A Study on Manufacturing of Microchannel Bipolar Plate in Fuel Cell by FDM 3D Printer

Abstract

A fuel cell bipolar plate having an active area of 250 mm x 250 mm was prepared using a 3D printer of Fused Deposition Modeling (FDM). The channel shape was made rectangular, and the channel's width and depth were 0.5 mm each. Altogether, 240 channels were arranged in the active area of the fuel cell bipolar plate. Three types of bipolar plates with different shapes (single type, double type, and triple type) in the bent areas were prepared. The printing speed is set to 50 mm/s, a nozzle diameter to 0.4 mm, a nozzle temperature at 200℃ and a bed temperature of 60℃. The working time of the 3D printer is about 14 hr. In the single type, filament was flown down from the end of the rib in the bent area, thereby blocking part of the channel. On the contrary, there were no blockages in the double type and triple type channels, and the shapes of the channels were produced close to the designed model. The precision of the bipolar plate channel designed as triple-type was 88.8%.

1. Introduction

As greenhouse gas emissions and climate changes were posed as issues worldwide, a low-carbon economic system emerged after the Paris Agreement in 2015. Alternative energies drew attention, and research and interest on fuel cell sharply increased [1,2].

A fuel cell is a device that converts energy generated through the chemical reaction between hydrogen and oxygen into electricity. Since hydrogen is used as a fuel, exhaust and toxic gases are not emitted; therefore, it draws attention as an alternative energy that can solve environmental issues [3,4].

A bipolar plate, one of the fuel cell’s key components, plays a role in transferring hydrogen and oxygen to Membrane Electrode Assembly (MEA). The bipolar plate is used in the cathode and anode in the single cell, accounting for more than 50% of the fuel cell stack’s weight and volume, and it occupies about 40% of the stack manufacturing cost [5,6]. The bipolar plate has various thermal, chemical, anti-corrosion, physical, shape, and machinability properties. The most ideal material for the bipolar plate could be graphite, but it is fragile in processability, consuming considerable manufacturing time and cost. Therefore, material for the bipolar plate has been concentrated on the metal, enabling mass production by changing the material for the bipolar plate to metal [7].

As the metal bipolar plate’s manufacturing process, the stamping, rubber forming, machining, vacuum die casting, and semi-solid forming were applied. These processes have merits, but drawbacks also exist. Vacuum die casting and semi-solid forming can make pieces with only aluminum, but the thickness cannot be reduced to lower than 0.8 m m. Stamping and rubber forming can make the die cavity rectangular, but a spring-back would form an elliptical channel. Also, since a micro-channel cannot be formed due to forming limit, many channels cannot be produced in the unit area [3-7].

A solution to resolve the problem of each process mentioned so far is 3D printing. 3D printers spray and laminate materials in the form of powder, liquid, and solid using the data obtained through the 3D modeling or digital 3D scanner and create a real-sized material with the same shape as the 3D data. The 3D printing technology was invented by Dr. Charles W. Hull of the United States in 1982 [8,9]. 3D printing can produce the shapes through various modes like Fused Deposition Modeling (FDM), Selective Laser Melting (SLA), and Stereo Lithography Apparatus (SLM). Currently, it is utilized in a variety of manufacturing segments, including medical, automobiles, machines, construction, and household goods [10,11].

This study prepares a bipolar plate with a rectangular micro-channel that could not be produced until recently, along with a micro-channel evenly arranged through the 3D printer. It did not exactly aim to prepare a metal bipolar plate with a metal 3D printer, but to try to see the potential of manufacturing a bipolar plate with an ideal shape by applying the polymer 3D printer. This research used an FDM type 3D printer to produce a bipolar plate having many rectangular micro-channels arranged in the active area of 250 mm x 250 mm.

2. Experimental Method

2.1 Modeling of the bipolar plates

First, the shape of the bipolar plate was modeled with the CATIA 3D modeling program’s help to print the bipolar plate with the 3D printer. Figure 1 shows the bipolar plate, which was modeled as 3D. The active area of the bipolar plate was 250 mm x 250 mm. Since many channels were arranged in the active area, the shape of the bipolar plate could be confirmed from the enlarged image. The bent area in the channel was designed at the right angle. Figure 2 shows the cut shape of the bipolar channel. The channel width (a), channel depth (d), and rib width (b) all were designed to 0.5 mm. Because the thickness of the bipolar plate was 1 mm, the total thickness of the bipolar plate became 1.5 mm. The number of channels in the active area of the bipolar plate was 240.

Fig. 1 3D modeling of the bipolar plate with micro-channels

Fig. 2 Section shape of the bipolar plate channels

A total of three bipolar plates having a different number of channels in the bent area were modeled. Figure 3 shows the modeling at the bent area of the bipolar plates with a different number of channels. Figure 3 (a) shows the single type having one channel in the bent area, Figure 3 (b) shows the double type having two channels, and Figure 3 (b) shows the triple type having three channels in its bent area. The lengths of the channel’s width in bent zones c_₁, c_₂, and c_₃ were designed to be 0.7 mm. After laminating the three types of bipolar plate, the channel’s cut length was measured to determine the most precisely laminated one. 

