奈米柱應用於燃料電池電極之技術開發

Abstract

摘 要 直接甲醇燃料電池(DMFC)是未來令人期待的科技，目前的發展方向在於3C產品的應用(如筆記型電腦、手機)、攜帶式電源供應器等。然而，目前發展DMFC仍有幾項瓶頸仍待克服，例如提升電極觸媒催化效能、減少甲醇不必要的穿透現象，這些負面影響均使其輸出功率依舊無法滿足實際應用的需求。由文獻可知，為了製作高低不平電極，以提升直接甲醇燃料電池效率，皆需使用感應耦合電漿離子蝕刻技術。然而，由於這些設備高價格之缺點，使得學術界與中小型企業難以投入相關的研究。根據上述，本研究將結合「自組裝奈米球微影」、「光輔助電化學蝕刻」、「精密電鑄」技術，預期將可成為低成本，並且用以製作出完美且具大規模排列之奈米柱狀陣列結構，藉由電極接觸表面積之大量增加，來提高反應性，以應用於直接甲醇燃料電池電極之開發。 實驗的結果証實結合薄光阻格狀結構製作及震盪塗佈法的方式，可將奈米球規則地排列於矽基板上，以得到大面積且趨近完美排列的奈米球陣列。而在光輔助電化學蝕刻的實驗中，當使用1 V的蝕刻電壓與HF濃度2.5 wt%的蝕刻液，蝕刻30分鐘後，能夠產生高度約為7.4 m，直徑約為90 nm，而孔洞的深寬比可達到67:1之高深寬比孔洞。並且証實加大蝕刻電壓機制使蝕刻孔洞擴孔及適當RIE蝕刻時間，即可製作出柱體高度約為1.56 µm，直徑約為250 nm~300 nm，因此柱體的深寬比可達6.2:1~5.2之奈米柱狀陣列。目前直接甲醇燃料電池電極測試性能後，結果顯示平板電極其開路電壓、極限電流密度、最大功率密度分別為105 mV、0.319 mA/cm2、0.0093 mW/cm2，柱狀電極其最大開路電壓、極限電流密度、最大功率密度分別為280 mV、1.044 mA/cm2、0.0584 mW/cm2，本實驗發現柱狀電極所製作燃料電池之最大功率密度優於平板電極6.3倍，顯示蝕刻電壓增加所製作之柱狀電極結構可提升觸媒與燃料接觸之表面積，使其性能也隨之提升。

關鍵字：奈米柱，光輔助電化學蝕刻，精密電鑄，直接甲醇燃料電池電極。Abstract A direct methanol fuel cell (DMFC) is the much-anticipated science and technology in the future, the present developing direction such as application of 3C products and portable power supplying device. However, the DMFC still has the following drawbacks to overcome, for example improve the catalyst efficiency of electrodes and reduce the methanol crossover phenomenon, it makes its output power is still unable to meet practical applications. According to the past approach, in order to make the rugged electrodes, it has to use the inductively coupled plasma reactive ion etching (ICP-RIE) technique, but these equipments are expensive. The research will integrate nanosphere lithography, photo-assisted electrochemical etching (PAECE) and electroforming techniques for fabricating perfect and high regular arrangement of the nanopillars array structure. If all techniques that we adopt can be integrated successfully, low cost, fabricating perfect and high regular arrangement of the nanopillars array can be realized. In addition, we can fabricate the electrodes in the direct methanol fuel cell by means of these techniques. Experiment results show that self-assembly nanosphere lithography, it can be used to define nano-pattern array by integrating thin photoresist and vibration method. The novel coating method of nanosphere can arrange nanosphere regularly. Nanohole can be easily obtained in PAECE process, which dimensions of width and height is 90 nm and 7.4 µm (aspect ratio, 67:1). We have finished the fabrication process of nanopillar array by using PAECE technique. Nanopillar array can be regularly arranged, which dimensions of width and height is 250 nm~300 nm and 1.56 µm (aspect ratio, 6.2:1~5.2.:1). The nanopillar electrode (9.3 mA/cm2) showed 3 times larger current density than that from planar electrode (3.1 mA/cm2) at electrode potential of 1V. We found the nanopillar electrode DMFC (58.4 µW/cm2) showed maximum 6.3 times higher power density than the planar electrode DMFC (9.3 µW/cm2) in fuel cell test.

Keywords: nanopillar, photo-assisted electrochemical etching, electroforming, direct methanol fuel cell.