Jihui Gao

Nitrogen-rich carbon spheres made by a continuous spraying process for high-performance supercapacitors

Supercapacitors have high power densities, high efficiencies, and long cycling lifetimes; however, to enable their wider use, their energy densities must be significantly improved. The design and synthesis of improved carbon materials with better capacitance, rate performance, and cycling stability has emerged as the main theme of supercapacitor research. Herein, we report a facile synthetic method to prepare nitrogen-rich carbon particles based on a continuous aerosol-spraying process. The method yields particles that have high surface areas, a uniform microporous structure, and are highly N-doped, resulting in a synergism that enables the construction of supercapacitors with high energy and power density for use in both aqueous and commercial organic electrolytes. Furthermore, we have used density functional theory calculations to show that the improved performance is due to the enhanced wettability and ion adsorption interactions at the carbon/electrolyte interface that result from nitrogen doping. These findings provide new insights into the role of heteroatom doping in the capacitance enhancement of carbon materials; in addition, our method offers an efficient route for large-scale production of doped carbon.

A high performance lithium ion capacitor achieved by the integration of a Sn-C anode and a biomass-derived microporous activated carbon cathode

Hybridizing battery and capacitor materials to construct lithium ion capacitors (LICs) has been regarded as a promising avenue to bridge the gap between high-energy lithium ion batteries and high-power supercapacitors. One of the key difficulties in developing advanced LICs is the imbalance in the power capability and charge storage capacity between anode and cathode. Herein, we design a new LIC system by integrating a rationally designed Sn-C anode with a biomass-derived activated carbon cathode. The Sn-C nanocomposite obtained by a facile confined growth strategy possesses multiple structural merits including well-confined Sn nanoparticles, homogeneous distribution and interconnected carbon framework with ultra-high N doping level, synergically enabling the fabricated anode with high Li storage capacity and excellent rate capability. A new type of biomass-derived activated carbon featuring both high surface area and high carbon purity is also prepared to achieve high capacity for cathode. The assembled LIC (Sn-C//PAC) device delivers high energy densities of 195.7 Wh kg−1 and 84.6 Wh kg−1 at power densities of 731.25 W kg−1 and 24375 W kg−1, respectively. This work offers a new strategy for designing high-performance hybrid system by tailoring the nanostructures of Li insertion anode and ion adsorption cathode.

Interactions between mercury and dry FGD ash in simulated post combustion conditions

Two different flue gas desulfurization (FGD) ash samples were exposed to a simulated flue gas stream containing elemental mercury vapor to evaluate the interactions and determine the effects of gas components, dry FGD ash samples, and temperature on adsorption and heterogeneous oxidation of mercury. Both samples were characterized for surface area, unburned carbon content, element content, and mineralogical composition. Mercury speciation downstream from the sample was determined using Ontario Hydro Method. Results showed that higher levels of mercury oxidation were associated with higher levels of mercury capture. The NO(2), HCl, and Cl(2) promoted mercury oxidation, while SO(2) and NO had inhibitory effects on mercury oxidation. Unburned carbon of dry FGD ash sample played an important role in mercury capture. Whether the surface area was caused by unburned carbon or by calcium-based sorbents might be more significant than the level of surface area. Extent of mercury oxidation and capture increased slightly and then decreased as the temperature rising due to the interaction of mass transfer and reaction rates control.

Influence of a reagents addition strategy on the Fenton oxidation of rhodamine B: control of the competitive reaction of ·OH

