Assessment of doped ceria as electrolyte

A model describing the performance of a fuel cell based on 10 mol% gadolinia-doped ceria, Ce_0.9Gd_0.1O_1.95−x (CG10), was formulated. The total electrical conductivity of CG10 was measured under very reducing conditions in the temperature range of 753 K to 948 K. Oxygen permeation experiments were carried out to measure the leak current through a ceria electrolyte. The results of the measurements are compared with predictions of the formulated model. Furthermore, the response of a fuel cell to changing operating conditions such as external load, temperature, electrode polarization resistances, and defect chemistry is investigated using the model. It is found that the maximum achievable efficiency of a CG10-based fuel cell is increased when (1) the temperature is decreased, when (2) the electrolyte thickness is increased, or when (3) the cathode polarization resistance is decreased. The efficiency can also in certain circumstances be increased by an increase of anode polarization resistance. Finally, the efficiency is reduced if the vacancy formation enthalpy is decreased to the level of fine-grained CG10. The performance of a CG10-based cell is evaluated by comparing it with a state-of-the-art zirconia-based cell. At 873 K, the efficiency of a fuel cell with a 10-μm CG10 electrolyte was limited to 0.74, whereas a cell with a perfect electrolyte would have an efficiency of 1. The power output of the CG10 cell at this efficiency is, however, four times larger than the zirconia-based cell at the same efficiency. This is due to the much lower cathode polarization resistance of -CG10 cathodes on CG10 compared to the (La_0.75Sr_0.25)_0.95MnO₃ cathodes on stabilized zirconia.     

Numerical Study of Electroconvective Instability of Binary Electrolyte in a Cell with Plane Parallel Electrodes

Electroconvective instability of a binary electrolyte in a cell with plane parallel electrodes is studied using a numerical method. When a direct current is passed through the cell, a space charge and an electrical force acting upon the solution form under certain conditions. If the electric force density reaches a critical value, convective instability emerges in the cell, leading to the formation of nonequilibrium dissipative structures similar to Rayleigh–Benard cells in a nonuniformly heated liquid. The critical parameter _crit, at which instability emerges in the system, is determined. Dependences of _crit on the ratio between the diffusion coefficients for electrolyte cations and anions and on the current passed through solution are determined.     

M/Xn (MAl,Mg; XBr,I) batteries based on anion transport mechanism

A new type of electrochemical storage energy batteries of M/X_n (MAl, Mg; XBr, I, n = 3, 2) were found. They can operate with nonaqueous electrolyte or solid electrolyte showing high voltage and moderate rate performances. The discharging mechanism is related to the formation and growth of the electrolyte phase MX_n on the surface of M electrode. The MX_n phase is X⁻ conductor and X⁻ anions play the transport role in electrode and electrolyte in these systems. Our finding shows that anion-conducting electrochemical batteries can be also promising for energy storage compared to cation-conducting systems in current batteries.     

Direct Synthesis of H2O2 by a H2/O2 Fuel Cell

Direct synthesis of H₂O₂ solutions by a fuel cell method was reviewed. The fuel cell reactor of [O₂, gas-diffusion cathode electrolyte solutions Nafion membrane electrolyte solutions gas-diffusion anode, H₂] is very effective for formation of H₂O₂. The three-phase boundary (O₂(g)–electrode(s)–electrolyte(l)) in the gas-diffusion cathode is essential for efficient formation of H₂O₂. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk electrolyte solutions are essential for H₂O₂ accumulation. The maxima H₂O₂ concentrations of 1.2 M (3.5 wt%) and 2.4 M (7.0 wt%) were accomplished by the heat-treated Mn-OEP/AC electrocatalyst with H₂SO₄ electrolyte and by the VGCF electrocatalyst with NaOH electrolyte, respectively, under short circuit conditions.     

Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer

One of the main challenges in metal–air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c?u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li⁺ and K⁺ and organic solvents that contain Li⁺, K⁺, and Mg²⁺, such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N‐methyl‐2‐pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 μL s^?1. This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O₂ decrease as the concentration of salt in the electrolyte was increased due to a “salting out” effect.     

