Performance and Stability of Nickelates Based Oxygen Electrodes for Solid Oxide Cells

The K2NiF4-type nickelates i.e. Ln2NiO4+δ (Ln = La, Pr, Nd) and La1.5Pr0.5NiO4+δ have shown promising behaviour as oxygen electrodes for solid oxide cells [1]. These compounds show high anionic bulk diffusion (D*) as well as surface exchange coefficients (k*), combined with good electrical conductivity and thermal expansion properties matching with those of other components (electrolyte, interconnect etc.) of the cell. In order to further enhance the physico-chemical properties, electrochemical performance of these nickelates as oxygen electrode, we performed the substitution of nickel by other transition element such as cobalt. Two compositions of Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ (x=0.1 and 0.2) were considered (as higher cobalt containing nickelates are difficult to synthesize under air atmosphere) and completely characterized using several methods. Finally, the symmetrical half-cells (8YSZ/GDC/electrode) as well as single cells (Ni-YSZ/8YSZ/GDC/electrode) were prepared and electrochemically characterized using DC- and AC-techniques in the temperature range 700 - 900 °C. The current-voltage characteristics of the single cells show an increase in the performance with increasing cobalt content in both SOFC and SOEC operation for Ln2Ni1-xCoxO4+ δ (Ln = La, Pr, Nd) and La1.5Pr0.5Ni1-xCoxO4+ δ electrodes. The best cell performance is obtained with 20% cobalt substitution for all four nickelates. However, the performance of Pr2Ni0.8Co0.2O4+ δ (PNCO20), La1.5Pr0.5Ni0.8Co0.2O4+ δ (LPNCO20) and Nd2Ni0.8Co0.2O4+ δ (NNCO20) are similar and far better than La2Ni0.8Co0.2O4+ δ (LNCO20) [2,3]. For instance under SOFC operation at 900 °C, the maximum power densities obtained at 0.6 V are 1.8, 1.9 and 1.91 W.cm-2 for NNCO20, LPNCO20 and PNCO20 single cells, respectively. Under SOEC conditions, the cell current densities obtained at 900 °C with an applied voltage of 1.5 V are 3.0, 2.84 and 2.8 A.cm-2, respectively for PNCO20, LPNCO20 and NNCO20 single cells with 50% H2O and 50% H2 feed gas mixture. The performance of these electrodes are better than the commercial single cell containing state of art (La,Sr)(Co,Fe)O3 oxygen electrodes. Moreover a large improvement in durability is also observed for the single cells containing cobalt substituted nickelates. Results including physico-chemical and electrochemical properties of these materials, electrochemical performance as oxygen electrode, long term stability under electrolysis conditions and post-test analysis (using SEM-EDX, XRD) will be presented and discussed in detail. References: [1] V.Vibhu et al., Journal of Energy Chemistry, 46 (2020) 62. [1] V.Vibhu et al., Journal of Power Sources, 444 (2019) 227292. [2] V.Vibhu et al., Journal of Power Sources, 482 (2021) 228909.