INTERFACIAL EFFECTS BETWEEN THE STRUCTURED NANOFILLERS AND NAFION MATRICES ON THE PERFORMANCE OF H2-PEM FUEL CELL GUO BING NATIONAL UNIVERSITY OF SINGAPORE 2012 INTERFACIAL EFFECTS BETWEEN THE STRUCTURED NANOFILLERS AND NAFION MATRICES ON THE PERFORMANCE OF H2-PEM FUEL CELL GUO BING (M. ENG., National University of Singapore) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 ACKNOWLEDGEMENT First, I wish to express my deepest appreciation and thanks to my supervisors, associate Professor Hong Liang and Dr. Liu Zhaolin from IMRE, for their guidance and encouragement throughout my candidature as a Ph.D student at the National University of Singapore (NUS). Professor Hong’s comprehensive knowledge and incisive insight on polymer materials, uncompromising attitude toward research as well as the insistence on quality works have deeply influenced me and will benefit my future study. His invaluable advice, patience and painstaking revisions of my manuscripts and this thesis are indispensable to the timely completion of this thesis. I am also grateful to Dr. Liu Zhaolin for his immense background and experience in electrochemical knowledge which enabled me to work through many problems smoothly. I would also like to express my gratitude to my colleagues Mr Chen Xinwei, Chen Fuxiang, Liu Lei, Sun Ming, Zhou Yien, Ms Wang Haizhen, and Dr Tay Siok Wei of IMRE for all the handy helps, invaluable discussion and suggestions. I am grateful for the Research Scholarship from NUS that enables me to pursue my Ph.D. degree. I am also indebted to the Department of Chemical & Biomolecular Engineering of NUS for the research infrastructure support. Last but not least, this thesis is dedicated to my parents, my husband and my lovely daughter for their great understanding and steadily moral support throughout my Ph.D program. i TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS . ii SUMMARY .v ABBREVIATION viii LIST OF FIGURES . xiii LIST OF TABLES .xvii LIST OF SCHEMES xviii CHAPTER INTRODUCTION 1.1 General background 1.2 Objectives and scope of this thesis .4 1.3 Organization of This Thesis .7 CHAPTER LITERATURE REVIEW .10 2.1 Proton exchange membrane Fuel Cell (PEMFC) and current status 10 2.1.1 Basic physical and chemical properties of SPFP-Nafion 16 2.1.2 Proton transport mechanism .23 2.1.3 Contemporary tactics for enhancing the cell performance of Nafion membrane 27 2.2 Nafion-based nanocomposite membranes .30 2.2.1 Nanoparticles dispersed in Nafion membranes .31 2.2.2 Nanotubes dispersed in Nafion membranes 36 2.2.3 Mesoposous materials dispersed in Nafion membranes 43 2.2.4 Other materials dispersed in Nafion membranes 46 2.2.5 Process technology 59 CHAPTER DOPING NAFION  MATRIX BY P-ARAMID FLAKES FOR A PROTON TRANSPORT LESS RELIANT ON MOISTURE .62 3.1 Introduction .62 3.2 Experimental 66 3.2.1 Materials .66 3.2.2 Synthesis and characterizations of oligomeric poly(p-phenylene terephthalamide) 66 3.2.3 Preparation of the Nafion-P105 composite membranes 67 3.2.4 Electron microscopy and 19F-NMR spectroscopy characterizations. 68 3.2.5 Thermal Analysis 69 3.2.6 Measurement of the properties of the colloidal suspensions. 69 3.2.7 Determination of water uptake and contact angle .70 3.2.8 Evaluation of electrochemical properties .71 ii 3.3 Results and discussion 72 3.3.1 Colloidal evidences for the interaction between P105 and Nafion molecule .72 3.3.2 Properties of the composite membrane composed of Nafion-P105 clusters 83 3.3.3 Proton transport in the composite matrix .89 3.4 Conclusions 94 CHAPTER SUBSTITUTED POLY (P-PHENLENE) OLIGOMER AS A PHYSICAL CROSSLINKER IN NAFION  MEMBRANE .95 4.1 Introduction .95 4.2 Experimental 99 4.2.1 Materials .99 4.2.2 Preparation of monomer 1, 4-dibromo-2,5-diacetoxybenzene (DBOAcB) .99 4.2.3 Preparation of poly-p-phenylene-2, 5, diacetoxy (POAc) (scheme 1) .100 4.2.4 Preparation of Nafion-POAc composite membranes 100 4.2.5 Structural characterizations .101 4.2.6 Measurement of intrinsic viscosity 101 4.2.7 The morphologies of membranes .102 4.2.8 Thermal analysis of the cast membranes 103 4.2.9 Determination of water uptake and ionic exchange capacity (IEC). .103 4.2.10 Evaluation of electrochemical properties .104 4.3 Results and Discussion .104 4.3.1 Synthesis of POAc and examination of the interactions between POAc and Nafion in a dilute colloidal suspension .104 4.3.2 Characterizations of the Nafion-POAc composite membranes .109 4.3.3 Electrochemical evaluation of the Nafion-POAc membranes .114 4.4 Conclusions. .118 CHAPTER ASSIMILATION OF HIGHLY POROUS SULFONATED CARBON NANOSPHERES INTO NAFION MATRIX AS PROTON AND WATER RESERVOIRS .120 5.1 Introduction .120 5.2 Experimental 122 5.2.1 Preparation of sulfonated porous carbon nanospheres (sPCNs) 122 5.2.2 Preparation of the Nafion-Carbon composite membranes 123 5.2.3 Structure characterization 124 5.2.4 Thermal Analysis of the membranes 124 5.2.5 Determination of water uptake and Ionic Exchange Capacity (IEC) .125 5.2.6 Evaluation of electrochemical properties .125 5.3 Results and discussions 126 5.3.1 Synthesis of sulfonated porous carbon nanospheres (sPCNs) .126 iii 5.3.2 The structure characteristics of the Nafion-sPCN composite membranes .130 5.3.3 Examination of hydrophilic phase in the Nafion-sPCN composite membranes 137 5.3.4 Electrochemical evaluation of the Nafion-Carbon membranes .139 5.4 Conclusions 144 CHAPTER EMBEDDING OF HOLLOW POLYME MICROSPHERES WITH HYDROPHILIC SHELL IN NAFION MATRIX AS PROTON AND WATER MICRO-RESERVIOR .146 6.1 Introduction .146 6.2 Experimental Section .148 6.2.1 Materials .148 6.2.2 Synthesis of SiO2-MPS nanoparticles 148 6.2.3 Synthesis of SiO2/polymer core-shell nanoparticles .149 6.2.4 Synthesis of hollow polymer nanopheres (HPS) 150 6.2.5 Fabrication of the composite membranes .150 6.2.6 Characterization. .151 6.3 Results and Discussion .153 6.3.1 Characteristics of the HPSs .153 6.3.2 Broadening hydrophilic channel of Nafion by hydrophilic HPS .155 6.3.3 Effects of water micro-reservoir in the composite membranes .158 6.3.4 Influence of Moisture Level on Proton Transport in the Composite Membranes 163 6.4 Conclusions. .169 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 170 7.1 Conclusive remarks on my Ph.D work 170 7.2 Recommendations for future work .174 REFERENCES 177 LIST OF PUBLICATIONS .201 iv SUMMARY The development of the proton exchange membrane fuel cell (PEMFC) has been an intense research area of which the goal is clear: to ensure a long service life without compromising performance (power density) and stable energy output at elevated temperatures (70-120oC) so as to meet the demands of commercialization. Since the application of traditional PEM (Nafion) was constrained by the operation temperature (below 80oC) and relative humidity (RH) level (above 80%), the current focus of membrane research is the pursuit of high proton conductivity at elevated temperatures with less reliance on water. Four types of special nanofillers were developed in this thesis with the aim of enhancing proton transport of Nafion. The fillers are: oligomeric poly (p-phenylene terephthalamide) (PPTA) nanoflakes and poly (pphenylene-2, 5, diacetoxy) (POAc) nanorods, sulfonated highly porous carbon nanospheres (sPCNs) and hollow polymeric nanospheres (HPSs) bearing different functional groups. They were assimilated into the Nafion matrix by means of solution dispersion and casting. The elaboration of physicochemical mechanisms behind the electrochemical behaviours, thermal/mechanical properties in the composite matrix constitutes the major part of this thesis. The main accomplishments of this thesis are highlighted below. Oligomeric PPTA Nanoflakes (about 20nm) were designed first. A low dose of such nanoflakes in the Nafion matrix causes a reduction in glass transition temperature and an increase in storage modulus of membrane