Evaluation of Sm_0.95Ba_0.05Fe_0.95Ru_0.05O₃ as a potential cathode material for solid oxide fuel cells

Abstract

The perovskite Sm_0.95Ba_0.05Fe_0.95Ru_0.05O₃ (SBFR) is investigated to assess its possible use as a potential cathode material for solid oxide fuel cells. SBFR shows good thermal stability and has good chemical compatibility with the yttria-stabilized zirconia (YSZ) electrolyte below 1000 °C. In the oxygen partial pressure (pO₂) range of 0.01–0.6 atm, the electrical conductivity of the SBFR sample follows a pO₂^1/6 dependency, attributed to p-type electronic conductor behavior. Polarization resistance (R_p) increases with a decrease in pO₂ (n = 0.53) and decreases with an increase in the amplitude of the applied direct current (DC) bias at 800 °C. The R_p values of the SBFR sample on the YSZ electrolyte in symmetrical cells are 0.28, 0.22, 0.15 Ω cm² at 700, 750 and 800 °C, respectively, and the maximum power density of the YSZ electrolyte-supported single cell with the Ni-YSZ anode and SBFR cathode reaches 465 W cm⁻² at 800 °C using pure H₂ as a fuel and ambient air as an oxidant.

---

1. Introduction

Solid oxide fuel cells (SOFCs), which can efficiently convert chemical energy of a gaseous fuel directly into electrical energy via electrochemical reactions in an environmentally friendly manner, have been considered as one of the most important power generation technologies.^1–4 An SOFC, based on a solid oxide electrolyte, is schematically shown in Fig. 1. Each individual cell consists of three components, a ceramic electrolyte acting as a membrane which permits only a specific type of charged particles to transport through, a porous anode where the fuel gas is electrochemically oxidized, and a porous cathode as the electrocatalyst for the oxygen reduction reaction (ORR). The SOFC generally works in high temperature (900–1000 °C) conditions, resulting in a series of problems such as high degradation rate of SOFC material systems and solid state reactions among cell components. In the past years, extensive efforts have been made to reduce the SOFC operating temperature, which can broaden choice of the electrode and interconnecting materials, reduce the system costs and increase the performance durability.^5–8 Nevertheless, with the reduction of temperature, the ohmic resistance from the electrolyte and interfacial polarizations from the electrodes will exponentially increase, giving rise to a dramatical decline of the cell performance.^9,10 The problem of ohmic resistance has been gradually solved via reducing the electrolyte thickness or applying alternative electrolyte materials, for instances, yttria-stabilized zirconia (YSZ), La(Sr)Ga(Mg)O₃ and Gd_0.1Ce_0.9O_2−δ (GDC).^11,12 For addressing the cathodic polarization, developing more cathode materials with high performance has become critical.

Fig. 1 Schematic diagram of solid oxide fuel cell.

ABO₃-type perovskites exhibit great technological versatility as their variable properties, which can be tailor-made to the needs of the application via substitution at the A-site, at the B-site, or both at the A- and the B-sites simultaneously. The amount and nature of the cations at A-site and B-site play an important role in tuning specific properties such as chemical stability, thermal stability, electrical conductivity and catalytic activity.^13–17 Mixed ionic-electronic conductors (MIEC) with ABO₃-type, such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, La_0.5Sr_0.5MnO₃ and La_0.3Sr_0.7Ti_1−xCo_xO₃ (x = 0.3–0.6), have been broadly investigated as potential cathode materials considering their high catalytic activity for ORR.^18–20

Among perovskites of the type ABO₃, SmFeO₃, as a rare-earth orthoferrite, has been widely investigated as a candidate material for SOFC and gas sensor for detecting O₃, CO and NO₂.^21,22 Doping a certain amount of Ce at the A-site of SmFeO₃ not only enhances electrical conductivity of the perovskite, but also improves the stability of it under reducing conditions.²³ Partial substitution at the B-site has a great influence on the stability of the perovskite structure under reducing conditions through changing the B–O bond strength.²⁴ Furthermore, the electrical conductivity of perovskite oxides is greatly affected by the B-site dopant since its ability to form oxygen vacancies. Generally, the formation of oxygen vacancies intimately relates to the temperature, reducing nature of the environment, the valance state of the cation doped at A-site and reducibility of the cation doped at B-site.²⁵ B-site substitution by Ni and Mg in SmFeO₃ has been assessed, leading to a better conductivity and exhibiting p-type conducting behavior.^26,27 Doping Co at the B-site of ferrite-based perovskites can easily induce the formation of vacancy, and the influence of cobalt substitution on the electrical conductivity of SmFeO₃ has been recently studied.^28,29 In oxidizing conditions, the conductivity of SmFeO₃ can be enhanced by doping cobalt at the B-site of SmFeO₃, but the chemical stability is reduced in reducing atmospheres attributing to the weaker Co–O bond compared with Fe–O bond.²⁴ Besides, the existence of cobalt can result in the phase separation of perovskites through the formation of Co⁰, Fe⁰ and Fe–Co alloys.^30,31

In this work, with an aim to develop a potential cathode material, perovskite oxide with composition of Sm_0.95Ba_0.05Fe_0.95Ru_0.05O₃ (SBFR) was synthesized by sol–gel method, and the feasibility of SBFR as a potential cathode material for SOFCs was examined based on yttria-stabilized zirconia (YSZ) electrolyte. Additionally, the phase composition, oxygen deficiency, chemical compatibility, thermal stability, rate-limiting steps for oxygen reduction reactions (ORR), electrochemical impedance spectra (EIS), electrical conductivity and cell performance were also investigated.

