Interconnected network of zinc-cobalt layered double hydroxide stick onto rGO/nickel foam for high performance asymmetric supercapacitors

Polyoxometalate Integrated with Conducting Polymer Nanocomposites for Supercapacitor and Biological Sensor Applications

Nanostructured redox-active composite electrode materials have been developed for energy storage applications to address conventional carbon-based supercapacitor's limited electrochemical performance. Polyoxometalates (POMs) and conducting polymers (CP) have significantly enhanced the pseudocapacitive activity of these electrode materials. In this study, we synthesized H4[PVW11O40]·xH2O (PVW11) and combined it with polypyrrole (PPy) and polyaniline (PAni) separately to improve energy performance and conduct electrochemical analysis. The PVW11-PPy outperformed the PVW11-PAni composite, achieving an energy density of 49.07 W h kg-1 and a specific capacitance of 405.16 F g-1. The supercapacitor cells showed a cyclic retention of 85.13% and 99.99% Coulombic efficiency after 6000 galvanostatic charge-discharge (GCD) cycles. The PVW11-PPy composite was fabricated into a supercapacitor device that powered a set of 10 LED bulbs for 2 min using an active mass of 76 mg. Additionally, the PVW11-PPy composite material was employed to sense glucose solutions with concentrations ranging from 0.04 to 0.4 mM, providing a sensitivity of 0.325 mA mM-1 cm-2, with limits of detection (LOD) and quantification (LOQ) of 0.381 mM and 1.270 mM, respectively.

Developing Outstanding Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries Using High-Purity Spinel-Type ZnCo2 Se4 Nanoparticles

Zinc-air batteries are gaining popularity as viable energy sources for green energy storage technologies. The cost and performance of Zn-air batteries are mostly determined by the air electrodes in combination with an oxygen electrocatalyst. This research aims at the particular innovations and challenges relating to air electrodes and related materials. Here, a nanocomposite of ZnCo₂ Se₄ @rGO that exhibits excellent electrocatalytic activity for the oxygen reduction reaction, ORR (E_1/2  = 0.802 V), and oxygen evolution reaction, OER (η₁₀  = 298 mV@10 mA cm^-2 ) is synthesized. In addition, a rechargeable zinc-air battery with ZnCo₂ Se₄ @rGO as the cathode showed a high open circuit voltage (OCV) of 1.38 V, a peak power density of 210.4 mW cm^-2 , and outstanding long-term cycling stability. The electronic structure and oxygen reduction/evolution reaction mechanism of the catalysts ZnCo₂ Se₄ and Co₃ Se₄ are further investigated using density functional theory calculations. Finally, a perspective for designing, preparing, and assembling air electrodes is suggested for the future developments of high-performance Zn-air batteries.

Alkali-Etched NiCoAl-LDH with Improved Electrochemical Performance for Asymmetric Supercapacitors

Hydrotalcite, first found in natural ores, has important applications in supercapacitors. NiCoAl-LDH, as a hydrotalcite-like compound with good crystallinity, is commonly synthesized by a hydrothermal method. Al3+ plays an important role in the crystallization of hydrotalcite and can provide stable trivalent cations, which is conducive to the formation of hydrotalcite. However, aluminum and its hydroxides are unstable in a strong alkaline electrolyte; therefore, a secondary alkali treatment is proposed in this work to produce cation vacancies. The hydrophilicity of the NiCoAl-OH surface with cation vacancy has been greatly improved, which is conducive to the wetting and infiltration of electrolyte in water-based supercapacitors. At the same time, cation vacancies generate a large number of defects as active sites for energy storage. As a result, the specific capacity of the NiCoAl-OH electrode after 10,000 cycles can be maintained at 94.1%, which is much better than the NiCoAl-LDH material of 74%.

