Experimental and Theoretical Investigation of the Effect of Oxygen Vacancies on the Electronic Structure and Pseudocapacitance of MnO2

Stabilizing α-MnO2 Tunnel Structure via Mo-Zn Synergistic Doping for Highly Efficient Zn2+ Storage

To address the rising demand for eco-friendly and efficient energy storage devices, rechargeable aqueous zinc ion batteries (AZIBs) are emerging as a promising candidate for large-scale energy storage. α-MnO2 has attracted extensive attention for its open channels and exceptional Zn2+ storage capacity. However, the electrochemical performance of α-MnO2 is significantly hindered by severe structural collapse and sluggish reaction kinetics. Herein, we propose a simple hydrothermal approach for co-doping Mo and Zn into tunnel-structured MnO2 (MZMO). The ion diffusion kinetics of MZMO are optimized due to ameliorated electrical conductivity by doped cations and introduced oxygen vacancies within the MnO2 lattice. Moreover, Mo and Zn co-doping stabilizes the MnO2 framework, significantly enhancing its electrochemical performance during prolonged cycling. Charge storage mechanism analysis further validates the extraordinary stability of the MZMO phase structure during the Zn2+/H+ co-intercalation and deintercalation. The MZMO cathode demonstrates rapid and reversible Zn2+ storage, with a high capacity of 395 mAh g-1 at 0.2 A g-1, and the capacity remains at 136 mAh g-1 after 1000 cycles at 2 A g-1. This study demonstrates Mo and Zn co-doping is an effective strategy to enhance the electrochemical performance of MnO2, offering valuable insights for developing other promising cathodes for AZIBs.

Reinforced supercapacitor electrode via reduced graphene oxide encapsulated NiTe2-FeTe2 hollow nanorods

Metal telluride-based nanomaterials have garnered considerable interest as positive electrode materials for supercapacitors due to their plentiful redox-active sites, robust chemical stability, and excellent electrical conductivity. In this work, these advantageous properties are further enhanced by hybridizing NiTe2-FeTe2 (NFT) hollow nanorods with reduced graphene oxide (RGO), resulting in an NFT@RGO composite suitable for supercapacitor applications. The hollow rod-like structure promotes efficient ion diffusion and maximizes the exposure of electroactive sites, while the RGO network boosts conductivity and mitigates nanomaterial agglomeration, thus preserving structural integrity and prolonging material durability. The NFT@RGO-based electrode exhibits a notable capacity of 1388.5 C g-1 at 1 A g-1, with 93.82% capacity retention after 10 000 cycles. This remarkable performance arises from the synergistic contributions of the Ni and Fe metals, the electrically conductive Te element, the RGO framework, and the unique hollow morphology of the nanorods. Furthermore, a hybrid device employing activated carbon (AC) as the negative electrode (NFT@RGO//AC) achieves an energy density of 61.11 W h kg-1 and retains 89.85% of its capacity over 10 000 cycles, underscoring the promise of NFT@RGO for next-generation supercapacitors. These findings position the designed nanomaterial as an excellent candidate for high-performance energy storage systems.

Ultrafast Charged MnO2 Nanosheets/Carbon Fiber with Mechanical and Postcharging Antibacterial Activity

Postcharging antibacterials have shown good application prospects in combating bacterial infections through electrical interaction. Herein, manganese oxide nanosheets in situ grown on carbon fibers (CM) are designed to perform the integration of mechanical intervention and postcharging therapy for efficient bacterial killing. This electrode disrupts bacterial membranes via sharp-edged microstructures. After charging at a low voltage in an ultrashort time, the charged CM affects the extracellular electron transfer (EET) of bacteria during the discharge process to kill the bacteria. Due to the dual-antibacterial mode, after charging at -1 V (vs saturated calomel electrode, SCE) for only 50.4 ± 3 s, the bacteria lethality rates of the CM against Escherichia coli and Staphylococcus aureus within 0.5 h both exceed 98%. Our developed ultrafast negatively charged CM exhibits high antibacterial activity and low cytotoxicity to fibroblast cells, providing a non-antibiotic approach to combat bacterial infection.

