"Unveiling marigold assembled micro flowers of tungsten oxide towards solid-state flexible pouch and coin cell supercapacitors"

Marigold analogues micro flowers of tungsten oxide (WO3) have been grown in thin film form through simple and cost-effective solution chemistry approach on stainless steel substrate. Aqueous precursor involving WO4-2 ions agglomerated as self-sacrificing template growing initially into the nano-petal, followed by self-assembly; leading to marigold analogues micro flower surface architecture. This enthralling morphology motivated us not only to fabricate supercapacitive electrode but also to design complete solid-state supercapacitor devices in dual configurations: flexible pouch cell and coin cell. Interestingly, both devices even in symmetric configuration yields remarkable potential window of 1.82 V when sandwiched by gel inclusive of Li+ ions dispersed in non-conducting polyvinyl alcohol matrix. Solid-state flexible pouch cell and coin cell delivered specific capacitances of 103.98 ± 3.59 and 30.09 ± 1.03 F/g respectively at a scan rate of 5 mV/s. Assembled electrode, coin-cell and flexible pouch-cells have been well assessed in-depth through specific capacitances using cyclic voltammetry and galvanostatic charge discharge, diffusive and capacitive contributions, mechanical bending tests, electrochemical active surface area, and electrochemical impedance analysis. Practical applicability has been demonstrated for designed flexible pouch cell to run small fan and light emitting diode panel whereas coin cell to run light emitting diode panel.

Exploring Vacuum-Assisted Thin Films toward Supercapacitor Applications: Present Status and Future Prospects

Demand for high-performance energy storage devices is growing tremendously. Supercapacitors possess an excellent candidature to fulfill the energy storage requisites such as high energy density when compared to conventional capacitors, high power density, and cycling stability as compared to batteries, though not only for large-scale devices for higher energy/power density applications but also for macro- to microdevices for miniaturized electrical components. With the aid of various routes, many materials have been explored with well-tuned properties with controlled surface architecture through various preparative parameters to find those best suited for supercapacitive electrodes. Growth of a thin film can be accomplished through chemical or physical (vacuum-assisted) routes. Vacuum-assisted (physical) growth yields high purity, precise dimensions with a line-of-sight deposition, along with high adhesion between the film and the substrates, and hence, these techniques are necessary to manufacture many macro- to microscale supercapacitor devices. Still, much effort has not been put forth to explore vacuum-assisted techniques to fabricate supercapacitive electrodes and energy storage applications. The present review explores the first comprehensive report on the growth of widespread materials through vacuum-assisted physical deposition techniques inclusive of thermal evaporation, e-beam evaporation, sputtering, and laser beam ablation toward supercapacitive energy storage applications on one platform. The theoretical background of nucleation and growth through physical deposition, optimization of process parameters, and characterization to supercapacitor applications from macro- to microscale devices has been well explored to a provide critical analysis with literature-reviewed materials. The review ends with future challenges to bring out upcoming prospects to further enhance supercapacitive performance, as much work and materials need to be explored through these routes.

Lamellar structured Ni3P2O8: first-ever use to design 1.8 V operated flexible all-solid-state symmetric supercapacitor

Increasing demand for microelectronic devices necessitates the development of highly flexible energy storage technologies with a wide operating voltage. Thus, flexible electrodes and their devices with the requisite mechanical and electrochemical characteristics have prime importance. In this regard, the present article demonstrates the feasibility of designing a flexible all-solid-state supercapacitor using a chemically grown Ni₃P₂O₈ lamellar microstructured electrode embedded with carboxy methyl cellulose-Na₂SO₄ (CMC-Na₂SO₄) gel electrolyte. The formed symmetric device impressively exhibited a maximum working voltage window of 1.8 V with a high specific energy of 44.7 W h kg^-1 and specific power of 3.3 kW kg^-1 along with prolonged cycle life. Also, the device's high deformation tolerance (95%) when bent at 170° with a flashing light-emitting diode (LED) working demonstration showcases its viability for advanced flexible energy storage applications.

