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A nest-like N-doped hierarchical porous active carbon (N-HAC) with higher specific surface area is achieved successfully by melamine cyanurate (MCA) sacrifice template molding and assistant chemical activation of NH4Cl and CaCl2. The results testify that the nest-like hierarchical porous character of N-HAC can form an interconnecting structure and introduce plenty of supercapacitor reactive sites, which can facilitate the ion diffusion/migration among the material, while nitrogen doping in N-HAC can change the charge distribution of N-HAC and then promote the electron transfer. Attributing to the synergistic effect of the hierarchical porous structure and nitrogen doping, N-HAC800 displays the highest specific capacitance (284.6 F g−1 at 1 A g−1 and 183.1 F g−1 even at 20 A g−1) and superior cycle stability (80.4% after 5000 cycles at 100 mV s−1). This study results prove that the N-doped hierarchical porous active carbon is a promising material for energy storage application in the future.

Inspired by the supramolcular chemistry assembly theory, carbon-coated MnO2 nanospheres (C@MnO2) were successfully synthesized by a two-step method. First, the supramolecular solution was prepared by β-cyclodextrin inclusion of manganese acetate, which was then treated by a hydrothermal reaction and the calcination in the following. The scan electron microscopy (SEM) and transmission electron microscopy (TEM) images present that the C@MnO2 composites have the nanosphere morphology, in which the carbon layer originated from the carbonization of β-cyclodextrin was uniformly coated on the MnO2 nanoparticles. The electrochemical characteristics were examined by galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results demonstrated that C@MnO2 exhibited high specific capacitance (396.4 F g−1 at 2 A g−1) and good rate capability. In addition, 84.1% of the initial capacitance was maintained after 10000 cycles, indicating the excellent electrochemical stability. The superior electrochemical performance of the C@MnO2 was attributed to the outstanding uniform nanospheres structure that facilitates ion mobility and the synergistic effect between conductive carbon and pseudocapacitive MnO2. Hence, C@MnO2 can be a potential candidate material for the applications in supercapacitors.

A new type of sandwiched nanosheets 3D MoS2/C@RGO is synthesized by supramolecular self-assembly methods, as a novel and excellence electrode material. The surface characterization techniques show that the sandwiched 3D MoS2/C@RGO possesses rich porosity and large specific area, which is convenient for electrolyte ion transportation and beneficial for the improved electrical conductivity. An outstanding specific capacitance of the prepared MoS2/C@RGO of 340.0 F/g at 1 A g−1 is achieved with an excellent cycle stability of 90% after 1000 cycles at the scanning rate of 40 mV/s. The sandwiched 3D MoS2/C@RGO is suggested to be an ideal candidate for supercapacitors.

A manganese dioxide (MnO2) film is electrodeposited onto a gold-coated quartz crystal electrode by a galvanostatic method. The scanning electron microscopy (SEM) image shows that the electrodeposited MnO2 film consists of nanorods with diameters of 15–20 nm. The cyclic voltammetric (CV) and electrochemical quartz crystal microbalance (EQCM) results demonstrate that the MnO2 electrodes exhibit similar specific capacitance (SC) and mass-to-charge ratios (MCR) in those electrolytes that contain the same cation but different anions. However, the SC and MCR are different in those electrolytes that contain the same anion but different cations. The SC of MnO2 electrodes increases with a decrease of the cationic radius and an increase of the cationic charge. The smaller the cationic radius, the smaller the deviation of the MCR from the equivalent weight (EW) of the cation. The results illustrate that the pseudocapacitive charge storage mechanism of MnO2 involves the cation in the electrolyte and its intercalation/deintercalation, which are affected by the charge number and the cationic radius. This work offers a theoretical basis for selecting suitable electrolytes to couple MnO2-based electrodes in supercapacitors.

Transition metal phosphides (TMPs) have attracted great interest owing to the metallic properties and high specific capacities. Here, we designed hollow NiCoP nanocubes with increased specific surface area using a Ni-Co Prussian blue analogue as a self-template and NH3·H2O as an etching agent. During the synthesis, both carbonization and phosphorization are completed in one step. The obtained hollow structure alleviates the volume variation of electrode material during reversible electrochemical reaction. Meanwhile, the residual carbon distributed uniformly in NiCoP at the molecular level, resulting in a high conductivity. DFT calculations further reveal that the electrical conductivity of NiCoP is superior to those of monometallic phosphide and metal oxide. Therefore, the optimized NiCoP-4–500 displays a high specific capacity (1590 F g−1 at 1 A g−1) and outstanding cycling stability (78.2% retention after 12,000 cycles). Moreover, a prepared hybrid supercapacitor device delivers an energy density of 38.4 W h kg−1 with a power density of 799.9 W kg−1 at 1 A g−1. The results indicate that the obtained high-performance TMPs with hollow structures have an application potential for energy storage devices.

