B-site Mo-doped perovskite Pr_0.4Sr_0.6 (Co_0.2Fe_0.8)_1−xMo_xO_3−σ (x = 0, 0.05, 0.1 and 0.2) as electrode for symmetrical solid oxide fuel cell

Highlights

• Pr_0.4Sr_0.6(Co_0.2Fe_0.8)_1−xMo_xO_3−σ is applied as symmetrical electrode materials SSOFCs.

• Cubic perovskite phase of PSCFM is formed after sintered at 1100 °C for PSCFM_x (x = 0, 0.05 and 0.1) samples.

• The structure of PSCFM can be repeatedly changed and recovered in dry H₂ and in air.

• PSCFM_0.05 symmetrical electrode shows the lowest polarization resistance (R_p) at 900 °C.

• The maximum power densities of a single SSOFC reach 493 and 160 mW cm⁻² at 850 °C in H₂ and CH₄, respectively.

Abstract

Pr_0.4Sr_0.6(Co_0.2Fe_0.8)_1−xMo_xO_3−σ (PSCFM_x, x = 0, 0.05, 0.1 and 0.2), which obtained by doping molybdenum at the B site of Pr_0.4Sr_0.6Co_0.2Fe_0.8O_3−σ (PSCF) cathode, have been synthesized by a solid state reaction method and studied towards the application as symmetrical electrode materials for symmetrical SOFCs (SSOFCs) in this study. It is found that cubic perovskite phase of PSCFM in the Pm/3 m space group is formed after sintered at 1100 °C for PSCFM_x (x = 0, 0.05 and 0.1) samples, and the main phase is converted to K₂NiF₄ structure identified as SrPrFeO₄ in the I4/m space group, and some new phases of Pr₂O₃ and CoFe-alloy appear after PSCFM_x is heat-treated in dry H₂ at 900 °C for 2 h. The K₂NiF₄ structure SrPrFeO₄ can be transferred to a pure cubic structure of PSCFM_x again by calcining it in air at 900 °C. The maximum power densities of a single SSOFC based on the PSCFM_0.05 symmetrical electrode, which shows the lowest polarization resistances (R_p), are 493 and 160 mW cm⁻² at 850 °C in H₂ and CH₄, respectively. No obvious degradation is observed during a 100 h stability test in CH₄, which suggests that PSCFM material is a potential symmetrical electrode for SSOFCs.

Introduction

Symmetrical solid oxide fuel cells (SSOFCs) has been developed as a novel concept of SOFCs in the past few years. [1], [2] In this configuration, the same material is used simultaneously as both the anode and cathode, which has several advantages comparing to the typical SOFCs design. One of the most important advantages is the possibility to address issues related to sulphur poisoning and carbon deposition by simply reversing the gas flows. Also, the development of SSOFCs simplifies significantly the process for the preparation of fuel cells, owing to the assembly of the electrolyte and the symmetrical electrodes can be performed just in one thermal step, hence decreasing the fabrication cost. Additionally, the chemical compatibility problems will also be minimized by using the same material as both the anode and cathode because two identical electrolyte–electrode interfaces are used in comparison with the typical electrolyte-anode and electrolyte–cathode interfaces in the traditional SOFCs. [3] Since the anode is in a reducing environment, while the cathode is in an oxidizing environment, the requirements for a material to be considered as a SSOFCs electrode are rather restrictive. The materials used for SSOFCs electrode should match all the conditions applicable to both of anode and cathode; furthermore, some other special requirements are imposed as following: 1) the chemical and structural stability in both reducing and oxidizing environments; 2) dual electrocatalytic activity for both oxygen reduction and fuel oxidation; 3) acceptable electronic conductivity and ionic conductivity in both conditions to reduce ohmic losses and promote electrochemical reactions, respectively. To date, there are only limited reports showing that an oxide can be effectively applied as the SSOFCs electrode.

