Question

hydrogen can be obtained economicallyas a bi-product in the electrolysis of

Answer 1

I would guess water.

Answer 2

In the short term, hydrogen can be produced economically by electrolysis of water in off-peak periods, enabling much greater utilisation of base-load generating plants, including nuclear ones. In future, a major possibility is decomposition of water by direct use of heat from nuclear energy, using a chemical process enabled by high-temperature reactors.
Source: http://www.world-nuclear.org/info/Non-Power-Nuclear-Applications/Transport/Transport-and-the-Hydrogen-Economy/