Electrocatalysis: Dioxygen reduction and carbon dioxide conversion.

摘要

Due to increasing greenhouse gas emissions and their link to global climate change, much attention has been given towards both reducing current, atmospheric greenhouse gas emissions and developing clean, renewable energy sources. The first section of this thesis focuses on reducing greenhouse gas emissions. Primarily focusing on CO2, routes towards reducing atmospheric levels include capture, sequestration, and conversion. For CO 2 conversion, Au electrocatalysts have demonstrated high CO2 reduction activity to CO which can then be further converted to various synfuels or commodity chemicals. In the first section of this thesis, Au electrocatalysts are further probed by utilizing a Ag-based model using N-containing additives, such as pyrazole and benzotriazole, and surface-enhanced Raman spectroscopy (SERS). SERS reveals that only in the presence of N-containing additives a stronger CO band is seen. These additives do not affect the CO2 reduction mechanism of Au, as found by Tafel and product distribution analyses. The enhancement of the CO2 reduction rate on Au is also demonstrated by utilizing a known CO2 scavenger, ethanolamine, adsorbed on the Au surface. This result suggests that improving CO2 reduction should focus on the reactant side of the Sabatier plot.;The next two sections of this thesis focus on the development of electrocatalysts for the oxygen reduction reaction (ORR), utilized in fuel cell applications. Due to the slow kinetics of the ORR, research has focused on studying how ORR catalysts reduce O2 and developing new cost-effective catalysts with improved activity. The second section of this thesis focuses on studying pyrolyzed Fe/N/C electrocatalysts, which have been shown to have high ORR activity similar to that of Pt, the benchmark catalyst. However, the active site of pyrolyzed Fe/N/C electrocatalysts for the ORR has been a source of debate since the initial discovery that these materials demonstrated activity towards the ORR. This has extremely limited systematic improvements to trial-and-error-based methods. In this section, a carbon-supported iron(II) phthalocyanine (FePc) that has been pyrolyzed at 800°C is utilized as a model catalyst. Studying the ORR on this material in the absence and presence of azide in acidic, neutral, and alkaline environments, the ORR activity and mechanism on pyrolyzed Fe/N/C materials can be further interrogated. The presence of azide served to enhance the ORR activity of this material in neutral electrolyte while having no effect in acidic or alkaline electrolytes. Tafel slope differences in addition to the azide enhancement suggest an Fe-centered active site for the ORR in pyrolyzed FePc and potentially other Fe/N/C electrocatalysts. This study provides both the first small molecule enhancement of the ORR with Fe/N/C catalysts and an additional route to further interrogate other electrocatalysts.;The last section of this thesis centers on the adsorption of O2 on dynamic electrode surfaces, most specifically Pd, Pt, and Pt-alloys, during the ORR in both acidic and alkaline electrolytes. Much work involving these catalysts involves relating electronic properties and adsorption energies to their ORR activities. However, most research focuses on the Pt-O or Pd-O bond, assuming a static Pt-Pt or Pd-Pd bond. It has previously been shown, utilizing in situ surface stress measurements and EXAFS, that the Pt-Pt bond is not static during the ORR in an acidic electrolyte, with changes in length from 5 to 10 mA due to O2 adsorption. By using in situ surface stress measurements, other electrode systems have been characterized and similar dynamics to what was demonstrated with Pt previously are seen and presented herein. In an alkaline electrolyte, the Pt surface expands less, potentially due to its initial expanded state caused by OH-. Pt alloy materials demonstrate an increased expansion over Pt when O2 adsorption occurs. Most interestingly, in an acidic electrolyte, Pd demonstrates a minimal change due to O2 adsorption while in an alkaline electrolyte, behaves similar to Pt. Understanding the surface dynamics of these systems will help to develop more effective ORR electrocatalysts by adding valuable insight into how O 2 adsorption alters the surface bonds.