Layered Li_0.88[Li_0.18Co_0.33Mn_0.49]O₂ Nanowires for Fast and High Capacity Li-Ion Storage Material

Layered Li0.88[Li0.18Co0.33Mn0.49]O2 nanowires are prepared using Co0.4Mn0.6O2 nanowires and lithium nitrate as precursors at 200 °C via a hydrothermal method for fast and high capacity Li-ion storage material. The obtained nanowires exhibit a reversible capacity of 230 mAh/g between 2 and 4.8 V, even at the high current rate of 3600 mA/g

<i>In Situ</i> Mn K-edge X-ray Absorption Spectroscopy Studies of Electrodeposited Manganese Oxide Films for Electrochemical Capacitors

In situ Mn K-edge fluorescence X-ray absorption spectroscopy (XAS) was used to analyze the manganese oxides electrodeposited on a porous carbon paper substrate for use in electrochemical capacitors in order to determine the local and electronic structural changes in the material as a function of the applied potential in a neutral electrolyte. Within the potential range from +0.1 to +0.8 V vs SCE (reversible region), the cyclic voltammogram (CV) showed ideal capacitive characteristics. On the other hand, large current tails were observed at near both ends of the potential window in the CV when the upper and lower potential limits were expanded to +1.0 and −0.3 V vs SCE (irreversible region), which is indicative of an irreversible reaction. According to the in situ X-ray absorption near-edge structure (XANES) results, the capacitive currents of the manganese oxides in 2 M KCl in the reversible region originated from the Faradaic pseudocapacitance. The average oxidation state and local structure of the manganese oxide changed reversibly during charging/ discharging within the reversible region. On the other hand, the local and electronic structure of manganese oxide changed in an irreversible manner in the irreversible region, particularly during the redox reaction within the potential range between +0.1 to −0.3 V vs SCE. This irreversible feature of the local and electronic structure changes was attributed to the formation of the electrochemically irreversible low valence manganese oxides such as Mn2O3 and Mn3O4, and the dissolution of Mn species from the electrode

Synthesis of Phase-Pure Interpenetrated MOF-5 and Its Gas Sorption Properties

For the first time, phase-pure interpenetrated MOF-5 (1) has been synthesized and its gas sorption properties have been investigated. The phase purity of the material was confirmed by both single-crystal and powder X-ray diffraction studies and TGA analysis. A systematic study revealed that controlling the pH of the reaction medium is critical to the synthesis of phase-pure 1, and the optimum apparent pH (pH*) for the formation of 1 is 4.0−4.5. At higher or lower pH*, [Zn2(BDC)2(DMF)2] (2) or [Zn5(OH)4(BDC)3] (3), respectively, was predominantly formed. The pore size distribution obtained from Ar sorption experiments at 87 K showed only one peak, at ∼6.7 Å, which is consistent with the average pore size of 1 revealed by single crystal X-ray crystallography. Compared to MOF-5, 1 exhibited higher stability toward heat and moisture. Although its surface area is much smaller than that of MOF-5 due to interpenetration, 1 showed a significantly higher hydrogen capacity (both gravimetric and volumetric) than MOF-5 at 77 K and 1 atm, presumably because of its higher enthalpy of adsorption, which may correlate with its higher volumetric hydrogen uptake compared to MOF-5 at room temperature, up to 100 bar. However, at high pressures and 77 K, where the saturated H2 uptake mostly depends on the surface area of a porous material, the total hydrogen uptake of 1 is notably lower than that of MOF-5

Synthesis of Phase-Pure Interpenetrated MOF-5 and Its Gas Sorption Properties

For the first time, phase-pure interpenetrated MOF-5 (1) has been synthesized and its gas sorption properties have been investigated. The phase purity of the material was confirmed by both single-crystal and powder X-ray diffraction studies and TGA analysis. A systematic study revealed that controlling the pH of the reaction medium is critical to the synthesis of phase-pure 1, and the optimum apparent pH (pH*) for the formation of 1 is 4.0−4.5. At higher or lower pH*, [Zn2(BDC)2(DMF)2] (2) or [Zn5(OH)4(BDC)3] (3), respectively, was predominantly formed. The pore size distribution obtained from Ar sorption experiments at 87 K showed only one peak, at ∼6.7 Å, which is consistent with the average pore size of 1 revealed by single crystal X-ray crystallography. Compared to MOF-5, 1 exhibited higher stability toward heat and moisture. Although its surface area is much smaller than that of MOF-5 due to interpenetration, 1 showed a significantly higher hydrogen capacity (both gravimetric and volumetric) than MOF-5 at 77 K and 1 atm, presumably because of its higher enthalpy of adsorption, which may correlate with its higher volumetric hydrogen uptake compared to MOF-5 at room temperature, up to 100 bar. However, at high pressures and 77 K, where the saturated H2 uptake mostly depends on the surface area of a porous material, the total hydrogen uptake of 1 is notably lower than that of MOF-5

