They don't? When the Toyota hydrogen cars were introduced here around 2015, one of the issues were that a full tank of gas would dilute through the tank walls within a week. From the marketing material of the latest Toyota Mirai, it seems that they still use Hydrogen stored in gas form, boasting improvements in a 3-layer tank that is tested for 235% of the pressure that the gas is stored at, compared to 150% for regular gas containers.