Direct methanol fuel cell catalysts with controllable interface

DIRECT METHANOL FUEL CELL CATALYSTS WITH CONTROLLABLE INTERFACE LIU FANG NATIONAL UNIVERSITY OF SINGAPORE 2007 DIRECT METHANOL FUEL CELL CATALYSTS WITH CONTROLLABLE INTERFACE LIU FANG (M.Eng., Beijing Univ. of Aero. & Astro.) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENTAL OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 Acknowledgement ACKNOWLEDGEMENT I am sincerely grateful to every individual who has helped me in one way or the other in my Ph. D study at the National University of Singapore. My greatest gratitude would go to my thesis supervisor, Professor Jim Yang Lee, for his unrelenting positivism and guidance throughout the course of this research project. He has imparted in me the skill of creative problem solving, the scientific rigor in critiquing experimental results from a myriad of angles, objectivity and optimism that transform apparent problems into new discoveries and opportunities. These skill sets have enabled me to establish the overall direction of the research and to identify the niche areas for more in-depth investigations. I also thank him for his generous support in both research activities and other aspects of my life experience. At the same time, I would like to express my sincere thanks to all of my friends and colleagues in the laboratory, in particular Dr. Weijiang Zhou, Dr. Qingfeng Yan, Dr. Jun Yang, Mr. Jianhuang Zeng, Mr. Qinjia Cai, Ms. Haiqin Pei, and Mr. Qingbo Zhang. Without their encouragement and collaboration, this work could not have been completed. Mr. Phai Ann Chia, Mr. Zeliang Yuan, Mr. Boey Kok Hong, Ms. Samantha Hwee Koong Fam, Ms. Sylvia Foon Kiew Wan, Mr. Ng Kim Poi, and Ms. Chai Keng Lee, are the unsung heroes whose technical support is behind the success of every graduate student’s work. I am indebted to them for all the services rendered. i Acknowledgement Finally, I acknowledge the generosity of the National University of Singapore for providing the research scholarship throughout my entire Ph.D candidature. ii Table of contents TABLE OF CONTENTS ACKNOWLEDGEMENT…………………………………………………… . i TABLE OF CONTENTS……………………………………………………… iii SUMMARY…………………………………………………………………… vii ABBREVIATIONS…………………………………………………………… ix LIST OF TABLES…………………………………………………………… . xii LIST OF FIGURES……………………………………………………………. xiii CHAPTER INTRODUCTION 1.1 Background……………………………………………………………… 1.2 Objectives and Scope…………………………………………………… . CHAPTER LITERATURE REVIEW 2.1 Direct methanol fuel cells (DMFC) ……………………………………… 2.1.1 Mechanisms of methanol electrooxidation on pure platinum……… . 2.1.2 Mechanisms of methanol electrooxidation on Pt-oxophilic metal catalysts………………………………………………………………………… 10 2.1.2.1 Bifunctional catalysis……………………………………………. 10 2.1.2.2 Ligand effects……………………………………………………. 16 2.2 Common multi-component methanol oxidation electrocatalysts…………. 19 2.2.1 Alloy nanoparticles………………………………………………… . 19 2.2.1.1 PtRu nanoparticles………………………………………………. 19 2.2.1.2 PtNi nanoparticles……………………………………………… 25 iii Table of contents 2.2.1.3 PtRe nanoparticles……………………………………………… 26 2.2.1.4 PtOs nanoparticles……………………………………………… 27 2.2.2 Unalloyed nanoparticles……………………………………………… 28 2.2.3 Pt(hkl) electrodes modified with oxophilic-metals by spontaneous deposition and electrodeposition………………………………………………… 30 2.3 Multi-segment nanorods………………………………………………… 33 2.3.1 Potential change method…………………………………………… . 34 2.3.2 Electrolyte change method…………………………………………… 36 2.4 Macroporous films……………………………………………………… . 38 CHAPTER TEMPLATE PREPARATION OF MULTI-SEGMENT PtNi NANORODS 3.1 Introduction……………………………………………………………… 43 3.2 Experimental section……………………………………………………… 44 3.2.1 Materials……………………………………………………………… 44 3.2.2 Synthesis of nanorods……………………………………………… . 45 3.2.3 Electrochemical measurements………………………………………. 47 3.3 Results and discussion……………………………………………………. 47 3.3.1 FESEM, XRD, XPS characterizations of nanorods………………… 47 3.3.2 Cyclic voltammetric studies………………………………………… 53 3.4 Conclusion……………………………………………………………… . 