Xie Xian Ning, et al.
Novel Energy-Storage Membrane: Performance Surpasses
Existing Rechargeable Batteries and Supercapacitors
A team from the National University of Singapore's Nanoscience and
Nanotechnology Initiative (NUSNNI), led by principle investigator
Dr Xie Xian Ning, has developed a novel energy-storage membrane.
Electrical energy storage and its management are becoming urgent
issues due to climate change and energy shortage. Existing
technologies such as rechargeable batteries and supercapacitors
are based on complicated configurations including liquid
electrolytes, and suffer from difficulties in scaling-up and high
fabrication costs. There is also growing public concern and
awareness of the impact of traditional energy sources on the
environment, spurring a continued search for alternative, green,
sustainable energy sources.
Recognising the issue, Dr Xie and his team have developed a
membrane that not only promises greater cost-effectiveness in
delivering energy, but also an environmentally-friendly solution.
The researchers used a polystyrene-based polymer to deposit the
soft, foldable membrane that, when sandwiched between and charged
by two metal plates, could store charge at 0.2 farads per square
centimeter. This is well above the typical upper limit of 1
microfarad per square centimetre for a standard capacitor.
The cost involved in energy storage is also drastically reduced.
With existing technologies based on liquid electrolytes, it costs
about US$7 to store each farad. With the advanced energy storage
membrane, the cost to store each farad falls to an impressive
US$0.62. This translates to an energy cost of 10-20 watt-hour per
US dollar for the membrane, as compared to just 2.5 watt-hour per
US dollar for lithium ion batteries.
Dr Xie said: "Compared to rechargeable batteries and
supercapacitors, the proprietary membrane allows for very simple
device configuration and low fabrication cost. Moreover, the
performance of the membrane surpasses those of rechargeable
batteries, such as lithium ion and lead-acid batteries, and
The research is supported by grants from the Singapore-MIT
Alliance for Research & Technology (SMART), and National
Research Foundation. Dr Xie and his team started work on the
membrane early last year and took about 1.5 years to reach their
current status, and have successfully filed a US patent for this
The discovery was featured in Energy & Environmental Science
and highlighted by the international journal Nature.
Potential applications: From hybrid vehicles to solar panels and
The membrane could be used in hybrid vehicles for instant power
storage and delivery, thus improving energy efficiency and
reducing carbon emission. Potentially, hybrid cars with the
membrane technology could be powered by the energy stored in the
membranes in conjunction with the energy provided by fuel
combustion, increasing the lifespan of car batteries and cutting
down on waste.
The membrane could also be integrated into solar panels and wind
turbines to store and manage the electricity generated. Energy
provided through these sources is prone to instability due to
their dependence on natural factors. By augmenting these energy
sources with the membrane, the issue of instability could
potentially be negated, as surplus energy generated can be
instantly stored in the membranes, and delivered for use at a
stable rate at times when natural factors are insufficient, such
as a lack of solar power during night-time.
The research team has demonstrated the membrane's superior
performance in energy storage using prototype devices. The team is
currently exploring opportunities to work with venture capitalists
to commercialise the membrane. To date, several venture
capitalists have expressed strong interest in the technology.
"With the advent of our novel membrane, energy storage technology
will be more accessible, affordable, and producible on a large
scale. It is also environmentally-friendly and could change the
current status of energy technology," Dr Xie said.
Xie Xian Ning. Energy technology: Supersizing a supercapacitor.
Nature, 477, 9; 01 September 2011 DOI: 10.1038/477009c
Xian Ning Xie, Kian Keat Lee, Junzhong Wang, Kian Ping Loh.
