Highly Efficient Solar Vapor Generator enabled by a 3D Hierarchical

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Highly Efficient Solar Vapor Generator enabled by a 3D Hierarchical Structure Constructed with Hydrophilic Carbon Felt for Desalination and Wastewater Treatment Zhen Yu, Shaoan Cheng, Chaochao Li, Longxin Li, and Jiawei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08480 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Highly Efficient Solar Vapor Generator Enabled by a 3D Hierarchical Structure Constructed with Hydrophilic Carbon Felt for Desalination and Wastewater Treatment

Zhen Yu a, Shaoan Cheng a*, Chaochao Li a, Longxin Li a, Jiawei Yang a a

State Key Laboratory of Clean Energy, Department of Energy Engineering,

Zhejiang University, Hangzhou 310027, PR China *

Corresponding author: [email protected]

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Abstract Solar vapor generation holds a great potential for seawater desalination and wastewater treatment. Although various efficient solar absorbers have been developed to enhance the performance of solar vapor generator in recent years, its efficiency is still limited by unnecessary heat loss. In this paper, a novel 3D hierarchical solar vapor generator (3DHG) was constructed with hydrophilic carbon felt. Different from interfacial solar vapor generator (ISVG) reported before, the porous and hydrophilic channels of 3DHG were exposed to the air directly, which probably resulted in a lower saturated vapor pressure of 3DHG. Therefore, this structure was beneficial for vapor escaping and leaded to a lower average temperature of 3DHG than that of the surroundings at the same time owing to negligible convection loss and radiation loss of 3DHG. The highest evaporation rate of 1.56 kg m-2 h-1 and efficiency of 98.1 % were obtained under 1 sun. In addition, 3DHG was also used for industry dyeing wastewater treatment and exhibited a minimum evaporation rate of 1.45 kg m-2 h-1 even after 7 days. This study presents a novel approach not only to design solar vapor generator with high efficiency but also widens its potential application in seawater desalination and practical wastewater treatment. Keywords: solar vapor generator, hydrophilic carbon felt (HCF), seawater desalination, practical wastewater treatment, photothermal

1 Introduction Solar vapor generation with little carbon feet, has been regarded as one of the most promising way to address increasing shortage and pollution of fresh water.1 Interfacial solar vapor generator (ISVG) with high efficiency, which consisted of absorber layer, thermal insulation layer, and many microchannels inside, was first reported by Chen 2 based on thermal capillarity. The high efficiency of this vapor generator was achieved under low optical concentration by localizing the solar energy and reducing conduction loss from absorber to bulk water. Since then, surface plasmon materials,3-4 carbon-based materials

5-7

and semiconductors material

8-9

were developed as the

absorber layer of ISVG and exhibited a good performance. However, the high optical absorption of absorber layer in ISVG resulted in pretty higher temperatures of absorber layers than those of the surroundings under solar irradiation, which inevitably brought about convection loss, radiation loss and conduction loss from 1 ACS Paragon Plus Environment

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absorber layer to the surroundings and water.10-12 To enhance the performance of ISVG by minimizing the heat loss mentioned above as far as possible, the ISVG with advanced structure must be developed for a reasonable management of photon transfer, heat flow and water transporting.13-14 Recently, Zhu 10

built a three-dimensional hollow conical evaporation structure with heat loss and

angular dependence of light absorption minimized, which enabled high efficiency over 85 % under 1 sun. In addition, a bio-inspired 3D photothermal cone was developed by Jiang

