Synthesis and Characterization of Nanosized Molybdenum Carbide and Molybdenum Selenide for Electrochemical Applications

Abstract

With the continuous increase in the world's population, the basic energy requirement has increased. Most of these requirements are fulfilled in a traditional way where fossil fuels are the primary source. However, the limited fossil fuel resources and environmental emissions have pressurized the scientific community to search for alternate energy sources. Electrochemical reactions such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the future hope of scientists to overcome such an energy crisis. A non-noble catalyst having high catalytic activity, high surface area, high stability, cost-effective and ecofriendly is the need for these reactions to perform. Conventional platinum (Pt) is the best electrocatalyst for HER, while Ir and Ru-based oxides are the ideal electrocatalysts for OER to date. However, its high cost and low abundance in nature hiders its large-scale usage. The supercapacitor, also known as electrochemical supercapacitor or ultracapacitor (ECs), can be a promising candidate for energy storage devices between conventional batteries and capacitors. They exhibit high power density, reliability, high cyclic stability, and a very short charging time. However, the energy density of supercapacitors is lower than batteries to date. Therefore, developing new electrode materials for supercapacitors to improve the energy density and other features such as flexibility and lightweight is an urgent and important requirement. In the present work, nanosized Mo2C and MoSe2 have been synthesized using different Mo and Se sources through a solid-solid reaction method. The prepared samples have been characterized and investigated for different electrochemical applications. The subject matter corresponding to the present study on electrochemical properties has been investigated and organized in eight separate chapters as listed below:  Chapter 1 describes the energy dilemma and the environmental concerns caused by the usage of fossil resources. The need for new renewable and environmental friendly energy sources and highly efficient energy storage devices are described. In this chapter, the role of electrocatalyst/anode material for electrochemical applications has been highlighted. The need for low-cost, high-efficient, and stable non-noble materials to replace the expensive noble metals for electrochemical applications is introduced. The various methods to improve the electrochemical performance of these compounds are also elaborated.  Chapter 2 reviews the latest literature on Mo2C and MoSe2 and their electrochemical properties towards HER, OER, supercapacitors, etc. The chapter also brings out the main gaps in the existing literature in this field.  Chapter 3 presents the design of the present research work. It discusses the formulation of research objectives based on gaps in the existing literature. The chapter establishes the need for the present research. The basic procedure adopted to synthesize the materials is described. The brief description of equipment used for synthesis (pot furnace), electrochemical measurements (electrode preparation, device fabrication, formulas used to calculate electrochemical parameters), and characterization (XRD, SEM, FESEM, EDS, TEM, XPS, Raman) of Mo2C and MoSe2 have also been discussed in this chapter.  Chapter 4 presents the one-step synthesis of pure phase molybdenum carbide (Mo2C and MoC) nanoparticles via the in-situ carburization reduction route using ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) and Hexamethylenetetramine ((CH₂)₆N₄) as precursors. The X-ray diffraction (XRD) results confirm the formation of pure phase Mo2C and MoC at 800 oC for 8h and 15h, respectively. The XPS analysis has also been discussed in this chapter. The TEM micrographs reveal that the graphitic carbon layers are stacked together, and the Mo2C particles are encapsulated inside the carbon cloth. Further, this chapter also discusses the Raman analysis of the samples, indicating higher crystallinity of graphitic carbon within MoC than Mo2C. The pure phase Mo2C shows high performance towards the hydrogen evolution reaction (HER) with a Tafel slope of 129.7 mV dec-1. However, MoC exhibits a low activity towards HER with a Tafel slope of 266 mV dec-1. Both the phases show high stability up to 5000 cyclic voltammetry cycles in the potential range of 0 to 0.4 V. In the case of MoC, the specific capacitance increases during the initial 2000 CV cycles, which may be attributed to the electrode activation during the CV test.  