Nitrogen-doped hierarchically porous carbon nanopolyhedras derived from core-shell ZIF-8@ZIF-8 single crystals for enhanced oxygen reduction reaction

It's significant to develop efficient and durable metal-free electrocatalysts for oxygen reduction reaction (ORR). The ORR performance of metal-free electrocatalysts are known to be limited by insufficient activity compared with platinum catalysts. Optimizing the configuration of porous texture or electronic structure can create and expose more active sites as well as facilitate mass transfer, which are effective ways to improve ORR activity. Here we fabricated a core-shell N-doped hierarchically porous carbon nanopolyhedras (CS-HPCNs) derived from core-shell ZIF-8@ZIF-8 single crystals. The pore characteristics and doped N species of catalysts were tuned by controlling the carbonization process for higher ORR activity. CS-HPCNs exhibited increased mesopore volume and surface area, higher contents of pyridinic and graphitic N compared with carbon nanocubes (HPCNs) without core-shell structure. Compared to commercial Pt/C catalyst, CS-HPCN1000-5 (carbonized at 1000 degrees C for 5 h) showed comparable onset potential, higher limiting current density and kinetic current density in an approximate four-electron ORR process. The large mesopore volume and surface area of such core-shell hierarchically porous structures, together with high contents of pyridinic and graphitic N, contributed to its good ORR activity. The construction of hierarchically porous structures with enhanced mesopore volume and surface area in metal-free N-doped carbon catalyst is proved to be an efficient approach to boost ORR activity.