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Multifunctional Structural-Energy Storage Nanocomposites 
for Ultra Lightweight Micro Autonomous Systems 

(First-year Report) 


by Mark L. Bundy, Daniel P. Cole, Monica Rivera, and Shashi P. Kama 



ARL-MR-0808 


February 2012 


Approved for public release; distribution is unlimited. 



NOTICES 

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The findings in this report are not to be construed as an official Department of the Army position 
unless so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or 
approval of the use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 



Army Research Laboratory 

Aberdeen Proving Ground, MD 21005 


ARL-MR-0808 


February 2012 


Multifunctional Structural-Energy Storage Nanocomposites 
for Ultra Lightweight Micro Autonomous Systems 

(First-year Report) 


Mark L. Bundy 

Vehicle Technology Directorate, ARL 

Daniel P. Cole and Monica Rivera 
Motile Robotics, Inc. 

Shashi P. Kama 

Weapons and Materials Research Directorate, ARL 


Approved for public release; distribution is unlimited. 








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12. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution is unlimited. 

13. SUPPLEMENTARY NOTES 

*Motile Robotics, Inc., Joppa, MD, 21085 

14. ABSTRACT 

Micro vehicles (MVs) are projected to play an increasing role in both civilian and military applications. However, even with 
minimal payload, present day battery powered micro aerial vehicles (MAVs) have time-of-flights measured in minutes, woefully 
short for military applications. Since battery mass already accounts for a significant portion of the overall system mass, 
increasing battery size to boost MV endurance is not the solution. On the other hand, if the energy storage device can be 
efficiently integrated into the vehicle structure, serving multiple functions, it could increase endurance by reducing parasitic 
mass. Unlike larger vehicles, MVs are generally made from lightweight, flexible materials; hence, an integrated power source 
should have similar characteristics. The research reported herein focuses on preliminary results and progress in a Director’s 
Research Initiative project focused on the design and fabrication of lightweight, flexible power sources, ultimately intended for 
integration into the structural features of MVs. 

15. SUBJECT TERMS 

Carbon nanotubes, CNTs, supercapacitor, multifunctional, energy, Structural-Energy 

17. LIMITATION 18. NUMBER 
OF OF 

ABSTRACT PAGES 

UU 40 

Standard Form 298 (Rev. 8/98) 
Prescribed by ANSI Std. Z3 


19a. NAME OF RESPONSIBLE PERSON 

Mark Bundy 


19b. TELEPHONE NUMBER (Include area code) 

(410) 278-4318 


16. SECURITY CLASSIFICATION OF: 


a. REPORT 

b. ABSTRACT 

c. THIS PAGE 

Unclassified 

Unclassified 

Unclassified 


1. REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 

February 2012 DRI 

4. TITLE AND SUBTITLE 

Multifunctional Structural-Energy Storage Nanocomposites for Ultra Lightweight 
Micro Autonomous Systems (First-year Report) 

6. AUTHOR(S) 

Mark L. Bundy, Daniel P. Cole, Monica Rivera, and Shashi P. Kama 


7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Army Research Laboratory 
ATTN; RDRL-VTM 

Aberdeen Proving Ground, MD 21005-5069 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 


3. DATES COVERED (From - To) 

FY2011 

5a. CONTRACT NUMBER 

NNL 09AA00A 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

5d. PROJECT NUMBER 

FY11 -VTD-009 

5e. TASK NUMBER 

5f. WORK UNIT NUMBER 

8. PERFORMING ORGANIZATION 
REPORT NUMBER 

ARL-MR-0808 


10. SPONSOR/MONITOR’S ACRONYM(S) 

11. SPONSOR/MONITOR'S REPORT 
NUMBER(S) 


11 



































Contents 


List of Figures v 

List of Tables vi 

Acknowledgments vii 

1. Objective 1 

2. Approach 1 

2.1 Introduction.1 

2.2 Carbon Nanomaterial-based Energy Storage Research.2 

2.3 Materials and Methods.3 

2.3.1 Randomly Oriented CNT-based Electrodes.4 

2.3.2 Vertically Aligned CNT-based Electrodes.5 

2.3.3 Current Collectors.7 

2.3.4 Separator Materials.8 

2.3.5 Electrolyte s.8 

2.3.6 Supercapacitor Assembly.8 

2.4 Electrical Characterization.9 

2.5 Mechanical Characterization.10 

3. Results 10 

3.1 Mechanical Results.10 

3.1.1 Randomly Oriented CNT-paper Electrodes.10 

3.1.2 Vertically Aligned CNT/Polymer Composite Electrodes.13 

3.2 Electrical Results.14 

3.2.1 Galvanostatic Charge-discharge Tests.14 

3.2.2 Self-discharge Tests.16 

3.2.3 C yclic Voltammetry.19 

4. Conclusions 21 


5. References 


22 






















6. Transitions 27 

6.1 Transitions into Developmental Army Programs.27 

6.2 Documentation.27 

6.3 Presentations.27 

List of Symbols, Abbreviations and Acronyms 28 

Distribution List 30 


iv 






List of Figures 


Figure 1. Images of DASH impacting the ground at a velocity of approximately 6.5 m/s. 

(b) and (c) show the great contortion that the body undergoes during impact. By (d), the 
body has almost fully recovered to its original shape (a). Image reproduced from 
reference 2. 2 

Figure 2. Conceptual drawing of a multifunctional structural-energy storage nanocomposite 
based on vertically aligned CNT-based electrodes. The inset shows a close up of the 
electrode/separator/electrode interface. 4 

Figure 3. CNT-based supercapacitor electrodes, (a) ROE on stainless steel current collector. 

(b) ROEs on commercial paper substrates processed via doctor blade technique. 5 

Figure 4. (a) SEM and (b) TEM images of vertically aligned CNTs grown via CVD (images 
provided by Rice University.) 6 

Figure 5. (a) Photograph displaying the flexibility of the aligned CNT-PDMS/PVDF 
composite electrode, (b) Photograph shows aligned CNT-PDMS/PVDF composite 
electrode with vacuum treatment (left) and without vacuum treatment (right). 7 

Figure 6. Electrical characterization equipment: (a) assembled Swagelok electrical test cell, 

(b) expanded view of the Swagelok electrical test cell with supercapacitor components, 
and (c) Arbin Instruments Supercapacitor Test System with the assembled Swagelok 


electrical test cell. 9 

Figure 7. (a) Instron Materials Characterization System with (b) pneumatic grips. 10 

Figure 8. Elastic modulus and tensile strength of commercial paper substrates as a function 

of strain rate. Error bars are located within the data points unless otherwise noted. 11 

Figure 9. Elastic modulus and tensile strength of CNT-coated paper electrodes obtained 
through bulk tensile tests. Note that all specimens were tested in the MD. Error bars are 
located within data points unless otherwise noted. 13 

Figure 10. (a) Schematic shows the direction of mechanical loading with respect to the 
aligned CNTs and (b) stress-strain curve for a neat polymer matrix and an aligned CNT 
composite electrode. 14 

Figure 11. Galvanostatic charge-discharge curves (current = ±0.5 mA). Red is the applied 
current; black is the measured voltage, (a) ROE 1 /G 2 supercapacitor, (b) ROE 2 /G 2 
supercapacitor, and (c) VAE 1 /G 2 supercapacitor. 15 

Figure 12. Charge/discharge rates with respect to applied current: (a) ROEj supercapacitor. 

