Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials

Ordered mesoporous metal oxides for electrochemical applications: correlation between structure, electrical properties and device performance

Ordered mesoporous metal oxides with a high specific surface area, tailored porosity and engineered interfaces are promising materials for electrochemical applications. In particular, the method of evaporation-induced self-assembly allows the formation of nanocrystalline films of controlled thickness on polar substrates. In general, mesoporous materials have the advantage of benefiting from a unique combination of structural, chemical and physical properties. This Perspective article addresses the structural characteristics and the electrical (charge-transport) properties of mesoporous metal oxides and how these affect their application in energy storage, catalysis and gas sensing.

Structural and chemical characterization of MoO2/MoS2 triple-hybrid materials using electron microscopy in up to three dimensions

This work presents the synthesis of MoO₂/MoS₂ core/shell nanoparticles within a carbon nanotube network and their detailed electron microscopy investigation in up to three dimensions. The triple-hybrid core/shell material was prepared by atomic layer deposition of molybdenum oxide onto carbon nanotube networks, followed by annealing in a sulfur-containing gas atmosphere. High-resolution transmission electron microscopy together with electron diffraction, supported by chemical analysis via energy dispersive X-ray and electron energy loss spectroscopy, gave proof of a MoO₂ core covered by few layers of a MoS₂ shell within an entangled network of carbon nanotubes. To gain further insights into this complex material, the analysis was completed with 3D electron tomography. By using Z-contrast imaging, distinct reconstruction of core and shell material was possible, enabling the analysis of the 3D structure of the material. These investigations showed imperfections in the nanoparticles which can impact material performance, i.e. for faradaic charge storage or electrocatalysis.

Microbe-Assisted Assembly of Ti3C2Tx MXene on Fungi-Derived Nanoribbon Heterostructures for Ultrastable Sodium and Potassium Ion Storage

As a typical family of two-dimensional (2D) materials, MXenes present physiochemical properties and potential for use in energy storage applications. However, MXenes suffer some of the inherent disadvantages of 2D materials, such as severe restacking during processing and service and low capacity of energy storage. Herein, a MXene@N-doped carbonaceous nanofiber structure is designed as the anode for high-performance sodium- and potassium-ion batteries through an in situ bioadsorption strategy; that is, Ti₃C₂T_x nanosheets are assembled onto Aspergillus niger biofungal nanoribbons and converted into a 2D/1D heterostructure. This microorganism-derived 2D MXene-1D N-doped carbonaceous nanofiber structure with fully opened pores and transport channels delivers high reversible capacity and long-term stability to store both Na⁺ (349.2 mAh g^-1 at 0.1A g^-1 for 1000 cycles) and K⁺ (201.5 mAh g^-1 at 1.0 A g^-1 for 1000 cycles). Ion-diffusion kinetics analysis and density functional theory calculations reveal that this porous hybrid structure promotes the conduction and transport of Na and K ions and fully utilizes the inherent advantages of the 2D material. Therefore, this work expands the potential of MXene materials and provides a good strategy to address the challenges of 2D energy storage materials.

Oriented Assembly of Anisotropic Nanosheets into Ultrathin Flowerlike Superstructures for Energy Storage

The hierarchical ultrathin nanostructures are excellent electrode materials for supercapacitors because of their large surface area and their ability to promote ion and electron transport. Herein, we investigated nine l-amino acids (LAs) as inductive agents to synthesize a series of CoNi-OH/LAs materials for energy storage. With the different amino acids, the assembled CoNi-OH/LAs form a lamellar, flower-shaped, and bulk structure. Among all materials, the ultrathin flowerlike CoNi₂-OH/l-asparagine (CoNi₂-OH/l-Asn) exhibits an excellent specific capacity of 405.4 mAh g^-1 (2608 F g^-1) and a 100% retention rate after 3000 cycles. We also assembled asymmetrical supercapacitor CoNi₂-OH/l-Asn//N-rGO devices, which demonstrated an energy density of 64.9 Wh kg^-1 at 799.9 W kg^-1 and superlong cycling stability (82.4% at 10 A g^-1) over 5000 cycles.

