Extracting Hydrogen from water

[Philip Miller Tate]

Close, but not quite there (no slight intended).

All of the energy (work) used to split H2O into H2 and O2 is 'stored'  
as potential energy in the isolated gases, ready to be released upon  
recombination. Unfortunately, the second law of thermodynamics  
demands that some of the energy stored is released as heat, because  
the product (a liquid) is less disordered than the reactants (two  
gases). Disorder, aka entropy, must increase in both the splitting  
process and the recombination process. In the splitting, the disorder  
is in the materials - liquid produces gas. In the recombination, the  
disorder is created in the surroundings as heat. If we just let all  
the energy out as heat to escape throughout the Universe, we can  
release the lot. However, if we extract the energy as useful work (we  
want an engine, not just an explosion), some has to be released as  
heat anyway to the environment. Hence, work out (usable energy) must  
be less than work in, in this and all similar situations.

The suggestion of a catalyst is a typical misdirection. A catalyst  
cannot improve the inherent efficiency of any process - it can only  
increase the rate at which a chemical process occurs. Even so, there  
is always a down side. No catalyst remains active forever, and will  
eventually need replacing. (It may be possible to regenerate it, but  
that will take more energy.) The most-recently announced fuel-cell  
cars will have this problem, and with precious metal catalysts,  
commonly platinum and/or palladium, replacement will be enormously  
expensive. Furthermore, there may not be enough catalyst metals for  
every car driver to have one...

Everything costs.

Phil M1GWZ








On 21 Mar 2010, at 19:25, Mike O'Dell wrote:

> splitting water into hydrogen and oxygen is simply a way
> of storing energy - the energy that went into splitting the
> molecule of water.
>
> the energy produced by the recombination of that hydrogen
> and oxygen is limited by the laws of thermodynamics to
> being *LESS* than the energy that went into splitting.
>
> what is going on here is mis-named in the headline;
> it is nothing like photosynthesis - it is simply electrolysis
> with the electricity coming from silicon PV cells.
>
> the contribution is a catalyst which improves the efficiency
> of water electrolysis. in normal circumstances, the process
> is not very efficient - it takes far more electrons to split
> the water molecule than will be produced in recombination.
>
> the catalyst acts by making it take less power to split
> the water molecules, so the round-trip loss is less.
> (round-trip meaning water to H2 and O2 and back to water)
>
> the net result is that for the same solar flux, more hydrogen
> is liberated than would be without the catalyst, but all of
> it is still subject to the laws of thermodynamics:
>       (1) you can't win
>    (2) you can't break even
>    (3) you can't get out of the game
>
> all of this ignores energy required to store hydrogen.
> a kilogram of hydrogen is *LARGE* at standard temp and pressure,
> while a litre of hydrogen at STP is bupkis when it comes to energy
> generation in a perfectly-efficient fuel cell (of which there
> ain't none). so storing H2 requires further energy for compression,
> further reducing the round-trip efficiency.
>
> even rocket engines that burn H2 and O2 (eg, Saturn V second and third
> stage engines and the reincarnation, the S1X for Ares) can't get
> enough density with just liquid hydrogen. it's actually stored as
> a "liquid hydrogen slushy". NASA discovered how to use nickel as a
> catalyst to make LH2 start to freeze into slush without lower
> temps or higher pressure. this increased the density of the LH2
> sufficiently to make it a viable rocket fuel.
>
> Hydrogen is just hard.
>
>    -mo
>