AbstractLarge Li2O2 aggregations can produce high-capacity of lithium oxygen (Li-O2) batteries, but the larger ones usually lead to less-efficient contact between Li2O2 and electrode materials. Herein, a hierarchical cathode architecture based on different discharge characteristics of α-MnO2 and Co3O4 is constructed, which can enable the embedded growth of large Li2O2 aggregations to solve this problem. Through experimental observations and first-principle calculations, it is found that α-MnO2 nanorod tends to form uniform Li2O2 particles due to its preferential Li+ adsorption and similar LiO2 adsorption energies of different crystal faces, whereas Co3O4 nanosheet tends to simultaneously generate Li2O2 film and Li2O2 nanosheets due to its preferential O2 adsorption and different LiO2 adsorption energies of varied crystal faces. Thus, the composite cathode architecture in which Co3O4 nanosheets are grown on α-MnO2 nanorods can exhibit extraordinary synergetic effects, i.e., α-MnO2 nanorods provide the initial nucleation sites for Li2O2 deposition while Co3O4 nanosheets provide dissolved LiO2 to promote the subsequent growth of Li2O2. Consequently, the composite cathode achieves the embedded growth of large Li2O2 aggregations and thus exhibits significantly improved specific capacity, rate capability, and cyclic stability compared with the single metal oxide electrode.

Supercapacitors using ionic liquids (ILs) as electrolytes have triggered great interest due to their much higher energy density when compared to aqueous supercapacitors. Although manganese oxides have obvious capacitive contribution in ILs and thus can be used as electrode materials for IL-based supercapacitors, they suffer from low specific capacitance in ILs. Here Mn3O4 nanodots loaded on nitrogen-doped graphene sheets (denoted as Mn3O4 NDs@NG) are prepared through a facile one-pot solvothermal method with the presence of octylamine as the surfactant. Octylamine plays an important role in obtaining quantum-sized Mn3O4 NDs and controlling their dispersion degree on the surface of NG. With an optimal loading mass of Mn3O4 NDs, the corresponding Mn3O4 NDs@NG material is able to achieve a high specific capacitance of 158.9 F g−1 in a given IL and shows excellent rate capability. On this basis, a symmetric supercapacitor is assembled based on such a Mn3O4 NDs@NG, which delivers a high energy density of 90.7 W h kg−1 in the IL electrolyte. Furthermore, an asymmetric supercapacitor is also built by using such a Mn3O4 NDs@NG and activated carbon as the negative and positive electrode, respectively. This asymmetric device shows a higher energy density of 124.4 W h kg−1 compared to the symmetric one, and it still can deliver 55.8 W h kg−1 at a large power density of 29.9 kW kg−1.

The effective shape-controlled synthesis of Co3O4 nanoarrays on nickel foam substrates has been achieved through a simple hydrothermal strategy. When they served as the binder- and conductive-agent-free porous cathodes for nonaqueous Li–O2 batteries, they sufficiently reflect the favorable catalytic characteristic of Co3O4 and alleviate the problems of serious pore blocking and surface passivation caused by insoluble and insulating discharge products. In particular, Co3O4 rectangular nanosheets exhibit superior electrocatalytic performance comparing with Co3O4 nanowires and hexagonal nanosheets, leading to higher specific capacity and better cycling stability over 54 cycles at 100 mA g–1, which relate to their good pore structure, large specific surface area, and highly active {112} exposed plane, effectively promoting the mass transport and reversible formation and decomposition of discharge products in the cathode. These comparisons further indicate the morphology effect of nanostructured Co3O4 on their performances as free-standing catalysts for Li–O2 batteries, which also have been proved through the further analysis of discharge products on different shapes of Co3O4 nanoarrays electrodes.Keywords: Co3O4; free-standing catalysts; lithium peroxides; lithium−oxygen batteries; nanostructures