58 CHAPTER TEMPLATE PREPARATION OF MULTI-SEGMENT PtRu NANORODS 4.1 Introduction……………………………………………………………… 60 iv Table of contents 4.2 Experimental section……………………………………………………… 61 4.2.1 Materials……………………………………………………………… 61 4.2.2 Synthesis of nanorods……………………………………………… . 62 4.2.3 Electrochemical measurements………………………………………. 64 4.3 Results and discussion……………………………………………………. 65 4.3.1 FESEM, XRD, XPS characterizations of nanorods…………………. 65 4.3.2 Electrochemical studies………………………………………………. 73 4.4 Conclusion……………………………………………………………… 77 CHAPTER TEMPLATE PREPARATION OF FIVE-SEGMENT Pt/Ru, Pt/Ni, AND Pt/RuNi NANORODS 5.1 Intrudution………………………………………………………………… 79 5.2 Experimental section……………………………………………………… 80 5.2.1 Synthesis of nanorods……………………………………………… . 80 5.2.2 Electrochemical measurement……………………………………… 82 5.3 Results and discussion……………………………………………………. 83 5.3.1 FESEM, XRD, XPS characterizations of nanorods………………… 83 5.3.2 Electrochemical studies……………………………………………… 93 5.4 Conclusion.……………………………………………………………… 101 CHAPTER TEMPLATE PREPARATION OF MULTI-SEGMENT PtRuNi NANORODS WITH DIFFERENT Ru AND Ni RATIOS 6.1 Introduction……………………………………………………………… 103 6.2 Experimental section……………………………………………………… 104 6.2.1 Materials……………………………………………………………… 104 v Table of contents 6.2.2 Synthesis of Nanorods……………………………………………… 105 6.2.3 Electrochemical measurements………………………………………. 107 6.3 Results and discussion……………………………………………………. 108 6.3.1 FESEM, XRD, XPS characterizations of nanorods………………… 108 6.3.2 Electrochemical studies……………………………………………… 117 6.4 Conclusion……………………………………………………………… 121 CHAPTER HIGH REGULARITY POROUS OXOPHILIC METAL FILMS ON Pt AS MODEL BIFUNCTIONAL CATALYSTS FOR METHANOL OXIDATION 7.1 Introduction……………………………………………………………… 123 7.2 Experimental section……………………………………………………… 124 7.2.1 Materials……………………………………………………………… 124 7.2.2 Fabrication of macroporous metal films on Pt quartz crystal substrate 125 7.2.3 Electrochemical measurements………………………………………. 127 7.3 Results and discussion……………………………………………………. 127 7.3.1 FESEM, XRD and XPS Characterizations………………………… . 127 7.3.2 Electrochemical Studies……………………………………………… 136 7.4 Conclusion……………………………………………………………… 142 CHAPTER CONCLUSIONS & RECOMMENDATIONS……………… . 144 vi Summary SUMMARY This thesis work focuses on developing model anode electrocatalysts for direct methanol fuel cell (DMFC) applications. Model catalysts in the form of multisegment nanorods and macroporous oxophilic metal films on Pt were fabricated and extensively characterized by field emission scanning electron microscopy, energy dispersive spectrometry, X-ray diffractometry, and X-ray photoelectron spectroscopy. The catalytic activities for room temperature electro-oxidation of methanol were measured in conventional three-electrode test cells using formulated mixtures of the model catalysts as the working electrodes. Multi-segment Pt-Ru (Pt-Ru, Pt-Ru-Pt, Pt-Ru-Pt-Ru, Pt-Ru-Pt-Ru-Pt, Pt-Ru-Pt-Ru-PtRu) and Pt-Ni (Ni-Pt, Ni-Pt-Ni, Ni-Pt-Ni-Pt, Ni-Pt-Ni-Pt-Ni) nanorods with controlled lengths of the individual metals were obtained by sequential electrodeposition of the metals into the pores of anodic aluminum oxide (AAO) membranes. The Pt-Ru nanorods were about 200 nm in diameter and 1.2 µm in length (with 900 nm of total Pt segment length); the Pt-Ni nanorods were about 170 nm in diameter and 1.6 µm in length (with 530 nm of total Pt segment length). These Pt-Ru and Pt-Ni nanorods were active catalysts for the room temperature electrooxidation of methanol under acidic conditions. Current-time curves and cyclic voltammograms showed a linear relationship between catalytic activity and the number of Pt-M (M = Ru and Ni) interfaces, thereby providing an unambiguous demonstration of the existence of bimetallic pair sites and bifunctional catalysis in the DMFC anode reaction. vii Summary Five segment Pt-Ru-Pt-Ru-Pt, Pt-Ni-Pt-Ni-Pt, and Pt-RuNi-Pt-RuNi-Pt (with three RuNi alloy compositions) nanorods with the same overall rod length and the same total Pt segment length were also produced by sequential electrodeposition of the metals into the AAO pores. Since these multi-segment nanorods were prepared with the same segment number, the same total Pt length, the same overall rod length and the same diameter, the observed difference in activities