2. Experimental

2.1. Sample fabrication

Sm_0.95Ba_0.05Fe_0.95Ru_0.05O_3−δ (SBFR) perovskite-type oxide was prepared by a citric acid method, with Fe(NO₃)₃·9H₂O (Sinopharm Chemical Reagent, 98.5%), Sm₂O₃ (Sinopharm Chemical Reagent, 99.9%), RuO₂ (Aladdin, 99.9%), and Ba(NO₃)₂ (Sinopharm Chemical Reagent, 99%) as original materials and C₆H₈O₇·H₂O (Fengchuan Chemical Reagent, 99.5%) as a complexant. Broadly speaking, Sm₂O₃ and RuO₂ were dissolved into nitric acid to form metal ion solution. Simultaneously, Fe(NO₃)₃·9H₂O and Ba(NO₃)₂ were added into de-ionized water mixing with nitrate solution when a transparent solution formed via rapid agitation. Then citric acid monohydrate was added into the solution at a mole ratio 1 : 1 of citric acid to total metal ions. The solution was gradually evaporated by stirring and heating at 80 °C on a magnetic heating stirrer till a gel obtained, which subsequently was moved into constant temperature air blowing oven for drying amorphous citrate precursors. Then, the solid precursor was calcined at 600 °C for 2 h (at a heating rate of 2 °C min⁻¹) and 950 °C for 1 h (at a heating rate of 2 °C min⁻¹) in air to achieve a single perovskite phase structure which was confirmed by XRD.

2.2. Cell fabrication

For a single cell fabrication, mixing the NiO powder with the as-made YSZ powder in a weight ratio of 7 : 3, and then intensively milling the mixture with prepared reagent (a mixture of terpilenol and turpentine with a v/v ratio of 20 : 1) in an agate mortar. The formed slurry was uniformly screen-printed onto one side of the dense YSZ disk (∼700 μm in thickness) as anode (∼30 μm in thickness), and the diameter of obtained circular electrode was ∼5 mm. Subsequently, the SBFR cathode slurry was screen-printed onto the other side of the YSZ electrolyte, accompanying with calcination at 950 °C for 2 h to form the working electrodes. Finally, Pt current collectors were adhered to both working electrodes by painting with Pt paste with following sintering at 600 °C for 1 h to obtain a better adhesion. When testing the performance of single cell, the as-fabricated cell was sealed onto one end of a ceramic tube using high temperature resistant sealant (C-2, Hubei Huitian Adhesion Enterprise Co., Ltd), and the anode was exposed to pure H₂ atmosphere with flow rate of 60 mL min⁻¹, while the cathode was exposed to ambient air. For symmetrical cells of SBFR|YSZ|SBFR, SBFR slurries were screen-printed on both sides of the YSZ electrolyte symmetrically, followed by calcination at 950 °C for 2 h.

2.3. Material characterization

The initial structural characterization and chemical compatibility of synthesized SBFR powder were performed by X-ray diffraction (XRD, Bruker D2 PHASER, Germany) operated at 30 kV and 10 mA using Cu Kα radiation with a wavelength of 1.54 Å at room temperature (RT). Exporting data was analyzed by commercially available software Jade 5.0. Scanning electron microscope (SEM, KEYENCE VE-9800, Japan) was carried out to examine the morphology of SBFR samples, which were coated with gold by high vacuum sputtering coating machine (Quorum Q150T, UK) before measurement. Moreover, transmission electron microscope (TEM) was performed using a Tecnai G2F20 S-Twin (USA) electron microscope. Specimens for TEM analysis were prepared by ultrasonically dispersing the powdered samples in acetone, and then dropping several drops of the suspension onto a copper grid which was placed on a holey carbon film. The obtained data were analyzed via Digital-Micrograph software.

