Institute

Liquid metal (LM) alloys are of increasing interest for the electro-reduction of CO₂. Besides avoiding coking effects, they often exhibit an electric field-induced accumulation of elements at their interface. In this study, we investigate the ternary GaInSn alloy in DMF-based electrolyte. Even small amounts of water in DMF significantly increase the CO₂ reduction rate, while the competing hydrogen evolution remains a minor side product. Analysis of the electrolyte reveals expelled tin and gallium. This points to an enrichment of the interface in tin, due the applied electrical field. Our study provides a mechanistic understanding of CO₂ reduction on GaInSn in the light of dynamic changes of the elemental surface composition.

Aya G. A. Mohamed, Daniel Lörch, Holger Euchner, Matthias M. May, Peter Bogdanoff; ChemCatChem 2025, 0, e202401740; doi.org/10.1002/cctc.202401740

To further explore our recently proposed concept that couples PEC water splitting to catalytic hydrogenation, we explored the techno-economic viability of seven different hydrogenation reactions for a hypothetical PEC-hydrogenation plant with a capacity of 1,000 kg H₂/day. We found that when assuming a solar-to-H₂ efficiency of 5% and a lifetime of 5 years, three of the seven reactions are economically viable, with the hydrogenation of acetophenone to 1-phenyl ethanol  offering the highest returns. The solar-to-H₂ efficiency and the fraction of the produced hydrogen used for hydrogenation are the most critical parameters.

X. Zhang, Z. Li, L.R. Sinaga, M. Schwarze, R. Schomäcker, R. van de Krol, F.F. Abdi, “Techno-Economic Assessment of Sustainable H₂ Production and Hydrogenation of Chemicals in a Coupled Photoelectrochemical Device”, ACS Sustain. Chem. Eng. 12 (37), 13783 (2024). DOI: 10.1021/acssuschemeng.4c03463.

The presence of gas bubbles at the surface of photoelectrodes can cause various efficiency losses, due to e.g. loss of active electrode area, resistive losses in the electrolyte, and optical scattering. Any gas bubbles formed should therefore detach as quickly as possible. We modelled the process of bubble detachment using multiphysics simulations and found that the presence of a boundary layer that is supersaturated with dissolved oxygen gas facilitates bubble detachment. Surface tension forces were found to be more important for bubble detachment than buoyancy effects. Based on these findings, we provide several suggestions for minimizing bubble-induces losses in practical PEC devices.

F. Liang, R. van de Krol, F.F. Abdi, “The influence of dissolved gas supersaturation on bubble detachment from planar (photo)electrodes”, Cell Rep. Phys. Sci. 5, 102069 (2024). DOI: 10.1016/j.xcrp.2024.102069.

Although PEC water splitting studies are almost always carried out at atmospheric pressure, operating at higher pressure may reduce optical and ohmic losses by reducing the size of the gas bubbles. We developed a multiphysics model to study these effects for a membrane-free PEC flow device. We found that by increasing the pressure to 6 – 8 bar, the combined energy losses (mainly due to light scattering at bubbles) can be reduced by a factor of 2, while a further increase in pressure offers little benefit. This is the first time that an optimum pressure range for PEC devices has been proposed.

F. Liang, R. van de Krol, F.F. Abdi, “Assessing elevated pressure impact on photoelectrochemical water splitting via multiphysics modeling”, Nat. Commun. 15 (2024) 4944. DOI: 10.1038/s41467-024-49273-2.

CuFeO₂ is a potentially interesting photocathode material for solar water splitting applications. It shows excellent stability in alkaline solutions and is a good light absorber, but there is a large spread in its reported opto-electronic properties. We prepared well-defined thin films of p-type CuFeO₂ with pulsed laser deposition, which allowed us to study its opto-electronic properties in detail. The band gap is 1.17 eV, smaller than previously reported, and the dielectric constant is 580, significantly larger than previously assumed. Carrier mobility measurements at GHz and THz frequencies indicate carrier localization at nm length scales, which may explain the relatively low photocurrents reported for this material.

