The electrodynamicsof nanoconfined water have been shownto changedramatically compared to bulk water, opening room for safe electrochemicalsystems. We demonstrate a nanofluidic "water-only" batterythat exploits anomalously high electrolytic properties of pure waterat firm confinement. The device consists of a membrane electrode assemblyof carbon-based nanomaterials, forming continuously interconnectedwater-filled nanochannels between the separator and electrodes. Theefficiency of the cell in the 1-100 nm pore size range showsa maximum energy density at 3 nm, challenging the region of the currentmetal-ion batteries. Our results establish the electrodynamic fundamentalsof nanoconfined water and pave the way for low-cost and inherentlysafe energy storage solutions that are much needed in the renewableenergy sector.