First-Principles Investigation on the Tunable Electronic Structures and Photocatalytic Properties of AlN/Sc2CF2 and GaN/Sc2CF2 Heterostructures.

Heterostructure catalysts are highly anticipated in the field of photocatalytic water splitting. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are proposed in this work, and the electronic structures were revealed with the first-principles method to explore their photocatalytic properties for water splitting. The results found that the thermodynamically stable AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are indirect semiconductors with reduced band gaps of 1.75 eV and 1.84 eV, respectively. These two heterostructures have been confirmed to have type-Ⅰ band alignments, with both VBM and CBM contributed to by the Sc2CF2 layer. AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures exhibit the potential for photocatalytic water splitting as their VBM and CBM stride over the redox potential of water. Gibbs free energy changes in HER occurring on AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are as low as -0.31 eV and -0.59 eV, respectively. The Gibbs free energy change in HER on the AlN (GaN) layer is much lower than that on the Sc2CF2 surface, owing to the stronger adsorption of H on AlN (GaN). The AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures possess significant improvements in absorption range and intensity compared to monolayered AlN, GaN, and Sc2CF2. In addition, the band gaps, edge positions, and absorption properties of AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures can be effectively tuned with strains. All the results indicate that AlN/Sc2CF2 and GaN/Sc2CF2 heterostructures are suitable catalysts for photocatalytic water splitting.

The Crucial Role of Charge Accumulation and Spin Polarization in Activating Carbon-Based Catalysts for Electrocatalytic Nitrogen Reduction.

Cost-effective carbon-based catalysts are promising for catalyzing the electrochemical N2 reduction reaction (NRR). However, the activity origin of carbon-based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon-based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom-doping-induced charge accumulation facilitates N2 adsorption on carbon atoms and spin polarization boosts the potential-determining step of the first protonation to form *NNH. Te-doped and Se-doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal-based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon-based materials paves the pathway for their NRR application.

Mesoporous ZnCo2O4 nanoflakes grown on nickel foam as electrodes for high performance supercapacitors.

ZnCo2O4 nanoflakes, as electrodes for supercapacitors, are grown on a cellular nickel foam using a cost-effective hydrothermal procedure. The mesoporous ZnCo2O4 nanoflakes have large electroactive surface areas with strong adhesion to the Ni foam, allowing fast ion and electron transport. The nanoarchitecture electrodes deliver an excellent specific capacitance of 1220 F g(-1) at a current density of 2 A g(-1) in a 2 M KOH aqueous solution and a long-term cyclic stability of 94.2% capacitance retention after 5000 cycles. The fabrication strategy is facile, cost-effective, and can offer great promise for large-scale supercapacitor applications.

Fabrication of a LiNi0.5Mn0.4Ti0.03Mg0.03Nb0.01Mo0.03O2 cathode for low-cost long-life lithium-ion batteries.

The high price of Co and Ni restricts the development of the lithium-ion battery industry. Reducing the Ni content and eliminating Co is an effective way to lower the cost. In this work, we eliminate the Co in NCM523 cathodes by using a complex concentrated doping strategy. LiNi0.5Mn0.4Ti0.03Mg0.03Nb0.01Mo0.03O2 shows an unparalleled cost advantage with relatively high specific energy (>720 W h kg-1) and significantly improved overall performance (96% capacity retained after 1000 cycles). This report offers an important pathway to fabricate cathode materials for low-cost and long-life LIBs.

Synthesis of highly twinned ZnSe nanorods for enhancing N2 electrochemical conversion to NH3.

In this work, we report an atomistic understanding of the hydrogenation behavior of a highly twinned ZnSe nanorod (T-ZnSe) with a large density of surface atomic steps and the activation of N2 molecules adsorbed on its surface. Theoretical calculations suggest that the atomic steps are essential for the hydrogenation of T-ZnSe, which greatly enhances its catalytic activity. As a result, the T-ZnSe nanorods exhibit a significantly enhanced NH3 production rate of 13.3 µg h-1 mg-1 and faradaic efficiency of 5.83% towards the NRR compared with the pristine ZnSe nanorods. This report offers an important pathway for the development of efficient catalysts for the NRR, and a versatile anion-exchange strategy for efficiently manipulating materials' functionalities.

Multiscale Structural Engineering of Ni-Doped CoO Nanosheets for Zinc-Air Batteries with High Power Density.

Zinc-air batteries offer a possible solution for large-scale energy storage due to their superhigh theoretical energy density, reliable safety, low cost, and long durability. However, their widespread application is hindered by low power density. Herein, a multiscale structural engineering of Ni-doped CoO nanosheets (NSs) for zinc-air batteries with superior high power density/energy density and durability is reported for the first time. In micro- and nanoscale, robust 2D architecture together with numerous nanopores inside the nanosheets provides an advantageous micro/nanostructured surface for O2 diffusion and a high electrocatalytic active surface area. In atomic scale, Ni doping significantly enhances the intrinsic oxygen reduction reaction activity per active site. As a result of controlled multiscale structure, the primary zinc-air battery with engineered Ni-doped CoO NSs electrode shows excellent performance with a record-high discharge peak power density of 377 mW cm-2 , and works stable for >400 h at 5 mA cm-2 . Rechargeable zinc-air battery based on Ni-doped CoO NSs affords an unprecedented small charge-discharge voltage of 0.63 V, outperforming state-of-the-art Pt/C catalyst-based device. Moreover, it is shown that Ni-doped CoO NSs assembled into all-solid-state coin cells can power 17 light-emitting diodes and charge an iPhone 7 mobile phone.

