Fabrication of helical SiO₂@Fe–N doped C nanofibers and their applications as stable lithium ion battery anodes and superior oxygen reduction reaction catalysts

Abstract

Helical silica nanofibers have been prepared based on a sol-gel transcription approach. After covering a layer of m-phenylenediamine formaldehyde resin and consequent carbonization, SiO₂@Fe–N doped C nanofibers with C content of 33.7 wt% and N content of 3.86 wt% were produced successfully. When applied as the electrode material for lithium-ion batteries (LIBs), they exhibit a high reversible capacity of 1274 mA h g⁻¹ in the 1000th cycle and superior rating capability, implying the possibility as outstanding candidate anode materials for LIBs. Additionally, owing to the doping of Fe₂O₃ and N, the nanofibers also exhibit a well-performed oxygen reduction reaction activity, which is comparable to the commercial Pt/C catalyst.

Introduction

Recently, mesoporous carbon nanomaterials have aroused great interest owning to their wide-ranging applications such as electrode materials[1], catalysts and supports [2], and adsorbent [3]. Generally, there are two methods to fabricate mesoporous carbon nanomaterials, the direct synthesis strategy is flexible in preparing mesoporous carbons from organic-organic self-assemblies [4]. The other is called nano-casting strategy, that is, hard templating method [5]. In 1999, Ryoo [6] and Hyeon [7] firstly prepared mesoporous carbons using a hard template of cubic mesoporous silica MCM-48. Thereafter, many other silicas such as SBA series[5], MSU-H [8] and HMS [9] were then utilized as the hard templates to construct mesoporous carbon materials. For example, Kim et al. [10] prepared mesoporous carbons with cubic structure using SBA-16 mesoporous silica template. Lee et al. [9] reported carbons with worm-hole framework meso-structures using hexagonal mesoporous silica template. Zhao et al. [11] have synthesized mesoporous carbon spheres with hierarchical foam-like structures using a dual templating method, spherical silica meso-cellular foams as the hard template and Pluronic F127 as the soft template. The obtained mesoporous carbon nanomaterials well replicated the morphology and meso-structure of the hard templates.

In recent years, considering the low intrinsic capacity (∼372 mA h g⁻¹) of commercial graphitic anodes, mesoporous and nano carbons have been considered as substituted LIBs anode materials. In order to further improve their electrochemical performance, besides designing hierarchical nanostructures, two ways can be realized, one is doping, the other is encapsulating. Among various dopants like N, B, S and P [[12], [13], [14], [15], [16]], N-doped carbon nanomaterials have attracted enormous interest [17,18]. As reported, N-doped carbon materials can increase electric conductivity, the surface polarity, as well as electron-donor tendencies. Previously, we reported helical carbonaceous nanofibers containing 12.4 wt% N content, which achieved a reversible discharge capacity of 499 mA h g⁻¹ after 100 cycles [19]. We also prepared carbonaceous nanorods with 7.87 wt% of N content and 551.3 m² g⁻¹of specific surface area. After 400 cycles, they gained a discharge capacity of 696.7 mA h g⁻¹ [20].

Some materials which possess high theoretical specific capacity such as Si (∼4200 mA h g⁻¹) and Fe₃O₄ (∼927 mA h g⁻¹) were encapsulated in N-doped carbons to fabricate nanocomposites. Kim et al. [21] reported N-doped carbon nanotube encapsulated Si composite electrode materials, which exhibited a high capacity retention up to 79.4% after 200 cycles and rather good rate capability showing 914 mA h g⁻¹ at a 10 C rate. Lee et al. [22] fabricated a N-doped carbon thin film encapsulated Fe₃O₄ composite, which showed a capacity of 850 mA h g⁻¹ after 50 cycles and stable cycling performance. Moreover, via precise designing of nanostructures, SiO₂ (∼1965 mA h g⁻¹) materials can show good electrochemical performance in lithium storage despite of their sluggish Li⁺ diffusion properties and poor intrinsic electronic conductivity. For example, Jiao et al. [23] found that SiO₂ spheres could achieve a specific capacity of 877 mA h g⁻¹ after 500 cycles. Nakashima et al. [24] fabricated SiO₂ hollow nanospheres with a uniform size of 30 nm, which gained a reversible capacity of 359 mA h g⁻¹ at the 100th cycle. Chen’s research group constructed the hollow SiO₂ nanocubes through a hard-templating method, and the hollow SiO₂ nanocubes showed a stable discharge capacity (334 mA h g⁻¹) after 500 cycles [25].

Furthermore, N-doped carbon nanocomposites can also act as attractive energy conversion catalysts, especially as the efficient electrocatalysts applied in oxygen reduction reactions (ORRs). Recently, N-doped carbon materials combined with metals (Pt, Fe and Pd) [[26], [27], [28]] or metal oxides (Fe₃O₄ and MnO₂) [29,30] have aroused much attention. Ferrero et al. [31] prepared a highly efficient Fe/N doped hollow carbon ORR electrocatalyst via the nano-casting approach, which exhibited an outstanding (comparable to Pt/C) performance in acidic media and an excellent activity in basic media. They also showed that both N-sites and Fe–N coordination sites were involved in the catalytic reactions. A novel nonprecious metal catalyst composed of Fe₃C nanoparticles entrapped in mesoporous Fe–N-doped carbon nanofibers was also reported [32]. They showed extraordinary ORR activity with −0.02 V of the onset potential and −0.140 V of the half-wave potential, which was comparable to the state-of-the-art Pt/C catalyst in alkaline and acidic medias. Liu et al. [33] fabricated Fe–N-doped porous carbon derived from petroleum asphalt by a templating method. Their remarkable ORR performance was attributed to the porous construction, high specific surface area, presence of pyridinic N, and the co-doping of Fe and N elements.

In this work, we prepared SiO₂@Fe–N doped C nanofibers from SiO₂ nanofibers encapsulated in Fe³⁺-doped m-phenylenediamine formaldehyde resin. The obtained SiO₂@Fe–N doped C nanofibers were used as LIBs anode materials, moreover were investigated for electrocatalytic performance.