Sweat powered fitness trackers. Bluetooth headphones work with atmospheric humidity. Thin air powered adhesive insulin sensors.
These are some of the potential applications for so-called “self-recharging batteries”, which harvest electrical energy from moisture.
Key points:
- “Self-charging batteries” convert the chemical energy of water into electricity
- UNSW researchers have developed units capable of powering small electronic devices
- A commercial version will be ready by the end of the year
The idea has been around for years, but early prototypes couldn’t generate usable amounts of electricity.
Earlier this year, a team from the University of New South Wales announced that they had found a way to dramatically increase electricity production – using atom-thick layers of graphene oxide. in a stack of pancakes a fraction of a millimeter thick.
When several of these small collapsible electric humidity generators (MEGs) were put together in series, they could power a pocket calculator.
Last month, Strategic Elements, an Australian company that had partnered with UNSW researchers, said it was bringing the technology to market.
He plans to have a demo version of an electronic moisture-powered skin patch by the end of the year.
So, are self-charging batteries on the doorstep – or are they too good to be true?
Where does the power come from?
Recharging batteries may seem like an impossible idea: like flying by lifting yourself off the soles of your shoes.
Dewei Chu is the head of the Nanoionic Materials Group at UNSW.
His team co-authored the paper describing novel graphene-oxide MEGs, which was published in the journal Nano Energy in April.
MEGs, he says, work by separating positively and negatively charged particles.
“If we can have more protons on one side than the other, that’s an ion gradient, and you can generate voltage and current.”
To understand what he means, imagine a solution full of positively and negatively charged particles.
Now imagine introducing a membrane that only lets positively charged particles through.
The uneven distribution of negatively and positively charged particles across the membrane causes an electrochemical gradient, which generates a voltage at the electrodes.
With the UNSW MEG design, these positively charged particles are hydrons or hydrogen ions.
So batteries don’t quite charge themselves – they really convert chemical potential energy into electrical energy.
But where do hydrons come from?
They come from the air — from water vapour.
Professor Chu and his team have developed a way to print very thin layers of graphene oxide nanomaterial.
A property of this material is that it is water-loving, or hydrophilic. Airborne water molecules readily bind to the surface.
On the surface, water (H2O) decomposes, releasing hydrons.
The trick, says Professor Chu, was to create a reservoir of hydrons large enough to generate usable amounts of electricity.
That’s where the nanomaterial comes in. Because it’s so thin, the stacked layers have a large combined surface area, which means they can pick up a lot of hydrons from ambient humidity.
In the Nano Energy paper, a single unit with the surface dimensions of a strip of chewing gum, but a fraction of a millimeter thick, generated 0.85 volts, or about half of a standard AAA battery.
Professor Chu and his team have developed better designs since the paper was submitted last year.
He says a matchbox-thick stack of these new MEGs is capable of generating 4 volts.
“Now the performance is much better. It’s at least 10 times better.”
Should it stay moist?
Yes, but they don’t need a lot of water and work well in low humidity conditions.
According to Professor Chu, MEGs operate with a minimum of 40% humidity, which in meteorological terms is a non-humid day.
Once the entire MEG unit has the same saturation level, it does not generate electricity – one side must be drier than the other to obtain an electrical potential.
But as one side dries out or gets wetter, it starts to generate electricity again.
However, this process of wetting and drying gradually wears down the surface area of the unit, Professor Chu said.
“We think it can last at least three months.”
What could it feed?
Jingwei Hou is a chemical engineer and nanomaterials expert at the University of Queensland.
He said the design of the UNSW MEG was “very interesting”.
“I think this device is the best so far,” said Dr. Hou, who was not involved in the research.
For now, these products are likely to be small disposable devices, such as adhesive electronic sensors that can monitor vital signs 24/7.
“If you have a big enough area, you could power a cellphone, but that would be huge – at the square meter level,” Dr Hou said.
Later this year, Professor Chu’s team plans to print a 3 square meter MEG.
If MEGs prove scalable, they could be applied as a thin film to the windows of high-rise buildings and used to power emergency services, Dr Hou said.
“You could create enough electron density to power the fire alarm sensors all over the building,” he said.
“Demo version ready in spring”
Strategic Elements has been working with Professor Chu’s UNSW team since 2015.
The work was partly funded by Commonwealth tax offsets and funding grants and assistance from CSIRO.
“We have received millions of dollars in support from the federal government,” said general manager Charles Murphy.
In the third quarter of this year, the company plans to release a commercial prototype, or what Murphy called, “a nice, improved demonstrator powering a commercial skin patch.”
“The main advantage of technology is that it is flexible,” he said.
“Skin patches are an obvious market we need to focus on first.”
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