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Manganese oxide materials for high-performance supercapacitors are as popular electrode materials of energy storage devices based on their high theoretical capacitance. However, its development is limited by its poor electrical conductivity and insufficient contact surface area, which causes the supercapacitor to fail to achieve its theoretical specific capacitance. In this paper, unique sea urchin-like MnO2/biomass carbon (BC) composite materials were prepared for supercapacitors, showing the lower resistance compared with pure MnO2, which possesses superior electrochemical performance due to the advances in outstanding electrical conductivity. The single electrode test results show that the composite material achieves a specific capacitance of 205.5 F·g−1 at the current density of 0.5 A·g−1; with the current density increasing by a factor of 20, the supercapacitor loaded with this composite still retained 63.2% of its initial capacitance, showing its high rate performance. Meanwhile, the constructed asymmetric supercapacitor can change the color of electrochromic devices and drive the light of electrochemiluminescent devices, indicating its promising application. This work provided a promising route for the rational construction of multiple dimensioned high-performance electrode materials for use in new energy storage devices. [ABSTRACT FROM AUTHOR]

Colorless‐to‐black switching has attracted widespread attention for smart windows and multifunctional displays because they are more useful to control solar energy. However, it still remains a challenge owing to the tremendous difficulties in the design of completely reverse absorptions in transmissive and colored states. Herein, we report on an electrochemical device that can switch between colorless and black by using the electrochemical process of hybrid organic–inorganic perovskite MAPbBr3, which shows a high integrated contrast ratio of up to 73% from 400 to 800 nm. The perovskite solution can be used as the active layer to assemble the device, showing superior transmittance over the entire visible region in neutral states. By applying an appropriate voltage, the device undergoes reversible switching between colorless and black, which is attributed to the formation of lead and Br2 in the redox reaction induced by the electron transfer process in MAPbBr3. In addition, the contrast ratio can be modulated over the entire visible region by changing the concentration and the applied voltage. These results contribute toward gaining an insightful understanding of the electrochemical process of perovskites and greatly promoting the development of switchable devices. [ABSTRACT FROM AUTHOR]

Selective hydroxylation of benzene to phenol over a visible light driven photocatalyst is a sustainable approach for the green synthesis of phenol. Iron vanadate (FeVO4) nanorods with an intrinsic photo‐Fenton catalytic behavior toward H2O2 activation is a promising candidate photocatalyst for the benzene hydroxylation reaction; however, its photocatalytic performance was greatly limited by the serious leaching of metals during the hydroxylation process. Herein, it has been demonstrated that this issue can be well addressed by the conformably coating of FeVO4 nanorods with an atomically‐precise thickness controlled dense Al2O3 layer (FeVO4@Al2O3) through the atomic layer deposition (ALD) technology. The Al2O3 nanolayer can well protect the FeVO4 nanorods from directly contacting with the reaction solution, thus effectively suppressing the leaching of metals during the photocatalytic hydroxylation process. In addition, the Al2O3 coating on FeVO4 also facilitates the kinetic control of heterogeneous benzene hydroxylation reaction over the catalyst surface, such as the benzene adsorption behavior and the photo‐Fenton activation of H2O2. As a result, FeVO4@Al2O3 exhibited an enhanced photocatalytic performance with a good durability in the long‐term production of phenol. [ABSTRACT FROM AUTHOR]

Selective hydroxylation of benzene to phenol over a visible light driven photocatalyst is a sustainable approach for the green synthesis of phenol. Iron vanadate (FeVO4) nanorods with an intrinsic photo‐Fenton catalytic behavior toward H2O2 activation is a promising candidate photocatalyst for the benzene hydroxylation reaction; however, its photocatalytic performance was greatly limited by the serious leaching of metals during the hydroxylation process. Herein, it has been demonstrated that this issue can be well addressed by the conformably coating of FeVO4 nanorods with an atomically‐precise thickness controlled dense Al2O3 layer (FeVO4@Al2O3) through the atomic layer deposition (ALD) technology. The Al2O3 nanolayer can well protect the FeVO4 nanorods from directly contacting with the reaction solution, thus effectively suppressing the leaching of metals during the photocatalytic hydroxylation process. In addition, the Al2O3 coating on FeVO4 also facilitates the kinetic control of heterogeneous benzene hydroxylation reaction over the catalyst surface, such as the benzene adsorption behavior and the photo‐Fenton activation of H2O2. As a result, FeVO4@Al2O3 exhibited an enhanced photocatalytic performance with a good durability in the long‐term production of phenol. [ABSTRACT FROM AUTHOR]

