Synthesis and characterization of α-MnO2 nanoneedles for electrochemical supercapacitors

Tuning the optical, electrical, anti-microbial, and swelling activity of nanowires manganese dioxide-loaded chitosan matrix

Skin wound healing is undoubtedly a complex and challenging process. Creating innovative bio-nanocomposites for skin care and wound healing/dressing applications are currently an urgent need. In this context, a mixture of chitosan (CS) and MnO2 nanowires (NWs), as potential wound dressing nano-film, are systematically investigated for optical, electrical, and antibacterial applications. The crystallinity of the materials was investigated through XRD, while FTIR was used to examine the interactions between the CS films and the MnO2 nanofiller. The resulting nanocomposite membranes exhibit superior swelling capacity than that of the pure CS membrane. Additionally, the thermal stability of the films was evaluated, demonstrating the highest stability of composite samples. The optical properties, including bandgap energies and refractive indices, can be adjusted by tuning the content of MnO2. Remarkably, the refractive index dropped from 3.76 in pure CS film to 3.4 in the highest fraction of MnO2 (CS-4 wt% MnO2). The impedance analysis was further improved for the (CS-4 wt% MnO2) sample, posing lower interfacial resistance. A comprehensive investigation of the antibacterial performance of the prepared films was carried out against five bacteria types. The pure CS film exhibited mostly inhibitory effects against Gram-positive bacteria (i.e., B. subtilis, and S. aureus) between the 19 and 14 mm range, while the Gram-negative bacteria (i.e., E. coli, and P. aeruginosa) showed activity between the 19 and 20 mm range, with an effect on A. niger and C. albicans 17 and 22 mm. However, the addition of MnO2 has significantly increased its antimicrobial activity. Overall, the prepared CS films demonstrate great potential for multifunctional applications, including antibacterial, wound healing, and optoelectronics.

MnO2 Nanowires with Sub-10 nm Thick Conjugated Microporous Polymers as Synergistic Triboelectric Materials

MnO2 nanowires coated with conjugated microporous polymers (CMP) are applied as triboelectric energy harvesting materials. The tribopositive performance of the CMP shells is enhanced with the assistance of MnO2 nanowires (MnO2 NW), likely due to cationic charge transfer from the tribopositive CMP layers to the surface Mn2+ and Mn3+ species of MnO2 NW. This is supported by model studies. The MnO2@CMP-2 with sub-10 nm thick CMP layers shows promising triboelectric output voltages up to 576 V and a maximum power density of 1.31 mW cm-2. Spring-assisted triboelectric nanogenerators fabricated with MnO2@CMP-2/PVP-3 films are used as power supplies to operate electronic devices.

Developing Z-scheme Bi2MoO6@α-MnO2 beaded core-shell heterostructure in photoelectrocatalytic treatment of organic wastewater

Photoelectrocatalysis (PEC) oxidation technology with the combination of electrocatalysis and photocatalysis is an ideal candidate for treatment of dyeing wastewater containing multifarious intractable organic compounds with high chroma. Constructing high-quality heterojunction photoelectrodes can effectively suppress the recombination of photo-generated carriers, thereby achieving efficient removal of pollution. Herein, a beaded Bi2MoO6@α-MnO2 core-shell architecture with tunable hetero-interface was prepared by simple hydrothermal-solvothermal process. The as-synthesized Bi2MoO6@α-MnO2 had larger electrochemically active surface area, smaller charge transfer resistance and negative flat band potential, and higher separation efficiency of e-/h+ pairs than pure α-MnO2 or Bi2MoO6. It is noteworthy that the as-synthesized Bi2MoO6@α-MnO2 showed Z-scheme heterostructure as demonstrated by the free radical quenching experiments. The optimized Bi2MoO6@α-MnO2-2.5 exhibited the highest degradation rate of 88.64% in 120 min for reactive brilliant blue (KN-R) and accelerated stability with long-term(∼10000s) at the current density of 50 mA cm-2 in 1.0 mol L-1 H2SO4 solution. This study provides valuable insights into the straightforward preparation of heterogeneous electrodes, offering a promising approach for the treatment of wastewater in various industrial applications.

