Synthesis of Fe₃C@C core-shell catalysts with controlled shell composition for robust oxygen evolution reaction

Highlights

• Fe₃C encapsulated in graphene-like layers were prepared via carbonation process.

• The role of the Fe₃C core and carbon shell were investigated for OER.

• Fe₃C@C-N exhibited a remarkable OER stability under alkaline conditions.

• The shell provides sites for OH⁻ adsorption leading to an enhanced OER activity.

Abstract

In this study, iron carbide (Fe₃C) particles were encapsulated in graphitic carbon shells by a facile carbonation process. These core-shell carbides, in which iron carbide is encapsulated in graphene-like layer (Fe₃C@C) and nitrogen doped graphene-like layer (Fe₃C@C–N) were investigated as oxygen evolution reaction (OER) catalysts to investigate the effect of the carbon shell on catalyst activity and stability. Due to the protective effect of the graphitic carbon shells, the performance of Fe₃C@C and Fe₃C@C–N were considerably better than the bare Fe₃C nanoparticles. The OER activity of Fe₃C@C–N is comparable with that of carbon-supported ruthenium oxide (RuO₂/C), and this core-shell carbide has a remarkable stability and high turnover frequency under alkaline conditions. To elucidate the real active sites of these core-shell carbides, the role of the Fe₃C core, Fe–N_x active sites, and effect of nitrogen doping in the shell were investigated in detail.

Introduction

The development of a catalyst that exhibits excellent activity and stability for the oxygen evolution reaction (OER) is essential to enable practical water splitting processes [1], [2], [3]. The electrochemical OER is kinetically challenging because it involves the transfer of four electrons and protons [4]. Transition metal oxides exhibit an extraordinary catalytic performance for OER [5], [6]; however, these catalysts have limited practical applications because of their instability irrespective of the pH of the solution. These oxides are easily solubilized into metal ion species [7], [8] and the dissolution rate largely depends on the metal. For example, the mass loss of RuO₂ was approximately 4.8 monolayers per hour at an applied potential of 1.8 V vs. RHE, while that of MnO_x was 3.9 monolayer per hour at 1.9 V vs. RHE [9]. Ir-, Ni-, and Co- based catalysts also displayed typical dissolution behaviors [10], [11], which led to a significant decrease in the OER activity [12], [13].

Recently, various studies have demonstrated that transition metals coordinated with nitrogen/carbon atoms exhibit an improved electrocatalytic performance because they have better electrical conductivity, nitrogen/carbon active centers, and exposed active sites [14], [15], [16]. Typically, iron carbide/carbon composites display enhanced catalytic performance and are promising candidates for OER [17], [18]. The electron-rich metal-carbon bonds and graphitic carbon encapsulation make iron carbide/carbon composites ideal matrices for catalysis [19]. In addition, owing to the formation of C–N active sites [20], the activity of a catalyst can be further enhanced by using a nitrogen–doped carbon support [21], [22]. Han et al. reported that an overpotential of 600 mV was required to achieve a current density of 10 mA cm⁻² using an Fe-N_x-C catalyst in the OER [23]. Jiang et al. encapsulated Fe₃C in Fe-N-doped graphene-like carbon hybrids, and the developed catalyst exhibited an excellent performance with a current density of 10 mA cm⁻² at an overpotential of 361 mV [18]. Numerous potential active sites, such as the C-N sites, Fe₃C moieties, and C surface are present in the Fe₃C-carbon composites. Therefore, it is necessary to further understand the actual catalytic sites.

Herein, core-shell iron carbide catalysts were prepared via carbonation for a proficient electrochemical OER catalyst. The OER performance of the synthesized core-shell carbides, viz. Fe₃C encapsulated in graphene-like layers (Fe₃C@C) and nitrogen-doped graphene-like layers (Fe₃C@C-N) under alkaline conditions was considerably better than those of bare Fe₃C nanoparticles and RuO₂/C catalysts. Typically, Fe₃C@C-N exhibited highly enhanced OER activity and stability owing to the synergetic effect of the active core and stable shell structure. The influence of the carbon shell, the effect of nitrogen doping in the carbon shell, and the role of the Fe₃C core were investigated in detail and the results were compared with those of Fe-N_x-based single-atom catalyst (Fe-SAC) and RuO₂/C.