U.S. patent application number 16/702737 was filed with the patent office on 2021-06-10 for osmotic energy conversion with mxene lamellar membrane-based system and method.
The applicant listed for this patent is KING ABDULLAH UNIVERSITY OF SCIENCE AND TECHNOLOGY. Invention is credited to Husam Niman ALSHAREEF, Seunghyun HONG, Peng WANG.
Application Number | 20210175789 16/702737 |
Document ID | / |
Family ID | 1000005608834 |
Filed Date | 2021-06-10 |
United States Patent
Application |
20210175789 |
Kind Code |
A1 |
HONG; Seunghyun ; et
al. |
June 10, 2021 |
OSMOTIC ENERGY CONVERSION WITH MXENE LAMELLAR MEMBRANE-BASED SYSTEM
AND METHOD
Abstract
An osmotic energy conversion system includes a housing having a
first inlet and a second inlet, an MXene lamellar membrane located
inside the housing and configured to divide the housing into a
first chamber and a second chamber, and first and second electrodes
placed in the first and second chambers, respectively, and
configured to collect electrical energy generated by a
salinity-gradient formed by first and second liquids across the
MXene lamellar membrane. The first chamber is configured to receive
the first liquid at the first inlet and the second chamber is
configured to receive the second liquid at the second inlet. The
first liquid has a salinity lower than the second liquid, and the
MXene lamellar membrane includes plural nanosheets of MXene stacked
on top of each other.
Inventors: |
HONG; Seunghyun; (Thuwal,
SA) ; WANG; Peng; (Thuwal, SA) ; ALSHAREEF;
Husam Niman; (Garland, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KING ABDULLAH UNIVERSITY OF SCIENCE AND TECHNOLOGY |
Thuwal |
|
SA |
|
|
Family ID: |
1000005608834 |
Appl. No.: |
16/702737 |
Filed: |
December 4, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K 44/085 20130101;
H02K 44/12 20130101 |
International
Class: |
H02K 44/12 20060101
H02K044/12; H02K 44/08 20060101 H02K044/08 |
Claims
1. An osmotic energy conversion system comprising: a housing having
a first inlet and a second inlet; an MXene lamellar membrane
located inside the housing and configured to divide the housing
into a first chamber and a second chamber; and first and second
electrodes placed in the first and second chambers, respectively,
and configured to collect electrical energy generated by a
salinity-gradient formed by first and second liquids across the
MXene lamellar membrane, wherein the first chamber is configured to
receive the first liquid at the first inlet and the second chamber
is configured to receive the second liquid at the second inlet,
wherein the first liquid has a salinity lower than the second
liquid, and wherein the MXene lamellar membrane includes plural
nanosheets of MXene stacked on top of each other, the plural
nanosheets of MXene forming a nanocapillary that extends from one
side of the MXene lamellar membrane to an opposite side, and a full
length of the nanocapillary is between 400 and 1000 nm.
2. The system of claim 1, wherein a thickness of the MXene lamellar
membrane is less than 3,000 nm.
3. The system of claim 1, wherein a thickness of the MXene lamellar
membrane is 400 nm.
4. The system of claim 1, wherein the MXene lamellar membrane
includes between 1,000 and 1,500 layers of nanosheets of MXene.
5. The system of claim 1, wherein the MXene includes
Ti.sub.3C.sub.2T.sub.x sheets.
6. The system of claim 5, wherein T.sub.x includes O and OH and
F.
7. The system of claim 1, wherein the MXene lamellar membrane has
nanoconduits between adjacent nanosheets.
8. The system of claim 1, wherein the first liquid is freshwater
and the second liquid is seawater.
9. The system of claim 1, further comprising: a heating element
configured to heat the first liquid.
10. A method for converting osmotic energy into electrical energy,
the method comprising: receiving a first liquid on a first side of
an MXene lamellar membrane; receiving a second liquid on a second
side of the MXene lamellar membrane, wherein the first side is
opposite to the second side; establishing a salinity-gradient
across the MXene lamellar membrane, between the first liquid and
the second liquid; converting the osmotic energy, due to the
salinity-gradient, into electrical energy; and collecting the
electrical energy at first and second electrodes placed in the
first and second liquids, respectively, wherein the first liquid
has a salinity lower than the second liquid, and wherein the MXene
lamellar membrane includes plural nanosheets of MXene stacked on
top of each other, the plural nanosheets of MXene form a
nanocapillary that extends from one side of the MXene lamellar
membrane to an opposite side, and a full length of the
nanocapillary is between 400 and 1000 nm.
11. The method of claim 10, wherein a thickness of the MXene
lamellar membrane is less than 3,000 nm.
12. The method of claim 10, wherein a thickness of the MXene
lamellar membrane is 400 nm.
13. The method of claim 10, wherein the MXene lamellar membrane
includes between 1,000 and 1,500 layers of nanosheets of MXene.
14. The method of claim 10, wherein the MXene includes
Ti.sub.3C.sub.2T.sub.x sheets.
15. The method of claim 14, wherein T.sub.x includes O and OH and
F.
