α-MnO2 Nanorods Research Articles

Synthesis of nanohybrids of polycarbazole with α-MnO2 derived from Brassica oleracea: a comparison of photocatalytic degradation of an antibiotic drug under microwave and UV irradiation.

The present work describes the synthesis of α-MnO2 nanorods using a natural extract of Brassica oleracea (cabbage) and the formulation of its nanohybrids with polycarbazole, i.e., α-MnO2/PCz. Synergistic interaction between PCz and MnO2 is revealed from infrared spectroscopy (IR) studies while the composition is determined by X-ray photoelectron spectroscopy (XPS). The formation of α-MnO2 nanorods is confirmed via high-resolution transmission electron microscopy (HRTEM). The indirect bandgap of α-MnO2 is reported as 2.5eV while for the nanohybrids it is found to be ranging between 2.3 and 2.5eV. Results show that 91% and 89% of degradation is achieved within 30min and 90min under the microwave and UV irradiation respectively. Hydroxyl radicals (•OH) and superoxide (•O2-) radicals are responsible for photocatalytic degradation of the drug Bactrim DS which is confirmed by radical scavenging experiments. The nanohybrids show promising catalytic activity under UV as well as microwave irradiation.

The effects of morphology, microstructure and mixed-valent states of MnO2 on the oxygen evolution reaction activity in alkaline anion exchange membrane water electrolysis

In this work, we focused on the evaluation of oxygen evolution reaction (OER) activity of three different shapes of α-MnO2 nanowires (NWs), nanorods (NRs) and nanotubes (NTs) in alkaline anion exchange water electrolyser. We have attempted to separate the effect of shape, surface area, Mn3+ content and crystal facets on OER activity and stability. X-ray Photoelectron Spectroscopy (XPS) measurements showed that NTs had the highest surface concentration of Mn3+ on the as prepared samples with average Mn oxidation state of 3.33. However, after activation an increase in the average oxidation state of all three shapes to 3.9 was confirmed by XPS. X-Ray Diffraction (XRD) showed surface restructuring after testing. MnO2 NWs showed the highest OER mass activity of 60.6 A g−1 (10 mA cm−2 at 1.67 V (RHE)) due to the higher surface area of 72.2 m2 g−1. While NTs showed the highest specific activity due to highest content of 211 facet, high Mn3+ surface concentration/surface defects. Similar trend was observed in electrolyser testing with 2 mg cm−2 loading. Poor electronic conductivity of MnO2 resulted in decrease in performance with increased loading to 4 mg cm−2. All the studied shapes showed good stability over 36 h of electrolyser testing.

Magnetic biochar supported α-MnO2 nanorod for adsorption enhanced degradation of 4-chlorophenol via activation of peroxydisulfate

A novel magnetic catalytic composite (MBM) was developed by compositing α-MnO2 with a magnetic biochar containing Fe3O4. XRD and EDS confirmed the crystalline structure and the chemical composition of MBM, while the one-dimensional α-MnO2 nanorods were observed on MBM by SEM. 4-chlorophenol as a typical toxic chlorinated organic compound was selected as the model pollutant. Even the MBM composite (MnO2 content: 0.2 g/L) needed the same time (120 min) as the pure α-MnO2 nanorods (0.2 g/L) to completely remove the 4-chlorophenol (10 mg/L) with overdosed peroxydisulfate (PDS), MBM indicated faster pollutant removal rate than the pure α-MnO2 nanorods in the first 100 min. It is possible that the adsorption of 4-chlorophenol by biochar might shorten the migration pathway of the generated active species to the pollutants, resulting the boosted removal rate. MBM was stable in the neutral environment which was desirable for the efficient pollutant removal. Both the radical quenching tests and the EPR spectra identified the main active specie generated by activation of PDS through MBM was singlet oxygen possibly generated by recombination of superoxide ions from the metastable manganese intermediates at neutral pH. TOC data of the effluent ensured 63.5% of the pollutant molecules were completely mineralized after the degradation. The applied magnetic field could recover MBM easily for reuse. This work shed lights on the preparation of highly efficient and environmentally friendly catalytic composites for PDS activation in persistent pollutant removal.

