Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe(100-y-z)Co(y)Ni(z)O(x) are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.
Photochemical metal-organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations.
Light-driven
decomposition of Ir(acac)3 spin-cast on
a conducting glass substrate produces a thin conformal film of amorphous
iridium oxide, a-IrO
x
. The decomposition process, which was carried out under an ambient
atmosphere at room temperature and tracked by Fourier transform infrared
(FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal
charge transfer (LMCT) process. The amorphous nature of the films
is based on the lack of any observable Bragg reflections by powder
X-ray diffraction techniques; the elemental composition was corroborated
by X-ray photoelectron spectroscopy (XPS) measurements. The films
are found to be excellent electrocatalysts for mediating the oxygen
evolution reaction (OER) in acidic media, as evidenced by the onset
of catalysis at 130 mV and a Tafel slope of 34 mV dec–1. These parameters enable current densities of 1 and 10 mA cm–2 to be reached at 190 and 220 mV, respectively. Exposing
the films to higher temperatures (500 °C) renders a film of crystalline
iridium oxide, c-IrO
x
, which displays a Tafel slope of 60 mV dec–1,
thus requiring an additional 50 mV to reach a current density of 1
mA cm–2. The film of a-IrO
x
reported here is among the best OER electrocatalysts
reported to date.
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Cobalt-based full-Heusler compounds with composition Co2M′ Z (where M ′ is a transition metal and Z is a main group element) are attracting attention due to their predicted half-metallic behaviour, a considerably desired property for spin-dependent electron transport devices. Knowledge of the basic magnetic properties of these materials, in particular in the form of thin films, is required both to fully exploit these promising materials, and to understand their underlying electronic structure and establish structure-property relationships. In this Topical Review, we present a survey of the magnetic anisotropy, exchange, and damping of Co2M ′ Z compounds. These properties are directly related to spin-spin and spin-orbit interactions.
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