Oxygen Bulk Diffusion Coefficients Research Articles

Tuning the ORR catalytic activity of LaFeO3-δ-based perovskite cathode for solid oxide fuel cells by doping with alkaline-earth metal elements

Co-based perovskite cathodes have demonstrated impressive catalytic activity for oxygen reduction reaction (ORR) at intermediate temperatures. However, these cathodes face the challenges such as high cost and thermal expansion coefficient (TEC) mismatch. On the other hand, Fe-based perovskite oxides like LaFeO3 exhibit more stable properties and lower TEC, but their ORR activity is not as remarkable as that of Co-based cathodes. In this study, La3+ at the A-site of LaFeO3 was substituted by aliovalent alkaline-earth metal cations. The research findings reveal that the doping of alkaline-earth metal ions induces the formation of structural defects and alters the valence state of Fe. This leads to an increase in the conductivity and oxygen vacancy concentration, which are critical for ORR kinetics in both H–SOFC and O–SOFC. Among the various dopants, Sr doped LaFeO3 (LSrF) exhibits the most outstanding ORR catalytic activity. This can be attributed to its highest total conductivity, O2− conductivity, oxygen surface exchange coefficient, and oxygen bulk diffusion coefficient. At 700 °C, single cell with LSrF cathode achieves peak power densities of 0.781 and 0.894 W cm−2, for O2--conducting solid oxide fuel cell (SOFC) and H+-conducting SOFC, respectively. These performances are essentially twice that of single cell with LaFeO3 cathode. The findings of this study provide valuable insights into doping strategies for Fe-based perovskite cathodes and contribute to the advancement of IT-SOFC cathode research.

A Strategy to Enhance the Catalytic Activity of Electrode Materials by Doping Bismuth for Symmetrical Solid Oxide Electrolysis Cells

Lanthanum chromate-based perovskites are potential ceramic electrodes for symmetrical solid oxide electrolysis cells (Sym-SOECs). In order to improve its electrochemical activity for CO2 reduction reaction (CO2RR), bismuth is introduced into the A-site. While all the components are stable in the anode atmosphere, the cathode application limits the doping content to 0.2, below which bismuth has the suboxidation state between 0 and +3. As revealed with experimental investigations and density functional theory calculations, doping bismuth could effectively increase the physicochemical properties for CO2RR, including oxygen vacancy concentration, CO2 adsorption capacity, conductivities in CO–CO2 atmospheres, oxygen bulk diffusion coefficient, as well as surface exchange coefficient. Consequently, Rp for CO2RR is reduced by 82% to 1.07 Ω cm2 at 800 °C. And the Sym-SOEC performance for CO2 electrolysis is increased by 46% to 0.79 A cm–2 at 800 °C, 1.5 V, indicating that bismuth-doped material is an excellent ceramic electrode material for Sym-SOECs.

Electrode properties of CuBi2O4 spinel oxide as a new and potential cathode material for solid oxide fuel cells

Herein, the spinel oxide CuBi 2 O 4 is prepared by solid-state reaction method and characterized for the first time as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). CuBi 2 O 4 (CBO) crystallizes in tetragonal structure and is chemically compatible with Ce 0.9 Gd 0.1 O 1.9 (GDC) electrolyte at 800 °C. Electrical conductivity relaxation is conducted to assess the oxygen surface exchange coefficient k chem and oxygen bulk diffusion coefficient D chem of CBO material. It is found that CBO exhibits better oxygen transport properties than the traditional La 0.8 Sr 0.2 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes. The electrochemical activity of CBO-based cathode is investigated by impedance spectroscopy on the symmetric cell with GDC electrolyte. The CBO cathode forms good contact with the GDC electrolyte and displays low polarization resistance ( R P ) values, e.g., 0.58 Ωcm 2 at 700 °C in air. The introducing GDC to form a composite cathode with CBO further reduces the R P to 0.39 Ωcm 2 . An anode-supported single cell with the CBO-GDC cathode exhibits a peak power density of 507 mWcm −2 at 750 °C, suggesting the viability of CBO oxide as a cathode material for IT-SOFC. • CuBi2O4 exhibits fast surface oxygen exchange and bulk diffusion properties. • CuBi2O4 forms good contact with GDC electrolyte at low sintering temperatures. • CuBi2O4 cathode shows improved electrochemical performance below 700 °C.

