Block copolymer derived uniform mesopores enable ultrafast electron and ion transport at high mass loadings

Redox molecule Alizarin red S anchored on biomass-derived porous carbon for enhanced supercapacitive performance

Biomass-derived porous carbon as a conductive framework in which the redox molecule Alizarin red S is anchored by strong interactions.

Self-Assembled Carbon Superstructures Achieving Ultra-Stable and Fast Proton-Coupled Charge Storage Kinetics

Designing ingenious and stable carbon nanostructures is critical but still challenging for use in energy storage devices with superior electrochemistry kinetics, durable capacitive activity, and high rate survivability. To pursue the objective, a simple self-assembly strategy is developed to access carbon superstructures built of nanoparticle embedded plates. The carbon precursors, 2,4,6-trichloro-1,3,5-triazine and 2,6-diaminoanthraquinone can form porous organic polymer with "protic salt"-type rigid skeleton linked by -NH₂ ⁺ Cl^- - "rivets", which provides the cornerstone for hydrogen-bonding-guided self-assembly of the organic backbone to superstructures by π-π plane stacking. The ameliorative charge density distribution and decreased adsorption energy in as-fabricated carbon superstructures allow the high accessibility of the build-in protophilic sites and efficient ion diffusion with a low energy barrier. Such superstructures thus deliver ultra-stable charge storage and fast proton-coupled kinetics at the structural-chemical defects, contributing to unprecedented lifespan (1 000 000 cycles), high-rate capability (100 A g^-1 ) for carbon-based supercapacitors, and an ultrahigh energy density (128 Wh kg^-1 ) for Zn-ion hybrid supercapacitors. The self-assembled carbon superstructures significantly improve the all-round electrochemical performances, and hold great promise for efficient energy storage.

Oxygen-vacancy abundant alpha bismuth oxide with enhanced cycle stability for high-energy hybrid supercapacitor electrodes

Bi₂O₃ is an outstanding electrode material due to its high theoretical specific capacity. Hence, the synthesis of δ-Bi₂O₃ materials with high oxygen-vacancy contents could improve their electrochemical performances but causes easy conversion to α-Bi₂O₃ with low oxygen-vacancy contents, leading to poor cycling stability and limited practical applications. To overcome these problems, an effective strategy for constructing high oxygen vacancies α-Bi₂O₃ on activated carbon fiber paper (ACFP) is developed in this study. To this end, ACFP/Bi(OH)₃ is first synthesized by the solvothermal method and then converted to ACFP/α-Bi₂O₃ by in situ electrochemical activation. The proposed innovative electrochemical method quickly and easily introduces oxygen vacancies while preserving the three-dimensional structure, thereby promoting the charge transfer and ions diffusion in ACFP/α-Bi₂O₃. Consequently, the specific capacity of ACFP/α-Bi₂O₃ reaches 906C g^-1 at 1 A g^-1, and the capacity retention remains above 70% after 3000 cycles, a value higher than that of δ-Bi₂O₃ (45%). Furthermore, the hybrid supercapacitor device assembled by ACFP/α-Bi₂O₃ delivers a maximum energy density of 114.9 Wh kg^-1 at 900 W kg^-1 and outstanding cycle stability with 73.56 % retention after 5500 cycles. In sum, the proposed ACFP/α-Bi₂O₃ with high performance and good stability looks promising for use as bismuth-based anode materials in supercapacitors and aqueous batteries.

Ultrafast Nonvolatile Ionic Liquids-Based Supercapacitors with Al Foam-Enhanced Carbon Electrode

