The biological hardware for producing hydrogen already exists; green plants make use of an oxygen-evolving complex to split water into oxygen and hydrogen, and

The biological hardware for producing hydrogen already exists; green plants make use of an oxygen-evolving complex to split water into oxygen and hydrogen, and the hydrogen then goes on to pump ATP synthase. However, storing hydrogen is probably infeasible for biology. It's difficult to store hydrogen, even using metal containers without it leaking out, because the molecules are so small.