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Computational Approach to the Problems of Electro- and Photo-Catalysis
Title
Computational Approach to the Problems of Electro- and Photo-Catalysis.
Creator
Zuluaga, Sebastian, Stolbov, Sergey, Schelling, Patrick, Roldan Cuenya, Beatriz, Masunov, Artem, University of Central Florida
Abstract / Description
The main objective of this work is to gain basis for rational design of catalysts used in fuel cells for conversion of chemical energy stored in hydrogen molecules into electric energy, as well as photo-catalysts used for hydrogen production from water under solar irradiation. This objective is achieved by applying the first principles computational approach to reveal relationship among compositions of materials under consideration, their electronic structure and catalytic activity. A major...
Show moreThe main objective of this work is to gain basis for rational design of catalysts used in fuel cells for conversion of chemical energy stored in hydrogen molecules into electric energy, as well as photo-catalysts used for hydrogen production from water under solar irradiation. This objective is achieved by applying the first principles computational approach to reveal relationship among compositions of materials under consideration, their electronic structure and catalytic activity. A major part of the work is focused on electro-catalysts for hydrogen fuel cells. Platinum (Pt) is widely used in the electrodes of fuel cells due to its good catalytic properties. However, Pt is an expensive and scarce element, its catalytic activity is not optimal and also it suffers from CO poisoning at anode. Therefore the search for new catalytic materials is needed for large scale implementation of fuel cells. The main direction of search of more efficient electro-catalysts is based in the design in which an active element monoatomic layer (AE) is deposited on a metal substrate (MS) made of a cost-effective material. Two goals are achieved by doing this: on the one hand, the cost of the catalytic system is reduced by reducing the amount of the AE in the system and on the other hand the catalytic properties of the AE can be tuned through its interactions with the MS. In the first part of this work the Pd-based alloys and layered structures have been studied as promising electro-catalysts for the ORR on the fuel cell cathodes, more precisely Pd-Co alloys and Pd/M/Pd (M=Co,Fe). There exists a robust model linking the activity of a surface toward ORR to computable thermodynamic properties of the system and further to the binding energies of the ORR intermediates on the catalyst surface. A more challenging task is to find how to tune these binding energies through modification of the surface electronic structure that can be achieved by varying the surface composition and/or morphology. To resolve this challenge, the electronic structure, binding energies of intermediates and the ORR free energies have been calculated within the density functional theory (DFT) approximation. The results presented in this work show that in contrast to the widely accepted notion, the strain exerted by a substrate on AE hardly affects the surface activity toward ORR, while the hybridization of the electronic states of the AE-and MS-electronic states is the key factor controlling the catalytic properties of these systems. Next it is shown that the catalytic activity of the promising anode electrocatalysts, such as Pt/M, M=Au, Ru and Pd, is also determined by the AE-MS hybridization with a minor effect of the strain. Furthermore, we have shown that, if AE is weakly bound to the substrate (as it is for Pt/Au), surface reconstruction occurs. This leads to the breaking of the relation between the electronic structure of the clean surface and the reactivity of the sytem. Other kind of promising ORR catalysts is designed in the form of Ru nanoparticles modified by chalcogens. In this work, I present the results obtained for small Ru clusters and flat Ru facets modified with chalcogens (S, Se and Te). The O and OH binding energies are chosen as descriptors of the ORR. The results on the two systems are compared, concluding that large clusters with relative large flat facets have higher catalytic activity due to the absence of low coordinated and thus high reactive Ru atoms. Regarding the problem of the hydrogen production via photo-catalytic splitting of water, one of the challenges is tuning the band gap of the photo-anodes to optimal levels. Graphitic carbon nitride (g-C3N4) is a promising material to be used as a photo-anode, however, a reduction of the band gap width by rational doping of the material would improve the efficiency significantly. This issue is addressed in the last chapter of this work. Two problems are considered: a) the stability of the doped system and b) the band gap width. To address the first problem the ab-initio thermodynamics approach has been used, finding that the substitution of C and N with the doping agent (B, C, N, O, Si and P) is thermodynamically preferred over the interstitial addition of dopant to the g-C3N4 structure. However, due to high kinetic energy barriers for the detachment of C and N atoms, involved in the substitution doping, the interstitial addition found to be kinetically more favorable. Since the density functional theory fails to reproduce the band gap of semiconductors correctly, the GW approximation was used to study the band gap of the system. The results indicate that the g-C3N4 system maintain its semiconductor character if doped with B, O and P under certain conditions, while reducing the band gap.