Thermal stability of the SBFR powder was inspected by means of thermogravimetric analyses (TGA, Mettler Toledo TGA/DSC 1, Switzerland) in the temperature range of 50–1000 °C at a heating rate of 2 °C min⁻¹. The oxygen non-stoichiometry (δ) of the SBFR perovskite at room temperature (RT) was surveyed by iodometric titration. Approximately 0.1 g of the as-synthesized powders was dissolved into 12 M HCl solution, followed by mixing with a certain amount of KI powder, the generated I₂ was then titrated with standardized 0.1 M Na₂S₂O₃ using starch solution as an indicator. Specific surface area of fully ground sample was determined by N₂ adsorption–desorption measurements at −196 °C by employing the Brunauer–Emmet–Teller (BET) method (Gold App V-sorb 2008p). Prior to N₂ adsorption, the sample was outgassed at 300 °C for 3 h to desorb moisture adsorbed on the surface and inside the porous network. Besides, X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA) was characterized to analyze the oxidation states of the elements at the surface of SBFR particles. The obtained binding energies (BEs) were analyzed by CasaXPS software and using the carbon (C1s) peak at 284.5 eV as a reference. A dilatometer instrument (Linseis L75H) was used to determine the thermal expansion coefficient (TEC) of SBFR material, and the ribbon-shape sample about 2 cm in length was tested under air at a programming rate of 5 °C min⁻¹.

Electrochemical measurement of cathode was investigated by impedance spectroscopy in air using a work station (Solartron Analytical SI 1260&1287, UK), subsequently, employing convenient Zview software to fit obtained results. To confirm the rate-limiting step in the oxygen reduction reactions (ORR) at the SBFR/electrolyte interface, a study of the oxygen partial pressure (pO₂) dependence of polarization resistance (R_p) was also investigated. For electrical conductivity measurement, which was determined by the standard four-probe DC method, densified pellet with a size of 10 × 10 × 3 mm was prepared by pressing the powders at 40 MPa and therewith sintered at 1250 °C for 12 h with a ramp of 2 °C min⁻¹. Additionally, the electrical conductivity as a function of pO₂ was also measured by the four-probe DC method to ascertain the type of electronic conductivity.

3. Results and discussion

The powder X-ray diffraction patterns of the as-synthesized SBFR after calcination at 850 °C for 20 h in air and SmFeO₃ (SF) oxides are displayed in Fig. 2(a). It can be observed that the SBFR powder presents an orthorhombic symmetry structure similar to SF belonging to space group Pnma(62), and all of the main peaks match well with those of SmFeO₃ (JCPDS no. 39-1490), indicating that a single phase was obtained and no phase separation appeared in the SBFR perovskite namely SBFR has good stability to be used as cathode material for SOFCs. After a qualitative analysis of the XRD data, the cell parameters of a (5.6128 Å), b (7.7334 Å) and c (5.4002 Å) were obtained. Density of 7.2488 g cm⁻³ was acquired by using formula D = (W × Z)/(V × 0.6022169), where W is the formula mass, Z is the number of formula units per unit cell and V is the volume calculated from unit cell constants.³² Specific surface area obtained from Brunauer–Emmet–Teller (BET) method for SBFR sample was 4.31 m² g⁻¹. It is worth noting that excellent surface area contributes to the migration of reactants and products occurring in or on electrode. Additionally, according to Goldschmidt formula t = (r_A + r_O)/sqrt2(r_B + r_O), where r_A, r_B and r_O are ionic radii of the A-cation, B-cation and oxygen anion presenting in the lattice, a ca. 0.82 of t for SBFR perovskite can be attained. The t value of 0.82 within the range of 0.75–1.00, implying a stable perovskite structure.^33,34

Fig. 2 (a) XRD patterns of the as-synthesized SBFR and SmFeO₃. (b) XRD profiles of the mixture of SBFR and YSZ with a weight ratio of 7 : 3 after oxidation in air at 1000 °C for 20 h, the patterns of pure YSZ and SBFR are also presented for comparison.

Interfacial reactions between cathode and electrolyte may result in the increase of interfacial polarization resistance. To evaluate the chemical compatibility of SBFR with YSZ electrolyte, a mixture of the as-prepared SBFR and YSZ powder with a weight ratio of 7 : 3 was adequately milled and followed by calcination at 1000 °C for 20 h in air. The well sintered sample was then examined by XRD. As displayed in Fig. 2(b), no observable reaction between the cathode material and electrolyte can be found, illustrating that the SBFR cathode material have a desirable chemical compatibility with YSZ electrolyte. In addition, for preventing excessive strain and cracking on thermal cycling, the thermal expansion coefficients (TECs) of the electrode and the electrolyte need to be comparable, since the single cell usually suffers from high temperature for a long time during SOFC testing. To evaluate the mechanical compatibility of the synthesized SBFR perovskite electrode with the YSZ electrolyte, a thermal expansion measurement was performed in air from 30 °C to 1000 °C, and the result is shown in Fig. 3(a). The TEC of SBFR perovskite is ∼11.4 × 10⁻⁶ K⁻¹, which is similar to that of the YSZ electrolyte,³⁵ implying that the SBFR material matches well with YSZ electrolyte.

Fig. 3 (a) Thermal expansion as function of temperature for SBFR sample measured in air from 30 °C to 1000 °C. (b) TGA curve of SBFR measured in air as a function of temperature with heating rate of 2 °C min⁻¹.