Tungsten-Modulated Molybdenum Selenide/Graphene Heterostructure as an Advanced Electrode for All-Solid-State Supercapacitors

Transition metal dichalcogenides (TMDs) have attracted widespread attention due to their excellent electrochemical and catalytic properties. In this work, a tungsten (W)-modulated molybdenum selenide (MoSe₂)/graphene heterostructure was investigated for application in electrochemistry. MoSe₂/graphene heterojunctions with low-doped W compositions were synthesized by a one-step hydrothermal catalysis approach. Based on the conducted density functional theory (DFT) calculations, it was determined that inserting a small amount of W (≈5%) into the MoSe₂/graphene heterostructure resulted in the modification of its lattice structure. Additionally, an increase in the distance between layers (≈8%) and a decrease in the adsorption energy of the potassium ions (K⁺) (≈−1.08 eV) were observed following W doping. Overall, the electrochemical performance of the MoSe₂/graphene hybrid was enhanced by the presence of W. An all-solid-state supercapacitor device prepared using electrodes based on the W-doped MoSe₂/graphene composite achieved excellent capacitance of 444.4 mF cm⁻² at 1 mV s⁻¹. The results obtained herein revealed that the MoSe₂/graphene hybrid exhibiting low W composition could be valuable in the field of energy storage and isoelectronic doping of TMDs.

Effect of H2SO4/H2O2 pre-treatment on electrochemical properties of exfoliated graphite prepared by an electro-exfoliation method

The effect of pre-treating graphite sheets in a H2SO4/H2O2 solution before electro-exfoliation is reported. It was revealed that the volume fraction of H2SO4 to H2O2 during pre-treatment could control the degree of exfoliation of the resulting exfoliated graphite (EG). X-ray diffraction (XRD), Raman, and Fourier transform infrared (FTIR) spectroscopy analyses have suggested that EG produced by first pre-treating the graphite sheet in H2SO4/H2O2 solution with the H2SO4 : H2O2 volume fraction of 95 : 5 demonstrates the highest exfoliation degree. This sample also demonstrated excellent electrochemical properties with good electrical conductivity (36.22 S cm-1) and relatively low charge transfer resistance (R ct) of 21.35 Ω. This sample also showed the highest specific capacitance of all samples, i.e., 71.95 F g-1 at 1 mV s-1 when measured at a voltage range of -0.9 to 0 V. Further measurement at an extended potential window down to -1.4 V revealed the superior specific capacitance value of 150.69 F g-1. The superior morphology characteristics and the excellent electrical properties of the obtained EG are several reasons behind its exceptional properties. The pre-treatment of graphite sheets in H2SO4/H2O2 solution allegedly leads to easier and faster exfoliation. The faster exfoliation is allegedly able to prevent massive oxidation, which frequently induces the formation of graphite/graphene oxide (GO) in a prolonged process. However, too large H2O2 volume fraction involved during pre-treatment seems to cause excessive expansion and frail structure of the graphite sheets, which leads to an early breakdown of the structure during electrochemical exfoliation and prohibits layer by layer exfoliation.

Lichen-like anchoring of MoSe2 on functionalized multiwalled carbon nanotubes: an efficient electrode for asymmetric supercapacitors

In the present study, we have developed a composite electrode of MSNT using a simple and scalable two-step scheme to synthesize a composite electrode material comprising MoSe₂/multiwalled carbon nanotubes (MoSe₂/MWCNTs) for supercapacitor applications. First, a MWCNT thin film was deposited on a stainless steel substrate by using a “dip and dry” coating technique. Subsequently, MoSe₂ was deposited onto the MWCNT thin film using the successive ionic layer adsorption and reaction method. The lichen-like growth of MoSe₂ on the MWCNT network provided dual charge storage and an effective ion transfer path. The composite electrode of MSNT has been studied systematically with different electrolytes and concentrations of electrolyte. As a result, the MoSe₂/MWCNT (MSNT) electrode exhibited excellent electrochemical properties such as a specific capacity of 192 mA h g⁻¹ and a capacitance retention of 88% after 2000 cycles in 1 M LiCl electrolyte. The results demonstrated the huge potential of the MSNT composite electrode for practical application in supercapacitors. The aqueous symmetric cell fabricated using the MSNT composite as both the anode and cathode showed an energy density of 17.9 W h kg⁻¹. Additionally, the energy density improved by designing an asymmetric device of MSNT//MnO₂ and notably, it reveals two-fold improvement in the energy density compared to a symmetric MSNT cell. The MSNT//MnO₂-based asymmetric cell exhibited a maximum specific capacitance of 112 F g⁻¹ with a high energy density of 35.6 W h kg⁻¹.

Simple and scalable chemical synthesis approach to develop a MoSe₂/MWCNTs composite thin film electrode for a highly efficient asymmetric supercapacitor cell.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.