Yongju Jung

Cathodic Deposition of Polypyrrole Enabling the One‐Step Assembly of Metal–Polymer Hybrid Electrodes

Conducting polymers combining the advantages of organic polymers and the electronic properties of semiconductors are attractive materials for use in energy conversion/storage, optoelectronics, coatings, and sensing applications. The polymerization process of conducting polymers initiates with chemical or electrochemical oxidation of monomers to radicals, which is followed by radical coupling and chain propagation. Chemical oxidation involves the use of oxidizing agents, such as FeCl3, while electrochemical oxidation is achieved by applying an anodic bias to a conducting substrate immersed in a monomer solution (anodic electropolymerization). 3] The electropolymerization method has been predominantly used to prepare filmor electrode-type conducting polymers, as it allows for polymerization confined to the working electrode with a facile control over film thickness and morphology. Conducting polymers have also been utilized as a matrix to embed or disperse metal particles (e.g., Cu, Au, Ag, Ni, Ru, Ir, Pt, Co, Pd, Fe) to form metal–polymer hybrid electrodes for use in sensors and electrocatalysts. Typically, these hybrid electrodes are prepared by a two-step electrodeposition process: electropolymerization followed by metal deposition. This two-step process not only makes the synthesis cumbersome and expensive but also limits the types and qualities of the metal–polymer composite architectures that can be assembled. However, no synthesis strategy that enables one-step synthesis of metal–conducting polymer hybrid films has been made available to date. This is because electropolymerization and metal deposition require an oxidation and a reduction reaction at the working electrode, respectively, with a significantly different range of potentials. Herein, we report an electrochemical method that allows cathodic deposition of polypyrrole (ppy) for the first time. The cathodic deposition method creates many new possibilities for assembling conducting-polymer and conductingpolymer-based hybrid electrodes that cannot be achieved by conventional anodic polymerization. First, conducting-polymer films or coatings can be deposited on substrates that are not stable under anodic deposition conditions. Second, the nucleation and growth processes of conducting polymers during cathodic deposition are different from those of anodic deposition, which results in new microand nanoscale polymer morphologies. Third, one-step electrodeposition of metal–conducting polymer hybrid electrodes becomes possible because both the polymerization and metal reduction reactions can occur under the same cathodic conditions. In this study, we demonstrate the use of cathodic polymerization for the production of high-surface-area ppy electrodes and the one-step synthesis of tin–ppy composite electrodes. The resulting tin–ppy electrodes were characterized for use as anodes in Li-ion batteries. The cathodic deposition of conducting polymers was achieved by coupling two redox reactions. The first reaction is electrochemical generation of an oxidizing agent, the nitrosyl ion (NO). The production of NO ions involves reduction of nitrate ions (NO3 ) to nitrous acid (HNO2) [Eq. (1)]. [14,15] HNO2 is amphoteric, and can generate various species in solution depending on the pH. Under mildly acidic conditions, HNO2 is the major species but it dissociates into NO2 and H as the pH increases (pKa = 3.3). [16] In strongly acidic conditions, HNO2 reacts with H + ions and generates the NO ion [Eq. (2), pKa of H2NO2 + = 7], which is a strong oxidizing agent.

Rational design of exfoliated 1T MoS2@CNT-based bifunctional separators for lithium sulfur batteries

Lithium-sulfur (Li-S) batteries are experiencing a design shift from a closed structure to an open structure to further improve their performance, expanding the design realm from the development of nanostructured materials for the cathode to the production of functional separators. Rational guidelines for preparing a bifunctional separator with exfoliated MoS2 and CNTs are suggested to deal with two conflicting issues: guaranteeing the electron pathway while strongly trapping polysulfide species. In addition, various exfoliation methods ranging from mechanical to chemical were investigated to identify an adequate method for preparing exfoliated MoS2 based-bifunctional separators. The electrochemical exfoliation method was found to be effective in not only exfoliating high quality MoS2 in terms of the lateral size and number of layers, but also providing a favorable MoS2 phase, 1T metallic MoS2. A bifunctional separator of 1T exfoliated MoS2@CNT in a tandem configuration (layer-by-layer structure)-coated Celgard rather than a hetero-configuration delivered an excellent electrochemical performance of ∼670 mA h g−1 after 500 cycles at a high current density of 1C. In addition, the separator was highly effective in trapping polysulfide species and facilitating electron transfer to the irreversible discharge products. The rational guidelines suggested in this study will be extended to other two-dimensional transition-metal dichalcogenides, and applied to the development of other functional membranes.

