Understanding how ions flow in and out of the smallest pores holds promise for better energy storage devices

Modern life relies on electricity and electrical devices, 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, a supercapacitor. My engineering team is working to make 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 a matter of seconds to a minute, but generally store less energy. They are used in devices that require storing or delivering a pulse of energy in a short time. In the car and in the elevator, they can help recover energy when braking. They help meet fluctuating energy demands in laptops and cameras and stabilize energy loads in electrical networks.

Batteries work through reactions in which chemical compounds lose or gain electrons. Supercapacitors, on the other hand, do not rely on reactions and resemble 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, which means they have a high ability to store energy 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 and my colleague Paweł Zuk describe how ions move in a network of nanopores, which are tiny pores only nanometers wide. This research could one day improve the energy storage capabilities of supercapacitors.

All about leeks

Scientists can increase the material’s capacity, or ability to store charge, by making its surface porous at the nanoscale. The nanoporous material can have 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).

Over the past 20 years, scientists have been investigating how to control this porous structure and the flow of ions, which are small charged particles, through the material. Understanding ion flow can help researchers control the rate at which a supercapacitor charges and releases energy.

However, 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 positive and negative ions. The pore opening connects to a reservoir of positive and negative ions. These ions come from the electrolyte, a conductive fluid.

For example, if you add salt to water, each salt molecule will split into a positively charged sodium ion and a negatively charged chlorine ion.

When the pore surface is charged, ions flow from the reservoir to the pore and vice versa. If the surface is positively charged, negative ions flow into the pores from the reservoir, and positively charged ions leave the pores as they are repelled. This flow creates capacitors that hold the charge in place and store energy. Once the surface charge is discharged, the 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 pores to these branches?

Think of ions as cars and pores as roads. Traffic flow on one road is simple. But at the intersection we need regulations to prevent accidents or traffic jams, that’s why we have traffic lights and roundabouts. However, scientists do not fully understand the rules that govern ions flowing through the junction. Understanding these principles can help scientists understand how a supercapacitor will be charged.

Modifying the laws of physics

Engineers typically use a set of laws of physics called “Kirchoff’s laws” to determine the distribution of electric current in a connector. However, Kirchhoff’s circuit laws were derived for electron transport, not ion transport.

Electrons only move when there is an electric field, while ions can move without an electric field, through diffusion. In the same way, a pinch of salt slowly dissolves in a glass of water, the ions move from areas of higher concentration to areas of lower concentration.

Kirchhoff’s laws are like accounting rules for circuit junctions. The first law states that the current flowing into a node must be equal to the current flowing out of it. The second law states that the voltage, or pressure pushing electrons through current, cannot change rapidly across a junction. Otherwise, additional current would be generated and disturb the balance.

Because ions also move by diffusion and not just by electric fields, my team modified Kirchhoff’s laws to fit ion currents. We replaced the voltage V with an electrochemical voltage φ, which combines voltage and diffusion. This modification enabled the analysis of the pore network, which was previously impossible.

We used a modified Kirchoff’s law to simulate and predict the flow of ions through a large network of nanopores.

The road lies ahead

Our study showed that splitting the current from pores to junctions can slow the rate of charged ion flowing into the material. But it depends on where the divide is. The way the pores are arranged in the materials also affects the charging speed.

This research opens new doors to understanding the materials from which supercapacitors are made and developing better ones.

For example, our model can help scientists simulate different pore networks to see which best fits their experimental data and optimize the materials used in supercapacitors.

Although our work focused on simple networks, researchers could apply this approach to much larger and more complex networks to better understand how the material’s porous structure affects its performance.

In the future, supercapacitors could be made from biodegradable materials to power flexible portable devices, and could also be customized using 3D printing. Understanding ion flow is a key step toward improving supercapacitors for faster electronics.