Polarizable energy-storage membrane based on ionic condensation
and decondensation. Energy & Environmental Science, 2011; 4
(10): 3960 DOI: 10.1039/C1EE01841H
A Supercapacitive Energy Storage Device Based On
In the fight against environment pollution and global warming,
clean energy generation and storage is vital to the sustainability
of Singapore. Supercapacitors are the major energy storage devices
due to their extremely high capacity in storing electric charges
and energy. They are superior to batteries because their power
density is up to 100 times that of batteries. As clean energy
sources, supercapacitors have important applications in electric
vehicles and consumer electronic devices including iPods and
iPhones. The increasing concerns on energy and environment call
for new generation supercapacitors with improved performances and
This project aims to use the research team’s patent-pending
nanomaterial to develop novel supercapacitors for energy storage
and management. In contrast to commercial supercapacitors
containing liquid electrolyte, our recently invented
superhydrophilic nanomaterial allows for a different mechanism of
energy storage, and thus create the possibility of a new type of
solid-state supercapacitor. According to our background IP, the
preliminary performance of the proposed supercapacitor is
comparable to that of commercial carbon-based devices. Moreover,
it has a lot of space for further improvement by optimizing the
chemical compositions of the active nanomaterials. To convert the
background IP into a commercial reality, further R&D work is
proposed in this project to deliver a new generation
supercapacitor which has market viability owing to its simpler
configuration, lower cost, and higher performances. image
Dr. Xie Xian Ning, PI, Lab
Manager, NUS Nanoscience & Nanotechnology Initiative –
Dr. Xie received his PhD degree in Chemistry from National
University of Singapore. He is currently Lab Manager/Principle
Investigator with NUSNNI-NanoCore. Dr. Xie has more than ten
years’ experience in nanoscience and nanotechnology. His research
interest is in advanced nanomaterials for energy and environment
sustainability. As principle author, Dr. Xie has published more
than 40 scientific papers since 2000. He also served as reviewer
for premier journals including Adv. Mater., Adv. Funct. Mater. and
Small, etc. His current research effort is focused on the
development and commercialization of his patent-pending
nanomaterial in the water and energy market.
Polarizable Ion-Conducting Membrane for Energy
A highly polarizable ion-conducting energy-storage membrane
capacitor demonstrates simplicity, easy device scaling up and low
fabrication cost for electrical energy storage. This
material system also presents a high cycle life at maintained
The membrane-based capacitor can have an average capacitance
of ~0.2 F/cm2, energy of 0.33 J/cm2 and charge of 0.39 C/cm2
across various constant resistances from 1.2 to 8.0 k?.
It can be seen that a linear increase of membrane area
corresponds to a scaling factor of 0.2 F/cm2 in capacitance. This
demonstrates that simple massive device scalability is readily
achieved due to the linear relationship as established in Fig 1.
1: Capacitance of membrane pieces with different area A.
Inset: Variation of capacitance
as a function of charge-discharge cycles.
The material system exhibits large open voltage (3.0V) as shown in
Figure 2: Comparison of
cost per Farad between membrane-based and double-layer
High polarization current at 105 times higher than that of typical
dielectric polarization. Fig. 1 shows a polarization current of
~250 mA/cm2 at 10 V.
Membrane-based capacitor incurs lower cost in storing energy at
$0.62/F than that of double-layer capacitors at a rate of ~$7.0/F
as seen in Fig. 3.
3: Discharge voltage V(t) of the device under constant current
Durable, negligible capacity fading after 1000 cycles at room
Simplicity in massive scale-up
Good cycle life, high ion conductivity and polarizability
Low cost of energy storage
Energy-storage; Supercapacitor; Battery; Fuel Cell; Data storage
Water-loving electrodes store more charge: Ionic
conductivity in supercapacitors
image: Water-loving electrodes
store more charge: Ionic conductivity in supercapacitors
A new type of electrode material for supercapacitors that, for the
first time, predominantly exploits ionic conductivity is reported
online in the Journal of Polymer Science: Polymer Physics.
Compared to conventional electrode materials, which require large
surface areas and high porosities, lead researcher Xian Ning Xie
explains that the new hydrophilized polymer network uses
ion-conducting channels for fast ion transport and charge storage.
Supercapacitors have an extremely high energy density compared to
regular capacitors and can be charged and discharged repeatedly
without degrading like batteries do. Their high capacitance and
stability in electrolyte solutions could make them ideal for
applications like uninterruptable power supplies and hybrid cars.
The polymer network that Xie and his colleagues use includes PSSH
(poly(styrene sulfonic acid)). PSSH has a high density of sulfonic
SO3- groups, which attract a large amount of water, creating
hydrated paths with increased ionic conductivity.
According to the authors, previous publications emphasized only
the importance of electronic conductivity and surface area in high
capacitance charge storage. For supercapacitor electrode material,
“both electronic and ionic conductivity of the material are
important in terms of high capacitance charge storage. It is this
work that, for the first time, points out the significance of
ionic conductivity in supercapacitive energy storage,” says Xie.
In their experiments, the researchers prepared PEDT:PSSH films on
a graphite substrate and tested both their capacitive behavior as
well as their stability in electrolyte solutions. Since electrode
materials for supercapacitors are often soaked in liquid
electrolyte, good stability is needed. The authors explain that
the network exhibits excellent stability due to the cross-linking
of sulfonic groups in PSSH, which stops it from dissolving. From a
practical standpoint, Xie says “This is an added advantage in
device fabrication and operation, as in current supercapacitors,
insulating binders have to be added to ensure the adhesion of
electrode materials on the collector surfaces.”