Atomically Thin Holey Two-Dimensional Ru₂P Nanosheets for Enhanced Hydrogen Evolution Electrocatalysis

The defect engineering of low-dimensional nanostructured materials has led to increased scientific efforts owing to their high efficiency concerning high-performance electrocatalysts that play a crucial role in renewable energy technologies. Herein, we report an efficient methodology for fabricating atomically thin, holey metal-phosphide nanosheets with excellent electrocatalyst functionality. Two-dimensional, subnanometer-thick, holey Ru2P nanosheets containing crystal defects were synthesized via the phosphidation of monolayer RuO2 nanosheets. Holey Ru2P nanosheets exhibited superior electrocatalytic activity for the hydrogen evolution reaction (HER) compared to that exhibited by nonholey Ru2P nanoparticles. Further, holey Ru2P nanosheets exhibited overpotentials of 17 and 26 mV in acidic and alkaline electrolytes, respectively. Thus, they are among the best-performing Ru–P-based HER catalysts reported to date. In situ spectroscopic investigations indicated that the holey nanosheet morphology enhanced the accumulation of surface hydrogen through the adsorption of protons and/or water, resulting in an increased contribution of the Volmer–Tafel mechanism toward the exceptional HER activity of these ultrathin electrocatalysts

A New Coating Method for Alleviating Surface Degradation of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material: Nanoscale Surface Treatment of Primary Particles

Structural degradation of Ni-rich cathode materials (LiNi_<i>x</i>M_1–<i>x</i>O₂; M = Mn, Co, and Al; <i>x</i> > 0.5) during cycling at both high voltage (>4.3 V) and high temperature (>50 °C) led to the continuous generation of microcracks in a secondary particle that consisted of aggregated micrometer-sized primary particles. These microcracks caused deterioration of the electrochemical properties by disconnecting the electrical pathway between the primary particles and creating thermal instability owing to oxygen evolution during phase transformation. Here, we report a new concept to overcome those problems of the Ni-rich cathode material via nanoscale surface treatment of the primary particles. The resultant primary particles’ surfaces had a higher cobalt content and a cation-mixing phase (<i>Fm</i>3̅<i>m</i>) with nanoscale thickness in the LiNi_0.6Co_0.2Mn_0.2O₂ cathode, leading to mitigation of the microcracks by suppressing the structural change from a layered to rock-salt phase. Furthermore, the higher oxidation state of Mn⁴⁺ at the surface minimized the oxygen evolution at high temperatures. This approach resulted in improved structural and thermal stability in the severe cycling-test environment at 60 °C between 3.0 and 4.45 V and at elevated temperatures, showing a rate capability that was comparable to that of the pristine sample

Cost-Effective Scalable Synthesis of Mesoporous Germanium Particles <i>via</i> a Redox-Transmetalation Reaction for High-Performance Energy Storage Devices

Nanostructured germanium is a promising material for high-performance energy storage devices. However, synthesizing it in a cost-effective and simple manner on a large scale remains a significant challenge. Herein, we report a redox-transmetalation reaction-based route for the large-scale synthesis of mesoporous germanium particles from germanium oxide at temperatures of 420–600 °C. We could confirm that a unique redox-transmetalation reaction occurs between Zn⁰ and Ge⁴⁺ at approximately 420 °C using temperature-dependent <i>in situ</i> X-ray absorption fine structure analysis. This reaction has several advantages, which include (i) the successful synthesis of germanium particles at a low temperature (∼450 °C), (ii) the accommodation of large volume changes, owing to the mesoporous structure of the germanium particles, and (iii) the ability to synthesize the particles in a cost-effective and scalable manner, as inexpensive metal oxides are used as the starting materials. The optimized mesoporous germanium anode exhibits a reversible capacity of ∼1400 mA h g^–1 after 300 cycles at a rate of 0.5 C (corresponding to the capacity retention of 99.5%), as well as stable cycling in a full cell containing a LiCoO₂ cathode with a high energy density (charge capacity = 286.62 mA h cm^–3)

