Showing papers by "Ikuya Yamada published in 2020"
TL;DR: This work provides a new insight into the design of bimetallic sulfide composites with high catalytic activities and shows a high cyclability in zinc-air batteries with Co9-xNixS8/NC catalyst.
Abstract: Bimetallic sulfides have been attracting considerable attention because of their high catalytic activities for oxygen reduction reaction (ORR) and oxygen evolution reaction; thus, they are considered efficient catalysts for important energy conversion devices such as fuel cells and metal-air batteries. Here, the catalytic activity of a novel catalyst composed of Co9-xNixS8 nanoparticles immobilized on N-doped carbons (Co9-xNixS8/NC) is reported. The catalyst is synthesized using a Ni-adsorbed Co-Zn zeolitic imidazolate framework (ZIF) precursor (NiCoZn-ZIF). Because of the porous structure of ZIF and the high intrinsic activity of the bimetallic sulfide nanoparticles, the Co9-xNixS8/NC catalyst exhibits high half-wave potential 0.86 V versus reversible hydrogen electrode for ORR and outstanding bifunctional catalytic performance. When Co9-xNixS8/NC is applied as a cathode catalyst in zinc-air batteries, considerably higher power density of about 75 mW cm-2 and discharge voltage are achieved compared to those of batteries with commercial Pt/C and other ZIF-derived catalysts. The zinc-air battery with the Co9-xNixS8/NC catalyst shows a high cyclability more than 170 cycles for 60 h with almost negligible decline at 10 mA cm-2. Our work provides a new insight into the design of bimetallic sulfide composites with high catalytic activities.
46 citations
TL;DR: In this article, highly active OER catalysts were reported in FeCo mixed oxides such as perovskite-tolerant peroxide, which is a crucial energy conversion reaction for achieving sustainable society.
Abstract: The oxygen evolution reaction (OER) is a crucial energy conversion reaction for achieving a sustainable society. Highly active OER catalysts were reported in Fe–Co mixed oxides such as perovskite-t...
27 citations
TL;DR: In this paper, the authors report a systematic study on predominant factors affecting oxygen/hydrogen evolution reactions (OERs/HERs) for Co-containing perovskite-related oxides.
Abstract: Efficient hydrogen evolution on water splitting is a crucial issue to achieve a sustainable society based on renewable energy. Highly active and cost-effective catalysts for oxygen/hydrogen evolution reactions (OERs/HERs) are desired to suppress the intrinsically significant overpotential of these reactions. Perovskite-related transition-metal oxides have been widely investigated as promising candidates for electrochemical catalysts, whereas their complex composition and structure inhibit the elucidation of essential factors for activating HER. Herein, we report a systematic study on predominant factors affecting HER catalysis for Co-containing perovskite-related oxides. The A-site-ordered double perovskite oxide PrBaCo2O6−δ exhibits HER activity with a volcano-type plot associated with oxygen deficiency content, and shows the highest activity at a moderate value of δ = 0.2, in addition to the significant superiority to the simple ABO3-type perovskite, ACoO3 (A = La, La0.5Ca0.5, Ca). Based on the Tafel slope, electric conductivity, and charge-transfer resistance analyses, we have found that complementary factors dominate the HER catalysis; Co–O covalency and water dissociation site, which are respectively induced by high Co valence and oxygen deficiency. This finding provides new insight into the rational design of transition-metal oxide catalysts for HER.
17 citations
TL;DR: The quadruple perovskite oxides RCu3Fe4O12 (R): rare-earth metals exhibit large latent-heat capacities with variable transition temperatures between 254 and 368 K, whereas their transition entropies are almost completely retained.
11 citations
TL;DR: A novel quadruple perovskite oxide CeCu3Co4O12 has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.
Abstract: A novel quadruple perovskite oxide CeCu3Co4O12 has been synthesized in high-pressure and high-temperature conditions of 12 GPa and 1273 K. Rietveld refinement of the synchrotron X-ray powder diffraction pattern reveals that this oxide crystallizes in a cubic quadruple perovskite structure with the 1:3-type ordering of Ce and Cu ions at the A-site. X-ray absorption spectroscopy analysis demonstrates the valence-state transitions in the ACu3Co4O12 series (A = Ca, Y, Ce) from Ca2+Cu3+3Co3.25+4O12 to Y3+Cu3+3Co3+4O12 to Ce4+Cu2.67+3Co3+4O12, where the electrons are doped in the order from B-site (Co3.25+ → Co3+) to A'-site (Cu3+ → Cu2.67+). This electron-doping sequence is in stark contrast to the typical B-site electron doping for simple ABO3-type perovskite and quadruple perovskites CaCu3B4O12 (B = V, Cr, Mn), further differing from the monotonical A'-site electron doping for Na1-xLaxMn3Ti4O12 and A'- and B-site electron doping for AMn3V4O12 (A = Na, Ca, La). The differences in the electron-doping sequences are interpreted by rigid-band models, proposing a wide variety of electronic states for the complex transition-metal oxides containing the multiple valence-variable ions.
9 citations
TL;DR: In this paper, the effect of size and crystallinity of a quadruple perovskite oxide, CaCu3Fe4O12, was investigated for its catalytic activity for the oxygen evolution reaction (OER).
