Perovskite of Ba_0.2 Sr_0.8 Ni_0.8 Fe_0.2 O_3-δ as a cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC): Electrochemical performance and micro-structural characteristics

 

Vandani Rawat, Ravindra Kumar Gautam, Sushmita Banerjee, Puja Rai,

Mahesh Chandra Chattopadhayaya*

Fuel Cell & Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad-211002, India

*Corresponding Author E-mail: mcchattopadhyaya@gmail.com

Nickel containing and cobalt free Perovskite oxides of Ba_0.2 Sr_0.8 Ni_0.8 Fe_0.2 O_3-δ (BSNF) were synthesized as a new cathode material for intermediate temperature – solid oxide fuel cell (IT-SOFC) by glycine nitrate process (GNP). Nitrates of Ba, Sr, Ni and Fe were used as precursor and glycine as self combustion reaction matter. Electrochemical impedance, conductivity, X-rays diffraction (XRD), and scanning electron microscopy (SEM) measurements were carried out of newly synthesized BSNF cathode material. Electrochemical impedance was measured at different temperatures by sintering the BSNF pellets at 900°C for 2 h. The conductivity measurement showed the highest value as 0.081 S cm^-1 at 650°C. Temperature dependence area specific resistance (ASR) of BSNF cathode material was also calculated and the minimum ASR value measured was 0.783 Ω cm² at 650°C. The results shows that BSNF can work as a good alternative of cobalt free, low-cost and intermediate temperature cathode material for solid oxide fuel cell.

 

 

 

1. INTRODUCTION:

Solid oxide fuel cell (SOFC) is a solid state electrochemical device that converts chemical energy into electricity with high conversion efficiency and low pollution emission¹. Usually SOFC works at a high temperature of 800-1000 °C which increases the operation budget. Reduction in operating temperature of SOFC from high range to intermediate temperature range of 500-800°C will accelerate the commercialization of SOFC technology in near future. Perovskites, which are based on alkaline earth and rare earth containing cobaltites, contain materials that can serve as cathode for SOFC at lower temperature². Many cobalt containing cathode have been reported due to its high electrocatalytic activity of oxygen reduction and it works as a good catalysis for the activation of oxygen molecule³. In material like Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃ (BSCF), there is a large concentration of oxygen vacancies and the electrochemically active sites can extended the whole porous cathode which make BSCF cathode to exhibit higher performance then it is found in traditional compound like La₀.₈Sr₀.₂MnO₃ for making a cathode.

 

Unfortunately, the cobalt-containing cathodes exhibit high thermal expansion coefficients (TECs) due to the flexible redox behavior of cobalt. Moreover, the temperature dependent coupled valence and spin state transition of cobalt can lead to the transition from cubic to hexagonal structure under long-term operation at intermediate temperature. In addition, these cobalt-containing cathodes encounter some other problems like easy evaporation and diffusion. Therefore, it is desirable to develop cobalt-free cathodes with excellent electrocatalytic activity for oxygen reduction reaction at intermediate temperatures. Efforts have been made to develop new cobalt-free materials, such as LaNi_0.6Fe_0.4O₃, SrCo_0.2Fe_0.6Ni_0.2O_3-δ, Ba_0.5Sr_0.5Zn₀.₂Fe_0.8O₃ (BSZF), Ba₀.₅Sr₀.₅Fe₀.₈Cu_0.2O₃ (BSFCu) and Bi_0.5Sr_0.5MnO₃ for the cathodes of ITSOFC^4-9. They were considered as promising alternatives to cobaltites and attracted much attention in recent years.

 

In the present work, preparation of cathode nanopowder of perovskite Ba_0.2Sr_0.8Ni_0.8Fe_0.2O_δ (BSNF) by glycine nitrate process (GNP) was attempted. The performance of this cobalt free cathode material for intermediate temperature SOFCs was investigated. Besides its microstructural characterization such as composition, structure, morphology, and grain size its electrical performance and temperature dependent ionic conductivity, area specific resistance was also discussed.

 

2. MATERIALS AND METHODS:

2.1 Synthesis of BSNF

All the chemical used for the present study were analytical reagent grade and includes Sr(NO₃)₂ (Sigma Aldrich, 99%), Ba(NO₃)₂ (Merck, 99%), Ni(NO₃)₂.6H₂O (Sigma-Aldrich, 99%), Fe(NO₃)₃.9H₂O and Glycine (NH₂-CH₂-COOH) (Merck, 99%). Nanosize Ba_0.2 Sr_0.8Ni_0.8Fe_0.2O_δ was synthesized using glycine nitrate process (GNP). Glycine acts as a self combustion reaction matter, and could generate fine particle. GNP could achieve higher yield for the final product. First of all the materials, Sr(NO₃)₂, Ba(NO₃)₂, Ni(NO₃)₂.6H₂O and Fe(NO₃)₃.9H₂O in a predecided molar ratio was dissolved in de-ionized water, and then glycine was added (in a molar ratio of 2 to all the metal cation). The solution mixture was stir over a magnetic stirrer at 250°C temperature until the excess water evaporates and a precursor gel formed. On further heating the gel was self-ignited and a fine powder was obtained. Then the powder was calcinated for 5 h at 850°C to obtain BSNF.

 

This calcinated mixed oxide were mixed with a binder Poly vinyl alcohol (PVA). The slurry was then heated for drying and grounded by using mortar-pestle. A certain amount of powder were pressed under 20 MPa. Obtained pellets were sintered at 900°C for 2 h at a heating rate of 5 °C/min.

