Immobilising and Interfacing Poly(Heptazine Imide) Ionic Carbon Nitride for Photo(electro)catalytic Fuel and Chemical Production
The production of fuels and other value-added chemicals from sunlight is one of the proposed sustainable pathways to fulfil the constantly increasing energy demand while pushing towards a carbon-neutral circular economy. Photocatalytic (PC) and photo(electro)catalytic (PEC) systems, based on a semiconductor/liquid electrolyte junction, are capable of converting and storing the energy from the sun into chemical bonds. Over the years, poly(heptazine imide) ionic carbon nitride, a cheap, non-toxic, noble metal-free polymeric semiconductor has been exploited mainly as photocatalyst, and more recently, as photoelectrocatalyst. Although the performance of this material in PEC systems has been improving, its application is still hindered by low photocurrent response and poor long-term stability due to its notably high recombination rates, inefficient charge separation and transport, and consequent photo-degradation. Moreover, its compatibility with bio-based systems for green fuel and chemical production has been poorly exploited and rationalised. This thesis tackles these challenges by introducing a versatile and facile method to synthesise highly performant carbon nitride photoanodes, which can be interfaced with metal- and enzyme-based catalysts for CO2-reduction and hydrogen production with record photocurrents and stabilities. State-of-the-art cyanamide-functionalised poly(heptazine imide) (PHI) ionic carbon nitride (NCNCNx) electrodes were produced by co-deposition with indium tin oxide (ITO) nanoparticles, binding agents and conductive bridges, on a thin alumina-coated FTO glass substrate. The Al2O3|ITO:NCNCNx photoelectrodes displayed remarkably low onset potential and an outstanding 1.4 ± 0.2 mA cm–2 at 1.23 V vs the reversible hydrogen electrode (RHE) when selectively oxidising 4-methylbenzyl alcohol. Detailed spectroscopic studies shine a light on electron extraction kinetics within the photoanode and show that the addition of the ITO nanoparticles significantly improves the extraction of electrons from the carbon nitride, which otherwise remain trapped in the material, whilst the alumina underlayer reduces the electrical resistance between the ITO nanoparticles and the FTO substrate leading to record photocurrents. Furthermore, record stability of over 51 hours under continuous operation was achieved by systematically studying the effect of applied potential and light intensity on the ITO:NCNCNx photoanode long-term performance. Voltage-dependent spectroscopic analysis revealed irreversible changes in the carbon nitride morphology and electrochemical behaviour after applying any potential higher than 0.4 V vs RHE and low light intensity. Moreover, operating under concentrated solar light proved fundamental in ensuring high stability. To take advantage of the local temperature increase, the photoanode was coupled to a thermoelectric (TEG) unit, capable of converting the otherwise wasted heat into additional voltage, and employed in a TEG-PEC cell to drive glycerol oxidation coupled to CO2-to-CO reduction for over 70 hours under no external applied bias. As a proof of concept, the ITO:NCNCNx photoanode was also employed in an unassisted 2-electrode photoelectrochemical setup wired to a formate dehydrogenase (FDH) enzyme bio-cathode to perform selective CO2-to-formate conversion. Even under 1 sun, the bio-hybrid PEC system could withstand 10 hours of operation. Finally, the use of a [FeFe]-hydrogenase (H2-evolving enzyme) possessing a positive surface charge around the active site made it possible to directly interface it to the negatively charged cyanamide-modified graphitic carbon nitride (NCNCNx) as photocatalyst powder in solution, without the need for an electron mediator. In conclusion, this thesis showcases significant advancements in addressing the inherent challenges of poly(heptazine imide) ionic carbon nitride for photo(electro)catalytic applications. Through a systematic and fundamental approach, state-of-the-art photoanodes with record-breaking photocurrents and long-term stability were developed for the selective oxidation of organic waste-derived substrates. The integration of NCNCNx in PEC devices and a bio-hybrid PC system demonstrated its potential to drive un-assisted CO2 reduction to green fuels. By improving the efficiency, stability, and versatility of this promising carbon-based polymeric semiconductor, this thesis aims to serve as a platform for further research on – and application of - carbon nitride materials for photo(electro)catalytic and biohybrid systems.
solar-to-chemical, biohybrid systems, photoelectrochemistry, solar fuels, sustainability, Carbon nitride, Renewable energy
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