Browsing by Author "Choi, YongMan"

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A first-principles analysis for sulfur tolerance of CeO2 in solid oxide fuel cells
(2007-07-26)
Chen, Hsin-Tsung
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Choi, YongMan
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Liu, Meilin
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Lin, M. C.
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應用化學系分子科學碩博班
The mechanism for H2S-CeO2(111) interactions in solid oxide fuel cells (SOFCs) has been investigated by using periodic density functional theory (DFT) calculations. In order to properly characterize the effect of the localization of Ce-4f states on the interactions, DFT + U calculations were applied. Adsorption of H2S, SH, and atomic S was initially examined to locate energetically favorable intermediates. The species adsorb favorably at the Ce-top, O-top, and Ce-O bridging sites, respectively. Potential energy profiles for the H2S-CeO2 (111) interactions along the three product channels producing H-2, H2O, and SO2 were constructed using the nudged elastic band (NEB) method. Calculations show that H2S weakly bounds on CeO2(111) with a small binding energy, followed by dehydrogenation processes, forming surface sulfur species with an exothermicity of 29.9 kcal/mol. Molecular-level calculations demonstrated that the SO2-forming pathway is energetically most favorable.
Composition-balanced trimetallic MOFs as ultra-efficient electrocatalysts for oxygen evolution reaction at high current densities
(2020-12-15)
Raja, Duraisamy Senthil
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Huang, Chun-Lung
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Chen, Yu-An
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Choi, YongMan
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Lu, Shih-Yuan
Engineering synergistic effects of multi-component catalysts is the key for breakthrough catalyst design. Here, a maximized-entropy approach was proposed to maximize the synergistic effects for maximum enhancements in electrocatalytic efficiencies of multi-component catalysts. Accordingly, composition-balanced iron, cobalt, and nickel based trimetallic MOFs was developed and demonstrated outstanding oxygen evolution reaction (OER) performances with ultra-low overpotentials of 196 and 284 mV achieved at current densities of 10 and 1000 mA cm(-2), respectively, as well as an ultra-low Tafel slope of 29.5 mV dec(-1) in alkaline aqueous media. The catalyst was ultra-stable even when operated at ultra-high current densities, experiencing only 5% loss in current densities, when chronoamperometrically tested at an industrially relevant current density of 1000 mA cm(-2) for over 50 h. in situ Raman spectroscopy study and density functional theory simulations were conducted to explore the OER mechanism and to illustrate the validity of the proposed maximized-entropy approach.
Computational study on the catalytic mechanism of oxygen reduction on La0.5Sr0.5MnO3 in solid oxide fuel cells
(2007)
Choi, YongMan
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Lin, M. C.
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Liu, Meilin
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應用化學系分子科學碩博班
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Institute of Molecular science
Effective Promotion of Oxygen Reduction Reaction by in Situ Formation of Nanostructured Catalyst
(2019-08-01)
Chen, Yu
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Yoo, Seonyoung
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Zhang, Weilin
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Kim, Jun Hyuk
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Zhou, Yucun
Efficient electrocatalysts for oxygen reduction reaction (ORR) are critical to high-performance energy conversion and storage devices. As an important family of functional materials, alkaline earth metal oxides are generally considered inert toward electrochemical reactions. Here we report the remarkable enhancement of ORR activity and durability of electrodes enabled by surface modification with, a precursor of barium nitrate. During cell start-up process, an ORR active cobaltite catalyst was in situ formed on the electrode surface, as confirmed by scanning transmission electron microscopy (STEM) analysis. A combination of in situ/operando Raman and electrochemical impedance study suggests that the transition from nitrate to cobaltite may occur at similar to 565 degrees C. The peak power density of a single cell at 750 degrees C is increased from similar to 0.85 to similar to 1.15 W cm(-2) (35% increase) by the surface modification, demonstrating the remarkable enhancement of electrocatalytic activity under realistic operating conditions.
From Ni-YSZ to sulfur-tolerant anode materials for SOFCs: electrochemical behavior, in situ characterization, modeling, and future perspectives
(2011-11-01)
Cheng, Zhe
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Wang, Jeng-Han
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Choi, YongMan
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Yang, Lei
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Lin, M. C.
