Cocoon-based 3D Solar Steam Generator for High-performance Saline Desalination

Abstract

Interfacial solar steam generation (ISSG) provides a facile and sustainable strategy for freshwater production from the saline. To further enhance the steaming efficiency, exploration of novel photo-thermal materials and optimization...

Full text

Cocoon-based 3D Solar Steam Generator for High-
performance Saline Desalination
Journal:
Sustainable Energy & Fuels
Manuscript ID
Draft
Article Type:
Paper
Date Submitted by the
Author:
n/a
Complete List of Authors:
Song, Changyuan; Zhengzhou University, School of Materials Science
and Engineering
Chen, Xuying; Zhengzhou University, School of Materials Science and
Engineering
Hao, Rui; Zhengzhou University, School of Materials Science and
Engineering
Cai, Dongna; Hubei University of Technology, National “111" Center for
Cellular Regulation and Molecular Pharmaceutics , Key Laboratory of
Fermentation Engineering (Ministry of Education), Hubei Key Laboratory
of Industrial Microbiology, College of Bioengineering and Food
Zhu, Xiangwei; Hubei University of Technology, National “111" Center for
Cellular Regulation and Molecular Pharmaceutics , Key Laboratory of
Fermentation Engineering (Ministry of Education), Hubei Key Laboratory
of Industrial Microbiology, College of Bioengineering and Food
Liu, Hao; Zhengzhou University, School of Materials Science and
Engineering
Chen, Jinzhou; Zhengzhou University, Materials Science & Engineering
Liu, Wentao; Zhengzhou University, School of Materials Science and
Engineering
Sustainable Energy & Fuels

May 7, 2021
Dear Prof. Wang,
Please kindly find a manuscript entitled “Cocoon-based 3D Solar Steam Generator for
High-performance Saline Desalination”, which is submitted to Sustainable Energy & Fuels as
an original report for consideration. This is our original work, which is not published or
submitted to other journals.
Recently, solar steam generator driven by green and sustainable solar energy is deemed a
potential technology in the actual operation of seawater desalination. 3D structure like sphere or
hemisphere is designed to reduce the dependence of light absorption on the incident angle, and
the high efficiency of photothermal conversion can be ensured under the imperfect illumination
angles. However, the design of thermal management is neglected in 3D solar steam generators.
when the generators work on the water surface, a large number of heat converted from solar will
be directly transferred to the water, accounting for a low solar-to-steam efficiency.
In our work, a 3D steam generator with two functional areas of light absorption and thermal
management is obtained by local modification of cocoon at low temperature heating. The 3D
steam generator exhibits a high evaporation rate of 1.44 kg/m2/h. moreover, excellent thermal
management realizes the localized heating of 3D steam generator, resulting in heat conduction
loss of only 0.9%. The 3D structure not only remains the high evaporation rates at different light
incident angles, but also impedes the excessive accumulation of salt on generator surface in sea
desalination. This work not only provides a method of low-cost, environmentally friendly and
high-efficiency product in sea desalination, but also contributes to the optimization of 3D solar
steam generator.
We would like to recommend the following experts as the suggested reviewers:
(1) Prof. Cheng bing Wang
School of Materials Science and Engineering, Shaanxi University of Science and Technology,
China. He is an expert in the fields of advanced functional coating, including solar thermal
conversion coatings, solar steam generation.E-mail address: wangchengbing@gmail.com
(2) Prof. Jiang Gong
Page 1 of 15 Sustainable Energy & Fuels

School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, China.. He is an expert in the fields of carbon nanomaterials and solar steam
generation. E-mail address: gongjiang@hust.edu.cn
(3) Prof. Tao Tang
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences. E-mail address:ttang@ciac.ac.cn
If you have any questions or need further information, please feel free to contact me. We
appreciate your considerations.
Sincerely on behalf of all the authors,
Corresponding author: Wentao Liu
E-mail: wtliu@zzu.edu.cn
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ARTICLE
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a.
School o f Materia ls Scienc e and Engineerin g, Zheng zhou Uni versity, Henan Ke y
Laborat ory of Adva nced Ny lon Mat erials and Appli cation, Zh engzho u Univer sity,
Zhengzh ou, 4500 01, China .
b.
