Effects of cerium doping on the performance of LSCF cathodes for intermediate temperature solid oxide fuel cells

https://doi.org/10.1016/j.ijhydene.2018.08.041Get rights and content

Highlights

  • The electrochemical performance was significantly enhanced by Ce doping.

  • The reason of the enhanced electrochemical catalytic activities was revealed.

  • Oxygen surface exchange properties of the cathodes were characterized by ECR method.

Abstract

In this paper, effects of Ce substitution of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) on the crystal structure have been investigated by XRD Rietveld refinement and results show that Ce doping at La-sites leads to lattice expansion without changing centrosymmetric cubic space group of LSCF. The electrochemical results suggest that the electrocatalytic activity of LSCF cathodes is enhanced by Ce doping. The values of oxygen surface exchange coefficient of LSCF, LCSCF03, LCSCF06, measured by electrical conductivity relaxation (ECR) method, are 5.6 × 10−4 cm s−1, 1.0 × 10−3 cm s−1, 3 × 10−3 cm s−1 at 750 °C respectively, implying that oxygen surface exchange coefficients are increased by Ce doping compared with that of LSCF. The enhancement of electrochemical performance of LSCF cathodes can be ascribed to the increase of the concentration of oxygen vacancies obtained from the thermogravimetric results, which results in faster kinetics of oxygen transport by Ce doping.

Introduction

Solid oxide fuel cells (SOFCs) are promising devices which can convert the chemical energy directly into electrical energy and heat as long as sufficient fuels such as hydrogen or natural gas are supplied [1], [2], [3]. Nowadays, many researchers devote efforts to intermediate temperature SOFC (IT-SOFC) using metal interconnectors because of their better long time stability, low-cost compared with high temperature SOFC (HT-SOFC) [4], [5], [6]. Although the other components such as anode and interconnect have issues such as coarsening of nickel and interactions between cathode and interconnect, a great challenge originates from cathodes [7].

La1−xSrxMnO3 (LSM) [5], [8] and La1−xSrxCoyFe1−yO3 (LSCF) [9], [10], [11], [12], [13], [14] are accepted as two promising cathode materials worldwide. Though the satisfied performance can be achieved at high operation temperature (∼1000 °C), high operation temperature will cause faster degradation rate and higher requirement for components. Lowering temperatures from ∼1000 °C to 600–800 °C for commercial application of SOFC decreases the performance of LSM cathodes since their poor catalytic activities for oxygen reduction reaction [15], [16], while more attentions were attracted by LSCF cathode due to their excellent mixed ionic and electronic conductivities [17], [18], catalytic activities for oxygen reduction reaction [11], [19], [20] and better stability than LSM [15], [21].

Many studies have adopted various methods such as changing microstructure, impregnation, doping elements to improve the electrocatalytic activities of LSCF-based cathodes. Zhao et al. studied the performance of LSCF fiber which had high electrocatalytic activity due to their high porosity and the facile gas diffusion [22]. Our previous study proved that impregnating palladium on LSCF-GDC cathode decreased the polarization resistance [23]. Zhang et al. found that infiltration CaO to porous LSCF electrodes can increase the oxygen reduction reaction (ORR) catalytic activity [24]. Lakshminarayanan et al. demonstrated that B-site doping of LSCF with Zn, Ni and Cu enhanced the oxygen vacancy generation properties significantly, while LSCF still maintaining a stable perovskite structure [16]. Huang et al. clarified that adding Cu, Ag and Pt to LSCF could take in more oxygen from the gas phase because of higher oxygen affinity of the metals [25]. Mastrikov et al. concluded that Pd doping on LSCF could decrease oxygen vacancy formation energy, forming more oxygen vacancies [26]. Naumovich et al. confirmed that partial substitution by copper in B sublattice of LSCF resulted in a higher electrical conductivity below 800 °C [27]. Longo et al. reported Ni dopant on LSCF stabilized the oxygen vacancies in LSCF and delayed their formation [28]. Chen et al. found that Nb and Pd co-doped LSCF exhibited better electrocatalytic activity and excellent operation stability than that of LSCF [29]. All the work mentioned above enhanced the performance of LSCF cathodes. Up to now, cation doping is regarded as an effective strategy to develop high performance perovskite-type SOFC cathode materials. But most elements for doping are nobel metal, which are expensive and not suitable for commercial use of SOFC.

