Eunsu Paek

Long-life Li/polysulphide batteries with high sulphur loading enabled by lightweight three-dimensional nitrogen/sulphur-codoped graphene sponge

Lithium–sulphur batteries with a high theoretical energy density are regarded as promising energy storage devices for electric vehicles and large-scale electricity storage. However, the low active material utilization, low sulphur loading and poor cycling stability restrict their practical applications. Herein, we present an effective strategy to obtain Li/polysulphide batteries with high-energy density and long-cyclic life using three-dimensional nitrogen/sulphur codoped graphene sponge electrodes. The nitrogen/sulphur codoped graphene sponge electrode provides enough space for a high sulphur loading, facilitates fast charge transfer and better immobilization of polysulphide ions. The hetero-doped nitrogen/sulphur sites are demonstrated to show strong binding energy and be capable of anchoring polysulphides based on first-principles calculations. As a result, a high specific capacity of 1,200 mAh g−1 at 0.2C rate, a high-rate capacity of 430 mAh g−1 at 2C rate and excellent cycling stability for 500 cycles with ∼0.078% capacity decay per cycle are achieved.

On the influence of polarization effects in predicting the interfacial structure and capacitance of graphene-like electrodes in ionic liquids

The electric double layer (CD) and electrode quantum (CQ) capacitances of graphene-based supercapacitors are investigated using a combined molecular dynamics and density functional theory approach. In particular, we compare an approach that includes electronic polarization to one that is polarization-free by evaluating both CD and CQ using [EMIM][BF4] ionic liquid as a model electrolyte. Our results indicate that the inclusion of polarization effects can yield higher CD values-in this study by up to 40% around ±2 V-which we attribute primarily to the presence of charge smearing at the electrode-electrolyte interface. On the other hand, we find that the polarization-induced distortion of the electronic structure of graphene does not noticeably alter the predicted CQ. Our analysis suggests that an accurate description of the spatial charge distribution at the graphene interface due to polarization is necessary to improve our predictive capabilities, though more notably for CD. However, the conventional polarization-free approximation can serve as an efficient tool to study trends associated with both the CQ and CD at the interface of various graphene-like materials.

Large Capacitance Enhancement Induced by Metal-Doping in Graphene-Based Supercapacitors: A First-Principles-Based Assessment

Chemically doped graphene-based materials have recently been explored as a means to improve the performance of supercapacitors. In this work, we investigate the effects of 3d transition metals bound to vacancy sites in graphene with [BMIM][PF6] ionic liquid on the interfacial capacitance; these results are compared to the pristine graphene case with particular attention to the relative contributions of the quantum and electric double layer capacitances. Our study highlights that the presence of metal-vacancy complexes significantly increases the availability of electronic states near the charge neutrality point, thereby enhancing the quantum capacitance drastically. In addition, the use of metal-doped graphene electrodes is found to only marginally influence the microstructure and capacitance of the electric double layer. Our findings indicate that metal-doping of graphene-like electrodes can be a promising route toward increasing the interfacial capacitance of electrochemical double layer capacitors, primarily by enhancing the quantum capacitance.

Batch Fabrication of High‐Performance Planar Patch‐Clamp Devices in Quartz

The success of the patch-clamp technique has driven an effort to create wafer-based patch-clamp platforms. We develop a lithographic/electrochemical processing scheme that generates ultrasmooth, high aspect ratio pores in quartz. These devices achieve gigaohm seals in nearly 80% of trials, with the majority exhibiting seal resistances from 20-80 GΩ, competing with pipette-based patch-clamp measurements.

A computational analysis of graphene adhesion on amorphous silica

We present a computational analysis of the morphology and adhesion energy of graphene on the surface of amorphous silica (a-SiO2). The a-SiO2 model surfaces obtained from the continuous random network model-based Metropolis Monte Carlo approach show Gaussian-like height distributions with an average standard deviation of 2.91 ± 0.56 A, in good agreement with existing experimental measurements (1.68–3.7 A). Our calculations clearly demonstrate that the optimal adhesion between graphene and a-SiO2 occurs when the graphene sheet is slightly less corrugated than the underlying a-SiO2 surface. From morphology analysis based on fast Fourier transform, we find that graphene may not conform well to the relatively small jagged features of the a-SiO2 surface with wave lengths of smaller than 2 nm, although it generally exhibits high-fidelity conformation to a-SiO2 topographic features. For 18 independent samples, on average the van der Waals interaction at the graphene/a-SiO2 interface is predicted to vary from Evd...

Anomalous Stagewise Lithiation of Gold-Coated Silicon Nanowires: A Combined in Situ Characterization and First-Principles Study

Through a combined density functional theory and in situ scanning electron microscopy study, the effects of presence of gold (Au) spreading on the lithiation process of silicon nanowire (SiNW) were systematically examined. Different from a pristine SiNW, an Au-coated SiNW (Au-SiNW) is lithiated in three distinct stages; Li atoms are found to be incorporated preferentially in the Au shell, whereas the thin AuSi interface layer may serve as a facile diffusion path along the nanowire axial direction, followed by the prompt lithiation of the Si core in the radial direction. The underlying mechanism of the intriguing stagewise lithiation behavior is explained through our theoretical analysis, which appears well-aligned with the experimental evidence.

First-principles assessment of CO2 capture mechanisms in aqueous piperazine solution.

Piperazine (PZ) and its blends have emerged as attractive solvents for CO2 capture, but the underlying reaction mechanisms still remain uncertain. Our study particularly focuses on assessing the relative roles of PZCOO- and PZH+ produced from the PZ + CO2 reaction. PZCOO- is found to directly react with CO2 forming COO-PZCOO-, whereas PZH+ will not. However, COO-PZCOO- appears very unlikely to be produced in thermodynamic equilibrium with monocarbamates, suggesting that its existence would predominantly originate from the surface reaction that likely occurs. We also find production of H+PZCOO- to be more probable with increasing CO2 loading, due partly to the thermodynamic favorability of the PZH+ + PZCOO- → H+PZCOO- + PZ reaction; the facile PZ liberation may contribute to its relatively high CO2 absorption rate. This study highlights an accurate description of surface reaction and the solvent composition effect is critical in thermodynamic and kinetic models for predicting the CO2 capture processes.

Stochastic Plasma Charging of Nanopatterned Dielectric Surfaces

A 2-D simulation of the plasma bombardment of high aspect ratio dielectric structures is used to demonstrate stochastic charging behavior arising as absolute dimension decreases from 500 to 50 nm. Statistical analyses are then performed after the system has evolved beyond initial charging to indicate the regions of high variability in potential and to provide representative snapshots of mean and extreme potentials within the structure.