Oxygen Chemical Diffusion Research Articles

Temperature-Dependent Oxidation Behavior of Silicon Carbide Surface: Reactive Molecular Dynamics Simulations.

The high-temperature oxidation mechanism of silicon carbide (SiC) is crucial for designing thermal protection systems in aircraft. This study explored the oxidative chemical reaction processes and mechanism on SiC surface and interfaces within the temperature range of 300-2300 K by using reactive molecular dynamics simulation. The oxygen impact on the silica (SiOx) growth of the SiC surface was analyzed, which shows a progressive increase of silica thickness. The simulation results indicated that the oxidation process of SiC was a typical passive oxidation mechanism. With the environmental temperature rising and oxygen impact, the increase of oxidation thickness on the SiC surface undergoes three oxidizing reaction processes: little chemical adsorption of oxygen molecules on the initial surface, rapid oxidation of silicon and carbon, and dramatic oxidation of the interface between the oxidation layer and SiC. Additionally, this work studied the mechanism of oxidation thickness growth and chemical diffusion of oxygen. The oxidation rate is weakened according to the oxygen atom diffusion barrier effect of silica repulsion. Moreover, the kinetic parameters were statistically calculated by fitting the growth of Si-O bonds and their reaction rate constants. Subsequently, the activation energy and pre-exponential factors were derived by using the Arrhenius equation to model the chemical reaction kinetics of the thermal oxidation process. The chemical reaction behaviors of the two stages could be concluded as follows: (i) in stage I, the initial oxidation is reaction rate limiting; (ii) in stage II, SiC oxidation is limited by both the oxidation reaction rate and the oxygen diffusion coefficient of the oxidation layer. The activation energy of stage II increased compared with stage I due to the oxygen atoms diffusion barrier between the oxidation layers. This study on the oxidation and ablation mechanism of the SiC surface at the atomic scale would provide insight into understanding thermal oxidation behavior and the design of ceramic materials.

Realizing extraordinary bifunctional electrocatalytic performance of layered perovskite through Ba-site Gd doping toward oxygen reduction and evolution reactions

Herein, we propose a Ba-site Gd doping protocol in layered perovskites, demonstrating highly active bifunctional PrBa1-xGdxCo2O5+δ electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Benefiting from enhanced oxygen surface exchange and chemical diffusion kinetics, resultant PrBa0.95Gd0.05Co2O5+δ (PBG0.05CO) exhibits an area-specific resistance (ASR) of 0.038 Ω cm2 at 700 °C, reduced by ∼46.5% relative to undoped PrBaCo2O5+δ (PBCO). When evaluated as the air electrode, the as-fabricated single coin fuel cell delivers a peak power density (PPD) of 1230 mW/cm2 at 700 °C. For the CO2-electrolysis at 750 °C, the large current density (J) of 2630 mA/cm2 is achieved in the PBG0.05CO anode-based electrolysis cell at a voltage (V) of 1.8 V. Furthermore, exceptional operating stability is realized in both ORR and OER manners. Our findings highlight an effective strategy to optimize the electrocatalytic properties of layered perovskites, endowing potential applications in energy storage and conversion aspects.

Synergistic effects of A-site deficiency and anion doping on Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode for H2O electrolysis and CO2 electrolysis

In this work, a strategy of A-site deficiency and F-doping was proposed to prepare Pr0.4Sr0.55Ni0.2Fe0.7Mo0.1F0.1O2.9 (PSNFMF) cathode. Compared with the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 (R-PSNFM), the reduced PSNFMF facilitate the exsolution reaction and generate more oxygen vacancies. Moreover, at 800 °C, the chemical oxygen surface exchange coefficient (kchem) of R-PSNFMF is 4.31 × 10−4 cm s−1, 21.8% higher than the value of 3.54 × 10−4 cm s−1 for R-PSNFM, and the oxygen chemical bulk diffusion coefficient (Dchem) of R-PSNFMF is 34.21 × 10−5 cm2 s−1, 24.3% higher than the value of 27.53 × 10−5 cm2 s−1 for R-PSNFM, suggesting faster oxygen transport kinetics are obtained for R-PSNFMF. Meanwhile, the synergistic effect of A-site deficiency and anion doping can also promote the electrolysis of H2O and CO2. For H2O electrolysis, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.75 A cm−2, 31.6% higher than the value of −0.56 A cm−2 for R-PSNFM based cell. And for CO2 electrolysis in harsh condition at 800 °C, the current density of R-PSNFMF based cell at 0.4 V (vs. OCV) is −0.43 A cm−2, 38.7% higher than the value of −0.31 A cm−2 for R-PSNFM based cell.