In this work, Xie and coworkers demonstrate that
hydrophilizer-based ionic conductivity can greatly improve
supercapacitor performance. In the future, they plan to “combine
the ionic-conduction network to other electronic-conduction
materials, and thus to develop composite materials for better
Macromolecular Chemistry and
Volume 211, Issue 20, pages
2187–2192, October 15, 201
A Nanosegregant Approach to Superwettable and
1. Xian Ning Xie1,*,
2. Sharon Xiaodai Lim2,
3. Yuzhan Wang2,
4. Xingyu Gao2,
5. Kian Keat Lee1,
6. Chorng Haur Sow2,
7. Xiaohong Chen3,
8. Kian Ping Loh1,3,
9. Andrew Thye Shen Wee1,2
A simple and economical approach to the preparation of
superhydrophilic and water-capturing surfaces is reported. In this
method, a common polymer aqueous blend is drop-cast onto a
substrate, and the natural drying of the aqueous drop allows for
the formation of phase-segregated nanosegregants between the bulk
film and substrate surface. The nanosegregant is then exposed by
dissolving the bulk film using water. The nanosegregant is stable,
water-insoluble and optically transparent, and exhibits
superhydrophilicity with a minimum contact angle below 10°. It
also displays strong water-attracting ability. The mechanism of
superwettability and water-capturing behavior is discussed in
terms of the self-organization and functional group of the
Inventor(s): XIE XIAN NING [SG]; LOH KIAN PING
[SG] + (XIE, XIAN NING, ; LOH, KIAN PING)
Applicant(s): UNIV SINGAPORE [SG]; XIE XIAN
NING [SG]; LOH KIAN PING [SG] + (NATIONAL UNIVERSITY OF SINGAPORE,
; XIE, XIAN NING, ; LOH, KIAN PING)
Classification: - international:
C09K3/00; C09K3/18; H01G4/14
A coated substrate includes a substrate and a coating containing a
water insoluble polymer and a water soluble polymer, the two
polymers, due to different water affinity, forming a nanosegregant
on the substrate. Also disclosed are a method of preparing the
above-described coated substrate and the use of this coated
substrate in a solid-state supercapacitor.
BACKGROUND OF THE INVENTION
Typically, superhydrophilic surfaces have a water contact angle of
less than 25[deg.] and water-capturing surfaces retain water as a
uniform film having a thickness of millimeters.
Many techniques have been developed to prepare superhydrophilic
surfaces. See I.P. Parkin et. al., J. Mater. Chem. 2005, 15, 1689.
For example, superhydrophilic surfaces are obtained by
UV-irradiation of oxide semiconductor films such as Ti02 and ZnO.
In this method, the superhydrophilicity, induced by
photon-generated short-lived charges, gradually disappears without
continuous UV illumination. See, e.g., X.M. Li et. al., Chem. Soc.
Rev. 2007, 36, 1350, A. Lafuma et. al., Nat. Mater. 2003, 2, 457.
A method has been devised for producing water-capturing surfaces
that mimic the water harvesting wing surfaces of the Namib Desert
beetle. See L. Zhai, Nano Lett. 2006, 6, 1213. This method is not
suitable for large-scale production as it involves layer- by-layer
multi-step patterning and deposition of both hydrophilic and
Of note, superhydrophilic surfaces do not necessarily possess
Surfaces that are superhydrophilic and/or water-capturing have
many industrial applications. Superhydrophilicity prevents fog
formation, as condensed water spreads across a superhydrophilic
surface. On the other hand, water-capturing surfaces can be used
to draw water from dew in arid areas. A surface that possesses
both superhydrophilicity and water-capturing capacity is ideal for
use in a solid-state supercapacitor.
There is a need for cost-efficient methods of preparing enduring
superhydrophilic and/or water-capturing surfaces.
SUMMARY OF THE INVENTION
One aspect of this invention relates to a coated substrate having
a surface that possesses both superhydrophilicity and
water-capturing capacity. The coated substrate includes (i) a
coating having a water insoluble polymer and a water soluble
polymer, and (ii) a substrate covered by the coating.
The two polymers, due to their different water affinity, form a
nanosegregant having a thickness of 1-10 nm (e.g., 1-5 ran). More
specifically, in the nanosegregant, the water insoluble polymer
adheres onto the substrate and the water soluble polymer adheres
onto the adhered water insoluble polymer. The nanosegregant has a
water contact angle of 0-25[deg.] (e.g., 5-20[deg.]) and a
capability to capture a water film having a thickness of 0.1-10 mm
(e.g., 0.5-3.0 mm).
