Defense Date
2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Indika Arachchige
Abstract
Molecular hydrogen production (H2) via electrocatalytic water splitting presents a unique opportunity to significantly decrease global dependence on fossil fuels. Since water splitting is a nonspontaneous reaction, careful consideration of the materials used for application to the hydrogen evolution reaction (HER). To date, the best HER catalysts contain varying concentrations of noble metals with Pt being the most common noble metal with its favorable hydrogen binding, high corrosive tolerance, and stable electronic properties. Unfortunately, noble 2 metals are expensive due to their scarcity and high demand, making them a poor choice for large scale implementation. Transition metal electrocatalysts offer an abundant alternative and their electronic properties can be effectively manipulated through heteroatom doping, making them a potential option. Several transition metals have been studied; however, this work will feature two classes of transition metal electrocatalysts. The first class are Fe-based materials, iron phosphides. Iron phosphides, specifically Fe2P nanorods (NRs), offer a tunable crystal structure leading to a more versatile catalyst for study. Furthermore, iron phosphides in general exhibit a polarized surface stemming from electronegativity differences between Fe and P. A polarized surface alters the binding energy between the substrate and H more effectively than Fe, which exhibits strong inherent H binding properties. The surface polarization and H binding capabilities can be further manipulated through heteroatom doping Fe2P with Mo to generate a more effective catalyst. The second class of materials are Ni-based alloy NCs. This work will feature a composition dependent focus on the H binding capabilities when Cr and Mo are doped on their own and simultaneously, resulting in a series of binary and ternary electrocatalysts. In this dissertation, the colloidal synthesis of Fe2-xMoxP, Ni1-xCrx, Ni1-yMoy, and Ni1-x-yCrxMoy NCs was investigated for control over crystal structure, morphology, and composition for application to HER as cost effective and earth-abundant catalysts. The physical properties and their composition-dependent HER activities under alkaline conditions were systematically examined. Iron phosphides have emerged as earth-abundant catalysts for the hydrogen evolution reaction (HER), where performance can be enhanced by admixing synergetic metals to produce bimetallic catalysts. Herein, we report a theoretical and experimental study that reveals the influence of dopant-induced hexagonal to orthorhombic phase transition on the catalytic activity and stability of Fe2P nanorods (NRs) for HER. Among eight metal dopants computationally 3 studied, Mo has been identified as the most promising dopant owing to its optimum hydrogen binding free energy (ΔGH) on the Fe2P (210) surface. Accordingly, hexagonal and orthorhombic Fe2-xMoxP NRs (x = 0 – 14%) with average lengths and widths ranging from 50.9 ± 22.1 to 92.4 ± 43.8 nm and 3.8 ± 1.0 to 6.3 ± 1.8 nm, respectively, were colloidally synthesized to investigate the structure- and composition-dependent HER activity. Upon incorporation of Mo, the underlying hexagonal Fe2P phase transformed into orthorhombic Fe2-xMoxP when x ≥ 0.11 (5.41%). The admixture of Mo caused variations in surface chemistry, leading to a significant decrease in Fe+ and P- charges. The HER performance was observed to be both phase- and composition-dependent with mixed-phase Fe2-xMoxP NRs (x = 0.03, 0.06, and 0.09) exhibiting superior catalytic activity and overpotentials (ɳ-10) of 298, 267, and 222 mV, respectively at a current density (j) of -10 mA/cm2 compared to hexagonal Fe2P (-10 = 378 mV) and orthorhombic Fe2-xMoxP (-10 = 331 – 459 mV for x = 0.12 – 0.28) catalysts. The highest HER performance was achieved for Fe1.91Mo0.09P NRs with a dopant composition of 4.58%, consistent with composition-dependent ΔGH calculations. Although all compositions displayed a Volmer-Heyrovsky HER mechanism, the admixture of Mo improved the HER kinetics, producing the lowest Tafel slope (167.08 mV/dec) for Fe1.91Mo0.09P NRs. The incorporation of Mo improves the charge transfer resistance and preserves the stability of hexagonal and orthorhombic NRs in alkali with a negligible increase in ɳ-10 after 10 h of HER. This study advances the understanding of dopant-induced crystal structure transitions and paves the way for efficient and stable catalytic material design. Similarly, Ni-based alloys have received increased attention as cost-effective and earth-abundant catalysts for the hydrogen evolution reaction (HER), where performance is optimized through admixing synergistic metals, specifically Cr and Mo, to produce binary and ternary alloy 4 catalysts. Herein, we report an integrated theoretical and experimental study that reveals the influence of Cr and Mo doping on the electrocatalytic activity and stability of Ni1-xCrx, Ni1-yMoy, and Ni1-x-yCrxMoy alloy NCs for HER. Theoretical calculations predicted that co-doping of Cr and Mo would significantly improve the HER activity of cubic Ni NCs owing to the lowest ΔGH achieved for Ni1-x-yCrxMoy NCs. Accordingly, a series of cubic Ni1-xCrx, Ni1-yMoy, and Ni1-x-yCrxMoy alloys were colloidally synthesized to investigate composition-dependent HER activity. Structural analysis of NiMo and NiCr alloys revealed average size of 10.2 ± 2.0 and 13.4 ± 2.8 nm, respectively while ternary alloys exhibited significantly larger diameters (33.3 ± 8.9 to 37.4 ± 17.6 nm). Nonetheless, the cubic structure of Ni was retained upon doping with Cr and Mo. Surface analysis of Ni-Cr-Mo alloys showed variations in surface chemistry, leading to shifting of electron density from Ni towards Cr and Mo after initial doping. Of the binary and ternary allows studied, HER performance revealed three high performing samples with overpotential (-10) values at current density (j) at -10 mA/cm2 of 65, 84, and 107 mV for Ni0.914Cr0.057Mo0.029, Ni0.903Cr0.035Mo0.062, and Ni0.935Cr0.031Mo0.034 compositions respectively. The highest performing HER catalyst, Ni0.914Cr0.057Mo0.029 NCs outperformed the HER activity of commercial Pt/C (10 wt%) at j ≥ -30 mA/cm2. Despite all binary and ternary compositions exhibiting a Volmer-Heyrovsky HER mechanism, the co-doping of 5.7% Cr and 2.9% Mo (i.e. Ni0.914Cr0.057Mo0.029 NC) produced the lowest Tafel slope (95.69 ± 0.29 mV/dec) suggesting faster rate kinetics. Moreover, the incorporation of Cr and Mo improves the charge transfer resistance (Rct), ECSA, and stability of cubic Ni to rival commercial Pt/C in alkali with a comparable increase in ɳ-10 after 10 h of HER. This study advances the understanding of composition dependence on catalytic activity of Ni-based alloys and demonstrates how the computational-guided catalyst design and 5 synthesis can be used to produce efficient, earth abundant catalysts for application in water electrolysis.
Rights
© Jordon Baker
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-8-2026