In their search to make better fuel cells, a research team from the National Institute of Standards and Technology (NIST), in collaboration with the University of Liverpool, has discovered a new structure that moves oxygen ions through the cell at substantially lower temperatures than previously thought possible. This discovery has been announced in this month’s Nature Materials, and it may be a solution to more reliable fuel cells. This may also lead to reducing the operating costs in high-power stationary fuel cells.
Fuel cells produce electricity by using the electrochemical reaction between hydrogen and oxygen. That reaction produces electric current and water. Stationary fuel cells operate at up to 70% efficiency and provide up to 100 megawatts of electricity, enough to power small cities, hospitals, military installations or airports without being connected to the power grid. They planned to make smaller versions of this type of fuel cell for auxiliary power units in applications such as refrigeration trucks to reduce the engine’s idling.
They are called “solid oxide” fuel cells (SOFCs) because the cell has a solid electrolyte in its core. The solid oxide transports oxygen ions extracted from surrounding air so they meet with the hydrogen atoms, from the hydrogen fuel tank. This reaction normally takes place at about 850 degrees Celsius in conventional SOFCs. That is a high temperature to get and there are long startup times needed, ranging from 45 minutes to eight hours.
The high temperatures need expensive materials and high operating costs, so stationary fuel cell research is focused on reducing the operating temperatures and startup times. The U.S. Department of Energy’s goal is to reach to a two minutes start-up time.
Chemists from the University of Liverpool made a new oxygen ion electrolyte material out of lanthanum, strontium, gallium and oxygen and sent it to the NIST Center for Neutron Research (NCNR) to investigate with collaborators from NIST, the University of Maryland and University College London. Neutrons provide an atomic-scale view of materials so scientists can “see” what is happening at that level.
The new materials have oxygen ions that become mobile at 600 degrees Celsius, much lower than previously studied materials. Researchers are presuming this is happening due to the location of the oxygen ions in the crystal framework of the material. The neutron probes allowed them to determine the basic crystal structure that held the lanthanum, strontium, gallium and oxygen atoms, however the exact nature of the extra oxygen ions was unclear.
The NCNR researchers recommended borrowing a method from radio astronomy called maximum entropy analysis. “When astronomers are not able to visualize a specific part of an image because it constitutes such a small part of the total information collected, they utilize a part of applied mathematics called information theory to reconstruct a sharper image,” explained NCNR researcher Mark Green.
“The combination of neutron diffraction and maximum entropy analysis not only allowed us to determine the location of additional oxygen ions outside of the basic framework, but revealed a new mechanism for ion conduction.”
“It allows us to take a fundamentally different approach in the design of future materials, so that we can harness this new mechanism for oxide ion conduction and produce lower operating fuel cells,” says Green. “This type of work is very important to us, which is why as part of the NCNR expansion we are developing a new materials diffractometer that will greatly enhance our capabilities in energy related research.”
(adapted from NIST press release)