High-Performance Battery-Type Supercapacitors Based on Self-Oriented Growth of Nanorods/Nanospheres Composite Assembled on Self-Standing Conductive GO/CNF Frameworks

MnOx-based materials have limited capacity and poor conductivity over various voltages, hampering their potential for energy storage applications. This work proposes a novel approach to address these challenges. A self-oriented multiple-electronic structure of a 1D-MnO2-nanorod/2D-Mn2O3-nanosphere composite was assembled on 2D-graphene oxide nanosheet/1D-carbon nanofiber (GO/CNF) hybrids. Aided by K+ ions, the MnO2 nanorods were partially converted to Mn2O3 nanospheres, while the GO nanosheets were combined with CNF through hydrogen bonds resulting in a unique double binary 1D-2D mixed morphology of MnO2/Mn2O3-GO/CNF hybrid, having a novel mechanism of multiple Mn ion redox reactions facilitated by the interconnected 3D network. The morphology of the MnO2 nanorods was controlled by regulating the potassium ion content through a rinsing strategy. Interestingly, pure MnO2 nanorods undergo air-annealing to form a mixture of nanorods and nanospheres (MnO2/Mn2O3) with a distinct morphology indicating pseudocapacitive surface redox reactions involving Mn2+, Mn3+, and Mn4+. In the presence of the GO/CNF framework, the charge storage properties of the MnO2/Mn2O3-GO/CNF composite electrode show dominant battery-type behavior because of the unique mesoporous structure with a crumpled morphology that provides relatively large voids and cavities with smaller diffusion paths to facilitate the accumulation/intercalation of charges at the inner electroactive sites for the diffusion-controlled process. The corresponding specific capacity of 800 C g-1 or 222.2 mAh g-1 at 1 A g-1 and remarkable cycling stability (95%) over 5000 cycles at 3 A g-1 were considerably higher than those of the reported electrodes of similar materials. Moreover, a hybrid supercapacitor device is assembled using MnO2/Mn2O3-GO/CNF as the positive electrode and activated carbon as the negative electrode, which exhibits a superior maximum energy density (∼25 Wh kg-1) and maximum power density (∼4.0 kW kg-1). Therefore, the as-synthesized composite highlights the development of highly active low-cost materials for next-generation energy storage applications.

Tuning Proton Insertion Chemistry for Sustainable Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) have emerged as an alternative to lithium-ion batteries due to their affordability and high level of safety. However, their commercialization is hindered by the low mass loading and irreversible structural changes of the cathode materials during cycling. Here, a disordered phase of a manganese nickel cobalt dioxide cathode material derived from wastewater via a coprecipitation process is reported. When used as the cathode for aqueous ZIBs , the developed electrode delivers 98% capacity retention at a current density of 0.1 A g-1 and 72% capacity retention at 1 A g-1 while maintaining high mass loading (15 mg cm-2). The high performance is attributed to the structural stability of the Co and Ni codoped phase; the dopants effectively suppress Jahn-Teller distortion of the manganese dioxide during cycling, as revealed by operando X-ray absorption spectroscopy. Also, it is found that the Co and Ni co-doped phase effectively inhibits the dissolution of Mn2+, resulting in enhanced durability without capacity decay at first 20 cycles. Further, it is found that the performance of the electrode is sensitive to the ratio of Ni to Co, providing important insight into rational design of more efficient cathode materials for low-cost, sustainable, rechargeable aqueous ZIBs.

Resent Development of Binder-Free Electrodes of Transition Metal Oxides and Nanohybrids for High Performance Supercapacitors - A Review

The entire world is aware of the serious issue of global warming and therefore utilizing renewable energy sources is the most encouraging steps toward solving energy crises, and as a result, energy storage solutions are necessary. The supercapacitors (SCs) have a high-power density and a long cycle life, they are promising as an electrochemical conversion and storage device. In order to achieve high electrochemical performance, electrode fabrication must be implemented properly. Electrochemically inactive and insulating binders are utilized in the conventional slurry coating method of making electrodes to provide adhesion between the electrode material and the substrate. This results in an undesirable "dead mass," which lowers the overall device performance. In this review, we focused on binder-free SCs electrodes based on transition metal oxides and composites. With the best examples providing the critical aspects, the benefits of binder-free electrodes over slurry-coated electrodes are addressed. Additionally, different metal-oxides used in the fabrication of binder-free electrodes are assessed, taking into account the various synthesis methods, giving an overall picture of the work done for binder-free electrodes. The future outlook is provided along with the benefits and drawbacks of binder-free electrodes based on transition metal oxides.

