Recent Advances in the Design of 3D PGM-Free Molecular Catalysts for ORR

In the past couple of decades great advancements have been made in the development of PGM-free catalysts based on earth-abundant elements, nitrogen, carbon and transition metals (usually Fe or Co), inspired by biological systems such as porphyrins and phthalocyanines. 1-6 In order to overcome the poor stability and low catalytic activity of transition-metal complexes, a new class of high temperature-treated (HT-treated) catalysts, composed of the same elements, i.e. , a transition metal, carbon and nitrogen, was developed. Although HT-treated PGM-free catalysts exhibit improved activity and stability, their performance remains inferior to PGM catalysts, calling for further improvements to make them a viable alternative to the state-of-the-art materials. In this work, we designed, synthesized and characterized ORR catalysts based on iron, carbon and nitrogen in a well-defined, high surface-area covalent framework (COF) of aerogels. Aerogels are ultralight, porous materials, with ultra-low density and high void volume (> 97%), also known for their unique physicochemical properties such as high porosity, controllable pore size and surface area, low thermal conductivity, just to name a few. 7 The variety of precursors used for aerogel synthesis makes them promising candidates for a wide range of applications in catalysts, capacitors, insulators, absorbents, and many more. 8-9 In the context of electrocatalysis of fuel cell reactions, carbon-aerogels have been mostly used so far as catalyst supports for PGM and PGM-free catalysts. 10-11 In their in form, aerogels can have ultra-high catalytic site density, high surface area, and tunable physical and chemical structures - all very important features for a heterogeneous catalyst. In this talk, we will discuss the synthesis and electrocatalytic properties of an iron-porphyrin aerogel. 5,10,15,20-(tetra-4-aminophenyl)porphyrin (H 2 TAPP) and Fe(II) were used as the building blocks of the aerogel, which was later heat-treated at 600 °C to enhance electronic conductivity and catalytic activity while preserving its macro-structure. The resulting material has a very high concentration of atomically dispersed catalytic sites (4.01∙10 19 sites cm -3 ), capable of catalyzing the oxygen reduction reaction in alkaline solution ( E onset = 0.93 V vs. RHE, TOF = 0.2 e - site -1 s -1 at 0.8 V vs. RHE). Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J., A review on non-precious metal electrocatalysts for PEM fuel cells. Energy & Environmental Science 2011, 4 (9), 3167-3192. Zion, N.; Friedman, A.; Levy, N.; Elbaz, L., Bioinspired Electrocatalysis of Oxygen Reduction Reaction in Fuel Cells Using Molecular Catalysts. Advanced Materials 2018 , 1800406. Jasinski, R., A new fuel cell cathode catalyst. Nature 1964, 201 (4925), 1212-1213. Levy, N.; Mahammed, A.; Friedman, A.; Gavriel, B.; Gross, Z.; Elbaz, L., Metallocorroles as Non-Precious Metal Electrocatalysts for Highly Efficient Oxygen Reduction in Alkaline Media. ChemCatChem 2016, 8 (17), 2832-2837. Levy, N.; Mahammed, A.; Kosa, M.; Major, D. T.; Gross, Z.; Elbaz, L., Metallocorroles as Nonprecious-Metal Catalysts for Oxygen Reduction. Angew. Chem.-Int. Edit. 2015, 54 (47), 14080-14084. Snitkoff, R. Z.; Levy, N.; Ozery, I.; Ruthstein, S.; Elbaz, L., Imidazole decorated reduced graphene oxide: A biomimetic ligand for selective oxygen reduction electrocatalysis with Metalloporphyrins. Carbon 2019, 143 , 223-229. Hrubesh, L. W., Aerogel applications. Journal of Non-Crystalline Solids 1998, 225 , 335-342. Mayer, S.; Pekala, R.; Kaschmitter, J., The aerocapacitor: An electrochemical double‐layer energy‐storage device. Journal of the Electrochemical Society 1993, 140 (2), 446-451. Aegerter, M. A.; Leventis, N.; Koebel, M. M., Aerogels handbook . Springer Science & Business Media: 2011. Marie, J.; Berthon-Fabry, S.; Achard, P.; Chatenet, M.; Pradourat, A.; Chainet, E., Highly dispersed platinum on carbon aerogels as supported catalysts for PEM fuel cell-electrodes: comparison of two different synthesis paths. Journal of Non-Crystalline Solids 2004, 350 , 88-96. Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y., Highly efficient nonprecious metal catalysts towards oxygen reduction reaction based on three-dimensional porous carbon nanostructures. Chem Soc Rev 2016, 45 (3), 517-531.