POROUS SILICA MATERIALS-POLYMER COMPOSITE AS PROTON CONDUCTING MEMBRANE FOR FUEL CELL SYED MUHAMMAD AL-AMSYAR UNIVERSITI TEKNOLOGI MALAYSIA

Transcription

1 PRUS SILICA MATERIALS-PLYMER CMPSITE AS PRTN CNDUCTING MEMBRANE FR FUEL CELL SYED MUHAMMAD AL-AMSYAR UNIVERSITI TEKNLGI MALAYSIA

2 UNIVERSITI TEKNLGI MALAYSIA PSZ 19:16 (Pind. 1/97) BRANG PENGESAHAN STATUS TESIS JUDUL: PRUS SILICA MATERIALS-PLYMER CMPSITE AS PRTN PRTN CNDUCTING MEMBRANE FR FUEL CELL SESI PENGAJIAN: 2006/ 2007 Saya : SYED MUHAMMAD AL-AMSYAR (HURUF BESAR) mengaku membenarkan tesis ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut : 1. Hakmilik tesis adalah di bawah nama penulis melainkan penulisan sebagai projek bersama dan dibiayai oleh UTM, hakmiliknya adalah kepunyaan UTM. 2. Naskah salinan dalam bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran bertulis daripada penulis. 3. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian mereka. 4. Tesis hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar yang dipersetujui kelak. 5.*Saya membenarkan/tidak membenarkan Perpustakaan membuat salinan tesis ini sebagai bahan pertukaran di antara institusi pengajian tinggi. 6. **Sila tandakan ( ) SULIT (Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia seperti yang termaktub dalam AKTA RAHSIA RASMI 1972) TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi/ badan di mana penyelidikan dijalankan) TIDAK TERHAD Disahkan oleh (TANDATANGAN PENULIS) Alamat Tetap : 27 JALAN BARLI 1/12 BANDAR BARU UDA, (TANDATANGAN PENYELIA) PRF. DR HALIMATN HAMDAN (NAMA PENYELIA) JHR BAHRU, JHR Tarikh : 7 MEI 2007 Tarikh : 7 MEI 2007 CATATAN: * Potong yang tidak berkenaan. ** Jika Tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

3 PRUS SILICA MATERIALS-PLYMER CMPSITE AS PRTN CNDUCTING MEMBRANE FR FUEL CELL SYED MUHAMMAD AL-AMSYAR A PRJECT REPRT SUBMITTED IN PARTIAL FULFILLMENT F THE REQUIREMENT FR THE AWARD F THE DEGREE F BACHELR F SCIENCE (CHEMISTRY) FACULTY F SCIENCE UNIVERSITI TEKNLGI MALAYSIA 2007

4 ii I hereby declare that I have read this project report and in my opinion this project report is sufficient in terms of scope and quality for the award of the degree of Bachelor of Science (Chemistry). Signature : Supervisor : Prof. Dr. Halimaton Hamdan Date : 7 th May 2007

5 iii I declare that this project report is the result of my own research except as in cited references. This project report has not been accepted for any degree and is not concurrently submitted in candidature of any degree. Signature : Name of Author : Syed Muhammad Al-Amsyar Date : 7 th May 2007

6 iv Untuk keluarga yang dicintai, Untuk kawan seperjuangan yang dikasihi, Untuk sahabat yang diingati, Untuk seseorang yang disayangi,

7 v ACKNWLEDGEMENT Bismillahirrahmanirrahim. Alhamdullilahirabbila lamin. Wassala tuwassala mua la asyrafil anbiya iwalmursalin. Waa la alihi wasohbihi ajmai n. My greatest gratitude to my supervisor, Prof. Dr. Halimaton Hamdan, for her thoughtful advices, encouraging words, and continuous supervision Sincere gratitude also for anybody who have been contributed in this research especially to Gui Lee Kee for her patience, guidance, and ideas. To all Zeolite and Porous Materials Group members, thanks for your support. I would also like to express my appreciation to all lecturers and lab assistants especially in Department of Chemistry for sharing their knowledge and expertise with me in order to complete this project. It has been such a pleasure of knowing you all. Finally, I would like to thank my family members and friends that have given their full support and encouragement during this project was executed. Gracias...

8 vi ABSTRACT Proton Exchange Membrane for Fuel Cell (PEMFC) application is an alternative source of energy for both mobile and stationary application due to its high efficiency, zero emission and environmental friendliness. Conventional PEMFC operates with Nafion membrane. Besides the high cost and the environmental hazards related with its disposal, Nafion also needs to be hydrated to be an effective proton conductor. The failure for Nafion to be operated above 100 C further complicates the design of fuel stack system. Therefore, it is desirable to explore an alternative, PEMFC that is capable to operate above 100 C in order to improve its efficiency. Porous silica materials are identified as potential candidate as PEMFC due to their moderate proton conductivity, excellent water retention at high temperature and high molecular sieving capabilities. In order to drive water out from the electrode, thus preventing flooding; surface modification of porous silica materials with hydrophobic chains is essential. In the studies, the development of porous silica materials composite which are zeolite A, MCM-41, and aerogel loaded with sulfonated poly (styrene-co-divinylbenzene) as proton conducting membrane was carried out. The results show that PEMFC from functionalized silica based materials show appreciable conductivity compared to Nafion. The result indicate that the proton of MCM-41 sulfonated poly (styrene-codivinylbenzene) shows similar proton conductivity with aerogel sulfonated poly (styrene-co-divinylbenzene) as X 10-3 Scm -1 and X 10-3 Scm -1 respectively.

