Characteristics, durability and degradation mechanism of CO2 electrolysis in solid oxide cells

Abstract

With the development of technology, environmental problems are becoming increasingly serious. It is crucial to use reliable electrochemical technology to treat carbon emissions to achieve the goals of “carbon peak” and “carbon neutrality”. Solid oxide electrolysis cells (SOECs) have become one of the reliable methods for dealing with carbon emissions due to various advantages such as high selectivity, high conversion rate, high efficiency, fast response, controllable reaction and multi fuel adaptability.
Flat-tube SOECs are a novel structure, in which fuel is introduced from the middle of the cells and diffused to the fuel electrode, which may alleviate the diffusion of macromolecules such as CO2 in the electrode. However, the research on this structure is currently very scarce. In this regard, this study attempted to address this research gap. The innovative points of this paper are as follows:
(i) The utilization of solid oxide electrolysis cells for CO2 electrolysis may generate by-products such as coke, thereby reducing Faraday efficiency. Therefore, in this thesis, the suitable reaction conditions for long-term operation were first calculated from thermodynamic theory, and then the stability of CO2 electrolysis operation under high temperature was verified through short-term experiments. The effects of different types and contents of reducing gases on the performance and products of solid oxide electrolysis cells were also studied. To avoid the impact of sealing on test results, the thesis further explored the types of sealing materials and assembly processes, and determined appropriate process parameters. These exploratory works have laid the foundation for extending the lifespan of flat-tube SOECs.
(ii) Feed gas compositions of 25 vol.% H2-75 vol.% CO2 and 23.8 vol.% CO-76.2 vol.% CO2 with same oxygen partial pressure were selected for long-term durability test under no air conditions for investigating the impact of air, with a focus on analyzing the efficiency changes, impedance changes, and potential degradation mechanisms of SOEC, including degradation of electrodes and electrolyte. The flat-tube SOECs were stable operated for more than 1000 hours under no air conditions, which exceeded that of most current planar SOECs. Through comparative experiments, it was found that strontium segregation at the interface between the air electrode and electrolyte is the main cause of degradation.
(iii) Focusing on the demand for “energy storage”, the durability and degradation mechanism of SOEC under fluctuating currents of -100-300 mA/cm2 were studied in this paper, after the cells successfully ran for 808 hours. Subsequently, the feasibility of the “power-gas-power” conversion technology was verified using the RSOC (reversible solid oxide cells) system concept in a 50 vol.% CO-50 vol.% CO2 fuel electrode atmosphere, and over 100 reversible charge-discharge cycles were achieved.
(iv) Consequently, this thesis also conducted research on the electrolysis stack to verify the durability of constant current electrolysis under different currents, and simultaneously analyzed the degradation of various components. The stack finally successfully achieved stable CO2 electrolysis operation for over 1200 hours under the high current density of -500 mA/cm2. Through the analysis of the degradation mechanism under high current density, the influence of interconnects on the overall stability of the stack was discovered, and coating improvements were performed on the interconnects for durability verification of the stack under intermittent pulsed current.
(v) In the assembled two-unit SOEC stack, a manganese-cobalt spinel coating was employed as the protective layer for the interconnects, and lead wire was used to monitor the real-time degradation of various parts in the stack. During over 900 hours of high-temperature CO2 electrolysis operation, the toxic effect of chromium on the air electrode interconnects seemed to have been alleviated, this work provides ideas for the development of in-situ monitoring technology for stacks.
In summary, this work aims to utilize flat-tube SOECs for CO2 electrolysis, achieving significant breakthroughs in durability, scalability, and degradation mechanism, which also presented great significance in the development of renewable energy storage. The novel flat-tube cells used in this paper greatly improved the mechanical strength and antioxidant reduction stability of the cells at high temperatures, which provided great help for long-term CO2 electrolysis.

Date of Award	Oct 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Svenja Hanson (Supervisor), Jing Wang (Supervisor) & Wanbing Guan (Supervisor)