R. Präg, M. Kölbach, F.F. Abdi, I.Y. Ahmet, M. Schleuning, D. Friedrich, R. van de Krol, “Photoelectrochemical Properties of CuFeO₂ Photocathodes Prepared by Pulsed Laser Deposition”, Chem. Mater. 36, 7764-7780 (2024). DOI: 10.1021/acs.chemmater.4c00903

To investigate the influence of strain on the photoelectrochemical properties of BiVO₄, we deposited epitaxial BiVO₄ films on YSZ/ITO substrates with pulsed laser deposition. By varying the concentration of yttrium in YSZ, we could tune the lattice parameters and, thereby, the amount of strain in the BiVO₄ films. Films with compressive hydrostatic strain showed significantly higher internal quantum efficiencies, which we attribute to a lower degree of carrier localization and a larger gap between the energy levels of the electron and hole polarons. This shows that strain engineering is a feasible strategy to improve the efficiency of photoabsorbers.

E.N. Fernandez, R. van de Krol, F.F. Abdi, “Tuning the Optical and Photoelectrochemical Properties of Epitaxial BiVO₄ by Lattice Strain”, Small Struct. 2400097 (2024). DOI: 10.1002/sstr.202400097.

Glycerol, a by-product of biodiesel, can be converted into valuable chemicals using photoelectrochemical (PEC) devices, improving their economic viability. While much research has focused on photoelectrode materials and co-catalysts, the role of electrolytes in this process has been less studied. Our research investigates the impact of different acidic electrolytes on PEC glycerol oxidation using a nanoporous BiVO₄ photoanode. We found that electrolyte composition significantly influences performance metrics like photocurrent, stability, and, to some extent, product selectivity. These findings underline the importance of electrolyte engineering in optimizing solar-driven biomass reforming.

Heejung Kong, Siddharth Gupta, Andrés F. Pérez-Torres, Christian Höhn, Peter Bogdanoff, Matthew T. Mayer, Roel van de Krol, Marco Favaro*, Fatwa Abdi*; Chem. Sci.; https://doi.org/10.1039/D4SC01651C

We investigate synthesis routes for a cerium-incorporated GaInSn-based liquid metal catalyst, focusing on the electrochemical production of graphitic carbon. Preparation and preconditioning of the catalyst are found to be crucial for carbon production, while trace amounts of H₂O and OH in the organic electrolyte play a decisive role in the efficiency of the electrocatalytic process. Finally, for a better understanding of the reaction mechanism and the involved active species, experimental findings and density functional theory-based calculations are combined, suggesting a two-step reduction pathway with Ce(OH)_x as the catalytically active surface species.

Daniel Lörch, Aya G. A. Mohamed, Holger Euchner, Janick Timm, Jonas Hiller, Peter Bogdanoff, Matthias M. May; J. Phys. Chem. C 2024, 128, 20827−20840; doi.org/10.1021/acs.jpcc.4c05482

This study explores the potential of indium phosphide (InP) as a reference material for photoelectrochemical (PEC) and photovoltaic (PV) applications, particularly in green hydrogen production. Using time-resolved two-photon photoemission (tr-2PPE) spectroscopy, we analyze the electronic states of phosphorus-rich p-doped InP(100). We identify nine distinct states, including conduction band resonances and a surface defect state pinning the Fermi level. Additionally, the decay constants of five conduction band states are determined, providing insight into electron relaxation. These findings contribute to optimizing InP-based III–V compounds for more efficient PEC water splitting and high-performance solar cells.

Diederich, J.; Velasquez Rojas, J.; Pour, M.A. Z.; Alvarado, I. A. R.; Paszuk, A.; Sciotto, R.; Hoehn, C.; Schwarzburg, K.; Ostheimer, D.; Eichberger, R.; Schmidt, W.G.; Hannappel, T.; van de Krol, R.; Friedrich, D.: Unraveling Electron Dynamics in p-type Indium Phosphide (100): A Time-Resolved Two-Photon Photoemission Study. Journal of the American Chemical Society 146 (2024), p. 8949-8960

Indium phosphide (InP)-based photo-absorbers, protected by TiO₂ layers grown via atomic layer deposition (ALD), hold efficiency records for solar water splitting. InP is also crucial for photonic computing, where ultrafast surface behavior is essential. This study investigates electron dynamics at the phosphorus-rich (P-rich) InP(100) surface modified with TiO₂. Electrons migrate through surface states into the TiO₂ conduction band, with discrete states maintained up to 10 nm TiO₂ deposition. Annealing at 300 °C alters interfacial states, affecting charge transfer. These findings support advancements in hot-carrier extraction, surface engineering, and improved PEC and photovoltaic device performance.