16. The method of claim 10, wherein the MXene lamellar membrane has
nanoconduits between adjacent nanosheets.
17. The method of claim 16, wherein the first liquid is freshwater
and the second liquid is seawater.
18. The method of claim 10, further comprising: heating the first
liquid.
19. An osmotic energy conversion system comprising: a housing; a
Ti.sub.3C.sub.2T.sub.x lamellar membrane located inside the
housing; and first and second electrodes placed on opposite side of
the Ti.sub.3C.sub.2T.sub.x lamellar membrane, and configured to
collect electrical energy generated by a salinity-gradient formed
by first and second liquids across the Ti.sub.3C.sub.2T.sub.x
lamellar membrane, wherein the first liquid has a salinity lower
than the second liquid, and wherein the Ti.sub.3C.sub.2T.sub.x
lamellar membrane includes plural nanosheets of
Ti.sub.3C.sub.2T.sub.x stacked on top of each other, the plural
nanosheets of Ti.sub.3C.sub.2T.sub.x forming a nanocapillary that
extends from one side of the Ti.sub.3C.sub.2T.sub.x lamellar
membrane to an opposite side, and a full length of the
nanocapillary is between 400 and 1000 nm.
20. The system of claim 19, wherein a thickness of the
Ti.sub.3C.sub.2T.sub.x lamellar membrane is 400 nm and the
Ti.sub.3C.sub.2T.sub.x lamellar membrane includes between 1,000 and
1,500 layers of nanosheets of Ti.sub.3C.sub.2T.sub.x.
Description
BACKGROUND
Technical Field
[0001] Embodiments of the subject matter disclosed herein generally
relate to a system and method for using a salinity-gradient to
generate electrical power, and more particularly, to using an MXene
lamellar membrane having nanoconfined channels for converting the
salinity-gradient into electrical power.
Discussion of the Background
[0002] Salinity-gradient is in ubiquitous existence on Earth and
has been extensively studied as a renewable and sustainable source
of energy, popularly known as the blue energy. Salinity-gradient
technologies generate electricity from the chemical pressure
differential created by differences in ionic concentration between
freshwater and seawater. Seawater has a higher osmotic pressure
than freshwater due to its high concentration of salt. The
extractable free energy of mixing of a concentrated salt solution
with pure water is promising because the energy yield from this
process is estimated to be 3 kJ per liter mixed, which is
equivalent to 0.8 kWhm.sup.-3.
[0003] To date, semipermeable, especially ion exchange, membranes
have been explored for reverse electrodialysis (RED) to harness
electricity from the Gibbs free energy of mixing under salinity
gradient. Recently, nanoporous structures such as MoS.sub.2
nanopores and boron nitride nanotubes have been developed as a new
class of RED membranes. Because of its size being close to the
Debye screening length and its surface charges, the nanoconfined
spacing in these nanostructures boosts the charge-selective osmotic
current. However, despite their superior electricity generation
performances, when compared to the conventional RED systems, the
fabrication of these nanostructures is poorly scalable, which
hinders their practical applications. In this regard, note that in
order to be able to have an industrially suitable device that is
capable to generate electricity from the salinity-gradient, the
fabrication of the nanostructures used in this device should be
available for large scale manufacturing, which is not yet the case
for the existing devices.
[0004] Lamellar nanostructures, which can be fabricated by stacking
two-dimensional (2D) nanosheets on top of each other, may provide a
promising and scalable alternative to efficiently harvest the blue
energy. Interplanar nanocapillaries between neighboring sheets are
densely interconnected in the lamellar membranes and provide
precise subnanometer fluidic channels that can facilitate ultrafast
ion transport (see [1]-[7]). Equally importantly, the charges of
the individual 2D nanosheet building blocks lead to
surface-charge-governed ion transport behaviors within the lamellar
membranes, which have been observed in graphene oxide- or carbon
nitride-based lamellar membranes.
[0005] These membranes have outperformed their counterparts used in
commercial RED systems [1], [3]. The simplicity and scalability of
lamellar membrane fabrication makes it even more attractive for
practical osmotic power generation. However, the membranes
currently used for converting the osmotic energy still suffer from
poor energy conversion and/or difficult manufacturing
processes.
[0006] Thus, there is a need for a new lamellar membrane that
solves the above noted problems and is capable to efficiently
convert the osmotic energy into electrical energy.
BRIEF SUMMARY OF THE INVENTION
[0007] According to an embodiment, there is an osmotic energy
conversion system that includes a housing having a first inlet and
a second inlet; an MXene lamellar membrane located inside the
housing and configured to divide the housing into a first chamber
and a second chamber; and first and second electrodes placed in the
first and second chambers, respectively, and configured to collect
electrical energy generated by a salinity-gradient formed by first
and second liquids across the MXene lamellar membrane. The first
chamber is configured to receive the first liquid at the first
inlet and the second chamber is configured to receive the second
liquid at the second inlet. The first liquid has a salinity lower
than the second liquid, and the MXene lamellar membrane includes
plural nanosheets of MXene stacked on top of each other.