Construction of the POMOF@Polypyrrole Composite with Enhanced Ion Diffusion and Capacitive Contribution for High-Performance Lithium-Ion Batteries

Polyoxometalate (POM) as an "electronic sponge" can store a great number of electrons; however, shortcomings of poor conductivity and solubility in electrolytes cause a significant decrease in specific capacity and poor rate capability. To address the aforementioned disadvantages, a dual strategy was proposed, including coating the conductive polypyrrole (PPy) and utilizing nitrogenous ligands (1,10-phenanthroline monohydrate = 1,10-phen) for metal-organic frameworks (MOFs) to fabricate a [Cu(1,10-phen)(H₂O)₂]₂[Mo₆O₂₀]@PPy (Cu-POMOF@PPy) composite, effectively confining the POM in MOFs to avoid dissolution of POM in the electrolyte and improve electrochemical stability. Simultaneously, the PPy shell could improve the conductivity, contribute extra capacity, and alleviate volume variation of Cu-POMOF during cycling. Therefore, the final Cu-POMOF@PPy composite provides an excellent specific capacity of around 769 mA h g^-1 at 0.1 A g^-1 after 160 cycles and good rate performance, associated with great cycling stability (319 mA h g^-1 at 2 A g^-1 after 500 cycles). Moreover, the electrochemical reaction mechanism of Cu-POMOF@PPy was investigated by ex situ XPS measurements, indicating that storage of electrons results from the reduction/oxidation of Mo atoms (Mo⁶⁺ ↔ Mo⁴⁺) and Cu atoms (Cu²⁺ ↔ Cu⁰). As a consequence, this work not only proposes a novel method for preparing POM-based lithium-ion batteries but also expands the variety of anode materials.

Adjusting the Valence State of Vanadium in VO2 (B) by Extracting Oxygen Anions for High-Performance Aqueous Zinc-Ion Batteries

VO₂ generally has a higher theoretical capacity and layered structure suitable for the intercalation/extraction of zinc ions. However, Zn²⁺ ions with high charge density interact with the crystal lattice and limit further improvement in electrochemical performance. Defect engineering is a potential modification method with very promising application prospects, but the established procedures for preparing defects are complicated. In this study, VO_2-x (B) with oxygen deficiency is prepared by a simple solution reaction with NaBH₄ . The presence of oxygen deficiencies is confirmed by positron annihilation lifetime spectroscopy, UV/Vis absorbance spectroscopy and others. Owing to the presence of oxygen defects, the aqueous Zn/VO_2-x (B) battery exhibits improved specific capacity, excellent reversibility, and structural stability. Ex situ characterization techniques are employed to demonstrate the reversible insertion-extraction mechanism of Zn²⁺ ions from and into the host material. In addition, the Zn/VO_2-x (B) batteries still exhibit considerable electrochemical performance, even with high-loading electrodes (about 4 mg cm^-2 ).

Two-step polyaniline loading in polyelectrolyte complex membranes for improved pseudo-capacitor electrodes

A two-step polyaniline (PANI) loading procedure has been developed to produce polyelectrolyte complex composite membranes (CPECs) to be used as supercapacitor electrodes. In the first step, CPECs were prepared by co-precipitation of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) mixed with various amounts of PANI as a filler. CPECs were formed by compression molding into 100 micron membranes using NaCl as a plasticizer and characterized for their electrochemical properties. In the second step, the highest capacitance CPEC membranes with 60% PANI loading were further modified and doped by crossflow polymerization of aniline through the composite membranes. By using a two-compartment crossflow reactor containing aniline and ammonium persulfate on each side, the PANI content of the composite membrane was further increased. Cyclic voltammetry showed a doubling in the capacitance of the membranes after the crossflow polymerization. The resulting electrodes were flexible with high capacitance and could be used to improve pseudocapacitor performance.

