Electrochemical impedence spectroscopy versus optical interferometry techniques during anodization of aluminium

In the present investigation, holographic interferometry was utilized for the first time to measure in situ the thickness of the oxide film, alternating current (AC) impedance, and double layer capacitance of aluminium samples during anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out by the electrochemical impedance spectroscopy (EIS), in different concentrations of sulphuric acid (1.0–2.5% H₂SO₄) at room temperature. In the mean time, the real-time holographic interferometric was used to measure the thickness of anodized (oxide) film of the aluminium samples in aqueous solutions. Also, mathematical models were applied to measure the AC impedance, and double layer capacitance of aluminium samples by holographic interferometry, during anodization processes in aqueous solution. Consequently, holographic interferometric is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film, the AC impedance, and the double layer capacitance of the aluminium samples can be determined in situ. In addition, a comparison was made between the electrochemical values obtained from the holographic interferometry measurements and from measurements of EIS. The comparison indicates that there is good agreement between the data from both techniques.     

Measurement of the a.c. impedance of aluminium samples by holographic interferometry

In the present investigation, holographic interferometry was utilized for the first time to measure the alternating current (a.c.) impedance of aluminium samples during the initial stage of anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H₂SO₄) at room temperature. In the mean time, a method of holographic interferometric was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the a.c. impedance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by the real-time holographic interferometry. Consequently, holographic interferometry is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the a.c. impedance of the aluminium samples can be determined in situ. In addition, a comparison was made between the a.c. impedance values obtained from the holographic interferometry measurements and from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.     

Measurement of the double layer capacitance of aluminium samples by holographic interferometry

In the present investigation, holographic interferometry was utilized for the first time to measure the double layer capacitance of aluminium samples during the initial stage of anodization processes in an aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H₂S0₄) at room temperature. In the meantime, a method of holographic interferometry was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the double layer capacitance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by real-time holographic interferometry. Consequently, holographic interferometry is found to be very useful for surface finish industries, especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the double layer capacitance of the aluminium samples can be determined in situ. In addition, a comparison was made between the obtained data of the double layer capacitance from the holographic measurements and the double layer capacitance data obtained from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.     

Interferometric Sensor for Electrochemical Studies of Metallic Alloys in Aqueuos Solution

In a previous study a mathematical model relating surface and bulk behaviors of metals in aqueous solution was developed. The model was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e., mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e., corrosion current. In the present work, an interferometric sensor was built based on the above model, and the corrosion current density of coated copper and brass in seawater were obtained using this sensor. The interferometric sensor was also utilized for the first time to measure the initial stage of the anodization process (oxidation) of aluminium samples in aqueous solution. This was carried out chemically in different acid concentrations (3.125–25% H₂SO₄) at room temperature. The sensor was further used for observation of catalytic activities, i.e., pitting corrosion, which occurred subsequent to the anodization of the aluminium samples in aqueous solutions, after an oxide film had been formed.     

Holographic interferometry as electrochemical emission spectroscopy of a carbon steel in 5–20 ppm TROS C-70 inhibited seawater

In the present investigation, holographic interferometry was utilized for the first time to determine the rate change of the number of the fringe evolutions during the corrosion test of a carbon steel in blank seawater and seawater with different concentrations of a corrosion inhibitor. In other words, the anodic dissolution behaviors (corrosion) of the carbon steel were determined simultaneously by holographic interferometry, as an electromagnetic method, and by the electrochemical impedance (EI) spectroscopy, as an electronic method. So, the abrupt rate change of the number of the fringe evolutions during corrosion tests, EI spectroscopy, of the carbon steel is called electrochemical emission spectroscopy. The corrosion process of the steel samples was carried out in blank seawater and seawater with different concentrations, 5–20 ppm, of TROS C-70 corrosion inhibitor using the EI spectroscopy method, at room temperature. The electrochemical-emission spectra of the carbon steel in different solutions represent a detailed picture of the rate change of the anodic dissolution of the steel throughout the corrosion processes. Furthermore, the optical interferometry data of the carbon steel were compared to the data, which was obtained from the EI. spectroscopy. Consequently, holographic interferometry is found very useful for monitoring the anodic dissolution behaviors of metals, in which the number of the fringe evolutions of the steel samples can be determined in situ.     