National “111" Center fo r Cellula r Regula tion and M olecula r Pharma ceutics , Key
Laborat ory of Ferm entatio n Engine ering (M inistry of Educa tion), Hu bei Key
Laborat ory of Indu strial Mic robiolog y, Colleg e of Bioen gineerin g and Foo d, Hubei
Univers ity of Tech nology, Wuha n, 43006 8, China .
Email add ress: xiangwei@ ksu.ed u (Xiangw ei Zhu), hli u@zzu.edu.cn (Hao Li u),
cjz@zzu. edu.cn (Jin zhou Che n) and wtliu @zzu.ed u.cn (Wen tao Liu)
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Cocoon-based 3D Solar Steam Generator for High-
performance Saline Desalination
Changyuan Songa, Xuying Chena, Rui Haoa, Dongna Caib, Xiangwei Zhub,*, Hao Liua,*, Jinzhou
Chena,* and Wentao Liua,*
Abstract: Interfacial solar steam generation (ISSG) provides a facile and sustainable strategy for freshwater production
from sea. To further enhance the steaming efficiency, exploration of novel photo-thermal materials and optimization of
their heat transfer are two important considerations. However, for most steam generators, the exquisite design of their
intrinsic evaporation architecture was less discussed, which in turn seriously reduced their solar-to-vapor conversion
efficiency. In this study, a 3D steam generator that demonstrating intrinsic bi-functional areas (light absorption and
thermal management) is prepared by local modification of cocoon at low temperatures. The obtained steam generator
exhibits a evaporation rate as high as 1.44 kg/m2/h, and its heat conduction loss was reduced significantly to only 0.9%.
More importantly, the intrinsic 3D structures of cocoon can help to maintain high evaporation rates of generators as the
light incident angles varied from 0-90°, which facilitates their practical uses, i.e. under moving sun. This work provides a
paradigm shift to design cost-effective, high-efficiency and eco-friendly steam generators for high-performance saline
desalination.
Keywords: solar steam generation; 3D structure; cocoon; sea desalination;
1. Introduction
The increasing shortage of fresh water resources has seriously
restricted the development of human society. Although the
water source covers about 75% of the earth surface, 97.5% of
this is the sea with high salinity.1Thus, how to obtain fresh
water from sea has drawn a great deal of attention. Among
kinds of desalination technologies, interfacial solar steam
generation, driven by clean energy, is sought after by
researchers. Solar steam generator, fabricated from
photothermal materials, is located at the interface of water
and air, which directly transfer the heat conversed from solar
to foster evaporation. It can decrease heat loss in the bulk
water by localized heating, promoting the solar steam
efficiency.2-4
More recently, multifarious 2D solar steam generators
prepared from metal plasmonic (e.g., TiO2,5Au,6CuS7and
MnO28nanoparticles), polymer (e.g., polypyrrole9and
polydopamine10), or inorganic nonmetallic materials (e.g.,
graphene,11-14 carbon nanotube (CNT) ,15-17 MXene18 and
amorphous porous carbon19-21) exhibit high photothermal
conversion efficiency and good stability in ISSG due to localized
heating effect. However, these excellent performances are
usually displayed under the condition that the solar adsorption
of 2D solar steam generator is perpendicular to the light
source and obtains the maximum photon density. While in
outdoor conditions, the position of the sun in the sky is
constantly changing, accounting for the sunlight irradiating on
the horizontal plane at different angles, and sunlight hardly
irradiate the surface of 2D solar steam generator vertically.22
This issue seriously restricts the light absorption of solar steam
generator in practical application.
Solar steam generator with 3D sphere or hemisphere structure
is designed to reduce the dependence of light absorption on
the incident angle. By far, 3D spherical solar steam generator
constructed using the Co3O4/Ti3C2MXene based fabric18 and
3D hemispheric solar steam generator composed of nano
carbon black and PMMA composite22 show the excellent
performance both in light absorption and evaporation.
