Research progress of the new high-performance MIRAI fuel cell stack

In 2014, Toyota Motor Corporation launched the world's first commercial fuel cell vehicle (FCV)   MIRAI  .  Compared  with the fuel cell (FC) stack used in the first-generation MIRAI model, the fuel cell stack used in the new MIRAI model adopts a new bipolar plate flow channel and improved electrodes, becoming one of the products with the highest volume power density in the world, with  a power density of 5.4 kW/L (excluding end plates), which is 1.5 times higher than the fuel cell stack of the first-generation MIRAI model. Increasing current density is an important means to improve the power performance of the power system and reduce the volume. The role of the bipolar plate is to reasonably distribute gas and drain water inside the battery to stabilize the generation of current. The traditional direct current path is easily flooded by water, and it is difficult to maintain a stable current. In order to improve the diffusion efficiency of oxygen, a local narrow flow channel is designed without the need to use a three-dimensional grid flow field in the original fuel cell stack. This new technology can make the oxygen concentration in the catalyst layer 2.3 times higher than that of the traditional direct current channel. In addition to the bipolar plate, the catalyst carrier and ionomer have also been improved. Ionomers on the surface of platinum (Pt) can cause sulfonic acid poisoning, resulting in decreased catalytic activity. To solve this problem, mesoporous carbon was developed as a catalyst carrier, carrying Pt in the pores of the mesoporous carbon, and sulfonic acid poisoning was suppressed by reducing the contact between the ionomer and the Pt surface. A high oxygen permeability ionomer was used, which has an oxygen permeability that is three times that of the ionomer used in previous fuel cell stacks.

0 Introduction

The first MIRAI was launched in 2014 as the world's first commercial fuel cell vehicle (FCV). The new MIRAI has been developed for mass production with higher performance and lower costs, promoting the promotion of FCVs.

Compared with the fuel cell (FC) stack used in the first-generation MIRAI model, the fuel cell stack of the new MIRAI model adopts a flow field structure composed of bipolar plates and electrodes, becoming one of the products with the highest volume power density in the world, with a power density of 5.4 kW/L (excluding end plates), which is 1.5 times higher than the fuel cell stack of the first-generation MIRAI model. The components of the FC system (such as direct current (DC)/DC converter, FC controller, etc.) are integrated with the FC stack, making the FC system more compact and able to be installed in the engine compartment (Figure 1).

Figure 1 Layout of fuel cell stack in engine compartment

In addition, the researchers developed an electrode with a small amount of platinum (Pt), a bipolar plate with a role-by-role surface treatment scheme, and a high-speed adhesion sealed battery, which reduced the cost of the FC stack. The cost of the new FC stack has dropped by 25% compared to the previous generation of stacks, and the production efficiency has been greatly improved.

This article introduces the key technologies of the new MIRAI FC stack, such as the flow field channel and new electrodes. These are developed to increase the power density of the FC stack and reduce its size.

1. Improvement of FC stack performance

Improving current density is an important means to improve the power performance of FC stacks and reduce their size. In the new MIRAI model, researchers used newly developed flow field structures and electrode component materials to improve the drainage performance and oxygen dispersion of the electrode catalyst layer, and the power performance was improved by 15%.

2 Innovation of fine-pore flow field channels

The role of the bipolar plate is to distribute gas and water in the battery and stabilize the generation of current, but the straight flow channel will produce water vapor (Figure 2). For the FC stack of the first-generation MIRAI model, researchers developed a three-dimensional (3D) fine grid flow channel that uses the structural hydrophilic effect to quickly move the generated water from the gas diffusion layer (GDL), thereby helping to improve the power performance of the FC stack (Figure 3).

Figure 2 X-ray computed tomography (CT) image of residual water in the direct current path

Figure 3 X-ray CT image of residual water in 3D fine grid flow channel

The 3D fine mesh flow channel structure will increase the number of parts, thereby increasing costs and generating additional pressure losses. Compared with the traditional flow channel composed of one bipolar plate, the 3D fine mesh flow channel is composed of a bipolar plate and a fine pore flow channel. In order to solve the problem caused by pressure loss, the researchers arranged an air manifold at the long side of the battery pack unit. The newly developed fuel cell stack has some narrow flow channel points to balance oxygen diffusion and pressure loss. Some narrow flow channel points in the new flow field can push air into the GDL through pressure resistance (Figure 4). The researchers adopted this new flow channel method to help reduce the number and size of FC stack parts and adjust the air manifold layout from 4 directions to 2 directions. Due to the drainage of this design, the oxygen concentration in the newly developed flow channel GDL is 2.3 times that of the traditional straight-through flow channel, which is the same as the 3D fine mesh flow channel (Figure 5).