Holographic interferometry as electrochemical emission spectroscopy of carbon steel in seawater with low concentration of RA-41 corrosion inhibitor

In the present investigation, holographic interferometry was utilized for the first time to determine the rate change of the number of the fringe evolutions during the corrosion test of carbon steel in blank seawater and with seawater with different concentrations of a corrosion inhibitor. In other words, the anodic dissolution behaviors (corrosion) of the carbon steel were determined simultaneously by holographic interferometry, an electromagnetic method, and by the electrochemical impedance (EI) spectroscopy, an electronic method. So, the abrupt rate change of the number of the fringe evolutions during corrosion test (EI) spectroscopy, of the carbon steel is called electrochemical emission spectroscopy. The corrosion process of the steel samples was carried out in blank seawater and seawater with different concentrations, 5–20 ppm, of RA-41 corrosion inhibitor using the EI spectroscopy method, at room temperature. The electrochemical emission spectra of the carbon steel in different solutions represent a detailed picture of the rate change of the anodic dissolution of the steel throughout the corrosion processes. Furthermore, the optical interferometry data of the carbon steel were compared to the data, which were obtained from the EI spectroscopy. Consequently, holographic interferometric is found very useful for monitoring the anodic dissolution behaviors of metals, in which the number of the fringe evolutions of the steel samples can be determined in situ.     

Measurement of surface resistivity and surface conductivity of anodised aluminium by optical interferometry techniques

Optical interferometry techniques were used for the first time to measure the surface resistivity and surface conductivity of anodised aluminium samples in aqueous solution, without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out in different sulphuric acid solutions (1.0–2.5% H₂SO₄), by the technique of electrochemical impedance spectroscopy (EIS), at room temperature. In the mean time, the real-time holographic interferometric was carried out to measure the thickness of anodised (oxide) film of the aluminium samples during the anodization process. Then, the alternating current (AC) impedance (resistance) of the anodised aluminium samples was determined by the technique of electrochemical impedance spectroscopy (EIS) in different sulphuric acid solutions (1.0–2.5% H₂SO₄) at room temperature. In addition, a mathematical model was derived in order to correlate between the AC impedance (resistance) and to the surface (orthogonal) displacement of the samples in solutions. In other words, a proportionality constant (surface resistivity or surface conductivity=1/surface resistivity) between the determined AC impedance (by EIS technique) and the orthogonal displacement (by the optical interferometry techniques) was obtained. Consequently the surface resistivity (ρ) and surface conductivity (σ) of the aluminium samples in solutions were obtained. Also, electrical resistivity values (ρ) from other source were used for comparison sake with the calculated values of this investigation. This study revealed that the measured values of the resistivity for the anodised aluminium samples were 2.8×10⁹, 7×10¹², 2.5×10¹³, and 1.4×10¹²  Ω cm in 1.0%, 1.5%, 2.0%, and 2.5% H₂SO₄ solutions, respectively. In fact, the determined value range of the resistivity is in a good agreement with the one found in literature for the aluminium oxide, 85% Al₂O₃ (5×10¹⁰ Ω cm in air at temperature 30 °C), 96% Al₂O₃ (1×10¹⁴  Ω cm in air at temperature 30 °C), and 99.7% Al₂O₃ (>1×10¹⁴ Ω cm in air at temperature 30 °C).     

Electrodeposited ruthenium oxide thin films for supercapacitor: Effect of surface treatments

Amorphous and porous ruthenium oxide thin films have been deposited from aqueous Ru(III)Cl₃ solution on stainless steel substrates using electrodeposition method. Cyclic voltammetry study of a film showed a maximum specific capacitance of 650 F g⁻¹ in 0.5 M H₂SO₄ electrolyte. The surface treatments such as air annealing, anodization and ultrasonic weltering affected surface morphology. The supercapacitance of ruthenium oxide electrode is found to be dependent on the surface morphology.     

