Los Alamos National Laboratory

Better Fuel Cell Membrane Materials

Laboratory researchers Rangachary Mukundan, Melinda Einsla (now working at Rohm and Haas, a Dow Chemical subsidiary), and Fernando Garzon were impressed when Japanese scientists discovered a new material, tin pyrophosphate, that could potentially replace the proton-conducting polymer that formed the membrane in proton exchange membrane (PEM) fuel cells. The new low-cost, conductive ceramic material has a number of possible cost and performance advantages over the currently used fluorocarbon polymer membranes.

Picture of study area.

A PEM fuel cell's membrane (magenta) is impermeable to electrons (e–), forcing them to travel to through a wire to the cell's cathode (green), while the protons (hydrogen ions, H+) go through the membrane. As shown in the figure, the protons and electrons separate when incoming hydrogen (H2) encounters the catalyst mixed into the anode (blue). They meet again at the cathode, where a catalyst causes them to combine with oxygen (O2) to form water.

The membrane is central to PEM fuel cells, which produce electricity from hydrogen. At one end of the cell, a catalyst causes the hydrogen to split into protons and electrons, which are then routed along separate paths: the protons through the membrane (an electrolyte) and the electrons through an outside circuit as electrical current. At the cell's other end, the protons and electrons combine with oxygen to form water, the fuel cell's only emission.

But there's a temperature problem. Current PEM fuel cells operate at 100 degrees Celsius and below, the temperature the polymer membrane needs to conduct protons. That relatively low temperature requires an expensive material, platinum, as the catalyst, making the fuel cell too expensive to be used in cars. In addition, it causes the water to form as a liquid rather than vapor. The liquid interferes with cell performance.

There are fuel cell technologies that use ceramic materials (other than tin pyrophosphate) that operate at greater than 700 degrees Celsius. Those allow for less-expensive catalysts and produce water in the form of vapor, but in cars the very high operational temperature would require exotic alloys and would make cold startup rather difficult and energy intensive.

The Japanese researchers found that the tin pyrophosphates could conduct protons at 200 to 300 degrees Celsius—hot enough to open the door for new catalysts and turn water to vapor but cool enough for common automobile materials. But the original way of making the new material had its own problems. The scientists were heating a mix of tin-containing oxide or salt with phosphoric acid to a molten state to evaporate off excess phosphorous oxides. That method produced the desired product, but the high heat left the product so nonuniform that only a small portion was usable. In addition, the process produced a very corrosive and acidic byproduct, phosphorous pentoxide.

The Los Alamos scientists found that they could start with different precursors— pyrophosphate salt and inorganic acid salt—dissolve them separately (several solvents worked, including water), mix them, and then evaporate out the solvent, this time at a lower temperature. The resulting product was a uniform tin pyrophosphate, and there were no corrosive byproducts. The newly patented process makes the synthesis of new membrane materials commercially possible.

—Eileen Patterson

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