Mesoporous CuFe2O4 Photoanodes for Solar Water Splitting Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

Marcus Einert^a, Arslan Waheed^a, Dominik Moritz^a, Stefan Lauterbach^b, Anna Kundmann^c, Sahar Demi^c, Helmut Schlaad^d, Frank Osterloh^c, Jan Philipp Hofmann^a

^aDepartment of Materials and Earth Sciences, Surface Science Laboratory, Technical University of Darmstadt, Otto-Bernd-Strasse 3, 63287 Darmstadt, Germany

^bInstitute for Applied Geosciences, Geomaterial Science, Technical University of Darmstadt, Schnittspahnstrasse 9, 64287 Darmstadt, Germany

^cDepartment of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USA

^dInstitute of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany

Proceedings of The Future of Hydrogen: Science, Applications and Energy Transition (H2Future25)

Production

Ibiza, Spain, 2025 May 5th - 7th

Organizers: Teresa Andreu, Bahareh Khezri and Jose Mata

Oral, Marcus Einert, presentation 002
Publication date: 27th March 2025

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While nanostructuring is well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronicâ F-127), polyisobutylene-block-poly(ethyleneoxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquidTM-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO present the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate is observed for the ordered KLE-based mesoporous CFO, which retain spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order. These findings will be of importance for the fabrication of improved metal oxide photoelectrodes and address the issue that nanostructuring of photoanodes with low minority charge carrier diffusion lengths are not generally beneficial for solar water oxidation.

References:

[1] Einert, Marcus, et al. "Mesoporous CuFe2O4 photoanodes for solar water oxidation: impact of surface morphology on the photoelectrochemical properties." Chemistry–A European Journal 29.24 (2023): e202300277.

Acknowledgements:

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation, Walter Benjamin Programm to M. Einert) under project no. 469377211. DCM and JPH acknowledge DFG for funding under project no. 424924805. Support for surface photovoltage spectroscopy measurements was provided by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DOE-SC0015329.

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