[0008] According to another embodiment, there is a method for
converting osmotic energy into electrical energy, and the method
includes receiving a first liquid on a first side of an MXene
lamellar membrane; receiving a second liquid on a second side of
the MXene lamellar membrane, wherein the first side is opposite to
the second side; establishing a salinity-gradient across the MXene
lamellar membrane, between the first liquid and the second liquid;
converting the osmotic energy, due to the salinity-gradient, into
electrical energy; and collecting the electrical energy at first
and second electrodes placed in the first and second liquids,
respectively. The first liquid has a salinity lower than the second
liquid, and the MXene lamellar membrane includes plural nanosheets
of MXene stacked on top of each other.
[0009] According to still another embodiment, there is an osmotic
energy conversion system that includes a housing; a
Ti.sub.3C.sub.2T.sub.x lamellar membrane located inside the
housing; and first and second electrodes placed on opposite side of
the Ti.sub.3C.sub.2T.sub.x lamellar membrane, and configured to
collect electrical energy generated by a salinity-gradient formed
by first and second liquids across the Ti.sub.3C.sub.2T.sub.x
lamellar membrane. The first liquid has a salinity lower than the
second liquid, and the Ti.sub.3C.sub.2T.sub.x lamellar membrane
includes plural nanosheets of Ti.sub.3C.sub.2T.sub.x stacked on top
of each other.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawings, in which:
[0011] FIG. 1 is a schematic diagram of an osmotic energy
conversion system;
[0012] FIGS. 2A and 2B illustrate an MXene lamellar membrane for
use in an osmotic energy conversion system;
[0013] FIGS. 3A and 3B illustrate a top surface and a
cross-section, respectively, of the MXene lamellar membrane;
[0014] FIGS. 4A to 4C show the XRD patterns, XPS spectra, and Raman
spectra of the MXene lamellar membrane;
[0015] FIG. 5A illustrates an osmotic energy conversion system,
FIG. 5B illustrates the current-voltage characteristic for the
system, FIG. 5C illustrates the conductance versus salinity of the
system, and FIG. 5D illustrates the conductance and surface charge
versus pH for the system of FIG. 5A;
[0016] FIG. 6A illustrates the current-voltage characteristic for a
given gradient across the membrane, FIG. 6B illustrates the osmotic
current and potential for various concentration differences of the
two fluids that wet the membrane, FIG. 6C illustrates the output
power density versus the concentration difference of the two fluids
of the osmotic energy conversion system, and FIG. 6D illustrates
the output power density versus the thickness of the membrane;
[0017] FIG. 7A illustrates the ionic conductance at various
temperatures in the system, FIG. 7B illustrates the maximum output
power density as a function of temperature in the system, FIG. 7C
illustrates the thermal dependence of the osmotic current, and FIG.
7D illustrates the output power density of the system versus the
apparent thickness of the membrane;
[0018] FIG. 8 is a flowchart of a method for making the MXene
lamellar membrane; and
[0019] FIG. 9 is a flowchart of a method for converting osmotic
energy into electrical energy with the system shown in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The following description of the embodiments refers to the
accompanying drawings. The same reference numbers in different
drawings identify the same or similar elements. The following
detailed description does not limit the invention. Instead, the
scope of the invention is defined by the appended claims. The
following embodiments are discussed, for simplicity, with regard to
an osmotic energy conversion system that uses a lamellar membrane
based on Ti.sub.3C.sub.2T.sub.x. However, the embodiments to be
discussed next are not limited to such material but may use other
MXene nanosheets.
[0021] Reference throughout the specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with an embodiment is
included in at least one embodiment of the subject matter
disclosed. Thus, the appearance of the phrases "in one embodiment"
or "in an embodiment" in various places throughout the
specification is not necessarily referring to the same embodiment.
Further, the particular features, structures or characteristics may
be combined in any suitable manner in one or more embodiments.
[0022] According to an embodiment, a novel osmotic energy
conversion system includes an MXene lamellar membrane that
separates a first high-saline medium from a second low-saline
medium. The osmotic energy between the first and second mediums is
converted into electrical energy.
[0023] Such a system 100 is illustrated in FIG. 1 and includes a
housing 102 that is configured to receive the first low-salinity
fluid 104 at a first inlet 102A, and the second high-salinity fluid
106, at a second inlet 1028. The first low-salinity fluid 104 may
be fresh water and the second high-salinity fluid 106 may be
seawater. In one application, both fluids 104 and 106 have a
salinity, but the first fluid 104 has a salinity lower than the
salinity of the second fluid 106. The first fluid 104 may be stored
in a first storage container 110 while the second fluid 106 may be
stored in a second storage container 112. In one embodiment, the
first storage container 110 is a part of the ocean while the second
storage container is a part of a river.