A new multimodal magnetic nanozyme and a reusable peroxymonosulfate oxidation catalyst: Manganese oxide coated-monodisperse-porous and magnetic core-shell microspheres

Monodisperse-porous, polydopamine and manganese oxide coated, core-shell type, magnetic SiO2 (MagSiO2@PDA@MnO2) microspheres 6.4 μm in size were synthesized for the first time, using magnetic, monodisperse-porous SiO2 (MagSiO2) microspheres 6.2 μm in size as the starting material. MagSiO2 microspheres were obtained by a recently developed method namely "staged shape templated hydrolysis and condensation protocol". In the synthesis, MagSiO2 microspheres were consecutively coated by polydopamine (PDA) and then by a MnO2 layer in the aqueous medium. The pore volume and the specific surface area of monodisperse-porous MagSiO2@PDA@MnO2 microspheres were measured as 0.59 cm3 g-1 and 154 m2 g-1, respectively. Their Mn and Fe contents were determined as 66 ± 1 mg g-1 and 165 ± 5 mg g-1 respectively. MagSiO2@PDA@MnO2 microspheres exhibited multimodal enzyme mimetic behavior with highly superior catalase-like, oxidase-like and peroxidase-like activities. The effective production of singlet oxygen (1O2) and superoxide anion (O2-*) radicals in MagSiO2@PDA@MnO2-peroxymonosulfate (PMS) system was demonstrated by ESR spectroscopy. By evaluating this property, MagSiO2@PDA@MnO2 microspheres were tried as a reusable catalyst for dye removal via peroxymonosulfate (PMS) activation in batch experiments for the first time. The degradation runs were made with, rhodamine B (Rh B), methyl orange (MO) and methylene blue (MB) as the pollutant. The core-shell type design allowing the deposition of porous MnO2 layer onto a large surface area provided very fast, instant removals with all dyes, via both physical adsorption and degradation via PMS activation. In the reusability experiments, the removal yields of MO and Rh B decreased 1.8% and 8.9% over five consecutive runs in batch fashion. MagSiO2@PDA@MnO2 microspheres exhibited very good functional and structural stability in consecutive dye degradations. No significant change was observed in Fe content of microspheres while Mn content exhibited a decrease of 7.4% w/w over 5 consecutive degradation runs.

Electrochemical sensor based on carbon nanotube decorated with manganese oxide nanoparticles for naphthalene determination

In this work, an electrochemical sensor was developed for the determination of naphthalene (NaP) in well water samples, based on a glass carbon electrode (GCE) modified as a nanocomposite of manganese oxides (MnOx) and COOH-functionalized multi-walled carbon nanotubes (MWCNT). The synthesis of MnOx nanoparticles was performed by the sol-gel method. The nanocomposite was obtained by mixing MnOx and MWCNT with the aid of ultrasound, followed by stirring for 24 h. Surface modification facilitated the electron transfer process through the MnOx/MWCNT/GCE composite, which was used as an electrochemical sensor. The sensor and its material were characterized by cyclic voltammetry (CV), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Important parameters influencing electrochemical sensor performance (pH, composite ratios) were investigated and optimized. The MnOx/MWCNT/GCE sensor showed a wide linear range of 2.0-16.0 μM, a detection limit of 0.5 μM and a quantification limit of 1.8 μM, in addition to satisfactory repeatability (RSD of 7.8%) and stability (900 s) in the determination of NaP. The determination of NaP in a sample of water from a gas station well using the proposed sensor showed results with recovery between 98.1 and 103.3%. The results obtained suggest that the MnOx/MWCNT/GCE electrode has great potential for application in the detection of NaP in well water.

Sodium Ion Pre-Intercalation of δ-MnO2 Nanosheets for High Energy Density Aqueous Zinc-Ion Batteries

With the merits of low cost, environmental friendliness and rich resources, manganese dioxide is considered to be a promising cathode material for aqueous zinc-ion batteries (AZIBs). However, its low ion diffusion and structural instability greatly limit its practical application. Hence, we developed an ion pre-intercalation strategy based on a simple water bath method to grow in situ δ-MnO₂ nanosheets on flexible carbon cloth substrate (MnO₂), while pre-intercalated Na⁺ in the interlayer of δ-MnO₂ nanosheets (Na-MnO₂), which effectively enlarges the layer spacing and enhances the conductivity of Na-MnO₂. The prepared Na-MnO₂//Zn battery obtained a fairly high capacity of 251 mAh g^-1 at a current density of 2 A g^-1, a satisfactory cycle life (62.5% of its initial capacity after 500 cycles) and favorable rate capability (96 mAh g^-1 at 8 A g^-1). Furthermore, this study revealed that the pre-intercalation engineering of alkaline cations is an effective method to boost the properties of δ-MnO₂ zinc storage and provides new insights into the construction of high energy density flexible electrodes.

