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Commercial purity Mg, Mg-RE alloys ZE10A (UNS M11600), and EV31A (UNS M12310) were evaluated in the as-received condition for potential application as anode for Mg-air battery. Anodic and cathodic polarization studies were performed in 0.6 M NaCl, 0.5 M NaNO3 and 0.1 M Na2SO4 electrolytes. Hydrogen evolved during anodic polarization was measured at different potentials. Electrochemical impedance spectroscopy was carried out at open circuit conditions. The results showed that commercial purity magnesium had higher corrosion resistance than the Mg-RE alloy specimens in all the electrolytes tested. All the specimens showed negative difference effect (NDE) where the rate of hydrogen evolution increased with increased anodic polarization. The NDE was more pronounced on ZE10A than on other materials. The rate of hydrogen evolution was about three times higher in NaCl solution than in other two solutions. The results suggest that ZE10A could be a potential anode material for Mg-air battery due to its high anodic activity in spite of the observed NDE.

Passivation kinetics of two Mg-RE alloys, such as Mg-Nd-Gd-Zn-Zr (EV31A), and Mg-Y-Nd-Gd-Zr (WE43C) were investigated in two different heat treated conditions (solution treated and overaged) in 0.01 - 1.0 M NaOH solutions under potentiostatic conditions. Negative reaction order was observed in dilute NaOH which transitioned to positive values as the passivation time increased and in the 1 M NaOH as well. The passive layers showed platelet morphology and the size of the platelets decreased with increase in the NaOH concentration. The hydrogen evolution reaction (HER) kinetics was not improved on the passive layer covered surface of the Mg-RE alloys in contrast to the improvements reported on the hydroxide covered pure magnesium. The electrochemical impedance increased with increase in the NaOH concentration in the solution treated condition of both Mg-RE alloys, whereas the overaged EV31A alloy showed a reverse trend. The passive layer of EV31A showed almost 100% higher charge carrier density than the film formed on the WE43C in the overaged condition. A better passivation behavior was observed in the solution treated condition than that in the overaged condition which could be attributed to the uniform distribution of the RE elements in the solution treated specimens. The WE43C alloy revealed better corrosion resistance in the alkaline solution than the EV31A alloy. Keywords: Magnesium rare earth alloy, Passivation, Alkaline solution, Hydrogen evolution

Mg–Zn–Gd–Nd–Zr alloy (EV31A) is a heat-treatable magnesium (Mg) cast alloy that can be used up to 200 °C for automobile and aerospace applications. This alloy has excellent mechanical properties (ultimate tensile strength: 280 MPa at room temperature, and ~230 MPa at 200 °C) and improved corrosion resistance. Electrochemical corrosion studies were conducted on this alloy under different heat treatment conditions in 0.1 M NaOH solution with the addition of 0–1000 ppm of chloride. The alloy showed excellent passivity in the 0.1 M NaOH solution. The passive potential range typically extended to more than 1.2 VAg/AgCl. The transpassive potential was observed to be dependent on heat treatment condition of the alloy. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 °C for 16 h imparted better resistance for localized corrosion than other heat-treated conditions. The alloy in the as-received condition showed the highest passivation kinetics due to its smaller grain size that possibly increased the diffusion of reactive elements to form protective oxide. The passive film of the EV31A alloy showed n-type semiconductivity with a charge carrier density of ~2 × 1021 cm−3 with no chloride addition. The charge carrier density increased with chloride addition in the electrolyte which could be correlated with the susceptibility to localized corrosion.

Cyclic voltammetry CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy measurements were carried out in 1 and 5 wt% TeCl4 containing LiCl-KCl eutectic molten salt at 450 °C using tungsten and glassy carbon as working electrodes. Reduction of Te(IV) to Te(0) occurred in three steps at potentials more negative than −0.4 VAg/AgCl. The Te(0) was further reduced to Te2− at more negative potentials. Formation of Te2− was observed at potentials more negative than −2.2 VAg/AgCl. Diffusivities of Te4+, Te2+, and Te2− and their formal potentials were estimated from the CV data. The diffusion coefficients of Te4+, Te2+ and Te2− were 0.9 × 10−5, 3.8 × 10−5, and 1.5 × 10−5 cm2 s−1, respectively in the 1 wt% TeCl4 containing LiCl-KCl molten salt. The diffusivity of Te species generally decreased with increase in the concentration of TeCl4.