Effects of crystal structure on the activity of MnO2 nanorods oxidase mimics

The crystal structures would directly affect the physical and chemical properties of the surface of the material, and would thus influence the catalytic activity of the material. α-MnO2, β-MnO2 and γ-MnO2 nanorods with the same morphology yet different crystal structures were prepared and tested as oxidase mimics using 3,3’,5,5’-tetramethylbenzidine (TMB) as the substrate. β-MnO2 that exhibited the highest activity had a catalytic constant of 83.75 μmol·m−2·s−1, 2.7 and 19.0 times of those of α-MnO2 and γ-MnO2 (30.91 and 4.41 μmol·m−2·s−1), respectively. The characterization results showed that there were more surface hydroxyls as well as more Mn4+ on the surface of the β-MnO2 nanorods. The surface hydroxyls were conducive to the oxidation reaction, while Mn4+ was conducive to the regeneration of surface hydroxyls. The synergistic effect of the two factors significantly improved the activity of β-MnO2 oxidase mimic. Using β-MnO2, a β-MnO2-TMB-GSH system was established to detect the content of glutathione (GSH) rapidly and sensitively by colorimetry. This method had a wide detection range (0.11-45 μM) and a low detection limit (0.1 μM), and had been successfully applied to GSH quantification in human serum samples.

Morphology and phase transformation of α-MnO2/MnOOH modulated by N-CDs for efficient electrocatalytic oxygen evolution reaction in alkaline medium

Engineering non-precious electrocatalysts with excellent water oxidation performance is imperative for developing renewable and clean energy. Herein, we realize the simultaneous morphology and phase transformation from α-MnO2 nanorods, large-size MnOOH nanorods to MnOOH nanoparticles by adding different content of N-CDs in the synthesizing α-MnO2 process. Benefitting from the unique structures, synergistic effect between Mn-based oxides/oxyhydroxides and N-CDs, various active oxide species and large specific surface areas, the as-prepared Mn-based oxides/oxyhydroxides@CDs with different morphologies show excellent electrocatalytic performance for oxygen evolution reaction (OER) in 1.0 M KOH. Impressively, the large-size MnOOH@CDs0.2 display high electrocatalytic performance with the overpotential of 302 mV to reach to 10 mA cm−2 along with a Tafel slope of 51.3 mA dec−1. More importantly, by coupling the MnOOH@CDs0.2 with Pt/C for overall water splitting, a cell potential of merely 1.56 V is required to reach to 10 mA cm−2. Additionally, in the process of stability studies, the key role of the (111) crystal plane in the catalytic process is established. Meanwhile, the MnOOH@CDs0.2 also shows excellent stability for continuous 72 h chronopotentiometry test. The proposed strategy showed here represents a facile and suitable method for the simultaneous transformation of the morphology and phase of Mn-based oxides/oxyhydroxides with the assistance of N-CDs for efficient energy conversion.

Promoting Effect of the Core-Shell Structure of MnO2@TiO2 Nanorods on SO2 Resistance in Hg0 Removal Process

Sorbent of αMnO2 nanorods coating TiO2 shell (denoted as αMnO2-NR@TiO2) was prepared to investigate the elemental mercury (Hg0) removal performance in the presence of SO2. Due the core-shell structure, αMnO2-NR@TiO2 has a better SO2 resistance when compared to αMnO2 nanorods (denoted as αMnO2-NR). Kinetic studies have shown that both the sorption rates of αMnO2-NR and αMnO2-NR@TiO2, which can be described by pseudo second-order models and SO2 treatment, did not change the kinetic models for both the two catalysts. In contrast, X-ray photoelectron spectroscopy (XPS) results showed that, after reaction in the presence of SO2, S concentration on αMnO2-NR@TiO2 surface is lower than on αMnO2-NR surface, which demonstrated that TiO2 shell could effectively inhibit the SO2 diffusion onto MnO2 surface. Thermogravimetry-differential thermosgravimetry (TG-DTG) results further pointed that SO2 mainly react with TiO2 forming Ti(SO4)O in αMnO2-NR@TiO2, which will protect Mn from being deactivated by SO2. These results were the reason for the better SO2 resistance of αMnO2-NR@TiO2.

Ultrasonic synthesis of α-MnO2 nanorods: An efficient catalytic conversion of refractory pollutant, methylene blue.

