It's more like using them as an electrolyte, but it acts like a battery (you charge by adding waste, discharge by extracting electrons), I guess? You run the bioreactor with some special electrodes that bacteria like to touch, and then directly harvest electrons from the medium as the little ones grow.

Mohammed and Melania (the lead researchers on the paper) are both super talented scientists. I had the pleasure of meeting them both in SF last year when they were on the west coast for a conference.