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11/24/2003 08:00:00

Caltech engineers announce new, more promising type of electrolyte for fuel cells

PASADENA, Calif.—The quest for a cheap and robust fuel cell for future cars may be a bit closer this week to the "grail" moment. Scientists at the California Institute of Technology have announced that they're getting promising results with a new material that solves various limitations of previously tested fuel cells.

In an article published online November 20 by the journal Science on the Science Express Website, associate professor of materials science and chemical engineering Sossina Haile and her colleagues report that they have created a new phosphate-based electrolyte to go inside the fuel cells. The new substance, formally named cesium dihydrogen phosphate is, for a variety of reasons, better than the team's previously favored electrolyte, which was based on a sulfate.

"It's a whole new way of doing fuel cells that opens up tremendous possibilities for system simplification," says Haile, a leading authority on fuel cell technology. Haile's most spectacular results in recent years have been with the "solid acid" electrolytes, such as both the phosphate and the sulfate materials, that ferry current along the fuel cell in a way that minimizes the use of expensive parts that rapidly wear out.

Fuel cells have for some time been promoted as a way to help wean global society away from its addiction to gasoline and internal-combustion engines. Like a combustion engine, a fuel cell uses some sort of chemical fuel as its energy source, but like a battery, the chemical energy is directly converted to electrical energy, without a messy and inefficient combustion step.

The components in a fuel cell that make this direct electrochemical conversion possible are an electrolyte, a cathode, and an anode. In the simplest example hydrogen fuel is brought into the anode compartment and oxygen is brought into the cathode compartment. There is an overall chemical force driving the oxygen and the hydrogen to react to produce water.

In the fuel cell, however, the direct chemical reaction is prevented by the electrolyte that separates the fuel (H2) from the oxidant (O2). The electrolyte serves as a barrier to gas diffusion, but it will let protons migrate across it. In order for the reaction between hydrogen and oxygen to occur, the hydrogen molecules shed their electrons to become protons. The protons then travel across the electrolyte and react with oxygen atoms at the cathode, where they also pick up electrons to produce neutral water. An additional requirement for these electrochemical reactions to occur is that there be some external path through which the electrons migrate; it is precisely this electron motion that provides usable electricity from the fuel cell.

Traditional fuel cells, which utilize polymer electrolytes, are hampered by a number of problems. The most notable are the cells' inability to operate at high temperatures, their requirement for complicated water regulation systems, and their failure to control fuel diffusion.

Haile and her associates have addressed these shortcomings by creating a novel fuel cell with a solid-acid electrolyte. Solid acids have unique properties that lie between those of normal acids and normal salts. Importantly, solid acids are very efficient at conducting protons when they are heated to "warm" temperatures.

However, their use for any application was largely ignored because they are water-soluble and difficult to fabricate into useful forms. In previous work, Haile explored the applicability of the solid acid CsHSO4 as a fuel cell electrolyte and demonstrated the successful operation of such a fuel cell. She found that the key to creating a functional solid-acid fuel cell is an operation temperature above 100 degrees C, which ensures that water in the system, which would otherwise dissolve and leach away the solid acid, is present as harmless steam.

The CsHSO4 electrolyte fuel cell suffered from a serious problem that prohibited its use for power generation. Specifically, the output of the fuel cell decreased over time as the hydrogen fuel reacted with the solid acid in the presence of the catalyst. As reported in their Science paper, Haile and her colleagues circumvented this problem by replacing the CsHSO4 solid acid with CsH2PO4, which does not react with hydrogen.

According to Haile, they were initially hesitant to use this material because it decomposes via dehydration into a nonuseful salt. However, they found that humidifying the fuel cell anode and cathode chambers with a relatively low level of water vapor could prevent the dehydration reaction and thereby maintain the fuel cell for long-term power generation.

Haile's humidity-stabilized CsH2PO4 fuel cells solve several critical problems that have plagued polymer fuel cell development. First, these solid-acid fuel cells can be operated at higher temperatures than those built with polymer electrolytes, which are limited to temperatures less than 100 degrees C. Operation at "warm" temperatures, 100-–300 degrees C, brings a number of benefits to fuel cell technology. Most directly, catalyst activity is enhanced, resulting in higher-efficiency fuel cells and allowing one to use less of the expensive catalyst.

In addition, the susceptibility of the catalyst to poisoning from carbon monoxide contamination of the fuel decreases. As a consequence, the fuel stream need not be purified as thoroughly as for polymer fuel cells, reducing the overall system complexity. Perhaps most significantly, operation at warm temperatures opens up the possibility of using less-expensive base-metal catalysts, which are not active enough to be considered for low temperature applications.

Additional system simplifications come about from the fact that the radiator necessary for maintaining a fuel cell at about 200 degrees C is much smaller than the one required for maintaining a temperature of about 90 degrees C. This has significant implications for automotive applications. Warm-temperature operation can furthermore be easily integrated with onboard hydrogen-generation systems that produce hydrogen also at warm temperatures. For a polymer electrolyte fuel cell, the hydrogen stream from these generators has to be cooled before it can be introduced into the cell.

Solid-acid fuel cells can be operated in the temperature range of 100–300 degrees C because, unlike polymers, they do not rely on water molecules to transport protons from one side of the membrane to the other. This "dry" proton transport results in additional advantages. In particular, there is no longer a need to remove water that accumulates at the cathode and replenish it at the anode. As a consequence, the overall system is, again, significantly simplified.

In the case of CsH2PO4, a small amount of water partial pressure, equivalent to about 10 percent relative humidity at 100 degrees C, is required in order to prevent dehydration of the material, but no water recirculation is necessary. The dry, solid-acid electrolytes are furthermore much less corrosive than their hydrated, polymer counterparts. This allows for much more flexibility in the choice of materials for the other components of the fuel cell system.

Where solid-acid fuel cells have tremendous advantages over polymer electrolyte fuel cells is in the use of alcohol (e.g., methanol) fuels. Hydrogen "stored" as methanol results in a liquid fuel with a high energy density, which is much easier to transport, store, and carry on board than hydrogen, says Haile. Polymer-based fuel cells do not work well with alcohol fuels because the fuel diffuses across the electrolyte, consuming fuel without generating electrical output. The solid-acid electrolytes are entirely impermeable to methanol, which means very high power outputs are possible—much higher than from polymer fuel cells running on methanol.

While the solid-acid fuel cells solve many of the problems of polymer fuel cells, there are still a few obstacles standing in the way of extensive fuel cell use. A continuing problem of the solid-acid fuel cells is the water solubility of the electrolytes. Haile suggests that clever engineering could circumvent this drawback. However, she plans to solve this problem by developing new solid-acid materials that are water-insoluble.

In developing humidity-stabilizing CsH2PO4 fuel cells, Haile was assisted by the lead author Dane Boysen, a graduate student in materials science; and Tetsuya Uda and Calum Chisholm, both postdoctoral scholars in Haile's lab.