Degradation Study on Solid Oxide Steam Electrolysis

Abstract

Steam solid oxide electrolysis (SOE) is a method to transform electrical into chemical energy in the form of hydrogen with the prospect of very high conversion efficiencies, which could play a major role as storage capacity in future electricity systems. However, its longevity is limited by numerous degradation processes which need to be addresses before. This thesis elucidates degradation of solid oxide electrolysis cells (SOEC) by systematically investigating the influence of three key operating parameters – temperature, hydrogen gas humidity and current density – on cell deterioration. A detailed understanding of rate limiting processes governing cell performance was achieved based on experimental results as well as the application of a physico-chemical model approach. Five processes, separable by electrochemical impedance spectroscopy were identified: the ohmic resistance accounting for the conduction of ions through the electrolyte as well as contact resistances, a hydrogen electrode process representing oxygen ion conduction within the hydrogen electrodes’ YSZ backbone coupled with the charge transfer reaction, the hydrogen electrode charge transfer reaction, the oxygen electrode’s charge transfer reaction coupled with its oxygen ion transport and finally mass transport limitations. In order to study the influence of operating conditions on degradation, a series of 20 SOECs were operated for 1000 h under identical conditions, while only the investigated parameters deviated. The influence of temperature was investigated between 750 °C and 850 °C, the humidity ranged from 40 %MH to 80 %MH, while the current density varied between OCV and 1.5 A/cm2. This systematic parameter study allowed for the separation of four independent degradation processes, two of which contribute to the ohmic resistance and would be inseparable otherwise. Furthermore, it provides in-sight into the mechanism of each degradation process. One major source of degradation is caused by unidirectional transport of Ni away from the hydrogen electrode | electrolyte interface, leaving behind a porous YSZ layer virtually deprived of Ni, which effectively translates into an increase of the electrolyte’s thickness. This Ni-depletion is driven by current density and only occurs significantly at humidities of 80 %MH and temperatures of 800 °C and above. A second degradation process, also contributing to the ohmic resistance, was identified to be a partial change of the electrolyte’s crystallographic phase from cubic to tetragonal. This process shows a linear progression with the square root of time and is more pronounced at lower temperatures within the investigated range of parameters. Another important source of degradation could be linked to a change in the crystallographic structure of the oxygen electrode, which is dependent on operating temperature but largely unaffected by current density. Finally, the hydrogen electrode also exhibits degradation for most investigated conditions. While it is exposed to a series of degradation mechanisms, it is shown that the degradation mainly originates from a change of the ionic conductivity in the YSZ backbone while the charge transfer plays a minor role. Thus, observed microstructural changes resulting in a reduction of the triple phase boundary (TPB) length are not primarily responsible for hydrogen electrode degradation.