In this work, uniform α-MnO2 nanorods were synthesized via a simple hydrothermal followed by ultrasonication method using ultrasonic bath (20 kHz, 100 W) without using any surfactant and template. The crystallographic phases and surface morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transition electron microscopy (TEM) analysis, respectively. Functional group identification and chemical states of α-MnO2 nanorods were confirmed by Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The as-synthesized uniform nanorods of α-MnO2 exhibit excellent catalytic conversion of toxic organic contaminant (methylene blue (MB)) in the presence of NaBH4 as reductant. The α-MnO2 exhibits excellent stability up to four repeated catalytic cycles with nearly 92% conversion. The kinetic rate constant (k), and turnover frequency (TOF) were 0.736 min-1 and 0.02 mmol mg-1 min-1, respectively. In addition, the fast electron transfer mechanism were investigated and discussed. These results open a new avenue for developing various metal oxide catalysts, which are expected to be very useful catalytic conversions.

Synthesis of a novel hybrid anode nanoarchitecture of Bi2O3/porous-RGO nanosheets for high-performance asymmetric supercapacitor

Novel bismuth oxide (Bi2O3) nanoparticles dispersed on porous reduced graphene oxide nanosheets is prepared using the facile hydrothermal reaction followed by a calcination process in the air atmosphere. At electrochemical study, the electrode materials of Bi2O3/porous-RGO display the capacitance retention to be 81.1% at a current density of 0.5 Ag-1 and α-MnO2-NRs exhibit about 80.7% at a scan rate of 10 mVs−1 for 3000 cycles in 6 M KOH electrolytes of three-electrode configuration. Moreover, the outstanding capacitance retention of anode and cathode materials mainly due to the porosity (porous-RGO), thermal stability with maximal weight loss rate temperature T(mwlr) reach of 623 °C, a smaller size of Bi2O3 (~7.5 ± 0.5 nm), and aspect ratio of α-MnO2 nanorods for 5.1 ± 0.9 nm. The assembled asymmetric supercapacitor (ASC) achieves the specific capacitance of 84 Fg-1 at a scan rate of 5 mVs−1 and capacitance retention of 91.4% at a current density of 1 Ag-1 by the Bi2O3/porous-RGO//α-MnO2-NRs in PVA/KOH gel electrolyte of two-electrode configuration. Notably, the ASC delivers an energy density of 86 Wh kg−1 (787 mF cm−2) at a power density of 9000 W kg−1. As a result, Bi2O3/porous-RGO and α-MnO2-NRs is considered as a promising candidate for future anode/cathode material in ASC energy storage device.

Electrochemical and magnetic properties of α-MnO2:Dy nanorods

A facile hydrothermal technique with optimized condition used to synthesize single crystalline, α-MnO2 and α-MnO2:Dy nanorods of diameter 40 and 20 nm, respectively. We observe two fold magnitude higher specific capacitance (94 F g−1 at a scan rate of 10 mV/sec) in α-MnO2:Dy than that of α-MnO2 nanorods. X-ray diffraction (XRD) patterns while confirm the tetragonal structure in both compounds, decrease in crystallinity as well as aspect ratio of nanorods in α-MnO2:Dy contribute towards high capacitance. A decrease in Neel temperature (TN) from 24.5 K in bulk to 18 K in α-MnO2 nanorods further decreases to 11 K in α-MnO2:Dy nanorods. Addition to TN at 11 K, another magnetic anomaly at 4 K indicates Dy-Dy interaction. Although Dy incorporation enhances the antiferromagnetic strength as indicated from high Curie-Weiss temperature (θcw), exchange bias field significantly decreases and spin-glass behaviour is absent unlike in bare α-MnO2. Such magnetic features are well explained on the basis of core shell structure of nanorods where core is contributed by antiferromagnetic frozen spins and rotatable spins are contributed towards the surface. The competition between core and surface spins depending on the size of nanorods thus decides the spin-glass behaviour, EB field observed in these nanorods. Change in exchange bias field with consecutive cycles showing the training effect has been discussed after fitting with phenomenological models like Power law and multiple exponent function.

Effect of Dy on electrochemical supercapacitive behaviour of α-MnO2 nanorods

α-MnO2 nanorods doped with different molar concentrations of Dysprosium (Dy) have been successfully synthesised via hydrothermal method as an electrode material. The electrochemical properties are evaluated using cyclic voltammetry, Galvanostatic charge discharge studies where we demonstrate an increase in capacitance with increase in Dy concentration. For α-MnO2: Dy (15 mol %), capacitance enhances by twice than that of bare α-MnO2. XRD (X-Ray diffraction) and FE-SEM (Field Emission- Scanning Electron Microscopy) measurements show that although the structure of Dy doped α-MnO2 remains tetragonal, the crystallinity degrades along with the reduction in aspect ratio of nanorods. In addition, BET (Brunauer-Emmett-Teller) measurement shows that the surface area and size of the pores increase with increase in Dy concentration. Poor crystallinity, large surface area and smaller nanorods are an evidence of high capacitance observed in α-MnO2: Dy (15 mol %). The effect of Dy concentration on structure, morphology, surface area and electrochemical properties are discussed in detail for the application of supercapacitor.