[0024] A lamellar membrane 120 is placed inside the housing 102 to
separate a first chamber 122 from a second chamber 124. The first
chamber 122 is fluidly connected to the first inlet 102A to receive
the first fluid 104 and the second chamber 124 is fluidly connected
to the second inlet 1028 to receive the second fluid 106. In one
application, the first chamber 122 has a first outlet 102C and the
second chamber 124 has a second outlet 102D. The first fluid 104
may be discharged from the first chamber 122 into a first discharge
storage tank 114, through the first outlet 102C, and the second
fluid 106 may be discharged from the second chamber 124 into a
second discharge storage tank 116, through the second outlet 102D.
in one application, the first discharge storage tank is also the
second discharge storage tank. Corresponding valves 114A and 116A
may be located between the corresponding outlets and the discharge
storage tanks to control an amount of fluid that is discharged from
the chambers 122 and 124.
[0025] Two or more electrodes 130 and 132 are placed inside the
housing 102, one in each of the chambers 122 and 124, and these
electrodes are connected to an energy storage device 134. The
electrodes may be placed directly into the first and second fluids.
The energy storage device 134 may be a battery or similar device.
The energy storage device 134 may be connected to a controller 136
and/or a motor 138. The controller 136 may include a processor,
memory and communication means (e.g., receiver, transmitter, or
transceiver) for exchanging data and/or commands with the various
elements shown in FIG. 1, but also with a remote server (not
shown). The controller 136 may be programmed to control the motor
138, based on the energy generated by the system 100, and also to
control the movement of the fluids 104 and 106, from the storage
tanks 110 and 112, through the housing 102, and into the discharge
storage tanks 114 and 116. Motor 138 may be any device, for
example, an engine, a turbine, etc.
[0026] The lamellar membrane 120 may be made from one or more
materials. FIG. 2A shows an example of a lamellar membrane 120 that
includes two-dimensional nanosheets 201, 202, 211, 212, 221, and
222 (only six of them are labeled for simplicity) disposed on top
of each other to form plural layers I, II, III, etc. The 2D
nanosheets may be made of metal carbide and nitride (MXene), which
has recently joined the 2D materials family, and is emerging as a
promising material to construct lamellar ion channels [7], [8].
MXene has typically a formula of Mn.sub.+1X.sub.nT.sub.x with n=1,
2, and 3, where M is an early transition metal and X is carbon
and/or nitride, and is synthesized by selectively etching an
A-group layer from the Mn.sub.+1AX.sub.n phase, as discussed in [9]
to [11]. The A-layer is replaced with surface terminal groups
T.sub.x, which may be T.sub.x: --O, --OH, and --F, during an
aqueous etching and exfoliation process. These functional groups
endow the MXene nanosheets 201 to 222 with surface charges and help
create interplanar spacing 209, 219 at subnanometer scale within
the MXene nanosheets of the lamellar membrane 120.
[0027] In one embodiment, the lamellar membrane 120 is made of
stacked Ti.sub.3C.sub.2T.sub.x sheets 201 to 222, which are
separated by an interlayer distance (d).about.16.2 .ANG. in a fully
hydrated state. Taking into account that a theoretical thickness
(a) of a monolayer Ti.sub.3C.sub.2T.sub.x sheet is about 9.8 .ANG.,
the empty space between two sheets in the same layer, which is
available for ions to diffuse, is estimated to
.delta.=(d-a).about.6.4 .ANG.. This effective interplanar spacing
for ion transport is corresponding to the height of a
nanocapillary. In one application, a thickness of a monolayer
Ti.sub.3C.sub.2T.sub.x sheet 201 is about 1 to 2 nm, and there are
1000 to 1500 monolayers in a lamellar membrane 120, so that a total
thickness of the membrane 120 is between 100 nm and 3000 nm, with a
preferred value of 400 nm. In one embodiment, the thickness of the
membrane 120 is less than 3000 nm.
[0028] As shown in FIG. 2B, a full length of a single nanocapillary
250 with a thickness h involves a number of turns (hid), and each
turn involves a capillary length (w). In this embodiment, it is
assumed that the single Ti.sub.3C.sub.2T.sub.x sheet 201 possesses
a same width and length w, and those are approximated from
experimentally averaged lateral sizes to be about 3.4 .mu.m of the
MXene sheets. Therefore, the complete length of the single
nanocapillary 250 is given by w.times.h/d. The total number of
parallel 2D channels 209 and 219 per unit area can be estimated to
be about 1/w.sup.2, and a resulting number of channels is about
10.sup.7 across an employed membranes with a full area of 0.196
cm.sup.2. The effective areal fraction of the nanocapillaries on
the total membrane area is estimated to be approximately 0.1%.
[0029] Thus, the scalable MXene lamellar membrane 120 may be used
as a nanofluidic platform to harness the salinity-gradient energy.
The subnanometer channels 209, 219 in the MXene membrane 120
exhibit strong surface-charge-governed ion transport and
consequentially excellent osmotic energy conversion efficiency up
to 40.6% at room temperature. The thermal-dependent osmotic energy
conversion is discussed later at elevated temperature, giving rise
to an electricity generation of 54 Wm.sup.-2 at 331 K. These
performances all transcend the state-of-the-art RED devices. These
results indicate the practical feasibility and viability of the
MXene laminar membranes as a large-scale osmotic energy-harvesting
platform.
