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I was trying to use a $500~\mathrm V$ AC power supply to split water, but nothing happened. I tested everything with a $9~\mathrm V$ battery, and it all worked fine.

Why did it not work with the high voltage? (Do not worry, I was taking the proper precautions, such as insulation and three pairs of gloves, and I did not touch any live wires.)

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From the comments, mcocdawc:

Was it DC or AC?

OP:

AC. I was trying to see if it was possible.

There's your problem. Alternating current spends half its time pushing electrons forward through the circuit, and half the time pushing them in reverse through the circuit. Thus, on average the net current pushed through the cell is zero.

So, leaving aside any metal dissolution/deposition, you were forming tiny, tiny amounts of electrolysis gases at the surface of your electrodes while there was current moving through the cell in one direction, but immediately consuming them again when the current reversed. The total number of coulombs passed in each direction was presumably so small that no gas bubble nucleation occurred before the polarity reversed.


Regardless: STOP USING $\mathbf{500V}$ FOR ELECTROCHEMISTRY EXPERIMENTS. It's way more potential than you need, and terrifically unsafe. In most cases, anything above $40~\mathrm V$ is overkill, and for most experiments that $9~\mathrm V$ battery you mentioned will more than suffice.

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