Coarse-grained molecular dynamics modeling and analysis of graded porous electrodes of reversible solid oxide cells sintered in two steps

Abstract

Low performance, durability and degradation-related issues are major challenges in the long-term operation and commercialization of reversible solid oxide cells (rSOCs). One solution is to develop symmetrically structured electrodes with integrated functional layers composed of gradient pores, to enhance thermo-mechanical compatibility, mass transport processes and electrochemical reactions. It is critical to understand the graded nanoparticle migration and sintering behaviors during rSOC preparation. In this study, a sub-micron scale of the graded nanoparticle system for yttrium stabilized zirconia (YSZ) and perovskite catalyst (La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ, LSCM) materials, together with a YSZ dense electrolyte layer, is modeled by a coarse-grained molecular dynamics method, aiming to address the sintering mechanism and important parameters affecting the graded YSZ skeleton and YSZ/LSCM composite layer in a two-step sintering process. The diffusivities of LSCM and YSZ decrease by 57.4% and 80.5% in the second-step sintering. The two-step sintering process is more appropriate for preparing graded porous structures with a higher triple-phase boundary length than the one-step process (the so-called co-sintering). A lower stress (250–410 MPa) is obtained in the two-step sintering compared with that (680 MPa) predicted in the co-sintering, avoiding the excessive densification and mechanical fault caused by plastic flow under high stress. 900–950 °C is appropriate in the second-step sintering to form a gradient porous LSCM/YSZ layer with a higher triple-phase boundary length. A mass fraction of YSZ > 0.6 may cause a too dense skeleton in the first-step sintering and then suppress the gradient pores and triple-phase boundary formed in the second-step sintering.