June 16, 2011
Skeptoid #87, titled Water: Alternative Fuel of the Future? Some claims about the electrolysis of water, into hydrogen and oxygen, involve Catalysis — a term which is sometimes invoked as if it were a magic solution to the problem. (I like this one, for example, with its description of water as a "battery"!) To dispel such nonsense, all it takes is a basic understanding of Catalysis and its limitations - which are very real, even after assuming conservation of energy.I have a few ideas for posts on the topic of Energy, and I'd like to follow up on something the other Brian touched on in
When you burn hydrogen in oxygen, a certain known amount of energy is released: you can visualize the water as having a lower energy "state" (a.k.a. "enthalpy") than the hydrogen gas, simply because energy was released as it was created. Logically, then, you might think, you could reverse the reaction, and split water in to hydrogen and oxygen, by pumping in that much energy - the process which we call electrolysis. Something like this:
Would that work? Well, no, for several reasons. The first and most obvious reason has to do with efficiency; you'll inevitably lose some energy in the process. As with conservation of energy, we'll take that as given - it's a practical problem, one we can work on, though it will never go away.
However, we're about to run in to a more fundamental theoretical problem. We've pictured the electrolysis operation as running in a straight line from the low energy state (water) to a higher energy state (H2 gas). That isn't what actually happens. Electrolysis of water, in to hydrogen and oxygen, looks more like this:
(This is simplified, and not to scale, in case there are any real chemists reading this!)
Even if we had 100% efficiency (which we wouldn't); more energy is required, to make the process work, than is actually retained in the products of the process. A good analogy might be to imagine that chart as a hill you're trying to climb to get to a destination: you have to go higher than you want to, just to get where you want to go, because there is no straight path from A to B. The scientific term for this energy "hump" is "overpotential".
This is where Catalysis comes in to play: Catalysts are substances that help other reactions along without being changed themselves. The Catalytic Converter in a car exhaust is a familiar example: the platinum coating allows further reactions to take place in the exhaust gases, using just the remaining heat in those gases - which would otherwise not occur. To show the effect of Catalysis simply on the energy chart, we flatten out the "hump" so it's easier to "climb over":
This shows (in outline) how a catalyst can significantly reduce the overpotential required for an electrolysis reaction. As you might imagine, many scientists are currently working on catalysts that will make a significant difference to the electrolysis of water in to hydrogen and oxygen - see here for an example. There are additional complications that can arise when the water is not pure, such as sea water (which contains chlorine compounds).
In summary, though, it's worth re-iterating that Catalysis, while potentially extremely useful, does not offer any way around the fundamental energy requirements associated with electrolysis: energy is still conserved, and you won't get more energy out than you put in. What it can offer, however, is greater efficiency, and a way of using free energy sources (such as Solar energy) where it was not possible to do so before, simply because it can reduce the overpotential problem. The first company to enjoy a breakthrough in this field could make a mint, but it will take more than some speculative claims about "Brown's Gas" to make it work. Catalysis, within its limitations, is not "snake oil" - which makes it all the more annoying when the word is used to peddle "snake oil".
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