在以Nafion與碳支撐鉑製備的電極上氫氣氧化與氧氣還原動力學之研究

Abstract

本研究以旋轉圓盤電極及循環伏安法探討Nafion與Pt/C混合製備的電極上的氫氣氧化與氧氣還原的電催化反應。結果顯示，隨著Nafion/Pt比例的增加，電化學活性表面積會下降。而白金負載量與Nafion/Pt比例的改變則對氫氣氧化的電流密度幾乎沒有影響。藉由伏安圖譜，應用修正型的Koutecky-Levich方程式，可求出反應動力學參數。本研究顯示，氫氣在電極上氧化反應的化學反應皆非常快速 (reversible)，反應為電解質溶液中氫氣的質傳控制，所測得之電流密度皆為質傳速率。 由Tafel圖 (未對外界質傳效應作修正) 可知氧氣還原反應的速率會隨著白金負載量增加而增加；然而由修正的Tafel圖則可知氧氣還原反應機制在低過電位和高過電位分別受到化學反應控制及反應與擴散之混合控制。應用Koutecky-Levich方程式，可求出反應動力學參數。根據真實的白金面積，所求得的交換電流密度為6.07×10-3 mAcm-2。 本研究之結果有助於使用Pt/C觸媒的燃料電池的電極之設計。

The hydrogen oxidation reaction and oxygen reduction reaction on electrodes prepared with Nafion and Pt/C were investigated by the rotating disk electrode (RDE) and cyclic voltammetry (CV) techniques. From this work, electrochemical active surface decreased as the Nafion/Pt content increased. The effects of Pt-loading and Nafion/Pt ratio are slight for the current density of hydrogen oxidation reaction. The hydrodynamic voltammograms were analyzed to determine the kinetic parameters by using the modified Koutecky-Levich equation. This work showed that the electrocatalytic reaction of hydrogen on electrodes was very fast (reversible), and the reactions were controlled by the diffusion of hydrogen in electrolyte solution. The measured current densities here were all the mass-transfer rates. From the Tafel plots (uncorrected for diffusion), the reaction rate of oxygen reduction reaction increased as Pt-loading increased. However, from the corrected Tafel plots, we could understand the mechanisms of oxygen reduction reaction were chemical reaction control and mixed kinetic-diffusion control at low and high overpotential, respectively. The hydrodynamic voltammograms were analyzed to determine the kinetic parameters by using the Koutecky-Levich equation. The exchange current density for Pt/C was determined to be 6.07×10-3 mAcm-2 based on the real Pt surface area. The results of this study are useful for designing the electrodes for fuel cells using Pt/C electrocatalyst.