Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Chemical Engineering

Advisor/Mentor

Greenlee, Lauren F.

Committee Member

Chen, Jingyi

Second Committee Member

Thoma, Gregory J.

Third Committee Member

Coridan, Robert H.

Fourth Committee Member

Ford, David M.

Keywords

Electrode-Electrolyte Engineering; Electrodeposition; Hydrogen Evolution Reaction; Iron; Metal-based catalysts; Nickel

Abstract

Electrochemical energy conversion technologies need to be economically viable in order to be adopted. Earth-abundant transition metals (i.e., Fe, Co, and Ni) have attracted attention due to their high availability, low cost, and catalytic characteristics that are in many cases found in and exploited by nature. Herein, we synthesize FexNi100-xOy electrocatalyst films and study how the electrocatalyst-electrolyte interactions alter the hydrogen evolution reaction (HER).In this work, FexNi100-xOy films were synthesized to study the composition, chemical state, and morphology. Characterization results provided a well-established electrocatalyst film platform to then proceed with electrochemical evaluations. The effects of the electrode composition and electrochemically active surface area on HER current density and overpotential were initially evaluated in alkaline electrolyte. We found that the HER activity of FexNi100-xOy films is greatly influenced by the active surface area, which is dependent on the composition. Therefore, the HER activities observed for the electrodeposited FexNi100-xOy films can be attributed to changes in surface area, rather than by an enhancement of the intrinsic catalytic activity of the components. To further build on our work, we studied the effects of the alkaline electrolyte composition for the application of FexNi100-xOy/Si films.

A simple two-sided electrode was used, one side composed of a FexNi100-xOy film, and another composed of Si, were evaluated in LiOH, NaOH, and KOH. The FexNi100-xOy was implemented as the electrocatalyst, while the Si was used as a substrate and as a method to enhance hydrogen production. The electrodes were evaluated using electrochemical methods coupled with the quantification of hydrogen both with applied voltage and at open circuit voltage. The activation of Si was only demonstrated in 0.1 M KOH and NaOH but not in LiOH under the time scales studied. At the same time the Fe80Ni20/Si system performed at lower overpotential and allowed the faster activation of Si to boost the production of hydrogen compared to Fe20Ni¬80/Si. The results demonstrate that the FexNi100-xOy composition and the electrolyte composition influence the performance of the FexNi100-xOy/Si system.

Next, we investigated the application of FexNi100-xOy electrodes in neutral buffered conditions, 0.1 M sodium phosphate at pH of 7.2. The surface chemistry was studied post-electrochemistry with x-ray photoelectron spectroscopy (XPS). At the same time, the HER electrochemical activity and hydrogen production were evaluated for different FexNi100-xOy electrodes. The post-electrochemistry surface chemistry analysis demonstrates a compositional dependence of the FexNi100-xOy electrocatalyst on electrolyte species chemical deposition during electrocatalytic HER studies. Deposition of both sodium and phosphate were observed at high iron content compositions. At the same time, the electrochemical activity, hydrogen production, and faradaic efficiency were demonstrated to be dependent on the FexNi100-xOy composition under neutral buffered conditions.

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