Defects evolution of Ca doped La2NiO4+δ and its impact on cathode performance in proton-conducting solid oxide fuel cells

As a typical Ruddlesden-Popper oxide, La2NiO4+δ draws special attention for its high oxygen ion conducting behavior and special interstitial oxygen defects which enables it a promising electrocatalyst toward oxygen reduction reaction. In this work, Ca-doped La2NiO4+δ samples are prepared and their structure and defect evolution are investigated as Ca content. Electrical conductivity and electron conduction relaxation (ECR) investigations suggest that La1.9Ca0.1NiO4+δ has the great electronic conductivity and the highest oxygen surface exchange coefficient and oxygen bulk diffusion coefficient at intermediate temperatures. These results may imply that more gas oxygen has inserted into La1.9Ca0.1NiO4+δ sample, suggesting the native interstitial oxygen defects in it. X-ray photon spectroscopy (XPS) results confirm that La1.9Ca0.1NiO4+δ have more active oxygen species when compared with La2NiO4+δ and La1.8Ca0.2NiO4+δ. The great oxygen exchange and bulk diffusion properties along with its great stability in steam involved atmosphere enables it a promising for cathode for H–SOFCs. Compared with that using La2NiO4+δ cathode, peak powder density of H–SOFC using La1.9Ca0.1NiO4+δ single phase cathode improves about 30.5% at 700 °C, suggesting that accelerating oxygen reduction reaction can effectively improve cathode performance of H–SOFCs.

Structural, Electrical, and Electrochemical Characteristics of LnBa0.5Sr0.5Co1.5Fe0.5O5+δ (Ln=Pr, Sm, Gd) as Cathode Materials in Intermediate‐Temperature Solid Oxide Fuel Cells

AbstractLayered perovskite oxides have received great attention as prospective cathode materials for intermediate‐temperature solid oxide fuel cells (IT‐SOFCs) because of their high electrical conductivities and fast oxygen kinetics. Herein, we focus on the structural, electrical, and electrochemical properties of layered perovskites LnBa0.5Sr0.5Co1.5Fe0.5O5+δ (LnBSCF, Ln=Pr, Sm, and Gd) as a cathode material for IT‐SOFCs. Among the evaluated perovskites, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) showed the highest electrical conductivity, the highest oxygen content, and the highest values of the oxygen bulk diffusion coefficient (D*) and surface exchange coefficient (k) measured by using isotope oxygen exchange. The composite cathode of PBSCF and gadolinium‐doped ceria is the most suitable cathode material for IT‐SOFCs among the materials investigated because of its excellent electrochemical properties and fast oxygen kinetics.

An efficient electrocatalyst as cathode material for solid oxide fuel cells: BaFe0·95Sn0·05O3−δ

Abstract The B-site substitution with the minor amount of tin in BaFeO3−δ parent oxide is expected to stabilize a single perovskite lattice structure. In this study, a composition of BaFe0·95Sn0·05O3−δ (BFS) as a new cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is synthesized and characterized. Special attention is paid to the exploration of some basic properties including phase structure, oxygen non-stoichiometry, electrical conductivity, oxygen bulk diffusion coefficient, and surface exchange coefficient, which are of significant importance to the electrochemical activity of cathode materials. BFS holds a single cubic perovskite structure over temperature range of cell operation, determined by in-situ X-ray diffraction and scanning transmission electron microscope. A high oxygen vacancy concentration at cell operating temperatures is observed by combining thermo-gravimetric data and iodometric titration result. Furthermore, electrical conductivity relaxation measurement illustrates the fast oxygen bulk diffusion and surface exchange kinetics. Accordingly, testing cells based on BFS cathode material demonstrate the low polarization resistance of 0.033 Ω cm2 and high peak power density of 1033 mW cm−2 at 700 °C, as well as a relatively stable long-term operation for ∼300 h. The results obtained suggest that BFS perovskite oxide holds a great promise as an oxygen reduction electrocatalyst for IT-SOFCs.