A series of bifunctional ReBaCo2O5+δ perovskite catalysts towards intermediate-temperature oxygen reduction reaction and oxygen evolution reaction

High-performance electrocatalysts are crucial to accelerate the commercialization of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). Herein we develop a series of bifunctional ReBaCo2O5+δ (Re = La, Pr, Nd, Sm, Eu, and Gd) perovskite catalysts, demonstrating both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity. When varying the rare earth ions, orthorhombic-tetragonal-cubic transformation can be identified in the ReBaCo2O5+δ (ReBCO) family, along with enhanced electrical conductivity, improved oxygen surface exchange and chemical diffusion rates. Through symmetrical half-cell test, as-obtained perovskite electrodes exhibit the area-specific resistances (ASRs) as low as 0.048–0.096 Ω cm2 at 700 °C. In the ORR and OER manners, LaBaCo2O5+δ (LBCO) shows the largest current densities of 314 and 323 mA cm−2 at an overpotential (η) of 75 mV at 700 °C among all compositions. Anode-supported fuel cell with the LBCO cathode delivers a peak power density (PPD) of 1212 mW cm−2 at 750 °C. Aiming at the CO2 electrolysis, a current density of 1.24 A cm−2 is achieved in the LBCO anode-containing cell at 750 °C. From first-principle density functional theory (DFT) calculations, it is inferred that the O 2p-band centers for electron-filled states may be closely correlated with electrocatalytic activity. Our results highlight promising intermediate-temperature applications of LBCO in both SOFC and SOEC models, endowing rational design of the perovskite catalysts in energy conversion field.

Considerable oxygen reduction activity and durability of BaO nanoparticles-decorated Ln0.94BaCo2O5+δ electrocatalysts

Owing to unique catalytic properties and durability, nanoparticles-containing heterostructured catalysts have attracted much interest in energy storage and conversion fields. Here we report a multi-composite electrocatalyst consisting of BaO nanoparticles and Ln0.94BaCo2O5+δ (Ln = La, Pr, Nd, Sm, and Gd) (e-Ln0.94BCO) though in situ exsolution route. As-obtained e-Ln0.94BCO catalysts exhibit desirable oxygen reduction reaction (ORR) activity and durability toward solid oxide fuel cells (SOFCs) cathode application. When varying Ln3+ from Gd3+ to La3+, the phase transformation of orthorhombic-tetragonal-cubic can be identified in Ln0.94BaCo2O5+δ, significantly demonstrating improved electrochemical performance as well as electrical conductivity, oxygen surface exchange and chemical diffusion coefficients. Resultantly, the e-L0.94BCO cathode exhibits the lowest area-specific resistance (ASR) of 0.03 Ω cm2 at 700 °C, reduced by ≈ 43% relative to pristine La0.94BaCo2O5+δ (L0.94BCO) cathode. Under realistic operating conditions, the e-L0.94BCO cathode-based button cell delivers a peak power density (PPD) 0f 1.05 W cm−2 at 700 °C, along with outstanding long-term stability. Furthermore, an exceptional CO2 durability is achieved in e-L0.94BCO with negligible degradation rate. Density-functional theory (DFT) calculations imply that interfacial BaO makes a movement of O 2p-band center to the Fermi level, which is closely related to promoted electrochemical performance.

