Electrolytic resistance of solution layers at hydrogen and oxygen evolving electrodes in alkaline solution

During alkaline water electrolysis, additional energy losses occur owing the presence of bubbles in the solution, particularly close to both the gas-evolving electrodes.For both hydrogen and oxygen-evolving disc electrodes (diameters from 0.2 to 2.0 mm) in KOH solutions, the reduced increase in ohmic resistance, ΔR*, has been determined by the alternating current—impedance method.It has been found that, for hydrogen-evolving electrodes, log ΔR* = 1₁ + b log i, where the exponent b at 0.1 A cm^?2 < i < 5 A cm^?2 does not depend on the diameter, position and material of the electrode, pressure and temperature but does significantly depend on KOH concentration. The factor a₁, however, being dependent on the position, height and material of electrode, temperature and KOH concentration. ΔR* cannot be expressed for the oxygen-evolving electrode by a general equation, due to the coalescence behaviour of oxygen bubbles.Moreover, it has been established that the Bruggemann equation is useful to determine the ohmic resistance of a solution layer containing gas bubbles of different size at which each bubbles adheres to the electrode surface.     

Steady-state mass transfer in radial flow-through thin layer cells—I. Reversible metal systems

The convective diffusion model, prevailing in radial thin layer flow cells (thickness 2h ? 10^?2 cm) with central solution inlet at ultra-low flow rates J ? 10^?2 cm³ s^?1, has been investigated by numerical finite-difference techniques for the transport-controlled conversion of metal ions at a coulometric asymmetrical or symmetrical metal detector, mounted on one or on both sides of the thin layer, respectively. At symmetrical detectors, the concentration profiles and the conversion efficiency have been evaluated by assuming negligible radial diffusion and are in good agreement with previous eigenvalue solutions. At unsymmetrical detectors, the mass transfer study has been carried out in consideration of radial diffusion. The diffusional contribution to the radial flux becomes increasingly apparent in the concentration profiles and the conversion efficiency, as the internal detector boundary r₀ is shifted downstream at constant h and J, or as J and h are lowered proportionally at constant r₀. This effect can be diminished by choosing appropriate correlations between r₀, h and J, which are established together with the limiting conditions for coulometric detector operation.     

Bubble parameters and efficiency of gas bubble evolution for a chlorine-, a hydrogen- and an oxygen-evolving wire electrode

The effect of current density used for total gas evolution, i_g, or for gas in bubbles, i_b, and the effect of solution—flow velocity, v_s,0, and temperature, T, on the detached bubbles parameters, viz average Sauter bubble radius, R_s,d,av, average bubble radius, R_d,av, average of the square of the bubble radii, (R²_d)_av, average of the third power of the bubble radii, (R³_d)_av, detached bubble frequency, ω, and efficiency of gas bubble evolution, η_b, for a chlorine-, a hydrogen- and an oxygen-evolving electrode at forced convection have been determined experimentally. The electrodes consist of a thin wire electrode with a small height. It has been found that:

• 1.The average Sauter bubble radius is proportional to in4b and decreases linearly with increasing vs,0 at constant ig. The constant n4 depends on the temperature, the nature of the gas evolved, the electrode material and the electrolyte and is independent of vs,0. Similar correlations were obtained for Rd,av, (R2d)av and (R3d)av.
• 2.The frequency of detached bubbles is proportional to in5b and increases linearly with increasing vs,0 at constant ig. The constant n5 depends on the temperature, the nature of the gas evolved, the electrode material and the electrolyte and is independent of vs,0.
• 3.The efficiency of gas bubble evolution ηb increases with increasing ig and approaches a limiting value.

     

Morphological controlling of CTA forward osmosis membrane using different solvent-nonsolvent compositions in first coagulation bath

