Shrinivas. B. Kulkarni

We report a systematic study on the angular and thickness-dependent phonon mode behaviour in SMS, single crystals using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The SMS crystal was rotated over a full 0°–360° range with its crystallographic a-axis initially aligned parallel to the THz electric field and the THz wave propagating along the [001] direction. A strong correlation is observed between the polarization angle, emergence of phonon modes' and spectral splitting. An increase in crystal thickness results in enhanced phonon mode splitting: the 0.18 mm and 0.35mm-thick crystals exhibit a single resonance within the 0.7–1.2 THz range, while the 0.65, 1.06 and 1.27 mm-thick crystals display well-resolved mode splitting. To interpret this thickness-dependent behavior, we consider the formation of bulk phonon-polaritons. The single resonance at 0.94 THz, which dominates the thin-sample spectra, is the TO phonon mode (ω _TO ) of the SMS lattice. In a polar crystal, this TO phonon couples strongly to photons, creating a hybrid polariton with a characteristic dispersion relation defined by a LPB and an UPB, separated by the Reststrahlen band. In the thin crystals (L ≤ 0.35 mm), the interaction path length is insufficient to fully resolve the bulk polariton dispersion, resulting in a single absorption feature. However, as the thickness increases (L ≥ 0.65 mm), the system transitions to a bulk-like response where the propagating LPB and UPB modes are clearly resolved. The observed splitting increases with thickness, consistent with other polar material systems and models of polaritons in finite-sized media, where the vacuum Rabi splitting scales with the interaction volume. This transition from a single-phonon peak to a split polariton doublet demonstrates how sample thickness gates the strong light-matter coupling regime. The 0.65 mm-thick crystal exhibits four distinct anisotropic lobes centred near 0.94 THz, emphasizing the inherent anisotropic phonon response of the SMS crystal.

This study presents a comparative analysis of cobalt oxide (Co₃O₄) thin films synthesized via electrodeposition using two different alkaline reagents—ammonium hydroxide (NH₄OH) and sodium hydroxide (NaOH)—on stainless steel substrates. The film prepared with NH₄OH exhibited enhanced crystallinity, confirmed by sharper (311) XRD peaks, and a highly porous morphology favourable for charge transport. FTIR and XPS characterizations confirmed the successful formation of spinel Co₃O₄ and the appropriate oxidation states of cobalt. Electrochemical evaluation in 1 M KOH showed that the electrode synthesized using NH₄OH delivered a specific capacitance of 1323 F/g at 10 mV/s, while maintaining 95.74% of its capacitance after 2000 charge–discharge cycles. These findings highlight that the type of alkaline reagent plays a crucial role in tailoring the structural and electrochemical behaviour of Co₃O₄ thin films, offering a viable route to optimize electrodes for enhanced energy storage supercapacitor applications.

The escalating global energy demand necessitates the development of efficient, sustainable, and robust energy storage solutions. Supercapacitors, bridging the gap between conventional capacitors and batteries, have attracted significant attention due to their high-power density, fast charge–discharge capability, and long cycle life. In this study, a novel approach was employed for the direct growth of mixed quaternary metal oxides (QMOs) comprising cobalt, nickel, copper, and zinc on various conductive substrates, including carbon cloth, stainless steel mesh, and nickel foam, using an electrodeposition method. The fabricated hierarchical 3D nanostructures were systematically characterized to evaluate their electrochemical performance. The CoNiCuZn oxide electrode exhibited outstanding capacitive behavior, achieving specific capacitances of 885.13 F/g, 2965.91 F/g, and 3824.34 F/g at 5mV/s. Specific capacity for carbon cloth, SS mesh, and Ni foam as 288 mAh/g, 908.33 mAh/g, and 2078.33 mAh/g, respectively, at 5mV/s, with capacitance retention of 80%, 89%, and 94% after 2500 cycles. The symmetric CoNiCuZn oxide/CoNiCuZn oxide device delivers an energy density of 14.32 Wh/kg at 1.36 kW/kg with 60.27% capacitance retention, supported by low Rs and Rct values. These results confirm excellent rate capability, stability, and strong potential for practical supercapacitor applications. These findings highlight the potential of mixed quaternary metal oxides as cost-effective and eco-friendly electrode materials for the next generation of flexible, high-performance supercapacitors.

