Impact of carbon pores size on ionic liquid based-supercapacitor performance

Self-Templating Synthesis of Mesoporous Carbon Cathode Materials for High-Performance Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) have attracted considerable interest because of their excellent power and energy densities. However, the development of LICs is limited by the low capacity of the cathode and the kinetics mismatch between the cathode and anode. In this work, mesoporous carbon materials (MCs) with uniform pore sizes were prepared using magnesium citrate as the raw material through a self-templating method. During the carbonization process, MgO nanoparticles generated from magnesium citrate act as a template, resulting in a more orderly pore structure. The resultant MCs demonstrate a high specific surface area of 1673 m2 g-1 and an abundance of small mesopores, which significantly accelerated ion migration within the electrolyte and expedited the formation of electric double layers. Benefiting from these advantages, the MCs cathode demonstrates a high reversible specific capacity, excellent cycling stability, and rate performance. The assembled MCs-based LIC provides a high energy density of 152.2 Wh kg-1 and a high power density of 14.3 kW kg-1. After 5000 cycles, a capacity retention rate of 80 % at the current density of 1 A g-1 is obtained. These results highlight the excellent potential of MCs as a cathode material for LICs.

Electrodeposition of Manganese (III) Phthalocyanine/ Reduced Graphene Oxide for Asymmetric Supercapacitor Devices

In this study, two novel tetra-substituted manganese (III) phthalocyanines bearing (9H-carbazol-2-yl)oxy groups on peripheral (1) or non-peripheral (2) positions were prepared and used for modification of reduced graphene oxide (rGO) by applying a simple one-step electrodeposition technique for the first time. The manganese (III) phthalocyanines (MnPcs) were electropolymerized and graphene oxide was electrochemically converted into reduced graphene oxide simultaneously. Subsequently, an rGO-MnPc hybrid structure was formed directly on the NiF electrode (substrate) via layer-by-layer assembly. Additionally, the effect of substituent position on the charge storage capacity of the prepared hybrid capacitive candidates was investigated. The fabricated hybrid electrodes exhibited remarkable electrochemical performance due to the combination of manganese (III) phthalocyanines and reduced graphene oxide. The NiF/rGO2-2 electrode exhibited the highest specific capacitance (512.4 F g-1) at 0.5 A g-1 and the remained specific capacitance was obtained 88.1 % after 5000 consecutive charge-discharge cycles. An asymmetric supercapacitor (ASC) was constructed from rGO2-2 as the positive electrode and rGO as the negative electrode with a working potential of 1.5 V. The as-prepared device delivered a specific energy of 17.4 Wh kg-1 at 350 W kg-1. Hence, manganese (III) phthalocyanine-reduced graphene oxide electrodes can be considered outstanding materials for energy storage applications in the future.

Raman Spectroscopy Measurements Support Disorder-Driven Capacitance in Nanoporous Carbons

Our recent study of 20 nanoporous activated carbons showed that a more disordered local carbon structure leads to enhanced capacitive performance in electrochemical double layer capacitors. Specifically, NMR spectroscopy measurements and simulations of electrolyte-soaked carbons evidenced that nanoporous carbons with smaller graphene-like domains have larger capacitances. In this study, we use Raman spectroscopy, a common probe of local structural disorder in nanoporous carbons, to test the disorder-driven capacitance theory. It is found that nanoporous carbons with broader D bands and smaller ID/IG intensity ratios exhibit higher capacitance. Most notably, the ID/IG intensity ratio probes the in-plane sizes of graphene-like domains and supports the findings from NMR that smaller graphene-like domains correlate with larger capacitances. This study supports our finding that disorder is a key metric for high capacitance in nanoporous carbons and shows that Raman spectroscopy is a powerful technique that allows rapid screening to identify nanoporous carbons with superior performance in supercapacitors.

Structural disorder determines capacitance in nanoporous carbons

The difficulty in characterizing the complex structures of nanoporous carbon electrodes has led to a lack of clear design principles with which to improve supercapacitors. Pore size has long been considered the main lever to improve capacitance. However, our evaluation of a large series of commercial nanoporous carbons finds a lack of correlation between pore size and capacitance. Instead, nuclear magnetic resonance spectroscopy measurements and simulations reveal a strong correlation between structural disorder in the electrodes and capacitance. More disordered carbons with smaller graphene-like domains show higher capacitances owing to the more efficient storage of ions in their nanopores. Our findings suggest ways to understand and exploit disorder to achieve highly energy-dense supercapacitors.

