Anomalous currents under cyclic polarization of magnetite electrode in acidic medium

An anomalous phenomenon was observed in acidic solutions in cyclic voltammetric curves in the studies of electrochemical behavior of magnetite. It consisted in appearance of the reverse current: a cathodic response under the anodic potential sweep. It was shown that the direct and reverse currents follow the same regularities. Analysis of experimental data allows concluding that both currents correspond to the same process: Fe₃O₄ + 8H⁺ + 2e ?? 3Fe _aq ²⁺ + 4H₂O. Appearance of anomalous currents on polarization curves of magnetite occurs if the following conditions are observed: irreversibility of the given electrochemical reaction under the given mode experimental parameters; incompleteness of reaction at the cathodic halfcycle stage; layer-by-layer dissolution when the electrode surface is continually refreshed.     

A Novel Electrode for Value-Generating Anode Reactions in Water Electrolyzers at Industrial Current Densities

Hydrogen generated in electrolyzers is discussed as a key element in future energy scenarios, but oxygen evolution as the standard anode reaction is a complex multi-step reaction requiring a high overpotential. At the same time,it does not add value-the oxygen is typically released into the atmosphere. Alternative anode reactions which can proceed at similar current densities as the hydrogen evolution are, therefore, of highest interest. We have discovered a high-performance electrode based on earth-abundant elements synthesized in the presence of H₂O₂, which is able to sustain current densities of close to 1 A cm⁻² for the oxidation of many organic molecules, which are partly needed at high production volumes. Such anode reactions could generate additional revenue streams, which help to solve one of the most important problems in the transition to renewable energy systems, i.e. the cost of hydrogen electrolysis.     

Die kathodische Reduktion von Ozon in alkalischen Elektrolyten

The cathodic reduction of ozone according to the overall reaction O₃+H₂O+2e→O₂+2OH^? was studied on bright platinum electrodes in KOH electrolytes. The rest potentials deviate from the theoretical values by ?300 to ?350 mV. They are determined by a mixed potential mechanism involving anodic evolution of O₂ and cathodic reduction of O₃ as half reactions. Steady-state polarization measurements were carried out. Extrapolation of Tafel-lines to zero over-voltage and the determination of the charge transfer resistance give current densities at the rest potential, which are analogous to exchange current densities. A single electron transfer reaction is found to be the rate controlling step, which is occurring twice for the reduction of one molecule of ozone. A cathodic reaction order of approximately zero is evaluated with respect to OH^?-ion concentration. The reaction mechanism is proposed according to $$\begin{gathered} O_3 + e \to O_3 - / \cdot 2 \hfill \\ 2O_3 - + H_2 O \to 2 OH - + O_2 + O_3 \hfill \\ \end{gathered} $$ which is consistent with experimental data.     

Adsorption and electrooxidation of dimethyl ether on platinized platinum electrode in sulfuric acid solution

Adsorption and oxidation of dimethyl ether (DME) on the Pt/Pt electrode from 0.5 M H₂SO₄ is studied by measuring transients of current and potential, charging curves, and curves of electrooxidation in the adsorbed layer and also by cyclic voltammetry and steady-state polarization measurements. The DME adsorption is accompanied by dehydrogenation and destruction of its molecules to form a chemisorbed adsorbate that mainly consists of C1 species (HCO_ads and/or CO_ads) with (under certain conditions) a small amount of species that desorb at cathodic polarization. The adsorption and electrooxidation of DME are inhibited by adsorbed oxygen. The possible schemes of DME oxidation, where the reaction of DME chemisorption products with adsorbed oxygen-containing species is the limiting stage, are discussed.     

A study of the cathodic reduction of ozone at the rotating disc electrode in acid electrolyte

The cathodic reduction of ozone according to O₃+2H⁺+2e^?→H₂O+O₂ was studied at a rotating disc electrode consisting of bright platinum in 1 M HClO₄ at 20°C. The current-potential curves obtained potentiostatically are determined by slow charge transfer in the range of low polarization (Tafel region) and diffusion as rate controlling step at high overvoltage. From the exchange current densities (i₀), obtained by extrapolation of Tafel lines to the reversible potentials, the heterogeneous rate constant k was evaluated giving an average value of 2.5×10^?7 cm s^?1. Likewise the diffusion coefficient (D_O3=1.7×10^?5 cm² s^?1) was calculated from the limiting currents measured at various partial pressures of ozone and different values of rotation rate. An equation for the total current-polarization curve was developed. The computed curves are in good agreement with the data which were obtained experimentally.     

