Large Ion-accessible Surface Area Research Articles

Hierarchical porous N-doped carbon fibers enable ultrafast electron and ion transport for Zn-ion storage at high mass loadings

Zinc-ion capacitors (ZICs) are a promising and safe energy storage system for portable electronics but usually limited by relatively low areal capacity and energy density. Although increasing the mass loading has been suggested, thick electrode layers and clogged pores unavoidably lead to sluggish electron transport and ion diffusion as well as “dead volume” of active materials. Herein, a millimeter-scale 3D thick-network electrode, composed of closely intertwined hierarchical porous N-doped and free-standing carbon nanofibers (HPNCFs), was fabricated via carbonization of nanoscale zeolitic imidazolate framework (ZIF-8) particles embedded in electrospun polyacrylonitrile (PAN) nanofibers. With fast electron/ion transport, large ion-accessible surface area and high utilization rate of active materials, the mass loading of the HPNCFs electrode can reach 48.67 mg cm−2 with a thickness of 4.46 mm. Moreover, the HPNCFs-based ZICs can yield a superior areal/gravimetric capacity of 4.13 F cm−2 /280 F g−1, superior rate capability up to 193.7 F g−1 even at 100 A g−1, high energy density of 99.6 Wh kg−1, and long-term stability for 40,000 cycles. The straightforward approach can provide an opportunity to prepare efficient thick electrodes that could be applied in electrocatalysis, energy storage/conversion, and other electrochemical energy-related techniques.

Facile wet mechanochemistry coupled K2FeO4 activation to prepare functional coal-derived hierarchical porous carbon for supercapacitors

The high power characteristics of supercapacitors (SC) are attractive for grid scale energy storage. The core to achieve the large scale application of SC lies in the preparation of high performance electrode materials. Herein, a facile wet mechanochemistry (WMC) coupled K2FeO4 activation is used to fabricate O/N/S doped coal-derived hierarchical porous carbon (CHPC). K2FeO4 generates high-energy sputtering O2 under mechanical force to impact precursor, leading to an increase in lattice defects, and full exposure of active sites, which facilitates activation of precursor and coupling of heteroatoms. Furthermore, K2FeO4 has the pore forming ability of K-based component and the catalytic graphitization effect of Fe-based component, which could synergistically regulate pore structure and graphitization of CHPC. Experimental exploration combined with density functional theory calculation showed that the unique micro-mesopores structure of CHPC provides a fast ion transport channel and a large ion accessible specific surface area (SSSA). Meanwhile, the high heteroatom doping and graphitization synergistic promote ion storage (high adsorption energy, Eads) and electron transport. The specific capacitance (C) of optimized CHPC900 can obtain 335 F g−1 under 0.5 A g−1. The C retention is 96.2% after 10,000 cycles at 1 A g−1. CHPC900//CHPC900 delivers remarkable energy/power characteristics (9.9 Wh kg−1 of 125 W kg−1).

Silica-Assisted Controlled Engineering of Nitrogen-Doped Carbon Cages with Bulges for High-Performance Supercapacitors.

The bulge structure of N-doped carbon cages is beneficial to improving the specific surface area and increasing the active sites of a chemical reaction. Therefore, this structure plays a role in increasing capacity in energy storage. However, the precise and most effective method of ensuring the bulge structures is still a challenge. Herein, a silica-assisted method is used to prepare N-doped carbon cages with bulges. The effective assembly of a nitrogen-rich resin and silica precursor is employed to construct the bulge structure on the surface. The reaction temperature of the assembly system and the amount of silica precursor are the key influences on the number and degree of bulges. In contrast to conventional carbon materials that have a smooth surface, the bulge structure allows for exposure and accessibility of the activity sites. Due to the N-doping features, a rich mesoporous structure and controllable bulges, the synergism of the high density, large ion-accessible surface area, and fast charge transfer, lead to high performance under the premise of high rate capability in supercapacitor. This silica-assisted strategy can also work on other preprepared corresponding templates that have a different architecture to prepare core-shell carbon tubes, carbon spheres, and carbon rods with a bulge structure.

In-situ redox reaction derived porous nanosheets of MnO2 for supercapacitors

The manganese dioxide (MnO2) nanosheets with numerous in-plane nanopores are prepared via an in-situ redox reaction, then followed by an etch process. Originating from the reaction between Fe2+ ions and negatively charged MnO2 nanosheets, the obtained porous MnO2 nanosheets (P-MnO2) exhibit high micro/mesoporous structure, which provide a large ion-accessible surface area for efficient charge storage. Moreover, the designed nanopores of MnO2 nanosheets facilitate the electrolyte ions diffusion across the 2D MnO2 nanosheets. As a result, in comparison to the pristine MnO2, the P-MnO2 achieves a higher specific capacitance and desirable rate capability.