15

for efficient solar vapor generation with minimum light

reflection and heat loss to bulk water. And the solar conversion efficiency up to 93.8% for evaporation was achieved under 1 sun. Recently, a cylindrical vapor generator composed of cotton cores wrapped with carbon black-coated cellulose paper was fabricated by Zhu’s group.11 With the lower average temperature of the generator than that of the surroundings, the generator can harvest energy from the warmer environment through convective and radiative heat transfer processes. And the high efficiency about 104 % was obtained finally. Here we reported a 3D hierarchical solar vapor generator (3DHG) constructed with hydrophilic carbon felt. Different from ISVG reported before, the hydrophilic and porous channels of 3DHG were exposed to the air directly, which probably resulted in a lower saturated vapor pressure of 3DHG. Therefore, this structure was beneficial for vapor escaping and leaded to a lower average temperature of 3DHG than that of the surroundings at the same time, which means negligible convection loss and radiation loss of 3DHG. The high evaporation rate of 1.56 kg m-2 h-1 and efficiency of 98.1 % were achieved by the optimal 3DHG under 1 sun. In addition, 3DHG was also used for industry dyeing wastewater treatment and the condensed water collected in every cycle reached the national environmental quality standard. In conclusion, this study provides a general guideline for the design and application of efficient solar vapor generator.

2 Method 2.1 Preparation of solar vapor generator All chemicals were from Aladdin Chemical Reagents (Shanghai, China) and used without further purification. Carbon felt (CF, from Phychemi Company Limited, Hongkong, China) was treated with HNO3 at 80 ℃ in water bath for 6 h to prepare 2 ACS Paragon Plus Environment


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hydrophilic carbon felt (HCF). The scheme in Figure 1 illustrates the design process of conventional interfacial solar vapor generator (ISVG) and 3D hierarchical solar vapor generator (3DHG). 3DHG mainly consisted of thermal insulation layer, channels and absorber layer. Cylindroid polystyrene foam (PS foam) with a diameter of 7.0 cm ± 0.1 cm and a thickness of 1.0 cm ± 0.01 cm was used as a thermal insulation layer and support. Then cuboid HCF (and CF) with an area of 0.5 cm × 0.5 cm and a length varying from 3 cm to 7 cm, used as the channels for water transporting

and

vapor

escaping

in

the

evaporation

process,

was

distributed uniformly across PS foam. A piece of HCF (and CF) with a diameter of 7.0 cm ± 0.1 cm and a thickness of 0.3 cm ± 0.01 cm, used as an absorber layer, was fixed on the top surface of channels. Considering different wettability between CF and HCF, 3DHG consisting of CF or HCF, respectively, possessed different hydrophilic and hydrophobic structure. To compare the performance of 3DHG with different hydrophilic and hydrophobic structure, 3DHG-CC (CF used as channels and CF used as absorber layer), 3DHG-CH (CF used as channels and HCF used as absorber layer), 3DHG-HC (HCF used as channels and CF used as absorber layer) and 3DHG-HH (HCF used as channels and HCF used as absorber layer) were built here, respectively. In addition, the height of channels here was defined as the distance between the top surface of thermal insulation layer and the bottom surface of absorber layer, which varied from 0 cm to 4 cm. Especially, 3DHG with the height of channels about 0 cm can be regarded as ISVG. And pure water without ISVG or 3DHG was named PW as a control group for comparing the performance of ISVG and 3DHG.

Figure 1. Schematic diagram of the preparation process of ISVG and 3DHG. 3DHG with the height of channels about 0 cm was regarded as ISVG. 3 ACS Paragon Plus Environment

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2.2 Solar vapor generation. A homemade optical measurement system mainly consisting of water container with a diameter of 72 mm ± 1.0 mm and a height of 40 mm ± 0.1 mm, Xe lamp (PLSSXE300/300UV) and electronic analytical balance (Mettler Toledo, MR420) were used to carry out solar vapor generation experiments (Figure S1). Xe lamp with AM 1.5 G was used as a simulated illuminant and the light intensity of Xe lamp was measured by light intensity meter (Spectronics-3000 UV-AB). Electronic analytical balance was used to record the mass change of the water container to calculate the evaporation rate of different vapor generators. In addition, temperature distribution of different vapor generators before and after illumining were recorded by IR-camera (FLTR-S65, US). The efficiency of different vapor generators was calculated by Eq.1 7, 16

where

(MJ kg-1),

is evaporation rate (kg m-2 h-1),

is total enthalpy for water evaporation

is solar irradiation (1 kW m-2). (1)