TGA measurement was performed on the synthesized SBFR powders in the temperature range of 50–1000 °C at a heating rate of 2 °C min⁻¹ in air, and the TG curve is shown in Fig. 3(b). It can be observed that the total weight loss for SBFR sample was 2.17% during the measurement, ascribing to the formation of oxygen vacancies namely the reduction of metal oxides and escape of oxygen ions in the lattice.³⁶ More significantly, up to 800 °C, the highest temperature for electrochemical performance measurement in present study, a weight loss of 1.62% was obtained, implying that SBFR possesses good thermal-stability. In order to investigate the oxygen nonstoichiometries (δ) of the SBFR and SmFeO₃ (SF, synthesized with a same route of SBFR) oxides at room temperature, iodometric titration method was performed. For credibility, five parallel experiments were carried out for each sample, and the δ values of 0.088 and 0.111 for SF and SBFR perovskites were acquired, respectively. The increase of δ value indicated that more oxygen vacancies were formed in SBFR oxide, relating closely to the conductivity and catalytic activity of perovskite materials.

As shown in Fig. 4, XPS characterization was performed on the SBFR powder. In the fitting, the ratio of Lorentzian–Gaussian was fixed and the Shirley background subtraction method was utilized, moreover, all spectra were also calibrated by using the carbon binding energy of 284.8 eV. From the broad Fe2p peaks in the spectrum displayed in Fig. 4(a), it is evident that the peaks contain several peaks. The spin–orbit splitting of Fe2p electron spectra were fitted using 3 valence states of Fe, Fe²⁺, Fe³⁺ and Fe⁴⁺, respectively. For Fe2p_3/2, the peak around at 709.89 eV can be assigned to Fe²⁺, 710.68 eV is ascribed to Fe³⁺ and 712.38 eV is attributed to Fe⁴⁺, respectively.³⁷ From peak fitting results, which are similar to the data reported by other authors,^38,39 37.80% Fe²⁺, 47.01% Fe³⁺ and 15.19% Fe⁴⁺ were obtained. Therefore, the oxygen nonstoichiometry δ value is determined to be 0.109, which is close to 0.111 of SBFR measured in powder sample by iodometric titration at room temperature. Additionally, as present in Fig. 4(b), spectrum of O1s is also given, since oxygen bounded to metals, especially in metal oxide catalysts, plays an important role in their catalytic performances. Two peaks including one weak peak and one intense peak can be observed for the O1s core level, and the high peak (528.78 eV) stands for oxygen ions (O²⁻) in the crystal lattice (O_lat) which are bounded by several metal atoms, additionally, surface adsorbed oxygen (O_ads) species (O²⁻ or O⁻), OH groups and oxygen vacancies promote the formation of low peak (530.58 eV).^40,41

Fig. 4 XPS spectra of (a) Fe2p and (b) O1s of the synthesized SBFR.

Fig. 5(a) displays a representative SEM image of the as-synthesized SBFR powder. The microstructure reveals the well sintered nature of the powder with average grain size of ∼450 nm, and the sample has desirable porous, facilitating to the gas transfer. Particles are also well connected to each other leading to the formation of a continuous network, which may facilitate to the increase of electrical conductivity of the electrode and be liable to thermal diffusion. The typical HR-TEM image of the as-prepared SBFR sample is displayed in Fig. 5(b). The legible parallel lattice fringes of HR-TEM image provide the evidence for high degree of crystallinity of the synthesized material, which can also be depicted from high intensity, narrow XRD peaks for the sample. The lattice fringe spacing was ∼0.221 nm, corresponding to the distance between the (022) planes of SBFR.

Fig. 5 (a) SEM view of the synthesized SBFR powder. (b) HR-TEM image of SBFR powder.

A cracking representation of the catalytic activity of the fuel cell cathode for oxygen reduction reactions is the area specific polarization resistance (R_p), which can be received from the electrochemical impedance spectroscopy (EIS) based on a symmetrical cell composed of cathode/electrolyte/cathode. The impedance spectra of SBFR cathode measured in a temperature range from 700 to 800 °C with a configuration of SBFR|YSZ|SBFR are shown in Fig. 6(a). The values of R_p were acquired from fitting of an equivalent circuit model LR(QR)(QR), which was done by using Zview software. As displayed in the inset of Fig. 6(a), the inductance (L) appears in high-frequency; the serial resistance of the circuit R₀ signifies the whole ohmic resistance due to the electrolyte, electrodes and the connection wires; QPE is the constant phase element, arising from the inhomogeneity of the surface of electrodes, with the impedance Z = 1/Q(jω)ⁿ, where ω is angular frequency, the related parameter “Q” can be defined as a non-ideal capacitor and “n” stands for its similarity to a true capacitor;⁴² R₁ corresponds to the high frequency arc implying the charge transfer of oxygen ions at the electrode/electrolyte interface, and R₂ corresponds to the low frequency arc relating to the procedure of the oxygen adsorption or dissociation process.^43–45 The temperature dependence of polarization resistances (R_p = R₁ + R₂) for SBFR cathode obtained from the data fitting with equivalent circuit LR(QR)(QR) are summarized in Table 1. The relative errors of fitted parameters are less than 10%, illustrating the fitted data are reliable.⁴⁶

Fig. 6 (a) Impedance spectra of the symmetrical cell SBFR|YSZ|SBFR measured in air at various temperatures from 700 °C to 800 °C. The ohmic resistance has been subtracted from the impedance for direct comparison, and inset is the equivalent circuit employed for data fitting. (b) The influence of applied DC bias on the impedance spectra of the SBFR-based symmetrical half cells at 800 °C.

