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
CNTs, b) Temperature profile in furnace, c) CNT electrode on gold
substrate. Figure from [6]...... 42
2-6 Three electrode cell set up. a) Beaker schematic, b) potentiostat
schematic adapted from [8]. The potential, VWE is monitored be-
tween the reference (RE) and working electrode (WE), while current
flows between the working and counter electrode (CE). WE-CNTs, RE-Ag/AgCl, CE-Pt mesh...... 43
8 2-7 Potential profile across an electrochemical cell. The potential drops at
the EDL of the working electrode, measured by the potential between
the reference and working electrode. There is negligible potential drop
in the solution, and then an additional potential drop at the EDL of
the counter electrode...... 44
2-8 CV scan schematic of electrode materials ...... 45
2-9 Potentiostatic testing: current response to a held potential value .. . 46
2-10 CV scan of CNT carpet, height 21 Mm. Electrode, tested in varying
solutions of NaCl, did not exhibit Faradaic peaks. v = 30 mV/s. .. . 47
2-11 Potentiostatically-measured capacitance normalized by electrode plane area for (a) 20 mM, (b) 50 mM, (c) 70 mM and (d) 90 mM. The red
circle, green triangle and blue square symbols correspond to step po-
tentials of 100 mV, 200 mV and 300 mV above the OCV, respectively. Figure from [6]...... 48
2-12 FIB cross section of transferred CNT carpet. The clean, tubular ge-
ometry of the CNTs is not maintained during the diffusion-bonding
process...... 49
3-1 Critical force to induce first-order buckling of column ...... 51
3-2 CNT slipping. a) Yap et al. [9] showed the deformation of CNTs under
uniaxial compression. b) Our SEM of transferred CNTs exhibiting
buckling similar to step 6...... 52
3-3 CNT carpet does not show buckling under small applied torque . . . 52
3-4 CNT transfer at various temperatures. There is 100% transfer at 540*C
and 270 'C. Carpet looks more aligned at 270 *C. 30 N-cm torque was applied...... 53
3-5 The bending of the plate under a load from the bolt. The 1/8" plates
bend upto 50 tim, damaging the carpet. Doubling the plate thickness
reduces the deflection, 6, to less than 1 micron...... 54
9 3-6 Successful transfer of CNTs using gold diffusion bond. T=270*C, torque=30 N-cm...... 55 3-7 Gold-gold bond. High temperature melted the gold, forming a weak bond. Scale bar: 1cm ...... 56 3-8 Delaminated carpet taped with copper tape to substrate. Scale bar: 1cm 56 3-9 AuSn bonding process. Similar to Au-Au bonding, except with an additional piece of tin foil ...... 57 3-10 Epoxying CNT carpet to substrate. Nickel and Silver epoxies were studied...... 57 3-11 Transfer methods that were successful: gold diffusion bonding, epoxy methods, and gold-tin bonding ...... 58 3-12 RC circuit model of a 3-electrode cell. a) Circuit model, b) Impedance plot for circuit ...... 59 3-13 Impedance testing results. Model fitting is shown with lines...... 60 3-14 Contact resistance for bonding methods. Error bars depict the varia- tion in calculation from the model fitting...... 61 3-15 CV scan for different bonding methods. 20 mM NaCl, sweep rate: 15 m V /s...... 62
4-1 Specific charge of CNT electrodes. a) CNT height 5 pm, b) CNT height 22 pm, c) CNT height 52 pm...... 65 4-2 Specific charge as a function of height in 5 and 20 mM NaCl solutions. Electrode applied potential was 300 mV. Error bars arise from variation in the calculation of charge...... 67 4-3 Fitting GCS model to charge data. a) GCS fit with am = 540 m 2 /g, b) GCS fit with am = 320 m 2 /g. CNT heights: El = 5, Q = 22, and A = 52 ym. Specific capacitance is 7-8 F/g and CST = 3-6 /LF/cm 2 . . 6 8 4-4 CV scans of various counter electrodes in 20 mM NaCl. Counter elec- trodes: a) Y-Carbon cloth, b) Pt mesh, c) CNT, d) Pt foil. WE: CNT electrode and RE: Ag/AgCl electrode...... 70
10 4-5 Current response due to a potential step VWE = 250 mV in 125 mM
NaCl solution ...... 72
4-6 Mean capacitance of CNT electrodes. Samples were tested in NaCl
solution...... 73
4-7 Charging time constant for CNTs for varying a) carpet heights and b)
NaCl solution concentrations...... 74
4-8 Optimizing capacitance and charging time constant. a) Capacitance
per planar electrode area divided by time constant, b) Capacitance
per mass divided by time constant. Data collected in 125 mM NaCl
solution. ACF data from [64]...... 75
A-1 Set up of CNT samples in double growth furnace ...... 89
11 List of Tables
2.1 CVD growth parameters for chapter 2 experiments ...... 38
3.1 Appled torque on jig translated to axial force ...... 51
3.2 Experiment summary for transfer methods ...... 63
12 Chapter 1
Introduction
Over 1.2 billion people lack access to potable freshwater, and an additional 2.6 billion people have little or no sanitation for their water [10]. Twenty-six countries do not have an adequate supply of safe drinking water for its people [1]. Industrial over- consumption of water is leading to overdrawn and contaminated aquifers, creating saltwater intrusion into the already limited potable water supply. It is predicted that in the next 30 years as global climates warm, glaciers in the Himalayas will recede, severely limiting access to drinking water for an additional 1.5 billion people in In- dia, China, and Southeast Asia [10]. Domestically, while the US has had a steady consumption of water in recent decades, a growing concern for droughts and urban population growth has resulted in efforts to increase our freshwater supply [11]. 98% of the world's water is in the form of brackish and seawater, defined as having 1,000-
35,000 ppm salt (concentrations of 10-600 mM NaCl) respectively. Desalination is one method to increase our fresh water supply, generate access for all and develop water security.
In this chapter, we outline existing desalination technologies, assessing their ben- efits and shortcomings for energy-efficient and clean desalination. We consider capac- itive deionization, a newer technique for desalination, reviewing the research that has been done and the opportunities to develop and optimize this technology.
13 1.1 Motivation
Desalination generates over 37 million cubic meters of potable water per day world- wide [11]. In order to serve populations that need potable water, its important to keep the costs of water desalination low. This can be achieved by minimizing the en- ergy consumed and the pretreatment required for production of drinking water. The minimum energy required to desalinate arises from reversing the Gibbs free energy of mixing salt and water, which is approximately 1.1 kWh/m 3 for seawater and 0.1 kWh/M 3 for brackish water. If the efficiency of a theoretical system (actual energy consumed compared to the minimum) is even 50%, the system will consume 2.2 kWh for every m 3 seawater desalinated. Assuming a per capita consumption of water is 50 liters, then the per capita energy consumption is only 0.25 kWh. This is only a small fraction of energy consumption in many countries (3.2 kWh per capita in China and
30 kWh per capita in the US) [10]. In addition to removing salt, desalination sys- tems must remove metals, bacteria, and other pollutants found in seawater. This can be done either through the desalination process or through additional pretreatment, which can generate pollution or reduce the efficiency of the plant.
There are three main approaches to desalination technology: phase change, seper- ation mechanisms, and charge-based desalination [1]. Within these approaches, the most widely used technologies are multi-stage flash, reverse osmosis, and electrodial- ysis.
1.1.1 Multi-Stage Flash Desalination
Multi-stage flash (MSF) is a phase change method for desalination, comprising 44% of the world's desalination capacity. In MSF systems, seawater enters low pressure
chambers, where it is flashed to steam and the vapor is collected seperate from the brine, as shown in Figure 1-1. Pretreatment of water for this type of system is minimal, because the vapor is seperated from the contents in the brine, but this process is very inefficient, consuming 23.9-96 kWh/m 3 [1].
14 Multi Stage Flash Steam inSewtrI
Distill te out
Figure 1-1: Multi-Stage flash system. Seawater enters low pressure chambers, is flashed and the vapor is collected to produce freshwater. Adapted from [1].
1.1.2 Reverse Osmosis
A less energy intensive method for desalination is reverse osmosis (RO), making up
41% of the world's desalination capacity, but 80% of the number of installed plants.
RO is a seperation method, where high pressure water flows across a semi-permeable membrane which allows water to pass through but sterically excludes solvated salt ions, as shown in Figure 1-2. In RO, the energy consumed for desalination is pro- portional to the salt concentration, minimally requiring enough energy to reverse the
Gibbs energy of mixing. RO is much more efficient than MSF, consuming an average of 3.6-5.7 kWh/m 3 for brackish to seawater, respectively, operating at 20-30% effi- ciency [1]. The best reported RO membrane, desalinating seawater, consumes 1.58 kWh/M 3 with 42% efficiency [10]. However, RO has many challenges: there is bio- fouling and scaling of the membranes, flow rates are limited due to concentration polarization at membranes, and there are challenges with the membrane in rejecting salt ions at high feed concentrations [10, 1, 12]. In order to prevent fouling of mem- branes, the feedwater undergoes intense pretreatment which can be energy-consuming and additionally contaminate water [101.
15 Reverse Osmosms Seawa Il H ~
sh water out """"""""""Sri
Figure 1-2: Reverse osmosis system. High pressure water flows across a semi- permeable membrane rejecting salt ions. Adapted from [1].
1.1.3 Electrodialysis
Desalination can be challenging because its very process and energy intensive. Charge- based methods for desalination could reduce some of these demands, requiring little pretreatment. Currently, charge-based methods such as electrodialysis comprise 6.1% of the world's desalination capacity. Electrodialysis is a method where electric fields are applied across membranes, driving salt ions to migrate across ion-exchange mem- branes, and desalinating the bulk stream, see Figure 1-8. This method reduces effects from scaling and concentration polarization through the applied electric field and mixing [1]. However, pretreatment is required to prevent fouling of the membranes.
1.1.4 Capacitive Deionization
Capacitive deionization (CDI), as shown in 1-5 is an emerging charge-based desalina- tion method. In CDI, a potential is applied across high-surface area electrodes. This polarizes the electrode, causing salt ions to adsorb on the surface, and the charge is stored in the electric double layer (EDL). The electric double layer will be de-
16 Feed Water Cation selective j Anion selective membrane membrane
=-P'Fresh Water
+Concentrate
Figure 1-3: Electrodialysis. An electric field is applied across ion-selective membranes, seperating anion and cations from freshwater. Adapted from [1]. scribe in detail in section 1.4. The bulk, desalinated water is purged through, and the potential is removed regenerating the cycle. Generally, the flow of water is past these porous electrodes, though research has also examined the development of flow- through electrodes [13] or by dipping/removing anode-cathode wires into a stagnant solution [14]. The main difference between electrodialysis and CDI is that the former is a flow- through continuous process whereas the latter is a charge-discharge process. CDI is advantageous over electrodialysis because it does not require membranes and can remove charged molecules of higher molecular weights, such as bacteria. This can significantly reduce the amount of pretreatment necessary. Because it requires lower pressure than RO, CDI can be more efficient (reported efficiencies are as high as 75- 90% [15]) in brackish water regimes and more portable. The potential required for operation is minimal, usually less than 1.23 V to prevent electrolysis. It has been demonstrated that this energy could be provided sustainably by microbes, similar in concept to a microbial fuel cell [16].
