Neutral Sodium Sulphate Research Articles

Cerium-based metal–organic framework-conducting polymer nanocomposites for supercapacitors

Nanocomposites consisting of a cerium-based metal–organic framework (Ce-MOF-808) and a conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), are synthesized by performing the pulse electrodeposition of PEDOT within the Ce-MOF-808 thin films. Ratios between MOF and PEDOT in the composites are tunable by simply adjusting the charge density used for electropolymerization, and the crystallinity, morphology, and elemental distributions of the resulting nanocomposites are characterized. Electrochemical behaviors and capacitive performances of the pristine MOF, pristine PEDOT thin films, and composite thin films are investigated with the use of the neutral sodium sulfate aqueous electrolytes. The reversible electrochemical reactivity of the highly porous Ce-MOF-808 provides a pseudocapacitance, and the electronically conducting PEDOT can not only offer a remarkable double-layer capacitance but also facilitate the electronic conduction between the redox-active cerium sites present in the MOF. As a result, the composite can outperform both the pristine MOF and pristine electrodeposited PEDOT as the active materials for supercapacitors. Findings here suggest that the highly porous and redox-active cerium-based MOF thin films are capable to enhance the capacitive performances of the electrodeposited PEDOT thin films.

High-capacity Bi2O3 anode for 2.4 V neutral aqueous sodium-ion battery-supercapacitor hybrid device through phase conversion mechanism

Aqueous battery-supercapacitor hybrid devices (BSHs) are of great importance to enrich electrochemical energy storage systems with both high energy and power densities. However, further improvement of BSHs in aqueous electrolytes is greatly hampered by operating voltage and capacity limits. Different from the conventional intercalation/de-intercalation mechanism, Bi2O3 implements charge storage by a reversible phase conversion mechanism. Herein, taking Bi2O3 electrode with wide potential window (from −1.2 to 1 V vs. saturated calomel electrode) and high capacity as battery-type anode, we propose that the overall performance of aqueous BSHs can be greatly upgraded under neutral condition. By paring with stable layer-structured δ-MnO2 cathode, a sodium-ion Bi2O3//MnO2 BSH with an ultrahigh voltage of 2.4 V in neutral sodium sulfate electrolyte is developed for the first time. This hybrid device exhibits high capacity (~215 C g−1 at 1 mA cm−2), relatively long lifespan (~77.2% capacity retention after 1500 cycles), remarkable energy density (71.7 Wh kg−1@400.5 W kg−1) and power density (3204.3 W kg−1@18.8 Wh kg−1). Electrochemical measurements combining a set of spectroscopic techniques reveal the reversible phase conversion between bismuth oxide and metallic bismuth (Bi2O3⇋ Bi0) through Bi2+ transition phase in neutral sodium sulfate solution, which can deliver multielectron transfer up to 6, leading to the high-energy BSHs. Our work sheds light on the feasibility of using Bi2O3 electrode under neutral condition to address the issue of narrow voltage and low capacity for aqueous BSHs.

Oxygen reduction reaction kinetics on pure copper in neutral sodium sulfate solution

Oxygen reduction reaction (ORR) kinetics were investigated on different copper surface conditions in 10 – 2 M Na2SO4 solution. On pre-reduced and passivated copper surfaces, the ORR is suggested to occur by a parallel pathway involving HO2− formation followed by its diffusion to the solution or rapid reduction to OH−. A significant difference in ORR activity is observed in the kinetic and mixed control regions as copper surface becomes more oxidized. On a surface covered by ammonium pyrrolidine dithiocarbamate (PDTC), ORR rate is inhibited due to the adsorption of PDTC and formation of a protective film, which limits O2 deposition on copper surface regardless of pH values and oxygen concentration in the solution.

