Postdoctoral Associate, Virginia Tech/Chemistry
Controlling the Pore Size of Mesoporous Carbon Thin Films through Thermal and Solvent Annealing
Mesoporous carbon thin films have broad applications in filters, catalyst supports, electronics, gas separators and adsorbents, and energy conversion and storage devices. One of the most important features is the tunable pore size that determine the performance of mesoporous carbon. As a result, a technique that enables precise control of the pore size of mesoporous carbon thin films is highly desirable.
Towards this goal, Dr. Zhengping Zhou and Prof. Guoliang Liu have used thermal and solvent annealing to control the pore size of mesoporous carbon thin films from poly(acrylonitrile-block-methyl methacrylate) (PAN-b-PMMA) block copolymers. They have synthesized PAN-b-PMMA using a metal-free polymerization method and spin-coated the block copolymer into thin films. The block copolymer thin films can self-assemble into various morphologies via thermal or solvent annealing. After pyrolysis, the block copolymer thin films form mesoporous carbon thin films with tunable pore sizes depending on the thermal and solvent annealing conditions. In this rationally designed block copolymer, PAN serves as a carbon precursor and can be directly carbonized into mesoporous carbon, PMMA behaves as a sacrificial phase and can be removed easily to create pores. This work demonstrates that through thermal annealing the temperature can easily tune the pore size and center-to-center spacing of mesoporous carbon thin films. In addition, the choice of solvent in solvent annealing strongly influenced PAN-b-PMMA nanostructures and the pore size of mesoporous carbon thin films. The work provides two simple strategies to control the pore size of mesoporous carbon thin films instead of synthesizing a series of block copolymers of various molecular weights and compositions.
Abstract: Porous carbon materials with high surface areas were prepared using a hypercrosslinked porous organic polymer as porous precursor with potassium hydroxide activation. Nitrogen adsorption analysis revealed that the porous carbon materials have high surface area of up to 3101 m2 g-1 and high pore volume of 1.84 cm3 g-1, which can provide more accessible surface and sites for gas adsorption and electrochemical energy storage. The porous carbons with high surface area show high H2 uptake ability of up to 3.25 wt% at 77 K and 1.13 bar and CO2 uptake ability of 6.69 mmol g-1 at 273 K and 1.13 bar. The lithium ion battery fabricated from CHCPB-K-600 shows a high specific discharge capacity of 1221 mA h g−1 at 100 mA g-1, which could still retain 833 mA h g−1 after 50 cycles at 100 mA g-1. CHCPB-K-600 also exhibits a high capacitance of 379 F g-1 at 0.5 A g-1 for supercapacitor, and the capacitance retains 91.2% after 3000 cycles at 2 A g-1. Considering the high gas uptake ability, the excellent electrochemical performance, and the facile preparation strategy, these porous carbons hold a great potential for gas adsorption and energy storage.
Pub.: 26 Dec '16, Pinned: 30 Jun '17
Abstract: A type of unusual interconnected graphitized carbon nanosheets (GCNS) was fabricated from biomass waste, i.e., inner shaddock skins using a facile combined method of simultaneous carbonization-activation and post-vacuum-annealing processes. The obtained GCNS has an optimized integration of a cage-like high-aspect-ratio nanosheet structure (∼8 nm thickness), a large surface area of 2327 m2 g−1 and hierarchical meso/micropore systems (82.3% mesopore volume). Given the effect of post-vacuum-annealing process, enhanced graphitization degree and excellent electronic conductivity (7.9 S cm−1) were obtained for the GCNS. The enhanced graphitization degree not only effectively improves electronic/ionic-transport kinetics in favor of rate capability but also prevents electrolyte degradation thus benefitting cyclic life. The ionic liquid-based supercapacitors assembled with symmetric GCNS electrodes exhibited an ultrahigh rate capability of 87% at current density of 100 A g−1 (holding 132 F g−1) and a long cyclic life of 97.6% capacitance retention after 10,000 cycles. The excellent rate capability resulted in an integrated high energy-power property at an energy density of 56 Wh kg−1 (29.2 Wh L−1), corresponding to a power density of 93 kW kg−1 (48.4 kW L−1).
