Experimental investigation of processing parameters and effects on chloralkali products in an electrolysis based chloralkali reactor

In the present study, the effects of different processing parameters on rate of hydrogen, chlorine and sodium hydroxide production are experimentally studied in a newly designed chloralkali process reactor. It is a multi membrane reactor which can also be coupled with a photochemical hydrogen production process. The parameters, which are studied in the present study, include applied voltage varied from 3 V to 20 V, brine concentration varied from 150 g/L to 225 g/L, electrolyte concentration in catholyte compartment varied from 35.3 g/L to 58.8 g/L, electrode height, temperature and different anode materials. A factorial design of experiments is applied and an analysis of variance (ANOVA) is used to analyze the experimental results. Energy and exergy efficiencies are also determined and discussed. The results show that graphite appears to be the most stable anode material. Brine and electrolyte concentration do not affect the production rate of products. Increasing the temperature increases the production but also increases the energy input to the system. Varying electrode height changes the current density which has a significant effect on rate of hydrogen production.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon-steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. II. Identification of potential fluctuation sources for unstressed electrodes

In this second part of the study of electrochemical noise (EN) generated by hydrogen evolution on a vertical cylindrical AISI 4140 steel electrode under galvanostatic control in 0.5 M sulphuric acid, the potential fluctuations induced by the growth and detachment of hydrogen bubbles at the electrode surface were analysed. They could be related to fluctuations of various quantities: electrode active surface due to bubble screening effects, concentration of dissolved hydrogen in the electrolyte close to the electrode surface, and metal-hydrogen interactions (MHI) on or beneath the electrode surface. The existence of MHI and their influence on faradaic potential fluctuations could be revealed by comparing the noise features on steel and platinum. The influence on EN of the charging cathodic current density, the presence of dissolved oxygen in the solution, and the electrode rotation speed was investigated in the absence of stress applied to the electrode. In the third paper of this series, the effect of hydrogen embrittlement on potential fluctuations of stressed electrodes will be examined.     

The Electrochemical Generation of Ozone: A Review

This article reviews work on the electrochemical generation of ozone from the original studies by Schönbein in the early 1800s to the present day, and is intended for specialists and nonspecialists in the field of electrochemistry. The experimental techniques employed to study the mechanism of electrochemical ozone generation are described, as is the most commonly quoted mechanism and the experimental evidence for the mechanism is summarized and discussed. The types of electrochemical cells employed are described, and the effects of temperature, anode type and composition, current density and electrolyte composition and pH are discussed.     

Electrochemical characterisation and oxygen evolution at a heavily boron doped diamond electrode

Characterisation of a commercial heavily doped BDD electrode demonstrated it contains a small sp² content, which on anodic potential scanning, is oxidised to CO/CO₂. This surface modification alters the electrode activity, increasing the overpotential for the hydrogen and oxygen evolution reactions (HER and OER). Ex situ and in situ investigations indicate film morphology is mainly composed of “chain of hills”, presenting relatively high differential capacitance values and morphology factor, which is attributed to the effect of surface states and high surface roughness of the BDD film. The voltammetric behaviour depends on the applied potential; the heavily doped BDD electrode behaving as a metallic electrode at more anodic potentials. Polarisation curves (potentiostatic (1 mV s⁻¹) or galvanostatic (point-by-point)), recorded at different temperatures and H₂SO₄ concentrations, lead to the same conclusions. The high Tafel coefficients and low apparent electronic transfer coefficient (α_A) are independent of overpotential and temperature but show a dependence on H₂SO₄ concentration. The linear relationship observed between the apparent electrochemical enthalpy of activation (ΔH^#_η) and overpotential supports α_A is constant. An OER mechanism was proposed taking into account the absence of adsorption sites at the BDD surface. The OER is inhibited, explaining the high overpotentials and elevated ΔH^#_η values.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. III. Influence of hydrogen absorption for stressed electrodes

