Jeng-Kuei Chang

Nano-architectured Co(OH)2 electrodes constructed using an easily-manipulated electrochemical protocol for high-performance energy storage applications

A simple, low-cost, and efficient electrochemical strategy, which includes the co-deposition of a Ni–Cu layer, selective etching of Cu from the film (leaving nano-porous Ni), and electrodeposition of Co(OH)2 nano-whiskers on the obtained Ni substrate, is used to construct a nano-structured electrode. This process can be conducted on many conductive surfaces, which can be cheap, flexible, and wearable, and can be integrated into advanced mobile micro-power systems. Due to its unique nano-architecture, the prepared Co(OH)2 electrode shows exceptional energy storage performance as compared to that of the conventional version of the electrode. The optimum specific capacitance obtained in this study, evaluated using cyclic voltammetry (CV), was as high as 2800 F/g. When the CV scan rate was increased from 5 to 200 mV/s, only a 4% decay in the capacitance was found, indicating excellent high-power capability. These characteristics make the nano-structured Co(OH)2 electrode a promising candidate for supercapacitor applications.

An entirely electrochemical preparation of a nano-structured cobalt oxide electrode with superior redox activity

A nano-structured Co oxide electrode (with a Ni substrate) was successfully prepared using an entirely electrochemical process, which included the co-deposition of a Ni-Cu alloy film, selective etching of Cu from the film, and anodic deposition of Co oxide on the obtained nano-porous Ni substrate which had an average pore size of approximately 100 nm and a pore density of about 10(13) m(-2). The excellent electrochemical activity of the prepared electrode was demonstrated in terms of its pseudocapacitive performance, which was evaluated using cyclic voltammetry (CV) in 1 M KOH solution. The specific capacitance of the nano-structured Co oxide measured at a potential scan rate of 10 mV s(-1) was as high as 2200 F g(-1), which is over ten times higher than that of a flat oxide electrode (209 F g(-1)). The highly porous Co oxide also had superior kinetic performance as compared to a flat electrode. At a high CV scan rate of 50 mV s(-1), the two electrodes retained 94% and 59%, respectively, of their specific capacitances measured at 5 mV s(-1).

Graphene nanosheets, carbon nanotubes, graphite, and activated carbon as anode materials for sodium-ion batteries

The electrochemical sodium-ion storage properties of graphene nanosheets (GNSs), carbon nanotubes (CNTs), mesocarbon microbeads (MCMBs), and activated carbon (AC) are investigated. An irreversible oxidation occurs for the AC electrode during desodiation, limiting its use in sodium-ion batteries. The MCMB electrode shows a negligible capacity (∼2 mA h g−1), since the graphitic structure has a low surface area and is thus not capable of storing a sufficient amount of Na+. In contrast, the CNT and GNS electrodes exhibit reversible capacities of 82 and 220 mA h g−1, respectively, at a charge–discharge rate of 30 mA g−1. The high electro-adsorption/desorption area, large number of Na+ entrance/exit sites, and a large d-spacing of GNSs contribute to their superior Na+ storage capacity. At a high rate of 5 A g−1, the GNS electrode still delivers a capacity of as high as 105 mA h g−1, indicating great high-power ability. The charge storage mechanism of the electrode is examined using an ex situ X-ray diffraction technique.

Tightly connected MnO2–graphene with tunable energy density and power density for supercapacitor applications

MnO2 nanoparticles uniformly distributed and tightly anchored on graphene are prepared using an ethanol-assisted graphene-sacrifice reduction method, producing composite electrodes with tunable energy density (up to 12.6 Wh kg−1) and power density (up to 171 kW kg−1) via rational design of the oxide/graphene ratio.

Pseudocapacitive Mechanism of Manganese Oxide in 1-Ethyl-3-methylimidazolium Thiocyanate Ionic Liquid Electrolyte Studied Using X-ray Photoelectron Spectroscopy

The electrochemical behavior of anodically deposited manganese oxide was studied in pyrrolidinium formate (P-HCOO), 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6), and 1-ethyl-3-methylimidazolium thiocyanate (EMI-SCN) ionic liquids (ILs). The experimental data indicate that the Mn oxide electrode showed ideal pseudocapacitive performance in aprotic EMI-SCN IL. In a potential window of approximately 1.5 V, the oxide specific capacitance, evaluated using cyclic voltammetry and chronopotentiometry, was about 55 F/g. The electrochemical energy storage reaction was examined using X-ray photoelectron spectroscopy (XPS). It was confirmed that the SCN- anions, instead of the EMI+ cations, were the primary working species that can become incorporated into the oxide and thus compensate the Mn3+/Mn4+ valent state variation upon the charge-discharge process. According to the analytical results, a pseudocapacitive mechanism of Mn oxide in the SCN- based aprotic IL was proposed.

Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes

Rechargeable Na/NaFePO4 cells with a sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-incorporated butylmethylpyrrolidinium (BMP)–TFSI ionic liquid (IL) electrolyte are demonstrated with an operation voltage of ∼3 V. High-performance NaFePO4 cathode powder with an olivine crystal structure is prepared by chemical delithiation of LiFePO4 powder followed by electrochemical sodiation of FePO4. This IL electrolyte shows high thermal stability (>400 °C) and non-flammability, and is thus ideal for high-safety applications. The effects of NaTFSI concentration (0.1–1.0 M) on cell performance at 25 °C and 50 °C are studied. At 50 °C, an optimal capacity of 125 mA h g−1 (at 0.05 C) is found for NaFePO4 in a 0.5 M NaTFSI-incorporated IL electrolyte; moreover, 65% of this capacity can be retained when the charge–discharge rate increases to 1 C. This ratio (reflecting the rate capability) is higher than that found in a traditional organic electrolyte. With a 1 M NaTFSI-incorporated IL electrolyte, a 13% cell capacity loss after 100 charge–discharge cycles is measured at 50 °C, compared to the 38% observed in an organic electrolyte under the same conditions.

Ionic-liquid-enhanced glucose sensing ability of non-enzymatic Au/graphene electrodes fabricated using supercritical CO2 fluid

Nano-sized Au particles (approximately 10nm in diameter) are uniformly distributed on both graphene and carbon nanotubes (CNTs) using a supercritical CO₂ fluid (SCCO₂), which has gas-like diffusivity, low viscosity, and near-zero surface tension. Since the Au nanoparticles are highly dispersed and tightly anchored on the carbon supports, the obtained nanocomposites exhibit an improved electro-oxidation ability toward glucose as compared to that of the control electrodes prepared using a conventional chemical deposition process (without SCCO₂). The Au/CNT electrode shows a higher glucose sensing current than that of the Au/graphene counterpart, which is due to the three-dimensional architecture interwoven by the CNTs creating a larger number of reaction sites. However, with ionic liquid (IL) incorporation, the detection sensitivity of the latter electrode significantly improved, becoming noticeably greater than that of the former. The synergistic interactions between Au/graphene and IL that lead to the superior electrochemical detection performance are demonstrated and discussed.

Improved Corrosion Resistance of Magnesium Alloy with a Surface Aluminum Coating Electrodeposited in Ionic Liquid

A metallic aluminum (Al) layer was successfully electrodeposited onto a magnesium (Mg) alloy in a Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl 3 -EMIC) ionic liquid under a galvanostatic condition at room temperature. Effects of deposition current density on material characteristics of the deposited layers were explored by means of a scanning electron microscope and an X-ray diffractometer. In addition, the improvement in corrosion resistance of the Mg alloy due to the Al coating was evaluated by electrochemical measurements and a salt spray test. The electrochemical impedance spectroscopic data indicated that a bare Mg alloy had a polarization resistance of only 470 Ω cm 2 in 3.5 wt % NaCl solution, whereas the Al-coated Mg sample showed its resistance as high as 8700 Ω cm 2 in the same environment. Moreover, it was also found that the Al layer deposited at a lower current density was more compact and uniform when compared to that deposited at a higher current density; consequently, this coating revealed a superior protection capability for the Mg substrate against corrosion.

Microstructure and Pseudocapacitive Performance of Anodically Deposited Manganese Oxide with Various Heat-Treatments

Amorphous, hydrous manganese oxide was prepared by anodic deposition in manganese acetate solution. The effect of heat-treatments (up to 600°C) on the material characteristics of the oxides was investigated. The results indicated that the as-deposited oxide, which was fully amorphous, was transformed into a fibrous shape with nanocrystallinity after annealing at 200°C for 2 h. Mn 3 O 4 and Mn 2 O 3 were formed within the nanocrystalline oxide when heating at 400°C. Furthermore, by increasing the temperature over 500°C, the spherical Mn 2 O 3 particles became the only phase present. In addition, atomic force microscopy was also carried out to explore the surface morphology of the oxide electrodes. This characterization method recognized condensation, rearrangement, reconstruction, and growth of the deposited manganese oxide as a function of temperature. The corresponding electrochemical performances of the oxides were evaluated by chronopotentiometry. The pseudocapacitive characteristics, reversibility, and cyclic stability of the deposited manganese oxide were improved by introducing the proper heat-treatment. However, high-temperature (> 200°C) heat-treatment promoted the formation of crystalline Mn 3 O 4 and Mn 2 O 3 and consequently resulted in the loss of the pseudocapacitive property of the oxides.

Improved supercapacitor performance of MnO2–graphene composites constructed using a supercritical fluid and wrapped with an ionic liquid

Supercritical CO2 (SCCO2) is used to synthesize MnO2. This process is found to be promising for producing oxide nanorods (∼5 nm in diameter) with little agglomeration. With suitable SCCO2 pressure and temperature, nanocrystalline α-MnO2 with an extremely high surface area of 245 m2 g−1 (versus 80 m2 g−1 for the oxide prepared in ambient air) was obtained. Introduction of graphene nanosheets and carbon nanotubes into MnO2 with and without the aid of SCCO2 is compared. SCCO2 can help debundle the graphene nanosheets and uniformly disperse the MnO2 nanorods in between, thus preventing graphene restacking. Since the oxide particles are sandwiched between the highly conductive sheets, a high electrochemical utilization is obtained. Accordingly, SCCO2-MnO2/graphene shows superior supercapacitor properties. The feasibility of using an ionic liquid (IL) as a conductive wrapping agent to improve the performance of the MnO2/graphene electrode is demonstrated. Because the intimate contact between the oxide and graphene can be ensured and the interfacial double-layer capacitance can be optimized, the electrode wrapped with an IL shows a higher capacitance, improved high-rate capability, and higher cyclic stability compared to those obtained without IL wrapping.