電化學交流阻抗法在直接甲醇燃料電池與光電化學矽蝕刻之研究; Investigation of the Electrochemical Impedance Spectroscopy on the Direct Methanol Fuel Cell and Silicon Photo-electrochemical Etching

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题名:	電化學交流阻抗法在直接甲醇燃料電池與光電化學矽蝕刻之研究;Investigation of the Electrochemical Impedance Spectroscopy on the Direct Methanol Fuel Cell and Silicon Photo-electrochemical Etching
作者:	賴建銘;Chien-Ming Lai
贡献者:	機械工程研究所
关键词:	電化學交流阻抗法;Ru溶解反應;介面阻抗;等效電路;巨孔洞.;光電化學矽蝕刻;直接甲醇燃料電池;Macro-pores.;Photo-electrochemical etching;Ru dissolution;Interfacial impedance;Direct methanol fuel cell;Electrochemical impedance spectroscopy
日期:	2008-05-22
上传时间:	2009-09-21 11:52:50 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本論文之主旨在運用電化學交流頻譜(Electrochemical impedance spectroscopy, EIS)分析技術，來解析(1)直接甲醇燃料電池(Direct methanol fuel cell, DMFC)之特性，與(2)矽單晶在氫氟酸中蝕刻之動力學，並分別探討其反應機理。研究結果摘要如下。 (1) 直接甲醇燃料電池之研究結果：直接甲醇燃料電池在不同電流下放電時，本論文藉由交流頻譜的監測與解析，提出一新見解之等效電路(Equivalent circuit)，用以代表電池中膜電池組(Membrane electrode assembly, MEA)之阻抗，其元件包含電池內阻(Internal resistance, Rs)、介面阻抗(Interfacial impedance, Rif /Cif)、電化學反應阻抗(Electrochemical reaction impedance, Rrxn /Crxn)與CO吸脫附反應阻抗(CO adsorption impedance, LCO /RCO)，本研究除了藉由實驗參數之改變，就其對應之結果，逐步來驗證此一等效電路之外，進而，佐以電化學理論，賦予各元件之物理意義。本研究所建立之等效電路，不僅適用於DMFC燃料電池性能之非破壞性檢測，更適用其劣化成因機理之探討。藉由比較電池劣化前後之EIS圖譜，進行等效電路模擬分析，並配合儀器分析之佐證，本論文對一加速劣化的DMFC電池試驗系統，推究出其劣化之主因如下：(a)電池中之陽極因觸媒釕(Ru)之溶解而使活性降低，溶解之Ru離子遷移至陰極附近的薄膜中析出，堵住了膜中的質子通道。Ru的溶解，不僅減少陽極觸媒的活性面積，也導致質子在陰極附近傳輸之阻礙，這些原因造成了等效電路中介面阻抗(Rif)與電化學阻抗(Rrxn)的增加。Ru的陽極溶解現象已藉由CO stripping、EPMA與XPS等分析證實。Ru在膜中的還原與堵塞，已由TEM與EPMA的觀察證實，此現象多少會造成電池內阻(Rs)的增加。(b)電池中之聚合物薄膜的裂解作用，造成膜上磺酸根的損失，因而導致電池內阻(Rs)增加。磺酸根的損失則藉由EMPA與XPS的分析證實。 (2) 矽單晶在氫氟酸中蝕刻之研究結果: n-型(100)矽單晶在2M氫氟酸中經定電位0.25V蝕刻3 h後，經掃瞄式電子顯微鏡(SEM)觀察蝕孔表面型貌顯示：若添加酒精於氫氟酸中將有助於使蝕孔具有平滑性，此平滑蝕孔尺寸較小(~3-4μm)，且蝕孔分佈更均勻。此結果歸因於酒精降低蝕刻液之表面張力，並使氫氣泡細緻化。經由直流電陽極動態極化曲線解析，分別對特定電位與電流如過渡電位(Transition potential, Etrans)、半波電位(half-wave potential, Ep/2)與極限電流密度(limiting current density, jlimit)等進行研究，配合電化學交流頻譜在上述特定電位下，探討單晶矽在含不同濃度酒精氫氟酸中之動力學，結果顯示：在含酒精蝕刻系統的EIS圖譜中低頻處，多出一電感迴路(L2/RL2)，此一迴路乃因酒精在矽晶表面的吸附/脫附反應所致。酒精添加雖會提高蝕刻液之電阻，但可促進氫氟酸對矽晶面的濕潤性(降低其接觸角)，因而酒精添加於蝕刻液中導致以下結果：在低電位下蝕刻，其極化電阻及電化學阻抗增大，而在高電位下蝕刻，其蝕刻速率增加。 The technique of electrochemical impedance spectroscopy (EIS) diagnosis has been used to investigate the electrochemical kinetics in the systems of (1) direct methanol fuel cell (DMFC) and (2) photo-electrochemical etching on silicon. The results and contributions of this work were summarized as follows. 1. EIS was carried out to monitor the performance of DMFC under a variety of current densities. Based on analysis of the EIS data that depend upon the performing conditions, an innovative model including the qualitative sketch and its quantitative description relying on postulated equivalent circuit (EQC) was established to delineate the reaction mechanism of DMFC on the membrane electrode assembly (MEA). This model provides a satisfactory diagnosis in the performance of DMFC in terms of the EQC sets. One EQC sets comprises elements such