Huachao Yang

Green preparation of reduced graphene oxide for sensing and energy storage applications

Preparation of graphene from chemical reduction of graphene oxide (GO) is recognized as one of the most promising methods for large-scale and low-cost production of graphene-based materials. This study reports a new, green, and efficient reducing agent (caffeic acid/CA) for GO reduction. The CA-reduced GO (CA-rGO) shows a high C/O ratio (7.15) that is among the best rGOs prepared with green reducing reagents. Electronic gas sensors and supercapacitors have been fabricated with the CA-rGO and show good performance, which demonstrates the potential of CA-rGO for sensing and energy storage applications.

Lithium insertion into CuO/carbon nanotubes

Abstract Carbon nanotubes are coated with a layer of copper by an electroless plating method. To prepare CuO/carbon nanotubes, Cu/carbon nanotubes are oxidized by heating to 160°C in air for 12 h. The lithium insertion properties of carbon and CuO/carbon nanotubes are tested by an electrochemical method. Carbon nanotubes can reversibly store 700 mA h Li g −1 carbon, while CuO in CuO/carbon nanotubes can reversibly store 268 mA h Li g −1 CuO. Li can insert into the CuO lattice at 1.7 to ∼1.0 V and be released at 2.3 to ∼2.5 V vs. Li according to CuO+ x e+ x Li↔CuOLi x .

Molecular Insights into Aqueous NaCl Electrolytes Confined within Vertically-oriented Graphenes.

Vertically-oriented graphenes (VGs) are promising active materials for electric double layer capacitors (EDLCs) due to their unique morphological and structural features. This study, for the first time, reports the molecular dynamics (MD) simulations on aqueous NaCl electrolytes confined within VG channels with different surface charge densities and channel widths. Simulation results show that the accessibility of ions and the structure of EDLCs are determined by the ion type/size, surface charging, and VG channel width. For relatively narrow VG channels with the same width, the threshold charge density (to compensate the energy penalty for shedding hydration shell) and the dehydration rate of Cl− ions are larger than those of Na+ ions. To achieve the highest ion concentration coefficient, the effective VG channel width should be between the crystal and hydration diameters of the ions. The results are further quantified and elucidated by calculating the electrolyte density profiles. The molecular insights obtained in the current work are useful in guiding the design and fabrication of VGs for advancing their EDLC applications.

Molecular Origin of Electric Double-Layer Capacitance at Multilayer Graphene Edges

Multilayer graphenes have been widely used as active materials for electric double-layer capacitors (EDLCs), where their numerous edges are demonstrated to play a crucial role in charge storage. In this work, the interfacial structure and capacitive behaviors of multilayer graphene edges with representative interlayer spacing are studied via molecular dynamics (MD) simulations. Compared with planar graphite surfaces, edges can achieve a 2-fold increase in the specific capacitance at a wider interlayer spacing of ∼5.0 Å. Unusually, the molecular origins for achieved charge storage are predominantly attributed to the structural evolutions of solvents occurring in the double layer, going beyond the traditional views of regulating the capacitance by ion adsorption/separation. Specifically, diverse ionic distributions exhibit similar screening ability and EDLC thickness, while water molecules can counterbalance the interfacial electric fields more effectively at edge site. The as-obtained findings will be instructive in designing graphene-based EDLCs for advanced capacitive performances.

Kinetic-Dominated Charging Mechanism within Representative Aqueous Electrolyte-based Electric Double-Layer Capacitors

The chemical nature of electrolytes has been demonstrated to play a pivotal role in the charge storage of electric double-layer capacitors (EDLCs), whereas primary mechanisms are still partially resolved but controversial. In this work, a systematic exploration into EDL structures and kinetics of representative aqueous electrolytes is performed with numerical simulation and experimental research. Unusually, a novel charging mechanism exclusively predominated by kinetics is recognized, going beyond traditional views of manipulating capacitances preferentially via interfacial structural variations. Specifically, strikingly distinctive EDL structures stimulated by diverse ion sizes, valences, and mixtures manifest a virtually identical EDL capacitance, where the dielectric nature of solvents attenuates ionic effects on electrolyte redistributions, in stark contradiction with solvent-free counterpart and traditional Helmholtz theory. Meanwhile, corresponding kinetics evolve conspicuously with ionic species, intimately correlated with ion-solvent interactions. The achieved mechanisms are subsequently illuminated by electrochemical measurements, highlighting the crucial interplay between ions and solvents in regulating EDLC performances.

Design of Supercapacitor Electrodes Using Molecular Dynamics Simulations

Electric double-layer capacitors (EDLCs) are advanced electrochemical devices for energy storage and have attracted strong interest due to their outstanding properties. Rational optimization of electrode–electrolyte interactions is of vital importance to enhance device performance for practical applications. Molecular dynamics (MD) simulations could provide theoretical guidelines for the optimal design of electrodes and the improvement of capacitive performances, e.g., energy density and power density. Here we discuss recent MD simulation studies on energy storage performance of electrode materials containing porous to nanostructures. The energy storage properties are related to the electrode structures, including electrode geometry and electrode modifications. Altering electrode geometry, i.e., pore size and surface topography, can influence EDL capacitance. We critically examine different types of electrode modifications, such as altering the arrangement of carbon atoms, doping heteroatoms and defects, which can change the quantum capacitance. The enhancement of power density can be achieved by the intensified ion dynamics and shortened ion pathway. Rational control of the electrode morphology helps improve the ion dynamics by decreasing the ion diffusion pathway. Tuning the surface properties (e.g., the affinity between the electrode and the ions) can affect the ion-packing phenomena. Our critical analysis helps enhance the energy and power densities of EDLCs by modulating the corresponding electrode structures and surface properties.

Reliability of Constant Charge Method for Molecular Dynamics Simulations on EDLCs in Nanometer and Sub‐Nanometer Spaces

The constant charge method (CCM) and constant potential method (CPM) are two major techniques to apply electric charges on electrodes for molecular dynamics (MD) simulations on electric double-layer capacitors (EDLCs). Compared with CCM, CPM is more realistic since the influence of electrode polarization is taken into account, although computationally more expensive. In this work, the reliability of CCM for MD simulations on EDLCs in nanometer and sub-nanometer spaces is investigated. Particular attention is paid to the comparison of CCM and CPM for modeling EDLCs in terms of ion concentration and EDL structures in such nano/sub-nano confined spaces, which are widely found in graphene-based materials. The deviation for ion concentration calculated from CCM and CPM was found to be significant in both neutral and charged confined spaces (e. g., deviation ratio of 40.27 % and 11.18 % for Cl ions in neutral and charged 9 Å space, respectively), which is different from previous observations in bulk solutions. This result could be attributed to the strong electrode polarization in nano/sub-nano confined spaces. Although CCM and CPM yielded essentially similar EDL structures, the time evolution of electrode charge with a potential difference could be conducted only with CPM. The findings of the current work could provide useful insights for choosing appropriate methods for MD simulation on EDLCs in nano/sub-nano confined spaces.

DC and Microwave Plasmas for Synthesis of Vertically Oriented Graphene

Plasma-enhanced chemical vapor deposition of vertically oriented graphene (VG) employing direct current glow discharge and microwave plasmas are presented and compared. The VG nanosheets with various morphologies can be obtained for diverse applications.