Thermal expansion and electrochemical properties of Ni-doped GdBaCo2O5+δ double-perovskite type oxides

Layered GdBaCo_2 −x Ni_xO_5 + δ (0 ≤ x ≤ 0.3) complex oxides were synthesized and investigated as cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFCs). All compositions formed an orthorhombic double-perovskite structure after calcination at 1000 °C for 5 h. The thermal expansion coefficient (TEC) was effectively decreased due to the partial substitution of Ni for Co, but the cathodic polarization resistance slightly increased with the increasing Ni content. Among the tested oxides, the GdBaCo_1.7Ni_0.3O_5 + δ composition showed a fairly reduced TEC (15.5 × 10⁻⁶ K⁻¹) and reasonably low polarization resistances (e.g., 0.54 Ωcm² at 600 °C), which was considered as a promising candidate for IT-SOFCs.     

Crystal structure,electrical and electrochemical properties of Cu co-doped Pr1.3Sr0.7NiO4+δ mixed ionic-electronic conductors (MIECs)

Room temperature crystal structure, electrical properties and electrochemical properties in the temperature range 25–700 °C of Cu co-doped Pr_1.3Sr_0.7NiO_4+δ prepared by acetate combustion is investigated from intermediate temperature solid oxide fuel cell cathode viewpoint. The Pr_1.3Sr_0.7Ni_1?xCu_xO_4+δ (PSNCO) solid solutions have a tetragonal I4/mmm K₂NiF₄-type structure which consists of a (Pr_1.3Sr_0.7) (Ni_1.xCu_x)O₃ perovskite unit and a (Pr_1.3Sr_0.7)O rock salt unit in the whole compositional range 0 ≤ x ≤ 0.4. A reduction in bond-length of Ni/Cu-O resulting from PSNCO lattice contraction eases hop of small polaron from Ni³⁺ to Ni²⁺/Cu²⁺ in (Ni_1?xCu_x)-O layer with low activation energy, which increases electron conductivity. The maximum electronic conductivity (σ = Ω cm^?1) with minimum activation energy (E_a = eV) is observed at x = 0.3. Lattice expansion along c-direction owing to Cu²⁺ doping facilitates hop of O^2? from its occupied interstitial site (O₃) to nearby equivalent site assisted by anisotropic thermal motion of apical oxygen O2 resulting in increase in ionic conductivity. The minimum polarization resistance value (R_p = 0.13 (2) Ω cm²) and activation energy (E_a = 1.321 (5) eV) at x = 0.3 is attributed to high electronic and ionic conductivities compared to other compositions. Complex impedance spectroscopy studies suggest that the ORR is co-limited by O^2? diffusion and O₂ surface exchange.     

Improved phase stability and electrochemical performance of (Y,In,Ca)BaCo3ZnO7+δ cathodes for intermediate temperature solid oxide fuel cells

With an aim to combine the performance-enhancing properties of Ca with the stability-promoting properties of In in the swedenborgite YBaCo₄O_7+δ-based cathodes for solid oxide fuel cells (SOFC), cation-substituted Y_1−x−yIn_xCa_yBaCo₃ZnO_7+δ (0.2 ≤ (x + y) ≤ 0.5) oxides have been explored. All samples presented in this work are stable in air after 120 h exposure to 600, 700, and 800 °C. Increasing In content shows a negligible impact on polarization resistances (R_p), but causes an increase in the activation energies (E_a) of (Y,In,Ca)BaCo₃ZnO_7+δ + Gd_0.2Ce_0.8O_1.9 (GDC) composite cathodes on 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte supported symmetric cells. Increasing Ca content shows a decrease in R_p and an increase in E_a on similar electrochemical cells. All (Y,In,Ca)BaCo₃ZnO_7+δ samples investigated here show superior performance compared to the unsubstituted YBaCo₃ZnO_7+δ + GDC cathode in the range of 400–800 °C. Especially, the Y_0.5In_0.1Ca_0.4BaCo₃ZnO_7+δ + GDC composite cathode exhibits good performance on GDC electrolytes in the range of 400–600 °C. With superior phase stability and electrochemical performance, the (Y,In,Ca)BaCo₃ZnO_7+δ series of oxides are attractive cathode candidates for intermediate temperature SOFCs.     

