Mesocarbon Microbeads Research Articles

Electrophoretic self-assembly of expanded mesocarbon microbeads with attached nickel nanoparticles as a high-rate electrode for supercapacitors

Expanded mesocarbon microbeads (EMCMBs) with graphene oxide (GO) sheets were prepared by expanding graphitized mesocarbon microbeads (MCMBs) using a simple solution-based oxidative process. EMCMB-supported nickel nanoparticles with an average size of 4.6 nm were fabricated by an electrophoretic deposition (EPD) method in the presence of nickel nitrate additive. Nickel ions were self-assembled on the fluffy GO sheets resulting in a more positively charged EMCMB particle for facilitating EPD and dispersion. After heat treatment at 300 °C, GO could be converted to graphene which could provide a conductive network for facilitating the transport of electrons. Well-dispersed nickel nanoparticles on graphene sheets could act as a redox center to allow storage of extra charge and a nanospacer to prevent the graphene sheets from restacking. The specific capacitance of EMCMB-supported nickel electrode could reach 491 F g(-1), which is much higher than that of EMCMB electrode (43 F g(-1)) and bare nickel electrode (146 F g(-1)) at a discharge current of 5 A g(-1). More importantly, the EMCMB-supported nickel electrode is capable of delivering a high specific capacitance of 440 F g(-1) at a discharge current of 50 A g(-1), and could pave the way towards high-rate supercapacitors.

Characterization of neutralized graphite oxide and its use in electric double layer capacitors

Spherical graphite oxide (GO) with high density was synthesized from mesocarbon microbeads (MCMB). The obtained GO was neutralized by various alkaline metal hydroxides and tetraalkylammonium hydroxides. The neutralized GO showed lower water content and much better thermal stability than those of pristine GO. The as-prepared neutralized GOs were investigated as electrode materials for electric double layer capacitor (EDLC) in organic electrolyte. From both the results of galvanostatic charge–discharge and cyclic voltammetry (CV) measurements, it was found that the tetrabutylammonium hydroxide (TBA-OH) together with lithium hydroxide neutralized GO (TBA-Li-GO) displayed a relatively ideal EDLC feature after electrochemical activation at the initial cycle. The positively polarized TBA-Li-GO with small surface area (16m2g−1) exhibited a high volumetric capacitance of 60Fcm−3 after 1000 cycles, which is much higher than that of commercial activated carbon (36Fcm−3).

Three-dimensional high resolution X-ray imaging and quantification of lithium ion battery mesocarbon microbead anodes

In order to improve lithium ion batteries it is important to characterise real electrode geometries and understand how their 3D structure may affect performance. In this study, high resolution synchrotron nano-CT was used to acquire 3D tomography datasets of mesocarbon microbead (MCMB) based anodes down to a 16 nm voxel size. A specimen labelling methodology was used to produce anodes that enhance the achievable image contrast, and image processing routines were utilised to successfully segment features of interest from a challenging dataset. The 3D MCMB based anode structure was analysed revealing a heterogeneous and bi-modally distributed microstructure. The microstructure was quantified through calculations of surface area, volume, connectivity and tortuosity factors. In doing so, two different methods, random walk and diffusion based, were used to determine tortuosity factors of both MCMB and pore/electrolyte microstructures. The tortuosity factors (2–7) confirmed the heterogeneity of the anode microstructure for this field of view and demonstrated small MCMB particles interspersed between large MCMB particles cause an increase in tortuosity factors. The anode microstructure was highly connected, which was also caused by the presence of small MCMB particles. The complexity in microstructure suggests inhomogeneous local lithium ion distribution would occur within the anode during operation.