17 a) b) re------C)
Figure 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.
In order to optimize CDI, there are many areas of investigation required: an ap- propriate electrode material needs to be designed for the process, the cell needs to be designed for continuous flow processes, and the design for energy recovery needs to be developed [17]. This thesis focuses on the design and optimization of appropri- ate electrode materials for CDI, focusing on the influence of electrode thickness on charging time scales. We will first examine the extensive work done to develop CDI, both in the literature and developing theory.
1.2 Background
1.2.1 History
The study of capacitive deionization (CDI) began in the early 1960s and 70s [18, 19, 20]. In 1961, Arnold and Murphy investigated demineralization, showing that graphite electrodes could remove salt from water through the adsorption of ions onto the electrode surface [18]. Caudle et al. studied demineralization using carbon elec- trodes finding that functionalization of the carbon surface with oxygen can make it more cation and anion responsive [19]. They conducted the first major series of bench- scale testing, finding that in low solution concentrations, NaCl and metals such as Fe (II) and Mn (II) typically found in brackish water, could be removed from feedwater and partially regenerated to prevent electrode fouling [19]. In 1971, Johnson and Newman studied porous carbon electrodes, determining
18 general criteria for CDI systems: the adsorption rate of ions should be fast, the ohmic drop in the material should be minimal, and the applied potential should be large enough for the carbon to behave as an ion exchanger [20]. The experimental work of Johnson and Newman was discontinued due to instablities in the anode [17].
However, they calculated that it would be possible to create low-cost desalination devices, with high surface area materials, estimating that materials with surface areas of 230 m 2/cm 3 would be sufficient [20].
In the 1980s, Oren conducted extensive studies of water desalting, using the same principles, under the name of electrochemical parametric pumping. Through his system, Oren et al. studied the influence of potential, electrolyte concentration, and the charging mechanisms in a carbon material with specific surface area of 100 m 2 /g [21, 22, 23, 24}. Oren also demonstrated that these electrodes could remove bacteria through electroadsorption [25]. E. Coli has a negatively charged surface membrane, which is adsorbed onto polarized electrodes. The bacteria can also be
desorbed during discharge, reducing fouling of the electrode [25]. These endeavors
showed that CDI could desalinate brackish water without the need for additional
pretreatment. However, the research was abandoned due to degradation of electrode
performance during experimentation.
As high surface area carbon materials were developed in the 1990s, scientists
renewed interest in CDI. At Lawerence Livermore National Laboratory (LLNL), re-
searchers built a prototypical CDI stack using carbon aerogel as the electrode material
[26, 27, 28, 29, 30]. Carbon aerogels do not require a binder, minimizing resistance in
the material. In addition, aerogels have between 400 and 1100 m 2 /g specific surface
area, promising for full-scale CDI. Richardson et al. estimated, based on labora-
tory experiment, that desalinating 1000ppm solution with carbon aerogel would only consume 0.02 kWh//m 3 [27].
Welgemoed and Schutte, developed a 1/40th size prototype of a CDI system [2].
At a flow rate of 50 mL/min, they were able to desalinate a feedstream from 1000
pS/cm to 23.4 pS/cm. The energy cost calculated at 0.6 kWh/M 3 is much higher
than what Richardson predicted [2, 27], due to inefficiencies which may be overcome
19 Figure 1-5: Test prototype developed by Welgemoed and Schutte [2]. This is 1 / 4 0th the size of an actual stack. Scale bar: 300 mm. through energy recovery.
In the past 40 years, it has been demonstrated that CDI can be effective for water desalination. The technology can remove salt, metals, and bacteria. At present, there are a few companies around the world using CDI in industry, with Voltea in the
Netherlands the most developed. Materials and design optimization has limited the development of CDI as a more widely used desalination technology. Understanding ion transport and adsorption on high surface area carbon electrodes is instrumental to the optimization of CDI.
1.2.2 Carbon Materials in Supercapacitors
The goal in CDI design is to maximize ion removal from the bulk of the solution. The goal in supercapacitor design is to increase electrical energy storage. These goals are both achieved by increasing the capacity of the double layer. Thus, electrode design in CDI and supercapacitors are similar. As a result, much of the work on material development and modeling for supercapacitors can inform design for CDI.
An electrode that has a capacitance of only 10 pF/cm2 at an applied potential
20 of 1V, would have an excess charge at the surface of 10 pC/cm2 . In terms of ions stored at the surface, this would be equivalent to 1.036 x 1010 ions/cm 2 . A material that has a specific surface area of 1000 m 2 /g, or a specific capacitance of 10 F/g, would only require 50 g of material to desalinate 1 L of 3000 ppm saline water [17].
This suggests that increasing capacitance of the material and its specific surface area available for ion adsorption, can maximize the salt removal per unit volume of a CDI cell. Extensive research on supercapacitors have studied the enhancement of these material properties.
In the late 1980s, carbon materials were documented as having capacitances be- tween 2 to 60 pIF/cm 2 [31], or for materials with specific surface areas of 100 m2 /g that is 2-60 F/g. These materials were tested in a variety of electrolytes, with a variety of testing methods (for more information on testing methods see section 2.3).
This variation of testing conditions can lead to a large variation of measured capaci- tance. In recent years, capacities of carbon materials have been documented as high as 100-200 F/g in organic electrolytes and upto 300 F/g in aqueous solutions [32].
Increasing the specific surface area, tuning the porous structure, surface treatment, and functionalization can enhance the capacity of carbon materials tremendously [32].
Adsorption of ions onto electrode materials can be specific and non-specific. Surfaces functionalized with oxygen groups provide sites for protons to ionically bond, leading to higher material capacitance but less cyclability [31].
Pore size can have dramatic effect on capacity. Salt dissolves in water when H2 0 molecules are attracted to the charge ions and surrounding it, forming a solvation shell. The solvated salt ion typically has diameters of 7-8 A. It has been previously found that 2-5 nm pores (mesopores) are optimal for using the available surface area of the electrode material [32]. However, sub-nm pores still can contribute to capaci- tance, suggesting that partial ion desolvation can occur [32]. Recent MD simulations show that this desolvation is possible in confined pores, due to charge compensation provided by the electrode surface [3], shown in Figure 1-6.
The chemical treatment of the surface can enhance capacitance substantially. Car- bon materials are typically hydrophobic; functionalization can increase wetting for
21 Figure 1-6: Ion confined in carbon structure (blue). Anion (red) density is higher at positively charged surface. Figure adapted from [3]. better capacity. Oxidation can enhance surface area and wettability, therefore enhanc- ing specific capacitance [33]. In the case of carbon nanotubes, nitric acid treatment or fluorine heat trematment can add functional groups enhancing capacitance seven- fold [33], due to higher specific adsorption and reduced hydrophobicity. In addition, creating carbon-oxide composites with MnO 2 and RuO 2, utilizes pseudo-capacitance to store charge in the double layer Faradaically. These composites yield specific ca- pacitances of 150 F/g and higher [32, 33], typically 10-50 times the material's original capacity [34]. Material developments in supercapacitor research can be directly adapted in CDI research. Increasing surface area, tuning pore size, and functionalizing carbon ma- terials can enhance capacitance and ultimately salt removal. These insights from supercapacitor work can inform the development of high performing CDI materials.
1.3 Carbon Materials for CDI
Studies over the past 20 years have examined a variety of carbon materials for CDI.
Through increasing surface area, tuning pore size, and functionalizing the surface, researchers have strived to find the best electrode material for CDI. Materials inves- tigated have included carbon aerogels, cloths, nanotubes, mesoporous and carbide-
22 2Um b
Figure 1-7: Carbon materials used in CDI. a) carbon nanotubes (inset: TEM image, 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]. derived materials. These carbonaceous materials have been shown to have typical double layer capacities between 10 - 35 pF/cm2 [35], which for high surface area materials (for example 500 m2/g) can lead to specific capacitances of 50-175 F/g. Increasing the electrical capacitance also increases its salt adsorption. Here we will discuss the research of materials used for CDI systems. Carbon aerogel (CA), shown in Figure 1-7 is a three-dimensional structure, typi- cally synthesized using a sol-gel process, transforming precurors such as resorcinol and formaldehyde into a highly porous network [36]. As Farmer found, CAs have high sur- face area (400-1100 m2/g), good structural strength, and high conductivity due to its interconnectivity. Aerogels typically have mesopores (3-30 nm) and interstitial pores.
Many researchers have continued to study CA for CDI [37, 13, 38, 39, 40, 41, 42]. Recent studies have shown capacitances of 130 F/g in 250 mM NaCl solution [13]. However, the large number of sub-nm pores, which are not necessarily optimal for ion adsorption, have driven researchers to investigate carbon materials with more
23 mesoporous features.
Activated carbon cloth (ACC), shown in Figure 1-7, is woven from activated carbon fibers that are synthesized from phenolic resin. Surface area can be as high as
2500 m 2 /g [43]. Surface treatment using acid etch, increases the hydroxyl, carboxyl, and carbonyl groups, leading to faster kinetics during desalination [44]. Incorporating titania into the material significantly enhances electrosorption which is reversible with polarity [45, 46]. Carbon cloths have been cited to have capacities ranging from 20 F/g [22] to 100 F/g [35].
Carbon nanotubes (CNTs), shown in Figure 1-7, are grown in a chemical vapor deposition process, typically having pores only in the mesoporous range (2-50 nm).
While CNTs have low surface area (1000 m 2/g), the mesopores suggest that the surface area is available for ion adsorption. CNTs have been studied in many electrode synthesis forms: in composites with nanofibers [47, 48], with binders [49, 50], and with ion-selective membranes [51]. While, CNTs have a lower surface area than ACC
(0.08 vs 0.5 M 2 ), Sun et al. showed that their CNT-CNF composite had the same performance. Yang et al. showed that CNTs have specific capacities from 100 F/g
(untreated surface) upto 140 F/g following surface functionalization [51]. CNTs could be promising for a material with appropriate pore structure for CDI.
Ordered mesoporous carbons (OMCs), showns in Figure 1-7 are synthesized to have high surface area (1500 m2 /g) and mesopores from 3-25 nm [1]. Li et al., showed that OMCs have upto double the specific capacitance than that of ACCs, with upto
180 F/g capacitance in 100 mM NaCl solution [52].
In contrast to OMCs, carbide-derived carbons (CDCs) have been studied to un- derstand the effect of sub-nm pores on capacitance [53]. Surprisingly, CDCs also have higher salt adsorption than ACCs, suggesting that sub-nm pores contribue substan- tially to ion storage.
Material geometry, surface properties, and the CDI operating parameters play a strong role in ion adsorption. While research has focused on optimizing surface area, it has become clear that the porosity and chemistry of the surface influence salt removal. In order to understand the physics of the charge storage in the double layer,
24 we turn to the theory for more insight.
