Many demonstrations of electrolysis show the use of baking soda and a 9 volt battery for electrolysis of water. As a science experiment, I tested different voltages by adding more batteries. In my experiment, I used distilled water. After letting the electrolysis happen for 5 minutes, the 18 volt test had yellow discoloration in the water. Because I couldn't find any places online that mention this or explain what it is, I thought to ask. Perhaps it is not a common thing?
[Edit] Answer: Most likely cause was degradation of poor electrode material with too much electricity.