Effect of DC current polarization on the electrochemical behaviour of La2NiO4+δ and La3Ni2O7+δ-based systems

The electrode performance of La₂NiO₄ and La₃Ni₂O₇ as cathode materials for solid oxide fuel cells (SOFC) was analyzed. The study was focused on the electrode polarization resistance of the interfaces formed by the cathodes with Ce_0.8Sm_0.2O_2−δ + 2%Co electrolyte. The study was extended to cathodes based on La₂NiO₄-Ce_0.8Sm_0.2O_2−δ composite and Pt to analyze the effect of changing the electronic and/or ionic transport properties on the electrode interface resistance. The electrode performance was studied in open circuit conditions and with DC current polarization. Important differences in the performance of the pure cathode materials were obtained as function of DC current flux. However, in La₂NiO₄-Ce_0.8Sm_0.2O_2−δ composite the DC current flux produces minor changes in the electrode polarization resistance. The aging process also affects the OCV electrode performance of cathodes based on Pt and pure ceramics, whereas the effect is practically invaluable in La₂NiO₄-Ce_0.8Sm_0.2O_2−δ composite. The electrode performance is higher for the composite cathode compared to pure ceramic electrodes for OCV or for low values of DC polarization. However, the important decrease in the interface resistance obtained for high values of DC current flux for La₂NiO₄ and La₃Ni₂O₇ cathodes increases their electrode performances to values close to those obtained in La₂NiO₄-Ce_0.8Sm_0.2O_2−δ composite. This retains the cathode overpotential with values as low as 140 mV at 750 °C for values of current load of 530 mA cm⁻² for both pure and composite La₂NiO₄-based cathodes. The low cathode overpotential allows to estimate values of power density between 300 and 350 mW cm⁻² at 750 °C for La₂NiO₄, La₃Ni₂O₇ and La₂NiO₄-Ce_0.8Sm_0.2O_2−δ composite, operating with Ce_0.8Sm_0.2O_2−δ + 2%Co electrolyte, with 300 μm in thickness, and a Ni-Ce_0.8Sm_0.2O_2−δ cermet anode with H₂ as fuel.     

Electrical conductivity,thermal expansion and electrochemical performances of Ba-doped SrCo0.9Nb0.1O3−δ cathodes for IT-SOFCs

Perovskite-type oxides Ba_xSr_1−xCo_0.9Nb_0.1O_3−δ (BSCNx, x = 0.0–0.8) were synthesized and investigated as cathodes for IT-SOFCs. Ba doping improves chemical compatibility between BSCNx oxides and Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte. Effects of Ba doping on electrical conductivity, thermal expansion and electrochemical performances were systematically elucidated and discussed. Both thermal expansion coefficient (TEC) and polarization resistance (R_p) decrease with increasing Ba doping level up to x = 0.6, attain a minimum at x = 0.6 and then increase with further increasing x > 0.6. The decrease of TEC with the incorporation of Ba can be attributed to the weakened chemical expansion and the decrease of R_p with Ba is due to the increase of oxygen vacancy concentration and oxygen vacancy diffusion coefficient. With a 300 μm-thick GDC as electrolyte and BSCN0.6 as the cathode, the maximum power density of a single-cell achieves 778 mW cm⁻² at 800 °C. All these results indicate that the BSCN0.6 oxide is a promising cathode material for IT-SOFC.     

Phase stability and electrical conductivity of Ca-doped LaNb1−xTaxO4−δ high temperature proton conductors

The electrical conductivity, crystal structure and phase stability of La_0.99Ca_0.01Nb_1−xTa_xO_4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La_0.99Ca_0.01Nb_1−xTa_xO_4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.     

Improved oxygen permeability of Ce0.8Sm0.2O2−δ–PrBaCo2O5+δ dual-phase membrane by surface-modifying porous layer

A porous PrBaCo₂O_5+δ or Ce_0.8Sm_0.2O_2−δ–50 vol.% PrBaCo₂O_5+δ (SDC–PBCO (5/5)) layer was deposited on dense Ce_0.8Sm_0.2O_2−δ–40 vol.% PrBaCo₂O_5+δ (SDC–PBCO (6/4)) membrane (450 μm) to enhance the oxygen permeability by increasing the surface area contacting with air. The oxygen permeation flux was measured in the temperature range of 825–945 °C. The results revealed that the oxygen permeation performance of Ce_0.8Sm_0.2O_2−δ–PrBaCo₂O_5+δ membranes can be significantly enhanced by coating SDC–PBCO (5/5) porous layer alone on the surface of feed side. The thickness of modification layer has obvious effect on the permeability of surface modified membrane. The modification on the feed side has much better effect than that on the permeate side. At 945 °C, the oxygen permeation flux of dense SDC–PBCO (6/4) membrane modified by porous SDC–PBCO (5/5) layer is 3.56 × 10⁻⁷ mol cm⁻² s⁻¹, 26% higher than that of the unmodified one.     

