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Article
Membrane-encapsulated,
 moisture-desorptive
passive cooling for high-performance, ultra-low-
cost, and long-duration electronics thermal
management
Graphical abstract
Highlights
d Passive thermal management based on a moisture
desorption-absorption process
d Hygroscopic salt confined by a porous membrane to prevent
leakage and corrosion
d Outstanding performance/cost competitiveness compared
to existing passive strategies
d Device performance improved by 32.65% by equipping the
strategy in a real CPU
Authors
Zengguang Sui, Yunren Sui,
Zhixiong Ding, Haosheng Lin, Fuxiang Li,
Ronggui Yang, Wei Wu
Correspondence
ronggui@hust.edu.cn (R.Y.),
weiwu53@cityu.edu.hk (W.W.)
In brief
Although passive thermal management
strategies have shown considerable
potential to alleviate the ever-increasing
energy demand for electronics cooling,
they have been suffering from low
efficiency and high cost. We address
these critical challenges using a
desorption-absorption process of
moisture based on a low-cost
hygroscopic salt solution confined by a
membrane encapsulation method. Our
strategy significantly outperforms the
current passive thermal management
strategies with a record-high cost
effectiveness, which can be useful for
various cooling applications with few
technological barriers.
Sui et al., 2023, Device 1, 100121
December 22, 2023 ª 2023 The Author(s).
Published by Elsevier Inc.
https://doi.org/10.1016/j.device.2023.100121
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mailto:ronggui@hust.edu.cn
mailto:weiwu53@cityu.edu.hk
https://doi.org/10.1016/j.device.2023.100121
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Article
Membrane-encapsulated, moisture-desorptive passive
cooling for high-performance, ultra-low-cost, and
long-duration electronics thermal management
Zengguang Sui,1 Yunren Sui,1 Zhixiong Ding,1 Haosheng Lin,1 Fuxiang Li,1 Ronggui Yang,2,* and Wei Wu1,3,*
1School of Energy and Environment, City University of Hong Kong, Hong Kong 999077, China
2School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
3Lead contact
*Correspondence: ronggui@hust.edu.cn (R.Y.), weiwu53@cityu.edu.hk (W.W.)
https://doi.org/10.1016/j.device.2023.100121
THEBIGGERPICTURE With the unprecedented demand for data generation and communication, the enor-
mous energy consumption for electronics cooling presents an increasingly significant environmental issue.
Passive thermal management has been drawing increasing interest to reduce this rising environmental
impact. However, the existing technologies need great improvements in efficiency and cost effectiveness
for wider adoptions. Herein, we demonstrate a passive cooling strategy that utilizes moisture desorption
from a low-cost hygroscopic salt solution to extract heat and prevent devices from overheating. Impor-
tantly, it can autonomously absorb moisture from the surrounding air during off hours to recover cooling
capacity. This strategy can improve the performance of a real computing device by 32.65%, exhibiting a re-
cord-high cost effectiveness. This strategy provides a new method for intermittent thermal regulation, with
applications including electronics, batteries, solar cells, and buildings, with few technological barriers.
SUMMARY
Passive thermal management strategies are one of the most promising ways to reduce energy consumption
for intermittent heat dissipation. However, the existing strategies encounter tough obstacles on their way to
commercialization due to their low efficiencies and high costs. Herein, we propose a passive thermal man-
agement strategy that relies on moisture desorption from hygroscopic salt solutions through a protective
membrane that only allows water vapor to pass through; importantly, it can spontaneously recover cooling
capacity during off hours. We selected lithium bromide as a cost-effective sorbent while avoiding crystalliza-
tion. Outstandingly, the strategy can provide an effective cooling capacity (DTmax = 11.5
�C) for �400 min,
while the measured heat flux can reach 75 kW/m2. By employing the strategy in a real computing device,
its performance is improved by 32.65% with a record-high cost effectiveness. The strategy can be useful
for various applications that need intermittent thermal regulation, with few technological barriers.
INTRODUCTION
With the ever-increasing demands for high-performance elec-
tronic and communication technologies, as well as the constant
push to reduce the sizes of electronic components, the power
density of electronics has continued to rise.1–5 This development
leads to higher heat fluxes that are challenging to heat dissipa-
tion. Poor thermal management can cause tremendous heat
accumulation within electronic devices, resulting in the loss of
functionality and, eventually, device failure.6,7 It is estimated
that the chip performance can degrade by �10% for every 2�C
rise when its operating condition exceeds 70�C–80�C.8 Hence,
developing an effective thermal management strategy is essen-
Device 1, 100121, Dec
This is an open access article und
tial for electronics.9 While active cooling technologies with
forced air or liquid circulation can achieve high cooling perfor-
mance, the ever-increasing energy demand for electronics cool-
ing has caused an increasingly significant environmental issue.10
Also, the active cooling strategies commonly require external
power sources and complex auxiliary controls, resulting in a
low adaptability in the application scenarios with strict limitations
on weight and space.11–14 By contrast, passive cooling strate-
gies have attracted increasing interest due to their zero global-
warming potential, higher compactness, and lower maintenance
cost.15 Solid-liquid phase change materials (PCMs) are the most
commonly used in the existing passive thermal management
strategies for electronics. Studies have demonstrated that the
ember 22, 2023 ª 2023 The Author(s). Published by Elsevier Inc. 1
er the CC BY license (http://creativecommons.org/licenses/by/4.0/).
mailto:ronggui@hust.edu.cn
mailto:weiwu53@cityu.edu.hk
https://doi.org/10.1016/j.device.2023.100121
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temperature rise of electronics can be suppressed for a period of
time due to the fact that PCMs absorb and store the latent heat
duringmelting.16–19 Despite extensive efforts in the improvement
of PCMs, low phase change enthalpy (commonly lower than 200
J/gPCM) remains a fundamental limitation.
