In the effort to develop an efficient and cost effective photoelectrochemical device for water splitting driven by sunlight only, transition metal oxides are promising candidates to catalyze the oxygen evolution reaction (OER) at the anode. We used X-ray photoelectron spectroscopy (XPS) to characterize very active manganese and cobalt oxide thin films deposited on FTO substrates before and after the application of different anodic potentials, in order to investigate the bias potential dependent changes on the catalysts’ surfaces. α-Mn2O3 undergoes a reversible partial oxidation from Mn3+ to Mn4+ under high anodic potentials, while the transition from Co2+ to Co3+ in amorphous CoOx samples occurs already at a potential well below the OER onset potential. This Co3+ state is then stable throughout the investigated potential range and no clear evidence for a Co4+ state at or above the OER onset potential could be found. We conclude that the OER reaction mechanism on the surface of these oxide films might be significantly different.
A procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanaodes was adopted to produce the doped oxide films by admixing the tungstic acid sol with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid–stabilized hematite (Fe2O3). Such physicochemical properties as structure, morphology and spectroscopic identity of the resulting hybrid (doped) WO3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectrocopies. When using a solar cell system operating in 0.5 mol dm-3 H2SO4 in the three-electrode configuration, the doped WO3 films acting as photoanodes yielded (following illumination with visible light) significantly (up to 80%) larger water oxidation photocurrents (at potentials higher than 0.75 V vs. RHE), in comparison to the analogous undoped mesoporous tungsten oxide films. The effect was the most pronounced in a case of the WO3 film doped with borotungstic acid–stabilized hematite. The observed enhancement effects could be rationalized either in terms of appearance of new conduction band structures in doped WO3 films or a marked increase in the degree of hydration of the doped oxide structures regardless annealing at high temperatures (450 oC). Judging from our experiments in 0.5 mol dm-3 NaCl, the systems are also applicable in the sea-water-type environments. Present results are consistent with the view that the doped tungsten oxide structures do not exhibit significant undesirable electron-hole recombination effects. Optimization of the system is underway aiming at further improvement of the photocurrent efficiency and the systems' utility under different conditions. Any increase in the photoelectrochemical efficiency of WO3–based films is of importance to the development of photoanodes for the visible-light driven water splitting.
In photosynthesis nature uses transition metal complexes as catalysts to evolve oxygen and hydrogen from water. In this process the catalytic centers are separated from the light capturing and absorbing co-factors in photosystem I and II integrated in the thylakoid membrane. To develop bio-inspired catalysts and to mimic e.g. the Mn3CaO3MnO complex in PSII, different manganese oxides as well as alkaline metal and earth alkali metal manganates were investigated with respect to the oxygen evolution reaction (OER) in the process of water oxidation. In this contribution special attention will be turned to the structure - morphology– function relationship of these materials. Manganese oxide electrodes have been prepared by reactive magnetron sputtering from a Mn target in an Ar/O2 atmosphere as well as by anodic electrodeposition and a subsequent annealing step. Besides amorphous MnOx obtained at low temperatures, crystallized oxides, such as γ-MnO2 and α-Mn2O3, were tested as electrocatalysts. Thin (60-70 nm) and dense layers were deposited by reactive magnetron sputtering using conductive glass (FTO) slides as substrates. Cross section transmission electron micrograph (Figure 1a) clearly revealed that the sputtered α-Mn2O3 layers consist of nanocrystals of 10 - 20 nm edge length. These layers show current densities of 10 mA/cm-2 at an overpotential of 370 mV (pH 13.8). Similar overvoltages were also obtained with electrodeposited α-Mn2O3 layers (10mA/cm-2 at 340 mV overvoltage). In contrast, the electrochemically deposited α-Mn2O3 layers are several 100 nm thick and appear highly porous (Figure 1b). Obviously, the specific activity related to the active surface area of the sputtered material is about one order of magnitude higher than the one of the electrodeposited samples. Nevertheless, higher current densities can be achieved with the electrodeposited material because high electrochemically active surface areas can be provided. This behavior will be discussed as a function of defect chemistry and will be compared with the structure – function relationship of other OER electrocatalysts. References [1] A. Ramírez, P. Hillebrand, D. Stellmach, M. May, P. Bogdanoff, S. Fiechter; J. Phys Chem. C, 118 (2014) 14073-14081 and SI. [2] M.M. Najafpour, T. Ehrenberg, M. Wiechen, P. Kurz; Angew. Chem. Int. Ed., 49 (2010) 2233-2237. Figure 1: α-Mn2O3 deposited a) by reactive magnetron sputtering and b) by electrochemical deposition. Figure 1
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