[0030] The Ti.sub.3C.sub.2T.sub.x nanosheet 201 was synthesized in
one embodiment by selective etching the Al from the MAX phase
Ti.sub.3AlC.sub.2 using in situ HF-forming etchant. A transmission
electron microscopic (TEM) image of the exfoliated
Ti.sub.3C.sub.2T.sub.x nanosheets clearly shows (see FIG. 3A) well
defined edges 300 as well as plain surfaces 302 with no wrinkles.
Its high crystallinity with no obvious defects and hexagonal
structure is also confirmed by the high-resolution TEM image and
selected area electron diffraction (SAED) pattern. An atomic force
microscopic (AFM) measurement indicates that the exfoliated
monolayer Ti.sub.3C.sub.2T.sub.x nanosheet possesses a thickness of
.about.1.5 nm. The average lateral size of the generated nanosheets
is approximately 4.2.+-.1.8 .mu.m and 2.6.+-.1.1 .mu.m in length
and width, respectively. Additionally, the high aspect ratio
(micrometer lateral width to nanometer thickness) of MXene sheets
is a favorable feature for creating uniform 2D interlayer channels
in a well-aligned stacked manner.
[0031] The 2D lamellar nanosheets 201 to 222 were assembled by
vacuum assisted filtration of Ti.sub.3C.sub.2T.sub.x dispersion on
porous polymeric support, to form the lamellar membrane 120. The
stacked nanosheets can be easily peeled off from the support
without damage after drying in air, leading to free-standing
flexible MXene membranes. The SEM image (see FIG. 3B) displays
highly oriented MXene nanosheets 201, 212, parallel to a support
surface 310. Additionally, the insert of FIG. 3B indicates that, at
the macroscopic level, the laminate membrane 120 has an outer
smooth surface with no detectable pinholes or cracks. The thickness
of the membrane 120 is controlled by the mass of MXene in the
filtrating dispersion.
[0032] The ordered stacked structure 120 is further characterized
by X-ray diffraction (XRD). The results of this analysis are shown
in FIG. 4A, which indicate a pronounced (002) peak 402 in the X-ray
diffraction pattern 400 of the Ti.sub.3C.sub.2T.sub.x membrane when
compared to the peak 404 of the XRD pattern 406 of the
Ti.sub.3AlC.sub.2 material. Note the shift of the (002) peak 402 of
the Ti.sub.3C.sub.2T.sub.x material to a lower angle than the
9.6.degree. value for the peak 404 of the MAX phase, which
indicates the introduction of the functional groups T.sub.x and
water in between adjacent MXene nanosheets. Also note that the
shift in the (002) peak for the Ti.sub.3C.sub.2T.sub.x material
when exposed to ambient 403 or in water 405, as illustrated in the
inset of FIG. 4A.
[0033] The surface functional groups of the Ti.sub.3C.sub.2T.sub.x
nanosheets are examined by X-ray photoelectron spectroscopy (XPS)
and Raman spectroscopy as shown in FIGS. 4B and 4C. The XPS spectra
420 in FIG. 4B show that abundant surface terminal groups,
including --O, --OH, and --F, are bonded to the surface of the
Ti.sub.3C.sub.2 sheets. These functional groups, evidenced by the
Raman spectra 430 in FIG. 4C as well, ensure that the MXene
lamellar membrane 120 is hydrophilic and negatively charged.
[0034] In a hydrated state, these terminal functional groups, which
act as spacers to keep neighboring nanosheets apart, allow water
molecules to be intercalated inside the interplanar channels 209
and 219 while preventing the laminates 201 to 222 from being
disintegrated. The enlarged channel height is verified by the shift
of the (002) peak to 28=5.46.degree. in its XRD pattern in the
inset of FIG. 4A, corresponding to an interlayer spacing of 1.615
nm. The effective interplanar nanocapillary is estimated to be
about 0.64 nm, which is large enough for hydrated small ions to
diffuse. For instance, the reported diameter of hydrated K.sup.+
varies from 0.4 to 0.66 nm. Additionally, the lamellar structure of
the MXene membrane 120 is stable in water under all experimental
conditions employed, showing its high aqueous stability [7].
[0035] To determine the intrinsic ionic transport properties of the
MXene membrane 120, a current-voltage (I-V) response for the
Ti.sub.3C.sub.2T.sub.x lamellar membrane under various salt (e.g.,
KCl) concentrations and pH values was measured. These measurements
provide information about the surface charges of the
Ti.sub.3C.sub.2T.sub.x nanochannels. Unless otherwise mentioned,
all ion transport experiments were carried out with a membrane
having a thickness of 2.7 to 3.0 .mu.m. The approximated length of
a single nanocapillary 250 is derived from the thickness of the
membrane, and the width is approximated to be the averaged lateral
sizes (.about.3.4 .mu.m) of the MXene nanosheets illustrated in
FIG. 2A [4], [6]. The electric current passing through the MXene
membrane 120 was measured by using a pair of Ag/AgCl electrodes
130, 132, with 10 pA precision, as illustrated in FIG. 5A. A
sourcemeter 500 was connected to the two electrodes to measure the
voltage and corresponding current in this electrical circuit. Note
that the positive ions 502 move through the membrane 120 along the
arrow 504, due to the salinity-gradient between the first
low-salinity fluid 104 and the second high-salinity fluid 106. The
salinity of the two fluids is indicated by the symbol "C" in FIG.