15 Additionally, their
cooling efficiency will decline rapidly as the melting front is
constantly away from heat sources.20,21
One of the most promising candidates for passive thermal
management is the liquid-vapor phase transition of water (i.e.,
desorptive cooling), which is inspired by the natural process
that mammals regulate their body temperature through sweat-
ing.22 Strategies for developing desorptive cooling in passive
thermal management are commonly based on hydrogels
and metal-organic frameworks (MOFs).15,23–25 Specifically, the
hygroscopic sorbents (hydrogels or MOFs) provide highly
porous structures that enable high water uptake. The water
stored in the sorbents can be evaporated during peak hours of
electronic devices, extracting a large amount of heat to prevent
the devices from overheating (i.e., desorptive cooling). During
off-peak hours, the hygroscopic sorbents can autonomously
absorb moisture from the surrounding atmosphere to recover
their cooling capacities (i.e., self-regeneration). However,
the low mass diffusion coefficients of hydrogels (�10�12–
10�11)26,27 and MOFs (�10�13–10�11)15 lead to their extremely
slow regeneration rates. Generally, hydrogel-based cooling stra-
tegies rely on active replenishment of water to maintain a high
heat flux,26 while MOF-based cooling strategies commonly suf-
fer from an irreversiblereduction in heat andmass flux due to the
binders between adsorbent particles and substrates.28 Further-
more, the high cost of MOFs (over 10,000 USD/kg29) also limits
their large-scale applications.30 In contrast, hygroscopic inor-
ganic salts, such as LiCl and LiBr, show promise in addressing
these issues due to their extremely high moisture sorption
capacity and low cost.31–35 These salts have been widely used
in absorption cooling,36 dehumidification,37 and atmospheric
water harvesting.38 Recently, researchers have impregnated
hygroscopic salts into porous hydrogels to improve their water
uptake and self-regeneration capability.23,26,39 However, salt-
based hydrogels commonly suffer from the salting-out effect
and hydrogel collapse, resulting in corrosion risks and poor
moisture sorption capacities.40,41
Herein, we develop an efficient and economical passive ther-
mal management strategy that uses a porous membrane to
confine a low-cost hygroscopic salt into a heat sink, achieving
an ultra-high equivalent enthalpy of 2,370 kJ/kg compared with
the traditional PCMs. We discuss the working principle and
heat/mass transfer process of the hygroscopic salt-loadedmem-
brane-encapsulated heat sink (HSMHS) in detail. Two commonly
used hygroscopic salts (i.e., LiCl and LiBr) are characterized and
analyzed, demonstrating that LiBr is the optimal selection for this
strategy. We also conduct proof-of-concept experiments to test
the effects of heat flux, solution concentration, and solution layer
thickness on the cooling performance. Through these experi-
ments, we demonstrate that the proposed strategy is capable
of offering a long-duration stable cooling capacity without solu-
tion leakage and corrosion, which can suppress the temperature
rise of an emulated heater with a record-high cost effectiveness
compared with state-of-the-art passive cooling strategies. The
2 Device 1, 100121, December 22, 2023
cooling capacity of the strategy is further demonstrated in a cen-
tral processing unit (CPU) of a real computing device, achieving
an improvement of 32.65% in the device performance. Addition-
ally, an analytical and finite element model is developed and vali-
dated by the experimental data, which can guide further optimi-
zation design.
RESULTS
Design and working principle of HSMHSs
Figure 1A illustrates the schematic working cycle of the passive
thermal management strategy. The cycle consists of desorption
and absorption processes, depending on the vapor pressure
difference between the salt solution (pm,s) and the surrounding
atmosphere (pv), where pm,s is determined by the solution tem-
perature and concentration. Figure S1 depicts the vapor partial
pressure and theoretical sorption equilibrium lines of LiBr solu-
tion. The reaction equilibrium equations can be found in Note
S1. In a typical operating condition (i.e., 60% relative humidity
[RH] and 25�C), the initial solution absorbs heat (from point a to
point b), desorbing moisture from the solution to prevent the sur-
face temperature of electronic devices from rising. Then, the
desorption process ismaintained for a certain duration as the so-
lution temperature gradually decreases due to a high pm,s (from
point b to point c). Once pm,s is lower than pv, the concentrated
solution spontaneously absorbs moisture from the surrounding
atmosphere (from point c to point a) to recover cooling capacity.
The cycle shows a fully passive desorption-absorption process,
where the salt solution is confined by a porous polytetrafluoro-
ethylene (PTFE) membrane that offers a vapor transfer interface
while preventing solution leakage and corrosion risks.