2.3 Analysis The morphologies of CF and HCF were characterized by the scanning electron microscope (SEM, SU-8010, Japan). The contact angles between the water and CF (HCF) were measured by Contact Angle Measuring Device (OCA20, German). The optical absorption of as-prepared absorber layer ranging from 2500 nm to 500 nm was recorded by UV-Vis-NIR spectrophotometer equipped with an integrating sphere (UV-3101, Japan). In addition, 3DHG was used for solar desalination and wastewater treatment experiments where 5 g L-1 Methylene blue (MB) solution and two different kinds of industry dyeing wastewater (provided by Xiao Shan sewage Treatment co. LTD (Hangzhou, China)) were used as typical wastewater. Condensed water collected after every experiment was saved for further experiment. The absorption peak at about 665 nm in UV-vis absorption spectra recorded by UV-vis spectrophotometer (UV-2600, China) was used for measuring the concentration of MB in condensed water and MB solution. The dye removal ratio of 3DHG was calculated based on Eq.2 where Ct is the concentration of MB in condensed water (g L-1), C0 is the initial concentration of MB solution (5 g L-1). Similarly, dye concentration ratio of 3DHG was calculated based on Eq.3 where St is the concentration of MB solution in water container after solar evaporation experiment (g L-1) and S0 is initial concentration of 4 ACS Paragon Plus Environment


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MB solution (5 g L-1). The concentrations of ions in seawater and condensed water were measured by inductively coupled plasma spectrometer (ICP- OES, ICP-6000, Britain), respectively. Dye removal ratio = Ct/C0

(2)

Dye concentration ratio = St/S0

(3)

pH and ionic conductivity of water samples (condensed water or two different kinds of industry dyeing wastewater) were measured by pH meter (FG20, Mettler-Toledo, Switzerland) and ionic conductivity meter (FG3, Mettler-Toledo, Switzerland), respectively. The concentrations of ammonia nitrogen of water samples were measured according to improved indigo blue method.17-18 Typically, 2 mL water sample was poured into the glass tube, followed by 2 mL of 1 M NaOH solution (containing 5 wt.% salicylic acid and 5 wt.% sodium citrate), 1 mL of 0.05 M sodium hypochlorite solution and 0.2 mL of 1 wt.% sodium nitro prussiate solution added, to form the homogeneous mixture. After the mixture being preserved in dark for 2 h at room temperature (25 ℃), the concentrations of ammonia nitrogen of water samples were measured by the optical absorption of water samples at 655nm in UV-vis absorption spectrum based on standard curve of ammonia nitrogen and absorption. The chemical oxygen demands (COD) of water samples were measured by spectrophotometric screening methods according to the manufacturer’s procedure (HACH Method 8000).19

3 Results 3.1 Characterization of CF and HCF The wettability of CF and HCF were examined first. As Figure 2a shown, plenty of irregular pores existed in CF, which can be used as paths for water transporting and vapor escaping in evaporation process. The pretty smooth surface of single carbon fibers in CF implied poor hydrophilia of CF (Figure 2b). As recorded in Video S1, 4 μL of water drop was added onto the surface of CF at the beginning and maintained original after 30 s. The contact angle between water and CF was measured to be 121.9° ± 0.5°, which is consistent with the inference above that CF possessed poor hydrophilia. To enhance the hydrophilia of CF, oxidation of CF by HNO3 was tried to prepare hydrophilic carbon felt (HCF). Although irregular pores in HCF remained 5 ACS Paragon Plus Environment

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same as those in CF (Figure 2c), the surface of single carbon fibers in HCF became rougher than those in CF (Figure 2d), which probably resulted in a great hydrophilia of HCF. As recorded in Video S2, 4 μL of water drop was absorbed by HCF as soon as the water reached the surface of HCF, which proved the excellent hydrophilicity and hygroscopicity of HCF.

Figure 2. The surface morphology of (a) CF, (b) single carbon fibers in CF, (c) HCF and (d) single carbon fibers in HCF (e) The optical absorption ranged from 500 nm to 2500 nm under AM 1.5G. Inset: The comparison of optical absorption of CF and HCF ranged from 500 nm to 800 nm.

