G95310003中英文摘要

摘要
在燃料電池中，改進陰極氧氣還原活性可有效的提升電池性能。

燃料電池中的陰極電極觸媒通常是使用碳黑作為載體，近年來許多學者紛紛製備中孔碳(ordered mesoporous carbon, OMC)做為觸媒載體來增進陰極之氧氣還原效能。

本文希望利用自製中孔碳具有的高表面積及良導電性特質，製備成Pt/OMC電極觸媒，使其Pt奈米粒子擁有高分散性，進而在陰極氧氣還原反應中改善其電化學觸媒活性。

利用一階段合成法，分別以TEOS(tetraethylorthosilicate)、蔗糖與磷酸作為矽源、碳源與活化試劑(擴孔劑)，改變H3PO4/TEOS莫耳比(P/Si)及碳化時間製備出具不同孔洞性質及石墨化程度之OMC作為電極觸媒載體。

接著，以醇類還原法製備不同負載量的Pt/OMC電極觸媒，進行氧氣還原反應之探討。

由BET分析表面積證實所製備出的OMC擁有高表面積的特性，其值皆介於614.98~1188.22 m2 g-1之間，改變H3PO4/TEOS莫耳比由0.4 ~ 0.55時，其主要孔洞從7.26 nm 增加至10.02 nm。

分析製備所得Pt/OMC觸媒中的表面積與孔洞體積，與中孔碳載體比較，發現其值大幅下降50 ~ 80 %與50 ~ 65 %，顯示Pt奈米粒子阻斷了中孔碳中的微孔與部份的中孔洞。

影響製備所得之Pt/OMC的電化學活性面積主要因素有Pt晶粒大小以及其分散性。

實驗結果顯示影響
Pt/OMC氧氣還原反應之質量活性(mass activity, MA)與比活性(specific activity, SA)為Pt晶粒尺寸與其晶面，其中Pt/OMC(P/Si = 0.5, Tc(碳化溫度) = 900 ℃)電極觸媒具有最大的質量活性及比活性，分別為15.54 A g-1及18.05 μA cm-2，其原因為擁有較低之Pt晶粒大小(3.175 nm)及較高之I Pt(111)/I Pt(200)比值(5.15)所致。

製備所得Pt/OMC電極觸媒，在氧氣還原之極化曲線法探討中，發現過電壓60 ~ 100 mV之間存在一明顯線性關係區，其塔弗斜率介於-58 ~ -71 mV dec-1之間，顯示各電極觸媒上的對氧氣還原機構應相同；其交換電流密度介於1.54 ~ 3.31×10-5 A cm-2之間。

在質量活性較佳之Pt/OMC(P/Si = 0.5, Tc = 900 ℃)電極觸媒上所得之塔弗斜率與交換電流密度分別為-59.12 mV dec-1與3.19×10-5 A cm-2。

關鍵字：燃料電池，陰極電極觸媒，中孔碳，氧氣還原反應
Abstract
The performance of the fuel cells can be improveed by promotion of the electroactivity of oxygen reduction reaction(ORR) on the cathode. The carbon black is generally used as the support of cathodic electrocatalysts for fuel cells. Recently, the preparation of ordered mesoporous carbon (OMC) used as the support of cathodic catalysts to promote the electroactivity of ORR has been received a great research interests. The electroactivity of ORR may be improved when the OMC is used as support to prepare Pt/OMC as the cathodic electrocatalyst due to the high surface area of OMC to increase in the Pt dispersibility on the support.
Using an one-step synthesis method with TEOS (tetraethyl orthosilicate), sucrose and phosphoric acid as the silicon and carbon sources, and the active agent (pore forming additive) to prepared OMC as the supports of electrocatalysts, the pore characteristics and the graphitic extend of OMC could be adjusted by controlling the molar ratio of phosphoric acid and TEOS (P/Si ratio) and the temperature of carbonation. Using the alcohol reduction method, the Pt/OMC electrocatalysts with various Pt loadings were prepared for ORR study.
The high surface area of home-made OMC were confirmed and analyzed by BET in the range of 614.98~1188.22 m3g-1. The main pore size of OMC increased from 7.26 to 10.02 nm by increasing the P/Si ratio from 0.4 to 0.55. Compared with the pristine OMC the surface area and pore volume of Pt/OMC electrocatalysts decreased in 50 ~ 80 % and 50 ~ 65 % were found due to the interdiction of the micro- and part of
meso-pores of the OMC support by the Pt nanoparticles. The electroactive area of Pt/OMC was mainly affected by the Pt particle size and dispersibility. The experimental results revealed that the mass activity (MA) and specific activity (SA) of ORR with Pt/OMC as electrocatalyst were affected by the Pt particle size and crystal structure. The maximum MA and SA of ORR on Pt/OMC (P/Si = 0.5, Tc (carbonation temperature) = 900 ℃) were experimentally found to be 15.54 A g-1及18.05 μA cm-2, respectively, due to the small Pt particle size (3.175 nm) and higher I Pt(111)/I Pt(200) ratio (5.15).
Using the home-made Pt/OMC as the cathodic electrocatalysts, the Tafel slopes and the exchange current densities were obtained by the I-E polarization method to be -58 ~ -71 mV dec-1 and 1.54~3.31× 10-5 A cm-2 in the overpotential of 60 ~ 100 mV. The almost of the same Tafel slopes for Pt/OMC prepared with the various conditions revealed that the similar ORR mechanisms were obtained for various Pt/OMC elctrocatalysts. The Tafel slope and the change current density of Pt/OMC (P/Si = 0.5, Tc = 900 ℃) with the maximum MA were experimentally found to be -59.12 mV dec-1 and 3.19 ×10-5 A cm-2, respectively.
Keywords：fuel cell, cathode electrocatalysts, ordered mesoporous carbon, oxygen reduction reaction。