Kinetic Analysis of Electrochemical Oxygen Reduction and Development of Ag-alloy Catalysts for Low Temperature Fuel Cells.

This dissertation applies insights from quantum chemical calculations and heterogeneous kinetic analysis to interpret macroscopic reactivity trends in electrochemical systems and design optimal electrocatalysts. Specifically we explore the mechanism of the electrochemical oxygen reduction reaction (ORR) on the surfaces of Pt (a near-optimal catalyst) and Ag electrodes. We have identified design criteria for improving the reaction rate in each case and developed cost-effective Ag-based alloy materials with activity approaching that of more costly Pt catalysts. We first demonstrate, using microkinetic modeling and density functional theory calculations, that deviations from ideal electrode kinetics (a linear potential vs. log current relationship) are inherent to the ORR and any multi-step heterogeneous electrocatalytic reaction. Deviations result from simultaneous changes in the rate of the rate-limiting elementary step and the number of available active sites on the electrode surface as potential is shifted. We show the ORR kinetic variations on Pt electrodes are well-reproduced by a simple description of changes in OH and H2O surface intermediate coverages, and that weaker binding materials exhibit higher rates due to higher active-site availability. In contrast, on Ag a very weak relation is found between adsorbate coverage and changes in the apparent rate law. This points to a strong role of under-coordinated active sites, which become poisoned at low potentials while the majority of the surface is still clean. Moving toward stronger binding on Ag should yield higher ORR activity by increasing turnover rates on the more predominant surface facets. Using the mechanistic insights mentioned, we illustrate the design of relatively inexpensive Ag-Co surface alloy nanoparticle electrocatalysts for ORR, with equivalent area-specific activity to commercial Pt-nanoparticles at realistic fuel cell operating conditions. The Ag-Co materials were identified with quantum chemical calculations and synthesized with a novel bimetallic precursor decomposition technique that generates a surface alloy, despite bulk immiscibility of the elements. Characterization studies show the origin of activity improvement comes from a ligand effect, in which Co perturbs Ag surface sites. We also explore bimetallic precursor decomposition to produce Ag-Ni and Ag-Fe alloys but find that the products exhibit substantial segregation and have ORR activities similar to monometallic Ag.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102423/1/ahole_1.pd

Insight into the oxidation mechanism of furanic compounds on Pt(111)

Partial oxidation of multifunctional oxygenates is of interest for the production of high-value chemicals, but the understanding of the mechanism on Pt catalysts is lacking. To probe the effects of oxygen in the reaction of furanics on Pt(111), temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), electrochemical catalytic experiments, and density functional theory (DFT) calculations were conducted. The presence of oxygen on the Pt(111) surface significantly altered the reaction paths for furan and furfuryl alcohol oxidation. Although furan desorbed without reaction from clean Pt(111) during TPD, on the O precovered surface, extensive oxidation and decomposition reactions were observed. The main volatile products were H2O, CO, and CO2. HREEL spectra and DFT calculations indicated that furan initially reacted through oxygen addition to the ring to produce surface-adsorbed furanone intermediates. For furfuryl alcohol, the same desorption products were formed, but maleic anhydride was also detected as a trace product. HREELS and DFT calculations suggested that alcohol oxidation proceeded through dehydrogenation at the hydroxyl and methylene groups and subsequent oxidation to produce a carboxylate intermediate. The carboxylate intermediate underwent decarboxylation to form CO2 and surface furyl intermediates, which then underwent deep oxidation and decomposition. Thus, the presence of the oxygenated side chain on the furan ring for furfuryl alcohol strongly influenced the oxidation chemistry. Comparison of furfural electrocatalytic oxidation results on Pd (collected here) and Pt (from previous work) indicated that Pt is less active for C&ndash;C activation but more active for insertion of oxygen atoms into the furan ring, in line with the current and previously published surface science results.</p

Abstracts from the 8th International Conference on cGMP Generators, Effectors and Therapeutic Implications

