Although rechargeable aqueous zinc-ion batteries have attracted extensive interest due to their environmental friendliness and low cost, they still lack suitable cathodes with high rate capabilities, which are hampered by the intense charge repulsion of bivalent Zn2+ . Here, a novel intercalation pseudocapacitance behavior and ultrafast kinetics of Zn2+ into the unique tunnels of VO2 (B) nanofibers in aqueous electrolyte are demonstrated via in situ X-ray diffraction and various electrochemical measurements. Because VO2 (B) nanofibers possess unique tunnel transport pathways with big sizes (0.82 and 0.5 nm2 along the b- and c-axes) and little structural change on Zn2+ intercalation, the limitation from solid-state diffusion in the vanadium dioxide electrode is eliminated. Thus, VO2 (B) nanofibers exhibit a high reversible capacity of 357 mAh g-1 , excellent rate capability (171 mAh g-1 at 300 C), and high energy and power densities as applied for zinc-ion storage.

Ultrafast Zn2+ Intercalation and Deintercalation in Vanadium Dioxide.

Free-standing two-dimensional (2D) crystals with atomic thickness have attracted extensive attention because of their novel electronic, optical, mechanical and biocompatible properties, and so on. In recent years, the study of atomically thick 2D crystals has mainly focused on the layered materials with weak van der Waals forces between the layers. For the lack of executable synthetic strategies, preparation of atomically thick 2D crystals with a nonlayered structure or quasi-layered structure with relatively strong bonds between the layers is still a great challenge. This review mainly focuses on recent advances in synthetic strategies for atomically thick 2D crystals with a nonlayered structure as well as the quasi-layered structure with relatively strong bonds between the layers. Furthermore, methods for the modulation of the electronic structures of 2D crystals along with assembly and transfer techniques of the 2D crystals are discussed. The key points of each strategy in preparation, electronic structure modulation, assembly and transfer processes are also presented.

Recent advances in free-standing two-dimensional crystals with atomic thickness: design, assembly and transfer strategies

The exponential increase in research focused on two-dimensional (2D) metal oxides has offered an unprecedented opportunity for their use in energy conversion and storage devices, especially for promising next-generation rechargeable batteries, such as lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs), as well as some post-lithium batteries, including lithium–sulfur batteries, lithium–air batteries, etc. The introduction of well-designed 2D metal oxide nanomaterials into next-generation rechargeable batteries has significantly enhanced the performance of these energy-storage devices by providing higher chemically active interfaces, shortened ion-diffusion lengths, and improved in-plane carrier-/charge-transport kinetics, which have greatly promoted the development of nanotechnology and the practical application of rechargeable batteries. Here, the recent progress in the application of 2D metal oxide nanomaterials in a series of rechargeable LIBs, NIBs, and other post lithium-ion batteries is reviewed relatively comprehensively. Current opportunities and future challenges for the application of 2D nanomaterials in energy-storage devices to achieve high energy density, high power density, stable cyclability, etc. are summarized and outlined. It is believed that the integration of 2D metal oxide nanomaterials in these clean energy devices offers great opportunities to address challenges driven by increasing global energy demands.

Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries.

Two-dimensional nanosheets for photoelectrochemical water splitting: Possibilities and opportunities

Under development for next-generation wearable electronics are flexible, knittable, and wearable energy-storage devices with high energy density that can be integrated into textiles. Herein, knittable fiber-shaped zinc-air batteries with high volumetric energy density (36.1 mWh cm-3 ) are fabricated via a facile and continuous method with low-cost materials. Furthermore, a high-yield method is developed to prepare the key component of the fiber-shaped zinc-air battery, i.e., a bifunctional catalyst composed of atomically thin layer-by-layer mesoporous Co3 O4 /nitrogen-doped reduced graphene oxide (N-rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co3 O4 and N-rGO nanosheets, the bifunctional catalyst exhibits high activity and superior durability for oxygen reduction and evolution reactions. Compared to a fiber-shaped zinc-air battery using state-of-the-art Pt/C + RuO2 catalysts, the battery based on these Co3 O4 /N-rGO nanosheets demonstrates enhanced and stable electrochemical performance, even under severe deformation. Such batteries, for the first time, can be successfully knitted into clothes without short circuits under external forces and can power various electronic devices and even charge a cellphone.

Atomically Thin Mesoporous Co 3 O 4 Layers Strongly Coupled with N-rGO Nanosheets as High-Performance Bifunctional Catalysts for 1D Knittable Zinc-Air Batteries

Room-temperature intercalation-deintercalation strategy towards VO2(B) single layers with atomic thickness.

