Metal Organic Frameworks (MOFs) as promising Electrocatalysts for Fuel Cells and Synthesis
Metal organic frameworks (MOFs) are an evolving class of porous materials. They can have a big variety of topologies and are prepared from different metals and organic linkers. MOFs have enormous surface area (up to 7000 m2/g), high robust porosity, thermal and chemical stability, and tunable properties. They can be post-synthetically modified, to improve their catalytic activity. Their structure can even be chiral, according to the synthesis and further treatment, leading to beautiful and unique morphologies. They found applications in drug delivery, photoelectronics, gas storage, sensing, gas separation, electrocatalysis and photocatalysis. Besides the synthesis and characterization of MOFs, I will discuss their utility as electrocatalysts for renewable energy sources. Two MOFs, with different morphologies, were studied for oxygen reduction reaction, the bottleneck reaction in H2-O2 fuel cells. Some of the studied MOFs were hexagonal and some are trigonal, with copper, nickel, and cobalt. All were studied at pH 8 and 13, revealing that the activity at pH 13 is better, but the stability is better at pH 8. In addition, the selectivity towards the desired four-electron pathway is best for the hexagonal copper catalyst. Measurements of the catalysts’ conductivity and their electrochemical surface area, gave a clue for the big difference in the activity and selectivity of the hexagonal relative to trigonal MOFs.
In addition, I will talk about electrosynthesis by MOFs. A bimetallic Ni-Fe MOF was prepared, pyrolyzed in different temperatures to form NiFeOx, and used for oxidation of alcohols and amines. It was found that oxidation of methanol is more efficient than oxidation of water in terms of onset potential and hydrogen evolution as a byproduct. The other studied molecules are also oxidized quite efficiently, without breaking the catalyst. For example, methanol can be oxidized by four-electrons to give formate, while hydrogen bubbles are formed on the counter electrode, that can be also used.