(b) ROE2 supercapacitor. Blue is Gi, green is G2, red is G3, and cyan is G5. 16 

Note: SC - supercapacitor. 16 

Figure 13. Self-discharge profile (I c = 1 mA, cycle 10). Fines colors refer to the amount of 
time that the supercapacitor was held at the charged potential (1 V): Blue is 0 min, green 


is 30 min, and red is 60 min. (a) ROE 1 /G 2 supercapacitor, (b) ROE 2 /G 2 supercapacitor. 
Insets: Truncated time frames. 19 


v 




Figure 14. Average cyclic voltammograms of randomly oriented CNT-based 

supercapacitors. The CV measurements were carried out at a scan rate of 50 mV/s at 
room temperature. Blue is Gi, green is Gi, red is G 3 , and cyan is G 5 . (a) ROEi 
supercapacitor, (b) ROE 2 supercapacitor. 20 

Figure 15. (a) Average cyclic voltammograms of randomly oriented CNT-based 

supercapacitors with 6 M KOH electrolyte. The CV measurements were carried out at a 
scan rate of 50 mV/s at room temperature. Blue is ROEi and green is ROEi. (b) Cyclic 
voltammogram of a polymer infiltrated vertically aligned CNT-based supercapacitor 
(VAEi) with organic electrolyte (LiPFr, in EC:DMC). The CV measurement was carried 
out at a scan rate of 0.5 mV/s at room temperature. 20 


List of Tables 


Table 1. Manufacturer supplied cellulose filter paper properties. 8 

Table 2. Galvanostatic charge-discharge data.16 

Table 3. Supercapacitor self-discharge data.18 


vi 








Acknowledgments 


We would like to thank Arava L. M. Reddy, Myung G. Hahm, Robert Vajtai, and Pulickel M. 
Ajayan of Rice University for providing us with the vertically aligned carbon nanotube (CNT) 
forests used in this project and for helpful discussions on CNT-based energy storage devices. 
We would also like to thank Matthew H. Ervin of the Sensors and Electronic Devices 
Directorate, U.S. Army Research Laboratory (ARL), for helpful feedback on the report and the 
research. 




Intentionally Left Blank. 



1. Objective 


The proposed research plan seeks to combine the emerging area of multifunctional lightweight 
energy storing materials with the existing large-scale structural battery/capacitor technology to 
accomplish the following: 

1. Develop ultra-light, structurally robust, nanocomposite energy storage materials. 

2. Characterize their structure-electrical/mechanical relationships, as well as the 
electromechanical coupling effects. 

3. Characterize environmental effects on electro-mechanical performance. 


2. Approach 


2.1 Introduction 

A reoccurring issue for lightweight (mass < 100 g), palm-sized, micro vehicle (MV) platforms, 
and in particular micro aerial vehicle (MAV) platforms, is the lack of sufficient onboard power. 
Stringent size and weight constraints and demanding voltage and power requirements 
significantly limit the number and type of energy storage devices that can be housed in MVs. 
While most commercial and developmental MVs currently use commercial-off-the-shelf (COTS) 
lithium polymer batteries for their energy storage needs, the capacity of these batteries can limit 
mission durations to the order of minutes and the weight of these batteries can account for up to 
60% of the overall system mass (/). One method to increase the vehicle endurance without 
adding mass to the system or sacrificing payload capabilities is to incorporate multiple functions 
into a single material or structure. For example, the body or chassis of a MV could be replaced 
with a multifunctional material that would serve as both the vehicle structure and the energy 
storage device. 

One of the primary structural characteristics of biomimetic MVs is their inherent flexibility. As 
seen in figure 1, the flexibility of the Dynamic Autonomous Sprawled Hexapod (DASH) 16-g 
hexapedal robot allows the small ground-based robotic platform to withstand falls from large 
heights (28 m or 280 body lengths) by absorbing energy on ground impact (2). Another example 
is the “flexible, twisty, wing structure” of the Defense Advanced Research Projects Agency 
(DARPA) Bug, which enables the vehicle to generate lift on both the upstroke and downstroke 
by reversing the twist and camber (3). The structural supports of biomimetic MVs are typically 
flexible, polymer-based materials, such as common plastics or cellulose. Hence, a suitable 
structure-serving energy source for MVs should also be flexible. 


1 







Figure 1. Images of DASH impacting the ground at a velocity of approximately 6.5 m/s. (b) and (c) show the 

great contortion that the body undergoes during impact. By (d), the body has almost fully recovered to 
its original shape (a). Image reproduced from reference 2. 


While the concept of a structural-energy storage device is not new (4-9), most structural-energy 
storage research has focused on high stiffness materials for large (meter-scale) devices. For 
instance, early structural-energy storage work by Luo et al. (4) focused on structural capacitors 
based on carbon fiber (tensile modulus = 221 GPa, tensile strength = 3.1 GPa), paper, and epoxy 
(flexural modulus = 3.7 GPa, flexural strength = 138 MPa). Within the U.S. Army Research 
Laboratory (ARL), lightweight, structural, energy-storing materials—derived from polymers, 
resins, and carbon-fiber electrodes—have been studied for potential applications in manned 
vehicles ( 10 - 18 ) and man-portable unmanned vehicles ( 19 ). In related research, ARL has also 
investigated using carbon nanotube (CNT)-based electrodes for general purpose ( 20 , 21 ) and 
flexible (22) energy storage applications. Likewise, the research reported in this Director’s 
Research Initiative (DRI) report relies on CNT-based electrodes for flexible energy storage 
devices, with the particular application of providing both structural support and power to MVs. 

2.2 Carbon Nanomaterial-based Energy Storage Research 

Since the discovery of CNTs, and more recently graphene, there has been a significant amount of 
research on using the high surface area and conductivity of these materials for battery and 
supercapacitor electrodes ( 20 - 31 ). Because CNT-based electrodes do not rely on thin, 
continuous films that are vulnerable to cracking, they retain conductivity even at large strains 
( 32 , 33 ). As a result, these nanomaterial-based energy storage devices are ideal candidates for 
flexible MV applications. 

Although carbon nanomaterial-based energy storage device research is an extremely active area 
of study, little is known about the complex interaction between the electrical and mechanical 
properties of these devices. While some researchers have begun to examine the electrical 
properties of CNT-based energy storage devices before, during, and after steady-state 
mechanical loading ( 30 , 34 ), little information is known about the electrical properties during 
time-variant mechanical loading. Since composites have also been explored as strain sensors 
( 35 - 39 ), the electromechanical coupling in carbon nanomaterial-based energy storage devices 
must be investigated if these devices are to be successfully incorporated into MV platforms. 
While in-situ mechanical and electrical characterization is a major part of the proposed DRI 


2 























research, the year 1 studies focused on non-coupled electrical and mechanical characterization 
techniques. 

2.3 Materials and Methods 

The electrodes of supercapacitors and batteries are similar in that the electrical conductivity and 
the available surface area of the electrode material strongly affect the overall device 
performance, with larger values typically leading to better electrical performance. Carbon 
nanomaterial-based supercapacitors, however, are easier to manufacture and test than carbon 
nanomaterial-based batteries as the mirrored device architecture of a supercapacitor facilitates 
device assembly and the reversible charge storage mechanism of electrical double layer 
capacitors (EDLCs) results in enhanced device stability. For these reasons, year 1 studies 
focused on the development of carbon nanomaterial-based supercapacitors. CNTs were selected 
as the carbon nanomaterial of choice in year 1 of the project as they are cheaper and more readily 
available than graphene sheets or flakes. 