9 vii ABSTRAK Penggunaan membran penukar proton untuk sel bahan api (PEMFC) ialah sumber tenaga alternatif untuk penggunaan bergerak dan pegun disebabkan tahap keberkesanan yang tinggi, pencemaran sifar dan mesra alam. Konvensional PEMFC menggunakan Nafion sebagai membran. Di samping kosnya yang tinggi dan juga berbahaya pada alam sekitar apabila dibuang, Nafion harus dihidratkan untuk menjadi pengkonduksi proton yang berkesan. Kegagalan Nafion untuk berfungsi pada suhu melebihi 100 C seterusnya menambahkan kerumitan dalam merekabentuk sistem bahan api berlonggok. Maka, jika kita berjaya merekacipta PEMFC yang boleh beroperasi pada suhu di atas 100 C, ianya akan memberi kelebihan dalam melebarkan lagi penggunaan di mana tahap piawaiannya adalah tinggi. Bahan silika berliang telah dikenalpasti sebagai bahan yang berpotensi untuk digunakan sebagai PEMFC disebabkan pengkonduksian proton yang sederhana, penahanan air yang baik pada suhu yang tinggi dan kebolehannya sebagai penapis molekul. Untuk memastikan air keluar daripada elektrod dalam mencegah kelimpahan air, pengubahsuaian pada permukaan bahan silika berliang dengan rantai hidrofobik adalah sangat penting. leh itu, kajian dan penciptaan bahan komposit silika berliang di mana zeolite A, MCM-41, dan aerogel yang diisi dengan poli (stirena-ko-divinilbenzena) tersulfat sebagai membran pengalir proton telah berjaya dihasilkan. Hasil kekonduksian proton daripada analisis rintangan menunjukkan komposit MCM-41 yang diisi dengan poli (stirena-kodivinilbenzena) tersulfat menunjukkan kekonduksian proton yang hampir sama dengan komposit aerogel yang diisi dengan poli (stirena-ko-divinilbenzena) tersulfat iaitu masing-masing mencatatkan bacaan X 10-3 Scm -1 dan X 10-3 Scm -1.

10 viii CNTENT CHAPTER TPIC PAGE TITLE F PRJECT REPRT SUPERVISR S CERTIFICATIN DECLARATIN DEDICATIN ACKNWLEDGEMENT ABSTRACT ABSTRAK CNTENT LIST F ACRNYMS/SYMBLS/TERMS LIST F TABLES LIST F FIGURES LIST F APPENDIXES ii iii iv v vi vii viii xi xiii xiv xvi CHAPTER I INTRDUCTIN 1.1 Research Background 1.2 Problem Statement 1.3 Literature Review Proton Conducting Membrane Zeolite A MCM Silica aerogel 1.4 bjectives

11 ix 1.5 Scope of study 1.6 Hypothesis CHAPTER II EXPERIMENTAL 2.1 Synthesis of Porous Silica Materials Synthesis of zeolite A Synthesis of MCM Synthesis of silica aerogel Post-synthesis grafting Polymerization Sulfonation 2.2 Characterization X-ray Diffraction Analysis Fourier Transform Infrared (FTIR) Analysis Impedance Analyzer Thermogravimetric (TGA) Analysis Water uptakes CHAPTER III RESULTS AND DISCUSSINS 3.1 X-ray Diffraction Analysis Zeolite A MCM Silica aerogel 3.2 Fourier Transform Infrared (FTIR) Analysis Zeolite A MCM Silica aerogel 3.3 Impedance Analyzer

12 x 3.4 Thermogravimetric (TGA) Analysis 3.5 Water uptakes CHAPTER IV CNCLUSIN AND RECMMENDATIN 4.1 Conclusion 4.2 Recommendation REFERENCES 39 APPENDIXES 44

13 xi LIST F ACRNYMS/SYMBLS/TERMS PEMFC - Proton Exchange Membrane for Fuel Cell MCM-41 - Mobile Crystalline Materials-41 EFAL - Extraframework Aluminium Al Aluminium oxide Si 2 - Silica H 2 - Water NaH - Sodium hydroxide Al - Aluminium g - Gram C - Celsius degree Al (H) 3 - Aluminium hydroxide CTABr - Cetyl N, N, N-trimethyl ammonium bromide H - Hour N 2 - Nitrogen gas K - Kelvin ml - milliliter K 2 S Potassium persulphate M - Molar λ - Wavelength 2θ - Bragg angle XRD - X-ray Diffraction FTIR - Fourier Transform Infrared KBr - Potassium bromide cm -1 - Wavenumber Hz - Hertz

14 xii V - Voltan σ - Proton conductivity R - Resistance A - Cross sectional area TGA - Thermogravimetric Analysis MPTS - Mercaptopropyltrimethoxysilane H 2 S 4 - Sulphuric acid SEM - Scanning Electron Microscope NMR - Nuclear Magnetic Resonance BET - Brunnauer, Emmet, and Teller

15 xiii LIST F TABLES TABLE TITLE PAGE 3.1 The proton conductivities of zeolite A, MCM-41, and aerogel The percentage of water uptakes for the composite membranes 35

16 xiv LIST F FIGURES FIGURES TITLE PAGE 1.1 PEMFC setup The structure of zeolite A The schematic diagram for the formation of Brønsted acid site The schematic diagram for the formation of Lewis acid site The structure of MCM The structure of silica aerogel The schematic diagram of FTIR X-Ray diffractograms of (a) as-synthesized zeolite A, (b) calcined zeolite A, and (c) zeolite A-PS-PDVB-S 3 H X-Ray diffractograms of (a) as-synthesized MCM-41, (b) calcined MCM-41, and (c) MCM-41-PS-PDVB-S 3 H X-Ray diffractograms of (a) as-synthesized aerogel, (b) calcined aerogel, and (c) aerogel-ps-pdvb-s 3 H The IR spectra of unmodified zeolite A for (a) as-prepared zeolite A, (b) calcined zeolite A, and (c) protonated zeolite A The IR spectra of unmodified MCM-41 for (a) as-prepared MCM-41, (b) calcined zeolite A, and (c) protonated MCM-41 24

