Double perovskite La_1.8Sr_0.2CoFeO_5+δ as a cathode material for intermediate temperature solid oxide fuel cells

Highlights

• LSFC studied as IT-SOFC potential electrode.

• TEC ≈ GDC-TEC.

• Viable electrochemical properties.

Abstract

Double perovskite La_1.8Sr_0.2CoFeO_5+δ (DP-LSCF) is prepared by a modified sol-gel method and investigated as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs, 600–850 °C). The compound is characterised by a variety of techniques, including TG/DTA, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and SEM-EDS. The formation of the double perovskite structure with rhombohedral symmetry after its calcination at 1000 °C for 6 h is revealed by X-ray diffraction. Excellent chemical compatibility with a Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte is found. The double perovskite structure is confirmed by Raman and X-ray photoelectron spectroscopy. The partial substitution of Fe cations at the Co site affects the physical-chemical properties of DP-LSCF. Compared with other double perovskite materials based on Co, DP-LSCF shows a lower thermal expansion coefficient (19.4 × 10⁻⁶ K⁻¹). The compound exhibits semiconductor behaviour and an area specific resistance of 0.42 Ω cm² at 800 °C when GDC is employed as the electrolyte. These results suggest that DP-LSCF is a promising cathode material for IT-SOFC applications.

Introduction

Solid oxide fuel cells (SOFCs) are promising systems for energy generation that can directly convert the chemical energy of fuel into electric energy with high efficiency, flexibility and low pollution emissions [1]. Nevertheless, conventional SOFCs with high operating temperatures of ~ 1000 °C have several disadvantages, such as thermal degradation, chemical reactivity between the electrodes (cathode and anode) and electrolyte, degradation of the components during operation and high cost fabrication [2].

Therefore, in recent decades, efforts to address these problems have been attempted by reducing the operating temperature to 600–850 °C through the development of intermediate temperature solid oxide fuel cells (IT-SOFCs). However, the conventional cathode, La_1−xSr_xMnO₃ [3], for SOFCs shows slow kinetics for the oxygen reduction reaction (ORR) [2] in the operating temperature range of IT-SOFCs and this is accentuated with further decreasing temperature. These slow kinetics result in increases in the polarisation resistance that cause an important reduction in cell performance. Hence, it is a significant challenge to find alternative cathode materials with high catalytic reaction activity for improving the ORR at ≤ 850 °C [4]. In this context, mixed ionic and electronic conductor materials are applied as alternative cathodes for IT-SOFCs, since their mixed conductivity enables oxygen ions to form on their surface, which then conduct to the electrolyte, thereby increasing the ionic conductivity [5]. Moreover, this also considerably extends the number of active centres, which rapidly increases the electrochemical reduction reaction of oxygen [6].

Recently, oxygen-deficient double perovskite oxides (DPOs) with the general formula A′A″B′B″O_5+δ (where A′ and A″ are lanthanum or alkali metals and B′ and B″ are transition metals) [7], [8] have been investigated as alternative cathode materials for IT-SOFCs [9]. The DPO structure consists of consecutive layers of [B′O₂]-[A′O]-[B″O₂]-[A″O_δ] ordered along the c axis. This stacking of lanthanum and alkali metal layers reduces the resistance of the oxygen retention in the A″O_δ layer and provides a remarkable increase in oxygen diffusivity [9] compared to other single perovskite ABO₃ materials [10], [11].

In particular, Co-based DPOs with the formula LnBaCo₂O_5+δ (where Ln is La, Pr, Nd, Sm or Gd) present excellent oxygen transport properties (oxygen surface exchange coefficient and oxygen-ion diffusivity) [9], [12], [13], [14]. However, these materials have high thermal expansion coefficients (TECs) of> 20.0 × 10⁻⁶ K⁻¹ [3], [9], [13], [15] compared to the electrolyte widely employed in IT-SOFCs, including Gd_0.1Ce_0.9O_2-δ (GDC, 13.08 × 10⁻⁶ K⁻¹ [16]), Sm_0.2Ce_0.8O_2-δ (SDC, 12.6 × 10⁻⁶ K⁻¹ [17]) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM, 11.4 × 10⁻⁶ K⁻¹ [18]), which can produce thermal stress at the cathode/electrolyte interface during thermal cycling, leading to delamination and their restricted use as IT-SOFC cathodes. The high TECs in LnBaCo₂O_5+δ are attributed to both the transition of Co³⁺ ions from low spin (or intermediate spin) to high spin and the variation of the oxygen vacancy content by increasing the temperature, causing the consequent expansion of the lattice expansion [6], [19].

