Controllable Design of Metal-Organic Framework-Derived Vanadium Oxynitride for High-Capacity and Long-Cycle Aqueous Zn-Ion Batteries.

Introducing N atoms in vanadium oxides (VOx) of aqueous Zn-ion batteries (ZIBs) can reduce their bandgap energy and enhance their electronic conductivity, thereby promoting the diffusion of Zn2+. The close-packed vanadium oxynitride (VON) generated often necessitates the intercalation of water molecules for restructuring, rendering it more conducive for zinc ion intercalation. However, its dense structure often causes structural strain and the formation of by-products during this process, resulting in decreased electrochemical performance. Herein, carbon-coated porous V2O3/VN nanosheets (p-VON@C) are constructed by annealing vanadium metal-organic framework in an ammonia-contained environment. The designed p-VON@C nanosheets are efficiently converted to low-crystalline hydrated N-doped VOx during subsequent activation while maintaining structural stability. This is because the V2O3/VN heterojunction and abundant oxygen vacancies in p-VON@C alleviate the structural strain during water molecule intercalation, and accelerate the intercalation rate. Carbon coating is beneficial to prevent p-VON@C from sliding or falling off during the activation and cycling process. Profiting from these advantages, the activated p-VON@C cathode delivers a high specific capacity of 518 mAh g-1 at 0.2 A g-1 and maintains a capacity retention rate of 80.9% after 2000 cycles at 10 A g-1. This work provides a pathway to designing high-quality aqueous ZIB cathodes.

Activating Inert Crystal Face via Facet-Dependent Quench-Engineering for Electrocatalytic Water Oxidation.

Developing a facile strategy to activate the inert crystal face of an electrocatalyst is critical to full-facet utilization, yet still challenging. Herein, the electrocatalytic activity of the inert crystal face is activated by quenching Co3O4 cubes and hexagonal plates with different crystal faces in Fe(NO3)3 solution, and the regulation mechanism of facet-dependent quench-engineering is further revealed. Compared to the Co3O4 cube with exposed {100} facet, the Co3O4 hexagonal plate with exposed {111} facet is more responsive to quenching, accompanied by a rougher surface, richer defect, and more Fe doping. Theoretical calculations indicate that the {111} facet has a more open structure with lower defect formation energy and Fe doping energy, ensuring its electronic and coordination structure is easier to optimize. Therefore, quench-engineering largely increases the catalytic activity of {111) facet for oxygen evolution reaction by 13.2% (the overpotential at 10 mA cm-2 decreases from 380 to 330 mV), while {100} facet only increases by 7.6% (from 393 to 363 mV). The quenched Co3O4 hexagonal plate exhibits excellent electrocatalytic activity and stability in both zinc-air battery and water-splitting. The work reveals the influence mechanism of crystal face on quench-engineering and inspires the activation of the inert crystal face.

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion.

Metal-organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

Tailoring Metal-Oxygen Bonds Boosts Oxygen Reaction Kinetics for High-Performance Zinc-Air Batteries.

Metal-oxygen bonds significantly affect the oxygen reaction kinetics of metal oxide-based catalysts but still face the bottlenecks of limited cognition and insufficient regulation. Herein, we develop a unique strategy to accurately tailor metal-oxygen bond structure via amorphous/crystalline heterojunction realized by ion-exchange. Compared with pristine amorphous CoSnO3-y, iron ion-exchange induced amorphous/crystalline structure strengthens the Sn-O bond, weakens the Co-O bond strength, and introduces additional Fe-O bond, accompanied by abundant cobalt defects and optimal oxygen defects with larger pore structure and specific surface area. The optimization of metal-oxygen bond structure is dominated by the introduction of crystal structure and further promoted by the introduction of Fe-O bond and rich Co defect. Remarkably, the Fe doped amorphous/crystalline catalyst (Co1-xSnO3-y-Fe0.021-A/C) demonstrates excellent oxygen evolution reaction and oxygen reduction reaction activities with a smaller potential gap (ΔE = 0.687 V), and the Zn-air battery based with Co1-xSnO3-y-Fe0.021-A/C exhibits excellent output power density, cycle performance, and flexibility.

Blocking Interfacial Proton Transport via Self-Assembled Monolayer for Hydrogen Evolution-Free Zinc Batteries.