Diederich, J.; Velazquez Rojas, J.; Paszuk, A.; Pour, M.A.Z.; Höhn, C.; Alvarado, I.A.R.; Schwarzburg, K.; Ostheimer, D.; Eichberger, R.; Schmidt, W.G.; Hannappel, T.; van de Krol, R.; Friedrich, D.: Ultrafast Electron Dynamics at the P-rich Indium Phosphide/TiO₂ Interface. Advanced Functional Materials 34 (2024), p. 2409455/1-13

Photoelectrochemical (PEC) devices, which convert sunlight, water, and CO₂ into fuels and valuable products, require advanced photoabsorber materials. Understanding interactions at the semiconductor/liquid interface is crucial for optimizing these materials. This perspective and technical paper highlights the use of operando Raman spectroscopy (RS) combined with electrochemical techniques to gain insights into photoelectrode behavior under working conditions. Despite challenges like low quantum efficiency and fluorescence, operando RS can reveal changes in photoabsorber structure and surface defects. It also aids in analyzing products from solar-driven biomass reforming. Our work discusses overcoming these challenges and introduces methods for real-time monitoring of PEC reactions, paving the way for improved photoelectrode design.

Marco Favaro, Heejung Kong, Ronen Gottesman, J. Phys. D: Appl. Phys. 2024, 57 103002; https://doi.org/10.1088/1361-6463/ad10d3

While almost all thin film photoelectrodes will experience a certain amount of lattice strain, surprisingly little is known about the influence of strain on opto-electronic properties. By combining X-ray reflectometry with optical transmission/reflection measurements, in-plane compressive strain was found to increase both the direct and indirect band gaps of epitaxial BiVO₄ films. Moreover, volume-preserving deviatoric strain was found to impact the optical properties BiVO₄ more than hydrostatic strain. These first insights can help guide future efforts to improve the efficiency of photoabsorbers through strain engineering.

E.N. Fernandez, D.A. Grave, R. van de Krol, F.F. Abdi, “Strain-Induced Distortions Modulate the Optoelectronic Properties of Epitaxial BiVO₄ Films”, Adv. Energy Mater. 13, 2301075 (2023). DOI: 10.1002/aenm.202301075.

We recently demonstrated that it is possible to directly use the hydrogen generated by solar water splitting for the catalytic hydrogenation of itaconic acid (IA) to methyl succinic acid (MSA). We have evaluated the net energy balance of this process, and found that by using even a small fraction (2%) of the produced H₂ for converting IA to MSA, the net energy balance of the overall process improves dramatically. This shows that our coupled PEC-hydrogenation concept makes sense from an energetic point of view and thus successfully passes this crucial ‘sanity’ check.

X.Y. Zhang, M. Schwarze, R. Schomacker, R. van de Krol, F.F. Abdi, “Life cycle net energy assessment of sustainable H₂ production and hydrogenation of chemicals in a coupled photoelectrochemical device”, Nat. Commun. 14, 991 (2023). DOI: 10.1038/s41467-023-36574-1.

Photoelectrochemical water splitting is an elegant pathway to produce green hydrogen, but difficult to make economically competitive due to the low value of hydrogen. Together with our TU Berlin partners in the UniSysCat Excellence Network, we demonstrate that it is possible to couple PEC water splitting to a homogeneous catalytic hydrogenation reaction. Up to 60% of the produced hydrogen can be used for the hydrogenation of itaconic acid to methyl succinic acid (MSA). The high value of the MSA can dramatically reduce the levelized cost of hydrogen, potentially down to 1.5 € / kg H₂ under the right conditions.

K. Obata, M. Schwarze, T.A. Thiel, X. Zhang, B. Radhakrishnan, I.Y. Ahmet, R. van de Krol, R. Schomäcker, F.F. Abdi, “Solar-driven upgrading of biomass by coupled hydrogenation using in situ (photo)electrochemically generated H₂”, Nat. Commun. 14, 6017, (2023). DOI: 10.1038/s41467-023-41742-4.

