A three-dimensional (3D) macroscopic network of manganese oxide (MnO2) sheets was synthesized by an easily scalable solution approach, grafting the negatively charged surfaces of the MnO2 sheets with an aniline monomer by electrostatic interactions followed by a quick chemical oxidizing polymerization reaction. The obtained structure possessed MnO2 sheets interconnected with polyaniline chains, producing a 3D monolith rich in mesopores. The MnO2 sheets had almost all their reactive centers exposed on the electrode surface, and combined with the electron transport highways provided by polyaniline and the shortened diffusion paths provided by the porous structure, the deliberately designed electrode achieved an excellent capacitance of 762 F g–1 at a current of 1 A g–1 and cycling performance with a capacity retention of 90% over 8000 cycles. Furthermore, a flexible asymmetric supercapacitor based on the constructed electrode and activated carbon serving as the positive and negative electrodes, respectively, was successfully fabricated, delivering a maximum energy density of 40.2 Wh kg–1 (0.113 Wh cm–2) and power density of 6227.0 W kg–1 (17.44 W cm–2) in a potential window of 0–1.7 V in a PVA/Na2SO4 gel electrolyte.Keywords: 3D network; electrostatic interaction; flexible devices; manganese oxide; two-dimensional sheets;

Lithium–sulfur batteries have many advantages, including their low material cost, high safety, and high energy density, and thus, they have recently been pursued as the most promising alternative to lithium-ion batteries in a multitude of applications, ranging from electric vehicles to stationary grid storage. However, these batteries are greatly hindered by certain problems, especially their low utilization of sulfur and fast capacity depletion due to the dissolution of the intermediate discharge product, polysulfide, and its diffusion across the separator to the anode side. In this study, we report the utilization of inorganic cationic sheets as a conformal modification layer on the separator, synergistically working as both a physical confinement barrier and chemical trap that efficiently blocked the crossover of polysulfide and accordingly improved the cycling life of Li–S batteries. Notably, due to the high percentage of surface atoms and the synergy of the physical and chemical trapping functions, the modification loading was minimized to a record low level (a density of ~0.018 mg cm−2 and a thickness of 20–30 nm), promising the unsacrificement of energy density when considering the entire cell. This proof-of-concept work of utilizing inorganic cationic sheets as a modification layer on a commercial separator provides a cheap and simple but effective strategy to enhance the cycle life of Li-S batteries.

Developing efficient but nonprecious bifunctional electrocatalysts for overall water splitting in basic media has been the subject of intensive research focus with the increasing demand for clean and regenerated energy. Herein, we report on the synthesis of a novel hierarchical hybrid electrode, NiFe-layered double hydroxide molecularly ultrathin sheets grown on NiCo2O4 nanowire arrays assembled from thin platelets with nickel foam as the scaffold support, in which the catalytic metal sites are more accessible and active and most importantly strong chemical coupling exists at the interface, enabling superior catalytic power toward both oxygen evolution reaction (OER) and additionally hydrogen evolution reaction (HER) in the same alkaline KOH electrolyte. The behavior ranks top-class compared with documented non-noble HER and OER electrocatalysts and even comparable to state-of-the-art noble-metal electrocatalysts, Pt and RuO2. When fabricated as an integrated alkaline water electrolyzer, the designed electrode can deliver a current density of 10 mA cm–2 at a fairly low cell voltage of 1.60 V, promising the material as efficient bifunctional catalysts toward whole cell water splitting.Keywords: chemical coupling; electrocatalyst; layered double hydroxide; molecular sheets; water splitting;

