The type of bipolar plate flow channel determines the state of the reactants and products in flow field. A flow field with reasonable design can make the reactants uniformly distributed on electrode and timely discharge water, fully ensure good stability and performance of battery stack. Different types of flow field of bipolar plates can be applied in cathode and anode, so that the oxidant and fuel can flow in channel in manners of forward flow, backward flow or cross flow as required. The design of flow field can make great impact on performance of battery.

Patterns of flow channel for bipolar plate

At present, the most common patterns of flow channel for bipolar plate mainly include serpentine channel, parallel channels, interdigitated channels, spiral channel, mesh flow field and bio-inspired flow field.

1）Serpentine channel

An older pattern of bipolar plate flow field, designed as a single serpentine channel in series arrangement, which can rapidly discharging fluid water to avoid blocking in flow channnel. However, if the area of electrode plate is large, this structure may cause the pressure difference due to the pressure decreases gradually in overlong flow channel. Reaction gas in rear channel is insufficient and may result in stagnation or poor drainage, which will great affect the performance of battery stack.

2）Parallel channels

There are many design types for this flow distribution on bipolar plates, increase the number of flow passages, distribution in a parallel can alleviate the pressure drop problem often occurs in serpentine structure, and improve the battery efficiency. However, in this flow field pattern, the fluid is is easier to gather or even block the flow channel, lead to insufficient reactants for some electrodes and affect the overall performance of battery stack.

3）Parallel serpentine channels

Combine the parallel flow distribution with serpentine flow distribution, and flexibly adjust the number, length and size of the flow passage as required to form a multi-channel flow passage form. In addition, in order to solve the problem of uneven current density, the gradual serpentine flow channel adopts multiple independent serpentine flow field designs to make the reaction gas enter different flow fields, converge after small circulation, recycle and converge again, and finally discharge from multiple outlets after such reciprocating. The number of flow channels at the inlet or outlet of this flow field can be increased or decreased as required. On the basis of inheriting the advantages of conventional serpentine flow channels, it greatly improves the uniformity of current density, and is widely used.

4）Interdigitated channels

Also known as discontinuous flow field, which is helpful to improve battery power density. The discontinuous flow channel forces the gas to pass through the diffusion layer, which means that more gas will enter into catalytic layer to participate in reaction, and the electrode is effectively used, improve power density and easily remove water. However, it requires high inlet pressure for gas, high pressure drop may damage the catalytic layer. Once design is unreasonable, short circuit easily occurs and will reduce the utilization efficiency of reactants and electrodes.

5）Spiral channel

It’s similar to the serpentine flow channel, the difference is that the channels of bipolar plate are designed alternately at inlet and outlet, which has strong drainage capacity and makes distribution of reaction gas and water more uniform in flow field, overcome the defects of traditional serpentine flow channel. However, the problem of high pressure drop and short circuit still exists, and machining process is relatively complex, so there are few practical applications.

6）Mesh flow field

Strictly speaking, it cann’t be called flow channel for bipolar plate, which are composed by two orthogonal sets of parallel channels. The grid like barriers are arranged between inlet and outlet, allow gas to flow around the gap between barriers. The reactant travel through any path with equal distance. Constant resistance can ensure uniform distribution of reaction gas, conducive to the transmission and keep strong moisturizing capacity. The disadvantage is that the gas velocity is very slow, the water is prone to accumulate or flood in the corner due to poor drainage capacity. In addition, anti-corrosion requirements for the wire mesh are very high. The clamping process for wire mesh should not only maintain uniform force, but also cann’t be pressed into the electrode material.

7）Bio-Inspired flow field

also called fractal flow channel. Based on the principle of bionics, the reaction gas can flow in the silky pipeline at a lower speed than serpentine or interdigital flow field, meanwhile keep very small pressure drop. With the help of computer simulation and optimization, the reaction gas can be distributed uniformly in branch channels, greatly improve the stability of battery stack performance. The common flow field patterns include lung bionics channels, mosquito repellent incense type channels, tree-like channels, etc.