The ultrafast frequency response supercapacitor is a promising candidate for alternating current line filtering. We report the fabrication of a special structured ionic liquid-based supercapacitor with an ultrafast response of only 1.5 ms. The three-dimensional aluminum (Al) foam in situ coated with carbon layer (∼500 nm) serves as the novel, highly efficient electrode-current collector. The high porosity (95%) of Al foam allows the rapid ion diffusion and the as-obtained Al/C interface with atomic-level mixing allows the fast electron transfer, two crucial factors for ultrafast response. Hence, it possesses an excellent specific mass capacitance of 68 mF g^-1 at 120 Hz, as well as an ultrahigh rate of up to 3000 V s ^-1. The supercapacitors exhibit frequency modulation performance in the range of 20 kHz to 16 MHz. They exhibit the similar even better alternating current filtering performance, as compared to the commercial aluminum electrolytic capacitors, detected at 10 Hz, 60 Hz, 100 Hz and 1 M Hz. These results suggest that, although ILs have high viscosity and low ion mobility, the IL-based supercapacitor has a great potential to be used as a device for alternating current line filtering, as well as providing nonvolatile and nonflammability safety.

Rational ion transport management mediated through membrane structures

Unique membrane structures endow membranes with controlled ion transport properties in both biological and artificial systems, and they have shown broad application prospects from industrial production to biological interfaces. Herein, current advances in nanochannel-structured membranes for manipulating ion transport are reviewed from the perspective of membrane structures. First, the controllability of ion transport through ion selectivity, ion gating, ion rectification, and ion storage is introduced. Second, nanochannel-structured membranes are highlighted according to the nanochannel dimensions, including single-dimensional nanochannels (i.e., 1D, 2D, and 3D) functioning by the controllable geometrical parameters of 1D nanochannels, the adjustable interlayer spacing of 2D nanochannels, and the interconnected ion diffusion pathways of 3D nanochannels, and mixed-dimensional nanochannels (i.e., 1D/1D, 1D/2D, 1D/3D, 2D/2D, 2D/3D, and 3D/3D) tuned through asymmetric factors (e.g., components, geometric parameters, and interface properties). Then, ultrathin membranes with short ion transport distances and sandwich-like membranes with more delicate nanochannels and combination structures are reviewed, and stimulus-responsive nanochannels are discussed. Construction methods for nanochannel-structured membranes are briefly introduced, and a variety of applications of these membranes are summarized. Finally, future perspectives to developing nanochannel-structured membranes with unique structures (e.g., combinations of external macro/micro/nanostructures and the internal nanochannel arrangement) for mediating ion transport are presented.

Printing Porous Carbon Aerogels for Low Temperature Supercapacitors

Maintaining fast charging capability at low temperatures represents a significant challenge for supercapacitors. The performance of conventional porous carbon electrodes often deteriorates quickly with the decrease of temperature due to sluggish ion and charge transport. Here we fabricate a 3D-printed multiscale porous carbon aerogel (3D-MCA) via a unique combination of chemical methods and the direct ink writing technique. 3D-MCA has an open porous structure with a large surface area of ∼1750 m² g^-1. At -70 °C, the symmetric device achieves outstanding capacitance of 148.6 F g^-1 at 5 mV s^-1. Significantly, it retains a capacitance of 71.4 F g^-1 at a high scan rate of 200 mV s^-1, which is 6.5 times higher than the non-3D printed MCA. These values rank among the best results reported for low temperature supercapacitors. These impressive results highlight the essential role of open porous structures for preserving capacitive performance at ultralow temperatures.

Modification of ultra-micropore dominated carbon by O/N-containing functional groups grafted for enhanced supercapacitor performances

In our study, a simple method was employed to prepare ultra-micropore-dominated carbon materials with controllable pore size. A mass of heteroatoms was introduced by surface functional group grafting, resulting in enhanced electrochemical performance: the maximum specific capacity of 327.5 F g-1 was obtained at 0.5 A g-1 in 6 M KOH, while that of un-grafted original ultra-microporous carbon was only 188 F g-1, with long-term cycle stability (90.5% of the initial after 10 000 cycles), and excellent rate performance (over 82% at the current density from 0.5 A g-1 to 10 A g-1). The mechanism behind the improved performance was due to the presence of the introduced functional groups that improved the surface wettability of the material and provided additional redox active sites. Their synergistic effects promoted the enhanced electrochemical performance of the ultra-microporous carbon. This study provides a basis for the study of the energy storage mechanism of ultra-microporous carbon and the grafted modification of carbon materials with heteroatom-containing functional groups.

