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Carbon Nanotube Electrodes for Capacitive Deionization Heena K

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Carbon Nanotube Electrodes for Capacitive Deionization Heena K Carbon Nanotube Electrodes for Capacitive Deionization ASSA SETINSTM E OF TECHNOLOGY by NOV J 2213 Heena K. Mutha UBRARIES Submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY September 2013 @ Massachusetts Institute of Technology 2013. All rights reserved. A uthor ....... ........... ....................................... Department of Mechanical Engineering August 20, 2013 Certified by... ............... ,.. .. Evelyn N. Wang Associate Professor Thesis Supervisor Accepted by.............. 1w David E. Hardt Chairman, Department Committee on Graduate Theses Carbon Nanotube Electrodes for Capacitive Deionization by Heena K. Mutha Submitted to the Department of Mechanical Engineering on August 20, 2013, in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Abstract Capacitive deionization (CDI) is a desalination method where voltage is applied across high surface area carbon, adsorbing salt ions and removing them from the water stream. CDI has the potential to be more efficient than existing desalination tech- nologies for brackish water, and more portable due to its low power requirements. In order to optimize salt adsorption in CDI, we need a better understanding of salt adsorption and the electrode properties involved in ion removal. Current materials are highly porous, with tortuous geometeries, overlapping double layers, and sub- nanometer diameters. In this work, we design ordered-geometry, vertically-aligned carbon nanotube electrodes. The CNTs in this study have 2-3 walls, inner diameter of 5.6 nm and outer diameter of 7.7 nm. The capacitance and charging dynamics were investigated using three-electrode cell testing in sodium chloride solution. We found that the material capacitance was 20-40 F/g and the charging time varies lin- early with CNT height. The data was matched with the Gouy-Chapman-Stern model indicating that porous effects were negligible. Charging rates of CNTs compared to microporous activated carbon fiber, show that CNTs are more efficient at charging by weight. However, densification and surface functionalization will be necessary to enhance CNT performance by planar area. Future work will involve investigating electrodes in a flow-through cell to use salt adsorption data to determine the influence on electrode thickness on salt adsorption in channel flow. Thesis Supervisor: Evelyn N. Wang Title: Associate Professor 2 Acknowledgments I would like to thank Professor Evelyn Wang for her guidance and encouraging support during this research study. I would also like to thank Jeremy Cho for discussing experimental set up, reviewing my thesis, and supporting me throughout this project. I would like to acknowledge Dr. Robert Mitchell and Professor Carl Thompson for the development of the carbon nanotube growth and use of instrumentation; Professor Gang Chen and Dr. Yuan Yang for use of their impedance analyzer; and Dr. Betar Gallant for her insights on the electrochemical work. I would also like to acknowledge Device Research Laboratory members and alumni Dr. Ryan Enright, Tom Humplik, and Dr. Nenad Miljkovic for their guidance regarding characterization of carbon nanotubes. Finally, I would like to wholeheartedly thank my family and friends for their support and love throughout this project. I would like to thank the King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia, for funding the research reported in this paper through the Center for Clean Water and Clean Energy at MIT and KFUPM. In addition, this material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374. 3 Contents 1 Introduction 13 1.1 M otivation .. .... .... ..... .... ... 14 1.1.1 Multi-Stage Flash Desalination ... ... 14 1.1.2 Reverse Osmosis ... .... .... ... 15 1.1.3 Electrodialysis ... .... .... .... 16 1.1.4 Capacitive Deionization ... .... ... 16 1.2 Background ... ... .... ... ... .... ........... 18 1.2.1 History .... .... .... ... .... .... ....... 18 1.2.2 Carbon Materials in Supercapacitors . .. ... ....... 20 1.3 Carbon Materials for CDI . ... .... ... .. .. .... .... 22 1.4 Electric Double Layer Theory . .... .... .. ....... .... 25 1.5 Thesis Outline ... ... ... ... .. ... ... .... ... ... 34 2 Synthesis of Electrode and Preliminary Testing 36 2.1 CNT growth and characterization .. ..... .. ...... ..... 37 2.2 Au-Au Self-Diffusion Bond .. ... .. ... ... ........ .. 40 2.3 Electrochemical Characterization ... ... ... ........... 41 2.3.1 Cell Set Up ... ... ... ... ... .. ...... ..... 41 2.3.2 Experiment: Cyclic Voltammetry (CV) . ........... 43 2.3.3 Experiment: Potentiostatic Testing . .. .... ... ... 