There are three kinds of symmetrical electrode materials according to the source from interconnect, anode and cathode materials, respectively. The anode materials are commonly prepared in oxidizing atmospheres, while operate under reducing environments, and exhibit high electronic conductivity and good catalytic activity towards fuel oxidation. Hence, the main concern to use an anode material as the symmetrical electrode material is to investigate whether the selected material will also have appreciable electronic conductivity in oxidizing conditions and good catalytic activity for oxygen reduction. La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−σ (LSCM) was reported as the first material in the proof of the concept in SSOFCs by Bastidas and Ruiz-Morales. [1], [2] A maximum power density of 300 and 230 mW cm⁻² were obtained at 900 °C, with the cell of LSCM as symmetrical electrode and yttria-stabilised zirconia (YSZ) as electrolyte, in wet H₂ and wet CH₄, respectively. And Ruiz–Morales et al. optimized the microstructure of the LSCM electrode, using poly methyl methacrylate (PMMA) microspheres as templates, achieved the maximum power density values of 500 and 300 mW cm⁻² at 950 °C in wet H₂ and CH₄, respectively. Zhu et al. [4] used the dry-pressing method to fabricate an SSOFC with a dense thin YSZ electrolyte film sandwiched between two symmetrical porous YSZ layers in which LSCM based anode and cathode were incorporated using wet impregnation techniques. The maximum power density of a single cell with 32 wt.% LSCM impregnated YSZ anode and cathode reached 333 and 265 mW cm⁻² at 900 °C in dry H₂ and CH₄, respectively. These values were further improved through the incorporation of 6 wt.% Ni during the impregnation process, and the performances were enhanced to 570 and 550 mW cm⁻² at 900 °C for dry H₂ and CH₄, respectively. El-Himri et al. [5] used the Pr to substitute La in LSCM, and investigated the performance of Pr_0.7Sr_0.3Cr_1−yMn_yO_3−σ (PSCM, y = 0.2, 0.4, 0.6 and 0.8) as the SSOFCs electrodes, and reached the modest values of power densities 250 and 160 mW cm⁻² at 950 °C, under humidified H₂ and CH₄, respectively. Canales-Vázquez et al. used La₄Sr₈Ti_12−xFe_xO_38−σ (LSTF) as the symmetrical electrode, and 100 mW cm⁻² at 900 °C in wet H₂ was obtained in a symmetrical configuration. Recently, Liu et al. reported a new promising symmetrical electrode material Sr₂Fe_1.5Mo_0.5O_6−σ (SFM), and the power densities obtained in SFM/LSGM/SFM single cell were 835 and 230 mW cm⁻² at 900 °C under wet H₂ and CH₄, respectively.

The interconnect materials, such as chromites, possess most of the requirements for a symmetrical electrode. Therefore, Ca-doped LaCrO₃ perovskite becomes a popular symmetrical electrode material in recent years. [6], [7], [8], [9] The maximum power density obtained in the SSOFC was 387 mW cm⁻² at 850 °C in dry H₂ using a 300 μm LSGM as the electrolyte. [7]

Only a few reports can be found on the symmetrical electrode from the cathode material of the traditional SOFCs, owing to the low electrical conductivity and worse stability of cathode materials in the reducing environment. [10], [11] Lai et al. [12] applied La_0.6Sr_0.4Co_0.8Fe_0.2O_3−σ (LSCF) directly as a symmetrical electrode but a rather low performance of 210 μW cm⁻² at 545 °C with a low OCV of 0.18 V was obtained because the LSCF possesses much lower electronic and ionic conductivities in a reducing environment than in an oxidizing environment. Zheng et al. [13] used the Sc-doped La_0.8Sr_0.2MnO_3−σ (LSSM) as the symmetrical electrode, and the maximum power densities of 310 and 130 mW cm⁻² at 900 °C in wet H₂ and CH₄, respectively, were achieved. Liu et al. [14] synthesized Sc-substituted La_0.6Sr_0.4FeO_3−σ (LSFSc) mixed conducting oxides as symmetrical electrode and applied thin La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−σ film with the impregnated nano-scale LSFSc catalysts as electrolyte for SSOFCs, and the maximum power densities of 560 mW cm⁻² in H₂ at 800 °C was reached.

So far, the main symmetrical electrode materials are chosen from the anode and interconnect materials in traditional SOFCs. Therefore, the candidates are very limit. In this study, a new symmetrical electrode Pr_0.4Sr_0.6(Co_0.2Fe_0.8)_1−xMo_xO_3−σ (PSCFM_x, x = 0, 0.05, 0.1 and 0.2), which was from the Mo-doped typical cathode material Pr_0.4Sr_0.6Co_0.2Fe_0.8O_3−σ was reported. An excellently redox recyclability in crystal structure, high cell performance and long-term stability during 100 h test in CH₄ were observed with this kind of perovskite oxide.