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 (H₂TAPP) 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 (4.0110¹⁹ 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).

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• 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.