All solid-state supercapacitor with phosphotungstic acid as the proton-conducting electrolyte

For the first time, the proton-conducting composite phosphotungstic acid PTA/Al₂(SO₄)₃·18H₂O was used as the electrolyte of symmetric supercapacitor based on PANI. The optimum weight ratio of PTA/Al₂(SO₄)₃·18H₂O for using in this supercapacitor was also reported. Electrochemical tests prove that the supercapacitor using this kind of composite as electrolyte has high capacitance performance. Its capacitance is as high as 240 F/g at 6 mA. It was more important that it has long cycle life. After 1000 cycles, the attenuation of the capacitance is less than 10% and the coulombic efficiency is still greater than 96%.     

Capacitance and H2SO4 adsorption in the pores of activated carbon fibers

H₂SO₄ adsorption was studied by solid-state ²H-NMRand temperature-programmed desorption (TPD) on a series of pitch-based activated carbon fibers (ACFs), and compared to their electric double-layer capacitance in H₂SO₄ and Et₄NBF₄. Three states of H₂SO₄ were found by magic angle spinning (MAS)²H-NMR, namely species adsorbed on the walls, trapped in the pores, and staying over the outer surface of the ACFs, respectively. Protons of H₂SO₄ are strongly fixed on the walls of small pores whereas in the larger pores they are rather mobile due to exchange. The adsorbed protons on the pore walls contribute to the capacitance while their exchange with the protons of H₂SO₄ in the pores appears to decrease their contribution to the capacitance. The capacitance of ACFs with smaller surface area is correlated to the amount of SO₂ desorbed by TPD up to 200 °C. By contrast, for other ACFs of larger surface area the amount up to the same temperature was found to be much larger than their capacitance. The contribution of H₂SO₄ adsorbed in the latter ACFs is much less effective for their capacitance.     

Growth process of atomically flat anodic films on titanium under potentiostatical electrochemical treatment in H2SO4 solution

The as-deposited titanium film on silicon wafer was electrochemically treated in potential sweep and potential step modes in 0.1 M H₂SO₄ solution at 30 °C. Under the anodization conditions of potential sweep and properly modulated cyclic voltammetry (CV), nanoscale grains, step-terrace structure and atomic images were clearly observed on the surface of anodic oxide film on titanium. Under potential step conditions, if the anodization time was short (1 s), no grains could be found on the anodic oxide film surface, even though the potential was high up to 9000 mV. Moreover, whatever potential sweep or potential step mode was performed, sufficient time (low sweep rate means a prolonged anodization time) was needed for the formation of nanoscale grains, atomically flat surface and step-terrace structure on the anodized titanium film.     

Trace analyses of gaseous products formed during heat treatment of high stage H2SO4-GICs and expanded graphite

H₂SO₄-GICs samples prepared by different methods were heated to 1000 °C under flow of air using heating rates of 2, 10 and 20 K/min Gaseous products of decomposition were analysed using FTIR spectra recorded during the thermal process. It was detected that S–O species started evolving from graphite gallery above 500 °C, not depending on the method of GIC preparation. High expansion of H₂SO₄-GIC was observed when the intercalate evolved from the intercalated graphite before its abrupt oxidation to CO₂. The disordered structure of graphite layers in H₂SO₄-GIC leads to easy release of S–O species without exfoliation effect. It was observed that using 2 K/min heating rate the time needed for intercalate to release from graphite gallery is shorter than that needed for its exfoliation. Therefore, the exfoliation process should be conducted using high heating rate.     

