The production of hydrogen is already 70-80% efficient. The conversion back to electricity seems like a bigger bottleneck for vehicles at ~40-60%. Wikipedia cl

The production of hydrogen is already 70-80% efficient. The conversion back to electricity seems like a bigger bottleneck for vehicles at ~40-60%. Wikipedia claims that fuel cells can hit 85% efficiency by using cogeneration to repurpose the waste heat, so to me it would seem that hydrogen is more interesting at grid scale.