Browsing by Author "Chiang, Mu-Sheng"

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Electrochemical reaction and performance of proton exchange membrane fuel cells with a novel cathode flow channel shape
(2007-04-15)
Chiang, Mu-Sheng
;
Chu, Hsin-Sen
;
Chen, Cha'o-Kuang
;
Jian, Sheng-Rui
;
機械工程學系
This study focuses on the investigation of the electrochemical reaction along a novel cathode flow channel of PEM fuel cells with various shoulder/channel (S/C) ratios at the outlet port. A three-dimensional mathematical model, considering conservation principles of mass, momentum, species and electric current is employed. Local variations of important model variables such as reactant concentration and local current density are presented by contour plots to elucidate the effects of channel geometry on transport process, catalyst reaction and cell performance. The potential fields of solid and membrane phases are also resolved in the cell domain and the driving force of the electrochemical reactions - the catalyst activation overpotential - is harnessed in modeling. Numerical calculations reveal the influence of the cathode channel configuration on the local distributions of various model variables. The results also show the dependence between optimal channel configuration and cell operating condition. At a medium reaction rate, the reaction sites underneath the shoulder region generate more current than the channel region. Therefore, a convergent channel configuration with a larger S/C ratio at the outlet port develops more current because such a design facilitates the electron transport and enhances local activation overpotential. However, as the cell voltage decreases and the reaction rate increases, such configuration loses its merit gradually as the requirement for a higher reactant concentration is more important and the reaction sites underneath the channel region have a higher reaction rate. Consequently, the divergent channel configuration with a lower S/C ratio of 0.67 perfonns better at a cell voltage of 0.22 V (c) 2007 Elsevier B.V. All rights reserved.
Localized electrochemical oxidation of p-GaAs(100) using atomic force microscopy with a carbon nanotube probe
(2006-08-14)
Huang, Wu-Ping
;
Cheng, Hung-Hsiang
;
Jian, Sheng-Rui
;
Chuu, Der-San
;
Hsieh, Jin-Yuan
The nanometre-scale oxidation characteristics of a p-GaAs(100) surface are investigated by atomic force microscope (AFM) electrochemical nanolithography with a multiwalled carbon nanotube (MWCNT) probe. The electrochemical parameters, such as anodizing voltages, scanning rate and modulated voltages, and how they affect the creation and growth of the oxide nanostructures are explored. The present results reveal that the initial growth rate (similar to 600 nm s(-1) for 10 V) decreases rapidly as the electric field strength is decreased. The oxide practically ceases to grow as the electric field is reduced to the order of similar to 1.2 x 10(7) V cm(-1). Also, the oxide growth rate depends not only on the electric field strength but also on the applied anodizing voltage. The present results show that the height of the oxide structures can be significantly improved at an applied anodizing voltage of 10 V by using a CNT probe. In addition, Auger electron spectroscopy (AES) measurements performed in the present work confirm that modified structures replace the form of anodizing p-GaAs(100).
Numerical investigation of transport component design effect on a proton exchange membrane fuel cell
(2006-09-29)
Chiang, Mu-Sheng
;
Chu, Hsin-Sen
;
機械工程學系
;
Department of Mechanical Engineering
A numerical investigation of the transport phenomena and performance of a proton exchange membrane fuel cell (PEMFC) with various design parameters of the transport component is presented. A three-dimensional fuel cell model, incorporating conservations of species, momentum, as well as current transport, is used. The Bulter-Volmer equation that describes the electrochemical reaction in the catalyst layer is introduced; the activation overpotential connects the solid phase potential field to that of the electrolyte phase. Through cell performance simulation with various channel aspect ratios and gas diffusion layer (GDL) thicknesses, a slender channel is found suitable for cells operating at moderate reaction rate, and a flat channel produces more current at low cell voltage. Plots of transverse oxygen concentration and phase potential variation indicate that these oppositely affect the local current density pattern. The relative strengths of these two factors depend on the transport component position and geometry, as well as on the cell operating conditions. Consequently, the curves of cell output current density demonstrate that the optimal GDL thickness increases as the cell voltage decreases. However, at the lowest considered cell voltage of 0.14 V optimal thickness decreases as that of a thick GDL. The oxygen deficiency caused by long traveling length and clogging effect of liquid water reverses this relationship. (c) 2006 Elsevier B.V. All rights reserved.