Joule Heating-Driven sp2-C Domains Modulation in Biomass Carbon for High-Performance Bifunctional Oxygen Electrocatalysis

Abstract

Natural biomass-derived carbon material is one promising alternative to traditional graphene-based catalyst for oxygen electrocatalysis. However, their electrocatalytic performance were constrained by the limited modulating strategy. Herein, using N-doped commercial coconut shell-derived activated carbon (AC) as catalyst model, the controllably enhanced sp²-C domains, through an flash Joule heating process, effectively improve the edge defect density and overall graphitization degree of AC catalyst, which tunes the electronic structure of N configurations and accelerates electron transfer, leading to excellent oxygen reduction reaction performance (half-wave potential of 0.884 V_RHE, equivalent to commercial 20% Pt/C, with a higher kinetic current density of 5.88 mA cm^-2) and oxygen evolution reaction activity (overpotential of 295 mV at 10 mA cm²). In a Zn-air battery, the catalyst shows outstanding cycle stability (over 1200 h) and a peak power density of 121 mW cm^-2, surpassing commercial Pt/C and RuO₂ catalysts. Density functional theory simulation reveals that the enhanced catalytic activity arises from the axial regulation of local sp²-C domains. This work establishes a robust strategy for sp²-C domain modulation, offering broad applicability in natural biomass-based carbon catalysts for electrocatalysis.

Keywords: sp 2-C domains; Carbon-based catalyst; Joule heating; Natural biomass; Oxygen electrocatalysis.

Fig. 1

a Schematic illustration of N-C_D synthesis by Joule heating N-C_D′ and many different natural biomasses were used to prepare N-C_D′. b High resolution transmission electron microscopy (HR-TEM) image of N-C_D′. c ACTEM image of N-C_D. d Enlarged images from the red square in (c), the yellow line area represents sp²-C domains with regular hexagon structures (inset, FFT pattern for the yellow area in (c))

Fig. 2

a–c N₂ adsorption–desorption isotherms, XRD patterns and Raman spectra of Pure C, Pure C_D, N–C and N-C_D. d-f XANES spectrum of C K-edge, high-resolution XPS C 1s spectra and N 1s spectra of N-C_D′ and N-C_D. g EPR signals of different samples

Fig. 3

a LSV curves without iR compensation in O₂-saturated 0.1 M KOH at the scan rate of 10 mV s⁻¹ at 1600 rpm with RDE for carbon catalysts and 20% Pt/C. b E_1/2 and J_k for different samples. c LSV curves of N-C_D at different rotation rates. d Electron transfer number (dotted line) and peroxide yield (H₂O₂%) (solid line) of N-C_D and 20% Pt/C measured with RRDE. e Tafel slopes based on LSV curves (a). f Radar plot for ORR performance comparison with the reported catalysts. g ORR durability measured for N-C_D by long-term chronoamperometric test in O₂-saturated 0.1 M KOH (inset, continuous 5000 times CV scanning). h Normalized ECSAs values with SSAs for different samples. i, j EIS spectra and conductivity diagram under different pressure. k OER LSV curves in 1.0 M KOH for carbon catalysts and commercial RuO₂ catalyst. l Comparison of catalytic performance with many reported ORR-OER bifunctional catalysts

Fig. 4

a–c Effect of Joule heating time and temperature on a 2θ degrees of (002) plane in XRD patterns, b I_D/I_G ratios in Raman spectra, and c ORR performance. d HR-TEM images of N-C_D under different Joule heating time from 1 to 10 s. e Schematic diagram of carbon nanostructure with prolonging Joule heating time and increasing heating temperatures. f, g LSV curves and onset potentials for different natural biomass-based carbon catalysts in O₂-saturated 0.1 M KOH electrolyte. h SSAs change before and after Joule heating at 800 °C for 1s

Fig. 5

a DFT calculation models of N-C_A, N-C_L, N-C_G. b Setup for measuring in-situ Raman spectra. c In-situ Raman spectroscopic study of the *OOH intermediate on N-C_D at various potentials vs. . RHE in 0.1 M KOH. d Scaling relationship between ΔG_*OOH and ΔG_*OH. e ORR volcano plots of theoretical onset potential versus ΔG_*OH. f Gibbs free energy of the ORR intermediates on different catalysts at 1.23 V. g Gibbs free energy of the OER intermediates on different catalysts at 1.23 V

Fig. 6

a Schematic configuration of the assembled ZAB. b Open-circuit voltages for ZABs using air cathodes of N-C_D or Pt/C + RuO₂. c Discharge polarization curves and corresponding power density plots. d Discharge–charge cycling curves for the ZABs at 5 mA cm⁻². e Photograph of a green LED powered by two N-C_D assembled ZABs