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The relationship between the ORR catalytic activity of LaMnO3+δ and its Mn–O bond length was examined by replacing La with Sr, and it was found that the shorter the bond length, the higher the activity.

The dissolution of substituted Zn2+ ions in Co–Al spinel oxides leads to cation vacancies at the tetrahedral sites, promoting the adsorption process of OH− ions for charge compensation and enhancing the activity for oxygen evolution catalysis.

We have prevented lattice collapse and developed an OER catalyst with both high activity and stability by slightly increasing Ir-dissolution and balancing the elemental dissolution in BaIr1−xMnxO3 for reconstructing the rigid catalytic surface.

Oxygen evolution reaction (OER) catalysts play an essential role in energy-conversion electrochemical reactions. High entropy oxides (HEOs) were recently investigated as promising candidates to realize highly active and cost-effective OER catalysts. The vast composition space for HEOs needs considerable efforts to find possible catalysts, which disturbs the further development beyond simple chemical compositions like equimolar ones. In this study, we conducted the fast and efficient design of the perovskite HEOs of La(Cr, Mn, Fe, Co, Ni)O3 with high OER catalytic activity using Bayesian optimization and elucidated the relationship between chemical compositions and OER catalytic activities. The HEOs with optimized compositions exhibited much higher activities than equimolar LaCr1/5Mn1/5Fe1/5Co1/5Ni1/5O3, which was previously reported as an active catalyst. Bayesian optimization adjusted the concentrations of OER active elements of Fe, Co, and Ni to enhance the catalytic activities and provided the insight that inactive elements (Cr and Mn) in HEOs even promoted the OER activities. These findings suggest the solution of data-based predictions to improve catalytic performances in multi-element transition metal oxides.

The quadruple perovskite ruthenate CaCu

Spinel oxides are promising catalysts for oxygen evolution reaction (OER). The catalytic activity can be enhanced by octahedral ions; however, the effects of tetrahedral ions on OER catalysis remain unclear. A study of the effects of tetrahedral ions on OER catalysis was conducted by synthesizing spinel oxides (ZnxCo3−xO4) with various Zn and Co ratios (x = 0–1). Zn0.4Co2.6O4 showed the highest OER activity, although Zn ions have lower catalytic activity than Co ions at both sites. In the structure of ZnxCo3−xO4, it was discovered via Rietveld analysis and X-ray absorption spectroscopy that Zn2+ ions mainly occupy the tetrahedral sites. The highest Zn0.4Co2.6O4 activity is attributed to a moderate amount of cation defects on the surface caused by Zn2+ dissolution; however, excessive Zn2+ ion amounts led to an unstable surface structure. This study elucidates the OER catalysis enhancement attributed to the ion type at tetrahedral sites in spinel oxides.

Noble metal alloys exhibit excellent catalytic activity in methanol oxidation reaction (MOR). However, noble metals are expensive and their synthesis is complicated. Thus, tremendous efforts have b...

One of the primary drawbacks in the development of Mg rechargeable batteries is their low operating voltage. Although electrolytes with a wide potential window have been used for high-voltage Mg rechargeable batteries, these electrolytes undergo oxidative decomposition at the surface of the positive electrode active materials at relatively low potentials. Moreover, the overpotential and kinetics of oxidative electrolyte decomposition significantly depend on the transition metal ion in spinel oxides (e.g., MgMn2O4, MgFe2O4, or MgCo2O4) used as positive electrode active materials. Because the catalytic activities of spinel oxides for electrolyte decomposition are different, electrolyte decomposition can be effectively suppressed by using transition metal ions with high overpotential for electrolyte decomposition in target spinel oxides. However, the mechanism of the catalytic reaction has not yet been elucidated. Herein, we determined that the direct electron transfer from the electrolyte to the electrode was slow, whereas the electron transfer via the oxidation reaction of spinel oxides was fast. Furthermore, we used experimental data and calculations to demonstrate that the catalytic activity for oxidative electrolyte decomposition was correlated with the valence band maximum (VBM) of spinel oxides; that is, low VBMs were correlated with high overpotentials for oxidative electrolyte decomposition.

Room-temperature operations of rechargeable Mg coin-cell batteries have been achieved using a Mg alloy negative electrode and a spinel MgMn2O4 (MMO)-positive electrode. The present work focuses on clarifying the effects of the physiochemical properties of the MMO powder including the specific surface area (SBET) and porosity of the positive electrode on Mg battery performances in practical applications. Finally, optimal specific surface area and porosity parameters were obtained that ensured excellent battery performances as a standard coin-cell at room temperature. A typical MMO powder synthesized using a modified sol–gel method with propylene oxide-driven complex polymerization had a large SBET > 200 m2 g−1, and more than 90% porosity with a triple-tiered 3D open-channel network. Here we evaluated the discharge capacity and the cyclability for room-temperature operation as a function of SBET in a full cell with a Mg negative electrode as well as half cells with a carbon graphite electrode. Irrespective of whether half cells or a full cell was used, the initial discharge capacity was found to depend linearly on the SBET of the porous MMO powder in a Mg tetrakis(hexafluoroisopropyl)borate/triglyme electrolyte with a wide potential window, ΔE > 3.6 V. Eventually, the maximum discharge capacity of 220 mA h g−1 was realized at 25 °C in the full cell using the 3D open-channel nanostructure with SBET = 236 m2 g−1. The cyclability in the full cell with the Mg alloy negative electrode, however, degraded with the increasing SBET, while no cyclability degradation was observed in the half cell with the carbon electrode. A possible mechanism is discussed regarding passivation of the Mg alloy electrode in discharge/charge cycles.