In addition, the optical absorptions of HCF and CF were measured in Figure 2e. Compared to CF, the optical absorption of HCF ranging from 500 nm to 800 nm had little increase, probably attributing to the rougher surface and lower optical reflection of HCF. The optical absorption of HCF and CF in the range from 500 nm to 2500 nm under AM 1.5G was calculated to be 91% and 89 %, respectively. The high optical absorption across full solar spectrum ensured the high photo-thermal conversion efficiency of absorber layer in some degree. 3.2 The performance of 3DHG 3DHG-HH with the height of channels about 2 cm was measured to be the optimal 3DHG under 1 sun (detailed comparison mentioned herein below). As excepted, 3DHG can probably become a promising option for future efficient solar vapor generation (Figure 3a). To demonstrate the superior structure of 3DHG quantitatively, the performance of PW, ISVG and optimal 3DHG were compared under 1 sun in Figure 3b. The evaporation rate of ISVG increased to 1.29 kg m-2 h-1 under 1 sun, which reached almost 2.4 times that of PW (0.53 kg m-2 h-1). And the evaporation rate 6 ACS Paragon Plus Environment


of optimal 3DHG further increased to 1.56 kg m-2 h-1, even approaching thermal limit of water evaporation under 1 sun.12 Similarly, the efficiency of ISVG and optimal 3DHG were calculated to be 81.8% and 98.1 %, respectively, increasing ca 1.5 times and 2.0 times those of PW (33.2%) under 1 sun. The temperature distribution of optimal 3DHG after running for 4 h in dark or under 1 sun was recorded in Figure 3c. The average temperatures of channels and absorber layer of optimal 3DHG were about 18 ℃ and 20 ℃ (RT 25 ℃) in dark while 19 ℃ and 25 ℃ (RT 25 ℃ increased to 33 ℃) under 1 sun, respectively. This phenomenon means optimal 3DHG could still maintain a lower average temperature than that of the surroundings even running for a long time, which implied negligible radiation loss and convection loss from optimal 3DHG to the surroundings.

Figure 3. (a) The photography of vapor generating from 3DHG under 1 sun. (b) The evaporation rate (ER) and efficiency of PW, ISVG and optimal 3DHG under 1 sun. (c) After running for 4 h, the temperature distribution of optimal 3DHG in dark or under 1 sun. The environmental temperature increased from 25 ℃ to 33℃ under 1 sun while environmental temperature maintains 25 ℃ in dark all the time.

In addition, the performance of 3DHG with different hydrophilic and hydrophobic structure under 1 sun was compared to investigate the effect of the wettability of the structure of 3DHG on its performance. Notably, the height of channels in 3DHG with different hydrophilic and hydrophobic structure here was all 2 cm. As Figure 4a shown, the evaporation rates of 3DHG-CC and 3DHG-CH were 0.17 kg m-2 h-1 and 0.18 kg m-2 h-1, respectively, even lower than those of PW (0.53 kg m-2 h-1). Compared to those of 3DHG-CC and 3DHG-CH, the evaporation rate of 3DHG-HC (0.65 kg m-2 h-1) increased remarkably, ca 1.23 times those of PW. The evaporation rate of 3DHG-HH further increased to be 1.56 kg m-2 h-1, ca 2.94 times those of PW. Similarly, the efficiencies of 3DHG-CC and 3DHG-CH were calculated to be 10.4 % 7 ACS Paragon Plus Environment

