Uniform MnCo2O4.5 porous nanowires and quasi-cubes for hybrid supercapacitors with excellent electrochemical performances

Evaluation of the effect of precursor ratios on the electrochemical performances of binder-free NiMn-phosphate electrodes for supercapattery

Binary metal phosphate electrodes have been widely studied for energy storage applications due to the synergistic effects of two different transition elements that able to provide better conductivity and stability. Herein, the battery-type binder-free nickel-manganese phosphate (NiMn-phosphate) electrodes were fabricated with different Ni:Mn precursor ratios via microwave-assisted hydrothermal technique for 5 min at 90 °C. Overall, NiMn3P electrode (Ni:Mn = 1:3) showed an outstanding electrochemical performance, displaying the highest specific (areal) capacity at 3 A/g of 1262.4 C/g (0.44 C/cm2), and the smallest charge transfer resistance of 108.8 Ω. The enhanced performance of NiMn3P electrode can be ascribed to the fully grown amorphous nature and small-sized flake and flower structures of NiMn3P electrode material on the nickel foam (NF) surface. This configuration offered a higher number of active sites and a larger exposed area, facilitating efficient electrochemical reactions with the electrolyte. Consequently, the NiMn3P//AC electrode combination was chosen to further investigate its performance in supercapattery. The NiMn3P//AC supercapattery exhibited remarkable energy density of 105.4 Wh/kg and excellent cyclic stability with 84.7% retention after 3000 cycles. These findings underscored the superior electrochemical performance of the battery-type binder-free NiMn3P electrode, and highlight its potential for enhancing the overall performance of supercapattery.

High-performance asymmetric supercapacitors assembled with novel disc-like MnCo2O4 microstructures as advanced cathode material

Herein, porous MnCo2O4 with disc-like (MnCo2O4-discs) and ring-like (MnCo2O4-rings) microstructures were respectively synthesized using an initial hydrothermal method at different temperatures and a calcination treatment in air. The electrochemical properties of these MnCo2O4 materials were investigated in three-electrode and two-electrode systems, and as such, MnCo2O4 presented a battery-like electrochemical response. The specific capacity of MnCo2O4-discs was determined to be 296.1C/g at 1 A/g, superior to 246.3C/g for MnCo2O4-rings. An asymmetric supercapacitor (ASC) was assembled with MnCo2O4 as the cathode and activated carbon (AC) as the anode to evaluate the potential for practical application. The MnCo2O4-discs//AC ASC exhibited an energy density (Ed) of 35.8 W h kg-1 at a power density (Pd) of 927.5 W kg-1. For the MnCo2O4-rings//AC ASC, an inferior Ed of 31.4 W h kg-1 under 890.9 W kg-1 was achieved. Furthermore, the two ASCs presented outstanding cyclic performance after 5000 cycles at 6 A/g. The exceptional properties of MnCo2O4 microstructures can be applied to the assembly of ASC devices, which can have promising potential for application in electrochemical energy storage. This synthetic method is straightforward, cost-effective, and can be extended to fabricate similar electrode materials with superior electrochemical performance.

Unraveling the Oxidation Kinetics Through Electronic Structure Regulation of MnCo2O4.5@Ni3S2 p-n Junction for Urea-Assisted Electrocatalytic Activity

A promising strategy to boost electrocatalytic performance is via assembly of hetero-nanostructured electrocatalysts that delivers the essential specific surface area and also active sites by lowering the reaction barrier. However, the challenges associated with the intricate designs and mechanisms remain underexplored. Therefore, the present study constructs a p-n junction in a free-standing MnCo2O4.5@Ni3S2 on Ni-Foam. The space-charge region's electrical characteristics is dramatically altered by the formed p-n junction, which enhances the electron transfer process for urea-assisted electrocatalytic water splitting (UOR). The optimal MnCo2O4.5@Ni3S2 electrocatalyst results in greater oxygen evolution reactivity (OER) than pure systems, delivering an overpotential of only 240 mV. Remarkably, upon employing as UOR electrode the required potential decreases to 30 mV. The impressive performance of the designed catalyst is attributed to the enhanced electrical conductivity, greater number of electrochemical active sites, and improved redox activity due to the junction interface formed between p-MnCo2O4.5 and n-Ni3S2. There are strong indications that the in situ formed extreme-surface NiOOH, starting from Ni3S2, boosts the electrocatalytic activity, i.e., the electrochemical  surface reconstruction generates the active species. In conclusion, this work presents a high-performance p-n junction design for broad use, together with a viable and affordable UOR electrocatalyst.

