Mechanism and Kinetics of Oxygen Reduction in Porous Nd2NiO4+δ Electrodes

Solid Oxide Fuel Cells (SOFC) cathodes must exhibit high catalytic activity for oxygen reduction, as well as good thermal and chemical compatibility with solid electrolytes in order to minimize resistance and promote stabile operation [1]. The Ln₂NiO_4+δ (Ln = La, Nd, Pr, for example) nickelates, belonging to the Ruddlesden-Popper series, have been proposed as cathode materials. These oxides have the aptitude to accommodate oxygen ions into interstitial sites in the LnO_x layer, which gives rise to a high ionic conductivity by interstitial oxygen diffusion [2]. In addition, the Ln₂NiO_4+δ materials present Thermal Expansion Coefficients (TEC) similar to the most widely used SOFC or IT-SOFC electrolytes: 8mol% Y₂O₃ stabilized ZrO₂ (YSZ), Ce_0.9Gd_0.1O_1.96 (CGO) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) [3-4].

In this work, we present a study of the reaction mechanism and the kinetic parameters of porous Nd₂NiO_4+δelectrodes. Three different microstructures have been obtained using different preparation methods. The conventional Solid State Reaction (SSR) and Pechini methods, and a soft chemical route based on a sol-gel method. Details of these syntheses are described in [5]. These materials have been deposited by spin coating on dense LSGM electrolytes and were fired at 1000°C. The resulting microstructures were quantified by 3D focused ion beam-scanning electron microscope (FIB-SEM) tomography. Figure 1 shows example 2D images from the 3D FIB-SEM data sets showing markedly different microstructures.

Electrochemical Impedance Spectroscopy (EIS) was used to measure the cells for temperatures ranging from 600 to 700°C and oxygen partial pressures (pO₂) between 1 atm and 6∙10^-4 atm in order to identify possible rate-limiting steps of the oxygen reduction reaction. The EIS spectra, which showed a low and a high frequency response, were fit using an electrical equivalent circuit model composed of two elements – the high frequency contribution was fit using a Gerischer-type element, whereas a parallel resistor/capacitor (RC) element was used for the low frequency arc. The low frequency contribution was only detected at low pO₂ and temperatures, with its size increasing with increasing particle size. The Gerischer element suggests a co-limitation by surface exchange kinetics and oxygen diffusion. The variation of the resistance (R_G) and relaxation time (τ_G) of the Gerischer-type contribution are plotted in Figure 2.

The 3D FIB-SEM data (porosity, specific surface area, tortuosity) was used in the ALS model [6] to find the characteristic kinetic parameters, oxygen surface exchange (k) and oxygen chemical diffusion coefficient (D_chem) that correctly predict the EIS-measured R_G and τ_G values. The results will be compared with reported values for Nd₂NiO_4+δ. Note that the main change in microstructure, the changes in particle size and specific surface area (Fig. 1), agree with the measured R_G values according to the ALS model. That is, high surface area (smaller particle size) led to lower R_G. The microstructure plays an important role in determining the mechanism of the reaction. Thus, the lower pO₂ dependence for R_G and τ_G and the high polarization resistance suggest that the kinetics of the oxygen reduction reaction is mainly limited by a bulk transport path for the SSR sample.

References
[1] S. B. S. Adler, Chem. Rev. 104, (2004) 4791.
[2] F. Mauvy, J. M. Bassat, E. Boehm, P. Dordor, J. C. Grenier, and J. P. Loup, J. Eur. Ceram. Soc. 24, (2004) 1265.
[3] Kharton VV, Viskup AP, Kovalevsky AV, Naumovich E, Marques FMB. Solid State Ionics 143 (2001); 337–53.
[4] Minervini L, Grimes RW, Kilner JA, Sickafus KE. Journal of Materials Chemistry 10, (2000), 2349–54.
[5] Montenegro-Hernández A, Mogni L, Caneiro A. International Journal of Hydrogen Energy 37(23) (2012) 18290-18301.
[6] Y. Lu, C. Kreller, and S. B. Adler, J. Electrochem. Soc. 156 (2009), B513.

Figure 1. Example 2D FIB-SEM images from the 3D data sets taken from epoxy-infiltrated Nd₂NiO_4+δ cathodes. The cathodes were prepared from powders synthesized by a) Solid State Reaction, b) Pechini and c) Sol-Gel methods. The electrolyte is on the right in each image.

Figure 2. Dependence of the Gerischer-type a) resistance (R_G) and b) relaxation time (τ_G) with pO₂.