Overview of transition metal-based composite materials for supercapacitor electrodes

Supercapacitors (SCs) can bridge the gap between batteries and conventional capacitors, playing a critical role as an efficient electrochemical storage device in intermittent renewable energy sources. Transition metal-based electrode materials have been investigated extensively as a class of electrode materials for SC application, but they have some limitations due to the sluggish ion/electron diffusion and inferior electronic conductivity, restricting their electrochemical performances towards energy storage. Developing advanced transition metal-based electrode materials is crucial for high energy density along with high specific power and fast charging/discharging rates towards high performance SCs. In this review, we highlight the state-of-the-art of transition metal-based electrode materials (transition metal oxides and their composites, transition metal sulfides and their composites, and transition metal phosphides and their composites), focusing on specific morphologies, components, and power characteristics. We also provide future prospects for transition metal-based electrode materials for SCs and hope this review will shed light on the achievement of higher performance and hold great promise in vast applications for future energy storage and conversion.

Structure-Enhanced Mechanically Robust Graphite Foam with Ultrahigh MnO2 Loading for Supercapacitors

With the fast bloom of flexible electronics and green vehicles, it is vitally important to rationally design and facilely construct customized functional materials with excellent mechanical properties as well as high electrochemical performance. Herein, by utilizing two modern industrial techniques, digital light processing (DLP) and chemical vapor deposition (CVD), a unique 3D hollow graphite foam (HGF) is demonstrated, which shows a periodic porous structure and robust mechanical properties. Finite element analysis (FEA) results confirm that the properly designed gyroidal porous structure provides a uniform stress area and mitigates potential structural failure caused by stress concentrations. A typical HGF can show a high Young's modulus of 3.18 MPa at a low density of 48.2 mg cm^-3. The porous HGF is further covered by active MnO₂ material with a high mass loading of 28.2 mg cm^-2 (141 mg cm^-3), and the MnO₂/HGF electrode still achieves a satisfactory specific capacitance of 260 F g^-1, corresponding to a high areal capacitance of 7.35 F cm^-2 and a high volumetric capacitance of 36.75 F cm^-3. Furthermore, the assembled quasi-solid-state asymmetric supercapacitor also shows remarkable mechanical properties as well as electrochemical performance.

Nitrogen-doped carbon nanotubes as an anode for a highly robust potassium-ion hybrid capacitor

Potassium ion hybrid capacitors (KIHCs) have drawn growing interest owing to their outstanding energy density, power density and excellent cycling stability. However, the large ionic radius of potassium triggers a huge volume change during continuous K⁺ insertion/extraction processes, restricting the development of KIHCs. Here, we report N-doped carbon nanotubes (NCNTs) for high-performance K⁺ storage. The NCNTs possess a hierarchical structure and N functional groups and not only offer sufficient space to relieve the volume expansion, but also provide highly efficient channels to transport electrons and ions. As a result, the NCNTs anode presents a high specific capacity and an excellent cycling stability with an average decay rate of 0.0238% per cycle (the lowest value among the reported carbon-based anodes for K-ions batteries) during 3600 continuous cycles. A potassium ion hybrid capacitor (KIHC) was also designed with the NCNT anode and a commercial active carbon cathode and achieved both a high energy/power density (117.1 W h kg^-1/1713.4 W kg^-1) and a long cycle life (2000 cycles at 1 A g^-1). Moreover, the in situ Raman and ex situ element mapping characterization demonstrate the outstanding electrochemical reversibility of the NCNTs. This work provides a superior strategy to design low-cost anode materials with excellent K⁺ storage electrochemistry.