Table 1 Polarization resistances as a function of temperature for SBFR cathode obtained from the data fitting with equivalent circuit LR(QR)(QR)

Sample	T (°C)	R1 (Ω cm^2)	R2 (Ω cm^2)	Rp (Ω cm^2)
SBFR-cathode	700	0.0249	0.2546	0.2795
	750	0.0200	0.1997	0.2197
	800	0.0137	0.1370	0.1507

---

To clearly show the difference in the electrode polarization behavior, all the spectra were moved to the origin of the Z′ axis after subtracting the series resistance, as shown in Fig. 6(a), the R_p value decreased with the increase of temperature, associating with the thermal interaction on the electrode reaction processes.⁴⁷ In this study, the SBFR showed the polarization resistance of 0.28, 0.22, 0.15 Ω cm² at 700, 750 and 800 °C under OCV conditions, respectively. Similar performance at intermediate temperatures under OCV conditions has also been reported for other compounds.^48–50 It should be noted that one must carefully compare the data from different authors as many factors act on them, such as degree of crystallinity, microstructure and porosity.⁵¹ Moreover, since the high frequency arc is much less than the low frequency arc, the rate-limiting step of the electrode reaction should be the molecular oxygen dissociation processes, as expressed in the following equations:

O_ad + e′ ↔ O^′_ad (1)

O_2,ad ↔ 2O_ad (2)

As presented in Fig. 6(b), R_p values of the SBFR-based half cells, obtained in air at 800 °C, decreased with the increase in amplitude of the applied DC bias, suggesting that the rate-limiting step of the electrode reaction occurring in the cell was an electrochemical reaction, not a pure chemical reaction.⁵²

To obtain more insights on the ORR at the SBFR/electrolyte interface, a study of the influence of oxygen partial pressure (pO₂) on the R_p of SBFR was investigated at different temperatures, the results are displayed in Fig. 7(a). It is worth noting that the ORR at mixed conducting electrode consists of many processes, such as gas diffusion, surface adsorption, gas dissociation, electron and ion transport, charge transfer at the interface, and so on. Each step has a different dependence on pO₂.⁵³ The most frequently-used approach to confirm the rate-limiting step in the ORR is to ascertain the slope of R_p as a function of pO₂, complied with the following formula:⁵⁴

R_p = k(pO₂)⁻ⁿ (3)

where k is the oxygen partial pressure independent constant. Different n values correspond to different rate-limiting steps, and the n value is expected to be 1, 1/2 or 1/4. In general, n = 1 is associated with molecular oxygen adsorption on the electrode surface or oxygen diffusion in the gas phase, n = 0.5 corresponds to oxygen dissociation, and n = 0.25 is related to charge transfer. It can be obtained that R_p increased with decrease in pO₂ and the n values were about 0.5, relating to the oxygen dissociation. Additionally, it can be observed from Fig. 7(b) that the polarization resistance exhibits an Arrhenius like temperature dependence, with activation energy of E_a = 0.60 eV, which can be calculated according to the formula:where R is the gas constant, T is the absolute temperature, and R₀ is the pre-exponential factor. Importantly, E_a has an intimate correlation with the reaction mechanism including gas adsorption, dissociation, bulk or surface diffusion, and the self-diffusion via electrolyte. The E_a value of this work is less than other E_a values reported in literature,⁵⁵ such as 1.70 eV of La_0.8Sr_0.2CoO_3−δ, 1.90 eV of La_0.8Sr_0.2FeO_3−δ, and 2.09 eV of La_0.8Sr_0.2Co_0.8Fe_0.2O_3−δ, suggesting that SBFR exhibits good activity for oxygen activation and mobility.

Fig. 7 (a) Polarization resistances versus oxygen partial pressure at 700 °C, 750 °C, and 800 °C. (b) Arrhenius representation of the SBFR cathode polarization resistance under OCV conditions.

The exchange current density defined as the absolute value of current density when the reactions of electrodes are in equilibrium, and it can be calculated from charge transfer resistances for SBFR cathode under different temperatures using the following formula:

i₀ = RT/nFR_ct (5)

where R is universal gas constant (8.314 J mol⁻¹ K⁻¹), T is temperature (K), n is the number of electron involved in the charge transfer process, F is Faraday constant (96 485 C mol⁻¹), and R_ct is charge transfer resistance. The i₀ results of 1444.78 mA cm⁻² at 700 °C, 1933.29 mA cm⁻² at 750 °C and 2974.06 mA cm⁻² at 800 °C can be obtained from SBFR cathode, respectively. Generally, a lower charge transfer resistance corresponds to a higher exchange current density, illustrating a better performance of the electrode. The increase in exchange current density as a function of temperature implies that a faster charge transfer appears in a higher temperature.

