MoS2/Nitrogen-doped graphene as efficient electrocatalyst for oxygen reduction reaction

Ternary (molybdenum disulfide/graphene)/carbon nanotube nanocomposites assembled via a facile colloidal electrostatic path as electrocatalysts for the oxygen reduction reaction: Composition and nitrogen-doping play a key role in their performance

Nanocomposites have garnered attention for their potential as catalysts in electrochemical reactions vital for technologies like fuel cells, water splitting, and metal-air batteries. This work focuses on developing three-dimensional (3D) nanocomposites through aqueous phase exfoliation, non-covalent functionalization of building blocks with surfactants and polymers, and electrostatic interactions in solution leading to the nanocomposites assembly and organization. By combining molybdenum disulfide (MoS2) layers with graphene nanoplatelets (GnPs) to form a binary 2D composite (MoS2/GnP), and subsequently incorporating multiwalled carbon nanotubes (MWNTs) to create ternary 3D composites, we explore their potential as catalysts for the oxygen reduction reaction (ORR) critical in fuel cells. Characterization techniques such as X-ray photoelectron spectroscopy, scanning electron microscopy, and X-ray diffraction elucidate material composition and structure. Our electrochemical studies reveal insights into the kinetics of the reactions and structure-activity relationships. Both the (MoS2/GnP)-to-MWNT mass ratio and nitrogen-doping of GnPs (N-GnPs) play a key role on the electrocatalytic ORR performance. Notably, the (MoS2/N-GnP)/MWNT material, with a 3:1 mass ratio, exhibits the most effective ORR activity. All catalysts demonstrate good long-term stability and methanol crossover tolerance. This facile fabrication method and observed trends offer avenues for optimizing composite electrocatalysts further.

Interfacial engineering of transition metal dichalcogenide/carbon heterostructures for electrochemical energy applications

To support the global goal of carbon neutrality, numerous efforts have been devoted to the advancement of electrochemical energy conversion (EEC) and electrochemical energy storage (EES) technologies. For these technologies, transition metal dichalcogenide/carbon (TMDC/C) heterostructures have emerged as promising candidates for both electrode materials and electrocatalysts over the past decade, due to their complementary advantages. It is worth noting that interfacial properties play a crucial role in establishing the overall electrochemical characteristics of TMDC/C heterostructures. However, despite the significant scientific contribution in this area, a systematic understanding of TMDC/C heterostructures' interfacial engineering is currently lacking. This literature review aims to focus on three types of interfacial engineering, namely interfacial orientation engineering, interfacial stacking engineering, and interfacial doping engineering, of TMDC/C heterostructures for their potential applications in EES and EEC devices. To accomplish this goal, a combination of experimental and theoretical approaches was used to allow the analysis and summary of the fundamental electrochemical properties and preparation strategies of TMDC/C heterostructures. Moreover, this review highlights the design and utilization of the interfacial engineering of TMDC/C heterostructures for specific EES and EEC devices. Finally, the challenges and opportunities of using interfacial engineering of TMDC/C heterostructures in practical EES and EEC devices are outlined. We expect that this review will effectively guide readers in their understanding, design, and application of interfacial engineering of TMDC/C heterostructures.

Effect of molybdenum doping on the catalytic activity of VS2/CNT for the oxygen reduction reaction in alkaline media

This study shows that chemical doping is a promising technique to improve the electrocatalytic activity of TMDs. The Mo-VS2-15/CNT/GCE catalyst with significant ORR activity could be an excellent alternative for platinum.