Notably, crystallization should be avoided in the desorption-ab-
sorption cycle, which can deteriorate the mass and heat transfer
performance.
We designed and fabricated the HSMHS based on a 3D-print-
ing heat sinkwith an anti-corrosion graphene coating (Figure 1B).
The HSMHS consists of a hollow plate, a porous membrane, a
3D-printing heat sink, and a hygroscopic solution, and the tested
prototype of the HSMHS is shown in Figure 1C. In the prototype,
a 0.25-mm-thick hydrophobic membrane with a pore size of
0.45 mmwas employed (Figures 1D and S2). A schematic illustra-
tion for an electronic device equipped with HSMHS is shown in
Figure 1E. The HSMHS was directly attached at the electronic
device. Compared to the conventional heat dissipation methods
(i.e., surface convection and radiation), the desorption process
of salt solution takes away a substantial part of heat generated
by the electronic device, releasing water vapor to the surround-
ing atmosphere through the porous membrane, and thus further
preventing the device from overheating. To understand the
working principle of the HSMHS, we have conducted a detailed
analysis of both heat andmass transfer processes (Note S3). Fig-
ure S3 depicts the simplified heat transfer process through the
porous membrane. The thermal resistances, which include the
external thermal resistances and the thermal resistance across
the porous membrane (sm/km), play a critical role in determining
the temperature difference between the solution-membrane
surface and the surrounding environment. The amount of heat
extracted from the solution (qcooling) can be expressed as
Figure 1. Overall working principle of the
proposed thermal management strategy
(A) Schematic illustration of the desorption and
absorption processes. In the desorption-cooling
process, moisture evaporates from the solution for
heat removal. In the absorption-regeneration
process, the concentrated salt solution with a
lower temperature absorbs moisture from the
surrounding atmosphere to autonomously recover
cooling capacity. The porous membrane prevents
the solution leakage and device corrosion while
allowing the water vapor to pass through.
(B) Schematic of the fabrication process for the
HSMHS.
(C) Photograph of the HSMHS prototype.
(D) High-magnification SEM image of the PTFE
membrane with a pore size of 0.45 mm to show the
nodes and fiber structures. The inset shows a
photograph of the contact angle for 39.4% LiBr
solution on the PTFE membrane. Details can be
found in Figures S2 and S4.
(E) Schematic illustration of electronic cooling with
the HSMHS. Except for the common natural
convection and radiation, the salt solution can
extract the heat generated by the electronic de-
vice, releasing water vapor through a protective
membrane and thus further preventing the device
from overheating.
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qcooling
�
1
hnat
+
1
hrad
+
1
hdes
+
sm
km
�
= Tsurf � Tamb (Equation 1)
where hnat is the natural convection heat transfer coefficient, hrad is
the effective radiation heat transfer coefficient, hdes is the equiva-
lent heat transfer coefficient due to the desorption process, sm is
the membrane thickness, km is the thermal conductivity of the
membrane, Tsurf is the temperature of solution-membrane inter-
face, and Tamb is the ambient temperature. It is evident that the
desorption process of salt solution extracts more heat compared
with a conventional natural convection of air, thus contributing to
themitigationof temperature rise inelectronicdevices.With regard
to cooling capacity involved in the desorption process (qdes), it is
related to thedesorptivemassflow (J) drivenby thevaporpressure
difference, and qdes can be expressed as
qdes = JhfgDm = Akm;m
�
pm;s � pv
�
hfgDm (Equation 2)
where A is the mass transfer area; hfg is the vaporization
enthalpy of water in the salt solution; km,m is the mass transfer
coefficient through the porous membrane (see details in Note
S3); pv is considered as an external boundary condition, which
can be calculated according to the ambient condition (RH and
Tamb); pm,s can be evaluated according to Figure S1; and Dm
is the water content variation of the salt solution in the desorp-
tion process. It can be seen from Equation 2 thatthe hygro-
scopic salts used in the passive cooling strategy should
feature fast desorption/absorption kinetics and high water
uptake.
Desorption-absorption performance for LiBr-loaded
heat sinks
As discussed in Figure 1, hygroscopic salts as the sorbents
directly determine the cooling performance of the proposed
strategy. LiCl and LiBr are themost commonly used hygroscopic
salts in absorption heat storage and cooling systems,36,42 both
of which have a high hygroscopic capacity and can be deli-
quesced at an extremely low RH (Table S1), especially for LiBr.
To test the desorption-absorption performance of the LiBr solu-
tion, we built proof-of-concept prototypes (see details in Note
S2). First, three LiBr solutions with mass fractions of 39.4%,
44.2%, and 49.4%were prepared to test the effect of initial water
content on the cooling performance of the strategy. These solu-
tionswere tested using a 3D-printing heat sink (Figure S5) coated
with a 0.01-mm-thick anti-corrosion graphene coating. A polyi-
mide film heater was used to emulate electronic devices, and
its heat flux could be controlled by a DC power. To reduce
heat loss from the bottom of the heat sink, the heat sink was
placed on an insulation foam. We then placed the entire
hygroscopic salt-loaded heat sink (HSHS) in an environmental
chamber (Figures 2A and S6), where Tamb and RH fluctuations
were kept below 1�C and 3%, respectively (Figure S7). To
analyze the effect of solution layer thickness, the HSHSs with so-
lution layer thicknesses of 1.0, 1.5, and 2.0 mm were prepared,
respectively. During the experiments, we first removed the
porous membrane to measure the desorption and absorption
Device 1, 100121, December 22, 2023 3
Figure 2. Desorption and absorption experiments with a typical ambient condition (RH 60% and 25�C)
(A) Schematic of the HSHS in an environmental chamber. In the chamber, the surrounding temperature and RH fluctuations can be kept below 1�C and 3%,
respectively.