Interface enhancement of carbon nanotube/mesocarbon microbead isotropic composites

Carbon nanotubes (CNTs) are incorporated into mesocarbon microbead (MCMB)-derived isotropic graphite to improve their mechanical properties. CNTs are homogenously distributed on the MCMB surface by acid-treatment and mechanical mixing. The composites are prepared by cold isostatic pressing, carbonization, and graphitization. The mechanical properties and isotropy ratios of the CNT/MCMB composites are determined by four-point bend tests and thermal expansion measurements, respectively. The addition of CNTs improves the flexural strength by ca. 20%, while keeps a low isotropy ratio. CNTs dispersed on particle interfaces improve the interfacial strength, this reinforcing mechanism is confirmed by a fracture mode analysis with scanning electron microscope.

Prickly structure of multi-walled carbon nanotube reinforced mesocarbon microbead composite with high strength at elevated temperature

High-density carbon blocks are extensively applied in bearing and sealing structures of some advanced aero-engines. Mesocarbon microbead (MCMB) based material, as a kind of high-density carbon, can potentially possess high mechanical strength at high temperature. Multi-walled carbon nanotube (CNT) reinforced mesocarbon microbead composite blocks were in situ fabricated with different amounts of incorporated CNT and further carbonized at 1500°C. High-temperature compressive strength testing of the composite blocks was performed on a thermal–mechanical system at 700°C in vacuum. The results indicated that addition of CNTs up to 5wt.% effectively improves the high-temperature compressive strength of CNT/MCMB composite. Morphology characterization showed that some CNTs are embedded beneath the microbead surface, creating a novel prickly structure which contributes to reinforcement. Moreover, excessive addition of CNTs leads to formation of large fragile hybrid spheres and consequently lowers composite strength at high temperature. The effect of CNTs on sphere structure along with the structure–property relationship revealed in this work would be helpful to further study of high-strength CNT/MCMB composite fabrication and prospective application.

Numerical Simulation of the Effects of Operating Parameters on the Electrical Performance of DEFC

The sintered body of tubular cathode for Direct Ethanol Fuel Cell (DEFC) is prepared by the gelcasting process using a certain proportion of graphite powder and mesocarbon microbead (MCMB). The electrical performance under different air flow rates, operating temperature, porosity of cathode diffusion layer, inlet pressure and different thickness of catalyst layer are analyzed by the combination of experimental and numerical simulation method. The results show that the increase of diffusion layer porosity, inlet pressure, thickness of catalyst layer and operating temperature are benefit for the improvement of the cell performance. Moreover, the electrical performance is improved when the air flow rate is 100 mL/min.

Preparation and Growth Characteristics of Mesocarbon Microbeads

Mesocarbon microbeads (MCMBs) have been synthesized from coal tar pitch. Scanning electron microscopy (SEM) and laser particle size analyzer were used to evaluate the structure and partical size. The effect of process parameters on the growth and morphologies of MCMBs was investigated. The results show that the optimum temperature range is 410-430 °C and the growth time has a suitable range. The diameter and yield of MCMBs will increase and the spherical degree does not change with the extension of time.

Porous mesocarbon microbeads with graphitic shells: constructing a high-rate, high-capacity cathode for hybrid supercapacitor

Li4Ti5O12/activated carbon hybrid supercapacitor can combine the advantages of both lithium-ion battery and supercapacitor, which may meet the requirements for developing high-performance hybrid electric vehicles. Here we proposed a novel “core-shell” porous graphitic carbon (PGC) to replace conventional activated carbon for achieving excellent cell performance. In this PGC structure made from mesocarbon microbead (MCMB), the inner core is composed of porous amorphous carbon, while the outer shell is graphitic carbon. The abundant porosity and the high surface area not only offer sufficient reaction sites to store electrical charge physically, but also can accelerate the liquid electrolyte to penetrate the electrode and the ions to reach the reacting sites. Meanwhile, the outer graphitic shells of the porous carbon microbeads contribute to a conductive network which will remarkably facilitate the electron transportation, and thus can be used to construct a high-rate, high-capacity cathode for hybrid supercapacitor, especially at high current densities.

