Investigation of Temperature Limitations During Rapid Thermal Cycling of a Micro-Tubular Flame-Assisted Fuel Cell

Abstract

Despite many efforts and improvements over the last few decades, two of the major challenges facing Solid Oxide Fuel Cells (SOFCs) are slow heating rates to operating conditions (typically < 5 °C.min−1) and a limited ability to thermal cycle (< 200 cycles). Recently a novel hybridized setup that combines a fuel-rich combustion reformer with a SOFC was developed and utilized to investigate rapid heating, cooling and thermal cycling of a micro-Tubular SOFC. The setup places the SOFC directly in the flame and exhaust of the high temperature combustion of methane, which allows for extremely rapid temperature rise in the SOFC. A SOFC with a (La0.8Sr0.2)0.95MnO3-x cathode was tested in the setup, but limitations on air preheating for the cathode resulted in low SOFC cathode temperatures (∼500°C) and low power density. Thermal insulation improved pre-heating of the air delivered to the cathode, increased the SOFC cathode temperature and, when a (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x cathode was applied to the SOFC, resulted in improved power density. After adjusting the thermal insulation, the air temperature near the cathode exceeded ∼750°C during testing. Over 3,000 thermal cycles were conducted at a heating rate exceeding 900°C.min−1 and a cooling rate that exceeded 300°C.min−1. The open circuit voltage was analyzed over the 150 h test and a low degradation rate of ∼0.0008V per 100 cycles per fuel cell was observed. Unlike the previous test, which was conducted at lower temperatures, significant degradation of the current collector was observed during this test. Electrochemical impedance spectroscopy shows that degradation in the SOFC was due to increases in ohmic losses, activation losses at the cathode and increased concentration losses. The setup demonstrates that rapid thermal cycling of micro-Tubular SOFCs can be achieved, but there are limitations on the maximum temperature that can be sustained depending on the current collector.