What if the concrete that holds up our cities could also store energy? MIT's ECO carbon concrete embeds a fractal network of carbon at the nanoscale that turns cement into a supercapacitor. In this episode we explain how hydration wires the network together, how researchers mapped it with FIB-SEM tomography, and how electrolyte choice—from organic quaternary ammonium salts to seawater—drives performance. We unpack the dramatic tenfold energy-storage leap (from tens of cubic meters to power a home down to a few cubic meters, even a fridge on 1 m^3), the prototype arch that powered an LED, and the surprising self-monitoring behavior under load. We weigh benefits against challenges: energy density gaps vs lithium-ion, safety and long-term stability of electrolytes, safety and demolition, and the sustainability upside of turning cement production’s footprint into a grid-friendly asset. Finally, we pose big questions for the design of future structures: how do you maintain or replace battery components inside a centuries-long building? And what changes must architects and engineers make when the foundation itself is part of the energy network.

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