Interestingly, platanus fruit23 with spherical structure was
carbonized as a solar steam generator which also presents
remarkable water evaporation. The whole generators or the
entire exterior of these 3D generators can be used as sunlight
absorber. However, when the generator floats and works on
the water surface, a number of heats from solar energy will be
directly transferred to water so that the heat could not be
concentrated at the interface, since the absorber is in contact
with water surface. The heat loss may be higher if the absorber
is constructed by using good thermal conductive material.
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Silk cocoon, one of the most abundant natural polymer
composites, exhibits an intrinsic 3D non-woven structure,
which is assembled by randomly arrangement of fibroin fibers
and fastened by sericin.24, 25 The oval cube structure and good
hydrophilicity make cocoon be a potential solar steam
generator with high property. In this contribution, a cocoon-
based 3D solar steam generator with two function (light
absorption and thermal management) regions is prepared by a
simple and green method of local modification. The generator
can maintain a good evaporation rate under different
illumination angles, and the advantage of 3D structure over 2D
is also explored. Localized heating effect in thermal
management region of materials during evaporation is
discussed. Besides, it is revealed that the oval cube structure
of cocoon prevents precipitated salts from unduly
accumulating on the generator surface after a long-term
desalination. We believe that cocoon-based generation
provide a novel route to the preparation of 3D solar steam
generator.
2. Experimental section
2.1 Materials
Silk cocoon (yellow) was obtained from demotic silkworm.
Ethanol and ferric chloride (FeCl3) and were purchased from
Sinopharm Chemical Reagent Co., Ltd. Seawater was obtained
from South Sea (near to Hainan).
2.2 Preparation of 3D solar steam generation
As shown in Fig. 1, the top of silk cocoon (ca. 1 cm) was
immersed into FeCl3(0.1 mol/L) solution and stirred for 2 h.
The resultant product (denoted as SC-Fe) was dry naturally.
HSC-Fe was prepared by heating SC-Fe in a muffle furnace at
180 °C (Fig. S1) for 10 min. The heating rate was kept as
10 °C/min. In HSC-Fe, the area modified by FeCl3is noted as
light absorption region (LAR), and the other part is named
thermal management region (TMR). For comparison,
untreated silk cocoon (SC) and whole silk cocoon modified by
FeCl3(SC@Fe) were heated at the same condition to obtain
HSC and HSC@Fe, respectively.
2.3 Characterization
The morphology was observed by means of a field-emission
scanning electron microscope (SEM, ZEISS GeminiSEM 500),
and the surface element distribution was analyzed by an
energy dispersive X-ray spectrometer (EDX, Genesis 2000). The
phase structure was analyzed by X-ray diffraction (XRD,
Shimadzu XRD6100). The effect of FeCl3on the pyrolysis of silk
cocoon was measured by thermogravimetric analysis (TGA,
SDT Q600) in air atmosphere at the heating rate of 5 °C/min.
The functional groups were characterized by Fourier transform
infrared spectroscopy (FT-IR, BRUKER Vertex 80). The
absorption spectra were tested by using a UV-Vis-NIR
spectrophotometer (Lambda 750 S) with an integrating sphere.
2.4 solar steam generation and sea desalination experiment
A solar light simulator (CEL, S500L) was used to carry out the
solar steam generation experiment. Firstly, a foam was stuffed
into the cavity of modified cocoon to obtain 3D solar steam
generation. The cone shaped foam was wrapped with
hydrophilic silk (Fig. S2) to make sure that the water could
directly reach the upper surface of modified cocoon (Fig. S3).
The surface temperature of water and membrane was
monitored by an infrared thermal imaging camera (DM-I220,
Dongmei). The mass change of water was measured by an
electronic balance (JA2003, Soptop). The evaporation rate (m,
kg/m2/h), and solar-to-steam conversion efficiency (η, %) were
calculated by Equation 1 and 2, respectively:
m= ∆m/ (S×t) (1)
η=m' × hLv /Pin (2)
Where ∆mis the mass change of water in 1 h (kg), S is the area
of cross section of LAR (m2), t is the time of solar irradiation (1
h), m' (kg/m2/h) is the evaporation rate after subtracting the
evaporation rate in dark, hLv is the latent heat of vaporization
of water, and Pin is the incident light power on the solar
absorber.
For the sea desalination experiment, the main ion
concentrations of the seawater and the condensed water were
tested using an inductively coupled plasma-optical emission
spectrometry (ICP-OES, HORIBA Scientific JY 200–2).
