Institute

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

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.

Figure: ©HZB

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

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.

Figure: ©HZB

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

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.

Figure: ©HZB

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

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.

Figure: ©HZB

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

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.

Figure: ©HZB

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.

Figure: ©HZB

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 )

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.

Figure: © pubs.acs.org/doi/10.1021/acsaem.0c01904

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)

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)