Facile synthesis of graphene-like copper oxide nanofilms with enhanced electrochemical and photocatalytic properties in energy and environmental applications.

Novel graphene-like CuO nanofilms are grown on a copper foam substrate by in situ anodization for multifunctional applications as supercapacitor electrodes and photocatalysts for the degradation of dye pollutants. The as-prepared CuO consists of interconnected, highly crystalline, conductive CuO nanosheets with hierarchical open mesopores and a large surface area. The CuO nanofilms supported on a copper foam are employed as freestanding, binder-free electrodes for supercapacitors, which exhibit wonderful electrochemical performance with a large specific capacitance (919 F g(-1) at 1 A g(-1)), an excellent cycling stability (7% capacitance loss after 5000 cycles), and a good rate capability (748 F g(-1) at 30 A g(-1)). The porous CuO nanofilms also demonstrate excellent photocatalytic activities for degradation of methylene blue, with a degradation rate 99% much higher than 54% of the commercial CuO powders after 60 min. This excellent energy storage and photocatalytic performance of the graphene-like CuO nanofilms can open a new avenue for large-scale applications in energy and environmental fields.

Hierarchical Core/Shell NiCo2O4@NiCo2O4 Nanocactus Arrays with Dual-functionalities for High Performance Supercapacitors and Li-ion Batteries.

We report the synthesis of three dimensional (3D) NiCo2O4@NiCo2O4 nanocactus arrays grown directly on a Ni current collector using a facile solution method followed by electrodeposition. They possess a unique 3D hierarchical core-shell structure with large surface area and dual-functionalities that can serve as electrodes for both supercapacitors (SCs) and lithium-ion batteries (LIBs). As the SC electrode, they deliver a remarkable specific capacitance of 1264 F g(-1) at a current density of 2 A g(-1) and ~93.4% of capacitance retention after 5000 cycles at 2 A g(-1). When used as the anode for LIBs, a high reversible capacity of 925 mA h g(-1) is achieved at a rate of 120 mA g(-1) with excellent cyclic stability and rate capability. The ameliorating features of the NiCo2O4 core/shell structure grown directly on highly conductive Ni foam, such as hierarchical mesopores, numerous hairy needles and a large surface area, are responsible for the fast electron/ion transfer and large active sites which commonly contribute to the excellent electrochemical performance of both the SC and LIB electrodes.

Solution growth of NiO nanosheets supported on Ni foam as high-performance electrodes for supercapacitors.

Well-aligned nickel oxide (NiO) nanosheets with the thickness of a few nanometers supported on a flexible substrate (Ni foam) have been fabricated by a hydrothermal approach together with a post-annealing treatment. The three-dimensional NiO nanosheets were further used as electrode materials to fabricate supercapacitors, with high specific capacitance of 943.5, 791.2, 613.5, 480, and 457.5 F g(-1) at current densities of 5, 10, 15, 20, and 25 A g(-1), respectively. The NiO nanosheets combined well with the substrate. When the electrode material was bended, it can still retain 91.1% of the initial capacitance after 1,200 charging/discharging cycles. Compared with Co3O4 and NiO nanostructures, the specific capacitance of NiO nanosheets is much better. These characteristics suggest that NiO nanosheet electrodes are promising for energy storage application with high power demands.

Hierarchical mesoporous nickel cobaltite nanoneedle/carbon cloth arrays as superior flexible electrodes for supercapacitors.

Hierarchical mesoporous NiCo2O4 nanoneedle arrays on carbon cloth have been fabricated by a simple hydrothermal approach combined with a post-annealing treatment. Such unique array nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and desirable cycle life at high rates. When evaluated as an electrode material for supercapacitors, the NiCo2O4 nanoneedle arrays supported on carbon cloth was able to deliver high specific capacitance of 660 F g-1 at current densities of 2 A g-1 in 2 M KOH aqueous solution. In addition, the composite electrode shows excellent mechanical behavior and long-term cyclic stability (91.8% capacitance retention after 3,000 cycles). The fabrication method presented here is facile, cost-effective, and scalable, which may open a new pathway for real device applications.

NiCo2O4 nanostructure materials: morphology control and electrochemical energy storage.

Three types of NiCo2O4 nanostructure, homogeneous NiCo2O4 nanoneedle arrays, heterogeneous NiCo2O4 nanoflake arrays and NiCo2O4 nanoneedle-assembled sisal-like microspheres are synthesized via facile solution methods in combination with thermal treatment. The NiCo2O4 nanoneedle arrays are evaluated as supercapacitor electrodes and demonstrate excellent electrochemical performances with a high specific capacitance (923 F g(-1) at 2 A g(-1)), good rate capability, and superior cycling stability. The superior capacitive performances are mainly due to the unique one dimensional porous nanoneedle architecture, which provides a faster ion/electron transfer rate, improved reactivity, and enhanced structural stability. The fabrication method presented here is facile, cost-effective and scalable, which may open a new pathway for real device applications.