Ammonium-Ion Storage Using Electrodeposited Manganese Oxides

NH₄ ⁺ ions as charge carriers show potential for aqueous rechargeable batteries. Studied here for the first time is the NH₄ ⁺ -storage chemistry using electrodeposited manganese oxide (MnO_x ). MnO_x experiences morphology and phase transformations during charge/discharge in dilute ammonium acetate (NH₄ Ac) electrolyte. The NH₄ Ac concentration plays an important role in NH₄ ⁺ storage for MnO_x . The transformed MnO_x with a layered structure delivers a high specific capacity (176 mAh g^-1 ) at a current density of 0.5 A g^-1 , and exhibits good cycling stability over 10 000 cycles in 0.5 M NH₄ Ac, outperforming the state-of-the-art NH₄ ⁺ hosting materials. Experimental results suggest a solid-solution behavior associated with NH₄ ⁺ migration in layered MnO_x . Spectroscopy studies and theoretical calculations show that the reversible NH₄ ⁺ insertion/deinsertion is accompanied by hydrogen-bond formation/breaking between NH₄ ⁺ and the MnO_x layers. These findings provide a new prototype (i.e., layered MnO_x ) for NH₄ ⁺ -based energy storage and contributes to the fundamental understanding of the NH₄ ⁺ -storage mechanism for metal oxides.

High-Energy and High-Power Pseudocapacitor-Battery Hybrid Sodium-Ion Capacitor with Na+ Intercalation Pseudocapacitance Anode

High-performance and low-cost sodium-ion capacitors (SICs) show tremendous potential applications in public transport and grid energy storage. However, conventional SICs are limited by the low specific capacity, poor rate capability, and low initial coulombic efficiency (ICE) of anode materials. Herein, we report layered iron vanadate (Fe₅V₁₅O₃₉ (OH)₉·9H₂O) ultrathin nanosheets with a thickness of ~ 2.2 nm (FeVO UNSs) as a novel anode for rapid and reversible sodium-ion storage. According to in situ synchrotron X-ray diffractions and electrochemical analysis, the storage mechanism of FeVO UNSs anode is Na⁺ intercalation pseudocapacitance under a safe potential window. The FeVO UNSs anode delivers high ICE (93.86%), high reversible capacity (292 mAh g^-1), excellent cycling stability, and remarkable rate capability. Furthermore, a pseudocapacitor-battery hybrid SIC (PBH-SIC) consisting of pseudocapacitor-type FeVO UNSs anode and battery-type Na₃(VO)₂(PO₄)₂F cathode is assembled with the elimination of presodiation treatments. The PBH-SIC involves faradaic reaction on both cathode and anode materials, delivering a high energy density of 126 Wh kg^-1 at 91 W kg^-1, a high power density of 7.6 kW kg^-1 with an energy density of 43 Wh kg^-1, and 9000 stable cycles. The tunable vanadate materials with high-performance Na⁺ intercalation pseudocapacitance provide a direction for developing next-generation high-energy capacitors.

Titanium Niobium Oxide Ti2 Nb10 O29 /Carbon Hybrid Electrodes Derived by Mechanochemically Synthesized Carbide for High-Performance Lithium-Ion Batteries

This work introduces the facile and scalable two-step synthesis of Ti₂ Nb₁₀ O₂₉ (TNO)/carbon hybrid material as a promising anode for lithium-ion batteries (LIBs). The first step consisted of a mechanically induced self-sustaining reaction via ball-milling at room temperature to produce titanium niobium carbide with a Ti and Nb stoichiometric ratio of 1 to 5. The second step involved the oxidation of as-synthesized titanium niobium carbide to produce TNO. Synthetic air yielded fully oxidized TNO, while annealing in CO₂ resulted in TNO/carbon hybrids. The electrochemical performance for the hybrid and non-hybrid electrodes was surveyed in a narrow potential window (1.0-2.5 V vs. Li/Li⁺ ) and a large potential window (0.05-2.5 V vs. Li/Li⁺ ). The best hybrid material displayed a specific capacity of 350 mAh g^-1 at a rate of 0.01 A g^-1 (144 mAh g^-1 at 1 A g^-1 ) in the large potential window regime. The electrochemical performance of hybrid materials was superior compared to non-hybrid materials for operation within the large potential window. Due to the advantage of carbon in hybrid material, the rate handling was faster than that of the non-hybrid one. The hybrid materials displayed robust cycling stability and maintained ca. 70 % of their initial capacities after 500 cycles. In contrast, only ca. 26 % of the initial capacity was maintained after the first 40 cycles for non-hybrid materials. We also applied our hybrid material as an anode in a full-cell lithium-ion battery by coupling it with commercial LiMn₂ O₄ .