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Date Issued
2013
Identifier
CFE0005288, ucf:50546
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0005288
Computational Approach to Electrocatalysis
Title
Computational Approach to Electrocatalysis.
Creator
Dhakal, Nagendra, Stolbov, Sergey, Rahman, Talat, Ishigami, Masa, Masunov, Artem, University of Central Florida
Abstract / Description
The main objective of this work is to understand the theoretical basis of the working principle of the Hydrogen fuel cell. We seek the physical basis of the Rational Design Technique, the smart way of preselecting materials from the material-pool, implemented in our study anticipating highly promising electrocatalysts for promoting the conversion of chemical energy stored in hydrogen molecules into the electrical energy. It needs the understanding of the relationship among the compositions of...
Show moreThe main objective of this work is to understand the theoretical basis of the working principle of the Hydrogen fuel cell. We seek the physical basis of the Rational Design Technique, the smart way of preselecting materials from the material-pool, implemented in our study anticipating highly promising electrocatalysts for promoting the conversion of chemical energy stored in hydrogen molecules into the electrical energy. It needs the understanding of the relationship among the compositions of the materials under consideration, their electronic structure and catalytic activities. We performed the first principle DFT calculations to achieve the goal.Our work is focused first on the issues in hydrogen oxidation reaction taking place in anode compartment of the cell. Next comes up with the issues with Oxygen Reduction Reaction taking place in cathode compartment. Finally, we focus on mechanisms underlying binding of small molecules on substrates.Platinum perfectly catalyzes hydrogen oxidation reaction on the hydrogen fuel cell anodes. However, it has at least two drawbacks: a) it is too expensive; b) it has a low tolerance to CO poisoning. Pt-Ru bi-functional catalysts are more tolerant to CO, but they are still very expensive. In this work, we performed first-principle studies of stability and reactivity of M/W (110) structures, where M = Pd, Ru, Au monolayers. All three systems are found to be stable: formation energy of MLs is significantly higher than cohesive energy of the M-elements. The calculated binding energies of H, H2, OH, CO, and H2O were used to obtain the reaction free energies. Analysis of the free energies suggests that Au-W bonding does not activate sufficiently Au monolayer, whereas Ru/W (110) is still too reactive for the CO removal. Meanwhile, Pd/W (110) is found to catalyze hydrogen oxidation and at the same time to be highly tolerant to the CO poisoning. The latter finding is explained by the fact that CO binds much weaker to Pd on W (110) than to Pt, while the OH binding is strong enough to ensure CO oxidation. The obtained results are traced to the electronic structure of the systems.Oxygen Reduction Reaction (ORR) is the heart core reaction in fuel cells, Proton Exchange Membrane Fuel cell and DEMFC. However, the reaction is not so obvious and need suitable electrocatalyst. Pt or Pt-based catalysts are found to be the best catalyst so far. But, its cost and shortage make it not feasible economically. Moreover, lower onset potential (maximal electrode potential at which the reaction can proceed) of such catalysts is offering another limitation to fuel cell performance. Research has been conducted in many directions for lowering the cost by replacing the Pt with some other elements of lower cost or reducing the Pt-load in the material; and even more finding the material performing better than Pt. In this paper, we've tried to understand the ORR mechanism and look for the material that could be potential option to Pt. Our calculations suggest that for monolayer of Pt on 5 layered slab of Nb or Mo the onset potential is the same as for Pt, while cost of these systems are much lower than that of Pt. Presence of water changes the reaction rate very minimum. Rational design method facilitates the research of selecting the appropriate catalyst and saves time and effort significantly. The result shows that the d-band center model is not accurate to describe the reactivity of the catalyst.For decades, adsorbates' binding energy (????) has been used as an indicator of the adsorbate-substrate bond strength (??????). Thus, although one can compute accurately any ?? models to gauge bond-strength are developed and applied to rationalize and anticipate ????'s because that is a key aspect in the rational search for efficient catalysts. Yet bond-strength alone fails to predict ???? trends. Therefore, quantifying and understanding the difference between ???? and ?????? is essential to catalysts design. Indeed, the adsorbate-substrate bond formation perturbs the substrate's electronic charge density, which reduces ???? by the energy attached to such perturbation: ??????????. Here, with the example of carbon monoxide adsorption on metal-doped graphene, we show that ?????????? may exceed 1 eV and render an unusual situation: although the EB of CO to the Au-doped graphene indicates that binding does not happen, we find evidence of a strong bond between CO and the substrate. Thus, in this case, the large ?????????? totally disrupt the equivalency between ?????? and ???? we also propose a method to compute ?????????? that bypasses dealing with an excited electronic state of the system.
