Understanding the Performance Enhancement of BiVO4 Photoanodes Decorated with Ultra-Thin MnOx

Rowshanak Irani^a, Paul Plate^a, Roel Van de Krol^a, Fatwa Firdaus Abdi^a

^aHelmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany, Berlin, Germany

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)

S1 Solar Fuel 18

Torremolinos, Spain, 2018 October 22nd - 26th

Organizers: Shannon Boettcher and Kevin Sivula

Poster, Rowshanak Irani, 278
Publication date: 6th July 2018

The development of metal oxide semiconductors as photoelectrode materials in solar water splitting is often hampered by the slow surface reaction kinetics for oxygen evolution reaction (OER). To overcome this, their surfaces are typically modified by depositing earth abundant, efficient, and inexpensive water oxidation co-catalysts, such as CoPi, FeOOH, NiFeO_x, and MnO_x[1, 2, 3, 4, 5]. As a result, the photocurrents of the catalyst-modified photoelectrodes are improved, but the enhancement mechanism is often not clear. Several studies have shed light onto the mechanism [6, 7, 8], but the charge transfer processes through the semiconductor-catalyst interface are not yet fully understood. In this study, we investigate the role of atomic layer deposited (ALD) MnO_x co-catalyst on BiVO₄ photoanodes. ALD allows conformal deposition of very thin layers and therefore an accurate control of the thickness of the co-catalyst. We found that there is an optimum MnO_x thickness of ~4 nm; the optimized MnO_x/BiVO₄ sample showed lower onset potential (by ~100 mV) as well as higher photocurrent (a factor of > 3) as compared to the bare BiVO₄. Intensity modulated photocurrent spectroscopy (IMPS) measurements revealed that the optimized MnO_x/BiVO₄ sample possess lower surface recombination rate constant, while the charge transfer rate constant is relatively unaffected. Through careful study of the dark current, their morphology as well as by applying Al₂O₃ as the interfacial back contact layer, we confirmed that no shunting effect of the co-catalyst is taking place [6]. Instead, we attribute the enhancement in the optimized sample to the competition between the passivation of surface states causing recombination and the intrinsic resistance of the MnO_x layer [9]. This is further confirmed by steady-state Hall conductivity and time-resolved microwave conductivity (TRMC) measurements. Overall, our study provides additional insights to the general understanding of the charge transfer processes occurring at the semiconductor-catalyst interface.

References:

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[2] Chemelewski, W. D.; Lee, H.-C.; Lin, J.-F.; Bard, A. J.; Mullins, C. B. Amorphous FeOOH oxygen evolution reaction catalyst for photoelectrochemical water splitting. J. Am. Chem. Soc. 2014, 136, 2843-2850.

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[4] Xu, D.; Stevens, M. B.; Rui, Y.; DeLuca, G.; Boettcher, S. W.; Reichmanis, E.; Li, Y.; Zhang, Q.; Wang, H. The role of Cr doping in Ni-Fe oxide/(oxy)hydroxide electrocatalysts for oxygen evolution. Electrochim. Acta. 2018, 265, 10-18.

[5] Ramírez, A.; Hillebrand, P.; Stellmach, D.; May, M. M.; Bogdanoff, P.; Fiechter, S. Evaluation of MnOx, Mn2O3, and Mn3O4 electrodeposited films for the oxygen evolution reaction of water. J. Phys. Chem. C. 2014, 118, 14073–14081.

[6] Qiu, J.; Hajibabaei, H.; Nellist, M. R.; Laskowski, F. A. L.; Oener, S. Z.; Hamann, T. W.; Boettcher, S. W. Catalyst deposition on photoanodes: the roles of intrinsic catalytic activity, catalyst electrical conductivity, and semiconductor morphologyNellist, M. R.; Laskowski, F. A. L.; Lin, F.; Mills, T. J.; Boettcher, S. W. Semiconductor-electrocatalyst interfaces: theory, experiment, and applications in photoelectrochemical water splitting. Acc. Chem. Res. 2016, 49, 733-740.. ACS Energy Lett. 2018, 3, 961–969.

[7] Nellist, M. R.; Laskowski, F. A. L.; Lin, F.; Mills, T. J.; Boettcher, S. W. Semiconductor-electrocatalyst interfaces: theory, experiment, and applications in photoelectrochemical water splitting. Acc. Chem. Res. 2016, 49, 733-740.

[8] Zacheaus, C.; Abdi, F. F.; Peter, L. M.; van de Krol, R. Photocurrent of BiVO4 is limited by surface recombination, not surface catalysis. Chem. Sci. 2017, 8, 3712-3719.

[9] Strandwitz, N. C.; Comstock, D. J.; Grimm, R. L.; Nichols-Nielander, A. C.; Elam, J.; Lewis, N. S. Photoelectrochemical behavior of n type Si(100) electrodes coated with thin films of manganese oxide grown by atomic layer deposition. J. Phys. Chem. C 2013, 117, 4931−4936.

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