Enhanced electrochemical redox kinetics of La0.6Sr0.4Co0.2Fe0.8O3 in reversible solid oxide cells

Due to its high conversion efficiency in fuel-to-power and power-to-fuel modes, reversible solid oxide cell (RSOC) is widely regarded as a promising device. In this study, La0.6Sr0.4Co0.2Fe0.7M0.1O3 (M=Fe, Mo, Nb) oxides are prepared and composited with Sm0.2Ce0.8O2-(Li0.67Na0.33)2CO3 to form reversible single component cell (RSCC). Among them, La0.6Sr0.4Co0.2Fe0.7Mo0.1O3 (LSCFM) shows the highest oxygen vacancy concentration, further promoting catalytic activity towards oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). Besides, Mo or Nb doping enhances the surface reaction rate constant (kchem) and oxygen chemical bulk diffusion coefficient (Dchem) at the temperature range of 550–700 °C. LSCFM shows the highest kchem of 39.3 × 10−5 cm s−1, approximate 3.5 times as 11.1 × 10−5 cm s−1 for La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), and Dchem for LSCFM is 31.1 × 10−6 cm2 s−1, approximate 2.5 times as 12.4 × 10−6 cm2 s−1 for LSCF at 700 °C. Furthermore, the rate determining steps (RDS) of ORR and HOR for LSCFM-based cell are the reduction of oxygen atoms to O− and the charge transfer reaction, respectively. Under SOFC/SOEC reversible operation, the RSCC composed of LSCFM exhibits the best cell performance. In the solid oxide fuel cell mode, the maximum power density reaches 236.2 mW cm−2 and in the solid oxide electrolysis cell mode the current density at 1.3 V is -312.4 mA cm−2.

Oxygen diffusion in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2

This study evaluates the self-diffusion and chemical diffusion coefficients of oxygen in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2 using point defect chemistry (oxygen vacancies and interstitials). The self-diffusion coefficient changed in proportion to the 1/n power of oxygen partial pressure, similar to the defect concentration. All parameters used to represent the diffusion coefficients were determined, and the experimental data were accurately stated. The defect formation and migration energies of the oxides were compared, and the change in Frenkel defect concentration was found to affect the high-temperature heat capacities of CeO2 and ThO2. The oxygen chemical diffusion was evaluated in the oxides, excluding the line compound ThO2, and the coefficients increased dramatically around the stoichiometric composition, i.e., the chemical diffusion coefficient was much higher at stoichiometric composition, with the oxygen-to-metal ratio equal to 2.00, than in low oxygen-to-metal oxides. This difference altered the mechanism of the reduction and oxidation processes. In the reduction process, the chemical diffusion control rate was dominant and a new phase with the oxygen-to-metal ratio equal to 2.00 was formed, which then expanded from the surface in the oxidation process from a low oxygen-to-metal ratio to the stoichiometric composition.

Oxygen potential, oxygen diffusion, and defect equilibria in UO2±x

Since the oxygen potential and the oxygen diffusion coefficient of UO2 have a significant impact on fuel performance, many experimental data have been obtained. However, experimental data of the oxygen potential and the oxygen diffusion coefficient in the high temperature region above 1673 K are very limited. In the present study, we aimed to obtain these data and analyze them by defect chemistry. the oxygen potentials and the oxygen chemical diffusion coefficient of UO2 were measured by the gas equilibrium method in the near stoichiometric region at temperatures ranging from 1673 to 1873 K. A data set of oxygen potentials was made together with literature data and analyzed by defect chemistry. The oxygen potential of UO2 was determined as a function of O/U ratio and temperature, and an equation representing the relationship was derived. The oxygen chemical diffusion coefficient values obtained in this study were reasonably close to the literature values. The oxygen partial pressure dependence of the oxygen chemical diffusion coefficients was predicted from the evaluated results of the oxygen potential data, but no clear dependence was observed.