CNT-based electrodes can be fabricated in a random or aligned fashion. Randomly oriented 
CNT-based electrodes (ROEs) typically consist of CNT inks or solutions that are directly 
deposited onto a solid matrix or current collector ( 20 - 22 , 29 ) or randomly dispersed within a 
polymer matrix ( 30 ). Vertically aligned CNT-based electrodes (VAEs), on the other hand, 
typically consist of vertically aligned CNT forests that are infiltrated with a matrix material ( 24 ) 
or inserted into a polymer electrolyte membrane ( 40 ). Figure 2 contains a conceptual drawing of 
multifunctional structural-energy storage device based on vertically aligned CNT-based 
electrodes. As both CNT-based electrode morphologies have exhibited promising electrical 
behavior, we chose to examine both morphologies in year 1 of the DRI project. The following 
subsections describe the fabrication, modification, and characterization of the supercapacitors 
constructed from these CNT-based electrode materials. 


3 




Figure 2. Conceptual drawing of a multifunctional structural-energy 

storage nanocomposite based on vertically aligned CNT-based 
electrodes. The inset shows a close up of the 
electrode/separator/electrode interface. 

2.3.1 Randomly Oriented CNT-based Electrodes 

The electrical properties of two different randomly oriented CNT electrode compositions were 
studied in year 1 of the DRI project. Randomly oriented electrode composition #1 (ROEi) 
consisted of a conductive CNT composite (Cheap Tubes, Inc., outer diameter = 60-80 nm) and a 
polyvinylidene fluoride (PVDF) binder material and random electrode composition #2 (ROE 2 ) 
consisted of multi-walled nanotubes (MWNTs) (Cheap Tubes, Inc., outer diameter <8 nm), 
carbon black, and PVDF. Each electrode composition was dispersed in dimethylformamide 
(DMF) and deposited onto one side of a stainless steel current collector (described in section 
2.3.3). After drying, the current collector/electrode systems were weighed and the effective 
electrode mass was calculated. Figure 3a contains a picture of a random CNT electrode 
deposited on a stainless steel current collector. 


4 



if) 

(a) _(b)_ 

Figure 3. CNT-based supercapacitor electrodes, (a) ROE on stainless 
steel current collector, (b) ROEs on commercial paper 
substrates processed via doctor blade technique. 

In order to determine the effect of separator porosity and thickness on device performance, 
randomly oriented CNT electrodes (ROEi and ROE 2 ) were processed in a batch-wise fashion. 

As four separator materials were used in our initial studies, each random CNT electrode batch 
consisted of eight electrodes. The separator materials are discussed in greater detail in 
section 2.3.4. 

The mechanical properties of randomly oriented CNT-paper electrodes were also studied in year 
1 of the DRI project. Flexible CNT-ink-based electrodes were processed through a technique 
first reported by Hu et al. (29) and also used by Anton et al. (22). Inks were processed by 
dispersing MWNTs (Cheap Tubes, Inc., outer diameter <8 nm) in various solvents, including 
deionized water (DI-HiO) and DMF. For the DI-HiO based inks, a surfactant, sodium 
dodecylbenzenesulfonic acid (SDBS), was used to help disperse the CNTs. The CNT-ink 
solution was mixed via magnetic stirring for 30 min and then sonicated using a Sonics VibraCell 
probe sonicator at 200 W for 30 min. Immediately following the sonication, the solution was 
mixed for an additional 30 min via magnetic stirring. The CNT-ink solution was cast onto 
commercial printing paper using both the Meyer rod technique (29) and doctor blade technique 
( 41 ). Figure 3b shows an image of three randomly oriented CNT ink-based electrodes processed 
via the doctor blade technique. From left to right, the image shows electrodes with one, two, and 
three CNT ink applications. Multiple depositions were required to fully coat the paper-based 
electrodes. This could potentially be due to CNT agglomerates that were still present in the ink 
after the stirring and sonication steps. 

2.3.2 Vertically Aligned CNT-based Electrodes 

Vertically aligned CNT forests were grown via a water-assisted chemical vapor deposition 
(CVD) method by collaborators at Rice University ( 42 ). A catalyst layer consisting of aluminum 
(-10 nm) and iron (-1.5 nm) was first sputter deposited onto silicon wafers. For the CVD 
process, the carbon source used was ethylene gas, while an argon/hydrogen mixture run through 
a water bubbler was used as the carrier gas. The aligned CNTs were grown in a tube furnace 
held at 775 °C. Figure 4 shows scanning electron microscope (SEM) and transmission electron 



5 









microscope (TEM) images of the vertically aligned CNT forests. The TEM images were used to 
show that the CNTs contained anywhere from 5-10 walls and had an outer diameter in the range 
of 5-15 nm. 



Figure 4. (a) SEM and (b) TEM images of vertically aligned CNTs grown via CVD (images 
provided by Rice University.) 

Flexible, aligned CNT composite electrodes were fabricated by infiltrating the forests with a 
mixture of 80-90 wt.% of polydimethylsiloxane (PDMS) and 10-20 wt.% of PVDF. PDMS was 
used to increase the flexibility of the composite, while the PVDF was chosen to bind the active 
electrode material together. The liquid polymer solution was magnetically stirred for 30 min, 
followed by probe sonication at 200 W for 15 min, and then magnetically stirred for another 
30 min. The solution was then put under a low vacuum for 30 min to remove air bubbles from 
the solution. The de-gas sed liquid polymer was then poured on top of the aligned CNTs and 
allowed to infiltrate the forest. After allowing the composite to cure overnight, the sample was 
lifted off from the substrate using a razor blade. Figure 5a displays the flexibility of the aligned 
CNT-PDMS/PVDF composite electrode. The mass of CNTs (Mcnt) in each composite 
electrode was approximated by weighing: (1) the CNT forest (A) and silicon wafer (B) prior to 
polymer infiltration (Mi = A+B), (2) the CNT forest-polymer (C) composite and silicon wafer 
after complete curing, (Mo = A+B+C) (3) the silicon wafer (after removing composite) 

(M3 = B), and then subtracting the mass in step 3 from the mass in step 1 (Mcnt= Mi - M3 = 
A+B-B = A). This information was used to approximate the CNT weight fraction in the 
composite electrode (CNT wt.% = (Mi-M 3 )/(M 2 -M 3 )). 


6 










Figure 5. (a) Photograph displaying the flexibility of the aligned CNT-PDMS/PVDF composite 
electrode, (b) Photograph shows aligned CNT-PDMS/PVDF composite electrode 
with vacuum treatment (left) and without vacuum treatment (right). 

We note that several combinations of polymers/solvents were attempted for the forest infiltration 
process; however, the solutions containing solvents tended to cause the forests to collapse. The 
collapsing effect was thought to be a result of relatively high capillary forces acting on the 
loosely rooted CNTs in the presence of a solvent. The selection of the solvent-free 
PDMS/PVDF solution allowed CNT architecture to remain mostly intact, although as figure 5b 
shows, the forest still collapsed in some areas. This was thought to be a result of air bubbles that 
formed in the liquid polymer during the magnetic stirring and probe sonication steps. The 
vacuum treatment step prior to infiltration was effective in removing the air bubbles and 
preserving the alignment of the forest. SEM of the composite cross section is still required in 
order to verify the alignment. The ability to preserve the CNT alignment during infiltration was 
a key step in the processing of the electrodes, as even a partially collapsed CNT forest was 
expected to result in a greatly reduced capacitance due to the large reduction in electrode surface 
area. 

