
Solar radiation is the most abundant renewable energy source but it is distributed and intermittent, thereby necessitating its storage via conversion to a fuel or chemical commodity (e.g. hydrogen or carbohydrates) for practical use. Solar thermo-chemical and photo-electro-chemical approaches (and combinations thereof) provide viable, non-biological routes for the direct synthesis of solar fuels and chemical commodities. While solar thermochemical approaches utilize heat at high temperature to support an endothermic reaction, photoelectrochemical approaches utilize the photons (with sufficient energy).
I will review the state of the art of photoelectrochemical and thermochemical solar fuel processing approaches and comment on their specific challenges. Given the economic and sustainability advantage of utilizing concentrated radiation in photoelectrochemistry , I will discuss the utilization of concentrated solar irradiation and thermal integration for photoelectrochemical water splitting and show that this design logic is equally valid for photoelectrochemical CO2 reduction. I will highlight engineering challenges to scale such solar fuel approaches to industrially relevant levels.
As the large kinetic, ohmic and transport overpotentials limit the current density and therefore the power density of typical photoelectrochemical approaches, I will explore approaches for close thermal integration and high current density operation. Specifically, I will discuss the development of a modeling framework that assesses the possibility of high-temperature operation (temperatures > 400 K) of the photoelectrochemical approach in a solid state equivalent design. Such devices would utilize ceramic electrolytes, earth abundant catalysts with small overpotentials, and high-temperature solar cells utilizing the thermionic emission principle. I will present the experimental implementation of such a solar-driven high-temperature electrolysis approach in a simpler integrational approach. I will highlight that combinations of solar thermochemistry and electrochemistry can help in enhancing the performance of solar thermochemical cycles and end with a discussion on some of our research highlights on optimization and analysis of porous reaction media as well as high temperature heat storage, relevant for continuous operation of solar thermochemical systems.
Sophia Haussener is an Associate Professor heading the Laboratory of Renewable Energy Science and Engineering at the Ecole Polytechnique Fédérale de Lausanne (EPFL). She received her MSc (2007) and PhD (2010) in Mechanical Engineering from ETH Zurich. Between 2011 and 2012, she was a postdoctoral researcher at the Joint Center of Artificial Photosynthesis (JCAP) and the Energy Environmental Technology Division of the Lawrence Berkeley National Laboratory (LBNL). She is a member of EPFL’s research award commission and of EPFL’s Academic Strategic Committee. She has published over 100 articles in peer-reviewed journals and conference proceedings, and 2 books. She has been awarded the ETH medal (2011), the Dimitris N. Chorafas Foundation award (2011), the ABB Forschungspreis (2012), a Starting Grant of the Swiss National Science Foundation (2014), the Prix Zonta (2015), the Global Change Award (2017), the Raymond Viskanta Award on Radiative Transfer (2019), and the Yellott award (2024). In 2024, she has been named one of Cell Press’ 50 Scientist that inspire. She is a co-founder of the startup SoHHytec aiming at commercializing photoelectrochemical hydrogen production. She is the former chair of the American Society of Mechanical Engineers’ (ASME) Solar Energy Division (2018), a former member of the Scientific Advisory Council of the Helmholtz Zentrum (2016-2022), a member of the scientific board of the Liquid Sunlight Alliance, and a member of the Ethics Board of Arete Ethik Invest.
Her current research is focused on providing design guidelines for thermal, thermochemical, and photoelectrochemical energy conversion reactors through multi-physics modeling and demonstrations. Her research interests include: thermal sciences and radiative transfer, fluid dynamics, charge transfer, and thermo/electro/photochemistry in complex multi-phase media on multiple scales.