Defense Date
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Chemistry
First Advisor
Indika U. Arachchige
Abstract
Hydrogen fuel (H2) production through electrocatalytic water splitting presents an opportunity to decrease the global dependence on fossil fuels. Since energy is required to drive this reaction, careful consideration must be given to the materials used in the design of the electrocatalyst to enable an efficient hydrogen evolution reaction (HER). To date, the best HER materials consist of noble metals such as Pt, which offer favorable hydrogen binding, stable electronic properties and high corrosion tolerance. However, these noble metals are expensive, rare and therefore not conducive for wide scale usage. Nickel based electrocatalysts offer an abundant alternative and their electronic properties can be manipulated through heteroatom doping. Two classes of Ni based HER platforms will be featured. The first class of Ni based materials are nickel phosphides, particularly hexagonal Ni2P and Ni5P4 nanocrystals (NCs) which possess a polarized surface due to the difference in electronegativities between Ni and P. This polarized surface alters the binding energy between the substrate and H more favorably than Ni, which has inherent strong H binding properties. We can further manipulate the surface polarization and H binding interactions by doping Ni2P and Ni5P4 with Zn. The second class of materials are nickel metal alloy NCs with a focus on altering the H binding interactions of Ni through doping with Cr. In this dissertation, colloidal methods were utilized to synthesize Ni5-xZnxP4, Ni2-xZnxP and Ni1-xCrx NCs with control over crystal structure, composition and morphology to be used as earth abundant electrocatalysts for HER. The physical properties of these materials and their composition dependent HER activity in an alkaline electrolyte were systematically examined.
Earth abundant Ni5P4 has emerged as an efficient catalyst for HER and its activity can be enhanced by admixing synergistic metals to modify the surface affinity and consequently kinetics of HER. Computational studies suggest that the HER activity of Ni5P4 can be improved by Zn doping, causing a chemical pressure-like effect on Ni3 hollow sites. Herein, we report a facile colloidal route to produce Ni5-xZnxP4 NCs with control over structure, morphology, and composition and investigate their composition-dependent HER activity in alkaline solutions. Ni5-xZnxP4 NCs retain the hexagonal structure and solid spherical morphology of binary Ni5P4 NCs with a notable size increase from 9.2-28.5 nm for x = 0.00-1.27 compositions. Elemental maps affirm the homogeneous ternary alloy formation with no evidence of Zn segregation. Surface analysis of Ni5-xZnxP4 NCs indicates significant modulation of the surface polarization upon Zn incorporation resulting in a decrease in Niδ+ and an increase in Pδ- charge. Although all compositions followed a Volmer-Heyrovsky HER mechanism, the modulated surface polarization enhances the reaction kinetics producing lower Tafel slopes for Ni5-xZnxP4 NCs (82.5-101.9 mV/dec for x = 0.10-0.84) compared to binary Ni5P4 NCs (109.9 mV/dec). Ni5-xZnxP4 NCs showed higher HER activitywith overpotentials of 131.6-193.8 mV for x = 0.02-0.84 in comparison to Ni5P4 NCs (218.1 mV) at a current density of -10 mA/cm2. Alloying with Zn increases the material’s stability with only a ~10% increase in overpotential for Ni4.49Zn0.51P4 NCs at -50 mA/cm2, whereas a ~33% increase was observed for Ni5P4 NCs. At current densities above -40 mA/cm2, bimetallic NCs with x = 0.10, 0.29, and 0.51 compositions outperformed the benchmark Pt/C catalyst, suggesting that hexagonal alloyed Ni5-xZnxP4 NCs are excellent candidates for practical applications that necessitate lower HER overpotentials at higher current densities.