Abstract: Effects of size and crystallinity of a quadruple perovskite oxide, CaCu3Fe4O12, were investigated for its catalytic activity for the oxygen evolution reaction (OER). Pristine CaCu3Fe4O12 powder sample was synthesized under high-pressure and high-temperature conditions of 8GPa and 1000°C, and a portion was treated by ball milling for 120min to obtain a powder sample with smaller particle size. The specific surface area significantly increased from 0.38 to 10.30m2g11 by ball-milling, leading to increased OER activity. However, it was found that the milling did not improve the OER activity efficiency in proportion to that expected from the increase in specific surface area as determined by BrunauerEmmett-Teller analysis of adsorption/desorption isotherms measured with nitrogen gas because the crystallinity was lowered by ball milling, which suppressed the catalytic activity. This study provides important information on how to achieve the best OER catalytic performance in terms of both size and crystallinity. [doi:10.2320/matertrans.MT-M2020147]
8 citations
TL;DR: In this article, a perovskite oxide with a new type of polar structure, called CuNbO3, was reported, thanks to the use of high-pressure and high-temperature synthesis.
Abstract: We report a perovskite oxide with a new type of polar structure, CuNbO3, thanks to the use of high-pressure and high-temperature synthesis. The perovskite-type CuNbO3 was previously obtained under ...
8 citations
25 Sep 2020
TL;DR: Maghemite, γ-Fe2O3, has regained importance as a magnetic tape storage material for the big data era due to its low cost and long-term durability.
Abstract: Maghemite, γ-Fe2O3, has regained importance as a magnetic tape storage material for the big data era due to its low cost and long-term durability. However, the conventional synthesis of γ-Fe2O3, wh...
7 citations
20 Apr 2020
TL;DR: π-FeOOH was synthesized using a high-pressure/high-temperature method and examined for the first time in nonaqueous Li and Na cells, finding that the rate capability of ϵ-FeooH was superior to that of α- FeOOH.
Abstract: The demand for eco-friendly materials for secondary batteries has stimulated the exploration of a wide variety of Fe oxides, but their potential as electrode materials remains unknown. In this cont...
6 citations
TL;DR: Ex situ XRD measurements clarified that the reversible electrochemical reaction can be attributed to the movement of vanadium ions from the tetrahedral 8a to octahedral 16c sites during the initial discharge reaction.
Abstract: Negative electrode materials with lower operating voltages are urgently required to increase the energy density of lithium-ion batteries. In this study, LiMgVO4 with a Na2CrO4-type structure, LiZnV...
5 citations
TL;DR: In this article, the catalytic activity on the oxygen evolution reaction (OER) was investigated for Fe3+-containing oxides with various crystal structures: spinel ZnFe2O4, post-spinel CaFe 2O4 and stuffed-tridymite BaFe 2 O4.
Abstract: The catalytic activity on the oxygen evolution reaction (OER) was investigated for Fe3+-containing oxides with various crystal structures: spinel ZnFe2O4, post-spinel CaFe2O4, and stuffed-tridymite BaFe2O4. The latter two oxides exhibited higher catalytic activity than ZnFe2O4, indicating that the differences in the coordination polyhedra and their connections are important factors to affect OER activity. [doi:10.2320/matertrans.MT-MN2019043]
TL;DR: In this article, the crystal structure evolution of solid solutions between giant-tetragonal (GT) perovskite-type PbVO3 and BiCoO3 is reported.
Abstract: In this study, the crystal structure evolution of solid solutions between giant-tetragonal (GT) perovskite-type PbVO3 and BiCoO3, namely, (1 – x)PbVO3–xBiCoO3, is reported. The all-proportional sol...
TL;DR: In this paper, the authors investigated the OER and HER catalytic activities of tetravalent FeCo mixed perovskite oxide CaFe0.5Co 0.5O3 and showed that the mixing of Fe4+ and Co4+ ions is an efficient way to activate OER.
Abstract: Electrochemical water splitting is a useful way for sustainable hydrogen production, whereas sluggish kinetics of oxygen/hydrogen evolution reactions (OER/HER) limits the efficiency; thus, active electrocatalysts to reduce overpotentials are desired. Mixing of multiple transition-metal elements is a promising way to enhance electrochemical catalysis. In the present study, we investigated the OER and HER catalytic activities of tetravalent FeCo mixed perovskite oxide CaFe0.5Co0.5O3. CaFe0.5Co0.5O3 demonstrated a higher OER activity than those of the parent compounds CaFeO3 and CaCoO3. In contrast, the HER activity of CaFe0.5Co0.5O3 was not significantly enhanced. These observations suggest that the mixing of Fe4+ and Co4+ ions is an efficient way to activate OER. [doi:10.2320/matertrans.MT-MN2019032]
TL;DR: In this article, the authors 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.
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 e-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 e-FeOOH strongly depended on the FeOOH sources, synthesis conditions, and composite electrodes. Nevertheless, one of the e-FeOOH samples exhibited a low OER overpotential compared with α-FeOOH and its parent β-FeOOH, which are widely used as OER catalysts. Hence, e-FeOOH is a potential candidate as a next-generation earth-abundant OER catalyst.