Modern life depends on electricity and electrical appliances, from cars and buses, to phones and laptops, to the electrical systems in homes. Behind many of these devices is a type of energy storage device called a supercapacitor. My team of engineers is working to make these supercapacitors even better at storing energy by studying how they store energy at the nanoscale.
Supercapacitors, like batteries, are energy storage devices. They charge faster than batteries, often in seconds to a minute, but generally store less energy. They are used in devices that need to store or deliver a burst of energy in a short time. In your car and in elevators, they can help recover energy when braking to slow down. They help meet the fluctuating energy demands of laptops and cameras, and stabilize energy loads in electricity grids.
Batteries work through reactions in which chemical substances give or take electrons. Supercapacitors, on the other hand, do not rely on reactions and are a bit like a charge sponge. When you immerse a sponge in water, it absorbs the water because the sponge is porous: it contains empty pores where water can be absorbed. The best supercapacitors absorb the most charge per unit volume, meaning they have a high capacity for energy storage without taking up too much space.
In research published in the journal Proceedings of the National Academy of Sciences in May 2024, my student Filipe Henrique, collaborator Pawel Zuk and I describe how ions move in a network of nanopores, or small pores that are only nanometers wide. This research could one day improve the energy storage capabilities of supercapacitors.
Everything about the pores
Scientists can increase a material’s capacity, or ability to store charge, by making its surface porous at the nanoscale. A nanoporous material can cover a surface area of up to 20,000 square meters (215,278 square feet) – the equivalent of about four football fields – while weighing just 10 grams (one-third of an ounce).
For the past two decades, researchers have studied how to control this porous structure and the flow of ions, tiny charged particles, through the material. By understanding ion flow, researchers can control the rate at which a 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 small 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 an electrolyte, 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 or vice versa. If the surface is positively charged, negative ions flow from the reservoir into the pore, and positively charged ions leave the pore as they are repelled. This current forms capacitors, which hold the charge in place and store energy. When the surface charge is discharged, the ions flow in the opposite direction and energy is released.
Now imagine that a pore divides into two different branched pores. How do the ions flow from the main pore to these branches?
Think of the ions as cars and pores as roads. The flow of traffic on a single road is simple. But at an intersection you need rules to prevent an accident or traffic jam, that’s why we have traffic lights and roundabouts. However, scientists don’t fully understand the rules that follow ions flowing through an intersection. Figuring out these rules could help researchers understand how a supercapacitor will charge.
Changing a law of nature
Engineers generally use a set of physical laws, called “Kirchoff’s laws,” to determine the distribution of electrical current across an intersection. However, Kirchhoff’s circuit laws are derived for electron transport, not for ion transport.
Electrons only move when there is an electric field, but ions can also move without an electric field, through diffusion. In the same way that a pinch of salt slowly dissolves in a glass of water, ions move from more concentrated areas to less concentrated areas.
Kirchhoff’s laws are like accounting principles for circuit connections. The first law states that the current entering an intersection must be equal to the current leaving the intersection. The second law states that voltage, the pressure that pushes electrons through the current, cannot change abruptly across an intersection. Otherwise an additional current would arise and the balance would be disturbed.
Because ions also move by diffusion and not just by the use of an electric field, my team adapted Kirchhoff’s laws to ion currents. We replaced the voltage V with an electrochemical voltage φ, which combines tension and diffusion. This adjustment allowed us to analyze networks of pores, which was previously impossible.
We used modified Kirchoff’s law to simulate and predict how ions flow through a large network of nanopores.
The road ahead
Our research found that splitting current from a pore into nodes can slow the rate at which charged ions flow into the material. But that depends on where the split takes place. And the way these pores are arranged throughout the materials also affects the loading rate.
This research opens new doors for understanding the materials in supercapacitors and developing better ones.
For example, our model can help scientists simulate different pore networks to see which best matches their experimental data and optimize the materials they use in supercapacitors.
While our work focused on simple networks, researchers could 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 possibly be customized through 3D printing. Understanding ion flow is an important step toward improving supercapacitors for faster electronics.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It is written by: Ankur Gupta, University of Colorado Boulder
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Ankur Gupta receives funding from NSF.