Electrochemical corrosion tests were carried out on AZ31 magnesium alloy specimens in pH: 4.5, 9.5, and 13.0 solutions with 0-2000 ppm of chloride additions at room temperature. No passivity breakdown was observed during cyclic polarization in pH:13 solutions containing up to 1500 ppm of chloride. Addition of sodium sulfate and sodium dihydrogen phosphate as supporting electrolytes offset the chloride effect on the corrosion of AZ31 in pH 4.5 and 9.5 solutions. The Mott-Schottky analysis showed the presence of a duplex surface layer consisting of an n-type MgO1−x inner layer (x = 0.024-0.05), and a p-type outer layer which thickened with time at the expense of the inner layer.

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85 mV and −230 mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021 cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03 mF/cm2 at a scan rate of 10 mV/s and the lowest was 1.93 mF/cm2 at 1 V/s in 1 M sulfuric acid solution.

Nanoporous oxide layer was formed on the NdFeB permanent magnets by a simple anodization process in fluoride containing ethylene glycol at 50 V for 15 minutes. The electrochemical polarization behavior of the commercial permanent magnetic material was evaluated in 0.1 M NaH2PO4 solution (pH: 4.75) containing 0–0.017 M of chloride at room temperature. The material was evaluated in magnetized and demagnetized condition with different surface preparation conditions such as: Ni-Cu-Ni coated, uncoated (bare), and anodized. The nickel coated magnetic sample showed reasonable pitting corrosion resistance up to 0.0017 M chloride addition. The uncoated samples showed passivity in chloride free phosphate solution. Chloride additions caused pitting and the pitting protection potential was equal to the corrosion potential with 0.0017 M chloride addition. Demagnetization did not alter the electrochemical polarization. However, the passive film formed on the magnetic sample showed higher charge carrier density than the film formed on the demagnetized sample.

Formation of iron oxide nanotubes on to pure iron substrate by an electrochemical anodization method was investigated in fluoride containing electrolytes. Anodization of iron foil in fluoride containing borate solution resulted in stacked nano-ring type oxide morphology. Nanoporous oxide layer was observed at low pH and a granular oxide layer was formed at higher pH of phosphate + fluoride solutions. Formation of either nanoporous or nanotubular oxide layer was observed in ethylene glycol (EG) solution containing 0.05–0.1 M fluoride + 1.5–3.0 vol.% water. Transition from nanoporous structure to nanotubular structure was critically controlled by anodization potential, water addition and fluoride concentration of the EG solution. The potential required for this transition decreased with increase in the water content up to 7 vol.% beyond which enhanced dissolution occurred. Annealing of the nanotubes at 500 °C resulted in predominantly α-Fe2O3 crystal structure. The annealed Fe2O3 samples consisting of a single layer of nanotubular structure showed a photo current density of 0.4 mA/cm2 at 0.5 V Ag/AgCl in 1 M KOH solution under simulated solar light illumination.

Highly ordered TiO 2 nanotube arrays generate a considerable interest for hydrogen generation by an electrochemical photocell, since ordered architecture of nanotube arrays provides a unidirectional electric channel for electron's transport. Here, we report the hydrogen generation by highly ordered TiO 2 nanotube arrays under actual sunlight in KOH electrolyte. The two-electrode electrochemical cell included an adjustable anode compartment capable of tracing the trajectory of the sun and a set of alkaline batteries connected with a rheostat for application of external bias. The results showed that the photocurrent responses of nanotube arrays match well with the intensity of solar irradiance on a clear summer day. Addition of ethylene glycol into KOH electrolyte as a hole scavenger enhanced the rate of hydrogen generation. A maximum photocurrent density of 31 mA/cm 2 was observed at 13:30 h, by focusing the sunlight with an intensity of 113 mW/cm 2 on the surface of the TiO 2 nanotube arrays in 1 M KOH electrolyte with 10 vol% ethylene glycol under an applied bias of 0.5 V. The observed hydrogen generation rate was 4.4 mL/h cm 2 under the focalized solar irradiance with an intensity between 104 mW/cm 2 and 115 mW/cm 2 from 10:00 to 14:20 h.