(B) Temperature andmass evolutions of the HSHSswith different solution layer thicknesses at a heat flux of 2.41 kW/m2 (xLiBr= 39.4%). The temperature recorded
by a thermocouple (TC1) shown in Figure S5 is treated as the temperature of the HSHS. For comparison, another 3D-printing heat sink was prepared as the
baseline sample.
(C) IR images of the HSHSs with different solution layer thicknesses.
(D) Temperature evolutions at different heat fluxes. The initial solution concentration and thickness are 39.4% and 1.5 mm, respectively.
(E) Performance comparison in terms of the maximum transient cooling power and temperature. Error bars are calculated depending on the propagation of
instrument uncertainty.
(F) Variation of thermal resistances induced by the salt solution in the desorption process.
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performances of LiBr solutions, and thermocouples and high-ac-
curacy mass balance were used to monitor the temperature
evolution and mass change of the HSHS, respectively.
Experimental data from Figure 2B show the temperature and
mass evolutions of the HSHSs at a heat flux of 2.41 kW/m2. Dur-
ing the initial period, using salt solution significantly slows down
the temperature rise rate, resulting in a lower temperature
compared with the baseline. The fast desorption process of
the salt solution is responsible for this phenomenon, as
confirmed by the corresponding mass curves (Figure 2B). As
the heating continues, the desorption rate (i.e., desorptive
mass flow shown in Equation 2) gradually decreases due to the
increasing solution concentration. The temperatures of the
HSHSs are fixed around the maximum values establishing
quasi-stable states, achieving a lower temperature (between
58.7�C and 60.6�C) compared with the baseline (64.5�C). The
temperature reductions can be ascribed to the alternate desorp-
tion-absorption process (insets in Figure 2B) and enhanced heat
transfer caused by a high solution concentration with a thin solu-
tion layer.43,44 On the basis of the experiments, it is found that the
maximum temperature difference between the HSHS and the
baseline further increases as the solution layer thickness in-
4 Device 1, 100121, December 22, 2023
creases from 1.0 to 2.0 mm, with DTmax,1.0mm = 10.8
�C,
DT max,1.5mm = 11.7
�C, and DT max,2.0mm = 13.0�C, respectively.
Additionally, nearly the same desorption rate (green dashed line
shown in Figure 2B) is observed for the three HSHSs at the initial
periods, regardless of the solution layer thickness. The reason is
that the water vapor transport occurs at the solution-air interface,
and the desorption rate is controlled by the ambient conditions un-
der a given heat flux.45 However, the HSHS (2.0 mm) can evapo-
rate more water vapor than the other HSHSs (1.0 and 1.5 mm)
due to its high initial water content, achieving a better cooling per-
formance (Equation 2). To further understand the physical mecha-
nism during the quasi-stable states, the temperature and mass
curves were magnified (insets in Figure 2B). The magnified mass
evolution shows an alternating process of desorption and absorp-
tion, which can be attributed to a delicate balance between the
heat flux and the heat dissipation by natural convection, radiation,
and desorption (Equation 1). Figure 2C illustrates the temperature
distribution of the baseline and the HSHSs obtained from infrared
(IR) images, and a uniform temperature distribution is observed.
Generally, electronic devices should operate at temperatures
below 70�C Figure 2D shows the temperature evolutions of the
HSHS with a 1.5-mm-thick solution layer, illustrating that the
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maximum heat flux allowed is around 3.28 kW/m2 for the HSHS,
corresponding to a maximum cooling power of �462.5 W/m2
(Figure S8).
To understand the effect of initial water content of the salt
solution on the cooling performance, we conducted a series of
experiments with 39.4%, 44.2%, and 49.4% LiBr solutions. Fig-
ure S9 shows the temperature and mass evolutions for the ex-
periments conducted at a given heat flux of 2.41 kW/m2. As
the solution concentration increases from 39.4% to 49.4% at a
fixed solution layer thickness, the temperature of the quasi-sta-
ble state increases due to the decreasing cooling performance.
By deriving themass curves (Figure S9), the transient cooling po-
wer is obtained (Figure S10). The cooling power increases with a
similar tendency in the initial several minutes for the HSHSs with
different solution layer thicknesses and fluctuates during quasi-
stable states. The maximum cooling powers for the various
HSHSs are shown in Figure 2E, with the corresponding average
temperatures at quasi-stable states shown above the bars.