2.3.3 Current Collectors 

Although copper and silver are predominantly used as current collectors in the commercial 
battery industry ( 43 ) and aluminum and nickel current collectors have been used in previous 
ARL studies ( 20 - 22 ), stainless steel 304 sheets were used as the current collectors of the 
randomly oriented CNT-based electrode (ROEi and ROE 2 ) devices in an effort to match the 
electrical resistivity of the supercapacitor current collector to the Swagelok electrical test cell. 
While this material substitution precludes a direct comparison with COTS supercapacitor 
devices, it will allow us to directly compare the performance of our prototypes and gauge 
whether or not the design is worth pursuing. 

Electron Microscopy Sciences silver adhesive 503 (62 wt.% solid) was used as the current 
collectors of the vertically aligned CNT-based (VAEi) supercapacitors. The silver paint was 
applied to the electrode surface and allowed to dry overnight. 


7 







2.3.4 Separator Materials 

Whatman qualitative cellulose filter papers were used as the separator material for the CNT- 
based supercapacitors. In order to determine the effect of porosity and thickness on 
supercapacitor performance, four different grades (1, 2, 3, and 5) of cellulose filter paper were 
examined in year 1 of the project. Manufacturer supplied material properties of the cellulose 
filters can be found in table 1. Filter paper separators were cut to size and weighed before being 
inserted into the Swagelok electrical test cell. 


Table 1. Manufacturer supplied cellulose filter paper properties. 


Sample 

ID 

Sample 

Grade 

Particle Retention 
(Liquid) 

(pm) 

Typical 

Thickness 

(pm) 

Dry Tensile 
Strength (MD) 
(N/15 mm) 

G, 

1 

11 

180 

39.1 

g 2 

2 

8 

190 

44.6 

g 3 

3 

6 

390 

72 

g 5 

5 

2.5 

200 

55.6 


Note: MD = machine direction and Particle Retention (Liquid) = particle size at which a retention level of 98% of 


the total number of particles initially challenging the filter is obtained ( 48 ). 

2.3.5 Electrolytes 

6 M potassium hydroxide (KOH) was used as the aqueous electrolyte in the randomly oriented 
CNT-based supercapacitors (ROEi and ROE 2 ). A 1M solution of lithium hexafluorophosphate 
(LiPF 6 ) in 1:1 volume/volume mixture of ethylene carbonate (EC) and dimethyl carbonate 
(DMC) was used as the electrolyte for the vertically aligned CNT-based supercapacitors (VAEi). 
The LiPFg-based electrolyte was prepared and handled in an Omni-lab glove box system 
(Vacuum Atmospheres Company) filled with argon gas. The amount of electrolyte used in 
individual tests was based on the amount of solution necessary to fully wet the separator 
(determined visually). 

2.3.6 Supercapacitor Assembly 

All supercapacitors were assembled in a modified two electrode Swagelok cell (figure 6a and b). 
The modified Swagelok electrical test cell consists of a stainless steel Swagelok ultra-torr 
vacuum fitting (SS-16-UT-6BT), a solid stainless steel cylinder, and a spring-loaded plate- 
cylinder assembly. KOH-based supercapacitors were assembled in an ambient environment and 
LiPF6-based supercapacitors were assembled in the inert environment of the glove box. The 
Swagelok ultra-torr vacuum fitting is ideal for basic supercapacitor and battery research as it 
allows for quick and easy single device assembly and prevents electrolyte evaporation and 
contamination during prolonged electrical characterization. The spring mechanism in the test 
cell applies a constant, even pressure to the current collectors throughout the test process, 
thereby ensuring that adequate contact is made between the supercapacitor sub-components. In 


8 




order to ensure that current only travels through the device under test (DUT) and not through the 
Swagelok fitting, the fitting is electrically isolated from the stainless steel cylinder, DUT, and 
spring-loaded plate-cylinder assembly via 3M transparency film (PP22500). 



Figure 6. Electrical characterization equipment: (a) assembled Swagelok electrical test cell, (b) expanded view of 
the Swagelok electrical test cell with supercapacitor components, and (c) Arbin Instruments 
Supercapacitor Test System with the assembled Swagelok electrical test cell. 


Supercapacitors are assembled in a bottom-up process by first placing the current 
collector/electrode into the test cell, with the current collector facing the solid stainless steel 
cylinder. Once the bottom current collector/electrode is in place, the separator material is 
carefully placed on top of the electrode inside the test cell. After wetting the separator with 
electrolyte, the top current collector/electrode is placed into the test cell with the electrode facing 
the wet separator. Once the top current collector/electrode is in place, the spring-loaded plate- 
cylinder assembly is inserted into the open end of the Swagelok vacuum fitting and placed on the 
exposed current collector. The Swagelok electrical test cell is then sealed and prepped for 
electrical characterization. 

2.4 Electrical Characterization 

To verify the electrical properties of the current collectors, the resistivity of a subset of the 
stainless steel current collectors was measured on a Cascade Microtech semiautomatic probe 
station with a Keithley 4200 Semiconductor Characterization System using the van der Pauw 
method. Because resistivity is dependent on the thickness of the sample, a number of steps were 
taken to minimize errors due to thickness. Specifically, the sample thickness used in the 
resistivity calculations was the average thickness of 10 measurements made on the same sample 
and the average resistivity value was calculated from 50 resistivity measurements made at five 
different xy locations on the sample. 

Cyclic voltammetry (CV), self-discharge, and galvanostatic charge-discharge test schedules were 
developed in the MITS Pro Testing Software (Arbin Instruments) and implemented on an Arbin 
Instruments Supercapacitor Testing System (figure 6c). During electrical characterization tests, 


9 








current, voltage, cell temperature, and ambient temperature readings were recorded at a 
frequency no less than 0.2 Hz, depending on the test step and/or the change in current or voltage. 
Cell temperature was measured by placing a flexible tip surface probe (type T) thermocouple on 
the exterior of the Swagelok electrical test cell. 

2.5 Mechanical Characterization 

The mechanical properties of the randomly oriented CNT-ink electrodes and the vertically 
aligned CNT-based electrodes were characterized via an Instron 5965 Materials Testing System 
with a 500 N load cell (figure 7). Pneumatic grips were used in order to apply a constant 
pressure to the materials and prevent slipping during loading. For the paper-based electrodes, 
groups of five specimens were tested at various strain rates, and the average tensile strength and 
elastic modulus were determined via the Technical Association of the Pulp and Paper Industry 
(TAPPI) T494om-96 standard for mechanical testing ( 44 ). All tests were run at room 
temperature. The effects of temperature and humidity on mechanical performance will be 
explored in year 2 of the project. 



(a) (b) 


Figure 7. (a) Instron Materials Characterization System with (b) pneumatic grips. 