Enhancing catalytic activity of CO2 electrolysis via B-site cation doped perovskite cathode in solid oxide electrolysis cell

Solid oxide electrolysis cell (SOEC) is considered as a high energy-efficient CO2 conversion method and a promising technology to achieve carbon neutralization. Thus, in this work, the reduced Co or Ni doped Pr0.4Sr0.6Fe0.9Mo0.1O3 perovskite oxides are used as the cathode of SOEC. After reduction, Co-Fe alloy nanoparticles are in situ exsolved from Pr0.4Sr0.6Co0.2Fe0.7Mo0.1O3 and Pr0.4Sr0.6Co0.45Fe0.45Mo0.1O3, and NiFe10.8 alloy nanoparticles are attached on the Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 matrix. The reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest oxygen vacancy concentration and CO2 adsorption ability, which further promote oxygen surface exchange and electrochemical surface exchange reaction. Besides, the reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 exhibits the highest surface reaction rate constant (kchem) of 3.54 × 10-4 cm s-1 and highest oxygen chemical bulk diffusion coefficient (Dchem) of 27.53 × 10-5 cm2 s-1, suggesting that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the best catalytic activity for CO2 reduction reaction. The symmetrical cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 shows the lowest polarization resistance (Rp) value, and at 800 °C, the electrolysis cell with reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode also exhibits the best performance of 0.81 A cm-2 in 50%CO-50%CO2 and 1.05 A cm-2 in pure CO2 at 1.5 V, indicating that reduced Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 is a promising cathode for CO2 electrolysis.

In-situ segregation of A-site defect (La0.6Sr0.4)0.90Co0.2Fe0.8O3-δ to form a high-performance solid oxide fuel cell cathode material with heterostructure

As a potential cathode material for solid oxide fuel cells, the commercial application of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has to face the challenges in insufficient oxygen reduction reaction (ORR) activity, segregation of Sr element, and CO2 poisoning. Therefore, the effect of A-site non-stoichiometry on the electrochemical performance of (LS)1-xCFs (x = −0.05, 0, 0.05, 0.10, 0.15) from the aspect of microstructure/elemental surface chemical environment evolution is investigated in this paper. The results show that (LS)0.90CF has the best ORR activity and the highest electrochemical performance. The excellent electrochemical performance is attributed to the (LS)0.90CF/Co2FeO4/CoFe2O4-type heterostructure, formed by in-situ segregation induced by A-site defects, which improves the surface oxygen diffusion coefficient and chemical bulk diffusion coefficient of the cathode. At the same time, the A-site defect can effectively inhibit the segregation of Sr element in (LS)0.90CF, thereby improving the tolerance of CO2. At 800 °C, the area-specific resistance (ASR) of (LS)0.90CF (0.023 Ω cm2) is 66.7% lower than that of LSCF (0.069 Ω cm2), and the peak power density (1.57 Wcm−2) is 1.8 times higher than that of LSCF (0.87 Wcm−2). Therefore, perovskite A-site defect-induced B-site segregation to form heterostructures provides an effective strategy for the preparation of high-performance cathode materials.

Surface kinetics and bulk transport in La2Ni0.5Cu0.5O4+δ membranes from conductivity relaxation

This work reports conductivity relaxation measurements on both uncoated (1.2 mm thick) and coated (2.0 mm thick) La2Ni0.5Cu0.5O4+δ membranes in the temperature range between 550 and 850 °C and oxygen partial pressures from 0.01 to 1.0 atm. The results show that surface kinetics has a significant effect on the relaxation profiles, especially at low temperatures and should not be neglected when extracting transport parameters. Oxygen chemical diffusion and surface exchange coefficients have been determined by transient conductivity with surface modification. Higher activation energy of surface exchange compared to bulk diffusion is observed for La2Ni0.5Cu0.5O4+δ, similar to that for La2NiO4+δ. Based on the oxygen partial pressure dependence of the surface exchange coefficient, it has been revealed that oxygen dissociative adsorption rate-limits the surface exchange.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