5A.
[0036] FIG. 5B shows representative I-V characteristics for a range
of KCl concentrations (10.sup.-1 to 10.sup.-3 M in the figure,
where M stands for mol per liter) at pH 5.7. The Ohmic conductance
G of the MXene membrane 120 at smaller biases (<30 mV), where
the I-V curve is linear, is plotted in FIG. 5C as a function of the
salt concentration at pH 5.7. The linear response 510, which is
typical of charge-neutral channels, at 1 M agrees well with the
bulk conductivity 512 of the KCl solution for the given channel
geometry. However, starting from 100 mM, the conductance G deviates
from the linear regime 512, implying the presence of surface
charges in the interplanar space. The surface charge effect was
previously reported to dominate at low salt concentrations through
nanoconfined channels. Particularly, overlapped electrical double
layers in nanochannels, derived from slit size close to Debye
screening length as well as surface charges explain the observation
of the higher-than-bulk ionic conductance 514.
[0037] Furthermore, a scaling behavior is observed at low salt
concentration. It is believed that salinity-dependent surface
charges may be responsible for such monotonic decrease in
conductance, which was previously predicted by the chemical
equilibrium model in the SiO.sub.2 nanochannel or nanopore. From
the measured conductance G for KCl 10 mM at pH 6.3, it was found
that the surface charge density is as high as 100 mCm.sup.-2, which
is higher than the values for graphene oxide laminate (50-60
mCm.sup.-2) as well as the values for perforated graphene
(.about.40 mCm.sup.-2) or MoS.sub.2 nanopores (20-80 mCm.sup.-2) at
pH 5.
[0038] In addition, the surface terminal groups are randomly
distributed in the basal planes of the MXene sheets. It was noted
that this property plays a key role in the highly cation-selective
ion flow through the MXene nanochannels. The conductance of the
membrane 120 can be further modulated by controlling the pH as
shown in FIG. 5D. The conductance G gradually increases with
increasing the pH value above 6, suggesting more accumulation of
negative surface charges in the MXene nanosheets at higher pH. The
estimated charge density above pH 9 reaches up to .about.130
mCm.sup.-2, corresponding to 0.84e nm.sup.-2. When the pH is
increased, the dissociation of the terminal groups leads to more
negative surface charges on the individual MXene nanosheets,
following a chemical equilibrium as:
(Ti.sub.3C.sub.2).sub.n(OH).sub.x(O.sup.-).sub.yF.sub.z+aH.sub.2O(Ti.sub-
.3C.sub.2).sub.n(OH).sub.x-a(O.sup.-).sub.y-aF.sub.z+aH.sub.3O.sup.+
[0039] Zeta potential (the zeta potential measurement is a
technique for determining the surface charge of nanoparticles in a
colloidal solution) values obtained from colloidal nanosheets and
stacked membranes indicate the strong dependence of the surface
charges of the Ti.sub.3C.sub.2T.sub.x membrane on the pH, see inset
of FIG. 5D. In contrast, the conductance G sharply declines at a
pH<6. This can be associated with fewer counterions inside
channels and narrowed interlayer spacing due to the protonation of
the surface functional groups.
[0040] To study the influence of the chemical gradient across the
lamellar membrane 120, different KCl concentrations are tested, for
example, in the range of 1 mM to 1 M in the two chambers 122 and
124. Charge separation by interplanar channels 209, 219 is
responsible for harvesting the electrical energy from the chemical
potential gradient. The selective passage of the cations 502 from
high to low concentrations, whereas the transport of the anions 506
is electrostatically impeded, as illustrated in FIG. 5A, results in
a positive net current across the lamellar membrane 120. In this
regard, FIG. 6A illustrates a current-voltage response 600 under a
variable concentration gradient, which is defined as the ratio
c.sub.high/c.sub.low. A direction of the short circuit current
(I.sub.sc) in the absence of bias is consistent with a net flow of
positive charges, and this charge-selective osmotic flow produces
an open circuit voltage (V.sub.oc) across the lamellar membrane
120. Note that the inset of FIG. 6A illustrates the electrical
diagram associated with the osmotic energy conversion system 100.
The pure electroosmotic current-voltage 602 can then be calculated
from the osmotic current (I.sub.os) and potential (V.sub.os),
corrected for redox potentials (V.sub.redox) emanating from unequal
potential drops at the electrode-solution interfaces in different
salt concentration. More specifically, the redox potential is
calculated using the Nernst equation in combination with the Pitzer
model, taking into account a temperature-dependent ion activity
coefficient. FIG. 6B shows the osmotic potential and current
obtained for different salt concentration gradient and pH
conditions.