Zeolitic imidazolate framework (ZIF)-derived porous carbon materials for supercapacitors: an overview

The present analysis focuses on the synthetic methods used for the application of supercapacitors with various mysterious architectures derived from zeolitic imidazolate frameworks (ZIFs). ZIFs represent an emerging and unique class of metal–organic frameworks with structures similar to conventional aluminosilicate zeolites, consisting of imidazolate linkers and metal ions. Their intrinsic porous properties, robust functionalities, and excellent thermal and chemical stabilities have resulted in a wide range of potential applications for various ZIF materials. In this rapidly expanding area, energetic research activities have emerged in the past few years, ranging from synthesis approaches to attractive applications of ZIFs. In this analysis, the development of high-performance supercapacitor electrodes and recent strategies to produce them, including the synthesis of various heterostructures and nanostructures, are analyzed and summarized. This analysis goes via the ingenuity of modern science when it comes to these nanoarchitecture electrodes. Despite these significant achievements, it is still difficult to accurately monitor the morphologies of materials derived from metal–organic frameworks (MOFs) because the induction force during structural transformations at elevated temperatures is in high demand. It is also desirable to achieve the direct synthesis of highly functionalized nanosized materials derived from zeolitic imidazolate frameworks (ZIFs) and the growth of nanoporous structures based on ZIFs encoded in specific substrates for the construction of active materials with a high surface area suitable for electrochemical applications. The latest improvements in this field of supercapacitors with materials formed from ZIFs as electrodes using ZIFs as templates or precursors are discussed in this review. Also, the possibility of usable materials derived from ZIFs for both existing and emerging energy storage technologies is discussed.

The present analysis focuses on the synthetic methods used for the application of supercapacitors with various mysterious architectures derived from zeolitic imidazolate frameworks (ZIFs).

The Application of Metal-Organic Frameworks and Their Derivatives for Supercapacitors

Supercapacitors (SCs), one of the most popular types of energy-storage devices, present lots of advantages, such as large power density and fast charge/discharge capability. Being the promising SCs electrode materials, metal-organic frameworks (MOFs) and their derivatives have gained ever-increasing attention due to their large specific surface area, controllable porous structure and rich diversity. Herein, the recent development of MOFs-based materials and their application in SCs as the electrode are reviewed and summarized. The preparation method, the morphology of the materials and the electrical performance of various MOFs and their derivatives (such as carbon, metal oxide/hydroxide and metal sulfide) are briefly discussed. Most of recent works concentrate on Ni-, Co- and Mn-MOFs and their composites/derivatives. Conclusions and our outlook for the researches are also given, which would be a valuable guideline for the rational design of MOFs materials for SCs in the near future.

New Insight into the Mechanism of Multivalent Ion Hybrid Supercapacitor: From the Effect of Potential Window Viewpoint

Multivalent ion hybrid supercapacitors have been developed as the novel electrochemical energy storage systems due to their combined merits of high energy density and high power density. Nevertheless, there are still some challenges due to the limited understanding of the electrochemical behaviors of multivalent ions in the electrode materials, which greatly hinders the large scale applications of its based hybrid supercapacitors. Herein, the long-term electrochemical behaviors of MnO₂ -based electrode in the divalent Mg²⁺ ions electrolyte are systematically studied and linked with the morphological and electronic evolution of MnO₂ by cycling at different potential windows (spanning to 1.2 V). It reveals that the different potential windows result in the different electrochemical behaviors, which can be divided into two ranges (below and above -0.2 V). And, the electrode cycled at a potential window of 0-1.2 V delivers the highest capacitance of 967 F g^-1 at a scan rate of 10 mV s^-1 , in which the MnO₂ is transformed into a uniformly distributed and nonagglomerated nanoflake morphology promoting the intercalation and deintercalation of Mg²⁺ ions. This study will enrich the understanding of the charge storage mechanism of multivalent ions and provide significant guidance on the performance improvement of the hybrid supercapacitors.