17 xv 3.6 The IR spectra of modified MCM-41 for (a) MCM-41- MPTS (b) MCM-41-PS-PDVB and (c) MCM-41-PS- PDVB-S 3 H The IR spectra of unmodified aerogel for (a) asprepared aerogel and (b) calcined aerogel The IR spectra of modified aerogel for (a) aerogel MPTS (b) aerogel-ps-pdvb and (c) aerogel-ps- PDVB-S 3 H Schematic diagram the formation of mercaptans on porous silica layers The schematic diagram for the formation of poly (styrene co-divinylbenzene) The schematic diagram of sulfonation process TGA for MCM-41-PS-PDVB-S 3 H TGA for aerogel-ps-pdvb-s 3 H 34

18 xvi LIST F APPENDIXES APPENDIX TITLE PAGE 1 The graph from impedance analyzer of zeolite A at 25 C, 50 C, 75 C, and 100 C 44 2 The graph from impedance analyzer of MCM-41 at 25 C, 50 C, 75 C, and 100 C 45 3 The graph from impedance analyzer of aerogel at 25 C, 50 C, 75 C, and 100 C 46 4 The graph from impedance analyzer of MCM-41-PS- PDVB-S 3 H at 25 C, 50 C, 75 C, and 100 C 47 5 The graph from impedance analyzer of aerogel-ps- PDVB-S 3 H at 25 C, 50 C, 75 C, and 100 C 48

19 CHAPTER I INTRDUCTIN 1.1 Research Background A fuel cell is an electrochemical device that produces energy by converting fuel into electricity via a chemical reaction between hydrogen and oxygen to yield water, heat, and electrical energy. A fuel cell recently received a great attention for their application because of its high efficiency and zero emission power plants. In fuel cell, the redox reaction occurs under nearly thermodinamically reversible condition, thus leads the efficiency approaching 75 %. These fuel cell systems can also reduces primary energy consumption by 20 %. At the mean time, it can decrease C 2 and N X emission. It can be used in producing continuous energy especially in automobiles, laptop, and compact heating system. The increasing of oil prices in the world market gives us an indicator that fuel cell will be used as primary energy to replace conventional energy sources in the future. Compared to the alternatives energy sources like solar energy, nuclear energy, biomass energy and many more, fuel cell is better in term of cost, reliability, efficiency, and environment-friendly. As little as 10 years ago, vehicles powered by fuel cells seemed more science fiction than fact. Today, development of fuel cell technology for transportation is made

20 2 possible due to the polymer electrolyte membrane fuel cell. This type of fuel cell is also known as the proton exchange membrane fuel cell, the solid polymer electrolyte (SPE) fuel cell and simply, polymer electrolyte fuel cell. The center of the fuel cell is the polymer electrolyte membrane. For all five families of fuel cells, it is the electrolyte that defines the type of fuel cell. An ordinary electrolyte is a substance that dissociates into positively charged and negatively charged ions in the presence of water, thereby making the water solution electrically conducting. The electrolyte in a polymer electrolyte membrane fuel cell is a type of plastic, a polymer, and usually referred to as a membrane. The appearance of the electrolyte varies depending upon the manufacturer, but the most prevalent membrane, Nafion produced by DuPont, resembles the plastic wrap used for sealing foods. Typically, the membrane material is more substantial than common plastic wrap, varying in thickness from 50 to 175 microns. In an operating fuel cell, the membrane is well humidified so that the electrolyte looks like a moist piece of thick plastic wrap. Figure 1.1: Proton Exchange Membrane Fuel Cell (PEMFC) setup

21 3 Polymer electrolyte membranes are somewhat unusual electrolytes in that, in the presence of water, which the membrane readily absorbs, the negative ions are rigidly held within their structure. nly the positive ions contained within the membrane are mobile and are free to carry positive charge through the membrane. In polymer electrolyte membrane fuel cells these positive ions are hydrogen ions, or protons, hence the term proton exchange membrane. Movement of the hydrogen ions through the membrane, in one direction only, from anode to cathode, is essential to fuel cell operation. Without this movement of ionic charge within the fuel cell, the circuit defined by cell, wires, and load remains open, and no current would flow. Because their structure is based on a Teflon backbone, polymer electrolyte membranes are relatively strong, stable substances. Although thin, a polymer electrolyte membrane is an effective gas separator. It can keep the hydrogen fuel separate from the oxidant air, a feature essential to the efficient operation of a fuel cell. Although ionic conductors, polymer electrolyte membranes do not conduct electrons. The organic nature of the polymer electrolyte membrane structure makes them electronic insulators, another feature essential to fuel cell operation. As electrons cannot move through the membrane, the electrons produced at one side of the cell must travel, through an external wire, to the other side of the cell to complete the circuit. It is in their route through the circuitry external to the fuel cell that the electrons provide electrical power to run a car or a power plant. Very recently, porous materials based polymer composites as solid electrolyte for proton exchange membrane fuel cell (PEMFC) and direct ethanol fuel cell (DEFC) reportedly fulfill the requirement to become ideal solid electrolyte because of some advantages by having a pore, moderate conductivity, and large surface area. These progresses hopefully contribute to ensure environmental sustainable development in fuel cell technology.

22 4 1.2 Problem Statement Well-established PEMFC such as Nafion are expensive and only useful at operating temperature below 100 C, but extensively used in fuel cells and others electrochemical devices. Since Nafion is perfluorinated polymer, it contributes to environmental disposal hazard. The failure of Nafion to operate above 100 C encounter some drawbacks including water management at the electrodes and C poisoning at the Pt anode catalyst, hence make the development of reliable fuel cell system become more complicated. Although organic-inorganic composite membranes have been developed, however it still failed to meet the requirement due to low proton conductivity. If we can increase the operating temperature above 100 C, therefore these entire problems can be alleviated, yet maintaining the humidity and perform moderate proton conductivity. 1.3 Literature Review Proton Conducting Membrane The first fuel cell was developed in England in 1839 by Sir William Grove [1]. During this time, the experiment focusing on electrolysis which using electricity to split water into hydrogen and oxygen. So, he proved this theory by developing two Pt electrodes in separate container, one containing hydrogen and another containing oxygen. When these containers were immersed in dilute H 2 S 4, a current began to flow between the two electrodes and water was produced in the gas bottle. Since his breakthrough until now, there are a lot of improvement and modification on developing the best fuel cell. This growing interest has been caused by increasing awareness about pollution and possible anthropogenic global warming and the consequent research for alternative that must satisfy the energy demand in cleaner way, probably based on hydrogen. Since hydrogen is obtained from other chemical species, thus the electrolyte that is used as proton conducting membranes must have high

