Printed into a staircase slab that carries 23 megapascals of structural load without cracking, a concrete block etched with carbon fingers stores electricity on its own, which means a building’s stairs can double as its battery
The slab on the bench looks like scrap from a stairwell pour.
Flip it over and a tight grid of black interlocking fingers runs across one face.
They were laid down in a paste of carbon nanotubes and carbon black, mixed straight into wet concrete.
Supercapacitors store less energy than batteries but take it in and release it fast, for millions of cycles, which matters for a slab tied to daily solar cycling.
A small LED near the edge glows when the two halves are wired together.
So what is actually happening inside that gray block to make a light turn on?
Two interlocking combs, and why the short gap is everything
The fundamental problem with earlier cement storage devices was geometry. Stacking electrodes in layers forces ions to travel the full thickness of the slab, and that trip only gets slower as the material grows thicker.
So researchers solved it by printing two sets of electrodes as interlocking combs on the same face rather than stacked in layers. Because neighboring fingers sit a very short distance apart, ions only cross a narrow gap between teeth, and the cement itself, once its pores filled with water and ions, became the medium they moved through.
That arrangement also makes the whole piece monolithic, since electrodes and separator form one continuous cement based structure, removing the weak interface where separately assembled layers tend to pull apart over time.
Carbon ink on a concrete slab, and what the printer actually deposited
The researchers mixed carbon nanotubes, carbon black and cement into a printable ink, then deposited it onto a small concrete slab in an interlocked finger pattern using direct ink writing, which extrudes thick paste through a nozzle much like a piping bag forces icing. No special curing oven or photochemical step is needed.
Once the ink was in place, the cement set normally while water and ions filled its pores, carrying charge between the fingers. The nanotubes gave the printed lines high electrical conductivity, and carbon black filled the gaps to form an unbroken conductive web through each tooth.
That is why the result charges and discharges not through a slow chemical reaction like a lithium ion cell, but by physically rearranging ions at a surface. Supercapacitors store less energy than batteries but take it in and release it fast, for millions of cycles, which matters for a slab tied to daily solar cycling.
What the measurements showed in the laboratory
The printed piece stored 162 millifarads of charge per square centimeter, measured at 0.46 milliamps per square centimeter across a 1 volt window, while its compressive strength held at 23 megapascals, comparable to commercial concrete. Both figures matter equally, since a layer that cracks under a floor load is not a building material, and one that cannot hold a charge is not a battery.
Three devices printed on the same slab and wired together lit a small row of LEDs, a modest demonstration by grid standards. The point was not to power a city block: emergency lighting, occupancy sensors, smart locks and fire alarm nodes are exactly the low draw devices a building runs continuously, the kind a stairwell slab could plausibly feed from rooftop solar.
Corresponding author Jing Zhong put it plainly: “If renewable energy is available to recharge the supercapacitors frequently enough, they could meet some energy needs through repeated charging and discharging.” That conditional matters, since the technology is a distributed buffer that trims peaks rather than a replacement for grid power.
Where the device struggles, and what still needs solving
The most honest number in the paper describes a limit rather than a triumph. The device kept working through moderate heat and cold, but near minus 18 degrees Celsius the electrolyte began to freeze and performance dropped sharply.
The next task, researchers say, is making the devices hold up in freezing conditions, which restricts near term deployment to temperate and warm climates unless the electrolyte chemistry is reformulated.
There is also the question of scale: the published slab is small enough to handle on a bench, while pouring a full staircase flight or road surface with hundreds of interlocking electrodes at consistent spacing is a manufacturing problem not yet resolved.
Even so, the work fits a broader pattern of generation hardware moving into unexpected geometries, the way raindrops moving through narrow tubes delivered a steep efficiency gain by rethinking the container rather than the fluid. The logic here is similar: the building was already there, so make it carry two jobs at once.
What it opens for buildings that already store solar power
The most important implication is not the device itself but the design permission it grants. A structural engineer who once set aside floor space for a battery cabinet now has a research backed reason to treat the floor slab as a candidate for that role.
So by building these devices into cement, researchers hope solar power generated near a building could be stored inside the structure itself, rather than in bulky batteries on the roof or in a utility room.
That shift from cabinet to slab is not cosmetic. A battery cabinet is a discrete object that can be damaged or replaced, while the slab is the building, with a useful life measured in decades.
Meanwhile, the team is working on electrolyte formulas that survive freezing, which would open cold climate markets. For a sense of how fast unusual power hardware can move from bench demonstration to working grid connection, the path a Utah granite geothermal project followed offers a useful comparison.
So the stairwell nobody looked twice at just became one of the more interesting components in the building, though whether the concrete agrees once temperatures drop well below freezing is a question the next paper still has to answer.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