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Date Issued
2017
Identifier
CFE0006583, ucf:51336
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0006583
INVESTIGATION OF NANOCERIA-MODIFIED PLATINUM-GOLD COMPOSITE ELECTRODES FOR THE ELECTROCHEMICAL REDUCTION OF OXYGEN IN ALKALINE MEDIA
Title
INVESTIGATION OF NANOCERIA-MODIFIED PLATINUM-GOLD COMPOSITE ELECTRODES FOR THE ELECTROCHEMICAL REDUCTION OF OXYGEN IN ALKALINE MEDIA.
Creator
Hegishte, Rahul, Diaz, Diego, University of Central Florida
Abstract / Description
Platinum-gold and nanoceria-modified platinum-gold electrodes were prepared on a platinum surface via electrochemical reduction of solutions of platinum and gold salts in the dispersion of nanoceria. The molar ratios of Pt and Au were varied in both PtAu and PtAu/CeO2 electrodes while the total concentration of the metals was maintained at 2 x 10-3M and the concentration of nanoceria was maintained constant at 5 x 10-3M. The electrodes were characterized by their cyclic voltammetry curves in...
Show morePlatinum-gold and nanoceria-modified platinum-gold electrodes were prepared on a platinum surface via electrochemical reduction of solutions of platinum and gold salts in the dispersion of nanoceria. The molar ratios of Pt and Au were varied in both PtAu and PtAu/CeO2 electrodes while the total concentration of the metals was maintained at 2 x 10-3M and the concentration of nanoceria was maintained constant at 5 x 10-3M. The electrodes were characterized by their cyclic voltammetry curves in 0.5M sulfuric acid solution. The electrochemically active area of the electrodes was determined using the copper underpotential deposition method. The linear sweep voltammograms of the PtAu and PtAu/CeO2 electrodes were plotted from -1V to 0V vs. Ag/AgCl, 3M KCl reference electrode using the rotating disk electrodes for the rotation speeds from 200 to 3600rpm in an oxygen saturated 0.1M sodium hydroxide solution. The values of the kinetic controlled current density were determined from the rotating disk voltammetry. The values of the limiting current density for each rotation speed were used to plot the Koutecky-Levich plots for the electrodes. The rate constants were obtained from the Koutecky-Levich plots for each composition of the electrode. The values of kinetic current density and the rate constants indicated that the addition of Au enhances the ORR rates in both the PtAu and the PtAu/CeO2 electrodes. The values of the kinetic current densities of the PtAu/CeO2 were lower than that of the PtAu electrodes owing to the poor electrical conductivity of ceria. The Koutecky-Levich plots for the PtAu and the PtAu/CeO2 electrodes are linear for the four-electron reduction of oxygen in the alkaline media, which indicates that the overall reaction follows the first order kinetics. The electron transfer rate constants obtained from the Koutecky-Levich plots for the PtAu and the PtAu/CeO2 electrodes both were found to increase in values with the addition of Au. The Tafel plots were plotted for the PtAu and PtAu/CeO2 electrodes and the values of Tafel slopes were found to be in a small range for lower amounts of Au which indicated that the ORR rates were enhanced in lower amounts of Au. The values of Tafel slopes were found to be much higher for the ceria-modified PtAu electrodes as compared to the PtAu electrodes, which indicate the lower rates of ORR after the modification with ceria. Also, the ORR rates for the electrodes with smaller amounts of Au in PtAu/CeO2 were higher than those in the larger amounts of Au.
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Date Issued
2011
Identifier
CFE0003639, ucf:48860
Format
Document (PDF)
PURL
http://purl.flvc.org/ucf/fd/CFE0003639