Electrochemical behavior of acidic acetonitrile solutions at platinum and gold electrodes

Hydrogen evolution reactions from acidic MeCN solutions were studied at Pt and Au electrodes. The acids studied were HCl and HPic in a wide range of concentrations. LiClO₄ and Et₄NClO₄ (0.4 M) were used as supporting electrolytes. Quasi steady state polarization curves show that the reaction is irreversible with a Tafel slope of 2 RT/F after diffusion polarization and pseudo-ohmic drop have been corrected. Non-stationary techniques reveal that the electrochemical reaction is preceded by a chemical reaction attributed to slow dissociation of ACN·2 HCl and ACN·3 HPic species. Equilibria and chemical rate constants for both species were evaluated by chronopotentiometry. The values obtained explain satisfactorily the results accomplished under potentiodynamic conditions. Data at the RDE confirm the existence of a previous chemical reaction. MeCN electroadsorption in the double layer potential region at Pt accounts for the solvent levelling effect towards H adsorption between Pt and Au. Furthermore, an inhibitory effect over hydrogen evolution and hydrogen oxidation is reported at more negative potentials at both Pt and Au electrodes.     

Hydrogen diffusivity in the massive LaNi5 electrode using voltammetry technique

The Randles–Sev?ik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi₅ intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 °C), and then voltammetry measurements were carried out at various, very slow potential scan rates (υ?=?0.01–0.1 mV?·?s^?1). At potentials more noble than the equilibrium potential of the H₂O/H₂ system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor α?=?½. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I _a ^(peak) ?=?f(υ ^1/2) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi₅ material after cathodic exposure to be C _0,H?=?0.071 mol?·?cm^?3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D _H?=?2.0?·?10^?9 cm²s^?1. Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H₂O/H₂ system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi₅ alloy are close to 1 mA?·?cm^?2, irrespective of the electrode potential scan rate.     

Hydride formation at Ni-containing glassy-metal electrodes during the H2 evolution reaction in alkaline solutions

A group of Ni-containing glassy metal alloys that are of interest as electrocatalysts for cathodic hydrogen production in water electrolysis have been examined with regard to the role of hydride formation and the participation of sorbed H during cathodic H₂ evolution in alkaline solutions. Hydride formation is important as it may determine, in part, the electrocatalytic properties of the alloys at their surfaces, for cathodic H₂ evolution. Characteristic time-dependences of currents observed near, but negative to the H₂ reversible potential in response to a stepwise change of potential, together with potential-relaxation measurements on open-circuit following current interruption, lead to the conclusion that a three-dimensional hydride is formed in the near-surface region of the metals during the H₂ evolution process. Complementary cyclic-voltammetry experiments show that hydrogen deposition-absorption, or H oxidation processes, can be distinguished from possible spontaneous oxidation of the glassy metals, over somewhat overlapping potential regions.     

Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles

Nanoparticles of cobalt phosphide, CoP, have been prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER) under strongly acidic conditions (0.50 M H₂SO₄, pH 0.3). Uniform, multi‐faceted CoP nanoparticles were synthesized by reacting Co nanoparticles with trioctylphosphine. Electrodes comprised of CoP nanoparticles on a Ti support (2 mg cm^?2 mass loading) produced a cathodic current density of 20 mA cm^?2 at an overpotential of ?85 mV. The CoP/Ti electrodes were stable over 24 h of sustained hydrogen production in 0.50 M H₂SO₄. The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long‐term viability under operating conditions. CoP is therefore amongst the most active, acid‐stable, earth‐abundant HER electrocatalysts reported to date.     

Influence of chemisorbed carbon monoxide on hydrogen evolution at smooth platinum electrodes in sulfuric acid solution

Cathodic current—potential curves were measured potentiostatically on two smooth platinum electrodes of different shape in 0.5 M H₂SO₄ at room temperature in the presence of chemisorbed carbon monoxide. Coverages of carbon monoxide between 0 and 0.6 lead to a relatively small decrease of the current density at constant hydrogen overvoltage. Mass transport processes remain largely rate-controlling. A rapid decrease of current follows, with a further increase of the carbon monoxide coverage. A transition from mass transport to kinetic control takes place. The rapid decrease is followed by a slower decrease at coverages above 0.85. This leveling-off results from hydrogen evolution on top of the chemisorbed layer of carbon monoxide. The retarding effect of chemisorbed carbon monoxide on hydrogen evolution is compared with the similar effect on hydrogen dissolution for the same electrodes.     