All solar steam generation and sea desalination experiments
were carried out under the condition of air temperature of
30 °C and humidity of 40%, and the intensity of simulated
sunlight was 1 kW/m2(one sun).
3. Results and discussion
3.1 The morphology of solar steam generator
A relatively complete silk fiber structure can be observed in
both SC (Fig. 2a) and HSC (Fig. 2b), and the micromorphology
of them is similar, but the photos of them (Insets in Fig. 2a and
Fig. 1 Schematic diagram of the preparation of cocoon-based
solar steam generator.
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2b) show that the color of cocoon changed from bright yellow
to light yellow after heat treatment. The reason for difference
in apparent color of SC and HSC may be that the sericin layer,26,
27 wrapping the outer of cocoon, is primarily decomposed
during heating, which results in the destruction of pigment in
the layer. While the silk fibroin, as the main structural support
of cocoon, is not destroyed at 180 °C, which also explains why
there is no big disparity in microstructure between them. For
HSC-Fe, the color of TMR is the same as that of HSC, but the
top turns deep brown (Inset in Fig. 2c) after modified by FeCl3.
And as shown in its SEM image (Fig. 2c), partial silk fibroin
fibers appeared of fracture, indicating the facilitation of FeCl3
in peptide degradation during heating. In addition, high
magnification SEM images (Fig. 2d and 2e) exhibit that a large
number of nanoparticles are loaded on the surface of the fiber.
Meanwhile, Fig. 2f-h display that N and Fe are the major
elements which are evenly distributed on the fiber surface.
3.2 Effect of FeCl3on the pyrolysis of cocoon
As a Lewis acid, FeCl3which possesses strong dehydration
effect is conducive to pyrolysis and carbonization of
biomass.28-30 In Fig. 3a, the pyrolysis of cocoon in air can be
divided into three stages: 1) From room temperature to 150 °C,
weight loss is caused by evaporation of adsorbed water and
unstable bound water in cocoon; the second stage occurs from
150 °C to 350 °C, in which some segments of peptide chain are
decomposed and crosslinked; the third stage (350~600 °C)
witnesses the largest weight change of cocoon due to violent
oxidation of cocoon in air. After high temperature pyrolysis,
the weight of the residue is about 3.6 wt%, which is mainly the
ash produced by high temperature pyrolysis of mineral
elements in cocoon. In the same vein, DTG curve (Fig. 3b)
demonstrates the maximum thermal degradation
temperatures of cocoon in the second and third stage are
309 °C and 571 °C, respectively. However, with the addition of
FeCl3, the thermal degradation rate of cocoon clearly increases
at high temperature, and the maximum thermal degradation
temperatures decreases to 243 °C and 448 °C, indicating that
FeCl3may promote the decomposition of peptide chain and
hinder peptide segments crosslinking during heating. It is
worth mentioned that the weight loss of the SC-Fe is not
obvious below ca. 180 °C. It is speculated that FeCl3reacted
with the water in the cocoon, and the product is converted
into Fe2O3after calcination at high temperature.31 This is also
the reason why the weight of the residue (14.4 wt%) is
evidently higher than that of SC. To further explore the effect
of FeCl3on cocoon pyrolysis at low temperature, XRD test was
conducted. In Fig. 3c, characteristic diffraction peaks of β-
sheet crystalline structure of silk (2θ = 9.2° (010) and 20.8°
(020))32 in SC are observed. After heat treatment at 180 °C, the
intensity of β-sheet (010) increased obviously and new
diffraction peaks appear at 25-30° in HSC, which suggests the
decomposed peptide segments will be crosslinked again during
pyrolysis, resulting in structural rearrangement of peptide
chain. The peaks of α-Fe2O3(110)31 in HSC-Fe shows that FeCl3
converted into Fe2O3and loaded on the cocoon successfully.
Interestingly, no other peaks of β-sheet except (020) could be
observed in HSC-Fe, confirming that FeCl3can facilitate the
thermal decomposition of peptide chain and impede peptide
segments crosslinking. Furthermore, the results of FT-IR (Fig.