Solid oxide fuel cells (SOFCs) offer great promise for the most efficient and cost-effective conversion to electricity of a wide variety of fuels such as hydrocarbons, coal gas, and gasified carbonaceous solids. However, the conventional Ni-YSZ (yttria-stabilized zirconia) anode is highly susceptible to deactivation (poisoning) by contaminants commonly encountered in readily available fuels, especially sulfur-containing compounds. Thus, one of the critical challenges facing the realization of fuel-flexible and cost-effective SOFC systems is the development of sulfur-tolerant anode materials. This perspective article aims at providing a comprehensive review of materials that have been studied as anodes for SOFCs, the electrochemical behavior of various anode materials in H(2)S-contaminated fuels, experimental methods for ex situ and in situ characterizations of species and phases formed on anode surfaces upon exposure to H(2)S-containing fuels, mechanisms for the interactions between H(2)S and anode surfaces as predicted from density functional theory (DFT) calculations, and possible strategies of minimizing or eliminating the effect of sulfur poisoning. While significant progress has been made in developing alternative anode materials with better sulfur tolerance, in probing and mapping electrode surface species relevant to sulfur poisoning, and in unraveling the mechanisms of H(2)S-anode interactions using both computational and experimental approaches, many challenges still remain to bridge the gaps between models at different scales or between theoretical predictions and experimental observations. An important new direction for future research is to develop a predictive multiscale (from DFT to continuum) computational framework, through a rigorous validation at each scale by carefully-designed experiments performed under in situ conditions, for rational design of better sulfur-tolerant anode materials and structures for a new generation of SOFCs to be powered by readily available fuels.
Oxygen reduction on LaMnO3-based cathode materials in solid oxide fuel cells
(2007-04-03)
Choi, YongMan
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Mebane, David S.
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Lin, M. C.
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Liu, Meilin
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應用化學系分子科學碩博班
Cubic perovskite LaMnO3 surface models were constructed to elucidate the mechanism of oxygen reduction using quantum chemical calculations with molecular dynamics (MD) simulations. Calculations predict that both dissociative and molecular adsorption may occur, depending on adsorbate configurations. Superoxo- or peroxo-like species may locate at La, Mn, and O-sub active sites with different vibrational frequencies and atomic charges. A stepwise elementary reaction sequence via the superoxo- or peroxo-like intermediates at both perfect and defective LaMnO3 was constructed by mapping out minimum-energy paths (MEPs) using the nudged elastic band (NEB) method. Charge transfer for the O-2-LaMnO3 interactions was also explored by Bader charge analysis. In particular, ab initio MD simulations carried out to simulate solid oxide fuel cell conditions at 1073 K suggest that oxygen vacancies enhance O-2 dissociation kinetics.
Prediction of O(2) Dissociation Kinetics on LaMnO(3)-Based Cathode Materials for Solid Oxide Fuel Cells
(2009-04-30)
Choi, YongMan
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Lynch, Matthew E.
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Lin, M. C.
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Liu, Meilin
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應用化學系分子科學碩博班
First-principles and statistical-theory calculations were applied to examine the interactions between oxygen molecules and the (100) surfaces of LaMnO(3) and La(0.5)Sr(0.5)MnO(2.75), one of the most-used cathode materials in solid oxide fuel cells (SOFCs). To predict the rate constants for the interactions between O(2) and LaMnO(3) or La(0.5)Sr(0.5)MnO(2.75), potential energy profiles were constructed using the nudged elastic band (NEB) method. Predicted rate constants for the dissociation of adsorbed oxygen species on LaMnO(3) (lm) and La(0.5)Sr(0.5)MnO(2.75) (lsm) can be expressed as k(diss,lm) = 2.35 x 10(12) exp(-0.50 eV/RT) s(-1) and k(diss,lsm) = 2.15 x 10(12) exp(-0.23 eV/RT) s(-1), respectively, in the temperature range of 873-1273 K at 1 atm. Because the activation energy for oxygen dissociation on La(0.5)Sr(0.5)MnO(2.75) (0.23 eV) is much smaller than that on LaMnO(3) (0.50 eV), oxygen vacancies greatly enhance O(2) dissociation kinetics. The kinetic and mechanistic studies for the interactions at the molecular level are imperative to gaining a fundamental understanding of oxygen reduction kinetics on cathode materials and to providing important insight into the rational design of more catalytically active cathode materials for SOFCs.