1.4 Electric Double Layer Theory
In CDI, salt removal occurs through the storage of ions in the electric double layer (EDL). The physics of the EDL can provide insight into the relevant material pa- rameters for desalination. The theory provides insight into how much salt can be removed during a CDI cycle. In this section, we study the basic Gouy-Chapman theory [54], its limitations, and work that has extended the model. Finally, we look at implications of the theory on electrode design. When a potential is applied at an electrode surface, 0o, charge builds up at the surface. At the electrode-solution interface, at x = 0, counterions from the solutions build up at the surface to balance the excess electrical charge of the electrode. Moving away from the surface, as x -+ o, this ion concentration decreases, returning to bulk concentration, co,. This is the formation of the double layer, depicted in Figure 1-9. The distribution of ions at the surface can be characterized by considering the Gibbs free energy of the solution:
dG = -sdT +vdP + pjdNj + E dqi (1.1)
where G is the Gibbs free energy, s is enthalpy, T is temperature, v is volume, P is pressure, p is chemical potential, Nj is the number of particles of type j, V) is potential, and q is charge. The electrical charge can also be equated in terms of the ion concentration, qj = zeNi, where z is the ion valence and e is the elementary charge. In a solution at equilibrium, the Gibbs free energy should be constant. Assuming no change in temperature and pressure, we find that equation 1.1 can be simplified to a sum of the electrochemical potential:
0 = E (pi + zei)dN (1.2)
For dilute, ideal solutions the chemical potential is a function of concentration, given
25 from chemistry as:
p (x = + kT In (c(x)) (1.3) where 1L is the reference chemical potential and k is the Boltzmann constant. The chemical potential is a function of x because the ion concentration is varying in x.
At equilibrium, combining equations 1.2 and 1.3, for equilibrium at two points in solutions, x 1 and X2 :
W + kTln(c(xi)) + zey(xi) = +± kTln(c(x 2 )) + zeP(x2 ) (1.4)
Simplifying, we arrive at the Nernst equation:
n C(2) -ze [(V)(x2) - (Xi)]( Lc(x1)J kT and the Boltzmann distribution:
C(X2) = c(xi)exp {Tze[(X 2 ) - ?P(Xl)} (1.6)
In order to relate charge and potential we use Poisson's equation. Derived from
Gauss' law, Poisson's equation gives:
d 2Pe (1.7) dx2 E where charge density pe = ciziF and e is the permittivity. The boundary conditions for this system are: (x- 00) = 0 (1.8)
P(X = 0) = 00 (1.9)
Summing from the surface of the electrode to the bulk of the solution (x = 0 to x -+ oo), and combining equations 1.7 and 1.6, we arrive at the Poisson-Boltzmann equation. For an ion solution where the valency is 1:1, such as sodium chloride, the
26 Poisson-Boltzmann equation is:
d2p Fcooz . zep d 2 = sinh( Ze) (1.10) dx2 6 kT
The above analysis was conducted independently derived by Gouy and Chapman, and comprises the Gouy-Chapman (GC) theory. To calculate the surface charge density, we integrate equation 1.10
if d2i4 0Pedx = -el dx (1.11)
While the Poisson-Boltzmann can be solved numerically, we can solve it analytically in the low potential limit, when the surface potential is much smaller than the thermal voltage b0 < 0T = - = 25 mV. The Poisson-Boltzmann can be linearized: e
d2 4b _?$ -x = dx2 A2 (1.12) where AD is the thickness of the double layer, or the distance over which the surface charge is screened. In an electrolyte, the Debye length is given by:
kTe AD VzeFc,,,,f ke-TE (1.13) or the thickness of the double layer. Integrating this, we find the surface charge density is: (1.14) AD
Figure 1-8 depicts the ion distribution at a polarized surface. At a surface where the applied potential is 10 mV we see larger counterion concentration at the surface, but co-ions also remain. The power required to adsorb counterions is also used to repel co-ions. Finally, we observe that the double layer thickness, or the region where there is an excess of ions compared to the bulk, has a physical thickness of about 10 nm. The Debye length varies from 1-10+ nm depending on the concentration of solution. For porous materials, where pore diameters are smaller than this length,
27 Ion Distribution 16 Na+ M14 cr
~12 E
0
0 5 10 15 Distance x (nm)
Figure 1-8: Ion distribution at a charged surface. 5o = 10 mV, c, = 10 mM there may be overlapping double layers which limit salt adsorption for the measured surface area. The Gouy-Chapman theory makes many assumptions. One assumption is that ions are infinitely small. At larger applied voltages k0 > K, an absurdly large number of ions can pack onto the surface, which is not physically possible due to the molecular size of ions. In order to correct for this, Stern introduced a compact layer, as shown in Figure 1-9. Stern's modification gives a compact layer, where there is a linear drop in potential and specific adsorption of ions on the surface, and a diffuse layer, which follows Gouy-Chapman theory. There are two potential drops in the double layer:
AV = A*D ± A+s8 (1.15)
The potential drop in the Stern layer is given by Gauss' law:
Aot =F (1.16) Ct where or = 4ADco sinh ( zke)(1.17)
28 A4'ST L4D +
vwI~
+ + q *+iX +
U-
+ tM47; + IkAI
I + I II ++
r- 'it - '2
Stern Diffuse Bulk
Figure 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 which we get by integrating 1.11 over the diffuse layer.
While the Gouy-Chapman-Stern model makes some improvement over the Gouy-
Chapman theory, the assumptions that hold in dilute solutions at low potentials break down at higher voltages. The GCS model gives some insight into the physics of the electric double layer, for example its thickness and capacity at an electrode surface for a given potential. However, there are more effects that need to be considered to apply this model to analyzing salt removal in CDI.
The salt stored on the electrode surface was derived by Bazant's group, using
GCS theory and mass conservation [55]. By integrating the concentration of ions stored in the double layer, as the double layer forms over time, they arrive at the salt adsorption density, w [56]:
W = 8ADco sinh 2 A D (1.18)
This accounts for the effects of ions diffusion from the neutral bulk solution to the excess charge in the EDL. The efficiency, or the amount of salt ions stored compared to the electrical charge density is then just [56, 571:
WAPD a= = tanh (1.19) 0- 4
This efficiency shows that for increasing salt concentration solutions, where the po-
tential drop in the diffuse layer decreases, the efficiency of the system also decreases.
This leads the CDI cell to become very inefficient at large bulk solution concentra-
tions. Thus CDI is considered efficient in the brackish water regime. Charge efficiency
can also be used to study the dynamics of adsorption and desorption of ions in CDI
[57], as well as determine the specific area of a material available for salt storage [56].
The Gouy-Chapman-Stern model is a dilute solution theory, which has a very
limited account for steric effects. The Boltzmann distribution is used to describe
ion concentration in solution as logarithmic; as potential exceeds the thermal voltage
there are non-linear effects which break the assumption that the solution is dilute at
the surface. Consider this: the maximum concentration of ions at a highly charged
30 surface is cmax = a- 3 where a is the molecular spacing [4].Using the Nernst equation, eqn 1.5, with x 1 = 0 and x 2 = oo, the maximum potential at the surface is given as [4]: kT i~)=kT Cma C ln(a3coo) = - ln(max) (1.20) ze ze coo
For a bulk solution concentration of 10 mM, spacing of 3 A, and z = 1, the maximum potential is only 330 mV [4]. In fact, the solution is no longer dilute beyond surface potentials between 25-200 mV. In CDI, voltages are typically applied between 500-
1000 mV across the electrode surface. The basic double layer theory does not remain valid in this regime; newer models and numerical tools are necessary to characterize the double layer.
In addition, the Dukhin number which determines the relative importance of con- ductivity in the diffuse layer compared to the bulk, breaks down at larger voltages.
The Dukhin number, assuming Gouy-Chapman theory, has the form:
Du - = AD sinh2(zeVD (1.21) aL 4 L 4kT
A large Dukhin number where the surface conductivity is higher than the bulk, but with thin double layers (AD < L), requires that the potential V)D be much larger than the maximum potential possible. The assumptions of the GCS do not hold for this thin double layer assumption [4].
Recent modeling efforts have tried to account for steric effects [4, 5]. Bazant's group derived a formula for the compact double layer based on steric effects at the surface [4]:
q = -sgn(VD)2zecOOAD In [I + 2v sinh ( 2kT (1.22) where v = 2a3 coo is the volumetric parameter. Figure 1-10 shows how their compact double layer accounts for charge build up at the surface for a given applied potential.
Unlike the Poisson-Boltzmann, this model asymptotes to limited charge build up at the surface. They also show that when steric effects are large the weakly non-linear approximations made from the Nernst-Planck equations do not hold, as shown in
31 04 mPB .. .,CDL 10 '/ V= 0.00005
162 v = 0.005
10 V= 0.5 -0
10
101 0 10 20 30 40 ze I'DI / kT
Figure 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].
Figure 1-11 [5]. The model begins to account for steric effects, but is only valid for flat parallel plate electrodes, not accounting for the complex geometry of porous electrodes.
Biesheuvel and Bazant [581 developed models to describe the nonlinear dynamics of desalination in porous electrodes. Due to the complexity of porous geometry, they simplified their model by using the Gouy-Chapman-Stern model, and neglecting steric effects. Typically, the double layer model assumes that the bulk concentration stays constant. However, in capacitive deionization, there is a large depletion of ions from the solution. By studying the dynamics in pores, and assuming that there is a mass-transfer layer where ions are transported from solution into porous electrodes, Biesheuvel and Bazant find that there are two time scales for charging electrodes: a supercapacitor, short charging-time regime, TC, and desalination, a long charging-time scale rD:
rc - 2 (1.23) De hp
32 (II) 4
3 0.4 - *2 S - 0 .1 0* , 'u=10 0.2 0.1
0 0 0 10 20 30 0 10 20 30 (a) t (b) t 0.06 0.4 e-0.01,- 3 0.3 0.04 MPB 0.2 e = 0.01 0.02 - Weakly Nonlinear 0.1 V-10 0 0 0 10 20 30 0 10 20 30 (C) t (d) t
Figure 1-11: The diffuse layer charge given by the modified Poisson-Boltzmann (MPB), over time. Variables: e = ', v = 2a0cO. As double layer thickness and/or steric effects increase, the weakly nonlinear approximation no longer holds. Adapted from [5].
L 2 TD e (1.24) De where hp is the ratio of pore volume to pore area, De is the diffusivity of the ions in bulk solution, and Le is the pore thickness. In the desalination regime, the ions stored in pores are largely comprised of counterions, and there is almost complete co-ion depeltion leading to desalination [58]. This model can inform the selection of porous materials, though they do make the assumption that the double layer thickness is smaller than the pore diameters.
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- nm diameters. In this thesis, we aim to investigate porous materials and identify appropriate double layer models to describe the salt adsorption.
33 1.5 Thesis Outline
CDI is a promising, portable desalination technology, especially for demineralization of brackish water. Thus far, development has focused on synthesizing high-surface
area materials and modifying the surface properties to maximize ion storage and re-
moval from the bulk. However, the physics of double layer charging can limit the
charge storage: overlapping double layers; steric effects due to inaccessible pores, high solution concentrations, large applied voltages, and charge inefficiencies as so-
lution concentration increases. In order to develop parameters for the optimal CDI
electrode, we strive to use a simple, characterizable geometry, to experimentally in-
vestigate capacitance and charging/discharging dynamics of an electrode material. Here, we study vertically aligned carbon nanotubes that have mesoporous diameters
and interspacing, and are ideal to study the effects of pore lengths and solution con-
centrations on capacitance. The goal of this work is to describe the experimental
results with an appropriate double layer model.
