Study of solid electrolyte polarization by a complex admittance method

The polarization behavior of zirconia-yttria solid electrolyte specimens with platinum electrodes has been studied over a temperature range of 400° to 800°C and a wide range of oxygen partial pressures. The complex admittance of these specimens was determined over a frequency range from d.c. to 100kHz. An analysis of these data in the complex admittance plane indicated the presence of three polarizations: (1) an electrode polarization characterized by a double layer capacity and an effective resistance for the overall electrode reaction, O₂(gas) + 2e(platinum) O²⁻ (electrolyte); (2) a capacitive-resistive electrolyte polarization, probably corresponding to a partial blocking of oxygen ions at the electrolyte grain boundaries by an impurity phase there; and (3) a pure ohmic electrolyte polarization.     

Understanding the Effects of Ultrasound (408 kHz) on the Hydrogen Evolution Reaction (HER) and the Oxygen Evolution Reaction (OER) on Raney-Ni in Alkaline Media

The hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) occurring at the Raney-Ni mesh electrode in 30 wt.-% aqueous KOH solution were studied in the absence (silent) and presence of ultrasound (408 kHz, ∼54 W, 100% acoustic amplitude) at different electrolyte temperatures (T = 25, 40 and 60 °C). Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) experiments were performed to analyse the electrochemical behaviour of the Raney-Ni electrode under these conditions. Under silent conditions, it was found that the electrocatalytic activity of Raney-Ni towards the HER and the OER depends upon the electrolyte temperature, and higher current densities at lower overpotentials were achieved at elevated temperatures. It was also observed that the HER activity of Raney-Ni under ultrasonic conditions increased at low temperatures (e.g., 25 °C) while the ultrasonic effect on the OER was found to be insignificant. In addition, it was observed that the ultrasonic effect on both the HER and OER decreases by elevating the temperature. In our conditions, it is suggested that ultrasound enhances the electrocatalytic performance of Raney-Ni towards the HER due to principally the efficient gas bubble removal from the electrode surface and the dispersion of gas bubbles into the electrolyte, and this effect depends upon the behaviour of the hydrogen and oxygen gas bubbles in alkaline media.     

On the behavior of a dense indium oxide/yttria-stabilized zirconia electrode

The electrical, structural and electrochemical properties of a dense In₂O₃ layer in contact with a single crystal YSZ electrolyte were studied. As a result of dc and ac investigations, it was found that under anodic polarization the rate of Faraday reaction at the surface of the In₂O₃ electrode is as low as in an ionically blocked electrode. Under cathodic polarization, however, the electrochemical activity of the electrode improves depending on the magnitude of the polarization voltage. Likewise, the electrode polarization resistance decreases after platinum or praseodymium oxide having been deposited on the surface of the In₂O₃ layer. The possible mechanism responsible for such a peculiar behaviour and the limiting step of the electrode reactions are discussed.     

Electrode reaction and microstructure of La0.6Sr0.4CoO3−δ thin films

Dense La_0.6Sr_0.4CoO_3−δ film electrodes were deposited by pulsed laser deposition (PLD) on Ce_0.9Gd_0.1O_1.95 electrolytes. The grain size of one film was 300–500 nm, and the other was 30–50 nm. DC polarization and AC impedance measurements were performed at 873 K–1073 K in O₂–Ar gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion.     

Study of adsorption of oxygen on β-Al2O3 + Au and β-Al2O3 + Pt: Work function measurements—proposition of a model

To improve the understanding of the electrochemical effects observed on an original potentiometric gas sensor, interactions of oxygen with the device were investigated. This gas sensor is made of a solid electrolyte (treated Na-β-alumina) associated with two metallic electrodes (gold and platinum) located in the same gas mixture. Adsorption of charged oxygen species, considered responsible for the electrical response developed by the sensor, was investigated by work function measurements. Results showed that charged oxygen species only form on partially gold or platinum covered solid electrolyte. Comparison of these results with those obtained in a previous calorimetric study of interactions between oxygen and the same materials suggests the existence of at least two different oxygen species adsorbed on the surface of the sensitive element. The first one, located on the solid electrolyte surface, is neutral and characterized by an endothermal reaction of formation. The second one is charged and probably produced at the gas/solid electrolyte/metallic electrode interface. A mechanism based on the concept of “three phase boundary” and similar to the “reverse spillover” phenomenon is proposed to account for the adsorption of these oxygen species.     

