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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CO2 Intermediates (From CO2, Water and Sunlight to Valuable Solar Fuels: Tracking Reaction Intermediates in Solar Fuel Generation with Ultrafast Spectroscopy for More Efficient Catalysis)

Teaser

Summary

Decarbonising our industrial, transport and energy sectors has become one of the most urgent challenges of modern society and research. The way we produce fuels and chemicals and store energy has to become sustainable and carbon neutral in the coming years. Integrating renewable energy sources such as solar and wind is the pathway forward to make this transition happen and this requires more knowledge and education in the development of new, clean, efficient and scalable technology based on photo- and electrocatalysis.
The objectives of this Marie Curie project and my future research are to design efficient devices for the generation of valuable fuels and chemicals from CO2, water and sunlight by understanding electro- and photoelectrocatalysis on the molecular/atomic scale and on the relevant timescales such as time of solar energy absorption (=charge generation), of charge transport to the catalyst surface and of breaking and making bonds to form a valuable product.
The Marie Curie fellowship allowed me to acquire knowledge and expertise in a set of spectroscopy techniques that allow me to track each of these processes by taking picosecond, nanosecond, microsecond, millisecond and second snap shots of the system after light absorption. Together with modern computational modelling techniques, the aim is to be able to assemble these snap shots to a full picture and understand reaction mechanisms, bottlenecks and energy losses in the device and ultimately establish guidelines that further the development of better catalysts.

Work performed

In this project I fabricated photoelectrodes for CO2 reduction and investigated these with a set of transient absorption spectroscopies and gas chromatography techniques. I found that buried junction type photoelectrodes provide higher photovoltages and hence driving force for the reactions and hence would be more promising for the photoelectrochemical approach to fuel generation from water, CO2 and sunlight. This finding is published in DOI: 10.1039/C8TA07036A, J. Mater. Chem. A, 2018, 6, 21809-21826 (open access). I fabricated polymer photocathodes and decorated those with suitable hydrogen evolution and CO2 reduction catalysts to study reaction dynamics, track intermediates and understand reaction mechanisms. These works require some further experiments due to the complexity of transient IR spectroscopy, instability of some polymer blend materials and novelty of this class of photoelectrochemical cells for CO2 reduction. I will continue my research at Imperial College London in this field and will benefit from my experience, expertise and collaboration network I established during the Marie Curie Fellowship.

In the transient absorption studies performed on photoabsorber materials such as CIGS, polymer bulkheterojunctions, La,Sr:SrTiO3 and other oxide materials, it became clear that the efficiency of catalytic systems is strongly affected by charge carrier (and energy) loss mechanisms within the photoabsorber material. Photoabsorber material properties can cause charge carriers to slow down on their way (i.e. polaron formation, defect trapping) or loose their energy entirely via electron-hole recombination by generating heat (non-radiative recombination) or light (radiative recombination). Hence, the challenge is to use these carriers for catalysis before these are lost to recombination.

In the study on La,Rh:SrTiO3 I could understand important mechanisms of co-doping in perovskite materials that transform the materialâ€™s electronic structure and eliminate mid-gap trap levels when the oxidation state of the dopant is varied. This study revealed the unique properties of La,Rh:SrTiO3 to be able to store charge very well up to timescales relevant for catalysis (second timescale) and proved the suitability of such material and doping mechanisms for CO2 reduction photocatalysts. The work will be submitted shortly.

Working with CIGS photoabsorber materials, I have learned that band gap gradients are useful to extract charge carriers and improve the mobility of carriers in polycrystalline materials. Using transient absorption spectroscopy as a tool to track photo-generated charge carriers and calculate mobility within the photoabsorber was introduced into the field of CIGS research. Two works will be submitted soon.

Working with a newly developed technique called pump-push-photocurrent spectroscopy in collaboration with the Bakulin group at Imperial College London, we could observe polaron formation (self-trapping) in oxide materials. Using a short infrared light pulse we could prove that these self-trapped charges can be re-excited to the conduction band and eventually be extracted as photocurrent. This work is just accepted in Nature Communications.

Final results

The project has provided me time and resources to expand my expertise in photocatalysis and gave me the opportunity to step into the field of renewable energy-driven electrocatalysis. It gave me opportunities to enhance my international visibility, grow as a scientist and leader in renewable energy research. Being part of several consortia on renewable energy research and related policies, I have been translating my knowledge and findings to industry (via the
H2FCSUPERGEN HUB, Climate KIC and WEInnovate programmes), policy makers (via SUNRISE Action and Energy Futures Lab at Imperial College London) and contribute to the education of a new generation of citizens who are needed in developing and deploying emerging cleantech technologies in the decades to come (via training of 6 PhD, 2 MRes and 4 undergraduate students during the time of this project).

The scientific insights gained at this point are initiating important directions for research and innovation in the field of solar energy conversion to fuels and chemicals. Having understood charge carrier dynamics within a series of absorber materials and catalysts, has provided guidelines for next-generation catalyst and device designs using earth-abundant materials. Combining these insights with charge carrier dynamics at the catalyst surface will be detrimental for the development of catalysts suitable for large-scale production of affordable clean technology.