Enhancement in the supercapacitive storage performance of MnCO<Subscript>3</Subscript> using SiO<Subscript>x</Subscript> nanofluid-based electrolyte

Generally adopted strategies to improve capacitance of the electrode materials are tuning various properties of the electrode material or increasing the cell voltage. While tuning the properties of the electrode material is tedious, increasing the cell voltage is restricted by the stability of the electrolyte. Herein, we report a facile approach to improve the capacitance of MnCO₃ by the influence of SiO_x nanofluid in the electrolyte. The capacitance properties of MnCO₃ are studied in 0.1 M Mg(ClO₄)₂ electrolyte in the presence and in the absence of SiO_x nanofluid. The presence of small amount of SiO_x nanofluid in the electrolyte provides higher diffusivity and more conductive percolation paths for ions and thus decreases internal resistance and increases ionic conductivity of the electrolyte. As a result, 60% enhancement in the capacitance is witnessed for MnCO₃. Further, nanofluid containing electrolyte is found to be stable over a month.

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Graphical abstract Improvement in the capacitance properties of MnCO by the influence of SiO nanofluid

     

Effect of anodic current on carbon dioxide reforming of methane on Pt electrode in a cell with solid oxide electrolyte

The carbon dioxide reforming of methane in a cell with a solid oxygen-conducting electrolyte:

PVDF/PAN/SiO<Subscript>2</Subscript> polymer electrolyte membrane prepared by combination of phase inversion and chemical reaction method for lithium ion batteries

PVDF/PAN/SiO₂ polymer electrolyte membranes based on non-woven fabrics were prepared via introducing a chemical reaction into Loeb-Sourirajan (L-S) phase inversion process. It was found that physical properties (porosity, electrolyte uptake and ionic conductivity) and electrochemical properties were obviously improved. A favorable membrane structure with fully connective porous and uniform pore size distribution was obtained. The effects of PVDF/PAN weight ratio on the morphology, crystallinity, porosity, and electrochemical performances of membranes were studied. The optimized PVDF/PAN (70/30 w/w) (designated as M_pc30) polymer electrolyte membrane delivered excellent electrolyte uptake of 246.8 % and the highest ionic conductivity of 3.32 × 10^?3 S/cm with electrochemical stability up to 5.0 V (vs. Li/Li⁺). In terms of cell performance, the Li/M_pc30 polymer electrolyte/LiFePO₄ battery exhibited satisfactory electrochemical properties including high discharge capacity of 149 mAh/g at 0.2 C rate and good discharge performance at different current densities. The promising results reported here clearly indicated that PVDF/PAN/SiO₂ polymer electrolyte membranes prepared by the combination of phase inversion and chemical reaction method were promising enough to be applied in power lithium ion batteries.     

Electroinitiated polymerization of acrylamide in acetonitrile medium

The electroinitiated polymerization of acrylamide (AA) has been studied in acetonitrile medium using tetrabutylammonium perchlorate (TBAP) as the electrolyte. Split-cell experiments showed that the polymer formation takes place both in the anode and the cathode compartments. The polymer yield depends on several factors such as the magnitude of the current flow, the duration of the electrolysis, the monomer concentration, the electrolyte concentration, the temperature of the solution, presence or absence of air, and finally whether or not the cell content was stirred. The current exponent of the polymerization was 0.28 with a reaction rate constant of 1.06 reaction % per hour. The IR and NMR spectra of the polymers suggest that the anodic polymer is polyacrylamide and the cathodic polymer is poly-β-alanine (? CH₂? CH₂? CO? NH? ). Based on the experimental results, a radical mechanism for the anodic polymerization and an anionic mechanism for the cathodic polymerization have been proposed.     

