Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification

Author’s Accepted Manuscript Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification Feng (Frank) Gong, Hao Li, Wenbin Wang, Jigang Huang, Dawei (David) Xia, Jiaxuan Liao, Mengqiang Wu, Dimitrios V. Papavassiliou www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(19)30053-9 https://doi.org/10.1016/j.nanoen.2019.01.044 NANOEN3391

To appear in: Nano Energy Received date: 29 November 2018 Revised date: 31 December 2018 Accepted date: 14 January 2019 Cite this article as: Feng (Frank) Gong, Hao Li, Wenbin Wang, Jigang Huang, Dawei (David) Xia, Jiaxuan Liao, Mengqiang Wu and Dimitrios V. Papavassiliou, Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.01.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification

Feng (Frank) Gonga*, 1, Hao Lia,1, Wenbin Wanga, Jigang Huangb, Dawei (David) Xiaa,c, Jiaxuan Liaoa, Mengqiang Wua , Dimitrios V. Papavassilioud* a

School of Materials and Energy, University of Electronic Science and Technology of China,

Chengdu 611731, P. R. China b

School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.

R. China c

Department of NanoEngineering, University of California San Diego, La Jolla, California

92093, United States d

School of Chemical, Biological, and Materials Engineering, University of Oklahoma,

Norman, Oklahoma, 73019, United States

[email protected] [email protected] *

Corresponding authors.

1

These authors contributed equally to this work.

1

Abstract Solar steam generation can be a practical and sustainable technology for wastewater purification and seawater desalination. However, both the inefficient utilization of solar energy and high complicity/cost of current solar steam generators hinder the scalable application of this technique. Herein, we demonstrate a facile, scalable and low-cost approach to produce highly-efficient solar steam generator via a one-step calcination of commercial melamine sponges (MS) in air. The in-air calcinated MS (AMS) with thermal insulator achieves an ultrafast solar evaporation rate (1.98 kg m-2 h-1) and a high photothermal efficiency (~92%) under one sun illumination (1 kW m-2), superior to most reported values. This high solar evaporation rate is attributed to the effective heat localization and adequate water supply in AMS, caused by the low bulk thermal conductivity, high porosity and hydrophilicity of AMS, as well as the set-up of a thermal insulator. The AMS is found to be highly efficient and stable, and it can be used to purify various types of waste water, including river water, seawater, and strong acid/alkaline water. Performance analysis of a large-scale prototype device based on the AMS design for field tests promises significant opportunities for highly-efficient, reusable, portable and low-cost water purification systems.

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Graphic abstract

Keywords: solar steam generator, scalable production, calcinated melamine sponge, anti-fouling, fresh water production

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1． Introduction The scarcity of clean water is one of the most important global challenges as industrialization and urbanization grow across the world today [1-3]. Significant effort has been devoted to developing solutions to address this challenge. As a green and sustainable source for renewable energy, solar energy has been arousing both academic and industrial attention, leading to advances in the fields of photocatalysis [4-6], photovoltaics [7, 8] and photothermal conversion [9-11]. In terms of photothermal conversion, power generation [12-15], desalination [16-20], sewage treatment [18, 21], and solar hot water systems [22, 23] have been extensively investigated. In recent years, harvesting solar energy for water vapor generation using photothermal conversion materials (PCMs) has attracted increasing attention [24, 25]. Various PCMs have been developed to enhance photothermal conversion efficiency, such as plasmonic nanoparticles [20, 26-30], semiconductors [31-33], diverse carbon materials [15, 18, 21, 34-39], polymer based materials [40-43], and carbonized biomass [34, 44]. For instance, Chen et al. [45] fabricated composite films using plasmonic Au nanoparticles and poly (pphenylene benzobisoxazole) nanofibers, achieving an evaporation rate of 1.42 kg m-2 h-1 and a photothermal efficiency of 83% under 1 sun illumination. A hydrophilic

aerogel

with

aligned

graphene

nanosheets

was

obtained

using

an

antifreeze-assisted technique and O2-plasma treatment, which exhibited an evaporation rate of 1.62 kg m-2 h-1 and a photothermal efficiency of ~87% under 1 sun [46]. Xu et al. carbonized mushroom at 500 ℃ in Ar for 12 hours toward solar steam generation, achieving an evaporation rate of 1.48 kg m-2 h-1 and a photothermal efficiency of 78% under 1 sun [34]. Despite intensive efforts to design PCMs, challenges still remain, including: i) The 4