In chapter 2, we discuss the synthesis and characterization of vertically aligned
carbon nanotube electrodes for water desalination. We grow multi-walled carbon
nanotubes (CNTs) using chemical vapor deposition. Electrodes are synthesized using
a gold-gold diffusion bond method to transfer the CNTs onto a conductive substrate.
Electrodes are characterized using electrochemical testing.
In chapter 3, we optimize electrode bonding methods for the material. We inves-
tigate the use of gold diffusion bonding, conductive epoxies, and gold-tin bonding to
transfer CNTs onto a current collector. The resistance and corrosion reactions are in-
vestigated using impedance spectroscopy and cyclic voltammetry. A low temperature gold-gold diffusion bond is best for CDI studies.
In chapter 4, we characterize the electrodes electrochemically. Potentiostatic test-
ing is used to determine the capacitance and double layer charging behavior of the
CNTs. The results are fitted to the Gouy-Chapman-Stern model. The charging ki-
netics are studies as a function of solution concentration and varying CNT height.
Comparing to a previous study, we find that CNTs are competitive with existing
34 high-surface area carbon materials. In chapter 5, we summarize conclusions and propose future work.
35 Chapter 2
Synthesis of Electrode and Preliminary Testing
This study seeks to investigate the physics of double-layer charging, in order to opti- mize CDI electrode materials. Ultimately, we are interested in determining how pore length and diameter affect the ion removal in CDI, and use that to inform models for double layer charging in porous electrode. In this study, we investigate vertically aligned carbon nanotubes (VACNTs) which have an ordered, easily-modeled geome- try; pores with diameters of 1-10+ nm; tunable heights between 1-1000 pm; and high conductivity. In addition, they have relatively high surface area of ~500 m 2 /g, which allows for measuring electrical response easily.
In this chapter, we cover the synthesis and characterization of VACNTs. We then
D
H
Figure 2-1: Parameters for electrode material. The electrode should have a tunable height, H and diameter, D. It should also be highly conductive.
36 use this material as an electrode, and conduct beaker testing to study its capacitive properties.
2.1 CNT growth and characterization
VACNTs were grown using a chemical vapor deposition process on a silicon wafer growth substrate, shown in Figure 2-2. Silicon wafers with 140 nm of SiO 2 were cleaned using 3:1 Piranha acid, rinsed with DI water, and spun dry. The cleaned wafers were prepared using electron-beam deposition, by sequentially depositing alu- mina (A120 3), the diffusion barrier, and iron, the growth catalyst. For the experiments conducted in this chapter, the alumina thickness was 20 nm and the iron was 5 nm. The wafer was cleaved into 1 cm x 1 cm pieces using a diamond scribe. 4-5 pieces were then loaded into a 1 inch tube furnace. In the furnace, the temperature is elevated between 650 *C and 750 'C. Hydrogen and helium flows over the substrate, removing oxides from the surface and forming catalyst sites. Once the furnace reaches the target growth temperature, the samples are annealed for 3-5 minutes. Growth begins when ethylene is introduced to the gas mixture, and carbon burns onto the catalyst sites forming nanotubes. In some cases, oxygen is also used to enhance the growth of CNTs. Following growth, the CNTs are cooled down for ~10 minutes in helium, and then removed from the furnace. Gas flow rates, growth temperature, and annealing times can all have an effect on the height and diameter of the CNTs. In this chapter, CNTs were grown using a single furnace set up. However, samples studied in subsequent chapters were grown using a double furnace set up. While the growth process remains the same, the additional furnace is used to preheat the flowing gases and to thermally crack the ethylene, breaking the carbon bonds, for high quality CNT growth. The CVD parameters for this experiment are given in Table 2.1 This CVD procedure was developed by Professor Carl's Thompson group at MIT. The detailed growth procedure is given in Appendix A. The CNTs were characterized using transmission electron microscopy (TEM)
37 a) Fe
A120 3 Si
b) -He
H2
T=650-750C
c) ====. C2H4 -= He
-= H2
T=650-7500C
Figure 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.
growth parameter value furnace temp 750 *C anneal time 3 min H2 flow rate 400 sccm He flow rate 100 sccm
C2H4 flow rate 200 sccm Table 2.1: CVD growth parameters for chapter 2 experiments
38 a ab I 0 0 0.8 0
0.6
0.4 0
0.2
1 2 3 4 5 6 7 walb c ' o dNo. 1 00
0.8- 0 0 0 0.6
0.4 0 0
0.2 0 0
0 0 0 5 10 Diameter (mn)
Figure 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].
(JEOL 2010 FEG Analytical Electron Microscope), Figure 2-3a. The CNTs are 2-3 walled, with an inner diameter of 5.6 ± 2 nm and outer diameter 7.7 +2 nm, shown in Figure 2-3b and c. The TEM characterization was conducted by Dr. Robert Mitchell. Carpet heights were measured using scanning electron microscopy (SEM) (FEI XL30 FEG ESEM and Zeiss Ultra55 systems). Brunauer Emmett Teller (BET) surface analyzer was used to characterize the specific surface area of the CNT carpets, which is approximately 540 m2/g. The VACNTs have been easily characterized using microscopy and materials anal- ysis methods. We have synthesized high-surface area, mesoporous, ordered carbon structures for the study. However, the iron catalyst that the CNTs are grown on is a challenge for the electrochemical testing. Studying the Pourbaix diagram given in Figure 2-4, we see that for a pH between 5 and 7 (typical pH of saline solutions), and
39 2.0-
Fe 4 1.2- Fe 3+ 0.8-
> 0.4- Fe26gn H20 WP 0 - Fe 2+
pH
Figure 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 03. Figure from [7] for typical operating potentials between 0-1 V with respect to the standard hydrogen electrode (SHE), the iron reacts with water producing rust. This Faradaic reaction creates two problems: 1) the capacitance measurements are adversely affected by on- going corrosion reactions which draw large currents, and 2) the catalyst is corroded away easily, leaving no binding material between the CNTs and the substrate. In order to avoid corrosion of the electrode material, the iron catalyst must be removed. We developed a transfer method using gold-gold self-diffusion bonding in order to re- move the CNTs from the growth substrate and bind it to a different, clean, conductive substrate.
2.2 Au-Au Self-Diffusion Bond
Gold-gold diffusion bonding was used to transfer CNTs from the growth substrate onto a non-corrosive current collector [6]. Gold-gold diffusion bonding, also referred to as thermocompression in the literature, has been used previously by several groups
40 using pressure and temperatures as low as 150 *C to bond CNTs to a substrate [59, 60, 61]. To transfer the CNTs, a 20 nm adhesion layer of Ti and a 200 nm of Au was ebeam deposited onto the VACNTs and the transfer substrate, a piece of titanium.
The gold interfaces on the CNTs and transfer substrate were then placed in contact, and pressure was applied using a stainless-steel pressure jig. The pressure jig was made up of 1/8" stainless steel plates, machined with holes for two 1/4-20" bolts in- house. The jig was tightened down with 2 bolts, with an applied torque of 120 N-cm.
The jig was placed in a hydrogen furnace, where the temperature was ramped to 540
0C at a ramp rate of 8 0 C/min. The temperature was held for 30 min. Following cooling, the jig was removed from the furnace, and the growth substrate containing the iron catalyst was lifted-off using a blade. This leaves behind uncapped, catalyst- free CNTs on a Au current collector. This procedure is depicted in Figure 2-5.
2.3 Electrochemical Characterization
Having synthesized carbon nanotube electrodes our goal is to study its electrochemical properties. In order to characterize the double layer charging of the electrodes, we need to ensure no Faradaic reactions are occuring. In addition, we want to study the capacitance of the electrodes, in order to understand the optimal electrode properties for CDI. Here we will discuss the experimental set up and testing methods in order to evaluate the electrode.
2.3.1 Cell Set Up
In this study, we used beaker testing to study the CNT electrodes. In a simple two- electrode cell, the sample being tested, the working electrode, is dipped in solution with a reference electrode which completes the cell. A potential is applied across this cell and the current is measured. However, in this system we only know the cell potential, not the potential drop across the electrode. The reference electrode must be capable of maintaining the applied potential and drawing the current required to
41 a b C A
Td 540 *C
Si CL) I CNT away I EU Au P_
...... - A120 3 + Fe t A 8 */IminTf125*'C T I TM T= 25 * IND ti Time t2
blade
Figure 2-5: Gold-gold diffusion bond process. a) Process steps for transfer of CNTs, b) Temperature profile in furnace, c) CNT electrode on gold substrate. Figure from [6]. a) b)
Potentiostat
WE CE
RE V AgAgCL VWE vs. ref
NaCI
Figure 2-6: Three electrode cell set up. a) Beaker schematic, b) potentiostat schematic adapted from [8]. The potential, VWE is monitored between the reference (RE) and working electrode (WE), while current flows between the working and counter elec- trode (CE). WE-CNTs, RE-Ag/AgCl, CE-Pt mesh. match the demand of the working electrode. If this is not achieved, then it is difficult to assess what the capacity of the electrode is and if Faradaic reactions are occurring.
In order to decouple the potential measurement and the current response, we use a three-electrode set up, as shown in Figure 2-6. Here the potential is measured between the surface of the electrode and the reference, which is maintained at a standard potential. The current is measured between the working and counter electrode. The expected voltage profile is shown in Figure 2-7 [8].
In these experiments we use a three-electrode with a Ag/AgCl reference electrode
(Beckman Coulter) and a platinum mesh as the counter (Sigma Aldrich). They are dipped with our working electrode in sodium chloride solutions of varying concentra- tions. These electrodes are connected through a potentiostat (Eco Chemie Autolab
PGSTAT 100), which can conduct experiments for studying the capacitance and corrosion-resistance of the electrode.
2.3.2 Experiment: Cyclic Voltammetry (CV)
Cyclic voltammetry can be used to assess the presence of Faradaic reactions as well as the capacitance of the electrode [8]. An input ramping voltage is applied across
43 Reference V :Electrode (RE)
* IN * I I I I I I I * I * I I I Working Counter Electrode Electrode (WE) (CE)
Figure 2-7: Potential profile across an electrochemical cell. The potential drops at the EDL of the working electrode, measured by the potential between the reference and working electrode. There is negligible potential drop in the solution, and then an additional potential drop at the EDL of the counter electrode. the electrode with a sweep rate, v:
_dV (2.1) dt
For a capacitor, with capitance C, charge Q, voltage V and current I, we can measure
the capacitance, very simply:
CV = Q = Idt (2.2)
dV C- = I (2.3) dt
C - (2.4) V
Thus, for an ideal capacitor, with constant capacitance for a given constant sweep
rate, we expect the current to be measured constant. Figure 2-8 shows typical CV
responses. An ideal capacitor will have a box-like curve, immediately responding to a
change in the voltage ramp, and maintaining constant current. A more typical curve
will show some delay, seen in the curved edges of the box, due to resistances in the
material and solution. Finally, a material undergoing corrosion, will exhibit Faradiac
44 Electrode Response
deal Typical :i Faradaic
Voltage (V)
Figure 2-8: CV scan schematic of electrode materials peaks at potentials corresponding to the corrosion half-cell potentials.