Intermediate-temperature electrochemical performance of a polycrystalline PrBaCo2O5+δ cathode on samarium-doped ceria electrolyte

A-site cation-ordered PrBaCo₂O_5+δ (PrBC) double perovskite oxide was synthesized and evaluated as the cathode of an intermediate-temperature solid-oxide fuel cell (IT-SOFC) on a samarium-doped ceria (SDC) electrolyte. The phase reaction between PrBC and SDC was weak even at 1100 °C. The oxygen reduction mechanism was investigated by electrochemical impedance spectroscopy characterization. Over the intermediate-temperature range of 450–700 °C, the electrode polarization resistance was mainly contributed from oxygen-ion transfer through the electrode–electrolyte interface and electron charge transfer over the electrode surface. An area-specific resistance as low as ∼0.4 Ω cm² was measured at 600 °C in air, based on symmetric cell test. A thin-film SDC electrolyte fuel cell with PrBC cathode was fabricated which delivered attractive peak power densities of 620 and 165 mW cm⁻² at 600 and 450 °C, respectively.     

Structural and electrochemical characterization of La2−xSrxNiTiO6−δ

Materials of the series La_2−xSr_xNiTiO_6−δ (0 ≤ x ≤ 0.5) have been characterized by both structural and electrochemical methods in order to assess their possible use as electrodes for SOFCs. Neutron and X-ray powder diffraction experiments have shown that they are stable under both oxidizing and reducing conditions while chemically compatible with YSZ at SOFC operating temperatures. Moreover, the thermal expansion has been determined to be isotropic with α_L = 10.0(3) × 10⁻⁶ K⁻¹. This value is similar to that found for other perovskite materials used in SOFCs. However, polarization resistances reveal modest values of electrochemical response under oxygen (1.5 Ω cm² at 900 °C) and are quite poor in 5%H₂/N₂ mixtures (15 Ω cm²) at the same temperature. Nevertheless, microstructure has not been optimized fully enough to discard the material as a potential SOFC cathode.     

Chemical compatibility, thermal expansion matches and electrochemical performance of SrCo0.8Fe0.2O3−δ-La0.45Ce0.55O2−δ composite cathodes for intermediate-temperature solid oxide fuel cells

The chemical compatibility, thermal expansion and electrochemical property measurements of the SrCo_0.8Fe_0.2O_3−δ (SCF)-La_0.45Ce_0.55O_2−δ (LDC) composite cathodes for solid oxide fuel cells (SOFCs) were investigated by X-ray diffraction (XRD), thermal expansion coefficients (TECs) and cathodic polarization measurements together with electrochemical impedance spectroscopy (EIS). The results indicated that LDC had good chemical compatibility with SCF and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM), and the addition of LDC to SCF markedly reduced the polarization resistance. When the content of LDC reached 50 wt%, the SCF50 cathode showed the best electrochemical performance, with a cathodic overpotential of 0.1 V at the current density of 1102.0 mA cm⁻², together with a polarization resistance of 0.149 Ω cm² at 800 °C. The improved electrochemical performance was attributed to the expansion of the electrochemical reaction region into the electrode, and offering an easier path for the oxygen ion transport. Furthermore, the SCF-LDC composite cathodes match better with the LSGM electrolyte.     

Chemical diffusivity and ionic conductivity of GdBaCo2O5+δ

The transport properties of layered perovskite GdBaCo₂O_5+δ (GBCO), which has recently been proposed as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs), are investigated as a function of oxygen partial pressure (OPP) over the oxygen partial pressure range of 10⁻⁴ ≤ pO₂ (atm) ≤ 0.21 at 1073 ≤ T (K) ≤ 1323. The increase in total conductivity with increasing temperature below the low-temperature, order-disorder transition indicates a semiconductor-type behaviour with an activation energy of 0.42 eV. When OPP is increased to air pressure at a fixed temperature, the total conductivity increases with an apparent slope (∂log σ/∂log pO₂) of 1/10 to 1/22. The maximum oxygen ion conductivity, as extracted from the oxygen permeation measurements, is around 0.01 S cm⁻¹ under the nitrogen condition, which strongly supports the potential for cathode application. The chemical diffusion coefficient () and surface exchange coefficient (κ) are also calculated from the d.c. conductivity relaxation measurement and the values are best fitted by the following two equations:

     

Structure, chemical stability and mixed proton-electron conductivity in BaZr0.9−xPrxGd0.1O3−δ

