Development of High-Performance FC Stack for the New MIRAI

The first MIRAI was launched in 2014 as the world’s first commercial fuel cell vehicle (FCV) [1]. Compared to the FC stack used in the first MIRAI, the FC stack in the new MIRAI achieved one of the highest volumetric power densities in the world (5.4 kW/L, excluding end plates, 1.5 times higher than the FC stack in the first MIRAI) by adopting a new flow channel for the bipolar plate and an improved electrode [2]. Enhancing the current density is an important means of increasing power performance and reducing size. The bipolar plate functions to distribute gas and drain water inside the cells to stabilize current generation. However, a conventional straight flow channel tends to cause flooding, which makes it difficult to maintain stable current generation. A partially narrow flow channel was developed to enhance oxygen diffusion without the 3D fine-mesh flow field that was adopted in the previous FC stack. With this new technology, the oxygen concentration in the catalyst layer is 2.3 times higher than a conventional straight channel and the same as the previous model. In addition to the bipolar plate, the catalyst support and ionomer were also improved. Catalytic activity deteriorates due to sulfonic acid poisoning derived from the ionomer covering the Pt surface. Mesoporous carbon was developed as a catalyst support to resolve this issue. Pt is carried in the holes of the mesoporous carbon, which inhibits sulfonic acid poisoning by reducing contact between the ionomer and the Pt surface. A highly oxygen-permeable ionomer was also newly adopted with an oxygen permeation performance three times higher than the ionomer used in the previous FC stack.