Before I present some facts, let's look at five sources of confusion surrounding water and hydrogen:
1st source of confusion: Water can be construed as a fuel in the context of nuclear fusion. However, that is not the context of this discussion, nor is it the context within which believers of water as a fuel are promoting their ideas. I have observed, though, that many people make statements regarding the enormous amount of energy stored in water in the incorrect context. For example, they will state that there is enough energy stored in a gallon of water to supply all the electrical needs of the U.S. for one year (which may be true in the context of theoretical nuclear processes, i.e. e = mc2) and then they will make a statement regarding electrolysis.
The chemical process of combusting hydrogen gas, which was manufactured by the electrolysis of water, is in no way related to the nuclear process which converts mass to energy. The amount of energy released by the former is infinitesimal when compared to that of the latter.
2nd source of confusion: The word hydrogen is commonly used to refer to two different things.
Hydrogen is used to mean both the hydrogen atom and the diatomic hydrogen molecule, H2, which is hydrogen gas. (At very cold temperatures it exists as liquid hydrogen.) Both uses are correct, but they refer to very different things. Unfortunately, many people fail to make the distinction and that leads to mistaken conclusions.
For example, "Water contains hydrogen and hydrogen is a fuel," is a misleading statement because it implies that water contains a fuel. It does not. Water contains hydrogen atoms (within the water molecules), not diatomic hydrogen molecules. Hydrogen molecules are a fuel, but the hydrogen atoms in water are not.
3rd source of confusion: The word "separate," as used to describe the process of electrolysis, is misleading.
"Separate" has historically been used by some texts to state that water is separated into hydrogen and oxygen, meaning hydrogen and oxygen molecules. Though they are the final result, stating that they were separated implies that they were formerly together, which is not true. There are no hydrogen or oxygen molecules in water.
4th source of confusion: The chemical energy in a hydrogen molecule comes from the electrical bond between the two atoms of hydrogen. The bonds between the hydrogen and and two oxygen atoms in a water molecule have no available chemical energy.
5th source of confusion: A lot of reputable people are preaching something that is not true.
I have no doubt that many of these people are well-meaning and sincere. The confusion surrounding water as a fuel has historically been one of the most prominent instances of pseudoscience since electrolysis was discovered, and has pulled in everyone from the science fiction author Jules Verne to prominent politicians and successful businessmen.
When there is so much potential for confusion combined with so much hope for a better source of energy, conditions are ripe for intelligent people to make premature conclusions, especially if many others have gone down that road before them. Thinking is hard. It is much easier to believe in what you want to be true, than to set aside hopes and honestly dig for the truth.
Now that you've had a chance to ponder some of the sources of confusion that are producing fiction, let's take a fresh look at the facts:
Hydrogen atoms are rarely found alone, at least not for long. A single hydrogen atom is very unstable and will always form a bond with one or more other atoms.
Like all atoms, hydrogen atoms can exist at different energy states, depending on what other atom(s), if any, they might be bonded to. The type of bond determines the atom's energy state.
A molecule is made up of two or more atoms bonded together. They can be the same atom, as in the diatomic hydrogen molecule, H2, or they can be made up of two or more different kinds of atoms, as in water, H2O.
Many molecules contain hydrogen atoms.
Indeed, hydrogen is the most abundant element in the universe.
Chemical energy is stored in a molecule if the bonds of that molecule can be broken, allowing the atoms to release energy and move to a lower energy state by forming new and different molecules with lower energy bonds.
Electrolysis of water requires an energy input. It requires energy to break the low energy bonds between the hydrogen and oxygen atoms found in water and make higher energy bonds between the two hydrogen atoms in a molecule of hydrogen gas. Another way of stating this is that energy is required to boost the two hydrogen atoms to the higher energy state required to leave their bond with an oxygen atom and form a bond between each other.
Combustion releases energy. The chemical reaction that breaks the high energy bond between two hydrogen atoms and allows them to move to a low energy state by bonding with an oxygen atom releases energy.
Hydrogen atoms in water are not fuel. The hydrogen atoms in water are at a low energy state and there is no chemical reaction available to recombine them into molecules with even lower energy.
Hydrogen molecules are fuel. Hydrogen gas, H2, is made up of diatomic (two atoms) molecules of hydrogen atoms. The bond between these two hydrogen atoms is very high energy and unstable. As such, these molecules are prone to quickly reacting with an oxygen molecule and giving up their energy. Hydrogen gas can be burned in an internal combustion engine or it can be oxidized in a hydrogen fuel cell. Either technology can and has been used to fuel cars, airplanes, etc.
