Boosting the Energy Density of Carbon-Based Aqueous Supercapacitors by Optimizing the Surface Charge

Chitosan-based high-performance flexible supercapacitor via "in-situ co-doping/self-regulation-activation" strategy

The construction of N, P co-doped hierarchically porous carbons (NPHPC) by a facile and green approach is crucial for high-performance energy storage but still an enormous challenge. Herein, an environment-friendly "in-situ co-doping, self-regulation-activation" strategy is presented to one-pot synthesize NPHPC using a phytic acid-induced polyethyleneimine/chitosan gel (PEI-PA-CS) as single precursor. NPHPC displayed a specific surface area of up to 1494 m2 g-1, high specific capacitance of 449 F g-1 at 1 A g-1, outstanding rate capability and cycling durability in a wide temperature range (-20 to 60 °C). NPHPC and PEI-PA-CS electrolyte assembled symmetric quasi-solid-state flexible supercapacitor presents superb energy outputs of 27.06 Wh kg-1 at power density of 225 W kg-1. For capacitive deionization (CDI), NPHPC also exhibit an excellent salt adsorption capacity of 16.54 mg g-1 in 500 mg L-1 NaCl solution at a voltage of 1.4 V, and regeneration performance. This study provides a valuable reference for the rational design and synthesis of novel biomass-derived energy-storage materials by integrating phytic acid induced heteroatom doping and pore engineering.

Different metal cation-doped MnO2/carbon cloth for wide voltage energy storage

Aqueous gel supercapacitors, as an important component of flexible energy storage devices, have received widespread attention for their fast charging/discharging rates, long cycle life and high electrochemical stability under mechanical deformation condition. However, the low energy density of aqueous gel supercapacitors has greatly hindered their further development due to the narrow electrochemical window and limited energy storage capacity. Therefore, different metal cation-doped MnO₂/carbon cloth-based flexible electrodes herein are prepared by constant voltage deposition and electrochemical oxidation in various saturated sulphate solutions. The influence of different metal cations as K⁺, Na⁺ and Li⁺ doping and deposition conditions on the apparent morphology, lattice structure and electrochemical properties are explored. Furthermore, the pseudo-capacitance ratio of the doped MnO₂ and the voltage expansion mechanism of the composite electrode are investigated. The specific capacitance and pseudo-capacitance ratio of the optimized δ-Na_0.31MnO₂/carbon cloth as MNC-2 electrode could be reached 327.55 F/g at 10 mV/s and 35.56% of the pseudo-capacitance, respectively. The flexible symmetric supercapacitors (NSCs) with desirable electrochemical performances in the operating range of 0-1.4 V are further assembled with MNC-2 as the electrodes. The energy density is 26.8 Wh/kg at the power density of 300 W/kg, while the energy density can still reach 19.1 Wh/kg when the power density is up to 1150 W/kg. The energy storage devices with high-performance developed in this work can provide new ideas and strategic support for the application in portable and wearable electronic devices.

Binder-Free Supercapacitors Based on Thin Films of MWCNT/GO Nanohybrids: Computational and Experimental Analysis

This work reports an innovative approach to the fabrication of free-standing thin films of multiwalled carbon nanotubes (MWCNTs)/graphene oxide (GO) nanohybrids by using dimethyl formamide (DMF) and n-hexane as a solvent–antisolvent system for the growth of thin films of MWCNTs/GO nanohybrids. The synthesis of the GO was carried out by using the modified Hummers method, while the synthesis of MWCNTs/GO nanohybrids was done by the intermixing of the carboxylic acid functionalized MWCNT and GO using the solution-mixing method. The growth of the thin film of MWCNTs/GO nanohybrids was done by obeying the surface-tension-driven phenomena which occur mainly due to the coalescence of bubbles due to the solvent–antisolvent interfacial tension. Furthermore, density functional theory (DFT)-based first-principles simulations were performed to understand the structural, electronic, and capacitive aspects of MWCNT/GO nanohybrids. The computational results demonstrated excellent quantum capacitance in the MWCNT/GO nanohybrid electrodes. Inspired by the computational results, the same process elaborated above has also been employed to develop binder-free supercapacitor devices utilizing the MWCNT/GO nanohybrid as an electrode material. The electrochemical performance of this electrode in 1 M aqueous H2SO4 demonstrates a good energy density of 21.63 WhKg−1 at a current density of 0.5 Ag−1, with a high specific capacitance of 369.01 F/g at the scan rate of 2 mVs−1 and excellent cyclic stability of 97% for 5000 charge–discharge cycles.

