鈀基奈米觸媒應用於葡萄糖催化與燃料電池之應用

Abstract

本論文主要研究方向著重於開發新型鈀基奈米觸媒並應用於葡萄糖電催化。首先，使用具有不同表面性質之奈米碳材料作為鈀奈米粒子的載體，並進而找到適合使用在多元醇還原法之碳載體。隨後採用一鍋式和兩步驟式多元醇還原法在酸化多壁型奈米碳管上合成出鈀-鉍雙金屬觸媒及鈀-鎳殼核型觸媒。本研究所開發出之鈀基雙金屬觸媒能有效增加觸媒表面活性面積、提升葡萄糖催化能力和改善觸媒使用的長效穩定性。此鈀基觸媒研究結合廣泛的物理化學性質鑑定和電化學分析方法，並在一系列的研究中探討不同鈀基奈米觸媒的材料特性和其對葡萄糖的催化性能，最終期望找到具有較高催化活性的鈀基觸媒。本論文研究宗旨為開發具低成本、高效能和高穩定性之鈀基奈米觸媒陽極。 第一部分，在研究初期將鈀奈米粒子還原修飾在不同碳載體表面，並探討所合成之觸媒材料對葡萄糖電催化的性質。所選用的碳載體材料包含未修飾型多壁奈米碳管、酸化多壁奈米碳管、胺基化多壁奈米碳管、氫氧基化多壁奈米碳管、酸化石墨烯和碳黑。鈀奈米粒子藉由使用乙二醇作為還原劑的多元醇還原法將鈀前驅物還原至碳載體表面。並使用電化學分析法探討碳載體表面官能基對所合成之鈀奈米觸媒在鹼性溶液中對葡萄糖電催化的影響性。根據穿透式電子顯微鏡觀察可知鈀奈米粒子尺寸介於4.5 nm至7.4 nm。電化學循環伏安法分析結果顯示具表面修飾之碳載體可有效提升鈀奈米觸媒之催化活性。在此研究使用的碳載體中，酸化多壁奈米碳管相較於未修飾型多壁型奈米碳管可提升6.2倍的葡萄糖催化電流並降低100 mV的過電位值。此外，鈀修飾酸化多壁型奈米碳管亦具有最低的塔佛斜率92 mV和最佳的觸媒長效穩定性。由此研究結果可知，藉由乙二醇還原法可成功地將鈀奈米粒子還原至不同碳載體的表面，其中酸化多壁型奈米碳管是較佳的碳載體選擇。故在後續鈀基奈米雙金屬觸媒的研究中，採用酸化多壁型奈米碳管作為碳載體材料。 第二部分，採用一鍋式多元醇還原法製備鈀-鉍奈米粒子修飾酸化多壁奈米碳管。在X射線繞射圖譜顯示鉍金屬是以非晶型態存在於鈀-鉍雙金屬中，並由X射線光電子能譜結果可知鉍金屬是以三氧化二鉍之化學組態存在。在此鈀基雙金屬觸媒研究中，藉由循環式伏安法、塔佛分析法和計時安培分析法找到一較佳鈀-鉍含量比例之雙金屬觸媒。相較於單一鈀金屬修飾酸化多壁奈米碳管；鈀-鉍雙金屬觸媒最高可提升40 %之葡萄糖催化電流密度。由觸媒毒化速率研究中可知，鉍金屬的添加可有效改善中間產物的毒化程度。在200圈循環伏安法掃描結果，發現鈀-鉍(1:0.14)具較高的活性維持度97 %。此研究亦對不同葡萄糖濃度、氫氧化鈉濃度和操作溫度對催化電流的影響性作完整的探討。在此章節，鈀-鉍(1:0.14)修飾酸化奈米碳管在一最佳化條件下可獲得的最高葡萄糖催化電流密度為29.5 mA cm-2。 第三部分， 採用兩步驟還原法合成鈀-鎳修飾酸化多壁奈米碳管之核殼型奈米觸媒。此核殼型結構可有效提高鈀觸媒的使用率、增加觸媒表面的活性面積和觸媒的使用穩定性。此核殼結構藉由高解析電子穿透顯微鏡和掃描式電子穿透顯微鏡分析，可證實內核鎳奈米粒子包覆在外殼鈀金屬中。並由葡萄糖電催化分析結果顯示鈀-鎳核殼型觸媒可提升表面活性面積、催化電流密度和觸媒穩定性。其中，鈀-鎳(1:0.06)觸媒具有最高的表面活性面積(78.0 m2 g-1)和葡萄糖催化電流密度(21.2 mA cm-2)。並由觸媒毒化速率研究和200圈循環伏安法掃描可知，鈀觸媒可由鎳金屬添加後產生協同效應進而改善鈀基觸媒的穩定性。在此章節的研究結果顯示，最高葡萄糖電催化電流密度42.5 mA cm-2可在1.0 mol L-1 氫氧化鈉和0.5 mol L-1葡萄糖溶液於313 K環境中達到。 最後，將所合成之鈀基雙金屬奈米觸媒作為陽極電極，應用於質子交換膜式直接型葡萄糖燃料電池。鈀-鉍(1:0.14)雙金屬觸媒和鈀-鎳(1:0.06)核殼型觸媒之陽極搭配白金鈮網陰極，在自製鹼性葡萄糖燃料電池模組下進行探討。由線性掃描伏安法結果可知，在槽式鹼性直接型葡萄糖燃料電池架構下，使用鈀-鉍(1:0.14)雙金屬觸媒作為陽極可達到3.0 mW cm-2的輸出功率。