23 5 proton conductivity can be operated at high temperature, and adequate mechanical strength [2]. All these properties belong to commercial PEMFC, which is Nafion. Nafion is perfluorinated polymer. It offers good performance below 80 C and show ideal characteristic for PEMFC which are high proton conductivity (0.1 Scm -1 ), adequate mechanical strength, good thermal and chemical stability [3]. But, Nafion still faces significant technology roadblocks that need to be overcome such as water management at the electrodes, C poisoning on the anode catalyst, slow cathode kinetics, and high cost of the Pt electrodes and Nafion itself. Researches have shown that once the operating temperature can be increased up to above 100 C, all these problems can be eliminated [4]. Researchers around the world try to use organic-inorganic composite membrane [5-7]. However, the conductivities of these membranes were still lower compare to commercial Nafion. In this case, porous materials like zeolite A, Al-MCM-41, and aerogel meet these requirements because they are tolerant proton conductor yet maintain hydration at very low relative humidity. However, the possible effect of the crystal type, crystal size, and pore shape on the permeation characteristic of the zeolite filled polymeric membrane has not been done yet [8]. Sulfonated polystyrene is a prospect for PEM applications owing to their good thermal stability and appropriate conductivity at high sulfonation degrees. The 1 H NMR analysis of sulfonated polymers suggests the formation of H 3 + groups at earlier stages of the hydration. These groups trigger an appreciable increase on conductivity when proton exchange between water molecules is produced [9]. However, a number of investigations have demonstrated that water uptake increased with the increase of sulfonation degrees. The presences of porous material materials are believed to overcome the increasing of water uptakes due to better water management and retention [10]. Another problem is the benzylic position, that is very

24 6 sensitive to oxidize and any free-radical created on the carbon of the monomer repeat unit will benefit from a double stabilization as tertiary radical as well as delocalized by resonance effect on the aromatic ring [2] Zeolite A Zeolite is a polymeric porous crystalline hydrated aluminosilicate based on an infinite three-dimensional structure [11] built of (Si 4 ) 4- and (Al 4 ) 5- tetrahedral linked by sharing of an oxygen atom, giving a framework with a regular system of interconnected cavities and channels of molecular dimension. The type of zeolites can be characterized by the distinct topology of their three-dimensional, the relative ratio of silicon and aluminium, the arrangement of the silicon and aluminium atoms in the tetrahedral sites of the framework, the type and distribution of cations. The general formula for zeolite is described below: Where Mx/n [(Al 2 )x(si 2 )y]. zh 2 Mx/n the non-framework exchangeable cation of valence n; [ ] the aluminosilicate framework zh 2 the number of moles of zeolitic water The aluminosilicate framework of zeolite A consists of an array of truncated octahedral linked via double 4-rings. Its chemical formula is Na 12 [(Al 2 ) 12 (Si 2 ) 12 ]. 27H 2, cubic in symmetry with a unit cell constant a o = Å. The framework consists cavities of 11.4Å in diameter and Al/Si ratio of 1 with highly ordered and obeyed the Loewenstein s rule which states that Al--Al bonds are forbidden [12]. Zeolite A are widely used as adsorbents and ion-exchangers [13]. The sorption and diffusion properties of zeolite A are due to the existence of channels and cavities

25 7 [14]. Sorption capacity of adsorption is related to the free space or void volume. Up to 50% of the intracrystalline can be occupied by water. Figure 1.2: The structure of zeolite A Na + - Al Si NH 4+ NH 4+ - Al Si 500 C -NH 3 H Al Si Figure 1.3: The schematic diagram for the formation of Brønsted acids site

26 8 Both Brønsted and Lewis acidities can be found in zeolites [15]. Brønsted acidity is generated from acidic hydroxyl groups known as bridging hydroxyls which are bonded to the Si 4 and Al 4 tetrahedral. Each aluminium in the framework structure induces active site when the charge balancing cations are replaced by proton. Brønsted acid sites are created when the cation balancing the framework anionic charges are H +. Brønsted are acids also known as proton donour. Lewis acidity originates from extraframework aluminium having trigonal aluminium atoms structure at aluminium deficient. Lewis acids are known also as electron donour. The number of active sites and the strength of the acidity depend on the number of aluminium or Si/Al ratio of the framework. The strength of acidity of zeolite increases with an increasing of Si/Al ratio in the framework. H -H Al Si Al Si Al Si Al + - Al Si Si Figure 1.4: The schematic diagram for the formation of Lewis acid site

27 MCM-41 Molecular sieves with a zeolite structure and pore diameter less than 15 Å exhibit a shape selectivity that enables them to be used as adsorbents or catalysts in a variety of processes. However, reactions involving bulky molecules require structures with channel diameters at mesopore scale. In the early nineties, scientists from Mobil il Corporation synthesized ordered mesoporous materials of the M41S type, family to which MCM-41 belongs [16,17]. Figure 1.5: The structure of MCM-41 This material possesses a porous system consisting of hexagonally arranged channels with diameters varying from 15 to 100 Å. MCM-41 has attracted the attention of scientists due to its elevated specific surface area, high thermal and hydrothermal stability, possibility of controlling its pore size and its hydrophobicity and acidity. These characteristics have made MCM-41 a promising material as catalyst and/or support [18] and to be used in industrial processes of adsorption [19], ion exchange [20], high thermal and hydrothermal stability if properly prepared [21]. The main advantages of MCM-41 are their high surface area, typically greater than 800 m 2 /g and pore volume of 0.7 cm 3 /g or greater [22].

