I put a hydrogen generator on my pick up a couple years ago. Several problems occurred. Since the hydrogen was generated from water you also put additional oxygen into the fuel. The O 2 sensors on the exhaust were telling the computerized fuel system that there was extra oxygen indicating that the fuel to air mixture was too lean so the computer richened the mixture calling for more gas thus decreasing mileage. I tried to mitigate this with an electronic device attached between the sensors and the computer that actually tried to fool the vehicle's computer by supplying false oxygen data. This worked while idling but when engine rpms were increased the data had to be dialed up manually and this just wasn't practical for driving. Also the hassle of filling the device when it got low and adding dangerous chemicals to prevent freezing became a deal breaker. Perhaps the fuel cells have some similar concerns.