Battery‐type electrode materials with exceptionally high specific energy have been recognized as prospective materials for hybrid supercapacitor (HSC). In this study, a ternary NiMoO ₄ ‐rGO@V ₂ O ₅ composite is synthesized for high performance HSC applications. The strategic integration of nickel molybdate (NiMoO ₄ ), reduced graphene oxide (rGO), and vanadium pentoxide (V ₂ O ₅ ) combines the fast‐redox activity of NiMoO ₄ , the high electrical conductivity and surface area of rGO, and the excellent pseudocapacitive behavior of V ₂ O ₅ . The NiMoO ₄ ‐rGO@V ₂ O ₅ composite obtains pore structure ranging from 20–30 nm with a higher specific surface area of 176.2 m ² g ⁻¹ and superior conductivity, demonstrating enhanced capabilities regarding the transfer of charge and diffusion of ions. Electrochemical measurements demonstrate that the NiMoO ₄ ‐rGO@V ₂ O ₅ composite exhibits a high specific capacity of 361.5 mAh g ⁻¹ at 1 A g ⁻¹ , excellent rate capability, and remarkable cyclic stability of 75% after 10,000 cycles. Furthermore, an HSC device assembled using the optimized composite as the positive electrode and activated carbon as the negative electrode and it delivered a capacitance of 207.5 F g ⁻¹ at 2 mA cm ⁻² with a specific energy of 73.77 Wh kg ⁻¹ at a specific power of 640 W kg ⁻¹ . These outcomes underscore the potential of NiMoO ₄ ‐rGO@V ₂ O ₅ composites as cutting‐edge electrode materials for next‐generation energy storage devices.

The demand for high-performance supercapacitors has driven extensive research into novel electrode materials with superior electrochemical properties. This study explores the supercapacitive behavior of quaternary CuNiCoZnO (CNCZO) films engineered into a three-dimensional (3D) flower-like morphology and developed on versatile substrates, including carbon cloth, stainless steel mesh, and nickel foam. The unique structural design, comprising interconnected nanosheets, enhances the electroactive surface area, facilitates ion diffusion, and improves charge storage capability. The synergistic effect of the multi-metallic composition contributes to remarkable electrochemical characteristics, including high specific capacitance, excellent rate capability, and outstanding cycling stability. Furthermore, the influence of different substrates on the electrochemical performance is systematically investigated to optimize material–substrate interactions. Electrochemical evaluations reveal outstanding specific capacitance values of 2318.5 F/g, 1993.7 F/g, and 2741.3 F/g at 2 mA/cm² for CNCZO electrodes on stainless steel mesh, carbon cloth, and nickel foam, respectively, with capacitance retention of 77.3%, 95.7%, and 86.1% over 5000 cycles. Furthermore, a symmetric device of CNCZO@Ni exhibits a peak specific capacitance of 67.7 F/g at a current density of 4 mA/cm², a power density of 717.4 W/kg, and an energy density of 25.6 Wh/kg, maintaining 84.5% stability over 5000 cycles. The straightforward synthesis of CNCZO on multiple substrates presents a promising route for the development of flexible, high-performance energy storage devices.

Transition metal molybdates are one of the most prominent materials for energy storage devices. The present investigation establishes a strong correlation between the structure and electrochemical performance of NiMoO ₄ through Density Functional Theory (DFT). Initially, the NiMoO ₄ microspheres are directly deposited on nickel foam using a hydrothermal method by tuning experimental parameters. When employed as electrode materials, the NiMoO ₄ microspheres deliver a specific capacity of 168.9 mAh g ⁻¹ at 1 A g ⁻¹ . In addition, the material retains 80% capacity over 7000 charge‐discharge cycles with 98.3% coulombic efficiency, implying its excellent stability. DFT calculations are used to determine specific capacity and potassium ion diffusion for 5 layers of [110] planes of NiMoO ₄ . The potential energy landscape is created for [110] plane using the potassium atom minimum hopping algorithm and atomic simulation environment. The DFT results clearly align with the theoretical capacity of 203 mAh g ⁻¹ close to the experimental results. A hybrid supercapacitor (HSC) is also developed with NiMoO ₄ //AC cell delivers a specific energy of 56.3 Wh kg ⁻¹ at a specific power of 421 W kg ⁻¹ with negligible capacity loss over 15 000 cycles. This investigation offers the development of battery‐type electrodes for hybrid supercapacitors using the fundamental understanding of ion‐diffusion in the materials’ structure.

Transition metal molybdates (AMoO₄ where A = Ni, Co, Mn, Fe, and Zn) have attracted much attention as promising electrode materials for energy storage devices due to their multi‐electron redox capability, higher electrical conductivity, good chemical and thermal stability, and stable crystal structure to get superior electrochemical performance. Transition metal molybdates and their graphene‐based composites possess multidimensional morphology for supercapacitors. The morphology‐dependent supercapacitor behavior has been reviewed in the present article. The formation mechanism of AMoO₄ nanostructures in the form of 1D, 2D, and 3D has been identified and respective supercapacitor behavior is outlined. The density functional theory based on the calculated electronic properties of AMoO₄ has been discussed. Additionally, the application of machine learning techniques in predicting and analyzing the relationships of AMoO₄ has been discussed for the first time. By leveraging ML algorithms, we identify key parameters influencing their energy storage capabilities, providing insights into the rational design of molybdate‐based composites. Integrating experimental results with ML‐driven optimization offers a novel pathway for accelerating the development of next‐generation energy storage devices. In conclusion, future perspectives and challenges have been discussed.