Fig. 3 Bend zone for three bipolar plates : (a) single type, (b) double type and (c) triple type

The part file of the 3D-modeled bipolar plate was converted to a stereolithography (STL) file for the 3D printing, and the STL file was again converted to a G-code using IdeaMaker.

2.2 3D Printing of the bipolar plates

A bipolar plate was printed out using an FDM 3D printer. FDM type is a method of laminating a material layer by layer after melting the thermoplastic material to a semi-solid state followed by extrusion. The filament used in this study was Polylactic Acid (PLA), an eco-friendly polymer resin, which was mainly used in a 3D printer. Table 1 shows the physical properties of the PLA filament.

Table 1. Properties of PLA filament

The bipolar plate was then printed under the conditions shown in Table 2. The printing speed was set to 50 mm/s, a nozzle diameter to 0.4 mm, a nozzle temperature at 200℃ and a bed temperature of 60℃. The time to laminate the bipolar plate from the 3D printer was about 13 hrs and 54 min.

Table 2. Printing condition of 3D printer

The three printed bipolar plates were cut to section A-A and section B-B, as shown in Figure 1. The lengths of channel cross-sections (a, h, b, c₁, c₂, and c₃) were observed through the digital microscope as shown in Figure 1.

3. Experimental Results

The plane figure of the bipolar plate printed from the 3D printer is shown in Figure 4. Since there were 240 bipolar plates inside the active area, it was difficult to check the channels with naked eyes. Therefore, magnified zone ① (bend) and zone ② (center) in Figure 4 were observed through 50X magnification using the digital microscope. Figure 5 shows the magnified images of zone ① zone and zone ② of the three types of bipolar plates. Figure 5 (a) shows the single-type bipolar plate, Figure 5 (b) is the double-type bipolar plate, and Figure 5 (c) is the triple-type bipolar plate.

Fig. 4 Printed bipolar plate by 3D printer

Fig. 5 Magnification image in bend zone of three printed bipolar plates : (a) single type, (b) double type and (c) triple type

In the single type in Figure 5 (a), filament flew down from the rib end of the bent area, which blocked part of the channel in the bent area. Filament flowing down was not found from the double type plate (Figure 5 [b]) and triple type plate (Figure 5 [c]), showing good printing performance without blocking channels at all.

The magnified images of the cross-sectional area of the channels from section A-A and section B-B, as in Figure 1, are shown in Figure 6 and Figure 7, respectively. Figure 6 shows section A-A images of three types of bipolar plates (single type, double type, and triple type). Figure 7 shows the image of section B-B. Because three types of bipolar plates had different shapes only in the bent areas and had the same shapes in the rest of the parts, section B-B’s cross-sectional shapes were the same. It was also confirmed that the section B-B channel area was almost rectangular, as in the modeling shown in Figure 2. However, the side of the rib became a bulged layer due to repeated lamination. The channel’s cut shapes of section A-A were irregular in all three types compared to those in section B-B.

Fig. 6 Channel section in section A-A of three printed bipolar plates : (a) single type, (b) double type and (c) triple type

Fig. 7 Channel section in section A-A

The cut lengths of the channels of the three printed bipolar plates (a, b, d, c₁, c₂, and c₃) are shown in Table 3. The cross-sectional length of the printed channels as a 3D print was compared with the cross-sectional length of the channels modeled as 3D in Figure 2 and Figure 3, and the differences were treated as errors.

Table 3. Section shape length of channel for three printed bipolar plates

The error values of the cross-sectional length (c₁, c₂, and c₃) of the bent area were far more extensive than the cross-sectional length of the channel in other zones. The same result can be confirmed from the images for the cross-sectional area in Figure 6.

The single type’s average error was 13.85% and 12.26% for the double type, and 11.17% for the triple type, showing the highest error level in the single type, with the triple type having the lowest. Such a result might be due to the different retraction frequencies according to the material’s extruder and shape movement route. The output of the single type was more frequent than that of the double type and triple type. Therefore, the filament was flowing down during printing, and the channel was blocked, which might have created the error. The retraction frequency differs according to the number of channels in the bent area and the extruder’s movement route. The higher the retraction frequency was, the higher the error value was for the channel’s cross-sectional shape. Therefore, if the retraction frequency is reduced, the precision will be increased.

The triple type seems to be the most suitable shape to print through the 3D printer, as indicated by the measurement results of the cross-sectional length of the channel in Table 3.

4. Conclusions

In this study, a fuel cell bipolar plate with rectangular micro-channels arranged in the active area of 250 mm x 250 mm was prepared. Next, three types of bipolar plates with different channels in their bent areas were prepared, and respective cross-sectional lengths of the channels were measured to search the most precisely manufactured bipolar plate structure. The experimental results are summarized as follows:

(1) From the 3D printer, rectangular micro-channel bipolar plates almost identical to the modeling shape could be printed.

(2) The bipolar plate with a single type bent area had the filament flowing down from the rib end, while this phenomenon was not found from the double type and the triple type variants.

(3) The most precise channel could be printed in the bipolar plate designed as triple type compared to the bipolar plates of a single type and double type. The precision of the triple-type bipolar plate was 88.8%.

(4) From the PLA filament 3D printer, a bipolar plate with rectangular micro-channels (width and depth: 0.5 mm) could be printed. It is expected that the same shaped metal or graphite bipolar plate could be printed using a 3D printer based on the experimental data in this study.