3. Results 


3.1 Mechanical Results 

3.1.1 Randomly Oriented CNT-paper Electrodes 

Initial mechanical tests were performed on commercial printing paper to determine the baseline 
substrate properties prior to CNT ink deposition. Commercial paper is typically manufactured 
from a slurry composed of water and 0.5-1.0% pulp fiber. The free standing paper is formed on 
a moving wire mesh that drains the water. The fibers tend to align with the direction of motion 
of the wire mesh, known as the machine direction (MD); the direction orthogonal to the MD is 


10 













referred to as the cross-machine direction (CD) ( 44 ). Figure 8 shows the orthotropic properties 
of the paper substrates used in this study. The tensile strength and elastic modulus of the paper 
loaded in the MD was approximately 250% higher than the paper loaded in the CD. The results 
also indicate that the mechanical behavior of the paper substrates is dependent on the strain rate. 
As the strain rate increased, the tensile strength and elastic modulus increased by approximately 
10% and 15%, respectively. 


ro 

o. 




10 

3 

TJ 

O 

u 

’+-» 

l/) 

_ro 

LLi 




Figure 8. Elastic modulus and tensile strength of commercial paper substrates as a function of 
strain rate. Error bars are located within the data points unless otherwise noted. 


11 











Tests on the CNT-coated paper samples (processed through the doctor blade technique) were 
performed in the MD in order to determine the maximum mechanical properties. The specimens 
were loaded at both 1 and 5 mm/min. Figure 9 shows the average tensile strength and elastic 
modulus of the CNT-coated paper electrodes. A control sample coated with pure DMF (0 layer 
of CNT ink) demonstrates the effects of the solvent on the mechanical properties of the paper. 
The tensile strength is largely unaffected; however, the elastic modulus of the sample decreases 
by approximately 50% as a result of the DMF treatment. Tests on the CNT-coated specimens 
also indicate that the CNT deposition process degrades the mechanical properties of the paper. 
The application of a single layer of the CNT ink caused the average elastic modulus and tensile 
strength of the coated paper to decrease by approximately 15%, with respect to the pure DMF- 
coated paper. This could potentially be due to additional damage caused to the paper substrate 
from the more viscous CNT-DMF solution, which was more difficult to spread across the paper 
substrate. Multiple applications of the ink were required to fully coat the paper substrates. A 
second application of CNT ink caused the average mechanical properties of the composite to 
degrade further. The application of a third layer did not cause a further decrease in the 
mechanical performance. 


12 




Layers CNT Ink 



Layers CNT Ink 


Figure 9. Elastic modulus and tensile strength of CNT-coated paper electrodes 

obtained through bulk tensile tests. Note that all specimens were tested in 
the MD. Error bars are located within data points unless otherwise noted. 

3.1.2 Vertically Aligned CNT/Polymer Composite Electrodes 

Uniaxial tensile tests were also performed on the vertically aligned CNT composite electrodes. 
Figure 10b compares the mechanical behavior of a neat PDMS (85 wt.%)-PVDF (15 wt.%) 
matrix and the same matrix loaded with 8 wt.% aligned CNTs. The specimens were loaded at a 
rate of 5 mm/min. The initial nonlinear portion of the loading curve corresponding to the 
specimen flattening was ignored; the initial linear behavior was used to calculate the elastic 
modulus. 


13 











The average elastic modulus of the composite electrode was approximately 2.5 times higher than 
the neat matrix. While the 250% increase in elastic modulus (E) is notable, the configuration of 
the composite electrode is not currently designed to maximize the mechanical properties. As the 
schematic in figure 10a shows, the CNTs are aligned transverse to the direction of loading. As a 
result, the configuration does not take full advantage of the stiffness of the CNTs, which is 
typically realized through aligning the filler with the direction of mechanical loading ( 45 , 46 ). 



Figure 10. (a) Schematic shows the direction of mechanical loading with respect to the aligned CNTs and 
(b) stress-strain curve for a neat polymer matrix and an aligned CNT composite electrode. 


3.2 Electrical Results 

In an effort to characterize the electrical properties of the randomly oriented and vertically 
aligned CNT-based supercapacitors, a number of electrical characterization tests were 
conducted. The following sections outline the results of the galvanostatic charge-discharge, self¬ 
discharge, and CV tests. 

3.2.1 Galvanostatic Charge-discharge Tests 

In an effort to characterize the performance of the CNT-based supercapacitors, a number of 
constant current (or galvanostatic) charge-discharge measurements were made. Figure 11 
contains the charge-discharge behavior of a randomly oriented and vertically aligned CNT-based 
supercapacitor. The charge-discharge curves of the randomly oriented CNT-based 
supercapacitors are similar to those reported in the literature ( 24 , 29 , 34 ). 


14 






















(a) 



(b) 



(c) 


Figure 11. Galvanostatic charge-discharge curves (current = +0.5 mA). Red is the applied current; black is the 
measured voltage, (a) ROE 1 /G 2 supercapacitor, (b) ROE 2 /G 2 supercapacitor, and (c) VAE 1 /G 2 
supercapacitor. 

Table 2 contains the average charge times, discharge times, and specific capacitance with respect 
to full electrode mass (i.e., total mass of CNTs, carbon black, and PVDF in the two-electrode 
supercapacitor device) for the randomly oriented CNT-based supercapacitors. Because the 
supercapacitor behavior is cycle dependent, data from cycles >3 was averaged and reported 
(unless otherwise noted). As table 2 and figure 12 illustrates, the charge/discharge rate is 
dependent on the applied current, the separator material, and the electrode composition. ROEi 
supercapacitors exhibited significantly faster (6-27 times) charge and discharge times than ROE 2 
supercapacitors. Of those randomly oriented CNT-based supercapacitors tested during the initial 
study, ROE 2 /G 1 had the highest specific capacitance (5.8 F/g). While additional studies are 
currently underway, the diameter of the CNTs in ROEi and ROE 2 are believed to play a 
significant role in the specific capacitance of the device, with larger diameter CNTs having more 
“dead weight” than smaller diameter, fewer walled CNTs. Electrode composition and dispersion 
and deposition techniques are also being investigated as potential sources for specific 


15 



















































































capacitance variations. The vertically aligned CNT composite electrodes displayed a much 
lower specific capacitance (-15 mF/g). This could potentially be the result of the poor ionic 
conductivity of the polymer matrix, which has not yet been optimized. 

Table 2. Galvanostatic charge-discharge data. 


Ic/d 

(mA) 

Sample 

ID 

T 

A charge 

(s) 

T . 

A discharge 

(S) 

c sp 

(F/g) 

Sample 

ID 

T 

A charge 

(s) 

T 

A discharge 

(s) 

c sp 

(F/g) 

0.1 

ROEyGi 

26+1 

17.26 + 0.02 

0.8 

ROE 2 /G, 

311 + 16 

152.4 + 0.4 

5.8 

0.5 

ROEyG! 

3.43 + 0.07 

2.98 + 0.01 

0.8 

ROE 2 /G! 

48 + 5 

28.5 + 0.1 

5.7 

1.0 

ROEyGi 

1.42 + 0.03 

1.29 + 0.01 

0.6 

ROE 2 /G! 

