Surface Scale Formation on SOFC Proximal Balance of Plant Components

Conference · Sat Sep 01 04:00:00 EDT 2007

OSTI ID:923742

Gerdes, K R; Johnson, C

The efficiency and performance of solid oxide fuel cells (SOFC) are known to degrade when chromium species are present at the cathode/electrolyte interface. Chromium containing alloys generate volatile chrome species that deposit as Cr2O3(s) under modest current densities (~0.5 A/cm2). Volatilization of chrome species from SOFC proximal balance of plant (BoP) components has received little attention. These components can reach temperatures of 600-800°C through direct thermal contact with the hot cell stack. Simple calculations show that a BoP chromium source can generate enough chromium to affect the power output of a 25-Watt cell in less than 50 hours of operation. In this work, materials representative of BoP component alloys were exposed to dry air at temperatures between 600°C and 800°C for 72 hours. 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 by XRD, SEM, and EDS. Thin surface scales were formed that included Cr-, Fe-, Al-, and Si- oxides as well as Mn-Cr spinel. The identity and relative abundance of the formed surface species were unique to each alloy. 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. A chrome load of 6.5 mg is calculated to deposit over 40,000 hours of operation assuming a 1% efficiency of volatilization of chrome species and a 1% efficiency of deposition of Cr2O3. This mass is equivalent to a 10 molecule deep layer spread evenly over 0.27 m2 of the SOFC active area, and is estimated to be sufficient to fully deactivate the affected region. The alloys tested are reviewed for suitability of service as a function of temperature. Alloy performance is discussed with respect to the formation rate of the BoP surface species and material cost. Cell experiments are anticipated which will directly test SOFC performance in the presence of these common BoP alloys.

• Research Organization: National Energy Technology Laboratory (NETL), Pittsburgh, PA, Morgantown, WV, and Albany, OR (United States)
• Sponsoring Organization: USDOE - Office of Fossil Energy (FE)
• OSTI ID: 923742
• Report Number(s): DOE/NETL-IR-2007-167
• Country of Publication: United States
• Language: English