In this study, amorphous manganese oxide (MnO₂) nanostructured thin films were synthesized by a simple hydrothermal method. It is well known that the nanostructure plays a crucial role in energy storage applications. Herein, MnO₂ nanostructures ranging from plates to flakes were synthesized without the use of any hard or soft templates. The 4+ oxidation state of Mn was confirmed by X-ray photoelectron spectroscopy. The MnO₂ nanoflake structure has a specific surface area of 46 m²g⁻¹, which provides it with an excellent rate capability and an exactly rectangular cyclic voltammogram (CV) curve. The MnO₂ nanoflake electrode has a high specific capacitance of about 433 Fg⁻¹, an energy density of 60 Whkg⁻¹ at 0.5 mAcm⁻², and an excellent cyclic stability of 95% over 1000 CV cycles in 1 M aq. Na₂SO₄. Kinetics analysis of the charge storage in the nanoflake MnO₂ sample shows a 55.6% diffusion-controlled contribution and 44.4% capacitive-controlled contribution to the total current calculated at a scan rate of 100 mVs⁻¹ from the CV curve.

The hydrothermal method has been utilized to synthesize graphitic carbon nitride (g-C₃N₄) polymers and cobalt oxide composites effectively. The weight percentage of g-C₃N₄ nanoparticles influenced the electrochemical performance of the Co₃O₄-g-C₃N₄ composite. In an aqueous electrolyte, the Co₃O₄-g-C₃N₄ composite electrode, produced with 150 mg of g-C₃N₄ nanoparticles, revealed remarkable electrochemical performance. With an increase in the weight percentage of g-C₃N₄ nanoparticles, the capacitive contribution of the Co₃O₄-g-C₃N₄ composite electrode increased. The Co₃O₄-g-C₃N₄-150 mg composite electrode shows a specific capacitance of 198 F/g. The optimized electrode, activated carbon, and polyvinyl alcohol gel with potassium hydroxide were used to develop an asymmetric supercapacitor. At a current density of 5 mA/cm², the asymmetric supercapacitor demonstrated exceptional energy storage capacity with remarkable energy density and power density. The device retained great capacity over 6k galvanostatic charge–discharge (GCD) cycles, with no rise in series resistance following cyclic stability. The columbic efficiency of the asymmetric supercapacitor was likewise high.

In this work, Ni₃V₂O₈ (NVO) and Ni₃V₂O₈-reduced graphene oxide (NVO-rGO) are synthesized hydrothermally, and their extensive structural, morphological, and electrochemical characterizations follow subsequently. The synthetic materials’ crystalline structure was confirmed by X-ray diffraction (XRD), and its unique marigold-like morphology was observed by field emission scanning electron microscopy (FESEM). The chemical states of the elements were investigated via X-ray photoelectron spectroscopy (XPS). Electrochemical impedance spectroscopy (EIS), Galvanostatic charge–discharge (GCD), and cyclic voltammetry (CV) were used to assess the electrochemical performance. A specific capacitance of 132 F/g, an energy density of 5.04 Wh/kg, and a power density of 187 W/kg were demonstrated by Ni₃V₂O₈-rGO. Key electrochemical characteristics were b = 0.67; a transfer coefficient of 0.52; a standard rate constant of 6.07 × 10⁻⁵ cm/S; a diffusion coefficient of 5.27 × 10⁻⁸ cm²/S; and a series resistance of 1.65 Ω. By employing Ni₃V₂O₈-rGO and activated carbon, an asymmetric supercapacitor with a specific capacitance of 7.85 F/g, an energy density of 3.52 Wh/kg, and a power density of 225 W/kg was achieved. The series resistance increased from 4.27 Ω to 6.63 Ω during cyclic stability tests, which showed 99% columbic efficiency and 87% energy retention. The potential of Ni₃V₂O₈-rGO as a high-performance electrode material for supercapacitors is highlighted by these findings.

Surface microstructure modification of metal oxides also improves the electrochemical performance of metal oxide nanoparticles. The present investigation demonstrates how varying the urea molar content during the hydrothermal process altered the surfaces of MnCo₂O₄ nanoparticles. Successive increases of 0.1 M in urea concentration transformed the surface shape of MnCo₂O₄ nanoparticles from flower-like to sheet-like microstructures. Excellent electrochemical performance of MnCo₂O₄ nanoparticles was demonstrated in an aqueous 1 M KOH electrolyte. The improved MnCo₂O₄ nanoparticles have been employed to develop an asymmetric supercapacitor (ASC). The ASC device exhibits an energy density of 13 Wh/kg at a power density of 553 W/kg and a specific capacitance of 29 F g⁻¹ at a current density of 4 mA/cm². The MnCo₂O₄ nanoparticle electrode demonstrates remarkable electrocatalytic activity in both HER and OER. The MnCo₂O₄ electrode shows overpotential for HER and OER at 356 mV and 1.46 V, respectively. The Tafel slopes for HER and OER of the MnCo₂O₄ electrode are 356 mV/dec and 187 mV/dec, respectively.

Gas sensors based on graphene have gained considerable attention because of graphene’s remarkable properties, such as its extensive surface area, impressive electrical conductivity, and exceptional mechanical strength. This review critically analyzes recent developments in functionalization strategies designed to enhance the sensitivity, selectivity, and stability of graphene-based sensors. It discusses various chemical, physical, and hybrid functionalization methods, illustrating how surface alterations affect graphene’s interaction with target gas molecules. The paper also investigates the fundamental sensing mechanisms, including charge transfer, carrier mobility modulation, and Schottky barrier modification, to provide a thorough understanding of sensor response characteristics. Additionally, it highlights emerging applications in environmental monitoring, healthcare diagnostics, and industrial safety, demonstrating the transformative potential of these sensors in real-world settings. Finally, the review addresses challenges concerning reproducibility, long-term stability, and large-scale production, while also offering future insights on utilizing innovative nanomaterials and artificial intelligence to advance the next generation of graphene-based gas sensing technologies.

Electrochemical water splitting is a feasible and effective method for attaining hydrogen, offering a mechanism for renewable energy solutions to combat the world’s energy crises due to the scarcity of fossil fuels. Evidently, the viability and stability of the electrocatalysts are fundamental to the electrochemical water-splitting process. However, the net efficiency of this process is noticeably hindered by the kinetic drawbacks related to the OER. Hence, NiFe LDH has been widely used as a highly efficient OER and HER catalyst material due to its unique nanostructure, tunable composition, and favorable electronic structure. This review offers a systematic analysis of the latest progress in the fabrication of functional NiFe LDH catalysts and associated fabrication strategies, structure optimizations, and performance improvements. Special emphasis is given to understanding the role of nanostructure engineering in increasing active site accessibility, enhancing the effectiveness of subsequent electron transfer, and boosting the intrinsic catalytic activity for HER and OER. Moreover, we discuss the influence of doping, defects, and the formation of heterostructures with other materials on the OER and HER activities of NiFe LDHs. Additional accounts of basic structures and the OER and HER catalytic activities are provided, along with an enhanced theoretical understanding based on DFT studies on the NiFe LDH. Moreover, the limitations and potential developments of the work focus on the need for existing synthesis approaches, the stability of the NiFe LDH catalysts, and their insertion into working electrochemical processes. This review is a comprehensive analysis of the current state of research and developments in the use of NiFe LDH catalysts for the electrochemical water-splitting process to foster improved development of sustainable hydrogen sources in the future.