Effect of Pore Size Distribution on Energy Storage of Nanoporous Carbon Materials in Neat and Dilute Ionic Liquid Electrolytes

This study investigates three carbide-derived carbon (CDC) materials (TiC, NbC, and Mo2C) characterized by uni-, bi-, and tri-modal pore sizes, respectively, for energy storage in both neat and acetonitrile-diluted 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A distribution of micro- and mesopores was studied through low-temperature N2 and CO2 adsorption. To elucidate the relationships between porosity and the electrochemical properties of carbon materials, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy measurements were conducted using three-electrode test cells. The ultramicroporous TiC-derived carbon is characterized by a high packing density of 0.85 g cm-3, resulting in superior cathodic and anodic capacitances for both neat ionic liquid (IL) and a 1.9 M IL/acetonitrile electrolyte (93.6 and 75.8 F cm-3, respectively, in the dilute IL). However, the bi-modal pore-sized microporous NbC-derived carbon, with slightly lower cathodic and anodic capacitances (i.e., 85.0 and 73.7 F cm-3 in the dilute IL, respectively), has a lower pore resistance, making it more suitable for real-world applications. A symmetric two-electrode capacitor incorporating microporous CDC-NbC electrodes revealed an acceptable cycle life. After 10,000 cycles, the cell retained approximately 75% of its original capacitance, while the equivalent series resistance (ESR) only increased by 13%.

Synthesis, characterization and study of electrochemical applicability of novel asymmetrically substituted 1,3-dialkyl-1,2,3-benzotriazolium salts for supercapacitor fabrication

Here we report the successful synthesis, fabrication, and testing of novel asymmetrically substituted 1,3-dialkyl-1,2,3-benzotriazolium-based ionic liquids. Their applicability in energy storage is tested as gel polymer electrolytes (ILGPE) immobilized in poly(vinylidene fluoride-co-hexa-fluoropropylene) (PVDF-HFP) copolymer as a solid-state electrolyte in electric double layer capacitors (EDLC). Asymmetrically substituted 1,3-dialkyl-1,2,3-benzotriazolium salts of tetrafluoroborates (BF₄^-) and hexafluorophosphates (PF₆^-) are synthesized by anion exchange metathesis reaction using 1,3-dialkyl-1,2,3-benzotriazolium bromide salts. N-Alkylation followed by quaternization reaction results in dialkyl substitution on 1,2,3-benzotriazole. The synthesized ionic liquids were characterized with ¹H-NMR, ¹³C-NMR, and FTIR spectroscopy. Their electrochemical and thermal properties were studied using cyclic voltammetry, impedance spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. The 4.0 V potential windows obtained for asymmetrically substituted 1,3-dialkyl-1,2,3-benzotriazolium salts of BF₄^- and PF₆^- are promising electrolytes for energy storage. ILGPE tested with symmetrical EDLC with a wide operating window from 0-6.0 V gave an effective specific capacitance of 8.85 F g^-1 at a lower scan rate of 2 mV s^-1, the energy density of 2.9 μW h and 11.2 mW g^-1 power density. The fabricated supercapacitor was employed for lighting red LED (2 V, 20 mA).

Graphene Film with a Controllable Microstructure for Efficient Electrochemical Energy Storage

The agglomeration of graphene sheets and undesired pore size distribution usually lead to unsatisfactory electrochemical properties of reduced graphene oxide (RGO) film electrodes. Herein, crumpled exfoliated graphene (EG) sheets are adopted as the microstructure-regulating agent to tune the morphology and micro-/mesopore amounts with the aim of increasing active surface sites and ion transportation paths in electrodes. With the optimum ratio between EG and GO, the resulting 75%-EG/RGO shows significantly improved specific gravimetric capacitance (C_s) and rate capability when compared with pure RGO electrodes in a symmetrical supercapacitor system. Moreover, when coupling the 75%-EG/RGO cathode with a Zn anode to form a Zn ion hybrid supercapacitor (ZHS), the 75%-EG/RGO exhibits a much higher C_s of 327.39 F g^-1 at 0.1 A g^-1 and can maintain 91.7% capacitance after 8000 cycles. Systematic ex situ X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) measurements reveal that the charge storage mechanism is based on both reversible physical adsorption and dual ion uptake. Furthermore, the quasi-solid-state flexible ZHS also presents high capacitive performance and can maintain ∼100% capacitance under various bending states, demonstrating potential application in wearable electronics. This strategy opens up a new path for constructing high-performance graphene film electrodes.

