Surface Scale Formation on Solid Oxide Fuel Cell Proximal Balance of Plant Components

Abstract

Chromium containing alloys used as solid oxide fuel cell (SOFC) interconnects can generate volatile chrome species that deposit as Cr2O3(s) at the SOFC cathode/electrolyte interface under modest current densities (∼0.5 A/cm2). Deposition of chromic oxide at this interface increases overpotential losses, thereby degrading fuel cell performance and efficiency. Balance of plant components have not received attention as a chromium source but can produce volatile Cr species through direct thermal contact with the hot cell stack. In this work, materials representative of BoP component alloys were exposed to dry air at temperatures between 600°C and 800°C for 72 h. The material classes tested include austenitic steel, ferritic steel, alumina formers, silica formers, and a specialty ferritic with elevated alumina and silica content. The surface scales formed on each alloy were identified using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Thin surface scales were formed that included Cr-, Fe-, Al-, and Si-oxides as well as Mn–Cr spinel. The surface composition estimated from the analytical data is used to thermodynamically calculate the abundance of volatile chromium species over the alloys. Using the calculated vapor composition and assumed rate efficiencies, it is possible to calculate the mass of Cr2O3 that will deposit on the SOFC surface. Up to 4.3 cm2 of SOFC active area can be deactivated in 2000 h of operation assuming a 1% efficiency of volatilization of chrome species and a 1% efficiency of deposition of Cr2O3. This is estimated to be approximately 5% of the starting active area of an ∼20 W SOFC. Degradation of SOFC performance is expected to scale almost linearly with stack size.