Degradation Mechanisms of SOECs

Solid oxide electrolyzer cells (SOECs) have great potential in energy conversion with high efficiency and synthesizing hydrogen with low cost. Commercial use of SOECs requires long operational lifetime, which implies low degradation rate.

One of the most commonly used electrolytes in SOECs is yttria-stabilized zirconia(YSZ) because of its high oxygen conductivity and negligible electronic conductivity. However, when operating at high current density or low temperature, delamination is observed in the experiments due to the high oxygen partial pressure introduced by the high overpotential, at the interface of oxygen electrode and electrolyte. Recently, Lanthanum strontium cobalt ferrite(LSCF) has emerged as an alternative material for conventional lanthanum strontium manganite (LSM)-YSZ oxygen electrodes in SOECs because of its better transport properties and lower polarization resistance. However, LSCF reacts with YSZ at the temperatures that SOECs are manufactured and operated. Experimentally, in order to solve this problem, a gadolinium-doped ceria(CGO) barrier layer is introduced between YSZ electrolyte and LSCF-CGO oxygen electrode. It worth to notice that a YSZ layer is still needed in the electrolyte to suppress electronic leak currents.

Moreover, it is found in experiments that SOECs with an CGO inter-diffusion barrier sandwiched between the YSZ electrolyte and an LSCF-CGO oxygen electrode shows a significantly lower degradation rate, including less severe crack propagation in YSZ electrolyte and less severe Ni-YSZ electrode degradation, comparing with cells with pure YSZ electrolyte and an LSM-YSZ oxygen electrode. According to former experimental and theoretical works, some degradation mechanisms are connected to the distribution of oxygen partial pressure and concentration of oxygen vacancies in the electrolyte under solid oxide electrolysis operating mode. However, these quantities are hard be measured in the experiments. In this project, we are working on quantitative modeling on gas transportation and chemical reactions at the  electrodes, which determines the polarization resistance, and the transport of species (oxygen ion, electrons and holes) in the multi-layer electrolyte of solid oxide electrolyzer cells, in order to understand the degradation mechanisms and optimize the design of the structure of electrolyte and electrodes to achieve expected durability under required operation condition (low temperature and high current density). This is a joint project with Prof. Scott Barnett’s group.