Ion-exchange membranes (IEMs) are vital for devices like fuel cells and batteries, enabling selective ion transfer to sustain electrochemical processes. Understanding their molecular behavior is crucial for optimization. Traditionally, ion transfer is believed to occur through diffusion and electromigration. However, using in situ ambient pressure hard X-ray photoelectron spectroscopy combined with finite element analysis, we found that ion transport in IEMs at the liquid/gas interface is driven mainly by diffusion through ionized functional groups. Additionally, we detected unwanted polarization fields that negatively impact device performance. This study highlights the importance of in situ investigations to improve the efficiency of (photo)electrochemical devices.

Maryline Ralaiarisoa, Senapati Sri Krishnamurti, Wenqing Gu, Claudio Ampelli, Roel van de Krol, Fatwa F. Abdi, and Marco Favaro*; J. Mater. Chem. A 2023, 11, 13570-13587; https://doi.org/10.1039/D3TA02050A

Metal oxides are promising, low-cost photoelectrodes for solar water splitting but suffer from poor charge transport, limiting efficiency. This study compares CuFeO₂, α-SnWO₄, BaSnO₃, FeVO₄, CuBi₂O₄, α-Fe₂O₃, and BiVO₄, analyzing charge carrier dynamics over 100 fs–100 µs using terahertz and microwave spectroscopy. All materials show low mobilities (<3 cm²V⁻¹s⁻¹) and partial carrier localization, which hinders diffusion. CuFeO₂ exhibits electrostatic barriers around crystallographic domains, while BaSnO₃ shows the highest mobility after H₂ annealing. Reducing carrier localization is key to improving metal oxide photoabsorbers for efficient hydrogen production.

Schleuning, M.; Kölbach, M.; Ahmet, I.; Präg, R.; Gottesman, R.; Gunder, R.; Zhang, M.; Wargulski, D.R.; Abou-Ras, D.; Grave, D.A.; Abdi, F.F.; van de Krol, R.; Schwarzburg, K.; Eichberger, R.; Friedrich, D.; Hempel, H. : Carrier Localization on the Nanometer-Scale limits Transport in Metal Oxide Photoabsorbers. Advanced Functional Materials 33 (2023), p. 2300065/1-11

Efficient charge separation is a crucial step in any photoelectrochemical (PEC) or photocatalytic (PC) process. The mechanism that drives charge separation is, however, still widely misunderstood in the PEC and PC communities. Specifically, it is not the electric field that drives charge separation, but the presence of selective contacts and the gradient in the electrochemical potential. In this paper we discuss these important concepts and present a ‘toolbox’ for selective contact design in photoelectrochemical devices.

M. Schleuning, I.Y. Ahmet, R. van de Krol, M.M. May, “The role of selective contacts and built-in field for charge separation and transport in photoelectrochemical devices”, Sust. Energy Fuels 6, 3701 (2022). DOI: 10.1039/d2se00562j.  

In this invited roadmap paper, more than 40 leading scientists in the field discuss recent progress and remaining challenges for the development of solar fuel generating devices. Aspects such as system benchmarking, device scaling, photoelectrode design approaches, materials discovery, and catalysis are discussed in 17 sections. The roadmap, which was coordinated by G. Segev, F. Houle and R. van de Krol, was announced a few months before publication through an online international webinar that was attended by more than 170 experts.

G. Segev, J. Kibsgaard, C. Hahn, Z.J. Xu, W.-H. Cheng, T.G. Deutsch, C. Xiang, J.Z. Zhang, L. Hammarström, D.G. Nocera, A.Z. Weber, P. Agbo, T. Hisatomi, F.E. Osterloh, K. Domen, F.F. Abdi, S. Haussener, D.J. Miller, S. Ardo, P.C. McIntyre, T. Hannappel, S. Hu, H. Atwater, J.M. Gregoire, M.Z. Ertem, I.D. Sharp, K.-S. Choi, J.S. Lee, O. Ishitani, J.W. Ager, R.R. Prabhakar, A.T. Bell, S.W. Boettcher, K. Vincent, K. Takanabe, V. Artero, R. Napier, B.R. Cuenya, M.T.M. Koper, R. van de Krol, F. Houle, “The 2022 solar fuels roadmap”, J. Phys. D Appl. Phys. 55, 323003 (2022). DOI: 10.1088/1361-6463/ac6f97.

