Fuel Cell Diagnosis

Backgrounds:

CO2 Bubble Problems

The Direct Formic Acid Fuel Cell (DFAFC) is a promising candidate for portable power applications owing to its high energy efficiency and high power density. During the DFAFC operation, formic acid is oxidized at the anode electrode producing electrons, protons, and carbon dioxide (CO₂). These protons are transported from the anode to the cathode through the polymer electrolyte membrane (PEM), and then combine with oxygen and the electrons to produce water at the cathode electrode. Additional water is moved from the anode to the cathode by both a molecular diffusion and an electro-osmotic drag processes. The excess water in the cathode is either evaporated or diffused back to the anode. The proton conductivity of PEM and thus performance of the DFAFC decreases dramatically when the water content of the membrane decreases. Therefore, to operate the cell with a high performance, a proper water management to maintain the membrane hydrated is so important. On the other hand, too much water in the cathode would cause the water flooding and limit the oxygen transport to the cathode electrode.

MRI Image of Human Brain

Having a proper fuel (formic acid) distribution at the DFAFC’s anode is another important aspect for achieving a good cell performance. CO₂ bubbles are generated as formic acid is oxidized at the anode electrode. This CO₂ evolution in the liquid feed anode of the DFAFC results in strongly two-phase flow, making the transport processes of fuel supply and product removal more complicated. If the CO₂ bubbles are not properly removed from the anode, they will block the catalyst sites from oxidizing formic acid and create a large pressure drop across the flow channels. Understanding both the water and fuel transport processes in DFAFC under the operating conditions is essential for developing its advanced fuel cell materials and designs.

Research Objectives:

We will use MRI technology for measuring both the water and fuel distributions at the cathode and anode of the operating DFAFC respectively. Additionally, we will correlate the time dependant performance of the DFAFC with time-resolved MRI measurements of the water content in the cathode electrode.

Experimental Method:

Figure 1: MRI Test Fuel Cell

Construction of Test Fuel Cell. The MRI system generates strong magnetic fields. To prevent any distortions of these strong magnetic fields, we build a test fuel cell from nonmagnetic materials as it is shown in Figure 1. Both the anode and cathode current collectors are constructed out of titanium foil. In order to protect the titanium from being corroded by the formic acid solutions, they are electrochemically coated with 5 microns of gold.

MRI Test System. To acquire MRI images of water and fuel contents in the operating DFAFC, we place the cell in the cylindrical test section of our MRI system (14 T). The experimental setup is shown in Figure 2. The nucleus observed in our MRI experiment is hydrogen atom (¹H). The experiments are performed at room temperature.

Figure 2:The MRI experimental Setup

Preliminary Results:

Figure 3: The MRI image of 5M formic acid flowing through the anode flow channels after operating the cell for 135 min

Figure 3 shows the MRI image of the anode flow field after operating the DFAFC with 5M formic acid for 135 minutes. In Figure 3, the bright image represents the 5M formic acid, which is flowing through the anode flow channels. These anode flow channels are oriented vertically (up and down). According to Figure 3, the bottom portion of the anode flow channels is occupied by the fuel (a high NMR signal which gives a bright image), while the top portion has almost no fuel (no NMR signal which gives a dark image). This would be explained in terms of CO₂ formation and orientation of the anode flow channels. As the formic acid is oxidized at the anode, CO₂ bubbles are formed which would rise up through the flow channels. Consequently, these CO₂ bubbles are accumulated in the top portion of the anode flow channels and occupy most of their volumes. Since CO₂ has no hydrogen atom, there would be no NMR signal to image CO₂ bubbles.

To investigate if there is any water flooding in the cathode of the operating DFAFC, we measured the MRI images of the water content in the cathode electrode of the operating cell in every 15 minutes (Figure 4). From Figure 4, at zero minute, there is no water at the cathode electrode. According to Figure 4, as the cell operation time increases, the number of the water flooding spot also increases at the cathode electrode. Figure 4 shows that sizes of these water flooding spots also grow bigger as the cell operation time increases. According to Figure 4, the cathode electrode of the DFAFC is severally flooded by water after operating it for 135 minutes.

Figure 4: The MRI images of the water distribution in the cathode electrode after the DFAFC starts to operate at 0 min