Wen-Ta Tsai

Material Characterization and Electrochemical Performance of Hydrous Manganese Oxide Electrodes for Use in Electrochemical Pseudocapacitors

Hydrous manganese oxide with promising pseudocapacitive behavior was deposited on a carbon substrate at anodic potentials of 0.5-0.95 V vs. saturated calomel electrode ~SCE! in 0.25 M Mn(CH3COO)2 solution at 25°C. The effects of the deposition potential on the material characteristics and electrochemical performances of the hydrous manganese oxide prepared were investigated. Porous manganese oxide with higher crystallinity was formed at a lower deposition potential. When the deposition potential was 0.8 VSCE or higher, the deposited oxide consisted of an inner layer with a laminated structure and a rough outer layer with nodules on the surface. X-ray photoelectron spectroscopy was also carried out to examine the chemical state of the deposited oxide. Analytical results indicated that the oxide was composed of both trivalent and tetravalent manganese oxides at a deposition potential of 0.5 VSCE. However, the tetravalent manganese oxide became the dominant species in the film deposited at above 0.65 VSCE. The manganese oxide formed at 0.5 VSCE exhibited a specific capacitance as high as 240 F/g, as evaluated by cyclic voltammetry ~CV! with a potential scan rate of 5 mV/s in 2 M KCl at 25°C. Increasing the CV scan rate reduced the specific capacitance. Only about 70% of the capacitance at 5 mV/s could be maintained when the CV scan rate was increased to 100 mV/s, for all the manganese oxide electrodes prepared. Moreover, a high deposition potential gave rise to a low specific capacitance of the manganese oxide formed.

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).

Role of phosphorus in the electrochemical behavior of electroless Ni-P alloys in 3.5 wt.% NaCl solutions

Abstract Electroless Ni-P deposits with phosphorus contents ranging from 4.8 to 12.8 wt.% were examined using d.c. polarization curves and a.c. electrochemical impedance spectroscopy techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in neutral and alkaline 3.5 wt.% NaCl solutions. The anodic polarization behavior of the Ni-P alloys in NaCl electrolyte was very similar for all deposits except Ni-4.8P. According to the electrochemical impedance data, the charge transfer resistance R ct and film resistance R f of these deposits have subsequently been studied as functions of the phosphorus content. It was shown that the R ct and R f values increased with increasing phosphorus content of the electroless nickel coating. The Ni-P deposits remained bright in appearance during the periods of electrochemical experiment, and no evidence of pitting was observed after testing. Based on X-ray photoelectron spectroscopy analysis, the passivation of Ni-P alloys was attributed to the formation and adsorption of a layer of hypophosphite.

Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl2 solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.

The Electrochemical Behavior of Electroless Plated Ni‐P Alloys in Concentrated NaOH Solution

Electroless Ni-P deposits with phosphorus content ranging from 4.8 to 12.8 weight percent (w/o) were examined using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in deaerated 40 w/o NaOH solution. Anodic polarization of the electroless Ni-P alloys in caustic NaOH solution shows that the passive current density decreases with increasing phosphorus content in the deposits. At an applied potential of -1.2 V vs. saturated calomel electrode (V SCE ) (close to their E corr ), EIS data indicate that the R ct for Ni-P alloys in NaOH solution increases with increasing phosphorus content. X-ray photoelectron spectroscopy (XPS) results suggest that the primary constituent formed on the Ni-P surface after EIS measurement in 40 w/o NaOH solution at an applied potential of -0.4 V SCE (in the passive region) is Ni(OH) 2 , which is responsible for the passivity of the Ni-P alloys. The polarization resistance of Ni-P alloys in NaOH solution at -0.4 V SCE also increases with increasing phosphorus content

Effect of heat treatment on anodization and electrochemical behavior of AZ91D magnesium alloy

The anodic films formed on AZ91D magnesium alloy after heat treatment were analyzed and their electrochemical properties were investigated. The results showed that the cooling rate had a significant influence on the microstructure evolution of the AZ91D magnesium alloy after solution heat treatment at 440 °C for 20 h in N 2 atmosphere. A single-phase microstructure was observed when the alloy was quenched in water after solution heat treatment. However, a duplex structure consisting of both α and β phases was found if the solution-annealed alloy was cooled in air. The differences in microstructure of the heat treated AZ91D magnesium alloy gave rise to a significant change in the property of the anodic film formed in 3 M KOH + 0.21 M Na 3 PO 4 + 0.6 M KF + 0.15 M Al(NO 3 ) 3 electrolyte. During the early stage of anodization, for the as-cast alloy, inhomogeneous anodic films were formed exhibiting relative rough surface appearances. A rather smooth anodic film was formed for the solution-annealed AZ91D magnesium alloy either followed by air cooling or water-quenched. The surface and cross section appearance was almost the same regardless of the prior heat treatment after anodizing for 20 min. The corrosion resistances of the various anodized AZ91D magnesium alloy were evaluated and compared by employing electrochemical impedance spectroscopy (EIS). The results demonstrate that the anodic film formed on the water-quenched AZ91D magnesium alloy had a slightly higher polarization resistance than that formed on the as-cast alloy. The highest polarization resistance of anodic film was found for that formed on annealed and air-cooled alloy. The presence of Al-rich β phase on the surface gave rise to the formation of a more protective anodic film which consisted of a great amount of Al 2 O 3 .

Stress corrosion cracking behavior of 2205 duplex stainless steel in concentrated NaCl solution

The stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in 26 wt% NaCl solution at 90°C was investigated. Slow strain rate testing was performed at a strain rate of 4.1 × 10−6 s−1. The susceptibility of 2205 DSS to SCC at open circuit potential and at various anodic potentials was evaluated. The experimental results showed that 2205 DSS was immune to SCC in the solution at potentials less than −160 mV (SCE). At potentials greater than −160 mV (SCE), a dramatic reduction in ductility was observed, indicating the susceptibility to SCC. The critical potential for the occurrence of SCC of 2205 DSS was coincident with its pitting potential in the above environment. Fractographical examination revealed that pitting corrosion assisted the initiation and selective dissolution was involved in the propagation of SCC of 2205 DSS in concentrated NaCl solution at 90°C.

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.

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.