Cellulose triacetate (CTA) membranes were fabricated via a modified nonsolvent induced phase separation (NIPS) method. Different solvent-nonsolvent compositions in first coagulation bath (FCB) were introduced to optimize CTA membrane structures. The effects of FCB compositions, immersion time and mass ratio of solvent (N-methyl-2-pyrrolidone, NMP) and nonsolvent (water, ethanol, ethylene glycol and glycerol) on membrane morphology and performance were systematically investigated. Prospective membranes with a dense bottom layer and a scaffold-like top layer were obtained under room temperature, owing to the low relative energy difference (RED) between nonsolvent and polymer as well as the high viscosity of FCBs. A high water flux J_v (12.6 L m^?2 h^?1) and a low reserve salt flux J_s (1.32 g m^?2 h^?1) were obtained in the optimized membrane, with a structural parameter S of 119 μm. Compared with membranes prepared via conventional NIPS method and commercial CTA forward osmosis (FO) membranes, a remarkable improvement of J_s/J_v value and S value was achieved, indicating membranes with single dense-layer structure might suffer less from internal concentration polymerization (ICP) which is the main obstacle for the development of FO process. This study might help us pave the way to design superior CTA membrane structures for forward osmosis application.     

Supersaturation of oxygen in acidic solution in the vicinity of an oxygen-evolving platinum anode

The concentration of O₂ in 1 M H₂SO₄ solution in the vicinity of the O₂-evolving smooth Pt anode was measured as a function of anodic cdi_a using the galvanostatic potential—transient method.The solution near the O₂-evolving anode was supersaturated with O₂. When the anodic current was interrupted, supersaturated concentration C^* decreased at a rate proportional to C^*  C₀, where C₀ is the solubility of O₂ in the electrolyte at 1 atm. The rate constant of the decay of the supersaturation under the open circuit condition was measured to be 0.069 sec^?1 at 25°C.At i_a < 40 mA/cm² there was a linear relation between log(C^*  C₀) and log i_a. At i_a > 200 mA/cm², however, the supersaturation exhibited a limiting value of 9.0 × 10^?2 mol/l.     

Transfer of mass or heat to an electrode in the region of hydrogen evolution—II.: Experimental verification of mass and heat transfer equations

Radioactive mercury, ²⁰³Hg, was deposited both on smooth Pt and amalgamated Pt electrodes in 1 N KOH, in the region of hydrogen bubble evolution. The relation found between the cd for deposition of mercury, i_R, and for evolution of hydrogen, i_G, is in accord with the equation derived previously[1] for systems with a surface incompletely covered with hydrogen bubbles. The heat transfer coefficient on a platinum electrode located on the wall of a rectangular channel was measured under defined hydrodynamic conditions during hydrogen evolution in 0·5 N KOH. The measured dependence of the heat flow and cd of hydrogen evolution is in accord with the relationship obtained earlier[1] for systems with fully covered surface by the evolved gas bubbles.     

On the electrochemical behavior of H2O2 AT Ag in alkaline solution

Electrochemical oxidation and reduction of H₂O₂ on Ag were studied in alkaline solution of 10^?3?0.3 M H₂O₂ and 2 × 10^?3 ?1.0 M KOH under N₂ bubbling. Steady i-φ curves obtained by a cyclic potential sweep method in a potential range where no electrode oxidation takes place, lead to the following results: (1) i^c_d (A cm^?2) (cathodic limiting current density) = 1.0 × [H₂O₂]^1.0_T (M), (2) i¹_d (A cm^?2 (anodic limiting one) = i^c_d ([KOH] ? [H₂O₂]_T) or 1.0 × [KOH] < [H₂O₂]_T), (3) φ_m (V) (mixed potential) = 0.126-0.060 log [KOH]^1.0 and (4) (?φ/?i)_φ=φm (Ωcm²) (reaction resistance at φ = φ_m) = 0.057 × [H₂O₂]^?1.0_T (M^?1), where [H₂O₂]_T designates a total H₂O₂ concentration and the others have their usual meanings.The above results are explained by the following mechanism; HO^?₂ formed by the reversible chemical reaction, H₂O₂ + OH ? HO^?₂ + H₂O, is oxidised in anodic reaction by two steps: HO^?₂

HO₂ (a) + e^? and HO₂(a) + OH^? → O₂ + H₂O + e^?, whereas in cathodic reaction, H₂O₂ is reduced by H₂O₂ + e^?

OH(a) + OH^?, OH(a) + e^? → OH^?. Here,

designates a rate determining step,Catalytic decomposition of H₂O₂ on the electrode is also discussed.     

