Photoelectrosynthetic processes in continuous-flow under concentrated sunlight: combining efficiency with selectivity

Project Information

SunFlower

Grant agreement ID: 101043617

DOI

10.3030/101043617

EC signature date 4 May 2022

Start date 1 June 2022

End date 31 May 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 999 750,00

EU contribution

€ 1 999 750,00

Coordinated by

SZEGEDI TUDOMANYEGYETEM
Hungary

Periodic Reporting for period 1 - SunFlower (Photoelectrosynthetic processes in continuous-flow under concentrated sunlight: combining efficiency with selectivity)

Reporting period: 2022-06-01 to 2024-11-30

Summary of the context and overall objectives of the project

To be the first CO2-neutral continent by 2050, Europe needs to develop and implement disruptive new technologies, based on scientific breakthroughs. In this regard, utilization of CO2 and organic waste as feedstock to generate valuable products will play a key role in turning the chemical industry on a more sustainable, circular path. In the SunFlower project, we are going to demonstrate that two high-value processes (CO2 or CO reduction and glycerol oxidation will be studied first) can be synergistically coupled to produce chemicals (such as ethylene and lactic acid) and fuels, using novel photoelectrode assemblies (both photocathodes and photoanodes), original photoelectrochemical (PEC) device architectures, and automated processes.

The SunFlower project is based on the following three hypotheses:
1. Proper engineering of continuous-flow PEC cells operating under concentrated sunlight will allow current densities similar to the electrochemical (EC) methods.
2. One semiconductor alone can supply the necessary energy input for bias-free operation of PEC cells, while generating two high-value products.
3. PEC methods can provide superior selectivity compared to their EC counterparts, even at high current density operation (as the current density and potential can be decoupled).

To validate our hypotheses, we are going to use for the first time:
• The pairing of two high-value generating redox processes (none of them being H2 or O2 evolution).
• Concentrated sunlight (which has only been used for water-splitting so far).
• Custom-designed and developed PEC cells, elaborating on the photo-gas diffusion electrode concept.
• Machine learning, based on the broad dataset collected by the sensors built in the PEC system, optimizing the performance at a system level.

The proposed combination of these novel approaches will be of groundbreaking nature, therefore, it opens a whole new arena of solar energy conversion.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The most important achievement of this reporting period was the proof-of-concept publication of the project (Nature Catalysis, 7, 522–535 (2024)), we demonstrate that by using concentrated sunlight, we can achieve current densities similar to electrochemical methods, but with lower energy input.
A continuous-flow PEC cell was designed to perform the direct photooxidation of glycerol combined with the dark electroreduction of water or CO2. A photocurrent density of more than 110 mA cm−2 was achieved under 10 sun irradiation using a high-performance Si-based photoanode. The reaction rate and the selectivity of the oxidation process was controlled by the operational parameters. We found that simply by varying the cell temperature, the anode potential (the band bending and therefore the number of holes reaching the surface) and/or the concentration of glycerol (the amount of the reactant), we could shift the reaction pathway to produce larger amount of the desired product, although still a product mixture forms. We compared the PEC and electrochemical oxidation of glycerol at the same current density, and we found a notably different product distribution. The phenomenon can be explained by the at least 0.45 V lower cell voltages (rooted in the anodic half-cell) in the case of the illuminated photoelectrode that ultimately leads to higher selectivity (glycerol oxidation without parasitic OER). The PEC scenario is therefore an exceptional case of the Butler–Volmer relationship where the current and the overpotential can be decoupled to a certain extent. With properly paired semiconductor–catalyst assemblies, bias free operation can be possible at high current densities, and such studies are in progress. Further studies will also assess the challenges of limited stability, by engineering the semiconductor(–protecting layer)–catalyst interface.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The SunFlower project plan is based on the following three hypotheses:

1. Proper engineering of continuous-flow PEC cells will allow similar current densities to the EC methods, when operating under concentrated sunlight.

2. PEC methods can provide superior selectivity compared to their EC counterparts even at high current density operation.

3. One SC alone can supply the necessary energy input for the standalone (i.e. bias-free) operation of PEC cells, while generating two high value products.

Proving any of this will go beyond the state-of-the-art. During the first time period, we have been able to prove the first two. We are currently working on achieving the third one.

1. In our Nature Catalysis paper we have demonstrated 110 mA cm-2 for coupled CO2R and glycerol oxidation. With further improving the components, we were able to demonstrate 200 mA cm-2 (not yet published) for the same paired process, which is fully in the regime of EC methods. Now we are working towards even higher current densities, by upgrading the cell design and using higher solar concentrations.

2. Again, we have shown the first proof for this concept in our Nature Catalysis paper. We have shown notable differences in the product distribution between the electrochemical and PEC scenarios at the same current density. The main differences are the lower total FE of glycerol oxidation products in the case of the electrochemical process; and the higher FE of formate and lactate, as well as the lower FE of glycerate and oxalate in the PEC oxidation. Both are the direct consequences of the parasitic OER in the electrochemical scenario, competing with glycerol oxidation in the dark and the forming O2 reacts with the intermediate products.