Role of electrolytes on the electrochemical characteristics of Fe₃O₄/MXene/RGO composites for supercapacitor applications

Highlights

• Hybrid composite Fe₃O₄/MXene/RGO (FMR) is fabricated by a simple chemical process.

• FMR, FMX and F worked in the potential window -1.0 to 0 V.

• The order of supercapacitor performance is 5 M LiCl > 1 M Na₂SO₄ > 1 M KOH.

• FMR electrode exhibits 82.1 % of cyclic stability even after 5000 cycles.

Abstract

This study aims at developing hybrid composite materials consisting of iron oxide (Fe₃O₄) /MXene /reduced graphene oxide (RGO) without any impurities using optimized experimental parameters to be utilized in next-generation supercapacitor applications and the searching of suitable electrolyte for the developed hybrid electrode materials. We have generated Fe₃O₄-decorated MXene nanosheets on RGO by a simple chemical oxidation method. The initial grain size of Fe₃O₄ of about 43 nm is further reduced to 30 nm when prepared with MXene nanosheets. The as-prepared samples are used as a negative electrode material and their capacitive performance is analyzed in potassium hydroxide (KOH), sodium sulphate (Na₂SO₄) and lithium chloride (LiCl) electrolytes. Fe₃O₄/MXene/RGO nanocomposites showed the best performance. Regarding the electrolytes, the following order has been obtained 5 M LiCl > 1 M Na₂SO₄ > 1 M KOH, which matches well with the bare ion size order. Moreover, Fe₃O₄/MXene/RGO electrode exhibits an 82.1% of cyclic stability up to 5000 charge/discharge cycles at a current density of 5 A g^-1 demonstrating the best performance.

Introduction

Two-dimensional (2D) materials have various applications in the area of materials science such as energy storage, dielectrics, optical modulation, optoelectronics, electrocatalysis and sensors [1], [2], [3], [4], [5], [6], [7]. Among 2D materials, MXene nano-sheets can be considered as a novel class of 2D materials with an outstanding performance in several fields such as metal ion batteries, supercapacitors, water splitting, photocatalysis, lubrication and electronics [8], [9], [10], [11], [12], [13], [14]. MXene nano-sheets are typically synthesized based upon MAX-phases, which are three-dimensional layered ternary metal nitrides, carbides and any combination of them [15]. An increasing number of members of the MXene family with different early transition metal and variable stoichiometry has been recently observed and a greater number of members can be expected in the near future [16]. MXene nano-sheets have been used for electrochemical supercapacitor applications due to their enhanced volumetric capacitance, which helped to improve the electronic conductivity, active intercalation sites, and surface area [17,18]. Among the MXene family, the most studied and prominent member is Ti₃C₂T_x, for which T_X stands for potentially existing surface terminations including -O₂, -(OH)₂ and -F₂ functional groups. Ti₃C₂T_x nano-sheets have been utilized for electrochemical energy storage applications [19], [20], [21], [22]. Owing to the existence of these surface terminations, Ti₃C₂T_x nano-sheets have proven to demonstrate an enhanced electronic conductivity. In this context, high purity Ti₃C₂T_x nano-sheets showed a conductivity of around 6500 S cm^-1, and even Ti₃C₂T_x prepared by HF etching presented a conductivity of about 1000 S cm^-1 [22], [23], [24]. The theoretical capacity of the MXene nano-sheets is predicted to be 615 C g^-1 [25]. It has been reported that the capacitance of MXene nano-sheets could be enhanced by proper cation interaction and suitable surface modifications [26]. Few composites using MXene nano-sheets have been developed to improve the electronic and surface characteristics of MXene with Ag, MnO₂, Fe₂O₃, Mn₃O₄, Bi₂MoO₆, carbon nanotubes (CNTs) and reduced graphene oxide (RGO) [27], [28], [29], [30], [31], [32], [33]. Concerning metal oxides, Fe₃O₄ magnetic nanoparticles (MNPs) seem to be interesting to be used as a negative electrode material for supercapacitor applications due to their high theoretical capacitance, which has not been experimentally achieved so far [34]. Magnetite stems from the ferrite family with an inverse spinel structure [35] can be easily prepared by chemical processes using low-cost chlorides. The formation of composites with Fe₃O₄ and carbon materials is promising due to their good chemical stability, better electrical conductivity, high electron mobility, larger surface area and tunable magnetic properties [36], [37], [38]. Regarding carbon-based materials, RGO is of special interest due to its superior electrochemical properties required for excellent electrode materials for electrochemical applications [39], [40], [41]. Various metal oxides along with RGO have been used to develop hybrid electrode materials. An improvement of the capacitive performance of the electrode materials with the addition of RGO has been verified [42], [43], [44], [45], [46], [47], [48], [49], [50], [51].