Synergy of Oxygen Vacancy and Surface Modulation Endows Hollow Hydrangea-like MnCo2O4.5 with Enhanced Capacitive Performance

Surface chemistry and bulk structure jointly play crucial roles in achieving high-performance supercapacitors. Here, the synergistic effect of surface chemistry properties (vacancy and phosphorization) and structure-derived properties (hollow hydrangea-like structure) on energy storage is explored by the surface treatment and architecture design of the nanostructures. The theoretical calculations and experiments prove that surface chemistry modulation is capable of improving electronic conductivity and electrolyte wettability. The structural engineering of both hollow and nanosheets produces a high specific surface area and an abundant pore structure, which is favorable in exposing more active sites and shortens the ion diffusion distance. Benefiting from its admirable physicochemical properties, the surface phosphorylated MnCo2O4.5 hollow hydrangea-like structure (P-MnCoO) delivers a high capacitance of 425 F g-1 at 1 A g-1, a superior capability rate of 63.9%, capacitance retention at 10 A g-1, and extremely long cyclic stability (91.1% after 10,000 cycles). The fabricated P-MnCoO/AC asymmetric supercapacitor achieved superior energy and power density. This work opens a new avenue to further improve the electrochemical performance of metal oxides for supercapacitors.

Zn-doped MnOx nanowires displaying plentiful crystalline defects and tunable small cross-sections for an optimized volcano-type performance towards supercapacitors

MnOx-based nanomaterials are promising large-scale electrochemical energy storage devices due to their high specific capacity, low toxicity, and low cost. However, their slow diffusion kinetics is still challenging, restricting practical applications. Here, a one-pot and straightforward method was reported to produce Zn-doped MnOx nanowires with abundant defects and tunable small cross-sections, exhibiting an outstanding specific capacitance. More specifically, based on a facile hydrothermal strategy, zinc sites could be uniformly dispersed in the α-MnOx nanowires structure as a function of composition (0.3, 2.1, 4.3, and 7.6 wt.% Zn). Such a process avoided the formation of different crystalline phases during the synthesis. The reproducible method afforded uniform nanowires, in which the size of cross-sections decreased with the increase of Zn composition. Surprisingly, we found a volcano-type relationship between the storage performance and the Zn loading. In this case, we demonstrated that the highest performance material could be achieved by incorporating 2.1 wt.% Zn, exhibiting a remarkable specific capacitance of 1082.2 F.g-1 at a charge/discharge current density of 1.0 A g-1 in a 2.0 mol L-1 KOH electrolyte. The optimized material also afforded improved results for hybrid supercapacitors. Thus, the results presented herein shed new insights into preparing defective and controlled nanomaterials by a simple one-step method for energy storage applications.

The synthesis of CoS/MnCo2O4-MnO2 nanocomposites for supercapacitors and energy-saving H2 production

In this study, CoS/MnCo₂O₄-MnO₂ (CMM) nanocomposites were synthesized by hydrothermal and then electrochemical deposition. Their electrochemical properties were systematically investigated for supercapacitors and energy-saving H₂ production. As an electrode material for supercapacitor, CMM demonstrates a specific capacitance of 2320F g^-1 at 1 A/g, and maintains a specific capacitance of 1216F g^-1 at 10 A/g. It also shows 72.8 % capacitance retention after 8000 cycles. The aqueous asymmetric supercapacitor exhibited high energy storage capacity (887.86F g^-1 specific capacitance at a current density of 1 A/g), good rate performance and cycling stability. Besides, CMM shows outstanding urea oxidation reaction(UOR) and glycol oxidation reaction (MOR) performances for H₂ production. Compared to oxygen evolution reaction (OER) (1.635 V) at 20 mA cm^-2, the potentials were reduced by 213 mV for UOR and 233 mV for MOR, respectively. Therefore, this study shows the promising practical applications of CMM nanocomposites for energy storage and energy-saving H₂ production.