In this third part of the study of the electrochemical noise generated by hydrogen evolution on cathodically-polarised AISI 4140 steel specimens in 0.5 M sulphuric acid, a tensile stress was applied to the specimens. The two components of the measured potential fluctuations ΔV, namely ohmic-drop fluctuations ΔR_eI and faradaic potential fluctuations ΔE, were analysed. After bubble size homogenisation, the effect of an applied stress was an increase in the level of the power spectral densities (PSD) Ψ_V and Ψ_E of the ΔV and ΔE fluctuations, indicating a change in metal-hydrogen-interactions related to damages in the metal induced by the combined actions of stress and hydrogen embrittlement. In the meanwhile, the PSD Ψ_ReI of the ohmic-drop fluctuations did not exhibit any change, revealing that the departure rate and size of hydrogen bubbles were not modified by the internal damages in the specimen. The time evolution of Ψ_E up to fracture could be explained by the enhancement of hydrogen penetration into the metal induced by the increase in the density of microdefects or crack advances inside the metal.     

Electrochemical discharges—Discovery and early applications

Electrochemical discharge phenomenon, as well known as plasma electrolysis, electrode effects or contact glow discharge electrolysis, was first described in literature more than 150 years ago. Today this effect is studied only in quite specialised applications. This was not always the case. During the 19th century and early 20th century, electrochemical discharges were intensively studied and a broad spectrum of applications, ranging from X-ray imaging to wireless telegraphy, was developed. This communication retraces this early history of electrochemical discharges by highlighting the interesting electrochemical effects and applications linked to them. The paper ends by shortly mentioning today's applications of electrochemical discharges which are in the field of micro-machining, surface engineering, nanotechnology and waste water treatment.     

Distortions in Electrochemical Impedance Spectroscopy Measurements Using 3‐Electrode Methods in SOFC. I – Effect of Cell Geometry

The use of a reference electrode (RE) is necessary to independently measure the overpotential of each electrode in solid oxide fuel cells (SOFC). This type of set‐up, known as the 3‐electrode (or 3‐terminal) configuration, can give erroneous results if the RE does not effectively separate the potential of the two active electrodes. In this work, calculations and experiments were performed to verify the effectiveness of the 3‐electrode configuration used in electrochemical impedance spectroscopy (EIS) measurements for studying the kinetics of anodes and cathodes in SOFC. Initially, a theoretical analysis of the impedance distortions in relation to the electrode geometry and configuration is presented and the main causes of distortions are elucidated. Then, this analysis is corroborated by experimental results obtained using two specially designed cells. Calculations and experiments reconfirm that configurations characterised by electrodes of equal area and symmetrical placement do not produce EIS distortions when the electrodes have similar area‐specific polarisation resistances and time constants. Moreover, distortions can be low even in considerably misaligned configurations when electrodes are small and relatively inactive.     

Electrochemical energy storage of Co powders in alkaline electrolyte

The electrochemical charge and discharge behavior of Co powders has been investigated by using X-ray diffraction (XRD), charge and discharge testing, and electrochemical impedance spectroscopy (EIS) at room temperature. Mechanical milling (MM) has been used to treat the Co powders for a comparative experiment. Mechanical milling induces a phase transition of fcc phase to hcp phase, an increase in particle size, and a decrease in grain size. The results of the XRD indicate a reversible reaction between Co and Co(OH)₂. The non-milled Co has a higher discharge capacity at a current density of 60 mA g⁻¹ as compared to the milled one. However, the milled Co presents a better HRD, in spite of the discrepancy in particle size. The results of EIS show that the electrochemical reaction process of Co powders consists of three steps, that is the charge-transfer of Co/Co(OH)₂ or Co/CoH_x, the mass-transfer of HCoO₂⁻, and hydrogen diffusion within Co, depending on the depth of discharge.     

