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December 14, 2005

High Temperature PEM Fuel Cells being developed

Source: Fuel Cell Review

Atlanta, GA: Heat is often a problem in fuel cells. Sometimes there's too much of it (think ceramic fuel cells) for portable applications in cars or consumer-electronic devices; sometimes there's too little (think proton-exchange-membrane (PEM) fuel cells) to achieve optimum efficiency.

Proton power
That needn't always be so, say scientists at the Center for Advanced Fuel Cell and Battery Technologies at Georgia Institute of Technology (GaTech), who have pinpointed a chemical that allows PEM fuel cells to operate at higher temperatures and with enhanced efficiency. Working in partnership with the Japanese car maker Toyota, Meilin Liu and his colleagues (Zhen Zhou, Siwen Li and Yuelan Zhang) have developed a PEM architecture in which a heterocyclic organic compound called triazole replaces water, yielding a chemistry that they say "opens a new way to practical application of PEM fuel cells".

In conventional PEMs, such as Nafion and sulphonated engineering polymers, water functions as the proton-conducting group. The trouble is, using water has several adverse side-effects. First, the PEM operating temperature has to be kept so low (<100 °C) in order to retain moisture that even trace amounts of carbon monoxide will poison the fuel cell's platinum catalyst. At the same time, heat must be removed from the fuel cell to ensure the correct water balance, in particular adequate levels of PEM hydration (~ 100% humidity). This water and thermal management comes at a price - increased complexity, which has a big impact on overall cell efficiency.

"For practical use," said Liu, "it is crucial to develop PEMs of high conductivity under low humidity (0-50%) at high temperatures (100-200 °C)." Two key approaches are being investigated in this regard. The first attempts to increase the water-holding ability of the Nafion (or similar) membrane at temperatures greater than 100 °C by doping the membrane with inorganic materials. The second looks to replace Nafion-like membranes and to exploit alternative chemical species for proton conduction.

Until now, scientists have had some limited success pursuing the Nafion-replacement strategy with systems such as PBI-H3PO4 and imidazole (another heterocyclic organic compound), where H3PO4 and imidazole are the respective proton conductors. On the plus side, these systems enable fuel cells to operate at temperatures above 100 °C. The downside is that they have problems of their own: the propensity of small molecules like H3PO4 to leak out with water, and the poor conductivity and electrochemical stability of imidazole.

Liu explained: "Based on these previous studies, we started our search for new proton-conducting groups which have high proton-conducting ability and adequate electrochemical stability for fuel-cell operation."

It was during this research that the GaTech group hit upon 1H(2H)-1,2,3-triazole (a molecule that's similar to imidazole, but that has one C atom substituted by N). "The conductivity and electrochemical stability of triazole are much better than imidazole systems," said Liu. So far, the researchers have been able to increase the PEM operating temperature to over 120 °C, dramatically simplifying the cooling system and eliminating the need for a water-management system.