Minoh Lee

One-step hydrothermal synthesis of graphene decorated V2O5 nanobelts for enhanced electrochemical energy storage

Graphene-decorated V2O5 nanobelts (GVNBs) were synthesized via a low-temperature hydrothermal method in a single step. V2O5 nanobelts (VNBs) were formed in the presence of graphene oxide, a mild oxidant, which also enhanced the conductivity of GVNBs. From the electron energy loss spectroscopy analysis, the reduced graphene oxide (rGO) are inserted into the layered crystal structure of V2O5 nanobelts, which further confirmed the enhanced conductivity of the nanobelts. The electrochemical energy-storage capacity of GVNBs was investigated for supercapacitor applications. The specific capacitance of GVNBs was evaluated using cyclic voltammetry (CV) and charge/discharge (CD) studies. The GVNBs having V2O5-rich composite, namely, V3G1 (VO/GO = 3:1), showed superior specific capacitance in comparison to the other composites (V1G1 and V1G3) and the pure materials. Moreover, the V3G1 composite showed excellent cyclic stability and the capacitance retention of about 82% was observed even after 5000 cycles.

Flexible organo-metal halide perovskite solar cells on a Ti metal substrate

Organo-metal halide perovskite solar cells have received much attention in the field of photovoltaics in recent years. Herein, we report a flexible perovskite solar cell based on a metal substrate. It showed a power conversion efficiency of over 6% for the first time using a silver thin film as a semi-transparent top electrode on a Ti substrate.

Silver Nanowire Top Electrodes in Flexible Perovskite Solar Cells using Titanium Metal as Substrate

Flexible perovskite solar cells (FPSCs) have various applications such as wearable electronic textiles and portable devices. In this work, we demonstrate FPSCs on a titanium metal substrate employing solution-processed silver nanowires (Ag NWs) as the top electrode. The Ag NW electrodes were deposited on top of the spiro-MeOTAD hole transport layer by a carefully controlled spray-coating method at moderate temperatures. The power conversion efficiency (PCE) reached 7.45 % under AM 1.5 100 mW cm(-2) illumination. Moreover, the efficiency for titanium-based FPSCs decreased only slightly (by 2.6 % of the initial value) after the devices were bent 100 times. With this and other advances, fully solution-based indium-free flexible photovoltaics, advantageous in terms of price and processing, have the potential to be scaled into commercial production.

Efficient, durable and flexible perovskite photovoltaic devices with Ag-embedded ITO as the top electrode on a metal substrate

Efficient flexible perovskite solar cells based on a Ti substrate have been fabricated using indium tin oxide (ITO) as the top electrode. Furthermore, an ultra-thin Ag layer embedded between spiro-MeOTAD and ITO led to lower sheet resistance and a highly durable electrode compared with bare ITO. Inclusion of the Ag thin film has also provided enhanced electromagnetic fields on the surface of Ag for devices, which caused an increase of short circuit current.

Efficient fiber-shaped perovskite photovoltaics using silver nanowires as top electrode

Methylammonium lead iodide (CH3NH3PbI3) perovskite solar cell in a flexible fiber shape is developed via a fully dipping process with a mixed solvent of N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) followed by toluene dipping. We introduce the first-ever effective n-type compact layer through facile anodizing of titanium wire, achieving a considerable power conversion efficiency of 3.85%, which remains stable during bending; spray-deposited silver nanowires (Ag NWs) are used as the top electrode instead of gold. The ease of fabrication, low cost of materials, and all-solid-state structures result in a simple approach to developing electronic textiles for harvesting solar energy and blazes a new trail in the field of fiber-shaped photovoltaics.

Effect of sulphur vacancy on geometric and electronic structure of MoS2 induced by molecular hydrogen treatment at room temperature

Investigations into the interaction between molecular hydrogen and molybdenum disulphide have been in increasing demand to improve the understanding of the hydrodesulphurisation process, especially the creation of sulphur vacancies which result in coordinatively unsaturated sites in MoS2. Here we present comprehensive studies of the structural and electronic modulation caused by exposure of MoS2 to H2 over a low temperature range, which may be helpful for industrial applications. Detail investigations were conducted with Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), and electrical transport properties as a function of H2 gas pressure up to 24 bar from 295 K to 350 K. Upon exposure to H2, we observed bond-softening using Raman spectroscopy, a decrease in d-spacing from the TEM results, and an increase in conductance, all of which are consistent with the first-principles calculations. The results demonstrate the formation of sulphur vacancies even under low H2 pressure at low temperature.