There are no hydrogen molecules in water. Water does not contain hydrogen gas. It contains hydrogen atoms bonded to oxygen atoms, otherwise known as H2O. Breaking these bonds does not release any energy. In fact, it requires energy, in the form of electricity, to do so.
There are (almost) no hydrogen molecules (H2) on Earth. Although hydrogen gas is abundant in stars, it is rare on Earth. The Earth's atmosphere was once rich in hydrogen gas, but all of the hydrogen atoms in those molecules have long since bonded with other elements and are mostly locked up in hydrocarbons and water, which we have in abundance. All the hydrogen we use we must somehow manufacture, using an energy source in the process, from something else containing hydrogen atoms.
Hydrocarbons are fuels. Hydrogen atoms are commonly bound to carbon atoms in many forms. The resultant hydrocarbon molecules are fuels because they have high energy bonds that will break and give up their energy when combined with oxygen. This happens when gasoline is combusted in an engine and when carbohydrates are digested in the body.
Hydrocarbons occur naturally. They exist in many forms, and are stored in fossil form as coal, crude oil, shale oil, natural gas, frozen methane on the sea floor, etc. They are constantly being produced as plant material via photosynthesis.
Hydrocarbons can be manufactured. Hydrocarbons such as gasoline, propane, and hydrogen gas are refined from fossil hydrocarbons. Hydrocarbons are commonly manufactured from other hydrocarbons, such as the production of ethanol from corn, and donuts from flour and sugar and grease (all hydrocarbons).
The product of combusting hydrocarbons is energy and a combination of many other molecules. These can include water vapor, nitrous oxides (air pollution), and carbon oxides (greenhouse gases.)
The product of combusting hydrogen gas is only energy and water vapor. This obviously has environmental benefits.
Chemically, this is what happens when hydrogen gas is combusted:
H2 + ½ O2 —› H20 + energy (237 kJ)
Note that no energy is required to sustain this reaction, though a spark or flame is required to initiate the process. Once this reaction begins, it requires no energy input, and supplies about 237 kJ worth of energy per hydrogen gas molecule. If this happens in an efficient internal combustion engine, some of the energy is converted to mechanical energy and is available to do work, but most (typically 70 to 80%) gets lost as heat. If it happens in a hydrogen fuel cell, most of it is available as electrical energy (which must still be converted to mechanical energy via an electric motor), but some (typically 10-20%) still gets lost as heat.
Hydrogen gas can be manufactured from fossil fuels. This is the cheapest and most common method used to manufacture hydrogen gas. It is a win-win situation because in fossil fuels the hydrogen atoms needed to make hydrogen gas are bound up in molecules (in this case hydrocarbons) which contain more energy chemically locked in them (in the form of high-energy electrical bonds), than is required to extract the hydrogen atoms and put them back together as hydrogen gas molecules. So the net result is a product that contains more energy than it took to produce it. This is possible because the raw material was a fuel and had chemical energy stored in it to begin with.
Hydrogen gas can be manufactured from water. This is not a cheap or common way to produce hydrogen gas because it is not an energy gain scenario. This is because the process of production of hydrogen gas via electrolysis of water, unlike refining it from methane, requires more energy than is available from combusting the hydrogen that was produced. Electrolysis is chemically the inverse of the process of combustion of hydrogen gas.
H20 + energy (237 kJ) —› H2 + ½ O2
In theory, it takes the same amount of energy to manufacture hydrogen gas from water as is then available from the combustion of the resultant gas. In practice, the situation is much worse, and is only about 20 to 40 per cent efficient. This means you are using 10 kilowatts of energy to produce something that contains only 2 to 4 kilowatts of energy.
Water is not a fuel. There is no chemical energy stored in water. There is no chemical energy stored in the hydrogen atoms in the water. Water cannot be burned or oxidized to produce heat or energy.
There is no energy available, "locked-in" or otherwise, in water. There are only hydrogen atoms available in water, not hydrogen molecules, and if you want to use them as building blocks to make hydrogen molecules you have to supply some energy to break the bonds in the H20 molecules and put the H atoms back together with newer higher energy bonds as H2 molecules. And, as can be seen from the stoichiometric equations above, all you can possibly get back when you burn this hydrogen gas is no more than you put in when you manufactured it. In practice, due to unavoidable losses in the conversion processes, it is much less.