The HER activity of Ni2P is lower compared to benchmark Pt group catalysts. To address this limitation, an integrated theoretical and experimental study was performed to enhance the HER activity of hexagonal Ni2P through doping with synergistic transition metals. Among the nine dopants studied computationally, Zn emerged as an ideal candidate due to its ability to modulate the hydrogen binding free energy (ΔGH) closer to a thermoneutral value. Consequently, hexagonal Ni2-xZnxP NCs, with a solid spherical morphology, variable compositions (x = 0–17.14%), and an average size ranging from 6.8 to 9.1 nm, were colloidally synthesized to investigate HER activity and stability in alkaline electrolytes. As predicted, the HER performance was observed to be composition-dependent with Zn compositions (x) of 0.03, 0.07, and 0.15 demonstrating superior activity with overpotentials (ɳ-10) of 188.67, 170.01, and 135.35 mV, respectively, at a current density of -10 mA/cm2, compared to hexagonal Ni2P NCs (216.19 mV). Conversely, Ni2-xZnxP NCs with x = 0.01, 0.38, 0.44, and 0.50 exhibited a decrease in HER activity, with corresponding ɳ-10 of 225.29, 269.89, 276.42 and 263.86 mV, respectively. The catalyst with the highest HER activity was determined to be Ni1.85Zn0.15P NCs, featuring a Zn concentration of 5.24%, consistent with composition-dependent ΔGH studies. The highest active Ni1.85Zn0.15P NCs showed improved HER kinetics, enhanced stability, and a marginal (~4%) increase of ɳ-10 after 10 h of continuous HER in alkaline electrolytes. This study offers valuable insights into enhancing the performance of metal phosphides through doping-induced electronic structure variation, paving the way for the development of high-efficiency, earth-abundant, and durable HER electrocatalysts.
Bimetallic nanoparticles (NPs) exhibit modified electronic properties compared to their monometallic counterparts due to synergistic effects that can be tuned by modifying the crystal structure and/or composition. Ni is a promising abundant metal alternative and has been demonstrated to be a more efficient electrocatalysts compared Mo, Co, W, Fe and Cu for HER, primarily due to its favorable H binding free energy ().1,2 By capitalizing on the synergistic features of metal alloys, the H binding properties of Ni can be further modified through admixing with a second metal. Computational analysis suggest that Cr would be an optimal dopant because it shifts the H binding free energy (ΔGH) of the Ni (001) surface closer to the thermoneutral range compared to other 3d-5d metals examined. In addition, a dopant dependent effect on the ΔGH value is predicted with the lowest ΔGH value achieved at an 8% Cr composition. Face centered cubic NiCr alloy NPs were synthesized using colloidal methods with the use of a strong n-BuLi reducing agent to reduce the high valent Cr precursor. As synthesized Ni1-xCrx NPs with Cr compositions ranging from x = 0.02-0.20 (1.93-20.29%) were found to have retained the cubic Ni structure with no evidence of impurities. Cr-doped NPs displayed a faceted morphology with a smaller average particle size ranging from 4.4- 6.0 nm compared to the solid spherical Ni NPs with an average size of 9.2 nm. Furthermore, d-spacing measurements calculated from the Ni (111) reflection of PXRD patterns and HR-TEM images suggest that Cr doping expands the cubic lattice, as Cr-doped NPs exhibited larger d-spacings compared to Ni NPs. Preliminary electrocatalytic measurements conducted in 1 M KOH confirm a Cr composition dependent HER activity as Ni0.98Cr0.02, Ni0.96Cr0.04 and Ni0.95Cr0.05 displayed improved HER activity with ɳ-10 of 173.60, 177.91 and 173.26 mV, respectively, compared to Ni NPs (198.71 mV). NiCr alloys with larger Cr compositions of Ni0.90Cr0.10, Ni0.88Cr0.12 and Ni0.80Cr0.20, demonstrated lower HER activity with ɳ-10 of 207.56, 306.99 and 379.71 mV, respectively. Zn-doped Ni2P, Ni5P4 and Cr-doped Ni NCs showcase the versatility of Ni-based electrocatalysts as these materials illustrate the tunable H-binding properties introduced through heteroatom doping and their subsequent HER response. Most promising is the improved HER performance exhibited by various compositions of Zn doped nickel phosphides at higher current densities compared to the Pt/C standard. This improved HER activity suggests that these compositions would be suitable materials for industrial alkaline electrolyzers and could further advance wide scale hydrogen production through electrocatalytic water splitting.
Rights
© The Author
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
5-9-2024