Preparation of polypyrrole-incorporated mesoporous carbon-based composites for confinement of Eu(III) within mesopores

Mesoporous polymer-carbon composite (CMPEI/CMK-3) materials were successfully prepared by incorporation of a chelating polymer, carboxymethylated polyethyleneimine (CMPEI), into a mesoporous carbon (CMK-3) and for immobilization of Eu(III) ions, commonly used surrogates for radioactive Am(III) ions. After Eu(III) ions were loaded onto the CMPEI/CMK-3 composite, they were subsequently confined by incorporation of polypyrrole (ppy) into the mesopores of the composites. Ppy was prepared as soluble short-chain polymers using NO+ ions as oxidizing agents in a mildly acidic solution. These polymer chains were easily adsorbed on the walls of Eu-CMPEI/CMK-3 composites, efficiently immobilizing the Eu(III) ions. The use of a metal-free oxidizing agent, NO+, in mildly acidic conditions (pH 6) ensured the minimal loss of Eu(III) ions from the composites during polymerization. The resulting ppy/Eu-CMPEI/CMK-3 composites were characterized by electron microscopy, X-ray diffraction and N2 sorption analysis. The results from these characterizations commonly supported the conclusion that ppy was incorporated into the mesopores of the composites, altering the mesoporous features and reducing the pore volumes of the CMK-3 supports. Eu(III)-leaching tests showed that the presence of ppy layers in the composites could significantly improve the retention of Eu(III) ions. This study demonstrated that chelating polymer-based composites can be used for removal and long-term confinement of radioactive actinide species by properly optimizing polymer incorporation processes.

Study on urea precursor effect on the electroactivities of nitrogen-doped graphene nanosheets electrodes for lithium cells

Nitrogen-atom doped graphene oxide was considered to prevent the dissolution of polysulfide and to guarantee the enhanced redox reaction of sulfur for good cycle performance of lithium sulfur cells. In this study, we used urea as a nitrogen source due to its low cost and easy preparation. To find the optimum urea content, we tested three different ratios of urea to graphene oxide. The morphology of the composites was examined by field emission scanning electron microscope. Functional groups and bonding characterization were measured by X-ray photoelectron spectroscopy. Electrochemical properties were characterized by cyclic voltammetry in an organic electrolyte solution. Compared with thermally reduced graphene/sulfur (S) composite, nitrogen-doped graphene/S composites showed higher electroactivity and more stable capacity retention.

Preparation and Electrochemical Behaviors of Sulfur-Containing Electrodes as a Function of Thermal Treatment Temperature

Zeolitic imidazolate framework-sulfur composites were synthesized by a simple solvothermal reaction. Furthermore, following thermal treatment enable electrochemical properties to be improved. In order to investigate optimal temperature, we conducted thermal treatment as a function of different temperature. The morphology of the composites was examined by Field Emission Scanning Electron Microscopy and Fourier Transform Infra-red Spectroscopy. Electrochemical characterizations were also conducted by cyclic voltammetry and Galvanostatic charge-discharge tests. Based on these electrochemical experiments, the sample treated at 900 °C indicated the highest initial specific capacity and retention property in this study. From the results of this study, sulfurcontaining composite treated at higher temperature showed the better characteristics of electrochemical performance.