We fabricated and evaluated four types of Li batteries, in this case a LTO/liquid electrolyte/Li battery, a LTO/LiPON/Li all-solid-state battery, and LTO/LiPON + liquid electrolyte/Li batteries with and without a separator to investigate and clarify the effects of each interface. Through the present research, it was found that a conventional polymer-based separator increases the impedance in the middle frequency region, resulting in an increase in the total cell resistance. After replacing the polymer-based separator with a thin-film solid electrolyte, the cycleability and capacity of the cell were comparable to those of a conventional Li-ion battery with a polymer separator. The all-solid-state Li thin-film battery without a liquid electrolyte exhibits the lowest capacity due to the large interfacial resistance between the Li metal and the LiPON solid electrolyte. However, we found that the insertion of an Al2O3 interlayer between the Li and LiPON improves the capacity.

Spinel-type MgMn2O4 nanoplates ~10 nm thick were prepared as a positive electrode for magnesium rechargeable batteries by transformation of metal hydroxide nanoplates. Homogeneous coating with a va...

application/pdf, Chemistry of Materials. 2020, 32 (9), P.3893-3903

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.

Amorphous cobalt oxides electrodeposited on VS2 grown on carbon paper show better catalytic activity for the oxygen evolution reaction than crystalline Co3O4 on carbon paper.

Postspinel-structured oxides, CaB2O4 (B = Cr3+, Mn3+, and Fe3+), have exhibited systematically higher catalytic activities in the oxygen evolution reaction (OER) than nominally conventional counterparts of perovskite LaBO3 and spinel ZnB2O4.

application/pdf, Materials Chemistry Frontiers. 2019, 3 (6), P.1209-1217

The oxygen evolution reaction (OER) plays a key role in the electrode reactions of emerging energy conversion technologies such as water splitting and rechargeable metal–air batteries. Herein, we report that stronger electron–electron correlation leads to higher OER activity by a systematic electrochemical study of Ca1−xSrxRuO3. Specifically, we find that when the conducting material is sufficiently near the metal–insulator boundary (Ca0.9Sr0.1RuO3 in this study), it leads to a remarkable activity enhancement by a factor of 17. Our finding suggests that electron correlation not only enhances the initial OER activity but also stabilizes the structure of the catalyst. Therefore, the coexistence of metallic conductivity and strong electron correlation is beneficial for developing highly active OER catalysts. Furthermore, the excellent OER activity at x ∼ 0.1 is accompanied by superior bifunctionality toward the OER and the oxygen reduction reaction (ORR). Our study also revealed that conducting materials close to the metal–insulator boundary are promising oxygen reaction catalysts for energy conversion technologies.

application/pdf, Inorganic Chemistry. 2021, 60 (10), P.7023-7030

In this chapter, it is shown that spinel oxides such as MgCo2O4 work as cathode materials for Mg rechargeable batteries with a high redox potential about 2–3 V versus Mg2+/Mg on the basis of the similarity between spinel and rocksalt structures (Okamoto et al., Adv. Sci., 1500072, 2015, [1]). The Mg insertion into spinel lattices occurs via “insertion and push-out” process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V versus Mg2+/Mg, and similarly, it occurs at around 2.3 V versus Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4. In addition, Mg2+ ions originally in MgMn2O4 and MgCr2O4 can be extracted to some extent because of the robust host structure. The “insertion and push-out” process proposed here provides a new design of cathode materials for Mg rechargeable batteries, and various approaches are introduced to develop cathode materials based on this mechanism in the subsequent chapters.

Molecularly imprinted polymers (MIPs) are a fascinating technology for the sensitive and selective detection of target molecules. However, in most situations, the need for complicated and expensive analytical devices for reading the responses of MIPs greatly limits their applications. For exploring low-cost and easy-to-use applications of MIPs, herein we have developed a MIP-modified extended-gate type organic field-effect transistor (MIP-OFET). Taurine was selected as a demonstrative analyte due to its biological roles and utility as a nutrient. We explored the rational design of the novel MIP with the aid of density functional theory and wave function calculations and characterized the electrochemically synthesized MIP using differential pulse voltammetry and electrochemical impedance spectroscopy. The mechanism of taurine detection by the MIP-OFET can be explained by the changes in the surface potential of the MIP-functionalized extended-gate electrode accompanied with the capture of taurine. The detection limit of taurine in complete aqueous media was estimated to be 0.33 μM, which was lower or comparable to those calculated by high-performance liquid chromatography. Furthermore, taurine in a commercial drink without any extraction was also successfully detected using the fabricated MIP-OFET. This study would broaden the scope of the applications of MIP-OFETS as chemical sensors for on-site detection of various daily nutrients.

