Question

Why does water electrolysis use direct current?

Answer 1

hydrogen and oxygen, a.c. will result in mixed generation as the anode and cathode are switching. Ideal voltage depends on the resistance of the water being used, all water is the same to be accurate but what is dissolved in that water makes it conductive, usually between 3 and 30 volts dc is my experience by way of hobby use and commercial salt water pool equipment (they use electrolysis to generate chlorine from table salt). If you're toying with it be sure to keep you voltage low and use your catalyst to regulate the current by mixing it in gradually until the desired result is achieved.