What kind of cellular structure converts hydrogen and oxygen to water and vice-versa. If have calculated the energy released by the combustion of oxygen and hydrogen to be 572 kJ enough to create 22 ATP molecules from ADP. If it is useful the atmospheric composition is: 0% CO2 20% O2 20% H2 50% N2 9% Xe 1% Kr
You cannot really have hydrogen breathing life forms, at least not in the way you want. Hydrogen in elemental form is way too reactive and thus not part of the environment. In fact, oxygen in its elemental form is too reactive, and the only reason it exists in the atmosphere is that it is a byproduct of photosynthesis.
The issue with your calculation is that you also need 572 kJ of energy to separate hydrogen and oxygen from water. And using energy from the sun to create hydrogen and then use it to create ATP is kinda inefficient. And life as a rule doesn't particularly like inefficiency.
If the question is how do they separate oxigen and hydrogen to breath hydrogen then : Some kind of electrosis process that would occur naturally, or natural reverse osmosis.
If it is molecular structure: its all theory so there really isnt anything right or wrong, its like asking what the best hot beverage might be, its Opinion based and grounds for Closure. (Not enough repo to vote/see closure pools)