as the internal resistance (Rs) at the highest frequency, the high-frequency impedance (Rif /Cif) that is a parallel combination of the interfacial resistance (Rif) and interfacial capacitance (Cif) resultant from the interfaces in the cell, the medium-frequency impedance (Rrxn /Crxn) that is a parallel combination of the resistance (Rrxn) and capacitance (Crxn) resultant from electrochemical reactions, and the low-frequency impedance (LCO /RCO) that is a parallel combination of the resistance (RCO) and inductance (LCO) resultant from the adsorption and relaxation of CO. This postulated model provides a useful tool to diagnose the degradation mechanism for a cell subject to a test of accelerating degradation. Through the diagnosing and the evidences supported by the examinations through instruments such as the electron probe microanalyzer (EPMA), transmission electron microscope (TEM) and X-ray photoelectron spectroscope (XPS), the degradation is major attributed to (a) the increase of Rif and Rrxn resultant from catalytic degradation that may arise from a series of processes including the dissolution of Ru from the anodic catalyst Pt-Ru, the migration of Ru ions to be reduced on the membrane nearby the cathode. The Ru-dissolution leads to a decrease of catalytic activity on the anode that could be confirmed by the technique of CO stripping in company with the observation through EPMA and XPS. The particles reduced on the membrane nearby the cathode were verified by the examination through TEM and EPMA. (b) The increase of internal resistance (Rs) is ascribed to the loss of sulfonic-acid group from the graded membrane near the anode. Membrane degradation possibly arisen from the heat accumulation in a severely acidic environment near the anode derived from cell reactions. The loss of sulfonic acid group was verified by EPMA and XPS analyses. 2. The photo-electrochemical etching on Si (100) surface reveals different SEM morphologies depending on whether or not the HF solution contains ethanol. Finer smooth pores (around 4 μm in diameter) were formed in the presence of ethanol but larger rough pores (around 8 μm in diameter) formed in 2 M HF solution alone during silicon etched at 0.250 V (vs. SCE) under 50W-illumination for 3 h. The characteristic potentials and current such as transition potential (Etrans), half-wave potential (Ep/2), and limiting current density (jlimit), resulted from dc anodic polarization, were the major parameters used in EIS to diagnose the etching system. There appears an extra low-frequency inductive loop in the Nyquist plot for the etching system in the presence of ethanol. This loop is attributed to relaxation of the adsorption of ethanol in the pores. The contact angle between the etching solution and the silicon decreases with increasing the ethanol concentration. Accordingly, ethanol plays a wetting role in the etching process thus forming fine smooth pores.
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