| 摘要: | 直接甲醇燃料電池因為具有工作溫度低、進料容易和高能量密度(6.09 kWh kg-1)等優點而引起了各界的關注，但是其亦有諸如較低的功率密度、白金觸媒易遭一氧化碳毒化(CO tolerance)、甲醇滲透(crossover)與觸媒價格相對高等缺點。本研究以改善直接甲醇燃料電池陽極觸媒之電化學效能、CO耐受性和使用壽命為主要目的。
由文獻得知傳統碳材載體具有易被腐蝕且與金屬之間作用力弱，進而造成觸媒效能和壽命下降的缺陷，因此本實驗使用具有較高電化學穩定性、熱穩定性和耐腐蝕性的金屬陶瓷材料，如二氧化鈦(TiO2)、碳化鈦(TiC)與氮化鈦(TiN)取代部份碳材。我們利用鈦陶瓷載體與碳黑的混合物作為直接甲醇燃料電池的觸媒載體，分別製備出PtRu/TiO2-C(1:1)、PtRu/TiC-C(1:1)和PtRu/TiN-C(1:1)觸媒。
實驗結果顯示PtRu/TiO2-C(1:1)、PtRu/TiC-C(1:1)和PtRu/TiN-C(1:1)觸媒與單純碳黑載體的觸媒(平均顆粒大於4.0 nm)相比，具有較高的PtRu金屬粒子分散性和較小的金屬顆粒(平均顆粒在3.5到4.0 nm之間)。而PtRu/TiO2-C(1:1) (1278.0 A/g Pt)、PtRu/TiC-C(1:1) (1598.5 A/g Pt)和PtRu/TiN-C(1:1) (1709.2 A/g Pt)觸媒在甲醇氧化的催化活性方面亦較商用的PtRu/E-TEK (667.3 A/g Pt)和單純碳黑載體之PtRu/CB (1128.1 A/g Pt)觸媒高出許多。其中催化活性最高的PtRu/TiN-C(1:1)觸媒其活性高出商用的PtRu/E-TEK大約2倍之多。
我們經由XPS測試得知三種鈦陶瓷載體具有較高的束縛能位移證實其和金屬之間具有較強的作用力，亦說明了為何能有較小的金屬顆粒與經過長時間的計時安培法下尚能保有較高穩定度的原因，同時此電子效應亦與甲醇氧化的催化活性有關。另一方面，TiO2表面的氫氧官能基(Ti-OH)加強了釕的雙功能效應，能將吸附在觸媒金屬上的一氧化碳去除，較表面沒有任何官能基的TiC和TiN更能提昇觸媒抗CO毒化的效能。我們的實驗結果證實TiO2、TiC和TiN作為觸媒載體確實具有傑出的效果，不但能有效防止觸媒金屬的聚集，亦能增加甲醇氧化的催化活性和穩定性。其中催化活性最好的是PtRu/TiN-C(1:1)觸媒。
在80°C的DMFC測試中，PtRu/TiO2-C(1:1)、PtRu/TiC-C(1:1)和PtRu/TiN-C(1:1)觸媒顯示出優秀的效能，功率密度可以達到98.1 mW/cm2、73.5 mW/cm2和80.0 mW/cm2，相較於PtRu /E-TEK的58.9 mW/cm2高出許多。其中PtRu/TiO2-C(1:1)因為擁有良好的甲醇親和力而具有較佳的CO耐受性，而使PtRu/TiO2-C(1:1)在MEA的測試中具有比PtRu/TiN-C(1:1)好的輸出功率。;Direct methanol fuel cells (DMFCs) have attracted considerable interest because of a variety of merits such as low operating temperatures, easy fuel-feeding, and the high energy density of methanol (6.09 kWh kg-1). However, it also has some disadvantages, for example, low power density, Pt poison by CO, methanol crossover, and high cost for catalysts. In this study, we focuse on the improvement in electrochemical performance, CO tolerance, and life time of catalysts.
Due to the research by the literature, the traditional carbon support is susceptible to corrosion under the harsh conditions, and has weakly interaction with catalytic metal, which will result in the degradation of catalyst performance and life time. So we try to use metal ceramic materials like titanium dioxide (TiO2), titanium carbide (TiC), and titanium nitride (TiN), which are plausible support materials due to their electrochemical and thermal oxidation stability and corrosion resistive nature under electrochemical oxidation condition. In this study, we present the studies using titania and carbon hybrids as supports to prepare PtRu/TiO2-C(1:1), PtRu/TiC-C(1:1), and PtRu/TiN-C(1:1) catalysts system.
PtRu catalysts prepared using PtRu/TiO2-C(1:1), PtRu/TiC-C(1:1), and PtRu/TiN-C(1:1) supports shows the nanoparticles are highly dispersive and exhibit smaller particle sizes (3.4 to 4.0 nm) on the TiO2, TiC, and TiN supports compared to that on Vulcan XC-72 carbon support (>4.0 nm). The Methanol oxidation reaction shows increasing activity from PtRu/E-TEK (667.3 A/g Pt), PtRu/CB (1128.1 A/g Pt), PtRu/TiO2-C(1:1) (1278.0 A/g Pt), PtRu/TiC-C(1:1) (1598.5 A/g Pt), and PtRu/TiN-C(1:1) (1709.2 A/g Pt). The maximum MOR activity in PtRu/TiN-C(1:1) is found to be nearly twice that of conventional PtRu/E-TEK catalysts.
Strong metal interaction with Titanium is identified by XPS studies which yielding higher binding energy shift of the three Titania supports, explains the production of smaller particle size and the stability of the nano-participles after I-T curve these systems. This electronic effect also partly accounts for the improved MOR activity. On the effects of CO tolerance, the OH groups on TiO2 surface combines with Ru to remove the CO intermediate, and shows improved CO tolerance over that of TiC and TiN supports where the surface is free of any functional groups. This study demonstrated TiO2, TiC, and TiN are outstanding supports which inhibits the aggregation of PtRu, results in significantly increases MOR catalytic activity and stability. The best MOR activity observed at the PtRu/TiN-C(1:1) catalyst.
The DMFC comprising the PtRu/TiO2-C(1:1), PtRu/TiC-C(1:1), and PtRu/TiN-C(1:1) anode shows an impressive power density of 98.1 mW/cm2, 73.5 mW/cm2, and 80.0 mW/cm2 compared to a peak power density of 58.9 mW/cm2 found in the MEA (Membrane electrode assembly) with a PtRu /E-TEK anode at 80°C. The better CO-tolerance in the anode catalyst of PtRu/TiO2-C(1:1) originated from its better methanol affinity, for example is likely to be responsible for the better fuel cell out-put compared to the anode made from PtRu/TiN-C(1:1). |
