Su Chan Lee

Enhanced Supercapacitive Performance of Chemically Grown Cobalt–Nickel Hydroxides on Three-Dimensional Graphene Foam Electrodes

Chemical growth of mixed cobalt-nickel hydroxides (CoxNi1-x(OH)2), decorated on graphene foam (GF) with desirable three-dimensional (3D) interconnected porous structure as electrode and its potential energy storage application is discussed. The nanostructured CoxNi1-x(OH)2 films with different Ni:Co (x) compositions on GF are prepared by using the chemical bath deposition (CBD) method. The structural studies (X-ray diffraction and X-ray photoelectron spectroscopy) of electrodes confirm crystalline nature of CoxNi1-x(OH)2/GF and crystal structure consists of Ni(OH)2 and Co(OH)2. The morphological properties reveal that nanorods of Co(OH)2 reduce in size with increases in nickel content and are converted into Ni(OH)2 nanoparticles. The electrochemical performance reveals that the Co0.66Ni0.33(OH)2/GF electrode has maximum specific capacitance of ∼1847 F g(-1) in 1 M KOH within a potential window 0 to 0.5 V vs Ag/AgCl at a discharge current density of 5 A g(-1). The superior pseudoelectrochemical properties of cobalt and nickel are combined and synergistically reinforced with high surface area offered by a conducting, porous 3D graphene framework, which stimulates effective utilization of redox characteristics and communally improves electrochemical performance with charge transport and storage.

Hierarchical MnCo-layered double hydroxides@Ni(OH)2 core–shell heterostructures as advanced electrodes for supercapacitors

Rational assembly and hetero-growth of hybrid structures consisting of multiple components with distinctive features are a promising and challenging strategy to develop materials for energy storage applications. Herein, we propose a supercapacitor electrode comprising a three-dimensional self-supported hierarchical MnCo-layered double hydroxides@Ni(OH)2 [MnCo-LDH@Ni(OH)2] core–shell heterostructure on conductive nickel foam. The resultant MnCo-LDH@Ni(OH)2 structure exhibited a high specific capacitance of 2320 F g−1 at a current density of 3 A g−1, and a capacitance of 1308 F g−1 was maintained at a high current density of 30 A g−1 with a superior long cycle lifetime. Moreover, an asymmetric supercapacitor was successfully assembled using MnCo-LDH@Ni(OH)2 as the positive electrode and activated carbon (AC) as the negative electrode. The optimized MnCo-LDH@Ni(OH)2//AC device with a voltage of 1.5 V delivered a maximum energy density of 47.9 W h kg−1 at a power density of 750.7 W kg−1. The energy density remained at 9.8 W h kg−1 at a power density of 5020.5 W kg−1 with excellent cycle stability.

Controllable sulfuration engineered NiO nanosheets with enhanced capacitance for high rate supercapacitors

NiO has been intensively studied as a promising electrode material for supercapacitors because of its high theoretical specific capacitance, well-defined redox behavior, and good chemical compatibility with nickel foam. However, it still suffers from inferior rate capability and cycling stability because of the simple component and random structural integration. Herein, we report a tunable sulfuration process of NiO nanosheets constructed on porous nickel foam for supercapacitor applications. The resulting NiO/Ni3S2 with distinct structural features exhibits an ultra-high specific capacitance of 2153 F g−1 at a current density of 1 A g−1, and the capacitance is retained at 1169 F g−1 even at a current density as high as 30 A g−1. An asymmetric supercapacitor device fabricated with NiO/Ni3S2 as the positive electrode and activated carbon as the negative electrode delivers high energy and power densities (52.9 W h kg−1 at 1.6 kW kg−1; 26.3 W h kg−1 at 6.4 kW kg−1), and good cycling stability (a capacitance retention of 92.9% over 5000 cycles).