45 2.4 Experimental Results and Discussion ... .... .... ....... 46 4 3 CNT Electrode Design 50 3.1 Bonding Methods .............. 50 3.1.1 Optimizing Au-Au Diffusion Bond. .. ..... ..... ... 50 3.1.2 Au ...................... ....... ........ 56 3.1.3 Conductive Tape ....... ..... ..... .... 56 3.1.4 Au-Sn Bond ......... .. ..... ..... ... 57 3.1.5 Conductive Epoxy ...... .... ..... ..... 57 3.1.6 Summary ............ ..... ..... .... 58 3.2 Contact Resistance Measurement ..... .......... 58 3.2.1 Impedance Spectroscopy ......... ...... 59 3.2.2 Results and Discussion . .. .............. 60 3.3 Corrosion Resistance of Electrodes ... .... ... ... .. 62 3.4 Summary .. ..... .... ... .... ... .... .... .. 63 4 Characterization of CNT Electrodes in NaCl Solutions 64 4.1 Experiment: Capacitance of CNT electrodes .. .... .... 64 4.1.1 Experimental Setup .... .... ..... .... .... 65 4.1.2 Results and Discussion .. .... ... .... .. ..... 66 4.2 Role of Counter Electrode in Setup . ... .... ... ..... 69 4.3 Charging Dynamics of Electrodes ... .... ... ... ..... 71 4.3.1 Experimental Setup . ... .. ... ... ... .. ..... 71 4.3.2 Results and Discussion ..... .... .... .. .. ... 71 4.4 Conclusions .. .... .... .... ... .... .... .... 75 5 Conclusions and Future Work 77 5.1 Future Work . .. .. .. .. .. 78 5.1.1 Increase Capacitance . 78 5.1.2 Further Parameterization . 78 5.1.3 Flow Cell .. .. ... .. 79 5 A CNT Synthesis 86 A.1 System Setup ........ ............................... 86 A.2 Safety ............................................ 87 A.3 Sample Preparation ....... ............................ 87 A.4 Tube Preparation ........ ............. ........ 87 A .4.1 Growth ........ ............ .......... 88 A.5 Shutting Down .. ............ ............. ... 90 6 List of Figures 1-1 Multi-Stage flash system. Seawater enters low pressure chambers, is flashed and the vapor is collected to produce freshwater. Adapted from [1]. ..... ....................... ......... 15 1-2 Reverse osmosis system. High pressure water flows across a semi- permeable membrane rejecting salt ions. Adapted from [1]. ..... 16 1-3 Electrodialysis. An electric field is applied across ion-selective mem- branes, seperating anion and cations from freshwater. Adapted from [1]. ........................ ............. 17 1-4 Capacitive deionization process. a) Water enters cell, b) Potential is applied, ions adsorbed, desalinated water purged through, c) Voltage is removed and cycle is refreshed. ........... ......... 18 1-5 Test prototype developed by Welgemoed and Schutte [2]. This is 1 / 4 0 th the size of an actual stack. Scale bar: 300 mm. ............. 20 1-6 Ion confined in carbon structure (blue). Anion (red) density is higher at positively charged surface. Figure adapted from [3]. ........ 22 1-7 Carbon materials used in CDI. a) carbon nanotubes (inset: TEM im- age, scale bar: 100 nm), b) activated carbon cloth (inset: TEM image, scale bar: 250 nm), c) carbon aerogel matrix, d) ordered mesoporous carbons (inset: TEM, scale bar: 100 nm). Figure adapted from [1]. 23 1-8 Ion distribution at a charged surface. 0 0 = 10 mV, c, = 10 mM .. 28 1-9 The double layer is divided into 3 parts: the Stern (or compact) layer, the diffus layer described by Gouy-Chapman, and the bulk solution. 29 7 1-10 The diffuse layer charge given by Poisson-Boltzmann (PB), modified PB (mPB) and compact double layer theory (CDL). mPB and CDL show that charge asymptotes at higher potentials, rather than infinitely increasing as with PB. Adapted from [4]. .......... ..... 32 1-11 The diffuse layer charge given by the modified Poisson-Boltzmann (MPB), over time. Variables: e = , = 2a'c_,. As double layer thickness and/or steric effects increase, the weakly nonlinear approxi- mation no longer holds. Adapted from [5]. ........... .... 33 2-1 Parameters for electrode material. The electrode should have a tunable height, H and diameter, D. It should also be highly conductive. 36 2-2 CVD growth of VACNTs. a) Growth substrate is prepared with alum- nia and iron, b) substrate is placed in furnace in He/H 2 environment at elevated temperature, c) VACNTs grown on catalyst with ethylene. 38 2-3 Characterization of CNTs. (a) TEM of CVD grown VACNT, Scale bar: 5 nm. (b) Cumulative distribution of CNT wall number. (c) Cumulative distribution of the CNT inner (circle) and outer (square) diameters. (d) SEM of CNT carpet. Figure adapted from [6]. .... 39 2-4 Pourbaix diagram of iron in water. In CDI operating conditions, with applied electrode potentials 0-1 V vs. SHE and solution pH between 5-7, iron corrodes forming rust, Fe2 0 3 . Figure from [7] ........ 40 2-5 Gold-gold diffusion bond process. a) Process steps for transfer of
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