28 10 But, the mesoporous purely siliceous MCM-41 has significant roadblock since its structure is neutral and neither has cation exchange capacities nor acidity [20]. Thus, aluminium is inserted to MCM-41 because of its similar size and the fact is that the presence of tetrahedral aluminium would create an ionic framework, hence can be protonated to form Brønsted acid sites. The negative charge increases since Al(III) replaces Si(IV) in the framework. Subsequent ion exchange of the sample with NH 4+ cations, followed by calcinations contribute an occupation of cationic sited almost dominantly by protons and thereby acid activity. The existence of Lewis-acid sites in Al-MCM-41 is not clearly understood, but possibly from extraframework Al species (EFAL) [23]. By MAS NMR spectroscopy, the presence of Lewis-acid site is due to both octahedral and tetrahedral EFAL species, formed by dehydroxylation of the hydrogen form of zeolites was confirmed [24] Silica Aerogel Aerogel is a silica-based solid with a porous, open cell, low density foam in which 99.8 % of the volume is empty space [25]. Aerogel is not like conventional foams, but is a special porous material with extreme microporosity on a micron scale [26]. Due to its microstructure, aerogel is a powerful desiccant [27], rapidly absorbing any moisture in the fingertips when held. Discovered in the 1930s by Steven Kistler [28], a Stanford University researcher, aerogel is the world's lightest solid. It is composed of individual features only a few nanometers in size. These are linked in a highly porous dendritic-like structure, in which spherical particles of average size 2-5 nm fuse together into clusters [25]. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores smaller than 100 nm. Aerogel can have surface areas ranging from 250 to 3000 m 2 /g [29], meaning that a cubic inch of aerogel flattened-out would have more surface area than an entire football field.

29 11 Figure 1.6: The structure of silica aerogel 1.4 bjectives 1. To synthesize zeolite A, MCM-41 and aerogel loaded in polymer to form the temperature tolerant proton conducting membrane. 2. To test the proton conductivity PEMFC that can operate at high temperature. 1.5 Scope of Study Areas of studies that have been conducted are; 1. Preparation and characterization of zeolite A, MCM-41 and silica aerogel 2. Modification by loading zeolite A, MCM-41 and aerogel with sulfonated poly (styrene-co-divinylbenzene). 3. Testing and optimization of the membrane

30 12 Synthesis of Zeolite A, MCM-41, and aerogel Modification and improvement Preparations of Zeolite A, MCM-41, and aerogel loaded with polymer Characterization of bulk and surface structures by XRD, FTIR, and TGA Proton conductivity testing via Impedance Analyzer 1.6 Hypothesis The development of PEMFC based on zeolite A, MCM-41, and aerogel could alleviate some technical issues associated when applying Nafion as proton conducting membrane due to the ability of zeolite A, MCM-41, and aerogel composite to stand above 120 C besides having good water retention.

31 CHAPTER II EXPERIMENTAL 2.1 Synthesis of Porous Silica Materials Synthesis of Zeolite A NaA zeolite synthesis gel was prepared using the procedure described previously. Synthesis runs were carried out in polyethylene bottle at autogeneous pressure in the oil bath. Agitation was achieved by a Teflon-coated magnetic stirrer. The reaction mixtures were prepared following molar composition of 8.67 Na 2 : Al 2 3 : Si 2 : 562 H g of the rice husk ash, g NaH and 50 g of water. The reaction mixture was sealed in stainless steel pressure vessel and heated in an oven overnight at 150 C at autogenous pressure. An aqueous solution of sodium aluminate; prepared separately by mixing calculated proportion of aluminum hydroxide Al(H) 3, sodium hydroxide (NaH) and water were then added to the alkaline solution, followed by vigorous stirring while heating to ensure formation of a solution. The aluminate and silicate solutions are mixed under vigorously stirring. The mixture was sealed in polyethylene bottle and heating in oil bath with stirring at 100 C for 5. The solid products were filtered, washed with distillate water, dried overnight at 100 C and calcined at 400 C for 1 h.

32 Synthesis of MCM-41 The Al-MCM-41 was synthesized according established method. First, a clear solution of sodium silicate was prepared by combining g of 1.00 M aqueous NaH solution with g rice husk ash (90 wt % Si 2 ) and the resulting solution (mixture A) was then heated under stirring for 2 h at 80 C. A mixture of 1.05 g of 25 wt % aqueous NH 3 solution, g of cetyl N,N,N-trimethyl ammonium bromide (CTABr) and g of NaAl 2 (54 wt % Al 2 3 ), were put in a polypropylene bottle and the mixture (mixture B) was then heated with stirring for 1 h at 80 C. Subsequently, mixture B was added drop wise to a polypropylene bottle containing mixture A with vigorous stirring at room temperature. After stirring for 1 h at 90 C, the gel mixture in the bottle was heated to 97 C for 24 h. The CTA aluminosilicate gel was then cooled to room temperature. The ph of the reaction mixture was then adjusted to 10.2 by adding 25 wt % acetic acid (CH 3 CH). Repeated ph adjustments were performed in order to increase thermal stability and textural uniformity of the product. The heating and ph adjustment procedures were repeated two times. The precipitated product, as-synthesized Al-MCM- 41 containing CTA-template was filtered, washed thoroughly with doubly-distilled water and dried in an oven at 97 C. Al- MCM-41 was calcined in air under static conditions in a muffled furnace. The calcinations temperature was increased from room temperature to 550 C for 10 h and maintained at 550 C for 6 h Synthesis of Silica Aerogel The sodium silicate solution was prepared by diluting g of rice husk ash with g of NaH pellet and 450 g of double distilled water. The solution was stirred overnight at 90 C for two days. Rice husk ash was used as the silica sources. Then, the solution was filtered to remove any impurities followed by hydrolysis with sulphuric acid. The resulting hydrogel was aged for seven days to obtain a strong silica

