why hydrogen is used to store energy instead of oxygen

Hydrogen (H2)

Hydrogen molecules consist of two hydrogen atoms bonded together. When the bond in a hydrogen molecule is broken, it usually occurs as part of a chemical reaction where the hydrogen reacts with another element (often oxygen) to form a new compound. The most common example of this is the reaction of hydrogen with oxygen to form water (H2O), which is highly exothermic—meaning it releases a significant amount of energy in the form of heat:

2H2 + O2 -> 2H2O + energy

This reaction releases energy because the total energy of the water molecules produced is less than the total energy of the hydrogen and oxygen molecules that reacted. The difference in energy is released according to the law of conservation of energy.

Oxygen (O2)

Oxygen molecules, similarly, consist of two oxygen atoms bonded together. The breaking of this bond by itself (without a subsequent reaction with another element) does not release energy. In fact, energy must be supplied to break the bond (this is endothermic). Oxygen molecules participate in combustion and oxidation reactions by accepting electrons (acting as an oxidizing agent) from other substances:

O2 + 4e^- -> 2O^{2-}

In typical reactions where oxygen is involved, such as the combustion of carbon or hydrogen, oxygen helps to release energy not by breaking its own bonds and releasing energy, but by forming stronger bonds with other atoms (like carbon in CO2 or hydrogen in H2O), which releases energy.

Why the Difference?

The key difference lies in the bond energies and the nature of the reactions:

• Hydrogen bonds in H2 are relatively weak compared to the bonds that hydrogen can form with oxygen. Breaking hydrogen bonds and forming water results in a net release of energy because the new bonds in water are much stronger than the original hydrogen bonds.

• Oxygen bonds in O2 are relatively strong, and breaking them requires energy. When oxygen forms new bonds (such as in CO2 or H2O), the new bonds are stronger than the oxygen-oxygen bonds, which results in a net release of energy, but the initial step of breaking oxygen bonds does not release energy.