When scaling solar fuel devices to larger areas, the resistivity of the transparent conducting substrate (TCO, typically F-doped SnO₂) may limit the achievable energy conversion efficiencies. To address this, metallic lines can be deposited on the TCO to improve the conductivity, but these often delaminate due to poor adhesion. We developed a new surface treatment method (“ReTreat”) that dramatically improves the adhesion for a variety of metals, incl. Ni, Ag, and Au, that are electrodeposited on FTO or ITO-coated PET. The films pass standardized tests for adhesion, thermal stability and electronic reliability, making them suitable for a variety of applications.

I.Y. Ahmet, F.F. Abdi, R. van de Krol, „Chemical Treatment of Sn-Containing Transparent Conducting Oxides for the Enhanced Adhesion and Thermal Stability of Electroplated Metals”, Adv. Mater. Interfaces 9, 2201617 (2022). DOI: 10.1002/admi.202201617.

We explored the stability of α-SnWO₄ photoanodes using a combination of ICP-OES, XPS, and spectro-photoelectrochemistry measurements. The material was found to be stable in acidic and neutral pH solutions, but not in alkaline solutions. In acidic and neutral solutions, a surface oxide layer composed of SnO₂ and WO₃ forms in the first few minutes, protecting the material against further dissolution. We could show that the oxide layer forms through a self-limiting reaction mechanism, providing a better understanding of the self-passivation behavior that we reported earlier for this material.

P. Schnell, J.M.C.M. Dela Cruz, M. Kölbach, R. van de Krol, F.F. Abdi, “pH-Dependent Stability of α-SnWO₄ Photoelectrodes”, Chem. Mater. 34 (4), 1590 (2022). DOI: 10.1021/acs.chemmater.1c03517.  

Our study explores how water interacts with a molybdenum-doped BiVO₄ surface, combining computational models and experimental techniques. We discovered that water splits into hydrogen and oxygen on this surface, changing its electronic structure. By comparing photoemission spectroscopy data with theoretical calculations, we found that this process stabilizes small electron polarons, which are localized charges that affect the material's properties. Our findings highlight how defects and dopants on oxide surfaces influence their reactivity with water, offering insights into designing better materials for applications like photocatalysis and sensors.

Wennie Wang, Marco Favaro, Emily Chen, Lena Trotochaud, Hendrik Bluhm, Kyoung-Shin Choi, Roel van de Krol, David E. Starr, and Giulia Galli; J. Am. Chem. Soc. 2022, 144, 37, 17173–17185; https://doi.org/10.1021/jacs.2c07501

With a band gap of 1.9 eV, α-SnWO₄ is a promising photoanode material for solar water splitting. The material becomes inactive upon contact with water, but we previously showed that this can be solved with a thin NiO_X protection layer. While this layer improves the stability and the photocurrent, it also reduces the photovoltage. Using synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we could now show that this is due to Fermi level pinning by Sn⁴⁺ species that form at the α-SnWO₄/NiO_x interface. To prevent these photovoltage losses, alternative techniques for the deposition of NiO_x should be considered.

P. Schnell, M. Kölbach, M. Schleuning, K. Obata, R. Irani, I.Y. Ahmet, M. Harb, D.E. Starr, R. van de Krol, F.F. Abdi, “Interfacial Oxide Formation Limits the Photovoltage of α-SnWO₄/NiO_x Photoanodes Prepared by Pulsed Laser Deposition”, Adv. Energy Mater. 2003183 (2021). DOI: 10.1002/aenm.202003183.  

We introduce a new facility at BESSY II for in situ spectroscopic analysis with tender X-rays, called SpAnTeX. This setup features a state-of-the-art electron spectrometer that performs efficiently under gas pressures up to 30 mbar and photon energies from 200 eV to 10 keV. It can capture photoelectron spatial distribution with high resolution and conduct time-resolved studies. An example experiment demonstrates its use with the dip-and-pull technique, highlighting its electrochemical capabilities. The end-station supports various interface investigations, including solid/liquid and solid/gas, making it versatile for advanced materials research.