Oxygen Vacancy Engineering in Titanium Dioxide for Sodium Storage

Titanium dioxide (TiO₂ ) is a promising anode material for sodium-ion batteries (SIBs) due to its low cost, natural abundance, nontoxicity, and excellent electrochemical stability. Oxygen vacancies, the most common point defects in TiO₂ , can dramatically influence the physical and chemical properties of TiO₂ , including band structure, crystal structure and adsorption properties. Recent studies have demonstrated that oxygen-deficient TiO₂ can significantly enhance sodium storage performance. Considering the importance of oxygen vacancies in modifying the properties of TiO₂ , the structural properties, common synthesis strategies, characterization techniques, as well as the contribution of oxygen-deficient TiO₂ on initial Coulombic efficiency, cyclic stability, rate performance for sodium storage are comprehensively described in this review. Finally, some perspectives on the challenge and future opportunities for the development of oxygen-deficient TiO₂ are proposed.

Electrochemical activation induced multi-valence variation of (NH4)2V4O9 as a high-performance cathode material for zinc-ion batteries

Electrochemical activation of (NH₄)₂V₄O₉ has been found in the first charging process. After this activation, a high capacity (477 mA h g⁻¹ at 50 mA g⁻¹) and stable cycling life (97.7% retention after 5000 cycles at 15 A g⁻¹) have been achieved.

The fluorination-assisted dealloying synthesis of porous reduced graphene oxide-FeF2@carbon for high-performance lithium-ion battery and the exploration of its electrochemical mechanism

Based on the dealloying conception, a porous rGO-FeF₂@C is attained and shows a great electrochemical performance. An intriguing phenomenon has been that the decrease in charge cut-off voltage contributes to the higher discharge plateau.

Ultralight Flexible Electrodes of Nitrogen-Doped Carbon Macrotube Sponges for High-Performance Supercapacitors

Light-weight and flexible supercapacitors with outstanding electrochemical performances are strongly desired in portable and wearable electronics. Here, ultralight nitrogen-doped carbon macrotube (N-CMT) sponges with 3D interconnected macroporous structures are fabricated and used as substrate to grow nickel ferrite (NiFe₂ O₄ ) nanoparticles by vapor diffusion-precipitation and in situ growth. This process effectively suppresses the agglomeration of NiFe₂ O₄ , enabling good interfacial contact between N-CMT sponges and NiFe₂ O₄ . More remarkably, the as-synthesized NiFe₂ O₄ /N-CMT composite sponges can be directly used as electrodes without additional processing that could cause agglomeration and reduction of active sites. Benefiting from the tubular structure and the synergetic effect of NiFe₂ O₄ and N-CMT, the NiFe₂ O₄ /N-CMT-2 exhibits a high specific capacitance of 715.4 F g^-1 at a current density of 1 A g^-1 , and 508.3 F g^-1 at 10 A g^-1 , with 90.9% of capacitance retention after 50 000 cycles at 1 A g^-1 in an alkaline electrolyte. Furthermore, flexible supercapacitors are fabricated, yielding areal specific capacitances of 1397.4 and 1041.2 mF cm^-2 at 0.5 and 8 mA cm^-2 , respectively. They also exhibit exceptional cycling performance with capacitance retention of 92.9% at 1 mA cm^-2 after 10 000 cycles under bending. This work paves a new way to develop flexible, light-weight, and high-performance energy storage devices.

Reagent-assisted hydrothermal synthesis of NiCo2O4 nanomaterials as electrodes for high-performance asymmetric supercapacitors

Spinel nickel cobaltate nanoneedle arrays in situ synthesized by a CTAB assisted hydrothermal method show an energy density of 22.5 W h kg⁻¹ at 800 W kg⁻¹.