Prediction of O-2 Dissociation Kinetics on LaMnO3-Based Cathode Materials for Solid Oxide Fuel Cells
(2009-04-30)
Choi, YongMan
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Lynch, Matthew E.
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Lin, M. C.
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Liu, Meilin
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應用化學系分子科學碩博班
First-principles and statistical-theory calculations were applied to examine the interactions between oxygen molecules and the (100) surfaces of LaMnO3 and La0.5Sr0.5MnO2.75, one of the most-used cathode materials in solid oxide fuel cells (SOFCs). To predict the rate constants for the interactions between O-2 and LaMnO3 or La0.5Sr0.5MnO2.75, potential energy profiles were constructed using the nudged elastic band (NEB) method. Predicted rate constants for the dissociation of adsorbed oxygen species on LaMnO3 (lm) and La0.5Sr0.5MnO2.75 (lsm) can be expressed as k(diss,lm) = 2.35 x 10(12) exp(-0.50 eV/RT) s(-1) and k(diss,lsm) = 2.15 x 10(12) exp(-0.23 eV/RT) s(-1), respectively, in the temperature range of 873-1273 K at 1 atm. Because the activation energy for oxygen dissociation on La0.5Sr0.5MnO2.75 (0.23 eV) is much smaller than that on LaMnO3 (0.50 eV), oxygen vacancies greatly enhance O-2 dissociation kinetics. The kinetic and mechanistic studies for the interactions at the molecular level are imperative to gaining a fundamental understanding of oxygen reduction kinetics on cathode materials and to providing important insight into the rational design of more catalytically active cathode materials for SOFCs.
Rational design of novel cathode materials in solid oxide fuel cells using first-principles simulations
(2010-03-01)
Choi, YongMan
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Lin, M. C.
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Liu, Meilin
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應用化學系分子科學碩博班
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Institute of Molecular science
The search for clean and renewable sources of energy represents one of the most vital challenges facing us today. Solid oxide fuel cells (SOFCs) are among the most promising technologies for a clean and secure energy future due to their high energy efficiency and excellent fuel flexibility (e.g., direct utilization of hydrocarbons or renewable fuels). To make SOFCs economically competitive, however, development of new materials for low-temperature operation is essential. Here we report our results on a computational study to achieve rational design of SOFC cathodes with fast oxygen reduction kinetics and rapid ionic transport. Results suggest that surface catalytic properties are strongly correlated with the bulk transport properties in several material systems with the formula of La(0.5)Sr(0.5)BO(2.75) (where B = Cr, Mn, Fe, or Co). The predictions seem to agree qualitatively with available experimental results on these materials. This computational screening technique may guide us to search for high-efficiency cathode materials for a new generation of SOFCs. (C) 2009 Published by Elsevier B.V.
Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides
(2020-08-27)
Zhu, Yunmin
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He, Zuyun
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Choi, YongMan
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Chen, Huijun
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Li, Xiaobao
Developing highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is critical for many energy devices. While regulating the proton-coupled electron transfer (PCET) process via introducing additive into the system has been reported effective in promoting OER activity, controlling the PCET process by tuning the intrinsic material properties remains a challenging task. In this work, we take double perovskite oxide PrBa0.5Sr0.5Co1.5Fe0.5O5+delta (PBSCF) as a model system to demonstrate enhancing OER activity through the promotion of PCET by tuning the crystal orientation and correlated proton diffusion. OER kinetics on PBSCF thin films with (100), (110), and (111) orientation, deposited on single crystal LaAlO3 substrates, were investigated using electrochemical measurements, density functional theory (DFT) calculations, and synchrotron-based near ambient X-ray photoelectron spectroscopy. The results clearly show that the OER activity and the ease of deprotonation depend on orientation and follow the order of (100) > (110) > (111). Correlated with OER activity, proton diffusion is found to be the fastest in the (100) film, followed by (110) and (111) films. Our results point out a way of boosting PCET and OER activity, which can also be successfully applied to a wide range of crucial applications in green energy and environment. Proton-coupled electron transfer (PCET) has been observed in chemical, energy, and biological transformation processes. Here we demonstrate that the rate of PCET and oxygen evolution reaction can be dramatically enhanced by tuning crystal orientation and the correlated proton diffusion.