Kaixi Li

Disproportionation in Li–O2 Batteries Based on a Large Surface Area Carbon Cathode

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li-O2 cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li2O2 component with superoxide-like character to a Li2O2 component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2-3.5 V) than the Li2O2 component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li-O2 cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li-O2 cell.

Hierarchical porous carbon microtubes derived from willow catkins for supercapacitor applications

With willow catkins as highly accessible carbon sources, hierarchical porous carbon microtubes (denoted as HPNCTs) have been successfully prepared by a facile carbonization and subsequent KOH activation process. The resulting materials not only inherited the natural tubular morphology of willow catkins, but also developed a hierarchical porous structure by activation, with nitrogen from the biomass being self-doped in the resulting carbon. A maximum specific surface area of 1775.7 m2 g−1 with a pore volume of 0.8516 cm3 g−1 was achieved for HPNCT-800. When evaluated as an electrode by a three-electrode system in 6 M KOH aqueous solution, the material exhibited a high gravimetric capacitance of 292 F g−1 at a current density of 1 A g−1, with a good rate capability of 83.5% retention at 10 A g−1. HPNCT-800 was further employed in a coin-type symmetric device with 1 M LiPF6 electrolyte, and exhibited a high energy density of 37.9 W h kg−1 at a power density of 700 W kg−1, with excellent cycling stability with 90.6% retention after 4000 cycles. By taking advantage of the unique structure of abundant biomass from nature, this work sheds light on the creation of advanced porous carbon materials towards energy storage applications.

Nitrogen-Enriched Hierarchically Porous Carbons Prepared from Polybenzoxazine for High-Performance Supercapacitors

Nitrogen-enriched hierarchically porous carbons (HPCs) were synthesized from a novel nitrile-functionalized benzoxazine based on benzoxazine chemistry using a soft-templating method and a potassium hydroxide (KOH) chemical activation method and used as electrode materials for supercapacitors. The textural and chemical properties could be easily tuned by adding a soft template and changing the activation temperature. The introduction of the soft-templating agent (surfactant F127) resulted in the formation of mesopores, which facilitated fast ionic diffusion and reduced the internal resistance. The micropores of HPCs were extensively developed by KOH activation to provide large electrochemical double-layer capacitance. As the activation temperature increased from 600 to 800 °C, the specific surface area of nitrogen-enriched carbons increased dramatically, micropores were enlarged, and more meso/macropores were developed, but the nitrogen and oxygen content decreased, which affected the electrochemical performance. The sample HPC-800 activated at 800 °C possesses a high specific surface area (1555.4 m(2) g(-1)), high oxygen (10.61 wt %) and nitrogen (3.64 wt %) contents, a hierarchical pore structure, a high graphitization degree, and good electrical conductivity. It shows great pseudocapacitance and the largest specific capacitance of 641.6 F g(-1) at a current density of 1 A g(-1) in a 6 mol L(-1) KOH aqueous electrolyte when measured in a three-electrode system. Furthermore, the HPC-800 electrode exhibits excellent rate capability (443.0 F g(-1) remained at 40 A g(-1)) and good cycling stability (94.3% capacitance retention over 5000 cycles).

Catalytic removal of SO2 over ammonia-activated carbon fibers

Abstract Nitrogen-containing functional groups were introduced onto the surface of activated carbon fibers (ACF) by activating an ethylene tar pitch-based carbon fiber with ammonia water. The activity of the ACF for the conversion of SO2 to aq. H2SO4 in the presence of H2O and O2 is significantly higher than that of other commercial ACF studied before. Both the SO2 adsorption capacity and oxidation activity of ACF are enhanced very much by the nitrogen-containing functional groups.

Self‐Assembled 3D Graphene‐Based Aerogel with Co3O4 Nanoparticles as High‐Performance Asymmetric Supercapacitor Electrode

Using graphene oxide and a cobalt salt as precursor, a three-dimensional graphene aerogel with embedded Co3 O4 nanoparticles (3D Co3 O4 -RGO aerogel) is prepared by means of a solvothermal approach and subsequent freeze-drying and thermal reduction. The obtained 3D Co3 O4 -RGO aerogel has a high specific capacitance of 660 F g(-1) at 0.5 A g(-1) and a high rate capability of 65.1 % retention at 50 A g(-1) in a three-electrode system. Furthermore, the material is used as cathode to fabricate an asymmetric supercapacitor utilizing a hierarchical porous carbon (HPC) as anode and 6 M KOH aqueous solution as electrolyte. In a voltage range of 0.0 to 1.5 V, the device exhibits a high energy density of 40.65 Wh kg(-1) and a power density of 340 W kg(-1) and shows a high cycling stability (92.92 % capacitance retention after 2000 cycles). After charging for only 30 s, three CR2032 coin-type asymmetric supercapacitors in series can drive a light-emitting-diode (LED) bulb brightly for 30 min, which remains effective even after 1 h.

