Using the electrical potential created by the electron transport chain to power electrical systems My friend and I were discussing the extremely low energy density of modern batteries and read that food has a much higher energy density. Thus, we were considering the possibility of using biological processes to extract energy from food and use that to create an electric potential or do work in some other way.

The electron transport chain creates an electrochemical gradient and thus and electric potential using protons. If some sort of system could be devised to mimic/perform glycolysis and the citric acid cycle and also mimic the ETC membrane and necessary proteins, is it feasible/is there already research on using glucose and cellular respiration (or a similar biological process) to power electrical systems? How would this relate to fossils fuels and what scale could this be useful at?
There are fuel cells that run on alcohols such as ethanol or methanol and on glucose. 

That said, the energy density in current Li-ion cells is quite high - a 18650 cell holds ~30,000 joules, about 9 kCal. By comparison, a  "biochemically" powered shotgun shell (well, the nitrate and carbon of gunpowder can come from bio sources) packs ~2,000 J.