Mechanism of photo-ionic stoichiometry changes in SrTiO3

The impact of UV light (λ = 365 nm) on defect chemistry and composition of undoped SrTiO3 single crystals was investigated by means of impedance spectroscopy. In-plane conductivity measurements between 280 and 450 °C in air reveal a drastic increase of the bulk conductivity upon UV illumination. The measured time dependence of this increase is in accordance with oxygen chemical diffusion in SrTiO3. The corresponding photo-induced change in defect concentrations is caused by oxygen being pumped from the gas phase into the oxide under UV irradiation. This affects the entire SrTiO3 crystal rather than only the thin UV absorption zone and leads to very high oxygen chemical potentials with nominal oxygen pressures up to 106 bar. After switching the UV light off, the resulting conductivity increase relaxes extremely slowly due to slow surface exchange kinetics. A mechanistic model is introduced to explain the impact of UV light on the oxygen chemical potential as well as on the oxygen vacancy and hole/electron concentrations in semiconducting oxides, here SrTiO3. This model is based on the formation of oxygen quasi-chemical potentials in the illuminated region. It is discussed how the interplay of four kinetic parameters causes the observed time-dependent stoichiometry and thus conductivity changes.

Perovskite Oxyfluoride Ceramic with In Situ Exsolved Ni-Fe Nanoparticles for Direct CO2 Electrolysis in Solid Oxide Electrolysis Cells.

Solid oxide electrolysis cell (SOEC) is a potential technique to efficiently convert CO2 greenhouse gas into valuable fuels. Thus, there is significant interest in developing highly active and stable electrocatalysts for the CO2 reduction reaction (CO2RR). Herein, a Ni and F co-doping strategy is proposed to facilitate the exsolution reaction and form a new cathode, Ni-Fe alloy nanoparticles embedded in ceramic Sr2Fe1.5Mo0.5O6-δ (SFM) doped with fluorine. F-doping and Ni-Fe exsolution enhance CO2 adsorption by a factor of 2.4 and increase the surface reaction rate constant (kchem) for CO2RR from 6.79 × 10-5 to 18.1 × 10-5 cm s-1, as well as the oxygen chemical bulk diffusion coefficient (Dchem) from 9.42 × 10-6 to 19.1 × 10-6 cm2 s-1 at 800 °C. Meanwhile, the interfacial polarization resistance (Rp) decreases by 52%, from 0.64 to 0.31 Ω cm2. At 800 °C and 1.5 V, an extremely high current density of 2.66 A cm-2 and a stability test over 140 h are achieved for direct CO2 electrolysis in the SOEC.

Tailoring the structural stability, electrochemical performance and CO2 tolerance of aluminum doped SrFeO3

Stabilizing SrFeO3-δ in cubic structure is of great significance for its application in solid oxide fuel cells. Herein, a rational way is reported to tune the structure and performance of SrFeO3-δ perovskite as oxygen reduction electrodes by embedding aluminum cation in B-site. Compared with their parent oxide, the obtained stabilized cubic perovskites SrFe1-xAlxO3-δ (x = 0.1 and 0.2) show much improved electrocatalytic activity, achieving area specific resistance values of 0.57, 0.15 and 0.34 Ωcm2 at 700 °C in air for SrFeO3-δ, SrFe0.9Al0.1O3-δ and SrFe0.8Al0.2O3-δ, respectively. Such improved performance is a result of the exceeding 2-fold increase in oxygen chemical bulk diffusion and surface exchange kinetics due to Al3+ doping. Moreover, favorable CO2-resistance is also demonstrated. DFT calculations are carried out to reveal the accelerated oxygen reduction reaction and enhanced CO2 tolerance. This work indicates an aluminum dopant in B-site may provide a highly attractive strategy for the future development of cathode materials.

Pr-Doping Motivating the Phase Transformation of the BaFeO3-δ Perovskite as a High-Performance Solid Oxide Fuel Cell Cathode.