3d) deeply analyzed the variations of functional groups in the
samples. For SC, the band at 3275 cm-1 are associated with the
stretching vibration of -OH, the bands at 1642 and 1517 cm-1
are owing to C=O stretching vibration of Amid I and aromatic
ring vibration, respectively. The bands at 1160 and 1057 cm-1
are due to stretching vibration of C-O. Furthermore, the bands
at 1441, 1362 and 1321 cm-1 are ascribed to the deformation
vibration of CH2and CH3, respectively.33 For HSC, the blue shift
of C=O (1618 cm-1) is due to the increase of intermolecular
force caused by the rearrangement of peptide chain structure.
And in HSC-Fe, the weak intensity of -OH and red shifted of
C=O (1674 cm-1) affirming that FeCl3accelerates the
Fig. 2
SEM images of (a) SC, (b) HSC and (c-e) HSC-Fe (Insets are the photos of resultants, respectively. The dashed boxes
show the selected section for SEM observation). EDX maps of C (f), N (g) and Fe (h) of HSC-Fe.
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decomposition of peptide chains during pyrolysis by
dehydration, inhabiting the secondary crosslinking of peptide
segments and weakens the interaction between chains.
Collectively, in the process of heating, FeCl3could promote the
degradation of peptide chain through dehydration, and
transform itself into Fe2O3uniformly loaded on the top of the
cocoon, realizing the local modification of the cocoon.
3.3 Light absorption for photothermal conversion
The light absorption capacity is cruel for photothermal
conversion which is the key to solar evaporation rate.34 Fig. 4a
shows that The absorption of SC is ca. 75% in the ultraviolet
visible region and ca. 47% in the infrared region. Since the
pigment protein is destroyed during heating, the absorption of
HSC decreased to ca. 50% in ultraviolet visible region. Due to
the load of Fe2O3, HSC-Fe manifests the highest absorption
performance in both ultraviolet visible (ca. 93%) and infrared
region (ca. 75%). Because of the excellent light absorption, the
surface temperature of HSC-Fe reach to 79.1 °C (Fig. 4b) under
1 kW/m2solar light irradiation in air after 10 min irradiation,
far outnumbering SC (60.3 °C), HSC (56.2 °C) and water
(32.2 °C).
Notably, infrared images of SC (Fig. 4c) and HSC (Fig. 4d) show
that there is no obvious temperature difference in whole
cocoon under solar irradiation, while the heat is concentrated
on LAR of HSC-Fe due to the local modification of cocoon
(Detailed discussion in 3.5). The solar evaporation
performance of solar steam generation floating on the water
surface was quantitatively investigated by recording the mass
change of water as the solar irradiation time. Fig. 4a reveals
the water mass decreases approximately linearly with
irradiation time. HSC-Fe presents an extremely high
evaporation rate of 1.44 kg/m2/h, which is 3.3, 1.7 and 1.3
times as that of pure water (0.43 kg/m2/h), HSC (0.85 kg/m2/h)
and SC (1.08 kg/m2/h), respectively. The solar-to-vapor
conversion efficiency of HSC-Fe (83.1%) exceeds that of SC
(60.2%) or HSC (45.1%). Furthermore, the evaporation rate
maintains at 1.41 kg/m2/h after 10 cycles, indicating the
remarkable stability of HSC in solar steam generation.
3.4 3D structure for solar evaporation
To investigate the advantages of 3D structure in solar
evaporation, 2D thin film cut from LAR of HSC-Fe was used as a
comparison (2D generator), and solar evaporation experiment
was carried out under the irradiation at different angles.
Previous studies18, 23 demonstrate that the irradiation area
affects the rate of absorption through the surface temperature
of generator. For 2D structure, when the incident light angle is
0°, almost no light will irradiate on the surface of generator.
