A Cool Fuel Cell

A new electrolyte for solid-oxide fuel cells, made by researchers in Spain, operates at temperatures hundreds of degrees lower than those of conventional electrolytes, which could help make such fuel cells more practical.

Jacobo Santamaria, of the applied-physics department at the Universidad Complutense de Madrid, in Spain, and his colleagues have modified a yttria-stabilized zirconia electrolyte, a common type of electrolyte in solid-oxide fuel cells, so that it works at just above room temperature. Ordinarily, such electrolytes require temperatures of more than 700 °C. Combined with improvements to the fuel-cell electrodes, this could lower the temperature at which these fuel cells operate.

Solid-oxide fuel cells are promising for next-generation power plants because they are more efficient than conventional generators, such as steam turbines, and they can use a greater variety of fuels than other fuel cells. They can generate electricity with gasoline, diesel, natural gas, and hydrogen, among other fuels. But the high temperatures required for efficient operation make solid-oxide fuel cells expensive and limit their applications. The low-temperature electrolyte reported by the Spanish researchers could be a "tremendous improvement" for solid-oxide fuel cells, says Eric Wachsman, director of the Florida Institute for Sustainable Energy, at the University of Florida.

In a solid-oxide fuel cell, oxygen is fed into one electrode, and fuel is fed into the other. The electrolyte allows oxygen ions to migrate from one electrode to the other, where they combine with the fuel; in the simplest case, in which hydrogen is the fuel, this produces water and releases electrons. The electrolyte prevents the electrons from traveling directly back to the oxygen side of the fuel cell, forcing them instead to travel through an external circuit, generating electricity. Via this circuitous route, they eventually find their way to the oxygen electrode, where they combine with oxygen gas to form oxygen ions, perpetuating the cycle.

The electrolyte--which is a solid material--typically only conducts ions at high temperatures. Santamaria, drawing on earlier work by other researchers, found that the ionic conductivity at low temperatures could be greatly improved by combining layers of the standard electrolyte materials with 10-nanometer-thick layers of strontium titanate. He found that, because of the differences in the crystal structures of the materials, a large number of oxygen vacancies--places within the crystalline structures of the materials that would ordinarily host an oxygen atom--formed where these two materials meet. These vacancies form pathways that allow the oxygen ions to move through the material, improving the conductivity of the materials at room temperature by a factor of 100 million.

The material is still some way from being incorporated into commercial fuel cells. For one thing, the large improvement in ionic conductivity will require further verification, Wachsman says, especially in light of the difficulty of measuring the performance of extremely thin materials. Second, the direction of the improved conductivity--along the plane of the material rather than perpendicular to it--will require a redesign of today's fuel cells. What's more, the limiting factor for the temperature in fuel cells now is the electrode materials. Before room temperature solid-oxide fuel cells are possible, these will also need to be improved.

Yet if initial results are confirmed by future research, the new materials will represent a significant advance. Ivan Schuller, a professor of physics at the University of California, San Diego, says that this represents a major change in the performance of electrolytes. He adds, "It will surely motivate much new work by others."