Fig. 8(a) shows the electrical conductivities (σ) of SBFR samples measured in air and N₂ at different temperatures using a DC four-probe configuration. Before measurement, the SBFR sample was heated to 800 °C at a heating rate of 10 °C min⁻¹ and stayed several minutes till to the stabilization of σ value, then starting the measurement at an interval of −50 °C. As shown in Fig. 8(a), the conductivity in N₂ is lower than that in air, following small polaron semiconducting behavior, in other words, the electron–hole is the charge carrier in this perovskite. To obtain more information about the conducting behavior of SBFR material, the pO₂ dependence of electrical conductivity of SBFR at various temperatures is displayed in Fig. 8(b). It can be found that the σ of the SBFR sample follows a pO₂^1/6 dependency, suggesting typical p-type electronic conductor behavior, as expressed in equation:

Fig. 8 (a) Temperature dependence of electrical conductivity of SBFR in air and N₂. (b) pO₂ dependence of electrical conductivity of SBFR at various temperatures. (c) Arrhenius plot of electrical conductivity of SBFR sample. (d) Electrical conductivity of SBFR under different thermal cycles in air.

As presented in Fig. 8(c), the Arrhenius representation of conductivity exhibits an almost-linear behavior in the testing temperature range, implying that the oxygen stoichiometry is mainly dependent on temperature, and the activation energy (E_a) value of 0.23 eV can be derived from the equation:⁵⁶

where A is the pre-exponential factor, T is the absolute temperature (K), E_a stands for the activation energy, k is the Boltzmann constant (8.62 × 10⁻⁵ eV K⁻¹), respectively. Low activation energy usually is an indication of the facility of oxygen ion migration.⁵⁷ For assessing the performance stability of SBFR material during the thermal cycling process, σ values of SBFR sample were investigated under different cycles, and the corresponding results are displayed in Fig. 8(d). Similar electrical conductivity values were obtained for the different cycles, indicating SBFR possesses desirable reversibility and good thermal cycling performance as the cathode material for SOFCs.

Fig. 9(a) shows the open circuit voltages (OCVs) of the cell Ni-YSZ|YSZ|SBFR measured in the temperature range of 500–800 °C. OCVs of 1.09, 1.11, 1.07 V at 700, 750 and 800 °C were attained, suggesting a good pressure tightness of the single cell was obtained during the measurement. The non-linear relationship between OCVs and temperature is ascribed to multi-stage oxidation of hydrogen on the anode surface and reduction of oxygen on the cathode surface and establishment of equilibrium between hydrogen oxidation and oxygen reduction products at different temperatures.⁵⁸ Fig. 9(b) shows the I–V and I–P curves of the single cell measured in a temperature range of 700–800 °C using wet H₂ (∼3% H₂O) as the fuel and ambient air as the oxidant. Power densities of 322 mW cm⁻² at 700 °C, 375 mW cm⁻² at 750 °C and 465 mW cm⁻² at 800 °C were achieved, respectively. Taking into account the thickness of electrolyte, these power densities are still approving. It's worth noting that more competitive performances may be achieved through decreasing the thickness of electrolyte and optimization of the electrode processing, such as enhancing electrode performance by surface modification through impregnation method^59,60 or fabricating thin film electrode via various approaches, e.g., electro-spray deposition (ESD),^61,62 physical vapor deposition (PVD),^63,64 and atomic layer deposition (ALD).⁶⁵ Moreover, the cell was operated in pure H₂ at 800 °C for 24 h with a constant current of 0.9 A cm⁻², and the result is displayed in Fig. 10. It can be observed that no wide degradation occurred in voltage, revealing a reasonable stability of electrolyte-supported single cell with the SBFR cathode, which is a strong indication of its potential as a cathode material for SOFCs.

Fig. 9 (a) OCVs of the cell Ni-YSZ|YSZ|SBFR measured at different temperatures. (b) Cell voltage (left) and power density (right) as a function of current density for the cell with the configuration of Ni-YSZ|YSZ|SBFR using pure H₂ as fuel and ambient air as oxidant measured at 700–800 °C, the thicknesses of SBFR electrode and YSZ electrolyte were 30 and 700 μm, respectively.

Fig. 10 Galvanostatic test of Ni-YSZ|YSZ|SBFR cell at a constant current of 0.9 A cm⁻² at 800 °C.

