Yongpeng Cui

Extremely high-rate aqueous supercapacitor fabricated using doped carbon nanoflakes with large surface area and mesopores at near-commercial mass loading

Achieving a satisfactory energy–power combination in a supercapacitor that is based on all-carbon electrodes and operates in benign aqueous media instead of conventional organic electrolytes is a major challenge. For this purpose, we fabricated carbon nanoflakes (20–100 nm in thickness, 5-μm in width) containing an unparalleled combination of a large surface area (3,000 m2·g−1 range) and mesoporosity (up to 72%). These huge-surface area functionalized carbons (HSAFCs) also had a substantial oxygen and nitrogen content (~10 wt.% combined), with a significant fraction of redox-active carboxyl/phenol groups in an optimized specimen. Their unique structure and chemistry resulted from a tailored single-step carbonization-activation approach employing (2-benzimidazolyl) acetonitrile combined with potassium hydroxide (KOH). The HSAFCs exhibited specific capacitances of 474 F·g−1 at 0.5 A·g−1 and 285 F·g−1 at 100 A·g−1 (charging time < 3 s) in an aqueous 2 M KOH solution. These values are among the highest reported, especially at high currents. When tested with a stable 1.8-V window in a 1 M Na2SO4 electrolyte, a symmetric supercapacitor device using the fabricated nanoflakes as electrodes yielded a normalized active mass of 24.4 Wh·kg−1 at 223 W·kg−1 and 7.3 Wh·kg−1 at 9,360 W·kg−1. The latter value corresponds to a charge time of <3 s. The cyclability of the devices was excellent, with 93% capacitance retention after 10,000 cycles. All the electrochemical results were achieved by employing electrodes with near-commercial mass loadings of 8 mg·cm−2.

Sorghum core-derived carbon sheets as electrodes for a lithium-ion capacitor

We use sorghum core-derived carbon sheets (SCDCS) as electrodes for a lithium-ion capacitor due to their large specific surface area (1122 m2 g−1) and sheet-like structure. A lithium-ion capacitor with high energy density is fabricated using SCDCS as both the negative and positive electrodes, in which the potential of the electrode is adjusted to obtain equal capacity in both the negative and positive electrodes. The lithium-ion capacitor demonstrates a high energy density of 124.8 W h kg−1 at a power density of 107 W kg−1, and maintains 59.5 W h kg−1 at a high power density of 10 508 W kg−1. The capacity retention ratio is 66% for the fabricated lithium-ion capacitor after 5000 cycles at a current of 10 A g−1, indicating that this lithium-ion capacitor is promising for fast energy storage.

Nitrogen-doped porous carbons derived from a natural polysaccharide for multiple energy storage devices

Designing advanced carbon electrodes is considered as one of the most promising directions for energy storage. Herein, we report a facile approach to produce porous carbon nanomaterials. The carbon nanomaterials were prepared via KOH activation using natural polysaccharide-sodium alginate as the precursor with the subsequent introduction of additional nitrogen heteroatoms achieved by further reaction with urea. The optimal electrodes with a high specific surface area (up to 3313 m2 g−1), interconnected porosity, and rich nitrogen (∼7.2 wt%) and oxygen (∼7.4 wt%) doping can achieve an excellent electrochemical performance in supercapacitors and lithium ion batteries. When these materials are employed as supercapacitor electrodes, they achieved an outstanding specific capacitance of 267 F g−1 at 1 A g−1 and an extremely high rate performance with 76.8% capacitance retention ratio in an alkaline electrolyte. In addition, a high capacitance of 197 F g−1 at 0.5 A g−1 with a high capacitance retention ratio of 52.9% at 100 A g−1 can be achieved in an ionic liquid electrolyte. When tested as lithium ion battery anodes, an extraordinarily high specific capacity of 1455 mA h g−1 and a stable energy storage performance up to 500 cycles were observed. The present study highlights that high-performance carbon electrodes can be produced by using sustainable precursor and can be used in multiple energy storage systems.