Efficiency Enhancement of Dye-Sensitized Solar Cells by the Addition of an Oxidizing Agent to the TiO2 Paste

The addition of various amounts of a strong oxidizing agent (3,5-dinitrosalicyclic acid, DNSA) to TiO2 paste enhances the solar-to-electrical-energy conversion efficiency of the corresponding dye-sensitized solar cells (DSSCs). Maximum performance was obtained from a device that was fabricated by using a TiO2 paste with 2 wt % DNSA, which showed a short-circuit current density of 17.88 mA cm(-2) , an open-circuit voltage of 0.78 V, and an overall conversion efficiency of 9.62 %, which was an improvement in comparison to reference cells without DNSA. This improvement was rationalized in terms of the amount of residual carbon (formed due to the oxidation of binders) remaining on the TiO2 surface. Addition of a larger amount of oxidizing agent led to a smaller amount of residual carbon on the TiO2 surface. This smaller amount of residual carbon enhanced the adsorption of a larger number of dye molecules on the TiO2 surface. The addition of an oxidizing agent facilitated the removal of more residual organic species during the high-temperature calcination process while causing no change in the surface morphology and microstructure of the TiO2 film.

A self-generated and degradation-resistive cratered stainless steel electrocatalyst for efficient water oxidation in a neutral electrolyte

An electron-mediated CO2-to-chemical conversion system is regarded as one of the effective solutions for the depletion of fossil fuels and the accumulation of atmospheric CO2. In this process, the protons and electrons generated from the water-oxidation reaction at an anode are used during the reduction of CO2 at a cathode, in order to produce high-value hydrocarbon chemicals. Therefore, water oxidation is also a key reaction for the overall electron-mediated CO2-to-chemical conversion. In this work, a facile preparation method is developed for a highly efficient water oxidation electrocatalyst which stably operates in a neutral bicarbonate electrolyte optimized for CO2-reduction conditions. Ni-rich cratered structures were spontaneously formed on the stainless steel surface by harsh electro-oxidation, and the chemical composition changes of Fe and Ni on the catalyst surface dramatically enhance water-oxidation activity showing an overpotential value of 504 mV at 10 mA cm−2 in a CO2-saturated bicarbonate electrolyte. In contrast to a severe degradation in the phosphate electrolyte, the cratered stainless-steel (CSS) catalyst is very stable for an 18 h reaction in the bicarbonate electrolyte. Surface spectroscopic analyses of CSS consistently revealed that the active-surface structure of the NiOOH and adsorbed water molecules is remarkably stable throughout water-oxidation in the neutral bicarbonate electrolyte, while the destruction of Ni structures by the phosphate electrolyte is proposed to cause concomitant activity loss for water oxidation.

Sloughing a precursor layer to expose active stainless steel catalyst for water oxidation

Hydrogen production by water electrolysis has been regarded as a promising approach to wean away from sourcing energy through fossil fuels, as the produced hydrogen gas can be converted to electrical or thermal energy without any harmful byproducts. However, an efficient hydrogen production is restricted by the sluggish oxygen evolution reaction (OER) at the counter anode. Therefore, the development of new OER catalysts with high catalytic activities is crucial for high performance water splitting. Here, we report a novel sloughing method for the fabrication of an efficient OER catalyst on a stainless steel (SS) surface. A chalcogenide (Fe-S) overlayer generated by sulfurization on the SS surface is found to play a critical role as a precursor layer in the formation of an active surface during water oxidation. Interestingly, a newly exposed catalytic layer after sloughing off the Fe-S overlayer has a nanoporous structure with changed elemental composition, resulting in a significant improvement in OER performance with an overpotential value of 267 mV at a current density of 10 mA cm-2 (in 1 M KOH). Our novel method for the preparation of OER catalyst provides an important insight into designing an efficient and stable electrocatalyst for the water splitting community.

Magnetic Frustration Driven by Itinerancy in Spinel CoV 2 O 4

Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. They exhibit a complex interplay between localized spins and itinerant electrons. In this paper, we study the origin of the unusual spin structure of the spinel CoV2O4, which stands at the crossover from insulating to itinerant behavior using the first principle calculation and neutron diffraction measurement. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state even without orbital orderings and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhances the frustration.