Kinetics of layer formation and corrosion processes of passive iron in acid solutions

The kinetics of the layer formation process and the corrosion process have been determined from the relation between the stationary (i_c,0) and non-stationary (i_c) corrosion rates and the rate of layer formation (i_ι) at passive iron in acid solutions. The rates of both processes depend on the potential difference ε_2,3 at the passive-oxide/electrolyte-solution interface. The fact that i_c and i_c,0 are independent of the electrode potential is explained by a nearly constant composition of the passive oxide in contact with the solution (Fermi level within the band gap).Linear relations, such as log i_ι⁺ = a + (α_ι⁺/α_c⁺)·log i_c, are obtained, which are explained by charge-transfer hindrance. The apparent charge-transfer coefficients are α_ι⁺ = 1·43 and α_ι^? = 0·57 for the layer formation and removal reaction, and α_c⁺ = 0·84 for the corrosion reaction. From the pH dependence (pH = 0.35–2·90) of i_ι and i_c,0 the reaction orders of the hydrogen ions are found to be approximately μn_ι⁺ = ?1 and ν_c = 0. The corrosion cd depends on the sulphate concentration, with a reaction order ν_s = ß = 0·16. From this, the kinetics of the oxide formation H₂O·aq ? OH^? · ox + H⁺·aq (preceding equilibrium), OH^?·ox?O^2?·ox + H⁺·aq (rate-determining) and the kinetics of the corrosion process Fe³⁺·ox + SO₄^2?·aq ? FeSO₄⁺·ad (adsorption equilibrium, Temkin conditions), FeSO₄⁺·ad → FeSO₄⁺·aq (rate-determining), with following dissociation of the complex can be deduced. The apparent charge-transfer coefficients are interpreted by α_ι⁺ = 1 + α_ι, α_ι^? = 1 ? α_ι and α_c⁺ = α + 2ßγ_s (ga_ι, α, true charge-transfer coefficients).     

The anodic dissolution process on active iron in alkaline solutions

Steady state anodic polarization curves were taken for Armco iron and in some experiments for high purity Puratronic iron in KOH solutions in the concentration range 5 x 10^?2-5M. After the 30 min cathodic pretreatment, well reproducible anodic Tafel plots are obtained. The experimentally obtained diagnostic criteria b_a = 67–70 mV dec^?1, n_OH? = 1.1 and nHFeO₂^?= ?0.45 are interpreted by the anodic reaction mechanism in which FeOH_adsand Fe(OH)_2,ads appear as the intermediates adsorbed under Temkin conditions, the primary stable product of the electrode reaction being HFeO₂^?, and the final product Fe (OH)₂, formed by precipitation.     

Effect of electrolytes in aqueous solutions on oxygen transfer in gas–liquid bubble columns

The effects of inorganic electrolytes (NaCl, MgCl₂, CaCl₂) in aqueous solutions on oxygen transfer in a bubble column were studied. Electrolyte concentrations (c) below and above the critical concentrations for bubble coalescence (c_tc), and six superficial gas velocities (v_sg), were evaluated. The volumetric mass transfer (k_La) and the mass transfer (k_L) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the k_L, but the interfacial area (a) increased enough to result in a net increase of k_La. Using as independent variable a normalizing variable (c_r = c/c_tc), and maintaining fixed v_sg, similar values of k_La were observed regardless the kind of electrolyte; the same happened for k_L. This suggests that c_r quantifies the structural effects that these solutes exert on mass transfer. Also, once c_r = 1 was reached, no significant variations were found in k_La and k_L for constant v_sg. It is concluded that the gradual inhibition of bubble coalescence (c_r < 1) governs the significant changes in hydrodynamics and mass transfer via the reduction of bubble size and the consequent increment of a and gas holdup (?_g). Finally, regarding the effects of v_sg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.     

Linear sweep voltammetry of manganate(VII), manganate(VI), and manganate(V) in alkaline media

Electrochemical reactions were assigned to the voltammetric waves obtained in alkaline KMnO₄ and K₂MnO₄ solutions. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that at slow potential scan rates in MnO^?₄ solutions ranging from 0·07 to 0·19 F in NaOH the first step in MnO^?₄ reduction is a l-electron reversible charge transfer with no coupled chemical reaction; at fast scan rates (? V s^?1 the process becomes quasi-reversible. The standard rate constant and the activation energy for the MnO^?₄/MnO^2?₄ charge transfer step were estimated. Plots of i_p (AC°ν^{$\text{1}\text{2}$})^?1 and i_api_cp^?1 νs log ν indicate that in 4·O F KOH and at potential scan rates between 0·005 and 2·V s^?1 the reduction of MNO^2?₄ to MnO^3?₄ is a reversible charge transfer with no coupled chemical reaction. The diffusion coefficients of MnO^?₄ and MnO^2?₄ in alkaline solutions are reported.     