should mirror the intrinsic chemistry of the pair sites. From voltammetric and chronoamperometric measurements the Pt-RuNi pair sites with a Ru:Ni atomic ratio of 2.46:1 had the highest and most sustainable catalyst activity in methanol oxidation because of their effectiveness in water dissociation and the oxidative removal of COad intermediates. The Pt-RuNi pair sites with a Ru:Ni ratio of 7.52:1 were the next active, followed by the Pt-RuNi pair sites with Ru:Ni ratio of 1:2.49 and the Pt-Ru pair sites. The Pt-Ni pair sites were the least active among the five types of pair sites. Macroporous Ru, Os, Re, RuOs (co-deposited), and RuRe (co-deposited) films on Pt were obtained by the electrodeposition of metals into the interstices of annealed, closely-packed uniform spheres of polystyrene arranged on a Pt substrate. The number and the size of the pores on the Pt substrates were both controllable, and were kept constant throughout the experiments. From CO stripping voltammetry in dilute acids and chronoamperometry in acidified methanol solutions, the Pt-RuOs pair sites showed the best CO tolerance and the most sustainable catalyst activity in methanol oxidation, followed by Pt-RuRe pair sites, Pt-Ru pair sites, and Pt-Os pair sites in that order. The Pt-Re pair sites were unstable because of the selective etching of Re by the electrolyte. viii Chapter 4. The activities of the Pt-RuNi interface were further investigated using fivesegment Pt-RuNi-Pt-RuNi-Pt nanorods with the same geometrical attributes (diameter of 209 nm, overall rod length of 1.448 µm and total Pt segment length of 1.058 µm) differing only in their RuNi compositions. The codeposited RuNi adopted either the Ru hcp structure or the Ni fcc structure depending on the Ni content. XPS analysis showed that Pt(0) was the predominant surface oxidation state of Pt in the Pt segments, while Ru(0) and NiO predominated on the surface of the RuNi segments. Since the segmented nanorods were produced with the same number of bimetallic interfaces (five per nanorod), the observed difference in activities should mirror the intrinsic chemistry of the pair sites. Voltammetric and chronoamperometric measurements showed that Pt-RuNi pair sites with a substantial presence of Ni in the RuNi phase while maintaining an overall Ru hcp structure was beneficial. Excess Ni, on the other hand, which resulted in the formation of a solid solution of Ru in Ni in the RuNi phase, was counterproductive. 5. Macroporous Ru, Os, Re, RuOs, and RuRe films were deposited on Pt substrate covered with annealed closely packed polystyrene (PS) spheres. Electrodeposition of the oxophilic metal was therefore confined to the interstices in the PS packing. XRD indicated that Pt was fcc.; Ru and Os were hcp.; Re was H(ReO4)H2O in tetragonal close packed (tcp); and codeposited RuOs and RuRe adopted Ru’s hcp structure with some negative shifts in the Bragg angles, suggesting that they were solid solutions of Os and Re in Ru. XPS detected the following major oxidation states on the metal surface: Pt0 on Pt surface; Ru0 on Ru, RuOs and RuRe surface; Os0 on Os and RuOs surface; 147 Chapter ReVII on Re and RuRe surface. Both CO stripping voltammetry and chronoamperometrtic measurements indicated that the Pt-RuOs interface is most promising for sustained CO oxidation, followed by Pt-RuRe, Pt-Ru, and Pt-Os in that order. The trend could be rationalized in terms of the strength of OH adsorption on the oxophilic metals and the contributions of OHad to the overall methanol oxidation reaction. Most of the comparative measurements on catalytic activities leading to the above conclusions were carried out at steady state where the contributions from the Pt sites remote from the bimetallic interface were minimal because of strong deactivation of the Pt-only sites by tenaciously held adsorbed methanol residues. In principle the effect of the bimetallic interfaces can be accentuated if the model catalysts are prepared with a proliferation of the pair sites. For example, the ratio of pair sites to Ptonly sites in multi-segment nanorods can be approximated as follows: πDrod ×N D Pt Nπ D Pt ratio = = × πDrod × Lrod Lrod D Pt π( ) (8-1) Where Drod is the nanorod diameter; DPt is the Pt atomic diameter (0.278nm); N is the number interfaces of Pt-X per rod; Lrod is the totally Pt length per rod. The equation suggests that the ratio can be increased through an increase in the number of interfaces or the decrease in the total Pt segment length. However, experimentally it was difficult to control the length of each Pt segment to within 100 nm by the constant current density electrodeposition method, and it was very time-consuming to 148 Chapter produce a large number of segments per rod because of the extensive cleaning procedures required between each switching of the plating solutions. The situation appears to be improved in the case of macroporous films where the ratio of Pt-X interfaces to Pt-only sites is approximated by: πD pore ratio = π( DPt D pore =π ) D π ( Pt ) 2 DPt D pore (8-2) Where Dpore is the average diameter of oxophilic metal(s) pores. While the ratio may be improved by reducing the oxophilic metal pore size, the number of pair sites on a Pt QCM electrode is however limited and this can lead to unreliable activity measurements by the potentiostat/galvanostat. The shortcoming may be circumvented by using a larger Pt substrate to provide more pair sites per Pt disc with the same PS sphere size and the same pore size, but the experimental cost would be prohibitively high. The thesis explored two very effective methods for the direct comparison of the activities of different binary and ternary pair sites. Although more variants of pair sites need to be investigated in order to establish a systemic trend, some simple rules did surface from the experimental results in regard to the selection of the oxophilic metal for the Pt-X pair sites. All oxophilic metals (those with higher binary bond dissociation energies for gas phase X-O than that of Pt-O) can provide OH species at a lower overpontential than Pt. Among the oxophilic metals which are stable in the acidic electrolyte, those with lower X-O bond dissociation energies are a better choice 149 Chapter as Pt promoters. It also appears that a prudent pre-alloying of two oxophilic metals can deliver a synergistic effect in regard to the promoting capability of the individual metal. 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Liu, F.; Lee, J. Y.; Zhou, W. J., Template Preparation of Multisegment PtNi Nanorods as Methanol Electro-Oxidation Catalysts with Adjustable Bimetallic Pair Sites. Journal of Physical Chemistry B. 2004, 108, (46), 17959-17963. 2. Liu, F.; Lee, J. Y.; Zhou, W. J., Multisegment PtRu Nanorods: Electrocatalysts with Adjustable Bimetallic Pair Sites. Advanced Functional Materials 2005, 15, (9), 1459-1464. 3. Liu, F.; Lee, J. Y.; Zhou, W. J., Segmented Pt/Ru, Pt/Ni, and Pt/RuNi Nanorods as Model Bifunctional Catalysts for Methanol Oxidation. Small 2006, 2, (1), 121-128. 4. Liu, F.; Lee, J. Y.; Zhou, W. J., Multi-segment Pt-RuNi Nanorods for Methanol Electro-oxidation at Room Temperature. Journal of the Electrochemical Society 2006, 153, (11), A2133-A2138. 5. Liu, F.; Lee, J. Y.; Zhou, W. J., High Regularity Porous Oxophilic Metal Films on Pt as Model Bifunctional Catalysts for Methanol Oxidation. Chemistry of Materials 2006, 18, (18), 4328-4335. 161 Publications 6. Yan, Q. F., Liu, F.; Wang, L. K.; Lee, J. Y.; Zhao, X. S., Drilling Nanoholes in Colloidal Spheres by Selective Etching. Journal of Materials Chemistry, 2006, (22), 2132-2134. Conference 7. Liu, F.; Lee, J. Y.; Zhou, W. J., Template preparation of multi-segment Pt and Pt/Ru nanorods as DMFC anode electrocatalysts in acid electrolytes. 2005 Materials Research Society Spring Meeting 2005, San Francisco, USA. 162 [...]