AlSb is a group III–V compound semiconductor material that is conventionally grown by high temperature processes such as Czochralski and Bridgman methods. Development of a method to synthesize AlSb at room temperature will be more economical to help modulate the electronic properties. In this investigation, a pulsed potential electrodeposition method using a room temperature molten salt system (aluminum trichloride, AlCl 3 /1-methyl-3-ethylimidazolium chloride, EMIC) with an addition of SbCl 3 is discussed. The potential pulse parameters were established by carrying out cyclic voltammetry at different concentrations of SbCl 3 and with varying molar ratios of AlCl 3 /EMIC. Stoichiometric AlSb deposits were obtained from an acidic AlCl 3 /EMIC (1.5:1 molar ratio) melt containing 4 × 10 −3 mol/l of SbCl 3 onto an ordered TiO 2 nanotubular template. The AlSb compound was predominantly amorphous in as-deposited condition and annealing at 350 °C for 2 h in argon transformed into crystalline phase. The AlSb deposit showed a high resistivity in the order of 10 9 Ω-cm and a defect concentration of 10 16 cm −3 which was attributed to presence of carbon. The deposits obtained from a basic melt (0.67:1 molar ratio of AlCl 3 /EMIC) were enriched with antimony.

Electron−hole (e-h) recombination loss is a major practical problem in using TiO2 as a photocatalyst. This paper describes the use of organic additives to reduce e-h recombination losses which significantly improved the photocurrent density of the integrated TiO2 nanotube/Ti photoanode. Studies on photoelectrochemical hydrogen generation using nanotubular arrays of TiO2 photoanodes were carried out in 1 M KOH with the addition of three different organic additives, namely, methanol (one hydroxyl group), ethylene glycol (two hydroxyl groups), and glycerol (three hydroxyl groups) using a simulated solar light. Ethylene glycol was found to be the best among the investigated organic additives to reduce electron hole recombination. The addition of ethylene glycol produced a photocurrent density of 3.3 mA/cm2 at 0.2 VAg/AgCl compared to 0.87 mA/cm2, using a 87 mW/cm2 light intensity in 1 M KOH solution. On the other hand, methanol and glycerol showed a photocurrent density of 2.43 and 2.55 mA/cm2 under the same ...

Self ordered arrays of titanium manganese mixed oxide nanotubes were prepared by anodization of Ti8Mn alloy (UNS R56080) under ultrasonication in diluted ethylene glycol containing fluoride. The dimensions of the nanotubes (diameter: 20–100 nm and length: 0.5–2.0 μm) could be tuned by changing the synthesis parameters. The as-anodized nanotubes showed a stoichiometry of (Ti,Mn)O 2 . Upon annealing at 500 °C in oxygen atmosphere, the nanotubes contained a mixture of anatase + rutile phases of TiO 2 and Mn 2 O 3 . The composition of the oxide nanotubes was influenced by the chemistry of the phases present in the alloy. More manganese content was observed in the oxide formed on the β-phase than in the oxide layer of α-phase. Anodization in the ultrasonic field increased the kinetics of nanotubular oxide formation and resulted in homogeneous ordering of the nanotubular arrays as compared to the anodization by conventional stirring in the fluoride containing ethylene glycol solution. Whereas, anodization in aqueous acidified fluoride solutions resulted in severe attack of the β-phase and did not show presence of nanotubular oxide structure.

Anodization of Ti using fluoride containing polyhydric alcohols such as ethylene glycol or glycerol as electrolyte results in ordered arrays of TiO2 nanotubes with a smooth surface and a very high aspect ratio. However, the reproducibility of the result is affected by many experimental parameters, notably the water content. In this investigation, anodizations of Ti foil in anhydrous ethylene glycol +0.2 wt% NH4F solution (EG solution) with 0–1.0 wt% water additions were carried out at 20 V for 45 min in a dry-argon filled controlled-atmosphere glove box. It was observed that a minimum amount of 0.18 wt% of water addition was required to form a well ordered TiO2 nanotubular arrays. When the anhydrous EG solution was reused for third time, ordered arrays of nanotubes started to form. When the water addition to the EG solution was more than 0.5 wt%, formation of ridges was observed on the nanotubes. XPS results showed presence of un-anodized Ti element in the anhydrous condition and presence of organic and (NH4)2TiF6 type compounds in all the anodized samples in addition to the regular TiO2 phase. The results underline the influence of water content and local pH condition to form the ordered nanotubular arrays. Keywords: Nanotubular TiO2, Anodization, Ethylene glycol, Oxide film

