Reason did a policy study last month exploring the impact of switching cars over to hydrogen on greenhouse gas emissions. Turns out that we use so much energy to make hydrogen that emissions are at best unchanged.
A new paper by CATO looks at the same issue, with similar conclusions.
Given current technology, switching from gasoline to hydrogen-powered fuel cells would greatly increase energy consumption even if the hydrogen were extracted from water rather than from fossil fuels. That's because it takes a tremendous amount of electricity to harvest hydrogen and to deliver it to consumers. Moreover, a transition from gasoline to hydrogen would nearly double net greenhouse gas emissions attributable to passenger vehicles, given the current fuel mix in the electricity sector.
Hydrogen may be one of the fuels of the future, but forcing a change in fuels when it is doubtful we can yet realize benefits from it is folly. Once the switch is beneficial, there will be plenty of incentive to switch, without mandates.