|摘要: ||本研究為製備直接甲醇燃料電池 (Direct Methanol Fuel Cell, DMFC) 之鉑-三氧化鎢/碳 (Pt-WO3/C) 陽極催化劑與其性質之研究，目的為開發對甲醇氧化反應 (Methanol oxidation reaction, MOR) 效能有提升之電催化劑。|
一般 DMFC 使用以 Pt 為主成份之電催化劑進行 MOR，往往因為 CO 毒化現象的發生，造成 Pt 表面吸附一層不易移除之 CO 分子，使得 DMFC 的發電效能快速衰減。本研究嘗詴於傳統 Pt/C 電催化劑中加入三氧化鎢 (WO3) 奈米顆粒，以期能提高電催化劑抵抗 CO 毒化的能力，進而提升 MOR 效能。本研究以六氯化鎢 (tungsten hexachloride, WCl6) 為起始物，使用二甲基甲醯胺 (N,N-dimethyl-formamide, DMF) 為溶劑，以經過 NaOH 前處理或未經前處理之 Vulcan XC-72 為碳載體，先將 WO3 製備於碳載體上成為 WO3/C 奈米複合材料，然後利用多元醇還原法 (Polyol method)，以乙二醇同時做為還原劑與溶劑，將 Pt 奈米顆粒製備於 WO3/C 上，取得 Pt-WO3/C 電催化劑。本研究使用 X 光繞射分析儀 (X-ray diffractometer, XRD)、穿透式電子顯微鏡 (transmission electron microscope, TEM)、掃描式電子顯微鏡 (scanning electron microscope, SEM)、傅立葉轉換紅外線吸收光譜儀 (fourier transform infrared spectroscopy, FT-IR) 與元素分析 (energy dispersive X-ray spectrometer, EDS) 等儀器，測量製備所得電催化劑之表面形態與成份，並使用循環伏安法 (cyclic voltammetry,CV)，計時安培法 (chronoamperometry, CA) 進行 MOR 效能、電化學活性表面積 (electrochemical active surface area, EASA)、CO 脫附與反應穩定性等電化學特性分析。 使用不同濃度 WCl6/DMF 溶液為起始物，以未經前處理之碳載體製備 WO3/C，經 450℃ 燒結所得之 PtW(0.06 ~ 0.14)/C450 系列電催化劑，WO3 奈米顆粒形成約 50 ~ 150 nm 之聚集，且其 MOR 效能低於市售 E-TEK Pt/C 電催化劑。使用經 NaOH 前處理過之碳載體，於相同條件下製得之 PtW(0.06 ~ 0.14)/Cs450 系列電催化劑，雖然 WO3 奈米顆粒仍呈現聚集狀態，但其 MOR 效能可獲得明顯提升。使用經前處理之碳載體，並改變 WO3/C 之燒結溫度為 200℃ 製備所得之 PtW(0.06 ~ 0.14)/Cs200 系列電催化劑，可得非晶形狀態(amorphous) 之 WO3 奈米顆粒均勻分佈於碳載體上，且其 MOR 效能皆高於市售 E-TEK Pt/C 電催化劑。其中 PtW(0.08)/Cs200 電催化劑於本研究中展現最佳之 MOR 效能，於相同 CV 實驗條件下，其 MOR 峰電流值為 873 ± 96 A/g-Pt，約 2.2 倍高於市售 E-TEK Pt/C 電催化劑 (389 ± 56 A/g-Pt)。於計時安培法之測量下也顯示 PtW(0.08)/Cs200 電催化劑具有最佳抵抗 CO 毒化的能力，於 1000 秒後之穩定電流密度值 (28.0 A/g-Pt) 約 11 倍高於市售 E-TEK Pt/C 電催化劑。
The major goals of this research are the preparation and characterization of Pt-WO3/C electrocatalysts for the methanol oxidation reaction (MOR) at the anode of a direct methanol fuel cell (DMFC). Pt-based electrocatalysts were generally employed as the anode electrocatalysts for DMFCs. However, the surface of Pt could be easily occupied by intermediates generated during MOR (especially CO), which are difficult to be removed and leading to a rapid decrease in the DMFC performance. Such phenomenon is known as “CO-poisoning” phenomenon. In this study, WO3 nanoparticles were synthesized and added to conventional Pt/C to give Pt-WO3/C electrocatalysts in order to enhance the MOR efficiencies as well as the CO-tolerance abilities.
The Pt-WO3/C electrocatalysts were prepared by two-step procedure. N,N-Dimethyl-formamide (DMF) solution of tungsten hexachloride (WCl6) was used as the precursor, and the WO3/C nanocomposites were firstly synthesized. Then, Pt nanoparticles were decorated onto the WO3/C nanocomposites by polyol method using ethylene glycol (EG) as both the reducing agent and solvent to obtain the Pt-WO3/C electrocatalysts. X-ray diffractometer (XRD)、transmission electron microscope (TEM)、scanning electron microscope (SEM)、fourier transform infrared spectroscopy (FT-IR) and energy dispersive X-ray spectrometer (EDS) were used to investigate the surface morphologies and compositions of the electrocatalysts. The MOR efficiency, electrochemical active surface area (EASA), CO stripping and MOR stability of the electrocatalysts were evaluated by cyclic voltammetry (CV) and chronoamperometry (CA) experiments.
For the PtW(0.06 ~ 0.14)/C450 series electrocatalysts using pristine Vulcan XC-72 carbon black as the support and calcined at 450 oC, the WO3 nanoparticle aggregates with sizes of 50 ~ 200 nm were obtained, and their MOR efficiencies were all lower then the commercial E-TEK Pt/C electrocatalyst. The MOR efficiencies were sighificantly improved for the PtW(0.06 ~ 0.14)/Cs450 series electrocatalysts using NaOH-pretreated Vulcan XC-72 carbon black as the support, however the aggregates of WO3 nanoparticles were still observed. In order to avoid the aggregation of WO3 nanoparticles, the PtW(0.06 ~ 0.14)/Cs200 series electrocatalyst were synthesized using NaOH-pretreated Vulcan XC-72 carbon black and calcined at 200 oC. Evenly distributed amorphous WO3 nanoparticles on carbon support were obtained and confirmed by XRD and TEM investigates. The MOR efficiencies as well as the CO-tolerance abilities of these electrocatalysts were all higher than the commercial E-TEK Pt/C electrocatalyst. The PtW(0.08)/Cs200 electrocatalyst achieved the highest MOR efficiency of 873 ± 96 A/g-Pt among all the electrocatalysts prepared in this study, and which was about 2.2 times higher than that of the commercial E-TEK Pt/C electrocatalyst (389 ± 56 A/g-Pt).