A General Electrodeposition Strategy for Fabricating Ultrathin Nickel Cobalt Phosphate Nanosheets with Ultrahigh Capacity and Rate Performance

Transition-metal phosphates/phosphides possess promising theoretical electrochemical characteristics and exhibit great potential in advanced supercapacitors. Unfortunately, limited by the processing techniques and overall structure, their specific capacity and rate performance are still unsatisfactory. Herein, we report the fabrication of transition-metal phosphate electrodes with an ultrathin sheetlike array structure by one-step electrodeposition at room temperature. As a proof-of-concept, a transition-metal phosphate member of NiCo(HPO₄)₂·3H₂O with an ultrathin nanosheet structure (thickness ∼2.3 nm) was synthesized and investigated. The as-prepared NiCo(HPO₄)₂·3H₂O electrode showcases an ultrahigh specific capacity of 1768.5 C g^-1 at 2 A g^-1 (the highest value for transition-metal phosphates/phosphides reported to date), superb rate performance of 1144.8 C g^-1 at 100 A g^-1, and excellent electrochemical stability. Moreover, the transition-metal phosphate nanosheet array can be uniformly deposited on various conductive substrates, demonstrating the generality of our strategy. Therefore, this simple electrodeposition strategy provides an opportunity to fabricate ultrathin transition-metal phosphate nanosheet materials that can be used for energy storage/conversion, electrocatalysis, and other electrochemical energy-related devices.

Porous organic materials offer vast future opportunities

In light of the surging research on porous organic materials, we herein discuss the key issues of their porous structures, surface properties, and end functions. We also present an outlook on emerging opportunities, new applications, and data science-assisted materials discovery.

One-pot solvothermal incorporation of graphene into chain-engineered polyquinones for metal-free supercapacitors

Solvothermal reduction of graphene oxide and Schiff-base condensation between anthraquinone and dialdehydes were simultaneously performed to produce well-dispersed composites as metal-free supercapacitor electrodes, which rationally combine conductive graphene and pseudocapacitive polyquinones that allow for fast electron/ion transport and efficient redox reactions, affording great rate capability and high power/energy densities.

2D nanosheet/3D cubic framework Ni-Co sulfides for improved supercapacitor performance via structural engineering

The construction of multi-dimensional structured battery-type electrode materials is a promising strategy to develop high performance electrodes for supercapacitors. Herein, a series of battery-type Ni3S2@Co3S4 electrodes with different morphologies are synthesized by controlling the hydrothermal reaction time. Owing to the unique structure with independent but interconnected 2D nanosheets and 3D cubic frameworks, NCS-60 displays high conductivity, numerous active sites and good wettability behavior. It can deliver a high specific capacity of 388.9 mA h g-1 (3500 F g-1) at 1 A g-1, an outstanding rate capacity of maintaining 88.6% at 10 A g-1 and long cycle stability. The battery-type supercapacitor hybrid (BSH) device with active carbon (AC) as the negative electrode delivers an energy density of 41.8 W h kg-1 at the power density of 800 W kg-1. This study provides a feasible route for regulating the morphologies of in situ growth materials that improve the electrochemical performance of supercapacitors.

Controllable Carbonization of Plastic Waste into Three-Dimensional Porous Carbon Nanosheets by Combined Catalyst for High Performance Capacitor

Polyethylene terephthalate (PET) plastic has been extensively used in our social life, but its poor biodegradability has led to serious environmental pollution and aroused worldwide concern. Up to now, various strategies have been proposed to address the issue, yet such strategies remain seriously impeded by many obstacles. Herein, waste PET plastic was selectively carbonized into three-dimensional (3D) porous carbon nanosheets (PCS) with high yield of 36.4 wt%, to be further hybridized with MnO₂ nanoflakes to form PCS-MnO₂ composites. Due to the introduction of an appropriate amount of MnO₂ nanoflakes, the resulting PCS-MnO₂ composite exhibited a specific capacitance of 210.5 F g^-1 as well as a high areal capacitance of 0.33 F m^-2. Furthermore, the PCS-MnO₂ composite also showed excellent cycle stability (90.1% capacitance retention over 5000 cycles under a current density of 10 A g^-1). The present study paved an avenue for the highly efficient recycling of PET waste into high value-added products (PCSs) for electrochemical energy storage.

