| 1. |
- Abrashev, Miroslav V., et al.
(författare)
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Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation
- 2019
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 7:28, s. 17022-17036
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Tidskriftsartikel (refereegranskat)abstract
- Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure-activity relations, we investigate the influence of moderate annealing (100-300 degrees C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure-reactivity relations. Oxide structures featuring both di-mu-oxo bridged Mn ions and (close to) linear mono-mu-oxo Mn3+-O-Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O-2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like zeta-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 degrees C) and this is the first report where it was stable under ambient conditions.
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| 2. |
- Görlin, Mikaela, et al.
(författare)
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Formation of unexpectedly active Ni-Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides
- 2019
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Ingår i: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 55:6, s. 818-821
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Tidskriftsartikel (refereegranskat)abstract
- We present an unusual, yet facile, strategy towards formation of physically mixed Ni-Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)(2) and Fe(OOH) catalysts self-assemble into atomically intermixed Ni-Fe catalysts with unexpectedly high activity.
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| 3. |
- Loos, Stefan, et al.
(författare)
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Electromodified NiFe Alloys as Electrocatalysts for Water Oxidation : Mechanistic Implications of Time-Resolved UV/Vis Tracking of Oxidation State Changes
- 2019
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Ingår i: ChemSusChem. - : Wiley-VCH Verlagsgesellschaft. - 1864-5631 .- 1864-564X. ; 12:9, s. 1966-1976
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Tidskriftsartikel (refereegranskat)abstract
- Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. During cyclic voltammetry and after sudden potential jumps between noncatalytic and catalytic potentials, Ni oxidation/reduction was tracked with millisecond time resolution by a UV/Vis reflectance signal. Optimal catalysis at intermediate Ni/Fe ratios is explained by two opposing trends for increasing Fe content: a)pronounced slowdown of the Ni2+/Ni3+ oxidation step and b)increased reactivity of the most oxidized catalyst state detectable at catalytic potentials. This state may involve an equilibrium between Ni4+ ions and Ni2+ ions with neighboring ligand holes, possibly in the form of bound peroxides.
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| 4. |
- Mousazade, Younes, et al.
(författare)
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Water oxidation by a manganese-potassium cluster : Mn oxide as a kinetically dominant "true" catalyst for water oxidation
- 2018
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Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:17, s. 4390-4398
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Tidskriftsartikel (refereegranskat)abstract
- Nature uses an Mn cluster for water oxidation, and thus, water oxidation using Mn clusters is interesting when used in artificial water-splitting systems. An important question is whether an Mn cluster is a true catalyst for water oxidation or not. Herein, an Mn-K cluster was investigated for electrochemical water oxidation to find the true and the kinetically dominant catalyst using X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electrochemical methods. The experiments showed that conversion into nanosized Mn oxide occurred for the cluster, and the nanosized Mn oxides are the true catalyst for water oxidation.
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| 5. |
- Pham, Long Vo, et al.
(författare)
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Unequal misses during the flash-induced advancement of photosystem II : effects of the S state and acceptor side cycles
- 2019
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Ingår i: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 139:1-3, s. 93-106
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (S-i states, i=0-4) resulting in O-2 evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the S-i-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S-2, S-3 and S-0 state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of Q(A)/Q(A)(-) in PSII is significantly less negative in C. merolae (E-m=-104mV) than in spinach (E-m=-163mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the E-m of Q(A)(-)/Q(A) plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from Q(A)(-) to Pheo slows down the charge recombination reaction with the S-3 and S-2 states considerably. In addition, our data support the concept that the S-2 S-3 transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.
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| 6. |
- Safdari, Rasoul, et al.
(författare)
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A mononuclear cobalt complex for water oxidation : new controversies and puzzles
- 2018
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 47:46, s. 16668-16673
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Tidskriftsartikel (refereegranskat)abstract
- Herein the role of a mononuclear cobalt(iii) complex, [Co-III(DPKOH)(2)]ClO4 (DPK = di(2-pyridyl)ketone), in the water electrooxidation process is investigated with scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction studies, NMR, chronoamperometry, cyclic voltammetry, extended X-ray absorption fine structure and X-ray absorption near edge structure determination. Our experiments show that, in comparison to the reported literature, other cobalt-containing structures on the surface of the FTO electrode could also be the true catalyst for water oxidation.
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| 7. |
- Schuth, Nils, et al.
(författare)
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K alpha X-ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S-3 State
- 2018
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:16, s. 10424-10430
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Tidskriftsartikel (refereegranskat)abstract
- The unique manganese calcium-catalyst in photosystem II (PSII) is the natural paragon for efficient light driven water oxidation to yield O-2. The oxygen-evolving complex (OEC) in the dark-stable state (S-1) comprises a Mn4CaO4 core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S-3) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn K alpha X-ray emission spectra of the S-0, S-1, S-2, and S-3 intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S-3 unlikely.
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| 8. |
- Zaharieva, Ivelina, et al.
(författare)
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Room-Temperature Energy-Sampling K beta X-ray Emission Spectroscopy of the Mn4Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O-2 Formation
- 2016
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 55:30, s. 4197-4211
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Tidskriftsartikel (refereegranskat)abstract
- In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn K beta(1,3) line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The K beta(1,3) line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)(3)Mn(IV) in S-0, Mn(III)(2)Mn(IV)(2) in S-1, Mn(III)Mn(IV)(3) in S-2, and Mn(IV)(4) in S-3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S-4 state unlikely. The difference spectra of both manganese K beta(1,3) emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S-2 -> S-3 transition when compared to Mn(III) oxidation in the S-1 -> S-2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S-2 -> S-3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal ligand bonding patterns at the Mn4Ca complex in the S-2 -> S-3 transition are discussed.
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| 9. |
- Zand, Zahra, et al.
(författare)
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Nickel-Vanadium Layered Double Hydroxide under Water-Oxidation Reaction : New Findings and Challenges
- 2019
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Ingår i: ACS Sustainable Chemistry and Engineering. - : AMER CHEMICAL SOC. - 2168-0485. ; 7:20, s. 17252-17262
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Tidskriftsartikel (refereegranskat)abstract
- Nickel-vanadium layered double hydroxide has recently been considered as a highly active, low-cost electrocatalyst and as a benchmark non-noble metal-based electrocatalyst for water oxidation. The material showed a current density of 27 mA/cm(2) at an overpotential of 350 mV, which is comparable to the best-performing nickel-iron-layered double hydroxides for water oxidation in alkaline media. The enhanced conductivity and facile electron transfer were suggested among important factors for the high activity of nickel-vanadium layered double hydroxide. In the present study, the stability of an Ni-V catalyst was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electrochemical characterization methods. These methods show that the initial Ni-V catalyst during water oxidation in alkaline conditions is converted from an initial alpha-Ni(OH)(2) phase to a partially oxidized alpha-Ni(OH)(2/)NiOOH phase and VO(4)(3-)ions. We carefully evaluate the stability of the catalysts and analyze the compositional changes during prolonged water-oxidation conditions using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The experiments using both Fe-free electrolyte and Fe-free nickel-vanadium layered double hydroxide reveal that vanadium do not affect the water-oxidizing activity of alpha-Ni(OH)(2).
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