Effect of electrodeposition modes on ruthenium oxide electrodes for supercapacitors

With developments in energy storage devices, supercapacitors are gaining more attraction because of their potential to excel batteries shortly. In this work, ruthenium oxide (RuO₂) has been deposited on stainless steel and studied the influence of surface modification of solid electrodes on capacitance properties. Hydrous ruthenium oxide was plated by different modes such as potential sweep method (cyclic voltammetric), constant potential method (chronoamperometry) and optimised potential pulse method using a recently reported precursor material namely ruthenium nitrosylsulfate (RuNS). The structural information and morphology of electrodeposits were characterised by X-ray diffractometer and scanning electron microscope respectively. The XRD studies indicate a poor crystalline state for RuO₂ in all the modes of deposition but can contribute to a higher surface area when compared to a highly crystalline form. The SEM analysis revealed the formation of surface modification concerning the change of potential mode. Mud-cracked morphology, spherical particles and dendrimeric morphology observed on chronoamperometry, potential pulse and cyclic voltammetry respectively. Electrochemical studies were also conducted on the samples to assess their performance for supercapacitor applications. The spherical particles of hydrous RuO₂ show high performance of capacitance behaviour 1180 F/g in 0.5 M H₂SO₄ at the scan rate of 5 mV/s. Dendrimeric morphology and mud-cracked morphology shows 573 F/g and 546 F/g respectively in same 0.5 M H₂SO₄ at the scan rate of 5 mV/s. The studies reveal that RuO₂ electrodes can be exploited for their outstanding capacitive behaviour by properly controlling the morphology of the deposits.     

Novel structure formation at the bottom surface of porous anodic alumina fabricated by single step anodization process

A simple approach for the growth of long-range highly ordered nanoporous anodic alumina film in H₂SO₄ electrolyte through a single step anodization without any additional pre-anodizing procedure is reported. Free-standing porous anodic alumina film of 180 μm thickness with through hole morphology was obtained. A simple and single step process was used for the detachment of alumina from aluminum substrate. The effect of anodizing conditions, such as anodizing voltage and time on the pore diameter and pore ordering is discussed. The metal/oxide and oxide/electrolyte interfaces were examined by high resolution scanning transmission electron microscope. The arrangement of pores on metal/oxide interface was well ordered with smaller diameters than that of the oxide/electrolyte interface. The inter-pore distance was larger in metal/oxide interface as compared to the oxide/electrolyte interface. The size of the ordered domain was found to depend strongly upon anodizing voltage and time.     

Pseudo-capacitance on exfoliated carbon fiber in sulfuric acid electrolyte

The specific capacitance of exfoliated carbon fibers (ExCF) which were synthesized from pitch-based carbon fibers showed a strong dependence with the concentration of sulfuric acid electrolyte and reached 1.4 F/m² in 18 M H₂SO₄ solution. Since the capacitance value is quite large compared with the case of conventional activated carbons, faradic reactions (charge transfer reactions) are the cause of pseudo-capacitance. ExCF, however, gave a featureless cyclic voltammogram in 18 M H₂SO₄ solution. In the case of exfoliated natural graphite, the intercalation of H₂SO₄ molecules is evidenced by redox peaks observed in the voltammograms in the same conditions. Therefore, a strong interaction between the H₂SO₄ molecules and the ExCF surface might be the reason for the origin of pseudo-capacitance with ExCF in H₂SO₄ electrolyte. PACS 81.05.Uw; 82.45.Fk; 82.47.Uv     

Synthesis of Y-junction carbon nanotubes within porous anodic aluminum oxide template

Y-junction carbon nanotubes with the average diameter about 200 nm were successfully synthesized within porous anodic aluminum oxide template, which was prepared by anodic anodizing aluminum sheet in 1.0 mol/l H₃PO₄ solution at a constant anodization voltage 90 V.     