3-Dimensional Porous Carbon with High Nitrogen Content Obtained from Longan Shell and Its Excellent Performance for Aqueous and All-Solid-State Supercapacitors

Three-dimensional porous carbon is considered as an ideal electrode material for supercapacitors (SCs) applications owing to its good conductivity, developed pore structure, and excellent connectivity. Herein, using longan shell as precursor, 3-dimensional porous carbon with abundant and interconnected pores and moderate heteroatoms were obtained via simple carbonization and potassium hydroxide (KOH) activation treatment. The electrochemical performances of obtained 3-dimensional porous carbon were investigated as electrode materials in symmetric SCs with aqueous and solid electrolytes. The optimized material that is named after longan shell 3-dimensional porous carbon 800 (LSPC800) possesses high porosity (1.644 cm3 g-1) and N content (1.14 at %). In the three-electrode measurement, the LSPC800 displays an excellent capacitance value of 359 F g-1. Besides, the LSPC800 also achieves splendid specific capacitance (254 F g-1) in the two electrode system, while the fabricated SC employing 1 M Li2SO4 as electrolyte acquires ultrahigh power density (15930.38 W kg-1). Most importantly, LSPC800 electrodes are further applied into the SC adopting the KOH/polyvinyl alcohol (PVA) gel electrolyte, which reaches up to an outstanding capacitance of 313 F g-1 at 0.5 A g-1. In addition, for the all-solid-state SC, its rate capability at 50 A g-1 is 72.73% and retention at the 10,000th run is 93.64%. Evidently, this work is of great significance to the simple fabrication of 3-dimensional porous carbon and further opens up a way of improving the value-added utilization of biomass materials, as well as proving that the biomass porous carbons have immense potential for high-performance SCs application.

Exceptional capacitive deionization rate and capacity by block copolymer-based porous carbon fibers

Capacitive deionization (CDI) is energetically favorable for desalinating low-salinity water. The bottlenecks of current carbon-based CDI materials are their limited desalination capacities and time-consuming cycles, caused by insufficient ion-accessible surfaces and retarded electron/ion transport. Here, we demonstrate porous carbon fibers (PCFs) derived from microphase-separated poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as an effective CDI material. PCF has abundant and uniform mesopores that are interconnected with micropores. This hierarchical porous structure renders PCF a large ion-accessible surface area and a high desalination capacity. In addition, the continuous carbon fibers and interconnected porous network enable fast electron/ion transport, and hence a high desalination rate. PCF shows desalination capacity of 30 mgNaCl g-1 PCF and maximal time-average desalination rate of 38.0 mgNaCl g-1 PCF min-1, which are about 3 and 40 times, respectively, those of typical porous carbons. Our work underlines the promise of block copolymer-based PCF for mutually high-capacity and high-rate CDI.

Two-Dimensional Materials for High-Energy Solid-State Asymmetric Pseudocapacitors with High Mass Loadings

A porous nanostructure and high mass loading are crucial for a pseudocapacitor to achieve a good electrochemical performance. Although pseudocapacitive materials, such as MnO₂ and MoS₂ , with record capacitances close to their theoretical values have been realized, the achieved capacitances are possible only when the electrode mass loading is less than 1 mg cm^-2 . Increasing the mass loading affects the capacitance as electron conduction and ion diffusion become sluggish. Achieving fast ion and electron transport at high mass loadings through all active sites remains a challenge for high-mass-loading electrodes. In this study, 2D MnO₂ nanosheets supported on carbon fibers (MnO₂ @CF) as well as MoS₂ @CF with high mass loadings (6.6 and 7.2 mg cm^-2 , respectively) were used in a high-energy pseudocapacitor. These hierarchical 2D nanosheets yielded outstanding areal capacitances of 1187 and 495 mF cm^-2 at high current densities with excellent cycling stabilities. A pliable pseudocapacitive solid-state asymmetric supercapacitor was designed using MnO₂ @CF and MoS₂ @CF as the positive and negative electrodes, respectively, with a high mass loading of 14.2 mg cm^-2 . The assembled solid-state asymmetric cell had an energy density of 2.305 mWh cm^-3 at a power density of 50 mW cm^-3 and a capacitance retention of 92.25 % over 11 000 cycles and a very small diffusion resistance (1.72 Ω s^-1/2 ). Thus, it is superior to most state-of-the-art reported pseudocapacitors. The rationally designed nanostructured electrodes with high mass loading are likely to open up new opportunities for the development of a supercapacitor device capable of supplying higher energy and power.

