Browsing by Author "Wang, Jeng-Han"

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Chemical stability and electrical conductivity of BaCe0.4Zr0.4Gd0.1Dy0.1O3-delta perovskite
(2015-11-01)
Tsai, Yen-Chang
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Chen, San-Yuan
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Wang, Jeng-Han
;
Wu, Pu-Wei
;
Chen, Po-Chun
A proton-conducting BaCe0.4Zr0.4Gd0.1Dy0.1O3-delta (BCZGD) perovskite is fabricated via a wet chemical route using precursors including Ba(NO3)(2), Ce(NO3)(3)center dot 6H(2)O, ZrO(NO3)(4)center dot 6H(2)O, Gd(NO3)(3)center dot 6H(2)O, and Dy(NO3)(3)center dot 5H(2)O. The synthesis process involves the dissolution of relevant precursors and glycine in deionized water at proper molar ratios. Subsequently, the mixture undergoes a heat treatment at 1300 degrees C for 10 h in air to form the perovskite powders. The as-synthesized BCZGD powders are filtered and pressed at 440 MPa, following by a sintering at 1450 degrees C for 24 h in air to render a free-standing proton-conductive disk with a thickness of 850 mu m. Material characterization including scanning electron microscopy (SEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), and electrical conductivity measurements are performed. The electrical conductivity of the BCZGD sample is recorded at 1.93 x 10(-3) S cm(-1) at 700 degrees C in a humid hydrogen atmosphere with an activation energy of 0.41 eV. TGA results confirm chemical stability of the BCZGD against carbon dioxide without any decomposition. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Correlation of surface processes with characteristic sensing responses of PdO thin films to ethanol
(2019-04-15)
Cheng, I-Kai
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Wang, Jeng-Han
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Tsai, Cheng-Yu
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Chen, Yi-Sheng
;
Pan, Fu-Ming
Gas sensing characteristics of PdO nanoflake thin films exposed to ethanol (EtOH) at temperatures below 250 degrees C was studied. The PdO thin film responses distinctly to 0.15 ppm EtOH in dry air at temperatures above 100 degrees C. A characteristic valley-shaped response feature develops in the early stage of the exposure to EtOH at 150 degrees C and above. Formation of the valley feature is a result of the combined effects of two types of surface processes, which successively modify the sensor conductance in the opposite way. Dehydrogenation of adsorbed EtOH induces reduction of preadsorbed oxygen anions and the PdO substrate, resulting in the drop of the conductance. On the other hand, subsequent adsorption of EtOH and oxygen on newly growing Pd nanoclusters, which develop due to the reduction of the PdO substrate, causes the increase in the conductance. The Pd nanoclusters can be later reoxidized in the EtOH gas mixture, thereby modifying the sensing behavior of the PdO sensor. X-ray photoelectron spectroscopy (XPS), diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) were used to study adspecies and gaseous species formed in the EtOH gas sensing reactions. On the basis of the characteristic electrical response and chemical characterizations, we proposed the formation mechanism for the characteristic response feature.
Effectiveness of organic molecules for spin filtering in an organic spin valve: Reaction-induced spin polarization for Co atop Alq(3)
(2015-01-21)
Tu-Ngoc Lam
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Lai, Yu-Ling
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Chen, Chih-Han
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Chen, Po-Hung
;
Chan, Yuet-Loy
The spin polarization of organic-ferromagnetic interfaces in an organic spin valve critically affects the efficiency of spin injection or detection. We examined the chemical and electronic properties of ferromagnetic Co deposited on organic Alq(3) and the interfacial spin-polarized capability of the electronic states. Our x-ray photoemission spectra and calculations with density-functional theory indicate a sequential and unequal distribution of charge from Co clusters to N and then to O atoms in Alq(3). The preferential orbital hybridization at specific functional sites produces efficient spin polarization of organic molecules. Element-specific measurements of x-ray magnetic circular dichroism demonstrate the preferential spin polarization in the lowest unoccupied molecular orbital state of N atoms at the complex interface for Co atop Alq(3), which agrees satisfactorily with calculation. Our results indicate that an induced interfacial spin polarization on engineering the dominant reaction of Co with mainly N and O atoms in Alq(3) might pave a way for effective spin filtering in organic spintronics.