An asymmetric supercapacitor with an interpenetrating crystalline Fe-MOF as the positive electrode and its congenetic derivative as the negative electrode

Interpenetrating crystalline Fe-based materials of FeSC and congenetic derivative FeSC# have been assembled into an asymmetric supercapacitor, which can offer an excellent supercapacitor performance.

Three-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability

Faradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g^-1 (h-Co(OH)₂)·min^-1 at 100 mA·g^-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity (117 ± 6 mg (NaCl)·g^-1 (h-Co(OH)₂) at 30 mA·g^-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.

Boosting electrochemical performance of activated carbon by tuning effective pores and synergistic effects of active species

Clean energy conversion/storage techniques have become increasingly significant because of the increasing energy consumption. Regarding practical applications like zinc-air batteries and supercapacitors, electrode materials are essential and often require both porous networks and active species to enhance their electrochemical performance. Nitrogen-doped porous carbon (NPC) is a kind of promising material, which provides efficient active sites and large surface areas for energy conversion/storage applications. However, rational modulation of properties for maximizing NPC performance is still a challenge. Herein, a promising NPC material derived from natural biomass is successfully synthesized by following a stepwise preparation method. Physisorption and X-ray photoelectron spectroscopy (XPS) analyses demonstrate both pore structures and nitrogen species of the NPC have been delicately tuned. The optimized sample NPC-800-m exhibits excellent performance in both oxygen reduction reaction (ORR) and three-electrode supercapacitor measurement. Moreover, the homemade zinc-air battery and symmetric supercapacitor assembled with NPC-800-m also display outstanding energy and power density as well as durable stability. Density functional theory (DFT) calculations further confirm the synergistic effects among graphitic, pyridinic and pyrrolic nitrogen. The existence of multispecies of nitrogen combined with the optimized pore structure is the key to the high electrochemical performance for NPC-800-m. This work not only provides feasible and green synthetic methodology but also offers original insights into the effective pores and the synergistic effects of different nitrogen species in the NPC materials.

A multi-layered composite assembly of Bi nanospheres anchored on nitrogen-doped carbon nanosheets for ultrastable sodium storage

Bismuth (Bi) is a promising anode candidate for sodium ion batteries (SIBs) with a high volumetric capacity (3765 mA h cm-3) and moderate working potential but suffers from large volume change (ca. 250%) during the sodiation/desodiation process, resulting in pulverization of the electrode, electrical contact loss, excessive accumulation of solid electrolyte interfaces, etc., devastating the cycling stability of the electrode seriously. Addressing this issue significantly relies on rational micro- and nano-structuring. Herein, we prepared a 3D multi-layered composite assembly of Bi/carbon heterojunctions with 0D bismuth nanospheres distributed and anchored on 2D nitrogen-doped carbon nanosheets (NCSs), using a preorganization strategy by taking full advantage of the strong complexation ability of Bi3+. The multi-layered composite assembly is periodic and close-packed, with Bi nanospheres <25 nm, carbon nanosheets ∼30 nm, and an average interlayer space of ∼75 nm. Such a specific architecture provides abundant electrochemically active surfaces and ion migration channels as the Bi nanospheres are attached to the 2D nitrogen-doped carbon nanosheets via a point-to-surface pattern. Moreover, the mono-layer Bi nanospheres oriented along the 2D-surface of NCSs are kinetically favorable for the recognition of Na+ by the active sites of Bi nanospheres as well as for avoiding the long distance migration of Na+ (external diffusion of Na+). Furthermore, thermodynamically, the small size and high surface energy of ultrasmall Bi nanospheres could contribute to high ion mobility (internal diffusion of Na+) and promote electrochemical reactions as well. The multi-layered composite assembly of Bi@NCSs (ML-Bi@NCSs) not only provides a robust 3D framework guaranteeing the whole structural stability but also ensures direct and full contact of each active nano-building block with electrolyte, thereby forming a high-throughput electron/ion transport system. When evaluated as the anode for SIBs, ML-Bi@NCSs deliver superior high-rate capability up to 30 A g-1 (specific capacity: 288 mA h g-1) and long-term cycling stability (capacity retention: 95.8% after 5000 cycles at 10 A g-1 and 90.6% after 10 000 cycles at 20 A g-1, respectively).