Obviously, the HSHS with low solution concentration outper-
forms other HSHSs. The maximum cooling power has a positive
correlation with the solution layer thickness, especially for the
HSHS with a low solution concentration. For example, by
increasing the solution layer thickness from 1.0 to 2.0 mm, the
maximum cooling power of the HSHS with a solution concentra-
tion of 39.4%, 44.2%, and 49.4% is improved by 73.1%, 67.8%,
and 54.1%, respectively. The maximum cooling power achieved
is up to 409.1 W/m2 with the lowest average temperature of
58.7�C. Meanwhile, we discussed the solution concentration at
the quasi-stable states (Figure S11). The solution concentration
is between 57.6% and 62.6%, demonstrating that the salt solu-
tion would not crystalize in the desorption processes (Figure S1).
Figure 2F shows the variation of the solution thermal resistance
at the starting and ending points of the desorption processes
(Figure S9). The thermal resistance is linearly related to the solu-
tion layer thickness. Moreover, the thermal resistance is lower at
the ending points than that at the starting points due to the thin-
ning solution layer thickness during desorption process.
Working performance under intermittent heatgeneration
To achieve a high-efficiency thermal management in some appli-
cations with intermittent heat generation, it is important for
HSHSs to not only have excellent cooling performance but
also enable high self-regeneration capability. We conducted ex-
periments at a heat flux of 2.41 kW/m2 for 10, 20, and 30 min to
analyze the effect of desorption time on regeneration time (Fig-
ure S12). It can be clearly seen that the desorption process
continued for a period of time even after stopping heating, which
can be attributed to a high pm,s caused by a high solution tem-
perature. As the solution temperature further decreases, the ab-
sorption process occurs when pm,s is lower than pv (Equation 2).
Also, Figure S12 shows that the temperature evolutions are un-
affected by the absorption process, indicating that the dilution
heat caused by the absorption process is negligible in small-
size HSHSs. The relationship between the desorption time and
the regeneration time is depicted in Figure 3A, demonstrating
that the salt solution enables self-regeneration capability. At a
given desorption time, the regeneration time is decreased as
the solution concentration increases from 39.4% to 49.4%, indi-
cating that a high solution concentration can endow the HSHSs
with rapidly self-regeneration capability, which is critical for
practical applications of the proposed strategy. It is worth noting
from Figures 2E and 3A that the conflict between the cooling po-
wer (low solution concentration) and the regeneration time (high
solution concentration) is unavoidable. Considering that the
solution concentration eventually depends on the surrounding
atmosphere, increasing the heat and mass transfer area can
provide an effective method to reduce the regeneration time
for achieving a more efficient thermal management in some
application scenarios.
In addition, the HSHS with a solution concentration of 39.4%
(1.5-mm-thick solution layer) was tested under a periodic heat
flux to discuss the effect of regeneration periods on the cooling
performance. Figure 3B compares the temperature evolutions
of the HSHS and the baseline (desorption, 3.28 kW/m2 for
10 min; regeneration, 0 kW/m2 for 10 min). In the initial several
cycles, the cooling efficiency slightly decreases due to incom-
plete solution regeneration. After several cycles, a stable cooling
efficiency is achieved and the peak temperature can become
nearly constant with a temperature drop of around 16.8�C. The
variation of the cooling efficiency can be validated by Figure 3C,
where the mass change of the desorption and absorption pro-
cesses within each cycle gradually reaches an equilibrium (i.e.,
green column tends to 0 after several cycles). In Figure S13,
we also conducted the cyclic experiments with the regeneration
periods of 20 and 30 min, respectively. Results indicate that,
even if the initial solution of HSHSs is in an unsaturated state
(i.e., pm,s s pv), the HSHSs can still provide efficient thermal
management. Figure 3D illustrates that the difference between
the peak temperature of the first cycle and the peak temperature
of the last cycle (i.e., black dashed boxes shown in Figure 3B) is
8.0�C, 6.5�C, and 3.6�C with the regeneration periods of 10, 20,
and 30 min, respectively. Apparently, the degradation of the
cooling efficiency becomes insignificant as the regeneration
time increases. Notably, benefiting from the anti-corrosion char-
acteristics of the graphene-based coating, no visible signs of
corrosion were observed during the experiments.
Cooling performance of LiBr-loaded membrane-
encapsulated heat sinks
Porous PTFE membranes feature vapor permeability and strong
chemical stability, which play a key role to confine the salt solu-
tion and achieve a long-duration stable cooling performance. An
HSMHS with 5.0-mm-thick solution layer was developed to
demonstrate these characteristics. The mass and temperature
variations of the tested devices with and without the membrane
were compared at a given heat flux of 2.41 kW/m2. From Fig-
ure 4A, the mass evolutions show nearly the same tendency dur-
ing the initial periods (black dash line depicted in Figure 4A), indi-
cating that the mass transfer resistance caused by the
membrane is insignificant. Eventually, the HSMHS shows larger
mass loss compared with the strategy without the membrane,
and we attribute this to the higher temperature of the HSMHS
in the quasi-stable state. As shown in Figure 4B, the maximum
temperature differences between the baseline and two samples
(i.e., HSMHS and HSHS) are 11.5�C and 12.2�C, respectively.
