A hollow carbon sphere having a carbon shell and an inner core is disclosed. The hollow carbon sphere has a total volume that is equal to a volume of the carbon shell plus an inner free volume within the carbon shell. The inner free volume is at least 25% of the total volume. In some instances, a nominal diameter of the hollow carbon sphere is between 10 and 180 nanometers.
 This invention was made with government support under Contract No. DE-AC52-06NA25396 ordered by the U.S. Department of Energy. The government has certain rights in the invention.