This work was supported by a restricted research grant of Bayer AG

Dynamic Electrocatalysis: Examining Resonant Catalytic Rate Enhancement Under Oscillating Electrochemical Potential

It has been shown through recent simulations that heterogeneous catalysts with dynamic properties – for example, the ability to vary adsorbate binding energy with time – could, in principle, reach higher turnover frequencies (TOFs) than an optimized catalyst operating at steady state (i.e. the “volcano curve” maximum, assuming typical scaling relations between elementary steps of the reaction). The enhancements are maximized near resonant frequencies in line with the time scales of the elementary steps. In this work, we perform a microkinetic analysis on a generalized electrochemical mechanism in order to evaluate the extent to which electrochemical potential can be used as a lever to achieve resonant catalytic rate enhancement. We illustrate that, because changing the electrochemical potential changes the free energy of reaction, the approach is conceptually distinct from oscillating binding energies of catalytic intermediates in isolation. However, benchmarks for rate and efficiency gains relative to potentiostatic operation can still be defined. We show that for faradaic reactions in series, no enhancements relative to the maximum steady-state TOF (within the potential range spanned by oscillation) can be achieved, even in cases where the dynamic potential limits favor adsorption and desorption, respectively. Enhancements relative to the average steady-state TOF (weighted by time at each potential), can be achieved at specific frequency/amplitude/duty cycle combinations, but only if the elementary reactions show disparate symmetry factors. In contrast, if a faradaically-driven parallel reaction controls the coverage of a strongly-adsorbed blocking species on a surface, significant dynamic rate enhancements over the maximum steady-state TOFs can be achieved, albeit at a significant cost of thermodynamic efficiency. We discuss how these simulations elucidate the possible sources of rate enhancements observed experimentally for dynamic electrocatalytic systems

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO2 reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend: Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface’s propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ~10, which is independent of potential for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length), as well as factors such as SAM-induced surface reconstruction, and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule

Impact of electrolyte composition on bulk electrolysis of furfural over platinum electrodes

Partial oxidation of furanic biomass derivatives such as furfural is of interest for the sustainable production of chemicals including furoic acid, maleic acid, and 2,5-furandicarboxylic acid (FDCA). The oxidative bulk electrolysis of furfural is here investigated on platinum electrodes in acidic media. The effects of potential, concentration, pH, and supporting anion are studied, and selectivity trends are coupled with attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to illuminate adsorbate structures that influence the catalysis. Increasing potential is found to shift selectivity from primarily C5 products to C4 products, coincident with oxidation of the Pt surface. Selectivity changes are also observed moving from pH 1 to pH 4, with an increase in C5 products at higher pH. Changing from the weakly adsorbing perchlorate anion to the specifically-adsorbing phosphate anion results in a number of changes that manifest differently depending on potential and pH. Selectivity to furoic acid is found to be highest above the pKa of phosphoric acid due to the strongly adsorbed phosphate ions suppressing flat-lying configurations of furfural that lead to C-C cleavage. These results point toward opportunities to use electrolyte engineering to tune selectivity and optimize surface conditions to disfavor binding of inhibitory products

Thermal Enhancement of Product Conductivity Permits Deep Discharge in Solid State Li-O2 Batteries

Li-O2 batteries are mainly limited by the poor conductivity of their discharge products as well as parasitic reactions with carbon-containing electrodes and electrolytes. Here, Li-O2 cells utilizing inorganic solid state electrolytes are investigated as a means to operate at elevated temperature, thereby increasing the conductivity of discharge products. Growth of dense, conductive LixOy products further removes the need for high surface area support structures commonly made of carbon. Patterned Au electrodes, evaporated onto Li7La3Zr2O12 (LLZO) solid electrolyte, are used to create a triple phase boundary for the nucleation of discharge product, with growth outward into the cell headspace with gaseous O2. Through capacity measurements and imaging, discharge product growths are estimated to reach a critical dimension of approximately 10 microns, far exceeding what would be possible for a conformal film based on its room temperature electronic conductivity. Raman spectroscopy and electrochemical mass spectrometry (EC-MS) are used to characterize the discharge chemistry and reveal a mixed lithium oxide character, with evidence of trace lithium hydroxides and initial carbonate contamination. These results showcase that thermal enhancement of Li-O2 batteries could be a viable strategy to increase capacity when paired with solid electrolytes

