The dioxygen reduction reaction (ORR) is a key process for renewable energy technologies such as batteries and fuel cells. Reliance on dioxygen as the terminal oxidant in fuel cell technologies requires the development of electrocatalysts that proceed at low overpotentials with fast rates, while maintaining adequate selectivity for H2O production over thousands of hours of operation. While many advances have been made, the discovery of efficient and inexpensive ORR electrocatalytic materials remains a holy-grail of energy science. The study of soluble, molecular electrocatalysts allow for more detailed structure : activity analyses to be made than for electrocatalytic materials, providing an atomic level understanding of catalytic barriers and opportunities for improvement.
Chapters 1-5 of this thesis develop a rational approach for analyzing, comparing, and improving homogeneous and molecular ORR electrocatalysts in non-aqueous solvents, and use this approach to study the reduction of dioxygen to water using iron porphyrin electrocatalysts. Quantification of the equilibrium potential for O2/H2O under the conditions of study allowed, for the first time, accurate estimations of the reaction overpotential in non-aqueous solvents. Knowledge of the reaction overpotential proved critical for comparing electrocatalysts under the somewhat diverse conditions encountered for homogeneous catalysts with varying solubilities in organic solvents.
A detailed kinetic and mechanistic study was then conducted on iron tetraphenyl porphyrin, Fe(TPP), which revealed that the turnover limiting step is protonation of the iron(III) superoxo adduct, formed via pre-equilibrium dioxygen binding to FeII(TPP). The protonation step was found to have a large kinetic barrier, suggesting that targeting proton delivery to the active site may improve the ORR activity of iron porphyrin electrocatalysts.
Studies of eleven substituted iron porphyrin ORR electrocatalysts all showed high selectivity for the 4H+/4e - reduction to water. The turnover frequencies (TOFs) were found to correlate with the reduction potential required to initiate electrocatalysis, in two log(TOF):overpotential linear free energy relationships (LFERs). The iron porphyrin electrocatalysts with well-positioned proton donors above the active site fell upon the same LFER as those without such proton relays, suggesting that the second coordination sphere does not directly participate in the rate-limiting proton transfer. These results contradict the general sense that well-positioned proton relays should decrease kinetic barriers. However, some iron porphyrin catalysts in the series can break the LFER, leading to more efficient catalysis. Computational studies suggest that, rather than directly participating in an intramolecular proton transfer, the second coordination sphere of some iron porphyrins can hydrogen bond with the O2 adduct to influence the thermochemistry for proton transfer. Importantly, the presence of these LFERs was shown to stem from the electrocatalyst E1/2 influencing the thermodynamics for O2 binding and proton transfer. Analogies are drawn between these linear free energy relationships and the scaling relationship analyses used for electrocatalytic materials for the ORR.
Using the mechanism, rate law, and thermochemistry, the log(TOF) : overpotential correlations were then derived for ORR catalyzed by iron porphyrins. Given that the TOF is a function of the catalytic rate law (TOF = k cat[O2][HA]) and the overpotential is a function of the reaction conditions, the predicted correlation between log(TOF) and effective overpotential is independently. derived for changes in the reaction conditions or for changes to the catalyst E1/2. For each parameter varied, a unique correlation coefficient was identified and shown to agree with experimental data. The very shallow dependence between log(TOF) and the pK a of the acid used was used to enable Fe(TPP) catalyzed ORR to break the prior LFERs by 104 s-1 in TOF. These scaling relations highlight how decoupling the ET, PT and substrate binding events can lead to diverse scaling relationships, providing opportunities for improving the activity of a catalytic system by targeting the medium, as opposed to the catalyst.
In chapter 6, an exploratory research project on driving the ORR using sunlight to produce hydrogen peroxide (H2O2) is discussed. H2O2 is a commodity chemical with diverse applications in water purification, as an oxidant, and as a liquid fuel. Preparation of nickel(II) oxide photocathodes sensitized with simple dyes revealed that these photocathodes are surprisingly active for H2O2 production, proceeding to produce H2O2 with unity faradaic efficiency at low overpotentials (<20 mV). The reaction is found to proceed via outer sphere electron transfer from reduced dyes to O2, forming superoxide, which disproportionates in solution, forming H2O2. Remarkably, these unoptimized systems are among the most active photocathodes for H 2O2 production. These results are promising for developing the delocalized production of H2O2 using dye-sensitized photoelectrosynthesis cells.
|Advisor:||Mayer, James M.|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 79/12(E), Dissertation Abstracts International|
|Keywords:||Electrocatalysis, Oxygen Reduction, Thermodynamics|
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