| 摘要: | 本次研究利用次磷酸(NaH2PO2)作為磷來源以電鍍方式沉積鎳磷、鈷磷、鎳鈷磷化合物，並搭配循環伏安儀（cyclic voltammetry, CV）、掃描式電子穿隧顯微鏡（in-situ scanning tunneling microscopy, in-situ STM）及X光光電子能譜儀（X-ray photoelectron spectroscopy, XPS），探討電鍍的反應機制、鍍層的組成及其在鹼性溶液中（1 M KOH）氫氣析出反應（hydrogen evolution reaction, HER）的活性。
金屬鍍層中摻入磷形成合金可以提高載體的機械強度及抗腐蝕特性，同時今年的理論計算及實驗的研究證實Ni2P(001)、CoP及NiCoP(0001)等固體材料具有極高的電解水活性。本研究所製備之金屬磷化物，在pH 4的硫酸水溶液中，其氧化剝除電位最大向正移將近1.0 V，顯示其抗腐蝕的特性，而電沉積研究發現，次磷酸根可提升薄膜的沉積速率最高約8倍，且電鍍效率(或還原消耗電量用於金屬的沉積)也達到80 %以上。綜合所獲結果顯示鎳鈷磷薄膜的形成大部分是經過磷化氫(PH3)中間物來還原鎳鈷離子，最後同時沉積於金電極上。
鎳磷與鎳鈷磷化合物的HER活性和薄膜中磷的含量及金屬氧化態有關，隨著鍍層中的磷含量增加，其鎳的束縛能產生明顯的正移，表示其帶有較多的正電，加強它與水解反應產物氫氧根的作用，導致電極活性位置無法有效釋出，最終導致活性的降低；相反的，鈷磷化合物隨磷含量上升，其中鈷的組成接近金屬態，因此減低氫氧根之吸附力，有助於氫氧根的脫附，及活性位置的釋出，且此時帶負電的磷扮演著氫吸附的角色，有助於水分解的反應及提升HER的活性。
鎳鈷磷與鈷磷化合物展現電化學還原水的電流隨電壓增加的幅度(塔菲爾曲線之斜率)相近，暗示此電化學反應是以相同的機制進行，從STM的結果中觀察到少量不同於鎳鈷之六方最密堆積的扭曲結構，可能就是鎳鈷磷化合物與鈷磷相同的HER活性位置。;In this study, nickel phosphide, cobalt phosphide and nickel-cobalt phosphide compounds were electrodeposited onto an Au(111) electrode in pH4 electrolyte containing sodium hypophosphite, nickel sulfate, and cobalt sulfate. The mechanism of the electroplating, compositions of the plating, and the activity toward the hydrogen evolution reaction (HER) in alkaline solution were studied by cyclic voltammetry (CV), in-situ scanning electron tunneling microscopy (in-situ STM) and X-ray photoelectron spectroscopy (XPS).
In addition to their mechanical properties and corrosion resistance properties, some metal phosphides such as Ni2P (0001), CoP and NiCoP (0001) show great activity toward electrolyzing water, according to recent theoretical calculations and experimental results. Indeed, the oxidation or corrosion potential of these as-prepared films shifts positive by nearly 1.0 V. Hypophosphite also catalyzed the reduction rate of Ni and Co cations by 8 times, resulting in a more efficient use of electricity in film fabrication. From the obtained results, it is concluded that all films are formed by reaction with metallic cations with PH3, the product from the reduction reaction of hypophosphite.
The HER activity of nickel phosphide and nickel-cobalt phosphide is inversely proportional to the content of phosphorus in the deposit. According to the XPS results, the binding energy of 2p electron of nickel shifts positively, indicating a positively charged Ni sites. This would enhance the adsorption of OH-, a product from the H2O reduction reaction, which could impede the release of surface active sites to water molecules. By contrast, the cobalt in the CoP deposit is less positive, which exerts a weaker attraction to OH-, and a fast release of active sites. Therefore, the activity of HER increased with the rise of the proportion of phosphorus in the cobalt phosphide.
The Tafel slopes obtained with HER at the nickel-cobalt phosphide and cobalt-phosphide are similar (~90 mV/dec), which suggests that the twisted structure from the STM was likely to be the active site of nickel-cobalt phosphide and cobalt phosphide. |