Bio-inspired and Eco-friendly Synthesis of 3D Spongy Meso-Microporous Carbons from CO2 for Supercapacitors.

Inspired by the spongy bone structures, three-dimensional (3D) sponge-like carbons with meso-microporous structures are synthesized through one-step electro-reduction of CO2 in molten carbonate Li2 CO3 -Na2 CO3 -K2 CO3 at 580 °C. SPC4-0.5 (spongy porous carbon obtained by electrolysis of CO2 at 4 A for 0.5 h) is synthesized with the current efficiency of 96.9 %. SPC4-0.5 possesses large electrolyte ion accessible surface area, excellent wettability and electronical conductivity, ensuring the fast and effective mass and charge transfer, which make it an advcanced supercapacitor electrode material. SPC4-0.5 exhibits a specific capacitance as high as 373.7 F g-1 at 0.5 A g-1 , excellent cycling stability (retaining 95.9 % of the initial capacitance after 10000 cycles at 10 A g-1 ), as well as high energy density. The applications of SPC4-0.5 in quasi-solid-state symmetric supercapacitor and all-solid-state flexible devices for energy storage and wearable piezoelectric sensor are investigated. Both devices show considerable capacitive performances. This work not only presents a controllable and facile synthetic route for the porous carbons but also provides a promising way for effective carbon reduction and green energy production.

Two-dimensional metal-organic framework-derived selenium-doped cobalt Sulfide@Graphene nanofoam for oxygen electrocatalysis

Herein, we report an in-situ recrystallization strategy for preparing two-dimensional metal-organic framework uniformly anchored on graphene sheets, and further design selenium-doped cobalt sulfide@graphene nanofoam via the anion doping as a highly efficient bifunctional oxygen catalyst. Doping selenium atoms into cobalt sulfide not only is favourable for accelerating the transition from ∗OH to ∗O via increasing the electron density around cobalt centers, but also induces the formation of effective metal-oxygen and selenite/selenate species, resulting in significantly increasing the oxygen evolution reaction activity without the obvious deterioration of the oxygen reduction reaction activity. Moreover, graphene nanofoam supports with hierarchical porosity and high crystallinity provide large ion-accessible surface area for sufficiently exposing active sites to the electrolyte and prevents the deactivation of active sites via the formation of a 3–5 layered graphene at the surface as a protective layer. Hence, a superior catalytic performance is achieved with a comparable mass activity to Pt/C catalyst and an overpotential of 347 mV at 10 mA cm−2 for the oxygen evolution reaction. This work gives an insight into the design of efficient and robust first-row transition-metal electrocatalysts via structural engineering and anionic doping.

Dual-template endowing N, O co-doped hierarchically porous carbon from potassium citrate with high capacitance and rate capability for supercapacitors

Engineering hierarchically porous architecture with large ion accessible surface area and abundant pseudocapacitive sites is highly desired for carbon materials to achieve high energy density and high rate capability in supercapacitors. In particular, designing a facile strategy for one-step synthesis of such carbon materials with multiple synergistic effects is full of challenges. Herein, we develop a one-pot strategy with dual template to synthesize N, O co-doped hierarchically porous carbon nanosheets (NOPC). The optimal NOPC possesses a large surface area (2557 m2 g−1) with hierarchical pore network and high pore volume (1.6 cm3 g−1), as well as abundant pseudocapacitive active species. Consequently, in the three-electrode cell, the resultant NOPC electrodes produce an outstanding specific capacitance of 527F g−1 at a current density of 1 A g−1, and a high rate capability of 246F g−1 even up to 200 A g−1. In addition, the assembled symmetric supercapacitors yield a high energy density of 14.5 Wh kg−1 at 250 W kg−1, and remain 8.5 Wh kg−1 even the power density as high as 25 kW kg−1. Such performance can outperform those of most recently reported carbon-based materials. In view of the facile synthetic route, available raw material, and encouraging performance, this work paves a new prospect to construct advanced carbon-based materials toward practical energy-related applications.

