Keunsik Lee

Vertical alignments of graphene sheets spatially and densely piled for fast ion diffusion in compact supercapacitors.

Supercapacitors with porous carbon structures have high energy storage capacity. However, the porous nature of the carbon electrode, composed mainly of carbon nanotubes (CNTs) and graphene oxide (GO) derivatives, negatively impacts the volumetric electrochemical characteristics of the supercapacitors because of poor packing density (<0.5 g cm(-3)). Herein, we report a simple method to fabricate highly dense and vertically aligned reduced graphene oxide (VArGO) electrodes involving simple hand-rolling and cutting processes. Because of their vertically aligned and opened-edge graphene structure, VArGO electrodes displayed high packing density and highly efficient volumetric and areal electrochemical characteristics, very fast electrolyte ion diffusion with rectangular CV curves even at a high scan rate (20 V/s), and the highest volumetric capacitance among known rGO electrodes. Surprisingly, even when the film thickness of the VArGO electrode was increased, its volumetric and areal capacitances were maintained.

Anti‐Solvent Derived Non‐Stacked Reduced Graphene Oxide for High Performance Supercapacitors

An anti-solvent for graphene oxide (GO), hexane, is introduced to increase the surface area and the pore volume of the non-stacked GO/reduced GO 3D structure and allows the formation of a highly crumpled non-stacked GO powder, which clearly shows ideal supercapacitor behavior.

Highly Air‐Stable Phosphorus‐Doped n‐Type Graphene Field‐Effect Transistors

Phosphorus-doped double-layered graphene field-effect transistors (PDGFETs) show much stronger air-stable n-type behavior than nitrogen-doped double-layered graphene FETs (NDGFETs), even under an oxygen atmosphere, due to strong nucleophilicity, which may lead to real applications for air-stable n-type graphene channels.

Highly Bendable, Conductive, and Transparent Film by an Enhanced Adhesion of Silver Nanowires

Recently, silver nanowires (AgNWs) have attracted considerable interest for their potential application in flexible transparent conductive films (TCFs). One challenge for the commercialization of AgNW-based TCFs is the low conductivity and stability caused by the weak adhesion forces between the AgNWs and the substrate. Here, we report a highly bendable, conductive, and transparent AgNW film, which consists of an underlying poly(diallyldimethyl-ammonium chloride) (PDDA) and AgNW composite bottom layer and a top layer-by-layer (LbL) assembled graphene oxide (GO) and PDDA overcoating layer (OCL). We demonstrated that PDDA could increase the adhesion between the AgNW and the substrate to form a uniform AgNW network and could also serve to improve the stability of the GO OCL. Hence, a highly bendable, conductive, and transparent AgNW-PDDA-GO composite TCF on a poly(ethylene terephthalate) (PET) substrate with Rs ≈ 10 Ω/sq and T ≈ 91% could be made by an all-solution processable method at room temperature. In addition, our AgNW-PDDA-GO composite TCF is stable without degradation after exposure to H2S gas or sonication.

Can Commonly Used Hydrazine Produce n‐Type Graphene?

A simple chemical method to obtain bulk quantities of N-doped, reduced graphene oxide (rGO) sheets (see figure) as an n-type semiconductor through the treatment of as-prepared GO sheets with the commonly used reducing reagent hydrazine, followed by rapid thermal annealing (RTA) is described.

Highly Stretchable and Conductive Silver Nanoparticle Embedded Graphene Flake Electrode Prepared by In situ Dual Reduction Reaction

The emergence of stretchable devices that combine with conductive properties offers new exciting opportunities for wearable applications. Here, a novel, convenient and inexpensive solution process was demonstrated to prepare in situ silver (Ag) or platinum (Pt) nanoparticles (NPs)-embedded rGO hybrid materials using formic acid duality in the presence of AgNO3 or H2PtCl6 at low temperature. The reduction duality of the formic acid can convert graphene oxide (GO) to rGO and simultaneously deposit the positively charged metal ion to metal NP on rGO while the formic acid itself is converted to a CO2 evolving gas that is eco-friendly. The AgNP-embedded rGO hybrid electrode on an elastomeric substrate exhibited superior stretchable properties including a maximum conductivity of 3012 S cm-1 (at 0 % strain) and 322.8 S cm-1 (at 35 % strain). Its fabrication process using a printing method is scalable. Surprisingly, the electrode can survive even in continuous stretching cycles.