Fabrication of 3D self-supported MoS2-Co-P/nickel foam electrode for adsorption-electrochemical removal of Cr(Ⅵ)

Electrochemistry is considered to be one of the most efficient and environment-friendly methods for removing highly toxic Cr (Ⅵ). In this study, a 3D self-supported MoS₂-Co-P/nickel foam (NF) electrode was prepared via a calcination-hydrothermal process to remove the Cr (Ⅵ) in aqueous medium. Scanning electron microscope (SEM) analysis indicated that the pine-needle-like Co₂P nanoneedle and flower-like MoS₂ nanosheets were successfully loaded on the three-dimensional (3D) framework of NF, which provided abundant active sites. The electrode modified by Co, P and MoS₂ exhibited high removal efficiency of Cr (Ⅵ) (96.9%) at pH 3.0, current of 0.128 mA and voltage of 2.5 V. Co, P and MoS₂ have the synergistic promotion on the catalytic performance of electrodes, and the reduction efficiency of Cr (Ⅵ) was greatly improved by 127.5 times relative to pure NF. The enhanced removal of Cr (Ⅵ) was related to the coupling effect of adsorption and electrocatalytic reduction. The mechanism study indicated that electron (e^-) is the active species of Cr (Ⅵ) reduction. The Cr (Ⅵ) removal rate was maintained at 90 ± 1% after five successive cycle experiments, demonstrating good stability and potential industrial applications of MoS₂-Co-P/NF.

Glassy Carbon Electrode Modified with N-Doped Reduced Graphene Oxide Sheets as an Effective Electrochemical Sensor for Amaranth Detection

Amaranth is one of the synthetic azo colorants used to improve the appearance and to increase the appeal of some foods and soft drinks. The excessive consumption of amaranth can be associated with health side effects, emphasizing the need to monitor this food dye. Accordingly, the present study aimed to introduce an electrochemical sensor of glassy carbon electrode (GCE) modified with N-doped reduced graphene oxide (N-rGO), N-rGO/GCE, to detect the amaranth sensitively and rapidly. Several electrochemical techniques such as differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), chronoamperometry (CHA), and cyclic voltammetry (CV) are exploited for the evaluation of the efficiency of the developed electrode for the detection of amaranth. We found that N-rGO/GCE enhanced amaranth oxidation, thus significantly elevating the current signal. Amaranth showed that calibration curves ranged from 0.1 to 600.0 µM, and the limit of detection (LOD) (S/N = 3) was 0.03 μM. Finally, the developed sensor was effectively applied for real samples (tap water, apple juice, and orange juice) with acceptable recovery values from 96.0 to 104.3%.

Green Synthesis of Flowerball-like MoS2/VC Nanocomposite and Its Efficient Catalytic Performance for Oxygen Reduction Either in Alkaline or Acid Media

Opening up electrocatalysts for oxygen reduction reaction (ORR) is essential for practical application in fuel cells and metal-air batteries; however, how to make the catalysts with both good performance and low cost is difficult. Recently, research on the ORR of molybdenum disulfide-based catalysts in alkaline electrolytes has been on the rise. However, the development of MoS2 catalyst for acidic ORR is still in its infancy. Herein, without using reductant and morphology control reagent, we firstly obtained flowerball-like MoS2/Vulcan XC-72R (VC) nanocomposites via hydrothermal method. The designed composite exhibits a nearly 4e− ORR process with 0.78 and 0.92 V onset potentials in 0.1 M KOH and HClO4, respectively. Furthermore, the flowerball-like composite shows utmost electrochemical stability judging by 87 and 80% current retention for about 5.5 h either in alkaline or acid media, long term durability for continuous 10,000 cycles, and stronger resistance to methanol than the commercial Pt/C catalyst. The abundant Mo edges as catalytic active centers of flowerball-like structure, high electron conductivity, and enhanced mass transport in either alkaline or acidic electrolyte are favorable for catalytic performance. The prepared catalyst provides great potential for the substitution of noble metal based catalysts in fuel cells and metal-air batteries.