This main research direction of this dissertation is to develop novel palladium (Pd) based nanocatalysts for alkaline glucose electrooxidation. The different carbon materials for Pd nanoparticle decoration are investigated to obtain a proper catalyst support for Pd decoration. The one-pot poly method and two-stage polyol synthesis process were proposed to obtain palladium-bismuth (Pd-Bi) bimetallic catalysts and palladium-nickel core-shell catalysts (Pd-Ni), respectively. The prepared carboxylated multi-walled carbon nanotubes (cMWCNT) supported Pd-based bimetallic catalysts aim to improve the active surface, catalytic performance, and catalysis durability toward glucose oxidation reaction (GOR). The electrochemical and physicochemical properties are comprehensively characterized and discussed to find out the idea Pd-based catalyst for the further discussion. The objective of this study is to develop Pd-based anode catalysts for the application of direct glucose fuel cell with the advantages of a low-cost, a high efficiency, and an improved stability. At first, a series of investigation was discussed on different carbon materials supported Pd nanocatalysts. The carbon materials used in this study are pristine multi-walled carbon nanotubes (pMWCNT), carboxylated MWCNT (cMWCNT), amine-modified MWCNT (nMWCNT), hydroxyl-modified MWCNT (oMWCNT), XC72 carbon black (XC72), and carboxylated graphene (cGraphene). Nanosized Pd particles were decorated on these carbon supports by an alkaline one-pot polyol method via the reduction of palladium chloride hydrate (PdCl2). The electrochemical behaviors of alkaline GORs on the prepared Pd nanocatalysts were studied in this part to understand the influences of carbon functionality. Among the functionalized MWCNTs, cMWCNT shows a 6.2-fold higher GOR current density and a 100 mV lower over-potential compared to pMWCNT. In addition, cMWCNT supported Pd nanocatalyst has the lowest Tafel slope of 92 mV dec-1 and the highest stability of The 500 continuous GOR cycles in this study. The results indicate that cMWCNT will be a promising carbon support for the decoration of Pd nanoparticles by a polyol method. In this part, cMWCNT had been known as an idea catalyst support material for the further studies of Pd-based catalysts catalyzed GOR. In the second part, the palladium-bismuth decorated cMWCNT catalysts (Pd-Bi/C) were prepared via a one-pot polyol method. The XRD data shows that Bi elements existed in the Pd-Bi/C are amorphous phase as Bi oxides. It was found that Pd-Bi/C (1:0.14) can significantly enhance the electrocatalytic activity on GOR about 40% times higher than Pd decorated cMWCNT (Pd/C) and as well as has a 3.7-fold lower poisoning rate. Moreover, the in-use stability of Pd-Bi/C (1:0.14) is remarkably improved. The effects of the operating temperature and the concentration of glucose and NaOH electrolyte on Pd-Bi/C were further investigated in this study. In this part, the highest Pd-Bi/C catalyzed GOR current density of 29.5 mA cm-2 is attained in alkaline medium. In the third part, the cMWCNT supported palladium-nickel bimetallic catalysts (Pd-Ni/C) with a core-shell structure were employed to increase the utilization of Pd nanocatalysts. The PdshellNicore catalysts decorated cMWCNT were prepared by a facile two-stage polyol method. High resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscope (STEM) were used to identify the core-shell structure and analyze the elemental distribution of Pd-Ni nanoparticles. From the results of the electrocatalytic studies, the prepared PdshellNicore can obviously improve the GOR electrocatalytic activity and catalyst stability. The electrochemical results indicate that Pd-Ni/C (1:0.06) exhibits the highest electrochemical active surface area of 78.0 m2 g-1 which is 4.5 times higher than that of Pd/C and as well as has a 1.5-fold higher GOR current density of 21.2 mA cm-2. In this part, the highest Pd-Ni/C (1:0.06) catalyzed GOR current density of 42.5 mA cm-2 is attained in 0.5 mol L-1 glucose and 1.0 mol L-1 NaOH alkaline medium at 313 K. The prepared Pd-based catalysts (Pd-Bi/C and Pd-Ni/C) coated glassy electrodes were applied to be anodes in the home-made direct glucose fuel cells. A platinum-niobium (Pt-Nb) cathode electrode and a Nafion® 117 proton exchange membrane were employed in this fuel cell system. In this study, 0.5 M mol L-1 glucose and 1.0 mol L-1 NaOH was used as a fuel in anode electrolyte and oxygen is used as an oxidizing agent. The power out of the constructed batch type direct glucose fuel cells and membrane electrode assemblies (MEA) were investigated in this study. From the results of the low-scan rate linear sweep voltammetry (LSV) investigation, the maximum power output of 3.0 mW cm-2 was attained in the Pd-Bi/C anode based batch type DGFC in this study.