Dinitrosyl iron complexes: From molecular electrocatalysts to electrodeposited‐film electrodes for hydrogen evolution reaction

AbstractClean and large‐scale production of hydrogen via water splitting triggered by active, robust, and low‐cost electrocatalysts is a promising and sustainable strategy for energy conversion and storage. In this study, a series of four‐coordinated chelating amine‐bound {Fe(NO)2}10 dinitrosyl iron complexes (DNICs) [(L)Fe(NO)2] were synthesized to investigate how the electronic structure of [Fe(NO)2] unit of DNICs was tailored to promote the electrocatalytic hydrogen evolution reaction (HER) triggered by the homogeneous DNICs' molecular catalysts and the heterogeneous DNIC‐derived electrodeposited‐film electrodes. The electrochemical studies demonstrate that HER onset potentials of those DNICs in neutral sodium sulfate aqueous solution are dependent on their IR ν(NO) stretching frequencies, indicating that the electron‐rich [Fe(NO)2] core modulated by the synergistic cooperation of the electron‐donating ability and steric effect of methyl‐/hydrogen‐substituted diamine‐coordinated ligands, presumably, benefits the formation of metal‐hydride intermediate to reduce the required onset potential. In contrast with homogeneous catalyst retaining its molecular integrity during the catalytic HER process, it is noticed that DNICs [(L)Fe(NO)2] act as the precursor of the active heterogeneous HER catalyst during the electrocatalytic HER process. It is presumed that the intermolecular hydrogen‐bonding interactions among DNICs [(L)Fe(NO)2] may control the particle sizes of DNIC‐derived electrodeposited film to modulate HER efficiency.

Effects of monomer solvent on the supercapacitance performance of PANI/TiO2 nanotube arrays composite electrode

Polyaniline/TiO2 nanotube arrays composite electrodes were achieved by depositing polyaniline onto TiO2 nanotube arrays through cyclic voltammetry method. Neutral sodium sulfate aqueous solution was used as a testing electrolyte and the effects of monomer solvent on the electrode performance were studied. Results show that the sample prepared in ethanol solution has much better capacitance performance than that in acetone or aqueous solutions. The potential window of the electrode could be enlarged to 2 V (−0.2–1.8 V) and the highest energy density of the active material reached 727.3 J/g. The composite electrode also has excellent rate and cycling performance.

Mechanically activated carbonized rayon fibers as an electrochemical supercapacitor in aqueous solutions

The activated carbon cloth (ACC), obtained by chemical/physical activation of carbonized rayon fibers, was grinded in a ball mill and studied from the aspect of double layer capacitance. The changes in pore structure, morphology and acid/basic properties caused by ball milling were studied by means of N2 adsorption/desorption, Fourier-transformed infrared spectrometry, Boehm’s titration and Scanning Electron Microscopy. Both potentiodynamic and galvanostatic cycling were used to evaluate the double layer capacitance in three alkaline, acidic and neutral aqueous solutions (KOH, H2SO4 and Na2SO4). While double layer capacitance of original ACC was found to be negligible, ball milled material (ACCm) displayed capacitance in the range of supercapacitors. In order to explain this huge capacitance improvement, we found that ball milling substantially increased the concentration of lactone, phenolic and quinone groups on the surface. We suggest that these groups, through improved hydrophilicity, enable faster ion diffusion into carbon micropores. The energy density stored by double layer was highest in neutral sodium sulphate solution. Namely, operational voltage of ∼2V and double layer capacitance of 220Fg−1 at 1A g−1, enable the energy density of ACCm/Na2SO4/ACCm capacitor of 31.7 Wh kg−1 at 2000Wkg−1, much higher than that of commercial EDLC carbon capacitors. According to the here presented literature survey in a tabular form, the energy density of the studied sample is also higher from that of numerous thus far published aqueous carbon capacitors.

Pathways for Hydrogen Absorption into Zircaloy-2 under Cathodic Polarization Assessed by Scanning Electrochemical Microscopy, Scanning Electron Microscopy, and Electrochemical Impedance Spectroscopy

Hydrogen absorption by Zircaloy-2 has been studied in neutral sodium sulfate solutions using steady-state polarization measurements, electrochemical impedance spectroscopy (EIS), and scanning electrochemical microscopy (SECM). For applied potentials < −1 V (vs. saturated calomel electrode, SCE), water reduction occurs in faults in the passive oxide covering the alloy. For potentials ≤−1.3 VSCE, SECM detects a distribution of reactive locations on the electrode surface. Matched SECM and SEM images of the same electrode surface show more reactive sites to be located in the β-phase grain boundaries than on the α-grains. The reactivity of surface locations was determined using SECM probe approach curves. Secondary phase particles incorporating impurities such as Fe, Ni, and Cr as Zr(Fe, Cr)2 and Zr2(Fe, Ni) are particularly reactive spots, which may act as “windows” for hydrogen absorption into the alloy. This claim is supported by the observation that the surface of the α-grains remains passive even at potentials as low as −2.0 VSCE. The need to include a Warburg impedance element in modeling EIS recorded at potentials ≥−1.3 VSCE suggests that H2O reduction is confined to tight flaws in the oxide at grain boundary locations.

