The hydrogen car, it appeared, had died. And many did not mourn its passing, particularly those who regarded the auto companies’ interest in hydrogen technology as a stunt to signal that they cared about the environment while selling millions of highly profitable gas guzzlers. Except the companies, including General Motors, Honda, Toyota, Daimler and Hyundai, persisted. After many years and billions of dollars of research and development, hydrogen cars are headed to the showrooms.