Pub.: 23 Apr '17, Pinned: 30 Jun '17
Abstract: We present a new protocol to grow large-area, high-quality single-layer graphene on Cu foils at relatively low temperatures. We use C60 molecules evaporated in ultra high vacuum conditions as carbon source. This clean environment results in a strong reduction of oxygen-containing groups as depicted by X-ray photoelectron spectroscopy (XPS). Unzipping of C60 is thermally promoted by annealing the substrate at 800ºC during evaporation. The graphene layer extends over areas larger than the Cu crystallite size, although it is changing its orientation with respect to the surface in the wrinkles and grain boundaries, producing a modulated ring in the low energy electron diffraction (LEED) pattern. This protocol is a self-limiting process leading exclusively to one single graphene layer. Raman spectroscopy confirms the high quality of the grown graphene. This layer exhibits an unperturbed Dirac-cone with a clear n-doping of 0.77 eV, which is caused by the interaction between graphene and substrate. Density functional theory (DFT) calculations show that this interaction can be induced by a coupling between graphene and substrate at specific points of the structure leading to a local sp(3) configuration, which also contribute to the D-band in the Raman spectra.
Pub.: 17 May '17, Pinned: 30 Jun '17
Abstract: We present electrical current assisted graphitization as an alternative to conventional high temperature annealing of carbon nanofibers. In-situ experiments were performed on individual vapor grown carbon nanofibers inside a transmission electron microscope to measure the changes in resistance as a function of current density while observing the microstructural changes in real time. About 1000 times decrease in resistivity was measured at current density below 106 A/cm2. Further increase in current density leads to the uniform exfoliation of mostly bi-layer graphene flakes from the skin of the graphitic nanofibers, which leads to further reduction of the electrical resistance. The uniformity of the graphene flake growth over the nanofiber surface area achieved in this study is difficult to achieve with conventional approaches. Further experiments on networked nanofibers suggest that the graphene can remarkably reduce the electrical Kapitza resistance of the nanofiber junctions. The demonstrated processing of such hierarchical nanostructured fibers lead to high surface area and high conductivity carbon nanofibers that can impact flexible electrodes in electrochemical energy conversion or high specific strength composites applications.
Pub.: 27 Feb '17, Pinned: 30 Jun '17
Abstract: A nickel incorporated carbon nanotube/nanofiber composite (Ni-CNT-CNF) was used as a low cost alternative to Pt as counter electrode (CE) for dye-sensitized solar cells (DSCs). Measurements based on energy dispersive X-rays spectroscopy (EDX) showed that the majority of the composite CE was carbon at 88.49 wt%, while the amount of Ni nanoparticles was about 11.51 wt%. Measurements based on electrochemical impedance spectroscopy (EIS) showed that the charge transfer resistance (R(ct)) of the Ni-CNT-CNF composite electrode was 0.71 Ω cm(2), much lower than that of the Pt electrode (1.81 Ω cm(2)). Such a low value of R(ct) indicated that the Ni-CNT-CNF composite carried a higher catalytic activity than the traditional Pt CE. By mixing with CNTs and Ni nanoparticles, series resistance (R(s)) of the Ni-CNT-CNF electrode was measured as 5.96 Ω cm(2), which was close to the R(s) of 5.77 Ω cm(2) of the Pt electrode, despite the significant difference in their thicknesses: ∼22 μm for Ni-CNT-CNF composite, while ∼40 nm for Pt film. This indicated that use of a thick layer (tens of microns) of Ni-CNT-CNF counter electrode does not add a significant amount of resistance to the total series resistance (R(s-tot)) in DSCs. The DSCs based on the Ni-CNT-CNF composite CEs yielded an efficiency of 7.96% with a short circuit current density (J(sc)) of 15.83 mA cm(-2), open circuit voltage (V(oc)) of 0.80 V, and fill factor (FF) of 0.63, which was comparable to the device based on Pt, that exhibited an efficiency of 8.32% with J(sc) of 15.01 mA cm(-2), V(oc) of 0.83, and FF of 0.67.