While this method is widely used to study the presence of half-cell reactions, it is not as accurate for measuring capacitance. The sweep rate can influence the kinetics of adsorption/desorption, overpredicting the electrical capacitance of the material.
Thus, steady-state testing, such as potentiostatic testing, can be more accurate for quantifying the capacitance of a material.
2.3.3 Experiment: Potentiostatic Testing
The capacitance of an electrode can be measured using potentiostatic testing. Because
CDI uses constant applied potentials to adsorb and desorb ions, this is the most characteristic experiment to characterize capacitance. In potentiostatic testing, the potential is held at a constant value, and the current is measured, shown in Figure
2-9. The capacitance is calculated using the simple electrical relation:
C = 1 Idt (2.5)
A background current is always present due to charge transfer through interfaces
45 I I 2
00.2 0)t
CO 60 12( time (s)
Figure 2-9: Potentiostatic testing: current response to a held potential value between the gold, carbon, and contaminants in the environment [62]. Therefore, to measure the double layer current, we subtract out the leakage current:
I = I - Iea (2.6)
These two electrochemical tests give the ability to determine the capacitor behav- ior of the electrodes. We can use these experiments to study trends of CNT electrodes in varying solution concentrations and at different carpet heights.
2.4 Experimental Results and Discussion
CV scans were used to assess the presence or lack of Faradaic reactions. For CNT carpets of 21 tim, in 20, 50, 70, and 90 mM solutions, no Faradaic peaks were observed in CV scans conducted with a sweep rate of 30 mV/s, shown in Figure 2-10. This indicates that the iron catalyst was successfully removed from the CNT surface, and that the gold and titanium do not react in sodium chloride solution.
Potentiostatic measurements are shown in Figure 2-11 comparing capacitance at varying heights and solutions concentrations. The capacitance, normalized by the
46 CV Scan X 0CNT Electrode 1 -20mM -50mM -70mM -9mM
0.5 (D
C 0-
-0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 Voltage (V)
Figure 2-10: CV scan of CNT carpet, height 21 jpm. Electrode, tested in varying solutions of NaCl, did not exhibit Faradaic peaks. v = 30 mV/s. planar area of the electrode shows samples varying between 0.5 to 2 mF/cm2 . In order to compare these values with existing carbon materials, the mass of the electrode must be measured in future experiments. The capacitance does not increase significantly with applied potential, indicating that the capacitance is not a function of potential.
The capacitance generally increases with height, which is more strongly shown in low concentration solutions. The discrepancy at higher solution times may be due to the deviation during long charging times shown in Figure 2-11e. During charing of the capacitor in larger salt concentrations, we do not observe the expected trend of current asymptotically approaching zero. This behavior is unexpected and must be inspected further.
In addition, SEM analysis of a post-diffusion bonded carpet shows that the align- ment is not maintained in the transfer, shown in Figure 2-12. The cross-section of a transferred carpet was milled using a Focus Ion Beam (Helios Nanolab 600 Dual
Beam FIB), in order to observe the geometry. Maintaining the geometry during the transfer of the CNTs is important for modeling of the double layer. This morphosis could give rise to the effects observed in electrochemical measurements, and make
47 a x le b Xl16, 10 2 2
~1.5 99 10
0.5 0.55 10 15 20 25 30 5 10 15 20 25 30 10 C x10' h-(m) d x h( V 2 2 10 1.5 1.5
10'
0.5 0.51 10 .2 5 10 15 20 25 30 5 10 15 20 25 30 10 10 h (pm) h (pm) t (s)
Figure 2-11: Potentiostatically-measured capacitance normalized by electrode plane area for (a) 20 mM, (b) 50 mM, (c) 70 mM and (d) 90 mM. The red circle, green triangle and blue square symbols correspond to step potentials of 100 mV, 200 mV and 300 mV above the OCV, respectively. Figure from [6]. it difficult to characterize the resistance of pores. The transfer method needs to be reconsidered in order to preserve the ordered-geometry of the VACNTs. This can be achieved by reducing pressure and temperatures applied during the bonding process, either by optimizing the gold-gold diffusion bond or by investigating other bonding materials.
In this chapter, we grew 2-3 walled VACNTs, with inner pore diameters of 5.6 nm and outer diameters of 7.7 nm. In this preliminary study, CNT heights were grown between 5-30 Mm. The CNTs were transferred onto a non-corrosive current collector using gold-gold diffusion bonding. Preliminary experiments were conducted in a three-electrode set up, verifying that the electrodes are non-corrosive. Finally, the
CNT electrodes have capacitances between 0.5-2 mF/cm2 (normalized by electrode planar area). However, the morphology is not preserved during transfer. In chapter
3, we redesign the transfer method to preserve the VACNT carpet.
48 Figure 2-12: FIB cross section of transferred CNT carpet. The clean, tubular geom- etry of the CNTs is not maintained during the diffusion-bonding process.
49 Chapter 3
CNT Electrode Design
We have designed VACNT electrodes to investigate the optimal electrode material properties for CDI. The ordered geometry is important for characterizing the influence of pore length and diameter on charging dynamics, without the influence of additional resistances from tortuosity of pores or variations in interconnectivity. In chapter 2, we observed that the CNT carpet following gold-gold diffusion bond did not maintain the as-grown geometric alignment.
In this chapter, we examine methods to transfer CNTs onto a conductive substrate.
The goal is to synthesize an electrode that maintains the alignment of the nanotubes, has low interfacial contact resistance between the carpet and current collector, and is free of Faradaic reactions. We will optimize the Au-Au self-diffusion bonding, explore other methods of transfer, and conduct electrochemical testing to determine which bonding method meets the criteria for the electrode design.
3.1 Bonding Methods
3.1.1 Optimizing Au-Au Diffusion Bond
In chapter 2, we introduced a method for gold-gold diffusion bonding: using a 120
N-cm torque to tighten down a stainless steel jig, applying pressure and elevating the temperature to 540 'C for diffusion of the gold films, shown in Figure 2-5. Both
50 Critical Force for CNTs at given heights 10'1 Ehr2 F,.=h2 10 0
S I1 CD 10'
0 LL
- le0~
.0 t 0 10 20 30 40 50 60 70 80 90 100 CNT height (um)
Figure 3-1: Critical force to induce first-order buckling of column
Torque (N-cm) Force (N) Force per tube (nN) 20 73 7.3 30 109 10.9 40 145 14.5 120 436 43.6
Table 3.1: Appled torque on jig translated to axial force the applied pressure and temperature are variables which can be tuned for optimal transfer of the CNTs onto a current-collector. First we consider the applied force.
The critical force for buckling of CNTs as a function of height is given in Figure 3-1.
Applying a torque of 120 N-cm on the bolts of the jig is equivalent to applying 436
N or approximately 43.6 nN of force per tube. This greatly exceeds the critical force.
However, others using Au-Au bonding have managed to maintain the carpet align- ment at large pressures [59]. It is possible that CNT alignment can be maintained, even beyond critical force, due to slipping of CNTs under axial compression [9], shown
in Figure 3-2. These effects have been observed in SEM images of CNTs following transfer. It is possible that by reducing the pressure applied during transfer, we can maintain the alignment, as shown in modes 1-3 in Figure 3-2.
The applied torque was reduced to 20-40 N-cm, with the corresponding forces outlined in Table 3.1.1. Reducing the load to 20-40 N-cm of torque on the jig, at room
51 io 0 0 0 I 3
1 2 4 s 7 o9 10 30 N-m30 -
qqs 1 Figure 3-2: CNT slipping, a) Yap et al. [9] showed the deformation of CNTs under uniaxial compression. b) Our SEM of transferred CNTs exhibiting buckling similar to step 6.
Figure 3-3: CNT carpet does not show buckling under small applied torque temperature, did not compress the CNT carpets, shown in Figure 3-3. Reducing the applied pressure helps maintain carpet alignment. However, elevated temperatures can still cause CNT misalignment. This is because high temperatures can lead to a significant reduction in the Young's modulus of the CNT, or basically the ability to yield buckling [63]. This could lead to buckling at lower pressures. Previously, Au-Au bonding with CNTs has been conducted at temperatures as low as 150 *C [61, 601. In chapter 2, we used much higher temperatures of 540 *C, annealing for 30 minutes. We conducted a study of temperatures at 540, 270, and 220 *C with applied torques of 30 N-cm, extending the anneal time to an hour to allow for sufficient diffusion of the gold films. The results are shown in Figure 3-4. At 270 *C there is 100% of carpet transfer, while at 220 *C there is not.While we do achieve complete carpet transfer
52 Figure 3-4: CNT transfer at various temperatures. There is 100% transfer at 540"C and 270 *C. Carpet looks more aligned at 270 *C. 30 N-cm torque was applied. at lower temperatures, we still see some collapse of the carpet. In addition to considering the role of pressure and temperature during the Au-Au process, there is also the design of the pressure jig. By modeling the pressure jig as a basic beam bending problem with a load at one end (where the bolt is placed), we can calculated the deflection, 6 the plate undergoes:
J W=3 (3.1) where W is the load from the bolt, E is the Young's modulus of steel, I is the moment of inertia: I = b,12'7 where b is the width of the jig (1 in) and t is the thickness of the plate, and 1 is half the plate length (1 in). Originally, we used 1/8" stainless steel plates to apply pressure on the carpet and the substrate. However, a plate loaded with -100 N of pressure at the edges, results in ~50 pm deflection, which is enough to crush the CNT carpets. By doubling the plate thickness to 1/4" plates that deflection
53 100 N
1/8" 101N thick plate thick plate 45 um 5= 0.22 um
Figure 3-5: The bending of the plate under a load from the bolt. The 1/8" plates bend upto 50 Mm, damaging the carpet. Doubling the plate thickness reduces the deflection, 6, to less than 1 micron. is reduced to < 1pm, see Figure 3-5.
Finally, we selected 30 N-cm, 270 *C anneal for 1 hour, and a jig design with 1/4" plates to generate Au-Au bonded CNT electrodes. The results in Figure 3-6 show that the alignment of the CNT electrodes is maintained across the majority of the carpet. We can also see that contacts between the carpet and the conductor are made through occasional joints bonding together, and conductivity is maintained through the gold film and interconnectivity of the CNT carpet.
The diffusion bonding process is useful for transferring bound CNTs from one substrate to another. However, the contacts between carpet and substrate are sparse.
In order to create more conformal contact between the carpet and the substrate, we investigated bonding methods that are wet-contact instead of dry. Wet-bonding has the advantage of using the viscosity of the bonding material to wet the CNT carpet, reducing the need for applied pressure. The diffusion bond process allows us to seperate the catalyst and CNTs during the blade lift-off step. Another method to remove the catalyst is to delaminate the CNT carpet from the growth substrate during synthesis. After the growth step during synthesis, C 2H 4 is removed, but 02 continues to flow at elevated temperature for 10 minutes (see Appendix A). The oxygen etches the CNT from the iron, generating free-standing, catalyst-free carpets.
