Degree

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Document Type

Dissertation

Abstract

In this dissertation, laser-based manufacturing was applied to develop NiFe-based alloys, with focus on their electrochemical characteristics toward oxygen evolution reaction during water electrocatalysis. The targeted electrocatalysts were designed by combining catalysis principles of absorption energy, electronic effect and geometry effect. Firstly, a one-step laser-based manufacturing method was applied to prepare a thin layer of NiFe-based electrodes with varied Ni to Fe ratios to replace high-cost noble metals as the electrocatalyst for oxygen evolution reaction (OER). Since each Ni has 0.6 d-vacancy and each Fe atom has 2.2 d-vacancies. The alloying of Ni-Fe allows electrons flowing from Ni to Fe, causing the increase of d-vacancy, resulting in a catalyst matching better to the reaction electron transfer coordination. The formation of ultra-small secondary FeNi3 phases effectively increased active sites, as well as the average lattice spacing to match better with the geometries of species involved in OER reactions. Secondly, laser processed alloys usually experience special thermal cycles that could produce anisotropic and heterogeneous microstructures significantly different from parts made by traditional casting. The electrocatalytic performances of laser-processed alloys differ significantly from traditionally processed alloys and provide interesting insights on novel electrocatalyst. The laser processed Ni-Fe alloys are prepared by varied laser scanning speeds as the electrocatalyst toward OER. In addition Ni-Fe alloys prepared by arc melting and spark plasma sintering are also prepared for comparison. Thirdly, monolithic nanoporous NiFe electrocatalyst is developed by dealloying Ni6Fe4Al10 prepared by laser-based manufacturing for the first time. The resulted nanoscale pores provided high surface areas and more active sites for catalytic reactions, while the microscale pores provided sufficient channels for gas and ion diffusion. Compared to the bulk NiFeelectrode, the nanoporous NiFe electrode exhibits an improved electrocatalytic activity. At last, Ni, Fe based high entropy alloys (HEAs), including FeNiMnCrCu and FeCoNiCrAl, were synthesized via arc melting and their electrochemical performance was studied. Due to the varied atomic radius and structure configurations of the elements in an HEA, the lattice parameters in the crystalline are usually seriously distorted and severe defects exist in the lattice with changed valence electron concentration, which are advantageous properties for catalyst performance.

Date

7-31-2018

Committee Chair

Guo, Shengmin

DOI

10.31390/gradschool_dissertations.4683

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