due to the adsorption of Nafion molecules to PPTA nanoflakes. The contacts between the –SO3H groups of Nafion v and PPTA nanoflakes constitute an alternative proton transfer channel that is less reliant on moisture levels. The 2% PPTA modified matrix sustains a power density of 450 mw/cm2 at 70oC in a dry gas operated single H2 PEM fuel cell (H2-PEMFC), much greater than what a pristine Nafion and Nafion-112 membrane would confer. The oligomeric POAc rigid rod was synthesized as the second type of filler. Both of the acetyl side groups and the π-system of POAc became acceptors of protons. Thereby, the side-chain -SO3H groups of Nafion molecules attached to POAc rods, creating an alternative proton transport channel. This association also led to a physical cross-linking network. It was supported by the variation of glass transition temperature of Nafion with the increase in POAc content, the UV-vis spectroscopic study of diluted colloidal system, the morphology of composite matrix as well as the fusion behaviour of matrix-bound water. The composite membrane with wt% POAc loading resulted in the highest proton conductivity and the superior power density (512 mw/cm2 at 70oC) over the pristine Nafion membrane in the single H2-PEMFC operated by dry H2. With respect to the third type of filler, highly porous sulfonated carbon nanospheres (sPCNs) were prepared from polypyrrole through pyrolysis, alkaline etching and sulfonation. The adsorption of Nafion molecules to the sPCNs generated a physical crosslinking network, which includes free Nafion molecules. As a result, a semiinterpenetrating network (sIPN) was accomplished. However, the sIPN was gradually replaced by a random assembly of Nafion-wrapped sPCN granules with raising the vi sPCN loading to 2wt%. The presence of free Nafion molecules in sIPN is critical to proton transfer. The porous scaffold of sPCN (1300 m2/g) is essential to promote water-capture and proton transport at elevated temperatures. The composite membrane with wt% sPCN loading could sustain a power-density of 571 mW/cm2 in a dry gas operated H2-PEMFC at 70oC, much greater than that of the pristine Nafion membrane. Finally, the hydrophilic hollow polymeric nanospheres (HPSs) carrying sulfonic acid groups or the carboxylic acid groups were synthesized using silica sub-microsphere as template. These HPSs are promising candidates because the hollow cavities act as micro water reservoir and the hydrophilic polymeric largely promotes proton hoping rate. With the exception of these two prominent effects, the adsorption of –SO3H groups of Nafion on HPSs also improved water preservation at elevated temperatures. The substantially low density of HPSs rendered HPSs a very high volume fraction. A loading of 0.2 wt% provided a surface area more than needed for accepting the sulfonic acid groups of Nafion. As a result, the composite matrix also contained HPSs free of adsorption, which contributed continuous proton transport channels. This chapter also scrutinized the freezable bound water and free water in the composite matrix by using DSC. The trend observed is coherent with ion-exchange capacity, proton-conductivity, water retention capability and single H2-PEMFC power density. 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Polymers 2012, 4(3), 1499-1516. 201 [...]... 106 Figure 4.3 Change of the UV-visible spectrum (300nm to 450nm) of a POAc-DMF solution (2mg/ml) on addition of various content of Nafion (based on the weight% of POAc) The insert shows the entire spectrum of POAc-1 %Nafion in DMF 107 Figure 4.4 FTIR spectra of POAc and the association of POAc-1 %Nafion colloidal dispersion 108 Figure 4.5 FETEM of Nafion (a) and Nafion/ 1P (b) membranes... conjugation between two adjacent benzene rings takes place through an amide bond The dispersion of a low dose of the PPTA nanoflakes in the Nafion matrix causes a reduction in glass transition temperature and storage modulus of membrane This matrix-softening phenomenon is attributed to the association of Nafion molecules to PPTA nanoflakes via the adsorption of the sulfonic acid groups of Nafion onto the hydrophilic... adsorption of Nafion molecules on P105 nanoflakes 79 Figure 3.9 19 F-NMR spectra of Nafion (A) and Nafion- 2%P105 (B) 80 Figure 3.10 Changes in the reduced viscosity (ηred ) of the colloidal suspensions consisting of Nafion, P105 (wt.