Ultra-thin layers of rust generate electricity from flowing water

This just popped up on my Phys.org feed and it’s frigging extraordinary.

In a nutshell, some geniuses at CalTech have discovered an entirely new way to generate electricity. Turns out when salt water moves across a very thin layer of iron oxide, it induces a current. The physics are way more complicated than that, but that’s the net effect.

Ten square meters of the stuff in a surf zone could power a household. At the limit… imagine a few skyscraper-sized Menger sponges of nano-rusted iron thrust into the Gulf Stream, powering the Eastern Seaboard.

Ocean currents and tides are driven by masses in motion at a planetary scale. From a human perspective they are a limitless source of energy, with salt water as its operating fluid.

This discovery promises to tap directly into that source… simply, passively, with no moving parts. Having a really hard time seeing a down side here.

Tell me why I'm full of shit!