BaZr_0.9−xPr_xGd_0.1O_3−δ (x = 0.3 and 0.6) was prepared by combustion synthesis and characterised with respect to conductivity and stability in an attempt to combine the desirable properties of the end members. The polycrystalline materials exhibit a cubic or pseudo-cubic structure as determined by X-ray synchrotron radiation and transmission electron microscopy. The chemical stability of the compositions is strongly dependent on the praseodymium content, the materials with more Pr present lower stability. Electron holes dominate the conductivity under oxidising atmospheres in BaZr_0.3Pr_0.6Gd_0.1O_3−δ, while BaZr_0.6Pr_0.3Gd_0.1O_3−δ exhibits a mixed electron hole-proton conducting behaviour. Substitution of Zr by Pr in acceptor-doped BaZrO₃ decreases the sintering temperature and increases the grain growth rate.     

Enhanced electrochemical performance of the solid oxide fuel cell cathode using Ca3Co4O9+δ

This paper reports on the electrochemical performance of an SOFC cathode for potential use in intermediate-temperature solid oxide fuel cells (IT-SOFCs) using the oxygen non-stoichiometric misfit-layered cobaltite Ca₃Co₄O_9+δ or composites of Ca₃Co₄O_9+δ with Ce_0.9Gd_0.1O_1.95 (CGO/Ca₃Co₄O_9+δ). Electrochemical impedance spectroscopy revealed that symmetric cells with an electrode of pure Ca₃Co₄O_9+δ exhibit a cathode polarization resistance (R_p) of 12.4 Ω cm², at 600 °C in air. Strikingly, R_p of the composite CGO/Ca₃Co₄O_9+δ with 50 vol.% CGO was reduced by a factor of 19 (i.e. R_p = 0.64 Ω cm²), the lowest value reported so far for the Ca₃Co₄O₉ family of compounds. These findings together with the reported thermal expansion coefficient, good compatibility with CGO and chemical durability of this material suggest that it is a promising candidate cathode for IT-SOFCs.     

Electrochemical performance of BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte based proton-conducting SOFC solid oxide fuel cell with layered perovskite PrBaCo2O5+δ cathode

BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb) exhibits adequate protonic conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered perovskite PrBaCo₂O_5+δ (PBCO) has advanced electrochemical properties. This research fully takes advantage of these advanced properties and develops a novel protonic ceramic membrane fuel cell (PCMFC) of Ni-BZCYYb|BZCYYb|PBCO. The performance of the button cell was tested under intermediate-temperature range from 600 to 700 °C with humified H₂ (∼3% H₂O) as fuel and ambient air as oxidant. The results show that the open circuit potential of 0.983 V and the maximal power density of 490 mW cm⁻² were achieved at 700 °C. By co-doping barium zirconate-cerate with Y and Yb, the conductivity of electrolyte was significantly improved. The polarization processes of the button cell were characterized using the complicated electrochemical impedance spectroscopy technique. The results indicate that the polarization resistances contributed from both charge migration processes and mass transfer processes increase with decreasing cell voltage loads. However the polarization resistance induced by mass transfer processes is negligible in the studied button cell.     

On the thermodynamic stability of La2Mo2O9−δ oxide-ion conductor

The stability of La₂Mo₂O_9−δ oxide-ion conductor was studied under Ar–H₂ and controlled oxygen partial pressure (pO₂) atmospheres within the range 608 ≤ T ≤ 1000 °C, by thermogravimetry (TG), X-ray Diffraction (XRD), Temperature Controlled X-ray Diffraction (TCXRD) and Scanning Electron Microscopy (SEM).     

The effect of doping in the electrochemical performance of (Ln1−xMx)FeO3−δ SOFC cathodes

A family of iron perovskites with the general formula AFeO_3−δ (A = Ln_1−xM_x; Ln = La, Nd and/or Pr; M = Sr or/and Ca) has been prepared keeping fixed the A cation radius 〈r_A〉 and cation size mismatch to isolate the effect of divalent dopant concentration from the A-cation steric effects. The electrochemical behaviour of these compounds for their application as SOFC cathodes was evaluated by using I-V curve measurements and ac impedance spectroscopy over three electrodes electrolyte supported cells processed under identical conditions. In contrast with the bulk behaviour, trends are more difficult to observe due to microstructural effects, but results seem to indicate that the doping level, x, does not influence in a significant way the electrochemical performance of iron perovskites with identical 〈r_A〉 and σ²(r_A).     

