It does look suspiciously like a potato battery! Those batteries consume the anode, though, so the power they "generate" actually comes at the cost of the metal you stick into them. The potato just enables you to harness the power of rusting.. According to the abstract of the paper linked in the article, though:

> The addition of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits the photocurrent, indicating that water oxidation is the primary source of electrons in the light.

If I'm reading this right, it means that, rather than consuming the iron anode like a potato battery would, the water molecules themselves are "consumed," producing hydrogen gas (and maybe oxygen gas?).

Biological systems are complex, though, and I've never even fully understood how a regular battery works (to my own satisfaction; I passed college physics courses well enough), so I could be understanding this incorrectly.

EDIT: At the risk of being even more wrong, it looks like (based on the diagram shown next to the abstract) what's happening is that the electrodes in the leaf are "short-circuiting" the normal photosynthesis process by catalyzing a reaction of NADH (an important molecule in photosynthesis) that generates capturable electricity and releases hydrogen gas as a byproduct.