Nanosheet-assembled porous MnCo2O4.5 microflowers as electrode material for hybrid supercapacitors and lithium-ion batteries

Herein, the MnCo₂O_4.5 microflowers (MFs) assembled by two-dimensional (2D) porous nanosheets were prepared through an initial solvothermal reaction with a subsequent annealing process. In this architecture, many interconnected 2D thin nanosheets were self-assembled together to form a 3D hierarchical MF with plenty of open channels. Such structure endows these MnCo₂O_4.5 MFs with large specific surface area of 156.85 m²/g for energy storage and provides rich ion diffusion pathways for ion transportation, thus the as-prepared MFs can exhibit good overall electrochemical performance in both hybrid supercapacitor (HSC) and lithium-ion battery (LIB). For the utilization in supercapacitor, the MFs deliver a specific capacity of 287.02 C/g at 1 A/g as well as a rate capability with 73.3 % capacity retention at 8 A/g. The energy density of the HSC assembled by MFs and activated carbon can reach up to 30.33 W h kg^-1 at 959.35 W kg^-1. When applied as the anode for Li-ion battery, a specific capacity of 1340.8 mA h g^-1 at 0.1 A/g and cycling performance with low capacity loss of 0.73 mAh/g per cycle after 200 cycles at 0.5 A/g can be achieved. This work uncovers a repeatable and facile synthetic strategy to prepare transition metal oxides with large specific surface area and good overall electrochemical property.

Construction of MnxCoyO4/Ti electrocatalysts for efficient bifunctional water splitting

In this work, we report the design and synthesis of non-noble metal-based electrocatalysts for effective overall water splitting in alkaline solutions for the development of hydrogen energy. The electrocatalysts were synthesized by a one-step hydrothermal method similar to microflower structure electrocatalysts. The synergistic effect between the special Echinops sphaerocephalus nanostructure and the nanowire can greatly improve the conductivity of the nanomaterial due to its high activity quality, fast ion transport, and exposure of more active sites, thus resulting in a better catalytic activity and a longer material stability of the electrocatalyst. For Mn_xCo_yO₄/Ti in alkaline aqueous solutions, a current density of 10 mA cm^-2 is required when the voltage is only 1.60 V. In addition, the hydrogen evolution activity of electrocatalysts is 168 mV at 10 mA cm^-2, the Tafel slope is 174 mV dec^-1, and the oxygen evolution activity of electrocatalysts is 229 mV at 10 mA cm^-2, which showed good long-term stability within 12 h, even better than that of previously reported electrocatalysts.

Structure-engineering of core-shell ZnCo2O4@NiO composites for high-performance asymmetric supercapacitors

The implementation of a structure-designed strategy to construct hierarchical architectures of multicomponent metal oxide-based electrode materials for energy storage devices is in the limelight. Herein, we report NiO nanoflakes impregnated on ZnCo2O4 nanorod arrays as ZnCo2O4@NiO core-shell structures on a flexible stainless-steel mesh substrate, fabricated by a simple, cost-effective and environmentally friendly reflux condensation method. The core-shell structure of ZnCo2O4@NiO is used as an electrode material in a supercapacitor as it provides a high specific surface area (134.79 m2 g-1) offering high electroactive sites for a redox reaction, reduces the electron and ion diffusion path, and promotes an efficient contact between the electroactive material and electrolyte. The binder-free ZnCo2O4@NiO electrode delivers a high specific capacitance of 882 F g-1 at 4 mA cm-2 current density and exhibits remarkable cycling stability (∼85% initial capacitance retention after 5000 charge-discharge cycles at 10 mA cm-2). The asymmetric supercapacitor device ZnCo2O4@NiO//rGO delivered a maximum energy density of 46.66 W h kg-1 at a power density of 800 W kg-1. The device exhibited 90.20% capacitance retention after 4000 cycles. These results indicate that the ZnCo2O4@NiO architecture electrode is a promising functional material for energy storage devices.

PPy coated nanoflower like CuCo2O4based onin situgrowth of nanoporous copper for high-performance supercapacitor electrodes

General CuCo₂O₄electrodes suffer a very low reversible capacity and poor cycling stability because of easily fading phenomena and volume change during cycling. To optimize the electrode, a facile method is conducted to fabricate a novel electrode of Cu@CuCo₂O₄@polypyrrole nanoflowers. Due to larger specific surface area and more electrochemical reactive areas of CuCo₂O₄@polypyrrole nanoflowers, the pseudocapacitance of thein situgrown CuCo₂O₄@polypyrrole (912 F g^-1at 2 A g^-1) is much higher than the pristine CuCo₂O₄(618 F g^-1at 2 A g^-1). Remarkably, the CuCo₂O₄@polypyrrole (cathode) and active carbon (anode) are used to assemble an asymmetric supercapacitor, which exhibits a relatively high energy density of 90 Wh kg^-1at a power density of 2519 W kg^-1and 35 Wh kg^-1at a high-power density of 9109 W kg^-1, and excellent cycling stability (about 90.4% capacitance retention over 10 000 cycles). The prominent performance of CuCo₂O₄@polypyrrole makes it as a potential electrode for supercapacitor.