Two-dimensional (2D) van der Waals (vdW) materials provide the versatile playground to stack two or more vdW layers for creation of superior materials with desired properties. Here we theoretically adopt a twisted stack-engineering of two LaBr 2 monolayers to break space inversion symmetry for ferroelectricity and ultimately multiferroism. The enhancement and reversal of electric polarization are accompanied with the transition from interlayer ferromagnetic and antiferromagnetic orderings, demonstrating an effective magnetoelectric coupling effect with a mechanism dissimilar to that of the conventional multiferroics. Magnetization dynamics simulations show that such magnetic phase transition can excite topologically protected bimeron, and the skyrmion Hall effect can be suppressed by bilayer-bimeron stabilized in both ferromagnetic and antiferromagnetic configurations. Moreover, in the small-angle twisted moiré superlattice, the uniform polarization will evolve into a staggered domain structure, accompanied with the appearance of bimeron, which forms a significant discrepancy with the non-twisted stack-engineered multiferroic LaBr 2 bilayer. This work provides a strategy for 2D multiferroic materials by twisted stack engineering of magnetic single layers.

Since the successful exfoliation of two-dimensional (2D) magnetic CrI3 film, an increasing interest of research is the 2D analog of fascinating physical property of 3D material, of which the most attractive for both fundamental research and practical applications is the achieving effective magnetoelectric coupling and manipulation in 2D van der Waals (vdW) multiferroic heterostructure (HS). Herein, we report the discovery of ferroelectrically tunable orbital reconstruction in alpha-RuCl3/CuInP2S6 2D vdW HSs, enabling the remarkable transitions of magnetic ordering from proximate quantum spin-liquid state to ferromagnetic as well as the easy magnetization axis tuning from in-plane to out-of-plane direction. In addition, Monte Carlo simulation verified that, alpha-RuCl3 would transform into a perpendicular ferromagnetic material with Curie temperature of 89 K when the ferroelectric polarization points to alpha-RuCl3. Furthermore, by analyzing the density of states and the d-orbital-resolved magnetocrystalline anisotropy energy (MAE) of Ru atoms based on the second-order perturbation theory we elucidate that the contribution to MAE from the spin-orbit coupling interaction between d orbitals of Ru atoms show a transition from positive to negative, and ultimately dominating the MAE variation from easy-plane to easy-axis magnetization upon the reversible FE polarization. Therefore, the CuInP2S6 nonvolatile ferroelectric switching enables the nonvolatile electrical control of magnetic ordering and anisotropy. This work paves the way for exploring high-efficiency nanodevices and nonvolatile information storage based on the multiferroic 2D vdW HSs.

AbstractDue to its high gravimetric energy density, abundance, wide applicability, and potential to be a zero‐carbon emission energy carrier, hydrogen is considered to be highly important for future sustainability. Currently, 96% of hydrogen is produced from fossil fuels, with relatively low purity and high associated CO2 emissions. Only 4% of hydrogen is produced by water electrolysis with high purity mainly due to the inefficiency of oxygen evolution reaction (OER). Therefore developing high performance OER electrocatalysts for water electrolysis powered by renewables is an urgent and crucial task for long‐term sustainability. Recently, low‐cost Ni‐Fe (oxy)hydroxides have demonstrated high OER activities and great potentials for hydrogen production through water electrolysis. This review commences with discussion of the mechanism of the OER and OER evaluation criteria. This is followed by a brief history of Ni‐Fe (oxy)hydroxides. Most importantly, the recent development of Ni‐Fe (oxy)hydroxide OER electrocatalysts in terms of material design, synthesis methods, morphology, and electrochemical performance, is comprehensively reviewed. In addition, in‐situ techniques for the characterization of Ni‐Fe (oxy)hydroxide OER electrocatalysts are introduced. Finally, strategies for the rational design of improved Ni‐Fe (oxy)hydroxide OER electrocatalysts for industrial water electrolysis are also discussed.

