Liang Zhan

Impedance of carbon aerogel/activated carbon composites as electrodes of electrochemical capacitors in aprotic electrolyte

Carbon aerogel (CAG), carbon black (CB), and graphite (G) were added to a high surface area activated carbon (HSAC), prepared by KOH activation, to form composites for use as electrodes of electrochemical capacitors (ECs). The performance of the ECs using these electrodes in (C2H5)4NBF4/propylene carbonate electrolyte were analyzed and compared by an electrochemical impedance spectrum (EIS). An equivalent circuit model with mixed kinetic and charge transfer control was assumed and a Marquardt fit procedure was applied to the EIS data, from which model parameters, such as, series solution resistance, R(subscript s), polarization resistance, R(subscript p), Nernst diffusion layer thickness, δ, and average value of the diffusion coefficients of electrolyte ions, D, were extracted and compared for CAG-, CB-, and Gbased composites. It was found that the ECs using CAG had the highest D values and a comparable R(subscript p) value with CB. The CAG-based composite had the lowest resistance and highest capacitance at the same frequencies among the three types of composites investigated.

Preparation of polystyrene-based activated carbon spheres and their adsorption of dibenzothiophene

Polystyrene-based activated carbon spheres (PACS) were prepared by steam activation and their adsorption performance to a sulfur-containing dibenzothiophene (DBT) was studied. The textural structure of PACS was characterized by scanning electron microscopy, N2 adsorption, thermal gravimetric, and aqueous adsorption. Results showed that PACS with BET surface areas up to 979-1672m^2/g were obtained. The BET surface areas and pore volumes increased with activation time and steam flow rate except for the volume of narrow micropores (＜0.7 nm). The maximum adsorption capacity of PACS to DBT was 109.36 mg/g and the adsorption capacity was related to the volume of narrow micropores, independent of surface area and total pore volume. Irreversible adsorption existed between DBT and PACS. The larger the volume of the narrow micropores, the higher the desorption temperature and the larger the amount of sulfur retained after heating.

Effect of pore structure on the electrochemical performance of coal-based activated carbons in non-aqueous electrolyte

Abstract Anthracite was activated by NaOH to prepare high-performance activated carbons as electrodes for electric double-layer capacitors. The porous structure and electrochemical characteristics of the carbons were investigated by nitrogen sorption and electrochemical methods. The effect of pore structure on the electrochemical performance of the carbons in a 1 mol/L (C 2 H 5 ) 4 NBF 4 /propylene carbonate (PC) electrolyte was investigated. The as-prepared activated carbons exhibit large surface areas (943-2479 m 2 /g) and high-specific capacitances (57–167 F/g). The specific capacitance depends not only on the surface area, but also on the pore size distribution (PSD) of the carbon. Pores with a size of 2–3 nm are crucial for the ions to penetrate inside them for the (C 2 H 5 ) 4 NBF 4 /PC electrolyte. Specific capacitance is higher and impedance is lower for the sample with a wider PSD due to the fact that electrolyte ions could easily enter the pores.

Synthesis of 3D hierarchical porous carbon as electrode material for electric double layer capacitors

Abstract A green and efficient method is presented to synthesize 3D hierarchical porous carbon from a metal organic framework (MOF) formed by 1,4-benzenedicarboxylic acid and zinc nitrate hexahydrate using glucose as the carbon precursor. Glucose was infiltrated into the external surface and/or voids of the cubic MOF, then polymerized and carbonized to form porous carbon. In the meantime, MOF was decomposed into ZnO, which was further reduced by carbon (or CO) into Zn that evaporated during carbonization. The morphology and pore characteristics of the products can be adjusted by changing the reaction time. When the synthesized porous carbon was used as the electrode material for electric double layer capacitors, it exhibited a high initial specific capacitance of 175 F·g−1 at 0.6 A·g−1 and a high capacitance retention of 94.2% at 12 A·g−1 in 1 mol/L NEt4BF4/propylene carbonate electrolyte.

Preparation of graphene nanosheets through detonation

Abstract Graphene nanosheets were synthesized using graphite oxide as a precursor by detonation. The composition, and structure of graphene nanosheets were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, and Raman spectroscopy. Results indicated that the as-prepared material was transparent and wrinkled, and comprised 2-5 graphenes with a highly crystalline structure. The exfoliation and reduction of graphite oxide to graphene nanosheets was induced by the self-generated thermal energy and shockwave of detonation.