exorbitant cost and complicated fabrication process of PCMs are obstacles for future industrial application. On one hand, some PCMs made from noble metal nanoparticles are unaffordable for most of the population in impoverished countries. On the other hand, the utilization of elaborate technologies [18, 36, 46-48] (e.g., hydrothermal reaction, freeze-drying, plasma treatment, high-temperature annealing in inert gas) further increases the cost. ii) The poor evaporation rate and low photothermal efficiency of PCMs induce a slow production rate of clean water. Due to heat dissipation from PCMs to bulk water, most developed PCMs exhibit unsatisfactory photothermal efficiency (below 85%) under 1 sun illumination [12, 17, 21, 38, 47-52]. Photothermal efficiency above 85% can be obtained by artificially utilizing higher solar intensity (e.g., up to 10 suns) [21, 28, 51, 53-55], however, solar intensity above 1 sun is not achievable in field applications. Therefore, novel techniques and designs are demanded to localize the solar heat in PCMs, thus to improve photothermal efficiency for fast steam generation. In this work, we demonstrate a cost-effective, scalable and environmentally-friendly fabrication of PCMs for highly-efficient solar steam generation by calcinating commercial melamine sponge (porous poly-melamine-formaldehyde sponge [56], denoted as MS) in air. The calcinated MS in air (AMS) exhibits strong solar absorption, ultra-low thermal conductivity, excellent hydrophilicity and a highly porous structure, favorable for solar steam generation. To effectively localize solar heat in AMS, a solar evaporation device is developed with air-laid paper providing water transfer channels and recycled polystyrene (PS) foam acting as thermal insulator. Under 1 sun illumination, AMS achieves an evaporation rate of 1.98 kg m-2 h-1 and a photothermal efficiency of up to 92%, surpassing those of most reported 5

studies. Comparison studies in evaporation rates of AMS with/without thermal insulator manifest the superiority of this solar evaporation device. Moreover, AMS also demonstrates highly-efficient solar steam generation, excellent cyclabilty and remarkable anti-fouling performance for various fluids (e.g., seawater, river water, strong acid/alkaline solution). This low-cost evaporation-condensation prototype with large-scale AMS indicates the potential of using AMS for efficient industrial wastewater treatment and cheap seawater desalination. 2． Results and discussion The one-step synthesis of AMS is schematically illustrated in Figure 1a. Within annealing at 400 ℃ in air, the white pristine MS (Figure 1b) transforms to the black AMS (Figure 1e). Our furnace (Figure S1) can simultaneously anneal 10 pieces of MS with size of 45 cm× 35 cm× 5 cm (Figure 1b) to AMS (Figure 1e), indicating the promising scalable fabrication of AMS. One can also easily fabricate AMS by calcinating MS using muffle furnaces. During the annealing of MS, white MS gradually changes to brown color (300-370 ℃) and finally turns to black AMS (370-400 ℃). The thermogravimetric analysis (TGA) of pristine MS in air (Figure S2) reveals the three weight-loss stages before 400 ℃: -5% at 50-175 ℃, -9.5% at 175-370 ℃ and -20.2% at 370-400 ℃. These three weight losses can be ascribed to the evaporation of water in MS, the volatilization of formaldehyde (CH2O) and the formation of melamine, and the transformation of melamine to melem, respectively (see Figure S3) [57, 58]. Fourier transform infrared spectroscopies (FTIR) of AMS and MS validate the formation process of AMS (Figure S4). The FTIR spectra of AMS and MS both show peaks at 3372 cm-1, 2997 cm-1, 1647 cm-1, and 1558 cm-1, corresponding to the characteristic peaks of NH/NH2, CH2, imine and triazine, respectively [56]. The reduced 6

intensity of CH2 peak in the spectrum of AMS than that of MS may be due to the formation of melamine, corroborating the proposed formation process of AMS in Figure S3. Scanning Electron Microscopy (SEM) images show that pristine MS possesses a highly-porous framework with uniform pores (Figures 1c-d). After calcination, the as-prepared AMS still inherits the porous structure but with reduced pore size (Figures 1f-g), which explains the ~60% volume shrink of AMS (Figure 1e) compared to that of pristine MS (Figure 1b). The XRD patterns of both AMS and MS display similar bread-like peaks at ~22o, indicating the amorphous structures of AMS and MS (Figure S5) [59-61].

