Researchers from the Karlsruhe Institute of Technology (KIT) and their Canadian partners have designed a low-cost photoreactor design for solar-driven synthesis. The photoreactors have a low level of complexity, are readily manufacturable via mass fabrication techniques in polymers, and are easy to adapt to diverse photocatalysts. An open-access paper on the work is published in the journal Joule.

In artificial photosynthesis, chemical reactions are carried out with the help of sunlight. As with the natural model, photons are absorbed by a photocatalytically active material in such a way that their energy directly drives a chemical reaction.

Different photocatalysts can be used, for example, to split water into hydrogen and oxygen, but climate-neutral fuels can also be produced from water and carbon dioxide, says Paul Kant from the Institute of Microprocess Engineering (IMVT) of KIT.

Until now, however, the technology has mainly been found in the laboratory because the costs of producing solar hydrogen were simply too high. However, with a concept for highly efficient photoreactor panels that can be installed in cost-effective modules, the research group has now taken a decisive step towards practice.

An efficient photoreactor module for practical use must essentially have two components: On the one hand, a suitable photocatalyst must be available that drives the actual chemical reaction. On the other hand, a photoreactor must be present—i.e. a “container” for the photocatalyst as well as the starting materials of the chemical reaction.

he photoreactor should ideally direct incident sunlight to the photocatalyst with low loss, no matter from which direction it occurs, or no matter where in the sky the sun is, explains Kant. It is also important that the photoreactor ensures optimal operating conditions for the photocatalyst due to its structure and the material used, such as the right temperature or the right intensity when absorbing light at the photocatalyst.

The photoreactor concept presented by the research team addresses this double challenge: It consists of microstructured polymer panels that are coated with aluminum for high reflectivity and enables both optimal operating conditions and efficient transport of light to the photocatalyst throughout the day. The researchers developed the system with the help of computer-aided geometry optimization and a photocatalytic model system and were already able to demonstrate it on a laboratory scale.

The base design is an extrudable array of reaction channels… The cross section of a single channel comprises a V-shaped concentrator capturing light from various incident directions and guiding it into a tube-like, mirrored cavity, enclosing the reaction volume… The precise shape of a single concentrator cavity channel is optimized in a way that the achieved UV-vis photocatalytic efficiency… is maximized. The optimization is based on a 3D optical model that employs Monte Carlo ray tracing coupled to a plug flow reactor model that maps the chemical conversion. The optimization, thus, considers the specific optical and reaction engineering properties of the employed reaction system and reactor component materials. Kant et al.

Computer-aided design (CAD) model rendering of the single-channel lab photoreactor employed for the demonstration of the proposed photoreactor concept. Kant et al.

On the basis of a generally valid guideline, which was developed by the researchers on the basis of a detailed analysis of their reactor concept, future photoreactor modules can now be designed relatively easily for maximum efficiency for different purposes.

However, high efficiency in the chemical reaction is only part of the challenge to establish artificial photosynthesis as an economic technology. For relevant product quantities, extremely large areas must be covered with photo reactor panels. According to initial calculations, the researchers estimate the price at about US$2 per square meter of photoreactor module.

Concept for highly efficient photo reactor panels for equipping inexpensive modules. (Photo: Amadeus Bramsiepe, KIT)

In further work under the leadership of Anselm Dreher, a suitable photocatalyst will now be developed in the next steps at the IMVT in Karlsruhe and in the working group led by Professor Geoffrey Ozin in Toronto, which efficiently splits water into hydrogen and oxygen. The photocatalyst is then integrated into the presented photoreactors. In addition, current work includes studies on the mass production of the presented panels.

Resources

• Paul Kant, Shengzhi Liang, Michael Rubin, Geoffrey Alan Ozin, Roland Dittmeyer (2023) “Low-cost photoreactors for highly photon/energy-efficient solar-driven synthesis.” Joule doi: 10.1016/j.joule.2023.05.006