|MadSci Network: Chemistry|
It is possible to use a resonant frequency technique to put a lot of energy into the bonds of a water molecule.
One way of doing this is to use microwaves to match a resonant rotational frequency of water. That is exactly how a microwave cooker works. The microwaves are the right frequency to be absorbed by water molecules in the food you are cooking and really heat them up.
In principle, you could do the same thing with vibrational frequencies using an infrared laser. But because there is hydrogen bonding in liquid water, the different molecules are coupled together, and water molecules absorb infrared energy in broad bands of frequency, rather than at sharp resonance frequencies.
The technology for using this region of the infrared at high power is rather more difficult than for microwaves. Glass or quartz windows or containers absorb infrared light at the same frequencies you are wanting to use, so we normally work with salt windows. But salt containers do not work particularly well with water! There are other difficulties to be overcome as well.
Either of these techniques might be an "efficient" way of producing hydrogen from water, but efficiency is not the problem. Electrolysis of dilute sulfuric acid can easily be made about 70% efficient, possibly higher, and you are not going to beat that by a great deal. The real problem with producing hydrogen from water is not efficiency, but simply the total energy cost.
When you use hydrogen as a fuel, the reaction
2 H2 + O2 --> 2 H2O produces a certain amount of energy Q
When you make hydrogen from water, the reaction
2 H2O --> 2 H2 + O2 requires a minimum amount of energy Q to make it go.
In other words, if everything operated at 100% efficiency, the energy needed to produce a certain amount of hydrogen from water is exactly the same as the energy you get back by burning that hydrogen to make water.
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