Aqueous Zn-ion batteries (ZIBs) are promising next-generation energy storage devices, yet suffer from the issues of hydrogen evolution reaction (HER) and intricate side reactions on the Zn anode surface. The hydrogen (H)-bond networks play a critical role in interfacial proton transport that may closely relate to HER but are rarely investigated. Herein, we report a self-assembled monolayer (SAM) strategy which is constructed by anchoring ionic liquid cations on Ti3C2Tx substrate for HER-free Zn anode. Molecule dynamics simulations reveal that the rationally designed SAM with a high coordination number of water molecules (25-27, 4-6 for Zn2+) largely reduces the interfacial densities of H2O molecules, therefore breaking the connectivity of H-bond networks and blocking proton transport on the interface, by which the HER is suppressed. Then, a series of in situ characterizations demonstrate that negligible amounts of H2 gas are collected from the Zn@SAM-MXene anode. Consequently, the symmetric cell enables a long-cycling life of 3000 h at 1 mA cm-2 and 1000 h at 5 mA cm-2. More significantly, the stable Zn@SAM-MXene films are successfully used for coin full cells showing high-capacity retention of over 94 % after 1000 cycles and large-area (10×5 cm2) pouch cells with desired performance.

Chloride-Free Electrolytes for High-Voltage Magnesium Metal Batteries: Challenges, Strategies, and Perspectives.

The demand for high-energy-density and safe energy storage devices has spurred increasing interest in high-voltage rechargeable magnesium batteries (RMB). As electrolytes are the bridge connecting the cathode and anode materials, the development of high-voltage electrolytes is the key factor in realizing high-voltage RMBs. This concept presents an overview of three chloride-free electrolyte systems with wide electrochemical windows, together with the degradation mechanisms and modification strategies at the anode/electrolyte interphase. Finally, future directions in stabilizing Mg anodes and realizing high-voltage RMBs are highlighted.

Interfacial Engineering of Magnesiophilic Coordination Layer Stabilizes Mg Metal Anode.

Rechargeable magnesium batteries (RMBs) are seriously plagued by the direct exposure of the Mg anode to the electrolyte components, leading to spontaneous and electrochemical side reactions and interfacial passivation. Herein, a benign coordination layer is constructed at the Mg/electrolyte interface where aniline with a strong magnesiophilic amine group and high stability to Mg is chosen as representative, which has higher adsorption energy than DME (1,2-dimethoxyethane) and trace water. This Mg coordination environment mitigates side reactions, forming a non-passivating interface consisting of aniline and much fewer by-products after several cycles. Therefore, the Mg symmetrical cell operates with a low overpotential and uniform Mg0 deposition. This interfacial coordination can also be adopted for Mg anode protection in various electrolyte cases of Mg(TFSI)2 electrolyte systems.

Voltage-Mediated Water Dynamics Enables On-Demand Transport of Sugar Molecules in Two-Dimensional Channels.

The constructing of artificial channels with gating functions is an important undertaking for gaining insight into biological process and achieving efficient bionic functions. Typically, controllable transport within such channels relies on either electrostatic or specific interactions between the transporting species and the channel. However, for molecules with weak interactions with the channel, achieving precise gating of the transport remains a significant challenge. In this regard, this study proposes a voltage gating membrane of two-dimensional channels that selectively transport of neutral molecules glucose with a dimension of 0.60 nm. The permeation of glucose is switched on/off by electrochemically manipulating the water dynamics in the nanochannel. Voltage driven-intercalation of ion into the two-dimensional channel causes water to stratify and move closer to the channel walls, thereby resulting in the channel center being emptier for glucose diffusion. Due to the sub-nanometer size dimension of the channel, selective permeation of glucose over sucrose is also achieved in this approach.

Charging Metal-Organic Framework Membranes by Incorporating Crown Ethers to Capture Cations for Ion Sieving.

Protein channels on the biofilm conditionally manipulate ion transport via regulating the distribution of charge residues, making analogous processes on artificial membranes a hot spot and challenge. Here, we employ metal-organic frameworks (MOFs) membrane with charge-adjustable subnano-channel to selectively govern ion transport. Various valent ions are binded with crown ethers embedded in the MOF cavity, which act as charged guest to regulate the channels' charge state from the negativity to positivity. Compared with the negatively charged channel, the positive counterpart obviously enhances Li+ /Mg2+ selectivity, which benefit from the reinforcement of the electrostatic repulsion between ions and the channel. Meanwhile, theoretical calculations reveal that Mg2+ transport through the more positively charged channel needed to overcome higher entrance energy barrier than that of Li+ . This work provides a subtle strategy for ion-selective transport upon regulating the charge state of insulating membrane, which paves the way for the application like seawater desalination and lithium extraction from salt lakes.

Quench-Induced Surface Engineering Boosts Alkaline Freshwater and Seawater Oxygen Evolution Reaction of Porous NiCo2 O4 Nanowires.

