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.
The original paper says you'd need 45 cubic meters of this stuff to store the daily energy for one house, which is about 60 yards of concrete. But even a relatively thick 5" pad that's 1500 square feet only has 23 yards of concrete in it.
So they'd have to improve the energy density by 3x before this is commercially viable.