Periodically Pulsed Polarization Gas Sensors Based on Au|YSZ: Mechanism of NOx Detection.

Perovskites within the (La,Sr)(Fe,Co)O3 class of materials show variations in the oxygen stoichiometry depending on temperature and oxygen activity and can potentially be used as catalysts, electrodes in high-temperature solid oxide fuel cells, gas sensors or for oxygen transport membranes. These perovskites possess a mixed ionic and electronic conductivity (MIEC), which can be highly beneficial for the processes on oxygen electrode surfaces. The oxygen transport through a MIEC is determined by the rate of the oxygen exchange over the gas-solid interface and the diffusivity of oxide ions and electrons (or holes) in the bulk. The oxygen exchange process over the surface in general involves several reaction steps, O2 adsorption, dissociation, charge transfer and incorporation of ionic species. The Co-free end member of the material class; LSF (e.g. (La0.6Sr0.4FeO3-δ ) is fairly low cost and chemically stable in both mildly reducing and oxidizing atmosphere. The electronic conductivity is excellent (283 S/cm at 800 °C) but the ionic conductivity especially at low temperature is limited (0.014 S/cm, 800 °C). Due to these properties the material is a candidate for use in composite membranes in combination with a better ionic conducting material like CGO. Such systems are also excellent model systems for fundamental studies of the oxygen exchange process.The aim of this study is to characterize the oxygen transport properties of a dual-phase composite system as a function of temperature, and to elucidate the origin of the enhancement of the surface exchange reaction that has been reported to occur in such systems1by increasing the amount of ionic conductor. Additionally, the effect of surface decorations (desired) and impurities (undesired) on the surface exchange was studied to illustrate the importance of controlled conditions for the sample preparation and measurements. Electrical Conductivity Relaxation (ECR) was used to study the surface exchange reaction and the oxygen ion diffusion of single phase (La0.6Sr0.4)0.98FeO3-δ and dual phase composites of (La0.6Sr0.4)0.98FeO3-δ – Ce0.9Gd0.1O1.95 (LSF-CGO). From the relaxation curves, fitted by using Fick’s Laws of diffusion with appropriate boundary conditions, both the oxygen surface exchange coefficient (k ex) and the oxygen chemical diffusion coefficient (D chem) was derived. The results of ECR clearly show that the CGO surface is involved in the surface exchange reactions on the composite and that by increasing the fraction of CGO the surface exchange is significantly enhanced. The involvement of CGO in the surface exchange can arise from fast exchange in the triple phase boundaries and/or the spillover of oxygen ions from LSF to CGO, where they incorporate. Additional effects can also originate from the fact that LSF scavenges impurities from CGO and therefore activate the CGO surface for the surface exchange reaction as proposed by Druce et al. 2. This was separately studied by coating of the surface of the single phase LSF and the composite with different elements/oxides regarded as potential impurities. However, opposite to the trivial anticipation a strong positive influence of secondary phases (not normally considered to be good oxygen reduction catalysts) on the surface exchange coefficient was observed. For example depositing CaO/CaCO3 on the surface led to an enhancement of the oxygen incorporation rate for 3.4 times at 750 °C.The study concludes that the formation of a composite of MIEC and ionic conductor and controlled coating with a range of secondary phases on the surface (e.g. CaO, …) is highly beneficial for the oxygen reduction process.1. Hu, B.; Wang, Y.; Xia, C., J. Power Sources 2014, 269, 180-1882. Druce, J.; Kilner, J. A., J. Electrochem. Soc. 2014 , 161, F99-F104.

