Fluorine-containing oxygen electrodes of the nickelate family for proton-conducting electrochemical cells

Abstract

In the present work, the anionic doping of a Ba-containing Nd2NiO4+δ mixed conductor is proposed as an efficient means of tuning its functional properties for application as an oxygen/steam electrode material in protonic ceramic electrolysis cells (PCECs). Single-phase Nd1.9Ba0.1NiO4+δFγ (γ = 0, 0.03, 0.05, 0.07 and 0.1) nickelates having a K2NiF4-type structure were prepared and comprehensively characterised in the range from room temperature to 1000 °C. A combination of complimentary techniques, including 4-probe DC electrical measurements, an electron-blocking method, electrochemical impedance spectroscopy and analysis of equivalent circuit schemes and distribution of relaxation times, was employed to reveal the fundamental correlations between electrical properties, oxygen-ionic transport and electrochemical performance of fluorinated nickelates. The highest ionic conductivity in combination with the lowest electrode polarisation resistance was found for the composition with γ = 0.05. The enhanced transport properties of this material were attributed to mixed anion lattice effect. Electrochemical tests of an electrolysis cell based on a proton-conducting BaCe0.5Zr0.3Y0.1Y0.1O3–δ electrolyte with a Nd1.9Ba0.1NiO4+δF0.05 oxygen electrode demonstrated competitive performance compared to state-of-the-art PCECs, thus supporting the prospective viability of the proposed approach.