Origin of colossal dielectric behavior in hydrothermally prepared non-stoichiometric α-MnO2 nanorods

Abstract Precise incorporation of defects into transition metal oxides is gaining scientific interest among researchers as a pertinent variable for tuning their functional properties. Herein we report an unusual behavior in the dielectric properties of α-MnO2 nanorods prepared by hydrothermal method. Recently, we have reported the origin of ferroelectricity in α-MnO2 nanorods prepared by co-precipitation method which has been ascribed to the co-operative Jahn-Teller (CJT) distortion prevailing in the sample due to defect induced non-stoichiometry. However, ferroelectricity is found to be absent in the hydrothermally prepared sample. Instead the new non-stoichiometry has resulted in the origin of peculiar features like temperature independent dielectric response, very high dielectric constant and low dielectric loss which are the characteristics of colossal dielectric materials. This opens up the prospect of usage of α-MnO2 as a possible candidate for capacitive applications. A thorough investigation into the inter-link between the structural and dielectric properties has disclosed that the novel exotic properties are a result of the non-stoichiometry-triggered interplay between space charges and dipoles within the interior of the nanorods.

Investigating Cathode Dissolution By Homogeneous Sol-Gel Coating on Manganese Dioxide Nanofibers to Extend Battery Performance

The cathode dissolution is a known phenomenon for manganese dioxide (MnO2) based cathode materials in both aqueous and non-aqueous batteries. Upon battery discharge, reduced Mn3+ species disproportionate (2Mn3+→ Mn4+ + Mn2+) and Mn2+ dissolves into the electrolyte causing the loss of active material. While MnO2 is a promising cathode for rechargeable aqueous zinc ion batteries (ZIB) due to its availability and low toxicity, the electrochemical performance is limited during battery cycling due to the dissolution of the cathode. In this work, a homogeneous sol-gel SiO2 coatings with various thicknesses (0.5-5nm) on hydrothermally synthesized α-MnO2 nanorods are reported. In-depth characterization of SiO2 coated α-MnO2 are done by XRD, BET, XPS, TGA and SEM/TEM to analyze the surface of the silica coated α-MnO2 cathodes. The electrochemical performance of the SiO2 coated α-MnO2 cathodes are studied in two electrode zinc batteries at room temperatures using Zn metal anode and in 1 M ZnSO4 and 1 M TFSI electrolytes. The cathode dissolution is investigated by determining the total dissolved Mn content and characterization of the used electrodes. Electrochemical performances of SiO2 coated α-MnO2 are evaluated using Cyclic Voltammetry, EIS, and Galvanostatic Cycling tests. This study will further benefit all different battery types that have the same cathode dissolution problem.

Inhomogeneous structural evolution of silver-containing Alpha-MnO2 nanorods in sodium-ion batteries investigated by comparative transmission electron microscopy approach

Alpha-MnO2 is an attractive cathode candidate for sodium-ion batteries attributed to its unique one-dimensional 2 × 2 tunnels for facile sodium-ion diffusion, in addition to its incomparable cost advantage. In particular, α-MnO2 shows superior rate capability with silver stabilizing ions at the center of tunnels that improve electrical conductivity. In this work, we directly compare structural transformation of silver-containing α-MnO2 nanorods (Ag1.22Mn8O16-x or L-Ag-HOL and Ag1.66Mn8O16-y or H–Ag-HOL), containing higher and lower concentrations of oxygen vacancies respectively, by transmission electron microscopy (TEM). The elaborate comparative and statistical TEM studies eliminate concerns regarding generalization errors and facilitate rational structural development of nanorods with improved functionality. It is found that sodium ions favorably diffuse through the area where oxygen vacancies are concentrated, and the samples with more silver ions and fewer oxygen vacancies (H–Ag-HOL) show more significant structural deformation with more inhomogeneous sodiation. The difference in functional electrochemistry coupled with the observed difference in inter- and intra-nanorod inhomogeneous structural evolution emphasizes the significance of the uniform electrical conductivity of the electrode. This work helps to improve the α-MnO2 electrode material for sodium-ion batteries as well as suggesting the importance of delicate statistical approaches for TEM investigations.