Enhancement Strategy of Photoelectrocatalytic Activity of Cobalt-Copper Layer Double Hydroxide toward Methanol Oxidation: Cerium Doping and Modification with Porphyrin

Designing durable, high-active, and low-cost noble-metal-free photoelectrocatalysts for methanol electrooxidation is highly demanded but remains a challenge. Herein, the photoelectrocatalytic activity of cobalt-copper layer double hydroxide (CoCu-LDH) for methanol oxidation reaction (MOR) in the alkaline media under light was remarkably enhanced by cerium (Ce) doping and further by 5,10,15,20-tetrakis(4-carboxylphenyl)porphyrin (TCPP) modification. TCPP/Ce-CoCu-LDH exhibits a remarkable mass activity of 1788.2 mA mg^-1 in 1 mol L^-1 KOH with 1 mol L^-1 methanol under light, which is 2.3 and 1.8 times higher than that of CoCu-LDH (782.2 mA mg^-1) and Ce-CoCu-LDH (987.4 mA mg^-1). The UV-vis diffuse reflectance spectra and photoluminescence emission spectra reveal that TCPP/Ce-CoCu-LDH can effectively utilize the visible light and inhibit the electron-hole pairs' recombination because of the introduction of porphyrin. Furthermore, more active sites and the greater electrical conductivity of TCPP/Ce-CoCu-LDH also contributed to the high photoelectrocatalytic activity. Thus, TCPP/Ce-CoCu-LDH can be used as a low-cost alternative for Pt-based catalyst toward MOR.

Stability-Enhanced α-Ni(OH)2 Pillared by Metaborate Anions for Pseudocapacitors

α-Ni(OH)₂ is an ideal candidate material for a supercapacitor except for its low conductivity and poor stability. In this work, BO₂^--intercalated α-Ni_xCo_(1-x)(OH)₂ is synthesized by a hydrothermal method at a low cost. The Co dopant can decrease the charge-transfer resistance and enhance the cyclic stability. The special unsaturated electronic state of BO₂^- enhances the bonding with metal ions and attracts water molecules. Thus, the BO₂^- ions support the hydroxide layers as pillars and create efficient paths for proton transportation, optimizing the utilization of α-Ni(OH)₂. The three-dimensional (3D) flowerlike morphology supplies an enormous number of active sites, and r-GO is added to improve the conductivity. As a result, the modified α-Ni(OH)₂ exhibits the specific capacitance of 2179, 1592, and 1423 F·g^-1 at 1, 20, and 40 A·g^-1, respectively, showing improved rate performance. Matching with the commercial activated carbon (AC) as an anode, the asymmetric capacitor delivers an energy density of 40.66 W·h·kg^-1 when its power density is 187.06 W·kg^-1. Meanwhile, it retains 81.5% capacitance of the initial cycle at 5 A·g^-1 after 3000 cycles. With conductivity enhanced and structure stabilized, the modified α-Ni(OH)₂ confronts broader fields of application.

Self-Supported Sheets-on-Wire CuO@Ni(OH)2/Zn(OH)2 Nanoarrays for High-Performance Flexible Quasi-Solid-State Supercapacitor

Transition metal hydroxides have attracted a lot of attention as the electrode materials for supercapacitors owing to their relatively high theoretical capacity, low cost, and facile preparation methods. However, their low intrinsic conductivity deteriorates their high-rate performance and cycling stability. Here, self-supported sheets-on-wire CuO@Ni(OH)2/Zn(OH)2 (CuO@NiZn) composite nanowire arrays were successfully grown on copper foam. The CuO nanowire backbone provided enhanced structural stability and a highly efficient electron-conducting pathway from the active hydroxide nanosheets to the current collector. The resulting CuO@NiZn as the battery-type electrode for supercapacitor application delivered a high capacity of 306.2 mAh g−1 at a current density of 0.8 A g−1 and a very stable capacity of 195.1 mAh g−1 at 4 A g−1 for 10,000 charge–discharge cycles. Furthermore, a quasi-solid-state hybrid supercapacitor (qss HSC) was assembled with active carbon, exhibiting 125.3 mAh g−1 at 0.8 A g−1 and a capacity of 41.6 mAh g−1 at 4 A g−1 for 5000 charge–discharge cycles. Furthermore, the qss HSC was able to deliver a high energy density of about 116.0 Wh kg−1. Even at the highest power density of 7.8 kW kg−1, an energy density of 20.5 Wh kg−1 could still be obtained. Finally, 14 red light-emitting diodes were lit up by a single qss HSC at different bending states, showing good potential for flexible energy storage applications.