Chapter 5 presents a facile and cost-effective way to synthesize Mo2C by utilizing sodium molybdate dihydrate (Na2MoO4.2H2O and laboratory plastic waste as Mo and Se sources, respectively. Magnesium (Mg) metal powder has been used as a reducing agent. This chapter discusses the synthesis of Mo2C nanoparticles at different reaction temperatures of 500, 600, 700, and 800 oC. The XRD analysis indicates the formation of pure phase Mo2C and Mo2C/MnO2 heterostructure. The TEM analysis reveals that Mo2C nanoparticles are well encapsulated by graphitic carbon layers, which provide high cyclic and structural stability to Mo2C. The electrochemical HER activity and capacitive properties of these samples are discussed in detail. Mo2C synthesized at 500 oC exhibits higher HER activity with a Tafel slope of 79 mV dec-1. To further improve the electrochemical activity of the Mo2C nanoparticles, Mo2C/MnO2 heterostructure has also been developed. This chapter discusses the role of MnO2 nanoflakes in the electrochemical activity of the Mo2C/MnO2 heterostructure. The optimized Mo2C/MnO2 (2:1) heterostructure delivers a current density of 10 mA cm-2 at a low overpotential of 58.3 mV and shows a very low Tafel slope of 36 mV dec-1 in 0.5 M H2SO4. It exhibited long-term cyclic stability up to 5000 CV cycles. This chapter describes a novel and effective method for fabricating noble metal-free HER electrocatalysts utilizing laboratory waste as a carbon precursor.  Chapter 6 presents the synthesis of few-layered 2H-MoSe2 nanosheets via an in-situ selenization route using ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) and selenium metal powder as Mo and Se source, respectively, and their electrochemical charge storage performance. The role of Mg metal powder as a reducing agent/catalyst has been discussed. The structure and morphology of the as-synthesized samples have been discussed systematically. The indications of the formation of few-layered nanosheets are found through Raman and transmission electron microscopy. In this chapter, SEM FESEM and XPS analysis have also been discussed. Further, the electrochemical performance of the MoSe2 nanosheets towards the supercapacitor has been discussed in detail using a three-electrode cell configuration in a 2 M KOH electrolyte solution. The prepared MoSe2 nanosheets show excellent electrochemical performance with a specific capacity of 46.22 mAh g-1 at a current density of 2 Ag-1. The MoSe2 electrode exhibit remarkable cyclic stability up to 2000 charge-discharge cycles. In addition, the fabricated MoSe2 symmetric supercapacitor delivered a specific capacitance of 4.1 Fg-1 at a current density of 0.5 Ag-1. It exhibited high cyclic stability with capacitance retention of 105% and high coulombic efficiency of 100% even after 10000 cycles.  Chapter 7 presents a facile and cost-effective method to synthesize ultrathin MoSe2 nanoflakes using sodium molybdate dihydrate (Na2MoO4.2H2O) and selenium metal powder as Mo and Se sources, respectively. Mg metal powder has been used as a reducing agent or catalyst. The TEM and Raman analysis confirmed the formation of few-layered MoSe2 nanoflakes consisting of 2 to 3 layers. The influence of Se content on the OER and capacitive performance of MoSe2 are discussed in detail. The synthesized MoSe2 sample requires a low overpotential of only 320 mV to reach a current density of 10 mA cm-2 with a low Tafel slope of 45.3 mV dec-1. Also, it exhibited a specific capacity of 25 mAh g-1 at a current density of 1 A g-1. Moreover, the prepared MoSe2 nanoflakes show excellent cyclic stability up to 5000 CV cycles. This chapter suggests that the amount of Se in MoSe2 plays a significant role in achieving maximum electrochemical performance from MoSe2.  Chapter 8 concludes and summarizes the entire work of the present study. This chapter also discusses the major conclusions drawn from the present research. Overall, the study revealed that the carbon support over Mo2C plays a crucial role in the electrochemical activity. MnO2 can effectively enhance the electrochemical activity of Mo2C nanoparticles by providing a highly conductive path to the electrons/ions. Also, the electrochemical performance of MoSe2 can be improved by controlling the amount of Se within the sample. MoSe2 can be synthesized using a stainless-steel autoclave by solid-solid reaction method having excellent activity towards different electrochemical applications. Further, the scope for a possible extension of the present work is also discussed.