Hybrid windshield-glass heater for commercial vehicles fabricated via enhanced electrostatic interactions among a substrate, silver nanowires, and an over-coating layer

We introduce a transparent windshield-glass heater produced via transparent electrodes using silver nanowire (AgNW) networks for conventional use in the automobile industry. A high-quality conducting hybrid film is deposited on a plasma-treated glass substrate by spraying AgNWs, immersing the sprayed product in positively charged adhesive polymer solution, and then spraying negatively charged graphene oxide (GO) and a silane layer as an over-coating layer (OCL).The results of heating tests conducted after adhesion tests show that the sheet resistance changes with the application of polymer glue. Surprisingly, the transmittance of the film with the GO OCL is higher than that of the film without the GO OCL. Heating and defrosting tests are carefully conducted via infrared (IR) monitoring. Adhesive-polymer-treated and GO-protected AgNW transparent glass heaters exhibit the best performance with low sheet resistance; thus, through strong electrostatic interaction among the substrate, adhesive layer, and OCL, our AgNW hybrid glass heater can reach the target temperature with a standard vehicle voltage of 12 V in a short period of time.

Tunable Sub-nanopores of Graphene Flake Interlayers with Conductive Molecular Linkers for Supercapacitors

Although there are numerous reports of high performance supercapacitors with porous graphene, there are few reports to control the interlayer gap between graphene sheets with conductive molecular linkers (or molecular pillars) through a π-conjugated chemical carbon-carbon bond that can maintain high conductivity, which can explain the enhanced capacitive effect of supercapacitor mechanism about accessibility of electrolyte ions. For this, we designed molecularly gap-controlled reduced graphene oxides (rGOs) via diazotization of three different phenyl, biphenyl, and para-terphenyl bis-diazonium salts (BD1-3). The graphene interlayer sub-nanopores of rGO-BD1-3 are 0.49, 0.7, and 0.96 nm, respectively. Surprisingly, the rGO-BD2 0.7 nm gap shows the highest capacitance in 1 M TEABF4 having 0.68 nm size of cation and 6 M KOH having 0.6 nm size of hydrated cation. The maximum energy density and power density of the rGO-BD2 were 129.67 W h kg(-1) and 30.3 kW kg(-1), respectively, demonstrating clearly that the optimized sub-nanopore of the rGO-BDs corresponding to the electrolyte ion size resulted in the best capacitive performance.

Fast diffusion supercapacitors via an ultra-high pore volume of crumpled 3D structure reduced graphene oxide activation

In order to obtain a high performance supercapacitor, there are several factors that must be achieved including a high specific surface area (SSA), high electrical conductivity, and a high diffusion rate of the electrolyte due to an appropriate pore volume. Herein, we report a high performance supercapacitor using activated non-stacked reduced graphene oxide (a-NSrGO) that has a high SSA (up to 999.75 m2 g−1) with intrinsic high graphene conductivity (1202 S m−1) and fast diffusion of the electrolyte. Due to a high total pore volume (5.03 cm3 g−1) and a wide pore size distribution from macro- to micropores (main pore width: 0.61 – 0.71 nm) in the a-NSrGO sheets, the as-prepared a-NSrGO electrode shows high specific capacitance (105.26 F g−1) and a short relaxation time (τ0 = 1.5 s) in a propylene carbonate (PC)-based organic electrolyte. A maximum energy density of 91.13 W h kg−1 and a power density of 66 684.73 W kg−1 were estimated in a fully packaged coin cell. The high performance of the a-NSrGO supercapacitors is attributed to their specific appearance and enlarged pore distribution with high SSA.

Low-dimensional carbon and MXene-based electrochemical capacitor electrodes.

Due to their unique structure and outstanding intrinsic physical properties such as extraordinarily high electrical conductivity, large surface area, and various chemical functionalities, low-dimension-based materials exhibit great potential for application in electrochemical capacitors (ECs). The electrical properties of electrochemical capacitors are determined by the electrode materials. Because energy charge storage is a surface process, the surface properties of the electrode materials greatly influence the electrochemical performance of the cell. Recently, graphene, a single layer of sp(2)-bonded carbon atoms arrayed into two-dimensional carbon nanomaterial, has attracted wide interest as an electrode material for electrochemical capacitor applications due to its unique properties, including a high electrical conductivity and large surface area. Several low-dimensional materials with large surface areas and high conductivity such as onion-like carbons (OLCs), carbide-derived carbons (CDCs), carbon nanotubes (CNTs), graphene, metal hydroxide, transition metal dichalcogenides (TMDs), and most recently MXene, have been developed for electrochemical capacitors. Therefore, it is useful to understand the current issues of low-dimensional materials and their device applications.