In-Situ Temperature Sensing of SOFC during Anode Reduction and Cell Operations Using a Multi-Junction Thermocouple Network

Temperature driven performance degradations is one of the major problems that impedes the successful commercialisation of Solid Oxide Fuel Cell (SOFC) technology.  To mitigate such degradations as well as to further enhance stack’s performance, it is very important to understand the temperature distribution of a SOFC stack while in operation. Researches for experimental measurement of the temperature distribution are very limited in literature. The available efforts are also confined for measuring temperature in both (air/ fuel) channels and they mostly do not measure the temperature from a SOFC electrodes, which is more desirable than gas temperature measurement for investigating cells’ behaviour and the correlation to the stack’s performance. Further, the most widely adapted method for stack temperature measurement is to insert commercial thermocouples inside a fuel cell stack, which introduce disturbance to the normal operation of a stack and impose significant limitations on the number of independent point within a stack where temperature can be measured. Therefore, the in-situ cell temperature monitoring is a topical research area where a technical challenge is necessary. The proposed thermocouple network architecture can independently measure the temperature at N² number of points with only 2N number of thermo-elements/ external wires. Hence, it requires much less number of external wires than a set of thermocouples require for the same number of independent temperature measuring points. Reduction in the number of external wires requirement enables to measure the temperature with greater spatial resolution and lower disturbance to the normal operation of the cell/ stack. In addition to measuring the temperature, the propose thermocouple network can simultaneously collect the current from the electrodes while measuring temperature. The authors have experimentally validated and confirmed these abilities prior to applying the thermocouple network for in-situ temperature monitoring presented below.

A thermocouple network was fabricated by weaving K-type thermocouple wires (ϕ 0.25mm), and then spot welding its intersections to form a mesh where each spot welded point acts as a temperature measuring point. The intersection of 3 Alumel (Ni: Al: Mn: Si – 95:2:2:1 by wt.) and 3 Chromel (Ni:Cr – 90:10 by wt.) wires formed 9 independent temperature measuring points with a pitch of about 10mm. Using this thermocouple network, authors measured the cathode temperature during an anode reduction process and then measured the same during normal cell operation of a commercial SOFC test cell (50mmx50mm, NextCell-5) while the air/fuel ratio varies. In addition, Open Circuit Voltage (OCV) was also measured during the operation. The thermocouple network was in direct contact with the cathode and hence, measured the cathode temperature. The gas temperature was also measured simultaneously using a commercial K-type thermocouple from 5 mm adjacent to the electrode to investigate how effectively the gas temperature can represent the electrode’s temperature of a cell. Hydrogen and Air flow rates were altered to investigate cell’s response in terms of OCV change and temperature change. The measurements showed that the thermocouple network on cathode was very sensitive to even a trivial temperature changes whilst the commercial thermocouple adjacent to the cell was almost non-responsive to most of subtle temperature changes that occurred on the cell. During the monitoring, dramatic temperature fluctuation and uneven temperature distribution were detected during the anode reduction process. A correlation between the OVC and the cell temperature was also investigated.