This carpet, wet-bonded to a conductive substrate, can be a simple method to make electrodes.
54 I'
h= 5gm, d= 5 nm h= 22pm, d= 5 nm h= 52pm,. d= 5 nm
Figure 3-6: Successful transfer of CNTs using gold diffusion bond. T=270*C, torque=30 N-cm.
55 CNT Gold Si Wafer
Figure 3-7: Gold-gold bond. High temperature melted the gold, forming a weak bond. Scale bar: 1cm
Figure 3-8: Delaminated carpet taped with copper tape to substrate. Scale bar: 1cm
3.1.2 Au
In order to use a wet-bond, and avoid applying pressure to the CNT carpets, we explored bringing the gold film on the substrate upto melting temperature. We placed a delaminated carpet on a silicon wafer substrate with 1 micron of gold deposited, and put it into a tube furnace. The temperature was raised to 1100'C, as 95%Argon and 5% H2 flowed through. The carpet made a weak bond, as shown in Figure 3-7, which was not held when the sample was cleaved.
3.1.3 Conductive Tape
We considered simply taping the CNT carpet to a current collector using conductive tape, as shown in Figure 3-8. Unfortunately conductive tape is still fairly resistive compared to metals and epoxy bonds.
56 Tin foil Furnace T=270-C
Figure 3-9: AuSn bonding process. Similar to Au-Au bonding, except with an addi- tional piece of tin foil
CNT Epoxy
Hot Plate
T=80-200*C Time=5-80 min
Figure 3-10: Epoxying CNT carpet to substrate. Nickel and Silver epoxies were studied.
3.1.4 Au-Sn Bond
Closely related to Au-Au diffusion bonding, we also looked at introducing tin foil to the bond. At 270*C, tin at the gold-tin interface melts and forms a eutectic with gold. The wetting of the tin could create a more conformal bond than the hard bond formed by diffusion alone. The method is similar to Au-Au bonding, excep that a piece of tin foil (Sigma-Aldrich) was introduced between the gold films, shown in
Figure 3-9. In addition, the anneal time was only 10 minutes. The finished electrode is shown in Figure 3-11.
3.1.5 Conductive Epoxy
Similar to the tape concept, we studied nickel and silver epoxies (Epo-tek) as a bonding material. The delaminated carpets were placed on current collectors coated with wet epoxy, and cured on a hot plate, as shown in Figure 3-10. The wetting of the epoxy allows us to avoid applying pressure to the carpet and maintain the geometry of the carpet. The resulting carpet is pictured in Figure 3-11.
57 Au-Au diffusion bond Epoxy AuSn solder bond
CNT Silver Epoxy Gold Nickel Epoxy Si Wafer Tin Foil
Figure 3-11: Transfer methods that were successful: gold diffusion bonding, epoxy methods, and gold-tin bonding
3.1.6 Summary
We have developed several different methods to create aligned CNT electrodes. The methods which had successful transers of the carpet were: the Au-Au bond, epoxy methods, and AuSn bonding, shown in Figure 3-11. SEM images also indicate that alignment of the geometry is maintained for these transfer methods. In the following section, we describe how we used electrochemical testing to determine if there is low electrical contact resistance and negligible Faradaic reactions in these samples. We electrochemically characterized gold-diffusion bond samples, bonded at 540 *C and 270 "C, as well as nickel epoxy, silver epoxy, and gold-tin bonding. While carpet alignment is not maintained at 540 *C, we used this sample to compare how the contact resistance changes due to bonding temperature.
3.2 Contact Resistance Measurement
The potential drop between the current collector and the electrode should be mini- mal for accurate characterization of the double layer. This is achieved by minimizing
58 a) C b)
V1 f R4CMass diffusion RL Elimitations
Rs + Rc RL Re(Z)
Figure 3-12: RC circuit model of a 3-electrode cell. a) Circuit model, b) Impedance plot for circuit contact resistance. In the previous section, we developed several methods to bond electrodes to the substrate. In this section, we use electrochemical testing to deter- mine the contact resistance between CNTs and the substrate for each method. The system draws current on the order of milli to micro amperes. For an applied poten- tial of 1V, to keep the potential loss to less than 10% due to contact resistance, the interfacial resistance should be lower than 100 Q.
3.2.1 Impedance Spectroscopy
Impedance spectroscopy was used to measure contact resistance. In a three-electrode set up, depicted in fig 2-6, the circuit can be modeled as two resistances in series with a resistor-capacitor parallel circuit. The potential applied across the cell is the voltage, V, and current flows through a pathway crossing the contact resistance, Re, between the current collector and the material. There is also a solution resistance, Rs, associated with the conductivity of the solution. The capacitance, C, is the double layer capacitance, which is in parallel with a leakage current, RL. The circuit model is depicted in Figure 3-12a.
Mathematically, the circuit can be described in complex form:
Z = Rs + RC + . (3.2) ±iCRL + 1
59 400- AuAu 270 -- AuAu 540
300- -- Silver Epoxy .-.E -- Nickel 0 200- T- I E 100 1'-
0
-10 0 100 200 300 400 Re(Z) [ohms]
Figure 3-13: Impedance testing results. Model fitting is shown with lines.
For an AC potential input, varying the frequency w, yields the impedance plot given in Figure 3-12b. As w -+ 0, Z = Rc + Rs + RL and as w -* oo, Z = Rs + Rc. In the experiment, we used a Bio-Logic VSP-300 potentiostat, varying the input frequency between 500 kHz to 500 mHz. The three-electrode set up consisted of the CNT working electrode, Ag/AgCl reference electrode, and activated carbon (Y- Carbon) as the counter electrode. CNT samples were submerged in DI water overnight to fully wet before the experiment. Testing was conducted in 20 mM NaCl.
3.2.2 Results and Discussion
The impedance testing results are shown in Figure 3-13. The behavior follows the expected trend sketched in Figure 3-12 for Au-Au bonding, AuSn bonding, and the Ag epoxy. However, the Ni epoxy does not show a reasonable fit with a semi-circle approximation. There may be additional reactions which are not taken into account by the model. In order to decouple the contact and solution resistance, data was collected for
60 Contact Resistance using Different Bond Methods 100 Limit
E80-
E 60-
U'
0
4-A 4)20-
0 270-C 540 -C 270-C 540 oC AuSn Silver Au-Au Au-Au Epoxy h=50 pm h=30 pm
Figure 3-14: Contact resistance for bonding methods. Error bars depict the variation in calculation from the model fitting. various measured distances between the reference and working electrode. Increasing the distance, linearly increased the resistance, so we were able to extrapolate the contribution of the solution from the measurement. The calculated contact resistance is given in Figure 3-14. Regardless of height of CNT carpet and temperature, the gold- gold diffusion bonding is well under the limit. This is also true for gold-tin bonding and silver epoxy methods. The nickel epoxy contact resistance is not calculated from the data, due to the poor fit with the model. Diffusion bonded samples, AuSn bonded samples, and Ag epoxy samples have negligible contact resistance. Therefore, the loss of potential between the current collector and the electrode surface should be negligible. The double layer potential profile should be characterizable in our experiments. Finally, we must verify which electrodes do not have corrosion reactions. This final criteria is important for being able to measure the double layer capacitance and charging/discharging behavior.
61 0.15 -- AuAu 270 AuAu 540 0.1 - -AuSn -Silver EpMx 0.05 - NickW
0-
o -0.05-
-0.1 -
-.12 -0.1 0 0.1 0.2 0:3 OA Voltage (V)
Figure 3-15: CV scan for different bonding methods. 20 mM NaCl, sweep rate: 15 mV/s.
3.3 Corrosion Resistance of Electrodes
Corrosion resistance was measured using the same electrode cell set up described in section 3.2. CV scans were used to assess the presence of Faradaic reactions or other parasitic behavior, shown in Figure 3-15.
The gold-gold diffusion bonding methods show box-like CV scans, as does the nickel epoxy. The gold-tin bond has box-like behavior, but the overall slope of the curve indicates a background current, which suggests a parasitic reaction may be occuring. The silver epoxy has a strong reaction, deviating from ideal capacitor behavior.
Thus, the gold-gold diffusion bonding has minimal corrosion in sodium chloride solution. Because tin and silver draw parasitic reactions, AuSn and Ag epoxy are not ideal candidates for CNT transfer. Nickel, which does not show parasitic behavior in the CV scan, did have deviating behavior in the impedance testing. Therefore, Ni epoxy is not an ideal candidate for CNT transfer.
62 Method Geometry (Ordered) R, < 100Q Corrosion Resistant? Au-Au Diffusion Bond (270 *C) yes 38 ± 15 yes Au-Au Diffusion Bond (540 *C) no 37 ± 13 yes AuSn Solder Bond yes 47 ± 12 no Nickel Epoxy yes n/a no Silver Epoxy yes 42 ± 10 no
Table 3.2: Experiment summary for transfer methods
3.4 Summary
Bonding methods were evaluated based on the alignment of the geometry following transfer, low interfacial contact resistance between CNTs and current collector, and minimal corrosion reactions in sodium chloride solution. The results of the experi- ments conducted to determine optimal transfer methods is summarized in Table 3.4.
The gold-gold diffusion bond conducted at 270 *C meets the desired criteria for an ordered-geometry, minimal contact resistance, and corrosion-resistant CNT electrode.
This method is used to test characteristics of charging/discharging processes in CDI in the following chapter.
63 Chapter 4
Characterization of CNT Electrodes in NaCl Solutions
We have designed ordered-geometry, corrosion-resistant, conductive CNT electrodes. We characterized the CNT structure using SEM and TEM, finding that the CNTs have a inner diameter of 5.6 nm and outer diameter of 7.7 nm. BET characterization measured a specific surface area of -540 m2/g. Having determined the geometry of the electrodes, we can now study the influence of saline solution concentration and carpet thickness on capacitance and charging dynamics.
4.1 Experiment: Capacitance of CNT electrodes
Measuring the capacitance of the electrode has two functions. We can compare our capacitance to literature values to verify experimental set up. In addition, measuring the electrical capacitance allows us to compare results to double layer theory and determine an appropriate model for charging. This is an important parameter for predicting desalination in CDL.
64 a. b. C.
4 4''2 4-
3- - 3
0.5 a 5mM0
_0 620. 0.3 0A 0.1 02 0 . 0.1 0.2 0.1 O
Figure 4-1: Specific charge of CNT electrodes. a) CNT height 5 pm, b) CNT height 22 pim, c) CNT height 52 im.
4.1.1 Experimental Setup
In this experiment, we grew 5, 22, and 50 pm carpets and synthesized electrodes using the diffusion bond procedure developed in chapter 3. The mass of the electrodes were determined by extrapolating from previous growths in the same conditions at varying times (provided by Professor Thompson's group). The substrate used was a silicon wafer in order to simplify cleaving for imaging. The electrodes were taped using electroplating tape (3M), leaving only the CNT carpet exposed to solution. The substrate was lined with Teflon tape in order to prevent wetting of the alligator clips and shorting of the circuit. CNT electrodes were then soaked in DI water overnight in order to fully wet the surface before testing. The electrodes were tested in a 3-electrode set up using a Ag/AgCL reference electrode (Sigma) and a carbon electrode (Y-carbon) as the counter. Tests were con- ducted in 5 and 20 mM NaCl solution. Potentiostatic testing was used to determine the charge of the electrodes. The half-cell potential, the voltage drop measured be- tween the working and reference electrodes, VWE, was stepped in order to measure current response. Tests were conducted by holding the half-cell potential at 0 mV for 1 min, stepping the potential to 100 mV for 1 min, and then returning the potential to 0 mV. Then the cycle was repeated for 200, 300, 400 and 500 mV.
