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Q & A: how does water electrolysis work?

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Q:
I understand the process of water electrolysis - that water/an electrolyte can be decomposed into hydrogen and oxygen via an external energy source (an electrical current). I know that the reduction of hydrogen takes place on the cathode and the oxidation reaction takes place on the anode. Electrons are pulled from the water in this process. I also know that water, though hydrogen bonds are strong, are already partially split into H+ and OH- (though there are very few of these ions in pure water). So what is it that actually allows the electric current to split water? Why do the water molecules sparate/transfer electrons simply because of the current?
- Andie (age 20)
Mukilteo, WA, USA
A:
Think of the two electrodes with a voltage difference between them. That means that there's an electric field in the water between them. So any H+ ions get pulled one way, OH- ions get pulled the other. Look at the electrode where the H+  ions gather. It has a negative voltage- too many electrons. Electrons can leave the electrode and join up with a pair of H+ ions to make the H2 molecule. The reaction over where the OH-'s gather is a little more complicated, but by donating electrons to the positive electrode they can form water molecules and O2 molecules.

This process doesn't use up the ions, because more H2O water molecules keep falling apart to make up more H+ + OH- ions. When there's no voltage on, that's also happening but it's balance by ions recombining to make H2O.

The process gets a bit more complicated if there are other ions around, but this simple example may get you started.

Mike W.

(published on 04/10/2012)

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