Electrochemical noise analysis of the corrosion of AZ91D magnesium alloy in alkaline chloride solution

The corrosion behavior of AZ91D magnesium alloy in alkaline chloride solution was investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and power spectral density (PSD), had been employed to analyze the EN data. It was revealed that there exist three different stages of corrosion for AZ91D magnesium alloy in alkaline chloride solution, including an anodic dissolution process companying with the growth, absorption and desorption of hydrogen bubbles, a development of pitting corrosion, an inhibition process by protective MgH₂ film. The results demonstrated that energy distribution plot (EDP) was a powerful tool to provide useful information about the dominant process for the different corrosion stages.     

Distortions in Electrochemical Impedance Spectroscopy Measurements Using 3‐Electrode Methods in SOFC. II. Effect of Electrode Activity and Relaxation Times

The 3‐electrode configuration is commonly applied to quantify the overpotential of anodes or cathodes in solid‐oxide fuel cells (SOFC). In this type of set‐up, a reference electrode (RE) is used to isolate the potential loss of one electrode from that of the entire cell; however, erroneous results can be obtained whenever the RE does not precisely separate the potential drop between the two active electrodes. In this study, we present the results of a theoretical and experimental analysis focused on verifying the effectiveness of the 3‐electrode configuration in electrochemical impedance spectroscopy measurements for the kinetic characterisation of SOFC electrode reactions. The focus of this paper is on the distortion of impedance measurements caused by differences in the area‐specific polarisation resistance and impedance time constants of the working and counter electrodes. The results obtained numerically and experimentally, both for planar and tubular SOFC cell geometries, prove the reliability of the theoretical model used. From the systematic simulation presented here and in our previous work, it was possible to formulate general guidelines for the design of 3‐electrode experimental SOFC. The theoretical model used here can also be used to verify the consistency of EIS measurements obtained with thin planar cells.     

Electrochemical generation of hydrogen peroxide using surface area-enhanced Ti-mesh electrodes

Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 °C. The Ti-coated mesh electrode has a BET surface area of 3.5 m²/g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.     

Electrochemical investigation of the effects of hydrogen on the stability of the passive film on iron

The effects of hydrogen on the stability of passive films on iron were investigated by electrochemical methods: open circuit potential decay, cathodic galvanostatic reduction, electrochemical impedance spectroscopy, and breakdown potential measurements. The results show that hydrogen decreases the final static open circuit potential, the cathodic charge for reduction and the charge transfer resistance of the passive film, indicating that hydrogen decreases the stability of the passive film. The charge transfer resistance of the passive film formed on the charged specimen does not change with increasing the film formation potentials, suggesting that increasing film formation potentials under hydrogen charging conditions cannot improve the stability of the passive film. Hydrogen decreases the breakdown potential of the passive film, especially at lower chloride ion concentrations, confirming that hydrogen promotes the susceptibility of the passive film on iron to pitting corrosion. The reasons why hydrogen decreases the stability of the passive film were discussed.     

煤电化学气化法制氢的动力学研究

研究了煤电化学气化法由水制氢的反应机理,实验验证了非电化学反应4Fe~(3 ) C 2H_2O→4Fe~(2 ) 4H~ CO_2是存在的。同时,通过实验得到了过程的动力学模型或     

Photobiocatalytic hydrogen production in a photoelectrochemical cell

In this study the photobiocatalytic hydrogen production system was photoelectrochemically examined. Preliminary experiments with a mixed slurry system revealed the following five facts: direct, inter-phase, electron transfer from photocatalyst to enzyme in the absence of electron relay was a rate-determining step; the enzyme was deactivated by irradiated light (half after 10 days); physical properties (crystallinity, specific surface area, pore volume and radius) of the photocatalysts, obtained by two preparation methods (sol-gel and hydrothermal), were seldom correlated with the production rate; chemical properties were postulated to have been affected; and each material needed different reaction conditions such as pH and temperature. For the later salt-bridged system, the I–V relation for each prepared photocatalyst was measured (V _fb in the range of ca. −0.9∼−1.0 and V_∞ close to −1.1 V vs. Ag/AgCl in saturated KCl). The change in the amount of evolved hydrogen was also independently checked according to external bias (critical bias around 1.0 V). Then both reaction components were connected through a salt bridge to initiate light-induced hydrogen production for the hybrid system. The H₂ production exhibited an exponential trend and the Tris-HCl buffer showed the highest rate. A feasible reaction pathway was proposed.     