Introduction Nitrogen oxides (NO x ; NO and NO 2 ) are limited emissions from combustion processes. They are not only harmful to human health, but also to the environment. This makes it necessary to measure and reduce nitrogen oxide emissions. Pulse polarization is a novel method for NO x concentration measurements. In contrast to existing static principles, this method utilizes the dynamic response of the sensor, similar to cyclic voltammetry or impedance spectroscopy. For pulse polarization, the sensor is polarized with a constant voltage U pol for t pol (Fig. 1). After applying the voltage, the self-discharge of the sensor is recorded over a defined time t discharge . Charging and discharging phases are repeated continuously, with alternating change of the charging voltage polarity. It has been shown that NO x selectively accelerates the discharge of the Pt|YSZ system [1]. The accelerated discharge can be used as a sensor signal by evaluating the voltages at a fixed time during the discharge phase. Due to the faster discharge, these voltages are below the values without nitrogen oxides and thus indicate the concentration of the analyte gas. A semi-log dependency between U and c NOx was found. However, the effects that lead to faster and selective discharge have not yet been fully understood. In order to investigate the effect of the catalytically active Pt electrodes in particular, they were replaced by gold electrodes, which should yield a significantly lower catalytic impact. Experimental To prepare the sensors, two rectangular gold electrodes were screen printed on both sides of an 8YSZ substrate and then fired at 850 °C. The sensors were contacted with Au wires by gap welding. The sensors were operated at 400 °C in a tube furnace. For pulse polarization, a sourcemeter was periodically connected to the sensors via relays and the voltages were recorded. A polarization voltage of U pol = 1 V, a polarization duration t pol = 1 s and a discharge time t discharge = 10 s were chosen. This lead to a total cycle time t cycle = 22 s. To determine the gas concentration dependence in the discharge phase, a mixture of 10 % O 2 with 2 % H 2 O in nitrogen was defined as base gas. In addition, NO, NO 2 , and a mixture of 50 % NO and NO 2 (NO x ) in concentrations between 50 and 200 ppm were added to the gas flow. Results and discussion The sensor signals are shown in Fig. 2. The voltages U 4s_neg shown were measured during self-discharging 4 s after each negative polarization. The long measuring time of 18 h and the cycle duration of 22 s result in over 2900 cycles during the measurement. The baseline of the voltage curve shows that the cycles are very stable during the entire measuring period. It is also noticeable that NO and NO 2 have an opposite effect on the self-discharge of the sensor. While NO 2 as well as a 50/50 mixture of NO and NO 2 accelerate the discharge, NO gas decelerates it. This means, the resulting absolute value of U 4s_neg is lower than that in NO x -free gas in case of accelerated discharge and higher at decelerated discharge. The mixture of 50% NO and 50% NO 2 has an accelerating effect. The accelerated self-discharge in the presence of NO 2 was already observed for platinum electrodes. In contrast to this, the discharge decelerated by NO as found here for gold electrodes has not yet been observed on Pt electrodes. We attribute the behavior on platinum electrodes to the gas phase reactions that occur during diffusion through the electrode and to the assumption that the gases are almost in thermodynamic equilibrium at the three-phase contact between electrode, electrolyte and gas phase [2]. At a temperature of 400 °C and an oxygen content of 10 %, this equilibrium is about 50 % NO and 50 % NO 2 . This would result in conditions similar to those for NO x to be dosed, which also accelerates the discharge. The equilibrium thus explains the same signal for NO and NO 2 . The catalytically less active gold electrodes make it possible to separate the influence of NO and NO 2 . The strong oxidizing effect of NO 2 [3] is expected to play a key role in the sensor effect. By applying the voltages, oxygen is pumped from the cathode to the anode. This leads to a lack of oxygen at the cathode and an excess at the anode. After polarization, NO 2 probably supplies the electrode, which is depleted of oxygen due to polarization, with additional oxide. This helps to reduce the oxygen gradient and thus leads to an accelerated discharge. In contrast, NO seems to slow down the oxygen supply at the oxygen depleted electrode and thus decelerate the discharge. The removal of oxygen at the oxygen-rich electrode seems to play a minor role. If both the oxygen supply on one side and the oxygen removal on the other were equally important, NO x would provide the strongest acceleration of the discharge.