Figure 1. Schematic illustration of the one-step synthesis and morphologies of calcinated melamine sponge in air (AMS). (a) Within one-step annealing at 400 ℃ in air for 2 h, white melamine sponge (MS) transforms to black AMS. (b) Photograph of a large piece of MS (45 cm× 35 cm). (c-d) SEM images with different magnification show the highly-porous structure with uniform pores of MS. (e) Photograph of the as-prepared AMS by annealing the MS (b), which shows ~60% volume shrink (30 cm× 20 cm). (f-g) SEM images at different magnification exhibit the inherited highly-porous structure of AMS with reduced pore size compared that in MS. 7

Figure 2. Physical and mechanical properties of AMS. (a) AMS shows hydrophilicity with a water-contact angle of 0°. (b) Within 10 s, the white tissue paper on AMS can be wetted by the blue ink. (c) AMS displays excellent mechanical strength during compress-release cycles. (d) Raman spectra of MS and AMS. (e) Thermal conductivity and density of dry MS and AMS. (f) Thermal conductivity and density of wet MS and AMS.

The as-prepared AMS exhibits excellent hydrophilicity, verified by the direct soak of water droplets into AMS (Figure 2a). Owing to the highly-porous structure and hydrophilic character of AMS, water can transfer quickly in AMS. As demonstrated in Figure 2b, within 10 s, the white tissue paper on the top of AMS of 1 cm thick can be wetted by the blue ink. Besides, AMS is of superior mechanical strength, comparable to that of pristine MS. As shown in Figure 2c, after ~90% volume compression, AMS can still recover its original shape and size. The mechanical performance of AMS (also indicates compatibility and 8

tailorability) guarantees the applicability of AMS under different conditions. The Raman spectra of MS and AMS are seen in Figure 2d. The intense peak at 974 cm-1 for pristine MS is the characteristic peak of the breathing vibrational modes of the triazine ring in melamine, which is consistent with the previous report of Lin et al. [57]. After annealed at 400 ℃ in air, the intense peak at 974 cm-1 disappears and two new peaks appear at 1372 cm-1 and 1559 cm-1. These two new peaks are attributed to the D band and G band of carbon vibrational modes [62], indicating the cabonization of MS to AMS. Both MS and AMS in dry state exhibit ultralow thermal conductivity, below ~0.038 W m-1 K-1 (Figure 2e). Dry AMS has a density of 16.4 kg m-3, ~2 times that of pristine MS (8.3 kg m-3), which is consistent with the ~60% volume shrink of AMS. The wet AMS in the solar evaporation test has a density of ~300 kg m-3, lower than that of pure water (1000 kg m-3), indicating that AMS is not completed soaked by bulk water during the solar evaporation test. The incompletely soaked AMS reveals a much lower thermal conductivity (0.24 W m-1 K-1) than that of pure water (~0.6 W m-1 K-1) [63, 64]. This lower thermal conductivity of wet AMS benefits the heat localization in AMS during the solar evaporation, favoring a faster evaporation rate. When AMS is completely soaked by water (density of ~900 kg m-3), it exhibits a thermal conductivity of ~0.56 W m-1 K-1，close to that of pure water (Figure S6). To demonstrate the solar evaporation performance of AMS, indoor solar evaporation tests were conducted. The set-up of the solar evaporation test of AMS with a thermal insulator (AMS-TI) is illustrated in Figure 3a. AMS was cut into pieces with dimensions of 2 cm × 2 cm × 0.3 cm and placed on PS foam (Figure S7a). The PS foam acted as thermal insulator to prevent any direct contact between AMS and bulk water. Cheap air-laid paper ($0.25 m-2) 9