The doping and surface modification of graphene based carbon materials are evaluated for the energy storage and conversion processes [52,53]. The process of making RGO is also important as the final product properties, depending on the synthesis process [54]. Graphene, graphene derivative, 2D analogous materials and their composites are also attempted and found some improvement in the electrochemical applications [55,56]. Excellent capacitance retention has been showed by developing the graphene oxide sheets with simultaneous reduction and covalent grafting of polythiophene on graphene oxide sheets [57]. The addition of inorganic materials such as NiO, MnO₂, ZnO, Mn₃O₄, Fe₂O₃, Fe₃O₄ and MoS₂ along with graphene based carbon materials showed the improved physicochemical characteristics [58], [59], [60], [61].

Apart from the electrode material, the electrolyte is important regarding the resulting electrochemical performance. Only a limited number of studies have tried to address the effect of the electrolyte and the electrolyte's concentration on the electrochemical characteristics of the materials [62], [63], [64], [65]. However, the electrochemical response of each electrode material would depend on the respective electrolyte and their compatibility. Therefore, it is important to evaluate the performance of the electrode materials in different electrolytes.

This study aims at developing hybrid composite materials consisting of Fe₃O₄/MXene/RGO to be used in next-generation supercapacitor applications. We have generated Fe₃O₄-decorated MXene nano-sheets on RGO by a relatively simple chemical oxidation method. The structural, morphological, and magnetic characteristics of the Fe₃O₄/MXene/RGO composites have been assessed. The electrochemical characteristics of these novel composite electrode materials have been studied using three different electrolytes including KOH, Na₂SO₄ and LiCl to understand the role of the electrolyte on the electrochemical performance of the Fe₃O₄/MXene/RGO hybrid nanocomposites.

Fe₃O₄ MNPs were synthesized by chemical oxidation using NaOH and KNO₃ as additives [66]. RGO was prepared by a microwave-assisted synthesis process using l-ascorbic acid [67]. Regarding the synthesis of MXene nano-sheets, the initial Ti₃AlC₂-powder was purchased from FORSMAN SCIENTIFIC Co. Ltd., Beijing (China). In order to create multi-layer Ti₃C₂T_x-nano-sheets, 10 g of Ti₃AlC₂-powder were immersed in 100 ml of a 40% hydrofluoric acid solution. The mixture was stirred for three minutes and, then kept at room temperature for two hours. The as-prepared suspension was then washed using deionized water several times until reaching a pH above 6 and, subsequently, centrifuged to separate the powder. Afterward, the washed powder was filtered under vacuum conditions and dried at room temperature for 24 hours. Optimized amounts of ferrous chloride were dissolved in 100 ml of double distilled water and heated to 60 °C. Then, 200 mg of MXene nano-sheets dispersed in water were added to the solution to decorate the ferrous ions on the layered structure of MXene nano-sheets. In another experimental batch, ferrous ions were mixed with a MXene (200 mg) / RGO (100 mg) dispersion. Afterward, appropriate amounts of NaOH and aqueous KNO₃ were added to the solution. The chemical reaction was going on for 2 hours at 90 °C before cooling the reactor down to room temperature. The formed MNPs were washed several times with double distilled water thus removing the unreacted initial species. During washing, a magnetic bar was used to allow a fast separation of magnetic materials from unreacted species (excess of non-magnetic MXene and RGO). The synthesis process of the Fe₃O₄/MXene/RGO hybrid nanocomposites is schematically shown in Fig. 1. Bare Fe₃O₄, Fe₃O₄/MXene and Fe₃O₄/MXene/RGO samples are abbreviated as F, FMX and FMR.

