Engineering the crystal facets of α-MnO2 nanorods for electrochemical energy storage: experiments and theory

Crystal facet engineering is an effective strategy for precisely regulating the orientations and electrochemical properties of metal oxides. However, the contribution of each crystal facet to pseudocapacitance is still puzzling, which is a bottleneck that restricts the specific capacitance of metal oxides. Herein, α-MnO2 nanorods with different exposed facets were synthesized through a hydrothermal route and applied to pseudocapacitors. XRD and TEM results verified that the exposure ratio of active crystal facets was significantly increased with the assistance of the structure-directing agents. XPS analysis showed that there was more adsorbed oxygen and Mn3+ on the active crystal facets, which can provide strong kinetics for the electrochemical reaction. Consequently, the α-MnO2 nanorods with {110} and {310} facets exhibited much higher pseudocapacitances of 120.0 F g-1 and 133.0 F g-1 than their α-MnO2-200 counterparts (67.5 F g-1). The theoretical calculations proved that the {310} and {110} facets have stronger adsorption capacity and lower diffusion barriers for sodium ions, which is responsible for the enhanced pseudocapacitance of MnO2. This study provides a strategy to enhance the electrochemical performance of metal oxide, based on facet engineering.

Engineering nanostructured Ag doped α-MnO2 electrocatalyst for highly efficient rechargeable zinc-air batteries

Engineering of highly active, and non-precious electrocatalysts are vital to enhance the air-electrodes of rechargeable zinc-air batteries (ZABs). We report a facile co-precipitation technique to develop Ag doped α-MnO₂ nanoparticles (NPs) and investigate their application as cathode materials for ZABs. The electrochemical and physical characteristics of α-MnO₂ and Ag doped α-MnO₂ NPs were compared and examined via CP, CV, TGA/DTA, FT-IR, EIS, and XRD analysis. CV result displayed higher potential and current for ORR in Ag doped α-MnO₂ NPs than α-MnO₂; but, ORR performance decreased when the Ag doping was raised from 7.5 to10 mmol. Moreover, α-MnO₂ and Ag doped α-MnO₂ NPs showed 2.1 and 3.8 electron transfer pathway, respectively, showing Ag doped α-MnO₂ performance to act as an active ORR electrocatalyst for ZABs. The EIS investigation exhibited that charge-transfer resistance for Ag doped α-MnO₂ was extremely lower associated to the MnO₂ demonstrating that the successful loading of Ag in α-MnO₂. A homemade ZAB based on Ag–MnO₂-7.5 showed a high open circuit potential, low ohmic resistances, and excellent discharge profile at a constant current density of 1 mA/g. Moreover, Ag–MnO₂-7.5 show a specific capacity of 795 mA h g⁻¹ with corresponding high energy density ∼875 Wh kg⁻¹ at 1 mA cm⁻² discharging conditions.

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Ag doped α-MnO₂ electrode for rechargeable zinc–air battery was prepared via a facile co-precipitation technique.

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Ag doped α-MnO₂ electrode shows lower charge transfer resistance associated to un-doped MnO₂ electrode.

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Ag doped α-MnO₂ shows enhanced ORR kinetics in oxygen electrode potential.

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The capacitance performance of Ag doped α-MnO₂ electrodes was highly improved.

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Ag doped α-MnO₂ electrode showed energy density of 69.3 W h kg−1 and power density of 722.9 W kg−1.

Exploring flowery MnO2/Ag nanocomposite as an efficient solar-light-driven photocatalyst

An efficient approach was developed to boost the solar light driven photocatalytic efficacy of pristine flowery MnO2 NCs through the immobilization of Ag NPs, which in turn produces MnO2/Ag NCs.

Morphological reduction of Fe3O4 by single step hydrothermal synthesis using 1D MnO2 as a template and its supercapacitive behaviour

Nanomaterials have attracted wide attention due to its promising characteristics in contrast to its bulk counterpart. However, its synthesis techniques need to be explored further for its employability in the...

Synthesis of V-shaped MnO2 nanostructure and its composites with reduced graphene oxide for supercapacitor application

A unique V-shaped MnO₂ nanostructure is synthesized with a weak acid (acetic acid) using the microwave-assisted hydrothermal technique. To improve the performance of MnO₂ in supercapacitor applications, its composite was prepared with reduced graphene oxide (rGO), i.e., MnO₂/rGO, with different weight ratios of MnO₂ and rGO. The specific capacitance values of the as-synthesized V-shaped MnO₂ nanostructure and MnO₂/rGO nanocomposite were calculated to be 64.75 and 88.95 F g^-1 at a current density of 0.5 A g^-1, respectively, in 1 M Na₂SO₄ electrolyte. Furthermore, a two-electrode asymmetric supercapacitor device was fabricated using the MnO₂/rGO nanocomposite as a positive electrode and activated carbon as a negative electrode. The device has shown energy densities of 25.14 and 17.95 W h kg^-1 at 0.25 and 1 kW kg^-1 power densities, respectively. These values suggest that the MnO₂/rGO nanocomposite is a promising material for supercapacitor devices.