% based on Nafion) and IPAH2O solvent (v/v=7/5, pH=3) with the increase in concentration of Nafion 82 Figure 3.11 Dynamic light scattering test that shows the. .. image of a P105 particle as synthesized 76 Figure 3.6 Zeta potential scanning with the variation of pH of two colloidal suspensions: P105 in H2O and Nafion in IAP/H2O (v/v=7/5) 77 Figure 3.7 Zeta potential scanning with the variation of pH of the colloidal suspensions containing Nafion (5.46mg/ml) and P105 (wt.% based on Nafion) 77 Figure 3.8 Schematic illustration of the Nafion – P105... Nafion and Nafion- 2%P105 membranes 87 Figure 3.15 FE-SEM images of the cryofractured cross-section of the Nafion membrane (a) and the Nafion- 2%P105 composite membrane (b) 88 Figure 3.16 Evaluation of temperature effect on proton conductivity of the two membranes: measured in water (RH 100%) (a), and in the saturated vapor of the saturated LiCl(aq) (b), which has a narrow range of RH (10-11%) over the. .. Figure 5 5 The evolution of the structure of the Nafion- Carbon composite membrane from sIPN to random structure 131 Figure 5 6 DSC (a) and DMA (b) of various membranes The temperatures marked in (a) represent the temperatures of the lowest concave points 133 Figure 5 7 The cross-sectional FESEM of Nafion (a), Nafion- 0.5%%sPCN (b), Nafion- 1%sPCN (c), Nafion- 1.5%sPCN (d) and 2%sPCN (e) composite... and a quiet operation Fuel cells have been used for stationary power generation as well as for mobile power generation to power cars, trucks, and buses Research and development on fuel cells have been ongoing ever since the first th fuel cell was demonstrated in the mid 19 century Proton exchange membrane fuel cell (PEMFC) is one of the most promising options for fuel cells due to the high power density,... Figure 3.17 Examination of proton conductivity of the two memberanes at 115oC under a controlled humidity environment (10% RH) 91 Figure 3.18 The polarization curves and power outputs of H2 fuel cell at 25 oC (a) and 70oC (b) 93 Figure 4.1 Cross-sectional FESEM of Nafion membrane 96 Figure 4.2 The composition dependence of the intrinsic viscosity of the Nafion- POAc diluted mixture... the primary proton transport channel A specific pattern of the variation of glass transition temperatures of Nafion with an increase in POAc content in the Nafion matrix supports the occurrence of physical crosslinking Different from the PPTA nanofillers which need an intense 5 dispersing into the Nafion system because of the difficulty to fully utilize the πsystem of PPTA due to strong hydrogen bonding... voltages and then a power density that ranges from 10 W to 1 MW in order for them to be used in various domains, including portable, stationary and transportation uses Fuel cell includes proton exchange membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC) and molten carbonate fuel cell (MCFC) Compared to other types of fuel cells, PEMFCs . INTERFACIAL EFFECTS BETWEEN THE STRUCTURED NANOFILLERS AND NAFION MATRICES ON THE PERFORMANCE OF H 2 -PEM FUEL CELL GUO BING NATIONAL UNIVERSITY OF SINGAPORE 2012 INTERFACIAL. INTERFACIAL EFFECTS BETWEEN THE STRUCTURED NANOFILLERS AND NAFION MATRICES ON THE PERFORMANCE OF H 2 -PEM FUEL CELL GUO BING (M. ENG., National University of Singapore) A THESIS SUBMITTED. solution (2mg/ml) on addition of various content of Nafion (based on the weight% of POAc). The insert shows the entire spectrum of POAc-1 %Nafion in DMF 107 Figure 4.4 FTIR spectra of POAc and the