DOI
https://doi.org/10.25772/2BXC-Q236
Author ORCID Identifier
https://orcid.org/0000-0002-1600-5151
Defense Date
2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Indika U Arachchige
Abstract
Nanoscience or nanotechnology has witnessed a significant surge over the past few decades, driven by the excitement of understanding properties of nanoscale materials and their potential applications in various disciplines such as catalysis, sensing, electronics, photovoltaics, energy, medicine, and environmental science.1,2 Nanoparticles (NPs) exhibit distinctive physical and chemical properties attributed to their size range (1-100 nm) which results in high surface-to-volume ratios and a high degree of surface curvature. However, the majority of the practical applications envisioned do not rely on single NPs but require self-assembly of NPs into macrostructures where the nanoparticulate properties remain unchanged. Assembling techniques such as polymer or biomolecules mediate crosslinking of NPs, superlattice formation or layer by layer assembly, have been useful to produce tunable and ordered macrostructures. However, these methods typically require the use of polymer electrolytes, biomolecules or organic surfactants which separate the NPs from each other, hindering interparticle coupling and charge transport as well as efficient integration of low-dimensional properties. In contrast, the oxidation induced sol-gel assembly has emerged as a powerful tool to assemble NPs into macrostructures without the use of any intervening ligands or substrate supports. This process involves the oxidative removal of the surface ligands from the NP surface, resulting in low coordinate active sites which leads to produce 3-dimentionally connected wet gel structure. Upon supercritical drying, the resultant aerogels exhibit directly connected primary NPs, high active surface area, high porosity, excellent electron and thermal conductivity, making them highly efficient as catalysts or electrocatalysts.
In this dissertation research we have synthesized ultrasmall (3-6 nm), quasi-spherical and narrowly dispersed Au/Ag/Pt alloy NPs and transformed them into freestanding aerogel superstructures to investigate their catalytic efficiency for methanol electrooxidation reaction (MOR). The Au/Ag/Pt alloy NPs were produced by using galvanic replacement reaction (GRR) of glutathione (GSH) coated Ag NPs with H2PtCl6 and HAuCl4 precursors. As-synthesized NPs were later transformed into large, free-standing alloy aerogels via oxidation-induced sol-gel assembly. The alloy aerogels exhibit direct NP connectivity, high crystallinity, surface area (125 ± 0.43 to142 ± 0.93 m2/g), and meso-to-macroporosity with an average pore size of 21.6 ± 2.2 nm. The molar composition of the alloy aerogels was controlled by varying the oxidant (C(NO2)4)/GSH molar ratio, which governs the extent of Ag dealloying with in-situ generated HNO3 and assist in selectively increasing the exposure of Au and Pt on the aerogel surface. The alloy aerogels exhibit superior MOR mass activity (1699 mA/mg), which is ~21.4 and ~2.5 times higher than those of the corresponding precursor NPs and commercial Pt (40 wt.%)/C electrocatalysts (683 mA/mg), respectively. Additionally, the aerogels exhibit enhanced tolerance for carbonaceous byproducts, and maintained ~94% of the initial MOR activity at -0.3 V for 24 h in an alkaline medium. A similar approach has been developed to produce Ag/Pt/Pd alloy NPs and corresponding aerogels as bifunctional electrocatalysts for methanol and ethanol oxidation reactions (EOR). The Ag0.449Pt0.480Pd0.071 aerogel produces mass activities of 3179 mA/mg and 2444.5 mA/mg for MOR and EOR respectively, which are either superior or equivalent to literature reports on noble metals-based electrocatalysts. The synergistic effect of trimetallic alloying, pristine active surface of NPs, and interconnected porous superstructure of the aerogel promotes the dissociative adsorption of alcohols, providing facile pathways for the analytes and reaction intermediates transport throughout the macrostructure and enabling the Au/Ag/Pt and Ag/Pt/Pd alloy aerogels as high efficiency, durable electrocatalysts for next generation of energy conversion studies.
Despite a significant potential of alcohols to substitute fossil fuel and several advantages provided by noble metals as catalysts, the high cost and scarcity of the metals and the emission of CO2 during the production and oxidation of alcohols, motivated us to design electrocatalysts using low-cost and earth abundant transition metal catalysts for green and sustainable energy generation such as molecular hydrogen. Hydrogen carries the highest energy density per unit mass of any fuel and upon combustion produces water, making it the greenest energy source. However, current hydrogen production is largely based on the steam reforming of limited natural gas resources which results in significant CO2 emission, rendering the process unsustainable. In contrast, hydrogen generation via electrochemical water splitting is a sustainable and renewable process, however not widespread and economical because of the requirement of costly Pt group metal (Pt, Pd, Rh, and Ir) catalysts. Therefore, it is critical to develop low-cost and earth abundant metals-based catalysts with optimized surface affinity, reaction kinetics and mechanisms, and high chemical stability and durability. Nickel phosphides have emerged as promising catalysts for HER, of which the activity can be further improved by doping with synergetic transition metals (i.e., Mo, Zn, Co, Fe, and Mn). Theoretical studies have suggested that the introduction of a dopant (M) to Ni5P4 crystal, induces a chemical pressure-like effect that results in a contraction in the bond length of M-Ni in the M-Ni-Ni hollow site. DFT calculation were conducted to establish a relation of |∆GH| of first and second hydrogen absorption with the M-Ni distance, where Zn produced the lowest M-Ni bond and the lowest |∆GH| values. Therefore, Zn doped Ni5P4 (Ni5-xZnxP4)NCs were synthesized via a low-temperature colloidal chemistry route to evaluate HER activity as function of structure and composition. As-synthesized Ni5-xZnxP4 NCs retained the hexagonal crystal structure and solid spherical morphology of binary Ni5P4, with an average particle size from 9.2 -28.5 nm. The HER performance of Ni5-xZnxP4 NCs was found to have volcanic relationship with the Zn concentration, resulting in the lowest overpotential of 131.1 mV to achieve a current density of -10 mA/cm2 current density for Ni4.90Zn0.10P NCs which is significantly lower than the phase pure Ni5P4 NCs (218.1 mV). At higher current densities (> 40 mA/cm2), the Ni5-xZnxP4 with x = 0.10, 0.29 and 0.51, outperformed the HER activity of commercial Pt (40%)/C. Additionally, with Zn doping, bimetallic NCs showed improved stability in alkaline medium compared to binary Ni5P4 NCs. Results gained from this research provide new perspectives to develop high efficiency and durable electrocatalysts from earth abundant metals for water electrolysis and guide researchers to produce novel (electro)catalysts for a number of heterogeneous catalytic studies.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-11-2023