The electrochemical oxygen evolution reaction (OER) by efficient catalysts is a crucial step for the conversion of renewable energy into hydrogen fuel, in which surface/near-surface engineering has been recognized as an effective strategy for enhancing the intrinsic activities of the OER electrocatalysts. Herein, a facile quenching approach is demonstrated that can simultaneously enable the required surface metal doping and vacancy generation in reconfiguring the desired surface of the NiCo2 O4 catalyst, giving rise to greatly enhanced OER activities in both alkaline freshwater and seawater electrolytes. As a result, the quenched-engineered NiCo2 O4 nanowire electrode achieves a current density of 10 mA cm-2 at a low overpotential of 258 mV in 1 m KOH electrolyte, showing the remarkable catalytic performance towards OER. More impressively, the same electrode also displays extraordinary activity in an alkaline seawater environment and only needs 293 mV to reach 10 mA cm-2 . Density functional theory (DFT) calculations reveal the strong electronic synergies among the metal cations in the quench-derived catalyst, where the metal doping regulates the electronic structure, thereby yielding near-optimal adsorption energies for OER intermediates and giving rise to superior activity. This study provides a new quenching method to obtain high-performance transition metal oxide catalysts for freshwater/seawater electrocatalysis.

Tailoring Coordination in Conventional Ether-Based Electrolytes for Reversible Magnesium-Metal Anodes.

Rechargeable magnesium (Mg) batteries based on conventional electrolytes are seriously plagued by the formation of the ion-blocking passivation layer on the Mg metal anode. By tracking the Mg2+ solvation sheath, this work links the passivation components to the Mg2+ -solvents (1,2-dimethoxyethane, DME) coordination and the consequent thermodynamically unstable DME molecules. On this basis, we propose a methodology to tailor solvation coordination by introducing the additive solvent with extreme electron richness. Oxygen atoms in phosphorus-oxygen groups compete with that in carbon-oxygen groups of DME for the coordination with Mg2+ , thus softening the solvation sheath deformation. Meanwhile, the organophosphorus molecules in the rearranged solvation sheath decompose on the Mg surface, increasing the Mg2+ transport and electrical resistance by three and one orders of magnitude, respectively. Consequently, the symmetric cells exhibit superior cycling performance of over 600 cycles with low polarization.

Activating Metal Oxides Nanocatalysts for Electrocatalytic Water Oxidation by Quenching-Induced Near-Surface Metal Atom Functionality.

Developing a reliable strategy for the modulation of the texture, composition, and electronic structure of electrocatalyst surfaces is crucial for electrocatalytic performance, yet still challenging. Herein, we develop a facile and universal strategy, quenching, to precisely tailor the surface chemistry of metal oxide nanocatalysts by rapidly cooling them in a salt solution. Taking NiMoO4 nanocatalysts an example, we successfully produce the quenched nanocatalysts offering a greatly reduced oxygen evolution reaction (OER) overpotential by 85 mV and 135 mV at 10 mA cm-2 and 100 mA cm-2 respectively. Through detailed characterization studies, we establish that quenching induces the formation of numerous disordered stepped surfaces and the near-surface metal ions doping, thus regulating the local electronic structures and coordination environments of Ni, Mo, which promotes the formation of the dual-site active and thereby affords a low energy pathway for OER. This quenching strategy is also successfully applied to a number of other metal oxides, such as spinel-type Co3O4, Fe2O3, LaMnO3, and CoSnO3, with similar surface modifications and gains in OER activity. Our finding provides a new inspiration to activate metal oxide catalysts and extends the use of quenching chemistry in catalysis.

Flexible and High-Voltage Coaxial-Fiber Aqueous Rechargeable Zinc-Ion Battery.

Extensive efforts have been devoted to construct a fiber-shaped energy-storage device to fulfill the increasing demand for power consumption of textile-based wearable electronics. Despite the myriad of available material selections and device architectures, it is still fundamentally challenging to develop eco-friendly fiber-shaped aqueous rechargeable batteries (FARBs) on a single-fiber architecture with high energy density and long-term stability. Here, we demonstrate flexible and high-voltage coaxial-fiber aqueous rechargeable zinc-ion batteries (CARZIBs). By utilizing a novel spherical zinc hexacyanoferrate with prominent electrochemical performance as cathode material, the assembled CARZIB offers a large capacity of 100.2 mAh cm-3 and a high energy density of 195.39 mWh cm-3, outperforming the state-of-the-art FARBs. Moreover, the resulting CARZIB delivers outstanding flexibility with the capacity retention of 93.2% after bending 3000 times. Last, high operating voltage and output current are achieved by the serial and parallel connection of CARZIBs woven into the flexible textile to power high-energy-consuming devices. Thus, this work provides proof-of-concept design for next-generation wearable energy-storage devices.