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and 11.3 %, respectively, lower than those of PW (33.2%). The efficiency of 3DHGHC increased to be 40.6 % while the efficiency of 3DHG-HH further increased to be 98.1 %. In addition, the temperature distributions of 3DHG-CC, 3DHG-CH, 3DHGHC and 3DHG-HH were recorded under 1 sun in Figure 4b. The temperatures of the absorber layer (and channels) were 42.7 ℃ (and 25.1℃), 43.6 ℃ (and 25.0 ℃), 41.8 ℃ (and 18.8 ℃) and 27.8 ℃ (and 19 ℃), for 3DHG-CC, 3DHG-CH, 3DHG-HC and 3DHG-HH, respectively. Notably, the temperature of absorber layer always exceeded the channels temperature, which was attributed to the excellent photothermal conversion performance of absorber layer and low thermal conductivity of channels and absorber layer (0.095 W m−1 K−1). Average temperatures of 3DHG-CC, 3DHGCH, 3DHG-HC and 3DHG-HH as calculated by weighting the area proportions of the absorber layer and channels were 35.1 ℃, 35.6 ℃, 32.0 ℃ and 24.0 ℃, respectively while environmental temperature was maintained at 25 ℃ all the time. The lower average temperature of 3DHG-HH means less radiation loss and convection loss from 3DHG-HH to the surroundings, which probably resulted in a better performance of 3DHG-HH. Finally, the performance of 3DHG-HH with different channels height was studied under 1 sun. As Figure 4c shown, the evaporation rate of 3DHG-HH varied from 1.39 kg m-2 h-1 to 0.64 kg m-2 h-1 as channels height increased from 0 cm to 4 cm. Especially, the evaporation rate of 3DHG-HH with channels height about 2 cm reached 1.56 kg m-2 h-1, which was higher than 3DHG-HH with other channels height. Moreover, as recorded in Figure 4d, the average temperature decreased from 29.1 ℃ to 23.1 ℃ as channels height of 3DHG-HH increased from 0 cm to 3 cm. When the channels height increased further to 4 cm, the average temperature increased to 28.4 ℃ instead. Notably, the heat exchange area also increased as the height of channels increased. Therefore, the Enhancement factor

11

calculated based on Eq.4 was

introduced to assess the effect of channels height on 3DHG-HH. Enhancement factor = actual evaporation rate / thermal limit of water evaporation

(4)

The Enhancement factor varied from 0.83 to 0.38 with the channels’ height increasing from 0 cm to 4 cm. The maximum enhancement factor about 0.94 was also obtained by 3DHG-HH with channels height about 2 cm, which means less heat loss and higher evaporation rate of 3DHG-HH in this moment. Based on the results above, it 8 ACS Paragon Plus Environment


could be concluded that 3DHG-HH with channels height about 2 cm shows better performance than 3DHG-HH with other height of channels under 1 sun.

Figure 4. The performance of different 3DHG under 1 sun. (a) The evaporation rate (ER) and efficiency, and (b) the temperature distribution of 3DHG-CC, 3DHG-CH, 3DHG-HC and 3DHG-HH. (c) The evaporation rate (ER), (d) average temperature and Enhancement factor of 3DHG-HH with different channels height.

3.3 The application of 3DHG Optimal 3DHG was set up for solar desalination and wastewater treatment under 1 sun. And to make it simple, 3DHG mentioned below was all optimal 3DHG unless special statements. As shown in Figure 5a, evaporation rate of seawater with 3DHG (1.45 kg m-2 h-1) was 2.8 times that of seawater without 3DHG (0.51 kg m-2 h-1). During 15 cycles, 3DHG exhibited highly reproducible evaporation rates of 1.45 kg m-2 h-1 ~ 1.42 kg m-2 h-1 with unimpaired performances. And the concentration of Na+, K+, Mg2+, Ca2+ in condensed water collected by 3DHG decreased 3 ~ 5 orders of magnitudes (Table 1), even far lower than the drinking water limit of WHO.20

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Table 1. The concentration of Na+, K+, Mg2+, Ca2+ in seawater and condense water obtained by 3DHG (Units: mg L-1) Na+

K+

Mg2+

Ca2+

Saline

29198±2.1

936±1.6

2520±2.2

898 ±2.1

Condensed water

8.7±1.5

0.4±0.3

0.3±0.1

2.1±0.2

Similarly, the evaporation rate of MB solution with 3DHG (1.47 kg m-2 h-1) was 2.5 times those without 3DHG (0.59 kg m-2 h-1). During 15 cycles, 3DHG in MB solution exhibited the stable evaporation rates of 1.46 kg m-2 h-1 ~ 1.51 kg m-2 h-1, suggesting the excellent cycling stability of 3DHG for MB solution treatment. The MB removal ratio and MB concentration ratio of 3DHG were recorded in Table 2. Table 2. MB removal ratio and MB concentration ratio of 3DHG under 1 sun 1st cycle

