| Abstract: | 直接甲醇燃料電池(Direct methanol fuel cell, DMFC)具有很好的發展潛力，因為它具有高能量轉換效率和低環境影響之優勢，但陽極電催化劑的活性不高，阻礙了 DMFC 的發展。一般使用 Pt當作甲醇氧化反應(Methanol oxidation reaction, MOR)的電催化劑。然而在 MOR 的過程中會產生 CO ，它會吸附在 Pt 表面且不易移除，使得 Pt無法繼續催化MOR，稱為 CO 毒化現象。本研究使用的電催化劑以碳黑為載體，將 Pt和TiO2的奈米複合材料負載在載體上(Pt-TiO2/C)，以利於提升 MOR 效能和抵抗 CO 毒化的能力。
Pt-TiO2/C 電催化劑使用兩步法合成。第一步，使用 NaOH 處理過後的碳黑當作載體，再以溶膠/凝膠法製作 TiO2 奈米顆粒，將其負載在載體上，可以得到 TiO2/C 奈米複合材料。第二步，將 Pt 奈米顆粒負載在 TiO2/C 的表面上，即可得到 Pt-TiO2/C 電催化劑。TiO2 奈米顆粒是經由四異丙烷氧化鈦(titanium tetra-isopropoxide, TTIP) 加入含有丙酮和碘的乙醇中，發生水解與縮合反應製備而得。Pt 奈米顆粒是使用多元醇還原法所合成的，利用乙二醇當作還原劑，在酸性或鹼性的環境下進行還原反應而得。將所得的 Pt-TiO2/C 電催化劑經由穿透式電子顯微鏡 (transmission electron microscope, TEM)、X 光能譜分析儀 (energy dispersive X-ray spectrometer, EDS)、傅立葉轉換紅外線吸收光譜儀(Fourier transform infrared spectroscopy , FT-IR) 和X光繞射分析儀 (X-ray diffractometer, XRD) 去分析其特性，並使用循環伏安法 (cyclic voltammetry, CV) 和計時安培法 (chronoamperometry, CA) 進行 MOR 效能、電化學活性表面積 (electrochemical active surface area, EASA)、CO 脫附與穩定性等電化學特性分析。
Pt-TiO2/CS(2)(80, 200, 450)A 和 Pt-TiO2/C(2)(80, 200, 450)A 兩系列電催化劑是在酸性環境下，利用 NaOH 處理過或未經處理的碳黑當作載體，並使用不同的燒結溫度所製作而成。Pt 奈米顆粒大小大約在 5~8 奈米且有聚集的現象。Pt-TiO2/CS(2)(200, 450, 600)B 和 Pt-TiO2/C(2)(200, 450, 600)B 兩系列的電催化劑是在鹼性環境下所製作的。Pt 奈米顆粒大小大約在 2~4 奈米且均勻分散在載體上，而且他們的 MOR 效能有明顯的提升。所以 Pt-TiO2/CS(1-5)450B 系列的電催化劑選擇在鹼性環境下製作，並改變 TiO2 和 C 的組成，找出最好的 MOR 效能。結果顯示 Pt-TiO2/CS(1-5)450B 系列電催化劑的 MOR 效能均高於市售 E-TEK Pt/C。Pt-TiO2/CS(4)450B 可以得到最好的 MOR 效能 799 ± 62 A/g-Pt，大約為市售 E-TEK Pt/C 414 ± 59 A/g-Pt 的 1.93 倍。
Direct methanol fuel cell (DMFC) has been considered as a new potential energy source in the near future owning to its higher energy conversion efficiency and smaller environmental impact compared to conventional fossil-based energy generation systems. However, the low activity of the anode electrocatalysts is one of the major problems for the development of DMFC. Pt is widely used as the electrocatalyst for methanol oxidation reaction (MOR). However, Pt can be easily poisoned by the strongly adsorbed intermediates generated during MOR, such as CO, and lose its electrocatalytic activity rapidly. In this study, Pt-TiO2 nanocomposites supported on carbon black was used as the electrocatalyst, in order to enhance the MOR efficiency and CO-tolerance ability.
The Pt-TiO2/C electrocatalysts were prepared by two-step procedure. First step, XC-72 was pretreated with NaOH aqueous solution and decorated with solgel-derived TiO2 nanoparticles to get TiO2/C as the substrate. Second step, Pt nanoparticles were prepared onto the substrate surface to obtain Pt-TiO2/C electrocatalysts . The TiO2 nanoparticles were synthesized by catalyzed hydrolysis and inhibited condensation reaction of titanium tetra-isopropoxide in an ethanol solution containing acetone and Iodine. A polyol process using ethylene glycol as the reducing agent was employed at acidic or basic condition for the synthesis of the Pt nanoparticles. The resultant Pt-TiO2/C electrocatalysts have been characterized by means of transmission electron microscope (TEM), energy dispersive X-ray spectrometer (EDS), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffractometer (XRD) analysis. 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 acidic polyol condition, the Pt-TiO2/CS(2)(80, 200, 450)A and Pt-TiO2/C(2)(80, 200, 450)A series electrocatalysts used NaOH-pretreated and pristine Vulcan XC-72 carbon black as the support and annealed at different temperature, the Pt nanoparticle aggregates with sizes of 5~8 nm were obtained. For the basic polyol condition (Pt-TiO2/CS(2)(200, 450, 600)B and Pt-TiO2/C(2)(200, 450, 600)B series), the Pt nanoparticle uniformly dispersed with sizes of 2~4 nm were obtained, and their MOR efficiency were sighificantly improve. In order to optimize the composition of TiO2 and Vulcan XC-72 carbon, the Pt-TiO2/CS(1-5)450B series electrocatalyst were synthesized using NaOH-pretreated Vulcan XC-72 carbon black and calcined at 450oC. The MOR efficiency of these electrocatalysts were all higher than the commercial E-TEK Pt/C electrocatalyst. The Pt-TiO2/CS(4)450B electrocatalyst achieved the highest MOR efficiency of 799 ± 62 A/g-Pt among all the electrocatalysts prepared in this study, and which was about 1.93 times higher than that of the commercial E-TEK Pt/C electrocatalyst (414 ± 59 A/g-Pt). |
