UNVEILINGTHEMULTIFUNCTIONALITY OF INTERMETALLIC COMPOUNDS FOR ENERGY AND CATALYTIC APPLICATIONS

Abstract

Despite significant advancement in material science, developing cost-effective, environmentally sound, and robust materials capable of singularly addressing both interconnected challenges of energy crisis and drinking water scarcity remains a formidable research challenge. Prevailing research predominantly focuses on singular-purpose compounds, necessitating a shift towards multifunctional materials adept at addressing both energy crisis and water pollution challenges. The quest for energy crisis resolution led us to explore materials capable of direct energy generation via renewable sources, exemplified by hydrogen production from water, and harnessing waste heat via thermoelectric materials. Simultaneously, addressing water contamination concerns involved investigating materials conducive to catalytic degradation of pollutants, facilitating their transformation into valuable chemicals. In our pursuit, we aimed to identify such versatile materials capable of multi-faceted functionality. Our investigation centered on the synthesis and characterization of binary and ternary intermetallic compounds like FeSb2, CoVSn, and CoSb and demonstrate their potential across diverse applications encompassing thermoelectricity, hydrogen generation from water, and catalytic degradation of pollutants. These research findings were comprehensively outlined across four distinct chapters, accompanied by two additional chapters dedicated to the introduction and experimental methodologies. Chapter 1 encompasses the foundational aspects of this thesis, starting with a comprehensive exploration of the contemporary landscape surrounding the energy and drinking water crisis. It delves into an extensive analysis of potential remedies, notably focusing on thermoelectric applications, hydrogen generation through water electrolysis, advancements in spintronics, and the catalytic breakdown of pollutants. This chapter meticulously reviews recent advancements and scholarly contributions in these specific domains. Additionally, it concludes by elucidating the thesis objectives, research aims, and the breadth of its scope. Chapter 2 is dedicated to the experimental framework of this thesis, encompassing a comprehensive elucidation of the characterization methodologies employed. This segment encapsulates concise discussions on a range of characterization techniques, including X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet-Visible Spectroscopy (UV-Visible), X-ray Photoelectron Spectroscopy (XPS), Liquid Chromatography-Mass Spectrometry (LC-MS), Electron Backscatter Diffraction (EBSD), and Physical Property Measurement System (PPMS). Furthermore, it meticulously details the instrumentation employed for the characterization of synthesized samples. Additionally, this chapter elucidates the research methodology and synthetic approaches encompassing polyol synthesis, co-precipitation, and arc-melting techniques utilized in sample preparation. Chapter 3 investigates the impact of isoelectronic substitution (Bi) and hole doping (Pb) at the Sb site on the transport properties of cryogenic thermoelectric FeSb2, with a particular focus on thermal conductivity (κ). FeSb2 is a narrow band-gap semiconductor, known for its colossal Seebeck coefficient, holds promise for efficient cooling and power generation in applications ranging from superconductors to quantum computing, provided its thermal conductivity value can be reduced. Polycrystalline FeSb2 powder, along with Bi- and Pb-doped samples, were synthesized using a simple co-precipitation approach, followed by thermal treatment in an H2 atmosphere. XRD and SEM analysis confirms the formation of the desired phase pre- and post-consolidation using spark plasma sintering (SPS). The consolidation process resulted in a high compaction density and the formation of submicrometer-sized grains, as substantiated by electron backscattered diffraction (EBSD) analysis. Substituting 1% of Bi and Pb at the Sb site successfully suppressed the thermal conductivity (κ) from ~15 W/m-K in pure FeSb2 to ~10 and ~8.7 W/m-K, respectively. Importantly, resistivity measurements revealed a metal-to-insulator transition at around 6.5 K in undoped FeSb2 and isoelectronically Bi-substituted FeSb2, suggesting the existence of metallic surface states and provides valuable evidence for the perplexing topological behavior exhibited by FeSb2.Chapter 4 presents the new developments of multifunctional and highly stable intermetallic FeSb2 in three categories: adsorbent, catalyst, and self-supported HER electrocatalyst. FeSb2 particles were synthesized using the coprecipitation method and were found to be highly effective in the adsorptive removal of Congo red (CR) dye, followed by catalytic degradation into less toxic and useful naphthalene and biphenyl derivatives. For electrocatalytic studies, a FeSb2 working electrode with a density greater than 95% was fabricated by the spark plasma sintering process. Remarkably, the FeSb2 electrode generates a geometric current density of −10 mA cm−2 with an overpotential as low as 58 mV and exhibits a Tafel slope of 54.9 mV/dec compared to the benchmark Pt/C catalyst. A strong boost in electrocatalytic activity was found after chronoamperometry testing for 5 h. Overall, this study proves FeSb2 to be a promising and potential electrocatalyst for hydrogen production and an adsorbent cocatalyst for the removal and degradation of CR dye. Chapter 5 discusses the multifaceted capabilities of half-Heusler CoVSn in the realm of energy applications, specifically emphasizing its dual potential for hydrogen generation and thermoelectric functionalities. Theoretical calculations based on first principles underscore cubic-CoVSn as an exceptionally robust and promising candidate for thermoelectric power generation. The synthesis of CoVSn was achieved using the arc-melting technique and optimized successfully into a cubic structure  a previously unattained and highly challenging feat. The resulting electrode, cut from the obtained CoVSn pellet, served as a self-supported electrocatalyst and initially generates a current density of 10 mA cm2 at an overpotential of 244 mV. Remarkably, this overpotential decreased uniquely over time, reaches 202 mV after a durability testing of 12 hours, while maintaining its crystal structure integrity after the electrocatalysis process. This progressive enhancement in catalytic activity and structural stability underscores the significance of this research. However, our evaluation of its thermoelectric capabilities revealed less than optimal performance attributed to lower thermopower and electrical conductivity, largely due to the presence of minor impurity of Sn phase. Overall, our study offers insights into optimizing CoVSn towards attaining the pure cubic phase, thereby paving the way for novel applications wherein other Heusler alloys can potentially serve as electrocatalysts for hydrogen generation.In Chapter 6, the synthesis of binary compound CoSb through polyol approach and its utilization in water purification are explored. The primary objective of this research is to investigate binary compounds and comprehend their multifunctional roles in addressing various energy-related challenges. Additionally, the study delves into examining its efficiency as a catalyst for the efficient conversion of highly toxic p-nitrophenol (p-NP) from aqueous solutions into the valuable pharmaceutical ingradient p-aminophenol (p-AP). The shape control of CoSb without the presence of any reducing/capping agent makes the work interesting, as it was only the variation in stoichiometry which played the game. It was observed that offstoichiometry makes the conditions favorable for the growth of one dimensional structure based on diffusion growth mechanism. However, the catalytic performance observed for the dimensional structures was not much impressive, which was understood in terms of the residual solvent content that remained adsorbed on the surface of the nanoparticles. The approach employed here is also expected to yield similar control over the morphology of other intermetallic compounds also. Chapter 7 reports the concluding segment of this thesis encapsulates a concise summary of each chapter's key findings and contributions. Additionally, it outlines potential future directions for research, suggesting areas where the findings of this study could be further explored or applied to uncover new insights or develop innovative applications.