Scalable fabrication of micron-scale graphene nanomeshes for high-performance supercapacitor applications

Graphene nanomeshes (GNMs) with nanoscale periodic or quasi-periodic nanoholes have attracted considerable interest because of unique features such as their open energy band gap, enlarged specific surface area, and high optical transmittance. These features are useful for applications in semiconducting devices, photocatalysis, sensors, and energy-related systems. Here, we report on the facile and scalable preparation of multifunctional micron-scale GNMs with high-density of nanoperforations by catalytic carbon gasification. The catalytic carbon gasification process induces selective decomposition on the graphene adjacent to the metal catalyst, thus forming nanoperforations. The pore size, pore density distribution, and neck size of the GNMs can be controlled by adjusting the size and fraction of the metal oxide on graphene. The fabricated GNM electrodes exhibit superior electrochemical properties for supercapacitor (ultracapacitor) applications, including exceptionally high capacitance (253 F g−1 at 1 A g−1) and high rate capability (212 F g−1 at 100 A g−1) with excellent cycle stability (91% of the initial capacitance after 50 000 charge/discharge cycles). Further, the edge-enriched structure of GNMs plays an important role in achieving edge-selected and high-level nitrogen doping.

Efficient Direct Reduction of Graphene Oxide by Silicon Substrate

Graphene has been studied for various applications due to its excellent properties. Graphene film fabrication from solutions of graphene oxide (GO) have attracted considerable attention because these procedures are suitable for mass production. GO, however, is an insulator, and therefore a reduction process is required to make the GO film conductive. These reduction procedures require chemical reducing agents or high temperature annealing. Herein, we report a novel direct and simple reduction procedure of GO by silicon, which is the most widely used material in the electronics industry. In this study, we also used silicon nanosheets (SiNSs) as reducing agents for GO. The reducing effect of silicon was confirmed by various characterization methods. Furthermore, the silicon wafer was also used as a reducing template to create a reduced GO (rGO) film on a silicon substrate. By this process, a pure rGO film can be formed without the impurities that normally come from chemical reducing agents. This is an easy and environmentally friendly method to prepare large scale graphene films on Si substrates.

All-solid-state flexible asymmetric micro supercapacitors based on cobalt hydroxide and reduced graphene oxide electrodes

In-plane micro supercapacitors (micro-SC) have attracted interest due to their high areal and volumetric capacitance that is dependent upon their electrode structure. This study proposes simply-fabricated micro-SCs based on cobalt hydroxide and electrochemically-reduced graphene oxide (erGO). The Au electron collector was prepared via photolithography, and Co(OH)2 and erGO were deposited on the Au surface via electrodeposition. Using facile two-step fabrication method and cost-effective materials, the prepared micro-SCs exhibit good electrochemical performance in PVA–KOH–KI solid electrolyte. The in-plane interdigitated electrode maximizes the areal capacitance by increasing the facing area between the anode and cathode. The micro-SC presents good power density (100.38 μW cm−2) and a wide potential window due to the electric double-layer properties of erGO and pseudocapacitive performance of Co(OH)2. These fabricated micro-SCs are flexible and can be used in various wearable and small-scale energy storage devices.

Crack-Release Transfer Method of Wafer-Scale Grown Graphene Onto Large-Area Substrates

We developed a crack-release graphene transfer technique for opening up possibilities for the fabrication of graphene-based devices. Graphene film grown on metal catalysts/SiO2/Si wafer should be scathelessly peeled for sequent transferring to a target substrate. However, when the graphene is grown on the metal catalyst on a silicon substrate, there is a large tensile stress resulting from the difference of the coefficient of thermal expansion in the catalyst and silicon. The conventional methods of detaching graphene from metal catalysts were found to induce considerable mechanical damage on graphene films during separation processes including metal wet etching. Here we report a new technique wherein bubbles generated by electrolysis reaction separate thin metal catalysts from the SiO2/Si wafer. The dry attachment of graphene to the target wafer was processed utilizing a wafer to wafer bonding technique in a vacuum. We measured the microscopic image, Raman spectra, and electrical properties of the transferred graphene. The optical and electrical properties of the graphene transferred by the bubbles/dry method are better than those of the graphene obtained by mechanical/wet transfer.

Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values

The development of economical, proficient, and highly stable catalysts to substitute the expensive noble metal electrodes for electrocatalytic water-splitting applications is exceedingly desirable. In this context, the most fascinating and challenging approach is the rational design of a nanocomposite encompassing multiple components with unique functionalities. Herein, we describe the fabrication of a strongly catalytic and superb durable phosphorus-incorporated cobalt molybdenum sulfide electrocatalyst grown on carbon cloth (P-CoMoS/CC). The hybrid material exhibited excellent activity for hydrogen and oxygen evolution reactions over a wide range of pH (1-14) with extremely high stability (∼90% retention of the initial current density) after 24 h of electrolysis. Importantly, when P-CoMoS/CC was used as both cathode and anode for overall water splitting, a very low cell voltage of 1.54 V is required to attain the 10 mA cm-2 current density, and the hybrid material exhibited a long-term stability (89.8% activity retention after 100 h). The outstanding overall water-splitting performance compared to an electrolyzer consisting of the noble-metal-based catalysts Pt/C and RuO2 makes P-CoMoS one of the most efficient earth-abundant water-splitting catalysts. Phosphorus incorporation was proved to be a vital aspect for the improved charge-transfer properties and catalytic durability of the P-CoMoS/CC catalyst.

Graphene-Iodine Nanocomposites: Highly Potent Bacterial Inhibitors that are Bio-compatible with Human Cells.

Graphene-composites, capable of inhibiting bacterial growth which is also bio-compatible with human cells have been highly sought after. Here we report for the first time the preparation of new graphene-iodine nano-composites via electrostatic interactions between positively charged graphene derivatives and triiodide anions. The resulting composites were characterized by X-ray photoemission spectroscopy, UV-spectroscopy, Raman spectroscopy and Scanning electron microscopy. The antibacterial potential of these graphene-iodine composites against Klebsiella pneumonia, Pseudomonas aeruginosa, Proteus mirobilis, Staphylococcus aureus, and E. coli was investigated. In addition, the cytotoxicity of the nanocomposite with human cells [human white blood cells (WBC), HeLa, MDA-MB-231, Fibroblast (primary human keratinocyte) and Keratinocyte (immortalized fibroblast)], was assessed. DGO (Double-oxidizes graphene oxide) was prepared by the additional oxidation of GO (graphene oxide). This generates more oxygen containing functional groups that can readily trap more H+, thus generating a positively charged surface area under highly acidic conditions. This step allowed bonding with a greater number of anionic triiodides and generated the most potent antibacterial agent among graphene-iodine and as-made povidone-iodine (PVP-I) composites also exhibited nontoxic to human cells culture. Thus, these nano-composites can be used to inhibit the growth of various bacterial species. Importantly, they are also very low-cytotoxic to human cells culture.

Conceptual design of three-dimensional CoN/Ni3N-coupled nanograsses integrated on N-doped carbon to serve as efficient and robust water splitting electrocatalysts

Developing binder-free, low-cost, and efficient electrocatalysts for water splitting is very important to meet the ever-increasing global energy demands. We have judiciously designed a polyaniline (PANI)-mediated protocol for the synthesis of nickel–cobalt nitride (NCN) heterostructures on carbon cloth (CC) to be applied as catalysts for full electrochemical splitting of water. Controlled pyrolyzation of the nickel–cobalt precursor on PANI-coated CC generates assembled grass-like nanostructures of cobalt nitride (CoN) and nickel nitride (Ni3N) along with beneficial, conductive nitrogen-doped carbon layers on CC for improved electrochemical activity. The generation of numerous catalytically active centers with expeditious charge and mass transportation due to the incorporated nickel and high mechanical stability owing to the self-supporting nature of the designed material result in excellent and stable electrocatalytic performance. The designed NCN/CC electrode requires low overpotentials (η10) of 247 and 68 mV to attain a current density of 10 mA cm−2 during oxygen evolution and hydrogen evolution reactions, respectively, with appreciable stability (>90% retention of the initial current density) over a 24 h long electrochemical test in 1.0 M KOH. Finally, the NCN/CC electrocatalyst is utilized to demonstrate full alkaline water splitting at a cell voltage of 1.56 V to deliver the current density of 10 mA cm−2 with tremendous stability over 240 h. Moreover, NCN/CC could afford a stable current density of 10 mA cm−2 towards full water splitting at 1.59 V cell voltage in acidic electrolyte with 100 h long-term stability. These results suggest its prospects as a substitute for expensive noble-metal-based water splitting electrocatalysts in practical applications.