To make the graphene for its electrodes, Ruoff’s team starts by putting graphite oxide in a water solution. This causes the material to flake into atom-thin sheets of graphene oxide. Next, the oxygen atoms are removed, leaving the graphene behind. So far, Ruoff’s lab has made graphene ultracapacitors that match the performance of those made using activated carbon. With further refinements, he says, they should outperform activated carbon, although the steps that his company is taking to achieve this remain secret.
Based on a description of the graphene ultracapacitors published last September in the journal Nano Letters, John Miller of JME, a research and consulting firm that specializes in electrochemical capacitors, says that it should indeed be possible to improve their performance. The graphene electrode described in this paper is “wadded into a ball like a crumpled piece of paper,” says Miller. “You don’t have full access to the surface.”
If Graphene Energy can grow the electrodes in vertical arrays, like a row of perfectly flat sheets of paper standing on edge, Miller says that the power output could be increased dramatically. In this arrangement, every single carbon atom would be exposed and able to store energy, with virtually no waiting time for the charge to travel down the tunnels found in activated carbon.
However, in addition to improving the performance of its ultracapacitors, Graphene Energy must also develop a method for making them at larger scales–a common challenge across all graphene research.
Dileep Agnihotri, CEO of Graphene Energy, says that the company hopes to test its first prototype product incorporating graphene electrodes by the end of this year.
Another group of researchers hopes to make better ultracapacitor electrodes using carbon nanotubes–rolled-up tubes of graphene that have many of the same properties. “I think both approaches can work in principle,” says Joel Schindall, a professor of electrical engineering and computer science at MIT who is working on the nanotube electrodes. “The key will be getting the growth process right, then working on ways to manufacture it in a cost-effective manner.”