 

2.2 Characterization

The crystal structure of the powder was analyzed by X-ray powder diffraction (rigaku.mini Flex600) with Cu-Kα radiation (λ = 1.54 °A). The morphology of the cathode material was examined by Scanning electron microscope (JEOL JSM-6010LA). Impedance spectra were measured by means of an AITOLAB - PGSTAT 30, machine, performed at a temperature of 300-750 °C with temperature intervals of 50 °C. All data of this study were plotted by Origin 6.1.

 

3. RESULTS AND DISCUSSION:

3.1 XRD and SEM analysis

XRD pattern of BSNF powder after calcinations at 900 °C has been shown in Fig. 1. Sharp lines reflect well formed crystals and the peaks denote as a cubic perovskite structure of Pm3m.

 

Fig. 2(a) and (b) shows the SEM micrographs of BSNF cathode material after calcinations. EDX pattern indicated the presence of metal oxides (Fig. 2(c)). The material is porous and homogeneous as well as has uniform structure. The particle size of BSNF was ~0.5 μm. The microstructure is important for the good performance of fuel cell. The rate of charge transfer is related to particle size and homogeneity of cathode material. EDX pattern verify the compositional homogeneity and structure uniformity of BSNF cathode material.

 

3.2 Impedance studies

The impedance spectra for BSNF as SOFC cathode material was measured at 300-750°C (Fig. 3). Results show that resistance decrease with increasing temperature from 300°C to 450°C then a significant change was not found. A plot between real component Z’ and imaginary component Z’’ is called Nyquist plot. In the Nyquist plot three types of arcs could be observed. We have considered the resistance corresponding to arc and it was used to determine the Area specific resistivity (ASR). The impedance curve show the low and the high frequency depressed arcs which are due to cathode polarization resistance. The larger arc suggests larger polarization resistivity.

 

Fig. 1 XRD Pattern of calcinated BSNF cathode material.

 

Fig. 2 SEM micrographs (a) and (b), EDX spectra of BSNF cathode material (c).

 

Fig. 3 Impedance response of BSNF cathode material at different temperatures.

The ASR values of BSNF are shown in Fig. 4. ASR is more in lower operating temperature. The value of ASR decreases on increase the temperature. In the present study the minimum value of ASR calculated at 650°C operating temperature, is 0.783 Ω cm².

 

Fig. 4 Temperature dependence of Area specific resistance (ASR) of BSNF cathode material.

 

The temperature dependence of the ionic conductivity has been investigated for BSNF cathode material. The traditional Arrhenius equation and its logarithm form are given below¹⁰

 

Which are usually adopted to analyze the measured conductivity data, where E is the activation energy for ionic conduction, σ⁰ a material constant and k is Boltzmann constant. A plot of ln σT against 1/T, can be predicted by this equation. Fig. 5 shows result of plot of lnσT against 1/T. A non-linear Arrhenius behavior is commonly studied by fitting the experimental data on two separate straight lines in both the low and the high temperature ranges.

 

As far as the intragrain-intergrain conductivity model is concerned, the non-linear Arrhenius behavior is attributed to the change in dominance of ionic conductivity from intergrain conductivity at low temperature to intragrain conductivity at high temperature^11-13. At high temperature the intergrain conductivity contributes less to the total conductivity than the intragrain conductivity, whereas the intergrain conductivity plays a key role on the ionic conductivity at a low temperature¹⁰. At low and high temperature linearity of the plots change and thus the activation energy change with increasing temperature. The change in activation energy begins at 500 °C. Activation energy for low temperature is 2.57 eV and for high temperature is 0.94 eV. The activation energy reflects the migration energy E_m for the oxygen vacancies. The migration energy, Em is required energy for O^2- ions displace across a saddle point in a diffusion path.

 

 

 

 

 

Fig. 5 Arrhenius plot for BSNF cathode material.

 

In present study the Electrical conductivity of BSNF cathode material with respect to temperature was shown in Fig. 6. The electronic conductivity is created by transition of cations between the triple and the tetravalent state. The pattern showed that the conductivity increases with increasing temperature upto 650°C. The highest conductivity value is 0.081 S cm^-1 at 650°C. The concentration of oxygen ions increase when the operating temperature increases, because the release of lattice energy. This condition promotes the thermal reduction of cations to lower valence state, leading to increase in electrical conductivity. At high temperature the concentration of electron holes increase, which leads to high electrical conductivity.

 

 

Fig. 6 Electrical conductivity of BSNF cathode material.

 

 

4. CONCLUSION:

Ba_0.2 Sr_0.8Ni_0.8Fe_0.2O_δ (BSNF) as SOFC cathode material was prepared by glycine nitrate process (GNP). The perovskite product was characterized after sintering at 900°C by XRD and SEM, techniques. The study showed that the material has a suitable perovskite structure. The conductivity measurement showed the highest value as 0.081 S cm^-1 at 650°C. The minimum ASR value measured was 0.783 Ωcm² at 650 °C. Hence BSNF can be used as a good alternative of low-cost, cobalt free material for the use in SOFC as cathode material at intermediate operating temperature.

 

5. ACKNOWLEDGEMENTS:

VR and PR are thankful to University of Allahabad for providing fellowship. RKG is grateful to UGC for the award of Senior Research Fellowship. Authors are thankful to Prof. S. Basu and Dr. Dyuti Pandey, Department of Chemical Engineering, IIT Delhi, Delhi, to provide instrumentation facilities.

 

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Received on 09.01.2015         Modified on 25.01.2015

Accepted on 05.02.2015         © AJRC All right reserved