16.7 + 0.4 

14.86 + 0.05 

5.8 

0.1 

ROEyG, 

16.9 + 0.5 

12.1+0.2 

0.4 

ROE 2 /G 2 a 

1567 + 587 

192 + 1 

4.4 

0.5 

ROEyGz 

1.95 + 0.02 

1.81+0.01 

0.3 

ROE 2 /G 2 

55 + 3 

36.67 + 0.05 

4.4 

1.0 

ROE^Gz 

0.82 + 0.03 

0.74 +0.01 

0.2 

roe 2 /g 2 

21.9 + 0.2 

18.69 + 0.04 

4.5 

0.1 

ROE^Gj 3 

2570 

49 

X 

ROE 2 /G 3 a 

632 + 170 

112.1+0.8 

4.3 

0.5 

ROE!/G 3 

4.19 + 0.02 

3.83 + 0.02 

0.9 

ROE 2 /G 3 

24.7 + 0.4 

21.01+0.03 

4.2 

1.0 

ROE!/G 3 

1.79 + 0.03 

1.50 + 0.01 

0.7 

roe 2 /g 3 

12.5 + 0.1 

10.36 + 0.01 

4.2 

0.1 

ROEyGs 

20 + 2 

11.96 + 0.03 

0.5 

roe 2 /g 5 

282 + 9 

148.5 + 0.3 

4.9 

0.5 

ROE^Gs 

2.09 + 0.04 

1.82 + 0.01 

0.4 

roe 2 /g 5 

35.0 + 0.6 

29.50 + 0.02 

5.1 

1.0 

ROEyGs 

0.91+0.02 

0.79 + 0.01 

0.4 

roe 2 /g 5 

15.5 + 0.1 

14.45 + 0.03 

5.0 


a Did not reach 1 V during all cycles. Only those cycles that did reach 1 V were averaged. 

Note: I c/d - charge/discharge current, T charge - charge time, T discharge - discharge time, C sp - specific capacity 


(calculated with respect to full electrode mass) 



Figure 12. Charge/discharge rates with respect to applied current: (a) ROE! supercapacitor, (b) ROE 2 
supercapacitor. Blue is Gi, green is G 2 , red is G 3 , and cyan is G 5 . 

Note: SC - supercapacitor. 


3.2.2 Self-discharge Tests 

The self-discharge behavior of a supercapacitor is a key indicator of device performance as the 
spontaneous loss of voltage causes the capacitor to approach a condition where energy is 


16 






































required to reset the device to the charged, functional state. As a result, capacitors with lower 
self-discharge (or leakage) are typically more desirable, from an electrical standpoint, than those 
with higher self-discharge. 

The self-discharge behavior of the randomly oriented CNT supercapacitors was measured using 
two different methodologies. In the first method, the supercapacitor was charged to the desired 
potential (IV) and held at that potential for a given period of time (30 or 60 min). The residual 
current flow needed to keep the supercapacitor fully charged was averaged over the course of the 
constant voltage hold time (T h ) and was recorded as the supercapacitor float current (I floa t). The 
second method is similar to the first with the exception that instead of measuring the float current 
during the hold period, the voltage of the device is measured after the charging current is 
removed from the device. For this experiment, the voltage was recorded until the potential 
reached the lower cutoff threshold (50 mV) or until the voltage decay time (Tdecay) exceeded 2 h, 
whatever occurred first. Because the supercapacitor behavior is cycle dependent, data from 
cycles 3-10 were averaged and reported (unless otherwise noted). 

Table 3 contains the average float current and decay time for the randomly oriented CNT 
electrodes (ROEi and ROE 2 ). The leakage or float current is similar for all supercapacitors 
examined in this preliminary study, an indication that the value may be dependent on the device 
architecture or the testing setup. In the case of the latter, the internal circuitry of the Arbin 
Supercapacitor Testing System or the control law(s) used to regulate the DUT voltage may 
generate measurement artifacts. As this DRI is a multi-year project, this potential measurement 
issue will be investigated in year 2 of the DRI. As indicated in table 3, lower charging currents 
(0.5 mA) had a tendency to have slightly prolonged decay times (1.5-3 times longer). 


17 



Table 3. Supercapacitor self-discharge data. 


Ic 

(mA) 

T h 

(min) 

Sample 

ID 

Ifloat 

(mA) 

T 

A decay 

(min) 

v d 

(mV) 

Sample 

ID 

Ifloat 

(mA) 

T 

A decay 

(min) 

v d 

(mV) 

0.5 

0 

ROEjAV 

X 

3.6+0.7 

50 

ROE 2 /G! 

X 

67+1 

50 

1 

0 

ROEjAV 

X 

1.3 +0.0 

50 

ROE 2 /G! 

X 

60 + 2 

50 

2 

0 

ROEj/G, 

X 

X 

X 

ROE 2 /G! 

X 

56.9 + 0.5 

50 

1 

30 

ROE^G, 

0.5+ 0.3 

88 + 2 

50 

ROE 2 /G! 

0.8+ 0.2 

42 + 2 

50 

1 

60 

ROE^G, 

0.5 +0.3 

52 + 26 

50 

ROE 2 /G! 

0.7 + 0.2 

120 + 0 

80 

0.5 

0 

ROE,/G 2 

X 

0.9 +0.0 

50 

roe 2 /g 2 

X 

41+4 

50 

1 

0 

ROE,/G 2 

X 

0.6 +0.0 

50 

roe 2 /g 2 

X 

31 + 1 

50 

2 

0 

roe,/g 2 

X 

0.4+ 0.0 

50 

roe 2 /g 2 

X 

49.6 + 0.3 

50 

1 

30 

roe,/g 2 

0.5 +0.3 

120 + 0 

85 

roe 2 /g 2 

0.8 + 0.1 

63 + 12 

50 

1 

60 

roe,/g 2 

0.5 +0.4 

62 + 6 

50 

roe 2 /g 2 

0.8 + 0.1 

99 + 8 

50 

0.5 

0 

ROEVG, 

X 

2.3 +0.1 

50 

ROEj/G, 

X 

43 + 3 

50 

1 

0 

ROEj/Gd 

X 

1.3 +0.1 

50 

roe 2 /g( : 

X 

24 + 7 

50 

2 

0 

ROEVG, 

X 

1.6+ 0.0 

50 

ROE 2 /Gj 

X 

10.0 + 0.4 

50 

1 

30 

ROEVG, 

0.6+0.3 

120 + 0 

56 

ROE 2 /Gj 

0.7 + 0.2 

120 + 0 

60 

1 

60 

ROEVG, 

0.7 +0.2 

18 + 3 

50 

ROE 2 /G 3 

0.8+ 0.1 

71 + 10 

50 

0.5 

0 

ROE]/G 5 

X 

2.9 +0.0 

50 

roe 2 /g, 

X 

38 + 4 

50 

1 

0 

ROE,/G, 

X 

1.0+ 0.2 

50 

roe 2 /g, 

X 

26 + 2 

50 

2 

0 

ROE,/G, 

X 

1.4+0.0 

50 

roe 2 /g, 

X 

26+1 

50 

1 

30 

ROE^Gs 

0.6+0.3 

120 + 0 

93 

roe 2 /g 5 

0.7 + 0.2 

91 + 16 

50 

1 

60 

ROE^Gs 

0.5 +0.3 

45+4 

50 

roe 2 /g 5 

0.7 + 0.2 

86 + 26 

50 


“Did not reach 1 V during all cycles. Only those cycles that did reach 1 V were averaged. 


Note: I c - charge current, T h - hold time, Ifi oa , - float current, T decay - average decay time, V, t - decay voltage, X - not 
available. 