Design of Hybrid Zeolitic Imidazolate Framework-Derived Material with C-Mo-S Triatomic Coordination for Electrochemical Oxygen Reduction

The emergence of Mo-based hybrid zeolitic imidazolate frameworks (HZIFs) with MoO₄ units brings substantial advantages to design and synthesize complex Mo-based electrocatalyst that are not expected in their conventional synthesis path. Herein, as a newly proposed concept, a facile temperature-induced on-site conversion approach (TOCA) is developed to realize the transformation of MoO₄ units to C-Mo-S triatomic coordination in hierarchical hollow architecture. The optimized hybrid (denoted as MoCS_x 1000) shows accelerating oxygen reduction reaction (ORR) kinetics and excellent stability, which are superior to the most reported Mo-based catalysts. Extended X-ray adsorption fine structure (EXAFS) analysis and computational studies reveal that the near-range electronic steering at C-Mo-S triatomic-coordinated nanointerface guarantees moderate ORR intermediates adsorption and thus is responsible for the boosted ORR activity. This work sheds light on exploring the intrinsic activity of catalysts by interfacial electronic steering.

Synergistic catalytic effects of MoO2 and Vulcan carbon on the oxygen reduction reaction

The MoO₂/C composite exhibits superior electrocatalytic activity towards the ORR via a near four-electron reaction path.

Electrogenerated chemiluminescence of Ru(bpy)32+ at MoS2 nanosheets modified electrode and its application in the sensitive detection of dopamine

Electrogenerated chemiluminescence (ECL) of Ru(bpy)₃²⁺ was studied at a MoS₂ nanosheets modified glassy carbon electrode (MoS₂NS/GCE) in neutral condition. Electrochemical results revealed that MoS₂ nanosheets could significantly catalyze the electrochemical oxidation of Ru(bpy)₃²⁺, as a result, strong anodic ECL was obtained. Several impact factors, such as the modified amount of MoS₂ nanosheets suspension, the pH value, and the concentration of Ru(bpy)₃²⁺, were investigated to obtain the optimal experimental condition. Dopamine exhibited apparent inhibiting effect on ECL intensity of Ru(bpy)₃²⁺-MoS₂ nanosheets through energy transfer process, and could be sensitively detected in the range of 1.0 × 10^-9 to 1.0 × 10^-4 mol L^-1. The linear equation between the decrease of ECL intensity and the logthium of dopamine concentration was determined as ΔI = 9965.02 + 1077.03lgC (C in mol L^-1), with the detection of 8.5 × 10^-10 mol L^-1 (3σ). The modified electrode exhibited satisfactory sensitivity, selectivity, and stability, which can be used to detect dopamine in real samples.

Mechanism and activity of the oxygen reduction reaction on WTe2 transition metal dichalcogenide with Te vacancy

WTe2 transition metal dichalcogenide is a promising candidate for the cathode of proton-exchange membrane fuel cells. In this paper, we investigated the mechanism and activity of the oxygen reduction reaction on the monolayer of the WTe2 transition metal dichalcogenide with Te vacancy denoted as WTed 2. By using density functional theory calculations, we studied the reaction intermediates on the surface of WTed 2. The Gibbs free energy was calculated to clarify the thermodynamic properties of the reaction. We discovered that the ORR mechanisms are more favorable outside than inside the vacancy. The ORR activity was found to be comparable to that of the well-known transition metal electro-catalysts.

Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn-Air Batteries

The development of efficient non-noble-metal electrocatalysts is of critical importance for clean energy conversion systems, such as fuel cells, metal-air batteries, and water electrolysis. Herein, uniform Co₉S₈@MoS₂ core-shell heterostructures have been successfully prepared via a solvothermal approach, followed by an annealing treatment. Transmission electron microscopy, X-ray absorption near-edge structure, and X-ray photoelectron spectroscopy measurements reveal that the core-shell structure of Co₉S₈@MoS₂ can introduce heterogeneous nanointerface between Co₉S₈ and MoS₂, which can deeply influence its charge state to boost the electrocatalytic performances. Besides, due to the core-shell structure that can promote the synergistic effect of Co₉S₈ and MoS₂ and provide abundant catalytically active sites, Co₉S₈@MoS₂ exhibits a superior hydrogen evolution reaction performance with a small overpotential of 143 mV at 10 mA cm^-2 and a small Tafel slope value of 117 mV dec^-1 under alkaline solution. Meanwhile, the activity of Co₉S₈@MoS₂ toward oxygen evolution reaction is also impressive with a low operating potential (∼1.57 V vs reversible hydrogen electrode) at 10 mA cm^-2. By using Co₉S₈@MoS₂ catalyst for full water splitting, an alkaline electrolyzer affords a cell voltage as low as 1.67 V at a current density of 10 mA cm^-2. Also, Co₉S₈@MoS₂ reveals robust oxygen reduction reaction performance, making it an excellent catalyst for Zn-air batteries with a long lifetime (20 h). This work provides a new means for the development of multifunctional electrocatalysts of non-noble metals for the highly demanded electrochemical energy technologies.

Cobalt super-microparticles anchored on nitrogen-doped graphene for aniline oxidation based on sulfate radicals

Cobalt super-microparticles anchored on nitrogen-doped graphene (Co-NG) were prepared using an inexpensive method and were tested for heterogeneous oxidation of aniline with peroxymonosulfate (PMS) in aqueous solutions. The crystal structure, morphology, and textural properties of Co-NG hybrids were investigated by various characterization techniques, such as X-ray diffraction, Raman spectroscopy, dark-field scanning transmission electron microscopy and X-ray photoelectron spectroscopy. Electron paramagnetic resonance and classical quenching tests were conducted to investigate the mechanism of PMS activation and aniline oxidation. The catalyst Co-NG exhibits an unexpectedly high catalytic activity in the degradation of aniline in water by advanced oxidation technology based on sulfate radicals (SO₄^-), and 100% decomposition can be achieved in 10min. This paper offers new insights on heterogeneous catalysis.

Mass-Producible 2D-MoS2-Impregnated Screen-Printed Electrodes That Demonstrate Efficient Electrocatalysis toward the Oxygen Reduction Reaction

Two-dimensional molybdenum disulfide (2D-MoS₂) screen-printed electrodes (2D-MoS₂-SPEs) have been designed, fabricated, and evaluated toward the electrochemical oxygen reduction reaction (ORR) within acidic aqueous media. A screen-printable ink has been developed that allows for the tailoring of the 2D-MoS₂ content/mass used in the fabrication of the 2D-MoS₂-SPEs, which critically affects the observed ORR performance. In comparison to the graphite SPEs (G-SPEs), the 2D-MoS₂-SPEs are shown to exhibit an electrocatalytic behavior toward the ORR which is found, critically, to be reliant upon the percentage mass incorporation of 2D-MoS₂ in the 2D-MoS₂-SPEs; a greater percentage mass of 2D-MoS₂ incorporated into the 2D-MoS₂-SPEs results in a significantly less electronegative ORR onset potential and a greater signal output (current density). Using optimally fabricated 2D-MoS₂-SPEs, an ORR onset and a peak current of approximately +0.16 V [vs saturated calomel electrode (SCE)] and -1.62 mA cm^-2, respectively, are observed, which exceeds the -0.53 V (vs SCE) and -635 μA cm^-2 performance of unmodified G-SPEs, indicating an electrocatalytic response toward the ORR utilizing the 2D-MoS₂-SPEs. An investigation of the underlying electrochemical reaction mechanism of the ORR within acidic aqueous solutions reveals that the reaction proceeds via a direct four-electron process for all of the 2D-MoS₂-SPE variants studied herein, where oxygen is electrochemically favorably reduced to water. The fabricated 2D-MoS₂-SPEs are found to exhibit no degradation in the observed achievable current over the course of 1000 repeat scans. The production of such inks and the resultant mass-producible 2D-MoS₂-SPEs mitigates the need to modify post hoc an electrode via the drop-casting technique that has been previously shown to result in a loss of achievable current over the course of 1000 repeat scans. The 2D-MoS₂-SPEs designed, fabricated, and tested herein could have commercial viability as electrocatalytic fuel cell electrodes because of being economical as a result of their scales of economy and inherent tailorability. The technique utilized herein to produce the 2D-MoS₂-SPEs could be adapted for the incorporation of different 2D nanomaterials, resulting in SPEs with the inherent advantages identified above.