N-doped layered porous carbon electrodes with high mass loadings for high-performance supercapacitors

We report a porous carbon material (NPCM) with a high N content as a high-performance supercapacitor electrode material which was prepared by a simple activation-doping process using Metaplexis Japonica shell as the carbon precursor, ammonium chloride as the nitrogen source and zinc chloride as the activation agent. Its high electrical conductivity, large ion-accessible surface area and fast ion transport ability make it possible to achieve a high mass loading of NPCM per area of the electrode and a high energy and high power density supercapacitor. An electrode with a low NPCM mass loading of 1 mg cm−2 has a gravimetric specific capacitance of 457 F g−1 and an areal specific capacitance of 47.8 μF cm−2. At a much high NPCM loading of 17.7 mg cm−2 it has a high gravimetric capacitance of 161 F g−1. Furthermore, an assembled NPCM//NPCM symmetric supercapacitor with an optimal NPCM loading of 12.3 mg cm−2 delivered a high specific energy of 12.5 Wh kg−1 at an ultrahigh power of 80 kW kg−1 in 1 mol L−1 Na2SO4. The achievement of such high-energy and high-power densities using NPCM will open exciting opportunities for carbon-based supercapacitors in many different applications.

Achieving Ultrahigh Volumetric Energy Storage by Compressing Nitrogen and Sulfur Dual-Doped Carbon Nanocages via Capillarity.

High volumetric performance is a challenging issue for carbon-based electrical double-layer capacitors (EDLCs). Herein, collapsed N,S dual-doped carbon nanocages (cNS-CNC) are constructed by simple capillary compression, which eliminates the surplus meso- and macropores, leading to a much increased density only at the slight expense of specific surface area. The N,S dual-doping induces strong polarity of the carbon surface, and thus much improves the wettability and charge transfer. The synergism of the high density, large ion-accessible surface area, and fast charge transfer leads to state-of-the-art volumetric performance under the premise of high rate capability. At a current density of 50 A g-1 , the optimized cNS-CNC delivers a high volumetric capacitance of 243 and 199 F cm-3 in KOH and EMIMBF4 electrolyte, with high energy density of 7.9 and 93.4Wh L-1 , respectively. A top-level stack volumetric energy density of 75.3Wh L-1 (at power density of 0.7kW L-1 ) and a maximal stack volumetric power density of 112kW L-1 (at energy density of 18.8Wh L-1 ) are achieved in EMIMBF4 , comparable to the lead-acid battery in energy density but better in power density with 2-3 orders. This study demonstrates an efficient strategy to design carbon-based materials for high-volumetric-performance EDLCs with wide practical applications.

Self-assembled reduced graphene oxide films with different thicknesses as high performance supercapacitor electrodes

Graphene-based materials have attracted considerable attention as the electrochemical energy storage and conversion material due to their high electronic conductivity and large ion-accessible surface area. Herein, a series of self-assembled reduced graphene oxide (rGO) films are facilely prepared via a scalable vacuum filtration method, whose thickness is regulated by changing the concentration of the filtering rGO solution. The effect of the mass loading (thickness) of rGO film electrodes on its electrochemical performance is investigated. These rGO film electrodes with mass loading of 6.7 mg cm−2 exhibit a high specific capacitance of 173.4 F g − 1 (1.16 F cm−2) at 1 A g − 1 and show high utilization of the active material due to their large specific surface area and uniform pore distribution. Moreover, a quasi-solid supercapacitor fabricated with the as-prepared rGO film electrode shows a high area capacitance of 1.03 F cm−2 and large specific energy of 0.073 mWh cm−2 at the specific power of 3.3 mW cm−2, as well as excellent cycling stability of 85.6% retention after 10,000 cycles. Such remarkable performance suggests that this rGO film is promising electrode material for supercapacitor 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.

Hierarchical porous carbon derived from activated biochar as an eco-friendly electrode for the electrosorption of inorganic ions