The coating can further include, on top of the nanosegregant, a
conductive non- nanosegregant film containing the same water
soluble polymer and the same water insoluble polymer. Unlike the
nanosegregant, this film, having a thickness of
0.4-500 [mu][pi][iota], does not exhibit either
superhydrophilicity or water-capturing capacity.
The water soluble polymer contains identical or different
hydrophilic and hygroscopic groups, e.g., carboxylic groups or
sulfonic groups. An example of such a water soluble polymer is
poly(styrene sulfonic acid) ("PSSH"). The water insoluble polymer,
on the other hand, contains identical or different electrically
conductive groups, e.g., aniline groups or thiophene groups. An
example of such a water insoluble polymer is
poly(3,4-ethelynedioxythiophene ("PEDT"). In one embodiment of the
above- described coated substrate, PSSH and PEDT are used as a
water soluble polymer and a water insoluble polymer, respectively,
at a ratio of 2.5 : 1 to 20: 1 by weight (e.g., 6:1 by weight).
Another aspect of this invention relates to a method of preparing
the coated substrate depicted above. The method includes at least
three steps: (i) providing an aqueous dispersion containing a
water soluble polymer and a water insoluble polymer, (ii) applying
the aqueous dispersion onto a surface of a substrate, and (iii)
allowing the applied aqueous dispersion to dry to form on the
surface of the substrate a nanosegregant, due to different water
affinity of the two polymers. The excess applied polymers form a
non-nanosegregant film, which is mentioned above. The method can
further include, after step (iii), a step of removing the excess
applied polymers to expose the nanosegregant.
The coated substrate of this invention can be a positive electrode
having a coating that includes both a nanosegregant and a
non-nanosegregant film. This coated positive electrode can be used
to make a solid-state supercapacitor. Such a solid-state
supercapacitor includes (i) a positive electrode, (ii) a negative
electrode, (iii) a nanosegregant, as described above, formed from
an aqueous dispersion that contains an ionizable water soluble
polymer and a water insoluble polymer due to their different water
affinity of the two polymers, and (iv) a conductive film, as
described above, containing the water soluble polymer and the
water insoluble polymer. The nanosegregant is disposed between the
positive electrode and the film, and the film is disposed between
the negative electrode and the nanosegregant.
The details of one or more examples of the invention are set forth
in the description below. Other features, objects, and advantages
of the invention will be apparent from the detailed description of
the examples and also from the drawings and the appending claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings are first described.
is a schematic
diagram illustrating a process of preparing a superhydrophilic and
is a schematic
diagram illustrating a process of preparing a solid-state
DETAILED DESCRIPTION OF THE
Within the scope of this invention is a substrate having a coating
made of a water soluble polymer and a water insoluble polymer. The
coating includes a nanosegregant (with a thickness of 1-10 nm)
containing the two polymers and, optionally a conductive
non-nanosegregant film (with a thickness of 0.4-500
[mu][iota][eta]) also containing the two polymers. The
nanosegregant unexpectedly exhibits both superior
superhydrophilicity and superior water-capturing capacity. The
substrate can be a metal, an insulator, a semiconductor, a
polymer, or a combination thereof. Examples of a metal include
gold, silver, copper, iron, aluminum, lead and alloy (i.e.,
stainless steel). Examples of an insulator include Si02, wood,
porcelain, clay, alumina, silicon, and paper. Examples of a
semiconductor include silicon, silicon carbide, gallium arsenide,
silicon nitride, indium sulfide, zinc oxide, and diamond. Examples
of a polymer include P VC, Teflon, polycarbonate, polyester,
nitrocellulose, polyethersulfone, and polypropylene.
To prepare a coated substrate of this invention, one applies onto
the surface of a substrate an aqueous dispersion including a water
soluble polymer and a water insoluble polymer. An artisan can mix
the two polymers in water to obtain this dispersion, or purchase
it from a commercial supplier.
The term "water soluble polymer" refers to a polymer that contains
identical or different hydrophilic and hygroscopic groups (e.g.,
carboxylic groups and sulfonic groups), which make the polymer
soluble or dispersible in water. Each of the groups can exist in
the form of an acid (e.g., carboxylic acid and sulfonic acid), or
in the form of a salt (e.g., carboxylate and sulfonate). Note that
when the water soluble polymer is used to prepare a supercapacitor
as discussed below, such a polymer must also be ionizable.
The term "water insoluble polymer" refers to a conductive polymer
that does not contain ionizable or strong polar groups, but
contains identical or different electrically conducting groups
(e.g., aniline groups and thiophene groups). Each of the groups
can exist in the form of an unsubstituted group (e.g., aniline and
thiophene), or in the form of a substituted group (e.g.
substituted aniline and substituted thiophene).
