Solid Oxide Cell Performance: Spatially Resolved Oxygen Electrode Stability
Authors: Langerman Michiel , Stodolny Maciej , da Rosa Silva Eduardo , Lu Xiaoqian , Ferchaud Claire , van Berkel Frans 
Published: September 2023

Solid Oxide Cell (SOC) electrolysis will play an important role in the electrification of the chemical, fertilizer, and fuel industries, contributing to the reduction of their reliance on fossil fuel feedstocks. The integration of high temperature steam and CO2-electrolyzers in industrial processes with readily available waste heat and renewable electricity allows for highly efficient green hydrogen production and methods for CO2 utilization.

The development of next generation SOC electrolyzer technology at TNO aims to provide high performance and low-cost technological options for the electrification of the aforementioned chemicals and fuels industries. The ambition of the SOC technology development program is to assist the whole value chain, from cell manufacturing to end-users, by increasing the economic viability of the technology. In line with this ambition, the TNO R&D activities regarding SOC technology concentrate on cell and stack technology development coupled with techno-economic and business case studies on SOC technology integration in the industrial environment. Improvements in SOC performance and durability for electrolysis applications are of great importance for the successful industrial implementation of the technology. Hydrogen Europe targets for 2030 aim at low stack degradation rates of 0.5%/khr at thermoneutral voltage, while delivering current densities of at least 1.5 A/cm2. Therefore, lifetime improvements at high current density are one of the vocal points of SOC development efforts, which calls for a good understanding of the degradation phenomena and analytical methods to investigate the material stability in the SOC during electrolysis operation.

In this contribution, the current status of SOC development and testing within TNO will be presented. Several aspects of TNO SOC performance characteristics will be discussed, with a focus on water electrolysis performance and lifetime behaviour during endurance tests of several thousand hours. Excellent water electrolysis performance of the TNO SOC cells is observed at thermoneutral voltage (1.3 V), achieving current density values of 1.75 A/cm2 at an operating temperature of 750 ºC. Lifetime assessment and material degradation of the SOC will be discussed, with a focus on the analysis of observed degradation phenomena in the oxygen electrode material. As oxygen electrode degradation can be critical for SOC performance in a stack environment, it is important to have a good understanding of the degradation processes and the ability to monitor them. At TNO, a novel high-resolution 2D mapping of the lateral conductivity of the oxygen electrode have been used as an effective tool for quality control and validation of the air electrode development. Additionally, the combination of 2D sheet resistance mapping, SEM-EDX and XRD techniques (Figure 1) provides a powerful combination of tools to obtain a spatially resolved (3D) view of the localized degradation observed in the oxygen electrode after several thousand hours of water electrolysis.

Figure 1