Antioxidant behaviour of 2′‐hydroxy‐chalcones: a study of their electrochemical properties

The electrochemical behaviour of 13 chalcone analogues was systematically studied by means of cyclic voltammetry and chronoamperometry at a glassy carbon (GC), gold and platinum working electrodes using two different supporting electrolyte/solvent combinations. It was found that chalcone analogues can be easily oxidized at both GC and gold working electrodes, but not at a platinum electrode. Principal component analysis was further employed to reveal similarities/dissimilarities between oxidation potentials, chronoamperometric signals and ability of the compounds to scavenge the reactive oxygen species H₂O₂. The study reveals the inverse proportional relationship between the scavenging ability of H₂O₂, expressed as IC₅₀, and chronoamperometric signal at 800 mV using gold as working electrode. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P_O2) of 0.01–1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P_O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.     

Oxygen reduction on lanthanum strontium manganites investigated by the use of cone shaped electrodes

The oxygen reduction kinetics has been investigated by impedance spectroscopy on two different cone shaped electrodes of La_0.85Sr_0.15MnO₃ in contact with either YSZ or CGO electrolyte pellets. The contact area has been varied by varying the mechanical load on the electrode on both polarized and unpolarized electrodes. The experiments showed the activation of the electrode observed after a cathodic polarization to be caused by the creation of an active zone on the electrolyte close to the triple phase boundary. As a result of this, the oxygen reduction on polarized electrodes probably takes place by several parallel routes making a kinetic investigation complicated. Impedance analysis of unpolarized electrodes at different oxygen partial pressures showed a charge transfer process at high frequencies involving oxygen defects and a mass transport limited reaction at lower frequencies. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

The formation and electrochemical property of lithium-excess cathode material Li<Subscript>1.2</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>Mn<Subscript>0.54</Subscript>O<Subscript>2</Subscript> with petal-like nanoplate microstructure

Lithium-excess oxide shows great potential for its high specific capacity of exceeding 280 mAh g^?1. However, the poor rate capability caused by the poor electrochemical kinetics condition as well as the structure instability block the way of its application. Here, we aimed to improve the kinetics circumstance for lithium ion transference through the material bulk by synthesizing lithium-excess oxide with high specific surface area. Petal-like nanoplates and nanoparticles with excellent electrochemical performance were obtained at different sintering temperatures and times by the electrospinning-sintering method, which facilitates the sufficient contact of electrode and electrolyte and helps to reduce the polarization during the electrochemical reaction process. Cyclic voltammetry tests verify that a portion of oxidized oxygen is reduced reversibly at 3.0 V and the reduction of oxygen contributes to the discharge capacity. Electrochemical impedance spectroscopy plots illustrate the ameliorative electrochemical kinetics is conductive to the oxidation of oxygen at 4.5 V.     

Electrode materials, interface processes and transport properties of yttria-doped zirconia

The kinetics of the oxygen exchange reactions at the electrodes of a galvanic cell using yttria-doped zirconia single crystals (9.5 mole-% Y₂O₃) as solid electrolyte and Pt or Ag as electrode materials was studied by complex impedance spectroscopy. The electrode resistance when using silver was found to have negligible values over the temperature range 180 – 900 °C. In agreement with these results, oxygen sensors were tested successfully at temperatures as low as 200 °C. According to the performance of silver as electrode material, an electrochemical method was developed to determine the oxygen diffusion coefficient in doped zirconia. The results obtained, compared to those of conductivity and oxygen tracer diffusion measurements, have allowed us to obtain information both on the structure of the defects in yttria-doped zirconia and on the correlation factor. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Sensors measuring oxygen activity in melts: Development of impedance method for“in-situ” diagnostics and control electrolyte/liquid-metal electrode interface

An impedance method for periodic“in-situ” diagnostics of the solid electrolyte/liquid-metal electrode interface during the lifespan of yttria-stabilized zirconia (YSZ)-based sensors measuring oxygen partial pressure in melts was developed. It was found that the impact of polarization effects on YSZ, stipulated by corrosive measuring environments (molten alkaline metals), is increasing with increase of the working temperature, which may lead to the appearance blocking reaction layers at the electrolyte/liquid-metal electrode interface. The proposed impedance method allows obtaining necessary information about the electrolyte/liquid-metal electrode interface and about the character and the level of polarization of the liquid-metal electrode.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Interfacial effects in electrochemical cells for oxygen ionic conduction measurements I. The e.m.f. method