Recently, materials containing cerium have exhibited excellent catalytic activities for oxygen reduction reaction and hydrocarbon reformation. Chaudhari et al. investigated the catalytic activity of Ce-doped Sm2CuO4+δ, and results demonstrated that Sm1.9Ce0.1CuO4 showed higher catalytic activity than that of Sm2CuO4+δ [30]. Song et al. explored A-site ceria-substituted La0.75Sr0.25Cr0.5Mn0.5O3−δ and studied the catalytic activities for H2 oxidation and H2S reformation, determining that La0.75Sr0.125Ce0.125Cr0.5Mn0.5O3−δ exhibited better redox properties [31]. Bian et al. studied Ce-doped La0.7Sr0.3Fe0.9Ni0.1O3−δ in direct-methane solid oxide fuel cells and concluded that cerium doping enhanced methane reforming activity of La0.7Sr0.3Fe0.9Ni0.1O3−δ [32]. Therefore, Ce doping may enhance the performance of LSCF cathodes and its mechanism should be investigated in detail.

The present study focused on effects of Ce doping on electrochemical properties of LSCF cathodes. In this work, LSCF, La0.57Ce0.03Sr0.4Co0.2Fe0.8O3 (LCSCF03) and La0.54Ce0.06Sr0.4Co0.2Fe0.8O3 (LCSCF06) electrodes were synthesized and their performances were investigated. X-ray diffraction (XRD), Electrochemical impedance spectroscopy (EIS) and Electrical conductivity relaxation (ECR) were employed to evaluate the enhanced mechanisms of electrochemical catalytic activities of LSCF cathodes by Ce doping.

Section snippets

Preparation of cathodes and cells

LSCF, LCSCF03, LCSCF06, La0.51Ce0.09Sr0.4Co0.2Fe0.8O3 (LCSCF09) powders were synthesized by sol-gel method. Stoichiometric amount of La(NO3)3·6H2O(99.9%), Sr(NO)3(99.5%), Co(NO)3·6H2O(98.5%), Fe(NO)3·9H2O(98.5%) and Ce(NO3)3·6H2O(99.5%) (Sinopharm Chemical Reagent Co. Ltd.) were dissolved in deionized water simultaneously, followed by the addition of citric acid (CA). The ethylene-diamine-tetra-acetic acid (EDTA) was dissolved with ammonia. Both of solutions above were then mixed together. The

Phase composition

Ce3+/Ce 4+ are considered to occupy A-site positions rather than B-site positions in the ABO3-type of LSCF since the radius of Ce ions is more similar with that of La3+ [33]. XRD patterns of LSCF, LCSCF03, LCSCF06 powder, sintered at 900 °C for 6 h in the air are presented in Fig. 1. The results show that all the peaks of the samples are indexed as the cubic perovskite-type structures without any impurities except LCSCF09. Two peaks at 2θ around 27 and 57° suggests the formation of a second

Conclusions

In summary, the structures and performance of La0.6−xCexSr0.4Co0.2Fe0.8O3 (x = 0, 0.03, 0.06) were investigated in this work. Ce doping at La-sites leads to lattice expansion without changing centrosymmetric cubic space group of LSCF. Rp values of LSCF, LCSCF03, LCSCF06 cathodes are 0.21, 0.14, 0.09 Ω cm−2 at 750 °C, respectively, indicating that the electrochemical performance is significantly enhanced by the increase of Ce content. This performance enhancement can be explained that Ce doping

Acknowledgements

The project was supported by the National Natural Science Foundation of China (51402227) and the 111 Project (B17034). XRD, SEM and TG examinations were assisted by the Center of Material Research and Analysis of Wuhan University of Technology.

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