Basic Theory of Hydrogen Storage

Several chemical principles must be understood to understand how hydrogen storage and fuel cells work.

Bonding

Water (H2O) is made up of hydrogen (H) and oxygen (O). It exists as water because this is the most stable form which means it has the least energy. Hydrogen and oxygen release energy when they are combined to form water. To separate water back into its components, energy must be added. Bonding occurs to form the most stable compound (compound with the least energy). Hydrogen and oxygen by themselves are most stable as the gases H2 and O2.

Atoms

Atoms make up elements (like hydrogen and oxygen) and combine to form compounds. Some compounds are H2 (two hydrogen atoms), O2 (two oxygen atoms), and H2O (an oxygen atom bonded to two hydrogen atoms). Bonding occurs through sharing or trading electrons. Electrons are negatively charged particles surrounding the nucleus of the atom. The nucleus is made up of positively charged protons and neutral neutrons. This leaves the nucleus with a positive charge that cancels the negative of the electrons if the number of protons and electrons are equal. However, when compounds are formed this balance is disturbed.

Types of Bonding

Water can demonstrate both forms of bonding. Generally water is covalently bonded where electrons are shared. The atoms are held together because all the same electrons are surrounding the different nucleus. In water, the oxygen attracts the electrons more than the hydrogen. This causes the hydrogen end of the molecule to be slightly positive and the oxygen end to be slightly negative. A molecule with charged ends, or poles, is called polar.

Small amounts of water become so charged that the electrons totally leave the hydrogen and go to the oxygen leaving OH- and H+ ions. Ions are atoms with a charge. These are still bonded but ionically because one of hydrogen's electrons has been traded to the OH-. The OH- has an extra electron making the charge negative, and H+ is missing an electron making the charge positive. These opposite charges attract like the opposite ends of a magnet; this is called ionic bonding.

Electrochemistry

When water divides into its ions an oxidation-reduction reaction takes place. Production of OH-, where an electron is gained, is called reduction. Production of H+, where an electron is lost, is called oxidation. Since H20 produces both it was oxidized and reduced. The part that became H+ was oxidized and is called the reducing agent because it produced the electron used in reduction. Similarly, the part of water that became OH- was reduced and is called the oxidizing agent because it took the electron which oxidized the other part of water. Usually this occurs randomly within a sample of water.

However, the oxidation half of the reaction can be separated from the reduction half of the reaction. The electron trade still must occur, so the locations of each half-reaction is connected by a wire and some kind of porous material also connects the two half reactions to allow for the positive ions to flow to keep this circuit running. If the two reactions produce more stable products an electric flow will occur. At one end with an electrode called the anode, the reducing agent is oxidized and produces an electron that goes across the wire and produces a positive ion that passes through the porous material. At another electrode (the cathode), the electrons are added to the oxidizing agents which produce negative ions.

Electrolysis

When the products are not more stable in the new form, electricity must be added to the system to force the reaction, called electrolysis. This is the process where water is forced into hydrogen and oxygen. Enough electricity is added to break the chemical bonds in the water. The electricity could be from the electricity from a battery, an outlet, a photovoltaic solar cell, a windmill, fossil fuel driven turbine, or anything. This electricity is "stored" in the situation where the water is separated into less stable compounds that contain more energy.

The water is divided by the following reactions:

Here the water at the anode is oxidized because it loses electrons. Water at the cathode takes these electrons and becomes reduced. The total reaction shows the products are hydrogen and oxygen gas and the H+ and OH- combine back to form more water.

This process is commonly 65-67% efficient, but 80-85% efficiency is possible. Unfortunately, cryogenically cooling the gas into a liquid is very energy intensive and makes the entire process of electrolysis, cooling, and combusting very inefficient (25% efficiency).

The electric energy is now stored in the chemical energy of the bonds in hydrogen and oxygen. This is how the energy can be stored from any electric generating source (solar cells, windmills, dams, fossil fuel, or anything else that makes electricity). The energy can be retrieved from the hydrogen and oxygen in two different ways. One is by fuel cells where basically the electrolysis process is reversed. The other way is combustion.

Combustion

Combustion is a chemical reaction where a substance is violently reacted with oxygen gas producing a great deal of energy. The combustion of hydrogen gas follows this reaction:

Notice, the only product is water and energy. The only negative is that the great energy produced, heats the surrounding air to make nitrogen oxides. This process is simple, but fuel cells are more controlled and produce a higher efficiency.

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