2.3. Solar Steam Generation Performance
Firstly, the performance of solar steam generation using pure water as
analyte is evaluated to optimize the product composition. The impacts of
aerogel
density, loading amount and composition of the photothermal materials
are respectively studied.
The density of an aerogel mainly affects
saturation
water content of the solar steam generator. As seen from Figure
2 a, the density of solar steam generator becomes larger with increased
mass ratio of Janus nanofibers to dispersion medium in freeze drying
process, while the saturation water content is decreased. Figure 2b
shows the water evaporation performances of the solar steam generators.
As the density increases, the solar steam generators display increased
first and then decreased water evaporation rates. The increase in water
evaporation rates can be attributed to alleviative thermal dissipation
from evaporation surface to bulk water caused by decreased water
contents in the solar steam generators. The thermal conductivities of CA
and PVB are determined to be 0.17 W m-2K-2 and 0.23 W m-2K-2, respectively, which are substantially lower than
that of water (0.59 W m-2 K-2).
Hence, decreased water content in the solar steam generator is
beneficial to reduce heat dissipation and increase the temperature of
evaporation surface. As seen in Figure 2c, the temperature (after
one-hour irradiation) of evaporation
surface increases with increased density of the solar steam generator,
whereas that of the bulk water is reduced. However, the increased
density of solar steam generator simultaneously results in increased
resistance of water transfer, which is negative to water evaporation.
Therefore, appropriate aerogel density is an important basis to ensure
high-efficient water evaporation.
Secondly, optimized loading amount of photothermal materials is
explored, as shown in Figure 2d. It can be seen that the solar steam
generator with the photothermal materials of 0.016
g·cm-2 exhibits the best performance. Insufficient
photothermal materials cannot produce enough heat to sustain rapid water
evaporation. On the contrary, because the pores in the photothermal
materials are small (at the nanoscale), excessive photothermal materials
leads to too high resistance of water transfer and thus decreases water
evaporation rate.
Thirdly, the dosages of raw materials for preparing photothermal
materials are optimized. To start with, the concentration of dopamine is
fixed (y=2.4), while the dosage of TEOS is varied (x=0.1, 0.2, 0.3, 0.4,
0.5), and relevant SSG results are shown in Figure 2e. With the
increased dosage of TEOS (i.e. enlarged content of SiO2NPs), the water evaporation rates reveal an increasing trend at the
beginning and then decrease when x>0.3. To explain this
phenomenon, at first, the surface temperatures of the solar steam
generators are measured. As seen from Figure 2f, with the gradual
increase of
SiO2NPs content, the surface temperatures of the solar steam generators show
a falling trend, which can be ascribed to the fact that the
SiO2 NPs, although have been modified with dark-colored
PDA, possess relatively lower photothermal conversion performance than
CNTs. The light absorption spectra of the solar steam generators shown
in Figure 2g can prove the degradation of light absorption performance
with increased SiO2 NPs content. Therefore, from the
perspective of surface temperature, the performance enhancement of the
products within x=0.1 to 0.3 cannot be well explained. It has been known
that there are three kinds of water in a material: bound water (BW),
intermediate water (IW) and free water (FW).[42]Among them, BW is the water molecule in direct contact with the
hydrophilic materials. Because strong chemical bonds are formed between
BW and the materials, BW is difficult to evaporate. The FW is relatively
far from the adsorption materials, and its property is consistent with
the bulk water. The IW is sandwiched between the BW and FW. Due to the
interaction between the adjacent BW and adsorption materials, the
hydrogen bonds between IW and BW are weakened, leading to the lowest
energy required for the evaporation of IW. That is, IW has the lowest
evaporation enthalpy among the three kinds of
water.[43] In this study, SiO2 NPs
can fill in the gaps among CNTs and thus regulate the water state in the
photothermal materials. Figure 2h visually depicts the increased
proportion of IW and reduced proportion of FW when more
SiO2 NPs are introduced. In addition, the variation of
BW can be ignored because BW is in monomolecular type and its proportion
is very low among the three kinds of water. It has been reported that
the ratio of IW to FW in a material can be quantitatively measured by
Raman spectroscopy, and Gaussian function can be used to fit the peaks
at 3233 cm-1, 3401 cm-1, 3514
cm-1 and 3630
cm-1.[44] The peaks at 3233
cm-1 and 3401 cm-1 correspond to the
in-phase and out-of-phase vibration modes of O-H bonds in water that
forms four hydrogen bonds with surrounding water molecules, which
represent the existence of FW. The peaks at 3514 cm-1and 3630 cm-1 are assigned to the symmetric and
asymmetric stretching of O-H bonds in the weakened hydrogen bonds, which
are the characteristic peaks of IW that is relatively weakly affected by
hydrogen bond. The ratio of the fitted peak areas of the two kinds of
water is the molar ratio of the two kinds of water. The Raman spectra of
the hydrated photothermal materials are given in Figure S4 .