Nitrogen-enriched and hierarchically porous carbon macro-spheres – ideal for large-scale CO2 capture

A facile and efficient “spheridization” method is developed to produce nitrogen-enriched hierarchically porous carbon spheres of millimeters in diameter, with intricate micro-, meso- and macro-structural features. Such spheres not only show exceptional working capacity for CO2 sorption, but also satisfy practical requirements for dynamic flow in post-combustion CO2 capture. Those were achieved using co-polymerized acrylonitrile and acrylamide as the N-enriched carbon precursor, a solvent-exchange process to create hierarchically porous macro-sphere preforms, oxidization to induce cyclization of the polymer chains, and carbonization with concurrent chemical activation by KOH. The resulting carbon spheres show a relatively high CO2 uptake of 16.7 wt% under 1 bar of CO2 and, particularly, an exceptional uptake of 9.3 wt% under a CO2 partial pressure of 0.15 bar at 25 °C. Subsequent structural and chemical analyses suggest that the outstanding properties are due to highly developed microporous structures and the relatively high pyridinic nitrogen content inherited from the co-polymer precursor, incorporated within the hierarchical porous structures.

A large-scale synthesis of heteroatom (N and S) co-doped hierarchically porous carbon (HPC) derived from polyquaternium for superior oxygen reduction reactivity

A simple, large-scale and green synthetic route is demonstrated for the preparation of polyquaternium derived heteroatom (N and S) co-doped hierarchically porous carbon (HPC). Our protocol allows for the simultaneous optimization of both porous structures and surface functionalities of (N and S) co-doped carbon (N–S-HPC). As a result, the obtained N–S-HPC shows a superior catalytic ORR performance to the commercial Pt/C catalyst in alkaline media, including high catalytic activity, remarkable long-term stability and strong methanol tolerance. Even in acidic media where most non-precious metal catalysts suffer from high overpotential and low durability, our N–S-HPC exhibits an amazing ORR activity with a half-wave potential of 0.73 V, and 40% enhanced limited diffusion-current density when compared to the Pt/C catalyst. Particularly, when used for constructing a zinc–air battery cathode, such an N–S-HPC catalyst can give a discharge peak power density as high as 536 mW cm−2. At 1.0 V of cell voltage, a current density of 317 mA cm−2 is achieved. This performance is superior to all reported non-precious metal catalysts in the literature for zinc–air batteries and significantly outperforms the state-of-the-art platinum-based catalyst.

Improvements of the DA equation for application in hydrogen adsorption at supercritical conditions

Abstract The Dubinin and Astakhovs equation (DA equation) is a privileged model for describing the behaviors of gas adsorption in microporous adsorbents. However, the behaviors of hydrogen adsorption at supercritical conditions reported are quite different from the normal gas adsorption. For example, the saturation vapor pressure P S of hydrogen does not exit above its critical temperature T C (33.2 K), and the specific amount of saturated adsorbed molecules n 0 and the characteristic energy of adsorption system E vary with equilibrium temperature. Whether the DA equation is adequate for hydrogen adsorption at supercritical conditions or not is well worth discussing. The authors have measured systematically the adsorption isotherms of hydrogen over a wide range of temperatures and pressures. In present works, an interesting phenomenon was reported that hydrogen adsorption exists an utmost value at supercritical conditions. Based on the new adsorption phenomenon, the parameters of the DA equation are analyzed and further modified for application in hydrogen adsorption. The results show that the DA equation can be applied to describe the behaviors of hydrogen adsorption at supercritical conditions, if it is modified as the following, (I) n=f(T C ) RT ln (P S /P) g(T C ) 2 , in which, (II) ln P S = −134.5 T −4.3362, (III) f(T C )=α+βT C , (IV) g(T C )=λ+γT C .

Influence of CO-evolving groups on the activity of activated carbon fiber for SO2 removal

An investigation was made into the influence of CO-evolving and CO2-evolving groups on the activities of activated carbon fibers (ACFs) for the oxidative conversion of SO2 into aq. H2SO4 in the presence of O2 and H2O. The results indicated that the amount of evolved CO determined the SO2 removal activity of ACFs, whereas, the amount of evolved CO2 did not correlate with the ACFs activity for SO2 removal. A direct proportionality between the amount of evolved CO and the enhanced activity of SO2 removal was confirmed by using different oxidizing agents for changing the types and amount of oxygen functional groups in ACFs.

Polybenzoxazine-based nitrogen-containing porous carbons for high-performance supercapacitor electrodes and carbon dioxide capture

Nitrogen-containing porous carbons with a high surface area, well developed micro/mesoporous structure, high nitrogen and oxygen contents, and good conductivity were synthesized from a novel nitrile-functional polybenzoxazine through a soft-templating method and KOH chemical activation. The introduction of a soft-templating agent into the precursors and the activation temperature have a significant effect on the pore structure and the surface chemistry. The existence of mesopores enhances the accessibility to the surface of the carbon framework and adequate utilization of the active adsorption sites. The material NPC-2 activated at 700 °C shows the highest gravimetric capacitance of 362.4 F g−1 at a current density of 1 A g−1 in KOH aqueous electrolyte, high rate capability and excellent cycling stability with a capacitance retention of 94.7% after 5000 cycles. As a solid adsorbent, the NPC-1 activated at 600 °C shows the highest CO2 uptakes at 1 bar of 6.20 and 3.95 mmol g−1 at 0 and 25 °C, respectively. It exhibits a remarkable selectivity for CO2/N2 separation and excellent recyclability. Such high performance as supercapacitor electrode materials and CO2 adsorbents can be attributed to the high surface area, optimal pore size distribution, superior electrical conductivity and the surface nitrogen functionalities in the carbon matrix.