Boosting electrochemical hydrogen evolution by coupling anodically oxidative dehydrogenation of benzylamine to benzonitrile

The electricity-driven water splitting acts as a promising pathway for renewable energy conversion and storage, yet anodic oxygen evolution reaction (OER) largely hinders its efficiency. Seeking the alternatives to OER exhibits the competitive advance to address this predicament. In this work, we show a more thermodynamically and kinetically favorable reaction, electrochemical oxidative dehydrogenation (EODH) of benzylamine to replace the conventional OER, catalyzed by a cobalt cyclotetraphosphate (Co₂P₄O₁₂) nanorods catalyst grown on nickel foam. This anodic reaction lowers the electricity input of 317 mV toward the desired current density of 100 mA/cm², together with a highly selective benzonitrile product of more than 97%. More specifically, when coupling it with cathodic hydrogen evolution reaction (HER), the proposed HER||benzylamine-EODH configuration only requires a cell voltage of 1.47 V@100 mA/cm², exhibiting an energy-saving up to 17% relative to conventional water splitting, as well as the near unit selectivity toward cathodic H₂ and anodic benzonitrile products.     

Recent research progress of bimetallic phosphides-based nanomaterials as cocatalyst for photocatalytic hydrogen evolution

Hydrogen energy（H2） has been considered as the most possible consummate candidates for replacing the traditional fossil fuels because of its higher combustion heat value and lower environmental pollution.Photocatalytic hydrogen evolution（PHE） from water splitting based on semiconductors is a promising technology towards converting solar energy into sustainable H2fuel evolution. Developing high-activity and abundant source semiconductor materials is particularly important to realize highly effici...     

Electrochemical studies on Zn deposition and dissolution in sulphate electrolyte

The electrochemical deposition and dissolution of Zn on Pt electrode in sulphate electrolyte was investigated by electrochemical methods in an attempt to contribute to the better understanding of the more complex Zn–Cr alloy electrodeposition process. A decrease of the Zn electrolyte pH (from 5.4 to 1.0) so as to minimise/avoid the formation of hydroxo-products of Cr in the electrolyte for deposition of alloy coatings decreases the current efficiency for the Zn reaction, but the rate of the cathode reaction increases significantly due to intense hydrogen evolution. The results of the investigations in Zn electrolytes with pH 1.0–1.6 indicate that Zn bulk deposition is preceded by hydrogen evolution, stepwise Zn underpotential deposition (UPD) and formation of a Zn–Pt alloy. Hydrogen evolution from H₂O starts in the potential range of Zn bulk deposition. Data obtained from the electrochemical quartz crystal microbalance (EQCM) measurements support the assumption that electrochemical deposition of Zn proceeds at potentials more positive than the reversible potential of Zn. Anodic potentiodynamic curves for galvanostatically and potentiostatically deposited Zn layers provide indirect evidence about the dissolution of Zn from an alloy with the Pt substrate. The presumed potential of co-deposition of Cr (−1.9 V vs. Hg/Hg₂SO₄) is reached at a current density of about 300 mA cm⁻².     

Rare earth hydrogen storage alloy used in borohydride fuel cells

Fuel cell using the borohydride as the fuel has attracted much attentions because of high energy density and working potential. In this work, LaNi_4.5Al_0.5 hydrogen storage alloy used as the anodic material to replace noble metals has been investigated. Experimental results showed that H₂ evolution was unavoidable during discharge process because of the hydrolysis of , but the utilization of the fuel increased with the increasing current densities. At high discharge current, the alloy electrode showed the lowest hydrogen generation rate and higher utilization of the fuel because, the generated hydrogen was absorbed and oxidized to produce electric energy similar to the behavior of hydrogen storage alloy in nickel–metal hydride batteries. The reaction mechanism of borohydride on the surface of electrode made of hydrogen storage alloy also has been discussed. Hydrogen storage alloy would be a promising material as the anodic catalyst in borohydride fuel cell.     