15th cycle

dye removal

dye concentration

dye removal

dye concentration

ratio (%)

ratio

ratio (%)

ratio

0h

100

1.00

100

1.00

1h

100

1.05

100

1.24

8h

100

1.92

100

1.83

There was no contaminant measured in condensed water obtained from 3DHG during 15 cycles. In addition, the concentrations of MB solution in water container were significantly increasing, which meant there was no MB in water container volatilizing to air at this moment. Notably, a higher MB concentration ratio (1.24) appeared in the first hour of 15th cycle compared to those of 1st cycle (1.05), probably attributed to residual MB in 3DHG dissolving in water container again at the beginning of 15th cycle. In the 8th hour, the concentration of MB in water container became high, more MB trended to be adsorbed by 3DHG as cycling,21-22 while the residual MB in 3DHG was hardly dissolved into water container solution with high concentration of MB.23 These two factors might result in a lower MB concentration ratio (1.83) appearing in the 8th hour of 15th cycle compared to those of 1st cycle (1.92). Therefore, it could be concluded the 3DHG possessed a unique self-regenerating ability where the MB within 3DHG rapidly dissolved to the surrounding MB solution in next experiment. In 10 ACS Paragon Plus Environment


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addition, it is important to note that the evaporation rates of 3DHG in MB solution and seawater were just lower than those of pure water, which can be attributed to the lower vapor pressure of seawater and MB solution, associated with stronger cohesive force between water molecules and components.

Figure 5. (a) The performance of 3DHG in MB solution and seawater during 15 cycles under 1 sun. Inset: The evaporation rate of pure wastewater with or without 3DHG in different wastewater under 1 sun (b) The performance of 3DHG in industry dyeing wastewater (dye 1 and dye 2) in 7 days.

Other than MB solution, the two different kinds of industry dyeing wastewater (named dye 1 and dye 2) were also used for examining performance of 3DHG in raw wastewater treatment under 1 sun. In 7 days, the evaporation rate of 3DHG in dye 1 decreased from 1.59 kg m-2 h-1 to 1.54 kg m-2 h-1 while the evaporation rate of 3DHG in dye 2 increased from 1.42 kg m-2 h-1 to 1.52 kg m-2 h-1 instead (shown in Figure 5b). The variation in evaporation rate of 3DHG for dye 1 and dye 2 should be attributed to the sensitization effect and contamination of practical wastewater. The pH, ionic conductivity, ammonia nitrogen and COD of condensed water collected by 3DHG were recorded in Table 3. The pH, ionic conductivity, ammonia nitrogen and COD of condensed water collected by 3DHG in dye 1 reached Class IV surface water standards

24

while in dye 2 reached to Class III surface water standards

24,

which

indicated an outstanding performance of 3DHG for industry dyeing wastewater treatment.


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Table 3. pH, ionic conductivity, ammonia nitrogen and COD of different water samples. Ionic Conductivity

Ammonia

Chemical oxygen

(μS cm-1)

nitrogen (mg L-1)

demand (mg L-1)

6.3 ± 0.2

5293.0 ± 1.1

50.3 ± 0.4

1980.0 ± 0.2

6.8 ± 0.1

34.1 ± 0.7

1.3 ± 0.2

4.1 ± 0.3

6~9

—

< 1.5

＜30

7.5 ± 0.2

1528.0 ± 0.3

25.6 ± 0.4

4956.0 ± 2.3

7.0 ± 0.4

12.9 ± 0.2

1.0 ± 0.1

17.8 ± 1.8

6~9

—

< 1.0

＜20

pH Dye 1 Condensed water Class IV surface water standards Dye 2 Condensed water Class III surface water standards