was utilized as channels to continuously transport water to AMS via capillary effects. The AMS-TI is expected to localize heat within AMS for highly-efficient evaporation. For comparison, solar evaporation tests were also conducted for pure water and AMS without thermal insulator (AMS-NTI). In AMS-NTI, AMS (2 cm × 2 cm × 0.3 cm) was mounted in the center of a piece of PS foam and floated on bulk water, where AMS was in direct contact with bulk water (Figure S7b). Under 1 sun irradiation (1 kW m-2), pure water had a solar evaporation rate of 0.39 kg m-2 h-1 (Figure 3g), which is consistent with prior reported values [21, 53, 65-67]. The solar evaporation rate for AMS-NTI was measured at 1.35 kg m-2 h-1, about 3.5 times that of pure water, attesting to the capability of AMS for solar steam generation. Furthermore, AMS-TI achieved a remarkable evaporation rate of 1.98 kg m-2 h-1, which is 5 times above that of pure water and ~50% higher than that of AMS-NTI. This ultrafast solar evaporation rate of AMS with our set-up is mainly attributed to the highly-efficient heat localization and adequate water supply in AMS due to the presence of PS thermal insulator and capillary water channels. When AMS was in direct contact with bulk water, an amount of heat would transfer from AMS to bulk water, lowering the surface temperature of AMS (Figure 3b). With the assistance of a thermal insulator, heat could be localized in AMS with negligible heat loss to bulk water (Figure 3c). Meanwhile, air-laid paper supplied fast water transfer channels via capillary effects (Figure 3a). For instance, the set-up in Figure 3a without AMS exhibits a solar evaporation rate of 0.36 kg m-2 h-1 under 1 sun.

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Figure 3. Solar evaporation performance of AMS under 1 sun. (a) Schematic plot of set-up of solar evaporation test with thermal insulator (AMS-TI). A piece of AMS (2 cm × 2cm × 0.3 cm) is placed on a piece of PS foam (thermal insulator). Air-laid paper (2 cm wide), providing continuous water transfer channels through capillary effect, is threaded through the gaps in PS foam and soaked in bulk water. The inserted figures in (a) are the schematic plots of the micro-structure in AMS and the capillary action in air-laid paper. (b) Schematic illustration of the solar evaporation process of AMS without thermal insulator (AMS-NTI). Heat is transferred to bulk water due to the direct contact between AMS and bulk water. (c) Schematic plot of the solar evaporation process of AMS-TI. Direct heat loss from AMS to bulk water is hampered with the presence of thermal insulator. (d-f) Infrared images of the solar evaporation processes of (d) pure water, (e) AMS-NTI, and (f) ANS-TI under steady state, with color code indicating degrees oC. The evaporation mass loss (g) and average temperature profiles (h) of pure water, AMS-NTI and AMS-TI for 1 h under 1 sun. (i) The solar evaporation rates and photothermal efficiency of pure water, AMS-NTI and AMS-TI.

The steady-state temperature profiles (Figures 3d-f, h) also manifest the superiority of AMS and the set-up for solar steam generation. AMS-TI attains a steady-state temperature of ~43 ℃ after 5 min under 1 sun (Figure 3f, h), which is 5 ℃ higher than that of AMS-NTI 11

(Figure 3e, h), and 10 ℃ above that of pure water (Figure 3d, h). Without thermal insulator, there is heat loss from AMS to bulk water, lowering the temperature of AMS (Figure 3e). The PS thermal insulator blocks heat losses from AMS to bulk water, elevating the temperature of AMS for more highly-efficient evaporation (Figure 3f). In Figure 3i we present the solar evaporation rate and photothermal efficiency of pure water, as calculated for AMS-NTI and AMS-TI (Calculation S1). Compared to pure water (24.4%), AMS achieves much higher photothermal efficiency at 69.7%. With PS thermal insulator, the photothermal efficiency is enhanced to 92.0%, superior to most reported values to-date (Figure S8). A macro-scale computational model based on finite volume method (FVM) was developed to predict the solar evaporation rate of AMS under solar intensity above 1 sun (Experimental section). The calculated solar evaporation rates of AMS under 0.5-8 sun demonstrate the functionality and superiority of our set-up with thermal insulator (Figures S9-S11). In summary, the highly-efficient solar steam generation of AMS-TI is ascribed to the unique properties of AMS and the innovative set-up of the solar evaporation system: (i) AMS has strong and broadband absorption to solar energy (Figure S12). (ii) Solar energy is subsequently converted to thermal energy, elevating thus the temperature in AMS because of the fast photothermal response of AMS. (iii) The low thermal conductivity of AMS and the PS thermal insulator localize heat within AMS [68]. (iv) The air-laid paper quickly and continuously transports water to AMS.

