(Nanowerk Spotlight) Supercapacitors sit at the heart of modern energy storage, charging and discharging rapidly to power everything from electric buses to industrial equipment. Their performance depends on their electrodes, which are made from carbon materials filled with microscopic pores where electric charges collect. Creating these pores at precisely the right size – less than one nanometer – has proven a persistent challenge in energy storage technology.
Scientists made a surprising discovery in 2006: the smallest pores could store more electric charge than larger ones. This finding upended the conventional approach of maximizing surface area through bigger pores. The catch? When nanopores become too small, less than half a nanometer, they block the movement of charged particles. The ideal pore size sits in a narrow sweet spot between 0.5 and 1.0 nanometers.
Current production methods cannot reliably create pores of such specific sizes. Chemical activation produces pores of many different sizes. Using hard templates like silica requires complex and expensive processing. Soft templates made from organic molecules often collapse during manufacturing. As a result, commercial supercapacitors use carbon materials with inconsistent pore sizes, limiting their energy storage capacity and charging speed.
Researchers at Fudan University have created carbon materials with precisely sized pores by using carbon dots – particles just a few nanometers across – as templates. These dots create pores, add performance-enhancing nitrogen atoms to the material, and become part of the final product, eliminating the need for template removal.
Schematic diagram of the synthetic procedure for Carbonized Polymeric Derived Hydrogels.
The team synthesizes carbon dots by heating common chemicals – citric acid and diethylenetriamine – to 170 °C. Mixing these dots into a polymer solution creates a gel. Treating this gel with potassium hydroxide breaks specific chemical bonds in the carbon dots. Heating to 800 °C then produces carbon material with uniform pores between 0.64 and 0.80 nanometers – ideal for storing electric charge.
This material stores 515.5 farads of charge per gram while maintaining a density of 0.81 grams per cubic centimeter – 30% more storage capacity than commercial activated carbons in the same volume. This density advantage means electric vehicles could store more energy in their existing supercapacitor spaces, extending their range between charges.
The material solves several practical problems that have limited supercapacitor adoption. It works with water-based electrolytes, which cost 50-70% less than organic alternatives and pose fewer safety risks. It maintains 99.9% of its capacity after 10,000 charge-discharge cycles, exceeding the durability of many commercial devices. Most significantly, it performs well in electrodes thick enough for commercial use – 10 milligrams per square centimeter – where many laboratory materials fail.
When assembled into a supercapacitor, the material operates at 1.4 volts, 40% higher than typical water-based systems, allowing more energy storage per device. It also retains its charge longer than conventional materials, losing only 48% of stored energy over 24 hours compared to typical losses of 70-80%. These improvements enable the device to store 22.3 watt-hours of energy per kilogram while delivering 3,500 watts of power per kilogram – performance that matches or exceeds current commercial devices.
The carbon dot template method could improve other technologies requiring precise nanoscale structures. Water filtration systems could use similar materials to remove specific contaminants. Gas separation systems could better isolate different molecules. Catalysts for chemical production could become more efficient. This approach demonstrates how controlling material structure at the smallest scales can enhance performance in multiple practical applications.
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