[0041] The osmotic potential is increased from 28 to 139 mV at pH
11.53 with varying the gradients from 10-fold to 1000-fold. The
osmotic current reaches up to 14.2 .mu.A at a higher pH under the
gradient of 100. A slight current drop is also observed under the
gradient of 1000, which is likely due to relatively stronger ion
concentration polarization effect at the surface of membranes.
Calculated by the equation: t+=0.5(1+V.sub.os/V.sub.redox), the
cation transference number (t.sub.+) approaches 0.95 under
1000-fold difference and highly alkaline conditions, nearly close
to ideal unity cation selectivity. Note that the transference
number is defined as the fraction of the current carried either by
the anion (J.sub.-) or the cation (J.sub.+) to the total electric
current (i.e., t.sub.+=J.sub.+/(J.sub.++J.sub.-)). A significant
increase in the osmotic current and voltage is observed at a higher
pH, implying that the surface charge plays a critical role in the
osmotic power generation process.
[0042] Based on the estimated I.sub.os and V.sub.os from the curve
602, a maximum output power density (PD.sub.max) 610 and its
corresponding electrochemical energy conversion efficiency
(.eta..sub.max) 612 were calculated and plotted in FIG. 6C. The
PD.sub.max 610 reaches up to 20.85 Wm.sup.-2 which is higher by a
factor of around 20 than those from the existing commercial ion
exchange membranes, and the .eta..sub.max 612 is as high as 40.6%
at a pH value of 11.5 for the salinity gradient of 1000, as shown
in FIG. 6C. Note that in this figure, the PD.sub.max is represented
on the Y axis, on the left hand side of the graph while the
electrochemical energy conversion efficiency .eta..sub.max is also
represented on the Y axis, but on the right hand side of the graph.
The MXene membrane 120 shows a higher output power by 2 orders of
magnitude, compared to other 2D materials such as graphene oxide or
carbon nitride. Such an enhancement of the MXene membranes could be
associated with their lower membrane resistance through the
structurally regular and straight Ti.sub.3C.sub.2T.sub.x
nanocapillaries, contrasting to conventional irregular ones from
other lamellar membranes.
[0043] The inventors have found that the osmotic energy conversion
depends on the thickness of laminar membrane under ambient pH
conditions. The power density exhibits a strong decay with
increasing membrane thickness, as illustrated in FIG. 6D. Above a
certain thickness, a longer channel length derived from a thicker
membrane is found to impair the ionic flux. This implies that
further enhancement of the osmotic power density can be achieved by
reducing the nanocapillary length of the membrane. It was observed
that the longitudinal length of the nanocapillaries in several
nanometer-thick layered membranes can be coincident with the
characteristic length scale (400-1000 nm) of the optimum
nanofluidic channels, to maximize the power generation while
balancing the energy conversion efficiency. Under these conditions,
an excellent performance may ideally occur in an ultrathin
Ti.sub.3C.sub.2T.sub.x laminar membrane at several nanometer scale.
From a technical perspective, emerging techniques such as a
roll-to-roll process, beneficial for a controlled large-scale 2D
sheet assembly, may be used to realize the uniform deposition of
such ultrathin membranes.
[0044] To improve the osmotic energy conversion performance, the
inventors have studied the thermal effect on the ionic transport
and its consequential impact on the power generation effect. As
shown in FIG. 7A, the ionic conductance G at a temperature in the
range of 294 to 341 K shows a linear dependence 700 on the
temperature and furthermore follows the Arrhenius behavior. As
previously demonstrated for a silica nanopore array, the fluid
temperature can affect not only the surface charge and chemistry,
but also the properties of the liquid media such as viscosity. As
the temperature of the membrane's ambient rises, the ionic mobility
increases by a factor of 2.35 in response to a reduced water
viscosity.
[0045] The estimated mobility enhancement is fairly consistent with
the observed increase in the conductance. As expected, the output
power shows a strong thermal dependence, reaching up to 54
Wm.sup.-2 at 331 K, as shown in FIG. 7B. Note that FIG. 7B shows
the maximum output power density as a function of temperature,
under a KCl concentration gradient of 100 at a pH value of 5.7. The
inset in FIG. 7B shows representative I-V characteristics at
different temperatures. Further, the inventors found that the
thermal effect increases the surface charge as well, which is
evidenced by the incremental cation transference number shown in
FIG. 7C (see Y axis, right hand side of the figure). An ionic
clogging, possibly arising from bubble nucleation in the
capillaries, was not observed at elevated temperatures.
[0046] Accordingly, the temperature-dependent enhancement of the
output power is an understandable result of the increase in local
concentration and mobility of cations on the charged surfaces.