This new method describes the application of sonoelectrochemistry to quickly synthesize well-ordered and robust titanium dioxide (TiO2) nanotubular arrays. Self-ordered arrays of TiO2 nanotubes of 30–100 m in diameter and 300–1000 nm in length can be rapidly synthesized under an applied potential of 5–20 V. The rate of formation of the TiO2 nanotubes by the sonoelectrochemical method is found to be almost twice as fast as the magnetic stirring method. It also demonstrates that high-quality nanotubes can be prepared using high viscous solvents like ethylene glycol under ultrasonic treatment. The TiO2 nanotubes prepared in the organic electrolytes (ethylene glycol) are then annealed under H2 atmosphere to give TiO2−xCx types material having a band gap of around 2.0 eV. This process is found to be highly efficient for incorporating carbon into TiO2 nanotubes. Various characterization techniques (viz., FESEM, GXRD, XPS, and DRUV–vis) are used to study the morphology, phase, band gap, and doping of the nanotubes. The photoelectrocatalytic activity of these materials to generate H2 by water splitting is found to be promising at 0.2 V vs Ag/AgCl.

Anodization of Ti in acidified fluoride solution results in an ordered nanotubular titanium oxide surface. In this study, vertically oriented arrays of TiO 2 nanotubes were prepared by incorporating nitrate and phosphate species during the anodization process. These nanotubes were annealed at 650 °C in a carbonaceous atmosphere using a chemical vapor deposition (CVD) furnace for a brief period. The carbon-modified nanotubular TiO 2 produced a photo-current density of more than 2.75 mA cm −2 at 0.2 V Ag/AgCl under solar light illumination. This photo-current density corresponds to a hydrogen evolution rate of about 11 l h −1 using a photo-anode of 1 m 2 area. The enhanced hydrogen evolution behavior of carbon-modified nanotubular TiO 2 is highly reproducible and sustainable for long duration. Annealed (at 350 °C in nitrogen atmosphere) TiO 2 nanotubes showed improved photo-activity as compared to the as-anodized or thermally oxidized TiO 2 photo-anodes.

A single step non-aqueous electrodeposition of cadmium zinc telluride (CZT) nanowires on nanoporous TiO 2 substrate was investigated under pulsed-potential conditions. Propylene carbonate was used as the non-aqueous medium. Cyclic voltammogram studies were carried out to understand the growth mechanism of CZT. EDAX and XRD measurements indicated formation of a compound semiconductor with a stoichiometry of Cd 1− x Zn x Te, where x varied between 0.04 and 0.2. Variation of the pulsed-cathodic potentials could modulate the composition of the CZT. More negative cathodic potentials resulted in increased Zn content. The nanowires showed an electronic band gap of about 1.6 eV. Mott-Schottky analyses indicated p-type semiconductor properties of both as-deposited and annealed CZT materials. Increase in Zn content increased the charge carrier density. Annealing of the deposits resulted in lower charge carrier densities, in the order of 10 15 cm −3 .

Room temperature anodization of titanium foil specimens was carried out in 0.5 M phosphoric acid solution with addition of various halide ions. Addition of 0.138 M HF or NaF resulted in self-ordered nano-tubular oxide structure. Addition of bromide and chloride ions initiated only pitting and nano-pores were not observed during anodization. Acidified fluoride solution is found to be necessary to obtain ordered nano-structure as neutral fluoride solution did not form nano-pores. Instability of the oxide layer during anodization and formation of the self-ordered structure can be explained by the perturbation theory. Separation of individual nano-tubes of titanium oxide layer from the inter-connected nano-pores could be attributed to the possible repulsion forces of the cation vacancies.

Nanotubular anodic oxide layer was grown on titanium surface by room temperature anodization in 0.5 M H3PO4 + 0.138 M fluoride solution. The inner surface of the nanotubular oxide walls contained adsorbed anions from the solution that were predominantly phosphate. Hydroxy apatite was electrodeposited on the anodized surface of Ti. Adsorbed phosphate ions facilitated nucleation of nanophase calcium phosphate material inside the nanotubular structure that resulted in vertical growth of apatite crystals. Bond strength of the calcium phosphate coating was found to be good, and cohesive mode of failure of the coating was observed after tensile testing.