Twinned Tungsten Carbonitride Nanocrystals Boost Hydrogen Evolution Activity and Stability.

Synergistic integration of two active metal-based compounds can lead to much higher electrocatalytic activity than either of the two individually, due to the interfacial effects. Herein, a proof-of-concept strategy is creatively developed for the successful fabrication of twinned tungsten carbonitride (WCN) nanocrystals, where W2 C and WN are chemically bonded at the molecule level. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy analyses demonstrate that the intergrowth of W2 C and WN in the WCN nanocrystals produces abundant N-W-C interfaces, leading to a significant enhancement in catalytic activity and stability for hydrogen evolution reaction (HER). Indeed, it shows 14.2 times higher and 140 mV lower in the respective turn-over frequency (TOF) and overpotential at 10 mA cm-2 compared to W2 C alone. To complement the experimental observation, the theoretical calculations demonstrate that the WCN endows more favorable hydrogen evolution reaction than the single W2 C or WN crystals due to abundant interfaces, beneficial electronic states, lower work function, and more active W sites at the N-W-C interfaces.

Freestanding Carbon Nanotube Film for Flexible Straplike Lithium/Sulfur Batteries.

Flexible lithium/sulfur (Li/S) batteries are promising to meet the emerging power demand for flexible electronic devices. The key challenge for a flexible Li/S battery is to design a cathode with excellent electrochemical performance and mechanical flexibility. In this work, a flexible strap-like Li/S battery based on a S@carbon nanotube/Pt@carbon nanotube hybrid film cathode was designed. It delivers a specific capacity of 1145 mAh g-1 at the first cycle and retains a specific capacity of 822 mAh g-1 after 100 cycles. Moreover, the flexible Li/S battery retains stabile specific capacity and Coulombic efficiency even under severe bending conditions. As a demonstration of practical applications, an LED array is shown stably powered by the flexible Li/S battery under flattened and bent states. We also use the strap-like flexible Li/S battery as a real strap for a watch, which at the same time provides a reliable power supply to the watch.

In situ electrochemical oxidation of electrodeposited Ni-based nanostructure promotes alkaline hydrogen production.

Highly active and stable electrocatalysts based on non-precious metals for hydrogen evolution reaction (HER) in alkaline solution are urgently required for enabling mass production of clean hydrogen in industry. Herein, core-shell NiOOH/Ni nanoarchitectures supported on the conductive carbon cloth have been successfully prepared by a facile electrodeposition process of Ni, and a subsequent in situ electrochemical oxidation. When explored as an alkaline HER electrocatalyst, the as-synthesized NiOOH/Ni nanoarchitecture requires only a low overpotential of ∼111 mV to attain a current density of -10 mA cm-2, demonstrating its strong catalytic capability of hydrogeneration. The excellent HER activity could well be attributed to the decreasing charge transfer resistance and competitive electrochemical active area of the amorphous NiOOH, compared with inactive Ni substrate. The feasible methodology established in this study can be easily expanded to obtain a series of nano-sized metal oxyhydroxide materials for various energy conversion and storage applications, where Ni-based nanomaterials are among the highly active ones.

In Situ Electrochemically Derived Amorphous-Li2 S for High Performance Li2 S/Graphite Full Cell.

High-capacity Li2 S cathode (1166 mAh g-1 ) is regarded as a promising candidate for the next-generation lithium ion batteries. However, its high potential barrier upon the initial activation process leads to a low utilization of Li2 S. In this work, a Li2 S/graphite full cell with the zero activation potential barrier is achieved through an in situ electrochemical conversion of Li2 S8 catholyte into the amorphous Li2 S. Theoretical calculations indicate that the zero activation potential for amorphous Li2 S can be ascribed to its lower Li extraction energy than that of the crystalline Li2 S. The constructed Li2 S/graphite full cell delivers a high discharge capacity of 1006 mAh g-1 , indicating a high utilization of the amorphous Li2 S as a cathode. Moreover, a long cycle life with 500 cycles for this Li2 S/graphite full cell is realized. This in situ electrochemical conversion strategy designed here is inspired for developing high energy Li2 S-based full cells in future.

Ultra-Broadband Flexible Photodetector Based on Topological Crystalline Insulator SnTe with High Responsivity.