4 Discussion Based on the results above, high efficiency about 98.1 % of 3DHG was achieved under 1 sun, ca 1.1 times (89.2%) ~ 2 times (48 %) those of solar vapor generator reported previously (Table S1, Supporting Information).5-6,

25-30

The good

performance of 3DHG under 1 sun can be explained by its superior structure with reasonable management of heat transfer and water flow. Firstly, the heat transfer process between solar vapor generators and the surroundings was analyzed in Figure 6. The excellent photothermal conversion performance of absorber layer in ISVG resulted in its higher heating speed than the surroundings under solar irradiation. Therefore, an obvious temperature difference between absorber (T1) and surroundings (TR) appeared, which made inevitable heat transfer from absorber to environment in the form of radiation and convection. In addition, little conduction loss from absorber to bulk water via channels still existed after running for a long time. The heat loss mentioned above made the efficiency of ISVG 12 ACS Paragon Plus Environment


below 100 % all the time (Figure 6a).

Figure 6. (a) The heat transfer between ISVG and the surroundings (b) The heat transfer between 3DHG and the surroundings.

As Figure 6b shown, different from ISVG, the water transporting channels of 3DHG exposed to the air directly. The small droplets existing in porous channels and absorber layer leaded to a higher vapor pressure of 3DHG than ambient vapor pressure,31 which prompted water to escape from channels continually and cooled down the channels at the same time despite the same temperature of 3DHG as the surroundings.11-12 The outstanding hygroscopicity and low thermal conductivity of HCF probably resulted in higher cooling speed than heating speed under solar irradiation, leading to a lower temperature of channels in 3DHG than that of the surroundings. Therefore, 3DHG can obtain energy (QE) from environment in some degree. In addition, the water existing in the absorber evaporated rapidly under the solar irradiation while the bottom water entered the absorber layer quickly, which lowered the absorber layer temperature to a low temperature (T1) until the thermal balance between absorber layer and water existing in absorber was established. The lower absorber layer temperature (T1) of 3DHG meant a less radiation loss (Qr) and convection loss (Qc) compared to ISVG. Moreover, longer channels can reduce the conduction loss from absorber layer to water further. When Qr +Qc < QE, the evaporation rate of 3DHG approached or even exceeded the thermal limit of water in theory. It is the reason why optimal 3DHG can perform a high efficiency even approaching 100 %. Besides, the heat transfer process between 3DHG with the height 13 ACS Paragon Plus Environment

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of channels about 2 cm and surroundings under 1 sun was calculated (detailed in of water was

Section SI 1, Supporting Information). The phase change heat

approximately equal to heat energy (including solar energy and environmental energy) that 3DHG-HH gained, which was consistent with the discussion above. Secondly, the Capillary Model in COMSOL was used to describe water flow in channels with different wettability under capillarity (detailed in Section SI 2, Supporting Information). As simulated in Figure 7a, the water can rise along the hydrophilic channels to a definite height quickly in the first 1.0 s. However, the water was still at the original place for hydrophobic channels within the first 1.0 s (Figure 7b). This phenomenon can be explained by Eq.5

32

surface tension (N m-1) and density of water (kg m-3), interface between water and channels (°), and

is gravity constant (m s-2), and

where

and

are the

is contact angle at the

is equivalent diameter of channels (mm)

is the maximum height which the water within

channels can rise up to under capillary action (m). (5) As displayed in Figure 7c, the maximum height (H) was decreased gradually even to a negative height with the contact angle increasing from 0 ° to 180 °, which means water can’t rise along the channels under capillarity for poor hydrophilia channels. Therefore, for 3DHG with CF channels (3DHG-CC and 3DHG-CH), the water would be hindered in the bottom of the channels, which leaded to a far lower evaporation rates of 3DHG-CC and 3DHG-CH than those of PW. For 3DHG-HC, water would rise along the hydrophilic channels but hindered in the bottom of the hydrophobic absorber layer, which resulted in evaporation process occurring only on the channels. For 3DHG-HH, water could rise along the hydrophilic channels and enter the absorber layer quickly. Under the irradiation, evaporation process occurred not only on the channels but also the absorber layer. A better performance of 3DHG-HH than other 3DHG was thus obtained. In addition, the maximum height (H) was limited to contact angle and equivalent diameter of channels (Figure 7c). 3DHG with different height of channels exhibited different performance. The heat transfer area increased with the height of channels increasing from 0 cm to 2 cm, which enhanced the performance of 3DHG under 1 sun.



