Electrochemical Impedance Spectroscopy Investigation of the Anodic Functionalities and Processes in LSCM-CGO-Ni Systems

Full ceramic anodes have been developed over the last decade as an alternative to the state-of-the-art Ni/YSZ cermet. The objective is to overcome the technical limitations of the cermet, especially in high fuel utilization operational mode, when hydrocarbon fuels are used, or in systems where rapid heat up is required. Numerous oxide compositions were proposed and investigated. (La,Sr)Cr_0.5Mn_0.5O₃(LSCM) perovskite has been reported as a very promising anode material because of its high tolerance to carbon and sulfur, and its redox and thermal cycling stability. However, similarly to other oxide anode materials, its overall performance still needed further improvement to match that of Ni/YSZ. The relatively low performance of LSCM compared to Ni/YSZ, used to be attributed to its relatively low electronic conductivity. Recently, we have reported that the pre-coating LSCM with Ni nitrate results in a considerable decrease in the anodic polarization resistance and activation energy. Ni dissolves in the perovskite phase at high temperature under oxidizing atmosphere, and then exsolves in the form of nanoparticles under reducing atmospheres.

This type of anode composition with split functionalities (ionic conduction, electronic conduction, and catalytic activity) gives a unique opportunity to investigate the specific influence of each of the above functions on the anodic mechanisms and performance. In this work, electrochemical impedance spectroscopy was used to characterize anode compositions containing LSCM and different levels (15, 40, and 60 wt%) of gadolinia doped ceria (CGO), with and without additional submicron Ni, as well as Ni nanoparticles exsoluted from pre-coated larger LSCM particles. Impedance measurements were performed in a three electrode – half cell configuration from 700 to 900°C under two flow rates of 3% wet H₂(50 and 150 ml/min).

At 900°C and under 150 mL/min of 3% wet H₂, lower values of the polarization resistance (R_p) were measured for the samples containing CGO compared to pure LSCM ones. The R_p showed a slight decrease when the content of CGO was increased (15 to 40 and 60 wt%). This was characterized by a decrease of the high frequency part of the impedance diagrams, likely due to the higher ionic conductivity brought by the addition of CGO. The addition of 5 wt% of submicron Ni to LSCM-CGO led to a remarkable decrease in R_p. This effect was even more spectacular for the samples containing 5 wt% of exsoluted Ni nanoparticles, characterized by a decrease of the whole impedance diagram, and especially the high and the middle frequency parts, which suggests an enhancement of the electrochemical processes related to anode charge transfer. Further analysis of the impedance diagrams, measured for the above anodes at different temperatures and gas flow rates, is in progress in order to define the effect of different anode functionalities on the anodic processes involved, and thus its overall performance.