Date of Graduation

1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Chemical Engineering

Advisor/Mentor

Lauren Greenlee

Committee Member

Jingyi Chen

Second Committee Member

Greg Thoma

Third Committee Member

Christa Hestekin

Fourth Committee Member

Bob Beitle

Keywords

Characterization, Electrochemistry, Materials Science, Nanoparticles, Operando, X-ray Absorption Spectroscopy

Abstract

Hydrogen fuel is increasingly seen as an appealing alternative by both the scientific and the industrial communities in the drive towards a clean energy future. Hydrogen, unlike fossil-based fuels, does not release carbon dioxide, a chief component of greenhouse gases, upon combustion. However, more than 95% of the hydrogen in the world is still produced by burning fossil fuels as this method is currently the only economically feasible option at a large industrial scale.

Water electrolysis shows a lot of potential in both hydrogen generation and in the storage of energy from renewable sources such as wind and sunlight. Likewise, water electrolysis coupled with renewable energy can lead to substantially reduced-carbon emissions. One of the main hurdles for electrolysis to be a viable industrial process is the kinetically slow oxygen evolution half-reaction. Various electrocatalysts have been designed to accelerate the oxygen evolution reaction (OER). Currently, the industry standard catalysts for OER are precious metal based, namely ruthenium, iridium, and platinum. There is a great opportunity for development of cheaper substitutes to these precious metal-based catalysts. Iron-incorporated nickel bimetallic catalysts have shown great promise as viable alternatives.

In this work, bimetallic iron-nickel (Fe-Ni) nanoparticles were extensively studied. Our nanoparticle synthesis method developed as part of this work is aimed at replicating the iron-nickel hydroxide thin films that had been previously studied by the alkaline water electrolysis community. The advantages of nanocatalysts over thin films include the increase in surface area, active catalytic sites, and scalability. One of the critical issues regarding bimetallic catalyst design has been the ratio of iron and nickel in the catalysts. We synthesized three iron-nickel nanoparticles with ratios of varying ratios for our initial studies. We used various electrochemical characterization methods such as cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP) to quantify the activity and the stability of the as-synthesized nanoparticles. Our results indicated that the bimetallic nanoparticles with ≤ 50% Fe are the most active in terms of OER electrocatalysis. However, the bimetallic nanoparticles with lower Fe content were not as stable. Optimization of catalysts should focus on both activity and stability and thus bimetallic nanoparticles with high iron content should still be considered in further designs. To further build on OER catalyst design, we studied the effects of chemical composition and morphology on the bimetallic Fe-Ni catalyst performance and discovered their roles in OER electrocatalytic activity.

Next, we investigated the atomic scale chemistry of Fe-Ni nanoparticles and examined the changes occurring in the Fe-Ni nanoparticles during the OER. The OER community still has not come to an agreement on how the roles of electrocatalyst morphology, nanoscale structure, electrocatalyst synthesis route, and the route of Fe incorporation determine the operando atomic scale structure that exists in FeNiO(H)x electrocatalysts when subjected to the Faradaic OER voltage environment. Further, there remains an open debate around the active site structure(s) for FeNiO(H)x-type OER electrocatalysts. We used a powerful characterization technique, x-ray absorption spectroscopy (XAS), in our efforts to resolve the still outstanding questions as mentioned above. We designed and 3-D printed an operando electrochemical cell for the specific purpose of collecting spectroscopic data while performing OER electrocatalysis. Furthermore, we evaluated two Fe-Ni bimetallic nanoparticles using two different synthesis routes (aqueous-phase solution synthesis and organic-phase solution synthesis (in collaboration with Prof. Jingyi Chen)) to examine how the Fe-Ni bimetallic catalysts behave and how the operando chemistry changes when the synthesis method is varied.

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