Bismuth oxide has well-dispersed valence bands that show enhanced mobility of charge carriers, high refractive index, and large dielectric constant. These properties are attractive for photocatalysis. Bismuth oxide has widely been investigated for photo degradation of dyes for safeguarding the environment. However, not much work has been reported on bismuth oxide as a photo electrode material for solar water splitting. Photo decomposition of Bi2O3 is a concern when used as a photo cathode. However, Bi2O3 can be obtained as an n-type semiconductor by stabilizing other polymorph: β-Bi2O3, which is a metastable phase. Thin films of nanoporous bismuth oxide were synthesized by a simple electrochemical anodization of bismuth substrate. Annealing the anodic nanoporous Bi2O3 at 240 °C for 2 h resulted in stabilization of the β-Bi2O3 phase. Longer annealing times resulted in formation of the more stable α-Bi2O3 phase that showed monoclinic lattice structure and p-type semiconductivity. The photo stability of these materials is discussed based on the photoelectrochemical measurements.

Low-carbon steel is a commonly used structural material in a wide variety of applications. An anodic oxide layer of this inexpensive alloy has been noted to have interesting photoelectrochemical behaviour similar to that of α-Fe2O3 prepared using other expensive starting materials. An ordered nanoporous oxide layer has been grown on to the low-carbon steel surface by a simple electrochemical anodization process in different electrolytes such as ethylene glycol containing 0.05M NH4F and 3–10 vol% water and 0.5M phosphoric acid solution containing 0.05M NH4F. After anodization, the nanoporous anodic oxide layer has been transformed to α-Fe2O3 by a low-temperature annealing process. Photoelectrochemical characterization of the anodic iron oxide materials has been carried out in 1M KOH electrolyte under a solar simulated illumination using Air Mass (AM) 1.5. The ordered nanoporous oxide layer prepared in ethylene glycol-based electrolyte showed a photocurrent density of about 85 µA cm−2 at 0.4 VAg/AgCl. Whereas the anodic iron oxide prepared by anodization of the low-carbon steel in 0.5M H3PO4 + 0.05M NaF solution showed a photocurrent density of 800 µA cm−2 at 0.4 VAg/AgCl. The improved photoactivity of the phosphate-modified oxide layer could be attributed to the high charge carrier concentration, low charge transfer resistance and better ability to expend holes in the oxygen evolution reaction.

Ordered arrays of fluoride incorporated iron oxide nanotubes were synthesized by a simple electrochemical anodization technique. An iron foil was anodized at 50 - 60 V in ethylene glycol solution containing 0.3 wt% NH4F and 1.5 vol% distilled water for 15 - 30 minutes. During the anodization process, oxygen anions were partially substituted with fluoride anions. The self-ordered, vertically oriented, and free-standing iron oxide based nanotubes showed enhanced Li-ion storage capacity, and faster kinetics for Li-ion diffusion. The fluoride incorporation in the oxide was believed to expand the lattice, thereby increasing the ability of the lattice to accommodate enhanced lithium ion intercalation. Furthermore, increased conductivity was noted because of adsorption of carbonaceous species on the nanotubes while anodization and subsequent incorporation of carbon in the Fe-O-F lattice structure after thermal annealing. The proposed configuration of nanotubes showed a reversible nominal capacity > 700 mAh/g.

Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.

The effect of crystallization and surface chemistry of nanotubular titanium dioxide (TiO2) in connection with the photoelectrochemical process is reported in this investigation. TiO2 nanotubular arrays were synthesized by a simple anodization process in an acidified fluoride electrolyte at room temperature. The TiO2 nanotubes were amorphous in as-anodized condition; their transformation to crystalline phases was a function of annealing temperature and gaseous environment. The anatase phase was observed predominantly after annealing in non-oxidizing atmospheres, whereas annealing in an oxygen environment showed a mixture of anatase and rutile phases. X-ray photoelectron spectroscopy was used to determine the chemical environment of the surface, which revealed the presence of phosphate, oxygen vacancies and pentacoordinated Ti in hydrogen annealed samples. Diffuse reflectance photospectrometry of non-oxygen annealed samples showed long absorption tails extending in the visible region. The photoelectrochemical response of the TiO2 nanotubes annealed in different conditions was investigated. Photoelectrochemical performance under simulated solar light was improved by annealing the nanotubular TiO2 samples in non-oxidizing environment.