A galvanic gas sensor using poly (ethylene oxide) complex of silver trifluoromethane sulphonate electrolyte

Summary A galvanic sensor for monitoring nitrogen dioxide was developed by using a poly(ethylene oxide) complex of silver trifluoromethanesulphonate electrolyte. The sensor, which is expressed as Au/P(EO)_4.5 AgCF₃SO₃/Ag, is a small disk (i.d. 13 mm). The polymeric electrolyte film was made by casting the mixture of acetonitrile solutions of both P(EO) (MW 6×10⁵) and AgCF₃SO₃. The working electrode was made by sputtering of gold in argon. The thicknesses of the desposited gold, polymeric electrolyte film and silver are 25 nm, 30 m and 1 mm, respectively. When the sample gas containing nitrogen dioxide impinges at 20 ml min^–1 on the gold cathode, the current flowing in the external circuit was linearly related to the concentration of nitrogen dioxide from 20 ppb to about 10 ppm. The current efficiency of the cell was 0.051%. The cell's response time was about 2 min for 0.5 ppm of nitrogen dioxide.     

Der potentialverlauf galvanischer sensoren bei pulsförmiger probenaufgabe

It has been shown that by sample application as a pulse the voltage—time relation of a galvanic sensor can generally be described by the equation

where U_a is the voltage output change, U_o the maximum voltage change at open circuit, R_a and R the external and total resistance, k₁ and k₂=1/RC the rate constants of charging and discharging and C the total capacity of the cell (→condenser model). k₁ is related to the over-all reaction of charging and gives no immediate indication to the rate determining step. For instance k₁ can be the rate constant of chemisorption or an other first order reaction. In the case of a diffusion controlled reaction it is

where K is the partition and D the diffusion coefficient in the electrolyte, S the electrode surface, d the mean thickness of the electrolyte film, V₀ the cell and V_L the electrolyte volume above the catalyst. The deduced equation can also be extended to a quite chemical reaction, for instance when the measuring electrode is depolarized by oxygen. k₂ has then analogous meanings to k₁. The mentioned formulas give some indication to the optimalisation of the cell geometry and the preparation of the electrodes. The experimental treatment includes the response of an alcohol sensor, also the determination of k₁ is described and its kinetic meaning is discussed.     

Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon‐Nanotube‐Based Electrocatalyst

Ammonia is synthesized directly from water and N₂ at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half‐cell for the NH₃ synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half‐cell. A rate of ammonia formation of 2.2×10⁻³ g m⁻² h⁻¹ was obtained at room temperature and atmospheric pressure in a flow of N₂, with stable behavior for at least 60 h of reaction, under an applied potential of −2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH₃ electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N₂, making it more reactive towards hydrogenation.     

Computer Simulation of the Cathode Active Layer in Hydrogen–Oxygen Fuel Cell with Polymer Electrolyte: The Nature of the Overall Current Variations

Full computer simulation of the cathode structure in hydrogen–oxygen fuel cell with polymer electrolyte is performed. Both transport, support grains (agglomerates of carbon particles onto whose surface Pt-catalyst is deposited), and the current generation in active layer are simulated. The active layer operation in potentiostatic mode is studied. The effect of variations of the active layer and the fuel cell temperature (T_s and Т, respectively) on the cathode overall current I and the support grain flooding with water is calculated. The changes in the temperature difference T_s–Т was shown for the first time, experimentally and by the simulation, to generate variations of I and the degree of the support grain flooding with water. In particular, with the increasing of T_s–Т the current I increased, whereas the support grain flooding with water decreased; and vice versa, with the decreasing of T_s–Т the current I drops down, while, the support grain flooding with water grows. An explanation of the phenomena is presented, which takes account of structure of the support grains in which О₂ reduction and Н₂О generation occur. There exist intrinsic channels for protons and О₂ molecules transportation to the catalyst. Water releasing in the support grains is able to fill partially or even entirely the gas pores through which oxygen is supplied to the platinum. As a result, the current generated in the support grains can drop down significantly; at the same time, the value of I also drops down. The degree of the support grainfilling with water is determined by two processes, namely, the flooding and draining. The source of flooding is the current generation; that of draining, the water saturated vapor diffusion and water filtration in nanopores. The lower cathode potential, the higher the flooding rate, whereas the water removal rate grows or drops down with the increasing of decreasing of the temperature difference Т_s–Т, respectively. Thus, the temperature difference variations naturally lead to those of the quantity I.     

