Abstract

This project will focus on the photocatalytic reduction of carbon dioxide (CO2) to produce sustainable fuels and feedstock, namely carbon monoxide (CO) and formic acid. To overcome the thermodynamic and kinetic challenges associated with CO2 reduction the enhancement of photon flux is essential. The optical properties of biomass-derived carbon quantum dots (CQDs) in a host matrix will be optimised to produce a printable luminescent solar concentrator (LSC). Waveguides will be produced by 3D printing with the architecture being designed to concentrate incident light towards a photocatalytic reactor. The overarching motivation of this project is to overcome the challenges associated with CO2 reduction to contribute to the circular carbon economy by the clean conversion of CO2 to fuels and feedstocks.

Project description

The current energetic landscape is changing in response to the threat that finite fossil fuels pose to the environmental and economic climate. In the face of the devastating consequences of global warming, innovative energy and fuel strategies are required. The reduction of CO2 is currently limited by the thermodynamic and kinetic challenges reflected by the high negative value of its free energy of formation. A large energy input is required to overcome the high overpotential associated with the formation of higher order products (CO2/HCOO- = -0.67 V (vs SHE) and CO2/CO = -0.52 V (vs SHE)). To yield sufficient current densities required for kinetically favourable reactions; the operating voltage of the electrode must account for the overpotential for both the reduction of CO2 (> 1 V) and water oxidation (~ 0.4 V). By the assembly of waveguides, concentrated light will be directed towards a photocatalytic reactor, the increased photon flux could be sufficient to efficiently reduce CO2.

A luminescent species will be suspended in the host matrix poly(methyl methacrylate), waveguides will be designed and produced by 3D printing or slot die coating. Waveguides trap a fraction of the emitted luminescence by internal reflection and the radiation energy is concentrated at the edge of the waveguide. The ideal luminescent solar concentrator should have:
* broad spectral absorption
* matched spectral response of the emitted photons and photocatalytic reactor
* a high photoluminescence quantum yield (PLQY)
* minimized re-adsorption of emitted photons - large Stokes shift

The photoluminescent (PL) properties of CQDs will be investigated. The effect of size, doping and surface functionalisation on PL will be screened against this criterion. Then the architectural design of waveguides on the concentration factor (C) of light achieved will be investigated.

Bottom-up synthesis methods provide greater flexibility to synthesise CQDs with varied size, morphology and surface functionality. Microwave pyrolysis and hydrothermal carbonisation (HTC) will be used to produce CQDs. By microwave pyrolysis, biomass is thermochemically decomposed to carbonaceous material which then can be activated by microwave heating. HTC takes small organic molecules dissolved in water and by transfer to a Teflon-lined autoclave at high temperature CQDs are produced.

A suitable photocatalytic system will be selected by spectral matching with the designed waveguide. A successful photocatalyst for homogenous CO2 reduction must display adequate light harvesting, rapid charge separation and active catalytic sites. Efficiency is facilitated by sufficiently fast charge transfer/ migration, efficient photon utilisation and multiple active sites capable of surface absorption of CO2. There are a variety of photocatalysts that are of interest namely transition metal complexes (Ru, Re, Ir, Ni, Fe, Co, Cu Mn), plasmonic metal nanoparticles (Au, Ag) and metal-organic frameworks (MOFs).