A key challenge in current superhard materials research is the design of novel superhard nanocrystals (NCs) whereby new and unexpected properties may be predicted. Cubic boron nitride (c-BN) is a superhard material which ranks next to diamond; however, downsizing c-BN material below the 10 nm scale is rather challenging, and the interesting new properties of c-BN NCs remain unexplored and wide open. Herein we report an electrochemical shock method to prepare uniform c-BN NCs with a lateral size of only 3.4 ± 0.6 nm and a thickness of only 0.74 ± 0.3 nm at ambient temperature and pressure. The fabrication process is simple and fast, with c-BN NCs produced in just a few minutes. Most interestingly, the NCs exhibit excellent piezoelectric performance with a large recordable piezoelectric coefficient of 25.7 pC/N, which is almost 6 times larger than that from bulk c-BN and even competitive to conventional piezoelectric materials. The phenomenon of enhancement in the piezoelectric properties of BN NCs might arise from the nanoscale surface effect and nanoscale shape effect of BN NCs. This work paves an interesting route for exploring new properties of superhard NCs.Keywords: Cubic boron nitride; electrochemical shock; nanocrystals; piezoelectricity;

The strong interest in macroscopic graphene and/or carbon nanotube (CNT) fiber has highlighted that anisotropic nanostructured materials are ideal components for fabricating fiber assemblies. Prospectively, employing two-dimensional (2D) crystals or nanosheets of functionality-rich transition metal oxides would notably enrich the general knowledge for desirable fiber constructions and more importantly would greatly broaden the scope of functionalities. However, the fibers obtained up to now have been limited to carbon-related materials, while those made of 2D crystals of metal oxides have not been achieved, probably due to the intrinsically low mechanical stiffness of a molecular sheet of metal oxides, which is only few hundredths of that for graphene. Here, using 2D titania sheets as an illustrating example, we present the first successful fabrication of macroscopic fiber of metal oxides composed of highly aligned stacking sheets with enhanced sheet-to-sheet binding interactions. Regardless of the intrinsically weak Ti–O bond in molecular titania sheets, the optimal fiber manifested mechanical performance comparable to that documented for graphene or CNTs. This work provided important hints for devising optimized architecture in macroscopic assemblies, and the rich functionalities of titania promises fibers with limitless promise for a wealth of innovative applications.

Supercapacitors are important energy storage technologies in fields such as fuel-efficient transport and renewable energy. State-of-the-art supercapacitors are capable of supplanting conventional batteries in real applications, and supercapacitors with novel features and functionalities have been sought for years. Herein, we report the realization of a new concept, a smart supercapacitor, which functions as a normal supercapacitor in energy storage and also communicates the level of stored energy through multiple-stage pattern indications integrated into the device. The metal-oxide W18O49 and polyaniline constitute the pattern and background, respectively. Both materials possess excellent electrochemical and electrochromic behaviors and operate in different potential windows, −0.5–0 V (W18O49) and 0–0.8 V (polyaniline). The intricate cooperation of the two materials enables the supercapacitor to work in a widened, 1.3 V window while displaying variations in color schemes depending on the level of energy storage. We believe that our success in integrating this new functionality into a supercapacitor may open the door to significant opportunities in the development of future supercapacitors with imaginative and humanization features.

Noble metal catalysts are core materials for many energy conversion and storage technologies such as fuel cell and hydrogen storage, while the performances of the noble metals depend critically on the nature of support materials. Herein, for the first time, we report the use of sorted high-purity metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWNTs) as support materials for a typical noble metal catalyst, Pd, which has been believed to have strong interactions with SWNTs. Our results clearly suggest that electrocatalytic performances of the noble metals/SWNTs hybrid system, specifically in hydrogen electrosorption and formic acid electrooxidation, exhibit strongly sensitive dependence with respect to the electronic type of SWNT supports. Pd nanoparticles on m-SWNTs demonstrate much enhanced electrocatalytic activities compared with those on s-SWNTs or unsorted-SWNTs. Our in-depth mechanism studies indicate that the m-SWNTs in the nanocomposite tend to provide more effective charge-transfer interfaces with Pd nanoparticles, more likely leading to higher electron densities on Pd nanoparticles to boost their catalytic performance. The present study provides a novel window onto the design and synthesis of new feasible electrocatalyst system for efficient energy conversion and storage.