Anode Glow Discharge Electrolysis Synthesis of Flower-Like α-MnO2 Nanospheres: Structure, Formation Mechanism, and Supercapacitor Performance

A novel green synthesis strategy-anode glow discharge electrolysis (AGDE) was employed for one-step preparation of α-MnO2 in 2 g L-1 KMnO4 solution, in which Pt needle and carbon rod were regarded as anode and cathode, respectively. The optimal preparation condition is 400 V for 60 min and the power consumption is below 45 W. The XRD, Raman spectra, XPS and EPR proved that α-MnO2 with structural defects (oxygen vacancies) is obtained. SEM and TEM revealed that α-MnO2 shows a flower-like nanospheres with a diameter of 165 nm, which is assembled by many nanosheets. A possible formation mechanism is that the MnO2 is generated via the reduction of MnO4 - by H⋅ and eaq - in plasma-liquid interface. Electrochemical test found that MnO2 nanospheres exhibit a specific capacitance of 365 F g-1 at 1 A g-1, and capacity retention of 79.8 % after 10,000 cycles at 5 A g-1. The assembled asymmetric supercapacitor shows the maximum energy density of 23.1 Wh kg-1 at power density of 1.89 kW kg-1. In brief, AGDE is a simple, facile and green technique for the synthesis of α-MnO2 without adding extra chemicals, and prepared α-MnO2 can be considered as an excellent candidate of electrode materials for supercapacitor.

Triggering Redox Active Sites Through Electronic Structure Modulation in rGO Encapsulated Mixed Transition Metal Oxides Hybrid for Alkaline Hydrogen Evolution

Designing and developing noble-metal-free catalysts are of current interest in clean hydrogen generation via water splitting. As carbonaceous species are ideal choices as templates for various electrocatalysis, an improved synthetic route and an in-depth understanding of their electrochemical performance are essential. Herein, we have investigated the catalytic performance of rGO-encapsulated Mn and V mixed oxide hybrid structures (MVG) on a NiFeP matrix, focusing on their potential for catalyzing hydrogen evolution in an alkaline environment. The hierarchical MVG hollow microspheres hybrids are synthesized via a simple one-step in situ solvothermal method and MVG/NiFeP coatings are developed by facile electroless plating technique. As evidenced from the X-ray photoelectron spectroscopy, the multiple redox active sites in the 3d-band of Mn and V in MVG hybrid structural coatings serve as electron pumps, and rGO facilitates electronic conductions during catalytic reactions. The modulated electronic structure and strong synergistic effects between NiFeP and MVG facilitate rapid electron transfer kinetics, and the hybrids demonstrate superior HER performance. Consequently, the structural hybrid coatings possess an enhanced electronic conducting path (lower RCT = 545.3 Ω) and large ECSA values with a lower overpotential of 85 mV at 10 mA cm-2 and a reduced Tafel slope of 64.1 mV dec-1 with Volmer-Heyrovsky mechanism in alkaline media.

Advanced three-dimensional hierarchical porousα-MnO2nanowires network toward enhanced supercapacitive performance

Hierarchicalα-MnO2nanowires with oxygen vacancies grown on carbon fiber have been synthesized by a simple hydrothermal method with the assistance of Ti4+ions. Ti4+ions play an important role in controlling the morphology and crystalline structure of MnO2. The morphology and structure of the as-synthesized MnO2could be tuned fromδ-MnO2nanosheets to hierarchicalα-MnO2nanowires with the help of Ti4+ions. Based on its fascinating properties, such as many oxygen vacancies, high specific surface area and the interconnected porous structure, theα-MnO2electrode delivers a high specific capacitance of 472 F g-1at a current density of 1 A g-1and the rate capability of 57.6% (from 1 to 16 A g-1). The assembled symmetric supercapacitor based onα-MnO2electrode exhibits remarkable performance with a high energy density of 44.5 Wh kg-1at a power density of 2.0 kW kg-1and good cyclic stability (92.6% after 10 000 cycles). This work will provide a reference for exploring and designing high-performance MnO2materials.

