The Relation of Microstructure, Materials Properties and Impedance of SOFC Electrodes: A Case Study of Ni/GDC Anodes

Abstract

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of >2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells.

Excellent kinetics of single-phase Gd-doped ceria fuel electrodes in solid oxide cells

“…Perovskite-type materials as well as ceria-based electrodes deliver very promising results in terms of power density [4][5][6] and stability 7,8 . In most studies, a major difference between ceria and perovskite-type anodes is that perovskitetype materials are often used in a single phase functional layer, [9][10][11] while ceria-based anodes are usually used as a cermet with Ni to compensate the relatively low electronic conductivity 5,[12][13][14][15][16][17] . Noteworthy, in reducing conditions the electronic conductivity of many perovskites, such as SrTi 0.3 Fe 0.7 O 3-δ 18 is quite similar to that of Gd-doped ceria (GDC) [19][20][21] .…”

“…In this paper, we focus on electrochemical impedance testing and electrochemical modelling of single-phase porous GDC anodes. The 3D porous GDC electrodes were fabricated similarly to Ni-GDC cermets used in earlier studies 15 . Those were used in metal-supported SOFCs allowing an exceptionally high power density of 1.2 W/cm 2 at 0.9 V and 700°C 22 .…”

Theory of the electrostatic surface potential and intrinsic dipole moments at the mixed ionic electronic conductor (MIEC)–gas interface

“…where the delta symbols refer to the difference between the working and reference electrodes. 17 When fast gas diffusion is assumed Dm H2O = 0, and the shift in electrochemical potential for oxygen and oxygen vacancies are given as…”

“…The bulk transport in doped ceria is relatively fast, and as such, the bulk-tosurface transport is not considered to be rate limiting for electrode processes. 17,18 The popular electrode bracket notation…”

“…where the delta symbols refer to the difference between the working electrode and grounded electrode, which is in equilibrium with the gas phase. 17 The process of charge transfer at the MIEC electrodegas interface comprises ambipolar exchange of ions and electronic species. 23 The result of such a process causes charge separation and an associated dipole moment at the electrode surface.…”