Interface Engineering of Oxygen Vacancy-Enriched Ru/RuO₂–Co₃O₄ Heterojunction for Efficient Oxygen Evolution Reaction in Acidic Media

RuO2 is currently regarded as a benchmark electrocatalyst for water oxidation in acidic media. However, its wide application is still restricted by limited durability and high cost. Herein, we report a Ru/RuO2–Co3O4 catalyst for boosting the acidic oxygen evolution reaction catalytic performance via constructing a heterointerface between RuO2 and Co3O4 and vacancy engineering. The resulting Ru/RuO2–Co3O4 shows a 226 mV overpotential at 10 mA cm–2 and excellent stability with a small overpotential increase after continuous testing for 19 h, greatly surpassing that of commercial RuO2 in a 0.1 M HClO4 solution. Depth structure characterizations involved in XPS, XANES, and EXAFS indicate that the favorable catalytic performance of Ru/RuO2–Co3O4 is mainly ascribed to the interfacial charge transfer by heterojunction interfaces between Co species and Ru species. Co3O4 is adjacent to RuO2 and donates electrons, making the valence state of Ru lowered and the Ru–O covalency weakened, which greatly suppress the dissolution of Ru and thus enhance stability. Meanwhile, the existing oxygen vacancies improve the intrinsic catalytic activity. This study is highly expected to favor the design and synthesis of more highly efficient electrocatalysts applied in energy-related devices

Coiled Conformation Hollow Carbon Nanosphere Cathode and Anode for High Energy Density and Ultrafast Chargeable Hybrid Energy Storage

Lithium-ion batteries and pseudocapacitors are nowadays popular electrochemical energy storage for many applications, but their cathodes and anodes are still limited to accommodate rich redox ions not only for high energy density but also sluggish ion diffusivity and poor electron conductivity, hindering fast recharge. Here, we report a strategy to realize high-capacity/high-rate cathode and anode as a solution to this challenge. Multiporous conductive hollow carbon (HC) nanospheres with microporous shells for high capacity and hollow cores/mesoporous shells for rapid ion transfer are synthesized as cathode materials using quinoid:benzenoid (Q:B) unit resins of coiled conformation, leading to ∼5-fold higher capacities than benzenoid:benzenoid resins of linear conformation. Also, Ge-embedded Q:B HC nanospheres are derived as anode materials. The atomic configuration and energy storage mechanism elucidate the existence of mononuclear GeOx units giving ∼7-fold higher ion diffusivity than bulk Ge while suppressing volume changes during long ion-insertion/desertion cycles. Moreover, hybrid energy storage with a Q:B HC cathode and Ge–Q:B HC anode exploit the advantages of capacitor-type cathode and battery-type anode electrodes, as exhibited by battery-compatible high energy density (up to 285 Wh kg–1) and capacitor-compatible ultrafast rechargeable power density (up to 22 600 W kg–1), affording recharge within a minute

Coiled Conformation Hollow Carbon Nanosphere Cathode and Anode for High Energy Density and Ultrafast Chargeable Hybrid Energy Storage

Lithium-ion batteries and pseudocapacitors are nowadays popular electrochemical energy storage for many applications, but their cathodes and anodes are still limited to accommodate rich redox ions not only for high energy density but also sluggish ion diffusivity and poor electron conductivity, hindering fast recharge. Here, we report a strategy to realize high-capacity/high-rate cathode and anode as a solution to this challenge. Multiporous conductive hollow carbon (HC) nanospheres with microporous shells for high capacity and hollow cores/mesoporous shells for rapid ion transfer are synthesized as cathode materials using quinoid:benzenoid (Q:B) unit resins of coiled conformation, leading to ∼5-fold higher capacities than benzenoid:benzenoid resins of linear conformation. Also, Ge-embedded Q:B HC nanospheres are derived as anode materials. The atomic configuration and energy storage mechanism elucidate the existence of mononuclear GeOx units giving ∼7-fold higher ion diffusivity than bulk Ge while suppressing volume changes during long ion-insertion/desertion cycles. Moreover, hybrid energy storage with a Q:B HC cathode and Ge–Q:B HC anode exploit the advantages of capacitor-type cathode and battery-type anode electrodes, as exhibited by battery-compatible high energy density (up to 285 Wh kg–1) and capacitor-compatible ultrafast rechargeable power density (up to 22 600 W kg–1), affording recharge within a minute