4. Conclusions

In summary, the perovskite-type oxide Sm_0.95Ba_0.05Fe_0.95Ru_0.05O_3−δ (SBFR) has been synthesized via a citric-acid method and investigated as prospective cathode material for SOFCs. XRD analysis demonstrated that SBFR exhibited an orthorhombic symmetry structure belonging to space group Pnma(62). SBFR material has good thermal stability with a total weight loss of 2.04% up to 900 °C, and no observable reaction between the SBFR material and YSZ electrolyte can be found namely SBFR has desirable chemical compatibility with YSZ electrolyte. The oxygen partial pressure dependence of polarization resistance for SBFR electrode revealed that the n values were about 0.5, relating to the oxygen dissociation. SBFR exhibited desirable reversibility and good thermal cycling performance, and displayed polarization resistance value of ∼0.15 Ω cm² in air at 800 °C. In the pO₂ region of 0.01–0.6 atm, the data can be fitted to a straight line with a slope of ca. 1/6, suggesting a predominantly p-type conducting behavior of SBFR. Additionally, peak power density of 465 mW cm⁻² at 800 °C for Ni-YSZ|YSZ|SBFR single cell was obtained using H₂ as the fuel. More competitive performances may be achieved through optimization of the electrode processing and decreasing the thickness of electrolyte.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant No: 51507133), the China Postdoctoral Science Foundation (Grant No: 2015M572556), and the New faculty research support program of Xi'an Jiaotong University is gratefully acknowledged.