3D-Printed Structure Boosts the Kinetics and Intrinsic Capacitance of Pseudocapacitive Graphene Aerogels

The performance of pseudocapacitive electrodes at fast charging rates are typically limited by the slow kinetics of Faradaic reactions and sluggish ion diffusion in the bulk structure. This is particularly problematic for thick electrodes and electrodes highly loaded with active materials. Here, a surface-functionalized 3D-printed graphene aerogel (SF-3D GA) is presented that achieves not only a benchmark areal capacitance of 2195 mF cm^-2 at a high current density of 100 mA cm^-2 but also an ultrahigh intrinsic capacitance of 309.1 µF cm^-2 even at a high mass loading of 12.8 mg cm^-2 . Importantly, the kinetic analysis reveals that the capacitance of SF-3D GA electrode is primarily (93.3%) contributed from fast kinetic processes. This is because the 3D-printed electrode has an open structure that ensures excellent coverage of functional groups on carbon surface and facilitates the ion accessibility of these surface functional groups even at high current densities and large mass loading/electrode thickness. An asymmetric device assembled with SF-3D GA as anode and 3D-printed GA decorated with MnO₂ as cathode achieves a remarkable energy density of 0.65 mWh cm^-2 at an ultrahigh power density of 164.5 mW cm^-2 , outperforming carbon-based supercapacitors operated at the same power density.

Birnessite based nanostructures for supercapacitors: challenges, strategies and prospects

In the past few years, intensive attention has been focused on birnessite based electrodes for supercapacitors. Much progress has been achieved in developing birnessite based nanostructures with high electrochemical performance. However, challenges still remain in taking full advantage of birnessite and building smart structures to overcome the gap between the obtained capacitance and its theoretical capacitance. In this review, the basic information on birnessite and its preparation strategies are summarized and the current challenges are put forward. Finally, some new strategies for preparing high electrochemical performance birnessite based nanostructures are highlighted.

TiN Paper for Ultrafast-Charging Supercapacitors

Ultrafast-charging energy storage devices are attractive for powering personal electronics and electric vehicles. Most ultrafast-charging devices are made of carbonaceous materials such as chemically converted graphene and carbon nanotubes. Yet, their relatively low electrical conductivity may restrict their performance at ultrahigh charging rate. Here, we report the fabrication of a porous titanium nitride (TiN) paper as an alternative electrode material for ultrafast-charging devices. The TiN paper shows an excellent conductivity of 3.67 × 10⁴ S m^-1, which is considerably higher than most carbon-based electrodes. The paper-like structure also contains a combination of large pores between interconnected nanobelts and mesopores within the nanobelts. This unique electrode enables fast charging by simultaneously providing efficient ion diffusion and electron transport. The supercapacitors (SCs) made of TiN paper enable charging/discharging at an ultrahigh scan rate of 100 V s^-1 in a wide voltage window of 1.5 V in Na₂SO₄ neutral electrolyte. It has an outstanding response time with a characteristic time constant of 4 ms. Significantly, the TiN paper-based SCs also show zero capacitance loss after 200,000 cycles, which is much better than the stability performance reported for other metal nitride SCs. Furthermore, the device shows great promise in scalability. The filtration method enables good control of the thickness and mass loading of TiN electrodes and devices.

Ultra-high Areal Capacity Realized in Three-Dimensional Holey Graphene/SnO2 Composite Anodes

Nanostructured alloy-type electrode materials and its composites have shown extraordinary promise for lithium-ion batteries (LIBs) with exceptional gravimetric capacity. However, studies to date are usually limited to laboratory cells with too low mass loading (and thus too low areal capacity) to exert significant practical impact. Herein, by impregnating micrometer-sized SnO₂/graphene composites into 3D holey graphene frameworks (HGF), we show that a well-designed 3D-HGF/SnO₂ composite anode with a high mass loading of 12 mg cm⁻² can deliver an ultra-high areal capacity up to 14.5 mAh cm⁻² under current density of 0.2 mA cm⁻² and stable areal capacity of 9.5 mAh cm⁻² under current density of 2.4 mA cm⁻², considerably outperforming those in the state-of-art research devices or commercial devices. This robust realization of high areal capacity defines a critical step to capturing the full potential of high-capacity alloy-type electrode materials in practical LIBs.

