Apically Dominant Mechanism for Improving Catalytic Activities of N‐Doped Carbon Nanotube Arrays in Rechargeable Zinc–Air Battery

Abstract

has been widely regarded as one of the most promising energy conversion and storage technologies to meet the growing energy demands of largescale application for electric vehicles and other electricity-related devices. [1] The two prominent reactions involved in ZABs are oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), both of which determine the practical performance of ZABs. Thus, the use of highly efficient and stable electrocatalysts as air electrodes is of paramount significance for facilitating the sluggish ORR and OER, thereby attaining high power and long operating lifetime performance for ZABs. Typically, platinum group metals (PGM) are most favorable ORR and OER catalysts displaying the absolute superiority of catalytic activity. However, the high cost, poor stability/durability, and low poison resistance of PGM have been the primary barriers that hamper widespread commercialization of ZABs. [1b,2] With decades of intensive effort in developing the cost-effective catalysts that possess remarkable catalytic performance, a new generation of PGM-free electrocatalystsThe oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in zinc-air batteries (ZABs) require highly efficient, cost-effective, and stable electrocatalysts as alternatives to high cost and low poison resistant platinum group metals (PGM) catalysts. Although nitrogen-doped carbon nanotube (NCNT) arrays are now capable of catalyzing ORR efficiently, their hydrophobic surface and base-growth mode are found to limit the catalytic performance in the practical ZABs. Here, the concept of an apically dominant mechanism in improving the catalytic performance of NCNT by precisely encapsulating CoNi nanoparticles (NPs) within the apical domain of NCNT on the Ni foam (denoted as CoNi@NCNT/NF) is demonstrated. The CoNi@NCNT/NF exhibits a more excellent catalytic performance toward both ORR and OER than that of traditional NCNT derived from the base-growth method. The ZAB coin cell using CoNi@NCNT/NF as an air electrode shows a peak power density of 127 mW cm −2 with an energy density of 845 Wh kg Zn −1 and rechargeability over 90 h, which outperforms the performance of PGM catalysts. Density functional theory calculations reveal that the ORR catalytic performance of the CoNi@NCNT/NF is mainly attributed to the synergetic contributions from NCNT and the apical active sites on NCNT near to CoNi NPs.

Importance of Electrocatalyst Morphology for the Oxygen Reduction Reaction

“…CoNi NPs were encapsulated at the tips of NCNTs. [207] The CoNi@NCNT/NF has excellent ORR (E 1/2 = 0.87 V) and OER (η 10 = 310 mV) performance in 0.1 M KOH. Meng et al constructed a CoÀ NÀ C electrocatalyst by calcinating the ZIF-67/polypyrrole grown on the CC.…”

“…fabricated a NCNT array supported on the 3D NF. CoNi NPs were encapsulated at the tips of NCNTs . The CoNi@NCNT/NF has excellent ORR ( E 1/2 =0.87 V) and OER ( η 10 =310 mV) performance in 0.1 M KOH.…”

“…Generally, the enhanced ORR performance is mainly related with the M-N x species and N doping structure. FeÀ N 4 and CoÀ N 4 species of MÀ NÀ C structures derived from the metal [207] doping or direct pyrolysis are highly active species for ORR. [106,211,212] Furthermore, the ORR performance will be further enhanced when NPs such as Fe, Co, Cu, and Fe 3 C, were encapsulated inside the carbon shell.…”

NiFe Alloyed Nanoparticles Encapsulated in Nitrogen Doped Carbon Nanotubes for Bifunctional Electrocatalysis Toward Rechargeable Zn‐Air Batteries

“…al discovered an apically dominant mechanism, which can significantly improve the catalytic performance of nitrogen doped carbon nanotube (NCNT) by precisely encapsulating CoNi nanoparticles within the apical domain of NCNT on the Ni foam (CoNi@NCNT/NF). [22] The ZAB cell using CoNi@NCNT/NF as an air-electrode showed a peak power density of 127 mW cm À 2 with an energy density of 845 Wh kg Zn À 1 and rechargeability over 90 h, outperforming the performance of the PGM catalysts. [22] Herein, we report a facile method to fabricate NiFe alloyed nanoparticles encapsulated in carbon nanotubes named Ni x Fe 100À x @N-CNTs as oxygen reversible electrocatalysts for rechargeable ZABs.…”

“…[22] The ZAB cell using CoNi@NCNT/NF as an air-electrode showed a peak power density of 127 mW cm À 2 with an energy density of 845 Wh kg Zn À 1 and rechargeability over 90 h, outperforming the performance of the PGM catalysts. [22] Herein, we report a facile method to fabricate NiFe alloyed nanoparticles encapsulated in carbon nanotubes named Ni x Fe 100À x @N-CNTs as oxygen reversible electrocatalysts for rechargeable ZABs. The Ni 50 Fe 50 @N-CNTs sample stood out among the series, of which its OER performance was superior to the state-of-the-art IrO 2 catalyst, while it had slightly inferior ORR activity but markedly better medium-term durability and long-term stability than the commercial Pt/C catalyst.…”