Fig. 5a suggests that the incident light from different angles
(even 0o) could irradiate the surface of 3D generator, so the
surface temperature of 3D generator (Fig. 5b) retains at
70.0 °C when the angle is 0°, sightly lower than the results at
the angle of 60° (76.5 °C) and 90° (79.1 °C). Although the
surface temperatures of 2D generator at 60° (73.0 °C) and 90°
(77.6 °C) are approximate, temperature at 0° (40.9 °C) is far
below that of 3D structure because of the minimal irradiation
Fig. 4 (a) UV-Vis-NIR absorption spectra and (b) surface
temperature under solar irradiation in air of water (blank), SC,
HSC and HSC-Fe. Infrared images of (c) SC, (d) HSC and (e) HSC-
Fe. (f) Cumulative mass changes of water with time under
different conditions: water in dark (dark); water under solar
irradiation (blank); water with SC, HSC and HSC-Fe. (g) Stability of
HSC-Fe in solar steam generation for 10 cycles under solar
irradiation.
(Note: Only the top of the materials is cut to test the UV-Vis-NIR
absorption spectrum.)
Fig. 3 TGA curves (a) and DTG curves (b) of SC and SC-Fe. XRD
patterns (c) and FTIR curves (d) of SC, HSC and HSC-Fe.
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area (Fig. S4). And the difference of interface temperature,
caused by irradiation area, results in lower evaporation rate of
2D generator than that of three-dimensional structure at
different angles of incident light (Fig. 5c).
Therefore, under the condition of unsatisfactory illumination
angle, 3D structure can still maintain a higher surface
temperature by ensuring the illumination area, so as to
maintain an outstanding evaporation rate.
3.5 Localized heating for thermal management
In order to explore whether further increasing the illumination
area facilitate evaporation, the whole cocoon surface was
modified by FeCl3to obtain HSC@Fe. Unfortunately, the
evaporation rate of HSC@Fe is 11.9% lower than that of HSC-
Fe (Fig. 6a). Fig. 6b suggests that the average temperature of
the upper surface of HSC@Fe (ca. 55.0 °C) below that of HSC-
Fe (ca. 57.4 °C) under 1 kW/m2solar irradiation on the water.
After stopping illumination, it takes only 18 minutes for
HSC@Fe to return to room temperature, while HSC-Fe need
ca.26 minutes. The similar result is obtained after five cycles
(Fig. S5). The thermal conductivity test shows that the thermal
conductivity of HSC@Fe with homogeneous material is 0.61
W/m/K, so is that of LAR of HSC-Fe. However, the thermal
conductivity of TMR of HSC-Fe is only 0.12 W/m/K, which
greatly reduces the speed of heat convention from the top of
HSC-Fe to the water surface, keeping the top at a high
temperature. It can be concluded that thermal management is
key to the difference in evaporation rate between HSC-Fe and
HSC@Fe. The heat loss (detail in Note 1 in supplementary
information) diagram is shown in Fig. 6c. The surface
temperature of HSC-Fe present gradient distribution due to
difference of thermal conductivity in structure, which is
consistent with the result of Fig. S6a. This localized heating
design can concentrate the heat on the top of generator.
Although the heat radiation (7.1%) and heat convection (4.9%)
to the air are slightly higher, it speeds up the evaporation of
water in contact with LAR (High temperature field) and greatly
reduces the heat conduction (0.9%) to the water touching the
bottom of TMR (Low temperature field). For HSC@Fe, there is
no much temperature difference on the whole structure under
irradiation (conformed to the IR image shown in Fig. S6b).
Because of high thermal conductivity, the heat generated by
photothermal conversion at the top is rapidly transferred to
whole generator and the water at the bottom, accounting for a
high heat conduction (6.7%). More heat loss reduces the
efficiency of solar-to-steam conversion in HSC@Fe, resulting in
a lower evaporation rate than HSC-Fe.
Therefore, the local modification of cocoon can not only save
cost, but also improve the efficiency of solar-to-steam
conversion by localized heating strategy under irradiation.
3.6 Sea desalination
The application of HSC-Fe for the generation of clean water
from the seawater was also investigated. In the process of
seawater desalination, the precipitation of salts will hinder the
evaporation of water, accounting for degradation of
evaporation rate. evaporation rate. In previous work,35-37 the
generator with directional aperture was prepared so that the
salt falls back into the seawater from the inner pore by gravity
after precipitation. Fig. 7a shows that the salts continuously
Fig. 5
Photothermal conversion ability and solar vapor
generation performance at different incident angles. (a)
Schematic illustration of 3D generator (HSC-Fe) at
different incident angles. (b) Surface temperatures on
water and (c) evaporation rates of 2D and 3D generator.
