Waste cotton fabric derived porous carbon containing Fe₃O₄/NiS nanoparticles for electrocatalytic oxygen evolution

Abstract

Developing low-cost, active and durable electrocatalysts for oxygen evolution reaction (OER) is an urgent task for the applications such as water splitting and rechargeable metal-air battery. Herein, this work reports the fabrication of a metal and hetero atom co-doped fibrous carbon structure derived from cotton textile wastes and its use as an efficient OER catalyst. The free-standing fibrous carbon structure, fabricated with a simple two-step carbonization process, has a high specific surface area of 1796 m²/g and a uniform distribution of Fe₃O₄/NiS nanoparticles (Fe₃O₄/NiS@CC). The composite exhibits excellent OER performance with an onset potential of 1.44 V and a low overpotential of 310 mV at the current density of 10 mA/cm² in a 1.0 M KOH solution, which even surpass commercial RuO₂ catalyst. Additionally, this ternary catalyst shows remarkable long-term stability without current density loss after continuous operation for 26 h. It can be believed that the outstanding OER performance is attributed to the synergistic effect between the iron oxides and nickel sulphides, as well as the micro-meso porous carbon structure. This study demonstrates a new strategy to use conventional textile materials to prepare highly efficient electrocatalysts; it also provides a simple approach to turn textile waste into valuable products.

Introduction

With the ever-growing concerns over the global energy crisis and environmental pollution, it is of great importance to develop sustainable and clean energy technologies. Recently, hydrogen has attracted great attention due to its environmental friendliness and renewability as an energy carrier [1,2]. Electrochemical water splitting has been considered as the most promising technique for large-scale hydrogen production, due to the abundance of H₂O and low carbon emission of the process. However, the overall energy conversion efficiency of a water splitting unit is largely limited by the sluggish kinetics of oxygen evolution reaction (OER), which creates very high overpotentials. To address this issue, noble metal oxides (such as RuO₂ and IrO₂) are commonly used to improve the kinetics [[3], [4], [5]]. However, their wide industrial applications are hampered by their high price and rarity. Therefore, high efficiency OER electrocatalysts with earth-abundant materials and elements have become a research focus in recent decade [6].

Till now, extensive research has been devoted to the design of earth-abundant transition-metal-based catalysts, such as alloys, metal sulphides, metal oxides/hydroxides, metal phosphides, and metal nitrides [4,5,[7], [8], [9]]. In particular, Ni-based sulphides (eg. NiS [10,11], Ni₃S₂ [12], Ni₃S₄ [13] and Ni₉S₈ [14]) have drawn huge interest owing to their excellent OER activity, which can be even better than the benchmark catalysts. To optimize electrical conductivity and specific surface area of the prepared catalysts, a conductive substrate with 3D porous architecture is essential. For instance, Zhou et al. fabricated Ni₃S₂ nanorods on nickel foam (NF) structure through a one-step hydrothermal process, which exhibited excellent OER activity with an onset potential of 1.39 V vs. RHE in 0.1 M KOH [15].

Recent studies have also shown that the incorporation of additional metal elements, such as Fe [[16], [17], [18]], Co [19], Mo [20,21] or Zn [22], could effectively boost the OER performance of catalysts based on nickel sulphide by tuning their electronic structures. Doped with Co, a NiCo₂S₄/NF structure showed enhanced electrocatalytic activity and long-term stability compared to Ni₃S₂/NF, with the overpotential decreased from 300 to 260 mV at 10 mA/cm², and current density loss of 15 % after 50 h in comparison of 20 % after 10 h [19]. Cheng synthesized Fe-doped Ni₃S₂ particle film on NF with an overpotential of only 253 mV at 100 mA/cm² [23]. An electrocatalyst containing nickel iron sulphides on nickel foam (NiFeS/NF) showed a high water-splitting efficiency. This bi-functional catalyst led to a low OER overpotential of 231 mV at 100 mA/cm² [24]. Furthermore, ultrathin Fe hydroxides were electrodeposited on V-doped nickel sulphide nanowires to achieve enhanced OER property and stability, due to the strong interactions between nickel-based sulphides and Fe-based hydroxides [25]. Despite the significant progresses so far, it is still challenging to use low-cost material to prepare efficient OER electrocatalysts with ideal activity and durability.

As one of the most abundant agricultural resources in the world, cotton fibres, which mainly consist of cellulose microfibres, are being widely used in textile, fashion and biomedical industries. In the last few decades, owing to the steady increase of cotton consumption, a vast amount of cotton textile waste has been generated [26]. Similar to other wastes, textile wastes are usually disposed of through landfill or incineration, which have led to serious economic loss and environment pollution. Therefore, many strategies have been exploited to regenerate values from textile wastes. Inspired by the widespread application of porous carbon materials, lots of attention have been paid to transform cotton textiles into carbon structures due to their low cost, high carbon yields and sustainability [[27], [28], [29], [30]]. Many encouraging achievements have been made on the carbonization of cotton materials for the application such as supercapacitor electrode [[31], [32], [33]]. A few studies on OER electrocatalytic behaviour of fibrous cellulose derived carbon materials have also been reported. However, their fabrication methods either involve highly corrosive gas environment (NH₃) or tedious fabrication procedure, and more importantly, most of them demonstrated unsatisfying OER performance [[34], [35], [36], [37], [38]]. For example, Zhang et al. synthesized a nitrogen-doped carbon catalyst through a one-step carbonization process of polyacrylonitrile coated cotton cloth, which exhibited an OER overpotential of 351 mV at 10 mA/cm current density [34]. In another study, tissue paper was hydrothermally treated and carbonized into porous carbon with cobalt, nitrogen and sulphur doping. This tri-doped catalyst showed an overpotential of 373 mV at 10 mA/cm² for OER, and this number increased by 2% after 20 h operation [39].

In this work, we use cotton fabric offcuts as the raw material to fabricate highly active and durable OER electrocatalyst through a simple two-step carbonization process. With a CO₂ activation carbonization, cotton fabrics are transformed into a hierarchically porous carbon architecture with a high specific surface area of 1226 m²/g. During the second step of the carbonization process in N₂, Fe₃O₄/NiS nanoparticles are formed on the cotton carbon matrix to obtain fibrous catalyst of Fe₃O₄/NiS@CC. This composite catalyst exhibits excellent OER performance with a low overpotential of 310 mV at a current density of 10 mA/cm², along with a small Tafel slop of 82 mV/dec in a 1.0 M KOH solution. Additionally, this ternary catalyst exhibits remarkable long-term stability with no obvious loss in current density after continuous operation for 26 h. It is believed that the outstanding OER performance is contributed by the synergistic effect between the iron oxides and nickel sulphides, as well as the hierarchically micro-meso porous structure of the cotton carbon matrix. This work provides a new strategy to transform cotton wastes into highly active and stable OER electrocatalysts for water-splitting application.