Anodic Determination of Acetylsalicylic Acid at a Mildly Oxidized Boron-Doped Diamond Electrode in Sodium Sulphate Medium

Differential pulse voltammetry (DPV) and chronoamperometry (CA) were used to detect and determine acetylsalicylic acid (ASA) at a mildly oxidized boron-doped diamond (BDD) electrode in a neutral sodium sulphate solution as supporting electrolyte. ASA determination in unbuffered medium was achieved using neutralized standard and real samples. Over the concentration range of 0.01 mM–0.1 mM, linear calibration plots of anodic current peaks in DPV and anodic currents in CA experiments versus concentration were obtained with very high correlation coefficients and good sensitivity values. The limits of detection were situated around 1 μM. The association of DPV and CA techniques with standard addition method represented a suitable option for the determination of ASA in real samples such as pharmaceutical formulations.

The Mechanism of Oxide Film Formation on AISI 316 Stainless Steel in Sulphate Solutions

Oxide film formation on AISI 316 stainless steel (a material often used for building nuclear reprocessing plants) in acidic and neutral sodium sulphate solutions is investigated using electrochemical and surface analysis techniques. It is found that the corrosion potential varies with pH in the amount of -55 mV / pH unit and critical currents increase with decreasing pH and increasing temperature in neutral solutions. The activation energy for the reaction in the active region is 45.6 kJ mol-1. Potentiostatic experiments indicate that three different film building processes are involved in film formation: the initial active-passive transition, the logarithmic growth of the film and the parabolic growth of the film. AES shows chromium enrichment at the surface, and ESCA gives little difference for the relative amount of Fe2+ and Fe3+ for 0.2 and 0.6 V. A theoretical model is being developed to account for the observed kinetics.

Effect of Zn and Pb contents on the electrochemical behavior of brass alloys in chloride-free neutral sulfate solutions

Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of brass alloys with various Zn contents (5.5–38.0 mass%) and Cu–38.0Zn–Pb alloy with different Pb contents (1.0–3.4 mass%) in neutral sodium sulfate solutions. The influence of working conditions, e.g., immersion time, sulfate ions concentration and temperature on the electrochemical behavior of the different alloys was also studied. It was found that the initial corrosion rate is relatively high for alloys with the higher zinc content due to dezincification. The dezincification process initiates by selective dissolution of zinc and continues by a simultaneous dissolution of copper and zinc followed by re-deposition of copper. An increase in the lead content and immersion time in the sodium sulfate solution increases the corrosion resistance of the alloy and improves its stability. The stability of the leaded brass was considered to be due to the formation of an insoluble film of lead sulfate on its surface. The impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface. The mechanism of the alloy dissolution was discussed in view of the obtained results.

Electrolytic pickling of stainless steel studied by electrochemical polarisation and DC resistance measurements combined with surface analysis

A tightly adhering oxide scale is formed on stainless steels when they are annealed. The removal of the oxide scale and chromium-depleted subscale is one of the most important processes during the stainless steel production. Electrolytic pickling in neutral sodium sulphate is widely used for oxide scale removal. This study describes the different stages of the oxide scale removal on Polarit 725 (EN 1.4301) stainless steel in sodium sulphate solution. A mechanism of the scale dissolution is also proposed. The dissolution is proposed to proceed by the electrochemical reactions of the scale in three successive stages. At the beginning of the pickling process chromium and manganese of the outer oxide layer were preferentially dissolved. When the chromium content of the outer layer decreased, the scale was enriched of iron. The electrode potential was then increased and the scale thickness greatly reduced. Finally a steady state was obtained and a thin oxide layer, rich in iron and silicon, was left on the surface. Silicon could not be removed by the electrolytic pickling and post-treatment in nitric–hydrofluoric acid is required.