Grotthuss Proton-Conductive Covalent Organic Frameworks for Efficient Proton Pseudocapacitors

Herein, we describe the synthesis of two highly crystalline, robust, hydrophilic covalent organic frameworks (COFs) that display intrinsic proton conduction by the Grotthuss mechanism. The enriched redox-active azo groups in the COFs can undergo a proton-coupled electron transfer reaction for energy storage, making the COFs ideal candidates for pseudocapacitance electrode materials. After in situ hybridization with carbon nanotubes, the composite exhibited a high three-electrode specific capacitance of 440 F g^-1 at the current density of 0.5 A g^-1 , among the highest for COF-based supercapacitors, and can retain 90 % capacitance even after 10 000 charge-discharge cycles. This is the first example using Grotthuss proton-conductive organic materials to create pseudocapacitors that exhibited both high power density and energy density. The assembled asymmetric two-electrode supercapacitor showed a maximum energy density of 71 Wh kg^-1 with a maximum power density of 42 kW kg^-1 , surpassing that of all reported COF-based systems.

"Water-in-Salt" Electrolytes for Supercapacitors: A Review

"Water-in-salt" (WIS) electrolytes, which have more salt than the solvent in both mass and volume, show promising prospects for application in supercapacitors due to their wide electrochemical stability window (about 3 V), considerable ion transport, high safety, low cost, and environmental friendliness. This Review summarizes the advances, progress, and challenges of WIS electrolytes in supercapacitors. The working mechanisms, reason for the wide electrochemical stability window, typical systems, challenges, and modification strategies of the WIS electrolytes in supercapacitors are discussed. Moreover, the application of WIS electrolytes in symmetric and asymmetric supercapacitors are presented. Finally, perspectives and the future development direction of WIS electrolytes are given. This Review is expected to provide inspiration and guidance for designing WIS electrolytes with advanced performance and push forward the development of high-performance aqueous supercapacitors with high cell voltage, good rate performance, and thus high energy density and power density.

MnO2 Synergized with N/S Codoped Graphene as a Flexible Cathode Efficient Electrocatalyst for Advanced Honeycomb-Shaped Stretchable Aluminum-Air Batteries

Aluminum-air batteries possess high theoretical specific capacities and energy densities. However, the desired application performance in the field of flexible electronics is limited by the rigid battery structure and slow kinetics of the oxygen reduction reaction (ORR). To address these issues, flexible, stretchable, and customizable aluminum-air batteries with a reference to honeycomb shape are composed of multilayer single battery units to achieve large scalability and start-stop control. The single aluminum-air battery combines MnO₂ with N/S codoped graphene to improve the electrocatalytic activity. Benefiting from an efficient electrocatalyst and reasonable structural design, the single aluminum-air battery exhibits excellent electrochemical characteristics under deformation conditions with a high specific capacity and energy density (1203.2 mAh g^-1 Al and 1630.1 mWh g^-1 Al). Furthermore, the obtained honeycomb-shaped stretchable aluminum-air batteries maintain a stable output voltage over the 2500% stretching. More interestingly, the stretchable honeycomb structure not only can solve the start-stop control problem but also has the potential to reduce the self-corrosion in disposable metal-air batteries. In addition, owing to the customizable shapes and sizes, the honeycomb-shaped stretchable aluminum-air batteries facilitate the integrated application of flexible batteries in wearables.

Regulation of the cathode for amphi-charge storage in a redox electrolyte for high-energy lithium-ion capacitors

Operating the cathodes of lithium-ion capacitors in an extended potential window for amphi-charge storage was proposed to be combined with a redox electrolyte-enabled capacity for balancing the typical gap between the capacity of the anode and that of the cathode. As a result, the as-obtained lithium-ion capacitors showed a three-fold increase in energy density.

Synthesis of Sub-nanometer Porous Carbon Film for Energy Storage

Carbon-based supercapacitors are a kind of supercapacitors with very promising applications because of their low cost, good stability and adjustable properties. Simple and rapid syntheses of carbon materials with a high surface area and narrow pore size distribution are of great significance to practical applications of carbon-based supercapacitors. Here we report a new strategy to synthesize sub-nanometer porous carbon films (Snp-CF) via a condensation reaction under mild conditions. Carbon films exhibit a narrow pore size distribution (6.6 Å) and high surface area (508 m²  g^-1 ) after annealing at 700 °C. Snp-CF-700 displays a good specific capacity and excellent cycle performance (130 F g^-1 after 5000 cycles, 118 % of initial 110 F g^-1 ).