Modification of multi-walled carbon nanotubes for electric double-layer capacitors: Tube opening and surface functionalization

Multi-walled carbon nanotubes (MWCNTs) obtained opening the closed ends and using surface functionalization by means of a combination of partial oxidation in air and chemical modifications are characterized systematically in 0.3 M H₂SO₄ between 0 and 1.0 V, and these nanotubes were planned to be used as electrode materials in electric double-layer capacitors (EDLCs). Opening of MWCNTs, clearly observed by means of transmission electron microscopy (TEM), can be easily achieved by the partial oxidation in air through a seven-step temperature program identified by thermogravimetric/differential thermal analyses (TG/DTA). An increase in 175% specific capacitance is obtained for the MWCNTs, partially oxidized in air and chemically modified in H₂SO₄+HNO₃. The temperature-programmed desorption (TPD) data showed that evolutions of CO and CO₂ are, respectively, promoted by the application of partial oxidation in air and chemical modification in H₂SO₄+HNO₃. The above increase in specific capacitance for modified MWCNTs is attributed to an obvious increase in the BET surface area (double-layer capacitance) and the density of oxygen-containing surface functional groups (pseudocapacitance).     

Electrochemical behaviour of electrodeposited Fe—8P amorphous alloys

Mössbauer spectroscopy was used in order to study the influence of electrochemical behaviour of electrochemically deposited Fe—8P amorphous alloy. Electrochemically deposited amorphous samples were exposed to corrosion in the solution of H₂SO₄ and Na₂SO₄ containing NaHSO₃ at pH=3.5. CEM spectra of as-deposited samples exhibit a typical sextet of ferromagnetic amorphous state. Even with samples being kept under corrosion condition for 10 minutes, the Mössbauer spectra reveal an extra Fe(III) component.     

Electrochemical synthesis and performance of PANI electrode material for electrochemical capacitor

Polyaniline (PANI) electrode materials doped with sulfuric acid (H₂SO₄) were prepared by cyclic voltammetry (CV) method in different reaction conditions. The structure and morphology of PANI samples were characterized by Fourier transform infrared spectroscopy and scanning electron microscope. The electrochemical properties of PANI samples were studied by CV, galvanostatic charge/discharge, and electrochemical impedance spectroscopy tests. Additionally, the effects of reaction conditions including aniline concentration, voltammetry scan rate, and deposition time on the morphology and properties of PANI samples were investigated in detail. The results showed that the PANI synthesized under the optimal conditions of 0.2 mol?L^?1 aniline, scan rate 20 mV?s^?1, and deposition time 50 min is in the form of nanorods with a cross-linked network structure. It exhibits an outstanding capacitive performance with good cycle stability and high rate performance. Besides, the specific capacitance of PANI is as high as 757 F?g^?1.     

Surface characterisation of ethylene-propylene-diene rubber upon exposure to aqueous acidic solution

Two types of pure ethylene propylene diene rubbers were exposed to two different acids for varying period of time. Surface characterisation was carried out using X-ray photoelectron spectroscopy (XPS). Two EPDM rubbers selected for this study were comparable in co-monomer compositions but significantly different with respect to molar mass and the presence of long chain branching. Both rubbers contained 5-ethylidene-2-norbornene (ENB) as diene. Solution cast films of pure EPDM samples were exposed in two different acidic solutions, viz. chromosulphuric (Cr (VI)/H₂SO₄) and sulphuric acid (H₂SO₄) (20%, v/v) at ambient temperature from 1 to 12 weeks. XPS analysis indicated that several oxygenated species were formed on the surface of both rubbers after exposure. It was postulated from the XPS analyses that both aqueous acidic solutions attacked the olefinic double bonds (CC) of ENB. Furthermore, 20% Cr (VI)/H₂SO₄ also attacked the allylic carbon-hydrogen (CH) bonds of ENB resulting in more oxygenated species on the surface compared to 20% H₂SO₄ under identical conditions. Cr (VI) in the 20% Cr (VI)/H₂SO₄ was found to play an important role in alteration of surface chemistry. Studies using a model system consisting of EPDM mixed with Cr (VI) and Cr (III) salts revealed that the change of oxidation state from Cr (VI) to Cr (III) as a consequence of direct involvement of Cr (VI) in the chemical alteration of EPDM surfaces. Interestingly, the presence of long chain branching and molar mass did not significantly influence the chemical processes owing to the acid treatment.