65 4.1.2 Results and Discussion
Potentiatic testing was used to determine the charge of the electrodes. Figure 4-1 shows the results of testing different CNT heights in 5 and 20 mM sodium chlo- ride solution. The data shows an increase in charge as a function of voltage and solution concentration. From equation 2.2 we expect that for a constant capacity electrode, charge must increase proportionally with voltage. In addition, as solution concentration increases, the double layer thickness decreases. This increases electrical capacitance due to the reduction in the interspacing between the electrode surface and the solution bulk. We also examined the relationship between specific charge and height, shown in Figure 4-2.The specific charge does not vary significantly between the heights of the carpet. This is anticapted because CNT mass increases linearly with height, so a constant capacitance material should have constant specific charge. There is some discrepancy between the 5 Mm carpet and the taller carpets. This deviation may be due to uncertainty in the measurement of carpet height and mass. The height varies by 1-2 Mm, and for a 5 Mm carpet this is substantial variation. The mass, derived from this height, may also have some significant variation. The charging data shown in Figure 4-1 was matched with the GCS model laid out in equations 1.15, 1.16,and 1.17. The charge, Q is calculated by Q = amoF, where am is the specific area (calculated by BET) and F is Faraday's constant. This model leaves CST as the fitting parameter given in equation 1.16. The Stern layer capacitance allows us to determine the profile of the double layer. We use it to compare capacitance measurements to existing values in literature. Assuming that the specific surface area of the carpet is what was measured in the BET, we get good matching with the data for a Stern layer capacitance of 3 piF/cm2. However, we consider if the inner pores are not accessible. This situation may arise if the CNTs remain capped, even after transfer of CNTs from one substrate to another. In this situation, the Stern layer capacitance is 6 pF/cm2 . This discrepancy can be resolved by collecting data on how much salt is removed during the charging cycles,
66 3-
2.8-
3 2.6- 20- 24
02.2
1.8-
1.6 5mM
1.4 1'0 20 30 40 50 60 CNT height (pm)
Figure 4-2: Specific charge as a function of height in 5 and 20 mM NaCl solutions. Electrode applied potential was 300 mV. Error bars arise from variation in the cal- culation of charge. and using charge efficiency to determine available surface area [56].
The data fits well with the Gouy-Chapman-Stern (GCS) model, indicating that for a static system,with pore diameters of 5 nm at solutions of 5-20 mM NaCl, the
GCS is sufficient to characterize the capacitance. From GCS, we can extract infor- mation regarding the available surface area and the Stern layer capacitance, which are parameters that can be used to design a CDI system with appropriate electrode area for desalination.
From the model, we find that the specific capacitance of the carpets is between 7-8
F/g, which is low for a high surface area carbon material, predicted to have between
50-175 F/g capacitance [35]). In addition the Stern capacitance is fitted as only 3-6
PF/cm2 , which is small compared to the 20 pF/cm2 capacitance measured by a variety of high surface area carbon materials [31]. This may be due to the hydrophobic nature of the CNT surface or due to a limitation in the beaker measurements.
The data collected shows anticipated trends for charge and capacitance as a func- tion of height and solution concentration. There is some increase in charge for in- creasing solution concentration, and negligible change in specific capacitance of an
67 a. 4.5 am = 540 m2/g 4 - Cs7 = 0.03 F/m 2 0.002 3.5 ~3-
2.5
2--
0.5- 5 mM, C=6.9 F/g 0 0 0.1 0.2 0.3 OA V (V) b. 4.5 m2fg 4- am= 320 2 C.- = 0.06 F/m 0.002 3.5
3-
2--
0.5 - 5 mM, C=7.1 F/g
S0.1 0.2 0.3 0.O.4 VwE(V)
Figure 4-3: Fitting GCS model to charge data. a) GCS fit with am = 540 m2 /g, b) GCS fit with am = 320 m2 /g. CNT heights: E = 5, Q = 22, and A = 52 pm. Specific capacitance is 7-8 F/g and CST = 3-6 pLF/cm2
68 electrode material. The experimental results match well with the GCS model, indicat- ing this simple double layer model is sufficient in this experimental regime. However, we find that the specific capacitance and the Stern capacitance are lower than an- ticipated. This may be due to materials challenges or cell set up problems. In the following experiments, we address these issues to determine what gives rise to these small capacitances.
4.2 Role of Counter Electrode in Setup
The measured specific capacitances from the previous experiment were only 7-8 F/g, which is small compared to the 20-100 F/g range high surface area materials achieve. The measurement taken with the counter electrode (CE) may be limiting. The polarizability and the high surface area of the counter electrode is important to ensure that it draws the current that is required to completely operate the working electrode. The counter electrode should be sized to prevent limiting performance of the working electrode. In chapter 2, experiments were conducted with platinum mesh, but its surface area is very small. In chapters 3 and 4, experiments were conducted with a high surface area, over-sized carbon cloth (Y-Carbon). Here, we investigated the use of these materials, as well as a platinum foil (5 x 5 cm, Sigma Aldrich) and another CNT electrode. The CV scans are given in Figure 4-4. The CV scan for a CNT electrode with the Y-carbon material as the counter has very resistor-like behavior, and the scan does not reverse around 0 F, indicating that there is an additional resistance in the system. The platinum mesh and CNT electrode as the counters show more capacitive like behavior. However, looking closer, the Pt mesh capacitance is one order of magnitude smaller in measurement. This could be due to the mesh limiting the current drawn. However, experiments with platinum foil show similar resistance-like behavior though with larger capacity. Using a similar CNT electrode as the counter seems to be the best choice for making measurements in the beaker set up. Using this experimental set up, we will investigate charging-
69 a. b.
x10 -25mVIs 3 1.2 50mVW 2 -10a MV/S - 242 nmW ~0 IRW - 493 mV/s 12 997 mV/u as0 06 -3
4 4 4
-0.C o 02 CA CA 0 1 _0 02 CA 0.5 0.. valage (V C d. vbkwm o*
0.01 0.02
am cm. CA
4.*005 0
4)M5 .001
02 CA 0.6 02 0.2 0.25 0.3 0.35 0.4 OAS 0.5 V~bo CV) Vos (V)
Figure 4-4: CV scans of various counter electrodes in 20 mM NaCl. Counter elec- trodes: a) Y-Carbon cloth, b) Pt mesh, c) CNT, d) Pt foil. WE: CNT electrode and RE: Ag/AgCl electrode.
70 discharging behavior of the CDI electrode.
4.3 Charging Dynamics of Electrodes
Thus far, we have investigated the capacitance of the CNT electrodes. We found that the data matched well with the GCS model. However, for application in a CDI cell, we also want to consider the influence of charging dynamics in porous electrodes. We can use this ordered geometry to isolate the effects of pore length without significant effects from the tortuosity.
4.3.1 Experimental Setup
In this experiment, we grew CNT carpets ranging in height from 20-600 /Lm and synthesized electrodes using the diffusion bond procedure developed in chapter 3. The mass of the electrodes was measured using a microbalance (Mettler Toledo XP6U). The electrodes were prepared in the same manner described in section 4.1.1. In order to investigate the influence of surface properties, half the samples were plasma treated with air (mild treatment 30 min) and the rest were tested following the diffusion bond
(as grown samples). The experiment was set up similar to the method described in section 4.1.1, except the counter electrode was replaced with a CNT electrode of similar size and mass as the working electrode. In addition, experiments were conducted using an applied potential of 250 mV.
4.3.2 Results and Discussion
The current response collected for CNT electrodes was measured in sodium chloride solutions varying from 7-1000 mM concentration. A typical response is shown in Figure 4-5. The data was fitted using a double exponential equation of the form:
-t -t I(t) = A exp- + B exp- + C (4.1) Ta Tb
71 x 10- -- 672 pm pm 4 - 400 -+140 jim --- 50 Jim 20 pm Z1- S--0 pim 2 --- i
n
0 2 4 6 8 10 Time (s)
Figure 4-5: Current response due to a potential step VWE = 250 mV in 125 mM NaCl solution where A, B, C, r, rb are fitting parameters. The two time constants helps fit the data accounting for both the slow and fast responses to charging, typically due to the time charge of relaxation and conduction through the material. Constant C in equation 4.1 represents the leakage current.
The capacitance was calculated by integrating the fitted current response using equation 2.5. The fitting is used to improve the integration for capacitance, due to the ability to predict the initial current and to remove background noise arising from the leakage. The results are shown in Figure 4-6. These results show that the capacitance varies between 20-40 F/g, depending on the solution concentration. The taller carpets have slightly higher capacitances, which may be due to slight changes in the surface morphology in longer carpets.
The data fitting and the change in counter electrode has dramatically increased measurements from 10 F/g upto 40 F/g.
The current response plotted in Figure 4-5 clearly shows that the step response of the CNT electrodes is slower for taller carpets. We examined the time constants
(rb) in response to varying heights and solution concentration. Figure 4-7 shows the response for the slower time constant (rb), though the behavior for r is similar. As
72 + 15mM c 70 M 40 125 mM
310 mM 36 500 M 1000 mM 307
~25±
0 100 200 300 400 500 600 700 CNT Height (pm)
Figure 4-6: Mean capacitance of CNT electrodes. Samples were tested in NaCl solution. the solution concentration decreases the time scale for charging increases. In addition, as the CNT carpet height increases the charging time increases. Bazant and Biesheuvel [58] had identified the timescales for charging in porous electrodes given in equations 1.23 and 1.24. However, Figure 4-7 shows that the time constant varies approximately linearly with height of the carpet and proportionally to the Debye length 1.13. This suggests that the dominating charging effect is due to surface conduction through the CNTs and charge relaxation. There are a couple possibilities for this effect. It is possible that the CNTs were not uncapped during the diffusion bond process, but this seems unlikely because of the non-Faradaic CV curves observed in Chapter 3. The other possibility is that the CNTs, due to their mesoporous nature, do not exhibit confinement effects observed in sub-2nm CNTs. CDI researchers strive to develop high surface area materials, which typically have large percentages of microporous volumes (< 2 nm). These materials are capable of larger specific capacitances with the trade-off of longer charging time scales, compared to mesoporous materials. For a CDI cell, it is important to optimize the capacitance of an electrode material by weight and by the planar area, which ultimately defines the volume of the CDI cell. In Figure 4-8, we compare CNTs of varying heights to an activated carbon fiber (ACF) material studied by [64]. ACF is 500 pim thick with
73 a. b. 9 12 + 1mM 8- + + CNTh=Opm 125mM 10* CNT h=20 Am 7 21m0 + CNT h -50 pm 310mM 8 CNT h=140 pm So mMM CNT h =400 pm '5. 100 + CNT h=672 pm 4 - 3 4 2- 2 1
4 0 200 400 00 C 200 00 M00 1000 CNT Height (um) NaCI Concentration (mM)
Figure 4-7: Charging time constant for CNTs for varying a) carpet heights and b) NaCl solution concentrations. a BET measured micropore volume of 0.6. While ACF has a capacitace of 75 F/g in 125 mM solution, CNTs have a capacitance of 25-30 F/g. The measured capacitance was normalized by planar area and by weight. The area capacitance divided by the time constant shows that the microporous ACF has similar rates of charging as the CNTs. However, when we consider the specific capacitance charging rate, we see that the CNTs are an order of magnitude faster. Figure 4-8, suggests that there are two regimes for charging the CNTs. For shorter carpet heights, the charging time may be dominated by charge relaxation of the electrode material, while for taller carpets the charging is dominated by diffusion. The CNT electrodes are more efficient by weight at charging than ACF, as shown in Figure 4-8, due to their mesoporous nature. However, when comparing the trade-off between CNT charging rate by planar area, we find that the ACF and CNT performance are comparable. This is because the CNT carpets are extremely sparse, compared to the ACF. However, the CNT performance can be easily doubled by densifying the CNT carpets, to pack more material per planar area. In addition, modifying the surface of the CNTs would allow for more specific adsorption, increasing the material capacitance without sacrificing the charging time dramatically. This study suggests that tall, densified, CNT carpets could be a much better suited material for CDI, with reasonable capacitance and faster charging time constants than
74 a. b.
