Maruo, T., Toida, M., Ogawa, T., Ishikawa, Y. et al., "Development of Fuel Cell System Control for Sub-Zero Ambient Conditions," SAE Technical Paper 2017-01-1189, 2017, doi:10.4271/2017-01-1189.
Toyota Motor Corporation (TMC) has been developing fuel cell vehicles (FCVs) since 1992. As part of a demonstration program, TMC launched the FCHV-adv in 2008, which established major technical improvements in key performance areas such as efficiency, driving range, durability, and operation in sub-zero conditions. However, to encourage commercialization and widespread adoption of FCVs, further improvements in performance were required. During sub-zero operating conditions, the FC system output power was lower than under normal operating conditions. The FC stack in the FCHV-adv needed to dry the electrolyte membrane to remove unneeded water from the stack. This increased the stack resistance and caused low output power. In December 2014, TMC launched the world’s first commercially available FCV named the Mirai, which greatly improved output power even after start-up in sub-zero conditions. To enable start-up in sub-zero temperatures under high water content conditions, oxygen diffusion through the cell was enhanced and a 3D fine mesh cathode flow channel was developed to improve the drainage control. In addition, control countermeasures were incorporated, including a water content measurement technique and water purge control during parking (parking purge). The water content measurement technique uses low-frequency impedance, which accurately measures high water content levels. The parking purge was developed to purge liquid water that migrates from the cathode to the anode while the vehicle is parked. Without the purge, the water may clog the anode channel. As a result, the FC stack in the Mirai improved output power after start-up at sub-zero conditions.