微型甲醇燃料電池之陽極微氣泡觀測與電催化測試研究

Observation of Micro Bubbles and Electrocatalytic Test Research of Anode for Micro DMFC

陳順林(Soon-Lin Chen)

指導教授 ： 曾繁根

國立清華大學/原子科學院/工程與系統科學系/博士(2009年)

摘要

本研究主要著重在微型甲醇燃料電池陽極甲醇電催化測試和微氣泡的觀測及分析。我們可以由兩個部份來討論此一議題。第一部份係針對不同觸媒基材載體電極進行CO2微氣泡觀測同步量測電化學訊號的實驗；第二部份為在數種不同型態直徑80μm的白金薄膜上利用CCD攝影機及高速攝影機去觀測O2微氣泡的生長和脫離現象。
第一部份可以得到的結論為若電極以奈米碳管(CNTs)作為觸媒支撐層可以直接減少電化學觸媒反應中二氧化碳(CO2)氣泡在觸媒表面的累積，導致可以增進二氧化碳(CO2)氣泡移除的能力。同時由光學系統觀察到的較高氣泡脫離率和較小的氣泡脫離尺寸與電化學系統中較高的電流振動頻率間有很高的一致性。這些結果提供了有奈米結構的電極對包含了液體反應物和氣體產物之電化學反應的質傳有所貢獻的直接證據。Pt/CNTs/CC電極的二氧化碳氣泡表面的未覆蓋率，較Pt/CC電極及Pt/CP電極的未覆蓋率分別高34％和32％，意味著奈米碳管修飾電極(Pt/CNTs/CC)有34%和32%的性能增強是因快速的CO2氣泡移除能力。
　在第二部份的微氣泡觀測方面，使用高速數位相機的影像和模擬的結果來研究微結構觸媒上產生的化學O2微氣泡，在實心圓結構白金觸媒上有三階段的氣泡成長現象被實驗確認，第一階段是慣性控制階段(t < 0.2秒)；第二階段是氣體產生率固定和駐足面積逐漸增加的階段(0.2 < t < 2.5秒)；第三階段是氣體產生率和駐足面積兩者皆同時固定的階段(t > 2.5秒)。基於駐足面積(footing area)的觀測，這些實驗結果與理論的預測有很高的一致性。實驗結果顯示，當次級微結構完全被分開來及它們的特徵尺寸較其所產生的初始微氣泡的直徑(～5 um)為大時，此時非連續性的網狀(mesh)結構能夠有效的縮短氣泡的脫離時間。然而同心圓狀(concentric circular)結構則沒有此一現象。白金網狀結構觸媒的氣泡駐足面積較白金實心圓結構觸媒和白金同心圓狀結構觸媒的氣泡駐足面積為小，因此同時提昇了氣泡產生的速率並且降低了氣泡和觸媒的黏著力。這些發現對未來微型甲醇燃料電池陽極腔體的設計將有相當的助益。

關鍵字

直接甲醇燃料電池 ； 奈米碳管 ； 二氧化碳微氣泡

並列摘要

This research mainly focus on the observation of micro-bubble and electrocatalytic test of anode for micro direct methanol fuel cells (μDMFC). We can discuss this issue from two parts. The first part is observing the CO2 micro-bubble and measure electrochemical signal simultaneously on different catalyst substrate electrodes. The second part is using CCD camera and high speed camera to observe the O2 micro-bubble growth and detached phenomenon on different type patterns of the diameter 80 μm Pt film.
We can concluded from part one that using carbon nano tube (CNTs) as catalytic support can get less bubble accumulation on electrode surface and lead to improvement of the CO2 microbubble detaching capability. Also, the high bubble detachment rate and smaller detachment size observed in the optical system are consistent with the high frequency of current vibration in the electrochemical system. These results demonstrate that electrodes with nanostructures contribute to the mass transfer for electro-catalytic reactions composed of liquid reactants and gaseous products. The uncovered reaction area on the Pt/CNT/CC electrode was 34%, and was 32% higher than that on the Pt/CC and Pt/CP electrodes, respectively, which equates to a 34% and 32% performance enhancement, respectively, in the CNT-modified electrodes (Pt/CNT/CC) due to a faster CO2 bubble removal capability.
At the second part of micro-bubble observation, we investigate the growth and detachment of chemically formed O2 micro-bubbles on micro-textured catalyst using a high-speed digital camera and simulation results. Three stages of bubble growth on a solid circular Pt catalyst were determined experimentally to be as follows. The first stage is one of inertial control (time <0.2s); the second stage is one of constant gas generation rate and increasing footing area (0.2< time <2.5s), and the third stage is one of constant gas generation rate with constant footing area (time >2.5s). These results are highly consistent with theoretical predictions based on footing area visualization. Experimental results revealed that a discontinuous mesh catalyst can effectively shorten the bubble detachment time when the substructures are thoroughly separated and the bubbles are larger than their initial size (～5 um), while the concentric circular pattern does not. The footing area of the mesh catalyst is suggested to be smaller than that of the solid circular and concentric circular catalyst, promoting the generation of gas and simultaneously reducing the adhesive force between bubble and catalyst. These findings will be beneficial for the design of an anode chamber in μ DMFC in the future.

並列關鍵字

direct methanol fuel cell ； carbon nanotubes ； carbon dioxide microbubbles

參考文獻

6. B. Yang, A. Manthiram, "Multilayered membranes with suppressed fuel crossover for direct methanol fuel cells ", Electrochemistry Communications, 6; 231–236; 2004.

8. 王壽凱，"微型甲醇燃料電池之表面奈米化陽極電催化測試與微氣泡觀測研究"，國立清華大學 工程與系統科學系碩士論文，2006年7月。

9. P. Argyropoulos, K. Scott, and W. M. Taama, "Carbon dioxide evolution patterns in direct methanol fuel cells" ,Electrochimica Acta, vol. 44; 3575-3584; 1999.

10. P. Argyropoulos, K. Scott, W.M. Taama, "Gas evolution and power performance in direct methanol fuel cells", Journal of Applied Electrochemistry, 29; 661-669; 1999.

[11] K. Scott, W.M. Taama, P. Argyropoulos, "Material aspects of the liquid feed direct methanol fuel cell ", Journal of Applied Electrochemistry, 28; 1389-1397; 1998.