In situ growth of a carbon nanofiber/Si composite and its application in Li-ion storage

Abstract A CNF/Si composite was synthesized in situ by growing carbon nanofibers on the surface of silicon particles through catalytic chemical vapor deposition. Microstructures of the composites were characterized by scanning and transmission electron microscopy, and X-ray diffraction. Electrochemical measurements indicated that the composite showed higher reversible capacity (1 042 mAh/g) and better cyclability than did a CNF-Si mixture prepared by simple mechanical milling. A structural evolution mechanism is proposed to explain the superior electrochemical performance of the CNF/Si composite electrode in comparison with the CNF-Si mixture. Scanning electron microscopy and ac impedance analysis indicated that the electrochemical performance can be attributed to the good contact of the in situ grown carbon fiber with the silicon particles.

Synthesis of porous carbons derived from metal-organic coordination polymers and their adsorption performance for carbon dioxide

A series of porous carbons (PCs) was synthesized from nonporous metal-organic coordination polymers (MOCPs), using in-situ polymerized phenol resin as a carbon precursor. The optimized PC has a BET surface area of 2 368 m2/g and an equilibrium CO2 adsorption capacity of 2.9 mmol/g at 300 K and atmospheric pressure. The porous structure of the PCs can be controlled by the formulations of the carbon precursors and gelation/aging time. Meanwhile, the evaporated Zn from the thermal decomposition of the MOCPs acts as an activation agent during carbonization, which eventually improves the microporosity of the PCs. The CO2 equilibrium adsorption capacity increases with increasing Brunauer-Emmett-Teller surface area of the PCs.

Preparation of mesoporous carbon microsphere/activated carbon composite for electric double-layer capacitors

Mesoporous carbon microspheres (MCMs) with specific surface area and diameter of 1850 m2·g−1 and 1μm, respectively, were synthesized using the solvothermal method. When 20 wt% of these were added to a high surface area-activated carbon (HSAC: 3 200 m2·g−1) its interfacial resistance and ion diffusion resistance decreased significantly. The specific capacitance of the composite electrode is 230 F·g−1 in 6 mol·L−1 KOH electrolyte, whereas those of pure HSAC and MCMs are only 190 and 148 F·g−1, respectively, at a high current density of 12 A·g−1. The improvement in electrochemical performance of the HSAC electrode at high current densities is attributed to the small particle diameter, mesoporous structure and high specific surface area of the added MCMs.

Influence of nitrogen hetero-substitution on the electrochemical performance of coal-based activated carbons measured in non-aqueous electrolyte

Abstract Nitrogen-containing carbons were prepared by modification of activated carbons. The modified carbons were used as electrode materials with improved electrochemical performance. Precursor anthracite was activated by KOH (KOH: anthracite= 1:1), modified by melamine or urea and then treated at 1173 K to obtain the modified carbons. The porous structure, the chemical composition and the electrochemical characteristics of the carbons were investigated by nitrogen sorption, XPS and electrochemical methods respectively. Electrochemical experiments were performed in an organic electrolytic solution of 1 M (C 2 H 5 ) 4 NBF 4 /PC. The samples modified by the different methods showed differences in chemical composition that introduced varying degrees of electrochemical performance enhancement. The presence of nitrogen enhanced the electron donor properties and the surface wettability of the activated carbons: this ensured a sufficient utilization of the exposed surface for charge storage.

Preparation of mesophase pitch based mesoporous carbons using an imprinting method

Abstract Mesoporous carbons (MCs) were prepared by an imprinting method, using mesophase pitch and nanometer colloidal silica solution as carbon precursors and pore generators, respectively. The effects of the type of mesophase pitch, the imprinting temperature, and the silica content on the porous properties of MCs were investigated. It was found that the pore size distributions of MCs were similar irrespective of the imprinting temperature and the amount of silica used owing to an imprinting mechanism. The mesopore volume of MCs increased with an increasing amount of silica. There existed an appropriate imprinting temperature slightly higher than the softening point of the mesophase pitch used. An easy imprinting was found for naphthalene based mesophase pitch with a low softening point and fine average particle size as compared with coal based mesophase pitch. Mesoporous carbons were provided with a specific surface area and a total pore volume of 482 m2/g and 1.62 cm3/g, respectively.