NOx Detection By Pulse Polarization: Influence of Gold Electrodes

Effect of hetero-structured nano-particulate coating on the oxygen surface exchange properties of La0.6Sr0.4Co0.2Fe0.8O3-δ

Evaluating oxygen diffusion, surface exchange and oxygen semi-permeation in Ln2NiO4+δ membranes (Ln=La, Pr and Nd)

The oxygen reduction active sites were visualized around the O2/SOFC cathode/electrolyte triple phase boundaries (TPB) by the16O/18O exchange techniques and secondary ion mass spectrometry (SIMS) analysis. The higher18O concentration is observed on the cathode top surfaces (La0.9Sr0.1MnO3-mesh, Au-mesh, and Ag-porous), which suggested the promotion of oxygen adsorption and oxygen surface exchange at the cathode. The oxygen diffusion through the bulk of cathode occurred at the La0.9Sr0.1MnO3-mesh and the Ag-porous cathodes, not at the Au-mesh cathode. On the YSZ surfaces after removing the cathode, the active sites for oxygen incorporation were analyzed by SIMS. The active sites for oxygen incorporation were at the La0.9Sr0.1MnO3/YSZ interface as well as the TPB. On the other hand, the active sites for oxygen incorporation are limited to the TPB in the case of the Au-mesh removed YSZ surface. From the SIMS analysis, the expansion of the active sites for oxygen incorporation is less than a few μm from the TPB lines.

The high temperature oxygen surface exchange kinetics of mixed conducting oxides play a critical role in the efficiency of solid oxide fuel/electrolysis cells (SOCs). Recently, an in situ Optical Transmission Relaxation (OTR) approach has been applied to quantify the oxygen surface exchange coefficients (kchem ) of thin films, with the advantage of providing contact-free, in situ and continuous measurements of native surfaces. The technique relies upon the application of the Beer-Lambert law, where optical absorption is proportional to the concentration of absorbing species, e.g. oxidized Pr (~Pr4+) in PrxCe1-xO2- δ (PCO) [1 ,2 ] or oxidized Fe (~Fe4+) in SrTi1-xFexO3- δ (STF), and to the oxygen stoichiometry via electroneutrality. For example in STF, during reduction (oxygen evolution), absorbing Fe4+ is replaced by Fe3+, resulting in an increase in measured optical transmission through the STF with time, which can be described by the surface exchange-limited kinetics equation to determine kchem . [ 3 ]. As we know, the grain size and degree of crystallinity could affect the thin films’ electrical, optical and catalytic properties. In our previous work, we examined the impact of crystallinity, grain boundaries, orientation, and surface chemistry on kchem for STF (x=0.35, STF35) by preparing films of different structures by pulsed laser deposition under different conditions. We found that fast oxygen surface exchange needs both crystallinity (typically obtained at high growth temperatures) and low Sr surface concentration (typically obtained at low growth temperatures). Amorphous STF thin films were not able to exhibit optically measurable oxygen exchange, while excellent crystallinity, obtained at high growth temperatures, coexisted with high Sr segregation and therefore non-optimal kchem . However, crystallization at intermediate temperatures was used to obtain much faster surface exchange kinetics [ 4 ]. In this work, we further studied the effect of crystallization on oxygen surface exchange kinetics, by post-annealing as-grown amorphous STF35 thin films and observing the evolution of their oxygen exchange behavior in situ during crystallization. The impact of annealing temperature (500-800 °C) and time (2-100 h) on the crystallinity and microstructure was observed by scanning probe microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and in situ optical transmission changes. The consistent appearance of a significant decrease in optical transmission during heating suggested that crystallization began around 545-550 ºC. The best kchem was found for annealing at 600 ºC for 2 h. The thin film with lower (or perhaps no) crystallinity annealed at 500 °C was not able to exhibit detectable oxygen surface exchange, while higher annealing temperatures and/or times led to non-optimal kchem , which is consistent with our previous research. In order to assess whether the benefits of crystallization under mild conditions for rapid surface exchange also applied to other compositions and structures, we investigated the surface exchange kinetics during in situ crystallization of other materials, such as perovskite SrTi0.65Co0.35O3- δ (STC35), fluorite Pr0.1Ce0.9O2- δ (PCO), and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4± δ (RP-STF35). Optical relaxations were observed in these materials, enabling the quantification of their kchem . For the perovskite STC35 and RP-STF35 thin films, in-situ crystallization was observed to benefit the oxygen exchange kinetics. On the other hand, for a fluorite PCO thin film, we found that it was already crystalline after deposition at 25 ºC. Therefore, the annealing process for PCO thin film did not show a positive effect on kchem . Implications for use of thin film electrodes in intermediate temperature devices will be addressed. Acknowledgements Support from WPI-I2CNER and a JSPS Kakenhi Grant-in-aid for Young Scientists (B) project number JP15K18213 (to N. H. Perry) and JSPS Fellowship (201702103) are gratefully acknowledged. Reference [1] J. J. Kim, S. R. Bishop, N. J. Thompson, D. Chen and H. L. Tuller, “Investigation of nonstoichiometry in oxide thin films by simultaneous in s itu optical absorption and chemical capacitance measurements: Pr-doped ceria, a case study”, Chemistry of Materials , 2014, 26, 1374-1379. [2] J. J. Kim, S. R. Bishop, N. J. Thompson and H. L. Tuller, “Investigation of redox kinetics by simultaneous i n s itu optical absorption relaxation and electrode impedance measurements: pr doped ceria thin films”, ECS Transactions, 57 (1) 1979-1984 (2013) [3] I. Denk, F. Noll and J. Maier, “In situ profiles of oxygen diffusion in SrTiO3: bulk behavior and boundary effects”, Journal of the American Ceramic Society , 1997, 80, 279-285. [4] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach”, Journal of Materials Chemistry A , 2017, 5, 23006-23019