Electrolytic deposition of Sn-coated mesocarbon microbeads as anode material for lithium ion battery

Deposited of crystalline tin (Sn) coatings on mesocarbon microbead (MCMB) powder as anodes of lithium ion (Li-ion) battery was conducted in the SnSO4 solution by a cathodic electrochemical synthesis. The Sn-coated MCMB specimens were characterized by X-ray diffraction, scanning electron microscopy, and charge/discharge tests. The synthesis condition of Sn-coated MCMB was optimized by considering the agglomeration, size, and adhesion of the samples to the current collectors in the battery. The Sn-coated MCMB electrodes exhibit increased reversible capacity without sacrificing its cycling behavior, compared with bare MCMB electrodes. It is concluded that electrolysis-deposited Sn-coated MCMB electrodes may emerge as a practical and promising anode material for secondary Li-ion batteries.

A comparative study on electrochemical performances of the electrodes with different nanocarbon conductive additives for lithium ion batteries

Three nanocarbon materials (0 D acetylene black (AB), 1 D carbon nanotubes (CNTs) and 2 D reduced graphene oxide (RGO)) were used as conductive additives (CAs) in the mesocarbon microbead anodes for lithium ion batteries. The electrochemical performances of the electrodes were investigated. The results show that the CAs have a significant impact on the electrode performance because they can influence the electron conduction and lithium ion transportation within the electrode. The electrode with RGO achieves a maximum capacity of 387 mAh g−1 after 50 cycles at a current density of 50 mA g−1, much higher than those of the electrodes with AB (334 mAh g−1) and CNTs (319 mAh g−1). The improvement should be mainly ascribed to the “plane-to-point” conducting network formed in the electrode with 2 D RGO which can favor the electron conduction and enhance the lithium ion transportation.

Faceted shape-drive cathode particles using mixed hydroxy-carbonate precursor for mesocarbon microbeads versus LiNi1/3Mn1/3Co1/3O2 Li-ion pouch cell

Using a mixed hydroxy-carbonate (MHC) precursor, homogeneous LiNi1/3Mn1/3Co1/3O2 phase synthesized through a facile route yielded well dispersed, faceted, edge-blunted polyhedral particles (∼200 nm). These cathode particles revealed high degree of hexagonal ordering favorable for impressive electrochemical performance viz., high capacity 187 mAh g−1 in Li vs LiNi1/3Mn1/3Co1/3O2 cell, not fortuitously. Rather, can be clearly attributed to edge-blunted cathode particles that may facilitate enhanced charge mobility mimicking a process akin to corona discharge predominantly in charge concentrated regions which contrasts to moderate capacity from spherical particles explainable under the ambit of shape-drive hither to unknown. Li-ion pouch cell (∼500 mAh) fabricated with this activity enhanced cathode quite rugged enough to endure loads (0.12 Wh, 1.1 Wh), as-demonstrated.

Synthesis of porous graphitic carbon from mesocarbon microbeads by one-step route

We developed a one-step route to synthesize porous graphitic carbon (PGC) from mesocarbon microbeads. The MCMB was ground with NaOH and FeCl3 in an agate mortar, and then the mixture was heated in an Argon flow. Finally the PGC was obtained after washing with acid and deionized water. The sample was characterized by nitrogen adsorption, X-ray diffraction, SEM, TEM and Raman spectroscopy. The results showed that NaOH acted as activation reagent to increase surface area, meanwhile FeCl3 worked as the catalyst to accelerate the graphitization. In order to balance the two effects, the orthogonal experimental design was used to optimize the experiment parameters including temperature, the amounts of NaOH and FeCl3. The optimized result showed a surface area of 929 m2/g and a high degree of crystallization (Lc = 32.6 nm). As a result, the combination of chemical activation and catalytic graphitization upon MCMB created one kind of PGC with a surface graphitization structure which was helpful to contribute a conductivity network.

