DC to DC conversion is somewhat more complex than AC conversion. If you know the resistance of your load, you could calculate a series resistor to give you a voltage of 2.2 at your load using ohms law. (be sure to make sure the resistor is capable of the power dissipation!) For electrolysis, water generally has a fairly high resistance, so 2.2 volts may not be enough to force much current through it. With electrolyis, the amount of reaction depends on the current, rather than the voltage. If you are comfortable working with mains voltage, here is a website describing the modification of a transformer from a microwave oven to make a power supply for electrolysis: http://www.geocities.com/CapeCanaveral/Campus/5361/chlorate/winding.html but please only consider this if you have the appropriate electrical knowledge for working with potentially deadly mains! Otherwise, have a look in the classifieds in the paper, or any surplus electrical equipment places for low voltage bench top power supplys, these often have a variable voltage, and are good for small electrolysis experiments. Good Luck!