33 15 linkage. In order to remove any excess of acid, the gel was washed with double distilled water before the extraction with ethanol by using Soxhlet extraction was performed to ensure the water in silica network was exchanged with ethanol. Supercritical drying was carried out with ethanol as a medium using Parr Instrument autoclave equipped with thermocouple and temperature controller. The alcogel was placed into two-liter autoclave and filled with ethanol in excess. The mixture was heated in multiple steps until reached 275 C. After that, the temperature was kept constant for an hour and the solvent was slowly vented until the pressure decreased to 28 psi. Finally, the nitrogen gas was flushed into the autoclave for ten minutes Post-synthesis grafting. The sample was dried at 116 C under N 2 stream (10 20 ml/min) for 3 h within a three necked flask. Then, 10 ml of dry toluene per gram of solid were added to the flask and the suspension was kept under mild stirring for 1 h at constant temperature. Finally, 5 mmol of MPTS per gram of sample were added, and the resulting mixture was allowed to react at the same temperature for 24 h. Finally, the sample was filtered, washed, and dried in oven at 100 C Polymerization. The functionalized polystyrene was prepared by polymerizing 2 ml of styrene and 1 ml of divinylbenzene with 0.5 g of the sample in M of K 2 S 2 8 at 80 C. The solution was stirred continuously for 6 h. The functionalized polymer was recovered by filtering and washing with deionized water. Finally, the polymer was drying at room temperature.

34 Sulfonation 10 ml of sulphuric acid per gram of sample was reacted at 80 C for 6 h under mild stirring. Then, sufficient amount of methanol was added to the sample. After that, the sample was filtered and washed with deionized water to remove the contaminants. Finally, the sample was dried in oven at 100 C. 2.2 Characterizations X-ray Diffraction (XRD) Analysis The crystallinity and homogeneity of the sample was determined by using X-ray diffractometer. The X-ray diffractogram was recorded from Bruker D8 Advance X-Ray. Approximately 1 g of sample was carefully ground to a fine powder and then lightly pressed between two glass slides to get a thin layer. After placing and locking sample holder ina proper place, the samples was measured, sources from CuKα radiation with λ = Ǻ in the range of 2θ = 2-50 at room temperature and the step interval of 2θ = 0.05 with 1 s per step Fourier Transform Infrared (FTIR) Analysis Generally, FTIR is used to observe the vibration with the respect of attached atoms thus provide the existence of functional groups in the sample. For sample preparation, KBr pellet technique was chosen. A sample (1 mg) was grinded with potassium bromide (100mg) by using pestle and mortar.

35 17 Figure 2.1: The schematic diagram of FTIR After obtained very fine powder, then it was transferred to the dye and 5 tonne of pressure was applied for two minutes. The pellet was placed it the sample holder and the FTIR spectrum was recorded using Perkin Elmer FTIR spectrometer ranging from 400 cm -1 to 4000 cm Impedance analyzer The proton conductivity was measured using Autolab PGSTAT Impedance Analyzer with Galvanostat&Potentiostat and software Frequency Response Analyzer. The sample 0.05 g was transformed to the pellets by using dye and the applied pressure was 3 tonnes. The pellet was sandwiched between the circular copper electrodes. In order to maintain the hydration, the pellet was hydrated with deionized water then sealed with cellophane tapes to prevent leakage. The frequency range is 0.01 Hz-1MHz while the amplitude is 0.01 V. From the cole-cole plot, the resistance value associated with

36 18 sample s conductivity was determined from the highest frequency intercept of the x- axis. The proton conductivities are calculated using the equation given below. σ = t RA Where σ proton conductivity, t the thickness of the membrane, R the resistance A the cross-sectional area perpendicular to the current flow However, it should be considered that the obtained conductivities are subject to some uncertainties. There are: I. The changes in physical dimension of the membrane due to swelling or contracting. II. III. The membranes might be dehydrated especially at high temperature because no humidity controller is provided. The physical contact between the membrane and copper electrode will produce an internal resistance which affects the value obtained Thermogravimetric Analysis (TGA) The thermal behaviour of the sample was determined by using Perkin Elmer s Pyris Diamond Thermogravimetric in addition with Mettler software. The analyses were performed over the range of C at heating rate 10 C per minute in nitrogen gas flow.

37 Water uptakes The characterization of water adsorption capacity is essential, since the presence of water determine the capabilities to adsorp water and maintain the hydration at high temperature. The membranes were dried at 150 C for 24 h, weighed and immersed in deionized water at room temperature for 24 h in desiccators. The water uptake of the membranes is reported in wt. % as follows: Wwet - W Water uptake = Wdry dry x100% Where W wet and W dry are the weights of the wet and dry membranes respectively. This method provides an accurate measure of the water uptake and has been established.

38 CHAPTER III RESULTS AND DISCUSSINS 3.1 X-Ray Diffraction Analysis Zeolite A Figures 3.1 show the XRD diffractograms of as-synthesized zeolite A, calcined zeolite A, and funtionalized zeolite A (zeolite A-PS-PDVB-S 3 H) respectively (c) Lin (Counts) (b) (a) Figure 3.1: X-Ray diffractograms of (a) as-synthesized zeolite A, (b) calcined zeolite A, and (c) zeolite A-PS-PDVB-S 3 H

39 21 Figure 3.1 indicates that zeolite A is a highly crystalline material. The presence of intense peaks for calcined zeolite A and as-synthesized zeolite A indicates an ordered framework structure. The contrast, the peaks of zeolite A-PS-PDVB-S 3 H disappeared into a featureless diffractogram, suggesting that the framework of zeolite A has collapsed, as a consequent of treatment with H 2 S 4 use for sulfonation process since it is believed that the framework of zeolite are very sensitive to strong acids. Therefore, the proton conductivity of zeolite A-PS-PDVB-S 3 H will not be carried out MCM-41 Figures 3.2 show the XRD diffractograms of as-synthesized MCM-41, calcined MCM-41, and functionalized MCM-41 (MCM-41- PS-PDVB-S 3 H) respectively (c) Lin (Counts) (b) (a) Figure 3.2: X-Ray diffractograms pattern of (a) as-synthesized MCM-41, (b) calcined MCM-41, and (c) MCM-41-PS-PDVB-S 3 H. Figure 3.2 proved that MCM-41 is semi crystalline materials with hexagonal lattice structure with four peaks, indexed as (100), (110), (200), and 210) indices. It also

