Hydrogen is more cleanly obtained by splitting water, the best option to doing so is not electrolysis, but rather thermochemical splitting, of which many examples are know, my favorite being the oldest, since it is amenable to flow chemistry and heat networks, the sulfur-iodine cycle. Modern advances in materials science, particularly with respect to corrosion resistant refractories now make it far more accessible, and I can think of many improvements myself, which I hope to share with my son before I die.
Any organic chemical - and I do mean any - now obtained from petroleum is accessible from syngas. Organic chemistry is a very mature science, and a fine organic chemist should be able to loosely mimic the remark attributed to Archimedes about giving him a place to stand to move the world by saying "give me a carbon oxide, energy and water, and I can make anything organic."
One path to hydrogen that is carbon based is the thermal chemical splitting of carbon dioxide, a great deal of work has also been published on the electrochemical reduction of carbon dioxide, followed by the famous water gas reaction which is generally driven by dangerous natural gas but need not be. (I'm not a big fan of electrochemical methods in most cases, except in cases where electricity is a side product of heat networks designed to capture "waste" energy, as in a combined cycle dangerous natural gas powered plant. My favorite carbon dioxide splitting thermochemical method is the cerium catalyzed version, although I freely admitted the limitation: Cerium Requirements to Split One Billion Tons of Carbon Dioxide, the Nuclear v Solar Thermal cases.
The biggest concern for the food supply is not the ammonia now made from dangerous natural gas by steam reforming to yield hydrogen. It's phosphorous. This scares me, but I believe we may be able to recover it from seawater. I note that many phosphorous deposits now mined to extinction were formed by extraction from seawater, the mechanism being seabirds capturing fish and then shitting on islands, like the tragic island nation of Nauru.
There are many problems to be met with using seawater as a resource, but I think it's probably our best option left.
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