Modern life relies on electricity and electrical devices, from cars to buses, from phones to laptops, to electrical systems in homes. Behind most of these devices is a type of energy storage device called a supercapacitor. My team of engineers is working on making these supercapacitors even better at storing energy by studying how they store energy at the nanoscale.
Supercapacitors are energy storage devices, like batteries. They charge faster than batteries, usually within a few seconds to a minute, but generally store less energy. They are used in devices that require storing or delivering a burst of energy over a short period of time. They can help recover energy during braking to slow down in your car and elevators. They help meet fluctuating energy demand from laptops and cameras and balance energy loads on power grids.
Batteries work by reactions in which chemical species donate or gain electrons. On the contrary, supercapacitors do not depend on reactions and are like a charge sponge. When you dip a sponge in water, it absorbs water because the sponge is porous; It contains empty pores through which water can be absorbed. The best supercapacitors absorb the most charge per unit volume; This means they have high energy storage capacity without taking up too much space.
In research published in May 2024 in the Proceedings of the National Academy of Sciences, my student Filipe Henrique, my colleague Pawel Zuk, and I describe how ions move through a network of nanopores, or tiny pores only nanometers wide. This research could one day improve the energy storage capabilities of supercapacitors.
All about pores
By making a material’s surface porous at the nanoscale, scientists can increase its capacitance, or ability to store charge. A nanoporous material could have a surface area as high as 20,000 square meters (215,278 square feet) (equivalent to about four football fields) weighing just 10 grams (one-third of an ounce).
Over the past 20 years, researchers have investigated how to control this porous structure and the flow of ions, small charged particles, through the material. Understanding the flow of ions could help researchers control the rate at which the supercapacitor charges and releases energy.
But researchers still don’t know exactly how ions flow in and out of porous materials.
Each pore in a sheet of porous materials is a tiny hole filled with both positive and negative ions. The opening of the pore is connected to a reservoir of positive and negative ions. These ions come from the electrolyte, which is a conductive liquid.
For example, if you put salt in water, each salt molecule breaks down into a positively charged sodium ion and a negatively charged chloride ion.
When the surface of the pore is charged, ions flow from the reservoir into the pore and vice versa. If the surface is positively charged, negative ions flow from the reservoir into the pore and leave the pore as positively charged ions are repelled. This flow creates capacitors that hold the charge in place and store energy. When the surface charge is discharged, ions flow in the opposite direction and energy is released.
Now imagine a pore splitting into two different branched pores. How do ions flow from the main pore to these branches?
Think of ions as cars and pores as roads. Traffic flow on a single road is simple. But at an intersection, you need rules to prevent an accident or traffic jam; hence we have traffic lights and intersections. But scientists don’t fully understand the rules followed by ions flowing through a junction. Understanding these rules can help researchers understand how a supercapacitor will charge.
Changing a law of physics
Engineers often use a set of laws of physics called “Kirchoff’s laws” to determine the distribution of electric current at a junction. However, Kirchhoff’s circuit laws were derived for electron transport, not ion transport.
Electrons only move when there is an electric field, but ions can move by diffusion without an electric field. Just as a pinch of salt slowly dissolves in a glass of water, ions move from more dense areas to less dense areas.
Kirchhoff’s laws are like the accounting principles of circuit junctions. The first law says that the current entering a junction must be equal to the current leaving it. The second law states that the voltage, the pressure that pushes electrons through the current, cannot change suddenly at a junction. Otherwise, it will create an extra current and disrupt the balance.
Since ions move not only by the use of an electric field but also by diffusion, my team modified Kirchhoff’s laws to accommodate ionic currents. We replaced the voltage (V) with an electrochemical voltage (φ) that combines voltage and diffusion. This change allowed us to analyze pore networks, which was previously impossible.
We used the modified Kirchoff law to simulate and predict how ions flow through a large network of nanopores.
road ahead
Our study found that splitting the current through a pore into junctions can slow the rate at which charged ions flow into the material. But it depends on where the divide is. And how these pores are arranged throughout the material also affects the charging rate.
This research opens new doors to understand the materials in supercapacitors and develop better ones.
For example, our model can help scientists simulate different pore networks to see which one best fits their experimental data and optimize the materials they use in supercapacitors.
Although our work focused on simple networks, researchers can apply this approach to much larger and more complex networks to better understand how a material’s porous structure affects its performance.
In the future, supercapacitors could be made from biodegradable materials, power flexible wearable devices, and be customized via 3D printing. Understanding ion flow is an important step towards developing supercapacitors for faster electronics.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by: Ankur Gupta, University of Colorado Boulder
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Ankur Gupta receives funding from NSF.