Synthesis and Characterization of Molybdenum Carbide(s) for Electrocatalytic Applications

Abstract

The increasing demand for energy with the limited supply of resources of fossil fuels have led to the development of new materials for energy production from natural or renewable sources. The production of hydrogen by hydrogen evolution reaction (HER) through electrochemical water splitting can provide a clean and renewable energy resource. The high-performance electrocatalysts are required in order to increase the efficiency of water splitting to promote the HER. Various efforts have been made in last few decades for the development of low cost electrocatalysts for HER. Among the noble-metal free catalysts, molybdenum carbide (Mo2C) is the fascinating candidate for electrocatalysis because of its low cost, high chemical stability and structural similarity to Pt group metals. Carbon support in various forms enhances the HER and electrochemical double layer capacitance (EDLC) performance. In the present work carbon supported nano Mo2C has been synthesized using different molybdenum and carbon sources. The electrochemical HER activity and EDLC performance of the synthesized products has been studied in detail. The effect of synthesis parameters and the corresponding features of the synthesized products, which also affect the electrochemical properties has been studied. Based on work done the entire study is presented in 8 chapters. Chapter 1 presents the current status of hydrogen production through HER using different techniques like photocatalysis and electrocatalysis. In both the processes, the role of catalyst for efficient HER activity has been described in this chapter. The important parameters affecting the HER activity has also been highlighted. The limitations of various developed catalysts has been discussed to improve the performance of developed materials. The basic introduction of transition metal carbides (TMCs) along with their applications is presented. A detailed discussion of Mo2C has been done considering its nature of bonding and phase transitions (α- Mo2C, β- Mo2C and γ- MoC). The reason to achieve better catalyst resembling to platinum (Pt) is given. It also pave a path to replace Pt an efficient electrocatalyt in HER. The role of various carbon support on HER activity and EDLC performance has been elaborated. Moreover, the need of carbon supported nano scaled Mo2C for such specific applications are also highlighted. Chapter 2 describes the details of literature for the synthesis of Mo2C. Different properties of the synthesized product is also given in detail. The effect of synthesis parameters on the phase transition, composition of MoxCy and their electrochemical properties has been highlighted.Moreover, utilization of non-biodegradable wastes carbon source to obtain Mo2C has also been discussed. Based on literature, their limitations pertaining to synthesis of developed materials and lack of stability of the product phase in the reported results, the work plan for this thesis is also proposed. Chapter 3 presents the methodology adopted for synthesis of nano Mo2C. It also includes the characterization of the synthesized products, which has been obtained using different molybdenum and carbon sources. The brief description of characterizations techniques (XRD, FESEM/TEM, Raman spectroscopy, XPS, and BET) are also been presented in this chapter. The method adopted for electrode fabrication to study the electrochemical activity has been included in this chapter. The importance of the method adopted for synthesis using analytical grade polypropylene and carbonaceous wastes (waste plastics and polyethene) and their electrochemical performance has been described in the subsequent chapters. Chapter 4 describes a brief introduction of need of hydrogen as green and renewable energy source. It also represents the importance of Mo2C as low cost elctrocatalysts for HER and EDLC applications. The optimization of synthesis parameters (reaction temperature, time and amount of initial carbon content) using analytical grade polypropylene as carbon source is done to obtain nano Mo2C. The effect of temperature and holding time on the formation of β-Mo2C@C has been discussed in detail. Features observed in FESEM and TEM analysis of the synthesized β-Mo2C@C has been analyzed critically. TEM/HRTEM results revealed the formation of carbon supported nano β-Mo2C@C. Moreover, Raman spectroscopy and XPS were carried out to understand the nature of carbon network and elemental composition on the surface of optimized samples. The detailed synthesis mechanism associated with pure phase formation has been presented in this chapter. The electrochemical activity of the synthesized products having variation in synthesis temperature and time has been tested in acidic medium. The obtained results are compared with the reported data in literature. The effect of scan rate on cyclic voltammetry (CV) analysis to obtain EDLC has been presented. The potential dependent electrochemical impedance spectroscopy (EIS) plots supporting HER activity has been presented at the end of this chapter. Chapter 5 describes the synthesis of Mo2C/C composite utilizing used pipette tip (waste plastic) as carbon source. The environmental issues caused by the disposal of these plastic wastes and the limitations of recycling/reuse has been addressed. In this chapter, reaction parameters have beenoptimized to obtain pure phase Mo2C encapsulated in carbon matrix (Mo2C/C).The effect of initial amount of carbon content on pure phase formation and the amount of residual surface carbon in the synthesized product has been discussed in detail. The pure phase (Mo2C/C) synthesized under different experimental conditions has been characterized for structural, morphological and surface characteristic by XRD, Raman spectroscopy, FESEM, TEM/HRTEM, XPS, BET and TGA. The dependence on amount of carbon source along with the reaction temperature on the surface characteristics (specific surface area) has been studied. The amount of carbon present in Mo2C/C determined by TGA varying with respect to initial amount of carbon source plays imperative role in protecting the carbide particles from oxidation and firmly contributes to the electrochemical activity. The effect of initial carbon content and its influence on nature of residual surface carbon over Mo2C on HER activity and EDLC has been elaborated. In order to increase the electrochemical efficiency of the carbon coated Mo2C, incorporation of nitrogen (N) was done. The brief description regarding the role of nitrogen (N) in carbon matrix over Mo2C for enhancing electrochemical performance has been described in Chapter 6. The comparative study C-Mo2C and C/N-Mo2C utilizing waste polyethene as carbon source has been presented in this chapter. In this chapter reaction temperature and time have been optimized to obtain pure phase C-Mo2C and C/N-Mo2C in an autoclave. The MoO3 and ammonium hepta molybdate tetra hydrate (AHM) were used as Mo source and N/Mo source for the synthesis of C-Mo2C and C/N-Mo2C composites. The presence of N in the carbon matrix over Mo2C has been confirmed by XPS and STEM. The comparative electrochemical study (HER activity and EDLC performance) of C-Mo2C and C/N-Mo2C has been done to highlight the role of N encapsulation in the carbon matrix supported over Mo2C. Chapter 7 presents formation of Mo2C@C/N using waste plastics as carbon source. The nature of hydrocarbons present in the carbon source affect the pure phase formation of Mo2C (Mo2C@C/N). The role of processing parameters (temperature, time and carbon source) on pure phase formation has been described in detail. The HER activity and EDLC performance of Mo2C@C/N synthesized under different synthesis conditions has been described. Chapter 8 concludes and summarizes the entire work of the present study. The work done shows the prominent role of synthesis parameters on formation of Mo2C. The nature of carbon coating on the surface of the carbide depends on the synthesis parameters, which influence the electrocatalytic activity and efficiency. The importance of using waste carbonaceous species as carbon source and incorporation of nitrogen in the supported carbon matrix over Mo2C on HER activity and EDLC performance has been summarized. Suggestion for the future work is also given at the end of the chapter.