... combine with the oxidant to form the reaction products, primarily H2O and CO2 if the fuel is hydrocarbon based 6 Chapter 2 Fuel cells are classified by the types of electrolytes used into: Alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), proton-exchange membrane fuel cells (PEMFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) Among these five fuel cell types,... electrocatalysts in this thesis effort 2.1 Direct methanol fuel cells (DMFC) A fuel cell is an electrochemical device in which the chemical energy stored in a fuel is converted directly into electricity Specifically, a fuel cell consists of an anode to which a fuel is delivered, and a cathode to which an oxidant is supplied The two electrodes are separated by an ion-conducting electrolyte An input fuel, ... and Hogarth, 2002) They are also the first fuel cell type to be used in the Apollo Lunar Missions as on-board power sources (Bockris and Srinivasan, 1969) DMFC is a PEMFC variant in which methanol is used as a fuel directly (i.e without chemical reforming) A schematic for the operation of DMFC is shown in Figure 2.1 Methanol +Water Figure 2.1 Schematic of DMFC with a solid polymer electrolyte membrane... the direct use of a relatively safe fuel (liquid methanol as compared to gaseous H2), without the need for any fuel- processor sub7 Chapter 2 system that runs at elevated temperatures, several operational problems still remain: 1) the low electrocatalytic activity of the noble metals and their alloys, even at a high metal loading, for the electrooxidation of methanol, and 2) the cross-over of liquid methanol. .. recognizable and controllable interfaces between the oxophilic metal and Pt, but also offers a higher ratio of pair sites relative to the Pt sites than the multi-segment nanorods, which can then be used advantageously to emphasize the effects of the pair sites in methanol electrooxidation 1.2 Objectives and Scope This Ph D work is aimed at producing nanostructured DMFC model anode catalysts with controllable. .. relevant to this work These topics are discussed in four sections: section one introduces the technological background of direct methanol fuel cells (DMFC), and current progress on the mechanism of methanol oxidation Section two reviews the conventional multi-component electrocatalyts for methanol oxidation, focusing on the methods of preparation and their application performance Sections three and four... INTRODUCTION 1.1 Background There is strong growing interest in using direct methanol fuel cells (DMFC) as the power source for portable electronic products because of their high volumetric energy density, quick start-up, convenience of generating energy from a liquid fuel and low levels of environmental pollution (Hamnett, 1997; Antolini, 2003) While methanol can be easily activated on a platinum anode at room... ABBREVIATIONS AAO Anodic aluminum oxide Ag Silver AgCl Silver chloride aq aqueous phase AR Atomic ratio BE Binding energy CH3OH Methanol CO Carbon monoxide CO2 Carbon dioxide Cu Copper CuCl2 Copper chloride CuSO4·5H2O Copper sulfate pentahydrate DMFC Direct methanol fuel cells Dpore The average diameter of oxophilic metal(s) pores DPt Pt atomic diameter (0.278nm) Drod The nanorod diameter EDS Energy... and microemulsion (Zhang and Chan, 2003) methods, none of these techniques offers good control of the interface between Pt and the oxophilic metal Model catalysts with geometrically distinct and reproducible bimetallic interfaces can in principle be fabricated by “fusing” different metal nanorods with the same diameter end-to-end to form multi-segment nanorods This can be done most conveniently by... determining step for methanol oxidation is the oxidative removal of CO via reaction (2-6) (Christensen, et al., 1993; Gasteiger, et al., 1993) At higher potentials the interaction of water with the Pt surface increases (Iwasita and Xia, 1996) and competition of methanol with water for the adsorption sites become important (Iwasita et al., 1997) Hence at potentials above 0.7 V vs RHE, methanol adsorption . DIRECT METHANOL FUEL CELL CATALYSTS WITH CONTROLLABLE INTERFACE LIU FANG NATIONAL UNIVERSITY OF SINGAPORE 2007 DIRECT METHANOL FUEL CELL CATALYSTS WITH CONTROLLABLE. LITERATURE REVIEW 2.1 Direct methanol fuel cells (DMFC) ……………………………………… 6 2.1.1 Mechanisms of methanol electrooxidation on pure platinum……… 8 2.1.2 Mechanisms of methanol electrooxidation. This thesis work focuses on developing model anode electrocatalysts for direct methanol fuel cell (DMFC) applications. Model catalysts in the form of multi- segment nanorods and macroporous