Hierarchical porous carbon (HPC) as an eco-friendly electrode material was fabricated from rice husk biochar using the hard template method for electrosorption of ions. The porous structure in HPC materials can be controlled by different activation times to optimize the material physicochemical and electrochemical properties. The HPC sample achieved a high specific surface area (1839 m2 g−1), large pore volume (1.21 cm3 g−1) and 58% mesopore to total pore volume ratio. The HPC electrode exhibited excellent electrochemical properties with a high specific capacitance of 120.5 F g−1 at 5 mV s−1 in a 1 M NaCl solution as well as good reversibility for capacitive charge storage. The large ion-accessible specific surface area, interconnected pore structure among micropores and mesopores, and large mesoporosity of the HPC electrode played crucial roles in the enhancement of the electrosorption performance. The HPC electrode exhibited a high electrosorption capacity of 8.11 mg g−1 and mean deionization rate of 0.92 mg g−1 min−1 for 20 mM NaCl in single-pass capacitive deionization. The associated charge efficiency and energy consumption were 48.1% and 0.064 kWh mol−1, respectively, indicating a low energy requirement of water desalination. Furthermore, the HPC electrode showed good regeneration ability in consecutive cycles for the removal of inorganic pollutants, i.e., NH4+, Mg2+ and Cu2+, with electrosorption capacities of 1.54, 1.53 and 0.52 mg g−1, respectively. Consequently, HPC from tailored activated rice husk biochar can provide a new opportunity to achieve high-performance electrosorption in various water and wastewater treatment processes.

Hollow carbon spheres with artificial surface openings as highly effective supercapacitor electrodes

A novel kind of hollow carbon spheres with artificial surface openings is fabricated as highly effective supercapacitor electrode. This kind of carbon sphere possesses typical hierarchical mesoporous structure with surface area of 670.0 m2 g−1, much higher than that of the conventional hollow carbon spheres and solid carbon spheres. The electrochemical performance of the prepared hollow carbon spheres with surface openings, hollow carbon spheres and solid carbon spheres electrode are evaluated in acidic aqueous electrolyte. The hollow carbon sphere with surface openings electrode shows the optimal capacitive performance due to the specific surface openings and hierarchical mesoporous structure which can provide a large ion-accessible surface area and a loose channel for the electrolyte ions to diffuse into the internal of the spheres, making the internal charge storage sites work more effectively. The hollow carbon spheres with surface openings electrode achieves a remarkable specific capacitance of 272.2 F g−1, much higher than that of the hollow carbon spheres and solid carbon spheres. Besides, the hollow carbon spheres with surface openings preserves 96.9% of its initial capacitance after 10000 charge-discharge cycles, suggesting a perfect cycle life.

Facile construction of hierarchically porous carbon nanofiber aerogel for high-performance supercapacitor

Nanofibrillated cellulose with the features of nano-scale fibers and self-assembly has attracted significant attention to acquire porous structure for low-cost and high-performance electrode materials. Here, a carbon nanofiber aerogel was prepared by self-assembling the building-blocks of nanofibrillated cellulose into controlled macro and mesoporous structure. A typical activation was further applied to engineer abundant micropores, which led to narrowed carbon walls as well as improved surface area (1726 m2 g−1). Due to the facile-constructed hierarchical pore structure and large ion-accessible surface area, the resultant carbon aerogel exhibited comparable performance to reported electrodes from porous bio-carbons. It displayed a high specific capacitance of 169 F g−1 at a high current of 20 A g−1, retaining 73% of that at 0.2 A g−1 (231 F g−1). Furthermore, the symmetric supercapacitor showed a high capacitance retention during the long-term charge–discharge. This work provides a facile and renewable way to develop hierarchical porous bio-carbons with high charge storage capability.

Highly porous carbon nanofoams synthesized from gas-phase plasma for symmetric supercapacitors

Highly porous carbon nanofoams (CNFs) could be massively synthesized at room temperature from acetylene gas precursor by a plasma-enhanced chemical vapor deposition technique. The synthesized CNFs had a three-dimensional framework structure composed of aggregations of nano-sized spherical carbon particles, in which the open-connected micron- and nano-pores were hierarchically distributed. Afterwards, thermal treatment and nitrogen doping process were conducted to enhance both electrical conductivity and electrochemical properties of the CNFs. The nitrogen-doped CNFs (N-CNFs) exhibited very high specific surface area of up to 920.4 m2 g−1 and a pore volume of approximately 0.70 cm3 g−1. Due to their hierarchical porous structure, large ion-accessible surface area, and good conductivity for fast ion sorption and electron transfer, the N-CNFs exhibited excellent electrochemical performance for supercapacitor application, such as high specific capacitance of 362.6F g−1 at a current density of 1 A g−1 and super stability in cycle life test. In addition, a symmetric supercapacitor assembled with N-CNFs could deliver a maximum energy density of 24.4 Wh kg−1 and a power density of 8.8 kW kg−1.

