Air-powered battery makes its own electrolyte from humidity

A freshly made battery arrives completely dry, only springing to life once it encounters humid air. Instead of being filled in advance, it draws in water vapour and creates the liquid it needs to operate. In effect, it is a battery driven by little more than the air in its surroundings.

The concept functions in settings ranging from very dry deserts to humid tropical rainforests, and its performance stands up well against widely used commercial cells. Because it only becomes active when moisture is present, it loses very little charge while stored in its packaging.

This approach tackles two long-standing obstacles for small electronics. It offers wearable and internet-connected devices a power supply that is lightweight, flexible and avoids toxic metals.

Battery power from air

To work, a battery requires two electrodes and a wet layer between them - the electrolyte - that allows charge to move across the cell. In most batteries that liquid is sealed inside, adding mass and introducing the risk of leakage. This design is shipped dry and instead extracts its electrolyte from the air.

The research comes from the laboratory of Amay J. Bandodkar, an engineering professor at North Carolina State University, working with collaborators at Rice University. Their battery combines a magnesium anode with a silver chloride cathode, materials already familiar from medical devices.

A two-stage separator positioned between the electrodes is the key component. In the first region, a cellulose sheet infused with lithium chloride - a salt with a strong affinity for water - pulls water vapour directly from the air and condenses it into liquid.

In the second region, that newly formed water is absorbed and used to dissolve a preloaded amount of ordinary table salt. This produces the salty liquid required to carry charge: an electrolyte mixed on demand from airborne moisture and a small quantity of salt, without any need for manual wetting.

The moisture-capturing chemistry is borrowed from a very different area of research: harvesting drinking water from dry air. Previous studies have shown that cellulose loaded with lithium chloride can trap water across a broad range of humidity conditions, and Bandodkar’s group adapted that idea for batteries.

Once exposed to typical indoor air, the cell stabilises its output in roughly 7 minutes. It produces about 1.6 volts, placing it slightly above a standard AA battery.

From sweat to air

This is not the first time the team has built a battery that relies on a body fluid rather than storing liquid inside. A few years ago, they described thin batteries worn on the skin that remained inactive until the wearer began to sweat, then used perspiration as the electrolyte.

Sweat can do the job, but only when the battery is on a warm, active body. The tissue fluid the group later used for implant-style batteries is only available inside the body. Using ambient moisture removes those constraints.

Almost everywhere people live, there is at least some water vapour in the air. That makes this battery far more widely usable than either of its earlier fluid-powered predecessors.

Keeping the battery dry until it is needed also brings another benefit. Conventional batteries slowly lose charge while sitting on the shelf through unwanted chemical reactions - a process known as self-discharge.

With no liquid present in storage to sustain those reactions, a dry battery remains largely unchanged until humidity activates it.

Bending without breaking

A battery designed to adhere to skin or wrap around objects must flex without cracking. Many flexible battery designs place rigid components far apart, linked by stretchable connections; that wastes space and reduces how much energy the device can store.

The researchers took inspiration from the pangolin, the scaly anteater whose overlapping plates let it curl into a ball. They arranged rigid battery units close together like those scales, connecting them with springy S-shaped wires that deform when the sheet is stretched or folded.

Raudel Avila, a mechanical engineer at Rice University and a co-author of the study, carried out the computer modelling used to refine the arrangement.

“Our simulations predicted how the entire battery deforms, allowing us to engineer the architecture so the interconnects absorb the motion while the battery cells remain protected,” Avila said.

The layout is compact enough that working cells occupy 87% of the battery’s area, while the overall sheet can still stretch by up to 80% in two directions. Its resistance changes very little during bending and twisting because the wavy wires take up the movement and the cells remain in place.

In terms of basic performance, the cell compares well with batteries bought off the shelf. It equals or surpasses several single-use commercial cells for both voltage and energy per unit mass. The team also produced an AA-shaped version designed to fit existing devices.

Driving real gadgets

To demonstrate that the battery can power demanding electronics, the researchers built two very different devices.

“Our battery matches the capacity of many commercial AA and AAA alkaline batteries, making it a suitable power source for a wide range of electronics,” Bandodkar said.

The first device is a wireless pulse oximeter - the fingertip tool that measures blood oxygen level and heart rate by shining light through the skin.

Their design separates into a reusable electronics module and a disposable patch that includes the battery and a skin adhesive. Small magnets connect the two, allowing a depleted battery patch to be removed and a new one attached without tools.

Powered by the moisture-activated cell, the device transmitted oxygen and pulse data to a smartphone. In a breath-hold test, it recorded a volunteer’s heart rate at about 70 beats a minute.

Sensor that self-destructs

The second prototype leans into a more unusual capability. It is a covert monitoring unit: a small wireless sensor that detects chemical vapours in the air, equipped with a kill switch that destroys the device as soon as someone attempts to tamper with it.

Beneath the sensor sits a dry mixture of aluminium and iodine powder, topped with a lithium chloride membrane that steadily collects moisture from the air. A thin barrier keeps the damp cap separated from the dry powder, so the device remains stable during ordinary use.

A firm press - the sort of pressure an unsuspecting person might apply - forces the two layers together. Water seeps from the membrane into the powder and sets off a violent, self-sustaining combustion that destroys the sensor, its electronics and any stored data within 3 minutes.

Humidity by itself will not set it off. The reaction still requires that physical push to bring the wet membrane into contact with the dry powder.

The group has applied the same water-triggered principle in other projects. One recent example is a low-cost wound dressing that stays inactive until moisture turns it on, after which it delivers therapeutic electrical pulses.

What comes next

One ingredient still creates a compromise. Adding glycerol, a viscous liquid, significantly increases how long the battery runs by helping retain water in the electrolyte region.

It may achieve this by softening the polymer separator and reducing internal resistance, although the precise mechanism still requires investigation.

However, glycerol harms storage life. Cells made without glycerol retained nearly all their capacity after 15 days in sealed storage, whereas glycerol-containing cells lost most of theirs. The liquid likely encourages unwanted reactions at the magnesium electrode even when the battery is not in use.

To avoid that trade-off, the researchers aim to replace the liquid with a solid material that can hold moisture - such as the porous crystals used in water-harvesting systems - so they can extend run time without sacrificing shelf life. They also plan rechargeable versions and moisture-capture layers tuned for different climates.

The kill-switch concept may also be taken further. The team envisages a similar feature built into everyday consumer electronics.

“Future consumer electronics, such as smartphones, could integrate similar remote kill switches. In the event of theft, owners could activate this feature to prevent unauthorized access to their private information,” Bandodkar said.

Why it matters

The results make a clear point. A non-toxic, flexible battery can generate its own electrolyte from the air and still rival commercial cells on key performance measures.

That is the central breakthrough. Many batteries developed through green-chemistry approaches improve safety by sacrificing performance; this one does not need to make that trade.

It points towards powering the expanding range of wearable health patches and small connected sensors without sealing a rigid, toxic cell into each device. Products could be shipped dry and inert, then become active the moment they are opened and worn.

Photo credit: Rajaram Kaveti