Ti-Cu、Ti-Co二元耦合薄膜觸媒在0.5 M H2SO4中對氧還原之催化反應研究

Cataytic Activities of Ti -Cu and Ti -Co Films for Oxygen Reduction Reaction in 0.5 M Sulfuric Acid Solutions

本研究由磁控濺鍍法製備不同比例之鈦-銅（Ti-Cu）與鈦-鈷（Ti-Co）金屬二元薄膜觸媒。在0.5 M硫酸溶液中，以慢速極化掃描（SSV）、循環伏安法（CV）與鐵弗極化作圖法（Tafel-Plot）等電化學技術，研究其在0.5M H2SO4對氧還原反應（ORR）之催化作用。此外以能量散佈光譜儀（EDS）來分析薄膜所含之元素，以X光光電子質譜儀（XSP）等來解析反應前後薄膜化學狀態之變化，綜合以上結果來探討不同薄膜對氧還原反應。由SSV結果可知Ti-Cu與Ti-Co兩種二元薄膜觸媒均有良好催化作用。由SSV掃描圖中之可獲得ORR的氧還原之起始電位（Eonset）及其還原電流。比較兩種二元系統之還原起始電位得知：Ti-Cu系統之Eonset（0.6 ~ 0.7 V）低於Ti-Co系統之Eonset（1 V）。在0 V與0.2 V電位下之ORR反應之還原電流會隨Cu、Co濃度增加而增大。在同合金濃度下Ti-Co系統之還原電流大於Ti-Cu系統。CV結果顯示在0.5 M H2SO4中Ti-Co系統比Ti-Cu系統穩定。由鐵弗斜率與氧交換電流密度可知，對氧的還原作用主要發生在Cu與Co上，Ti-Cu與Ti-Co系統提升觸媒活性的機制與Ti對氧還原作用無關。XPS分析可知Ti在反應過程中，較易發生氧化。Ti-Cu與Ti-Co系統對氧還原催化能力的提升機制，可能是因為Ti氧化物增加了吸附氧氣的能力並打斷其鍵結，因而提升Co與Cu還原能力。添加Cu或Co也會使得反應的電子轉移數增加，有利於氧還原的反應。

Catalytic activities of Ti-Cu and Ti-Co films for the oxygen reduction reaction (ORR) in a 0.5 M H2SO4 solution were investigated in this work. Ti-Cu and Ti-Co films of various compositions were prepared by magnetron sputtering. Electrochemical techniques such as slow scan voltammetry (SSV), Tafel polarization (Tafel), and cyclic voltammetry (CV) were employed to study the electrochemical characteristics of the films. Energy dispersive spectrometry (EDS) and x-ray photoelectron spectrometer (XPS) were applied to analyze the composition of the films before and after the electrochemical tests.
The results of SSV indicated that both the Ti-Cu and Ti-Co films were catalytically active for the ORR. The Ti-Co films were more active than the Ti-Cu ones. The catalytic activity increased with increasing concentration of Co (from 40 ~ 60 at%) in the Ti-Co films and of Cu (from 50 to 90 at%) in the Ti-Cu films. With a higher catalytic activity, the Ti-Co film revealed a higher onset potential (i.e., at 1.0 V) than Ti-Cu (i.e., 0.6 ~ 0.7 V). At a constant potential of 0.2 V, the film of Ti-Co indicated a greater reduction current density (272.14 μA/cm2) than Ti-Cu (i.e., 28.57 μA/cm2). According to the CV results, there was a strong broad oxidation peak at 0.5V and 0.3 V on the first scan cycle of the Ti-Cu films. The strong broad peak was attributed to the oxidation of Cu to Cu+ and Cu2+. The cyclic voltammogram depicted that the Ti-Co film resulted in only a symmetrical and smooth loop in the potential range from 0 to 1.2 V. No peaks were present on the loop. This phenomenon implied that Ti-Co was more stable than Ti-Cu. The instability of Ti-Cu film was due to the dissolution of Cu in the 0.5 M H2SO4 solution .The Tafel polarization plots showed that the ORR occurred predominantly on Cu and Co. According to XPS, the Ti-Cu and Ti-Co films were composed of titanium oxide, Cu and Co. The titanium oxide became TiO2 after the SSV, indicating that titanium was oxidized during the ORR. The activities of the Ti-Cu and Ti-Co films increased probably because the titanium oxide enhanced the adsorption of oxygen and accordingly broke the bonds, which in turn promoted the reduction activities of Cu and Co. Adding Cu and Co would increase the number of electrons being transferred, promoting the ORR at the same time.