Nickel compounds, especially Ni(HCO3)2 (here denoted as NiC), have been widely combined with other materials to obtain composites with a more favorable structure that exhibit excellent electrochemical performance as supercapacitors. Unfortunately, the complicated processes for preparing such composites directly restrict their further application. Herein, we prepared a NiC/nickel tetraphosphate (Ni(P4O11)) nanocomposite (NiC/NiP) by introducing ions into the NiC reaction system; this composite can be applied in high-performance supercapacitors. The micromorphology of NiC/NiP material displayed an appropriate combination of NiP nanowires and thin NiC nanosheets, which provide sufficient active sites, short ion diffusion paths and fast ion diffusion speeds. NiC/NiP material exhibited an excellent rate performance of 70.2% retained capacity, although the current was increased by 15 times (1196 F g−1 at 2.0 A g−1 and 840 F g−1 at 30 A g−1). The energy density of a NiC/NiP//active carbon (AC) asymmetric supercapacitor fabricated in 6 M KOH was as much as 39.02 W h kg−1 and 26.67 W h kg−1 under corresponding power densities of 160 W kg−1 and 8000 W kg−1, respectively. The asymmetric supercapacitor delivered a stable cyclic performance of 78% capacitive retention after 5000 continuous charge/discharge cycles. More importantly, a 2.5 V light-emitting diode was lit successfully by two NiC/NiP//AC asymmetric supercapacitors in series. These results confirm that NiC/NiP nanocomposite has great potential in practical applications of electrochemical energy storage devices. [ABSTRACT FROM AUTHOR]

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO 2 through simply controlling the degree of redox reaction process, which could obtain a balance between Mn 3+ /Mn 4+ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO 2 including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO 4 to Na 2 SO 3 . The electrode materials of α-MnO 2 -0.25 with an appropriate Mn 3+ /Mn 4+ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g -1 at 0.5 A g -1 , beyond most reported MnO 2 -based materials. The asymmetric supercapacitors formed from α-MnO 2 -0.25 and activated carbon can present an energy density as high as of 36.33 W h kg -1 at 200 W kg -1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics.

Manganese oxides composed of various valence states Mn x + ( x = 2, 3, and 4) have attracted wide attention as promising electrode materials for asymmetric supercapacitor. However, the poor electrical conductivity limited their performance and application. Appropriate regulation content of Mn x + in mixed-valent manganese oxide can tune the electronic structure and further improve their conductivity and performance. Herein, we prepared manganese oxides with different Mn 2+ /Mn 3+ ratios through an over-reduction (OR) strategy for tuning the internal electron structure of mixed-valent manganese, which could make these material oxides a good platform for researching the structure-property relationships. The Mn 2+ /Mn 3+ ratio of manganese oxide could be precisely tuned from 0.6 to 1.7 by controlling the amount of reducing agent for manipulating the redox processes, where the manganese oxide electrode with the most appropriate Mn 2+ /Mn 3+ ratio, as 1.65 (OR4) exhibits large capacitance (274 F g -1 ) and the assembling asymmetric supercapacitors by combining OR4 (positive) and the commercial activated carbon (as negative) achieved large 2.0 V voltage window and high energy density of 27.7 Wh kg -1 (power density of 500 W kg -1 ). The cycle lifespan of the OR4//AC could keep about 92.9% after 10 000-cycle tests owing to the Jahn-Teller distortion of the Mn(III)O 6 octahedron, which is more competitive compared to other work. Moreover, a red-light-emitting diode (LED) can easily be lit for 15 min by two all-solid supercapacitor devices in a series.

Nickel compounds, especially Ni(HCO 3 ) 2 (here denoted as NiC), have been widely combined with other materials to obtain composites with a more favorable structure that exhibit excellent electrochemical performance as supercapacitors. Unfortunately, the complicated processes for preparing such composites directly restrict their further application. Herein, we prepared a NiC/nickel tetraphosphate (Ni(P 4 O 11 )) nanocomposite (NiC/NiP) by introducing [Formula: see text] ions into the NiC reaction system; this composite can be applied in high-performance supercapacitors. The micromorphology of NiC/NiP material displayed an appropriate combination of NiP nanowires and thin NiC nanosheets, which provide sufficient active sites, short ion diffusion paths and fast ion diffusion speeds. NiC/NiP material exhibited an excellent rate performance of 70.2% retained capacity, although the current was increased by 15 times (1196 F g -1 at 2.0 A g -1 and 840 F g -1 at 30 A g -1 ). The energy density of a NiC/NiP//active carbon (AC) asymmetric supercapacitor fabricated in 6 M KOH was as much as 39.02 W h kg -1 and 26.67 W h kg -1 under corresponding power densities of 160 W kg -1 and 8000 W kg -1 , respectively. The asymmetric supercapacitor delivered a stable cyclic performance of 78% capacitive retention after 5000 continuous charge/discharge cycles. More importantly, a 2.5 V light-emitting diode was lit successfully by two NiC/NiP//AC asymmetric supercapacitors in series. These results confirm that NiC/NiP nanocomposite has great potential in practical applications of electrochemical energy storage devices.