Boosting the capacitive property of nickel cobalt aluminum layered double hydroxide in neutral electrolyte

Layered double hydroxide (LDH) has shown great potential for energy storage due to their high theoretical specific capacitance, relatively low cost and eco-friendliness. LDH, however, always works in alkali aqueous electrolyte for supercapacitors, which brings serious environmental pollution. In this work, a reduced graphite oxide/Fe(CN)₆^3-- nickel cobalt aluminum LDH (RGO/Fe(CN)₆^3--LDH) composite has been prepared via ion-exchange reaction using RGO/LDH as precursor. RGO/Fe(CN)₆^3--LDH electrode provides a specific capacitance of 221 F g^-1 in a wide potential window of -1 ~ 0.8 V vs. SCE in Na₂SO₄ aqueous electrolyte, and which is much higher than that of LDH electrode (3.56 F g^-1). Owing to the wide potential window of RGO/Fe(CN)₆^3--LDH electrode, a symmetrical solid supercapacitor device (RGO/Fe(CN)₆^3--LDH//RGO/Fe(CN)₆^3--LDH) with a high voltage of 2.0 V can deliver a high specific energy of 25.2 Wh kg^-1 at a specific power of 250 W kg^-1, and a capacitance retention of 75% after galvanostatic charging/discharging at 5 A g^-1 for 5000 times. This work supplies enlightenment for boosting the capacitive performance of LDHs in neutral electrolyte.

Adsorption kinetic and thermodynamic studies of the 2, 4 - dichlorophenoxyacetate (2,4-D) by the [Co-Al-Cl] layered double hydroxide

[Co-Al-Cl] layered double hydroxide (LDH) obtained by co-precipitation at constant pH 8 presented a single phase in a hexagonal unit cell parameters similar to the hydrotalcite (JCPDS 14-191) belonging to the rhombohedral crystal system and space group R (-3)m . The adsorption kinetics of 2,4-D onto [Co-Al-Cl] LDH was better described by the Pseudo Second-Order (best adjust R2 = 0.9998 for 60 mg L-1 2,4-D adsorption). Intra-particle diffusion model was not the sole rate-controlling factor, indicating the adsorption of 2,4-D by the [Co-Al-Cl] LDH is a complex process for the experimental conditions performed, involving both boundary layer and intra-particle diffusion. The adsorption isotherm adjusted better to the Freundlich model (R2 = 0.9845) and the ΔH° value of - 51.18 kJ mol-1 indicated the predominance of the physical adsorption. The FT-IR spectrum of LDH after adsorption presented 2,4-D bands together with those of LDH and XRD showed an increase in the interlamellar distance (d 003) due to the intercalation of 2,4-D in the interlayer structure of the [Co-Al-Cl] LDH, corroborating inter and intra-particle adsorption data. Thus, [Co-Al-Cl] LDH, commonly used as electrodes in supercapacitors, can be effectively used as an adsorbent for the removal of 2,4-D from contaminated waters.

Current progress achieved in novel materials for supercapacitor electrodes: mini review

Supercapacitors are highly attractive for a large number of emerging mobile devices for addressing energy storage and harvesting issues. This mini review presents a summary of recent developments in supercapacitor research and technology, including all kinds of supercapacitor design techniques using various electrode materials and production methods. It also covers the current progress achieved in novel materials for supercapacitor electrodes. The latest produced EDLC/hybrid/pseudo-supercapacitors have also been described. In particular, metal oxides, specifically ZnO, used as electrode materials are in focus here. Eventually, future developments, prospects, and challenges in supercapacitor research have been elaborated on.