Abstract. Symmetrical Pt|YSZ|Pt sensors were produced by screen printing with frit-containing and fritless Pt pastes and fired at 950, 1100, and 1300 ∘C. Subsequently, the sensors were operated by pulsed polarization, and the NO sensitivity was investigated. The sensitivity of the sensors with fritless pastes was found to be significantly higher. The influence of the firing temperature was low in contrast to the influence of the paste. The low NO sensitivity of the frit-containing electrodes was attributed to a blocking effect that probably occurs at the triple-phase boundaries. Therefore, the oxygen transport through the sensor is inhibited, which, however, seems to be necessary for the sensor effect.

Introduction In solid electrolyte-based electrochemical devices such as fuel cells and gas sensors, the devices include hetero interfaces between solid electrolyte and metal, which play an important role for the device working mechanism. The interface between solid-electrolyte and solid-metal can be recognized as a two phase boundary, and the two phase boundary exposed to vacuum (i.e., gas phase) is called as triple (or three) phase boundary (TPB). TPB in electrochemical devices is a key boundary for electrochemical reactions because the boundary is an active site for charge transfer reactions, and thus for the production of electricity. In addition, a rate limiting step in electrochemical devices is often identified in the electrochemical reactions at TPB, although a specific investigation is required in each system. In this study, we carried out first principles electronic structure calculations for the precise understanding of electrochemical processes on Ni-YSZ TPB, and show how TPB models should be constructed in the atomistic level, and what is the suitable index for the characterization of the electrochemical activity of TPB. Method For the investigation of electrochemical reactions in the atomistic level, first principles electronic structure method is a powerful theoretical tool, but the calculation procedures are not so simple when the target system includes complicated interface TPBs. The TPB atomistic model must include the following ingredients: 1) metal region (i.e., slab), 2) solid electrolyte region (slab), 3) the metal-solid electrolyte interface, and 4) the interface exposed to vacuum [1,2]. When we construct a TPB model holding these conditions, the number of atoms included in the computational cell becomes large, and reaction analysis based on the large models is sometimes found to be inappropriate depending on the computational costs. Therefore, theoretical calculations for reaction analysis on TPB were sometimes carried out using small metal-cluster deposited on solid electrolyte slab. However, the electronic structures of metal-slab and metal-cluster are very different because of the finite size effect of the metal cluster model. At the first part of this study, we will present how the metal cluster models on solid electrolyte inappropriately affect computational results at TPB, and how an acceptable TPB model should be constructed based on metal-slab layers (see an example for TPB structure in Figure). The second step is the selection of the stoichiometry and the determination of impurity positions. For example in yttrium stabilized zirconia (YSZ) as solid electrolyte, the numbers of dopant Y and oxygen vacancy are determined depending on the stoichiometry and dopant concentration. Thus, we usually fix the system stoichiometry and dopant concentration at first (e.g., stoichiometric or oxygen-rich or oxygen poor in YSZ), and in turn determine the position of dopants from the calculated total energies. The reaction analysis using the atomistic models are thereby carried out as the final step by using first principles calculations with the nudged elastic band method. Since the standard calculations are done along the procedure, the computational results are usually characterized in terms of the system stoichiometry. However, in electrochemical reactions in TPB, the system stoichiometry is sometimes not a good index for the characterization of TPB activity. At the second part of this study, we will present the calculated reaction energies at TPB are highly scattered when those are classified with the system stoichiometry, but clearly characterized when those are classified with a new index, TPB local stoichiometry. We will present the detail of the new concept for the TPB local stoichiometry. [1] Tomofumi Tada, Shusuke Kasamatsu, and Satoshi Watanabe, First principles study on electronic structures of Ni/H/ZrO2 triple phase boundary, ECS Transactions 16(51) , 265 (2009). [2] Tomofumi Tada and Satoshi Watanabe, Chemically softened boundary of metal/vacuum/solid-electrolyte from first principles, J. Phys. Chem. C 113 , 17780 (2009). Figure 1

