Prof. Thomas Hannappel, TU Ilmenau about Depecor

5 Questions about DEPECOR

CO₂-WIN Connect: Prof. Hannappel, while reading through the DEPECOR project factsheet we learn that you aim at performing both water splitting and CO₂ reduction towards solar fuels. Could you explain which fuels exactly you plan to produce and how the selection is performed (in case that several catalysts are tested, or a product mix is obtained)?

Prof. Hannappel: As it is also in natural photosynthesis, hydrogen is the initial, simplest and most fundamental solar fuel in the series of potential products of artificial photosynthesis. In nature, this first output in the process of photosynthesis is then utilized to successively generate bio-chemical products of higher values when transforming CO₂. Our approach to reduce CO₂, the approach of a direct and efficient photoelectrochemical route, in short DEPECOR, combines so-called multi-absorber semiconductor structures with efficient catalysts, specifically selected according to their material, adsorption site, size, and shape. Since the redox potentials of the different feasible reaction products in our approach such as carbon monoxide, methane, formic acid, or ethanol, are in a narrow energetic range, they cannot be clearly selected only by adjusting the voltage, the electro-chemical potentials generated in the semiconductor device structure, the so-called Fermi-level-splitting. Thus, the selectivity of products needs to be accomplished by the choice of the appropriate catalyst properties. Whether a product mix might be utilized or refined suitably, and how to optimize this challenging problem, is one of our many different current research questions.

CO₂-WIN Connect: For achieving your goal of developing a demonstrator, you collaborate with a multitude of partners from science/academia and industry; not only from Germany but even on international scale. Could you explain on the complexity of current APS devices, and particularly the DEPECOR device?

Prof. Hannappel: The development of demonstrators or even prototypes is, of course, a long-term challenge with many facets starting from the heart of the device, the semiconductor structure, involving the functional layers and the catalysts, dynamics and transport issues of charge carriers, educts and products, and ending in the module architecture with all the gas treatment. In our extensive scientific interaction and exchange, we are currently discussing all these challenges with each other, not only with our direct partners in the project including our industry partners therein, but also with further national partners and with international associated partners, in particular, in collaboration with the U.S. In this consequence, and specifically motivated by the DEPECOR project, we have initiated a vital exchange with corresponding experts in the U.S. Since the last year, we have carried out a series of workshops and as the pandemics has not allowed for in person meetings since the beginning of the DEPECOR project both the collaborative exchange as well as the workshops have been performed virtually. Many options for the US-Germany solar fuels research communities to work together were identified, enthusiastically, during the meeting, and perspectives, opportunities and synergies have been elaborated extraordinarily fruitful.

CO₂-WIN Connect:How much can and will the DEPECOR technology add on existing knowledge on APS? Could it be the long-awaited break-through? Which are major milestones and risks of the project / device development?

Prof. Hannappel: In particular, in our intense exchanges with experts from the U.S. and Germany, we've identified and elaborated essential areas of challenges, such as general needs for architectures of efficient AP systems and prototypes and all the associated scientific and technological questions and problems such as catalyst development, critical interfaces, suitable multi-junction device structures, light management, etc., and specifically: Understanding! An understanding, which elements are actually key to achieving a positive energy return from AP, and how to balance efficiency with device costs are, of course, finally also essential. However, currently the principle, fundamental questions and challenges are of highest priority. The DEPECOR project can be regarded as a representation and as an initiator, which directly displays and addresses the ambitious targets in conjunction with AP.

CO₂-WIN Connect: What is the intended production volume of the demonstrator under development? Do you already have an envisioned production site for your demonstrator?

Prof. Hannappel: In comparable approaches, for instance in the development of solar cells, we've learned that feasibility studies on scales of some square centimeters already show very clear and dependable whether an integrated system or a demonstrator would be suitable to efficiently operate and which problems are obvious ones. Hence, we're planning length scales of our demonstrator device structures in spatial ranges of a few square centimeters. Demonstrators need to provide concerted optical, opto-electronic, chemical and ionic processes, while yielding high conversion efficiencies, catalytic selectivity, and photochemical stability. The coupling of all the elementary processes over such length scales within an integrated system already presents the challenging scientific and technological questions, as well as impediments to the realization of practical systems.

CO₂-WIN Connect: Beyond this particular research project: What is your overarching vision when it comes to APS? Can APS replace refineries on a long term, or will we have smaller decentralized fuel production sites?

Compared to water-splitting devices, for the reduction of CO₂ different material structures such as different semiconductor band gap combinations need to be integrated in a photoelectrochemical cell. Regarding the design and demonstration of efficient and durable solar fuel systems for CO₂ reduction, there are significant challenges both in integration of components into systems and in quantitative characterization of performance. Working devices must ensure the coupling of the involved components and elementary processes with minimized voltage and current losses preferentially utilizing abundant materials with sufficient stability for achieving economically viable systems. Our vision bridges and interconnects considerations of basic research and technology developments in different, highly relevant directions up to the large-scale application such as it has already been developed in the track record and success story of solar cells. It is certainly a fantastic future vision if we would be able to mimic nature, and to have stable, competitive and scalable devices available, which can be utilized to reduce the concentration of CO₂ in air, water, or directly from industrial processes using sunlight, and to renewably generate solar fuels out of it.

CO₂-WIN Connect: Prof. Hannappel, thank you for the interview!

Do you want to learn more about the BMBF-funded Project DEPECOR?

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