  The renewable energy sector critically needs low-cost and environmentally
neutral energy storage solutions throughout the entire device life cycle.
However, the limited performance of standard water-based electrochemical
systems prevents their use in specific applications. Meanwhile, recent
fundamental studies revealed dielectric anomalies of water near solid-liquid
interfaces of carbon-based nanomaterials. In contrast to the bulk water
properties, these anomalies of water under nano-confinement and in the presence
of electric fields have not yet been understood and used. Here, we
experimentally study the ability of the interfacial water layer to engender and
store charge in electrochemical double-layer capacitance. We demonstrate the
prototype of a 'water only' membrane-electrode assembly. The prototype exhibits
characteristics with a perspective of competing with existing batteries and
supercapacitors without using electrolytes as ionic carriers. The results
provide the impetus for developing high-energy-density electrochemical
double-layer capacitors and open up other avenues for ecologically-neutral
batteries, fuel cells, and nanofluidic devices.
