A 3D Simulation of DC-Biased Electrochemical Impedance of Solid Oxide Electrolysis Cell: Effects of Delamination

Abstract

The demand for green energy sources with near zero-carbon footprints is rising owing to global warming, pollution and increasing world population. Hydrogen is one of the capable energy sources with higher gravimetric energy density than conventional gasoline fuel. Clean hydrogen production can be achieved through water splitting electrolysis processes. High temperature solid oxide electrolysis cells (SOEC) have higher efficiency, which requires less overall electric power compared to other electrolysis cells [1]. Hitherto, SOEC has not been commercialized due to their faster degradation under higher current densities, compared against its performance in fuel cell mode [2]. Delamination of oxygen electrode (OE) from the electrolyzer (EL) is the major reason for the cell degradation [3,4].Half-cell configuration is simulated to understand the delamination of OE coupled with experimental results observed during polarization test ran for 1000 hours. The OE used here is a composite of La1-xSrxCo1-yFeyO3-δ (LSCF) and Gadolinium-doped Ceria (GDC). A buffer layer (GDC) is introduced between the electrolyte (EL) made of 8 mol% Y2O3 doped ZrO2 (YSZ) and composite OE. The OE, buffer layer and EL diameter are 7, 10 and 10 mm with thicknesses of 30, 1 and 10 μm, respectively. The oxygen evolution reaction here is considered as multi-step charge transfer reaction at two phase boundaries (2PB) accompanied by triple phase boundary (3PB) intersection of OE, EL and gas phase [5]. Charged species like vacancies, holes, and electrons in OE, especially inside LSCF, are solved using Nernst-Planck transport equations. Gas diffusion in pores is simulated by Fick’s law. The charge transfer in the transport pathways (2PB and 3PB) are given by Butler-Volmer model [6]. Eventually, the perturbation form and the steady state form of the physics-based model will be coupled to simulate DC-biased impedance under various operating current densities. We will use the model to study the evolution of impedance when the OE/EL interface delamination propagates under long-term tests.The preliminary steady state results are shown in Figure 1, operated at 1 A/cm2. The electronic potential distribution in LSCF can be seen in Figure 1a. This potential is close enough to the applied voltage and the variation of potential along the thickness is negligible. The trend of ionic potential along the half-cell thickness is illustrated in Figure 1b. The linearity in potential is changed to exponential correlation when we move from EL to OE region, which is because of 2PB and 3PB current contribution to GDC. The variation in the vacancy is also minimal in OE, owing to its micro scale dimension. The oxygen partial pressure is maximum at OE-Buffer layer interface due to oxygen evolution. All these trends are plotted as a function of cell thickness along its axis. We will validate the model by both the experimentally measured voltage-current curve and electrochemical impedance spectra (EIS) under DC bias. This study will unveil the underlying degradation mechanism of SOEC by relating EIS with delamination of SOEC under long-term tests.