Water electrolysis powered by renewable electricity will likely be critical to a future hydrogen economy. However, the typical use of strongly acidic or alkaline electrolytes necessitates the use of expensive materials, while bubbles add to capital and operational costs, due to blocking of the electrode surface and the necessary use of pumps and gas liquid separators. Here 'bubble-free' oxygen evolution at mild pH is carried out using an electrocatalyst that mimics photosystem II (PSII). The bubble-free electrode includes a gas extracting Gore-Tex (R) membrane. Edge-functionalised graphene (EFG) is included to mimic the metal-binding local protein environment, and the tyrosine residue, in the oxygen evolving complex (OEC) of PSII, while MnOx and Ca2+ are incorporated to mimic the Mn4CaO5 cluster. Interaction between EFG, MnOx, and Ca2+ results in a significant, 130 mV fall in the overpotential required to drive electrocatalytic oxygen evolution at 10 mA cm(-2), compared to the electrode without these biomimetic components. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Understanding of the operating mechanism of a ‘breathable’ water-splitting electrode, which extracts evolved gas without forming bubbles, is advanced.

Eco-friendly energy harvesters with high output for effectively harvesting mechanical energy over a broad temperature range are highly desirable. Here, a lead-free, flexible, and efficient hybrid energy harvester was demonstrated by the combination of a single electrode triboelectric energy harvester (TEH) and an interdigital electrode piezoelectric energy harvester (PEH), which are based on a 0-3 composite of high temperature BiFeO3-BaTiO3(BF-BT) piezoelectric ceramic particles and polyimide (PI) matrix. With an appropriate external connection, the hybrid piezo-triboelectric energy harvester (P-TEH) generates an open-circuit voltage and a short-circuit current of 175 V and 600 nA, respectively, together with high power density of 4.1 mW cm(-3). This work demonstrates that the combination of the interdigital electrode PEH with TEH can enable high electrical output due to minimum internal interference between the two energy harvesting parts. Of particular significance is that the flexible P-TEH shows great high temperature stability up to 200 degrees C with an open-circuit voltage of 150 V and a short-circuit current of 560 nA, exhibiting excellent potential for using at high temperature.

Regulating the electronic structure of catalysts is the most efficient strategy yet, despite its limitations, to improve their oxygen evolution efficiency. Instead of only adjusting the electronic structure, here we utilize ferroelectric polarization to accelerate the oxygen evolution reaction as well. This is demonstrated on a multiferroic layered perovskite Bi5CoTi3O15 with in-situ grown BiCoO3. Thanks to the superimposed effects of electronic regulation and ferroelectric polarization, the as-prepared multiferroic electrocatalysts are more efficient than the benchmark IrO2 (with a final 320 mV overpotential at the current density of 10 mA cm-2 and a 34 mV dec-1 Tafel slope). This work not only demonstrates a low-cost and high-efficient OER electrocatalyst, but also provides a strategic design for multi-component electrocatalytic material systems by consideration of both spin and polarization degrees of freedom.

A novel approach to achieving high areal capacitance and rate capability is demonstrated, whereby an active material is loaded onto a high surface area, filled Ni foam current collector. Micro/nano Ni-filled Ni foam (MNFNF) current collector was fabricated by initially filling commercial Ni foam with Ni slurry and sintering to yield micro Ni-filled Ni foam, followed by electrochemical deposition of nano Ni. This current collector has a greatly enhanced surface area compared to Ni foam, allowing for high mass loading of active material and thereby high areal capacitance and rate capability. Using NiCo2O4 prepared via hydrothermal reaction followed by annealing as the active material, an outstanding areal capacitance of 29.4 F cm(-2) at 5mA cm (2) discharge was achieved, while a high 80% of this capacitance (i.e. 23.5 F cm (2)) was retained at 50 mA cm (2) discharge, ascribed to the high surface area and high conductivity of the MNFNF current collector. Regarding stability, only a 2% decrease in areal capacitance after 1000 charge/discharge cycles was observed. The approach employed here might be used to enhance the performance of other active materials besides NiCo2O4. (C) 2018 Elsevier Ltd. All rights reserved.

