MIT engineers created a carbon-cement supercapacitor that can store large amounts of energy. Made of just cement, water, and carbon black, the device could form the basis for inexpensive systems that store intermittently renewable energy, such as solar or wind energy.
You’re assuming they can repurpose structural concrete with this stuff. It’s highly unlikely that this capacitor material could be structural. If it’s not a strength concern then it’ll certainly be an efficiency one. I doubt you want metal things and people walking on your capacitor.