Device 1, 100121, December 22, 2023 5
Figure 3. Cyclic experiments of HSHSs with a typical ambient condition (RH 60% and 25�C)
(A) Relation between the desorption duration and the regeneration duration at a given heat flux of 2.41 kW/m2. The regeneration duration is counted from stopping
heating. Details can be found in Figure S12.
(B) Cyclic stability test of the HSHS at a heat flux of 3.28 kW/m2 for 10 min, and the concentrated solution spontaneously absorbs water vapor from the sur-
rounding atmosphere for 10 min.
(C) Mass changes during the desorption and absorption processes within each cycle.
(D) Comparison between the peak temperature of the first cycle and the peak temperature of the last cycle (black dashed boxes shown in Figure 3B) for the HSHS
with different regeneration periods.
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The quasi-stable temperature of the HSMHS is slightly higher
than that of the HSHS, with an average temperature difference
of �1.3�C. To quantitatively discuss the effect of the membrane
on the duration of the thermal management, we defined an effec-
tive cooling time (teff) as themoment when the temperature of the
proposed strategies exceeds that of the baseline. It is found that
introducing membrane reduces the effective cooling time from
�520 to�400min (Figure 4B). Nevertheless, the HSMHS can still
significantly extend the effective cooling time by �10-fold
compared with an MOF-101(Cr)-based cooling strategy.15
Moreover, a 12 V direct-current (DC) fan was used to provide
the forced airflow (Figure S14). Comparison results indicate
that the forced convection affects the effective time insignifi-
cantly, which can be attributed to a lower desorption tempera-
ture.15 Furthermore, comparison between Figures 2B and 4B
demonstrates that increasing the solution layer thickness from
1.0 to 2.0 mm enhances the cooling capacity of HSHSs; howev-
er, the cooling capacity is decreased as the solution layer thick-
ness increases to 5.0 mm. It suggests that an optimal solution
layer thickness exists between 1.0 and 5.0 mm at the given
boundary conditions. In practical applications, users can deter-
6 Device 1, 100121, December 22, 2023
mine the optimal solution layer thickness by utilizing the analyt-
ical model developed in Note S3 while considering specific
working conditions.
Developing low-cost and high-performance cooling materials
is the key for realizing the large-scale application of passive cool-
ing strategies. We used an equivalent index (h) (i.e., the cooling
performance per USD) to quantitate the cost effectiveness of
cooling strategies, defined as
h =
Dmwhfg
ms
Cs;t (Equation 3)
where Dmw represents the mass loss of water in a desorption
process, ms is the mass of sorbents, and Cs,t is the cost of sor-
bents. Figure 4C shows the quantitative comparison re-
sults.15,17,46–55 In the comparison, the equivalent enthalpy (heq)
of the selected sample is �2370 J/g (i.e., the corresponding en-
ergy density [qeq] is �1.31 J/mm3) at a heat flux of 3.28 kW/m2
(corresponding to Dmw = 1.165 g and ms = 1.178 g), which is
higher than existing liquid-vapor passive cooling techniques
Figure 4. Cooling performance of HSMHS
(A) Mass evolutions of the tested devices with and without the membrane. The inset is a photograph of the HSMHS. A 5.0-mm-thick LiBr solution with a solutionconcentration of 39.4% was employed.
(B) Temperature evolutions of the tested devices with and without membrane at a heat flux of 2.41 kW/m2. A fan (12 V) was used to force ambient air to flow
through the upper surface of the heat sink (Figure S14). The temperature of the HSMHS is slightly higher than that of the HSHS, indicating that introducing the
membrane increases additional thermal resistance.
(C) Cost effectiveness of HSMHSs compared with that of existing cooling strategies reported in the literature (see Table S3 for details).
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(Table S2). The detailed price data can be found in Table S3. An
improvement of about three orders of magnitude in the cost
effectiveness can be achieved compared with the state-of-the-
art MOF-based cooling strategies, demonstrating that the
proposed cooling strategy is a promising technology toward
commercialization.
Thermal management for a real CPU
The regeneration rate is a key parameter that measures the ther-
mal management capacity of moisture-desorptive passive cool-
ing strategies. To demonstrate the superiority of the proposed
cooling strategy, we compared the regeneration rates of the
proposed strategy with those of the existingMOF-based and hy-
drogel-based cooling strategies. In Figure S15, the regeneration
rate of the proposed strategy is up to 25.14 g/(m2,h), which is 1.2
times and 1.3 times that of the MOF-based and hydrogel-based
cooling strategies,15,23 respectively. To further reduce the regen-
eration time and improve the practicality of the proposed cooling
strategy, we utilized a needle heat sink with hollow fiber mem-
branes to enhance the heat and mass transfer process. The
heat sink was made of aluminum to reduce weight and cost,
and its physical structure can be found in Figure S16. We used
an elastic PTFE hollow fiber membrane with a pore size of
0.45 mm to confine the LiBr solution (Figure S17). The hygroscop-
ic salt-loaded membrane-encapsulated needle heat sink
(HSMNHS) was tested at a heat flux of 2.41 kW/m2 for various
RH conditions (RH 60%, RH 70%, and RH 80%), with the tem-
perature and mass data recorded until the mass recovered to
0 (Figure S18). An obvious reduction in regeneration time for
RH 80% is observed. This can be ascribed to the fact that the
high RH accelerates the vapor absorption of the HSMNHS
from the surrounding environment, as elucidated in Equation
S15. In Figure 5A, we compared the mass curves of the
HSMNHS with those of Figures S11A–S11C at the ambient con-
dition of RH 60%, and the same desorption rate could be found
due to a given heat flux of 2.41 kW/m2. The mass curve of
HSMNHS rises faster than that of other samples benefiting
from the improved heat and mass transfer process, and the
regeneration rate of HSMNHS is improved by 335% compared
to the HSHS with a solution thickness of 2.0 mm (Figure S19).