Intermediate temperature solid oxide fuel cells (IT-SOFCs) have been extensively studied due to high efficiency, cleanliness, and fuel flexibility. To develop highly active and stable IT-SOFCs for the practical application, preparing an efficient cathode is necessary to address the challenges such as poor catalytic activity and CO2 poisoning. Herein, an efficient optimized strategy for designing a high-performance cathode is demonstrated. By motivating the phase transformation of BaFeO3-δ perovskites, achieved by doping Pr at the B site, remarkably enhanced electrochemical activity and CO2 resistance are thus achieved. The appropriate content of Pr substitution at Fe sites increases the oxygen vacancy concentration of the material, promotes the reaction on the oxygen electrode, and shows excellent electrochemical performance and efficient catalytic activity. The improved reaction kinetics of the BaFe0.95Pr0.05O3-δ (BFP05) cathode is also reflected by a lower electrochemical impedance value (0.061 Ω·cm2 at 750 °C) and activation energy, which is attributed to high surface oxygen exchange and chemical bulk diffusion. The single cells with the BFP05 cathode achieve a peak power density of 798.7 mW·cm-2 at 750 °C and a stability over 50 h with no observed performance degradation in CO2-containing gas. In conclusion, these results represent a promising optimized strategy in developing electrode materials of IT-SOFCs.

Enhanced accuracy of electrochemical kinetic parameters determined by electrical conductivity relaxation

Electrical conductivity relaxation (ECR) is a widely adopted technique for determination of oxygen surface exchange coefficient (kchem) and chemical oxygen diffusivity (Dchem) of mixed ionic and electronic conductors (MIECs). However, it has been argued that the fitting process of determining two kinetic parameters from a single conductivity relaxation curve inevitably leads to high error in the determined values. In this research, we demonstrate experiment-based analytical approaches to overcome the issue and obtain highly reliable kinetic parameters using ECR for the case of (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF), a representative MIEC. As a baseline, kinetic parameters of a standard LSCF bar sample with the thickness close to the critical thickness are obtained using a conventional ECR method along with error range and sensitivity analysis at oxygen partial pressures of 0.2–3.125 × 10−3 atm. Dchem with improved accuracy is obtained by ECR under primarily bulk diffusion-controlled condition, which is achieved by either increasing the thickness or coating porous LSCF on the baseline sample. With the Dchem determined, the baseline data are further refined to obtain kchem as the sole fitting parameter and the error range is reduced more than 70% of the original value under all test conditions. With the significantly improved accuracy and sensitivity of both kchem and Dchem, this demonstrated method is proven to be a practical approach to advance the application of ECR.

(Invited) Potentiometric Measurements of Defect Equilibrium of Nonstoichiometric Oxides Under Mechanical Load

Nonstoichiometric compounds are widely used for solid state electrochemical devices such as lithium ion batteries and solid oxide fuel cells. Their functions arise from point defects of which the equilibrium concentration varies with temperature, pressure, and chemical potential of the constituent elements in the surrounding environment. In an actual device, materials are used in combination with other compounds among which internal stress appears due to the mismatch of the thermal or chemical strain. The mechanical load, or stress tensor, applied to the solid may also affect the local defect equilibrium. In thermodynamic considerations by Johnson and Schmalzried [1], the modification of the defect equilibrium by the mechanical load was correlated by the chemical strain or the volume change upon variation of defect concentration.In our previous reports [2,3], we proposed a potentiometric method to monitor the shift of oxygen defect equilibrium of a mixed conducting oxides placed under mechanical load. The measurements were done using a small ball of stabilized zirconia (YSZ) which was used for a potentiometric oxygen sensor as well as for a pushing jig to apply compressive stress on the sample at the measurement point. As a mixed conducting oxide, (La,Sr)(Co,Fe)O3 was chosen and the measurements were done using a universal mechanical testing machine. The emf was monitored over the YSZ ball between the sample and a reference electrode which was placed on the free surface of the ball. The results were as expected, i.e. a negative voltage appeared on the sample side on the instance of applying load, and it gradually approached back to zero. On removing the load, a positive voltage appeared, and it decayed to zero with time. The observed phenomena were explained as follows: On application of the load, the local composition tends to shift to a new equilibrium under the load. However, it does not occur instantly since it accompanies mass transport. Instead, the sample behaves first as a closed system and the local chemical potential varies, which was monitored by the YSZ ball. Equilibration with the gas phase proceeds gradually via chemical diffusion of oxygen in the sample. The maximum voltage in the negative and positive directions showed good agreement with those estimated from the local stress and the chemical expansion coefficient for vacancy formation reaction. The similar measurements were made with (La,Sr)CoO3, (La,Sr)FeO3, and (La,Sr)MnO3, and the similar results were obtained when oxygen nonstoichiometry variation is large enough compared to the thermal fluctuation.Recently, the similar measurements were made by monitoring a voltage across a mixed conducting oxide itself on pressing its surface with an alumina ball. Gadlinia doped ceria, GDC, was used as the sample, and the surface was pushed with a small alumina ball coated with platinum paste. Reference Pt electrode was placed on the GDC sample at the place far from the applied load. The voltage across GDC was monitored using the Pt coat on alumina ball and Pt electrode on GDC. Under reducing atmosphere, the negative shift of potential followed by a gradual decay was observed. The maximum voltage was as expected if the transference number that is less than unity was taken into consideration. Under oxidizing atmosphere, a quick appearance of positive voltage was observed upon loading. It was similar to that observed with YSZ ball when it was pushed against Pt foil. The oxygen dissolved or adsorbed on Pt or at the interface could be the source of the response. With GDC, a persistent voltage was also observed in some cases, of which the reason is not clear yet.The similar approach as GDC was explored to a Ba(Ce,Y)O3 based proton conducting oxides. In this case, not only proton but also electron hole and oxide ion vacancy may contribute to the transport and thus the appearance of EMF. The results observed so far was the negative voltage on loading under oxidizing atmospheres, and negative voltage under reducing atmospheres. Rather large persistent volrage was also detected. Details of the results and the explanation will be discussed.

