Innovative routes towards structured materials: Metal-organic frameworks pave the way for new hierarchically porous carbons for energy storage

MARKUS KLOSE; MARTIN UHLEMANN; FLORIAN WOLKE; KATJA PINKERT; ROMY REINHOLD; JUAN BALACH; TONY JAUMANN; MARKUS HERKLOTZ; JÜRGEN ECKERT; LARS GIEBELER

Simposio; 2015 MRS Spring Meeting; 2015

Metal-organic frameworks (MOFs) constitute a peculiar new class of porous compounds and are currently being investigated for a great variety of applications such as gas separation and storage, catalysis or drug delivery. Besides their stellar performance in those fields, it has been shown recently that MOFs can also act as highly efficient precursors for porous carbon materials.Here we introduce a new class of porous MOF-derived carbons (MDCs) obtained from iron-containing MOFs such as MIL-100(Fe). As it will be shown, depending on the individual synthesis parameters, different pore structures and hierarchies can be obtained from those compounds. For example specific surface areas can be tuned from several hundred nanometers up to 1800 m2/g, thus surpassing even some commercially available activated carbons. One striking feature of this new approach in particular is the possibility to generate highly graphitic carbon-onions with diameters between 3-80 nm, exhibiting a mesoporous interior and small microporous openings in the shells which allow access to the inner voids of these structures. This type of topology has thus far not been achieved by any other approach for the fabrication of MDC materials. A detailed structural analysis of the intermediate compounds that are formed during the synthesis revealed the great importance of iron already being present in the MOF-precursor as a ?built-in? catalyst for the formation of graphitic carbon. Interestingly we find that, in contrast to common belief, carbide phases are not necessarily required in iron-MOF MDCs for the process of catalytic graphitization.The inherent structural features of those MDCs render them very promising candidates for their application in advanced energy storage systems that will help meeting the demands of tomorrow?s technology. An outstanding feature in this respect is that MDCs can not only be used as anode materials in lithium-ion batteries like conventional graphitic carbons, but also as cathodes in the lithium-sulfur-system. In this case reversible capacities of more than 600 mAh/g can be reached with coulombic efficiencies of more than 99% for over 100 cycles. Furthermore, when used as electrodes in supercapacitors, iron-MOF based MDCs show excellent capacities of up to 163 F/g and stable rate capabilities of several orders of magnitude of frequency in PEIS experiments. In addition, long-term measurements prove the stability of MDC-materials in aqueous electrolytes for as much as 10000 cycles without any relevant capacity deterioration. MDCs from iron-MOFs can be considered to hold great promise in the field of hierarchically structured carbon materials synthesis. Our findings provide a comprehensive insight into the extensive potential of this new synthesis route for MDCs and it is expected that the concepts presented here can easily be applied to similar material systems in the near future.