Przewodnictwo protonowe w sieciach metalo-organicznych : nowe możliwości w technologii ogniw paliwowych

Nowadays energy demands are huge and still increasing. This fact drives the search for modern technologies which are economically advantageous and environmentally friendly. A fuel cell technology is one of many solutions and hydrogen fuel cells are especially important. The essential element of such a cell is the electrolytic membrane which makes proton transfer possible. In this article, selected examples of metal-organic frameworks (MOFs) that can be used as proton-conducting membranes are described. Porous structure of such materials as well as the existence of proton-donating and accepting groups on their pore walls allow for creation of hydrogen bonding network enabling the proton hopping (Grotthuss’s conduction mechanism). The conduction can also occur on the way of diffusion of bigger ions, e.g. H₃O⁺ (vehicular conduction mechanism) Proton conducting MOFs can be divided according to temperature in which these materials can operate. There are two regimes – below 100°C – conductivity in MOFs is aided by the presence of water molecules, and above 100°C – conductivity does not depend on humidity. Important group among MOFs conductive under low-temperature conditions are oxalate-based frameworks. Taking into account synthetic methods, interesting case is the MOF reported by Matoga and co-workers, which was obtained on the way of economically and environmentally friendly mechanosynthesis. High proton conductivity in metal-organic frameworks can be achieved not only when channels are filled with water molecules but also by introduction of non- -volatile organic compounds to framework voids or by incorporating them into the framework. Imidazole, 1,2,4-triazole, pyrazoline or histamine may play the role of such compounds. Remarkable examples of this strategy include the MOF reported by Kitagawa and co-workers, where 1,2,4-triazole molecules are incorporated into the framework as well as the material in which proton conduction occurs owing to the presence of imidazole guest molecules.