As the Ifloat values did not indicate which electrode composition and/or separator material had the 
best charge retention behavior, the self-discharge profiles of the supercapacitors were also 
recorded (table 3). For I c = 1 mA and Th = 0 min, supercapacitors constructed from ROEo had 
significantly longer decay times (24 min < Tdecay < 60 min) than those constructed from ROEi 
(0.6 min < T de cay <1.3 min), further supporting the results from galvanostatic charge-discharge 
tests. With the exception of supercapacitor ROEo/Gi, which had a decay time of 60 min, 
randomly oriented CNT-based supercapacitors exhibited a weak charge retention behavior when 
the devices were not held in a charged state for an extended period of time. For ROE phased 
supercapacitors, the decay time was longest for hold times of 30 min. For ROE 2 -based 
supercapacitors, Td eca y varied depending on the separator material, with supercapacitors ROE 2 /G| 
and ROE 2 /G 2 having longer decay times for a hold time of 60 min and ROE 2 /G 3 and ROE 2 /G 5 
having a longer decay time for a hold time of 30 min. As an example of this phenomenon, 
figure 13 contains the self-discharge behavior of supercapacitor ROE 1 /G 2 and supercapacitor 
ROE 2 /G 2 at various hold times. Although the mechanism for performance degradation for some 
supercapacitors at prolonged hold times (60 min) is not yet known, it may be the result of 
electrolyte degradation due to overcharging, corrosion of the current collector, or variations in 
the temperature of the testing environment (18.4 °C < RT < 28.2 °C). In an effort to mitigate 
temperature fluctuations in the testing environment, a dedicated air conditioning unit was 
installed in the laboratory. As can be seen in table 3, the overall decay time appears to be 
dependent on the charge current, hold time, and device properties. Of those supercapacitors 


18 




tested in this preliminary study, ROEo/Gi exhibited the best charge retention behavior, further 
supporting the results from galvanostatic charge-discharge tests. Nevertheless, the charge 
retention time of even ROE 2 /G 1 is several orders of magnitude lower than commercial 
supercapacitors (e.g., reference 47). As this is a multi-year project, mechanisms for improving 
the leakage current of the CNT-based supercapacitors will be explored in year 2 of the project. 



(a) (b) 

Figure 13. Self-discharge profile (I c = 1 mA, cycle 10). Lines colors refer to the amount of time that the 

supercapacitor was held at the charged potential (1 V): Blue is 0 min, green is 30 min, and red is 
60 min. (a) ROE 1 /G 2 supercapacitor, (b) ROE 2 /G 2 supercapacitor. Insets: Truncated time 
frames. 

3.2.3 Cyclic Voltammetry 

Electrochemical measurements were carried out on the randomly oriented and vertically aligned 
CNT-based supercapacitors at room temperature. Cyclic voltammograms for ROEi- and ROE 2 - 
based supercapacitors can be found in figure 14. Due to system noise (which was later resolved, 
see figure 15b), each of the curves shown in figure 14 was constructed from the average of 
20-25 individual cyclic voltammograms. As the figure illustrates, the cyclic voltammograms 
have a rectangular core and do not contain the distinctive current peaks that are associated with 
redox reactions. The current increase around the voltage minima and maxima is believed to be 
due to electrolyte degradation or heterogeneous pore distribution in the separator material, with 
the grade 1 separator being the most heterogeneous of the four materials tested. 

The cyclic voltammograms of the randomly oriented (ROEi and ROE 2 ) and vertically aligned 
(VAEi) CNT-based supercapacitors can be found in figure 15. Of the three electrode materials 
tested, ROE 2 exhibited the best electrochemical performance, as indicated by the area enclosed 
by the cyclic voltammograms. The capacity and energy density of the vertically aligned CNT 
supercapacitor is negatively impacted by the interaction of the gel electrolyte (LiPF6 in 
EC:DMC) with the polymers contained in the electrode. As indicated by the slope and enclosed 
area of the cyclic voltammogram shown in figure 15b, the polymer matrix we initially chose to 
investigate is believed to limit the ionic mobility of the organic electrolyte contained within the 


19 






























device. As a result, we are currently exploring different polymer matrices and device 
architectures to improve the electrical behavior of the vertically aligned CNT-based 
supercapacitors. 



Voltage (V) 



(a) 


(b) 


Figure 14. Average cyclic voltammograms of randomly oriented CNT-based supercapacitors. The CV 

measurements were carried out at a scan rate of 50 mV/s at room temperature. Blue is Gi, green is 
Gi, red is G ( , and cyan is G5. (a) ROEi supercapacitor, (b) ROE 2 supercapacitor. 



(a) 



Voltage (V) 

(b) 


Figure 15. (a) Average cyclic voltammograms of randomly oriented CNT-based supercapacitors with 6M 
KOH electrolyte. The CV measurements were carried out at a scan rate of 50 mV/s at room 
temperature. Blue is ROEj and green is ROE 2 . (b) Cyclic voltammogram of a polymer infiltrated 
vertically aligned CNT-based supercapacitor (VAEi) with organic electrolyte (LiPF fi in EC:DMC). 
The CV measurement was carried out at a scan rate of 0.5 mV/s at room temperature. 


20 






































4. Conclusions 


A major challenge in unmanned MV development stems from the lack of suitable energy storage 
devices. Demanding voltage and power requirements and stringent size and weight constraints 
significantly limit the number and type of batteries that can be housed in the MV structures. As 
a result, vehicle payloads and endurance times are significantly compromised. While the 
conventional approach is to create batteries with higher energy densities, an alternative approach 
is to replace structural components with multifunctional structural-energy storage materials. 

This current research effort has leveraged recent work in the areas of (1) lightweight, flexible, 
energy storage materials and ( 2 ) large-scale and high-stiffness multifunctional structural 
batteries, capacitors, and supercapacitors to develop lightweight, flexible, multifunctional 
structural-energy storage devices for MV applications. 

The first year of this DRI effort explored several combinations of lightweight nanocomposites 
for multifunctional structural-energy storage applications. The energy storage devices studied 
initially were based on (1) randomly oriented CNT/polymer electrodes, (2) randomly oriented 
CNT/paper electrodes, and (3) vertically aligned CNT/polymer electrodes. 

The baseline mechanical behavior of the CNT-ink-coated paper electrodes and the aligned CNT 
composite electrodes was established through uniaxial tensile tests. The elastic modulus and 
tensile strength of the CNT-ink-coated electrodes was found to be highly dependent on the 
loading direction and loading rates. The mechanical properties of the CNT-coated electrodes 
were also found to decrease by up to 25% after the multiple ink depositions that were required to 
fully coat the substrates. Commercial paper substrates coated with randomly oriented CNTs 
displayed mechanical behavior highly dependent on the loading direction and strain rate. 
Electrodes designed with vertically aligned CNTs displayed inherent flexibility as well as an 
elastic modulus approximately 250% higher than the neat PDMS/PVDF matrix. 

The electrical behavior of supercapacitors fabricated with CNT-based electrodes was established 
through galvanostatic charge-discharge, self-discharge and CV tests. During year 1 of the 
project, three different electrode compositions (ROEi, ROE 2 , and VAEi) and four different 
separator porosities (Gi, G 2 , G 3 , and G 5 ) were studied. ROE 2 -based supercapacitors exhibited 
higher specific capacitance and better charge retention behavior than supercapacitors constructed 
from ROEi and VAEi electrodes. While preliminary, the results presented herein also indicate 
that an engineered separator material with a porosity and thickness similar to the Gi separator 
may be most effective for aqueous (KOH) electrolytes. The cyclic voltammograms indicate that 
the capacity and energy density of VAEi supercapacitor was negatively impacted by polymer 
matrix material. Research focused on improving the specific capacitance of these randomly 
oriented and vertically aligned CNT devices will be continued in year 2 of the project. 


