For the energy density problem already mentioned, consider that greater fuel volume = more drag = less efficiency. There is also accident safety to consider. But most pertinently in climate change terms there is the question of how the hydrogen is generated in the first place. It does not occur naturally in significant quantity and has to be produced. Whatever process is adopted, it will be less than 100% efficient so additional energy is needed from the first stage. Burning fossil fuels to generate electricity to do this, typically at lower efficiency than a jet engine can achieve, is clearly a nonsense unless all pollutants can be captured, in which case why the current angst? Nuclear fission might be credible in capacity terms but has cost and political issues; good luck getting the green lobby on board. And renewables need to get closer to existing electrical power demand before it becomes conceivable to add another huge requirement. The received wisdom in futurist writing used to be that nuclear fusion power would be the step which unlocked a ‘
hydrogen economy’ but that seems as far away as ever...