In-Situ XAFS Measurement of K:MnO x Oxygen Evolution Catalyst

Abstract

[Introduction] Water splitting using solar energy is a promising approach to produce renewable hydrogen gas toward a sustainable society. For the realization of efficient photovoltaic and photoelectrochemical systems for water splitting, oxygen evolution reaction (OER) electrocatalysts have been used to catalyze water oxidation. Thus, the development of highly active OER catalysts has attracted considerable attention. Recently, manganese oxide electrodeposited in a potassium phosphate electrolyte (K:MnO x ) was reported to function as an efficient OER electrocatalyst over a wide pH range.[1,2] Therefore, in this study, the K:MnO x catalyst was investigated by in-situ Mn K-edge XAFS measurements under potential control conditions. [Experimental] Electrochemical measurements were performed in a three-electrode Teflon cell equipped with a Pt counter electrode and an Ag/AgCl (saturated KCl) reference electrode. The K:MnO x catalyst was electrodeposited on a gold-coated film in a 50 mM methanesulfonic acid electrolyte containing 0.5 mM MnCl2. In-situ XAFS spectra of the prepared samples were measured at various potentials in a 0.1 M potassium phosphate buffer electrolyte at BL 9A of the Photon factory and BL01B1 of SPring-8 as fluorescence excitation spectra. [Result and discussion] Figure 1 shows the Mn K-edge XAFS spectra for the K:MnO x catalyst at various electrode potentials between 0.2 and 1.0 V. At the potential of 0.2 V, the position of half edge energy for K:MnO x was observed at ca. 6549.3 eV, similar to those of the Mn3O4 and Mn2O3 references, which indicates that the electronic structure of Mn species in the K:MnO x is most likely the same as that of Mn3+. As the potential was changed from 0.2 to 1.0 V, the position of half edge energy gradually shifted to the higher energy around 0.6 V and then remained constant at 6552.3 eV up to 1.0 V, which shows that the Mn species in K:MnOx catalyst was oxidized from Mn3+ to Mn4+ by positive electrode potential. Figure 2 shows Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra of the K:MnO x catalyst. The Mn-O and Mn-Mn peaks of the spectrum at 0.2 V were observed at the same positions with those of δ-MnO2, exhibiting that the structure of K:MnO x is similar to that of δ-MnO2. Moreover, those peaks increased when the electrode potential was changed to 1.0 V, which probably implies that the Jahn−Teller distortion with nonequivalent Mn−O and Mn−Mn distances was relaxed. Considering that the OER proceeds at higher electrode potential, the δ-MnO2 structure with lower distortion is likely to function as an active species for water oxidation catalysis. In this presentation, we will discuss more detailed assignment for the XAFS spectra and relationship with the OER activity. [1] Huynh, M.; Bediako, D. K.; Nocera, D., J. Am. Chem. Soc., 2014, 136, 6002. [2] Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G., J. Am. Chem. Soc. 2015, 2015, 137, 14887. Figure 1