The conversion of carbon dioxide to useful materials usually requires high energy input due to its ultrahigh stability. A heat-releasing reaction between carbon dioxide and sodium was developed to take carbon dioxide and turn it into 3D graphene with micropores across its surface.
The material’s surface is pockmarked with micropores, and folds into larger mesopores. Both increase the surface area available for adsorption of electrolyte ions, making it an excellent electrode material for energy storage devices.
A supercapacitor material needs to store — and release — a charge. The limiting factor is how quickly ions can move through the material. The supercapacitive properties of the structure of 3D surface-microporous graphene make it suitable for elevators, buses, cranes, and any application that requires a rapid charge/discharge cycle. Supercapacitors are an important type of energy storage device, and have been widely used for regenerative braking systems in hybrid vehicles.
Current commercialized supercapacitors employ activated carbon using swaths of micropores to provide efficient charge accumulation. Electrolyte ions, however, have difficulty diffusing into or through activated carbon’s deep micropores, increasing the charging time. In the new material, the interconnected mesopores are channels that can act as an electrolyte reservoir, and the surface micropores adsorb electrolyte ions without needing to pull the ions deep inside the micropore.
The mesopore is like a harbor, and the electrolyte ions are ships that can dock in the micropores. The ions do not have to travel a great distance between sailing and docking, which greatly improves charge/discharge cycles they can steer through. As a result, the material exhibited an ultrahigh areal capacitance of 1.28 F/cm2, which is considered an excellent rate capability as well as superb cycling stability for supercapacitors.
To synthesize the material from carbon dioxide, researchers added carbon dioxide to sodium, followed by increasing temperature to 520 °C. The reaction can release energy as heat, instead of requiring an energy input. During the process, carbon dioxide not only forms 3D graphene sheets, but also digs the micropores — the tiny dents are only 0.54 nanometer deep in the surface layers of graphene.