應用在燃料電池的甲醇濃度感測方法研究

本文的甲醇濃度感測方法研究，主要是針對水溶液中的甲醇濃度偵測，特別是微小型高分子質子交換膜直接甲醇燃料電池(direct methanol fuel cell, DMFC) 的應用。DMFC的技術瓶頸之一為甲醇會從陽極滲透至陰極，在陰極產生的氧化反應會造成電位降低與電池整體效能的衰退。一些研究報告指出，低濃度的甲醇可降低甲醇的滲透現象及和緩電池效能下降的情況。因此甲醇濃度感測技術與DMFC的發展有緊密的關聯性，也是引發本研究的初始動機。研究基本上分為兩個階段：第一階段為相關研究的文獻回顧，並藉由一般甲醇感測器的驗證實驗、甲醇滲透率量測，進行研究理論基礎的分析探討；第二階段則是創新概念的實驗研究，包含平面式電極設計的氧化鋁基材、矽基材兩種元件的實驗探討。綜觀目前常用的各種甲醇濃度偵測技術，包括利用物理、化學性質等各種方法，在微小尺寸的燃料電池系統都不太適用，而且感測器不能消耗過多能量，構造也不能夠太複雜。因此本文實驗乃選擇以電化學感測為研究的方向。實驗主要目標是要利用電化學的特性偵測甲醇濃度，發展一個構造簡單的甲醇感測器。電化學槽(cell)的設計是以金屬薄膜構成的平面金屬電極，加上高分子膜的固態電解質薄膜，藉由阻抗(EIS)、循環伏安法等電化學技術分析，瞭解不同的甲醇濃度會有不同程度的電化學反應，進一歩分析靈敏度(sensitivity)等感測性能。此外，在德國Forschunszeentrum Juelich研究中心，合作進行的甲醇滲透率(permeation rate) 量測實驗，則有助於實際甲醇感測解析度(resolution)與感測範圍(range)需求的瞭解，當然也可以間接推測甲醇濃度變化。本研究的結果顯示以氧化鋁基材的感測元件，甲醇氧化電流的感測靈敏度(sensitivity)可以達到~0.01mA/mole (or 0.1mA/mole 氫脫附電流)，偵測範圍(range)則是在0.5M~2M。證明了平面電極加上固態電質，藉由電流強度作為感測甲醇濃度的理論是可行的。而在以矽晶圓為基材的元件，則可以辨別甲醇濃度0.5M~1.5M的差異。雖然訊號較氧化鋁基材為弱(~0.01mA/mole)，但是若以微機電製程技術為基礎在訊號靈敏度應可有效改進，而且在建構系統時的製程整合性較佳。此一創新概念可作為未來微型甲醇燃料電池系統發展的一項參考。

關鍵字

燃料電池；甲醇；感測器；直接甲醇燃料電池

並列摘要

This thesis focuses on the development of sensors for the determination of aqueous methanol concentration in direct methanol fuel cell (DMFC) applications. The crossover of methanol from anodic compartment to the cathodic compartment and its subsequent oxidation in the cathodic compartment is the main reason for the low efficiency of DMFC, which is yet to be solved in the current DMFC technology. DMFC operated at controlled low concentration methanol feed is one of the approaches to avoid the cell voltage loss due to methanol crossover. Such an operation needs the development of a suitable methanol sensor which motivates the present study. The study consists of two major parts. In the first part, literature overview of DMFC, methanol crossover problem, methods to circumvent the methanol crossover are discussed. Apart from this, salient features of various methanol detection methods and the necessary features of a methanol sensor are discussed. Then, rationales for the selection of electrochemical sensor development as the subject matter are presented. Fabrication and operation of a fuel cell based sensor is presented in chapter 3. Even though, the sensitivity of the fuel cell based sensor is adequate for the methanol concentration in the range of 0.5 M to 1.5M, higher power consumption and bigger size are the problems with this design. Operating experience with the conventional methanol detection techniques like densitometer and refractive index detector showed the shortcomings of these techniques. An innovative concept of solid state planar structured methanol sensor design is introduced in chapter 4. Thick film screen printing process was used to print the gold working and counter electrodes on alumina substrate and process for the formation of the Ag/AgCl reference electrode was also developed. A thick recast Nafion film was the electrolyte. The sensor was characterized by Electrical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). The experimental results revealed that either hydrogen desorption (peak at ~0.22 VSHE) or methanol oxidation (~1.0 VSHE) could be used to evaluate the methanol concentration. The device should be used for the estimation of methanol concentration at 0.01mA/M by methanol oxidation current (or 0.1 mA/M by hydrogen desorption current) in the range of 0.5M to 2M. Silicon based microfabrication is introduced in chapter 5. This has the advantage of producing microsized structures in highly uniform and geometrically well defined manner. Platinum working and counter electrodes were prepared by sputtering. The process for the formation of recast nafion electrolyte film was the same as alumina-based device. The sensor could be used to monitor methanol concentration from the methanol oxidation current. The current linearly increased with methanol concentration in the range 0.5 to 1.5M (with 0.01mA/mole sensitivity). A summary of the work and recommendations for further activities were presented in chapter 6.

並列關鍵字

FUEL CELL ； METHANOL ； SENSOR ； DFMC

參考文獻

for reducing fuel crossover in a direct methanol fuel cell”, Journal of Power Source,

polymer electrolyte membrane for DMFC using Pd films formed by sputtering”, Journal

of Power Source, 106, p.215-223, 2002.

4 NASA Tech Briefs, p.38-40, January 2005

5 Fuqiang Liu, Guoqiang Lu, and Chao-Yang Wang, Journal of The Electrochemical

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應用在燃料電池的甲醇濃度感測方法研究

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