Sulfonated graphene-modified electrodes for enhanced capacitive performance and improved electro-oxidation of hydrogen peroxide

Highlights

• Study and characterization of sulfonated graphene oxide (SGO).

• SGO as promising electrode material for high performance supercapacitor application.

• SGO showed excellent electrocatalytic activity towards H${}_2$O₂ oxidation.

• SGO exhibited electrochemical sensing for H${}_2$O₂ with high sensitivity.

• Reaction kinetics depicts diffusion-controlled electrooxidation mechanism at SGO surface.

Abstract

Electrochemical studies of graphene oxide (GO) containing sulfur-oxygen functional group (sulfonated graphene) and its application as electrode material for supercapacitor and electrocatalytic oxidation of H${}_2$O₂ are demonstrated in this report. The sulfonated graphene (SGO) was synthesized using uniform heating of a homogeneous mixture of GO and ammonium sulfate at an elevated temperature $\sim$245 °C and characterized using FTIR, UV–Vis spectroscopy, and transmission electron microscopy. The electrochemical characterizations showed that the SGO exhibit the capacitance value as high as 248 F/g at current density 0.15 A/g along with superior cyclic stability ($\sim$80% retention of cyclic stability after 8000 cycles). Also, SGO showed excellent electrocatalytic activity towards H${}_2$O₂ oxidation and further, an electrochemical sensor for H${}_2$O₂ detection was fabricated in an aqueous medium with ultra-low detection limit and high sensitivity. The oxidation current was found to be increased linearly with H${}_2$O₂ concentration in the range of 10–100 $\mathrm{\mu }$M and 0.1–1 mM with a detection limit of 10.44 $\mathrm{\mu }$M and 0.038 mM respectively. The detailed kinetic study, including evaluation of kinetic parameters of the electro-oxidation process was also performed from the cyclic voltammetry study. We believe that this work will pave pathways towards diverse functional applications including electrocatalysis, non-enzymatic sensing in medical devices, energy harvesting and storage.

Introduction

The development of novel materials and techniques for energy storage is the utmost necessity for sustainable economic growth and survival of human civilization [1], [2], [3], [4]. For conserving the limited energy supply, electrochemical supercapacitors have attained considerable research attention because of their excellent power density and remarkable cycle life [5], [6]. To improve the performances, several new materials have been used as supercapacitor electrodes including, carbonaceous nanomaterials and their hybrids, conducting polymer, metal oxide/conducting polymer composites and several other [5], [7]. Graphene is a versatile material in the field of electrochemistry because of its eccentric properties and large potential application towards the supercapacitors [8], sensors [9], batteries [10], and solar cells [11], [12]. However, aggregation of the individual nanosheets due to strong stacking forces, limited wettability with aqueous and nonaqueous solvents and inert basal planes result in low capacitance value of the devices when pristine graphene is used as an electrode material. For improving the electrochemical performance of graphene, numerous strategies have been adopted in the past decades, such as, surface modification [13], doping [11], [14], [15], [16], the introduction of functional groups [17], hybrid formation with various other metal oxides [13], [18], [19], [20], functionalized with polymer [21], [22], [23], [24], and mixing with block copolymer [25], etc. Since direct chemical modification of graphene is difficult, graphene oxide is often used for such modification instead of pristine graphene. In particular, surface functionalization of graphene with functional groups such as COOH, NH₂, NO₃, and SO₃H can improve the surface wettability of the electrode with the solvents, increase the extent of double-layer formation and improve the electron transfer between electrode and electrolyte during charge–discharge process [10], [17]. Specifically, graphene oxide (GO) functionalized with SO₃H functional groups are known as sulfonated graphene oxide (SGO), which exhibits much better conductivity and electrochemical performance compared to its pristine analog (i.e. GO) [26], [27], [28], [29], [30]. To this point, sulfonated graphene has been found quite unique for diverse applications, including as proton exchange membrane in fuel cell [26], electrode material for energy storage devices [27], [31], [32], catalysis [28], [33], adsorption of ions and dye separation [29], drug delivery [30], [34], and corrosion inhibitor [30]. Typically, the anchored SO₃H groups to the graphene surface can further enhance the chemical adsorption of different ions, nanoparticle, small molecules, and polymers on the graphene surface to form hierarchical graphene-based composites with enhanced electrochemical performance [32]. For example, Lu et al. [35] developed a catalyst for the proton exchange membrane for fuel cells by using sulfur-doped reduced graphene oxide (S-rGO) as support for deposition of Pt nanoparticle. In a different example, Ravikumar et al. [26] fabricated sulfonated GO paper with good conductivity for fuel cell applications. SGO has also been integrated with conducting polymer like polypyrrole [31] or polyaniline [36] to prepare high-performance electrode material for supercapacitor. Although few studies have been carried out on sulfonated graphene-related materials, however, little work is devoted to the detailed electrochemical study of sulfonated graphene-based charge-storage and electrocatalysis studies. Specifically, electrocatalytic studies of SGO modified electrodes for electrochemical sensing applications have been little explored till date. Recently, it has been reported that sulfur-doped graphene can exhibit excellent electro-catalytic activity towards oxygen reduction though sulfur (electronegativity of sulfur: 2.58) has close electronegativity to carbon (electronegativity of carbon: 2.55) [37], [38], [39], [40], [41]. Such electro-catalytic activity may be attributed to the sulfur related active sites favorable for O${}_2$ adsorption [41]. Tagmatarchis et al. demonstrated the improved electrocatalytic activity of sulfur-doped graphene/MoS₂ or WS₂ towards hydrogen evolution reaction [37]. In another example, Xu, et al. has demonstrated that S-rGO sheets produced by hydrothermal synthesis using graphene oxide (GO) sheets and sodium sulfide (Na₂S) as precursor shows very high conductivity [42]. So, from the above discussion, it can be concluded that graphene or its derivative having CS bond may show catalytic activity towards electro-oxidation of H₂O₂. It should also be noted that sulfur, which is attached with more electronegative oxygen in the sulfonic group, will make sulfur atom more electron-deficient and will favorably act as adsorption sites for catalysis.

