Biomass-derived N-doped porous carbon supported single Fe atoms as low-cost and high-performance electrocatalysts for oxygen reduction reaction

Abstract

Single-atom catalysts (SACs) are promising for oxygen reduction reaction (ORR) on account of their excellent catalytic activity and maximum utilization of atoms. However, due to the complicated preparation processes and expensive reagents used, the cost of SACs is usually too high to put into practical application. The development of cost-effective and sustainable SACs remains a great challenge. Herein, a low-cost method employing biomass is designed to prepare efficient single-atom Fe-N-C catalysts (SA-Fe-N-C). Benefiting from the confinement effect of porous carbon support and the coordination effect of glucose, SA-Fe-N-C is derived from cheap flour by the two-step pyrolysis. Atomically dispersed Fe atoms exist in the form of Fe—Nx, which acts as active sites for ORR. The catalyst shows outstanding activity with a half-wave potential (E1/2) of 0.86 V, which is better than that of Pt/C (0.84 V). Additionally, the catalyst also exhibits superior stability. The ORR catalyzed by SA-Fe-N-C proceeds via an efficient 4e transfer pathway. The high performance of SA-Fe-N-C also benefits from its porous structure, extremely high specific surface area (1450.1 m2/ g), and abundant micropores, which are conducive to increasing the density of active sites and fully exposing them. This work provides a cost-effective strategy to synthesize SACs from cheap biomass, achieving a balance between performance and cost.

1. Introduction

The electrocatalysts are indispensable for metal-air batteries due to the sluggish kinetics of the oxygen reduction reaction (ORR) on cathodes. Although platinum (Pt)-based electrocatalysts possess high activity for ORR, their scarcity and high cost severely hinder their large-scale applications, thus restricting the commercialization of metal-air batteries [1−4]. Developing low-cost and high-performance alternatives to substitute Pt-based catalysts is still necessary but challenging [5−7]. Fe-based catalysts, especially Fe-N-C catalysts, have been considered to be the most promising class of candidates on account of their high activity, abundant source, and relatively low price [8]. Among them, single-atom catalysts (SACs) with Fe atomically dispersed on N-doped carbon supports have been a hot topic in catalysis in recent years for their excellent performance and nearly 100% atomic utilization of Fe atoms [9−11].

Among Fe-N-C SACs, the single atom Fe coordinated with N atoms (Fe—Nx, normally x=2, 4) has been thought to be the active sites for ORR catalysis [12−15]. The N can not only efficiently anchor and confine the Fe atoms on the carbon matrix, but also regulate the electronic structure of Fe atoms to optimize the adsorption/desorption of intermediates during the ORR process [16, 17]. However, the synthesis of SACs remains challenging because of the agglomeration tendency of Fe atoms during the preparation process, which severely hinders the exposure of active sites and leads to very low atomic utilization efficiency [18, 19]. In particular, metal atoms are more likely to migrate and aggregate at high temperatures. This makes it difficult to obtain SACs with single metal atoms loading, since high temperatures (over 700 ℃) are often required for the formation of the Fe─N bond. There are usually several strategies to inhibit the agglomeration, namely coordination effects, confinement effects, and chemical bonding, of which coordination engineering is the most effective and accessible strategy to overcome the agglomeration [20−22]. During the experimental design, the choice of ligands is a key factor. In the process of pyrolysis, metal atoms can be well dispersed on the carbon matrix through steric hindrance resulting from the coordination between metal atoms and organic ligands. Selecting a suitable carbon support is also critical for the preparation of SACs, which can effectively suppress agglomeration, increase atomic utilization, and improve electrical conductivity to facilitate electron transfer [18].