Solar Fuels

The technology of solar fuels has been proposed as an answer to this critical scientific and technological hurdle for the widespread development of solar systems. In essence, it consists in converting solar energy into chemical energy. More precisely, a solar fuel is a chemical, exhibiting one or several energetic but stable covalent bonds, and generated from abundant reagents under the action of solar energy. Generating solar fuels with high solar-to-chemical efficiencies would grant access to an energy-dense, stable and mobile energy vector, which could additionnaly be easily integrated in the current energy grid as gasoline replacement.

One promising way of generating solar fuels involves the use of a photoelectrochemical cell, which is described on Figure 3. This device is composed of two photoelectrodes: an n-type semiconductor (the photoanode) and a p-type semiconductor (the photocathode) connected in series. These device is then directly immersed in an aqueous electrolyte containing the desired substrate for solar fuel generation, where it can simultaneously accomplish light absorption, charge carrier generation and electrochemical solar fuel production. The "top" photoelectrode posseses a wide bandgap and only absorbs the most energetic photons of the incident sunlight, while the "bottom" photoelectrode exhibits a smaller bandgap and absorbs the remaining low-energy photons that were not absorbed by the "top" photoelectrode. Moreover the band edges of the semiconductors are chosen so that the valence band of the photoanode and the conduction band of the photocathode straddle the redox potentials involved in the desired electrochemical solar fuel generation process. Thus, the holes and electrons generated upon light absorption are spontaneously injected in the electrolyte at the photoanode and photocathode surface repectively, due to the band bending that naturally develops at the semiconductor-liquid junction. Finally, to improve the kinetics of the electrochemical reaction and reduce losses in the solar-to-chemical conversion process, suitable electrocatalysts are deposited on the surface of each photoelectrode to reduce as much as possible the associated overpotentials (η) and efficiently direct the selectivity of the reaction towards the desired product(s).

There exist several avenues of progress for the improvement of photoelectrochemical cells. Among them, the development of novel semiconductor materials exhibiting optimal bandgap values, optoelectronic properties and long-term stability is a vast field of investigation. Indeed, while binary materials have shown promising performance, they still fall short to meet the performance standard necessary to make the technology commercialy relevant. Therefore, the investigation of tertiary and quaternary semiconductors has gained increasing attention, particularly with the successful development of efficient BiVO4 or CIGS photoelectrodes. Because of the enormous amount of possible element combinations offered by the periodic table, there remains a lot of room to discover even more performant semiconductor materials for solar fuel generation.

Our research is mainly concerned with the development of materials and devices capable of generating solar fuels from abundant sources: water, carbon dioxyde and byproducts of biomass exploitation. Interestingly, while water can only be converted to oxygen and hydrogen, a well-studied solar fuel candidate in its own right, using water as a renewable source of protons and electrons in combination with carbon dioxyde or biomass-derived organic compounds allows to access more energy-dense liquid solar fuels.

Carbon dioxyde can be electrochemically reduced to a wide range of carbon-based compounds, including gaseous fuels such as methane or ethane or even more attractive liquid fuels such as methanol or ethanol. Importantly, a lot of equilibrium redox potentials associated with the reduction of CO2 are located within a very small range, as shown in Table 1. One can therefore appreciate that a major challenge when it comes to electrochemical CO2 reduction resides in controling the selectivity of the process towards a product of choice, even moreso as the reduction of CO2 falls within the same potential range than the one of water (E° = 0.00V vs RHE), therefore making hydrogen production a prevalent secondary process, which needs to be managed and potentially supressed.

Indeed, hydrogen production from water is even favored over CO₂ reduction on most metalic surfaces, as shown in Figure 4, using proton and carbonyl adsorption as . Only three groups of metals are capable of significant CO2 conversion in aqueous environments, with a selctivity depending on their adsorptive properties towards the radical intermediate CO₂^•- and the carbonyl indermediate CO^*:

(i) Sn, In, Hg, Pb: the surface of these metals do not coordinate CO₂^•- but interact with it through strongly enough to tranfer one electrons to it and convert it to a formate ion, which readily desorbs from the surface.

(ii) Au, Ag, Pd, Zn, Bi: these metals are able to corrdinate CO₂^•- and further reuce it to a carbonyl adsorbate CO*. This adsorbates interacts only weakly with the surface of these metals and therefore desorbs, producing a carbon monoxide molecule.

(iii) Cu: copper is the only metal that binds CO* strongly enough (without binding protons too strongly and therefore not favoring water reduction) to further reduce it to compounds such as formaldehyde, methanol or methane, and even multi-carbon compounds, such as ethylene or ethanol. Therefore it is the most promising and most studied electrocatalyst for the electrochemical conversion of CO₂ into carbon-based fuels

• Development of oxysulfide photoelectrodes (2022-2025)

A PhD thesis financed by the doctoral school of University of Lyon