To solve this problem, the most employed strategy is to partially substitute Co (B site) with other transition metals, such as Fe, Cu or Mn [3], [13], [19], [20]. Kim et al. [21] reported that the introduction of Fe at the Co positions in LnBaCo_2−xFe_xO_5+δ (where Ln is Nd or Gd) increases the oxygen content, thereby increasing structural stability and decreasing the TEC. However, they also detected an increase in electrical and polarisation resistance with increasing Fe content, which implies less electrochemical activity for the cathode. Zhao et al. [3] reported the partial substitution of Fe on to the Co site in PrBaCo_2−xFe_xO_5+ẟ (x = 0, 0.5, 1.0, 1.5 or 2.0) and they found that increasing Fe decreases the TEC, as well as the electrochemical properties of the compound. Furthermore, this compound presents a good chemical compatibility with GDC as the electrolyte. However, with an yttria-stabilised zirconia (YSZ) electrolyte, it showed chemical instability at 700 °C in air.

Some researchers have found that partial substitutions of Ba on Sr (A site) in LnBaCo₂O_5+δ (where Ln is La, Pr, Nd, Sm and Gd or Y) increase the electrical conductivity and electrocatalytic activity for the ORR [10], which results in chemical stability for IT-SOFCs [6], [9], [10], [15], [22]. However, RBaCo₂O_5+δ (R = Y or Ho) [6], [23] double perovskites are thermodynamically unstable at elevated temperature, decomposing into RCoO_3-δ and BaCoO_3-δ phases after calcining at 800 °C for 12 h in air, as well as they present high thermal expansion coefficient and chemical reactivity with GDC electrolyte [23]. Although an appropriate amount of Sr substitution into Ba site stabilised the double perovskite phase and improved the chemical stability with GDC electrolyte [23]. It is also reported that a partial substitution of Co by Cu and/or Fe improved the structural stability and reduce the thermal expansion, but the YBaCo_2/3Fe_2/3Cu_2/3O_5+δ compound presented chemical reactivity with the SDC and GDC electrolytes, as revealed the presence of the Sm₂CuO₄ and Gd₂CuO₄ impurity phases.

On the other hand, it is reported that the Sr substitution by La in double perovskite materials provoke an increase in the electrical conductivity and an enhancement of the electrochemical properties. Jun et al. [10] studied the effect of the incorporation of Sr on Ba in SmBa_1−xSr_xCo₂O_5+δ (x = 0, 0.25, 0.5, 0.75 or 1) and observed an increase in electrochemical properties compared with the undoped material. Gómez et al. [24] assessed the incorporation of Sr by La in La_2−xSr_xCoTiO₆ (0 ≤ x ≥ 0.5) and detected an increase in the electrical conductivity for all Sr contents, except for x = 0.5. In addition, they reported that La_1.6Sr_0.4CoTiO₆ exhibits similar electrochemical behaviour compared to the traditional cathode of Sr-doped lanthanum manganite employing a YSZ electrolyte.

Moreover, another effective process to enhance the electrochemical properties is to use an electrolyte with high ionic conductivity and that shows excellent chemical compatibility with the cathode. In this context, GDC is considered as a very promising electrolyte material due to its high ionic conductivity in the operating temperature range of IT-SOFCs [16] that can replace the standard electrolyte YSZ [17]. Generally, GDC presents chemical stability with the DPO structure [6], [10], [14], [20], resulting in an improvement in its electrochemical activity to the ORR. Based on these results, double perovskite La_1.8Sr_0.2CoFeO_5+δ (DP-LSCF) have been selected, since this composition may present stable phase and compatibility with GDC electrolyte at high temperature in contract to some LnBaCoO_5+δ oxides [6], [23]. In addition, the partial substitution of Sr by La could improve the electrical conductivity, and the incorporation of Fe at Co site could reduce the TEC. Then, DP-LSCF was is synthesised by a modified sol-gel method by a polymeric route developed in our group [25] with homogenous nanoparticles and high purity at a relatively low temperature. We present the structure, microstructure, chemical and thermal compatibility with a GDC electrolyte, electrical conductivity and electrochemical properties of the material. Our preliminary results reveal that La_1.8Sr_0.2CoFeO_5+δ DPOs could be promising cathodes for IT-SOFCs.