Besides, the laminar membrane 120 sustained its stable chemical
feature as well as mechanical integrity even after a temperature
rise. It should be noted that this thermal performance is promising
from a practical perspective, because widely available industrial
waste heat can be tapped into for further enhancing the osmotic
power generation. When comparing the osmotic energy conversion
system 100 with other power generators, as illustrated in FIG. 7D,
the resultant output power of the MXene Ti.sub.3C.sub.2T.sub.x
laminar membrane 120 at high temperature is higher than the
performances of state-of-the-art osmotic power generators. FIG. 7D
includes labels B to I, which correspond to the following existing
membranes: B is the mesoporous silica film (2017), C is the Janus
Carbon/alumina membrane (2014), D is Silica nanochannels (2010), E
is Janus 3D porous membrane (2018), F is Nafion-filled PDMS
microchannels (2016), G is polymeric carbon nitride laminate
(2018), H is BCP-coated PET conical nanochannels (2015), and I is
Janus nanokaolinite film (2017).
[0047] Furthermore, the inventors found that the osmotic power
performance of the system 100 can be stably maintained for more
than 20 h, even with Na.sup.+, the most abundant ion of seawater.
Based on these observations, the system 100 shown in FIG. 1 may be
configured to have a heating element 170 to heat one of the first
and second fluids. In one application, the heating element 170 is a
solar cell. In another application, the heating element 170 is a
heat exchanger that takes heat from industrial waste heat and
transfers it to one or both of the first and second fluids. The
amount of heat transferred from the heating element 170 to the
first and/or second fluids is controlled by controller 136.
[0048] A method for forming the MXene lamellar membrane 120 is now
discussed with regard to FIG. 8. The method starts in step 800 with
providing layered ternary carbide Ti.sub.3AlC.sub.2 (MAX phase)
powder, that is commercially procured (e.g., having particle size
<40 .mu.m). In step 802, Ti.sub.3C.sub.2T.sub.x MXene is
synthesized by selective etching of the Al from the
Ti.sub.3AlC.sub.2 powder using in situ HF-forming etchant. The
etching solution was prepared by adding 1 g of lithium fluoride to
20 mL of hydrochloric acid (HCl 35-38%) followed by stirring for 5
min. Then, 1 g of Ti.sub.3AlC.sub.2 powder was slowly added to the
above etchant at 35.degree. C. and stirred for 24 h. The acidic
suspension was washed in step 804 with deionized water using
centrifugation at 3,500 rpm for 5 min per cycle, and the
centrifugal washing of a supernatant collected after each cycle was
repeated until pH>6. At around pH.gtoreq.6, a stable dark green
supernatant of Ti.sub.3C.sub.2T.sub.x was observed, and then a
final supernatant was collected at step 806 by additional
centrifugation at 3,500 rpm for 5 min. The lamellar MXene
Ti.sub.3C.sub.2T membrane 120 was fabricated by filtering in step
808 specific amounts of MXene dispersion through a cellulose
acetate membrane (0.45 .mu.m pore size and a diameter of 43 mm).
All filtrated membranes were air-dried in step 810, at ambient
conditions, and could be easily detached from the support.
[0049] A method for converting osmotic energy into electrical
energy with the lamellar membrane discussed above is now presented
with regard to FIG. 9. The method includes a step 900 of receiving
a first liquid 104 on a first side of an MXene lamellar membrane
120, a step 902 of receiving a second liquid 106 on a second side
of the MXene lamellar membrane 120, where the first side is
opposite to the second side, a step 904 of establishing a salinity
gradient across the MXene lamellar membrane 120, between the first
liquid and the second liquid, a step 906 of converting the osmotic
energy, due to the salinity gradient, into electrical energy, and a
step 908 collecting the electrical energy at first and second
electrodes 130, 132 placed in the first and second liquids,
respectively. The first liquid has a salinity lower than the second
liquid and the MXene lamellar membrane includes plural nanosheets
of MXene stacked on top of each other.
[0050] In one embodiment, a thickness of the MXene lamellar
membrane is less than 3000 nm. In another embodiment, the thickness
of the MXene lamellar membrane is 400 nm. The MXene lamellar
membrane includes between 1000 and 1500 nanosheets of MXene and the
MXene includes Ti.sub.3C.sub.2T.sub.x sheets, wherein T.sub.x
includes O and OH and F. The MXene lamellar membrane has
nanoconduits between adjacent nanosheets, the first fluid is
seawater and the second fluid is freshwater. In one application,
the method may include a step of heating the first liquid.
[0051] The disclosed embodiments provide an osmotic energy
conversion system that transform osmotic energy into electrical
energy. It should be understood that this description is not
intended to limit the invention. On the contrary, the embodiments
are intended to cover alternatives, modifications and equivalents,
which are included in the spirit and scope of the invention as
defined by the appended claims. Further, in the detailed
description of the embodiments, numerous specific details are set
forth in order to provide a comprehensive understanding of the
claimed invention. However, one skilled in the art would understand
that various embodiments may be practiced without such specific
details.
[0052] Although the features and elements of the present
embodiments are described in the embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the embodiments or in various
combinations with or without other features and elements disclosed
herein.
[0053] This written description uses examples of the subject matter
disclosed to enable any person skilled in the art to practice the
same, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the
subject matter is defined by the claims, and may include other
examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims.
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