Abstract
Research on materials under extreme conditions such as high pressures provides new insights into the evolution and dynamics of the earth and space sciences, but recently, this research has focused on applications as functional materials. In this contribution, we examined high-pressure/high-temperature phases of β-FeO1−x(OH)1+xClx with x = 0.12 (β-FeOOH) and their catalytic activities of water oxidation, i.e., oxygen evolution reaction (OER). Under pressures above 6 GPa and temperatures of 100–700 °C, β-FeOOH transformed into ε-FeOOH, as in the case of α-FeOOH. However, the established pressure–temperature phase diagram of β-FeOOH differs from that of α-FeOOH, probably owing to its open framework structure and partial occupation of Cl− ions. The OER activities of ε-FeOOH strongly depended on the FeOOH sources, synthesis conditions, and composite electrodes. Nevertheless, one of the ε-FeOOH samples exhibited a low OER overpotential compared with α-FeOOH and its parent β-FeOOH, which are widely used as OER catalysts. Hence, ε-FeOOH is a potential candidate as a next-generation earth-abundant OER catalyst.
Highlights
IntroductionMars[12,13] and in the cannon shot of the Mary Rose, King Henry VIII's shipwrecked agship.[14] b-FeO1Àx(OH)1+xClx with x 1⁄4 0.12 (b-FeOOH) was generally synthesized by hydrolyzing FeCl3 solutions at [60–100] C,1,15–20 before it was discovered as a natural product in the Akagane metal mine, Japan.[21]
One of the polymorphs of FeOOH, namely, b-FeO1Àx(OH)1+xClx with x 1⁄4 0.12 (b-FeOOH), has attracted interest in highly diverse elds
We investigated the distribution of ClÀ ions in the particles of pristine b-FeOOH and sample [8-400] with an energy dispersive X-ray (EDX) spectrometer (Ultim Extreme, Oxford Instruments)
Summary
Mars[12,13] and in the cannon shot of the Mary Rose, King Henry VIII's shipwrecked agship.[14] b-FeOOH was generally synthesized by hydrolyzing FeCl3 solutions at [60–100] C,1,15–20 before it was discovered as a natural product in the Akagane metal mine, Japan.[21]. In this contribution, we revealed HP/HT phases of b-FeOOH under pressures of [2,3,4,5,6,7,8,9,10,11,12] GPa and temperatures below 800 C, as well as the OER activity of the obtained materials. We revealed HP/HT phases of b-FeOOH under pressures of [2,3,4,5,6,7,8,9,10,11,12] GPa and temperatures below 800 C, as well as the OER activity of the obtained materials Several methods such as synchrotron XRD measurements, X-ray absorption spectroscopy (XAS), and 57Fe Mossbauer spectroscopy were employed to clarify their structural and electronic properties. B-FeOOH a-Fe2O3 a-FeOOH a-Fe2O3 a-Fe2O3 3-FeOOH 3-FeOOH a-FeOOH a-Fe2O3 b-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH a-Fe2O3 a-Fe2O3 Fe3O4 b-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH b-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH 3-FeOOH a-Fe2O3 a-FeOOH 3-FeOOH 3-FeOOH a The potential vs. reversible hydrogen electrode (RHE) at which the anodic current density (j) was 1 mA cmÀ2
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