According to the calculation of fitted peak areas, the molar ratios of
IW to BW in these samples (x=0.1, 0.2, 0.3, 0.4, 0.5) are 0.38, 0.52,
0.67, 0.71, 0.73, respectively, which further demonstrates increased
proportion of IW when more SiO2 NPs are introduced.
However, excessive SiO2 NPs (x=0.4 and 0.5) cannot
substantially increase the proportion of IW, but make the light
absorption performance of the photothermal materials continuously
degrade, as revealed in Figure 2g, resulting in the decreased water
evaporation rate.
The impact of PDA is evaluated while the ratio of CNTs and
SiO2 NPs is fixed (x=0.3). Here, the PDA mainly acts as
an adhesive in the photothermal materials, and also contributes to
photothermal conversion due to its dark color. In order to study the
adhesive performance of the PDA, the solar steam generators with
different PDA contents were completely immersed in water for 1 h and
then naturally dried. The mass changes of the samples before and after
soaking are given in Figure 2i. When PDA is not enough (y=0.8 and 1.6),
some photothermal materials fall off the solar steam generators and
disperse into the water, and obvious mass losses of the samples are
found. When sufficient PDA is introduced (y≥2.4), the photothermal
materials can be stable on the solar steam generators. In spite of this,
excess PDA content is still detrimental because the light absorption
performance is impaired and the water evaporation rate is decreased, as
indicated in Figure 2i-l.
Based on the above, the optimized parameters for preparing the solar
steam generator are as follows: the mass ratio of nanofibers to water is
0.0500:1 in the dispersion medium for freeze drying; the dosages of TEOS
(x) and dopamine (y) are respectively 0.3 and 2.4 for preparing the
photothermal materials; the loading amounts of the photothermal
materials is 0.016 g·cm-2. The optimized solar steam
generator can provide a state-of-the-art water evaporation rate (2.944
kg m-2 h-1) under 1 sun irradiation.
Next, the impact of light irradiation intensity on water evaporation
rate and energy efficiency is investigated. Light irradiation intensity
is one of the important factors affecting the water evaporation rate. In
order to further explore the water evaporation performance of the
optimized solar steam generator under different light irradiation
intensities, 0.5, 1, 1.5 and 2 suns are respectively adopted. As shown
in Figure 3 a and b, water evaporation rates of 1.486
kg·m-2·h-1, 2.944
kg·m-2·h-1, 4.469
kg·m-2·h-1 and 5.899
kg·m-2·h-1 are achieved when the
irradiation intensities are 0.5, 1, 1.5 and 2 suns, while the energy
efficiencies are 88.74 %, 91.05 %, 91.94 % and 92.83 %,
respectively. In most reports on solar steam generators, energy
efficiency generally increases with the increase of irradiation
intensity.[10,32,45] Of the solar steam generator
prepared in this work, the evaporation surface temperature significantly
rises with increased irradiation intensity, whereas the temperature of
bulk water varies little, which indicates excellent thermal insulation
performance of the prepared solar steam generator. The experimental
results show that the product possesses superior SSG performance under
different irradiation intensities, suggesting its application potential
in the natural environment where sunlight intensity changes frequently
and the conditions with concentrated sunlight by using optical
concentrators such as convex lens and heliostats.
The stability of solar steam generators is also a key factor affecting
their practical application. Hence, the real-time water content,
continuous
working stability and reusability of the prepared solar steam generator
are studied. The test method of real-time water content is as following:
the solar steam generator is taken out from bulk water every 20 min
during SSG process, the water drops on the surface of solar steam
generator are wiped off by using filter papers, and then the weight of
the waterlogged solar steam generator is measured and compared with that
of the dry solar steam generator. Continuous working stability is
evaluated by ten-hour uninterrupted irradiation, and the mass changes of
water are recorded per hour to calculate hourly water evaporation rates.
The reusability is studied by twenty-cycle SSG processes. Continuous
ten-hour irradiation is employed for each cycle, and the solar steam
generator is dried after each cycle.
The test results are shown in Figure 4 a-c. The water content of
the solar steam generator is basically constant in SSG process,
indicating continuous and stable water pumping. The water evaporation
rates remain steady within both ten-hour continuous SSG process and
twenty -cycle reusing. These results manifest that the product possesses
excellent stability and has great application potential.