Imaging of oxygen transport at SOFC cathode/electrolyte interfaces by a novel technique

Introduction Recently, a new anode incorporating a BaCe0.8Y0.2O3-δ (BCY) proton conductor was proposed by the authors to increase a power density of SOFCs, and it was found that the anode overpotential was reduced by incorporating BCY particles to the conventional Ni/GDC cermet anode1. Moreover, it was found that BCY particles might contribute to supply the adsorbed hydrogen to the triple phase boundary (TPB) in anodic reaction2. Based on these results, a modified reaction model around the TPB was proposed to explain the mechanism of overpotential reduction in the anode incorporating the proton conductor, and analytical expression of current density with oxygen activity and anode overpotential with current density were obtained3. In this study, the analytical expression of interfacial conductivity of anode (the inverse of the anode polarization resistance) was obtained based on the reaction model3. In addition, the analytical results of overpotential or interfacial conductivity of conventional Ni/YSZ anode were compared with those obtained from experiments4, and the validity of the reaction model was discussed. Reaction model The details of the reaction model were shown in our previous work3. In this model, some finite areas, which contribute to species adsorption, are assumed around TPB, and hydrogen and oxygen are assumed to be adsorbed mainly on the surface areas of Ni and oxide ion conductor, respectively (We define this idea as “species territory adsorption model”). In addition, it is also assumed that the reaction rate in the anode is controlled by the surface reaction between Had and Oad, while all other reaction takes place under the condition of chemical equilibrium. Based on the model, under the condition that the pure oxygen (P O2=1atm) is used in the cathode side, a simple expression for current density i and anode overpotential ηa could be obtained explicitly as a function of an oxygen activity at the anode a O and a current density i, respectively. Here, a O is expressed as a O=exp(2FE/RT) (E is anode potential). Using the analytical expression of i as a function of a O, the interfacial conductivity (σE ) of anode, which is described as the inverse of the anode polarization resistance, is derived as follows. Eq. [1] Here, θV (1) and θV (2) means the coverage of vacant adsorption site on the surface areas of Ni and oxide ion conductor, respectively. These are described as follows. Eq. [2, 3] Here, P H2 and P H2O are partial pressure of hydrogen and water vapor at the anode side, respectively. Ki , K'i (i = species) and kaj , kcj (a, c = anodic or cathodic reaction for 2Had+Oad → H2Oad+2Vad ) show the equilibrium constant and the rate constant, respectively. A unique combination of fitting parameters, such as equilibrium constants and reaction rate constants were obtained from a comparison between analytical and experimental results4 for the case of Ni/YSZ anode. Analytical results and discussions with experiments Figure 1 shows the analytical and experimental4 results of anode overpotential of Ni/YSZ with current density at 1273K. From Figure 1, it was disclosed that the anode overpotential decreased with increasing humidity from 3% to 20%, while, it increased with increasing humidity from 20% to 40%. These results are in good agreement with those of experiments. Moreover, Figure 2 shows the analytical and experimental4 results of interfacial conductivity of Ni/YSZ as a function of water vapor pressure at 1273K under the open-circuit condition. In this case, the hydrogen partial pressure P H2 was equal to 1-P H2O atm. The analytical results were calculated from Eq. [1]. From Figure 2, it was disclosed that the interfacial conductivity had a local maximum value in the range of P H2O=0.15-0.25 atm in the experiments. These results successfully followed the analytical results. From the analytical results, it was disclosed that the term ∂i/∂a O decreases with increasing P H2O, while ∂a O /∂E increases. As a result, σE has a local maximum value. Conclusions Based on the reaction model of SOFC anode proposed by the authors, the overpotential and interfacial conductivity of conventional Ni/YSZ anode was discussed. The experimental results of humidity dependence of anode overpotential or interfacial conductivity is in good agreement with those obtained by the analytical results based on the proposed model.