Role of Cl− in breakdown of Ni Passivity in aqueous NaOH solutions

Repetitive potentio- and galvanokinetic traces of Ni rods yielded anodic peaks (or arrests) corresponding to probable formations of α and β Ni(OH)₂ and NiO(OH) prior to O₂ evolution. Cathodic half cycles consistently showed only two reduction processes that are attributed to NiO(OH)→ Ni(OH)₂åNi. Potentiostatic polarization resulted in multi-peaks i-t traces, in [NaOH] ? 1 moll^?1; the main peak exhibiting the general form characteristic of instantaneous nucleation processes. The maximum current intensity, i_p, of the potentiokinetic traces as well as the parameter i_mt_m of the i-t curves showed negligible dependence on NaOH in the range 0.01–1 moll^?1. In the range of 1–200 mVs^?1 scan rate, i_p ∝ v. Employing an aqueous media of mixtures of different concentrations of NaCl plus a constant [NaOH] resulted in: (a) significant increases of both i_p and i_mt_m and (b) nearly linear i-v^{$\text{sol1}\text{2}$} relation. Achievement of electrode passivity was retarted by Cl^? but eventually attained. As [Cl^?]/[OH^?] exceeded ～ 3 passivity showed signs of breaking down. The passivation current showed continous rise with increasing [Cl^?]. A mechanism of Cl^? attack involving surface adsorption, increasing solubility of an intermediate Ni hydroxide species that nucleates into passive film and peptization of the deposited oxide, by Cl^?, is discussed.     

The pressure beneath a growing rising bubble

On the basis of potential theory, the equations of motion and the associated pressure field are derived for an idealized growing spherical gas bubble rising in an inviscid liquid under the influence of gravity from a horizontal plate-orifice and from a free standing nozzle. The dimensionless pressure (p_N?p₀)/πga at the gas source N (where p₀ is the undisturbed ambient pressure, a the instantaneous bubble radius and π the liquid density) is found to rise to a maximum when the dimensionless bubble position h/a = 1·55 for bubbles formed at a plate orifice and (125/48)^{$\text{1}\text{3}$} ? 1·38 for bubbles formed at a free standing nozzle. These positions of maximum pressure correspond well to experimentally observed positions at which the gas supply to bubbles grown at constant flow rate in water is cut off by the collapse of the neck linking bubble and gas source. The volumes of bubbles at this instant are predicted theoretically and compare well with experimentally determined values.     

Potentiality of Bi and Mn co-doped lead-free NaNbO3 ceramics as a pyroelectric material for uncooled infrared thermal detectors

The pyroelectric materials have immense applications in the uncooled infrared thermal detectors. However, owing to increasing environmental concerns due to Pb element, it is required to explore novel, high-performance, environmental-friendly pyroelectric materials. This is the first study to report about the pyroelectric properties of lead-free NaNbO₃ ceramics, which displayed a high pyroelectric coefficient of 1.85 × 10^?8 C cm^-2 K^?1 and figures of merit as F_i = 0.67 × 10^?10 m V^?1, F_v = 3.33 × 10^?2 m² C^?1, and F_d = 5.32 × 10^-5 Pa^?1/2 at room temperature. Also, highly temperature-stable pyroelectric characteristics were also observed in NaNbO₃ ceramics due to the high depolarization temperature of 280 ℃. The high pyroelectric properties and temperature stability were a result of the electric field induced irreversible phase transition from antiferroelectric to ferroelectric. Hence, we can conclude that lead-free NaNbO₃ ceramics are a novel and promising candidate for pyroelectric detectors in a wide temperature range, especially for large area detectors and pyroelectric point detector.     

The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H₂SO₄ in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i_o2 follows the relation, log i = A - (E⁰_a - αFη)/2·3 RT - d/d_o. The experimental activation energy E_o = E^o_a = αFη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d_o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH^?_ad or O^2?_ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl^? and Ce⁴⁺ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.     