For the first time, a generalised enzyme-catalysed room temperature and atmospheric pressure method for synthesising metal carbonate hydroxides is shown. The enzyme urease was used to catalyse the hydrolysis of urea at room temperature and atmospheric pressure. Product CO32- and OH- anions were separated from urease and used to precipitate low crystallinity metal carbonate hydroxides from solutions containing Ni2+, Co2+, Zn2+ and/or Cu2+ cations. Ni-Co carbonate hydroxides with different Ni2+/Co2+ molar ratios were evaluated as supercapacitor electrodes. An optimised Ni2+/Co2+ molar ratio of 3:1 yielded a specific capacitance of 1499 F g-1 @ 1 A g-1 current density. Combining this material with graphene oxide increased the specific capacitance to 1656 F g-1 @ 1 A g-1 current density. In turn, using this composite as the positive electrode in an asymmetric supercapacitor with activated carbon as the negative electrode yielded a high energy density of 45.8 Wh kg-1 at a power density of 899 W kg-1, and 70% capacitance retention after 10,000 cycles. Our enzyme-catalysed room temperature and atmospheric pressure method may be promising for industrial-scale production of nano-materials for energy conversion and storage.

In this paper we report BiFeO3-Bi2Fe4O9 composite with negative permittivity at about 1 GHz, a necessary step to construct a real metamaterial with both negative permeability and negative permittivity. This purposely designed material is obtained by a self-assembly procedure with the same starting raw material. The composite material shows a dielectric resonance at about 1 GHz. The reason for the resonance is analyzed, based on the equivalent resistance-inductance-capacitance circuit, which is confirmed through the theoretical model. Furthermore, the frequency of the resonance can be easily tuned by varying the composition ratio. Our results show that the composite is applicable in the new field of metamaterials for the miniaturization of antennas. (C) 2017 Elsevier B. V. All rights reserved.

Plane-wave pseudo-potential methods based on density functional theory are employed to investigate the electronic structures, and the magnetic and half-metallic properties of the newly designed quaternary Heusler compounds ZrRhHfZ (Z = Al, Ga, In) without 3d transition metal elements. The calculated results show that ZrRhHfZ (Z = Al, Ga, In) compounds are half-metallic, with 100% spin polarization around the Fermi level. The structural stability of these compounds has been tested from the aspects of their cohesion energy and formation. The spin-flip/half-metallic gaps of ZrRhHfZ (Z = Al, Ga, In) compounds are quite large, with values of 0.2548 eV, 0.3483 eV, and 0.2866 eV, respectively. These compounds show Slater-Pauling behavior, and the total spin magnetic moment per unit cell (Mt) scales with the total number of valence electrons (Z(t)) following the rule: Mt = Z(t) - 18. The magnetization of ZrRhHfZ (Z = Al, Ga, In) compounds mainly comes from the 4d electrons of the Zr atoms and the 5d electrons of the Hf atoms. Furthermore, the effects of uniform strain and tetragonal deformation on the half metallicity has been investigated in detail, which is important for practical application. Finally, we reveal that the half-metallicity can be maintained when the Coulomb interactions are considered. (C) 2017 Elsevier B.V. All rights reserved.

Supercapacitors are important energy storage systems due to their high power densities compared to batteries, giving them unique applications.

