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 VRHE, 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.

1. Introduction

Dimensional carbon materials, such as graphene quantum dots, graphene, and carbon nanotubes, have emerged as highly versatile candidates for electrocatalysis, functioning as active catalysts or catalyst supports [1–3]. Over the past decade, substantial advancements have been achieved in enhancing their catalytic performance through heteroatom doping (e.g., N, S, P, B) and the introduction of geometric carbon defects [4]. These structural and chemical modifications create active, metal-free catalytic sites within the carbon matrix, enabling a synergistic interaction between heteroatom dopants and carbon defects, thereby demonstrating great potential across a variety of electrochemical reactions (e.g., oxygen reduction reaction, CO2 reduction reaction) [5, 6]. In addition, compared to traditional metal-based catalysts, metal-free carbon catalysts offer several distinct advantages, including high stability and low resistance [7, 8], making them promising candidates for practical applications. Despite these advancements, the catalytic performance of metal-free carbon materials remains suboptimal, limiting their broader adoption in industrial applications [9, 10].

Achieving high catalytic performance requires a high density of active sites with enhanced intrinsic activity, coupled with efficient electron and mass transfer pathways [11]. Essentially, the design of high-performance carbon catalysts depends on the rational tuning of critical parameters, including the local electronic environment of active sites, the degree of graphitization, and the pore structure [12, 13]. Specifically, to modulate the electronic environment of heteroatom-doped active sites, strategies such as introducing oxygen-containing functional groups or geometric defects like pentagonal carbon rings [14] have been widely employed [15]. These approaches enhance catalytic activity through multiple modulations of heteroatom dopants. Interestingly, heteroatom-doped graphene, enriched with sp2-C domains [16], consistently outperforms heteroatom-doped amorphous biomass-derived carbon in catalytic applications (e.g., low onset potential, low current density).