Ion conducting properties of imidazolium salts with tri-alkyl chains in organic electrolytes against activated carbon electrodes

Electrochemical double layer capacitors (EDLCs), a type of energy storage device, are currently receiving considerable attention. They have a high power density and good cycle ability. Furthermore, they operate on a simple mechanism where electrical charges in an electrochemical double layer are accumulated at the interface between the electrode and the electrolyte [1-3]. For these reasons, capacitors are used in a wide range of applications such as mobile phones, electrical vehicles, and industry power supplies. Capacitors generally consist of an electrode and an electrolyte. The electrode is prepared using carbon materials such as activated carbon, graphene, or graphite fibers [4-7]. Among the carbon materials, activated carbon, which has a high specific surface area and a large number of pores, is suitable for the capacitor electrode. In particular, it readily absorbs or desorbs electric charge. The electrolyte, meanwhile, can be divided into aqueous and non-aqueous electrolytes. Non-aqueous electrolytes have a wide electrochemical stability of op-erative voltage compare to aqueous electrolytes. Because the window potential is related to the energy density (E = 1/2V

Influence of ionic liquid additives on the conducting and interfacial properties of organic solvent-based electrolytes against an activated carbon electrode

This study reports on the influence of N-butyl-N-methylpyrrolidinium tetrafluoroborate (PYR14BF4) ionic liquid additive on the conducting and interfacial properties of organic solvent based electrolytes against a carbon electrode. We used the mixture of ethylene carbonate/dimethoxyethane (1:1) as an organic solvent electrolyte and tetraethylammonium tetrafluoroborate (TEABF4) as a common salt. Using the PYR14BF ionic liquid as additive produced higher ionic conductivity in the electrolyte and lower interface resistance between carbon and electrolyte, resulting in improved capacitance. The chemical and electrochemical stability of the electrolyte was measured by ionic conductivity meter and linear sweep voltammetry. The electrochemical analysis between electrolyte and carbon electrode was examined by cyclic voltammetry and electrochemical impedance spectroscopy.

Effects of phosphorus content and operating temperature on the electrochemical performance of phosphorus-doped soft carbons

A series of high capacity soft carbons with different phosphorus contents were successfully prepared by carbonizing petroleum cokes treated with hypophosphorous acid at 900°C. The effect of phosphorus content on the electrochemical performance of the soft carbons was extensively investigated. The P-doped soft carbons exhibited greatly enhanced discharge capacities and outstanding rate capabilities with increasing phosphorus content. In addition, the influence of temperature on the electrochemical behaviors of the soft carbons was investigated in a wide temperature range of 25°C to 50°C. Surprisingly, the electrochemical properties of the pristine and P-doped soft carbons were highly sensitive to the operating temperature, unlike conventional graphite. The pristine and P-doped soft carbons exhibited significantly high discharge capacities of 470 and 522 mAh/g, respectively, at a high temperature of 50°C.

Adsorption Characteristics of Uranyl Ions on Carboxymethylated Polyethyleneimine (CM-PEI) / Activated Carbon Composites

We present the adsorption characteristics of uranyl ions on a new and innovative composite which was composed of a carboxymethylated polyethyleneimine (CM-PEI) and an activated carbon (F400) with a nanopore less than 2 nm in diameter. In this study, we examined the adsorption phenomena of uranyl ions on the CM-PEI/F400 composite and evaluated the adsorption data using various isotherm models. It was found that the adsorption of uranyl ions on the CM-PEI/F400 composite obeys the Langmuir isotherm model. In addition, it was observed that pH of solutions had great influence on the adsorption capacity of uranyl ions on the CM-PEI/F400 composite. Specially, the adsorption capacity of uranyl ions was linearly increased with an increase of pH at pH > 3.0.

Study on nickel ferrite formation by using a simple method to simulate heat transfer surface

The condition of a heat transfer surface with boiling is composed of three environmental components; high temperature, high pressure and water vapor. In general an autoclave or a high temperature loop system is used for maintaining the required condition. The thermodynamic relationship of chemical reactions states that the free energy difference (ΔG) is clearly dependent on the reaction temperature and independent of the reaction pressure if the reaction has no volume change: (∂ΔG/∂P)T = ΔV ~ 0. Based on the above relationship, the high-pressure term was removed from the environmental components of the heat transfer surface. A vacuum quartz capsule with a moisturized metal oxide powder was used to study the formation of nickel ferrite. The feasibility of this simplified method to simulate a heat transfer surface was confirmed by an analysis of the FT-IR spectra and XRD pattern during the transformation of a nickel and iron mixed oxide into nickel ferrite.