Rational Design and in-situ Synthesis of Ultra-Thin β-Ni(OH)2 Nanoplates for High Performance All-Solid-State Flexible Supercapacitors

The all-solid-state flexible supercapacitor (AFSC), one of the most flourishing energy storage devices for portable and wearable electronics, attracts substantial attentions due to their high flexibility, compact size, improved safety, and environmental friendliness. Nevertheless, the current AFSCs usually show low energy density, which extremely hinders their practical applications. Herein, ultra-thin β-Ni(OH)₂ nanoplates with thickness of 2.4 ± 0.2 nm are in-situ grown uniformly on Ni foam by one step hydrothermal treatment. Thanks to the ultra-thin nanostructure, β-Ni(OH)₂ nanoplates shows a specific capacitance of 1,452 F g⁻¹ at the scan rate of 3 mV s⁻¹. In addition, the assembled asymmetric AFSC [Ni(OH)₂//Activated carbon] shows a specific capacitance of 198 F g⁻¹. It is worth noting that the energy density of the AFSC can reach 62 Wh kg⁻¹ while keeping a high power density of 1.5 kW kg⁻¹. Furthermore, the fabricated AFSCs exhibit satisfied fatigue behavior and excellent flexibility, and about 82 and 86% of the capacities were retained after 5,000 cycles and folding over 1,500 times, respectively. Two AFSC in series connection can drive the electronic watch and to run stably for 10 min under the bending conditions, showing a great potential for powering portable and wearable electronic devices.

Electrochemical Energy Storage: Questioning the Popular v/v1/2 Scan Rate Diagnosis in Cyclic Voltammetry

The v/v^1/2 scan rate diagnosis in electrochemical energy storage devices is based on application of the relationship i = k₁v + k₂v^1/2 (where k₁ and k₂ are two constants independent of the scan rate v) to the variation of the cyclic voltammetric responses with v. Several examples show that application of this scan rate diagnosis procedure leads to absurd results because the procedure is inappropriate under these conditions. It follows that the best approach is to simply forget about this v/v^1/2 scan rate diagnosis, concentrate on the maximum number of experimental observations of the scan rate dependency, and build models able to reproduce these data in each case.

Research Progress of Graphene-Based Materials on Flexible Supercapacitors

With the development of wearable and flexible electronic devices, there is an increasing demand for new types of flexible energy storage power supplies. The flexible supercapacitor has the advantages of fast charging and discharging, high power density, long cycle life, good flexibility, and bendability. Therefore, it exhibits great potential for use in flexible electronics. In flexible supercapacitors, graphene materials are often used as electrode materials due to the advantages of their high specific surface area, high conductivity, good mechanical properties, etc. In this review, the classification of flexible electrodes and some common flexible substrates are firstly summarized. Secondly, we introduced the advantages and disadvantages of five graphene-based materials used in flexible supercapacitors, including graphene quantum dots (GQDs), graphene fibers (GFbs), graphene films (GFs), graphene hydrogels (GHs), and graphene aerogels (GAs). Then, we summarized the latest developments in the application of five graphene-based materials for flexible electrodes. Finally, the defects and outlooks of GQDs, GFbs, GFs, GHs, and GAs used in flexible electrodes are given.