Fig. 6
(a) Cumulative mass changes of water versus time
with HSC-Fe and HSC@Fe under solar irradiation. Surface
temperature (b) and heat loss diagram (c) of HSC-Fe and
HSC@Fe under solar irradiation on the water.
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precipitate and accumulate on the top of HSC-Fe during the
first 5 h, which adsorbs and liquefies steam, resulting in the
decrease of evaporation rate (Fig. 7b). With the increase of
liquid water on the surface, the accumulated salts are slowly
dissolved and begins to flow, and the unique 3D structure
enables the salt to slide on the surface under the action of
gravity, and part of the salts gradually fall off. Then the newly
precipitated salt is difficult to accumulate in the flowing state,
and evaporation rate return to the average level. After
continuous seawater desalination for 10 hours, the residual
salts on the surface of HSC-Fe can be easily removed by using a
straw with deionized water, realizing semi-automatic salt
resistance of solar steam generator. Besides, the results of
seawater purification are shown in Fig. 7c, and the
concentrations of Na+, K+, Mg2+ and Ca2+ are notably decreased
from 22222.9 ppm to 25.4 ppm, 980.3 ppm to 1.9 ppm,
2858.4ppm to 2.0 ppm, and 1013.3 ppm to 1.7 ppm,
respectively. which is far lower than the drinking water
standard established by the World Health Organization (WHO).
Conclusions
The In summary, cocoon-based 3D solar steam generator
(HSC-Fe) with light absorption region and thermal
management region has been obtained by local modification
of silkworm cocoon with FeCl3at 180 °C. Hemispherical light
absorption areas with good photothermal conversion
performance retains high evaporation rates at different light
incident angle (0°, 60° and 90°). The thermal management area
with low thermal conductivity effectively reduces the heat
conduction loss, producing the localized heating in light
absorption area to improve the efficiency of solar-to-steam
conversion. In addition, the unique 3D structure of cocoon
impels salts precipitated from seawater to disperse on
generator surface or even slide down, which weakens the
adverse effect of salt accumulation on evaporation during
desalination. We hope this work can inspire the design and
preparation of 3D solar steam generator with high
performance.
Fig. 7
(a) Top-view of HSC-Fe during solar steam generation in sea water. (b) The evaporation cycle performance of HSC-Fe in
sea water. (c) Concentrations of Na+, K+, Mg2+ and Ca2+ in the seawater, the condensed water and WHO standard for the
healthy drinkable water.
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Author Contributions
The manuscript was written through contributions of all authors.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to thank Prof. Jiang Gong and Dr. Liang
Hao from Huazhong University of Science and Technology for
evaporation test.
Notes and references
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Page 9 of 15 Sustainable Energy & Fuels

Supplementary information
Cocoon-based 3D Solar Steam Generator for High-performance
Saline Desalination
Changyuan Songa, Xuying Chena, Rui Haoa, Dongna Caib, Xiangwei Zhub,*, Hao Liua,*,
Jinzhou Chena,* and Wentao Liua,*
a School of Materials Science and Engineering, Zhengzhou University, Henan Key
Laboratory of Advanced Nylon Materials and Application, Zhengzhou University,
Zhengzhou, 450001, China.
b National “111" Center for Cellular Regulation and Molecular Pharmaceutics, Key
Laboratory of Fermentation Engineering (Ministry of Education), Hubei Key
Laboratory of Industrial Microbiology, College of Bioengineering and Food, Hubei
University of Technology, Wuhan, 430068, China.
*Corresponding authors
Page 10 of 15Sustainable Energy & Fuels

Fig. S1. Photos of (a) SC-Fe and (b) HSC-Fe-200.
HSC-Fe-200 is obtained by heating SC-Fe at 200 oC. When the temperature above
180 oC, the silk fibroin in the inner layer of the cocoon begin to break in air. Fig. S1b
shows that HSC-Fe-200 is too fragile to use as generator.
Fig. S2. (a-c)Water transport properties of gauze wrapped PS foam, (b) hydrophilicity
of LAR.
