Freestanding electrodes for CO 2 electroreduction Hattie Chisnall QUML, UK
The electrochemical reduction of carbon dioxide is a promising route to reducing excess atmospheric CO2 emissions as well as sustainably producing commodity chemicals. The most thermodyanamically unfavourable process within this reaction, the first electron transfer of CO2 to CO2•−, proves difficult in the absence of a catalyst due to the intensive energy required for the bending of the thermodynamically stable, linear CO2 molecule. The success of CO2RR is therefore guided by efficient electrocatalysts which circumnavigate such high activation energies and promote favorable kinetics. Design of electrcatalysts which offer high partial faradaic efficiencies, reduced overpotentials and provide significant current densities has therefore become a highly researchered area. Supported heterogeneous molecular electrocatalysts with well-defined active sites are a promising option within this field. Phthalocyanines, being one such example, are aromatic, macrocyclic molecules with a central cavity surrounded by four pyrrolic nitrogen species which can play host to a metallic active site. This work focuses on utilisation of inexpensive transition metal-based active sites working in conjunction with these widely available, electron-rich compounds. Such distinct active sites are particularly favorable due to their ability to suppress competing HER and produce CO at high faradiac efficiencies. This research also explores the well-documented ability of copper to facilitate the electroreduction of CO2 to C2+ products for use as commodity chemicals or as chemical storage. Controlled growth of copper nanoparticles can be achieved via electrospinning polymers with optimised concentrations of copper precursors and subsequent temperature control. Facile methods of restraining particle size and regulating crystal facets allow the user significant control over product selectivity and overall catalytic efficiency. This research aims to optimise these features in order to produce high-performance catalysts. It is not only sufficient to provide a highly efficient active site to improve electrocatalytic performance. This work has also been inclined to finding scalable and conductive catalytic supports. This has been achieved through electrospinning the widely-available polymer, polyacrylonitrile, which is subsequently carbonised to provide conductive graphitic domains. Similarly, the use of the bio-polymer, lignin, allows introduction of oxygen-functionalities while avoiding the use of petrochemical-derived precursors. Additionally, freestanding electrocatalysts, which do not rely on the time-consuming synthesis of often expensive binder-supported catalytic inks and can be utilised in-situ are a promising branch of this field. These benefits are realised through this facile electrospinning method which produce fibrous mats with advantageous heteroatom functionalities. The covalent binding of molecular catalysts via the central transition metal onto such supports have demonstrated improved charge transfer when compared to those which rely on isolated molecular catalysts or those bound though pi-pi stacking. Axial covalent anchoring tunes the metal’s electronic structure to significantly reduce otherwise high (-1.9V vs SHE) CO2RR overpotentials with significant selectivity for CO. Additionally, both molecule and support can be functionalised through facile synthesis methods to tailor electron density at the active site and generate higher overall performance of the catalysts. Alternatively, such high-surface area supports allow controlled growth of metallic nanoparticles uniformly across their surface, generating reliable, active overall catalysts.
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© The Author(s), 2023
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