Investigating oxygen reduction pathways on pristine SOFC cathode surfaces by in situ PLD impedance spectroscopy.

Matthäus Siebenhofer,Alexander Karl Opitz,Christoph Riedl,Andreas Limbeck,Alexander Schmid,Jürgen Fleig,Markus Kubicek

doi:10.1039/d1ta07128a

Abstract

The oxygen exchange reaction mechanism on truly pristine surfaces of SOFC cathode materials (La0.6Sr0.4CoO3−δ = LSC, La0.6Sr0.4FeO3−δ = LSF, (La0.6Sr0.4)0.98Pt0.02FeO3−δ = Pt:LSF, SrTi0.3Fe0.7O3−δ = STF, Pr0.1Ce0.9O2−δ = PCO and La0.6Sr0.4MnO3−δ = LSM) was investigated employing in situ impedance spectroscopy during pulsed laser deposition (i-PLD) over a wide temperature and p(O2) range. Besides demonstrating the often astonishing catalytic capabilities of the materials, it is possible to discuss the oxygen exchange reaction mechanism based on experiments on clean surfaces unaltered by external degradation processes. All investigated materials with at least moderate ionic conductivity (i.e. all except LSM) exhibit polarization resistances with very similar p(O2)- and T-dependences, mostly differing only in absolute value. In combination with non-equilibrium measurements under polarization and defect chemical model calculations, these results elucidate several aspects of the oxygen exchange reaction mechanism and refine the understanding of the role oxygen vacancies and electronic charge carriers play in the oxygen exchange reaction. It was found that a major part of the effective activation energy of the surface exchange reaction, which is observed during equilibrium measurements, originates from thermally activated charge carrier concentrations. Electrode polarization was therefore used to control defect concentrations and to extract concentration amended activation energies, which prove to be drastically different for oxygen incorporation and evolution (0.26 vs. 2.05 eV for LSF).

Highlights

The kinetics of the oxygen exchange reaction (OER) is crucial for a multitude of applications in energy- and environment-related technologies, e.g. electrode materials for solid oxide fuel cells (SOFCs),[1,2,3] solid oxide electrolysis cells (SOECs)[4,5] or oxygen permeation membranes.[6,7] The overall reaction can be written asO2 þ 4eÀ#2O2À or O2 þ 2VO þ 4e0#2OxO; (1)in common notation or in Kroger–Vink notation, respectively.[8]
We present the results of in situ impedance spectroscopic measurements on pristine, dense thin lms immediately a er deposition, i.e. still in the PLD chamber.[36,37,38]
In the impedance spectra recorded during LSC deposition, the semicircle assigned to the counter electrode is initially well separated, which allows the determination of resistance and capacitance of the porous LSC counter electrode

Summary

Introduction

The kinetics of the oxygen exchange reaction (OER) is crucial for a multitude of applications in energy- and environment-related technologies, e.g. electrode materials for solid oxide fuel cells (SOFCs),[1,2,3] solid oxide electrolysis cells (SOECs)[4,5] or oxygen permeation membranes.[6,7] The overall reaction can be written asO2 þ 4eÀ#2O2À or O2 þ 2VO þ 4e0#2OxO; (1)in common notation or in Kroger–Vink notation, respectively.[8]. A comprehensive understanding of the processes occurring during these reactions ( of the rate determining steps) is essential with regard to the optimization of components for energy-related applications and to develop materials with satisfactory activity and stability. In the case of SOFCs, virtually all cathode materials are oxides and the predominant part of those possess perovskite structure.[3] For many of those materials, the bottleneck in SOFC operation is the aforementioned surface reaction, where oxygen is incorporated into the electrode.[9] From the nature of the reaction itself, it is evident that it must be comprised of several reaction steps: (a) diffusion of oxygen through the gas phase to the electrode surface, (b) adsorption of oxygen on the electrode surface, (c) dissociation of O2,ads, (d) charge transfer from the electrode to adsorbed oxygen and (e) incorporation into the lattice.[10,11,12,13,14] the exact order and the detailed kinetics of all steps a er (b) are difficult to unravel due to the complex interactions between electronic and ionic defects, especially with regard to the identi cation of the rate determining step. Even for a given cathode composition, the surface structure and chemistry can vary substantially and multiple degradation and activation phenomena may occur

Results

Discussion

Conclusion