voltaic couple

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Two dissimilar conductors in contact or in the same electrolytic solution, resulting in a difference of potential between them.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The pair of plates (made of different metals or other conductive materials), and separated by an electrolyte, which form a voltaic cell (in a battery, for example).