Thread: Electrolysis

Can the process of electrolysis (of water) be sped up by a catalyst? Could the reaction be made more efficient by one?

Originally Posted by The P-manator

Can the process of electrolysis (of water) be sped up by a catalyst? Could the reaction be made more efficient by one?

I think you could...as I recall you have to use significantly more voltage than is strictly necessary when doing electrolysis on water because there's a high activation energy, and it goes very slowly if you don't apply extra voltage. A catalyst could potentially lower the activation energy and allow you to do it with less voltage.

Note that this wouldn't change the absolute energy requirements for electrolysis of water.

This US patent may be of relevance to you.
http://www.freepatentsonline.com/6872286.html

Note that this wouldn't change the absolute energy requirements for electrolysis of water.

How then can I do that, and why doesn't it?

Originally Posted by The P-manator

How then can I do that, and why doesn't it?

Here's the long answer:

There are two things that you need to look at when considering the energy of chemical reactions: the activation energy and the enthalpy. The activation energy is (like the name describes) the energy required to start the reaction. The most intuitive example of this is lighting an explosive. An explosive will sit harmlessly until you provide it with enough energy to start the explosive reaction – perhaps with a spark. The reactants will initially absorb the activation energy, then release it back out by the time the reaction is completed. Reactions with a lower activation energy go faster.

Enthalpy is the internal energy of the reactants and products. If the products have a lower enthalpy than the reactants, energy will be released. The amount of energy released will be the difference between the enthalpy of the reactants and the enthalpy of the products. Similarly, if the products have a higher enthalpy than the reactants then the reaction will absorb energy.

That’s why you can use electrolysis of water to store energy. Hydrogen and oxygen gas have higher enthalpy than water, so when you turn water into H2 and O2 with electrolysis you add energy (enthalpy) to it. The added energy comes from the power source you're using for the electrolysis. When you turn it back into water (either with a fuel cell or by burning it) you release the enthalpy that you added with electrolysis. Note that you can’t get more energy out than you put in in the first place. Sadly, most people (including me) agree that it is impossible to produce energy by electrolysis, since you can’t get more energy out than you put in. At best, you can use it as a way to store energy.

A catalyst can only change the activation energy required to start a reaction – it can’t affect the overall enthalpy change in a reaction. Lowering the activation energy means that the reaction will start easier and go faster, but the energy change in going from reactants to products will be the same (since the activation energy is released back to you anyway when the reaction finishes).

But let's say if I electrolyse the water and I use x amount of electricity to start it without a catalyst, and then I use a catalyst (and if a catalyst lowers the energy needed to start a reaction), wouldn't I make more hydrogen with less electricity?

Originally Posted by The P-manator

But let's say if I electrolyse the water and I use x amount of electricity to start it without a catalyst, and then I use a catalyst (and if a catalyst lowers the energy needed to start a reaction), wouldn't I make more hydrogen with less electricity?

No. It would just allow you to make the same amount of hydrogen faster. Current would drain from your battery (or whatever you were using) faster, because since you were making H2 and O2 faster you would be drawing energy from your power source faster. But the total amount of energy needed to make a given amount of hydrgen wouldn't change. When you applied the catalyst you would see your rate of hydrogen generation suddenly increase, but there would be a coresponding energy drain from your power source.

Catalysts only allow reactions to go faster - they don't affect the overal enthalpy change of a reaction.

Remember - the energy that's released when you burn hydrgen is the energy that you put into it when you made it. That's why you can get energy from it; because you put the energy there. That's also why you can't get out more than you put in - once you drain the energy from the H2 and O2 that you put in, there's nothing left.

But I could make more hydrogen in a less amount of time.

Originally Posted by The P-manator

But I could make more hydrogen in a less amount of time.

Yes, that's correct.

Thank you very much. That'll help me design my fuel cell system.