CHEMICALLY SYNTHESIZED TIN/ARSENIC-BASED TERNARY COMPOUNDS AS ELECTROCATALYSTS FOR WATER SPLITTING APPLICATIONS

Abstract

The depletion of fossil fuel reserves and the global challenge of climate change highlight the need for alternative energy solutions. Renewable energy sources have emerged as promising options in this regard. One such technology is water splitting, which involves the electrolysis of water to produce hydrogen, a clean fuel. However, the high energy barrier of the oxygen evolution reaction (OER) in water splitting necessitates the use of electrocatalysts. While noblemetal- based electrocatalysts have shown efficiency, their practical application is hindered by economic and scalability constraints. Transition-metal-based intermetallic compounds (IMCs) present themselves as a potential alternative. Although binary IMC nanoparticles have been extensively studied in the realm of electrocatalysis, research on ternary IMC nanoparticles remains relatively limited. This thesis focuses on the cost-effective synthesis of ternary IMCs, including Ni2CuSn, FeNiAs, and CoNiAs nanoparticles, and their application as efficient electrocatalysts for the oxygen evolution reaction (OER) in water oxidation. Notably, the electrocatalysts were evaluated from a commercial standpoint, as they were utilized as selfsupported electrodes in compacted bar form. This approach eliminates the necessity for expensive binders like Nafion and conductive additives such as carbon, which are typically required for conventional electrocatalysts. The three working chapters of the thesis are structured around the investigation of the aforementioned ternary IMCs. Chapter 1 of the thesis begins with an introduction to the current energy crisis and potential solutions. It provides a concise overview of electrochemical water splitting and reviews previous literature on electrocatalytic water oxidation. The chapter also delves into the challenges and benefits associated with the chemical synthesis of ternary intermetallics, followed by an outline of the thesis objectives. Chapter 2 presents a comprehensive examination of the synthesis methodology for ternary intermetallics, followed by a detailed explanation of the process for fabricating selfsupported electrodes. Additionally, it provides an overview of various characterization techniques utilized in the study, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), induced plasma coupled-mass spectroscopy (ICP-MS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS).In Chapter 3, the investigation focuses on the utilization of the Heusler alloy Ni2CuSn, prepared through a cost-effective and straightforward reduction method, as an electrocatalyst for the oxygen evolution reaction (OER). Interestingly, Ni2CuSn exhibits excellent OER activity, achieving a current density of 10 mA cm-2 with minimal overpotential of 300 mV in alkaline conditions, surpassing the performance of previously Ni-based electrocatalyst. Furthermore, after a durability test lasting 70 hours, it is noted that only a 250 mV overpotential is required to sustain the same current density of 10 mA cm-2. Post-OER examination suggested that this heightened electrocatalytic activity is attributed due to the transformation of Ni2CuSn into its electrochemically active elemental oxides and oxy-hydroxides, specifically NiOOH. This study highlights the promising potential of Ni2CuSn as an OER electrocatalyst and opens new possibilities for other Heusler alloys as viable alternatives to conventional noble-metal electrocatalyst. Chapter 4 and 5 detail the oxygen evolution reaction (OER) activity of ternary intermetallics FeNiAs and CoNiAs, respectively, synthesized through a reduction approach. Both compounds exhibit a hexagonal crystal structure (P6̅2m). Remarkably, the FeNiAs and CoNiAs electrodes achieve a current density of 10 mA cm-2 at exceptionally low applied overpotentials of only 170 mV and 280 mV, respectively. Additional electrochemical measurements, such as Tafel slope and double layer charge capacitance, highlight accelerated kinetics and rapid electron transfer between the electrode and electrolyte in both cases. Subsequent post-OER characterization reveals structural and textural transformations in the electrodes, leading to a proposed mechanism for the enhanced catalytic activity. To summarize, the thesis work proves the electrocatalytic potential of ternary intermetallics for water splitting, laying the groundwork for further exploration of intermetallics to address the energy crisis and promote the growth of the hydrogen economy.