銅鋁合金之去合金化製程及奈米多孔結構之電催化應用

Dealloying Process of Copper-Aluminum Alloy and Electrocatalytic Application of Nanoporous Structure

吳東樺(Tung-Hua Wu)

指導教授 ： 鄭憶中

國立臺灣大學/工學院/機械工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100594

摘要

近年來地球暖化日漸嚴重，地球的生態開始受到威脅，設法減少二氧化碳排量已成為受各界注意的環境議題，而目前透過電化學將二氧化碳轉化為可再利用的燃料已成為達成該目的最有前景的方法之一。
奈米多孔銅(Nanoporous Copper，在本研究中簡稱為NPC)有著密度低、高表面積、低成本等優點，又擁有良好的導電性、導熱性以及高催化活性，可應用於包括感測、焊接、超級電容、催化、二氧化碳還原等等領域[1-5]，是一項具有無限應用前景的新興材料。
本研究主要是在探討銅鋁合金在不同去合金化條件下會對於奈米多孔結構的生成會有何影響，並利用不同條件(電解液、溫度、外加電位)的改變去控制奈米多孔結構的支架大小；在試片經過去合金化後，將其作為電極，測試奈米多孔銅結構的電化學二氧化碳還原的效率以及分析還原後的產物，並分析不同多層結構及支架尺寸的NPC在電化學二氧化碳還原上的優劣勢。首先選擇三種組成比例的銅鋁前驅合金(precursor alloy)：Cu18Al82、Cu30Al70以及Cu33Al67並使用真空電弧熔煉法進行融煉。此三種合金均含有不同比例的富鋁相(α相)及介金屬化合物(θ相)，也因此造就了不同的晶相分佈。接著使用不同種類的電解液、不同的環境溫度以及施加不同的電位，對前驅合金進行自由腐蝕去合金化，利用元素間還原電位的不同將活性較高的鋁成分選擇性去除，留下活性較低的銅擴散堆積形成奈米孔洞結構 (NPC)；再由掃描式電子顯微鏡、能量散射光譜儀以及X射線繞射儀進行分析，確認去合金化處理後所生成的奈米多孔結構及其的成分。
本研究藉由不同的腐蝕條件將Cu-Al合金去合金化，成功合成出不同多層結構與支架尺寸的NPC。Cu-Al合金的去合金化時，具有共晶組織的Cu18Al82，其反應時間相較於其他的前驅合金短；在加溫腐蝕時，隨著腐蝕溫度上升而加速其反應速率，且支架尺寸也會隨著溫度上升而變粗，而此現象在HCl溶液中較為明顯；在定電位腐蝕時，去合金化所需的反應時間除了有明顯縮短，並能夠在去合金化後保留完整的多層次結構。除此之外，NPC的支架尺寸較自由腐蝕小，且支架尺寸可藉由改變施加的電位來控制。在二氧化碳還原反應的部分，將不同條件下所合成的NPC製作電極，並透過循環伏安法(cyclic voltammetry, CV)、高效液相色譜法(HP-LC)及氣相色譜法(GC)，分析NPC的二氧化碳還原效果及檢測還原出的產物。目前結果顯示，支架分佈在70nm~80nm之間的NPC所量測的電流密度及電化學反應面積較大；在產物分析方面，目前的結果顯示雖然具有多層結構的NPC，在高電位時具有較高產物的法拉第效率，但在較低電位時卻不是如此。因此推測施加電位的改變及NPC的種類，是影響到二氧化碳還原法拉地效率的關鍵因素。

關鍵字

奈米多孔結構 ； 銅 ； 去合金化 ； 電化學腐蝕 ； 介金屬化合物 ； 二氧化碳還原反應

並列摘要

In recent years, the effects of global warming have threatened the ecology of earth. Reducing carbon dioxide emission is necessary to solve the environmental issue. At present, electrochemical conversion of carbon dioxide into reusable fuels has become one method to achieve this goal.
Nanoporous copper has the advantages of having light weight, large specific surface area, good electrical, thermal, and electrochemical stability. Its applications in carbon dioxide reduction, seawater desalination, lithium battery and super capacitor are increasingly important [1-5]. In addition, this material is an emerging research topic with wide application prospects.
The purpose of this study was to investigate the influence of different dealloying conditions on the formation of nanoporous structures and its ligament size. After dealloying, the specimens were used as electrodes to test the electrochemical carbon dioxide reduction efficiency of nanoporous copper structure, and the reduced products were analyzed.
In this study, three kinds of composite Cu-Al precursor alloys, Cu18Al82, Cu30Al70 and Cu33Al67 were prepared by vacuum arc melting. All three alloys contain different crystalline phase distributions which includes aluminum-rich phase (α phase) and metal-meshed compound (θ phase).The precursor alloys were then dealloyed under different corrosion conditions (electrolyte, temperature, applied potential). Aluminum, the element with high electrochemical activity of the two elements in the precursor alloy, was selectively removed to form nanoporous copper structure (referred as NPC in this study).
The results of dealloying were analyzed by scanning electron microscope, energy scattering spectrometer and X-ray diffraction to confirm the nanoporous structure and composition generated after the dealloying treatment. After confirming the results of dealloying, the NPCs were made into electrodes, and the CO2 reduction effect was analyzed by cyclic voltammetry (CV), high performance liquid chromatography (HP-LC), and gas chromatography (GC)
The results showed that the etching time of Cu18Al82 with eutectic structure was shorter than other precursor alloys. The corrosion rate is accelerated with the increase of corrosion temperature, and the size of the support also gets thicker. This effect is more obvious in HCl solution. In addition, in terms of potentiostatic dealloying, the ligament size of NPC becomes thicker with the increase of applied potential, while the dealloying time was shortened.
For carbon dioxide reduction reaction, NPCs made under different conditions were tested through cyclic Voltammetry (CV), high performance liquid chromatography (HP-LC), and gas chromatography (GC). Current results show that NPC with ligament size of between 70nm and 80nm has the highest current density and ECSA values . The product analysis results show that the NPC with hierarchical structure has higher Faraday efficiency of products at high potential, however, that is not the case at low potential. Therefore, it is speculated that the potential and the relative density of NPC are the key factors affecting the efficiency of CO2 reduction.

並列關鍵字

nanoporous copper ； dealloying ； electrochemical corrosion ； mesometallic compounds ； carbon dioxide reduction reaction

參考文獻

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