While hydrogen is the most abundant element in the Universe on the Earth its mostly found as water. Hydrogen can be easily separated from Oxygen in water via Electrolysis. This process is about 67 efficient .Burning hydrogen combines with oxygen to form water > no other combustion products except for small amounts of nitrogen oxides formed around high temperature combustion zone .

Website: http://zebu.uoregon.edu/1996/ph162/l11.html