Transition metal sulfides (TMS) are of great interest as promising battery-type electrode materials, however, the poor conductivity and sluggish reaction kinetics seriously limit their application. Here, we designed a hollow structured precursor of Ni-based metal-organic frameworks (Ni-MOFs) via Ostwald ripening mechanism. Based on this unique precursor, a hollow carbon-coated nickel sulfide nanocrystal (H-NiS1-X/C) with sulfur vacancies was further synthesized through an ion exchange strategy and thermal annealing. By optimizing the content of sulfur source, the sample with appropriate S-vacancies (H-NiS1-X/C-50) was developed. Benefiting from its hollow structure and S-vacancies, this H-NiS1-X/C-50 displayed a high reversible specific capacity (1728 F g−1, 1 A g−1), stable cycling (72% capacity retention over 8000 cycles) and superior rate capability. After assembling the asymmetric supercapacitor, a high energy density of 36.88 Wh kg−1 was achieved. Experimental results and DFT calculations demonstrate that introducing S-vacancies builds an embedded electric field and produces lattice distortions in H-NiS1-X/C, thus enhancing the conductivity of the material. Our strategy also provides a facile way to construct high-performance TMS with unique hollow structure and S-vacancies for developing advanced energy storage devices.

Nanotunnels inserted hierarchical-porous carbon (NTHPC) have been synthesized successfully via a MnO2-introduced-tunnels strategy using MnO2 nanorods as template, with agar and β-cyclodextrin serving as hybrid carbon precursors. The as-prepared NTHPC possesses a higher specific surface area of 1441 m2 g−1, a moderate pore volume of 1.23 cm3 g−1, and the hierarchical-porous structure with inserted nanotunnels. Transmission electron microscopy has demonstrated that the width of the nanotunnels is between 20 and 100 nm, and the length ranges from 0.2 to 2.0 μm. Tests in a three-electrode system showed that the NTHPC has high specific capacitance (253.1 F g−1, 5 mV s−1), as well as good rate capability (203.3 F g−1, 100 mV s−1) and excellent cycling stability. More importantly, an assembled symmetric supercapacitor with NTHPC electrodes delivered an outstanding energy density of up to 34.9 Wh kg−1 with power density of 755.2 W kg−1. The remarkable electrochemical performance of the NTHPC is ascribed to the nanotunnels, which act as ion reservoirs and liquid transfer channels that can increase the ion transport rate and shorten the ion transfer distance. This study provides a novel method for the preparation of high-performance hierarchical-porous carbon and guidance for its potential applications in supercapacitors.

A series of Zr4+ and/or Al3+ doped LiNi0.83Co0.11Mn0.06O2 are synthesized via a solid phase method. TEM shows that the co-doped specimen consists of a layered core and a coherent interface cation-mixed shell. XRD and XPS results clarify that Zr4+ doping increases the Li+/Ni2+ cation site exchange degree and the concentration of Ni2+. The morphology and structural integrity of the co-doped electrode maintains well, and it provides a higher cycle retention (89.7%) than blank (60.1%) after 150 cycles and a superior capacity of 152.8 mAh g−1 at 7C (only 57.7 mAh g−1 for blank) and an enhanced elevated temperature cycle retention (83.0%) than blank (33.7%) after 100 cycles at 55 °C. Generally, Zr4+ raises the concentration of Ni2+ to keep electric neutrality and then reconstruct a stable interfacial structure to inhibit side reactions, the structural stability could be also enhanced by the strong Zr–O and Al–O bonds. Meanwhile, Al3+ served as centers of positive charge can suppress the phase transformation. Benefiting from the synergistic effect of Zr4+/Al3+, as well as the induced coherent interface cation-mixed shell, co-doped specimen shows enhanced structural stability and the superior electrochemical performance. This study provides a route to prepare advanced layered cathodes for lithium-ion batteries.

Na-doped Ni-rich layered oxide LiNi 0.5 Co 0.2 Mn 0.3 O 2 cathode materials are synthesized via co-precipitation method in this work. The crystal structure, thermal stability as well as electrochemical performance are studied. Results show that when the Na-doping amount is less than and equal to 2%, the samples can exhibit better electrochemical performance than the parent compound. One reason is that the Na doping can reduce the cationic mixing in the compound, as confirmed by X-Ray powder diffraction (XRD). Beside, results from X-ray photoelectron spectroscopy (XPS) and galvanostatic intermittent titration technique (GITT) show that the introduced sodium can also impact the chemical environment of lithium, making the latter become more easily de-intercalated from the bulk material. However, as the Na-doping amount increases, the electrochemical properties of the samples were deteriorated. Furthermore, results from differential scanning calorimetry (DSC) measurement indicate that the added Na + is also beneficial for improving the thermal stability of the layered oxide.