Regulated electrochemical performance of manganese oxide cathode for potassium-ion batteries: A combined experimental and first-principles density functional theory (DFT) investigation

Potassium-ion batteries (KIBs) are promising energy storage devices owing to their low cost, environmental-friendly, and excellent K⁺ diffusion properties as a consequence of the small Stoke's radius. The evaluation of cathode materials for KIBs, which are perhaps the most favorable substitutes to lithium-ion batteries, is of exceptional importance. Manganese dioxide (α-MnO₂) is distinguished by its tunnel structures and plenty of electroactive sites, which can host cations without causing fundamental structural breakdown. As a result of the satisfactory redox kinetics and diffusion pathways of K⁺ in the structure, α-MnO₂ nanorods cathode prepared through hydrothermal method, reversibly stores K⁺ at a fast rate with a high capacity and stability. It has a first discharge capacity of 142 mAh/g at C/20, excellent rate execution up to 5C, and a long cycling performance with a demonstration of moderate capacity retention up to 100 cycles. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) simulations confirm that the K⁺ intercalation/deintercalation occurs through 0.46 K movement between Mn^IV/Mn^III redox pairs. First-principles density functional theory (DFT) calculations predict a diffusion barrier of 0.31 eV for K⁺ through the 1D tunnel of α-MnO₂ electrode, which is low enough to promote faster electrochemical kinetics. The nanorod structure of α-MnO₂ facilitates electron conductive connection and provides a strong electrode-electrolyte interface for the cathode, resulting in a very consistent and prevalent execution cathode material for KIBs.

Breaking the Electronic Conductivity Bottleneck of Manganese Oxide Family for High-Power Fluorinated Graphite Composite Cathode by Ligand-Field High-Dimensional Constraining Strategy

Primary lithium fluorinated graphite (Li/CF_x ) batteries with superior energy density are an indispensable energy supply for multiple fields but suffer from sluggish reaction kinetics of the CF_x cathode. Designing composite cathodes emerges as a solution to this problem. Despite the optimal composite component for CF_x , the manganese oxide family represented by MnO₂ is still faced with an intrinsic electronic conductivity bottleneck, which severely limits the power density of the composite cathode. Here, a cation-induced high-dimensional constraining strategy from the perspective of ligand-field stacking structure topological design, which breaks the molecular orbital hybridization of pristine semiconductive oxides to transform them into the high-conductivity metallic state while competitively maintaining structural stability, is proposed. Through first-principles phase diagram calculations, mixed-valent Mn₅ O₈ ( Mn 2 2 + Mn 3 4 + O 8 ${\rm{Mn}}_2^{2 + }{\rm{Mn}}_3^{4 + }{{\rm{O}}_8}$ ) is explored as an ideal high-dimensional constraining material with satisfied conductivity and large-scale production feasibility. Experiments demonstrate that the as-proposed CF_x  @ Mn₅ O₈ composite cathode achieves 2.36 times the power density (11399 W kg^-1 ) of pristine CF_x and a higher CF_x conversion ratio (86%). Such a high-dimensional field-constraining strategy is rooted in the established four-quadrant electronic structure tuning framework, which fundamentally changes the orbital symmetry under the ligand field to overcome the common conductivity challenge of wide transition metal oxide materials.

Orange G degradation by heterogeneous peroxymonosulfate activation based on magnetic MnFe2O4/α-MnO2 hybrid

Wastewater containing an azo dye Orange G (OG) causes massive environmental pollution, thus it is critical to develop a highly effective, environmental-friendly, and reusable catalyst in peroxymonosulfate (PMS) activation for OG degradation. In this work, we successfully applied a magnetic MnFe₂O₄/α-MnO₂ hybrid fabricated by a simple hydrothermal method for OG removal in water. The characteristics of the hybrid were investigated by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller method, vibrating sample magnetometry, electron paramagnetic resonance, thermogravimetric analysis, and X-ray photoelectron spectroscopy. The effects of operational parameters (i.e., catalytic system, catalytic dose, solution pH, and temperature) were investigated. The results exhibited that 96.8% of OG degradation was obtained with MnFe₂O₄/α-MnO₂(1:9)/PMS system in 30 min regardless of solution pH changes. Furthermore, the possible reaction mechanism of the coupling system was proposed, and the degradation intermediates of OG were identified by mass spectroscopy. The radical quenching experiments and EPR tests demonstrated that SO₄^•̶, O₂^•̶, and ¹O₂ were the primary reactive oxygen species responsible for the OG degradation. The hybrid also displayed unusual stability with less than 30% loss in the OG removal after four sequential cycles. Overall, magnetic MnFe₂O₄/α-MnO₂ hybrid could be used as a high potential activator of PMS to remove orange G and maybe other dyes from wastewater.

