Why not hydrogen? Hydrogen's weakness is volume and cost; but for shipping neither of those is a huge concern. Batteries are more efficient but given the scale for shipping that may not be as much
Shipping is more likely to use methanol, which is derived from hydrogen, than anything else. Cargo ships are already in the process of being converted over.
With this concept, you could replace the shipping containers containing batteries with ones containing hydrogen fuel cells. That way, as the technology improves the source of power can change.