Fig.4 Schematic model of gas diffusion in a narrow channel GDL

Figure 5 Visualization of residual water in the battery and gas diffusion performance

Figure 6 shows the unit components of the new MIRAI FC stack. To improve the gas distribution and drainage performance in the electrolyzer, the researchers used a corrugated flow channel as the anode separator compared to the serpentine flow channel in the original electrolyzer. In addition, in order to maintain stable current generation, the researchers adopted a countercurrent method for air and hydrogen in the cell. This countercurrent achieves automatic humidification by using the produced water alone, thereby reducing the operation of the humidifier.

Figure 6 Unit components of the FC stack in the new MIRAI car

3 Innovation of electrode materials

Figure 7 shows the characteristics of current density enhancement. The researchers used new electrode materials to improve the performance of catalysts and ionomers. The original fuel cell used low surface area carbon (LSAC) as a catalyst support to improve the utilization of Pt. However, the catalytic activity of the LSAC support is deteriorated by sulfonic acid poisoning caused by ionomers on the Pt surface. To solve this problem, the researchers developed mesoporous carbon as a catalyst support.

Figure 7 Electrode specifications of new fuel cell stack

Since about 80% of the Pt in the battery will be brought into the pores of the mesoporous carbon, the researchers suppressed sulfonic acid poisoning by reducing the contact between the ionomer and the Pt surface. By adopting this approach, as well as increasing the solid solubility of the PtCo alloy catalyst, the researchers were able to increase the catalytic activity by about 50% (Figure 8).

Figure 8 Schematic model of mesoporous carbon catalyst support

In addition, the researchers also used a highly oxygen-permeable ionomer whose oxygen permeability was three times that of the original fuel cell. By increasing the number of acidic surface functional groups, the proton conductivity was increased by 1.2 times (Figure 9).

Figure 9 Comparison of ionomer characteristics of 1st and 2nd generation FC stacks

The researchers studied the strength of the proton exchange membrane and found that strengthening the membrane by increasing the ratio of the cell backplane layer, increasing the membrane thickness (10 times compared to previous membranes) and reducing the thickness (about 29%) can reduce the amount of hydrogen crossover, while the thinner membrane can increase the proton conductivity by 1.5 times.

In GDL, reducing the density of carbon paper materials and increasing the pore size can improve gas diffusion characteristics by 25%. As mentioned above, the researchers can increase the power density per unit area of ​​electrode by 15% by innovating the flow field path and electrodes (Figure 10).

Figure 10 Characteristic curve

4 FC stack reduction and weight reduction (power density)

The maximum power of the new generation FC stack has been increased to 128 kW, which is 25% higher than the previous fuel cell stack with a maximum power of only 114 kW. The researchers reduced the thickness of the battery from 1.34 mm to 1.11 mm (Figure 11) by reducing the thickness of the separator (from 0.13 mm to 0.10 mm) and the number of flow channels (from 3 to 2). In addition, the researchers also increased the maximum current of the battery by 20%, thereby reducing the number of batteries from 370 to 330 (Figure 12), reducing the size of the battery pack from 33 L to 24 L (excluding end plates), and reducing the mass from 41 kg to 24 kg (excluding end plates).

Figure 11 Comparison of battery cell thickness

Figure 12 FC stack dimensions (excluding end plates)

By improving the power performance of the battery stack and reducing its size and mass in the above-mentioned way, the researchers have made the new generation FC stack equipped in the new MIRAI model one of the products with the highest volume power density in the world, with power increased from 3.5 kW/L to 5.4 kW/L (excluding end plates) and mass power density increased from 2.8 kW/kg to 5.4 kW/kg (excluding end plates) (Figure 13).

Figure 13 Power density of FC stack (excluding end plates)

5 FC stack cost reduction and mass production performance

In order to reduce the cost of FC stacks, researchers need to reduce the amount of high-cost materials such as Pt catalysts. The researchers increased the maximum current density and electrode area utilization, reducing the battery volume by 20%. In addition, the researchers reduced the cost of the new fuel cell stack by 75% by reducing the number of cells, using thinner ion exchange membranes and bipolar plates, reducing the number of parts, and reducing the amount of Pt (Figure 14). In order to facilitate large-scale production, the researchers used advanced high-speed battery sealing technology. The battery sealing process was changed from ethylene propylene diene monomer (EPDM) vulcanization bonding to thermoplastic bonding and ultraviolet light curing (UV).