Two of humanity's most ubiquitous historical materials, cement and carbon black (which
resembles very fine charcoal), may form the basis for a novel, low-cost energy storage
system, according to a new study.
The technology could facilitate the use of renewable energy sources such as solar, wind,
and tidal power by allowing energy networks to remain stable despite fluctuations in renewable
energy supply.
The two materials, the researchers found, can be combined with water to make a supercapacitor
— an alternative to batteries — that could provide storage of electrical energy.
As an example, the MIT researchers who developed the system say that their supercapacitor could
eventually be incorporated into the concrete foundation of a house, where it could store
a full day’s worth of energy while adding little (or no) to the cost of the foundation
and still providing the needed structural strength.
The researchers also envision a concrete roadway that could provide contactless recharging
for electric cars as they travel over that road.
The simple but innovative technology is described in the journal PNAS.
Capacitors are in principle very simple devices, consisting of two electrically conductive
plates immersed in an electrolyte and separated by a membrane.
When a voltage is applied across the capacitor, positively charged ions from the electrolyte
accumulate on the negatively charged plate, while the positively charged plate accumulates
negatively charged ions.
Since the membrane in between the plates blocks charged ions from migrating across, this separation
of charges creates an electric field between the plates, and the capacitor becomes charged.
The two plates can maintain this pair of charges for a long time and then deliver them very
quickly when needed.
Supercapacitors are simply capacitors that can store exceptionally large charges.
The amount of power a capacitor can store depends on the total surface area of its conductive
plates.
The key to the new supercapacitors developed by this team comes from a method of producing
a cement-based material with an extremely high internal surface area due to a dense,
interconnected network of conductive material within its bulk volume.
The researchers achieved this by introducing carbon black — which is highly conductive
— into a concrete mixture along with cement powder and water, and letting it cure.
The water naturally forms a branching network of openings within the structure as it reacts
with cement, and the carbon migrates into these spaces to make wire-like structures
within the hardened cement.
These structures have a fractal-like structure, with larger branches sprouting smaller branches,
and those sprouting even smaller branchlets, and so on, ending up with an extremely large
surface area within the confines of a relatively small volume.
The material is then soaked in a standard electrolyte material, such as potassium chloride,
a kind of salt, which provides the charged particles that accumulate on the carbon structures.
Two electrodes made of this material, separated by a thin space or an insulating layer, form
a very powerful supercapacitor, the researchers found.
The two plates of the capacitor function just like the two poles of a rechargeable battery
of equivalent voltage: When connected to a source of electricity, as with a battery,
energy gets stored in the plates, and then when connected to a load, the electrical current
flows back out to provide power.
Supercapacitors made of this material have great potential to aid in the world’s transition
to renewable energy.
The principal sources of emissions-free energy, wind, solar, and tidal power, all produce
their output at variable times that often do not correspond to the peaks in electricity
usage, so ways of storing that power are essential.
There is a huge need for big energy storage, and existing batteries are too expensive and
mostly rely on materials such as lithium, whose supply is limited, so cheaper alternatives
are badly needed.
That’s where this new technology is extremely promising, because cement is ubiquitous.
The team calculated that a block of nanocarbon-black-doped concrete that is 45 cubic meters in size — equivalent
to a cube about 3.5 meters across — would have enough capacity to store about 10 kilowatt-hours
of energy, which is considered the average daily electricity usage for a household.
Since the concrete would retain its strength, a house with a foundation made of this material
could store a day’s worth of energy produced by solar panels or windmills and allow it
to be used whenever it’s needed.
And, supercapacitors can be charged and discharged much more rapidly than batteries.
After a series of tests used to determine the most effective ratios of cement, carbon
black, and water, the team demonstrated the process by making small supercapacitors, about
the size of some button-cell batteries, about 1 centimeter across and 1 millimeter thick,
that could each be charged to 1 volt, comparable to a 1-volt battery.
They then connected three of these to demonstrate their ability to light up a 3-volt light-emitting
diode (LED).
Having proved the principle, they now plan to build a series of larger versions, starting
with ones about the size of a typical 12-volt car battery, then working up to a 45-cubic-meter
version to demonstrate its ability to store a house-worth of power.
There is a tradeoff between the storage capacity of the material and its structural strength,
they found.
By adding more carbon black, the resulting supercapacitor can store more energy, but
the concrete is slightly weaker, and this could be useful for applications where the
concrete is not playing a structural role or where the full strength-potential of concrete
is not required.
For applications such as a foundation, or structural elements of the base of a wind
turbine, the “sweet spot” is around 10 percent carbon black in the mix, they found.
Another potential application for carbon-cement supercapacitors is for building concrete roadways
that could store energy produced by solar panels alongside the road and then deliver
that energy to electric vehicles traveling along the road using the same kind of technology
used for wirelessly rechargeable phones.
A related type of car-recharging system is already being developed by companies in Germany
and the Netherlands, but using standard batteries for storage.
Initial uses of the technology might be for isolated homes or buildings or shelters far
from grid power, which could be powered by solar panels attached to the cement supercapacitors,
the researchers say.