Hierarchical porous carbon materials from bio waste-mango stone for high-performance supercapacitor electrodes

Porous activated carbon was successfully fabricated from bio waste-mango stone via a two-step chemical etching strategy. The optimized carbon material (denoted as MGKZ-2) displayed a high specific capacitance of 353.8 F g−1 at 0.5 A g−1 and a satisfactory rate capability in the three-electrode supercapacitor system. Moreover, MGKZ-2-based symmetric cell presented a high energy density of 27.6 Wh kg−1 at 159.9 W kg−1, as well as long-term cycling stability due to its hierarchical pores to offer large ion-accessible surface area, efficient ion diffusion and electron transport pathways.

Scalable fabrication of ultrathin free-standing graphene nanomesh films for flexible ultrafast electrochemical capacitors with AC line-filtering performance

The commercialized aluminum electrolytic capacitors (AECs) currently used for alternating current (AC) line-filtering have low specific capacitances, making them usually the largest components in electronic circuits. Herein, an ultrathin free-standing graphene nanomesh film (GMF) with uniform and dense nanoholes has been successfully fabricated by an in-situ carbothermal reduction strategy on the basis of spontaneously assembled graphene hybrid film. Due to the carbothermal reduction reaction between carbon atoms of graphene and the metal oxide nanoparticles in-situ generated during assembly process, in-plane nanopores are generated within graphene film. The pore size and density of GMF can be conveniently tuned by annealing temperature. With large ion-accessible surface area, efficient ion diffusion and electron transport pathways, the typical GMF-based electrochemical capacitor (EC) exhibits a high volumetric capacitance of up to 7.6 F cm−3 at 120 Hz, unprecedented power density of up to 3000 W cm−3, ultrafast frequency response with a phase angle of −82.3°, and a short resistor-capacitor time constant of 0.32 ms at 120 Hz, as well as long-term cycling stability. The performances of these ECs are superior to those of the state-of-the-art carbon-based ECs for this purpose; thus, it is promising to replace commercialized AECs for AC line-filtering in future electronics.

Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy Dense, High Voltage Supercapacitors

Supercapacitors, also known as electric double-layer capacitors (EDLCs), are able to store energy rapidly and reversibly through the formation of a double-layer of electronic and ionic charge, closely spaced, at the electrode/electrolyte interface.[1-2] Due to the combination of various advantageous properties, such as efficient operation at high power density, long cycle life and improved safety compared to Li-ion batteries,[3,4] supercapacitors are being increasingly used as alternative power sources to rechargeable batteries. However, the implementation of supercapacitors in practical application is still restricted by the limited energy density, typically 5-8 Wh/L,[5] which is much lower than that of lead-acid batteries which can achieve ~50-90 Wh/L.[6] Considering a symmetric configuration, the volumetric energy density (E V) of a supercapacitor is directly proportional to the volumetric capacitance (C V) of a single electrode and the square of operating voltage (U) following the equation, Ev = 1/8(C V·U 2). Therefore, in such a system, there are two ways to improve energy density: boosting capacitance and extending cell voltage window. The operating voltage of EDLCs is typically limited by the stability of electrolyte and thus room temperature ionic liquids (ILs) with large electrochemical stability windows (> 3-4 V) have become promising next-generation electrolytes. However, the relatively high viscosity of ILs results in lower ionic conductivity compared to traditional aqueous or organic electrolytes and also leads to challenges with pore wetting. On the electrode side, materials with a high intrinsic capacitance (C DL) per area and a large ion-accessible surface area (SSA) are needed to achieve high gravimetric capacitance (C G) since C G = C DL·SSA. The potentially high electrical conductivity, surface area, and chemical stability of graphene-based materials make them promising candidate electrode.[7,8] Theoretically, single layer graphene can exhibit SSA as high as 2675 m2/g. While pristine graphene is limited by its low quantum capacitance leading to C DL ~3-4 mF/cm2, more defective and functionalized graphene produced by the chemical or thermal reduction of graphene oxide (GO) have been shown to exhibit C DL > 17 mF/cm2 in non-aqueous electrolyte leading to theoretical gravimetric capacitance, C G,theoretical > 450 F/g if all of graphene’s surface area could be made ion-accessible. To prevent aggregation and restacking of graphene-based materials into lower SSA structures, we demonstrated an IL microemulsion system (Figure 1) that spontaneously assembles on the surface of GO, placing nanometer-sized droplets of a high-performance, hydrophobic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) directly onto the available surface of well-dispersed single GO layers. We first demonstrate that a common non-ionic surfactant, Tween 20 is capable of forming a stable microemulsion with EMImTFSI with a particle size on the order of several nanometers. These surfactant stabilized nano-droplets (microemulsion particles) spontaneously adsorb to GO sheets yielding a dispersion which can be cast directly onto current collectors leading to a dense nanocomposite of GO/IL/Tween 20. Tween 20 is then removed by evaporation, while the GO is thermally reduced leading to what we refer to as layered IL-mediated reduced GO (IM-rGO) electrodes. The approach is found to yield high ion-accessible SSA as evidenced by one of the highest C G ever reported (302 F/g) when the composite contains 80 wt% IL. These results indicate that the microemulsion particles formed were better able to deploy IL as a spacer to prevent rGO sheets from restacking. Reducing the IL content to 60 wt%, resulted in dense electrodes that exhibited a C V = 218 F/cm3, which is the highest value reported to date among all graphene-based supercapacitors leading to exceptional volumetric energy density. Reference (1) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Springer Science & Business Media 2013. (2) Miller, J. R.; Simon, P. Electrochemical capacitors for energy management. Science 2008, 321, 651. (3) Du Pasquier, A.; Plitz, I.; Menocal, S.; Amatucci, G. A comparative study of Li-ion battery, supercapacitor and nonaqueous asymmetric hybrid devices for automotive applications. J. Power Sources 2003, 115, 171. (4) Khaligh, A.; Li, Z. Battery, ultracapacitor, fuel cell, and hybrid energy storage systems for electric, hybrid electric, fuel cell, and plug-in hybrid electric vehicles: State of the art. IEEE transactions on Vehicular Technology 2010, 59, 2806. (5) Burke, A. R&D considerations for the performance and application of electrochemical capacitors. Electrochim. Acta 2007, 53, 1083. (6) Linden, D. Handbook of batteries. Fuel and Energy s 1995, 265. (7) Pope, M. A.; Aksay, I. A. Four-Fold Increase in the Intrinsic Capacitance of Graphene through Functionalization and Lattice Disorder. J. Phys. Chem. C 2015, 119, 20369. (8) Stoller, M. D.; Park, S.; Ruoff, R. S. Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498. Figure 1