An unavoidable problem with solar steam generators in practical outdoor
applications is the flip of the generators due to the effects of wind,
water wave and etc . In order to accommodate this situation, the
prepared cubic solar steam generator is fully covered with photothermal
materials on its six surfaces, so that its all surfaces can be used for
water evaporation, as depicted in Figure 4d. The performance recovery of
the solar steam generator after flipping is investigated, and the
specific experimental methods are as follows: in a sixty-minute SSG
experiment, the solar steam generator is randomly flipped every 10 min
to switch the evaporation surface. The temperature of evaporation
surface (Figure 4e) and water mass loss are recorded after flipping. In
order to meticulously investigate the effect of flipping on water
evaporation rate, the concept of “interval mean evaporation rate” is
proposed. As seen from the red dots in Figure 4f, every dot is also
calculated by Formula S1, and the irradiation time (T ) and
initial mass of water are reset at the initial point of each interval.
The results reveal that temperature of evaporation surface can quickly
rise to more than 90 % of the maximum temperature within 2 min, and
concurrently, the interval mean evaporation rate is recovered to near
maximum. The black line in Figure 4f presents the water mass change
(2.740 kg·m-2 h-1 in total) over the
course of sixty-minute SSG experiment with quintuplicate flipping of the
solar steam generator. It can be seen that the flipping of the solar
steam generator does not have a great influence on the total water
evaporation rate, meaning that the prepared solar steam generator is
adequate for dealing with dynamic water environment.
In order to clarify the superiority of using hydrophile/hydrophobe
amphipathic Janus nanofibers aerogels (CA//PVB-based product for short)
as host materials for solar steam generators, completely hydrophilic CA
nanofibers aerogel (CA-based product), hydrophobic PVB nanofibers
aerogel (PVB-based product) and CA-PVB blended nanofibers aerogel
(CA-PVB-based product) are also respectively fabricated and coated with
photothermal materials through the same freeze drying and air spraying
processes as those for preparing amphipathic Janus nanofibers aerogel.
The PVB-based product cannot be used as a solar steam generator at all
due to its inability to pump water. The CA-based product can pump water
but cannot float on water surface. Therefore, a scaffold has to be used
to immobilize the CA-based product on the water surface. CA-PVB-based
product also can float on water surface by itself. The water evaporation
performances of CA//PVB-based product, CA-PVB-based product and CA-based
product as well as the water without using any solar steam generators
(blank water), are compared. The results of water evaporation rates are
shown in Figure 5 a. The CA//PVB-based product exhibits much
higher water evaporation rate than CA-PVB-based product and CA-based
product (2.944 kg m-2 h-1 vs .
0.771 kg m-2 h-1 and 2.125 kg
m-2 h-1), and the evaporation rate
of blank water is only 0.414 kg m-2h-1. Compared with CA//PVB-based product, CA-PVB-based
product shows a much lower water evaporation performance, even though
they have the same composition in raw materials. Through carefully
observing the evaporation surface of CA-PVB-based product during SSG
process, it can be found that almost no water exists on the evaporation
surface, proving a poor water-pumping ability of CA-PVB-based product.
As illustrated in Figure 5b, the CA components of CA//PVB Janus
nanofibers facilitate rapid upward pumping of water molecules due to the
closely situated hydrophilic groups on CA. As for CA-PVB blended
nanofibers, the presence of PVB molecular chains results in an increased
distance between hydrophilic groups, leading to a much weaker
water-pumping ability. Thus, a simple blending of hydrophilic and
hydrophobic materials is not applicable for fabricating solar steam
generators. The relatively lower water evaporation performance of the
CA-based product can be attributed to its excessive internal water
content (~ 0.95 g cm-3), which
aggravates heat dissipation, as revealed in Figure 5c and d. In
addition, it is also found that the CA-based product exhibits poor
reusability, as shown in Figure S5 . In order to explain this
result, the physical photos of CA//PVB Janus nanofibers aerogel, CA
nanofibers aerogel, CA//PVB-based product and CA-based product before
and after water soaking are provided in Figure S6 . After 10
cycles, the photothermal materials are seriously peeled off the CA-based
product. This phenomenon is attributed to the swelling of CA nanofibers
aerogel in water. After water soaking, CA-based product experiences a
noticeable expansion in volume, which creates internal stress between
photothermal materials and the swollen CA nanofibers aerogel and thus
leads to the detachment of photothermal materials. As for the
CA//PVB-based product, the hydrophobic PVB components suppresses the
volume expansion of the CA components, and the entire aerogel retains
almost the same morphology before and after being placed in water. Thus,
photothermal materials can be stable on CA//PVB Janus nanofibers aerogel
during SSG process, thereby achieving excellent reusability. From the
above results, it is obvious that the hydrophile/hydrophobe amphipathic
Janus nanofibers aerogels are superior for preparing solar steam
generators in view of their high performance, reusability and ease of
use.