Time: July 28, 2017, 10:30-11:30 am
Venue: No.1 Conference Room, Energy Buildings Convention Center
Lecturer: Prof. Stjepko Krehula, Ru？er Bo？kovi？ Institute, Zagreb, Croatia
Abstract:
Mixed Ni-Fe(oxy)hydroxides of various Ni:Fe molar ratios (from 0:100 to 100:0) were synthesized by a hydrothermal precipitation in a highly alkaline medium and investigated using X-ray powder diffraction (XRPD), M？ssbauer spectroscopy, magnetic measurements, thermogravimetric analysis (TGA), scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FT-IR) spectroscopy and ultraviolet–visible–near infrared (UV-Vis-NIR) spectroscopy. Fe oxyhydroxide goethite (α-FeOOH) nanorods and Ni hydroxide β-Ni(OH)2 fluffy nanostructures were formed in a pure Fe or Ni system, respectively. Ni-doped α-FeOOH or Fe-doped β-Ni(OH)2 were formed at intermediate Ni:Fe ratios accompanied with Ni-ferrihydrite and Ni ferrite as minor phases. Incorporation of Ni in α-FeOOH and Fe in β-Ni(OH)2 by a cation substitution was confirmed from the shifts in the position of XRPD lines (due to a difference in ionic radius of Fe3+ and Ni2+) and from a decrease in the hyperfine magnetic field in α-FeOOH (due to a substitution of Fe3+ by Ni2+ with a lower magnetic moment). The Fe3+-for-Ni2+ substitution in β-Ni(OH)2 caused a formation of an interstratified structure with β-Ni(OH)2 and α-Ni(OH)2 structural units interconnected inside the same structural layers and in the same crystallites. An increased Fe content in Ni hydroxide caused an increase in amount in α-Ni(OH)2 structural units containing intercalated water molecules and nitrate ions. Mossbauer spectra showed the presence of some Fe3+ ions located in the highly distorted octahedral sites, presumably at the boundary between α-Ni(OH)2 and β-Ni(OH)2 structural units. Electrochemical measurements showed significantly higher oxygen evolution reaction (OER) activities of Ni-doped goethite and Fe-doped Ni hydroxide compared to pure phases. The highest activity (the lowest overpotential) was measured for sample Ni67 (67 mol% Ni, 33 mol% Fe) containing Fe-doped Ni hydroxide and Ni ferrite as a minor phase.
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