Whereas hydrogen fuel cells (e.g., proton exchange membrane (PEM) and other fuel cells) generate electricity from the chemical reaction between pure hydrogen and oxygen, direct carbon fuel cells (DCFCs) can use any number of carbon-based resources for fuel, including coal, coke, tar, biomass, and organic waste.
Because DCFCs make use of readily available fuels, they are potentially more efficient than conventional hydrogen fuel cells. But earlier DCFC designs have several drawbacks. They require high temperatures — 700 to 900 °C — which makes them less efficient and less durable. Further, as a consequence of those high temperatures, they are typically constructed of expensive materials that can handle the heat. Also, early DCFC designs are not able to effectively utilize the carbon fuel.
A new fuel cell was developed that incorporates innovations in three components: the anode, the electrolyte, and the fuel. Together, these advancements allow the fuel cell to utilize about three times as much carbon as earlier DCFC designs. The fuel cell also operates at lower temperatures and demonstrated higher maximum power densities than earlier DCFCs.
The true direct carbon fuel cell is capable of operating at lower temperatures — below 600 °C. The fuel cell makes use of solid carbon, which is finely ground and injected via an airstream into the cell. To address the need for high temperatures, an electrolyte was developed using highly conductive materials: doped cerium oxide and carbonate. These materials maintain their performance at lower temperatures.
Carbon utilization was increased by developing a 3D ceramic textile anode design that interlaces bundles of fibers together like a piece of cloth. The fibers themselves are hollow and porous. All of these features combine to maximize the amount of surface area available for a chemical reaction with the carbon fuel. A composite fuel was developed made from solid carbon and carbonate. At the operating temperature, the composite is fluid-like and flows easily into the interface. The molten carbonate carries the solid carbon into the hollow fibers and the pinholes of the anode, increasing the power density of the fuel cell.
The resulting fuel cell looks like a green, ceramic watch battery about as thick as a piece of construction paper. A larger square is 10 centimeters on each side. The fuel cells can be stacked on top of one another, depending on the application.