CNT -10 SACF, h= 600 pNoked 2009 10
102
A 10-10
107 2 10 101 10 10 10 Electrode Thickness (pm) Electrode Tckness (m)
Figure 4-8: Optimizing capacitance and charging time constant. a) Capacitance per planar electrode area divided by time constant, b) Capacitance per mass divided by time constant. Data collected in 125 mM NaCl solution. ACF data from [64]. microporous materials. One limitation of comparing to published data is the inability to characterize all aspects of the geometry. Studying microporous materials in our lab and comparing to CNTs with an understanding of planar surface area, BET surface area, weight, and electrode thickness would allow for a more conclusive comparison between CNTs and other structured materials.
4.4 Conclusions
The potentiostatic data collected using a three-electrode beaker set up shows that our CNT electrode capacitances vary between 20-40 F/g. Adjusting the counter electrode made a significant difference on the measurement. The GCS model was used to match the capacitance data, showing that this simple double layer model is suitable for our geometry. In addition, we found that the time scale for charging varies linearly with height and decreases with solution concentration. Due to the well-matching of the GCS with the CNTs and the linear relationship of charging time scale with height, we conclude that we do not observe any strong effects of porosity from the mesoporous nature of CNTs. We find that the CNTs are more efficient at charging than ACF, suggesting that by densifying and functionalizing the CNT surface, the mesoporous
75 structures could be better suited for CDI than microporous carbons.
76 Chapter 5
Conclusions and Future Work
In this work we investigated the capacitance, and charging-discharging dynamics of vertically aligned carbon nanotube (VACNT) electrodes. Thus far, in literature, experimental studies have largely focused on creating novel materials for CDI. Here we used an ordered-geometry VACNT electrode to better understand how electrode thickness plays a role in charging dynamics. This can allow us to inform the optimal design parameters for synthesizing materials for CDI.
We designed VACNT electrodes using a low-temperature gold-gold diffusion bond process. By applying pressure and annealing at 270 'C for 1 hour we achieve con- sistent, conductive bonds. This method allows us to use a flux-free, metallic bond, minimizing contact resistance and corrosion of the electrode. The electrodes were characterized using SEM, TEM, and BET. The carpet heights were grown between 5 and 700 Mm, with diameters of 5.6 nm inner and 7.7 outer. The CNTs were nominally
2-3 walled. The specific surface area of the material was 540 m 2 /g
Using the VACNT electrodes, we investigated the capacitance and charging dy- namics of CNTs. We found that the capacitance is 20-40 F/g. The GCS model matches well with the data, indicating that porous effects are negligible. In addi- tion, the charging time scales varies linearly with carpet height and decreases with solution concentration, suggesting that the dominating effect is due to surface con- duction. Finally, a comparison of charging rates between CNTs and activated carbon fiber suggests that CNTs are more efficient at charging by weight, and would be better
77 performing by densifying and functionalizing the CNT surface.
5.1 Future Work
5.1.1 Increase Capacitance
The CNT electrode materials presented in this work had capacitances of 20-40 F/g, roughly 7 IpF/cm 2 (normalized by BET surface area). Carbon materials can have upto 20 piF/cm2 . The hydrophobic nature of CNTs may be limiting the capacitance of the electrode. We can enhance capacitance by functionalizing the surface, either through acid etches or the introduction of pseudocapacitance. We would like to investigate how an acid etch in nitric acid enhances capacitance through the addition of oxygen at the surface of the CNTs. In addition, using atomic layer deposition
(ALD) to functionalize the surface with titania could increase the capacity by an order of magnitude, as shown by [46].
5.1.2 Further Parameterization
Here we studied a single pore-size CNT. By changing the growth parameters, we can modify the diameter of the CNTs [65]. This can allow us to explore CNT with diameters from 2-10+ nm, allowing us to investigate the effects of overlapping dou- ble layers. The charging dynamics may also change dramatically and confinement effects develop. In addition, testing needs to be done to ensure that the CNTs are uncapped when transferred onto the new electrode surface. This would also enable us to accurately estimate the available surface area of the material.
In addition, densification of CNTs would increase the capacitance per planare electrode area. Densifying the carpets, and comparing the charging rates to other electrode materials would allow us to understand the effects and trade offs of micro- porosity and capacitance.
78 5.1.3 Flow Cell
Ultimately, the goal of this work is to understand charging-discharging dynamics of porous electrodes. Recent work by Hidrovo's group has informed how flow rates and cell design can influence the desalination for flat plate electrodes [661. An important extension of this is to study the transport in porous electrodes. The developed CNT electrodes could be the perfect stepping stone for optimizing CDI.
In addition a flow cell allows us to measure the salt removal during charge/discharge.
With this information, we can characterize the charge efficiency of the system, and determine what the available surface area of the electrode material is using the GCS model [56]. Capacitive Deionization is a promising technology for water desalination. It re- quires minimal pre-treatment, is low power, and with the right material, portable.
CNT ordered-geometry electrodes can allow us to understand the optimal pore lengths and diameters for optimal CDI.
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85 Appendix A
CNT Synthesis
The CNT synthesis procedure was developed and written by Robert Mitchell in Pro- fessor Carl Thompson's group at MIT. This process is developed for a double furnace growth procedure. Additional steps are noted for delamination of CNT carpets.
A.1 System Setup
This CVD system was designed to grow tall carpets of carbon nanotubes (up to several millimeters). The system has two separate furnaces to independently control gas phase decomposition of the carbon precursor and also the temperature of CNT growth. Two separate gas circuits allow for independent gas flows to be delivered to the preheater or growth zone. Each gas circuit has an on/off valve for preventing air from entering the gas lines when the system is not being used. These valves should always be closed when the system is not in use, and an overpressure of He gas should be left in the system to prevent air from entering the system. Additionally, each mass flow controller (MFC) has a separate on/off valve for each independent gas circuit.
The MFC on/off valves should be used simultaneously with the MFC on/off switches.
86 A.2 Safety
This system presents a potential explosion risk because of the large volume of H2 and C2H4 that is involved in the CNT growth process. Always be sure that the gas lines and process tube have been adequately purged with He, removing 02, before introducing either explosive gas into the system. There are no safety interlocks to automatically prevent exposure of H2/C2H4 to 02, so always be aware of the state of the system. Always ensure that the tube end caps are securely in place and that the exhaust line is operational before proceeding with the growth.
A.3 Sample Preparation
Standard samples (catalyst on silicon wafers) can be prepared simply by scribing the wafer into small pieces (- x 1 cm) and then using the N2 gun to blow off any loose particles. Be sure to wear gloves and handle the silicon substrates with tweezers avoiding contact with your gloved hand as this will leave a residue on the substrate surface.
We also diced wafers using a diesaw to cleave the wafer, and cleaned the substrate with acetone, methanol, isopropynol, and water before use.
A.4 Tube Preparation
1. Open both furnaces and place a long (54) tube in the groove in the refractory brick.
2. Cleaning only: Securely cap both ends of the tube leaving the gas supply off and
the tube filled with air. Turn on the end-cap cooling fans and them ramp both
furnaces to greater than ~775 *C. An anneal in air for - 30 minutes should
remove any amorphous carbon from the sidewalls of the tube. The tube can
be flushed with He using the gas supply to remove any residual air and smoke
from the tube after annealing.
87 3. Turn on the He flow (75 scem is sufficient) to continuously purge air from the tube.
4. Remove the right end cap and load samples in the growth furnace using tweezers and the loading arm. The substrate position will have some effect on the growth of CNTs and adhesion to the growth substrate so be sure to note the substrate positions.
5. Securely cap the tube.
A.4.1 Growth
1. Log the process variables in the log sheet.
2. Set/verify the MFC controller so that appropriate gas flows will flow during the run. Set/verify the furnace temperatures.
3. Purge the tube for -10 minutes after loading samples to completely remove 02 from the process tube.
4. Turn on H2 gas (first the on/off valve (H2 Preheat), and then the MFC con- troller)
5. Close the preheater furnace and ramp to temperature (- 650-850 *C was used in these experiments). Begin the purge with He/H2 for >15 minutes.
6. After purging, close the growth zone furnace and begin ramping to temperature (~720 C), while starting a stopwatch to record the ramp time, which should be around (9:30min).
7. When the growth zone reaches temperature, start a countdown clock for the annealing step (~5 min).
8. Upon completion of the growth step, turn on the C2H4 supply and valve and start the growth timer.
88 CNT growth set up
Gas fk~
Figure A-1: Set up of CNT samples in double growth furnace
9. For improved delamination: After growth, turn off the C2H4 and anneal the sample in He/H2 for 10 minutes. (only use this step if the goal is to delaminate the carpet)
10. After growth/post-anneal steps, turn off the C2H4 supply and open the furnace and start the fan to rapidly cool the sample.
11. After ~3 min, turn off the H2 supply
12. Purge the furnace with He for L10 min to remove residual H2 from the system.
13. Open the furnace and unload samples using tweezers and the loading arm.
14. Log the ramp time and any abnormalities occurring during the growth in the process log.
15. If performing another run, load samples cap the tube and begin with step 3 above.
As discussed in the synthesis procedure, sample position in the heater an duration of growth are directly related to the height in the CNT carpet. Figure A-1 shows the positioning of samples in the double furnace set up.
89 A.5 Shutting Down
1. After completing your run(s) ensure that both furnaces are open, off, and cooling
to room temperature
2. With all gases off turn on the He gas (~75 sccm).
3. Close the gas outlet on/off valve for the preheater circuit and wait until the
MFC controller flow rate reads ~ 0 sccm.
4. Close the He supply on/off valve and turn off the MFC, leaving an overpressure
of He in the gas lines.
90