以銅離子電化學置換普魯士藍薄膜二價鐵之研究

Abstract

本研究建立以普魯士藍（Prussian Blue, PB）薄膜為初始層，經電化學方法使 Cu2+ 取代 PB 骨架中高自旋 Fe 位點，製備具亞鐵氰化銅（copper hexacyanoferrate, CuHCF）電化學特徵之 Cu@PB 薄膜電極。相較於傳統直接電沉積 CuHCF，此方法可先利用 PB 電沉積製程之成膜均勻性與載量可控性，再透過後續置換反應調整薄膜組成，藉以改善 CuHCF 薄膜在均勻性、膜厚控制與循環穩定性上的限制。PB 初始層以定電位電沉積法製備，並以沉積電量調控薄膜載量為 10 mC cm-2 ，兼具薄膜置換完整性與穩定性。Cu2+ 置換先以循環伏安法進行，較佳條件為 0.1 M CuCl2、0.1 M KCl、pH 2、掃描速率 50 mV s-1 並搭配 3 次預掃描；置換後循環伏安圖由 PB 特徵雙峰轉變為約 0.7 V（vs. Ag/AgCl）之 CuHCF 單峰，並經 EDX、SEM、 XPS、FTIR 與 UV–Vis 分析支持 Cu 導入 PB 骨架、Cu/(Fe+Cu) 原子比（EDX 半定量結果）可達 50% 以上。本研究亦使用定電位（−0.2 V、180 s）置換法，作為穩定性較佳之製備路徑。電化學分析顯示 Cu@PB 具良好離子傳輸與界面電荷轉移能力，CV峰電流與掃描速率0.6568次方成線性關係，電荷轉移阻抗由 PB 之 20.8 Ω 降至 Cu@PB 之約 9.6 Ω，且經 1000 圈循環後氧化還原電量不減反增、保留率 > 100%，具活化現象，但峰電位差略增，顯示長循環後仍有一定極化累積。電致色變方面，Cu@PB/ITO 單膜於 495 nm 處之初始光學對比度為 14%，著色與去色響應時間分別約為 4 s 與 5 s；經 1000 次著色／去色循環後，光學對比度仍維持原本的79%，顯示其具有良好的單膜電致色變循環穩定性。雖然其光學調變幅度低於 PB，但因其循環穩定性佳且操作電位較高，適合作為電致色變元件之輔助電極。

This study establishes a strategy in which copper ions (Cu2+) electrochemically substitute the high-spin Fe sites in the Prussian Blue (PB) framework, using PB thin films as precursor layers to prepare Cu@PB thin-film electrodes with copper hexacyanoferrate (CuHCF) electrochemical characteristics. Compared with conventional direct electrodeposition of CuHCF, this method first exploits the film uniformity and controllable loading of the PB electrodeposition process and then tunes the film composition through the subsequent substitution reaction, thereby mitigating the limitations of CuHCF thin films in uniformity, thickness control, and cycling stability. The PB precursor layer was prepared by potentiostatic electrodeposition, with the film loading controlled by the deposition charge at 10 mC cm-2, achieving both substitution completeness and stability. Cu2+ substitution was first carried out by cyclic voltammetry under the preferred conditions of 0.1 M CuCl2, 0.1 M KCl, pH 2, 50 mV s-1, and 3 pre-scan cycles; after substitution, the voltammogram changed from the characteristic PB double peaks to a single CuHCF peak at approximately 0.7 V (vs. Ag/AgCl), and EDX, SEM, XPS, FTIR, and UV–Vis analyses supported the incorporation of Cu into the PB framework, with a Cu/(Fe+Cu) atomic ratio (EDX semi-quantitative result) above 50%. A potentiostatic (−0.2 V, 180 s) substitution route was also employed as a preparation path with better stability. Electrochemical analyses showed that Cu@PB possesses favorable ion transport and interfacial charge transfer, with a linear relationship between the CV peak current and the scan rate raised to the power of 0.6568, and the charge-transfer resistance decreasing from 20.8 Ω for PB to approximately 9.6 Ω for Cu@PB; after 1000 cycles the redox charge increased rather than decayed (retention > 100%), exhibiting an activation behavior, although the peak separation increased slightly, indicating some polarization accumulation after long-term cycling. For electrochromic applications, the Cu@PB/ITO single film exhibited an initial optical contrast of 14 % at 495 nm, with coloring and bleaching response times of approximately 4 s and 5 s, respectively; after 1000 coloring/bleaching cycles the optical contrast retained approximately 79% of its original value, indicating good electrochromic cycling stability. Although its optical modulation is lower than that of PB, its good cycling stability, and higher operating potential make it suitable as an auxiliary electrode for electrochromic devices.