Electrochemical properties of ceramic membranes based on SrTi0.5Fe0.5O3−δ in reduced atmosphere

The present work aims at the investigation of the electrochemical properties of SrTi_0.5Fe_0.5O_3−δ as a membrane material for hydrogen production via electrochemical reforming. The dependence of the electrical conductivity on the oxygen partial pressure, as well as the oxygen permeability in the range of 10⁻²⁰ atm ≤ pO₂$p_{{\text{O}}_2}$ ≤ 10⁻¹⁴ atm is examined. The oxygen permeability is measured by an electrochemical method. The dependences of ion current as a function of the electromotive force (EMF) at various temperatures, oxygen partial pressures and the membrane surface conditions (rough and activated by PrO_x) are studied. Finally, the values of hydrogen flux at different temperatures are calculated and a long term investigation during 600 h at pO₂$p_{{\text{O}}_2}$ = 10⁻¹⁹ atm, T = 1173 K is carried out. According to the present results, the permeation current increases with the increase of temperature, oxygen partial pressure gradient and activation by PrO_x. The long term investigation shows that the electrical resistance of the SrTi_0.5Fe_0.5O_3−δ ceramic membrane increases by 10%, possibly due to the formation of micro-domains into the material's volume and the decrease in the grain boundary conductivity, because of the segregation of dopant-rich layers near the grain boundaries.     

The electrocatalysis of oxygen evolution reaction on La1−xCaxFeO3−δ perovskites in alkaline solution

The electrocatalytic oxygen evolution reaction (OER) on iron based perovskites with composition La_1?xCa_xFeO_3?δ (0.0 ≤ x ≤ 1.0) in alkaline solution has been investigated. The perovskite samples were synthesized by combustion method. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy were used to determine the bulk and the surface composition, respectively. The X-ray diffraction and iodometric titration method were employed to examine the phases and the oxidation state, respectively. It was observed that incorporation of calcium (Ca²⁺) ions in the lattice of LaFeO₃ decreases the lattice parameters and the cell volume systematically as evaluated by Rietveld method. Furthermore, increase in the degree of Ca²⁺ substitution from 0.0 to 1.0, increases the average oxidation state of iron from Fe³⁺ to Fe⁴⁺ in addition to creating oxygen vacancies. The evaluation of OER kinetics on a rotating disk electrode setup suggests that incorporation of Ca²⁺ decreases the activity initially (0.0 ≤ x ≤ 0.4), but further substitution increases the activity. The maximum activity was observed for x = 1.0. This change in the OER activity suggests an interplay between the bond lengths and angles, oxygen vacancy and the average oxidation state of Fe.     

SmBa0.5Sr0.5Cu2O5+δ and SmBa0.5Sr0.5CuFeO5+δ layered perovskite oxides as cathodes for IT-SOFCs

Novel Cobalt-free layered perovskite oxides SmBa_0.5Sr_0.5Cu₂O_5+δ (SBSCO) and SmBa_0.5Sr_0.5CuFeO_5+δ (SBSCFO) were investigated as cathode materials for intermediate-temperature solid fuel cells (IT-SOFCs). The thermal expansion coefficients (TEC) of SBSCO and SBSCFO were 14.1 × 10⁻⁶/°C and 14.9 × 10⁻⁶/°C in 50 °C–800 °C, which were more compatible with electrolyte than cobalt-based cathodes. When A′-site is partially substituted by Sr, the conductivity of SBSCO and SBSCFO had been improved. The max electrical conductivity of SBSCO was 277.7 S cm⁻¹, about one order of magnitude higher than SmBaCu₂O_5+δ. Polarization resistance of SBSCO is 0.25 Ω cm² at 650 °C, which is twice lower than that of SmBaCu₂O_5+δ (SBCO). This implies SBSCO has higher activity for oxygen reduction than SBCO. Preliminary results indicate that SBSCO are promising as cathodes for IT-SOFCs.     

In situ drop-coated BaZr0.1Ce0.7Y0.2O3−δ electrolyte-based proton-conductor solid oxide fuel cells with a novel layered PrBaCuFeO5+δ cathode

In order to develop a simple and cost-effective route to fabricate proton-conductor intermediate-temperature SOFCs, a dense BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte was fabricated on a porous anode by in situ drop-coating. The PrBaCuFeO_5+δ (PBCF) composite oxide with layered perovskite structure was synthesized by auto ignition process and examined as a novel cathode for proton-conductor IT-SOFCs. The single cell, consisting of PBCF/BZCY/NiO-BZCY structure, was assembled and tested from 600 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and the static air as the oxidant. An open-circuit potential of 1.01 V and a maximum power density of 445 mW cm⁻² at 700 °C were obtained for the single cell. A relatively low interfacial polarization resistance of 0.15 Ω cm² at 700 °C indicated that the PBCF is a promising cathode for proton-conductor IT-SOFCs.