must first overcome a number of crucial challenges to produce carbonaceous fuels at a scale that is both energy and cost efficient for use on an industrial scale. Some of the key challenges limiting the use of electrochemical CO2 reduction electrodes beyond laboratory scale include low electrocatalyst selectivity, high overpotentials for initial activation steps, and poor reduction activity resulting from the limited transfer of charge throughout the electrode material.

Cu-based electrocatalyst materials have consistently been shown to reduce CO2 to produce carbonaceous products at relatively mild overpotentials and mild current densities when compared with other transition metals [1–6]. Cu electrodes however, have often been shown to produce a mixture of reduction products at activity levels not currently economically viable for use on an industrial scale, with a great proportion of the charge transferred across the electrocatalytic surface being leached to fuel the competing hydrogen evolution reaction.

To overcome these key challenges, numerous methods have been explored over the years, including surface engineering of Cu, such as the production of oxidederived (OD-) electrodes to produce larger catalytic surface areas to facilitate the transfer of charge, as well as the formation of bi-metallic Cu-based composites to limit the influence of HER. The addition of non-noble metals such as In and Sn with Cu in particular, has demonstrated a clear suppression of HER in favour of the formation of lower order carbonaceous products such as carbon monoxide and formate.

Herein, we provide a thorough examination of Cu2O/In and Cu2O/Sn electrodes, overcoming key limitations in the often low current densities observed by such electrocatalyst materials through cutting-edge optimisation techniques, examining highly novel tri-metallic electrode materials via both experimental and computational methods.

The following thesis begins with a brief outline of the key concepts within the field of electrochemical CO2 reduction, as well as outlining the underlying principles behind density functional theory (DFT) simulations implemented throughout the PhD project. The subsequent chapter provides an overview of the key challenges facing the electrochemical CO2 research community, highlighting some of the more key published works of recent years. The 3rd chapter then describes the various experimental methods used throughout the project, including the underlying principles of key characterisation methods such as EDS and XPS. Chapters 4 and 5 examine the properties of cutting-edge Cu2O/In/Pt electrocatalytic systems from
computational, and experimental perspectives, respectively. This thesis then further extends its examination of electrocatalyst systems to characterise Cu2O/Sn and novel Cu2O/Sn/Pt electrodes produced via electrochemical spontaneous precipitation (ESP) methods, before finally examining the crucial role of chloride ions in the activity optimisation of electrochemical systems.

Specifically, chapter 4 develops upon prior analysis, examining the effect of monoatomically Pt doped CuIn surface, and sub-surface structures, observing facet dependent mechanisms in the alteration of adsorption energies for key intermediates in both the HER and CO2RR. Chapter 5 examines the electrochemical performance of synthesized Cu2O/In/Pt electrodes at a range of deposition depths, revealing drastic improvements in electrocatalyst activity through sub-surface Pt deposition, in addition to novel approaches incorporating Pt within the gas diffusion layer of the electrode itself to promote charge transfer. Chapter 6 outlines progress made in examining the effect of Pt doping on bi-metallic electrocatalysts produced using earth-abundant materials, presenting current characterisation data produced examining Cu2O/Sn and novel Cu2O/Sn/Pt electrodes prior to electrochemical testing. Finally, chapter 7 highlights the crucial influence of Cl ions when present at the electroactive surface of Cu-based electrodes, providing clear improvements in electrochemical activity through improved charge transfer mechanisms.

This thesis provides an overview of novel tri-metallic electrocatalyst designs, provi viding insight into possible future avenues within the field of electrocatalyst design through the incorporation of highly conductive Pt metal centres beneath the electroactive surface. Experimental results, supported by DFT studies herein, outline possible methods of tailoring electrocatalyst activity with Pt incorporation, and uncover mechanisms toward tailoring CO and H binding energies through defect manufacturing using tri-metallic electrocatalytic complexes, providing facile methods of drastic activity enhancement of prior electrode designs.

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