The synthesis and characterization of MnCo2O4 nanoflake/graphene nanoplatelets composite is reported here for high performance supercapacitor electrode applications. The MnCo2O4 nanoflakes with different morphologies were synthesized successfully via a hydrothermal technique by changing the amount of NH4F. The MnCo2O4 nanoflakes in combination with the graphene nanoplatelets was deposited on Ni foam using an electrophoretic deposition technique. The as prepared composite electrode showed superior performance in terms of specific capacitance and cycling stability, as compared to the pristine MnCo2O4 system, due to the enhanced electronic conductivity resulted from bond formation between carbon and MnCo2O4, A high specific capacitance of similar to 1268 F g(-1), was observed at 1 mV s(-1) scan rate. Noteworthy cycling stability was observed even at the end of 10,000 cycles of consecutive charging and discharging at a current density of 7.81 Ag-1. (C) 2016 Elsevier B.V. All rights reserved.

In this work, we report the structure and magnetic properties of DyFe0.6Mn0.4O3, which crystallizes in orthorhombic perovskite structure with a space group of Pnma. For DyFe0.6Mn0.4O3, a spin reorientation transition, from the canted antiferromagnetic (AFM) state to a collinear AFM state, is found to occur around 310 K, which is remarkably higher than the corresponding transition in DyFeO3. This will greatly facilitate its practical electronic applications near room temperature. More importantly, the AC susceptibility reveals that its frequency-dependent spin reorientation temperature decreases with increasing frequency, which is totally opposite to that of a spin glass. The relaxation time of this process is also much larger than conventional spin glass. The possible mechanism of this distinctive process is explored, which is based on the characteristic feature of spin reorientation for DyFe0.6Mn0.4O3. (C) 2016 Elsevier B.V. All rights reserved.

In this work, first-principles calculations have been used to investigate the electronic structures, magnetic properties, and half-metallic nature of the newly designed quaternary Heusler compounds ZrVTiAl and ZrVTiGa.

NH4F was used as a vital additive to control the morphology of Co3O4 precursors through hydrothermal reaction, some novel growth mechanisms are proposed. Co3O4 materials were obtained via thermal decomposition for supercapacitor application.

Fully compensated ferrimagnetic spin-gapless semiconductors (FCF-SGSs) have been recently proposed as a new class of materials for potential applications in spintronic devices. In this work, based on first-principles calculations, we have designed a new half-metallic fully compensated ferrimagnet (HM-FCF), Zr2MnAl, with a total magnetic moment that satisfies the M-t = Z(t) - 18 rule (where M-t is the total magnetic moment per unit cell and Z(t) is the total number of valence electrons). Importantly, uniform strains can drive Zr2MnAl to display diverse electronic and magnetic properties, from nonmagnetic semiconductor (NM)/fully compensated ferrimagnetic semiconductor (FCFS) -> HM-FCF -> FCF-SGS -> FCFS -> FCF-SGS -> HM-FCF -> metallic ferrimagnet (MFi) transitions, indicating high tunability of its electrons and magnetism near the Fermi surface. Our results show that Zr2MnAl not only is a potential candidate spin filter, but also can cater for various applications in spintronics devices subjected to strain tuning. Finally, the structural stability of Zr2MnAl and a possible rule to design FCF-SGSs are also discussed in detail. (C) 2016 Elsevier B.V. All rights reserved.

Plane-wave pseudo-potential methods based on density functional theory are employed to investigate the electronic structures, magnetic properties of newly designed DO3-type XO3 (X=Li, Na, K and Rb) compounds. Result shows they are d0 HM ferromagnets with total magnetic moment of 5.00 μB. Importantly, the d0 HM characteristic is originated from the polarization of the p-orbitals of O atoms in these hypothetical compounds. The structure stability in the aspects of cohesion energy and formation energy of these four compounds have been tested. The spin-flip gaps of the four XO3 compounds are quite large (>1.00 eV). Furthermore, the d0 HM behavior can be maintained in a wide range of lattice constants.

Ultrathin Co-Co(OH)(2) composite nanoflakes have been fabricated through electrodeposition on 3D nickel foam. As electrochemical capacitor electrodes, they exhibit a high specific capacitance of 1000 F g(-1) at the scan rate of 5 mV s(-1) and 980 F g(-1) at the current density of 1 A g(-1), respectively, and the retention of capacitance is 91% after 5000 cycles.