Metal‐organic frameworks (MOFs) are promising porous precursors for the construction of various functional materials for high‐performance electrochemical energy storage and conversion. Herein, a facile two‐step solution method to rational design of a novel electrode of hollow NiCo2O4 nanowall arrays on flexible carbon cloth substrate is reported. Uniform 2D cobalt‐based wall‐like MOFs are first synthesized via a solution reaction, and then the 2D solid nanowall arrays are converted into hollow and porous NiCo2O4 nanostructures through an ion‐exchange and etching process with an additional annealing treatment. The as‐obtained NiCo2O4 nanostructure arrays can provide rich reaction sites and short ion diffusion path. When evaluated as a flexible electrode material for supercapacitor, the as‐fabricated NiCo2O4 nanowall electrode shows remarkable electrochemical performance with excellent rate capability and long cycle life. In addition, the hollow NiCo2O4 nanowall electrode exhibits promising electrocatalytic activity for oxygen evolution reaction. This work provides an example of rational design of hollow nanostructured metal oxide arrays with high electrochemical performance and mechanical flexibility, holding great potential for future flexible multifunctional electronic devices.

A flexible quasi-solid-state Ni-Zn battery is developed by using tiny ZnO nanoparticles and porous ultrathin NiO nanoflakes conformally deposited on hierar chical carbon-cloth-carbon-fiber (CC-CF) as the anode (CC-CF@ZnO) and cathode (CC-CF@NiO), respectively. The device is able to deliver high performance (absence of Zn dendrite), superior to previous reports on aqueous Ni-Zn batteries and other flexible electrochemical energy-storage devices.

Highly active and durable air cathodes to catalyze both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are urgently required for rechargeable metal-air batteries. In this work, an efficient bifunctional oxygen catalyst comprising hollow Co O nanospheres embedded in nitrogen-doped carbon nanowall arrays on flexible carbon cloth (NC-Co O /CC) is reported. The hierarchical structure is facilely derived from a metal-organic framework precursor. A carbon onion coating constrains the Kirkendall effect to promote the conversion of the Co nanoparticles into irregular hollow oxide nanospheres with a fine scale nanograin structure, which enables promising catalytic properties toward both OER and ORR. The integrated NC-Co O /CC can be used as an additive-free air cathode for flexible all-solid-state zinc-air batteries, which present high open circuit potential (1.44 V), high capacity (387.2 mAh g , based on the total mass of Zn and catalysts), excellent cycling stability and mechanical flexibility, significantly outperforming Pt- and Ir-based zinc-air batteries.

Supercapacitor with ultrahigh energy density (e.g., comparable with those of rechargeable batteries) and long cycling ability (>50000 cycles) is attractive for the next-generation energy storage devices. The energy density of carbonaceous material electrodes can be effectively improved by combining with certain metal oxides/hydroxides, but many at the expenses of power density and long-time cycling stability. To achieve an optimized overall electrochemical performance, rationally designed electrode structures with proper control in metal oxide/carbon are highly desirable. Here we have successfully realized an ultrahigh-energy and long-life supercapacitor anode by developing a hierarchical graphite foam-carbon nanotube framework and coating the surface with a thin layer of iron oxide (GF-CNT@Fe2O3). The full cell of anode based on this structure gives rise to a high energy of ∼74.7 Wh/kg at a power of ∼1400 W/kg, and ∼95.4% of the capacitance can be retained after 50000 cycles of charge-discharge. These performance features are superior among those reported for metal oxide based supercapacitors, making it a promising candidate for the next generation of high-performance electrochemical energy storage.

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Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and core-branch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The self-supported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.

This study reports the design and fabrication of ultrathin MoS 2 nanosheets@metal organic framework-derived N-doped carbon nanowall array hybrids on flexible carbon cloth (CC@CN@MoS 2 ) as a free-standing anode for high-performance sodium ion batteries. When evaluated as an anode for sodium ion battery, the as-fabricated CC@CN@MoS 2 electrode exhibits a high capacity (653.9 mA h g −1 of the second cycle and 619.2 mA h g −1 after 100 cycles at 200 mA g −1 ), excellent rate capability, and long cycling life stability (265 mA h g −1 at 1 A g −1 after 1000 cycles). The excellent electrochemical performance can be attributed to the unique 2D hybrid structures, in which the ultrathin MoS 2 nanosheets with expanded interlayers can provide shortened ion diffusion paths and favorable Na + insertion/extraction space, and the porous N-doped carbon nanowall arrays on flexible carbon cloth are able to improve the conductivity and maintain the structural integrity. Moreover, the N-doping-induced defects also make them favorable for the effective storage of sodium ions, which enables the enhanced capacity and rate performance of MoS 2 .

Exploration of cheap, efficient, and highly durable transition-metal-based electrocatalysts is critically important for the renewable energy chain, including both energy storage and energy conversion. Herein, we have developed cobalt (Co) single atoms anchored in porous nitrogen-doped carbon nanoflake arrays, synthesized from Co-MOF precursor and followed by removal of the unwanted Co clusters. The well-dispersed Co single atoms are attached to the carbon network through N–Co bonding, where there is extra porosity and active surface area created by the removal of the Co metal clusters. Interestingly, compared with those electrocatalysts containing excess Co nanoparticles, a single Co atom alone demonstrates a lower oxygen evolution reaction (OER) overpotential and much higher oxygen reduction reaction (ORR) saturation current, showing that the Co metal clusters are redundant in driving both OER and ORR. Given the bifunctional electrocatalytic activity and mechanical flexibility, the electrocatalyst assembled on carbon cloth is employed as the air cathode in a solid-state Zn–air battery, which presents good cycling stabilities (2500 min, 125 cycles) as well as a high open circuit potential (1.411 V).