That is a very nice potato battery. With exactly those metal foils I'd expect you to instantly double output by using lemons instead. With lemons for acid and six cells in series, you should be able to get an LED to light.
Back to the topic which called me to this thread,
https://www.youtube.com/watch?v=5B6icvUBNzE
Parts of the presentation look not quite right but it does mention some interesting trials.
The University of Queensland design looks like a perfect swap-in fit for a CR3032 'coin cell' so rather than discourage their efforts I'll say that they seem to have a rechargable little battery which could be tested.
The MIT group also propose something interesting. If theirs contains sulphur then there are sure to be ways to get that to burn. I did not see enough metallic Aluminium in the explanations of either type, so they can't have high capacity.
For claims like these, I say always test.
Now, from the wizzo227 preposterous things which might blow up department, pick a fresh lemon and insert a foil of pure nickel into a slot carefully cut into the top left. Insert a slab of pure potassium into the top right. That should get a higher voltage than the potato experiment, until it blows up.