As the height of channels increased from 2 cm to 4 cm, water rose along the channels more slowly until can’t rise along the channels to reach the absorber layer, which deteriorated performance of 3DHG. Based on the discussion above, 3DHG-HH with the height of channels about 2 cm was proved to be optimal 3DHG. Later, the optimal 3DHG was set up for seawater desalination and wastewater treatment. The minimum evaporation rate of 3DHG in different water environment (seawater, MB solution and two different industry dyeing wastewater) was about 1.45 kg m-2 h-1 under 1 sun. And there was no contaminant measured in condensed water. It can be calculated that 580 kg of condensed water can be collected by 3DHG with the evaporation area about 50 m2 under 1 sun for 8 h, which means 8 persons’ daily water consumption.33

Figure 7. The simulation for water rising along (a) HCF channels and (b) CF channels by Capillary Model in COMSOL. (c) The maximum height which the water within channels can rise to under capillary action with contact angle and equivalent diameter of channels varying. Considering porous structure of channels, the equivalent diameter of channels was far less than the actual channels diameter about 5 mm, which made the maximum height (H) difficult to confirm here.

Although the high efficiency of 3DHG about 98.1 % was achieved under 1 sun, other factors lowering the efficiency of 3DHG still existed. For example, the efficiency of evaporator was limited by the relatively low optimal absorption of the absorber across the full solar spectrum (about 91 %) and limited capillary force that hydrophilic channels can provide. Notably, 3DHG assembled by other materials probably can perform high efficiency, too. The materials for channels in 3DHG must possess porous structure, excellent hydrophily and low thermal conductivity while the 15 ACS Paragon Plus Environment

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materials for absorber layer in 3DHG should possess high optical absorption and excellent hydrophily. Therefore, it can be expected that novel materials with high optical absorption across the full solar spectrum and porous moisture absorption function (such as metal-organic frameworks) could be used to enhance the performance of solar vapor generator. New physical mechanisms should be further developed to reduce the heat loss from the absorbers to the surroundings.

Conclusion In this work, a 3D hierarchical solar vapor generator (3DHG) with negligible convection loss and radiation loss was presented for highly efficient solar evaporation. The high evaporation rate of 1.56 kg m-2 h-1 and efficiency of 98.1 % were achieved under 1 sun, probably attributed to the negligible heat loss of 3DHG achieved by reasonable management of heat transfer and water flow. In addition, solar vapor generator was firstly used for industry dyeing wastewater treatment and 3DHG showed an excellent performance in industry dyeing wastewater even after multiple cycles. The condensed water collected by 3DHG has reached the national environmental quality standard. In a word, this work not only provides a novel strategy to construct high-efficiency solar vapor generator, but also represents a new avenue for large-scale, recyclable and eco-friendly wastewater treatment technology.

Supporting Information Additional figures included schematic for solar vapor generation experiment, the mass change of PW and optimal 3DHG in dark as a function of time, the photography of MB solution concentrated by optimal 3DHG under 1 sun, the photography of two different industry dyeing wastewater, the photography of CF and HCF working in different water environment. Additional Video included dynamic contact angle test of the CF and HCF. Additional Table included the comparison between efficiency of carbon-based solar vapor generator reported and our works. The calculation for heat transfer process between 3DHG and the surroundings, and the detailed process of COMSOL simulation for water rising along HCF channels and CF channels.

Acknowledgments This work was supported by the National Key Research and Development program of China (2018YFA0901300) and the National Science Foundation of China 16 ACS Paragon Plus Environment


(No.51778562).

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Highly Efficient Solar Vapor Generator enabled by a 3D Hierarchical

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