People on a busy sidewalk use lots of energy to get from one place to another. But some of that energy gets wasted. Shoes and the pavement absorb this mechanical energy. What if that energy instead could be put to better use?
“Harvesting mechanical energy is vitally important for wearable and portable electronics,” says Zhong Lin Wang. He’s a materials scientist at the Georgia Institute of Technology in Atlanta. A person’s phone, for instance, might be powered by their own movements. But first researchers have to figure out a way to gather that energy.
Sangtae Kim had an idea about how to do that. He’s a materials scientist at the Massachusetts Institute of Technology in Cambridge. He and his colleagues have come up with a way to use chemistry to convert wasted energy into electricity.
Their invention is similar to a battery. A battery has two electrodes, made from different materials. Between them sits a liquid or solid material called an electrolyte. When the battery is connected to an electric circuit, a reaction inside the electrolyte creates particles with an electrical charge, called ions. Those ions move from one electrode to another, through the circuit.
In the new device, the electrodes are made of the same material, a mixture of lithium and silicon. And they’re flexible. The electrolyte is sandwiched between them. As the device bends, it compresses the electrode on one side. At the same time, the bending stretches the other electrode. The uneven stresses make ions flow.
“Once we bend the device, lithium really wants to move to the other electrode,” says Kim. But it can only pass through the electrolyte in the form of lithium ions. “So lithium will separate into an electron and a lithium ion,” Kim explains. “And then lithium ions will move from the compressed electrode to the one that is stretched. Those lithium ions have a positive charge.”
The process leaves a lot of lone electrons, each of which has a negative charge. Those electrons want to join up again with the positively charged ions. However, they can’t move through the electrolyte. Instead, the electrons flow through wires that connect one side to the other. That movement is an electric current, and it can power a device. “The flow of electrons is the electricity,” Kim explains.
When the electrons reach the other electrode, they recombine with the lithium ions. That results in neutral lithium atoms.
The device unbends when the pressure is released. Now there’s again an imbalance between the two electrodes. So lithium ions travel back to the uncompressed electrode. And electrons flow through wires to rejoin them.
“This process basically repeats and repeats in reverse directions” as the device is bent and unbent, says Kim. His team reported its work January 6 in Nature Communications.
The inspiration
Kim got the idea for his team’s new invention after reading about a “smart road” project in Italy. That project used the mechanical energy from cars passing on the road to make electricity. It did that with piezoelectric (PEET-zoe-eh-LEK-trik) materials. These are crystals that produce an electric charge when pressure deforms their structure.
Piezoelectric devices work very well with a high frequency of mechanical energy, Kim notes. Those frequencies might range from 20 to several thousand times per second. However, the type of smart road project Kim read about is not yet very practical, he says. The reason: Costs for the equipment would be high. And especially if a road doesn’t have a lot of constant traffic, little electricity would be produced.
Kim’s approach instead harvests energy with chemistry. And it works best with actions that take place more slowly. “It doesn’t have to be regular motion,” he notes. “It can be any slow motion.” For instance, it could be something like walking at a rate of two steps per second.
The ions move relatively slowly in the new method. And that may be one reason why it works well with low-frequency actions, notes Wang, who was not involved in the research. However, he adds, piezoelectric and other types of energy harvesting devices also can work with a range of low to high frequencies.
The new technology “still needs some engineering breakthroughs” before it becomes practical, Kim says. The output from the prototype is only one microwatt per square centimeter. That’s less than a thousandth of the energy needed to light a typical LED light bulb.
Despite the challenges, Kim thinks the technology could one day power personal electronics or run a smart home. Maybe a sidewalk application could even add power to the electric grid.
“Someone might say it’s ambitious, that but that’s sort of the goal that we’re working towards,” Kim says.
Source- www.sciencenewsforstudents.org
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