Anchoring Co Porphyrins in N-Doped 3D Graphene/CNTs to Synergize Macrocyclic and Electronic Configurations for Boosted Electrocatalytic Oxygen Reduction

In the metal porphyrin-based electrocatalysts for oxygen reduction reaction (ORR), unraveling the electronic effect in different levels of coordination sphere of central metal ions is appealing to boost their intrinsic catalytic performance. To understand the long-range electronic effect, herein, we synthesize four cobalt porphyrin molecules featuring electron-donating/withdrawing substituents located in the third coordination sphere, and then, we π-π stack them onto three-dimensional nitrogen-doped graphene/carbon nanotubes (N-G/CNTs). The results demonstrate that the N-G/CNTs carrier orientates the macrocyclic planarity of porphyrin molecules for the extended π-conjugation, by which electron-donating substituents can substantially drive electron accumulation around the Co-N₄ centers. Consequently, electron density of intrinsic Co-N₄ centers is efficaciously compensated and spin crossover of Co²⁺ ion is induced from low spin to high spin states, as to optimize the absorption/desorption of reactive intermediates and lower the reaction energy barriers. The resultant Co-tetra(methoxyphenyl)porphyrin stacked on the N-G/CNTs has the best ORR catalytic performance. An assembled Zn-air battery showcases a peak power density of 151 mW cm^-2, a specific capacity of 807 mAh g^-1, and an outstanding charge/discharge cycle stability of more than 300 cycles, outlining the promising alternative to the Pt/C-based Zn-air battery.