α-MnO2 nanorod/boron nitride nanoplatelet composites for high-performance nanoscale dielectric pseudocapacitor applications

We demonstrate the synthesis of a composite of α-MnO2 nanorods and dielectric boron nitride nanoplatelets(BNNPs) as an electrode material for application in nanoscale dielectric pseudocapacitors. The optimize nanocomposite delivers a significantly high specific capacitance (890 F/g at 0.41 A/g), which is >72% of the theoretical specific capacitance of α-MnO2 at a mass loading of ca. 6 wt%. Pure BNNPs exhibit negligible charge storage capacity with a specific capacitance of 1.30 F/g at 0.024 A/g. The BNNPs increase the amount of K+ insertion/extraction and the conductivity of α-MnO2 nanorods by lowering the charge transfer resistance at the electrode-electrolyte interface. This is due to the electrical polarization of dielectric BNNPs during charging and discharging, which increases the rate and amount of K+ insertion or extraction induce by electrostatic force. The nanocomposite shows good capacity retention (94.12% after 2000 cycles) with high energy and power density. This research opens up a new avenue for the development of new types of nanoscale dielectric pseudocapacitors with high capacitance by exploring other suitable metal-oxides and nanoscale dielectric material composites.

Efficient removal of organic contaminant via activation of potassium persulfate by γ-Fe2O3/α-MnO2 nanocomposite

In this study, γ-Fe2O3/α-MnO2 nanocomposite, which possesses magnetism as well as excellent activation performance for potassium persulfate (PS) to degrade organic contaminants in water, was successfully fabricated through ultrasonic strategy. Noted, proportion between γ-Fe2O3 and α-MnO2 was optimized to achieve excellent magnetic and activation performance. Also, physicochemical properties of magnetic nanocomposite material (γ-Fe2O3/α-MnO2 1/7, FM1-7) are investigated by SEM, EDS, BET, XRD, TEM, XPS and vibrating sample magnetometer (VSM). Results show that γ-Fe2O3 nanoparticles is connected with α-MnO2 nanorods successfully, what’s more, FM1-7 nanocomposite crystal structure is intact and its crystallinity is excellent. Besides, in γ-Fe2O3/α-MnO2/PS reaction system, about 92.79% of Rhodamine B could be removed within 30 min. The result proves that when the γ-Fe2O3 content is 12.5%, γ-Fe2O3/α-MnO2 nanocomposite exhibits superior activation performance. γ-Fe2O3/α-MnO2/PS reaction shows the most excellent degradation towards Rhodamine B when it is compared with another reaction systems. Most importantly, in this study, the crucial role of γ-Fe2O3 and catalysis mechanism of γ-Fe2O3/α-MnO2 is also discussed in detail. In the Fenton-like system mediated by Fe(III) and Mn(IV), the redox reactions with the activation of PS happen on the surface of γ-Fe2O3/α-MnO2. Therefore, strong oxidizers of SRs (SO4−), hydroxyl radicals (OH) and even superoxide anion (O2−) may be generated and participated in the degradation of Rhodamine B. What’s more, the result of repetitive experiment illustrates that magnetic FM1-7 sample possesses good recycle and reusability capacities. This study not only provides a feasible strategy to prepare catalyst which could be used to active PS, but also is expected to be applied to deal with many kinds of contaminants removal problem according to the practical requirement.

Regenerable α-MnO2 nanotubes for elemental mercury removal from natural gas

The elemental mercury (Hg0) vapor removal ability of α-MnO2 nanotubes (NTs), nanorods (NRs) and nanowires (NWs) was evaluated in simulated natural gas mixtures. It was shown that NTs exhibited superior efficiency (~100%) at the temperature range of interest (25–100 °C). The Hg0 breakthrough (corresponding to 99.5% Hg0 removal efficiency) at ambient conditions and Hg0in = 870 μg∙m−3, occurred after 48 h, reflecting a Hg0 uptake of >10 mg∙g−1 (1 wt%). Most importantly, the developed nanosorbent was repeatedly, sufficiently regenerated at the relatively low temperature of 250 °C in a series of seven successive sorption-desorption tests, maintaining its capacity to remove all incoming Hg0in in each cycle. Interestingly, kinetic studies showed that at least 85% of total adhered Hg0 could be recovered within the first hour of a 3-h regeneration process. Moreover, the presence of CO2 and H2S did not affect its activity. The thermogravimetric analysis indicated significantly higher dehydration for NTs due to the loss of bound water inside its tunnels, likely indicating the presence of abundant surface adsorbed oxygen species which are believed to facilitate Hg0 adsorption. This was also confirmed by X-ray spectroscopy, possibly explaining the enhanced activity of NTs.