Ultrafast microwave synthesis of rambutan-like CMK-3/carbon nanotubes nanocomposites for high-performance supercapacitor electrode materials

Ordered mesoporous carbon materials show great potential for electric double-layer supercapacitors because of their high specific surface area, designable pore structure, and tunable morphology. However, low graphitic crystallinity nature and poor contact between particles lead to their high inherent resistance, which limits the supercapacitance performance. Herein, we report on a hierarchically rambutan-morphological design of carbon composites with ordered mesoporous carbon as the core and carbon nanotubes as the shell, which significantly improve the electric contact between mesoporous carbon particles and promote the electrochemical performance. By an ultrafast microwave process in a household microwave heater under ambient condition, carbon nanotubes grow out from the pores of ordered mesoporous carbon and are dispersed on its surface like the whiskers of rambutan. As-synthesized ordered mesoporous carbon CMK-3/carbon nanotubes nanocomposites show significantly enhanced specific capacitance (315.6 F·g^-1 at 1 A·g^-1, as compared with 172.1 F·g^-1 of CMK-3), high rate capability (214.6 F·g^-1 at 50 A·g^-1), and cycling durability (10,000 cycles, 99.32%). The structural design and microwave synthesis enable a facile preparation of the hybrid ordered mesoporous carbon CMK-3/carbon nanotubes nanocomposites, and show potential for easy and low-cost production of high performance electric double-layer supercapacitors materials.

High Capacity and Energy Density of Zn-Ni-Co-P Nanowire Arrays as an Advanced Electrode for Aqueous Asymmetric Supercapacitor

Developing multicomponent transition-metal phosphides has become an efficient way to improve the capacitive performance of single-component transition-metal phosphides. However, reports on quaternary phosphides for supercapacitor applications are still scarce. Here, we report high capacity and energy density of Zn-Ni-Co-P quaternary phosphide nanowire arrays on nickel foam (ZNCP-NF) composed of highly conductive metal-rich phosphides as an advanced binder-free electrode in aqueous asymmetric supercapacitors. In a three-electrode system using the new electrode, a high specific capacity of 1111 C g^-1 was obtained at a current density of 10 A g^-1. Analysis of this aqueous asymmetric supercapacitor with ZNCP-NF as the positive electrode and commercial activated carbon as the negative electrode reveals a high energy density (37.59 Wh kg^-1 at a power density of 856.52 W kg^-1) and an outstanding cycling performance (capacity retention of 92.68% after 10 000 cycles at 2 A g^-1). Our results open a path for a new design of advanced electrode material for supercapacitors.

Macromolecular Polyethynylbenzonitrile Precursor-Based Porous Covalent Triazine Frameworks for Superior High-Rate High-Energy Supercapacitors

Porous covalent triazine framework (CTF)-based carbon materials have gained increasing attention in energy-storage applications because of their tunable structure, high chemical stability, and rich heteroatom contents. However, CTFs have thus far been exclusively synthesized from small-molecular precursors and generally show unsatisfactory supercapacitive performance. We report herein the construction of a novel range of CTFs of significantly improved supercapacitive performance from polyethynylbenzonitrile (PEBN) as a unique macromolecular precursor for the first time by ionothermal synthesis. CTF-800 synthesized at the optimized condition (800 °C; ZnCl₂/PEBN mass ratio of 3:1) shows a nanosheet-like morphology with a high yield (∼90%), high nitrogen content (>5.8%), high specific surface area (1954 m² g^-1), and optimized micropore to meso/macropore surface area ratio (42:58). As the electrode material for supercapacitor application, CTF-800 exhibits a high specific capacitance of 628 F g^-1 at 0.5 A g^-1, high-rate performance (71% of capacitance retention at 50 A g^-1), and excellent cyclic stability (96% of capacitance retention over 20 000 cycles) in a three-electrode system with aqueous 1 M H₂SO₄ electrolyte. Symmetric supercapacitor devices have been further fabricated with CTF-800 in aqueous 1 M H₂SO₄, [EMIM][BF₄], and LiPF₆ electrolytes separately. The device with the aqueous electrolyte shows the highest capacitance of 448 F g^-1 (at 0.5 A g^-1) and a high energy density of 15.5 W h kg^-1. The devices with [EMIM][BF₄] and LiPF₆ electrolytes exhibit exceptional energy densities of 70 and 78 W h kg^-1, respectively, and retain energy densities of 41 and 45 W h kg^-1, respectively, even at the high power density of 15 000 W kg^-1, confirming their high-rate high-energy performance. Meanwhile, the device with [EMIM][BF₄] electrolyte has also been demonstrated to operate well at various temperatures ranging from -20 to 60 °C with remarkable energy-storage performance.