Efficient Flexible All-Solid Supercapacitors with Direct Sputter-Grown Needle-Like Mn/MnOx@Graphite-Foil Electrodes and PPC-Embedded Ionic Electrolytes

Recent critical issues regarding next-generation energy storage systems concern the cost-effective production of lightweight, safe and flexible supercapacitors yielding high performances, such as high energy and power densities as well as a long cycle life. Thus, current research efforts are concentrated on the development of high-performance advance electrode materials with high capacitance and excellent stability and solid electrolytes that confer flexibility and safety features. In this work, emphasis is placed on the binder-free, needle-like nanostructured Mn/MnOx layers grown onto graphite-foil deposited by reactive sputtering technique and to the polymer gel embedded ionic electrolytes, which are to be employed as new flexible pseudocapacitive supercapacitor components. Microstructural, morphological and compositional analysis of the layers has been investigated by X-ray diffractometer (XRD), Field Emission Scanning Electron Microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS). A flexible lightweight symmetric pouch-cell solid-state supercapacitor device is fabricated by sandwiching a PPC-embedded ionic liquid ethyl-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMIM)(TFSI) polymer gel electrolyte (PGE) between two Mn/MnOx@Graphite-foil electrodes and tested to exhibit promising supercapacitive behaviour with a wide stable electrochemical potential window (up to 2.2 V) and long-cycle stability. This pouch-cell supercapacitor device offers a maximum areal capacitance of 11.71 mF/cm2@ 0.03 mA/cm2 with maximum areal energy density (Ea) of 7.87 mWh/cm2 and areal power density (Pa) of 1099.64 mW/cm2, as well as low resistance, flexibility and good cycling stability. This supercapacitor device is also environmentally safe and could be operated under a relatively wide potential window without significant degradation of capacitance performance compared to other reported values. Overall, these rationally designed flexible symmetric all-solid-state supercapacitors signify a new promising and emerging candidate for component integrated storage of renewable energy harvested current.

Electrode Materials for Supercapacitors: A Review of Recent Advances

The advanced electrochemical properties, such as high energy density, fast charge–discharge rates, excellent cyclic stability, and specific capacitance, make supercapacitor a fascinating electronic device. During recent decades, a significant amount of research has been dedicated to enhancing the electrochemical performance of the supercapacitors through the development of novel electrode materials. In addition to highlighting the charge storage mechanism of the three main categories of supercapacitors, including the electric double-layer capacitors (EDLCs), pseudocapacitors, and the hybrid supercapacitors, this review describes the insights of the recent electrode materials (including, carbon-based materials, metal oxide/hydroxide-based materials, and conducting polymer-based materials, 2D materials). The nanocomposites offer larger SSA, shorter ion/electron diffusion paths, thus improving the specific capacitance of supercapacitors (SCs). Besides, the incorporation of the redox-active small molecules and bio-derived functional groups displayed a significant effect on the electrochemical properties of electrode materials. These advanced properties provide a vast range of potential for the electrode materials to be utilized in different applications such as in wearable/portable/electronic devices such as all-solid-state supercapacitors, transparent/flexible supercapacitors, and asymmetric hybrid supercapacitors.

High Power Energy Storage via Electrochemically Expanded and Hydrated Manganese-Rich Oxides

Understanding the materials design features that lead to high power electrochemical energy storage is important for applications from electric vehicles to smart grids. Electrochemical capacitors offer a highly attractive solution for these applications, with energy and power densities between those of batteries and dielectric capacitors. To date, the most common approach to increase the capacitance of electrochemical capacitor materials is to increase their surface area by nanostructuring. However, nanostructured materials have several drawbacks including lower volumetric capacitance. In this work, we present a scalable “top-down” strategy for the synthesis of EC electrode materials by electrochemically expanding micron-scale high temperature-derived layered sodium manganese-rich oxides. We hypothesize that the electrochemical expansion induces two changes to the oxide that result in a promising electrochemical capacitor material: (1) interlayer hydration, which improves the interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. When compared with a commercially available activated carbon for electrochemical capacitors, the expanded materials have higher volumetric capacitance at charge/discharge timescales of up to 40 s. This shows that expanded and hydrated manganese-rich oxide powders are viable candidates for electrochemical capacitor electrodes.

An in situ approach to functionalize metal–organic frameworks with tertiary aliphatic amino groups

Tertiary aliphatic amino modified UiO-67/66(Zr), IRMOF-n(Zn) and MIL-101(Fe) were synthesized by a facile and efficient one-pot strategy under the corresponding metal catalysis.