Strain-Induced Electronic Structure Modulation on MnO2 Nanosheet by Ir Incorporation for Efficient Water Oxidation in Acid

Oxygen electrochemistry plays a key role in renewable energy technologies, such as fuel cells and electrolyzers, but its slow kinetics limits the performance and the commercialization of such devices. Here, a strained MnO2 nanosheet induced by Ir incorporation is developed with optimized electronic structure by a simple hydrothermal method. With the incorporation of Ir, the strain induces elongated Mn─O bond length, and thereby tuning the electronic structure to favor the oxygen evolution reaction (OER) performance. The obtained catalyst exhibits an excellent mass activity of 5681 A g-1 at an overpotential of 300 mV in 0.5 m H2 SO4 , and reaches 50 and 100 mA cm-2 at overpotentials of only 240 and 277 mV, respectively. The catalyst is also stable even at 300 mA cm-2 in 0.5 m H2 SO4 . Using the nanosheet as the OER catalyst and the Pt/C as the hydrogen evolution reaction catalyst, a two-electrode electrolyzer achieves 10 mA cm-2 with only a cell voltage of 1.453 V for overall water splitting in 0.5 m H2 SO4 . This strategy enables the material with high feasibility for practical applications on hydrogen production.

Local Electric Field Induced by Atomic-Level Donor-Acceptor Couple of O Vacancies and Mn Atoms Enables Efficient Hybrid Capacitive Deionization

Transition metal oxides suffer from slow salt removal rate (SRR) due to inferior ions diffusion ability in hybrid capacitive deionization (HCDI). Local electric field (LEF) can efficiently improve the ions diffusion kinetics in thin electrodes for electrochemical energy storage. Nevertheless, it is still a challenge to facilitate the ions diffusion in bulk electrodes with high loading mass for HCDI. Herein, this work delicately constructs a LEF via engineering atomic-level donor (O vacancies)-acceptor (Mn atoms) couples, which significantly facilitates the ions diffusion and then enables a high-performance HCDI. The LEF boosts an extended accelerated ions diffusion channel at the particle surface and interparticle space, resulting in both remarkably enhanced SRR and salt removal capacity. Convincingly, the theoretical calculations demonstrate that electron-enriched Mn atoms center coupled with an electron-depleted O vacancies center is formed due to the electron back-donation from O vacancies to adjacent Mn centers. The resulted LEF efficiently reduce the ions diffusion energy barrier. This work sheds light on the effect of atomic-level LEF on improving ions diffusion kinetics at high loading mass application and paves the way for the design of transition metal oxides toward high-performance HCDI applications.

IrO2 Nanoparticle-Decorated Ir-Doped W18O49 Nanowires with High Mass Specific OER Activity for Proton Exchange Membrane Electrolysis

The oxygen evolution reaction (OER) severely limits the efficiency of proton exchange membrane (PEM) electrolyzers due to slow reaction kinetics. IrO₂ is currently a commonly used anode catalyst, but its large-scale application is limited due to its high price and scarce reserves. Herein, we reported a practical strategy to construct an acid OER catalyst where Iridium oxide loading and iridium element bulk doping are realized on the surface and inside of W₁₈O₄₉ nanowires by immersion adsorption, respectively. Specifically, W_0.7Ir_0.3O_y has an overpotential of 278 mV at 10 mA·cm^-2 in 0.1 M HClO₄. The mass activity of 714.10 A·g_Ir^-1 at 1.53 V vs. the reversible hydrogen electrode (RHE) is 80 times that of IrO₂, and it can run stably for 55 h. In the PEM water electrolyzer device, its mass activity reaches 3563.63 A·g_Ir^-1 at the cell voltage of 2.0 V. This improved catalytic performance is attributed to the following aspects: (1) The electron transport between iridium and tungsten effectively improves the electronic structure of the catalyst; (2) the introduction of iridium into W₁₈O₄₉ by means of elemental bulk doping and nanoparticles supporting for the enhanced conductivity and electrochemically active surface area of the catalyst, resulting in extensive exposure of active sites and increased intrinsic activity; and (3) during the OER process, partial iridium elements in the bulk phase are precipitated, and iridium oxide is formed on the surface to maintain stable activity. This work provides a new idea for designing oxygen evolution catalysts with low iridium content for practical application in PEM electrolyzers.

