In some respects a fuel cell is like a bettery. You use the fuel cell to combine Hydrogen with Oxygen and you capture that reaction energy in water.
Going the other way you split water into hydrogen and oxygen by adding that reaction energy.
They need to make these reactions into reversible half cell potentials and there has been a hunt for the proper catalytic membreans or surfaces that will do this for years and years and which will not gradually degrade or get poisoned by stray funny atoms in the reaction streams. This much like the chase to improve Lithium batteries over the past 20 years.
They make incremental progress and have functional systems now, but they do not have highly efficient ones. Hydrogen storage is one problem. They have made some hydride based hydrogen storage tanks, but they are not fast fill. They have made some that use air, but sulfur from SO2 in the air poison the membranes/catalysts they use.
All this said, after another 20 years they may well solve all the problems - there is a lot to be said for Japanese perseverance.