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3D holey graphene framework/SnO₂ composite electrode was designed and prepared

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Micrometer-sized SnO₂/graphene was impregnated into 3D holey graphene frameworks

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The 3D composite anode can deliver an ultra-high areal capacity up to 14.5 mAh cm⁻²

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This study defines a critical step in exploring the alloy-type electrode for LIBs

Cobalt-Containing Nanoporous Nitrogen-Doped Carbon Nanocuboids from Zeolite Imidazole Frameworks for Supercapacitors

Pyrolyzing metal-organic frameworks (MOFs) typically yield composites consisting of metal/metal oxide nanoparticles finely dispersed on carbon matrices. The blend of pseudocapacitive metal oxides and conductive metals, as well as highly porous carbon networks, offer unique opportunities to obtain supercapacitor electrodes with mutually high capacitances and excellent rate capabilities. Herein, we demonstrate nitrogen-doped carbon nanocuboid arrays grown on carbon fibers and incorporating cobalt metal and cobalt metal oxides. This composite was synthesized via pyrolysis of a chemical bath deposited MOF, cobalt-containing zeolite imidazole framework (Co-ZIF). The active materials for charge storage are the cobalt oxide and nitrogen-doped carbon. Additionally, the Co metal and the nanoporous carbon network facilitated electron transport and the rich nanopores in each nanocuboid shortened ion diffusion distance. Benefited from these merits, our Co-ZIF-derived electrode delivered an areal capacitance of 1177 mF cm^-2 and excellent cycling stability of ~94% capacitance retained after 20,000 continuous charge-discharge cycles. An asymmetric supercapacitor prototype having the Co-ZIF-derived hybrid material (positive electrode) and activated carbon (negative electrode) achieved a maximal volumetric energy density of 1.32 mWh cm^-3 and the highest volumetric power density of 376 mW cm^-3. This work highlights the promise of metal-metal oxide-carbon nanostructured composites as electrodes in electrochemical energy storage devices.

Two-dimensional hierarchically porous carbon nanosheets for flexible aqueous supercapacitors with high volumetric capacitance

Two-dimensional carbon nanomaterials can be assembled into a dense film and used for flexible supercapacitors, but the high packing density leads to restacking problems and poor volumetric capacitance. To address this challenge, we report two-dimensional hierarchically porous carbon (2D-HPC) consisting of micro-, meso-, and macro-pores on the 2D sheet, which shows superior capacitance and rate capability compared with a 2D carbon nanosheet consisting predominantly of micro-pores. A flexible supercapacitor fabricated with the 2D-HPC presents a high volumetric capacitance of 412 F cm^-3, a high volumetric energy density of 9.2 mW h cm^-3 and a power density of 120 mW cm^-3.

Block copolymers for supercapacitors, dielectric capacitors and batteries

Block copolymer-based energy storage emerges as an active interdisciplinary research field. This topical review presents a survey of the recent advances in block copolymers for energy storage. In the first section, we introduce the background of electrochemical energy storage and block copolymer thermodynamics. In the second section, we discuss the current understandings of block copolymer chemistry, processing, pore size, and ionic conductivity. In the third section, we summarize the design principles and state-of-the-art applications of block copolymers in three energy storage devices, namely, supercapacitors, dielectric capacitors, and batteries. Lastly, we present our perspectives on future possible breakthroughs and associated challenges that are essential to propel the development of advanced block copolymers for energy storage. We expect the review to encourage innovative studies on integrating block copolymers into energy storage applications.

Facile and scalable preparation of 3D SnO2/holey graphene composite frameworks for stable lithium storage at a high mass loading level

A binder-free 3D SnO₂/holey graphene composite framework electrode with high mass loading shows superior Li⁺ storage performance.