Mao-Sung Wu

Capacitive Behavior of Porous Nickel Oxide/Hydroxide Electrodes with Interconnected Nanoflakes Synthesized by Anodic Electrodeposition

Nanostructured nickel hydroxides are galvanostatically deposited onto a stainless steel substrate by a plating bath of nickel sulfate, sodium acetate, and sodium sulfate at room temperature. The anodically deposited nickel hydroxide electrodes are highly porous and composed of interconnected nanoflakes about 12-16 nm in thickness. Pore size of the deposited electrode increases with decreasing the depositing current density. X-ray diffraction patterns show that the deposited nickel hydroxide converts into nickel oxide at annealing temperature above 300°C. Annealing temperature influences both the electrical resistance and the grain size of the electrode and, consequently, determines the capacitive behavior of the electrode investigated by cyclic voltammetry in 1 M KOH aqueous solution. An optimal annealing temperature of 300°C is obtained in terms of the electrodes specific capacitance. An electrode with larger pores deposited at lower current density has a higher specific capacitance because larger pores facilitate the ion diffusion rate. An electrode with smaller pore size deposited at higher current density exhibits more kinetically reversible behaviors, resulting in a better high-rate capability.

Electrochemical Growth of Iron Oxide Thin Films with Nanorods and Nanosheets for Capacitors

Porous iron oxide films with nanorods and nanosheets were anodically deposited onto the nickel substrates by a plating bath of Fe(NH 4 ) 2 (SO 4 ) 2 -6H 2 O, sodium acetate, and sodium sulfate at room temperature. Nucleation and growth mechanism of the iron oxide film was found to be an instantaneous nucleation and growth of two-dimensional cylindrical iron oxide. X-ray diffraction patterns show that the as-deposited iron oxide is orthorhombic α-FeOOH, which converts into rhombohedral Fe 2 O 3 after annealing at temperature above 300°C. Annealing temperatures influences both the chemical composition and grain size of the film, and consequently determines the capacitive behavior of the film investigated by cyclic voltammetry in 1 M Li 2 SO 4 aqueous solution. An optimal annealing temperature of 300°C is obtained in terms of the films specific capacitance. Morphology of the film deposited at a current density higher than 0.125 mA cm -2 shows nanosheets, while the film deposited at a current density lower than 0.125 mA cm -2 shows nanorods. Specific capacitance of a film with nanosheets is higher than that of a film with nanorods.

Nanostructured Iron Oxide Films Prepared by Electrochemical Method for Electrochemical Capacitors

Nanostructured iron oxides were electrodeposited anodically onto nickel substrate in aqueous solution at room temperature. Deposited films after annealing at 300°C are characterized as rhombohedral Fe 2 O 3 . The morphology of the porous film deposited at high current density shows nanosheets of 10-16 nm in thickness. A film deposited at low current density has aggregates of nanorods. Electrochemical impedance results show that the film of randomly distributed nanosheets has a higher percentage of large pores than the film of aggregated nanorods. Consequently, specific capacitance of the film with nanosheets is higher than that of the film with aggregated nanorods at galvanostatic charge/discharge.

Iron Oxide Nanosheets and Nanoparticles Synthesized by a Facile Single-Step Coprecipitation Method for Lithium-Ion Batteries

The nanostructured iron oxides were synthesized by a simple coprecipitation technique at room temperature and tested as the anode materials for lithium-ion batteries. The iron salt precursor has a significant effect on the morphology evolution of the iron oxide. The nanosheet and nanoparticle samples were obtained by using ferrous ammonium sulfate and ferric chloride precursors, respectively. Both samples could be identified as α-Fe 2 O 3 after annealing at 400°C. The electrical conductivity of the nanosheet sample was higher than that of the nanoparticle sample due to its sheet morphology and small grain size. The galvanostatic charge/discharge results indicated that the α-Fe 2 O 3 nanosheet anode (1327 mAh g ―1 ) has a higher reversible capacity than the α-Fe 2 O 3 nanoparticle anode ( 1006 mAh g ―1 ) at 1 C current rate. More importantly, the nanosheet anode exhibited a high capacity of 1215 mAh g ―1 at 3 C current rate; this value is much higher than the nanoparticle anode (812 mAh g ―1 ). The improved performance of the iron oxide nanosheet toward lithium could be attributed to the high electrical conductivity and small grain size for facilitating the transport of the electrons and lithium ions through the nanosheet.