The first silicon ionistor

    Everyone knows the properties of ionistors - these electrochemical devices combine the properties of capacitors and chemical batteries. They are able to charge / discharge very quickly and store much more energy than conventional capacitors, due to a unique feature - a double layer of ions and counterions, which act as electrolyte plates.

    No one could have previously suggested that silicon ionistors could be created without the use of a chemical electrolyte. However, a scientific article in the journal Scientific Reports dated 10/22/2013 indicates that scientists from the University of Vanderbilt managed to do this. They were the first in the world to create a silicon ionistor by etching a silicon substrate and coating wafers with graphene.

    It is difficult to even describe what prospects this holds for mobile electronics, because now you can store the charge directly on the chip, without the need to charge a chemical battery! Imagine solar panels that store a charge and give out electricity around the clock. A mobile phone or laptop that charges in a few seconds and lasts a week without recharging or, conversely, can be discharged in a second, like a stun gun. And these are just the most obvious examples.

    “If you asked an expert whether it is possible to create a silicon ionistor, he will immediately tell you that this is a crazy idea,” saysCary Pint (Cary Pint), assistant professor from the Department of Engineering, Vanderbilt University, under whose leadership research was carried out (pictured). “But we managed to find a simple way to do this.”

    The unique properties of electrochemical ionistors have already allowed them to find commercial application. True, only in some narrow niches. For example, they are used to accumulate kinetic energy from braking in Formula 1 cars and some commercial cars, buses and electric cars. These are KERS-type systems: they soon give up the stored energy to the wheels, adding torque during acceleration. Ionistors put large windmills on turbines that need energy recharge at the moment of changing the strength and direction of the wind.

    Ionistors are still inferior in energy density to chemical batteries such as lithium-ion batteries, so they are too bulky for most mobile devices, but they quickly eliminate the backlog.


    The diagram shows the energy density (watt-hour per kg) and specific power (watt per kg) of porous silicon (P-Si) ionistors, carbon commercial ionistors and porous silicon ionistors with graphene coating

    It should be noted that recently experiments with ionizers made of graphene or nanotubes, so that the new work of engineers from the University of Vanderbilt is somewhat knocked out of the general flow.

    The simplicity of their approach is the use of porous silicon - a material with controlled properties, which can be easily obtained by etching "wafers". Engineers found that when a material is coated with a graphene layer, its properties as an ionistor dramatically improve.

    “We had no idea what would happen [when we started the experiments],” Pint says. “Usually, researchers grow graphene from silicon carbide at temperatures above 1400 ° C, so at lower temperatures of 500-600 ° C we did not expect that something like it would grow.”

    When the engineers removed the silicon wafer from the furnace, they saw that its color was no longer orange, but purple, sometimes black. An electron microscope study showed that porous silicon is coated with a thin layer of carbon a few nanometers thick.


    The structure of porous silicon without coating with graphene (left) and coated (right)

    Tests have shown that graphene coating acts as a protective layer, and when the ionizer is charged, the maximum energy density increased by 25 times.

    The authors of the study say that the purpose of their work is not the creation of ionistors with a record energy density, but their integration into conventional microcircuits, which are manufactured according to the standard technological process. The most logical option is to install ionistors on the back of solar panels and sensors. More and more devices around us require electrical energy: “The better we can integrate energy storage into existing materials and devices, the more compact and efficient they will become,” says Pint.

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