Pub.: 08 Aug '12, Pinned: 30 Jun '17
Abstract: A novel porous three dimensional (3D) hierarchical graphene-beaded carbon nanofibers with incorporated Ni nanoparticles (G/CNFs–Ni) were used for the first time as cost-effective counter-electrode for dye-sensitized solar cells (DSCs). G/CNFs–Ni was synthesized by electrospinning G/PAN/Ni(AcAc)2 precursor nanofibers, followed by carbonization and activation. The introduction of graphene nanosheets and Ni nanoparticles in CNF networks significantly increased the cells' stability and decreased the charge-transfer resistance at the interface between electrolyte and counter-electrode, leading to the high electrocatalytic activity/efficiency for triiodide reduction. The G/CNFs–Ni composite counter-electrodes possessed larger capacitance than that of Pt counter-electrodes due to larger specific surface area, leading to significantly higher electrocatalytic activity/efficiency for triiodide reduction at the interface between electrolyte and counter-electrode. The dye-sensitized solar cells (DSCs) fabricated using G/CNFs–Ni composite as counter-electrodes were tested at 100 mW/cm2 AM 1.5 illumination. The G/CNFs–Ni composite exhibited an overall power conversion efficiency of 7.14% as compared to 7.59% for reference platinum (Pt) counter-electrodes.
Pub.: 02 Mar '16, Pinned: 29 Jun '17
Abstract: A hierarchical mesoporous carbon foam (ECF) with an interconnected micro-/mesoporous architecture was prepared and used as a binder-free, low-cost, high-performance anode for lithium ion batteries. Due to its high specific surface area (980.6 m(2)/g), high porosity (99.6%), light weight (5 mg/cm(3)) and narrow pore size distribution (~2 to 5 nm), the ECF anode exhibited a high reversible specific capacity of 455 mAh/g. Experimental results also demonstrated that the anode thickness significantly influence the specific capacity of the battery. Meanwhile, the ECF anode retained a high rate performance and an excellent cycling performance approaching 100% of its initial capacity over 300 cycles at 0.1 A/g. In addition, no binders, carbon additives or current collectors are added to the ECF based cells that will increase the total weight of devices. The high electrochemical performance was mainly attributed to the combined favorable hierarchical structures which can facilitate the Li(+) accessibility and also enable the fast diffusion of electron into the electrode during the charge and discharge process. The synthesis process used to make this elastic carbon foam is readily scalable to industrial applications in energy storage devices such as li-ion battery and supercapacitor.
Pub.: 05 May '17, Pinned: 29 Jun '17
Abstract: Herein an approach to controlling the pore size of mesoporous carbon thin films from metal-free polyacrylonitrile-containing block copolymers is described. A high-molecular-weight poly(acrylonitrile-block-methyl methacrylate) (PAN-b-PMMA) is synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The authors systematically investigate the self-assembly behavior of PAN-b-PMMA thin films during thermal and solvent annealing, as well as the pore size of mesoporous carbon thin films after pyrolysis. The as-spin-coated PAN-b-PMMA is microphase-separated into uniformly spaced globular nanostructures, and these globular nanostructures evolve into various morphologies after thermal or solvent annealing. Surprisingly, through thermal annealing and subsequent pyrolysis of PAN-b-PMMA into mesoporous carbon thin films, the pore size and center-to-center spacing increase significantly with thermal annealing temperature, different from most block copolymers. In addition, the choice of solvent in solvent annealing strongly influences the block copolymer nanostructure and the pore size of mesoporous carbon thin films. The discoveries herein provide a simple strategy to control the pore size of mesoporous carbon thin films by tuning thermal or solvent annealing conditions, instead of synthesizing a series of block copolymers of various molecular weights and compositions.
Pub.: 02 Feb '17, Pinned: 29 Jun '17
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