Hierarchical N-Doped Porous Carbons for Zn-Air Batteries and Supercapacitors

Abstract

Nitrogen-doped carbon materials with a large specific surface area, high conductivity, and adjustable microstructures have many prospects for energy-related applications. This is especially true for N-doped nanocarbons used in the electrocatalytic oxygen reduction reaction (ORR) and supercapacitors. Here, we report a low-cost, environmentally friendly, large-scale mechanochemical method of preparing N-doped porous carbons (NPCs) with hierarchical micro-mesopores and a large surface area via ball-milling polymerization followed by pyrolysis. The optimized NPC prepared at 1000 °C (NPC-1000) offers excellent ORR activity with an onset potential (E_onset) and half-wave potential (E_1/2) of 0.9 and 0.82 V, respectively (vs. a reversible hydrogen electrode), which are only approximately 30 mV lower than that of Pt/C. The rechargeable Zn-air battery assembled using NPC-1000 and the NiFe-layered double hydroxide as bifunctional ORR and oxygen evolution reaction electrodes offered superior cycling stability and comparable discharge performance to RuO₂ and Pt/C. Moreover, the supercapacitor electrode equipped with NPC prepared at 800 °C exhibited a high specific capacity (431 F g^-1 at 10 mV s^-1), outstanding rate, performance, and excellent cycling stability in an aqueous 6-M KOH solution. This work demonstrates the potential of the mechanochemical preparation method of porous carbons, which are important for energy conversion and storage.

Keywords: Ball milling; Nitrogen doping; Oxygen reduction reaction; Porous carbon; Supercapacitor; Zn–air battery.

Fig. 1

Schematic representation of the preparation of N-doped porous carbon (NPC) via a mechanochemical route

Fig. 2

a, b SEM, and c TEM images of NPC-1000, d–f SEM image of NPC-1000 and corresponding elemental mapping images of C and N

Fig. 3

a XRD patterns and b Raman spectra of different NPCs, c nitrogen adsorption–desorption isotherms of NPC-1000 (inset: pore-size distribution), d XPS spectra, and e, f high-resolution N 1s and C1s spectra of NPC-1000

Fig. 4

a Cyclic voltammograms (CVs), b ORR polarization curves (sweep rate: 10 mV s⁻¹; rotation speed: 1600 rpm) of the as-prepared catalysts in an O₂-saturated 0.1-M KOH electrolyte, c Tafel plots derived from b, d comparison of the onset (E_onset) and half-wave (E_1/2) potentials of different catalysts, e LSV curves of NPC-1000 at different rotation speeds (insert shows the corresponding Koutecky–Levich plots), f peroxide yield and electron transfer number of the as-prepared catalysts at various potentials based on the RRDE data

Fig. 5

a Schematic illustration of the Zn–air battery with NPC-1000 as the air cathode, b image of the ZAB with a measured open-circuit voltage of 1.43 V, c discharge polarization curves and corresponding power-density plots, d, e charge/discharge cycling at 10 mA cm⁻² (0.5 h for each cycle) for a rechargeable ZAB with NPC-1000 and NiFe-LDH as the air cathode, f image of a Bluetooth headset powered by four ZABs in series

Fig. 6

a CV curves at 50 mV s⁻¹, b GCD curves at 0.5 A g⁻¹, c variations in the specific capacitances at different current densities and d Nyquist plots of NPC-700, NPC-800, NPC-1000, and NPC-1100 in a 6-M KOH electrolyte, e cycling performance of NPC-800 at a current density of 10 A g⁻¹, f deconvolution of the charge contribution as a function of the scan rate