Spray deposited Hausmannite Mn3O4 thin films using aqueous/organic solvent mixture for supercapacitor applications

Colloidal Stability, Sedimentation, and Aggregation of Crystalline Two-Dimensional Crumpled Birnessite Flakes, Their Dye Adsorption and Immune Cell Response

Highly crystalline two-dimensional (2D) flakes of birnessite, a polymorph of manganese oxide with a MnO2 chemistry, were synthesized by reacting manganese oxide, Mn3O4, at 80 °C with aqueous solutions of tetramethylammonium hydroxide (TMAH) for tens of hours. Their colloidal stability, aggregation, and sedimentation were studied as a function of ionic strengths of Na+ and Li+ cations. After reaction, a water-based stable colloidal suspension (ζ-potential ∼ -31 ± 1 mV) was obtained. Mixing the colloidal suspension with a LiCl or NaCl aqueous solution resulted in the sedimentation of crumpled flakes, as evidenced by electron microscopy (transmission and scanning). Concomitant with the sedimentation, the TMA+ cations present after synthesis are exchanged by the alkali ions, as evidenced by a decrease in the d-spacings between the 2D sheets illustrated by X-ray diffraction (XRD). Both Na and Li uptakes were quantified by elemental analysis via inductively coupled plasma tandem mass spectrometry, giving Li0.17Mn0.96O2 and Na0.16Mn0.96O2. Rhodamine 6G dye was also studied as a sedimentation agent, resulting in a maximum uptake of 550 mg (1.15 mmol) of dye per g of birnessite. To explore the immune response of the Li+-intercalated crumpled flakes, the activation of antigen-presenting cells by the flakes was investigated. It was found that the immune cells were slightly activated in a dose-dependent manner, indicating that the materials may have good biocompatibility and thus possibe applications in healthcare.

Recent advances on surface mounted metal-organic frameworks for energy storage and conversion applications: Trends, challenges, and opportunities

Establishing green and reliable energy resources is very important to counteract the carbon footprints and negative impact of non-renewable energy resources. Metal-organic frameworks (MOFs) are a class of porous material finding numerous applications due to their exceptional qualities, such as high surface area, low density, superior structural flexibility, and stability. Recently, increased attention has been paid to surface mounted MOFs (SURMOFs), which is nothing but thin film of MOF, as a new category in nanotechnology having unique properties compared to bulk MOFs. With the advancement of material growth and synthesis technologies, the fine tunability of film thickness, consistency, size, and geometry with a wide range of MOF complexes is possible. In this review, we recapitulate various synthesis approaches of SURMOFs including epitaxial synthesis approach, direct solvothermal method, Langmuir-Blodgett LBL deposition, Inkjet printing technique and others and then correlated the synthesis-structure-property relationship in terms of energy storage and conversion applications. Further the critical assessment and current problems of SURMOFs have been briefly discussed to explore the future opportunities in SURMOFs for energy storage and conversion applications.

Stereolithography-Derived Three-Dimensional Pyrolytic Carbon/Mn3O4 Nanostructures for Free-Standing Hybrid Supercapacitor Electrodes

The development of permeable three-dimensional (3D) macroporous carbon architectures loaded with active pseudocapacitive nanomaterials offers hybrid supercapacitor (SC) materials with higher energy density, shortened diffusion length for ions, and higher charge-discharge rate capability and thereby is highly relevant for electrical energy storage (EES). Herein, structurally complex and tailorable 3D pyrolytic carbon/Mn₃O₄ hybrid SC electrode materials are synthesized through the self-assembly of MnO₂ nanoflakes and nanoflowers onto the surface of stereolithography 3D-printed architectures via a facile wet chemical deposition route, followed by a single thermal treatment. Thermal annealing of the MnO₂ nanostructures concurrent with carbonization of the polymer precursor leads to the formation of a 3D hybrid SC electrode material with unique structural integrity and uniformity. The microstructural and chemical characterization of the hybrid electrode reveals the predominant formation of crystalline hausmannite-Mn₃O₄ after the pyrolysis/annealing process, which is a favorable pseudocapacitive material for EES. With the combination of the 3D free-standing carbon architecture and self-assembled binder-free Mn₃O₄ nanostructures, electrochemical capacitive charge storage with very good rate capability, gravimetric and areal capacitances (186 F g^-1 and 968 mF cm^-2, respectively), and a long lifespan (>92% after 5000 cycles) is demonstrated. It is worth noting that the gravimetric capacitance value is obtained by considering the full mass of the electrode including the carbon current collector. When only the mass of the pseudocapacitive nanomaterial is considered, a capacitance value of 457 F g^-1 is achieved, which is comparable to state-of-the-art Mn₃O₄-based SC electrode materials.

Spray Pyrolysis Synthesis of Pure and Mg-Doped Manganese Oxide Thin Films

Pure and Mg-doped manganese oxide thin films were synthesized on heated glass substrates using the spray pyrolysis technique. The surface chemical composition was investigated by the use of X-ray photoelectron spectroscopy (XPS). Structural and morphological properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscopy (AFM). Optical properties were characterized by UV-visible spectroscopy. XPS spectra showed typical Mn (2p3/2), (2p1/2) and O (1s) peaks of Mn3O4 with a slight shift attributed to the formation of different chemical states of manganese. XRD analysis revealed the tetragonal phase of Mn3O4 with a preferred (211) growth orientation that improved with Mg-doping; likewise, grain size is observed to increase with the Mg doping. SEM images of Mn3O4 films showed rough surfaces composed of uniformly distributed nanograins whose size decreases with the Mg-doping. The manganese oxide films surface observed in AFM show a textured, rough and porous surface. The combination of transmittance and absorption data in the UV-visible range allowed determining the energy values of the Eg band gap (1.5–2.5 eV). The decrease of the band gap with the Mg-doping increase is attributed to the influence of the greater size of the Mg2+ ion in the manganese oxide lattice.

In situ mass change and gas analysis of 3D manganese oxide/graphene aerogel for supercapacitors

Manganese oxide nanoparticles decorated on 3D reduced graphene oxide aerogels (3D MnO_x/rGO_ae) for neutral electrochemical capacitors were successfully produced by a rapid microwave reduction process within 20 s.

Rapid Production of Mn₃O₄/rGO as an Efficient Electrode Material for Supercapacitor by Flame Plasma

Benefiting from good ion accessibility and high electrical conductivity, graphene-based material as electrodes show promising electrochemical performance in energy storage systems. In this study, a novel strategy is devised to prepare binder-free Mn₃O₄-reduced graphene oxide (Mn₃O₄/rGO) electrodes. Well-dispersed and homogeneous Mn₃O₄ nanosheets are grown on graphene layers through a facile chemical co-precipitation process and subsequent flame procedure. This obtained Mn₃O₄/rGO nanostructures exhibit excellent gravimetric specific capacitance of 342.5 F g⁻¹ at current density of 1 A g⁻¹ and remarkable cycling stability of 85.47% capacitance retention under 10,000 extreme charge/discharge cycles at large current density. Furthermore, an asymmetric supercapacitor assembled using Mn₃O₄/rGO and activated graphene (AG) delivers a high energy density of 27.41 Wh kg⁻¹ and a maximum power density of 8 kW kg⁻¹. The material synthesis strategy presented in this study is facile, rapid and simple, which would give an insight into potential strategies for large-scale applications of metal oxide/graphene and hold tremendous promise for power storage applications.