Polarization resistance of the Ruddlesden-Popper nickelates Lan+1NinO3n+1 (n = 1, 2, 3): Comparative analysis using the distribution of relaxation times method

Abstract

The Ruddlesden-Popper layered nickelates La2NiO4+δ (n = 1, LN21), La3Ni2O7-δ (n = 2, LN32) and La4Ni3O10-δ (n = 3, LN43) were studied as electrodes in symmetrical cells based on Ce0.8Sm0.2O2-δ (SDC) at various temperatures (600 ≤ T ≤ 800 °C) and oxygen partial pressures (−3 ≤ log(P(O2)/P0) ≤ −0.67). Electrochemical Impedance Spectroscopy (EIS) of the selected Lan+1NinO3n+1/SDC cells with similar microstructure reveals that the polarization resistance of the electrodes gradually increases with increasing n, namely by ≈0.75 Ω cm2 at 800 °C in air. The Distribution Function of Relaxation Times (DFRT) analysis of the obtained data shows that the polarization resistance of Lan+1NinO3n+1 includes four distinguishable contributions RHF (1), RHF (2), RMF and RLF. The Ri resistances at fixed temperatures and P(O2) were determined by fitting the obtained impedance spectra using the equivalent circuit consisting of electrolyte resistance and a series of R-CPE elements. Based on the analysis of the Ri = f(T, P(O2)) dependences, the RHF (1), RHF (2) resistances are assigned to the charge transfer through the interfaces and RMF, RLF are interpreted as “chemical” part of the impedance, i.e. ionic transport in the electrode and oxygen dissociation at the electrode surface, respectively. Comparison of the obtained resistances shows that RMF dominates the polarization resistance meaning that ionic transport in the electrodes is the slowest stage of the electrochemical reaction at the studied temperatures in air. The increase in the number of perovskite layers deteriorates ionic diffusion and promotes oxygen dissociation. As RMF > RLF, this leads to an increase in the overall polarization resistance from 0.75 Ω cm2 for LN21 to 2.25 Ω cm2 for LN43 at 800 °C in air.