使用貴金屬微保護層延長操作壽命及耐高壓力結構強化設計應用於優化純水電解產氫電池

Abstract

氫氣是最乾淨也是最豐富的燃料，亦被稱為下一個世代的能源，因地球表面有百分之七十的面積是水，而水的組成就是氫氣跟氧氣。燃料電池是將氫氣與氧氣直接從化學能轉換成電能，效率可高達60%以上，比傳統的汽油內燃機發電效率高上許多，且這反應過程中，產生的只有水、電與熱。燃料電池於1950年代就已經被美國重度發展，並應用於提供太空梭，於2007年前後，燃料電池車已經可以在北美租賃了，距離取代傳統引擎來降低二氧化碳排放的日子已經越來越近了。可是，現今氫氣的來源有百分之九十以上都是從提煉石化燃料所產生的副產物，這對於要推廣綠色能源是相當矛盾的。所以，從水中取得氫氣是最直接也是取得純度最高的方式 - 純水電解產氫。SPE (Solid Polymer Electrolyte) or PEM (Proton Exchange Membrane) 膜式電解是效率最高也是最乾淨，其特點之一是可將氫氣與氧氣獨立分開，避免氫氧混和造成危險，而其產生的氫氣與氧氣純度可高達99.99%以上。但是，在電解反應過程中，容易對氧氣端的氣體擴散層或電極造成腐蝕及氧化作用，加速其性能的衰退。使用壽命對商用化是相當重要的，故我們找到適當的觸媒得以大大延長電極衰退時間，並進階強化純水電解電池結構，提高氫氣出口壓力，得以做更多的應用。 這篇論文的主要目的在於研究開發領先國內學術與業界的技術，從頭到尾都是我們自己設計與製作。先使用既有的燃料電池的架構 – PEM燃料電池，先驗證我們塗佈的膜電極組穩定性，之後進一步應用於PEM純水電解電池。使用貴金屬微保護層塗佈於碳製的氣體擴散層上，應用於常壓氫氣出口的PEM純水電解電池，保護氧氣端電極避免被腐蝕，使此PEM純水電解產氫電池得以壽命延長。更因應近年開發成功的合金粉末儲氫罐的及純水電解產氫的趨勢，強化純水電解結構設計，使新結構設計的高壓純水電解產氫電池壓力可以達到10 bar，能直接將產生的氫氣儲存到儲氫罐中，加上內部關鍵元件的升級，使用新的微保護層觸媒組合 (IrO2 / Ta2O5)，達到更穩定的性能表現。 首先開發製作燃料電池的心臟 - 膜電極組，並測試其製程的穩定性及性能是否與文獻可以比較 ; 待燃料電池的膜電極組已有相當穩定性後, 進而開發純水電解產氫的膜電極組。選用不含碳基材的白金與氧化銥觸媒，調配特定比例的Nafion，塗佈於質子交換膜上。待純水電解產氫膜電極組製程穩定後，針對延長氣體擴散層則使用1層的貴金屬微保護層，簡化製程並降低時間與成本，而該層貴金屬微保護層可以將純水電解產氫的過程中，將氧氣端所產生的活性氧原子催化成無害的氧氣。經過長久的實驗證實，在常壓氫氣出口的純水電解產氫電池操作於高電流密度 (1.4 A cm-2)下，使用1層微保護層的膜電極組與沒有微保護層的相比，壽命延長了10倍以上，操作時數亦超過2000個小時。甚至，為了更多的應用，氫氣需要被儲存是未來的趨勢，所以電池結構需要提升並具有耐壓的能力，才能對應也在發展的儲氫罐。為了強化電池結構，我們利用台灣卓越的CNC加工技術，將水流道與集電板結合，減少組裝元件以降低於高壓力操作環境下，水電解電池漏氣的風險，同時也可以簡化組裝的步驟。經實驗證實，氫氣出口壓力可以達到至少10 bar，可直接將氫氣填充到金屬粉末儲氫罐中；而基於前一個實驗，經過長時間測試所得到微保護層的效用，新的微保護層可讓耐高壓的純水電解產氫電池穩定的連續操作超過600小時，其電壓震盪幅度不超過0.02 V。

Hydrogen is the cleanest and most sufficient fuel on earth and also called the energy of the next generation. Fuel cells convert the chemical energy into electricity, generating only heat and water. The proton exchange membrane fuel cell (PEMFC) is one type of fuel cells and has been regarded as one of the most promising alternative power sources due to its low emissions and high efficiency which can achieve more than 60%. However, 90% of hydrogen we use today is obtained from petroleum products. To solve the global warming issue, every country plans to reduce the usage of gasoline. Pure water electrolysis with a proton exchange membrane (PEM) or solid polymer electrolyte (SPE) is the most effective and the cleanest method to produce hydrogen. The purity of hydrogen could achieve 99.99% because only de-ionized water (DI water) is used. However, a challenging problem for PEM water electrolysers is the corrosion and oxidation to the gas diffusion layer at anode side by active oxygen species (such as oxygen atoms and hydroxyl free radicals) during the reaction of water electrolysis. For the use of hydrogen fuel in a wide range of applications, high-pressure water electrolysers are owing to the pre-storage of hydrogen. In recent years, some studies have developed low-pressure hydrogen storage by metal hydrides, metal-organics, and carbon nanotubes. The minimum pressure to store hydrogen has been reduced to 10 bar or below. PEM water electrolysers have to provide high enough of hydrogen outlet pressure to store hydrogen directly into the hydrogen storage tank for more applications. The first purpose of this study is to extend the lifetime of the PEM water electrolyser. We repeat the process of catalyst coated membrane (CCM) fabrication to get uniform performances for PEM fuel cells. After that, a carbon-made gas diffusion layer (GDL) is coated a noble metal (IrO2) micro protective layer (MPL) to replace the micro porous layer, normally uses carbon black (XC-72). The functions of the MPL are used to transform active oxygen species into harmless oxygen gas and to prevent the carbon-made GDL from corrosion and oxidation during water electrolysis. The second purpose is to increase the outlet pressure of hydrogen of the high pressure PEM water electrolyser up to 10 bar. Our design is to combine the current collector and the flow field plate into one single component which is carried out by the mature computer numerical control (CNC) technique. For the lifetime extension, the MPL is working based on previous study. The advanced MPL is coated on the titanium porous disc with IrO2 / Ta2O5 composition. The titanium porous disc is used to replace the carbon-made GDL to support the thin membrane and prevent it from rupturing when operating at high pressures and stabilize the performance of the high pressure PEM water electrolyser when operating at high current density. We verify the noble metal MPL coated on carbon-made GDL can effectively extend the lifetime of the ambient pressure PEM water electrolyser more than 2000 h when operating at high current density (1.4 A cm-2) that is 10 times longer than that of a commercial sample coated only with carbon black as the micro porous layer. Moreover, the innovative structure of the high pressure PEM water electrolyser successfully eliminates the sealing risk of assembly and can operate at 10 bar of hydrogen outlet pressure and achieve a lifetime of over 600 h with the advanced MPL. The high pressure PEM water electrolyser with an advanced stabilizing MPL (IrO2 / Ta2O5 composition) remains the voltage within 0.02 V which shows excellent stability at high current density (1 A cm-2).