Mn-doped Co3O4 for acid, neutral and alkaline electrocatalytic oxygen evolution reaction

This work reports the application of Mn-doped Co3O4 oxides in the electrocatalytic oxygen evolution reaction (OER). The materials were characterized by structural, morphological, and electrochemical techniques. The oxides with higher Co : Mn molar ratio presented a lower electron transfer resistance, and consequently the most promising OER activities. Pure Co3O4 shows an overpotential at j = 10 mA cm-2 of 761, 490, and 240 mV, at pH 1, 7, and 14, respectively, and a high TOF of 1.01 × 10-1 s-1 at pH 14. Tafel slopes around 120 mV dec-1 at acidic pH and around 60 mV dec-1 at alkaline pH indicate different OER mechanisms. High stability for Co3O4 was achieved for up to 15 h in all pHs, and no change in the structure and morphology after the electrocatalysis was observed. The reported excellent OER activity of the Mn-Co oxides in a wide pH range is important to broaden the practical applicability in different electrolyte solutions.

Interfacial tension driven adsorption of MnO2 nanoparticles at the liquid/liquid interface to tailor ultra-thin polypyrrole sheets

An emerging aspect of research is designing and developing fully tunable metamaterials for various applications with fluid interfaces. Liquid/liquid interface-assisted methods represent an efficient and facile route for synthesizing two-dimensional (2-D) thin films of potential materials. The underlying mechanism behind thin film formation at the liquid/liquid interface involves the preferential adsorption of nano-sized particles at the interface to minimize high interfacial tension. Here, a water/chloroform interface-assisted method is employed for the one-pot synthesis of highly crystalline polypyrrole/manganese dioxide (PPy/MnO₂) sheets. The temporal evolution in the dynamic interfacial tension (from 32 mN m^-1 to 17 mN m^-1) observed in pendant drop tensiometry proved the preferential adsorption of MnO₂ atttached PPy oligomers at the water/chloroform interface. An ultra-thin sheet-like morphology and uniform distribution of ∼6 nm highly crystalline MnO₂ nanoparticles are evidenced by transmission and atomic force microscopy techniques. The predominance of interfacial polymerization in retaining the electrochemical activity of the PPy/MnO₂ sheets is elucidated for the electrochemical detection of nicotine. This study opens a new avenue for the realization of ultra-thin sheets of polymer-nanomaterial hybrids, enabling applications ranging from new classes of sensors to optics.

Recent progresses in the synthesis of MnO2 nanowire and its application in environmental catalysis

Nanostructured MnO2 with various morphologies exhibits excellent performance in environmental catalysis owing to its large specific surface area, low density, and adjustable chemical properties. The one-dimensional MnO2 nanowire has been proved to be the dominant morphology among various nanostructures, such as nanorods, nanofibers, nanoflowers, etc. The syntheses and applications of MnO2-based nanowires also have become a research hotspot in environmental catalytic materials over the last two decades. With the continuous deepening of the research, the control of morphology and crystal facet exposure in the synthesis of MnO2 nanowire materials have gradually matured, and the catalytic performance also has been greatly improved. Differences in the crystalline phase structure, preferably exposed crystal facets, and even the length of the MnO2 nanowires will evidently affect the final catalytic performances. Besides, the modifications by doping or loading will also significantly affect their catalytic performances. This review carefully summarizes the synthesis strategies of MnO2 nanowires developed in recent years as well as the influences of the phase structure, crystal facet, morphology, dopant, and loading amount on the catalytic performance. Besides, the cutting-edge applications of MnO2 nanowires in the field of environmental catalysis, such as CO oxidation, the removal of VOCs, denitrification, etc., have been also summarized. The application of MnO2 nanowire in environmental catalysis is still in the early exploratory stage. The gigantic gap between theoretical investigation and industrial application is still a great challenge. Compared with noble metal based traditional environmental catalytic materials, the lower cost of MnO2 has injected new momentum and promising potential into this research field.

Al-doped α-MnO2 coated by lignin for high-performance rechargeable aqueous zinc-ion batteries

Zn/MnO2 batteries, one of the most widely studied rechargeable aqueous zinc-ion batteries, suffer from poor cyclability because the structure of MnO2 is labile with cycling. Herein, the structural stability of α-MnO2 is enhanced by simultaneous Al3+ doping and lignin coating during the formation of α-MnO2 crystals in a hydrothermal process. Al3+ enters the [MnO6] octahedron accompanied by producing oxygen vacancies, and lignin further stabilizes the doped Al3+ via strong interaction in the prepared material, Al-doped α-MnO2 coated by lignin (L + Al@α-MnO2). Meanwhile, the conductivity of L + Al@α-MnO2 improves due to Al3+ doping, and the surface area of L + Al@α-MnO2 increases because of the production of nanorod structures after Al3+ doping and lignin coating. Compared with the reference α-MnO2 cathode, the L + Al@α-MnO2 cathode achieves superior performance with durably high reversible capacity (∼180 mA h g-1 at 1.5 A g-1) and good cycle stability. In addition, ex situ X-ray diffraction characterization of the cathode at different voltages in the first cycle is employed to study the related mechanism on improving battery performance. This study may provide ideas of designing advanced cathode materials for other aqueous metal-ion batteries.