Aging in lithium-ion batteries: Model and experimental investigation of harvested LiFePO4 and mesocarbon microbead graphite electrodes

This study investigates aging in LiFePO4/mesocarbon microbead graphite cells that have been subjected to either a synthetic hybrid drive cycle or calendar aging, at 22°C. The investigation involves detailed examination and comparison of harvested fresh and aged electrodes. The electrode properties are determined using a physics-based electrochemical impedance spectroscopy (EIS) model that is fitted to three-electrode EIS measurements, with input from measured electrode capacity and scanning electrode microscopy (SEM). Results from the model fitting provide a detailed insight to the electrode degradation and is put into context with the behavior of the full cell aging. It was established that calendar aging has negligible effect on cell impedance, while cycle aging increases the impedance mainly due to structural changes in the LiFePO4 porous electrode and electrolyte decomposition products on both electrodes. Further, full-cell capacity fade is mainly a consequence of cyclable lithium loss caused by electrolyte decomposition.

Supercapacitive behaviors of the nitrogen-enriched activated mesocarbon microbead in aqueous electrolytes

The activated nitrogen-enriched novel carbons (a-NENCs) have been prepared by direct carbonization of polyaniline/activated mesocarbon microbead composites and further activated by 16 M HNO3. The electrochemical performances and supercapacitive behaviors of the a-NENCs in 6 M KOH, 1 M H2SO4, and 0.5 M K2SO4 solutions are evaluated by cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy, cyclic life, leakage current, and self-discharge measurements. The results demonstrate that the supercapacitors perform definitely supercapacitive behaviors; especially in 6 M KOH electrolyte, the supercapacitor represents much better electrochemical performance with more excellent reversibility, shorter relaxation time of 1.11 s, and nearly ideal polarizability. The maximum specific capacitance of the supercapacitors using a-NENCs as active electrode material is 85.1 F g−1 at a rate of 500 mA g−1 in 6 M KOH. These outcomes indicate that the 6 M KOH aqueous solution is a promising electrolyte for the supercapacitor with a-NENCs as electrode material.

Preparation of a direct methanol fuel cell tubular cathode support from mesocarbon microbeads and graphite

A tubular cathode support green body for tubular direct methanol fuel cell was prepared by a gel casting molding method using a mixture of mesocarbon microbeads and natural graphite powder with a mass ratio of 3:2 as carbon precursor. It was sintered at 1000 °C for 5 h in a graphite boat covered by graphite powder. Subsequently, a tubular cathode and a planar anode were obtained by coating the supports with slurry containing a catalyst and a Nafion membrane in a PTFE suspension. The catalysts for the cathode and anode are Pt/C and Pt–Ru/C, respectively. Results show that the surface of the tubular cathode support is smooth with no distortion. Both the gas diffusion layer and catalyst layer of the cathode show a similar porous structure with most of the holes in the sub-micron scale, which is favorable for decreasing mass transfer resistance and improving catalytic efficiency. A single cell performance test indicates that the sintered cathode bodies from the mixture of mesocarbon microbeads and graphite can be used as a special-shaped cathode support. [New Carbon Materials 2012, 27(6): 433–439]

Electrochemical performance of MCMB/(AC+LiFePO4) lithium-ion capacitors

Lithium-ion capacitors (LICs) were fabricated using mesocarbon microbeads (MCMB) as a negative electrode and a mixture of activated carbon (AC) and LiFePO4 as a positive electrode (abbreviated as LAC). The phase structure and morphology of LAC samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The electrochemical performance of the LICs was studied using cyclic voltammetry, charge-discharge rate measurements, and cycle performance testing. A LIC with 30 wt% LiFePO4 was found to have the best electrochemical performance with a specific energy density of 69.02 W h kg−1 remaining at 4 C rate after 100 cycles. Compared with an AC-only positive electrode system, the ratio of practical capacity to theoretical calculated capacity of the LICs was enhanced from 42.22% to 56.59%. It was proved that adding LiFePO4 to AC electrodes not only increased the capacity of the positive electrode, but also improved the electrochemical performances of the whole LICs via Li+ pre-doping.