Furthermore, Figure 5B compares the temperature of the
HSMNHS with that of a conventional needle sink to demonstrate
the outstanding thermal management capacity of the HSMNHS
at the ambient condition of RH 60%. In the cyclic experiment
(2.41 kW/m2 for 10 min and 0 kW/m2 for 10 min), the maximum
peak-temperature difference between the HSMNHS and the
conventional needle heat sink is 15.1�C, which occurs in the sec-
ond cycle. The temperature difference decreases gradually and
is stabilized at �8.7�C ultimately. To demonstrate the thermal
management capacity of the proposed strategy at heavy loads,
we applied the HSMNHS to a high-power field effect transistor
(Figures S20A to S19C). Three heat sinks (i.e., needle heat sink
[NHS], fin heat sink [FHS], and HSMNHS) were tested in room
environment (Figures S20D to S19F), and their detailed compar-
ison is listed in Table S4. Figure S21 shows the temperature and
effective heat transfer coefficient evolutions for the three
heat sinks at various heat fluxes (i.e., 25, 50, and 75 kW/m2).
Compared to the NHS, the HSMNHS has overwhelming advan-
tages in cooling performance. HSMNHS shows a competitive
temperature reduction and heat transfer coefficient compared
to the FHS at a given heat flux (Figures 5C and S21), even if
the volume and heat transfer area of the FHS are �5 and �10
times that of the HSMNHS (Table S4), respectively. The results
demonstrated that the proposed HSMNHS enables outstanding
cooling performance at high heat fluxes. Notably, we did not
observe any solution leakage and corrosion phenomena during
the experiments.
To demonstrate the cooling capacity of the improved strategy
in a real scenario, we placed the HSMNHS on a CPU surface of a
real computing device (ODROID-XU4). In Figure 5D, we used
TCs to record the temperature of the CPU surface equipped
with the HSMNHS and the original heat sink, respectively. The
computing device was run on the Android system, which was
driven by a professional benchmark software (AnTuTu) to
Device 1, 100121, December 22, 2023 7
Figure 5. Practical application of the proposed thermal management strategy
(A) Mass evolutions of different samples with the same desorption time (10 min), solution concentration (39.4%), and heat flux (2.41 kW/m2). It clearly illustrated
that introducing hollow fiber membranes effectively improves the absorption-regeneration capacity.
(B) Cyclic experiments of the NHS with and without the membrane (2.41 kW/m2 for 10 min and 0 kW/m2 for 10 min).
(C) Comparison of temperature evolutions for the HSMNHS and FHS at heavy loads (25, 50, and 75 kW/m2).
(D) Photograph of ODROID-XU4 with the HSMNHS and the original heat sink. The temperature and RH during the testing periods are shown in Figure S22.
(E) Temperature evolutions of the tested devices with the two cooling strategies. A program forced the device to work at a maximum power for 15min. The insets
show IR images of the tested device with the two cooling strategies at the end of the testing.
(F) Transient input power evolutions of the tested devices with the two cooling strategies during testing periods.
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simulate a peak workload for 15 min in a room. A maximum
temperature drop of 11�C from 68.5�C to 57.5�C was achieved
during the testing periods (Figure 5E). As shown in the IR images
inserted in Figure 5E, the proposed thermal management strat-
egy enables a lower temperature for the electronic load board.
Meanwhile, we measured the input power during the experi-
ments using Power-Z KT002 (Figure S23). Benefiting from the
outstanding cooling capacity of our proposed strategy, the tran-
sient input power of the device equipped with the HSMNHS is
significantly higher than that of the device with the original heat
sink during the testing periods, and an improvement of 32.65%
in the average input power is achieved (Figure 5F). The experi-
ments confirm the enormous potential of our proposed thermal
management strategy for passive cooling.
Numerical simulation and analytical model
We first simulated the heat loss from the bottom of the tested de-
vice using a computational fluid dynamics (CFD) tool (see details
in Figure S24). The simulated temperature is consistent with the
experimental data, and the heat loss is �10.4% of the total heat
flux. Togainmore insights into the thermalmanagement strategy,
we carried out a numerical calculation using the CFD tool based
on the analyticalmodel developed inNoteS3. FigureS25demon-
8 Device 1, 100121, December 22, 2023
strates that a good agreement is found between the simulations
and the experimental results, indicating that the HSHS with a
solution layer thickness of 3.0 mm show an outstanding cooling
performance. Also, Figure S26 displays the cross-sectional tem-
perature contour over time. The solution shows an uneven tem-
perature distribution in the desorption process. We ascribe this
unstable behavior of the temperature to thermal convection in-
side the solution. Through the experimentally validated numerical
simulations, we provide guidelines for integratingthe passive
cooling strategy in various thermal management applications.
