Performance of SOFC coupled with n-C<inf>4</inf>H<inf>10</inf>autothermal reformer: Carbon deposition and development of anode structure

Abstract

The performance deterioration of solid oxide fuel cells (SOFCs, Nickel-Yttria stabilized zirconia (Ni-YSZ)/YSZ/lanthanum doped strontium manganite-YSZ (LSM-YSZ)) coupled with n-C4H10steam reformers (SR), autothermal reformers (ATR), or catalytic partial oxidation reformers (CPOX) was examined using an integrated system of a micro-reactor reformer and SOFC unit. The terminal voltage rapidly degraded in CPOX-driven SOFC (oxygen to carbon ratio (OCR) = 0.5) while it was fairly stable for SR-driven SOFC (steam to carbon ratio (SCR) = 2) over 250 h. For ATR-driven SOFC at near the thermoneutral point (OCR = 0.5 and steam to carbon ration (SCR) = 1.3), significant deterioration of the terminal voltage was observed in 100 h of operation. The main precursors of carbon deposition on the SOFC were identified by reformate gas analysis during the tests. In this study, we reveal that the carbon deposition on the SOFC anode can be affected by not only lower-order hydrocarbons (C1∼C4), but also by the CO/H2gas mixture. The change in electrical conductivity of the Ni-YSZ cermet used for the SOFC anode was investigated under different gas mixtures. To investigate the propensity for carbon deposition by each carbon-containing gas mixture, we defined the ratios of steam to specific carbon (C1∼C4lower-order hydrocarbons and CO) in the reformate gas (SSCR, steam to specific carbon ratio). To inhibit carbon deposition on SOFC anode, the SSCR must be sufficiently high. However, the reformer operates near its maximum efficiency at low SSCR value and the higher the SSCR value, the lower the open circuit voltage and operating power density due to Nernst potential. In this study, a metal-foam supported SOFC single cell (Ni-YSZ/YSZ/Gd-doped ceria (CGO) buffer layer/lanthanum strontium cobalt ferrite-samarium doped ceria (LSCF-SDC)), impregnated with catalyst was designed; this novel SOFC was then examined for operation at a low SSCR value of the autothermal reformer conditions (near maximum efficiency of n-C4H10reformer and thermal neutral point, SSCR = 0.5, OCR = 0.5 and SCR = 1.3). The voltage for the metal-foam supported SOFC impregnated with 0.5 wt% Rh/CGO remained at a nearly constant value, around 0.8 V, for 200 h under a constant temperature of 750 °C and current load of 250 mA cm-2. © 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.