Study on the Fabrication of Carbon Nanotube - Mesocarbon Microbead Composites

Carbon nanotube-Mesocarbon microbead composites were synthesized from coal tar pitch with carbon nanotubes (CNTs) as additive. The effect of CNTs addition and process parameters on the growth and morphologies of Mesocarbon microbeads (MCMBs) was investigated. The results show that adding CNTs enhances the nucleation and inhibits the growth and coalescence of MCMBs. Under the same thermal condensation conditions, the MCMBs made in the presence of CNTs tend to have smaller size, lower yield and more uniform size distribution, but more CNTs can lead to poor spherical degrees. Compared with the raw CNTs, the CNTs treated with blended acid can achieve better sphere and more uniform MCMBs with increasing CNTs ratio.

The lithium intercalation properties of SnSb/MCMB core-shell composite as the anode material for lithium ion battery

SnSb/MCMB composite material was prepared by multi-step synthesis methods. Mesocarbon Microbeads (MCMB) powders were modified by acid treatment firstly, and then SnSb nano particles were deposited on the surface of MCMB through chemical reduction method forming a core-shell composite structure. To characterize the phase and morphology of the composites material, X-ray diffraction (XRD), scanning electron microscope (SEM) were used. The constant current charge and discharge (CD) and cyclic volt ampere (CV) methods were also used to test the electrochemical performance of SnSb/MCMB. The results demonstrated that SnSb/MCMB presents a multiphase system of nanocrystalline and amorphous structure. The capacity attenuation of SnSb alloy is faster than that of SnSb/MCMB. For the SnSb/MCMB composite, the tiny alloy particles were dispersed on the surface of MCMB powders, preventing from the serious agglomeration of nano particles. At the same time, the inner core MCMB can also buffering the volume effect of the alloy compoites to improve the elecrtochemical cycling stability. The composite material was a first discharge specific capacity of 936.161 mAh/g and the first Coulomb efficiency 80.3%. The specific capacity was still up to 498.221 mAh/g after 50 cycles.

Changing of SEI Film and Electrochemical Properties about MCMB Electrodes during Long-Term Charge/Discharge Cycles

The reasons for the capacity loss of mesocarbon microbead (MCMB) in lithium ion cells during long-term charge-discharge cycles were evaluated in this paper. The MCMB electrodes were charged and discharged for 2000, 2500 and 3000 cycles after activation. Solid electrolyte interphase (SEI) film was deposited on the surface of MCMB particles after activation. The contents of O and F in SEI film tended to increase continuously. Li2O was generated firstly, then LiF was produced later and the content of LiF increased with the cycles proceeding. During the cycles, the voltage differences between charge and discharge platforms increased gradually and the specific capacity decreased. The cell resistances increased continuously and the peak currents of intercalation/de-intercalation reactions in cyclic voltammetry curves were reduced. The decline of electrochemical properties was related to both the growth of SEI films and the consumption of electrolyte salt LiPF6.

Optimization of a Single Lithium-Ion Battery Cell with a Gradient-Based Algorithm

This paper presents a numerical framework for automating the design of lithium-ion cells to maximize cell energy density while meeting specific power density requirements. The various processes in a battery cell are simulated using a physics-based electrochemistry model. The design is automatically performed by coupling the battery model with a gradient-based optimization algorithm. We demonstrate the potential for gradient-based optimization by applying this framework to optimize the design of a lithium-ion cell with spinel manganese dioxide cathode and meso-carbon micro beads (MCMB) anode for a range of power requirements. Results indicate that variations in the electrode thickness and porosity at optimal cell designs can be quantified via active mass ratios and it is found that the active mass ratios for optimal cell designs are independent of discharge rate.