Dirk Holtmann (KIT) about GAMES

Gas Diffusion Electrodes for Coupled Microbial Electrochemical Syntheses from CO₂

CO₂-WIN Connect: Professor Holtmann, in the "GAMES" project, you and your research partners are working on microbial electrosynthesis from CO₂ based on gas diffusion electrodes. Can you please briefly explain how a gas diffusion electrode (GDE) works and how the GDE developed within GAMES differs from those used in fuel cells, for example?

Dirk Holtmann: Gas diffusion electrodes (GDE) are special electrodes that are used in electrochemical systems such as fuel cells, electrolysers, and batteries. They enable the efficient conversion of gaseous reactants at the electrode surface. In general, gas diffusion electrodes play a central role in modern electrochemistry, especially in technologies based on the conversion of chemical energy into electrical energy and vice versa. The GDEs consist of various components where the catalyst layer is particularly important, as this layer is crucial for the electrochemical reaction. Gaseous reactants (such as hydrogen in a fuel cell or oxygen in an electrolyser) diffuse through the porous structure of the electrode to the catalyst layer. The porous structure ensures that the gas diffuses effectively and quickly. The electrochemical reactions take place at the catalyst layer. In a fuel cell, for example, hydrogen is split into protons and electrons at the anode, while oxygen reacts to form water at the cathode. The advantages of GDEs are:

• High reaction rates: Due to the large surface area of the porous structure and the effective diffusion of the gaseous reactants, high reaction rates can be achieved without the gases first having to be dissolved in the electrolyte.

• Efficient use of catalysers: The catalyst layer is usually thin, which enables efficient use of the expensive catalyst material.

• Scalability: Gas diffusion electrodes can be produced in different sizes and shapes, making them suitable for a wide range of applications.

In terms of their general design, GDE in electro-biotechnological applications do not differ from purely electrochemical applications. The differences relate to the electrolytes used and the electrochemical conditions, which must be biocompatible.

CO₂-WIN Connect: Your project consortium is developing processes for coupled electrochemical-microbial synthesis. Hereby CO₂ is reduced to formate at a GDE in a first step and this in turn is biotechnologically converted into industrially relevant valuable substances in a further step. So you combine electrochemical, microbiological, biotechnological and process engineering aspects. In which of these areas has the greatest progress been made? Which area is the most critical? Can the individual disciplines be clearly separated from one another, or are the decisive advances and innovations more likely to be realised at the interfaces?

Dirk Holtmann: These steps can never be considered separately; all steps must be considered holistically. This is what makes the GAMES project so special as researchers from all areas work together to achieve effective overall processes. On the one hand, our innovations stem from sub-areas such as the production of better GDEs or a deeper understanding of the microbial utilisation of formate. On the other hand, they also relate to the integration of all individual steps into an efficient overall process.

CO₂-WIN Connect: In your opinion, does coupled electrochemical-microbial synthesis on a large scale offer a viable technology for storing excess electrical energy? Or are the produced valuable materials primarily of interest for further industrial processing?

Dirk Holtmann: Both! We can store excess electrical energy very well as formate and use it to produce a wide range of intermediates and products, e.g. alkanes, alcohols, aldehydes, complex carbohydrates, or bioplastics. The technology platforms in GAMES can be used very flexibly and are easily transferable to other applications with different organisms.

CO₂-WIN Connect: Please explain to us how biocompatible drop-in electrolysis works and what role it plays in the project.

Dirk Holtmann:There are various configurations that combine the sub-steps of CO₂ reduction to formate and subsequent microbial conversion. This can, for example, take place in separate steps with intermediate purification of the formate. In biocompatible drop-in electrolysis (ex-cell), the formate solution is produced without an additional intermediate step in an electrolyte that has very favourable electrochemical properties and can be used in microbial cultures at the same time. This contributes to simplifying the process and reducing the use of raw materials.

CO₂-WIN Connect: One of the project objectives is to generate new added value for biogas plant operators. Can you please specify what is meant by this and what progress has been made in this regard?

Dirk Holtmann: This involves the electrochemical conversion of CO₂, particularly from biogas to formate in times of electricity excesses or very favourable electricity prices. The formate can then be converted into methane and fed into the gas grid as biomethane, for instance. Here we create an electricity buffer that can be fed back into the energy sector as needed.

CO₂-WIN Connect: The GAMES project will soon be completed. What are the next steps and which vision do you have for the future use and relevance of your new technology?

Dirk Holtmann: The project ends in March 2025, and we still have to finalise some research work. However, we are already in the final utilisation phase, some publications are in progress and the PhD theses are close to completion. However, most important is the transfer to real-life applications, which is to take place both in further publicly funded projects and in bilateral follow-up projects in collaboration with industry. This is also the vision of GAMES - to bring GDE-based bioprocesses into application - and we are already moving in the right direction. The particular relevance arises on the one hand from the coupling of the energy sector with chemical production and on the other hand from the utilisation of CO₂ as a raw material and the associated contribution to the sustainability goals of the United Nations. This will enable a modern circular economy in terms of a bioeconomy.

CO₂-WIN Connect: Thank you for your answers!

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