40 22 demonstrates that the structure of MCM-41 remained intact even under high temperature during calcinations. The increase in intensity of peaks of calcined MCM-41 compared to as-synthesized MCM-41 showed that the templates have been removed successfully hence more ordered framework structure was established. However, the reduced intensity of (100) and absence of peaks at (110), (200), and (210) after sulfonation process indicate lack of long range order or nearly becoming disorder due to sensitivity of the framework toward strong acids Silica aerogel Figures 3.3 represent the XRD diffractograms of as-synthesized silica aerogel, calcined silica aerogel, and functonalized silica aerogel (aerogel-ps-pdvb-s 3 H) respectively. 400 (c) (b) (a) Figure 3.3: X-Ray diffractograms pattern of (a) as-synthesized silica aerogel, (b) calcined silica aerogel, and (c) aerogel-ps-pdvb-s 3 H Figure 3.3 show that silica aerogel is totally amorphous. There are no significant peaks for as synthesized aerogel, calcined aerogel, and aerogel-ps-pdvb-s 3 H due to

41 23 lack of long range order of the solid, which indicates that there are no well-defined planes in the structure. 3.2 Fourier Transformed Infrared (FTIR) Spectroscopy Zeolite A (a) (b) (c) %T cm-1 Figures 3.4: The IR spectra of unmodified zeolite A for (a) as-prepared zeolite A, (b) calcined zeolite A, and (c) protonated zeolite A. Figures 3.4 shows the functional group presents in unmodified zeolite A. The broad band around 3453 cm -1 represents the existence of water in the sample, while the formation of silanol groups is unlikely due to the Si/Al ratio is 1. The absorption at 1653 cm -1 is assigned to bending mode of water. Assymmetric stretching of Si--Si appear at 1010 cm -1 while for symmetric stretching of Si--Si is observed at 669 cm -1. The absorption appears around 462 cm -1 corresponds to the Si- bending, which is observed in all samples.

42 MCM-41 (a) (b) %T (c) cm-1 Figures 3.5: The IR spectra of unmodified MCM-41 for (a) as-prepared MCM-41, (b) calcined MCM-41, and (c) protonated MCM-41 Figure 3.5 shows the functional group presence in unmodified MCM-41. The spectrum shows six main absorption bands. The broad band around 3453 cm -1 represents the existence of hydroxyl group in the sample. The absorption at 1631 cm -1 is assigned to bending mode of water. Assymmetric stretching of Si--Si appears at 1082 cm -1 while for symmetric stretching of Si--Si is observed at 669 cm -1. The Si-H stretching vibration occurs as a weak band at 960 cm -1. Symmetric stretching of Si--Al appears as a weak band around 790 cm -1. The absorption which appears at 450 cm -1 corresponds to the Si- or Al- bending, which are observed in all sample. The various C-H stretching vibrations associated with the surfactant molecules at 2922 cm -1 and 2856 cm -1 are not observed in IR spectra for calcined MCM-41, indicating that the templates have been successfully removed.

43 25 (a) (b) %T (c) cm-1 Figures 3.6: The IR spectra of modified MCM-41 for (a) MCM-41-MPTS, (b) MCM- 41-PS-PDVB, and (c) MCM-41-PS-PDVB-S 3 H Figures 3.6 prove the existence of functional groups in modified MCM-41. The absorption at 2929 cm -1 corresponds to the C-H stretching for MCM-41-MPTS was observed, indicates that the functionalized thiol groups have been attached to the silica surfaces. The sp 2 C-H stretching for MCM-41-PS-PDVB was observed at 3062 cm -1 and 3025 cm -1, proved the presence of phenyl group on the modified MCM-41. In addition, the stretching of aromatic C=C appeares at 1601 cm -1 and 1491 cm -1 confirming the formation of poly (styrene-co-divinylbenzene). For the analysis of sulfonic groups, the bonding of the sulfonic groups to the aromatic ring of polystyrene (out of plane) deformation bands is assigned to substituted aromatic ring γ (C ar H) at wavenumbers approximately from 830 cm -1 to 850 cm 1. The absorption at 1034 cm 1 results from the symmetric stretching vibration of S 3 H groups and the absorption at 1126 cm 1 results from an attached sulfonate anion to a phenyl ring. The decreasing of relative transmittance of MCM-41-PS-PDVB-S 3 H compared to MCM-41-PS-PDVB possibly due to the degradation of polymers by sulphuric acid.

44 Silica aerogel (a) (b) %T cm-1 Figures 3.7: The IR spectra of unmodified silica aerogel for (a) as-prepared silica aerogel and (b) calcined silica aerogel The IR spectra in Figures 3.7 show the functional groups that exist in unmodified aerogel. The spectra show six main absorption bands. The broad band around 3453 cm -1 represents the existence of hydroxyl group in the sample due to the interaction of -H groups with water in humid air via hydrogen bonding. The absorption at 1631 cm -1, assigned to bending mode of water, only appear for calcined aerogel due to the hygroscopicity of the aerogel after calcination. Because of that, ion exchange of aerogel is unnecessary. Assymmetric stretching of Si--Si appears at 1093 cm -1 while for symmetric stretching of Si--Si is observed at 801 cm -1. The Si-H stretching vibration occurs as a weak band at 960 cm -1.The absorption at 450 cm -1 corresponds to the Si--Si bending.

