(Invited) Shape Fixing By Salt Nanoreactor to Produce Nanocarbon Materials for the Catalysis of Oxygen Reduction Reaction

Abstract

Developing catalytic materials for oxygen reduction reactions (ORR) with high performance and low cost has been one of the major challenges for large-scale applications of fuel cells. We demonstrated a high-yield " shape fixing via salt recrystallization" method to efficiently synthesize N-doped carbon nanomaterials with a high density of active sites, as an ORR catalyst. The process of shape-fixing involves pouring a supersaturated sodium chloride (NaCl) solution onto a 3D polyaniline (PANI) carbon-based polymer in a beaker, which results in the water evaporating and NaCl recrystallizing around the PANI until the PANI is fully covered by crystals. The NaCl crystal functions as a fully sealed nanoreactor which facilitates the N incorporation and graphitization. The gas from gasification in such a closed nanoreactor produces a large quantity of pores in resultant samples, and the carbonized PANI retains its original 3D shape due to previously being shape-fixed by the NaCl crystal. This makes it possible for the active sites (activated by planar pyridinic and pyrrolic N atoms with sp2 hybrid orbitals, which only appear at the outside fringe of a graphene sheet) to appear in large quantities on the edges of numerous pores. Such favorable catalyst structure leads to the availability of efficient mass transport pathways and a high utilization of the active sites in fuel cell. It results in an excellent ORR activity with a half-wave potential only 58 mV behind that of Pt/C in an acidic medium. The proton-exchange membrane fuel cells with the CPANI-Fe-NaCl-catalyzed cathode outputs a peak power of 600 mW cm-2, which is among the best for non-precious metal catalysts reported so far for the ORR. Additionally, no other method is comparable to this invention, in terms of its high yield per batch of N-doped carbon catalysts. The “shape fixing via salt recrystallization” method represents a powerful approach for the preparation of high-performance carbon nanostructures by confined pyrolysis of nanopolymers and, more importantly, provides insight into the design of advanced PEMFC cathode catalysts.