The Fenton system (Fe2+/H2O2) generates ·OH with a high oxidation potential. However, as reactants themselves, H2O2 and Fe2+ can act as ·OH initiators as well as ·OH scavengers, leading to the need for a high dosage of reactants and increased costs. As a mixing-sensitive reaction, the ·OH-related reaction kinetics (·OH with Fe2+, H2O2, and RhB) was determined from the reaction rates (which were a constant in this work) and stoichiometry, in which the latter could be regulated by an addition strategy of Fenton reagents. This suggests that ·OH competitive reactions could be controlled by applying a macrolevel addition strategy. Herein, the effects of different addition approaches of Fe2+ and H2O2 on ·OH competitive reactions were quantitatively and systematically studied by analyzing the removal of the model pollutant RhB. We found that without stirring, and compared with a one-time addition, once H2O2 or Fe2+ was added in a step-wise pattern (e.g., one drop by one drop, 2 times, or 4 times), a high concentration of H2O2 or Fe2+ existed in a localized place for a longer period, resulting in a lower proportion of ·OH reacting with RhB, which we ascribed to an enhanced reaction between Fe2+, H2O2, and ·OH. However, when H2O2 and Fe2+ were added from two close points without stirring, a larger proportion of ·OH was scavenged by H2O2 and Fe2+; while under stirring, even a one-time addition of H2O2 or Fe2+ could cause severe scavenging of ·OH. The results also revealed a linear relationship between the RhB removal percentage and wavelength blue-shifts. This study showed that microlevel ·OH competitive reactions could be controlled by applying a macrolevel addition strategy of Fenton reagents without the addition of external chemicals. The results suggest this methodology can also offer an approach to lower ·OH invalid consumption by regulating the addition strategy in bigger reactors.

Highlighting the role of nitrogen doping in enhancing CO2 uptake onto carbon surfaces: a combined experimental and computational analysis

N-doped carbons with a gradient N content and consistent pore structure were prepared to independently determine the N doping effect on CO2 adsorption. Density functional theory calculations combined with noncovalent interaction analysis further highlight the importance of dispersion and electrostatic interactions for explaining the CO2 adsorption mechanism on N-doped carbon surfaces.

The role of quinone cycle in Fe2+–H2O2 system in the regeneration of Fe2+

ABSTRACT The reaction between Fe2+ and H2O2 generates highly reactive ·OH. However, the weak conversion from Fe3+ to Fe2+ limits its continuous reaction. Here, the difference between the Fenton system and modified Fenton system for the regeneration of Fe2+ was analyzed. A UV-vis spectrometer and redox potential measurements were used to detect Fe2+ concentration. Results indicated that Fe2+ could be better regenerated in the modified Fenton system. The regeneration of Fe2+ was facilitated by the consumption of NH2OH, while in hydroquinone (HQ)- and 1,4-bezoquinone (1,4-BQ)-modified Fenton systems, the quinone cycle could be built up and Fe3+ could be converted to Fe2+ continuously. However, results showed that HQ and 1,4-BQ reacted with ·OH, which caused a gradual decline in the enhancement effect. In order to keep Fe2+ concentration stable for a longer time, the influence of [HQ/1,4-BQ]0/[Fe2+]0 on Fe2+ concentration was carefully studied. When the mole ratio was 5:1, Fe2+ concentration remained nearly 90% of total iron at 40 min. But when the mole ratios were 0.5:1 and 0.1:1, Fe2+ concentration decreased to a very low level at 20 min. Oxidation–reduction potential (ORP) results further confirmed the role of quinone cycle. GRAPHICAL ABSTRACT

Effect of char structures caused by varying the amount of FeCl3 on the pore development during activation

A series of Fe-loaded coal chars is obtained by varying the load amount of FeCl3 additives (0, 6, 15 and 30 wt%). The effect of FeCl3 on the char structure during pyrolysis is presented; furthermore, the chars with different typical structures are activated under different burn-offs to study the pore development during activation. The results show that the mesopore size distribution is related to the amount of FeCl3. A small amount of FeCl3 additive (6 and 15 wt%) can promote depolymerization of the aromatic structure and formation of well-developed spatial crosslinks, and a greater amount of FeCl3 additive (30 wt%) can accelerate the graphitization tendency. The pore formation of 0Fe-900 without initial mesopores follows a hierarchical development from the surface to the core. The chars 15Fe-900H, 15Fe-900H and 30Fe-900H with some initial mesopores follows a non-hierarchical development from the external and internal particles simultaneously; however, the difference in microstructure can affect the penetration of the activated agent into the particle, resulting in different burn-offs and external carbon losses when the hierarchical porous structure has been produced.