Engineering Oxygen Vacancies on Mixed-Valent Mesoporous α-MnO2 for High-Performance Asymmetric Supercapacitors

Intrinsically poor conductivity and sluggish ion-transfer kinetics limit the further development of electrochemical storage of mesoporous manganese dioxide. In order to overcome the challenge, defect engineering is an effective way to improve electrochemical capability by regulating electronic configuration at the atomic level of manganese dioxide. Herein, we demonstrate effective construction of defects on mesoporous α-MnO₂ through simply controlling the degree of redox reaction process, which could obtain a balance between Mn³⁺/Mn⁴⁺ ratio and oxygen vacancy concentration for efficient supercapacitors. The different structures of α-MnO₂ including the morphology, specific surface area, and composition are successfully constructed by tuning the mole ratio of KMnO₄ to Na₂SO₃. The electrode materials of α-MnO₂-0.25 with an appropriate Mn³⁺/Mn⁴⁺ ratio and abundant oxygen vacancy showed an outstanding specific capacitance of 324 F g^-1 at 0.5 A g^-1, beyond most reported MnO₂-based materials. The asymmetric supercapacitors formed from α-MnO₂-0.25 and activated carbon can present an energy density as high as of 36.33 W h kg^-1 at 200 W kg^-1 and also exhibited good cycle stability over a wide voltage range from 0 to 2.0 voltage (kept at approximately 98% after 10 000 cycles in galvanostatic cycling tests) and nearly 100% Coulombic efficiency. Our strategy lays a foundation for fine regulation of defects to improve charge-transfer kinetics.

Manganese oxides activated peroxymonosulfate for ciprofloxacin removal: Effect of oxygen vacancies and chemical states

Ciprofloxacin (CIP) as an anti-inflammatory drug is frequently detected in various water resources. Recently, Sulfate Radical-based advanced oxidation processes with manganese oxides have been recognized as a highly effective method for CIP degradation. Herein, ε-MnO₂ was obtained through a convenient drying process. After different atmospheric treatments, MnO and Mn₂O₃ were fabricated for subsequent degradation experiments. The results show that MnO exhibits better catalytic activity than Mn₂O₃, with high removal efficiency of almost 84.3% for CIP. Quenching test and electron paramagnetic resonance spectra confirm that ¹O₂ is the dominant species during reaction, while ·OH and SO₄^·- play a supporting role. A related discussion about the role of valence states of Mn and oxygen vacancies is presented, which can provide a theoretical basis for further development of Mn/peroxymonosulfate system.

Synthesis of flower-like MnO2 nanostructure with freshly prepared Cu particles and electrochemical performance in supercapacitors

Four types of flowerlike manganese dioxide in nano scale was synthesized via a liquid phase method in KMnO₄-H₂SO₄ solution and Cu particles, wherein the effect of Cu particles was investigated in detail. The obtained manganese dioxide powder was characterized by XRD, SEM and TEM, and the supercapacity properties of MnO₂ electrode materials were measured. The results showed that doping carbon black can benefit to better dispersion of copper particles, resulting in generated smaller size of Cu particles, and the morphology of MnO₂ nanoparticles was dominated by that of Cu particles. The study of MnO₂ synthesis by different sources of Cu particles showed that the size of MnO₂ particles decreased significantly with freshly prepared fine copper powder compared with using commercial Cu powder, and the size of MnO₂ particles can be further reduced to 120 nm by prepared Cu particles with smaller size. Therefore, it was suggested that the copper particles served as not only the reductant and but also the nuclei centre for the growth of MnO₂ particles in synthesis process MnO₂, and that is the reason how copper particles worked on the growth of flower-like MnO₂ and electrochemical property. In the part of investigation for electrochemical property, the calculated results of b values indicated that the electrode materials have pseudo capacitance property, and the highest specific capacitance of 197.2 F g^-1 at 2 mV s^-1 and 148 F/g at 1 A/g were obtained for MCE electrode materials (MnO₂ was synthesized with freshly prepared copper particles, where carbon black was used and dispersed in ethanol before preparation of Cu particles). The values of charge transfer resistance in all types of MnO₂ materials electrodes were smaller than 0.08 Ω. The cycling retention of MCE material electrode is still kept as 93.8% after 1000 cycles.

Ti3C2 supported transition metal oxides and silver as catalysts toward efficient electricity generation in microbial fuel cells

The improved electricity generation performance of MFCs could be attributed to the Ti₃C₂ support and the synergistic effect between transition metal oxides and silver for ORR.

Mn-Doped material synthesized from red mud and rice husk ash as a highly active catalyst for the oxidation of carbon monoxide and p-xylene

Red muad and rice husk ash were treated without neutralization by acid to produce a support material (RR).