Electrochemical Behavior of a Copper Electrode in Solid RbCu4Cl3I2Electrolyte

The electrochemical behavior of a copper electrode in solid RbCu₄Cl₃I₂electrolyte is studied by galvanostatic and potentiostatic methods. It is found that a Cu₂O layer 1 m thick exists at the interface between the Cu electrode and RbCu₄Cl₃I₂. The layer blocks the electrochemical reaction Cu⁰– e Cu⁺, which involves with metallic copper. At low overpotentials, the Cu electrode acts as an inert redox electrode. At the Cu₂O/RbCu₄Cl₃I₂interface, the electrochemical reaction Cu⁺– e Cu²⁺occurs, which involves Cu²⁺ions. The reaction rate is limited by slow diffusion of Cu²⁺ions in RbCu₄Cl₃I₂. The initial concentration of Cu²⁺ions in the electrolyte near this interface is about 1.4 × 10¹⁷cm^–3. The exchange current density is (4 ± 2) × 10^–6A/cm². At potentials exceeding 8–10 mV, an electric breakdown of the Cu₂O layer occurs, and the reaction with metallic copper becomes unblocked. At 10 mV < < 100 mV, the rate of this reaction is limited by the nucleation of copper crystals and the nuclei growth. At > 120 mV, the reaction rate is limited by charge transfer.     

Throwing power of a dilute sulfuric acid copper plating electrolyte during intensive electrodeposition

A method of determination of the throwing power (TP) of electrolytes in conditions of forced convection in a Hull cell with a rotating cylindrical electrode is presented. The method is used to determine TP of a copper plating electrolyte containing 0.15–0.5 M CuSO₄ and 1 M H₂SO₄ at current densities of up to 75 mA cm^-2. The TP decreases with increasing current density at a constant i _av/i _d ratio, where i _av is the average working current density and i _d the limiting diffusion current density. With increasing the i _av/i _d ratio due to an increase in the cathodic polarizability conditions for an increase in TP may be achieved. In optimum (for uniform deposition) modes, i _av/i _d 0.4–0.5. The TP may increase upon diluting electrolyte with respect to its main component.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 91–96.Original Russian Text Copyright © 2005 by Dikusar, Bobanova, Yushchenko, Yakovets.To the Centennial of B.N. Kabanov.     

A lithium secondary battery using a thin film of polymer electrolyte as a separator

Ion-conductive polymer which shows an ionic conductivity (σ_i) of 1.4 × 10^?4S/cm at 25°C when mixed with LiClO₄ (molar ratio in Li/OE = 0.05) was used as a separator of electrodes in a lithium secondary battery. The effect of high ionic conductivity on the performance of the battery was studied. The polymer structure was and the cathode was comprised of poly(1,3,4-thiadiazole disulfide), graphite powder and the polymer electrolyte. The cell [(?)Li/polymer electrolyte/graphite–poly(disulfide) (+)] had an open circuit voltage (V_oc) of 3.25 V, a plateau voltage of 2.75 V, a discharge density (i_d) of 0.05 mA/cm² with the cathode utilization of 63%, and achieved over several tens of cycles at 25°C.     

Functional coatings of ternary alloys of cobalt with refractory metals

Finely crystalline cobalt–molybdenum–tungsten coatings were deposited in the nonsteady-state mode from a polyligand citrate-pyrophosphate electrolyte at a concentration ratio of the alloy-forming components Co²⁺/(WO ₄ ^2– + MoO ₄ ^2– ) = 1: 1 and citrate/pyrophosphate ligands of 1: 2. It was found that the quantitative composition and current efficiency by the ternary alloy depends on the current density. The effect of energy- and time-related parameters of the pulsed electrolysis on the surface morphology and roughness was studied. It was shown that, as the total content of refractory metals in Co–Mo–W coatings increases, the corrosion rate decreases in the acid medium and increases in the alkaline medium because of the instability of molybdenum and tungsten oxides.     

