According to the University of California, they consist of positive and negative electrode layers separated by a plastic separator a thin piece of plastic with little holes in it.
When the oil and gas reached the rig, they were diverted not overboard, as might have been wiser, but to a system called the mud-gas separator which spewed gas back onto the rig.
But for the process to result in a working battery, all five layers must stick together and work in synchrony, and the tricky step was finding a separator material that kept the whole stack in one piece.
To pull this off they took advantage of a number of recent engineering advances reduced anode-cathode spacing, evolved microbes and novel separator materials though the fundamental process is pretty straight-forward: special bacteria (those evolved microbes) are introduced to organic waste material.