CO2 electroreduction to high-energy-density C2+ products is highly attractive, whereas the C2+ selectivity under industrial current densities is still unsatisfying. Here, an anti-swelling anion exchange ionomer (AEI) was first proposed to optimize the local environment for promoting industrial-current-density CO2-to-C2+ electroreduction. Taking the anti-swelling AEI-modified oxide-derived Cu nanosheets as an example, in situ Raman spectroscopy and contact angle measurements revealed that the OH--accumulated -N(CH3)3+ groups and anti-swelling backbone of AEI could synergistically regulate the local pH level and water content. In situ Fourier-transform infrared spectroscopy and theoretical calculations demonstrated that the higher local pH value could lower the energy barrier for the rate-limiting COCO* hydrogenated to COCOH* from 0.08 to 0.04 eV, thereby boosting the generation of C2+ products. Owing to the anti-swelling backbone, the optimized water content of 3.5% could suppress the competing H2 evolution and hence facilitate the proton-electron transfer step for C2+ production. As a result, the anti-swelling AEI-modified oxide-derived Cu nanosheets achieved a C2+ Faradaic efficiency of 85.1% at a current density up to 800 mA cm-2 with a half-cell power conversion efficiency exceeding 50%, outperforming most reported powder catalysts.

Industrial-Current-Density CO2-to-C2+ Electroreduction by Anti-swelling Anion-Exchange Ionomer-Modified Oxide-Derived Cu Nanosheets.

Here, noble metal doped two-dimensional metal oxide nanosheets are designed to realize selective CO 2 photoreduction to CH 4 . As a prototype, Pd-doped CeO 2 nanosheets are fabricated, where the active sites of Pd δ+ (2 < δ < 4) and Ce 3+ -O v are revealed by quasi in situ X-ray photoelectron spectra and in situ electron paramagnetic resonance spectra. Moreover, in situ Fourier-transform infrared spectra of D 2 O photodissociation and desorption verify the existence of the Pd-OD bond, implying that Pd δ+ sites can participate in water oxidation to deliver H* species for facilitating the protonation of the intermediates. Furthermore, theoretical calculations suggest the Pd doping could regulate the formation energy barrier of the key intermediates CHO* and CH 3 O*, thus making CO 2 reduction to CH 4 become the favorable process. Accordingly, Pd-doped CeO 2 nanosheets achieve nearly 100% CH 4 selectivity of CO 2 photoreduction, with the raising CH 4 evolution rate of 41.6 μmol g -1 h -1 .

Selective CO2 Photoreduction to CH4 via Pdᵟ+-assisted Hydrodeoxygenation over CeO2 Nanosheets.

Designing catalysts with high selectivity toward C2 products in CO2 electroreduction is crucial to energy storage and sustainable development. Here, we propose a Cu foil kinetic model with abundant nanocavities possessing higher reaction rate constant k to steer the ratio of C2H4 to the competing CH4 during CO2 electroreduction. Chemical kinetic simulation demonstrates that the nanocavities could enrich the adsorbed CO surface concentration (θCOad), while the higher k helps to lower the C-C coupling barrier for CO intermediates, thus favoring the formation of C2H4. The commercial Cu foil treated with cyclic voltammetry is used to match this model, displaying a remarkable C2H4/CH4 ratio of 4.11, which is 18 times larger than that on the pristine Cu foil. This work offers a handy strategy for surface modification and provides new insights into the C-C coupling and the C2H4 selectivity in terms of mass transfer flux and energy barrier.

Surface Engineering on Commercial Cu Foil for Steering C2H4/CH4 Ratio in CO2 Electroreduction.

Arduous CO2 activation and sluggish charge transfer retard the photoreduction of CO2 to CH3OH with high efficiency and selectivity. Here, we fabricate ultrathin Ti-doped WO3 nanosheets possessing approving active sites and optimized carrier dynamics as a promising catalyst. Quasi in situ X-ray photoelectron spectroscopy and synchrotron-radiation X-ray absorption near-edge spectroscopy firmly confirm that the true active sites for CO2 reduction are the W sites rather the Ti sites, while the Ti dopants can facilitate charge transfer, which accelerates the generation of crucial COOH* intermediates as revealed by in situ Fourier-transform infrared spectroscopy and density functional theory calculations. Besides, the Gibbs free energy calculations also validate that Ti doping can lower the energy barrier of CO2 activation and CH3OH desorption by 0.22 eV and 0.42 eV, respectively, thus promoting the formation of CH3OH. In consequence, the Ti-doped WO3 ultrathin nanosheets show a superior CH3OH selectivity of 88.9% and reach a CH3OH evolution rate of 16.8 μmol g-1 h-1, about 3.3 times higher than that on WO3 nanosheets. This work sheds light on promoting CO2 photoreduction to CH3OH by rational elemental doping.

Zhiqiang Wang

Papers

Room-temperature intercalation-deintercalation strategy towards VO2(B) single layers with atomic thickness.

Industrial-Current-Density CO2-to-C2+ Electroreduction by Anti-swelling Anion-Exchange Ionomer-Modified Oxide-Derived Cu Nanosheets.

Selective CO2 Photoreduction to CH4 via Pdᵟ+-assisted Hydrodeoxygenation over CeO2 Nanosheets.

Surface Engineering on Commercial Cu Foil for Steering C2H4/CH4 Ratio in CO2 Electroreduction.

Ultrathin Ti-doped WO3 nanosheets realizing selective photoreduction of CO2 to CH3OH.