Template-cum-catalysis free synthesis of α-MnO2 nanorods-hierarchical MoS2 microspheres composite for ultra-sensitive and selective determination of nitrite

In this work, a novel composite constituted by α-MnO2 nanorods and hierarchical MoS2 microspheres is synthesized by a simple, template-cum-catalysis free two-step hydrothermal method for ultra-sensitive and selective detection of nitrite. The nitrite sensor (with optimal MnO2-MoS2 weight ratio) showed a limit of detection of 16 μM, the short response time (<5 s), outstanding selectivity, reproducibility, and sensitivity of 515.84 μA mM−1 cm−2 (R2 = 0.996) in the range of 100–800 μM in an optimized electrolytic pH of 4. The excellent sensing ability of the sensor can be attributed to the heterogeneous interface between two material constituents that results in unimpeded electrical transport due to the presence of intertwined nanosheets in MoS2 and 1D α-MnO2 nanorods, high number of active sites emanating from huge numbers of edges and defects in MoS2, high proportion of metallic (1T) phase than semiconducting (2H) phase in MoS2 and the combined effect of 1D α-MnO2 nanorods and MoS2 nanosheets. The fabricated sensor was also effectively assessed for the detection of nitrite in potable water. This novel, binder-free, MnO2-MoS2 composite based electrode material paves a new way for the development of simple, non-enzymatic and inexpensive electrochemical sensors for analytical applications.

α-MnO2 nanorods supported on three dimensional graphene as high activity and durability cathode electrocatalysts for magnesium-air fuel cells

Abstract Magnesium-air fuel cells are considered as a potential energy conversion device owing to a high theoretical specific capability, economic viability and environmental friendliness. In this work, α-MnO2 nanorods are supported on three-dimensional graphene (3D-G) which is fabricated with coal tar pitch as the carbon precursor and MgO as the template via hydrothermal reaction. The synergistic interactions between α-MnO2 nanorods and 3D-G improve the activity and durability performance for oxygen reduction reaction in 0.1 mol L−1 KOH solution. α-MnO2/3D-G exhibits a high half-wave potential (0.81 V) and superior to α-MnO2/C (0.72 V) and α-MnO2/rGO (0.76 V), and close to 20 wt% Pt/C (0.83 V). α-MnO2/3D-G also possesses higher durability than commercial Pt/C. Furthermore, the magnesium-air fuel cells based on α-MnO2/3D-G, air-cathode display the peak power density (106.2 mW cm−2), and continuously durability (discharge for 16 h at 50 mA cm−2 with the mere decay of cell voltage by 7.6%). These results prove that α-MnO2/3D-G catalyst with high activity and durability can be a promising electrocatalyst in magnesium-air fuel cells.

α-MnO2 nanorod/onion-like carbon composite cathode material for aqueous zinc-ion battery

Onion-like carbon (OLC) integrated α-MnO2 nanorods (α-MnO2/OLC) composite has been studied as a viable cathode material for potential development of high-performance zinc-ion batteries (ZIBs). XRD results revealed the tetragonal phase of the α-MnO2 materials, and the FE-SEM and HR-TEM images show nano-sized rod-shaped morphology with an average diameter of 30 nm. The BET surface area of the α-MnO2/OLC composite was almost 6 times (247.22 m2/g) higher than that of the pristine (42.48 m2/g) material and the thermogravimetric investigation was exposed 50% of MnO2 and 50% OLC is presented in the composite. The electrochemical performance of the cells was evaluated by galvanostatic cycling (GC), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using 1 M ZnSO4 plus 0.1 M MnSO4 additive as electrolyte and Zn foil as the anode. Cycling results indicate that α-MnO2/OLC composite exhibits a stable and high reversible capacity of 168 mAh g−1 (93% capacity retention) compared to the α-MnO2 material with a capacity of 104 mAh g−1 (81% capacity retention) after 100 cycles.

Effect of Thermally Induced Oxygen Vacancy of α-MnO2 Nanorods toward Oxygen Reduction Reaction.

MnO2 has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO2 crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO2 has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO2 nanorods were explored for the first time under thermal treatment with different temperatures (300, 400, and 500 °C). Under thermal treatment, oxygen vacancies were induced. The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO2 materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO2 nanorods were strengthened with the temperature increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO2 nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO2 nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.