Engineering Co3O4/MnO2 nanocomposite materials for oxygen reduction electrocatalysis

Stable and active electrocatalysts preparation for the oxygen reduction reaction (ORR) is essential for an energy storage and conversion materials (e.g. metal-air batteries). Herein, we prepared a highly-active MnO₂ and Co₃O₄/MnO₂ nanocomposite electrocatalysts using a facial co-precipitation approach. The electrocatalytic activity was examined in alkaline media with LSV and CV. Additionally, the physicochemical characteristics of the MnO₂ and Co₃O₄/MnO₂ composite materials were studied via SEM, XRD, BET, UV-Vis, TGA/DTA, ICP-OES and FTIR. Morphological studies indicated that a pure MnO₂ has a spherical flower-like architecture, whereas Co₃O₄/MnO₂ nanocomposites have an aggregated needle-like structure. Moreover, from the XRD investigation parameters such as the dislocation density, micro-strain, and crystallite size were analyzed. The calculated energy bandgaps for the MnO₂, Co₃O₄/MnO₂-1-5, and Co₃O₄/MnO₂-1-1 nanocomposites were 3.07, 2.6, and 2.3 eV, correspondingly. The FTIR spectroscopy was also employed to study the presence of M-O bonds (M = Mn, Co). The thermal gravimetric investigation showed that the Co₃O₄/MnO₂ nanocomposite materials exhibited improved thermal stability, confirming an enhanced catalytic activity of ORR for MnO₂/Co₃O₄-1-1 composite materials for ORR. These results confirm that the prepared Co₃O₄/MnO₂ composite materials are promising air electrode candidates for the energy storage and conversion technologies.

Tuning Electrocatalytic Aptitude by Incorporating α-MnO2 Nanorods in Cu-MOF/rGO/CuO Hybrids: Electrochemical Sensing of Resorcinol for Practical Applications

In this study, Cu-MOF/rGO/CuO/α-MnO₂ nanocomposites have been fabricated by a one-step hydrothermal method and used in the voltammetric detection of resorcinol (RS). The poor conductivity of MOFs in the field of electrochemical sensing is still a major challenge. A series of Cu-MOF/rGO/CuO/α-MnO₂ nanocomposites have been synthesized with varying fractions of rGO and with a fixed amount of α-MnO₂ via a facile method. These nanocomposites are well characterized using some sophisticated characterization techniques. The as-prepared nanohybrids have strongly promoted the redox reactions at the electrode surface due to their synergistic effects of improved conductivity, high electrocatalytic activity, an enlarged specific surface area, and a plethora of nanoscale level interfacial collaborations. The electrode modified with Cu-MOF/rGO/CuO/α-MnO₂ has revealed superior electrochemical properties demonstrating linear differential pulse voltammetry (DPV) responses from a 0.2 to 22 μM RS concentration range (R² = 0.999). The overall results of this sensing podium have shown excellent stability, good recovery, and a low detection limit of 0.2 μM. With excellent sensing performance achieved, the practicability of the sensor has been evaluated to detect RS in commercial hair color samples as well as in tap water and river water samples. Therefore, we envision that our hybrid nanostructures synthesized by the structural integration strategy will open new horizons in material synthesis and biosensing platforms.

Oxygen-generating glycol chitosan-manganese dioxide nanoparticles enhance the photodynamic effects of chlorin e6 on activated macrophages in hypoxic conditions

This study aimed to investigate the use of glycol chitosan (GC) for the synthesis of MnO₂ nanoparticles (NPs) and to evaluate whether the prepared GC-MnO₂ NPs enhance the light-triggered photodynamic effects of chlorin e6 (Ce6) via the generation of oxygen and alleviation of hypoxia in lipopolysaccharide (LPS)-activated macrophages (RAW 264.7), which produce excessive amounts of reactive oxygen species (ROS). GC-MnO₂ NPs were synthesized by a simple reaction between GC and KMnO₄ in water. The prepared GC-MnO₂ NPs were spherical in shape, with a mean diameter of approximately 60 nm. The particles effectively generated oxygen via H₂O₂-induced degradation under hypoxic conditions, which led to an increase in the singlet oxygen levels upon laser irradiation. Furthermore, GC-MnO₂ NPs significantly enhanced the light-triggered photodynamic effects of Ce6 on activated macrophages under hypoxic conditions, as shown by the increased levels of cell death and cell membrane damage in activated macrophages. Therefore, these results suggest that GC can be used as an alternative natural polymer for the synthesis of MnO₂ NPs and that oxygen-generating GC-MnO₂ NPs enhance the light-triggered photodynamic effects of Ce6 on activated macrophages by alleviating hypoxia.

