Thermal Imaging for Diagnosing Fuel Cells

NASA's Jet Propulsion Laboratory, Pasadena, California

Thermal (that is, infrared) imaging has been demonstrated to be an effective technique for the diagnosis of operating fuel cells and fuel-cell stacks. Thermal imaging can be used to identify a variety of phenomena, described below, that are associated with nonuniform generation of heat. Hence, thermal imaging is expected to be widely used in fuel-cell research, development, and manufacturing.

These Infrared Images of the cathode side of a six-fuel-cell flat pack were recorded under two operating conditions. The nonuniformities of temperature depicted in these images can be analyzed to extract information about processes in the fuel cells and their electrical interconnections.

The performances of fuel-cell stacks (especially micro-fuel-cell stacks) are commonly adversely affected by nonuniform distributions of fuel, high resistances of electrical interconnections, cell-to-cell variations, and other phenomena associated with nonuniform generation of heat. The analysis of such phenomena by means of individual cell measurements (e.g., point-probe electric-potential measurements) can be quite tedious, even for a short stack; in the case of flat-pack micro-fuel cells, such measurements are almost impossible. Under these circumstances, one is often left guessing as to the causes of reduced performance.

The generation of heat always accompanies the operation of a fuel cell. The generation of heat is due to inefficiency of the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel (usually, the fuel is methanol), and electrical heating of interconnection resistances. Temperature differences occur if any of these heat-generating processes occur differently in different parts of a fuel-cell stack. For example:

Nonuniform distribution of fuel across the surfaces of electrodes leads to nonuniform distribution of electric current and hence temperature differences;
High-resistance interconnections in a stack distinguish themselves by producing more heat than the others do; and
Variations among cells within a stack, arising from variables in fabrication, can cause one or more cells to be more efficient than the others are, leading to thermal gradients.

These examples illustrate how mapping the surface temperature of a fuel-cell stack during operation can yield useful information about the processes occurring in the stack.

Infrared cameras equipped with quantum-well infrared photodetectors (QWIPs) can detect temperature differences as small as 0.005 K. Such a camera has been found to be particularly useful for monitoring processes in fuel cells. For example, the figure shows the temperature variations on the cathode side of a six-fuel-cell flat pack, both in the open-circuit condition and with an electrical load connected. The cell marked 4 exhibits a temperature greater than do the others, even in the open-circuit condition: this is attributed to a high methanol-crossover rate in the particular cell. With the load connected, the interconnections also exhibit differences in temperature, and some of them can be identified to be substantially more resistive than others are.

This work was done by Sekharipuram Narayanan and Thomas Valdez of Caltech for NASA's Jet Propulsion Laboratory.

NPO-21177