References

1. N. Q. Minh, J. Am. Ceram. Soc., 1993, 76, 563.
2. H. C. Yu, F. Zhao, A. V. Virkar and K. Z. Fung, J. Power Sources, 2005, 152, 22.
3. T. Inagaki, K. Miura and F. K. Takehisa, J. Power Sources, 2000, 86, 347–351.
4. Y. H. Li, R. Gemmena and X. B. Liu, J. Power Sources, 2010, 195, 3345–3358.
5. Y. L. Yang, L. Qiu and A. J. Jacobson, J. Mater. Chem., 1997, 7, 937–941.
6. N. P. Bansal and Z. M. Zhong, J. Power Sources, 2006, 158, 148–153.
7. D. Pérez-Coll, A. Aguadero, M. J. Escudero, P. Núnez and L. Daza, J. Power Sources, 2008, 178, 151.
8. Z. Liu, M. F. Han and W. T. Miao, J. Power Sources, 2007, 173, 837.
9. B. Wei, Z. Lü, X. Huang, M. Liu, N. Li and W. Su, J. Power Sources, 2008, 176, 1.
10. T. Komatsu, R. Chiba, H. Arai and K. Sato, J. Power Sources, 2008, 176, 132.
11. Z. Tianshu, P. Hing, H. Huang and J. Kilner, Solid State Ionics, 2002, 148, 567–573.
12. T. Ishihara, H. Matsuda and Y. Takita, J. Am. Chem. Soc., 1994, 116, 3801–3803.
13. D. P. Fagg, V. V. Kharton, A. V. Kovalevsky and A. P. Viskup, J. Eur. Ceram. Soc., 2001, 21, 1831.
14. A. E. Giannakas, A. A. Leontiou, A. K. Ladavos and P. J. Pomonis, Appl. Catal., A, 2006, 309, 254.
15. T. Ishihara, H. Furutani, M. Honda, T. Yamada, T. Shibayama, T. Akbay, N. Sakai, H. Yokokawa and Y. Takita, Chem. Mater., 1999, 11, 2081.
16. V. V. Kharton, A. P. Viskup, E. N. Naumovich and N. M. Lapchuk, Solid State Ionics, 1997, 104, 67.
17. M. A. Pena and J. L. G. Fierro, Chem. Rev., 2001, 101, 1981.
18. Z. Shao and S. M. Haile, Nature, 2004, 431, 170–173.
19. M. Zhi, G. Zhou, Z. Hong, J. Wang, R. Gemmen, K. Gerdes, A. Manivannan, D. Ma and N. Wu, Energy Environ. Sci., 2011, 4, 139–1441.
20. Z. H. Du, H. L. Zhao, Y. N. Shen, L. Wang, M. Y. Fang, K. Swierczek and K. Zheng, J. Mater. Chem. A, 2014, 2, 10290.
21. B. V. Prasad, G. N. Rao, J. W. Chen and D. S. Babu, Mater. Res. Bull., 2011, 46, 1670–1673.
22. Y. Subramanian, L. Samar, S. M. Vidyavathyc, Y. Selvaraj, M. Danielle and R. K. Selvana, Mater. Res. Bull., 2015, 72, 77–82.
23. S. M. Bukhari and J. B. Giorgi, Solid State Ionics, 2009, 180, 198.
24. X. Zhu, H. Wang and W. Yang, Solid State Ionics, 2006, 177, 2917.
25. J. N. Kuhn and U. S. Ozkan, Catal. Lett., 2008, 121, 179.
26. L. Chen, J. Hu, S. Fang, Z. Han, M. Zhao, Z. Wu, X. Liu and H. Qin, Sens. Actuators, B, 2009, 139, 407–410.
27. X. Liu, J. Hu, B. Cheng, H. Qin and M. Jiang, Sens. Actuators, B, 2008, 134, 483–487.
28. R. Yinzhe, L. Erbao, W. Jianying and W. Yuxiang, Rare Met., 2002, 21, 190.
29. Y. Itagaki, M. Mori, Y. Hosoya, H. Aono and Y. Sadaoka, Sens. Actuators, B, 2007, 122, 315.
30. Y. Teraoka, H. Shimokawa, C. Y. Kang, H. Kusaba and K. Kasaki, Solid State Ionics, 2006, 177, 2245.
31. F. J. Berry and X. Ren, Czech J. Phys., 2005, 55, 771.
32. E. N. Maslen, V. A. Streltsov and N. Ishizawa, Acta Crystallogr., Sect. B: Struct. Sci., 1996, 52, 406.
33. V. M. Goldschmidt, Naturwissenschaften, 1926, 14, 477.
34. R. D. Shanon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751.
35. J. W. Stevenson, T. R. Amstrong, D. E. McCready, L. R. Pederson and W. J. Weber, J. Electrochem. Soc., 1997, 144, 3613–3620.
36. A. S. Harvey, F. J. Litterst, Y. Zhen, J. L. M. Rupp, A. Infortuna and L. J. Gauckler, Phys. Chem. Chem. Phys., 2009, 11, 3090–3098.
37. T. Dickinson, A. F. Povey and P. M. A. Sherwood, J. Chem. Soc., Faraday Trans. 1, 1977, 73, 327.
38. Y. F. Bu, Q. Zhong, D. D. Xu and W. Y. Tan, J. Alloys Compd., 2013, 578, 60–66.
39. T. Yamashita and P. Hayes, Appl. Surf. Sci., 2008, 254, 2441–2449.
40. L. Xi, L. Xiaoxun, X. Baokun and Z. Muyu, J. Alloys Compd., 1992, 186, 315.
41. J. L. G. Fierro and L. G. Tejuca, Appl. Surf. Sci., 1987, 27, 453.
42. S. L. Pang, X. N. Jiang, X. N. Li, Q. Wang and Z. X. Su, Int. J. Hydrogen Energy, 2012, 37, 2160.
43. A. Babaei, L. Zhang, S. L. Tan and S. P. Jiang, Solid State Ionics, 2010, 181, 1221.
44. S. Tao and J. T. S. Irvine, J. Electrochem. Soc., 2004, 151, 252.
45. S. B. Adler, J. A. Lane and B. C. H. Steele, Solid State Ionics, 1998, 111, 125–134.
46. M. J. Escudero, A. Aguadero, J. A. Alonso and L. Daza, J. Electroanal. Chem., 2007, 611, 107–116.
47. F. Mauvy, C. Lalanne, J. M. Bassat, J. C. Grenier, H. Zhao, L. Huo and P. Stevens, J. Electrochem. Soc., 2006, 8, 1547–1553.
48. Y. Shen, F. Wang, X. Ma and T. He, J. Power Sources, 2011, 196, 7420–7425.
49. A. Aguadero, D. Perez-Coll, C. Calle, J. A. Alonso, M. J. Escudero and L. Daza, J. Power Sources, 2009, 192, 132–137.
50. P. Zeng, R. Ran, P. Shao, H. Yu and S. Liu, J. Chem. Eng., 2009, 26, 563–574.
51. E. V. Tsipis and V. V. Kharton, J. Solid State Electrochem., 2011, 5, 1007–1040.
52. E. J. L. Schouler and M. Kleitz, J. Electrochem. Soc., 1987, 134, 1045–1050.
53. M. J. Escudero, A. Aguadero, J. A. Alonso and L. Daza, J. Electroanal. Chem., 2007, 611, 107–116.
54. Y. Takeda, R. Kanno, M. Noda, Y. Tomida and O. Yamamoto, J. Electrochem. Soc., 1987, 11, 2656–2661.
55. J. M. Ralph, A. C. Schoeler and M. Krumpelt, J. Mater. Sci., 2001, 36, 1161.
56. L. W. Tai, M. M. Nasrallah, H. U. Anderson, D. M. Sparlin and S. R. Sehlin, Solid State Ionics, 1995, 76, 259–271.
57. S. Y. Li, Z. Lü, X. Q. Huang and W. H. Su, Solid State Ionics, 2008, 178, 1853–1858.
58. O. A. Marina and M. Mogensen, Appl. Catal., A, 1999, 189, 117.
59. S. P. Jiang, Mater. Sci. Eng., A, 2006, 418, 199–210.
60. J. M. Vohs and R. J. Gorte, Adv. Mater., 2009, 21, 943–956.
61. D. Beckel, A. Dubach, A. R. Studart and L. J. Gauckler, J. Electroceram., 2006, 3, 221–228.
62. D. Marinha, C. Rossignol and E. Djurado, J. Solid State Chem., 2009, 7, 1742–1748.
63. A. Zomorrodian, H. Salamati, Z. G. Lu, X. Chen, N. J. Wu and A. Ignatiev, Int. J. Hydrogen Energy, 2010, 22, 12443–12448.
64. P. Plonczak, A. Bieberle-Hutter, M. Sogaard, T. Ryll, J. Martynczuk, P. V. Hendriksen and L. J. Gauckler, Adv. Funct. Mater., 2011, 14, 2764–2775.
65. M. Cassir, A. Ringuede and L. Niinisö, J. Mater. Chem., 2010, 20, 8987–8993.

---