Yao Zhou

Novel Sulfur Host Composed of Cobalt and Porous Graphitic Carbon Derived from MOFs for the High-Performance Li–S Battery

A composite consisting of cobalt and graphitic porous carbon (Co@GC-PC) is synthesized from bimetallic metal-organic frameworks and employed as the sulfur host for high-performance Li-S batteries. Because of the presence of a large surface area (724 m2 g-1) and an abundance of macro-/mesopores, the Co@GC-PC electrode is able to alleviate the debilitating effect originating from the volume expansion/contraction of sulfur species during the cycling process. Our in situ UV/vis analysis indicates that the existence of Co@GC-PC promotes the adsorption of polysulfides during the discharge process. Density functional theory calculations show a strong interaction between Co and Li2S and a low decomposition barrier of Li2S on Co(111), which is beneficial to the following Li2S oxidation in the charge process. As a result, at 0.2C, the discharge capacity of the S/Co@GC-PC cathode is stabilized at 790 mAh g-1 after 220 cycles, much higher than that of a carbon-based cathode, which delivers a discharge capacity of 188 mAh g-1.

Co3O4@(Fe-Doped)Co(OH)2 Microfibers: Facile Synthesis, Oriented-Assembly, Formation Mechanism, and High Electrocatalytic Activity

Cobalt oxide or hydroxide nanoarchitectures, often synthesized via solvothermal or electrodeposition or templated approaches, have wide technological applications owing to their inherent electrochemical activity and unique magnetic responsive properties. Herein, by revisiting the well-studied aqueous system of Co/NaBH4 at room temperature, the chainlike assembly of Co3O4 nanoparticles is attained with the assistance of an external magnetic field; more importantly, a one-dimensional hierarchical array consisting of perpendicularly oriented and interconnected Co(OH)2 thin nanosheets could be constructed upon such well-aligned Co3O4 assembly, generating biphasic core-shell-structured Co3O4@Co(OH)2 microfibers with permanent structural integrity even upon the removal of the external magnetic field; isomorphous doping was also introduced to produce Co3O4@Fe-Co(OH)2 microfibers with similar structural merits. The cobalt-chemistry in such a Co/NaBH4 aqueous system was illustrated to reveal the compositional and morphological evolutions of the cobalt species and the formation mechanism of the microfibers. Owing to the presence of Co3O4 as the core, such anisotropic Co3O4@(Fe-doped)Co(OH)2 microfibers demonstrated interesting magnetic-responsive behaviors, which could undergo macro-scale oriented-assembly in response to a magnetic stimulus; and with the presence of a hierarchical array of weakly crystallized thin (Fe-doped) Co(OH)2 nanosheets with polycrystallinity as the shell, such microfibers demonstrated remarkable electrocatalytic activity toward oxygen evolution reactions in alkaline conditions.

A Natural Biopolymer Film as a Robust Protective Layer to Effectively Stabilize Lithium-Metal Anodes

Li metal is considered as an ideal anode for Li-based batteries. Unfortunately, the growth of Li dendrites during cycling leads to an unstable interface, a low coulombic efficiency, and a limited cycling life. Here, a novel approach is proposed to protect the Li-metal anode by using a uniform agarose film. This natural biopolymer film exhibits a high ionic conductivity, high elasticity, and chemical stability. These properties enable a fast Li-ion transfer and feasiblity to accomodate the volume change of Li metal, resulting in a dendrite-free anode and a stable interface. Morphology characterization shows that Li ions migrate through the agarose film and then deposit underneath it. A full cell with the cathode of LiFPO4 and an anode contaning the agarose film exhibits a capacity retention of 87.1% after 500 cycles, much better than that with Li foil anode (70.9%) and Li-deposited Cu anode (5%). This study provides a promising strategy to eliminate dendrites and enhance the cycling ability of lithium-metal batteries through coating a robust artificial film of natural biopolymer on lithium-metal anode.

Onion-like metal–organic colloidosomes from counterion-induced self-assembly of anionic surfactants

Micelles or vesicles of surfactants, though being long utilized in material synthesis, are yet to be studied in terms of their evolutionary assembly behaviors and their role as a template in mesoscale morphology control. Nevertheless, restricted by their soft and changeable nature, so far investigations of pure micelles or vesicles often rely on state-of-the-art characterization techniques (e.g., in situ ones). Herein, we reported that the self-assembly behaviors of the anionic surfactant SDBS are profoundly affected by the presence of transition metal cations M2+ (M = Co or Ni) (which work as the counterion of the anionic DBS−) in a slightly alkaline environment. With well-controlled reaction kinetics, concentric multishelled M(OH)x(DBS)y colloidosomes are formed easily with meta-stability. The electrostatic forces between the anionic DBS− and the unsaturated cationic [M(OH)x]y+ species, and the van der Waals forces from the hydrophobic interaction among the DBS− molecular tails are the major forces organizing the overall structure. The as-observed colloidosomes on one hand are sufficiently stable which could be separated from the solution and characterized conveniently using ex situ techniques such as FESEM, TEM, XPS, FTIR and XRD. On the other hand, they could either evolve flexibly into a series of layered or shelled metal hydroxide nanostructures via the classical Ostwald ripening process, or react with another reactant to generate other shelled derivatives (e.g., metal silicates). The multishelled colloidosomes and their derivatives from simple thermal treatment were also explored as efficient electrocatalysts for the oxygen evolution reaction. The facile formation, the convenient characterization and the flexible transformation of such onion-like colloidosomes emphasize the important role of counterions in regulating the self-assembly behavior of ionic surfactants so as to construct novel functional materials that are thermodynamically difficult to achieve; they also contribute to a mechanistic study on surfactant-assisted morphology control in material synthesis.

Three-Dimensional Networks of S-Doped Fe/N/C with Hierarchical Porosity for Efficient Oxygen Reduction in Polymer Electrolyte Membrane Fuel Cells

Reasonable design and synthesis of Fe/N/C-based catalysts is one of the most promising way for developing precious metal-free oxygen reduction reaction (ORR) catalysts in acidic mediums. Herein, we developed a highly active metal-organic framework-derived S-doped Fe/N/C catalyst [S-Fe/Z8/2-aminothiazole (2-AT)] prepared by thermal treatment. The S-Fe/Z8/2-AT catalyst with uniform S-doping possesses a three-dimensional macro-meso-micro hierarchically porous structure. Moreover, the chemical composition and structural features have been well-optimized and characterized for such S-Fe/Z8/2-AT catalysts; and their formation mechanism was also revealed. Significantly, applying the optimal S-Fe/Z8/2-AT catalysts into electrocatalytic test exhibits remarkable ORR catalytic activity with a half-wave potential of 0.82 V (vs reversible hydrogen electrode) and a mass activity of 18.3 A g-1 at 0.8 V in 0.1 M H2SO4 solution; the polymer electrolyte membrane fuel cell test also confirmed their excellent catalytic activity, which gives a maximal power density as high as 800 mW cm-2 at 1 bar. A series of designed experiments disclosed that the favorable structural merits and desirable chemical compositions of S-Fe/Z8/2-AT catalysts are critical factors for efficient electrocatalytic performance. The work provides a new approach to open an avenue for accurately controlling the composition and structure of Fe/N/C catalysts with highly activity for ORR.