Understanding of Solid Oxide Electrolysis Cell Degradation: The Role of the Electrode Overpotential

Abstract

Solid oxide electrolysis cells (SOECs) can produce hydrogen, an increasingly important energy carrier, with considerably higher efficiency than other electrolysis technologies. A critical hurdle for the market penetration of SOECs is to ensure that degradation rates are sufficiently low to allow long-term operation. Degradation of the oxygen electrode has been shown to increase with increasing current density; however, the effects of other parameters such as temperature, electrode overpotential, and electrode composition have not been fully quantified. Moreover, significant degradation within the electrolyte as well as fuel electrode is observed during electrolysis operation, but the mechanisms are still not fully understood. Here, we report an experimental life test study of both full and symmetric cells with YSZ electrolytes along with various oxygen electrode materials including Sr(Ti0.3Fe0.7)O3 (STF), Sr(Ti0.3Fe0.63Co0.07)O3, (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF), and Ni-YSZ fuel electrodes. The observed dependences on temperature, current density, and electrode composition indicate that the electrode overpotential is the factor determining the electrode/electrolyte degradation and eventual cell failure. A theory that determines the electric potentials and oxygen partial pressure across the electrolyte is used to model the results, and can determine the condition for fracture or oxygen bubble formation. For example, fracture at the oxygen-electrode/electrolyte interface is observed when the electrode overpotential exceeds ~200 mV, corresponding to a critical oxygen partial pressure of 7410 atm; the model shows that the pressure is near its peak at this interface, and is sufficient to cause fracture because of the relatively low fracture toughness of the perovskite electrode.