Predictive Structure–Reactivity Models for Rapid Screening of Pt-Based Multimetallic Electrocatalysts for the Oxygen Reduction Reaction

Due to the immense phase space of potential alloy catalysts, any rigorous screening for optimal alloys requires simple and accurate predictive structure–reactivity relationships. Herein, we have developed a model that allows us to accurately predict variations in adsorption energy on alloy surfaces based on easily accessible physical characteristics of the metal elements that form the alloymainly their electronegativity, atomic radius, and the spatial extent of valence orbitals. We have developed a scheme relating the geometric structure and local chemical environment of active Pt sites to the local chemical reactivity of the sites in the electrochemical oxygen reduction reaction (ORR). The accuracy of the model was verified with density functional theory (DFT) calculations. The model allows us to screen through large libraries of Pt alloys and identify many potentially promising ORR alloy catalysts. Some of these materials have previously been tested experimentally and shown improved performance compared to pure Pt. Since the model is grounded on validated theories of chemisorption on metal surfaces, it can be used to identify the critical physical features that characterize an optimal alloy electrocatalyst for ORR and propose how these features can be engineered

Thermal Enhancement of Product Conductivity Raises Capacity in Solid-State Li‑O₂ Batteries

Li-O2 batteries are mainly limited by the poor conductivity of their discharge products as well as parasitic reactions with carbon-containing electrodes and electrolytes. Here, Li-O2 cells utilizing inorganic solid-state electrolytes are investigated as a means to operate at elevated temperature, thereby increasing the conductivity of discharge products. Growth of dense, conductive LixOy products further removes the need for high-surface area support structures commonly made of carbon. Patterned Au electrodes, evaporated onto Li7La3Zr2O12 (LLZO) solid electrolyte, are used to create a triple-phase boundary for the nucleation of the discharge product, with growth outward into the cell headspace with gaseous O2. Through capacity measurements and imaging, discharge product growths are estimated to reach a critical dimension of approximately 10 μm, far exceeding what would be possible for a conformal film based on its room temperature electronic conductivity. Raman spectroscopy and electrochemical mass spectrometry are used to characterize the discharge chemistry and reveal a mixed lithium oxide character, with evidence of trace lithium hydroxides and initial carbonate contamination. These results showcase that thermal enhancement of Li-O2 batteries could be a viable strategy to increase capacity when paired with solid electrolytes

Understanding Reactivity of Self-Assembled Monolayer-Coated Electrodes: SAM-Induced Surface Reconstruction

Thiolate self-assembled monolayers (SAMs) are often used to modify surface properties, including catalytic activity. These SAMs can also induce reconstruction of some metallic surfaces. Here we show, through formation and subsequent removal of thiolate SAMs from Au polycrystalline electrocatalysts, that irreversible changes to the underlying metal surface can lead to significant changes in catalytic properties, irrespective of specific interactions that might occur between thiolate molecules and various reactants. Using underpotential deposition of Pb as a surface probe, we find that across a range of different thiolates, SAMs tend to increase the proportion of (111)-facets on Au, but they simultaneously increase the defect density upon these and other facets. These changes lead to delayed onset but higher maximum activity toward formic acid oxidation, which is interpreted in terms of both the density of appropriate active site ensembles and changes to the binding isotherm for site-blocking hydroxyl species. The impacts of reconstruction are further illustrated through measured shifts in selectivity for electroreduction of crotonaldehyde, with reconstructed catalysts changing the favored product from butanal to crotyl alcohol. Thus, complex surface reorganization may play a significant role in catalytic behaviors of thiol-coated SAMs as well