Mixed Ionic-Electronic Conductivity, Redox Behavior and Thermochemical Expansion of Mn-Substituted 5YSZ as an Interlayer Material for Reversible Solid Oxide Cells.

Alejandro Natoli,Agnieszka Żurawska,Enrique Rodríguez-Castellón,Jorge R Frade,Blanca I Arias-Serrano,Aleksey A Yaremchenko

doi:10.3390/ma14030641

Abstract

Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO2)0.95(Y2O3)0.05]1−x[MnOy]x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn2+/3+→Mn2+ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x < 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion.

Highlights

Solid oxide cells (SOCs) are attractive high-temperature electrochemical systems that offer a solution for the efficient utilization of renewable energy
Solid oxide electrolysis cells (SOECs) can utilize excess electrical energy produced by renewable sources to generate green hydrogen as a fuel or energy storage, while solid oxide fuel cells (SOFCs) may convert chemical energy stored in H2 back into electricity with high conversion efficiency [1,2]
Powdered samples for X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) studies were prepared by grinding sintered ceramics in a mortar

Summary

Introduction

Solid oxide cells (SOCs) are attractive high-temperature electrochemical systems that offer a solution for the efficient utilization of renewable energy. Most reports agree that Mn additions (within the solid solubility limits) to 8YSZ result in a decrease in total electrical conductivity under oxidizing conditions, with an increase in activation energy [20,21,23,24,25,26,27,28] This is accompanied by a gradual increase in p-type electronic contribution [22,23,24], there is a substantial scattering in obtained oxygen-ion transference numbers: from >0.99 [20,21,23] to ~0.2 [24] for similar compositions under similar conditions. Materials 2021, 14, 641 characterization includes structural and microstructural studies, measurements of thermochemical expansion and electrical conductivity in controlled atmospheres, determination of ionic and electronic contributions to the total electrical transport, and assessment or redox behavior by thermogravimetry and conductivity relaxation experiments

Materials and Methods

Phase Composition and Crystal Structure

Thermochemical Expansion

Conclusions