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Surface-functionalised nanocrystal catalysts for the electrochemical reduction of carbon dioxide

Periodic Reporting for period 1 - SURFCAT (Surface-functionalised nanocrystal catalysts for the electrochemical reduction of carbon dioxide)

Reporting period: 2020-02-01 to 2022-01-31

Modern society faces a number of crucial challenges regarding energy and the environment. Firstly, the amount of carbon dioxide (CO2) entering the environment must be reduced to minimise its negative effects on global warming and ocean acidification. Secondly, advances in energy storage are required so that energy from intermittent, renewable sources are not wasted when produced in excess. In recent years, many efforts have been devoted to developing carbon capture, storage and utilisation technologies in order to address both of these issues simultaneously. One such example of CO2 utilisation is in the electrochemical CO2 reduction reaction (CO2RR), where CO2 is converted into energy dense, transportable fuels, using water and electrical energy. This conversion of CO2 into fuels, such as ethylene, ethanol or methane, involves the formation of new chemical bonds, which store significant amounts of energy. Therefore, using renewable energy sources (e.g. solar, wind, tidal etc) to power this process ultimately results in the storage of electrical energy in the form of chemical bonds, which can be reclaimed through existing combustion infrastructure. Electrocatalysts are materials that are responsible for facilitating the chemical processes in the CO2RR and determine the reaction outcome.
Copper is the only known pure metal that can convert CO2 into multicarbon products, such as ethylene and ethanol, which are highly desirable as they are more energy dense and have a much higher value than C1 products. One significant problem of using copper in the CO2RR is that 18 different products can be formed, and so more sophisticated materials based on copper are required to target specific products. The objective of this project was to synthesise organic molecules (ligands) that can be anchored onto the surface of a copper catalyst in order to fine-tune the CO2RR towards a more favourable outcome. These ligands would contain an imidazolium functional group that is well known to interact with CO2 molecules, which might concentrate CO2 molecules at the copper surface and increase the activity of the catalyst. The research carried out in this project concluded that indeed, functionalising metal surfaces with imidazolium ligands is a powerful tool for directing the outcome of the CO2RR.
Copper nanocrystals (Cu NCs) are an ideal platform to explore surface functionalisation approaches in CO2RR electrocatalysis, as they possess a high surface / volume ratio. They are also well established as powerful electrocatalysts in their own right, as their tuneable size and shape determines which crystal facets are exposed at the surface, which drastically influences the product distribution from the CO2RR. For example, cubic Cu NCs produce mainly ethylene because of the exposed 100 facets, whereas octahedral Cu NCs produce mainly methane due to the exposed 111 facets.
A simplified model system was first investigated using silver nanocrystals (Ag NCs), which only produce CO from the CO2RR, as well as H2 from the competing hydrogen evolution reaction (HER) from the reduction of protons in water. In this work, 12 new ligands containing different anchor and tail groups were synthesised and fully characterised. A ligand-exchange procedure was developed in order to introduce these ligands to the Ag NC surface without destroying the NC. It was found that electron-withdrawing anchor groups improved the activity of the catalyst, and that medium-length tail groups were optimal in hindering the HER without blocking CO2 molecules from accessing the surface. More than 90% CO selectivity was achieved using this approach and this work was published in Chemical Science. Applying the optimal ligand to the functionalisation of Cu NCs allowed us to study the impact on more complex product distributions. In that case, some HER suppression was also observed and we also discovered that the imidazolium ligand promoted the production of formate whilst suppressing ethylene. In this study on Cu NCs, the results allowed us to comment on a more fundamental aspect of catalyst design involving metallic NCs and functional surface ligands, as we discovered that cubic NCs were stable under CO2RR conditions, but spherical NCs underwent structural changes that ejected the surface ligands, meaning that their benefits are lost. Finally, in this study, we tested a cubic Cu NC / imidazolium hybrid catalyst in a larger, gas-fed cell and demonstrated that equivalent CO2RR performance was observed. This is significant as usual CO2RR testing is carried out in small “H”-cells at low current density. By moving to larger cells that operate at higher current density, we are starting to mimic the requirements of more economically viable devices. In this case we showed that these hybrid catalyst systems are appropriate candidates for such devices. This research was published in Inorganic Chemistry.
The uniqueness of the results from this project is that the imidazolium ligand platform allowed a comprehensive and systematic study to be carried out, where the impact of the anchor group, imidazolium group and tail group on the CO2RR could be deconvoluted and assessed individually. In comparison, other related contributions from the literature have focussed more on specific examples of a functional molecule that can alter the CO2RR outcome, in a more phenomenological approach. The advantage of the systematic approach undertaken in this project is that a more fundamental understanding of hybrid catalyst attributes could be obtained in a group of closely related catalysts. Specifically, how groups that interact with CO2 (such as imidazolium) impact the CO2RR, but also how the ligand-induced changes in NC electronic structure and the NC-electrolyte interface impact the CO2RR. This project has also compared the performances of similarly functionalised NC catalysts with different metals (Ag vs Cu) and different morphologies (Cu cubes vs Cu spheres). The results from this project have made a significant contribution to CO2RR catalyst design concepts, which will greatly benefit the CO2RR community. Furthermore, by undertaking fundamental studies regarding the stability of ligands under CO2RR conditions and by demonstrating that functional ligands can be used in larger gas-fed electrochemical cells at higher current densities, this project has contributed to advancing the use of hybrid catalysts in larger, more economically viable electrochemical devices that might be used in future energy technology.
The widespread integration of such devices in future energy infrastructures will not only impact a number of scientific and engineering communities, but will massively impact wider society as well. Fully developed CO2RR fuel cells in the future will be able to out-compete fossil fuel energy sources in terms of price and local energy production. We will be able to produce multi-carbon fine chemicals and fuels in conjunction with localised green energy sources, adding value to renewable energy technologies and securing feedstocks for traditionally fossil fuel-based hydrocarbons. Combined with carbon capture technologies, we may even be able to accelerate the reduction of atmospheric CO2 levels, or at least divert new emissions from the atmosphere, making an important step in attaining our targets for tackling climate change.
Overview of the main findings of the SURFCAT project