Mechanism of electrolytic pickling of stainless steels in a neutral sodium sulphate solution

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Technical Note:Localized Corrosion of Pure and Sulfur-Doped Nickel in Neutral Sodium Sulfate Solution

Abstract The localized corrosion of pure and sulfur-doped Ni has been investigated in 0.25 M Na2SO4 (pH 6.5), both with and without addition of chloride ions, at 25 C. Addition of more than 500 ppm l− to Na2SO4 solution is found to cause pitting of annealed pure Ni. Auger electron spectroscopy (AES) examination of the fracture surface of nickel previously exposed to SO2 at 800 for 10 min showed that sulfur had penetrated along the grain boundaries by ∼100 µm. Grain boundary sulfur coverage of more than about 0.01 monolayer causes pitting, with induction time and pitting potential independent of increasing sulfur coverage, in 0.25 M Na2SO4 (pH 6.5), both with and without l−. The diffusivity of sulfur along grain boundaries in Ni at 800 is calculated to be ∼4.6×10−10m2/S.

Characterization of n-AgIn 5Se 8 polycrystalline semiconductor electrodes by photoelectrochemical techniques

Photoelectrochemical techniques were employed to characterise the properties of polycrystalline n-AgIn 5Se 8 samples in aqueous solutions. Flatband potential, donor density, depletion layer width, majority carrier mobility, minority carrier diffusion length and absorption coefficient were determined and discussed in connection with the semiconductor performance. The material stability was examined under reverse bias both in the dark and under illumination. The electrode behaviour was determined for both iodide-polyiodide and neutral sodium sulphate electrolytes. The iodide-polyiodide electrolyte was found to be the most suitable for the formation of liquid junction, but did not stabilise the material during extended illumination and resulted in the growth of a selenium overlayer. The dark corrosion process in the sulphate solution is discussed with reference to the AgIn 5Se 8 electron energy band diagram and to the decomposition potential of possible bond-cleavage reactions.

Interpretation of low frequency ac impedance data for organic coatings on mild steel

Abstract A polyamide cured epoxy lacquer and a microporous polycarbonate membrane were examined on a mild steel substrate when immersed in neutral sodium sulphate and sodium chloride solutions, using an ac impedance method. The process of pore blocking and unblocking is seen to be reflected in changes in the low frequency impedance response.

Photopotentials and the corrosion of mild steel in sodium sulphate solution

Experiments were carried out to observe the effect of light on the photopotential of mild steel in a neutral sodium sulphate solution. It has been shown that the effect is a function of the presence of corrosion products on the specimen. The sign of the photopotential is thought to be related to the semi-conducting properties of the surface layers.

Zur kinetik der anodischen eisenauflösung im neutralen medium

The kinetics of iron dissolution in acidic and neutral sodium sulphate solutions have been investigated, using stationary and non-stationary galvanostatic techniques. The experimental results in acidic solution agree with a consecutive mechanism proposed by Bockris, assuming adsorption of (FeOH) ads following a Langmuir type isotherm. Such a dissolution mechanism is to be expected with the used material (low surface activity). For pH values higher than about five, stationary Tafel slopes increase to + ≃ + 65 mV, whereas the electrochemical reaction order with respect to pH decreases. These results also agree with the consecutive (Bockris-) dissolution mechanism, if for the adsorption of (FeOH) ads a Frumkin type isotherm is assumed.

Electrolytic pickling process and its effect on the physical properties of iron and steel

A tightly adhering oxide scale is formed on stainless steels when they are annealed. The removal of the oxide scale and chromium-depleted subscale is one of the most important processes during the stainless steel production. Electrolytic pickling in neutral sodium sulphate is widely used for oxide scale removal. This study describes the different stages of the oxide scale removal on Polarit 725 (EN 1.4301) stainless steel in sodium sulphate solution. A mechanism of the scale dissolution is also proposed. The dissolution is proposed to proceed by the electrochemical reactions of the scale in three successive stages. At the beginning of the pickling process chromium and manganese of the outer oxide layer were preferentially dissolved. When the chromium content of the outer layer decreased, the scale was enriched of iron. The electrode potential was then increased and the scale thickness greatly reduced. Finally a steady state was obtained and a thin oxide layer, rich in iron and silicon, was left on the surface. Silicon could not be removed by the electrolytic pickling and post-treatment in nitric–hydrofluoric acid is required.