Design of flow field on bipolar plate

1）Size optimization of the flow channel

The size of channel and land in bipolar plate flow field has a great impact on the performance of battery stack. The dimensions of channel include the length, width and depth of channel, the width of land, and the sectional shape of channel and land. The channel size will directly affect the drainage performance on cathode, the width will affect the pressure of the reaction gas to membrane. Longer channel means greater lose of gas pressure, and lead to lower concentration of the reaction gas in rear section, result in flooding problems. Generally, the shallow flow channel will accelerate flow rate of reaction gas to avoid flooding and improve battery performance.

Take serpentine channel as an example, some scholars found that the optimal channel width should be from 1.14 to 1.4 mm, and the channel depth should be from 1.02 to 2.04 mm. By varying the inclination angle, the sectional area of channel can be changed to increase the use area of membrane electrode. The common channel inclination angel is from 0 ° to 60 °. The size of land will also affect the contact resistance and gas diffusion performance, the ideal width width of land is from 0.89 to 1.4mm. Generally speaking, smaller land widths and larger channel widths are conducive to promote the transmission of reaction gas, further to improve the performance of fuel cell stack. The performance of fuel cell can be improved by changing land/channel width ratio. In a word, the size optimization of bipolar plates requires a comprehensive analysis of battery performance, stability and service life to seek for best equalization point.

2）Performance study on flow channel

It has become an important trend to improve the performance of cell stack by improving design of flow field. For example, change width and cross-sectional area of channel in parallel-pattern field can make flow distribution uniform and flow rate faster. During the continuous generation of electric energy by fuel cells, due to pressure drop problems, auxiliary systems such as pumps or fans must consume a certain amount of energy in order to maintain the gas flow, the design of parallel and grid flow fields can effectively reduce pressure drop and energy consumption, further to improve the operating efficiency of fuel cell. Another design of integral flow field can process the gas channel and cooling water channel on same bipolar plate. This design reduces the thickness of BP while increases the assembly density of battery stack, and greatly improves the specific power. The recent studies are focused on how to obtain a field with uniform temperature. Many fuel cells run stably at the beginning, after a few days or even hours, the performance will degrade. Therefore, it’s significant to keep stable flow rate and reaction rate by reducing the number of flow channels.

Better flow passage is beneficial to improving the mass transfer inside the cell, also allow reaction gas to fully pass through diffusion layer by changing the flow mode and size of gas channel, the electrode can be fully utilized and take away the water. In addition, if the reaction water continuously accumulates in a certain area, it will cause the temperature difference in that area, and also cause the difference in the water content in different areas when use cooling plates to balance the temperature, this requires that the design of bipolar plates is convenient to adjust the temperature in each area of flow field. During the flow of reaction gas in channel, part of the gas flows into the diffusion layer under the land. When the total amount of this part of gas exceeds the amount of gas flowing along the flow channel, it will cause the pressure difference between adjacent channels to reduce the gas flow rate, and the water can not be eliminated. This is called “gas short circuit”, which should be avoided in design of flow field.

In addition to guiding the flow of reaction gas, the bipolar plate also has fuctions of mechanically supporting for membrane electrode,thermal conductivity and electrical conductivity, fewer channels means more contact with membrane electrode and better support. From the perspective of chemical reaction, more channels can help reaction gas reach each area of membrane electrode. For solve this contradiction, it is necessary to optimize the channel structure, increase the contact area between bipolar plate and membrane electrode, and provide enough gas for the electrochemical reaction. For example, the flow channel and land with higher density are conducive to mechanical support, the water in narrower land is easier to transfer from diffusion layer to flow channel, the reaction gas is easier to diffuse into the catalytic layer, finally improve the performance of battery.

In summary, we should consider different electrode structures to determine geometric structure and flow field pattern when designing the bipolar plate flow field. Performance improvement on one hand is often at expense of performance degradation on the other hand, the best solution of bipolar plate flow channel should be designed according to various purposes, in order to reduce the overall cost of cell stack, and ensure the good performance and stability of whole system.