•A method is developed for efficient nitrogen doping and improved porous structure.•Nitrogen doped three-dimensional graphene is first applied in lithium oxygen cell.•This electrode exhibits hierarchical porous structure and high catalytic activity.•The free-standing cathode shows high specific capacity and good cycling stability.Nitrogen doped three-dimensional graphene (N-3DG) with improved catalytic activities and optimized pore structure is prepared through facile hydrothermal and subsequent annealing processes. When employed as a novel free-standing cathode for Li-O2 batteries, its special three-dimensional structure with increased active sites, large pore volume and numerous available macropores and mesopores effectively facilitates the discharge products deposition, oxygen diffusion and electrolyte infiltration. Compared with pristine three-dimensional graphene (3DG), N-3DG delivers a higher specific capacity of 7300 mAh g−1 at the current density of 50 mA g−1 and better cycling stability.

In this work, a series of graphene nanoscroll (GNS)-wrapped Fe3O4 nanoparticles (NPs) composites (denoted as Fe3O4@GNS) are prepared by cold quenching of mixed suspensions of water-dispersible Fe3O4 NPs and graphene oxide (GO) with different mass ratios in liquid nitrogen followed by a low-temperature thermal reduction. In all samples, it is interesting that Fe3O4 NPs are able to be in situ encapsulated completely in GNSs, forming a three-dimensional network consisting of a fiber-like structure. The amount of Fe3O4 NPs wrapped with GNSs is in proportion to the mass ratio between Fe3O4 NPs and GO in the initial mixed suspension. As a new anode material of lithium ions batteries (LIBs), these Fe3O4@GNSs exhibit outstanding Li-ion storage characteristics. Among them, the Fe3O4@GNS (Fe3O4:GO = 2:1) electrode shows the best electrochemical properties, including excellent cycling stability with a reversible capacity of 1172 mA h g−1 over 200 cycles at 100 mA g−1 and 525 mA h g−1 over 1000 cycles at 2 A g−1, as well as superior rate performance with a reversible capacity of 648 and 480 mA h g−1 at 2 and 5 A g−1, respectively. Such high performance is very close to the novel hybrid structure of Fe3O4@GNS as well as the optimal ratio between Fe3O4 and GO.

•Nitrogen-containing activated carbon fibers (N-ACFs) were derived from silk fibers.•N-ACFs had ultra-high specific surface area and abundance micropores.•N-ACFs exhibited excellent CO2 capture capacity.Nitrogen-containing activated carbon fibers (N-ACFs) with ultra-high specific surface area and abundance micropores are synthesized from natural silk fibers. The N-ACFs exhibited the CO2 uptake of 7.0 mmol/g and 4.8 mmol/g at 0 °C and 25 °C, 101 kPa. These outstanding features as a CO2 solid adsorbent were mostly attributed to the materials׳ physical properties (microporosity) in addition to its high content of basic nitrogen species.

Supercapacitors using ionic liquids (ILs) as electrolytes have triggered great interest due to their much higher energy density when compared to aqueous supercapacitors. Although manganese oxides have obvious capacitive contribution in ILs and thus can be used as electrode materials for IL-based supercapacitors, they suffer from low specific capacitance in ILs. Here Mn3O4 nanodots loaded on nitrogen-doped graphene sheets (denoted as Mn3O4 NDs@NG) are prepared through a facile one-pot solvothermal method with the presence of octylamine as the surfactant. Octylamine plays an important role in obtaining quantum-sized Mn3O4 NDs and controlling their dispersion degree on the surface of NG. With an optimal loading mass of Mn3O4 NDs, the corresponding Mn3O4 NDs@NG material is able to achieve a high specific capacitance of 158.9 F g−1 in a given IL and shows excellent rate capability. On this basis, a symmetric supercapacitor is assembled based on such a Mn3O4 NDs@NG, which delivers a high energy density of 90.7 W h kg−1 in the IL electrolyte. Furthermore, an asymmetric supercapacitor is also built by using such a Mn3O4 NDs@NG and activated carbon as the negative and positive electrode, respectively. This asymmetric device shows a higher energy density of 124.4 W h kg−1 compared to the symmetric one, and it still can deliver 55.8 W h kg−1 at a large power density of 29.9 kW kg−1.