Self-Diffusion of the Constituent Elements in Alpha-Alumina, Mullite and Aluminosilicate Glasses

Aluminium is a key element in geological and man-made materials which has only one stable isotope and no radionuclides with half-life times suitable for standard experimental diffusion studies. Here we report on our method using the radioisotope 26Al (t1/2 = 7.4×105 a) as a quasi-stable tracer for aluminium in combination with SIMS depth profiling. First, our data for the aluminium bulk diffusivity in a-alumina are discussed jointly with published oxygen bulk diffusion coefficients. They clearly show that the relation DAl>>D0 is valid in the temperature range 1200 °C ≤ T ≤ 1800 °C. In an analogous manner, the two rare stable isotopes 18O and 30Si are used together with 26Al in diffusion studies of generic examples of materials which either consist of aluminium, silicon and oxygen only, or where these three elements are key constituents of the structure. For the crystalline aluminium silicate mullite our diffusivity data for aluminium, oxygen and silicon are used to explain the kinetics of the solid state formation reaction of mullite and the segregation kinetics of alumina from mullite. Finally, the diffusivities of oxygen and aluminium in model aluminosilicate glasses are presented as a function of temperature for different Al3+/Na+ ratios. For the aluminium silicate mullite and for the aluminosilicate glasses the relation D0>DAl>DSi is valid regardless of the exact composition. For the glass system the activation enthalpies of aluminium and oxygen diffusion decrease with decreasing Al3+/Na+ ratio.

Structural Properties of Mechanically Activated Rare-Earth Manganites

The effect of mechanical processing in different types of activators on the structure and physicochemical properties of the oxides LnMnO3+δ (Ln = Sm, Nd, La) has been studied. The optimal modes of mechanical activation for transformation the oxides under consideration to nanostructured state have been defined. Mechanical processing has been shown to cause a significant change of Jahn-Teller distortion parameters and temperatures of the cooperative Jahn-Teller phase transition. The presence of connection between redox processes and destruction of orbital ordering has been demonstrated. Very low values (10 -21 -10 -24 m 2 /s) of the oxygen bulk diffusion coefficient have been obtained for the oxides under consideration in the temperature range of 400 - 750°C. The diffusion activation energies are close to 1 eV, indicating the oxygen diffusion proceeds through structural defects at these temperatures.

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

AbstractIn this study, La0.4Sr0.6CoO3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (Dv) and surface exchange coefficient (Kex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La0.4Sr0.6CoO3‐δ membrane was ∼4.0 × 10−7 mol cm−2 s−1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10−5 mol cm−2 s−1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Growth mechanism of Cr 2O 3 scales: oxygen and chromium diffusion, oxidation kinetics and effect of yttrium

The growth mechanism of Cr 2O 3 scales was studied at 900 and 800 °C by determining diffusivities, both in bulk and along the grain boundary, of 18O and 54Cr in Cr 2O 3 scales formed on a Ni 70Cr 30 alloy with and without yttrium implants (10 16 ions cm −2). The diffusion of 18O and 54Cr was carried out simultaneously which allowed us to obtain a direct comparison of self-diffusion. All the concentration profiles were established by SIMS. Without yttrium in Cr 2O 3 scales, the oxygen bulk diffusion coefficient is close to that of chromium, but the chromium grain boundary diffusion coefficient is greater than that of oxygen. The presence of yttrium in Cr 2O 3 scales slightly decreases the grain boundary diffusivities of 18O and 54Cr, but enhances the oxygen lattice diffusion. The parabolic oxidation constant rates calculated from our diffusion data according to Wagner theory, were close to the experimental ones determined by oxidation kinetic tests. It is concluded that the Cr 2O 3 scale growth is controlled by counter-current diffusion of oxygen and chromium for the unimplanted and implanted alloys, mainly by grain boundary diffusion. Yttrium does not change the growth mechanism.