Ionothermal Carbon Nanochemistry and Its Use in Energy Conversion

Nanoporous carbon materials are known for their applicability in important areas such as sorption, catalysis and electrochemistry (e.g. fuel cell catalysts, supercapacitor or battery electrodes) It was shown that mesoporous nitrogen doped carbons (NDCs) act as an inexpensive and highly active non-metal catalyst in the oxygen reduction reaction (ORR), with the potential to reach performances of practical need one day.1More importantly nitrogen doped carbons are acting as solid-state ligand and support material for non-noble catalysts using low cost transition metals like iron or cobalt. This class of materials has the potential for a disruptive technology in fuel cells e.g. for automotive applications. A key strategy towards improvement for this aim comprises the generation of carbons with advantageous porosity, which typically means high surface area and mass transport pores. My group is developing a novel nanochemistry strategy that is using inorganic salt melts as unconventional reaction medium for the porogenesis, thereby revisiting classic activation techniques.2 Carbon materials with extra high surface area and pore volumes, four times as high as in commercial activated carbons, are obtained.3 In our most recent work we introduce an in-situ template transformation as a new tool to develop anisotropic tubular porosity towards highly porous NDCs. While we previously observed that residual solid salt can introduce additional macroporosity, we herein purposely in-situ crystallize nanoscopic fibrous salt crystals acting as anisotropic templates for mass transport nanopores.4 The utilization of magnesium chloride in contrast to the more commonly used zinc chloride supports the scalability and shows the more general applicability of the salt templating strategy. The obtained NDCs show very high specific surface areas up to 2780 m2 g 1 and outstanding total pore volumes up to 3.86 cm3 g-1, because of the present unique nanochannel pores. In a next step, we exploit the unique porosity for the development of a mild low temperature formation of non-noble oxygen reduction catalysts with the conservation of the doped carbon chemistry. Very active catalysts with a half wave potential of up to 0.76 V vs. RHE in 0.05 M H2SO4 are obtained after metalation with iron. The catalyst shows four electron selectivity and exceptional stability with only a very low performance degradation of down to ∆E1/2 = 5 mV after 1000 cycles between 0.4-1.0 V at 50 mV s-1 in O2-saturated electrolyte. The Fe K-edge fourier transform EXAFS profile of the most active catalyst is almost completely matching that of the iron(II) phthalocyanine, indicating the possibility to create active and stable FeN4 sites for advanced non-noble oxygen reduction electrocatalysts at chemically non-destructive low temperatures. References: 1. (a) Yang, W.; Fellinger, T.-P.; Antonietti, M., Efficient Metal-Free Oxygen Reduction in Alkaline Medium on High-Surface-Area Mesoporous Nitrogen-Doped Carbons Made from Ionic Liquids and Nucleobases. Journal of the American Chemical Society 2010, 133 (2), 206-209; (b) Jaouen, F.; Proietti, E.; Lefevre, M.; Chenitz, R.; Dodelet, J.-P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P., Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy & Environmental Science 2011, 4(1), 114-130. 2. (a) Fechler, N.; Fellinger, T.-P.; Antonietti, M., “Salt Templating”: A simple and sustainable pathway towards highly porous functional carbons. Advanced Materials 2012, just accepted; (b) Elumeeva, K.; Fechler, N.; Fellinger, T. P.; Antonietti, M., Metal-free ionic liquid-derived electrocatalyst for high-performance oxygen reduction in acidic and alkaline electrolytes. Mater. Horiz. 2014, 1(6), 588-594. 3. Pampel, J.; Fellinger, T. P., Opening of Bottleneck Pores for the Improvement of Nitrogen Doped Carbon Electrocatalysts. Advanced Energy Materials 2016, 6(8). 4. Pampel, J.; Mehmood, A.; Antonietti, M.; Fellinger, T. P., Ionothermal template transformations for preparation of tubular porous nitrogen doped carbons. Materials Horizons 2017, DOI: 10.1039/C6MH00592F.