PNNL

Abstract

Interconnects in solid oxide fuel cells (SOFCs) provide the electrical connection between the individual cells, as well as separate the anode air from the cathode fuel for each cell. The recently developed anode-supported planar SOFCs design allows the cell working temperature to be 800oC and lower, which led to the proposed use of ferritic stainless steel as interconnects and current collectors in the SOFC systems. Many experimental studies have shown that metal based interconnects will go through surface oxidation under the typical operating temperature and environment of an SOFC system. Since the oxide scale is formed at the outer surface of interconnect, its microstructure (morphology, texture), properties (electrical, thermal and mechanical properties), and structural integrity/stability will have a significant impact on the performance of interconnect. In this paper, we study the oxide growth kinetics and stresses generated during scale growth the thermal cycling for Cr-Fe based alloy, e.g. Crofer 22 APU. The goal is to predict the interconnect life under typical operating conditions and thermal cycles. First, a mathematical model is developed based on the growth of scale and coupling relation between diffusion and stress field. The following phenomena are considered: 1) The predominant cationic diffusion and cation distribution within the oxide scale in transient state and steady state; 2) Chemical reaction within the scale as well as the growth kinetics of the oxide scale; 3) Stresses distribution during the oxidation, especially for mid- and long-term treatment; 4) Coupling relationship or interaction between oxidation induced stresses and mass diffusion within the scale. Next, thermal stresses generated due to temperature changes in thermal cycling are predicted using finite element analyses for different substrate and scale thickness. In general, very high compressive in-plane stresses are predicted for the oxide layer. Also, high shear stress is also predicted on the oxide/substrate interface. The predicted shear stress on the interface is used to compare with the experimentally determined bond strength to predict delamination. Finally, scale spallation and IC life prediction are predicted by comparing the compressive stress in the scale with the critical buckling load for the delaminated scale.