Improved activity of oxygen in Ni–Ce0.8Sm0.2O2-δ anode for solid oxide fuel cell with Pr doping

Ni–Ce0.8PrxSm0.2-xO2-δ (x = 0–0.15) is studied as an anode of solid oxide fuel cells. The doping of Pr facilitates the chemical oxidation of CH3OH on the ceramic phase because the strength of Pr–O bond is weaker than that of Ce–O bond. With the addition of Pr, the electronic conductivity of the ceramic phase increases in an oxidizing atmosphere but decreases in a reducing atmosphere. The electrochemical conductivity relaxation results indicate that Pr improves oxygen surface exchange coefficient and oxygen chemical diffusion coefficient of Ce0.8Sm0.2O2-δ simultaneously. The doping of Pr shows negligible influence on the performance of the cell fed with H2 due to the high reactivity of H2. However, the maximum power density of the cell with methanol as the fuel increases significantly with the addition of Pr in the anode, demonstrating the high importance of the oxygen activity of the anode when a less active fuel is used. A Ce0.8Sm0.2O2-δ-carbonate electrolyte-supported single cell with Ni–Ce0.8Pr0.1Sm0.1O2-δ anode shows a maximum power density of 792 mW cm−2 at 650 °C with methanol as the fuel.

Investigation of structural, electrical and electrochemical properties of La0.6Sr0.4Fe0.8Mn0.2O3-δ as an intermediate temperature solid oxide fuel cell cathode

La0.6Sr0.4Fe0.8Mn0.2O3 (LSFM) compound is synthesized by sol-gel method and evaluated as a cathode material for the intermediate temperature solid oxide fuel cell (IT-SOFC). X-ray diffraction (XRD) indicates that the LSFM has a rhombohedral structure with R-3c space group symmetry. The XRD patterns reveal very small amount of impurity phase in the LSFM and Y2O3-stabilized ZrO2 (YSZ) mixture powders sintered at 600, 700, 800 and 850 °C for a week. The maximum electrical conductivity of LSFM is about 35.35 S cm−1 at 783 °C in the air. The oxygen chemical diffusion coefficients, DChem, are increased from 1.39 × 10−6 up to 1.44 × 10−5 cm2 s−1. Besides, the oxygen surface exchange coefficients, kChem, are obtained to lie between 2.9 × 10−3 and 1.86 × 10−2 cm s−1 in a temperature range of 600–800 °C. The area-specific resistances (ASRs) of the LSFM symmetrical cell are 7.53, 1.53, 1.13, 0.46 and 0.31 Ω cm2 at 600, 650, 700, 750 and 800 °C respectively, and related activation energy, Ea, is about 1.23 eV.