Modification of the Pitzer model to calculate the mean activity coefficients of electrolytes in a water-alcohol mixed solvent solution

The approach tested in this work consists in adapting the Pitzer model, initially designed for aqueous solutions of electrolytes, to the case of solutions with a mixed solvent, without systematically readjusting the coefficients. This modified model was applied successfully to the calculation of the mean activity coefficients of NaBr in the mixed solvent H₂O+MeOH, H₂O+EtOH and compared with the experimental values obtained from electrode potential measurements.General D dielectric constant - G ^ex excess Gibbs energy - I ionic strength - k Boltzman's constant - m _i molality - N _o Avogadro's number - n _w number of kilograms of water - R gas constant - T temperature (K) - T _c critical temperature - T _cm critical temperature of mixed solvent - x _i molar fraction - z _i ionic charge - electronic charge - osmotic coefficient - ± mean activity coefficient of electrolyte - number of moles of ions given by one mole of electrolyte - _M number of moles of cations given by one mole of electrolyte - _X number of moles of anions given by one mole of electrolyte - _W density of water Debye- Hückel Model A Debye-Hückel parameter - a distance of closest approach of ions in solution - B Debye-Hückel parameter Pitzer Model b numerical parameter of model - numerical parameter of model - ₁, ₂ numerical parameters of model, used in the case of 2:2 electrolyte - _MX ⁽⁰⁾ numerical parameter of electrolyte MX - _MX ⁽¹⁾ numerical parameter of electrolyte MX - _MX ⁽²⁾ numerical parameter of 2:2 electrolyte MX - C _MX numerical parameter of electrolyte MX - binary interaction parameter - ternary interaction parameter     

Low-temperature performance of Li-ion cells with a LiBF<Subscript>4</Subscript>-based electrolyte

The effect of LiBF₄ on the low-temperature performance of a Li-ion cell was studied by using a 1:1:1 (wt) EC/DMC/DEC mixed solvent. The results show that the LiBF₄-based electrolyte has a 2- to 3-fold lower ionic conductivity and shows rather higher freezing temperature compared with a LiPF₆-based electrolyte. Owing to electrolyte freezing, cycling performance of the Li-ion cell using LiBF₄ was significantly decreased when the temperature fell below –20 °C. However, impedance data show that at –20 °C the LiBF₄ cell has lower charge-transfer resistance than the LiPF₆ cell. In spite of the relatively lower conductivity of the LiBF₄-based electrolyte, the cell based on it shows slightly lower polarization and higher capacity in the liquid temperature range (above –20 °C) of the electrolyte. This fact reveals that ionic conductivity of the electrolytes is not a limitation to the low-temperature performance of the Li-ion cell. Therefore, LiBF₄ may be a good salt for the low-temperature electrolyte of a Li-ion cell if a solvent system that is of low freezing temperature, high solubility to LiBF₄, and good compatibility with a graphite anode can be formulated. Electronic Publication     

Anodic Electrocatalysts for Fuel Cells Based on Pt/Ti<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ru<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

The electrocatalytic activity of materials in the 10% Pt/Ti_1–xRu _x O_2–δ system, where x = 0–0.3 (0 ≤ Ru ≤ 30 mol %), in the reactions of hydrogen electrooxidation in the presence of CO is studied in the liquid three-electrode cell and a model of fuel cell. It is shown that the tolerance of the electrocatalysts towards CO is determined by the crystal structure of the support: the support with the rutile structure provides a higher rate of CO desorption than the support with the anatase structure. The potential of the onset of CO oxidation decreases with increasing concentration of dopant in the support from 650 mV for 10% Pt/TiO₂ to 480 mV (NHE) for 10% Pt/Ti_0.91Ru_0.09O_2–δ (rutile). The use of these materials as the anodic catalysts of fuel cell operating with hydrogen containing 30 ppm CO enabled us to obtain a current density by 7 times higher as compared with the 20% PtRu/C E-Tec catalysts.