Electrochemical regeneration of activated carbon loaded with p-nitrophenol in a fluidized electrochemical reactor

This paper described a novel electrochemical process for the regeneration of activated carbon (AC) loaded with p-nitrophenol (PNP), aiming to reduce regeneration time and improve cost-effectiveness of the process by adoption of a novel non-active anode of modified lead dioxide and operation of AC in a fluidized mode. The regeneration parameters such as current density, liquid flow rate, NaCl concentration, pH of the solution and regeneration time were systematically investigated. Under the optimum conditions, the regeneration efficiency of AC could reach 90% in 1.5 h, and no significant declination was observed after five-times continuous adsorption-regeneration cycles, confirming the reuse feasibility of the regenerated AC. The adsorption of organic pollutants was confirmed occurring in the micropore of AC, and AC regeneration was mainly due to the decomposition of organics by the attack of active species such as hydroxyl radical that were generated by electrochemical oxidation. The time-space production for AC regeneration has been greatly improved in the present modified process, indicating this regeneration process is much more potentially cost-effective for application.     

Electrochemical applications of net-benefit analysis via Bayesian probabilities

By means of three specific applications to electrochemical science, this paper demonstrates the usefulness of the net-benefit principle and Bayesian (posterior) probabilities in deciding whether equipment in an electrochemical laboratory or plant should be repaired or replaced.     

Structural and electrochemical properties of mixed calcium-zinc spinel ferrites nanoparticles

In the current work, we provide the electrochemical (EC) characteristics and considerable size of Ca-doped ZnFe₂O₄ nanoparticles. Mixed transition metal oxides are widely used as excellent electrode materials in superior supercapacitors because of their superior capacitance, low cost, and environmental friendliness. The prepared nanoparticles were characterized by X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and EC methods. The results exhibited that the as-synthesized nanoparticles had a cubic spinel crystal structure and efficient EC properties. The EC properties of the prepared electrodes were explored by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) studies. The Ca_0.1Zn_0.9Fe₂O₄ electrode demonstrated a specific capacitance (SC) ～208 Fg^-1 at a 2 mV/s scan rate due to significant morphological behavior. Therefore may be the prepared materials are the finest electrodes for supercapacitor applications.     

Electrochemical mass-flow control of hydrogen using a fullerene-based proton conductor

A membrane electrode assembly (MEA) was fabricated using proton conductive hydrogensulfated fullerenol (C₆₀(OSO₃H)_n(OH)_n). Rate-controlled mass flow of hydrogen was performed by applying voltage to both electrodes of the MEA without humidification. The amount of the electrochemically transported hydrogen through the MEA increased as the applied current increased, obeying Faraday's law. Residual gas analysis of the transported hydrogen showed that the transported hydrogen contains trace amounts of water less than 1%.     

Mass transport characterization of a novel gas sparged photoelectrochemical reactor

The photoelectrochemical treatment of waste water using immobilised TiO₂ electrodes has been demonstrated to be an attractive alternative to TiO₂ slurry reactors; however, it is generally believed that the diffusion of species to the surface of the catalyst imposes severe mass transfer limitations and hence is a disadvantage of the photoelectrochemical approach. To challenge this view, this paper reports the characterization of the mass transport properties of a novel gas sparged photoelectrochemical reactor. It is shown that passing a constant stream of nitrogen gas through the reactor increased the mass transfer coefficient by an order of magnitude above that in the absence of gas and this was attributed to the turbulent flow regime imposed by rising gas bubbles. It is also demonstrated that the gas–liquid transfer coefficient was greater than that for the rate of liquid diffusion and this has important implications for heterogeneous processes.