Investigation of La2-XSrxNi0.8Co0.2O4+d (0.0 ≤ x ≤ 0.2) Materials as Oxygen Electrodes for Solid Oxide Cells
Authors: Unachukwu Ifeanyichukwu Daniel , Vibhu Vaibhav , Vinke I. C. , Eichel Rüdiger-A. , De Haart L. G. J. (Bert) 
Published: July 2021

Lanthanide Nickelate Ln2NiO4+δ (Ln = La, Pr or Nd) has drawn significant attention as an oxygen electrode for solid oxide cells (SOCs) due to its mixed ionic and electronic conductivity (MIEC) property. They exhibit very good electro-catalytic activity as well as high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values [1]. Earlier reported results [2, 3] show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) lead to an enhancement in the transport and electrochemical properties of the material. The present study aims to investigate the effect of Sr2+ substitution at A-site in LNCO i.e. La2-xSrxNi0.8Co0.2O4+δ (LSNCO) in order to further investigate the structural, physico-chemical and electrochemical properties of the materials. Three different compositions i.e. LSNCO5, LSNCO10 and LSNCO20 were studied with x = 0.05, 0.10 and 0.20 respectively. First the materials were characterized by XRD, TGA and SEM. Then, the electrochemical characterizations were carried out by fabricating the symmetrical half-cells (8YSZ//GDC//LSNCO) as well as single cells (NiO-YSZ//8YSZ//GDC//LSNCO) using Electrochemical Impedance Spectroscopy (EIS) in 700 – 900 °C temperature range.

All these materials exist with orthorhombic structure and a decrease in the cell volume as well as oxygen content (4+δ) is observed with Sr-substitution. The TGA analyses as a function of temperature reveals that these materials show reversible oxygen exchange and are always over-stoichiometric in 25-1000 °C temperature range. However, the loss of oxygen with increasing temperature is decreased with increasing Sr-content. The EIS data analysis of the symmetrical half-cells shows an increase in polarization resistance (Rp) with increasing the Sr-content. For instance, the Rp values of 0.047 and 0.88 Ω.cm2 are observed for LSNCO5 and LSNC20 respectively at 800 °C under air at OCV conditions. The EIS measurements under different partial pressure of oxygen reveals that four electrode processes (as fitted by 4R//CPE along with a series resistance Rs) are involved in the oxygen reduction/evolution mechanism. The observed electrode processes are; ionic transfer of oxide ion through electrode electrolyte interface (R1), charge transfer process (R2), adsorption/desorption of atomic species (R3) and molecular gas diffusion process (R4). The impedance data analysis under polarization (-0.2 to +0.2 V) reveals a decrease in the exchange current density with the increase of Sr-content. For instance, the io value is 0.040 A.cm-2 for LSNCO5 which is decreased to 0.003 A.cm-2 for LSNCO20 at 800 °C. Current-Voltage characteristics obtained from the single cell measurements show a decrease in the current density with increasing Sr-content.

The detailed structural, physico-chemical and electrochemical properties of these materials will be presented and discuss in detail.

References:

[1] E. Boehm et al., Solid State Ionics, 176 (2005) 1895.

[2] J.A. Kilner, C.K.M. Shaw, Solid State Ionics, 154 (2002) 523.

[3] V. Vibhu et al., Journal of Power Sources, 444 (2019) 227292