Cathodic reduction of superconducting oxocuprates YBa2Cu3O7− x at high current densities

The cathodic reduction of YBa₂Cu₃O₇ in aqueous electrolytes at ambient temperature turns out to be strongly dependent upon current density with respect to the reaction mechanism. At low current density, topotactic electron/proton transfer is the dominant process, while at higher current densities, two competing reactions appear, i.e. the topotactic conversion and an irreversible reaction leading to products amorphous in terms of X-ray diffraction. Received: 21 April 1999 / Accepted: 7 June 1999     

The effect of cerium pre-treatment on the corrosion resistance of steel sheets

The steel samples have been coated with cerium layer by cathodic electrolytic deposition from the Ce(NO₃)₃·6H₂O solution in aqueous ethyleneglycol in the presence of hydrogen peroxide. The influence of the coating parameters (cathodic current density, pH, cerium concentration, hydrogen peroxide concentration, temperature, and treatment duration) on the surface properties; the optimum conditions of the formation of corrosion preventing coating have been elucidated. Hydrogen peroxide concentration and pH are the major factors influencing the deposition process. The corrosion resistance has been further enhanced after treatment with Na₃PO₄·12H₂O solution. The cerium-coated samples have been subsequently coated by cathodic electrostatic deposition from the colloidal solution of the paint. The coated materials have been subjected to mechanical testing (hardness, impact, cross cut, bending, and cupping tests), and their structure has been visualized by electron microscopy. The cerium coating has been found to improve the steel corrosion resistance by 15%.     

Computed current density in the case of slow charge-transfer coupled with a second order chemical reaction,under potentiostatic conditions (SnIV/SnII system)

Current-time curves at constant potential on plane and spherical electrode have been computed by numerical integration of the transport equations in the case of a slow charge-transfer coupled with a chemical reaction leading to a non-reactive product E₃, following the scheme:E₁+e→E₂E₁+E→E₂?E₃Results indicate that this mechanism is characterized by lower current densities than those of simple diffusion-transfer control. The current density is not proportional to depolarizer E₁ concentration. The main characteristic of these systems is that, when polarograms are analysed in terms of log k versus potential E, different straight lines are obtained depending on the concentration of the depolarizer; consequently, the apparent transfer coefficient α varies with concentration. Moreover all lines intersect at a same point lying at a potential near the equilibrium value, where the charge-transfer rate constant k is sufficiently small to be rate determining.     

Cathodic polarization behavior of self-supporting polyaniline film electrode

The preparation method of a self-supporting doped-polyaniline film electrode and its open-circuit potential (OCP) in NaClO₄ and Na₂SO₄ solutions with different pH value as well as cathodic polarization behavior have been investigated for the purpose of discussing the corrosion electrochemical behavior of polyaniline (PANI) in the acid solution. X-ray photoelectron spectroscopy (XPS) reveals that the lower pH corresponds to higher doping level of H⁺ in the film and a more positive OCP of PANI film electrode. OCP of the PANI film reached 0.35 V vs. SCE in 1M H₂SO₄, which is more positive than that of most metals, suggests that PANI would act as cathode when it couples with these metals. The cathodic polarization experiments indicate that the dominating cathodic polarization process of PANI is reversible doping and dedoping reaction and the reduction of dissolve oxygen has very little contribution to it. The potentiostatic current-time curves exhibit a large transient current density at initial stage of polarization, which should be attributed to the charge stored in the film and a relative less steady state current density at the subsequent stage of polarization, which is provided by its doping/dedoping equilibrium activity. Such a current characteristic of PANI electrode might be the force of PANI to provide the passivation protection for some active-passive metals. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 10, pp. 1205–1212. The text was submitted by the authors in English.     

Elektrochemische Methode zur quantitativen und kontinuierlichen Bestimmung von Ozon in Gasgemischen

The method is based on the measurement of the diffusion controlled limiting current obtained with the cathodic reduction of ozone according to O₃ + 2H⁺ + 2e^? → H₂O + O₂ in sulphuric acid. The electrolyte is vigorously mixed with the gas containing ozone and circulated through the electrolytic cell by the ascending gas flow operating as a gas-lift pump. The solution saturated with ozone moves along a cylindric electrode consisting of a smooth platinum foil in laminar flow. A constant potential is maintained at the electrode by means of a potentiostat in such a way, that the electrochemical reaction is proceeding under limiting current conditions. A lead anode of high capacitiy is used as a counter- and reference electrode. The current recorded continuously is directly proportional to the concentration of ozone in gas.