Electrochemical Capture and Storage of CO2 as Calcium Carbonate

A carbon dioxide capture, conversion, and utilization technology has been developed that can be powered by renewable energy with the potential to mitigate CO₂ emissions. This relies on an electrochemical process whereby the dissolution of carbon dioxide into carbonate ions is accelerated by a locally induced pH change at the cathode. The carbonate ions can then complex with metal cations, such as Ca²⁺ , Sr²⁺ , or Mn²⁺ , present in solution to form their respective metal carbonates, which precipitate out of solution. To ensure the cathode is not fouled by deposition of the insulating metal carbonate, the process is operated under hydrogen evolution conditions, thereby alleviating any significant attachment of the solid to the electrode. This process is demonstrated in CO₂ -saturated solutions while the possibility of direct air capture is also shown, where the precipitation of CaCO₃ from atmospherically dissolved CO₂ during electrolysis is observed. The latter process can be significantly enhanced by using 5 vol.% of monoethanolamine (MEA) in the electrochemical cell. Finally, the process is investigated using seawater, which is also successful after the initial precipitation of metal sulfates from solution. In particular, the use of renewable energy to capture CO₂ and create CaCO₃ while also generating hydrogen may be of particular interest to the cement industry, which has a significant CO₂ footprint.

The effects of Fe-doping on MnO2: phase transitions, defect structures and its influence on electrical properties

The composition of Mn_1−xFe_xO₂ (x = 0–0.15) was synthesized by a hydrothermal method at 140 °C for 5 hours of reaction time. Investigations were carried out including XRD, FTIR, Raman spectroscopy, FESEM, and TEM for crystallographic phase analysis. Furthermore, XPS and XAS were used to analyze the oxidation states of Mn and dopant Fe in the octahedron sites. For electrical characterizations, an impedance analyzer was used to explore the conductivity and dielectric properties. It was discovered that the undoped MnO₂ possessed an α-MnO₂ structure performing (2 × 2) tunnel permitting K⁺ insertion and had a nanorod morphology. The Fe ion that was doped into MnO₂ caused a phase transformation from α-MnO₂ to Ramsdellite R-MnO₂ after x = 0.15 was reached and the tunnel dimension changed to (2 × 1). Furthermore, this caused increased micro-strain and oxygen vacancies. An oxidation state analysis of Mn and substituted Fe in the octahedron sites found mixed 3+ and 4+ states. Electrical characterization revealed that the conductivity of Fe-doped MnO₂ is potentially electron influenced by the oxidation state of the cations in the octahedron sites, the micro-strain, the dislocation density, and the movement of K⁺ ions in the tunnel.

Phase transformation from initially α-MnO₂ to R-MnO₂ due to Fe-doping cause modification of interatomic distances affects to the electrical properties.

Synthesis of Hierarchical Porous Ni1.5Co1.5S4/g-C3N4 Composite for Supercapacitor with Excellent Cycle Stability

In this work, the hierarchical porous Ni1.5Co1.5S4/g-C3N4 composite was prepared by growing Ni1.5Co1.5S4 nanoparticles on graphitic carbon nitride (g-C3N4) nanosheets via a hydrothermal route. Due to the self-assembly of larger size g-C3N4 nanosheets as a skeleton, the prepared nanocomposite possesses a unique hierarchical porous structure that can provide short ions diffusion and fast electron transport. As a result, the Ni1.5Co1.5S4/g-C3N4 composite exhibits a high specific capacitance of 1827 F g-1 at a current density of 1 A g-1, which is 1.53 times that of pure Ni1.5Co1.5S4 (1191 F g-1). In particular, the Ni1.5Co1.5S4/g-C3N4//activated carbon (AC) asymmetric supercapacitor delivers a high energy density of 49.0 Wh kg-1 at a power density of 799.0 W kg-1. Moreover, the assembled device shows outstanding cycle stability with 95.5% capacitance retention after 8000 cycles at a high current density of 10 A g-1. The attractive performance indicates that the easily synthesized and low-cost Ni1.5Co1.5S4/g-C3N4 composite would be a promising electrode material for supercapacitor application.

