Is energy lost to heat?...or is lost to the oxygen?
Define "efficient". In what context do you ask?
In the context of producing hydrogen for fuel cells.
According to the wiki on the subject, electrolysis can be between 50% and 80% efficient. 80% seems pretty decent to me.
Just to head off a possible area of misunderstanding: you do understand that 100% efficiency just means it requires exactly as much energy to split the hydrogen and oxygen as you get back by buring it, right? You can't ever split it without at least putting that much energy into it. This is just conservation of energy in a chemical process: the equation is (note it can go in either direction):
H2O + Energy <=> H2 + (1/2)O2
Apparently by heat and within the process used to generate the electricity , accounting for both processes yields an efficiency of < 50 % at times.
The rate of stirring as it pertains to junction potentials , the efficiency of the machine itself , area of the electrode , other reaction processes that take up the charge were all problems when I was conducting electrochemical experiments for undergrad.
Separate names with a comma.