"The goal is to manufacture an artificial leaf"

Nature as a model for DEPECOR research. © DEPECOR

Professor Hannappel, you are doing research on artificial photosynthesis. What is that about?

We are trying to copy the most significant, famous and vital process for wildlife on Earth, by which I mean the extremely successful process of photosynthesis. Over the course of billions of years, nature developed a sophisticated mechanism for turning sunlight into chemical energy. Sunlight is in abundant supply for free. In a first step, water is broken down into its components hydrogen and oxygen. In a second step, the plant uses the hydrogen and carbon dioxide to produce a high-quality fuel. Our aim in artificial photosynthesis is to manufacture an artificial leaf. Dipped into water, this leaf will generate a fuel when exposed to sunlight without being linked to the outside world.

Thomas Hannappel is a physics professor at TU Ilmenau and studies the fundamentals of energy materials. ©TU Ilmenau

What must happen to ensure that this process can be applied in practice?

A general proof of feasibility has already been furnished and some of the results have been very encouraging. Of course, competitiveness is very important in this endeavor. Parameters such as efficiency, material, production costs and robustness play a major role. Research and development are indispensable for mastering this extremely complex challenge. Nobel prize-winning topics such as catalysis and optoelectronics are very important from a scientific perspective. Interfaces like the liquid-cell interface are of crucial significance. As Herbert Krömer, winner of the Nobel Prize for Physics in 2000, put it: "The interface is the device."

Your BMBF-funded project DEPECOR deals with precisely these interfaces. What is the project about?

Our project is about the solid-liquid interface and catalysis. Plants achieve a photosynthesis efficiency of around one percent. We can do much better in principle, using so-called semiconductor materials. This enables us to successively exploit sunlight in the way nature does – first its high-energy blue part, and then the low-energy red and infrared parts. Such a configuration is called tandem or multi-absorber set-up, which you need to ensure the efficiency of the fundamental process of water decomposition. Such a set-up can also be used to split carbon dioxide into its components. Our DEPECOR project pursues a systemic approach, using highly efficient solar sub-cells that are already common in photovoltaics.

That means you are trying to outwit nature. How much better do you want to become?

Photovoltaics has taught us that we cannot be competitive with low, solar-to-fuel efficiency. We must by no means remain under 10%. Ideally we should reach or even surpass an efficiency level of 20%. We have already succeeded in doing so in hydrogen generation, where we have cooperated with our partners at the Joint Center for Artificial Photosynthesis (JCAP) in the United States to develop an artificial leaf and achieved a world record efficiency of 19.3%. We are thus in a good position, but we can and must do even better.

BMBF-DoE workshop on artificial photosynthesis

On 24-26 June 2020, the German Federal Research Ministry (BMBF) and the US Department of Energy (DoE) are co-hosting a virtual workshop to strengthen US-German cooperation in the field of artificial photosynthesis. At the invitation of BMBF and DoE, fifteen high-level researchers from each of the two countries will participate to develop a common vision for artificial photosynthesis. The scientific co-chairs are Professor Thomas Hannappel of TU Ilmenau and Dr. Frances Houle from the Joint Center for Artificial Photosynthesis (JCAP) in Berkeley, CA. The workshop will be opened by Volker Rieke, Director-General "Provision for the Future – Basic Research and Research for Sustainable Development" at BMBF, and Dr. Harriet Kung, Deputy Director for Science Programs, Office of Science, U.S. Department of Energy.

Who is involved in the project?

We are working with excellent partners, including TU Ilmenau, the Fraunhofer Institute for Solar Energy Systems (ISE), Helmholtz-Zentrum Berlin (HZB), the Technical University of Munich and AZUR SPACE Solar Power GmbH as a partner from industry. The Joint Center for Artificial Photosynthesis (JCAP) is involved as our US partner and there are further associated partners like BASF, Evonik and EPFL Lausanne.

Under DEPECOR you are cooperating with US researchers just as you did for your world record. What is this cooperation expected to deliver?

Artificial photosynthesis requires research and development which is highly ambitious, complex and interdisciplinary. We are pooling all the available capabilities and expertise for this purpose. In a determined joint effort, we have successfully developed an artificial leaf which is currently the best of its kind in the world. We want to build on this success story and in this way benefit from our different strengths, potential and experience.

The BMBF and US DoE are co-hosting a virtual workshop on artificial photosynthesis on 24-26 June. What is the objective of this event?

We want to work out a research and development strategy as a basis for jointly addressing, and eventually solving, major issues of artificial photosynthesis. To achieve this, we need all the expertise and know-how from Germany and the United States. During the workshop, we will be discussing vital issues and identify complementary strengths. Excellent work has been done in Germany on topics such as crystal growth and solar energy conversion. Our US partners have achieved outstanding results in fields such as system architecture and prototype development.

© DEPECOR

What are you actually doing at the workshop?

The overall aim is to develop an integrated multifaceted component. We can significantly complement each other's efforts and improve our performance. We will address the variety of topics, jointly considering, discussing and elaborating them in small groups. That is, we are pursuing the ambitious goal of developing something in a truly joint effort.

What will be done to follow up on the workshop?

We must keep working in this field in a persistent long-term approach. There is good reason for considering this to be the holy grail of electrochemistry, which can provide sustainable solutions for global problems such as energy supply, storage and mobility. Of course, we also want to achieve milestones that are important for related technologies. These include competitive tandem solar cells, which are the focus of current research efforts. Efficient and stable catalytic processes that can be used in alternative hydrogen generation, the so-called Power-to-X technology, are another example. Optoelectronics in general may benefit from semiconductor material development. We are therefore talking about a whole set of breakthrough technologies, not just a single one. Much of this is still a long way off and a lot remains to be done. But our current work takes us nearer our goal.

Thank you for the interview, Professor Hannappel.

DEPECOR and CO2-WIN

The project entitled "DEPECOR – Direct efficient photoelectrocatalytic CO2 reduction" is part of the CO2-WIN funding measure (CO2 as a sustainable source of carbon – Pathways to industrial application). This measure of the German Federal Research Ministry supports projects which enable carbon dioxide to be used as a raw material in German industry. Twelve collaborative research and development projects are supported with a total of roughly €27 million over a period of three years. The projects started on February 1st 2020. Funding is not only provided for work on artificial photosynthesis but also for projects dealing with the mineralization and the chemical, electrochemical and biotechnological conversion of carbon dioxide. For more information about the funding measure please visit www.co2-utilization.net