It is critical that the two polymers can be well dispersed in
water as nanoparticles (e.g., a diameter of 20-120 nm) so as to
form an aqueous dispersion.
The ratio between the water soluble polymer and the water
insoluble polymer varies. It affects the properties of the
nanosegregant and the film prepared from the two polymers, e.g.,
superhydrophilicity, conductivity, and water-capturing capacity.
One can apply the aqueous dispersion onto a substrate by
drop-casting. Other methods include, but are not limited to, spin
coating, spray coating, roller coating, flow coating, roll-to-roll
coating, and electrospinning. Once an aqueous dispersion has been
applied onto a substrate, it is left to dry. During this process,
the water soluble polymer and the water insoluble polymer undergo
phase segregation. As a result, a nanosegregant forms on top of
the substrate and a conductive non-nanosegregant film forms on top
of the nanosegregant. The conductive film can be washed away to
expose the nanosegregant. Due to the intermolecular
reorganization, the nanosegregant adheres strongly to the
It has been found that nanosegregants formed on various substrates
remain on the substrates surfaces even after boiling in water or
other common solvents.
The phase segregation is driven by the different water affinity of
the water soluble polymer and the water insoluble polymer. The two
polymers segregate themselves on the substrate surface to reach a
thermodynamic equilibrium in order to minimize the Gibbs free
energy of the whole system.
The nanosegregant thus formed, not soluble in water, has a
thickness of 1-10 nm. Optionally, it also has a nanometer length
and a nanometer width, e.g., 20-150 nm. These dimensions can be
measured by an atomic force microscope (AFM). See X. N. Xie, et.
al., Macromol. Chem. Phys, 2010, 211, 2187. It exhibits two
unexpected superior features. First, it transforms various
substrate surfaces into superhydrophilic surfaces with water
contact angles of 0-25[deg.], regardless of the water wettability
of the substrate surfaces. Second, the nanosegregant demonstrates
a strong water-capturing capacity. With a thickness of nanometers,
it is capable of capturing a water film having a thickness of
In one embodiment, the density of the hydrophilic and hygroscopic
groups contained in the water soluble polymer are in the order of
10 groups/cm in the segregant and the density of the electrically
conducting groups contained in the water insoluble polymer are
also in the same order.
When PEDT is used as a water insoluble polymer and PSSH is used as
a water soluble polymer, the nanosegregant thus formed contains
both PEDT and PSSH. The sulfonic acid groups contained in PSSH are
distributed on the nanosegregant surface and provide the strong
hydration force for water-capturing after the above-described
phase segregation. Previous calculations have shown that one
sulfonic acid group can capture five water molecules to form a
[Eta]502<+> hydronium nanocluster. See H. M. Li, et. al.,
Polymer International, 2001, 50, 421-428. The hydronium
nanocluster is much more stable than bulk water because the
hydrogen bond length of the nanocluster is significantly shorter
than that of bulk water. The [Eta]502<+> nanocluster forms
the first hydration shell surrounding the sulfonic acid group and
provides the base for further water uptake through proton
derealization and hydrogen bonding. Thus, a nanometer- thick
nanosegregant can transform various substrate surfaces into
superhydrophilic surfaces, and it can also capture a water film
having a thickness of millimeters.
The conductive non-nanosegregant film contains the same water
soluble polymer and the same water insoluble polymer in the
nanosegregant. As the two polymers in the film do not undergo
phase segregation, the film, does not exhibit either
superhydrophilicity or water-capturing capacity. The thickness of
the film (0.4-500 [mu][pi][iota]) can be determined based on
measurement by an AFM or surface profiler.
When the above-described coated substrate is used as a positive
electrode having a coating that includes both the nanosegregant
and the conductive non-nanosegregant film, a negative electrode is
attached to the film to form a supercapacitor with an
electrode/film/electrode sandwich configuration.
Both the nanosegregant and the water absorbed in the film
contribute to the supercapacitive behavior, which can be explained
by a nanosegregant-mediated charge storage mechanism. The
nanosegregant is superhydrophilic as it is negatively charged,
attributable to the ionizable and hygroscopic groups (e.g.,
S03<"> sulfonic groups) in the water soluble polymer. The
nanosegregant containing immobile hygroscopic groups is surrounded
by mobile H<+> ions (due to electrostatic interactions), and
also by hydration water (due to its superhydrophilicity). When the
positive electrode is charged, H<+> cations are forced to
move through the film towards the negative electrode. At the same
time, the absorbed hydration water is dissociated into H<+>
and OH<"> ions by the charging voltage, which move in
opposite directions. When the supercapacitor is fully charged,
H<+> cations accumulate near the negative electrode, while
OH<"> (and S03<">) anions are located in the vicinity
of the positive electrode. This ionic relocation generates an
internal field that holds the positive charge near the negative
electrode and the negative charge near the positive electrode,
thus achieving charge storage in the supercapacitor. According to
this mechanism, the negatively charged nanosegregant (e.g.