Marco Favaro*, Pip C.J. Clark, Micheal J. Sear, Martin Johansson, Sven Maehl, Roel van de Krol, David E. Starr*; Surf. Sci. 2021, 713, 121903; https://doi.org/10.1016/j.susc.2021.121903

The electrochemical co-reduction of CO₂ and water to industrially usable products powered by electricity from renewable energy sources (e.g. photovoltaic, wind-power etc.) is a promising possibility to recycle CO₂ in an emission-neutral and energetically efficient process. However, the complex reaction mechanisms still require the development of efficient, cheap and stable electrocatalysts. Low cost carbon-based materials represent a relatively new and yet little-investigated approach in this research field. Especially, metal- and nitrogen-doped carbon, in which M‑N₄ centers are integrated in graphene like layers, were found to be active and highly selective towards the CO₂RR. In collaboration with the TU-Darmstadt, here we systematically investigated the influences of the transition metal ion in the catalytic center (Mn, Fe, Co, Ni, Cu, Zn, Sn) on the selectivity and activity of the electrochemical reduction of CO₂ in aqueous electrolyte.

S. Paul, Yi-Lin Kao, L. Ni, R. Ehnert, I. Herrmann-Geppert, R. van de Krol, R. W. Stark, W. Jaegermann, U. I. Kramm, and P. Bogdanoff, Influence of the Metal Center in M–N–C Catalysts on the CO2 Reduction Reaction on Gas Diffusion Electrodes; ACS Catalysis 11 (9), 5850-5864 (2021 )

Light absorption in strongly correlated electron materials can generate both mobile charge carriers and localized states that do not contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), quantifies the fraction of absorbed light that produces mobile carriers but is challenging to measure. This study introduces an empirical method to extract ξ(λ) using optical and quantum efficiency measurements of ultrathin films. The method is applied to haematite and aligns with photoconductivity data, showing increased carrier generation at higher energies. Low-energy absorption limits haematite’s efficiency, setting a lower photocurrent limit than expected. The approach is also validated for TiO₂ and BiVO₄.

Grave, D.A.; Ellis, D.S.; Piekner, Y.; Kölbach, M.; Dotan, H.; Kay, A.; Schnell, P.; van de Krol, R.; Abdi, F.F.; Friedrich, D.; Rothschild, A.: Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting. Nature Materials 20 (2021), p. 833-840

The performance of a number of metal oxide photoelectrodes and catalysts for solar fuel generation is improving significantly, however, their stabilities for long-term operation remains a significant challenge. In order approach, this problem systematically there needs to be a suitable tool set. Here we collaborated with researchers from the Max Planck Institute für Eisenforschung, HI-ERN, the University of Freiburg and Imperial College London and present the first operando stability study of high-purity BiVO₄ photoanodes during the photoelectrochemical oxygen evolution reaction (OER). This work shows how the stability of photoelectrodes and catalysts can be compared and enhanced in the future.

S. Zhang, I. Ahmet, Se-Ho Kim, O. Kasian, A. M. Mingers, P. Schnell, M. Kölbach, J. Lim, A. Fischer, K. J. J. Mayrhofer, S. Cherevko, B. Gault, R. van de Krol, and C. Scheu, Different Photostability of BiVO4 in Near-pH-Neutral Electrolytes; ACS Appl. Energy Mater. 3, 10, 9523–9527 (2020) pubs.acs.org/doi/10.1021/acsaem.0c01904

Obtaining pure, crystalline multinary metal oxide photoelectrodes requires higher temperatures than the thermal stability of glass-based transparent conductive substrates would typically allow, which makes the synthesis of these materials very challenging. We show that by combining pulsed laser deposition and rapid thermal processing, it is possible to form phase-pure, crystalline CuBi₂O₄ photocathodes by reacting Bi₂O₃ + CuO in 10 min at 650°C. These photocathodes show enhanced electronic properties and photoelectrochemical stability.

R. Gottesman, A. Song, I. Levine, M. Krause, A. T. M. Nazmul Islam, D. Abou‐Ras, T. Dittrich, R. van de Krol, A. Chemseddine, "Pure CuBi2O4 Photoelectrodes with Increased Stability by Rapid Thermal Processing of Bi2O3/CuO Grown by Pulsed Laser Deposition", Advanced Functional Materials, 1910832 (2020)