Enhancement mechanism of SO2 removal with calcium hydroxide in the presence of NO2

The enhancement mechanism of SO2 removal by the presence of NO2 under low temperature and humid conditions was studied in a fixed bed reactor system. The presence of NO2 in the flue gas can enhance SO2 removal. The interaction between SO2 and NO2 in gas phase could not explain the effect of NO2 on SO2 removal under low-temperature and humid conditions. When Ca(NO3)2 and Ca(NO2)2 as additive were added on the surface of sorbent, the desulfurization activity of sorbent decreased. However, the sorbent pretreated by NO2 for a moment has higher SO2 removal. The oxidization of SO32− to SO42− and the evolution of sorbent surface structure in the presence of NO2 can explain the enhancement of SO2 removal by the presence of NO2. HSO3− and SO3− reacted with NO2 to form sulfate, which can accelerate the hydrolysis of SO2. The reaction between NO2 and Ca(OH)2 can make the unreacted sorbet under the SO2 removal product exposed to the reactant gas.

In Situ High-Level Nitrogen Doping into Carbon Nanospheres and Boosting of Capacitive Charge Storage in Both Anode and Cathode for a High-Energy 4.5 V Full-Carbon Lithium-Ion Capacitor

To circumvent the imbalances of electrochemical kinetics and capacity between Li+ storage anodes and capacitive cathodes for lithium-ion capacitors (LICs), we herein demonstrate an efficient solution by boosting the capacitive charge-storage contributions of carbon electrodes to construct a high-performance LIC. Such a strategy is achieved by the in situ and high-level doping of nitrogen atoms into carbon nanospheres (ANCS), which increases the carbon defects and active sites, inducing more rapidly capacitive charge-storage contributions for both Li+ storage anodes and PF6- storage cathodes. High-level nitrogen-doping-induced capacitive enhancement is successfully evidenced by the construction of a symmetric supercapacitor using commercial organic electrolytes. Coupling a pre-lithiated ANCS anode with a fresh ANCS cathode enables a full-carbon LIC with a high operating voltage of 4.5 V and high energy and power densities thereof. The assembled LIC device delivers high energy densities of 206.7 and 115.4 Wh kg-1 at power densities of 0.225 and 22.5 kW kg-1, respectively, as well as an unprecedented high-power cycling stability with only 0.0013% capacitance decay per cycle within 10 000 cycles at a high power output of 9 kW kg-1.

A general and scalable approach to produce nanoporous alloy nanowires with rugged ligaments for enhanced electrocatalysis

Nanoporous metal nanowires with large surface areas, and a high density of defect sites play an important role in catalysis. Here, a general and scalable one-step dealloying strategy is developed to prepare nanoporous alloy nanowires with controllable compositions by manipulating the grain size, structure and composition of bulk Cu-based precursor alloys. We prepared PtCuAu nanoporous nanowires with a diameter of 200–500 nm and tunable composition by dealloying a diluted Pt1Au0.5Cu98.5 single-phase alloy with nanoscale column-like-structured grains. Material characterization suggests that the formation of separated nanowires is due to the large-scale shrinkage of the column-structured grains during dealloying of Cu, which also generates ultrafine nanopores and rugged alloy ligaments with a high density of defect sites in the nanowires. When used as a cathodic catalyst for the oxygen reduction reaction (ORR), the PtCuAu nanoporous nanowires exhibit a composition-dependent catalytic performance. The 8.0 M HNO3 dealloyed sample exhibits a specific activity of 4.12 mA cm−2 at 0.9 V, which is more than 14 times that of commercial Pt/C. With the advantages of being easy to scale up, highly reproducible and controllable, the fabrication strategy holds great promise to prepare nanocatalysts for fuel cells.