Synthesis of High Surface Area α-KyMnO2 Nanoneedles Using Extract of Broccoli as Bioactive Reducing Agent and Application in Lithium Battery

With the aim to reduce the entire cost of lithium-ion batteries and to diminish the environmental impact, the extract of broccoli is used as a strong benign reducing agent for potassium permanganate to synthesize α-KyMnO2 cathode material with pure nanostructured phase. Material purity is confirmed by X-ray powder diffraction and thermogravimetric analyses. Images of transmission electron microscopy show samples with a spider-net shape consisting of very fine interconnected nanoneedles. The nanostructure is characterized by crystallite of 4.4 nm in diameter and large surface area of 160.7 m2 g-1. The material delivers an initial capacity of 211 mAh g-1 with high Coulombic efficiency of 99% and 82% capacity retention after 100 cycles. Thus, α-KyMnO2 synthesized via a green process exhibits very promising electrochemical performance in terms of initial capacity, cycling stability and rate capability.

Electrochemical Adsorption of Cs+ Ions on H-Todorokite Nanorods

In this work, manganese oxide with a 3 × 3 tunnel structure was synthesized. Mg-todorokite was treated with HNO₃ to obtain H-todorokite, which was used as an electrode for adsorption of Cs⁺ from aqueous solution by an electrochemical reaction. H-todorokite was electrochemically reduced to lower oxidation state to enhance its negative charge and simultaneously increase the amount of cations because of the charge compensation. The Cs⁺ adsorption on H-todorokite was completed by a coupled electrochemical reaction (the redox reaction between Mn³⁺ and Mn⁴⁺) and an ion-exchange reaction between Cs⁺ and H⁺ ions. Cyclic voltammetry measurements at different pHs and Cs⁺ concentrations were performed. H-todorokite revealed high electrochemical adsorption capacity for Cs⁺ because of the high crystallinity and stability of the materials, which reached 6.0 mmol g^-1 in 0.1 mol·L^-1 Na₂SO₄ solution.

Effect of nickel ion doping in MnO2/reduced graphene oxide nanocomposites for lithium adsorption and recovery from aqueous media

Novel and effective reduced graphene oxide–nickel (Ni) doped manganese oxide (RGO/Ni-MnO₂) adsorbents were fabricated via a hydrothermal approach. The reduction of graphite to graphene oxide (GO), formation of α-MnO₂, and decoration of Ni-MnO₂ onto the surface of reduced graphene oxide (RGO) were independently carried out by a hydrothermal technique. The physical and morphological properties of the as-synthesized adsorbents were analyzed. Batch adsorption experiments were performed to identify the lithium uptake capacities of adsorbents. The optimized parameters for Li⁺ adsorption investigated were pH = 12, dose loading = 0.1 g, Li⁺ initial concentration = 50 mg L⁻¹, in 10 h at 25 °C. It is noticeable that the highest adsorption of Li⁺ at optimized parameters are in the following order: RGO/Ni3-MnO₂ (63 mg g⁻¹) > RGO/Ni2-MnO₂ (56 mg g⁻¹) > RGO/Ni1-MnO₂ (52 mg g⁻¹). A Kinetic study revealed that the experimental data were best designated pseudo-second order for each adsorbent. Li⁺ desorption experiments were performed using HCl as an extracting agent. Furthermore, all adsorbents exhibit efficient regeneration ability and to some extent satisfying selectivity for Li⁺ recovery. Briefly, it can be concluded that among the fabricated adsorbents, the RGO/Ni3-MnO₂ exhibited the greatest potential for Li⁺ uptake from aqueous solutions as compared to others.

Novel and effective reduced graphene oxide–nickel (Ni) doped manganese oxide (RGO/Ni-MnO₂) adsorbents were fabricated via a hydrothermal approach for lithium adsorption and recovery from aqueous media.

Preparation of Flexible Substrate Electrode for Supercapacitor With High-Performance MnO2 Stalagmite Nanorod Arrays

A large-area MnO₂ stalagmite nanorod arrays (SNAs) growing vertically on flexible substrates were successfully fabricated by an easy heat-electrodeposition method. The large specific capacitance (646.4 F g⁻¹ at 500 mA g⁻¹) and excellent rate capability (42.3% retention with 40 times of increase) indicate that the prepared MnO₂ SNAs flexible electrode has outstanding electrochemical performance. Furthermore, after 5,000 repetitions of CV tests, the overall specific capacitance could retain ~101.2% compared with the initial value meant a long cycling life. These outstanding properties could be ascribed to the effective conductive transport path between Ni substrate and MnO₂ nanorods, and owing to the stalagmite like structure of MnO₂ nanorods, the exposed sufficient active sites are beneficial to the electrolyte infiltration.