Topological crystalline insulators (TCIs) are predicted to be a promising candidate material for ultra-broadband photodetectors ranging from ultraviolet (UV) to terahertz (THz) due to its gapless surface state and narrow bulk bandgap. However, the low responsivity of TCIs-based photodetectors limits their further applications. In this regard, a high-performance photodetector based on SnTe, a recently developed TCI, working in a broadband wavelength range from deep UV to mid-IR with high responsivity is reported. By taking advantage of the strong light absorption and small bandgap of SnTe, photodetectors based on the as-grown SnTe crystalline nanoflakes as well as specific short channel length achieve a high responsivity (71.11 A W-1 at 254 nm, 49.03 A W-1 at 635 nm, 10.91 A W-1 at 1550 nm, and 4.17 A W-1 at 4650 nm) and an ultra-broad spectral response (254-4650 nm) simultaneously. Moreover, for the first time, a durable flexible SnTe photodetector fabricated directly on a polyethylene terephthalate film is demonstrated. These results prove the great potential of TCIs as a promising material for integrated and flexible optoelectronic devices.

Wrapping Aligned Carbon Nanotube Composite Sheets around Vanadium Nitride Nanowire Arrays for Asymmetric Coaxial Fiber-Shaped Supercapacitors with Ultrahigh Energy Density.

The emergence of fiber-shaped supercapacitors (FSSs) has led to a revolution in portable and wearable electronic devices. However, obtaining high energy density FSSs for practical applications is still a key challenge. This article exhibits a facile and effective approach to directly grow well-aligned three-dimensional vanadium nitride (VN) nanowire arrays (NWAs) on carbon nanotube (CNT) fiber with an ultrahigh specific capacitance of 715 mF/cm2 in a three-electrode system. Benefiting from their intriguing structural features, we successfully fabricated a prototype asymmetric coaxial FSS (ACFSS) with a maximum operating voltage of 1.8 V. From core to shell, this ACFSS consists of a CNT fiber core coated with VN@C NWAs as the negative electrode, Na2SO4 poly(vinyl alcohol) (PVA) as the solid electrolyte, and MnO2/conducting polymer/CNT sheets as the positive electrode. The novel coaxial architecture not only fully enables utilization of the effective surface area and decreases the contact resistance between the two electrodes but also, more importantly, provides a short pathway for the ultrafast transport of axial electrons and ions. The electrochemical results show that the optimized ACFSS exhibits a remarkable specific capacitance of 213.5 mF/cm2 and an exceptional energy density of 96.07 µWh/cm2, the highest areal capacitance and areal energy density yet reported in FSSs. Furthermore, the device possesses excellent flexibility in that its capacitance retention reaches 96.8% after bending 5000 times, which further allows it to be woven into flexible electronic clothes with conventional weaving techniques. Therefore, the asymmetric coaxial architectural design allows new opportunities to fabricate high-performance flexible FSSs for future portable and wearable electronic devices.

Constructing Ultrahigh-Capacity Zinc-Nickel-Cobalt Oxide@Ni(OH)2 Core-Shell Nanowire Arrays for High-Performance Coaxial Fiber-Shaped Asymmetric Supercapacitors.

Increased efforts have recently been devoted to developing high-energy-density flexible supercapacitors for their practical applications in portable and wearable electronics. Although high operating voltages have been achieved in fiber-shaped asymmetric supercapacitors (FASCs), low specific capacitance still restricts the further enhancement of their energy density. This article specifies a facile and cost-effective method to directly grow three-dimensionally well-aligned zinc-nickel-cobalt oxide (ZNCO)@Ni(OH)2 nanowire arrays (NWAs) on a carbon nanotube fiber (CNTF) with an ultrahigh specific capacitance of 2847.5 F/cm3 (10.678 F/cm2) at a current density of 1 mA/cm2, These levels are approximately five times higher than those of ZNCO NWAs/CNTF electrodes (2.10 F/cm2) and four times higher than Ni(OH)2/CNTF electrodes (2.55 F/cm2). Benefiting from their unique features, we successfully fabricated a prototype coaxial FASC (CFASC) with a maximum operating voltage of 1.6 V, which was assembled by adopting ZNCO@Ni(OH)2 NWAs/CNTF as the core electrode and a thin layer of carbon coated vanadium nitride (VN@C) NWAs on a carbon nanotube strip (CNTS) as the outer electrode with KOH poly(vinyl alcohol) (PVA) as the gel electrolyte. A high specific capacitance of 94.67 F/cm3 (573.75 mF/cm2) and an exceptional energy density of 33.66 mWh/cm3 (204.02 µWh/cm2) were achieved for our CFASC device, which represent the highest levels of fiber-shaped supercapacitors to date. More importantly, the fiber-shaped ZnO-based photodetector is powered by the integrated CFASC, and it demonstrates excellent sensitivity in detecting UV light. Thus, this work paves the way to the construction of ultrahigh-capacity electrode materials for next-generation wearable energy-storage devices.