Oxygen transport membranes (OTMs) are attracting great interest for the separation of oxygen from air in an energy efficient way. In the last decade one major driver was the development of CO2 mitigation scenarios utilizing carbon capture and storage (CCS) technology for large point sources. One very promising concept is the oxyfuel technology, which realizes the combustion of fossil fuels with oxygen-enriched recirculated flue gas thus requiring enormous quantities of oxygen. OTMs can deliver this oxygen efficiently if the required heat is delivered by the process itself as it is in the case of power, cement, glass, or steel plants. Another possible application is the operation of a membrane reactor, in which oxygen is directly consumed by a chemical reaction. In this context, the research focuses on the partial oxidation of methane or even the oxidative coupling of methane to higher hydrocarbons such as ethylene, propylene, aromatics etc. Such OTMs consist of mixed ionic electronic conductors (MIEC), which can consist of a single phase MIEC material or a composite of two separate phases, each phase providing electronic or ionic conductivity, respectively. In general, the bulk transport is based on defects in the crystal lattice, i.e. oxygen vacancies and electron holes. This leads to a trade-off between permeability requiring a high defect concentration and stability, requiring a low defect concentration. Therefore, materials with limited permeability have to be used in many applications to provide a long term stable operation. In order to still reach sufficiently high permeation rates, thin supported membrane layers are developed in different shapes, mainly planar or as capillaries. In case that the membrane thickness is below a characteristic thickness, surface exchange kinetics become rate limiting because of the relatively fast diffusion through the thin bulk membrane. Consequently, the solid-gas electrochemical interfaces become more and more important, making catalytically active layers, such as porous electrodes, a key element in the future development of long term stable, high performance membranes. In this presentation the development of thin (8µm – 400µm) supported membrane layers is described using high performance perovskites, i.e. Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6-xSr0.4Co0.2Fe0.8O3-δ (LSCF). Oxygen permeation measurements with varying conditions, i.e. temperature as well as feed gas composition and sweep gas flow rates, are used to identify limiting transport processes. The overall oxygen transport process of BSCF membranes is analyzed by combining different modelling approaches. While the gas diffusion in the gas phase and the porous support is modelled by cfd and binary friction model respectively, the transport through the bulk and the surface exchange is addressed in combination using a modified Wagner approach, accounting for the electrochemically active surface areas. According to this model, either gas diffusion in the porous support or the oxygen surface exchange is limiting the transport depending on experimental conditions. Surface exchange limitations can be partly overcome using porous electrode layers made of the membrane material. Due to the high electrochemical activity of the perovskites used, an additional permeation enhancement using noble metal catalysts cannot easily be realized at high temperatures. In addition, dual phase membranes were investigated, which are much more prone to surface exchange limitations because of the limited length of the active triple phase boundaries. Electrodes made of a single phase MIEC material, i.e. LSCF, show evidence of these limitations even using 1 mm thick samples.

