The article mentions hydrogen from electrolysis of water, but I think a bigger source in the future could be steam reforming of biomass. That is, when you heat biomass (plant matter, sewage sludge, maybe even municipal garbage) to about 300C in steam, the organic matter breaks down into simple molecules like hydrogen, carbon monoxide (highly flammable!), methanol, elemental carbon (biochar) and miscellaneous others. Some of those molecules can be recovered for important chemical feedstock (since we won't have petroleum or natural gas as feedstocks anymore, right?), and the gas can be fuel.

In the early days of natural gas use, towns would "reform" the methane (CH4) by reacting it with steam to make carbon monoxide (CO) and 3 molecules of H2 - a mixture known as "city gas". It is not new technology.