45 27 (a) %T (b) (c) cm-1 Figures 3.8: The IR spectra of modified aerogel for (a) aerogel-mpts, (b) aerogel-ps- PDVB, and (c) aerogel-ps-pdvb-s 3 H Figure 3.8 shows the presence of functional groups in modified aerogel. The absorption at 2922 cm -1 corresponds to the C-H stretching for aerogel-mpts, indicates that the functionalized thiol groups are covalently bonded to the silica surface. The sp 2 C-H stretching mode for MCM-41-PS-PDVB is observed at wave number 3062 cm -1 and 3025 cm -1, which prove the presence of phenyl group on the modified aerogel. In addition, the stretching of aromatic C=C appeares at 1601 cm -1 and 1491 cm -1 confirms the formation of poly (styrene-co-divinylbenzene). The bonding of the sulfonic groups to the aromatic ring of polystyrene (out of plane) deformation bands is assigned to the substituted aromatic ring γ (C ar H) at wave numbers of 830 cm -1 to 850 cm 1. The absorption at 1037 cm 1 is assigned to symmetric stretching vibration of S 3 H groups and the absorption at 1122 cm 1 resulted from a sulfonate anion attached to a phenyl ring. The decreasing transmittance of aerogel-ps-pdvb-s 3 H compared to aerogel- PS-PDVB might due to the degradation of poly (styrene-co-divinylbenzene) by sulphuric acid.

46 Impedance Analyzer The graph of resistance from impedance analyzer in determining proton conductivity is given in Appendix 1, 2, 3, 4, and 5. Table 3.1: Proton conductivities of different membranes at various temperatures Membranes Zeolite A MCM-41 Aerogel MCM-41-PS-PDVB- S 3 H Aerogel-PS-PDVB- S 3 H Nafion 25 C (Scm -1 ) 50 C (Scm -1 ) 75 C (Scm -1 ) 100 C (Scm -1 ) x x x x x x x x x x x x x x x x x x x x x x x x The proton conductivities of zeolite A, MCM-41, aerogel, MCM-41-PS-PDVB- S 3 H, and aerogel-ps-pdvb-s 3 H at 25 C, 50 C, 75 C, and 100 C are given in Table 3.1 above. Among porous silica materials, MCM-41 gives the best proton conductivity where the highest measurement recorded is x 10-6 Scm -1 at 100 C. It is because, it has larger surface area among zeolite A and aerogel, thus allows a high dispersion of the active species, that is, a high amount of the active centers is accessible for proton

47 29 mobility. n the other hand, having larger surface area meaning it has more silanol groups, useful to contribute for Brønsted acid property. As the temperature is increased, the proton conductivity also increases. This is confirmed by the observation that Si H groups do not dissociate under moderate conditions [30]. In protonated form, where protons are the charge compensating cations, bridged hydroxyl groups on each Al-site, so-called Brønsted acid sites, are formed. In these structures the probability and energetics of proton jumps between the oxygen sites, which directly surrounding the Al-centers (on-site motion) or between neighboring Al-sites (inter-site motion) are fundamental interest in the conductivity, since it is assumed that the mobility is directly related to the conductivity activity [31]. Zeolite A give the lowest conductivity because the formation of silanol groups is unlikely due to the Si/Al ratio is 1. Therefore, the contribution of Bronsted acid property is not observed. Similarly observed for porous silica materials loaded with sulfonated poly (styrene-co-divinylbenzene) again MCM-41- PS-PDVB-S 3 H exhibits higher proton conductivity than aerogel-ps-pdvb-s 3 H. The reason for these trends also can be explained in term of the surface area of these nanostructured materials. It is well-known that porous silica materials have remarkable high density of surface silanol groups. The importance for the surface modification of silica by post-synthesis grafting treatment, as these surface silanol groups can act as anchoring centers for functional silanes since organofunctional silanes create the strongest interfacial bond with the silanol groups of silica [32]. 3-mercaptopropyltrimethoxysilane (MPTS) was chosen as silane coupling agents since it has three methoxy groups to form the strongest achievable interfacial bonds. Furthermore, the mercaptofunctional silanes are chain-transfer agents in radical polymerization.

48 30 Porous Silica H H H H H CH 3 + H 3 C Si SH CH 3 H Porous Silica Si SH H Figure 3.9: Schematic diagram the formation of mercaptans on porous silica layers Thus, the greater surface area will contribute to the formation of more mercaptan group on the silica surfaces, hence the greater polystyrene coupled with the silanes groups will be formed. To enhance the acidity of the composites, sulfonation process was carried out in order to improve proton conductivity by producing Brønsted acid sites, in which the sulfonic acid groups would directly attach to the benzene ring, where para position is the most favourable.

49 31 H Porous Silica Si SH H styrene divinylbenzene H Porous Silica Si S [ ] [ ] H Figure 3.10: The schematic diagram for the formation of poly (styrene codivinylbenzene)

50 32 H Porous Silica Si S [ ] [ ] H H S S 4 H Porous Silica Si S [ ] [ ] H S 3 H Figure 3.11: The schematic diagram of sulfonation process The membrane must be fully hydrated because in the first case, proton mobility would be associated with that of water molecules, but in the second case, a long-term mobility of protons is established. In the presence of water, the polymers swell and the sulfonic groups dissociate into S 3 (fixed charge) and H + (mobile charge). Thus, the protons, which are responsible for conductivity, encounter a low resistance in moving across the membrane under a potential gradient [9]. Hence, it is easily understood that the water management of the membrane is a fundamental key. The performance of PEMFC is strongly influenced by conductivity, and the conductivity is strongly influenced by the state of hydration of the membrane. If the membrane is too dry, its conductivity falls resulting in a reduced cell performance.

51 33 An excess of water in the fuel cell can lead to cathode flooding problems, also resulting in less than optimal performance. The porous silica materials can play decisive roles where it has the capabilities to retain water in high humidity conditions since it has pores in their structures, and maintain the hydration since water trapped in the pores might be useful for hydration when the condition is too dry. Therefore, these phenomenons can explained why MCM- 41-PS-PDVB-S 3 H has a higher proton conductivity compared to aerogel-ps-pdvb- S 3 H. It is because of the surface area and the pore size in MCM-41 which allows for easier flow of water into and out of the system. 3.4 Thermogravimetric Analysis (TGA) The thermal stabilities of MCM-41-PS-PDVB-S 3 H and aerogel-ps-pdvb- S 3 H were examined in nitrogen atmosphere at a heating rate of 10 C per minute. $=MCM 41-PS MCM 41-PS, mg 2 mg C Figure 3.12: TGA for MCM-41-PS-PDVB-S 3 H