Page 11 of 15 Sustainable Energy & Fuels

PS foam, as a supporting substrate, can help the cocoon float on the water surface.
Meanwhile, gauze wrapped on foam surface has a strong ability to draw water and
transfers water from the bottom of the foam to its top in just 20 seconds. The cocoon
possesses good hydrophilicity and water permeability. In less than 3 seconds, the water
droplets completely wet and penetrate the cocoon.
Thus, It can ensure that the water can be quickly transferred from the bottom to the
top of the cocoon, and the water vapor could easily escape during the process of solar
evaporation.
Fig. S3. Photos of steam generator.
Page 12 of 15Sustainable Energy & Fuels

Fig. S4. Schematic illustration of 2D generator at different incident angles.
Fig. S5. Surface temperature of HSC-Fe and HSC@Fe under 1 kW/m2 solar
irradiation on the water.
Note 1:
Normally, the heat loss of water evaporation processes has three parts, i.e., radiation,
convection and conduction. The calculation details of heat loss are shown below [1-3].
(1) Radiation
The heat radiation loss was calculated through Stefan-Boltzmann equation:
Page 13 of 15 Sustainable Energy & Fuels

Φ = εAσ(T14-T24) (R1)
Where Φ represents heat flux, ε is the emissivity, and emissivity in the water
evaporation processes is supposed has a maximum emissivity of 1. A is the effective
evaporation surface area (ca. 400 mm2). σ is the Stefan-Boltzmann constant (the value
is 5.67×10-8 W/m2/K4). T1 is the surface temperature of the as-prepared materials after
stable steam generation under one-sun illumination, and T2 represents ambient
temperature upward the as-prepared materials (ca. 45 oC, or 318.15 K). According to
Equation R1 in this file, the radiation heat flux of HSC-Fe and HSC@Fe are 0.035 W
and 0.032 W. Therefore, the calculated heat radiation loss of HSC-Fe and HSC@Fe are
7.0% and 6.4%, respectively.
(2) Convection
The convection heat loss can be calculated by Newton's law of cooling as follows:
j = hAΔT (R2)
Where j is the convection heat flux, h represents the convection heat transfer coefficient,
which is approximately 5 W/m2/K in the early report [4]. ΔT is difference between the
surface temperature of as-prepared carbon materials and the ambient temperature.
Consequently, the connection heat loss of HSC-Fe and HSC@Fe are calculated through
Equation R2, and the value are 0.0243 W (4.9%) and 0.0226 W (4.5%).
(3) Conduction
The conduction heat loss is due to that the heat was transferred from the as-prepared
materials to water, which can be calculated by the following equation::
Qcond = CmΔT (R3)
Where Qcond is the heat energy, C represents the specific heat capacity of water (4.2 J/
oC/ g), and m denotes the weight of water (g). ΔT is the increased temperature of water
(K). In this work, m = 10 g, ΔTHSC-Fe = 30.4 oC - 30 oC = 0.4 oC, ΔTHSC-Fe = 32.7 oC -
30 oC = 2.7 oC, Wsolar = Psolart = 1800 J. Consequently, according to Equation S3, QHSC-Fe
= 16.8 J, QHSC-Fe = 113.2 J, the calculated conduction heat loss of HSC-Fe and
HSC@Fe are 0.9% and 6.3%.
Page 14 of 15Sustainable Energy & Fuels

Fig. S6. IR image of (a) HSC-Fe and (b) HSC@Fe under 1 kW/m2 solar irradiation on
the water.
[1] George N, Gabriel L, Svetlana B, et al, Steam generation under one sun enabled by
a floating structure with thermal concentration. Nat Energy, 2016, 1: 16126–16132.
[2] Xu N, Hu XZ, Xu WC, et al, Mushrooms as Efficient Solar Steam-Generation
Devices. Adv Mater, 2017, 29: 1606762.
[3] Li TT, Fang QL, Xi XF, et al, Ultra-robust carbon fibers for multi-media purification
via solar-evaporation. J Mater Chem A, 2019, 7: 586–593.
[4] Hadi G, George N, Amy MM, et al, Solar steam generation by heat localization. Nat
Commun 2014, 5: 5449–5455.
Page 15 of 15 Sustainable Energy & Fuels