Spinel-type MgMn

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 Co

application/pdf, Article, The Journal of Physical Chemistry C. 2018, 122 (49), p.27885-27892

Transition metal oxides have been extensively investigated as novel catalysts for oxygen evolution reaction (OER). Partial elemental substitutions are effective ways to increase catalytic performance and such electronic interactions between multiple elements are known as synergistic effects. However, serious issues such as random atomic arrangement and ambiguous roles of constituent elements humper theoretical investigations for rational materials design. Herein, we describe systematic study on OER activity of AA′3B4O12-type quadruple perovskite oxides, in which multiple transition metal ions are located at distinct crystallographic sites. Electrochemical measurements demonstrate that OER catalytic activities of quadruple perovskite oxide series, CaCu3B4O12 (B = Ti, V, Cr, Mn, Fe, and Co), are all superior to those of simple perovskite counterparts CaBO3. The order of activity of B-site transition metal ions for CaBO3 (Fe4+ > Co4+ ≫ Ti4+, V4+, Cr4+, Mn4+) is retained in CaCu3B4O12, indicating that B-site ...

Mesocrystals of MgCo2O4 spinel oxide exhibiting porous and single-crystalline structures were synthesized by the reaction of magnesium ions with calcite-type CoCO3 mesocrystal precursors grown in micrometer-sized agar gels. The mesocrystals consisted of single-crystalline porous grains measuring 200–300 nm in diameter with iso-oriented subunits of approximately 10 nm. The single-crystalline porous framework structure of the MgCo2O4 mesocrystals enhanced the discharge capacity and positively shifted the discharge potential compared with MgCo2O4 nanoparticles prepared by the inverse co-precipitation method. These enhancements were attributed to a combination of the high specific surface areas of the mesocrystals and the high connectivity of the magnesium ion diffusion paths in the solid-phase.

The oxygen evolution reaction (OER) is an important reaction in the field of renewable energy and is utilized in electrochemical water splitting for hydrogen fuel production and rechargeable metal–air batteries. Herein, we report a new oxygen evolution reaction mechanism originating from oxygen vacancies, which remarkably enhances the OER activity of oxygen deficient electrocatalysts. The OER activity of Sr2VFeAsO3−δ is drastically enhanced above the lattice oxygen vacancy of δ = 0.5, exhibiting ∼300 mV lower overpotential and 80 times higher specific activity at 1.7 V vs. RHE. Surprisingly, the initially low OER activity of Sr2VFeAsO3−δ (δ 0.5. As a result, the distance between OH− coupled oxygen-vacant sites becomes sufficiently short to enable direct O–O bond formation as in photosystem II. Thus, we found that the OER activity of oxygen deficient electrocatalysts is controllable by the variety of lattice oxygen vacancies, which suggests that oxygen deficient layered superconductors are promising OER catalysts for energy conversion technologies.

Perovskite-type oxides composed of earth-abundant elements have been extensively studied as possible candidates for oxygen evolution reaction (OER) catalysts. In our recent study, quadruple perovskite oxides (e.g., CaCu3Fe4O12 and LaMn7O12) displayed catalytic activity that was higher than that of simple perovskites (e.g., LaMnO3), but the reason has not yet been unveiled. We have conducted first-principles calculations of the several surface energies of LaMn7O12 and adsorption energies of OER intermediates on LaMn7O12 using slab models to clarify the reaction mechanism. The Mn-rich surfaces, i.e., the (001) with BO2 termination and (220) surfaces, are found to be more stable for LaMn7O12. It is found that all intermediates are preferentially adsorbed on the A′–B-bridge site on the LaMn7O12 (220) surface, although only the B-top site was a stable adsorption site on the (001) surface of LaMn7O12 and LaMnO3. The difference between theoretical overpotentials on the (220) surface of LaMn7O12 and the (001) sur...

We report the reversible intercalation of Mg2+ ions into monocrystalline layered FePS3 in a tetrahydrofuran electrolyte containing 0.8 M PhMgCl and 0.4 M AlCl3. Mg2+ ions were not intercalated into FePSe3 because of the lower interlayer spacings (FePS3: 1.24 A, FePSe3, 1.00 A). The contribution of the non-Faradaic (double layer formation) and Faradaic (electrochemical intercalation/deintercalation, adsorption/desorption) processes toward the energy storage mechanism of FePS3 and FePSe3 were quantified. The maximum specific capacity displayed by FePS3 was 232.7 mAh g–1 at a current density of 0.2 A g–1, which was higher than that of FePSe3 (153.1 mAh g–1). Even at high current density (2 A g–1), FePS3 exhibited a specific capacity of 100.7 mAh g–1 after 450 cycles; the specific capacity of the less robust FePSe3 was just 26.3 mAh g–1.