Mechanochemical Pretreated Mn+1AXn (MAX) Phase to Synthesize 2D-Ti3C2Tx MXene Sheets for High-Performance Supercapacitors.

Two-dimensional (2D) MXenes sheet-like micro-structures have attracted attention as an effective electrochemical energy storage material due to their efficient electrolyte/cation interfacial charge transports inside the 2D sheets which results in ultrahigh rate capability and high volumetric capacitance. In this article, Ti3C2Tx MXene is prepared by a combination of ball milling and chemical etching from Ti3AlC2 powder. The effects of ball milling and etching duration on the physiochemical properties are also explored, as well as the electrochemical performance of as-prepared Ti3C2 MXene. The electrochemical performances of 6 h mechanochemically treated and 12 h chemically etched MXene (BM-12H) exhibit an electric double layer capacitance behavior with an enhanced specific capacitance of 146.3 F g-1 compared to 24 and 48 h treated samples. Moreover, 5000-cycle stability tested sample's (BM-12H) charge/discharge show increased specific capacitance due to the termination of the -OH group, intercalation of K+ ion and transformation to TiO2/Ti3C2 hybrid structure in a 3 M KOH electrolyte. Interestingly, a symmetric supercapacitor (SSC) device fabricated in a 1 M LiPF6 electrolyte in order to extend the voltage window up to 3 V shows a pseudocapacitance behavior due to Li on interaction/de-intercalation. In addition, the SSC shows an excellent energy and power density of 138.33 W h kg-1 and 1500 W kg-1, respectively. The ball milling pre-treated MXene exhibited an excellent performance and stability due to the increased interlayer distance between the MXene sheets and intercalation and deintercalation of Li+ ions.

High-performance flexible transparent micro-supercapacitors from nanocomposite electrodes encapsulated with solution processed MoS2 nanosheets.

Two-dimensional molybdenum disulfide (MoS2) nanosheets have emerged as a promising material for transparent, flexible micro-supercapacitors, but their use in electrodes is hindered by their poor electrical conductivity and cycling stability because of restacking. In this paper, we report a novel electrode architecture to exploit electrochemical activity of MoS2 nanosheets. Electrochemically exfoliated MoS2 dispersion was spin coated on mesh-like silver networks encapsulated with a flexible conducting film exhibiting a pseudocapacitive behavior. MoS2 nanosheets were electrochemically active over the whole electrode surface and the conductive layer provided a pathway to transport electrons between the MoS2 and the electrolyte. As the result, the composite electrode achieved a large areal capacitance (89.44 mF cm-2 at 6 mA cm-2) and high energy and power densities (12.42 µWh cm-2 and P = 6043 µW cm-2 at 6 mA cm-2) in a symmetric cell configuration with 3 M KOH solution while exhibiting a high optical transmittance of ~80%. Because the system was stable against mechanical bending and charge/discharge cycles, a flexible micro-supercapacitor that can power electronics at different bending states was realized.

Electrodeposited Trimetallic NiFeW Hydroxide Electrocatalysts for Efficient Water Oxidation.

Tungsten-doped Ni-Fe hydroxides fabricated on a three-dimensional nickel foam through cathodic electrodeposition are proposed as effective oxygen evolution reaction (OER) catalysts for alkaline water oxidation. Incorporating an adequate amount of W into Ni-Fe hydroxides modulates the electronic structure by changing the local environment of Ni and Fe and create oxygen vacancies, resulting in abundant active sites for the OER. The optimized electrocatalyst, with a substantial number of catalytic sites, is found to outperform the well-established 20 wt% Ir/C electrocatalyst. The catalyst only requires small overpotentials of 224 mV and 251 mV to generate current densities of 10 mA cm-2 and 50 mA cm-2 , respectively, at an extremely low Tafel slope. Surface study after long-term chronopotentiometry (ca. 30 h) reveals that the tungsten dopant undergoes reduction to stabilize the Ni and Fe active sites for predominant water oxidation. This research provides new insight to apply optimum amounts of tungsten doping to enable more significant electronic coupling within Ni-Fe for the chemisorption of hydroxy and oxygen intermediates and greatly improved OER activity.

Cation intercalated one-dimensional manganese hydroxide nanorods and hierarchical mesoporous activated carbon nanosheets with ultrahigh capacitance retention asymmetric supercapacitors.