Significant electrode polarization leads to the underestimation of apparent oxygen ion transference numbers of mixed ionic–electronic conductors, determined by measurement of either e.m.f. of oxygen concentration cells or Faradaic efficiency. In addition to e.m.f. data under open-circuit conditions, the exact measurement of the transference numbers requires no less than two parameters describing the system such as oxygen permeation flux, bulk resistance, voltage or current under closed-circuit conditions. Selected examples of experimental routes, aimed at taking electrode polarization into account, are considered. An appropriate choice of experimental procedures used in e.m.f. or Faradaic efficiency measurements, in combination with the use of electrodes with high polarization resistance, enables significant accuracy improvement in the determination of very small electronic contributions to the total conductivity. For potentiometric oxygen sensors, the presence of minor electronic conduction was shown to result in great experimental errors in the oxygen partial pressure measurement if the polarization resistance of sensor electrodes is high. Analogously, minor electronic conductivity in solid electrolytes may lead to a large error of the polarization resistance determination by impedance spectroscopy in conditions where the polarization resistance significantly exceeds the bulk resistance of the electrolyte.     

A mechanistic study on the activation process of (La,Sr)MnO3 electrodes of solid oxide fuel cells

The mechanism of the activation process for the O₂ reduction on (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes is investigated by examining the electrochemical behavior of LSM under cathodic and anodic polarization conditions and the relaxation behavior of LSM under open circuit. A comparative study is also performed on a LSM electrode after dilute acid etching treatment. It has been shown that the segregated SrO has a significant inhibiting effect on the surface exchange process such as dissociative adsorption, incorporation and diffusion of oxygen species on the LSM surface, resulting in the initially very high impedance for the O₂ reduction on LSM electrodes. A mechanism involving the incorporation of SrO into LSM lattice with the concomitant removal of cation vacancies is proposed for the activation effect of cathodic current passage/polarization in solid oxide fuel cells.     

Electrochemical study of silver thiosulphate reduction in the absence and presence of ultrasound

The electrochemical reduction of silver thiosulphate was studied potentiostatically on platinum electrodes in the absence and presence of ultrasound (20 kHz). This system is irreversible and the reaction is both diffusion and kinetically controlled. The slowest step is the kinetic reaction especially the chemisorption of ions at the electrode surface. Ultrasound greatly improves the mass transport, which can be explained by changing from diffusion to mainly convection. This paper reports the effect of ultrasound upon electrode kinetic and mass-transport parameters at various RDE rotation speeds and ultrasonic intensities. It was found that the heterogeneous rate constant (kf) is improved in the presence of ultrasound due to the increase in the formal or standard heterogeneous rate constant (k0) (approximately by 10-fold under sonication).     

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HC's) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga₂O₃ were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C₃H₆, O₂ gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C₃H₆. The optimal composition is found to be at 20 mass-% Ga₂O₃. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrochemical and structural study of neodymium nickelate thick film deposited by spin coating on an oxyapatite electrolyte

The electrochemical characteristics of an Nd₂NiO₄ cathode, synthesized using the citric acid-nitrate combustion method and symmetrically deposited by spin coating on both surfaces of an La_9.33Si_5.3Al_0.7O_25.65 (LSAO) electrolyte, were studied using AC impedance spectroscopy. It was found that, varying the temperature (600 to 850 °C) and the oxygen partial pressure (0.2–10^?7 atm), the impedance characteristics of the electrode could be determined by three different processes. Said processes were ionic conduction in the electrode bulk, interfacial charge transfer, and gas-phase diffusion, all of which are based on the temperature and oxygen partial pressure dependence on the polarization resistance, which is associated with each individual processes as well as on the corresponding capacitance values.     

Electrocatalysis of oxygen reduction reaction on Nafion/platinum/gas diffusion layer electrode for PEM fuel cell

In this work a new membrane electrode based on Pt-coated Nafion membrane was fabricated. Chemical deposition process was used to coat platinum on Nafion 117 membrane and then Pt-coated Nafion membrane was hot pressed on gas diffusion layer (GDL) to make new membrane electrode. The electrochemical and chemical studies of the Pt-coated Nafions were investigated by electrochemical techniques, X-ray diffraction and scanning electron microscopy. The electrochemical results indicated that as the concentration of H₂PtCl₆ increased, the oxygen reduction reaction rate increased until the concentration was reached where the reduction reaction was limited by the problem of mass transport. The electrochemical results for oxygen reduction reaction showed that the new electrode which prepared by plating Nafion membrane with 0.06 M H₂PtCl₆ in electroless plating solution, has a higher performance than other electrodes. The XRD results showed that the average platinum particle size of the best sample was about 3 nm. The loading of platinum for this electrode was 0.153 mg cm⁻².