21 




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28. Guo, Q. H.; Zhou, X. P.; Li, X. Y.; Chen, S. L.; Seema, A.; Greiner, A.; Hou, H. Q. 
Supercapacitors Based on Hybrid Carbon Nanofibers Containing Multiwalled Carbon 
Nanotubes. Journal of Materials Chemistry 2009 ,19 (18), 2810-2816. 

29. Hu, L. B.; Choi, J. W.; Yang, Y.; Jeong, S.; La Mantia, F.; Cui, L. F.; Cui, Y. Highly 
Conductive Paper for Energy-storage Devices. Proceedings of the National Academy of 
Sciences of the United States of America 2009 ,106 (51), 21490-21494. 

30. Meng, C. Z.; Liu, C. H.; Chen, L. Z.; Hu, C. H.; Fan, S. S. Highly Flexible and All-solid- 
state Paperlike Polymer Supercapacitors. Nano Letters 2010 ,10 (10), 4025-4031. 

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ARL-TR-5451; U.S. Army Research Laboratory: Adelphi, MD, February 2011. 

32. Hu, L. B.; Yuan, W.; Brochu, P.; Gruner, G.; Pei, Q. Highly Stretchable, Conductive, and 
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25 



48. Whatman, “Whatman.” 

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26 



6. Transitions 


6.1 Transitions into Developmental Army Programs 

This research effort is primarily concerned with the development of lightweight structural/energy 
storage composites for MV applications. Within ARL, the Micro Autonomous Systems and 
Technology (MAST) program has identified energy storage as a key issue for the development 
of MVs. The broader impact of this work is the development of lightweight multifunctional 
building blocks for portable electronic systems, which could be of interest to such Army 
organizations as the Aviation and Missile Research, Development and Engineering Center 
(AMRDEC) and Communications-Electronics Research Development and Engineering Center 
(CERDEC). 

6.2 Documentation 

• Cole, D. P.; Rivera M.; Bundy M. Characterization of Mechanical Properties in 
Multifunctional Structural-energy Storage Nanocomposites for Lightweight Micro 
Autonomous Systems, In Proceedings of the ASME 2011 Conference on Smart Materials, 
Adaptive Structures and Intelligent Systems , Phoenix, AZ, 2011. 

• Rivera M.; Cole, D. P; Bundy M. Electrical Properties of Carbon Nanomaterial-based 
Structural-energy Storage Devices, In Proceedings of the ASME 2011 Conference on Smart 
Materials, Adaptive Structures and Intelligent Systems, Phoenix, AZ, 2011. 

• Rivera, M. Current and Next-Generation Energy Storage Devices for Micro Vehicle 
Applications, In Proceedings of the SAE 2011 AeroTech Congress and Exhibition, 
Toulouse, France, 2011. 

6.3 Presentations 

• Rivera, M.; Cole, D. P; Bundy, M. Electrical Properties of Carbon Nanomaterial-based 
Structural-energy Storage Devices,” ASME 2011 Conference on Smart Materials, Adaptive 
Structures and Intelligent Systems, Phoenix, AZ, September 20, 2011. 

• Cole, D. P; Rivera M.; Bundy, M. Characterization of Mechanical Properties in 
Multifunctional Structural-energy Storage Nanocomposites for Lightweight Micro 
Autonomous Systems,” ASME 2011 Conference on Smart Materials, Adaptive Structures 
and Intelligent Systems, Phoenix, AZ, September 20, 2011. 

• Rivera, M. Current and Next-Generation Energy Storage Devices for Micro Vehicle 
Applications,” SAE 2011 AeroTech Congress and Exhibition, Toulouse, France, October 
20 , 2011 . 


27 




List of Symbols, Abbreviations and Acronyms 


AMRDEC 

ARL 

ASME 

CERDEC 

CD 

CNT 

COTS 

C sp 

cv 

CVD 

DARPA 

DASH 

DI-H 2 0 

DMC 

DMF 

DRI 

DUT 

EC 

EDLC 

Gus 

Ic 

Id 

Ifloat 

KOH 


Aviation and Missile Research, Development and Engineering Center 
U.S. Army Research Laboratory 
American Society of Mechanical Engineers 

Communications-Electronics Research Development and Engineering Center 

cross-machine direction 

carbon nanotube 

commercial-off-the-shelf 

specific capacitance 

cyclic voltammetry 

chemical vapor deposition 

Defense Advanced Research Projects Agency 

Dynamic Autonomous Sprawled Hexapod 

deionized water 

dimethyl carbonate 

dimethylformamide 

Director’s Research Initiative 

device under test 

ethylene carbonate 

electric double layer capacitor 

cellulose filter paper grades 

charge current 

discharge current 

float current 

potassium hydroxide 


28 




LiPF 6 

MAST 

MAV 

MD 

MV 

MWNT 

PDMS 

PVDF 

ROE 

RT 

SC 

SDBS 

SEM 

TAPPI 

Tcharge 

Tdecay 

Tdischarge 

TEM 

T h 

VAE 

V d 


lithium hexafluorophosphate 

micro autonomous systems and technology 

micro aerial vehicle 

machine direction 

micro vehicle 

multi-walled nanotube 

polydimethylsiloxane 

polyvinylidene fluoride 

randomly oriented CNT-based electrode 

room temperature 

supercapacitor 

sodium dodecylbenzenesulfonic acid 
scanning electron microscope 

Technical Association of the Pulp and Paper Industry 
charge time 
decay time 
discharge time 

transmission electron microscope 
hold time 

vertically aligned CNT-based electrode 
decay voltage 


29 



No of. 

Copies Organization 
1 ADMNSTR 

ELEC DEFNS TECHL INFO CTR 
ATTN DTIC OCA 

8725 JOHN J KINGMAN RD STE 0944 
FORT BELVOIR VA 22060-6218 

10HCS US ARMY RESEARCH LAB 
ATTN RDRL VTM 
DANIEL P COLE (2 CPS) 

MONICA RIVERA (2 CPS) 

MARK BUNDY (3 CPS) 

DYLE 

MARK VALCO 
ATTN RDRL WM 
SHASHI KARNA 

ABERDEEN PROVING GROUND MD 21005 

9 HCS US ARMY RSRCH LAB 

ATTN IMNE ALC HRR MAIL & RECORDS MGMT 

ATTN RDRL CIO LL TECHL LIB 

ATTN RDRL CIO LT TECHL PUB 

ATTN RDRL SER L M ERVIN 

ATTN RDRL SER L M DUBEY 

ATTN RDRL SER L B PIEKARSKI 

ATTN RDRL-SED-C C. LUNDGREN 

ATTN RDRL SED C C XU 

ATTN RDRL SER P AMIRTHARAJ 

ADELPHI MD 20783-1197 

4 HCS US ARMY RSRCH LAB 

ATTN RDRL WMM E E NGO 
ATTN RDRL-WMM-A E WETZEL 
ATTN RDRL-WMM-A D. OBRIEN 
ATTN RDRL WMM G J SNYDER 
BLDG 4600 

ABERDEEN PROVING GROUND MD 21005 
TOTAL: 24 (23 HCS, 1 PDF) 


30