Enhanced Magnetic Anisotropy via Quasi-Molecular Magnet at Organic-Ferromagnetic Contact
(2013-01-17)
Hsu, Yao-Jane
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Lai, Yu-Ling
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Chen, Chih-Han
;
Lin, Ying-Chang
;
Chien, Hsiu-Yun
To realize the origin of efficient spin injection at organic-ferromagnetic contact in organic spintronics, we have implemented the formation of quasi-molecular magnet via surface restructuring of a strong organic acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), in contact with ferromagnetic cobalt. Our results demonstrate a spin-polarized F4-TCNQ layer and a remarkably enhanced magnetic anisotropy of the Co film. The novel magnetic properties are contributed from strong magnetic coupling caused by the molecular restructuring that displays an angular anchoring conformation of CN and upwardly protruding fluorine atoms. We conclude that the pi bonds of CN, instead of the lone-pair electrons of N atoms, contribute to the hybridization-induced magnetic coupling between CN and Co and generate a superior magnetic order on the surface.
Ethanol gas sensing mechanism of PdO nanoflake thin film at temperatures below 250 degrees C
(2017-04-02)
Cheng, I-Kai
;
Wang, Jeng-Han
;
Tsai, Cheng-Yu
;
Chen, Yi-Sheng
;
Pan, Fu-Ming
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
;
Yang, Lei
;
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.
Mechanism of Ethanol Reforming: Theoretical Foundations
(2009-04-23)
Wang, Jeng-Han
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Lee, C. S.
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Lin, M. C.
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應用化學系
;
應用化學系分子科學碩博班
The mechanism of ethanol reforming has been systematically studied by both energetic calculation to examine ethanol decomposition and electronic structure analysis to investigate the redox capability of the nine selected metals of Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au with the same crystal structure and surface orientation. The energetic calculation shows that most of the dissociation barriers are lower on Co(111), Ni(111), Rh(111), and Ir(111.) surfaces and higher on Cu(111), Ag(111), and Au(111) surfaces. The initial C-H bond dissociation, forming the doubly adsorbed *C(H(2))C(H(2))O(H)* and *C(H(2))C(H(2))O*, with a lower barrier than those in the initial C-C and C-O bond dissociations is considered as the most feasible decomposition route. In addition, the linear correlation between reaction barriers and d-band centers breaks down in the case of C-H bond dissociation due to the lower barriers on Rh(111) and Ir(111) surfaces. This result may be related to the suitable bond distances on Rh(111) and Ir(111) surfaces to form the more stable double adsorbates, *C(H(2))C(H(2))O(H):* and *C(H(2))C(H(2))O*. In the electronic structure analysis, Rh(111) and Ir(111) surfaces with higher density of state (DOS) distributions around the Fermi level can efficiently accept/donate electrons from/to the reacting ethanol and its fragments, showing better redox capability. Therefore, the excellent efficiency of Rh- and Ir-based catalysts, as observed from the reforming experiment, can be attributed to both the lower decomposition barrier and the higher DOS distribution around the Fermi level based on the first-principles calculation.
Palladium based cermet composite for hydrogen separation at elevated temperature
(2015-01-15)
Tsai, Yen-Chang
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Lin, Chien-Cheng
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Lin, Wei-Lin
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Wang, Jeng-Han
;
Chen, San-Yuan
A cermet composite consisting of palladium and BaCe0.4Zr0.4Gd0.1DY0.1O3-x (BCZGD) is fabricated by mixing palladium and BCZGD powders in a ball mill, followed by pressing and sintering at 1450 degrees C for 24 h in air. The Pd-BCZGD cermet demonstrates impressive hydrogen permeation flux in a mixture of hydrogen and carbon dioxide at elevated temperature, in which the palladium plays the predominant role of facile transport in the hydrogen atoms whereas the BCZGD provides channels for proton conduction. Material characterization including scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermo-gravimetric analysis (TGA) are performed. XRD patterns indicate pure phases of fcc palladium and perovskite BCZGD. SEM images and element mapping suggest a homogeneous mixture of cermet without noticeable defect and phase segregation. TGA results confirm stability of the cermet against carbon dioxide without chemical decomposition. The hydrogen permeation flux is determined via a gas chromatography from 400 to 700 degrees C at various hydrogen concentration gradients. We record a hydrogen flux of 1.25 cm(3) min(-1) cm(-2) in 50% hydrogen and 50% carbon dioxide at 700 degrees C, with a selectivity of H-2/CO2 approaching infinity. (C) 2014 Elsevier B.V. All rights reserved.