鈷-鐵-磷合金奈米線陣列結構之製備與電催化特性研究

Fabrication and Electrocatalytic Characterization of Co-Fe-P Nanowire Array Structures

劉希聖(Hsi-Sheng Liu)

指導教授 ： 廖建能

國立清華大學/工學院/材料科學工程學系/碩士(2018年)

摘要

電解水製氫因為具備永續、高效率以及低汙染的特性被視為替代能源中最具發展潛力的能源之一。電解水的反應透過電壓來驅動陽極的析氧反應(OER)與陰極的析氫反應(HER)，為了降低水分解的工作電壓以及簡化系統，發展具有析氫與析氧的雙功能電極材料是重要的關鍵。一般來說，在析氫反應最有效的觸媒為鉑系金屬，而析氧反應則為銥/釕基的化合物，然而這些金屬的稀少性使他們的價格居高不下，進而限制了實用性，因此人們致力於發展非貴金屬且具備高效分解水的電觸媒材料。其中過渡金屬(鐵鈷鎳)的地球存量高且具備一定的催化能力，且透過混摻不同的元素能進一步提高電解水活性而被廣泛研究。此外，調控電觸媒材料的微結構來增加表面積，讓更多的活性位點進行反應也成為重要研究方向。
本研究利用簡易且低成本的電沉積法，首先在陽極氧化鋁模板中電鍍鈷鐵二元合金奈米線陣列，並改變溶液中的亞鐵離子濃度來調控奈米線中的鐵含量，然後在1 M KOH溶液中以循環伏安法穩定電極表面後，進行線性掃描伏安法(LSV)來評估鈷鐵合金奈米陣列達到10 mA/cm2所需過電位，結果顯示Fe0.22Co0.78之奈米線具備最佳雙功能特性，HER與OER過電位分別為177 mV與306 mV。接著進一步以共鍍方法得到鈷鐵磷合金奈米線，磷的添加能降低兩電極的過電位且不改變鐵含量對過電位影響的趨勢，在相近的鐵分率下，(Fe0.26Co0.74)0.83P0.17在HER與OER的過電位分別為147 mV與284 mV。透過SEM的觀察發現鈷鐵磷合金奈米線在OER的CV掃描下產生了大量片狀氧化物，此片狀氧化物經由TEM、XPS鑑定為氧化鐵與氧化鈷所構成，而片狀的結構能提供更多的反應活性位點應是造成OER過電位下降的主因。最後將此鈷鐵磷合金奈米陣列進行長效性雙功能測試，結果顯示僅需1.66 V即能達到10 mA/cm2，且在12個小時內維持優良的穩定性。

關鍵字

鹼性水電解 ； 陽極氧化鋁 ； 合金奈米陣列 ； 過渡金屬磷化物 ； 電沉積法

並列摘要

Electrocatalytic hydrogen production is one of the most promising alternative energy sources because of its sustainability, high efficiency and low pollution. The reaction of electrolysis drives the oxygen evolution reaction (OER) at anode and the hydrogen evolution reaction (HER) at cathode by applying a specific overpotential. In order to minimize the overpotential of water decomposition and simplify the system, it is important to develop a bifunctional electrode material that is active for catalyzing both the HER and OER reactions. In tradition, the most effective catalyst for hydrogen evolution is Pt-based metals, and the oxygen evolution reaction are Ir/Ru alloys. However, the scarcity raises the cost and limits the wide employment of these metals accordingly. Therefore, people are committed to the development of precious-metal-free and efficient electrical catalytic materials. Among them, the earth-abundant transition metals (iron/cobalt/nickel) have decent catalytic ability, which have been extensively studied by mixing different elements to further improve the electrocatalytic capability for water splitting. In addition, engineering the microstructure of the electrocatalyst for large surface area and abundant active reaction sites has also become an important research field.
In this study, a low-cost electrodeposition method has been used to electrodeposit cobalt-iron nanowires in an anodized aluminum template (AAO). The iron content in the Co-Fe nanowires is modulated by adjusting the ferrous ion concentration in the electrolyte. After stabilizing electrode in 1 M KOH solution by cyclic voltammetry (CV), a linear scanning voltammetry method (LSV) was performed to evaluate the overpotential required for the iron-cobalt nanowire array at a current density of 10 mA/cm2. The results show that Fe0.22Co0.78 has the best bifunctional performance, with HER and OER overpotentials of 177 mV and 306 mV, respectively. Then, the iron-cobalt-phosphate nanowire is further obtained by the co-plating method. The addition of phosphorus can reduce the overpotentials of both anode and cathode with the optimized iron content in the alloy nanowires. Therefore, under the similar iron fraction, the overpotential of (Fe0.26Co0. 74)0.83P0.17 for HER and OER becomes 147 mV and 284 mV, respectively. It is also found that the iron-cobalt-phosphorus nanowires exhibit a large amount of flaky oxide after the CV potential sweeping during OER reaction according to the SEM images. The flaky oxides were identified by TEM and XPS as iron oxide and cobalt oxide. These oxide sheets can provide more active reaction sites, which is responsible for the decrease of OER overpotential. Finally, the complete cell made of the same electrode material only requires a low voltage of 1.66 V for water splitting at a current density of 10 mA/cm2, and maintains excellent stability within 12 hours.

並列關鍵字

Alkaline water electrolysis ； Anodic aluminum oxide ； Alloy nanowire array ； TMP ； Electrodeposition

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