Origin of low frequency inductive impedance loops of O2 reduction reaction of solid oxide fuel cells
Introduction
Inductive impedance loop or semicircle at low frequencies on electrochemical impedance spectroscopy curves is a common phenomenon in electrochemical reactions and has been observed in a number of electrochemical systems, such as corrosion [1], electrodeposition [2], batteries [3] and proton exchange membrane fuel cells (PEMFCs) [4], [5], [6], [7], [8]. An impedance semicircle is called inductive loop if its imaginary component is positive. In the case of metallic dissolution, a 2D Monte Carlo stimulation based on a flat surface model indicates that the low frequency inductive loop is caused by the surface adsorbate relaxation [9], while a 3D model shows that the roughening of surface due to the dissolution of interfacial atoms also contributes to the occurrence of low frequency loops [10]. For the methanol oxidation reaction on Pt/C catalyst, a well-accepted theory is that the low frequency impedance loop is due to the adsorption of the reaction intermediates such as CO species [11].
Low frequency inductive impedance loops are also frequently observed in the electrode reaction of high temperature solid oxide fuel cells (SOFCs) [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. SOFC is an energy conversion device to directly convert chemical energy of fuels into electrical energy with the highest efficiency among various fuel cells. The low frequency inductive loop occurs in the case of hydrogen oxidation reaction on Ni cermet anodes [13], [14], [15] and oxygen reduction reaction on the metallic and oxide cathodes such as Pt and Au [16], [17], [18], (La,Sr)MnO3 (LSM) [12], [19], [20], [21], [22], [23] and Sr0.25Bi0.5FeO3-δ [24]. The low frequency inductive loop is particularly pronounced in nanostructured electrodes such as LSM, (La,Sr)(Co,Fe)O3 (LSCF) and Pd infiltrated yttria-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) infiltrated LSCF [25], [26], [27]. There are attempts to understand the underlying mechanism of the occurrence of low frequency inductive loops. In the case of O2 reduction reaction at the Pt/YSZ interface, the inductive loop at low frequencies under dc bias may be caused by the enhanced three-phase boundaries at the electrode/electrolyte interface due to the onset of mixed conductivity in the YSZ electrolyte [16] or due to the presence of gas reservoirs at the Pt/YSZ interface [18]. For the reaction on Au electrode, Van Hassel et al. [17], [28] proposed that the inductive loop at low frequencies was due to an increase of the fraction of coverage of oxygen intermediates at a high cathodic dc bias. For the LSM cathode, Narita et al. [19] pointed out that the partial reduction of electrolyte or electrode at a high dc bias was responsible for the occurrence of low frequency inductive loop. In the case of nanostructured cathodes, the low frequency inductive loop may be a result of significant adsorption and dissociation of oxygen species on the electrode surface, enhanced by the nanostructure and the presence of catalytic promoters [26]. Nevertheless, there are considerable discrepancies in understanding the mechanisms of low frequency inductive loops in the electrode processes of SOFCs.
Herein, the occurrence and evolution of low frequency inductive impedance loops are studied on nanostructured and MIEC cathodes of SOFCs i.e., GDC infiltrated LSM and Pt and pristine LSCF. Nanostructured electrodes are selected due to the dominance of the nanostructured electrodes in the development of intermediate temperature SOFCs [29]. In the case of GDC infiltration, the presence of GDC nanoparticles (NPs) has been demonstrated to significantly enhance the electrocatalytic activity of cathodes of SOFCs, but also enhance the performance and operating stability of LSM oxygen electrodes of solid oxide electrolysis cells (SOECs) [30], [31]. The results indicate that the occurrence of low frequency inductive loops is closely related to the microstructure and electrocatalytic activity of the electrodes for the O2 reduction reaction.
Section snippets
Experimental
Electrolyte pellets (1.0 mm thick) were prepared by die pressing of 8 mol% Y2O3-stabilized ZrO2 powder (YSZ, Tosoh), followed by sintering at 1450 °C for 5 h. La0.8Sr0.2MnO3 (LSM, Fuel Cell Materials) cathodes were prepared on the YSZ pellets by slurry coating and sintered at 1100 °C for 2 h. The thickness of the LSM electrodes determined by SEM was 20–30 μm and the electrode geometric area confined by a circular template was 0.5 cm2. Gd0.2Ce0.8O1.9 (GDC) infiltrated LSM cathodes were prepared by
GDC infiltrated LSM
Fig. 1 shows the impedance responses for the O2 reduction reaction on as prepared LSM and GDC infiltrated LSM (GDC-LSM) cathodes at 800 °C. RE for the O2 reduction reaction on the pristine LSM electrode is 7.81 Ω cm2 (Fig. 1e). In the case of the infiltrated GDC-LSM electrodes, RE is 0.04, 0.07, 0.67 and 3.02 Ω cm2 after the heat-treatment at 700, 850, 950 and 1100 °C, respectively. This indicates that GDC impregnation significantly enhanced the electrochemical activity of LSM electrodes, consistent
Conclusions
The occurrence and transformation of low frequency inductive impedance loops were studied for the O2 reduction reaction on nanostructured GDC-LSM, GDC-Pt and conventional LSCF cathodes at 800 °C. The impedance responses with low frequency inductive loops are characterized by a single high frequency arc with no capacitive arcs at low frequencies, i.e., the low frequency inductive loop and low frequency capacitive arc do not exist at the same time. The occurrence of the low frequency inductive
Acknowledgements
The project is supported by Curtin Research Fellow program, Curtin University and Australian Research Council under Discovery Project scheme (project number: DP150102044 and DP150102025). The authors acknowledge the facilities, scientific and technical assistance of the Curtin University Microscopy & Microanalysis Facility, a facility partially funded by the University, State and Commonwealth Governments. The authors thank the insightful discussion with Prof Zhe Lu, Harbin Institute of
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