Biomass-Derived Carbon: A Value-Added Journey Towards Constructing High-Energy Supercapacitors in an Asymmetric Fashion

Currently, asymmetric supercapacitors (ASCs) produced from supercapacitors (SCs) offer more benefits for energy-storage applications because they display a high operational voltage in aqueous-based electrolytes that may enhance grid storage and zero-power transportation with high energy density in the future. At the same time, the realization of low-cost energy devices through the construction of cheap electrode materials deserves a permanent place in the market once the goals of high energy, extra power, and long cycling stability are achieved. Biomass-derived carbon retrieved from sources such as plants has attracted considerable attention because of the rich abundance, low cost, and environmentally friendliness. In addition, the utilization of porous hierarchical structures has achieved enhanced electrochemical performance with excellent capacitance, outstanding stability, and praiseworthy rate capability. However, issues still persist in procedures used to obtain biomass-derived carbon materials with a high yield and a high degree of carbonization/graphitization, surface functionality, and porous characteristics, wherein the materials are used as electrodes in ASC devices. The present review briefly addresses the need for biomass-derived carbon materials in ASCs, comprehensively categorizes SCs in the context of their historical background, and elucidates the SC mechanism. In addition, influencing factors, such as the pore size distribution, role of surface functional groups, surface area, active-material loading, heteroatom doping, and activation techniques used in the preparation of biomass-derived carbon, have been discussed in detail. Moreover, this review assesses other nanostructured carbon electrodes used in ASCs and advances made in the fabrication of ASCs by using biomass-derived carbon in aqueous electrolytes. Finally, existing challenges and mandatory solutions toward developing cost-effective and high-performance ASCs by using environmentally friendly biomass-derived carbon materials are discussed in detail.

High-Performance Symmetric Supercapacitor Constructed Using Carbon Cloth Boosted by Engineering Oxygen-Containing Functional Groups

Carbon materials display appealing physical, chemical, and mechanical properties and have been extensively studied as supercapacitor electrodes. The surface engineering further allows us to tune their capability of adsorption/desorption and catalysis. Therefore, a facile and inexpensive chemical-acid-etching approach has been developed to activate the carbon cloth as an electrode for supercapacitor. The capacitance of the acid-etched carbon cloth electrode can approach 5310 mF cm^-2 at a current density of 5 mA cm^-2 with remarkable recycling stability. The all-solid-state symmetric supercapacitor delivered a high energy density of 4.27 mWh cm^-3 at a power density of 1.32 W cm^-3. Furthermore, this symmetric supercapacitor exhibited outstanding mechanical flexibility, and the capacity remained nearly unchanged after 1000 bending cycles.

Redox-Mediator-Enhanced Electrochemical Capacitors: Recent Advances and Future Perspectives

Supercapacitors deliver exceptional power densities, high cycling stability, and inherent safety but suffer from low energy densities. Many methods to enhance the energy density are based on exploring electrode materials with well-developed structures and designing asymmetric systems with wide voltage windows. The energy density is substantially enhanced at the compromise of power density by utilizing the sluggish kinetics of pseudocapacitive materials. Redox-active electrolytes can contribute additional pseudocapacitance from the reactions of redox mediators at the interface, which have attracted increasing attention of researchers. Redox-mediator-enhanced supercapacitors deliver high energy densities while retaining high power densities. This Minireview highlights the recently prominent progresses of single-, dual-, and ambipolar-redox-mediator-enhanced supercapacitors, the challenges they face, and approaches to suppress self-discharge and develop high-concentration redox-active electrolytes for performance promotion.