Fabrication of zero to three dimensional nanostructured molybdenum sulfides and their electrochemical and photocatalytic applications

Transition metal dichalcogenides (TMDs) are emerging as promising materials, particularly for electrochemical and photochemical catalytic applications, and among them molybdenum sulfides have received tremendous attention due to their novel electronic and optoelectronic characteristics. Several review articles have summarized the recent progress on TMDs but no critical and systematic summary exists about the nanoscale fabrication of MoS₂ with different dimensional morphologies. In this review article, first we will summarize the recent progress on the morphological tuning and structural evolution of MoS₂ from zero-dimension (0D) to 3D. Then the different engineering methods and the effect of synthesis conditions on structure and morphology of MoS₂ will be discussed. Moreover, the corresponding change in the electronic and physicochemical properties of MoS₂ induced by structure tuning will also be presented. Further, the applications of MoS₂ in various electrochemical systems e.g. hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and supercapacitors as well as photocatalytic hydrogen evolution will be highlighted. The review article will also critically focus on challenges faced by researchers to tune the MoS₂ nanostructures and the resulting electrochemical mechanism to enhance their performances. At the end, concluding remarks and future prospects for the development of better MoS₂ based nanostructured materials for the aforementioned applications will be presented.

Synthesis of hybrid carbon spheres@nitrogen-doped graphene/carbon nanotubes and their oxygen reduction activity performance

The hybrid architecture of carbon spheres@nitrogen-doped graphene/carbon nanotubes (CS@N-G/CNT) was synthesized by a hydrothermal and ultrasonic-assisted method.

Layer-separated MoS2 bearing reduced graphene oxide formed by an in situ intercalation-cum-anchoring route mediated by Co(OH)2 as a Pt-free electrocatalyst for oxygen reduction

A significant improvement in the electrochemical oxygen reduction reaction (ORR) activity of molybdenum sulphide (MoS2) could be accomplished by its layer separated dispersion on graphene mediated by cobalt hydroxide (Co(OH)2) through a hydrothermal process (Co(OH)2-MoS2/rGO). The activity makeover in this case is found to be originated from a controlled interplay of the favourable modulations achieved in terms of electrical conductivity, more exposure of the edge planes of MoS2 and a promotional role played by the coexistence of Co(OH)2 in the proximity of MoS2. Co(OH)2-MoS2/rGO displays an oxygen reduction onset potential of 0.855 V and a half wave potential (E1/2) of 0.731 V vs. RHE in 0.1 M KOH solution, which are much higher than those of the corresponding values (0.708 and 0.349 V, respectively) displayed by the as synthesized pristine MoS2 (P-MoS2) under identical experimental conditions. The Tafel slope corresponding to oxygen reduction for Co(OH)2-MoS2/rGO is estimated to be 63 mV dec(-1) compared to 68 mV dec(-1) displayed by the state-of-the-art Pt/C catalyst. The estimated number of electrons transferred during oxygen reduction for Co(OH)2-MoS2/rGO is in the range of 3.2-3.6 in the potential range of 0.77 V to 0.07 V, which again stands out as valid evidence on the much favourable mode of oxygen reduction accomplished by the system compared to its pristine counterpart. Overall, the present study, thus, demonstrates a viable strategy of tackling the inherent limitations, such as low electrical conductivity and limited access to the active sites, faced by the layered structures like MoS2 to position them among the group of potential Pt-free electrocatalysts for oxygen reduction.