Transition Metal Doped Electrospun Fibers as Freestanding Electrodes for Carbon Dioxide Electroreduction

The electrochemical carbon dioxide reduction reaction (CO₂RR) is a promising route to reducing excess atmospheric CO₂ emissions and producing valuable commodity chemicals. While there are upwards of 16 products accessible via the multi-electron, multi-step reduction, each pathway is required to proceed via an initial electron transfer from CO₂ to the anion radical, CO₂*−. This first electron transfer step, being the most energetically demanding, proves difficult in the absence of a catalyst due to the high overpotential (-1.9V vs SHE) required to reduce the thermodynamically stable, linear CO₂ molecule. The success of CO₂RR is therefore guided by efficient electrocatalysts which circumnavigate such high activation energies and promote favourable kinetics. The design of selective electrocatalysts which offer high faradaic efficiencies, reduced overpotentials, and provide high current densities has thus inspired an extensive area of research.<br/><br/>Transition metals have been consistently established as highly efficient electrocatalysts within this field. Within this reach, we have investigated two promising morphologies of active site: molecular-based catalysts with well defined active sites which demonstrate high selectivity for the 2-electron reduction of CO₂ into CO and in situ formed copper nanoparticles, which facilitate CO₂ conversion into C2+ products. The former has inspired investigation into the electrocatalytic behaviour of phthalocyanine complexes: aromatic, macrocyclic molecules consisting of four isoindole subunits with a central cavity which supports a metal atom. While distinct transition metal sites within the molecular cavity offer improved selectivity and work well to suppress the competing hydrogen evolution reaction (HER), they fail to offer pathways beyond C1 products. Attempts to access more energy dense products have been made via controlled in situ growth of copper nanoparticles during simultaneous carbonisation of the graphitic matrix. While an intermediate binding energy to *CO is responsible for the well-known propensity of copper to facilitate electrochemical CO₂ reduction to C2+ products, restraint of particle size and regulation of crystal facets via temperature control allow the user significant control over product selectivity and overall catalytic efficiency.<br/><br/>Freestanding electrocatalysts offer a significant advantage over commonly used catalytic inks as they provide a larger surface area. We have elected to synthesise freestanding nanofibrous mats via electrospinning polymer solutions followed by a carbonisation step. Polyacrylonitrile, a widely-available synthetic polymer, used as a precursor allows fabrication of conductive fibres with advantageous nitrogen doping. Likewise, electrospinning bio-mass derived lignin can introduce oxygen-functionalities with the added advantage of avoiding petrochemical-derived precursors. Heteroatom functionalities may act to both bolster performance of the overall catalyst through their inherent activity and also to anchor active sites to the surface of the support. The covalent binding of molecular catalysts via the central transition metal onto such supports has demonstrated improved charge transfer when compared to those which rely on isolated molecular catalysts or those bound through pi-pi stacking. In the case of freestanding carbon fibers with in situ deposition of copper nanoparticles, the distribution of uniformly sized catalytic particles is enabled throughout the graphitic fibres.