X-ray diffraction (XRD) patterns were recorded using a Bruker D8 X-ray diffractometer equipped with a Cu-Kα radiation source for phase analysis. Fourier Transform Infra-Red spectroscopy (FTIR) was performed using a Perkin Elmer FTIR spectrophotometer. A vibrating sample magnetometer (VSM) (Model 7404, Lakeshore, USA) was utilized to record the magnetic hysteresis loop at room temperature. The morphology of the bare and composite materials was examined by field emission scanning electron microscopy (FESEM) (FEI Quanta 250 FEG) and transmission electron microscopy (TEM) (Tecnai F20 FEG TEM with 200 kV accelerating voltage).

The overall supercapacitor performance is mainly based on the interaction between the electrode and electrolyte, which is a crucial function apart from the operating voltage. The electrolyte plays a significant role in the electrochemical parameters including cycling stability, operating temperature, power density, equivalent series resistance and self-discharge rate of the supercapacitors. Therefore, it is highly important to investigate the electrochemical performance of the as-prepared materials in different electrolytes. Herein we have studied the electrochemical performance of as-prepared materials in different electrolyte ion size including 5 M LiCl, 1 M Na₂SO₄ and 1 M KOH. All electrochemical experiments were done using a PARSTAT MC-1000 electrochemical workstation with a VERSA STUDIO software. The electrochemical characterizations included cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). All experiments were done for three electrode systems, for which the synthesized composites, Hg/HgO or Ag/AgCl and platinum electrodes were used as working electrode, reference electrode and counter electrode respectively. In the case of electrolyte of 1 M KOH, Hg/HgO was used as a reference electrode, whereas Ag/AgCl was utilized for this purpose for 1 M Na₂SO₄ and 5 M LiCl electrolytes.

The electrodes were prepared based upon a binder approach. The active material, PVDF (as a binder) and carbon black (as conductive agent) were used in mass ratios of 75:15:10 and grounded finely using a mortar pastel to form a smooth slurry. The slurry was coated over an area of 1cm² of pre-cleaned stainless steel (SS) substrates of a total area of 2 cm × 1 cm. In order to circumvent the insulating effect of the binder, a conducting agent was used during the preparation of the slurry. In order to mix the slurry, N-methyl pyrrolidone (NMP) was used as a solvent and the coated electrodes were kept at 80 °C overnight. After drying, the mass of the electrodes was measured. The active mass of the electrode was obtained by subtracting the final mass of the coated SS substrate from the original mass of SS substrate.

Based upon the CV and GCD analysis, the specific capacitance of the as-prepared electrodes was calculated using the following equations:

$$\text{From}\text{CV},\text{Cs}=\frac{\int IdV}{2m\Delta V\nu }$$

$$\text{From}\text{GCD},\text{Cs}=\frac{I\Delta t}{m\Delta V}$$

where, $\int IdV$ is the area under the CV curve (A V), m is the active mass of the electrode (gm), ΔV is the working potential window (V), ν is the scan rate (mV s^-1), Δt is the discharge time (s) and I is the discharge current (A).

The CV and GCD analyses were accomplished in the potential window between -1.0 and 0 V. For CV, the electrodes were tested with various scan rates ranging from 5 to 125 mV s^-1. Similarly, GCD profiles were obtained for various current densities ranging from 0.2 to 1 A g^-1.