Why is it necessary to use direct current to conduct water electrolysis experiments?

First of all, we need to understand that H2 and O2 are generated as long as electricity is applied

And H2 is generated at the negative electrode and O2 is generated at the positive electrode

1. The laboratory needs to obtain pure H2, so use direct current because the positive and negative poles of DC power supply do not change, so the negative pole can collect pure H2

2. What is alternating current? This is because its positive and negative poles alternate all the time. If we use alternating current, what is collected at the two electrodes will be mixed gas H2 O2~