DISCUSSION
We proposed an efficient thermal management strategy with a
record-high cost effectiveness based on hygroscopic salt solu-
tions and demonstrated its feasibility under heavy loads. As pre-
viously discussed, LiCl is also a commonly used hygroscopic
salt, which possesses higher water uptake compared to LiBr
(Figures S1 and S27). Hence, we conducted an experiment to
evaluate the feasibility of LiCl solution for this strategy (see
more details in Note S4). An obvious crystallization during
desorption cooling is observed, resulting in a non-uniform tem-
perature distribution with a maximum temperature difference
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of 4.8�C (Figure S28). Apparently, crystallization challenges
should be taken into consideration during the selection of hygro-
scopic salts for the passive thermal management strategy. To
demonstrate the feasibility of the proposed cooling strategy, a
comprehensive evaluation was conducted by comparing it
with other emerging passive cooling technologies (MOFs and
hydrogels). Five representative indexes were selected in terms
of desorption performance, absorption performance, cost effec-
tiveness, cycling performance, and equivalent enthalpy, as
shown in Figure S29. It is found that the high cost is a major chal-
lenge for MOF-based cooling strategies, while hydrogel-based
cooling strategies have low cycling performance due to interior
structure collapse and low absorption capacity. In contrast, hy-
groscopic salts confined by porous membranes enable stable
thermophysical properties and high moisture sorption capacity,
which provide a promising alternative for the passive thermal
management of electronics.
In addition, introducing porous membranes introduces an
additional thermal resistance, which can cause the cooling
capacity of HSMHSs to slightly decline. It can be attributed to
the low thermal conductivity of PTFE membranes. With the
development of materials, some additives, such as graphene,
carbon nanotubes, and silica, can be added to porous mem-
branes to enhance their thermal conductivities.56 Meanwhile,
we noted that a thin air layer may be generated during the
desorption process, resulting in a potential reduction in heat
transfer, and thus optimal structural design warrants future
studies. It is also found that hygroscopic salts may leak and
can lead to corrosion risk of metal devices during long-term op-
erations. The leakage and corrosion risks can be avoided using
membrane encapsulation techniques,57,58 graphene-based
anti-corrosion coatings,59 and anti-corrosion containers.60
In summary, we proposed a passive thermal management
strategy based on low-cost hygroscopic salts.We demonstrated
that the strategy enabled high cooling performance without the
risk of solution leakage and corrosion by introducing the porous
PTFEmembrane. Benefiting from the high moisture-sorption ca-
pacity of hygroscopic salts, HSMHSs enable the outstanding
self-regeneration capability. Compared with the existing passive
thermal management strategies, the proposed strategy shows a
record-high cost effectiveness for commercialization. Further-
more, we developed a detailed heat and mass transfer model
validated through experiments, which could provide guidelines
for further optimization. Our cooling strategy is both cost effec-
tive and highly scalable, which can be useful for various cooling
applications with few technological barriers.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources and materials should be
directed to and will be fulfilled by the lead contact, Wei Wu (weiwu53@cityu.
edu.hk).
Materials availability
This study did not generate new unique materials.
Data and code availability
d All data that support the findings of this study are available from the lead
contact upon reasonable request.
d Full experimental procedures are provided in the supplemental informa-
tion.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.
device.2023.100121.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from the Research Grants
Council of Hong Kong (project numbers CityU 11212620, CityU 11215621,
CityU 11218922). R.Y. acknowledges financial support from the National Nat-
ural Science Foundation of China (NSFC) under grant no. 52036002.
AUTHOR CONTRIBUTIONS
Z.S. and W.W. conceived the concept of this work. Z.S., Z.D., H.L., and F.L.
synthesized the materials. Z.S., R.S., Z.D., H.L., and F.L. designed the exper-
iments. Z.S. performed the experiments. Z.S. and R.S. developed the heat and
mass transfer modeling. Z.S. wrote the original manuscript draft. Z.S., W.W.,
and R.Y. discussed and revised the manuscript. All authors checked the
manuscript. W.W. and R.Y. directed and supervised this project.
DECLARATION OF INTERESTS
AUS non-provisional patent (18/183,184) related to this work has been filed by
W.W., F.L., Z.S., Z.D., Y.S., C.Z., and H.L.
Received: July 3, 2023
Revised: September 21, 2023
Accepted: October 3, 2023
Published: October 31, 2023
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	Membrane-encapsulated, moisture-desorptive passive cooling for high-performance, ultra-low-cost, and long-duration electron ...
	Introduction
	Results
	Design and working principle of HSMHSs
	Desorption-absorption performance for LiBr-loaded heat sinks
	Working performance under intermittent heat generation
	Cooling performance of LiBr-loaded membrane-encapsulated heat sinks
	Thermal management for a real CPU
	Numerical simulation and analytical model
	Discussion
	Experimental procedures
	Resource availability
	Lead contact
	Materials availability
	Data and code availability
	Supplemental information
	Acknowledgments
	Author contributions
	Declaration of interests
	References