OH<"> and SO3<">) on the positive electrode dictates
that the electrode is positively charged. This is in agreement
with the observation that the supercapacitive behavior only takes
place on the positive electrode and the influence that small
H<+> cations can move through the film due to their high
mobility in the film. In contrast, negatively charging the
electrode coated with a nanosegregant requires the movement of
OH<"> anions through a thick film, which is not favorable
due to the poor mobility of OH<"> anions in the film. The
film, which does not contain a nanosegregant, just acts as a
typical conducting media for its electronic and ionic
The high Cs (e.g., about 65 mF/cm<2> as shown in an example
below) of the supercapacitor is related to the large surface area
of the nanosegregant. The nominal surface area of the
nanosegregant is estimated to be at least >10 m /g. The density
of the hygroscopic groups on the nanosegregant surface is also
very high, which explains the strong hydration force for the
water-capturing behavior. The strong electrostatic field of the
nanosegregant generates a large counter-ion (H<+>) zone and
attracts thick shells of hydration water. Therefore, the
supercapacitive characteristic of the supercapacitor is
attributable to the formation of an electric double layer around
the nanosegregant under a charging voltage. The absorbed hydration
water provides mobile H<+> cations for charge storage. Thus,
the supercapacitive behavior disappears when the absorbed water is
A supercapacitor of the invention can further include in the
nanosegregant and the film, a carbon-based nanomaterial that has a
high surface area (e.g., 2000 m<2>/g) and a high electric
conductivity (e.g., 10<4> S/cm). Graphene is an example of
such a nanomaterial. Specifically, graphene powders or flakes can
be added into an aqueous dispersion of a water soluble polymer and
a water insoluble polymer. This aqueous dispersion is then used to
form a supercapacitor by the same method described above. The
supercapacitor thus prepared exhibits even higher energy storage
The coated substrate provides numerous other industrial
applications, e.g., anti- fogging mirrors and traffic signs,
self-cleaning devices and water-harvesting devices.
Without further elaboration, it is believed that one skilled in
the art can, based on the disclosure herein, utilize the present
invention to its fullest extent. The following specific examples
are, therefore, to be construed as merely descriptive, and not
limitative of the remainder of the disclosure in any way
whatsoever. All publications cited herein are incorporated by
Preparation of superhydrophilic
and water-capturing surfaces
A PEDT:PSSH polymer blend (CLEVIOS(TM) P, Heraeus Clevios Gmbh,
Germany, w/w 1:6), as an aqueous dispersion, was used to prepare
superhydrophilic and water-capturing surfaces. In the dispersion,
the two polymers form nanoparticles, each having a diameter of
20-110 nm. It had a combined polymer concentration of 1.0-3.0 % by
As illustrated in Fig. 1, a PEDT:PSSH aqueous drop was first cast
onto the surface of a Si substrate and left to dry at ambient
temperature for about 3 hrs. During the drying process, the two
polymers formed a PEDT:PSSH nanosegregant on the substrate and a
PEDT:PSSH film on top of the nanosegregant. As discussed below,
AFM methodology confirmed the formation of both the nanosegregant
and the film. The film was then removed by gently rinsing it with
deionized water to expose the nanosegregant.
The water contact angles of the Si surface, both bare and
nanosegregant-covered, were measured in a manner similar to that
described in S. H. Lu, et. al., Langmuir 2009, 25, 12806. The
water capturing capacity of the nanosegregant was estimated by
measuring the average thickness of the absorbed water film. The
average thickness was obtained by measuring both the weight of the
water captured film and the size of the water captured film. The
measurements show that the nanosegregant was superhydrophilic
(exhibiting a minimum water contact angle of 8[deg.]) and
possessed a great water-capturing capacity (retaining a water film
as thick as 3.0 millimeters.
When the drying process was expedited, i.e., fast drying in vacuum
for several minutes, no nanosegregant could be observed after the
film was rinsed off. This indicated the importance of the drying
process during the nanosegregant formation. When the PEDT:PSSH
aqueous drop was left to dry at ambient temperature for more than
3 hrs, e.g., 4.0 or 5.0 hrs, the superhydrophilicity and
water-capturing capacity of the nanosegregant remained about the
same. AFM images revealed that the nanosegregant had a round or
elongated shape. The average width and height of the nanosegregant
were -70.0 and -2.4 ran,
respectively. The maximum length of the elongated nanosegregant
was -165 nm. The thickness of the film was determined to be -1.2
[mu][pi][iota] based on measurement of the height difference
between the film-covered Si surface and the nanosegregant-covered
Si surface. The film had the same width and the length as the
segregant. See X. N. Xie, et. al., Macromol. Chem. Phys, 2010,
The film was removed by water in seconds. The nanosegregant, on
the other hand, remained on the Si substrate after immersion in
water for half a year.
