A scanning electron microscope image of sodium-embedded carbon reveals its nanowall structure and pores. Image: Yun Hang Hu, Michigan Tech.
A scanning electron microscope image of sodium-embedded carbon reveals its nanowall structure and pores. Image: Yun Hang Hu, Michigan Tech.

A little sodium goes a long way, or at least it does for carbon-based energy technologies. In a paper in Nano Letters, a research team led by Yun Hang Hu, professor of materials science and engineering at Michigan Technological University, reports that embedding sodium in carbon materials can tremendously improve electrodes. This follows their creation of a brand-new way to synthesize sodium-embedded carbon nanowalls, which had previously been purely theoretical.

High electrical conductivity and a large accessible surface area, which are required for ideal electrode materials in energy devices, tend to be mutually exclusive in current materials. Amorphous carbon has low conductivity but a large surface area, while graphite has high conductivity but a low surface area. Three-dimensional graphene has the best of both properties – and the sodium-embedded carbon invented by Hu and his team is even better.

"Sodium-embedded carbon's conductivity is two orders of magnitude larger than three-dimensional graphene," Hu says. "The nanowall structure, with all its channels and pores, also has a large accessible surface area comparable to graphene."

These sodium-embedded carbon nanowalls are different from metal-doped carbon, where the metals are on the surface of the carbon and easily oxidized; embedding a metal in the actual carbon structure helps to protect it. To make such a dream material, Hu and his team had to develop a new process, which uses a temperature-controlled reaction between sodium metal and carbon monoxide to create a black carbon powder that traps sodium atoms. Furthermore, in collaboration with researchers at the University of Michigan and the University of Texas at Austin, they confirmed that the sodium was embedded inside the carbon instead of being merely on the surface. The team then tested the material in several energy devices.

In dye-sensitized solar cells, every tenth of a percent improvement in the conversion efficiency counts in making devices more commercially viable. In this study, a platinum-based solar cell reached a power conversion efficiency of 7.89%, which is considered standard. In comparison, a solar cell containing Hu's sodium-embedded carbon as an electrode reached efficiencies of 11.03%.

Supercapacitors can accept and deliver charges much faster than rechargeable batteries and are ideal for cars, trains, elevators and other heavy-duty equipment. The power of their electrical punch is measured in farads (F); the material's density, in grams, also matters.

Activated carbon is commonly used for supercapacitors; it packs a 71F/g punch. Three-dimensional graphene has more power, at 112F/g. Sodium-embedded carbon knocked them both out of the ring with 145F/g. Plus, after 5000 charge/discharge cycles, the material retained a 96.4% capacity, indicating that the electrode is highly stable.

According to Hu, innovation in energy devices is in great demand. He sees a bright future for sodium-embedded carbon and the improvements it offers in solar cells, batteries, fuel cells and supercapacitors.

This story is adapted from material from Michigan Technological University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.