Cathode flooding

Figure 15. Simulation results showing membrane dehydration (a) and cathode flooding (b). (a) 1 as a function of membrane position (cathode on the left) for different current densities. (Reproduced with permission from ref 14. Copyright 1991 The Electrochemical Society, Inc.) (b) Dimensionless oxygen mole fraction as a function of cathode-diffusion-medium position and cathode overpotential. (Reproduced with permission from ref 120. Copyright 2000 The Electrochemical Society, Inc.)...

Another good problem for modeling is the micro-DMFC system. Both anode carbon dioxide blockage and cathode flooding are especially acute in microsystems due to the small channel length scale involved, low operating temperature, dominance of surface tension forces, and requirement for low parasitic power losses in these systems. ... 

To avoid cathode flooding, a hydrophobic cathode backing and an efficient means to remove water droplets in the cathode flow field are required. We report here measurements of water flux in both liquid and vapor forms in the cathode... 

From the foregoing discussion, it is clear that, in a DMFC, the air cathode has to be operated under rather challenging conditions, that is, with a low air feed rate at nearly full water saturation. This type of operating conditions can easUy lead to cathode flooding and thus poor and unstable air cathode performance. To secure better air cathode performance, we have made great efforts to improve the ell cathode structure and cathode flow field design to facilitate uniform air distribution and easy water removal. The performance of our 30-cell DMFC stacks operated with dry air feed at low stoichiometry is reported in the following section. 

In order to study cathode flooding in small fuel cells for portable applications operated at ambient conditions, Tuber et al.81 designed a transparent cell that was only operated at low current densities and at room temperature. The experimental data was then used to confirm a mathematical model of a similar cell. Fig. 4 describes the schematic top and side view of this transparent fuel cell. The setup was placed between a base and a transparent cover plate. While the anodic base plate was fabricated of stainless steel, the cover plate was made up of plexiglass. A rib of stainless steel was inserted into a slot in the cover plate to obtain the necessary electrical connection. It was observed that clogging of flow channels by liquid water was a major cause for low cell performance. When the fuel cell operated at room temperature during startup and outdoor operation, a hydrophilic carbon paper turned out to be more effective compared with a hydrophobic one.81... 

In practice, trade-offs between optimization of different membrane functions have to be accepted. For instance, the immobilization of the proton solvent will impede the leaking out of solvent and, thus, help to avoid membrane dehydration and cathode flooding. On the other hand this may only be achievable at the cost of lower proton conductivity. A good theoretical understanding of mechanisms of proton mobility in various aqueous and non-aqueous environments is thus of vital importance. 

Micro fuel cell designs without polymeric membranes can overcome some PEM-related issues such as fuel crossover, anode dry-out or cathode flooding. In these membraneless laminar flow-based fuel cells (LF-EC) two or more liquid streams merge into a single microfluidic channel. The stream flows over the anode and the cathode electrodes placed on opposing side walls within the channel. The reaction of fuel and oxidant takes place at the electrodes while the two liquid streams and their liquid-liquid interface provide the necessary ionic transport [122,123]. 

It was found for PEFC applications that the hydration of the electrolyte membrane is critical for good performance. If the membrane is too dry high resistivity results, whereas, if the water flux is too great it can lead to cathode flooding. For proton transport to occur in Nafion -based membranes,... 

Optimum hydration level of electrolyte membranes is a key factor for normal fuel cell operation. If the electrolyte membrane is too dry its conductivity decreases, whereas an excess of water in the membrane can lead to cathode flooding. In both cases fuel cell performance drops. 

PTFE treatment of the GDL is one method to control internally the water content. Water management in PEM fuel cells is a key factor for the correct functioning of the device. Water management refers to the control of the water content inside the fuel cell. Low amounts of water within the boundaries of the MEA causes the membrane to dry, consequently reducing the ion transport properties. On the other hand, excessive amount of water particularly on the cathode side, known as cathode flooding, hinders the reaction on the catalyst surface. Both high and low water content may produce the shutdown of the cell. In direct alcohols fuel cells (DAEC), the water management is a key factor because as the fuel is introduced as an aqueous solution the amount of water inside the cell could be excessive. 

At present, the most widely used commercial PEM is Naflon produced by DuPont since 1992. Naflon is a plain perfluorosulfonic membrane that is thermally stable and is excellent for PEMFC because of its high proton conductivity. However, Naflon is not suitable for DMFC applications, partly due to its cost. This type of membrane has high permeability toward methanol even at low temperatures, which drastically reduces the DMFC performance (Neburchilov et al. 2007). This is worsened by high water permeability in perfluorinated membranes that can cause cathode flooding and thus lower cathode performance, which also contributes to lower DMFC performance. 

Cathode flooding, membrane drying, and anode catalyst poisoning by CO... 

Water management is one of the most important aspects for the proper operation of PEMFCs. If there is too much water in a PEMFC, it will cause cathode flooding. On the other hand, if there is too little water in a PEMFC, it wiU result in proton exchange membrane dehydration. The cathode... 

A highly water-permeable PEM would facilitate water removal via liquid transport toward the anode, alleviating the problem of cathode flooding and anode dehydration. Erom a system perspective, it is deemed beneficial to make use of internal humidification of CLs and PEM by water that is produced at the cathode. This mode of internal water management obviates the need for external humidifiers. It demands, however, precise control of water permeation rates through the PEM and of vaporization rates in partially saturated porous electrodes. Therefore, it is cmcial to know how relevant parameters of water transport (diffusion, hydraulic permeation, electro-osmotic drag, vaporization, and condensation) depend on PEM morphology and thermodynamic conditions. 

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