Change the frequency of AC voltage from 60 Hz to 2 Hz
According to the information given to the answer with links in a question in the chemistry stack exchange, you probably don't want AC, but rather pulsed DC. The pulses can be all of one polarity or both positive and negative, but the positive and negative voltages are not equal. If you want to design a power conversion circuit to do that, you should first define the waveform completely including the voltage, current, pulse width, pulse frequency and range of adjustment. Then do some research on power conversion circuit design.

You may be able to buy a packaged plating power supply. A VFD is not appropriate for this application.