Electrochemical behavior of poly(3-hexylthiophene). Controlling factors of electric current in electrochemical oxidation of poly(3-hexylthiophene)s in a solution

Electrochemical response of regio-random and regio-controlled poly(3-hexylthiophene), P3HexTh, was investigated by cyclic voltammetry. P3HexTh underwent electrochemical oxidation at about 0.4 V vs. Ag⁺/Ag in a THF solution, and the peak anode electric current, i_pa, was proportional to the sweeping rate v; i_pa=const×v^1/2. These data indicated that diffusion of the P3HexTh molecule in the solution was important to determine i_pa. Application of a Matsuda's equation with assumptions gave a diffusion coefficient, D, of about 1×10⁻⁷ cm² s⁻¹ at molecular weight of about 5000, and the D value steeply decreased with increase in the molecular weight.     

Optimization of parameters for competitive adsorption of heavy metal ions (Pb+2, Ni+2, Cd+2) onto activated carbon

This study investigates optimization of various competitive adsorption parameters for removal of Cd(II), Ni(II) and Pb(II) from aqueous solutions by commercial activated carbon (AC) using the Taguchi method. Adsorption parameters such as initial metal concentration of each metal ion (C_0,i ), initial pH (pH₀), adsorbent dosage (m) and contact time (t) in batch technique were studied to observe their effects on the total adsorption capacity of metals onto activated carbon (q _tot ). The adsorbent dosage has been found to be the most significant parameter. Interactions between C_0,Cd ×C_0,Ni , C_0,Cd ×C_0,Pb and C_0,Ni ×C_0,Pb have been considered for simultaneous metal ions adsorption. The optimum condition for adsorption of metal ions were obtained with C_0,i =100 mg L^?1, pH₀=7, m=2 g L^?1 and t=80 min. Finally, experimental results showed that a multi-staged adsorptive treatment would be necessary to reach the minimal discharge standards of metal ions in the effluent.     

Temperature dependence of the cathodic and anodic kinetics in a copper electrowinning cell based on reactive electrodialysis

The temperature dependence of various kinetic parameters for the copper electrodeposition reaction and for the ferrous to ferric ion oxidation has been experimentally determined. Potentiodynamic sweeps were carried out in a reactive electrodialysis cell in the 30- range in order to obtain expressions for each parameter as a function of temperature. In the conditions studied, for both reactions, the exchange current density depends on temperature as ln[i₀(A/m²)]=-a/T+b and the limiting current density, as |i_L(A/m²)|=cT-d. For the copper electrodeposition reaction, the charge transfer coefficient is constant, whereas for the ferrous to ferric ion oxidation it depends on temperature as α=fT²+gT+h (a-h are constants). These expressions can be used in a temperature-dependent model of a membrane-based copper electrowinning cell.     

Mass transfer in bubble columns packed with motionless mixers

The cocurrent upward mode was employed to absorb pure oxygen into water in bubble columns packed with Koch (Sulzer) motionless mixers. The liquid-side volumetric mass transfer coefficient, K_La, in the packed bubble column was found to be always larger than that in the unpacked bubble column. In the range of liquid velocities from 6.7 cm/sec to 39.9 cm/sec, the value of K_La in the packed bubble column increased with the increasing liquid velocity while that in the unpacked bubble column was almost independent of the liquid velocity. The equation of the formK_La= mν_l^β? was successfully adopted to correlate the K_La data.     

Thermal stimulation: I. Determination of activation enthalpy ΔHv for volume relaxation in polypropylene

Volume relaxation of isotactic polypropylene was studied at the temperature region of the α-relaxation by the technique of thermal stimulation (or temperature-jump). It is shown that the interpretation of volume relaxation experiments with viscoelastic models fails unless the limiting values of the bulk compliance are assumed to be temperature dependent. Much simplification in the theory of volume relaxation follows if the rigorous formalism of viscoelasticity is adopted, particularly if the treatment by one model is intended for the interpretation of controlled experiments at a single temperature, for both volume and mechanical relaxation. With this model an experiment is designed, based on the Boltzmann superposition principle. The activation enthalpy for volume relaxation, ΔH_v, can then be determined if the Boltzmann principle is applied at an appropriate reduced time, $\text{t}\text{a}{}_{\mathrm{v}}\text{(T)}$, where a_v(T) is the Arrhenius shift factor. For polypropylene at 40°C ΔH_v = 35 kcal mol¹, equal to ΔH_J the activation enthalpy for creep (at 40°C).