Resist-Dyed Textile Alkaline Zn Microbatteries with Significantly Suppressed Zn Dendrite Growth

The progress of electronic textiles relies on the development of sustainable power sources without much sacrifice of convenience and comfort of fabrics. Herein, we present a rechargeable textile alkaline Zn microbattery (micro-AZB) fabricated by a process analogous to traditional resist-dyeing techniques. Conductive patterned electrodes are realized first by resist-aided electroless/electrodeposition of Ni/Cu films. The resulting coplanar micro-AZB in a single textile, with an electroplated Zn anode and a Ni_0.7Co_0.3OOH cathode, achieves high energy density (256.2 Wh kg^-1), high power density (10.3 kW kg^-1), and stable cycling performances (82.7% for 1500 cycles). The solid-state micro-AZB also shows excellent mechanical reliability (bending, twisting, tailoring, etc.). The improved reversibility and cyclability of textile Zn electrodes over conventional Zn foils are found to be due to the significantly inhibited Zn dendrite growth and suppressed undesirable side reactions. This work provides a new approach for energy-storage textile with high rechargeability, high safety, and aesthetic design versatility.

Flexible Zn-Ion Batteries: Recent Progresses and Challenges

To keep pace with the increasing pursuit of portable and wearable electronics, it is urgent to develop advanced flexible power supplies. In this context, Zn-ion batteries (ZIBs) have garnered increasing attention as favorable energy storage devices for flexible electronics, owing to the high capacity, low cost, abundant resources, high safety, and eco-friendliness. Extensive efforts have been devoted to developing flexible ZIBs in the last few years. This work summarizes the recent achievements in the design, fabrication, and characterization of flexible ZIBs. Representative structures, such as sandwich and cable type, are particularly highlighted. Special emphasis is put on the novel design of electrolyte and electrode, which aims to endow reliable flexibility to the fabricated ZIBs. Moreover, current challenges and future opportunities for the development of high-performance flexible ZIBs are also outlined.

Suppression of self-discharge in solid-state supercapacitors using a zwitterionic gel electrolyte

The zwitterionic gel electrolyte developed here can be applied to minimize self-discharge whilst maintaining the closed circuit electrochemical performance of solid-state supercapacitors.

o-Benzenediol-Functionalized Carbon Nanosheets as Low Self-Discharge Aqueous Supercapacitors

Widening the voltage window is often proposed as a way to increase the energy density of aqueous supercapacitors. However, attempting to operate beyond the aqueous supercapacitor stability region can undermine the supercapacitor reliability due to pronounced electrolyte decomposition, which can lead to a significant self-discharge process. To minimize this challenge, charge injection by grafting o-benzenediol onto the carbon electrode is proposed through a simple electrochemical cycling technique. Due to charge injection from o-benzenediol into the carbon electrode, the equilibrium potential of the individual electrode can be reduced. In addition, due to its small molecular size, charge distribution, which is commonly faced by bulk pseudocapacitive materials, is also avoided. The assembled supercapacitor based on the o-benzenediol-grafted carbon demonstrated a maximum energy density of 24 Wh kg^-1 and a maximum power density of 69 kW kg^-1 , with a retention of 89 % after 10 000 cycles at 10 A g^-1 . A low self-discharge of about 4 h was recorded; this could be attributed to the low driving force arising from the lower equilibrium potential. Thus, the proposed technique may provide insight towards the tuning of the equilibrium potential to attain reliable, high-performing supercapacitors with a low self-discharge process.

Integrating Desalination and Energy Storage using a Saltwater-based Hybrid Sodium-ion Supercapacitor

Ever-increasing freshwater scarcity and energy crisis problems require efficient seawater desalination and energy storage technologies; however, each target is generally considered separately. Herein, a hybrid sodium-ion supercapacitor, involving a carbon-coated nano-NaTi₂ (PO₄ )₃ -based battery anode and an activated-carbon-based capacitive cathode, is developed to combine desalination and energy storage in one device. On charge, the supercapacitor removes salt in a flowing saltwater electrolyte through Cl^- electrochemical adsorption at the cathode and Na⁺ intercalation at the anode. Discharge delivers useful electric energy and regenerates the electrodes. This supercapacitor can be used not only for energy storage with promising electrochemical performance (i.e., high power, high efficiency, and long cycle life), but also as a desalination device with desalination capacity of 146.8 mg g^-1 , much higher than most reported capacitive and battery desalination devices. Finally, we demonstrate renewables to usable electric energy and desalted water through combining commercial photovoltaics and this hybrid supercapacitor.