Furthermore, there is a constant effort to develop photo or electro-catalyst which can decompose or degrade industrial pollutant and remediate environmental pollution  [43], [44], [45], [46], [47], [48]. Hydrogen peroxide (H${}_2$O₂) is a widely known industrial pollutant and also act as a side product of different enzymatic reactions and its excess presence in a living organism, which can even cause serious diseases like Parkinson and cancer [49], [50]. So, the sensitive and reliable detection of H${}_2$O₂ is of prime importance in the field of analytical chemistry to develop non-enzymatic sensors [51], [52]. Previous literature review suggests that the electro-oxidation of H${}_2$O₂ is challenging compared to its reduction because the oxidation on a solid substrate is difficult to reproduce [53], and generally performed by transition metal electrode (like Pt, Ir, or Pt-Au alloy) or nanoparticle modified electrode with low overpotential. Recently different nanomaterials of metal like Ni₂P nanosheets [54], Cobalt nitride nanowire [55], Nickel Borate [56], Copper-Nitride Nanowire [57], Fe₃N-Co₂N Nanowires [58] have been reported as a novel electrochemical catalyst electrode for non-enzymatic H₂O${}_2$ sensing application. To date, there are very few reports of graphene-based materials for H${}_2$O₂ detection using electro-oxidation of H${}_2$O₂, whereas either they are primarily based on metal, or metal oxide nanoparticles supported GO. Shumbha et al. [40] reported the electrocatalytic activity of sulfur-doped GO/cobalt phthalocyanine and GO/cobalt tetra-amino phthalocyanine nanocomposite towards oxidation of H${}_2$O${}_2$ using CV technique. Also, brominated graphene has been used as a mimic of catalase for electrochemical (electro-oxidation) detection of hydrogen peroxide [59]. However, there are very few reports, which describe the detailed electrochemical study of GO after functionalizations with the sulfur-oxygen functional group and demonstrating its effect on capacitance as well as electrocatalysis. To the best of our knowledge, there is no such report on the use of SGO as electrocatalytic material for the H${}_2$O₂ oxidation, and no such systematic kinetic studies are available for the electro-oxidation reactions.

In this work, we present the synthesis of sulfur-oxygen functionalized graphene oxide and its applications in enhanced supercapacitor devices as well as high-performance electrocatalytic sensors for H${}_2$O₂ detection. We did the detailed electrochemical studies, including cyclic voltammetry (CV), charge–discharge, and electrochemical impedance spectroscopy (EIS) to obtain the performance of the as-prepared SGO as an electrode for supercapacitor devices. We also fabricate the two-electrode assembled capacitance device to study the real-life performance of the materials for energy-storage applications. Similarly, we did the detailed electrochemical studies of the SGO as an electrocatalytic material for H${}_2$O₂ oxidation and further an electrochemical sensor for H${}_2$O₂ detection is fabricated in an aqueous medium with low detection limit and high sensitivity. The manuscript also reported detailed kinetic study of the electro-oxidation of H₂O₂ on the SGO modified electrode. We believe the material described here will be useful for electrochemical applications including sensors and energy storage.