Photoelectrochemical fuel synthesis using organic semiconductors with (bio)molecular catalysts

Photoelectrochemical (PEC) devices offer great promise for simultaneous solar light harvesting and chemical storage, converting water and CO2 into value-added products. While progress has been made to improve their solar-to-fuel conversion efficiencies, most conventional prototypes face challenges of insufficient photovoltage, moisture instability, high material cost or toxicity. This PhD thesis introduces PEC devices based on an organic π-conjugated bulk heterojunction (BHJ), protected by a carbon-based encapsulant to produce a range of solar fuels from H2 to syngas and formate. Starting from the rational design of organic photovoltaic (OPV) layers, conventional architecture devices based on the archetypal P3HT:fullerene BHJ were developed. The OPV fabrication techniques were subsequently extended to the BHJ PCE10:EH-IDTBR and photocathodes interfaced with a platinum catalyst achieved unprecedented long-term H2 production over 300 h, especially in a benign pH-neutral aqueous solution. This tackled the main bottleneck in the organic PEC field, proving that acidic buffers are not always needed for optimal performance, and expands the scope of reactions driven by organic semiconductors (OSCs). The devices were further interfaced with a molecular cobalt CO2 reduction catalyst, demonstrating tunable and selective CO production under 0.1 sun irradiation. Their early onset potentials and complementary light absorption with BiVO4 enable the assembly of PCE10:EH-IDTBR–BiVO4 standalone artificial leaves, operating over 96 h completely powered by sunlight. Besides synthetic electrocatalysts, this thesis also demonstrates the biocompatibility of organic PEC devices with enzymes. Through systematic design, the use of toxic elements and non-innocent external components (buffers, mediators or sacrificial reagents) were carefully avoided. To this end, OPVs were interfaced with hierarchically nanostructured inverse opal (IO) TiO2 electrodes, hosting the enzymes hydrogenase (H2ase) or formate dehydrogenase (FDH) for direct solar fuel synthesis. By co-immobilising carbonic anhydrase (CA), supported by simulations and spectroscopic investigations, the photobiocathodes generate unprecedented photocurrent densities of up to −8 mA cm−2 in a pH neutral bicarbonate solution, attaining stable H2 production or selective CO2-to-formate conversion over 10 h. Semi-artificial photosynthesis is achieved by assembling the photobiocathode with a BiVO4 photoanode in an artificial leaf device for unassisted CO2 reduction coupled to O2 evolution, attaining a faradaic yield (FY) of 87% over 24 h of operation. Extending beyond the O2 evolution reaction, an organic–inorganic PEC tandem device was constructed by coupling the organic photobiocathode with a hematite photoanode. The hematite photoanode achieved continuous and selective oxidation of alkaline pre-treated real-world polyethylene terephthalate (PET) plastics to formate, while the organic photocathode coupled to FDH showed selective CO2 photoreduction to formate under neutral pH conditions. The integrated PEC device operated without an external voltage input, driving the solar-powered comproportionation of plastic waste and CO2 into a single product, formate, with a FY approaching 200%. Finally, the semi-artificial organic PEC devices were integrated with engineered microbes for domino catalysis. Buffer optimisation proved crucial for the H2ase-Clostridium ljungdahlii system, while challenges of glycerol contamination and reactor design were addressed for the FDH-Escherichia coli catalytic cascade. The biohybrid assembly strategies herein may provide a general basis on which further developments could be made in the future. The overall findings provide a proof-of-concept for the integration of OSCs with synthetic and biological catalysts for solar fuels production. This establishes a new path for organic PEC devices, as we approach the composition, function, and efficiency of natural leaves.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Yeung, Celine Wing See

Advisor dc:contributor.advisor

• Reisner, Erwin

• organic photovoltaics
• CO2 reduction
• photoelectrochemistry
• solar fuels
• water splitting
• semi-artificial photosynthesis
• enzymes
• biohybrids
• organic synthesis
• computational modelling
• microbes
• artificial photosynthesis

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/94f2dce0-ae4f-4e01-bfba-e325da4dc445/download
• http://purl.org/NET/rdflicense/allrightsreserved

Author Identifier

0000-0003-4954-7288

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/386928