Graphene oxide/CoAl-layered double hydroxide (GO/CoAl-LDHs) composites were successfully prepared by a face-to-face electrostatic self-assembly method. Positively charged colloidal CoAl-LDHs nanosheets (CoAl-LDH-NS) and negatively charged colloidal GO nanosheets were uniformly mixed; then CoAl-LDH-NS were assembled on GO nanosheets via electrostatic attraction to prepare GO/CoAl-LDHs composites. The GO/CoAl-LDHs composites were then transformed to the reduced graphene oxide/CoAl-layered double hydroxide (RGO/CoAl-LDHs) composites with a hydrothermal method. The results showed that the content of RGO has a remarkable influence on the capacitive properties of composites. Among the composites with different RGO content, the RGO/CoAl-LDHs composite containing 12.0 wt% RGO displayed a maximum specific capacitance of 825 F g −1 at low current density of 1 A g −1 , while that of pure CoAl-LDHs was 552 F g −1 . The capacitance retention of the RGO/CoAl-LDHs composite from 1 A g −1 to 8 A g −1 is found to be 62.3%, whereas that of pure CoAl-LDHs is only 31.9%. Face-to-face self-assembling between positively charged CoAl-LDH-NS and negatively charged GO nanosheets can effectively reduce the self-agglomeration of GO nanosheets and avoid CoAl-LDH-NS stacking together, which lead to improvement of the capacitive performance.

Adding redox-active species into the electrolyte can supply extra capacitance to the carbon-based supercapacitors. However, the devices may suffer from serious self-discharge caused by the shuttling and intermixing of charged active species between the electrodes. Confining these active species in a solid-state electrode material is an effective strategy to avoid this. Inspired by this, we fabricate a three-dimensional reduced graphene oxide confined methylene blue composite via a supramolecular strategy, combined with a hydrothermal reduction and a freeze-drying process. Benefited from the porous and conductive structure of the product, the fast-reversible electrochemical reaction of methylene blue, and the spatial confinement effect of the methylene blue confined in reduced graphene oxide, the as-prepared product exhibits a high-rate capability (311 and 262 F g−1 at 1 and 20 A g−1, respectively) and an excellent cycle stability (96% capacitance retention after 10000 cycles) in a three-electrode system. Moreover, a solid-state symmetric supercapacitor device based on the as-prepared product delivers the maximum energy density of 8.2 Wh kg−1 and outstanding cycle stability. This supramolecular-induced confining active species in reduced graphene oxide strategy provides insight into the fabrication of other high-performance graphene-based pseudocapacitors.

In this paper, Li4Ti5O12 (LTO)/graphene (G) composites are synthesized by interfacial electrostatic self-assembly in a water-in-oil (W/O) microemulsion system, combining with high-temperature calcination. In the W/O microemulsion, the aqueous phase is dispersed into discontinuous uniform nanoscale ‘water pools’. Both the hydrolysis of titanium source and the growth of Ti(OH)4 are confined in those nanoscale ‘water pools’. The positively charged Ti(OH)4 colloid and the negatively charged graphene sheets present at the water-oil interface are tightly bound by electrostatic interaction. In the scanning electron microscopy (SEM) and transmission electron microscope (TEM) images of LTO/G (W/O) composite, LTO particles (less than 50 nm) are uniformly anchored on graphene. Electrochemical measurements show that the LTO/G (W/O) anode has a high reversible capacity of 174 mAh g−1 (304.5 mAh cm−3) and 152 mAh g−1 (266 mAh cm−3) at 1 C and 10 C respectively, and a 97% capacity retention after 600 cycles at 10 C rate. The above excellent electrochemical performance benefits from the fact that the uniformly dispersed nano-sized LTO particles are tightly anchored on the graphene, which can shorten the lithium ion migration path, expose more active sites and improve the conductivity.

A reduction of Fe3O4 nanowires in nanoscopic reactors of amorphous C:H nanotubes (a-CNTs) was taken to understand features of the chemical reaction mechanism in nanoscale reactors. Fe3O4 nanowires encapsulated in a-CNTs were reduced into iron at a relatively low temperature of 570 degrees C, producing iron nanoparticles encapsulated in CNTs accompanied by the crystallization of the a-CNT shell. It was found that carbon in the a-CNT shell rather than hydrogen (5.5 wt % in it) reduced Fe3O4, showing features different from those in a macroscopic system. The possible mechanisms behind this phenomenon are discussed.

The NO reduction features over a noble-metal-free NO(x) storage/reduction catalyst ([Ca24Al28O64](4+*)4O-/K, defined as C12A7-O-/K), including the NO conversion, the N2 selectivity, and sulfur tolerance, were investigated with hydrogen and C3H6 as the reducing agents in a fixed-bed continuous flow reactor. The NO conversion and the N2 selectivity on the C12A7-O-/K catalyst mainly depends on the sample temperature, the percentage of potassium, the reducing agents, and the composition of the mixture of gases. The C12A7-O-/10%K catalyst possessed the highest selective reduction ability (to N2) among the catalysts C12A7-O-/x%K. Over 50% of NO can be reduced to N2 with H2 as the reduction agent at 550-700 degrees C. The C12A7-O-/K catalyst also shows higher NO(x) storage capacity (183.9 micromol/g at about 550 degrees C) as well as sulfur tolerance for both the NO(x) storage and the reduction processes. The catalyst characteristics and the intermediate species formed in the NO storage and reduction processes were investigated by the X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and time-of-flight mass spectrometry. The mechanism of NO(x) reduction was addressed according to the above investigations.