Sulfuric acid etching for fabrication of porous MnO2 for high-performance supercapacitor

We developed a facile and efficient route to prepare highly porous nanostructure MnO2 by etching of proton-type layered manganese oxide (H-MnO2) with sulfuric acid (H2SO4). Results from TEM images and N2 adsorption showed that H2SO4 etching created porous MnO2 with average pore size of about 4 nm and high specific surface area (315 m2 g−1). With such porous structure, the obtained MnO2 exhibits a high specific capacitance of 253 F g−1 and enhanced rate capability (62.1% capacitance retention from 0.5 to 10 A g−1) when comparing with the H-MnO2 precursor (154 F g−1, 45.5%) and annealed H-MnO2 in the absence of H2SO4 (134 F g−1, 43.3%). The excellent capacitive properties demonstrate that creation of porous structure on H-MnO2 not only provides large ion-accessible surface area for efficient charge storage, but also to some extent promotes the kinetics of electrochemical reactions.

Achieving commercial-level mass loading in ternary-doped holey graphene hydrogel electrodes for ultrahigh energy density supercapacitors

Enabling fast ion diffusion in thick electrodes (100–200 µm, ~ 10 mg cm−2) is critical for their practical application in state-of-the-art supercapacitors (SCs). We developed a three-dimensional (3D) boron, nitrogen, and phosphorus ternary-doped holey graphene hydrogel (BNP-HGH) film to achieve an optimized porous structure with a high electrical conductivity, large ion accessible surface area, efficient electron and ion transport pathways, as well as high ion adsorption capacity. The binder-free BNP-HGH electrode can deliver a specific capacitance of 350 F g−1 and a volumetric capacity of 234 F cm−3, which are the best performance reported so far for graphene-based SCs using an organic electrolyte. Fully packaged SCs using the BNP-HGH electrodes with a commercial level graphene mass loading (150 µm, ~ 10 mg cm−2) can deliver ultrahigh stack gravimetric and volumetric energy densities of 38.5 Wh kg−1 and 57.4 Wh L−1, respectively, which are comparable to those of lead-acid batteries (35–40 Wh kg−1 and 50–90 Wh L−1) while maintaining an ultrahigh power density of 83 kW kg−1 (~ 55 kW L−1) as well as a long cycle life (81.3% capacitance retention over 50,000 cycles). The high energy and power densities bridge the gap between traditional SCs and batteries, and should be very useful in practical applications.