Toyota made a speech at the Automotive Technology Association Spring Meeting held in May 2015 (sponsored by the Japan Automotive Technology Association) and introduced the start-up of the "MIRAI" fuel cell vehicle (FCV) below the freezing point.
The fuel cell stack in the FCV generates hydrogen and oxygen in the air to generate electricity and water. Therefore, the water produced when starting below freezing point freezes. In the worst case, the flow path of gas and water in the battery cell is blocked, causing the fuel cell stack to stop working. In order to solve this problem, it is necessary to reduce the water, or increase the volume of the fuel cell stack so that the water content can be easily controlled.
The fuel cell stacks of the 2008 models ensure the startability below the freezing point through the drying control. However, when the drying control is performed, the output power of the fuel cell drops.
In the past, there was a problem that water generated in the previous model became retained water. For this reason, MIRAI used a 3D pore network with a three-dimensional flow path to improve the drainage of generated water. The 3D pore network flow path is a fine lattice-like flow path, and the diffusion of oxygen into the air electrode catalyst layer is improved by causing air to flow in the direction of the turbulent air flow and the electrodes.
In terms of the electrode of the battery cell, by changing the carbon structure and reducing the oxygen diffusion resistance, the substrate is designed with a thin layer. At the same time, the water repellent pressure of the water repellent layer is reduced, the drainage is improved, the water content is greatly reduced, and the oxygen diffusibility is improved compared to the previous models. Doubled. In this way, the battery current density after the heat engine operation can be increased to 2.4 times.
Through the above improvements, the battery capacity has been reduced from 64L in 2008 to 37L, heat capacity has been reduced to 2/3, and the amount of generated water has also been reduced to 2/3.
The structure of the flow path and the modification of the electrode allow the water content to be doubled, and therefore it can usually be started at -30°C after the heat engine. (Special Contributor: Ito Tatsuo, Shibaura Institute of Technology, Japan)
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