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Photoelectrochemical tandem cells with enzymes wired to hierarchically-structured electrodes for solar fuel synthesis



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Sokol, Katarzyna 


In photosynthesis, solar energy drives the conversion of CO2 and H2O into chemical energy carriers and building blocks, releasing O2 as a by-product. Artificial photosynthesis attempts to mimic this process to produce a renewable and storable fuel, such as H2. Semi-artificial photosynthesis combines the strengths of natural photosynthesis with synthetic chemistry and materials science to develop model systems that overcome Nature’s limitations, such as low-yielding metabolic pathways and non-complementary light absorption by photosystem (PS) I and II. PSII, the first photosynthetic enzyme, is capable of photocatalytic water oxidation, a bottleneck reaction in artificial photosynthesis. The study of PSII in protein film photoelectrochemical (PF-PEC) platforms sheds light into its biological function and provides a blueprint for artificial water-splitting systems. However, the integration of biomolecules into electrodes is often limited by inefficient wiring at the biotic−abiotic interface. In this thesis, a range of tuneable hierarchically-structured electrodes was developed, constituting a versatile platform to accommodate a variety of biotic guests for PF-PEC cells. A new benchmark PSII−electrode system was assembled, that combined the efficient wiring afforded by redox-active polymers with the high loading provided by hierarchically-structured inverse opal indium tin oxide (IO-ITO) electrodes. A fully-integrated host−guest system showed a substantially improved wiring of PSII to the IO-ITO electrode with an Os complex-based and a phenothiazine-based polymer. Subsequently, a bias-free tandem semi-artificial cell was assembled, that wired PSII to hydrogenase for overall solar-driven water splitting. This PEC cell integrated the red and blue light-absorber PSII with a green light-absorbing diketopyrrolopyrrole dye-sensitised TiO2 photoanode enabling complementary panchromatic solar light absorption. Effective electronic communication at the enzyme−material interface was engineered using an Os complex-modified polymer on a hierarchically-structured IO-TiO2. Finally, a semi-artificial tandem device was designed, which performed solar-driven CO2 reduction to formate with formate dehydrogenase by coupling to the PSII−dye photoanode. The system achieved a metabolically-inaccessible pathway of light-driven CO2 fixation to formate and demonstrated a precious metal-free model for solar-driven selective CO2 to formate conversion using water as an electron donor. These semi-artificial platforms demonstrate the translatability and versatility of coupling selective and efficient electrochemical reactions to create challenging models and proof-of- principle devices for solar fuel synthesis. They provide a design protocol for bias-free semi-artificial Z-schemes and an extended toolbox of biotic and abiotic components to reengineer photosynthetic pathways. The assembly strategies presented here may form the basis of all-integrated electrode designs for a wide range of biological and synthetic catalysts.




Reisner, Erwin




Awarding Institution

University of Cambridge