Hydrogen electrolysis and combustion cycle in hydrogren on demand engines?

Hello! Recently learned about some people using hydrogen on demand in their hydrogen engines, in which they use electrolysis to generate hydrogen from water which they can then use in the combustion cycle, whitout having to deal with the problems of storing hydrogen. My question is, if we at the same time electrolise 2H20 into 2H2+02, and later combust it from 2H2+02 back into 2H20. Isnt this just an infinite cycle that means that we cant produce any net positive amount of energy?