In fuel cells, the flow field plates are designed to provide an adequate amount of the reactants (hydrogen and oxygen) to the gas diffusion layer (GDL) and catalyst surface while minimizing pressure drop. The most popular channel configurations for PEM fuel cells are serpentine, parallel, and interdigitated flow. Some small-scale fuel cells do not use a flow field to distribute the hydrogen and air but rely on diffusion processes from the environment. Since the hydrogen reaction is not rate limiting, and water blockage in the humidified anode can occur, a serpentine arrangement is typically used for the anode in smaller PEM fuel cells.
The serpentine flow field design is shown in Figure 1. Its flow path is continuous from start to finish, and it is relatively efficient at providing flow distribution across the electrode surface of the fuel cell. This design may cause pressure loss due to the relatively long flow path. For high current density operation, large plates, or when using air as an oxidant, alternative designs are used based upon the serpentine design. An advantage of the serpentine flow path is that any obstruction in the path will not block all downstream activity of the obstruction. A disadvantage of serpentine flow is the fact that the reactant is depleted through the length of the channel, so an adequate amount of the gas must be provided. When air is used as an oxidant, problems may arise with the cathode gas flow distribution and cell water management. When the fuel cell operates for extended periods of time, the water accumulates in the cathode, and pressure is required to move the water out of the channels.
Figure 1: A serpentine flow field design
Figure 2 shows the multiple serpentine flow channel design. In this design, several continuous flow channels are used to limit the pressure drop and reduce the amount of power required to pressurize the air through a single serpentine channel. This design prevents stagnant area formation at the cathode surface due to water accumulation. The reactant pressure drop through the channels is less than the serpentine channel but is still high due to the long flow path of each serpentine channel.
Figure 2: Multiple serpentine flow channel design
In the parallel flow field design configuration (see Figure 3), the flow channels require less mass flow per channel and provide more uniform gas distribution with a reduced pressure drop. If air is used as the oxidant, low and unstable cell voltages may occur after prolonged periods of operation due to water buildup and cathode fuel distribution. When the fuel cell is operated continuously, the water accumulates in the flow channels. The disadvantage of the parallel flow configuration is that an obstruction in one channel results in flow redistribution among the remaining channels, and thus a dead zone downstream of the blockage. The amount of water in each channel could vary, which leads to uneven gas distribution. Another issue with this design is that the channels are short, and have few directional changes. Thus, the pressure drop in the channels are low, but the pressure drop in the stack distribution manifold may need to be higher. The first few cells near the manifold inlet often have a greater flow rate than the cells toward the end of the manifold.
Figure 3: A parallel flow field design
The reactant flow for the interdigitated flow field design is parallel to the electrode surface. Often, the flow channels are not continuous from the plate inlet to the plate outlet. The flow channels are dead-ended, which forces the reactant flow, under pressure, to go through the porous reactant layer to reach the flow channels connected to the stack manifold. This design can remove water effectively from the electrode structure, which prevents flooding and enhances performance. The interdigitated flow field promotes forced convection, which helps to avoid flooding and gas diffusion limitations. This design sometimes outperforms conventional flow field design, especially on the cathode side of the fuel cell. The interdigitated design is shown in Figure 4.
Figure 4: An interdigitated flow field design
Fluid flow channels are typically rectangular, but other shapes such as trapezoidal, triangular, and circular have been demonstrated. The change in channel shape can influence the water accumulation in the cell, and, therefore, the fuel and oxidant flow rates. In rounded flow channels, condensed water forms a film at the bottom of the channel, and with other channel shapes, the water forms small droplets. The shape and size of the water droplets are determined by the hydrophobicity and hydrophilicity of the porous media and channel walls. Channel dimensions are usually around 1 mm, but a large range exists for micro- to large-scale fuel cells (0.1 mm to 3 mm). Simulations have found that optimal channels dimensions for macro fuel cell stacks (not MEMs fuel cells) are 1.5, 1.5, and 0.5 mm for the channel depth, width, and land width (space between channels), respectively. These dimensions depend upon the total stack design and stack size. The channels dimensions affect the fuel and oxidant flow rates, pressure drop, heat and water generation, and the power generated in the fuel cell. Wider channels allow greater contact of the fuel with the catalyst layer, have less pressure drop and allow more efficient water removal. However, if the channels are too wide, there won’t be enough support for the MEA layer. If the spacing between flow channels is also wide, this reduces the area exposed to the reactants and promotes the accumulation of water.
The flow field plates have multiple functions and are designed to transport gases and liquids to and from the catalyst sites. Popular channel configurations include the serpentine, parallel, and interdigitated flow. Many flow field designs can be used, and the design chosen depends upon the stack size, design, and configuration.