ANSWER: What is the best design of Hydrogen Generator

There are four popular ways to make hydrogen gas

1.  HHO.  Basically un-separated electrolysis.  Makes oxy-hydrogen mixed gas.  and it has the least BTU per cubic foot.  The most convenient, but cannot be safely pressurized and stored as the oxygen in it can cause a spontaneous explosion.  See https://bigbay4bestbuys.com/content/forget-hho-better-way-150-mpg-moder…

2.  Aluminum and Lye reaction in water.  Makes pure gas assuming the initial reaction begins in a totally aqueous enclosed environment.  Working with Lye is extremely hazardous.  The heat and reaction can only be controlled by decreasing the Lye concentration or removing  the Aluminum from the Lye bath solution.  See e.g. https://bigbay4bestbuys.com/AL_Hydrogen_Generator

3.  PEM electrolysis.  This process uses a coated Nafion  plastic membrane in contact with a mesh electrode on both sides and distilled water on the positive side.  When a current is applied Hydrogen gas only forms as a migration through the membrane and the liquid on the other side then turns to Oxygen from the broken bond of the water and is allowed to escape.  It can achieve pressures upward of 100 psi of 99.9 percent pure hydrogen.

(you can click the image for a larger view)

The serious downside is heat or a small arc can burn a small hole in the membrane making the entire system worthless.    Also this process is only of value in a heated environment where the water cannot freeze.  This happens quite often and is what "kills" these kinds of Hydrogen Generators.  You cannot see this happen and depending on the use and pressure on the Hydrogen side - may keep liquid water out of the Hydrogen side as long as the unit is running.  Eventually it floods.  .See https://bigbay4bestbuys.com/content/pem-catalyst-coated-membrane-parker…

4.  Separated Hoffman generator.  This is the most complicated to build and tune but can produce pure hydrogen from many liquid stocks safely by electrolysis.  It can also use  liquid feedstocks that can survive moderate freezing temperatures. See https://bigbay4bestbuys.com/content/which-design-hydrogen-generator-bes…

There are other ways to make hydrogen all more complicated and beyond the average peson - such a s Hydrogen stripping from a Hydro-Carbon liquid and etc.