All-carbon lithium capacitor based on salt crystal-templated, N-doped porous carbon electrodes with superior energy storage

In the pursuit of a lithium ion capacitor (LIC) with higher energy density and lower cost, the all-carbon symmetric-like LIC (ACS-LIC) has recently risen to prominence. In this article, we report a successful example of ACS-LIC synthesized by constructing both anode and cathode with one designed porous carbon material, prepared by a one-pot method and the cheap precursor methyl cellulose. By employing the salt crystal templates and N-doping, the as-obtained materials possess a unique interconnected porous carbon network with hierarchical pores and abundant active sites, showing low internal resistance, good wettability, high conductivity, and rich pseudocapacitance. The resultant NPC//NPC ACS-LIC device exhibited outstanding energy-power characteristics. Even at the super-large power density of 66 000 W kg−1, it can still achieve a high energy density of 70 W h kg−1. More importantly, the NPC//NPC ACS-LIC device demonstrates state-of-the-art cycling performance. After 10 000 cycles at 2 A g−1, the performance is retained at nearly 100%; even when tested at 10 A g−1, the device can still deliver 80.0% retention after 20 000 cycles, with only 0.001% fading per cycle, which is superior or at least comparable to the current state-of-the-art LICs.

Non-carbon coating: a new strategy for improving lithium ion storage of carbon matrix

Coatings with non-carbon films to improve the electrochemical performance of carbon materials have recently begun to gain attention. As one attempt in this field, we fabricated a thin TiO2/C composite film coated on porous biomass-derived biocarbon (PBC) (denoted as PBC@TC) by a facile sol–gel strategy. The experimental results exhibit an obvious Li+ storage capacity enhancement of the as-obtained PBC@TC sample compared with the uncoated sample, demonstrating the significant positive effect of the TiO2/C films on improving the electrochemical performance of the carbon matrix. Notably, the cycling stability of carbon materials can also be improved greatly by coating with this TiO2/C film, delivering an 86% capacity retention even after 10 000 cycles at 5 A g−1. When employed as anode materials in lithium ion capacitors (LICs), the resultant PBC@TC activate carbon LICs (PBC@TC//AC LICs) present an energy density of 130 W h kg−1 at a power density of 66.6 W kg−1 in a voltage range from 0 to 4 V, and a 90% capacity retention after 10 000 cycles at 5 A g−1. Therefore, besides chemical or physical activation and heteroatom doping, coating with TiO2/C films could be a new facile and green approach for improving lithium ion storage properties of carbon materials. Owing to the flexibility of the carbon matrix and film species, this strategy can be extended to other carbon matrix and non-carbon films, which could have potential for applications in exploring high-performance electrode materials.

Marine-Biomass-Derived Porous Carbon Sheets with a Tunable N-Doping Content for Superior Sodium-Ion Storage

Synthesis of the electrode materials of sodium-ion storage devices from sustainable precursors via green methods is highly desirable. In this work, we fabricated a unique N, O dual-doped biocarbon nanosheet with hierarchical porosity by direct pyrolysis of low-cost cuttlebones and simple air oxidation activation (AOA) technique. With prolonging AOA time, thickness of the carbon sheets could be reduced controllably (from 35 to 5 nm), which may lead to tunable preparation of carbon nanosheets with a certain thickness. Besides, an unexpected increase in N-doping amount from 7.5 to 13.9 atom % was observed after AOA, demonstrating the unique role of AOA in tuning the doped heteroatoms of carbon matrix. This was also the first example of increasing N-doping content in carbons by treatment in air. More importantly, by optimizing the thickness of carbon sheets and heteroatom doping via AOA, superior sodium capacity-cycling retention-rate capability combinations were achieved. Specifically, a current state-of-the-art Na+ storage capacity of 640 mAh g-1 was obtained, which was comparable with the lithium-ion storage in carbon materials. Even after charging/discharging at large current densities (2 and 10 A g-1) for 10 000 cycles, the as-obtained samples still retained the capacities of 270 and 138 mAh g-1, respectively, with more than 90% retention. The assembled sodium-ion capacitors also delivered a high integrated energy-power density (36 kW h kg-1 at an ultrahigh power density of 53 000 W kg-1) and good cycling stability (90.5% of capacitance retention after 8000 cycles at 5 A g-1).