We have reported the electrochemical performance of K+ ion doped Mn(OH)4 and MnO2 nanorods as a positive electrode and a highly porous activated carbon nanosheet (AC) made from Prosopis Juliflora as negative electrode asymmetric supercapacitor (ASC) with high rate capability and capacity retention. The cation K+ doped Mn(OH)4 and MnO2 nanorods with large tunnel sizes allow the electrolyte to penetrate through a well-defined pathway and hence benefits from the intercalation pseudocapacitance and surface redox reactions. As a result, they exhibit good electrochemical performance in neutral aqueous electrolytes. More specifically, the K+-Mn(OH)4 nanorods exhibit higher capacitance values than K+-MnO2 nanorods due to the homogenous distribution of 1D nanorods and optimum amount of OH bonds. The fabricated K+-Mn(OH)4 symmetric electrochemical Pseudocapacitor shows very high energy density of 10.11 Wh/kg and high-power density of 51.04 W/kg over the range of 1.0 V in aqueous electrolyte. The energy density of AC||K+-Mn(OH)4 ASC is improved significantly compared to those of symmetric supercapacitors. The fabricated ASC exhibits a wide working voltage window (1.6 V), high power (143.37 W/kg) and energy densities (41.38 Wh/kg) at 0.2 A g-1, and excellent cycling behavior with 107.3% capacitance retention after 6000 cycles at 2 A g-1 indicating the promising practical applications in electrochemical supercapacitors.

Revealing the Self-Degradation Mechanisms in Methylammonium Lead Iodide Perovskites in Dark and Vacuum.

Organic-inorganic lead halide perovskite phases segregate (and their structures degrade) under illumination, exhibiting a poor stability with hysteresis and producing halide accumulation at the surface.In this work, we observed structural and interfacial dissociation in methylammonium lead iodide (CH3 NH3 PbI3 ) perovskites even under dark and vacuum conditions. Here, we investigate the origin and consequences of self-degradation in CH3 NH3 PbI3 perovskites stored in the dark under vacuum. Diffraction and photoelectron spectroscopic studies reveal the structural dissociation of perovskites into PbI2 , which further dissociates into metallic lead (Pb0 ) and I2- ions, collectively degrading the perovskite stability. Using TOF-SIMS analysis, AuI2- formation was directly observed, and it was found that an interplay between CH3 NH3+ , I3- , and mobile I- ions continuously regenerates more I2- ions, which diffuse to the surface even in the absence of light. Besides, halide diffusion causes a concentration gradient between Pb0 and I2- and creates other ionic traps (PbI2- , PbI- ) that segregate as clusters at the perovskite/gold interface. A shift of the onset of the absorption band edge towards shorter wavelengths was also observed by absorption spectroscopy, indicating the formation of defect species upon aging in the dark under vacuum.

Inhibition of Redox Behaviors in Hierarchically Structured Manganese Cobalt Phosphate Supercapacitor Performance by Surface Trivalent Cations.

The stability and performance of supercapacitor devices are limited by the diffusion-controlled redox process occurring at materials' surfaces. Phosphate-based metal oxides could be effectively used as pseudocapacitors because of their polar nature. However, electrochemical energy storage applications of Mn-Co-based phosphate materials and their related kinetics studies have been rarely reported. In this work, we have reported a morphology-tuned Mn x Co3-x (PO4)2·8H2O (MCP) spinel compound synthesized by a one-step hydrothermal method. Detailed physical and chemical insights of the active material coated on the nickel substrate are examined by X-ray diffraction, field-emission scanning electron microscopy, field-emission transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy analyses. Physiochemical studies reveal that the well-defined redox behavior usually observed in Co2+/Ni2+ surface-terminated compounds is suppressed by reducing the divalent cation density with an increased Co3+ and Mn3+ surface states. A uniform and dense leaflike morphology observed in the MnCo2 phosphate compound with an increased surface area enhances the electrochemical energy storage performance. The high polar nature of P-O bonding formed at the surface leads to a higher rate of polarization and a very low relaxation time, resulting in a perfect square-shaped cyclic voltagram and triangular-shaped galvanostatic charge and discharge curve. We have achieved a highly pseudocapacitive MCP, and it can be used as a vital candidate in supercapacitor energy storage applications.