Asymmetric supercapacitors based on SnNiCoS ternary metal sulfide electrodes

While metal sulfides have extensively investigated as electrode materials for supercapacitors, the further optimization of their material system is still necessary to achieve satisfied performance. In this work, we reported the synthesis of ternary metal sulfide SnNiCoS and its application as electrode material of asymmetric supercapacitors, in which active carbon is used as the other electrode. For control experiments, asymmetric supercapacitors based on single metal sulfide CoS and binary metal sulfide NiCoS are also fabricated and investigated. The results show that the nanospherical SnNiCoS achieves the best performance. Ternary sulphide materials offer more redox than corresponding single-metal sulphides due to the synergy among various transition metal elements. The specific capacitance is 18.6 F cm^-2at current density of 5 mA·cm^-2. An energy density of 937.2μWh cm^-2is achieved at a power density of 4000μW·cm^-2. After 8000 cycles, the capacity retention rate is 82.9%. Present work indicates that SnNiCoS ternary metal sulfide could be a promising composite for high performance supercapacitors.

Adsorption of heavy metal onto biomass-derived activated carbon: review

Due to the rapid development of the social economy and the massive increase in population, human beings continue to undertake processing, and commercial manufacturing activities of heavy metals, which has caused serious damage to the environment and human health. Heavy metals lead to serious environmental problems such as soil contamination and water pollution. Human health and the living environment are closely affected by the handling of heavy metals. Researchers must find several simple, economical and practical methods to adsorb heavy metals. Adsorption technology has been recognized as an efficient and economic strategy, exhibiting the advantages of recovering and reusing adsorbents. Biomass-derived activated carbon adsorbents offer large adjustable specific surface area, hierarchically porous structure, strong adsorption capacity, and excellent high economic applicability. This paper focuses on reviewing the preparation methods of biomass-derived activated carbon in the past five years. The application of representative biomass-derived activated carbon in the adsorption of heavy metals preferentially was described to optimize the critical parameters of the activation type of samples and process conditions. The key factors of the adsorbent, the physicochemical properties of the heavy metals, and the adsorption conditions affecting the adsorption of heavy metals are highlighted. In addition, the challenges faced by biomass-derived activated carbon are also discussed.

Synthesis of manganese molybdate/MWCNT nanostructure composite with a simple approach for supercapacitor applications

Recently, magnesium molybdate materials have attracted scientific attention for application in supercapacitor devices due to advantages like low synthesis cost and good redox reactions. Nevertheless, these materials endure low electrical conductivity leading to inferior electrochemical performance. To eliminate this drawback, we prepare a composite powder containing magnesium molybdate and functionalized carbon nanotubes (MMO/C) using a simple process to improve the supercapacitive properties. The results proved an electrostatic interaction between the two components of the composite powder, which contains 18-30 nm magnesium molybdate nanoparticles. A crystal model related to magnesium molybdate powder (MMO) was simulated, illustrating that MnO6 octahedra are formed next to MoO4 tetrahedra. The mesoporous structure of both powders was confirmed whereas the specific surface area of the MMO was enhanced by 69.9% to 36.86 m2 g-1 in the MMO/C powder with more electroactive sites. The higher electrical conductivity of the MMO/C electrode was proved using electrochemical impedance spectroscopy (EIS) results, with the MMO/C electrode achieving a specific capacitance of 571 F g-1 at 1 A g-1 current density, improved by more than 4.5 times that of the MMO. Furthermore, the rate performance and cycling stability of the MMO/C electrode reached 87% and 85.2%, respectively. Finally, a two-electrode energy storage device (MMO/C//AC) was assembled. It reveals a specific capacitance of 94.7 F g-1, a maximum energy density of 29.6 W h kg-1 at a power density of 660.1 W kg-1, and cycling performance of 84.3% after 2000 cycles. As a result, the resulting data demonstrate that the MMO/C electroactive material has promising abilities in capacitive energy storage systems.

High-performance flexible supercapacitor enabled by Polypyrrole-coated NiCoP@CNT electrode for wearable devices

As a pseudocapacitive electrode material, nickel-cobalt bimetallic phosphide has attracted wide attention with its advantage in capacitance and chemical activity. While, like Ni-Co oxides or sulfides, the application of nickel-cobalt bimetallic phosphide is generally hampered by its confined conductivity, low chemical stability and unsatisfactory cycle durability. Herein, this work demonstrates a NiCoP@CNT@PPy (NCP@CNT@PPy) composite that is obtained by polymerizing pyrrole monomer on the surface of NiCoP@CNT complex. According to density functional theory (DFT), it is theoretically demonstrated that the bimetallic Ni-Co phosphide (NiCoP) can exhibit more electrons near the Fermi level than single Ni or Co phosphide. Under the combined effects of carboxylic carbon nanotubes (c-CNTs) and polypyrrole (PPy), the NCP@CNT@PPy electrode exhibits excellent electrochemical performance. In addition, a flexible asymmetric supercapacitor (ASC) is prepared, which demonstrated high energy density and admirable heat-resistance and flexibility performance, showing huge potential in the application of heat-resistant storage energy systems and portable wearable devices.