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.

Morphological evolution process of δ-MnO2 from 2-D to 1-D without phase transition

Morphological evolution of δ-MnO₂ from nanosheets to nanofibers with direct pathways for electron transport.

Ultrasonication synthesis of PVA/PVP/α-MnO2-stearic acid blend nanocomposites for adsorbing CdII ion

In this research, the functionalization of α-MnO₂ nanorods (NRs) was conducted with stearic acid (SA) through solvothermal technique. The α-MnO₂-SA NRs were used as nanofiller for the preparation of blend nanocomposites (NCs) based on poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) by ultrasonic irradiation. The results exhibit that with increasing in α-MnO₂-SA quantity, the thermal stability of blend NCs was improved. Morphological studies revealed that α-MnO₂-SA with rod structure and a diameter size of 25-80nm was uniformly dispersed in the PVA and PVP matrices. The use of ultrasonic was responsible for these great homogeneities which could not be achieved by mechanical or magnetically stirring. The prepared blend NCs were exploited as an adsorbing for uptake of Cd^II ion from the aqueous system. The results indicated that they can be potentially utilized for the elimination of Cd^II ion from an aqueous system with q_m of 47(mgg^-1).

Precise control of morphology of ultrafine LiMn2O4 nanorods as a supercapacitor electrode via a two-step hydrothermal method

Ultrafine 1D LiMn₂O₄ and its promising galvanostatic charge/discharge profiles in KOH/K₃Fe(CN)₆ electrolyte.

Use of PVA/α-MnO2-stearic acid nanocomposite films prepared by sonochemical method as a potential sorbent for adsorption of Cd (II) ion from aqueous solution

Alpha manganese dioxide (α-MnO₂) nanorods have been covalently functionalized with stearic acid by solvothermal method. The α-MnO₂-stearic acid nanorods were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The amount of stearic acid grafted onto α-MnO₂ surface was determined about 41wt% by thermogravimetric analysis. The α-MnO₂-stearic acid nanorods was used as nanofiller for the preparation of poly(vinyl alcohol) (PVA) nanocomposites (NCs). The NCs with 1, 3, and 5wt% of nanofiller were prepared through ultrasound assisted technique as an economical, fast, eco-friendly, and effective method. The PVA/α-MnO₂-stearic acid NCs were characterized by different techniques. The results showed that with increasing the α-MnO₂-stearic acid content, the thermal stability and tensile properties were improved. Besides, the NC 5wt% was used as adsorbent for sorption of Cd (II) ion. The adsorption efficiency of NC 5wt% in different initial concentrations of Cd (II) ion (20-100mgL^-1) was 58-95%. All the isotherm data were well fitted by both Langmuir and Freundlich equations. The adsorption kinetics study demonstrated that pseudo-second-order model was the best fit with the experimental data. The results indicated that prepared NCs are promising adsorbents for the removal of Cd (II) ion from the aqueous solution.

Hierarchical branched α-MnO2: one-step synthesis and catalytic activity

The hierarchical pine tree-like α-MnO₂are controlled synthesized at room temperature without any template. The α-MnO₂displays superior activity in the catalytic combustion of dimethyl ether.

Facile synthesis of well-shaped spinel LiNi0.5Mn1.5O4nanoparticles as cathode materials for lithium ion batteries

High voltage spinel LiNi_0.5Mn_1.5O₄nanoparticles with polyhedral shapes were synthesized by a solid state reaction using α-MnO₂nanowires as precursors.

Synthesis of amorphous manganese oxide nanoparticles – to – crystalline nanorods through a simple wet-chemical technique using K+ ions as a ‘growth director’ and their morphology-controlled high performance supercapacitor applications

Highly crystalline MnO₂ nanostructures are fabricated by a simple acid reduction of KMnO₄ solution followed by air-annealing. Their excellent electrochemical properties are useful for combinatorial morphology-controlled high performance supercapacitor applications.

Covalent surface modification of α-MnO2 nanorods with l-valine amino acid by solvothermal strategy, preparation of PVA/α-MnO2-l-valine nanocomposite films and study of their morphology, thermal, mechanical, Pb(ii) and Cd(ii) adsorption properties

The surface of α-manganese dioxide (α-MnO₂) nanorods was modified chemically with l-valine amino acid by a solvothermal strategy.