S 2p core level spectra were compiled from the film and
nanosegregant by using photoelectron spectroscopy (PES) in the
manner described in X. N. Xie, et. al., ACS NANO 2009, 3, 2722; X.
N. Xie, et. zX.Macromol. Chem. Phys, 2010, 211, 2187.
For the film, there were two peaks located at 163.5 and 167.8 eV.
The peak at 163.5 eV arose from the thiophene ring of PEDT, and
the peak at 167.8 eV originated from the sulfonic group
(R-S(=0)2-OH) of PSSH. See G. Grecaynski, et. al., Thin Solid
Films, 1999, 354, 129-135.
The S 2p core level spectrum of the nanosegregant also consisted
of two peaks corresponding to the PEDT and PSSH. However, the two
peaks of the nanosegregant were systematically shifted towards
higher binding energy (by -1.4 eV). This was attributable to the
charging of the nanosegregant, as such a shift was also observed
in the PES measurement for other core levels including C I s and
Si 2p. The charging phenomenon indicated that the nanosegregant
was more insulating than the film.
Other than Si, four additional substrates, i.e., Au, glass, mica
and plastic transparency, were also used to prepare a
nanosegregant on their surfaces by the same method described
above. The water contact angles of bare and nanosegregant-covered
substrate surfaces were measured and the measurement results are
listed in Table 1 below.
Table 1 below shows that, regardless of water wettability of the
surfaces of these substrates, the nanosegregant transformed all of
them into superhydrophilic surfaces with water contact angles in
the range of 7[deg.] to 20[deg.].
Due to its superhydrophilicity, the nanosegregant showed an
anti-fogging effect. A glass substrate with a nanosegregant on its
surface was placed above a water boiler that generated water
steam. When the water steam condensed onto the glass substrate, it
formed a uniform water layer on the nanosegregant-covered surface
and droplets on the bare glass surface. In other words, the
nanosegregant-covered surface remained transparent and the bare
glass surface was foggy.
Other than the superhydrophilic feature, the water-capturing
capacity of the PEDT:PSSH nanosegregant was also tested. A Si
substrate with a nanosegregant on its surface was dipped into
water for a few seconds. A water film, having a thickness of
millimeters, was formed exactly where the nanosegregant was
located, indicating that the nanosegregant retained a great amount
of water. Similar water-capturing behavior was also observed for a
nanosegregant on a Au surface. The shape of the nanosegregant was
purposely made irregular. After being dipped into water, the
nanosegregant captured a water film that had exactly the irregular
shape. The other three coated substrates listed in Table 1 also
exhibited such water-capturing capacity.
Preparation of solid-state
As shown in Fig. 2, an aqueous drop of PEDT:PSSH blend described
above was cast onto a Au substrate and left to dry at ambient
temperature. As a result, a PEDT:PSSH film and a PEDT:PSSH
nanosegregant were formed, with the nanosegregant disposed between
the Au substrate and the film. AFM images showed that the
nanosegregant had an elongated structure, packed densely on the Au
surface and having a length of 50-120 nm and a height of 1.5-3.8
ran. In contrast, surface profiling indicated that the film was
much thicker, i.e., having a height of about 100 [mu][iota][eta].
Finally, Ag glue was pasted on the film, thus forming a
Current-voltage (IV) curves were compiled from this supercapacitor
by scanning the Au electrode voltage in the form of 0->8->0
V at different scan rates. The IV curves were characterized by
strong hysteresis between the forward (0->8 V) and reverse
(8->0 V) currents. The hysteresis depended on the voltage scan
rate v; namely, the higher the v, the greater the hysteresis. This
behavior is indicative of charge storage in the supercapacitor. By
integrating the area Aj, (the active area of Ag electrode was ~1.0
mm<2>) of the hysteresis and employing the equation Q =
^<h>/v, the typical amount of charges Q stored in the
supercapacitor was estimated to be about 14.4 mC.