Oxygen diffusivities and surface exchange coefficients in various porous mullite/zirconia composites were measured at oxygen partial pressures ranging from 20.2 to 2.02 kPa using the conductivity relaxation method. However, the oxygen diffusivities in porous high‐zirconia composites could not be determined because of the predominant surface exchange reaction. Oxygen diffusivities and surface exchange coefficients in low‐zirconia composites increased with the zirconia content, while the surface exchange coefficients in high‐zirconia composites were approximately constant. A percolation threshold of the surface exchange coefficients occured at ∼40 vol% zirconia for porous zirconia/mullite composites. The oxygen diffusivities in porous low‐zirconia composites were independent of the oxygen partial pressure, implying that oxygen diffusion in these composites was related to the migration of oxygen vacancies, whose concentration was independent of the oxygen partial pressure. The surface exchange coefficients of high‐zirconia composites decreased with increasing oxygen partial pressure. Finally, it was inferred that the rate‐limiting step for oxygen surface exchange could be the charge‐transfer process.

Investigations were conducted on Li/BrCl in SOCl2 cells to determine both the performance and shelf life characteristics after storage for a period of one year at various temperatures. Room temperature discharge test results indicate that there is little self-discharge in those cells stored at either room temperature or at −40° C. Even when the cells were subjected to elevated storage temperatures, the discharge results show that the cells exhibit a low self-discharge rate. However, cells stored at an elevated temperature do exhibit a decreased rate capability when compared to those results obtained from fresh cells. From the 56 ohm load discharge performance results, the self-discharge rate for cells stored at −40, 24, 50 and 72° C was calculated. Further, the activation energy over the temperature range was found to be 27.6±1.7 kJ. Also, the self-discharge rate was monitored under light load conditions. The results for Li/BrCl in SOCl2 D cells discharged under a current density of 2.38 to 0.016 mA cm−2, show that the operationally induced self-discharge rate reaches a maximum of 4.5μA cm−2 for cells discharged under a current density of 0.034 mA cm−2 and decreases to 2.9μA cm−2 as the discharge current density decreases to 0.016 mA cm−2. These data indicate that the self-discharge rate for cells discharged under very light loads may eventually approach that for cells stored under open circuit conditions.

This paper presents, for the first time, the results of studies of the electrochemical reaction of oxygen reduction on Sr2Fe1.5Mo0.5O6-δ and Sr-deficient Sr1.95Fe1.5Mo0.5O6-δ, as promising electrodes for solid state electrochemical devices, by the electrochemical impedance method with the subsequent interpretation of the data using the concepts outlined in the Adler et al. model. It was established that the oxygen reduction reaction for both electrodes is determined by two relaxation processes associated with oxygen diffusion, oxygen surface exchange, and Knudsen diffusion in the pores of the electrode. Strontium deficiency was found to have a positive effect on the electrochemical activity of the electrodes, enhancing the stage related to oxygen diffusion and surface exchange. Also, for the first time, for Sr2Fe1.5Mo0.5O6-δ, a quantitative correlation for the surface oxygen exchange and diffusion coefficients obtained from the impedance data and by the isotopic exchange method is demonstrated.