The supercapacitor was also charged by a constant current of 2.0
mA for 20 minutes with a charging voltage of 5.0 - 8.0 V applied
to the Au electrode. It was then discharged to measure the power
and the energy delivered by the supercapacitor. The fully charged
supercapacitor showed a typical open-circuit voltage Voc of 1.85 -
2.2 V. Under a discharge current of 1.0 mA, the voltage of the
supercapacitor dropped from 2.0 V to 0.02 V within 1.2 s. The
discharge time increased to 1335 s when a much smaller discharge
current of 0.001 mA was used. The discharge current was also
detected with a constant resistance R connected with the
supercapacitor. When R was 5.0 I D, the initial current was -0.4
mA and the discharge completed in 9.2 s. When R was 1000
i-[Omega], the initial current was in the order of several
[mu][Alpha] and the discharge took -2000 s.
A Ragone plot was constructed for the supercapacitor using data
obtained in the constant current and resistance methods described
above. The Ragone plot shows that the energy density was quite
stable (-3.5 W.h/g) during the entire discharge period. The
maximum power density was 1 1000 W/kg, comparable to that reported
for electrolyte solution-based supercapacitors. See, e.g., B. E.
Supercapacitor s: Scientific Fundamentals and Technological
Applications, Kluwer Academic/Plenum Publisher, New York, 1999; A.
S. Arico, et. al, Nature Mater. 2005, 4, 366. Table 2 below lists
the capacitance C, output energy E, specific capacitance Cm
specific capacitance Cs, and energy density Em of the
supercapacitor using the data obtained by the above-described
constant current and resistance methods. The Cm, Cs, and Em were
obtained by normalizing the C and E to the PEDT:PSSH film mass of
the supercapacitor. The mass of the film was determined by peeling
off the film and weighing it with a microbalance. It was about 0.1
mg for the film with an area of 1.0 mm . The m and Em were about
6.5 F/g (or Cs was 65 mF/cm ) and 3.5 W.h/kg, respectively.
In addition, the fully charged supercapacitor was used as an
energy source to power a digital thermometer (1.5 V, 0.06 mA) for
durations of up to 60 s. As listed in Table 2, the energy
delivered to the thermometer and the corresponding capacitances
were in agreement with those obtained in constant current and
constant resistance methods.
This supercapacitive behavior was found to depend on presence of
water in the film. Some water remained in the film as the
supercapacitor was prepared and measurement were performed both at
ambient temperature. When the supercapacitor was placed in a
vacuum chamber to remove absorbed water, it was found to lose the
charge storage function; namely, the IV curve of the film showed
no hysteresis. Only a negligible open circuit voltage (Voc) of 0.3
V and a short circuit current (7SC) of 0.2 [mu][Alpha] were
observed after fully charging the supercapacitor at 2.0 mA for 20
minutes. These results were in sharp contrast to a V[infinity] of
1.89 - 2.2 V and a 7SC of 0.8 - 3.0 mA as observed with the film
having absorbed water.
The supercapacitive behavior was found to also depend on presence
of a nanosegregant. In a control experiment, a thin layer (~20 nm)
of PEDT:PSSH was first formed on a Au surface by fast drying the
layer in vacuum for several minutes. No nanosegregant was formed
as the drying process was too short. This was confirmed by lack of
the superhydrophilicity of the surface covered by the thin layer.
An aqueous drop of PEDT:PSSH blend was then cast on top of the
thin layer to form a control film. The control film also did not
contain a nanosegregant. IV curve compiled from the control film
showed almost no hysteresis due to absence of a nanosegregant.
After charging the supercapacitor at 2.0 mA for 20 minutes, a Voc
of 0.30 - 0.40 V and a Isc of 0.1 - 1.0 [mu][Alpha] were observed,
indicating the lack of the supercapacitive behavior in the device
containing the control layer and the control film.
In addition to Au, transparent fluorinated tin oxide (FTO)
conducting glass and graphite were also used as the positive
electrodes to prepare supercapacitors. Table 3 below lists the
capacitance C, output energy E, specific capacitance Cm, Cs and
energy density Em of these three supercapacitors having different
Table 3 shows that Cs was in a range of 65 - 68 mF/cm and Em was
about 3.5 W.h/kg for all three different supercapacitors,
indicating that they had consistent supercapacitor performance.
More supercapacitors were made by the following method. Graphene
powders or flakes were added into the PEDT:PSSH aqueous
dispersion, this aqueous dispersion was then used to form a
supercapacitor by the same method described above. This new
supercapacitor showed an increase of energy storage capacity by
All of the features disclosed in this specification may be
combined in any combination. Each feature disclosed in this
specification may be replaced by an alternative feature serving
the same, equivalent, or similar purpose. Thus, unless expressly
stated otherwise, each feature disclosed is only an example of a
generic series of equivalent or similar features.
From the above description, one skilled in the art can easily
ascertain the essential characteristics of the present invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions. Thus, other examples are also
within the claims.