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Figure 4. Solar evaporation performance of AMS-TI on different water. (a) Evaporation mass loss of AMS-TI on different water under 1 sun for 1 h. (b) Surface temperature of AMS varying with time on different water. (c) Evaporation rate and photothermal efficiency of AMS on different water under 1 sun. (d) Photographs of pH papers tested in acid water before and after evaporation. (e) Photographs of pH papers tested in alkaline water before and after evaporation. (f) Anti-fouling performance of AMS on dirty water. Insert is the photograph of the polluted air-laid paper. (g) Cycling performance of AMS-TI on strong acid/alkaline water and seawater. (h) The design and composition of the solar evaporation-condensation prototype. (i) Photograph of the solar evaporation-condensation prototype floating on river water (15th, October, 2018 on Qingshui River, Chengdu, Sichuan Province, P. R. China). For practical application of AMS, it is considerably vital to investigate the capability of AMS to purify different types of water. The solar evaporation performance of AMS on different water is presented in Figure 4. Four types of water, including seawater, river water, strong acid and alkaline water, were utilized. As presented in Figure 4a, under 1 sun for 1 h, the evaporation mass losses of seawater (1.91 kg m-2) and river water (1.88 kg m-2) are close 13

to that of pure water (1.98 kg m-2). The mass losses of strong acid (1.81 kg m-2) and alkaline water (1.75 kg m-2) are slightly lower than that of pure water. The different solar evaporation rates of different water types may be caused by the presence of contaminants or other substances in water. Higher concentration of impurities appear to decelerate the water transfer to AMS, inducing a lower evaporation rate. Consistent with the mass loss, the surface temperature of AMS-TI on different water has no apparent difference (~43 ℃, Figure 4b). To demonstrate the significance of AMS for water purification, the ohmic resistance of different water before/after evaporation was measured and compared to indirectly characterize the water quality. As presented in Figure S13, all the purified water samples exhibit much higher ohmic resistances (above 1.2 MΩ) than those of initial water samples (below 92 KΩ). Moreover, the ohmic resistances of all purified water samples are even higher than that of potable water (~0.8 MΩ), indicating the purity of the evaporated water [69]. The comparable evaporation rates and photothermal efficiency (Figure 4c) of AMS-TI for different water, as well as the high purity of evaporated water (Figure S13) manifest the ability and stability of AMS-TI to generate fresh water in diverse conditions. The evaporated water from both acidic water (pH~1, Figure 4d, Figure S13b) and alkaline water (pH~14, Figure 4e, Figure S13b) displays a pH value close to 7, indicating promising anti-fouling capacity of AMS. To further characterize the anti-fouling performance of AMS, a 10 wt% NaCl solution mixed with HCl solution and 10 g of soil was utilized (Figure S14). The whole system was placed under 1 sun illumination for 30 h. As presented in Figure 4f, the solar evaporation rate of AMS-TI can remain above 1.7 kg m-2 h-1 during 30 h under 1 sun. After such long time under 1 sun, the air-laid paper was contaminated by the 14

dirty water (Insert in Figure 4f). However, there was no obvious contamination observed on AMS (Figure 4f and Figure S14), demonstrating a rather the excellent anti-fouling capability of AMS to purify the high-salinity, acidic and feculent water. The cycling performance of AMS with thermal insulator on alkaline water, acidic water and seawater is presented in Figure 4g. One can observe similar evaporation rates during 15 cycles of operation. The photothermal efficiencies of AMS on different water are calculated and presented in Figure S15, exhibiting the cyclability and stability of AMS to generate purified water from various polluted water. After 15 wetting-drying cycles, AMS maintained its original elastic properties, suggesting a long lifetime for AMS. Moreover, AMS also demonstrates excellent salt rejection property for seawater desalination (Figure S16). In order to verify that scalable production of fresh water is possible with this set-up, a solar evaporation-condensation prototype with larger area of AMS (40 cm× 40 cm) was constructed and tested in the field. The concept is illustrated in Figure S17. In Figure 4h, we present the design and composition of the device. Recycled PS foam (60 cm× 60 cm× 15 cm) was used as the main body, which floated the whole prototype on river water. A transparent cover made of polyvinyl chloride (PVC) film was utilized to condense steam. There were channels at the bottom of the PVC cover that guided the condensed water to the water-collection bottle. Air-laid paper was applied to transport water via capillary effect and to avoid the direct contact between AMS and water. Specifically, a layer of active carbon was placed at the bottom of the PS foam to filter river water: it blocked particles and pollution substances, thus protecting the air-laid paper and AMS. A photograph of the prototype floating on river water (Qingshui River, Chengdu, P. R. 15

China) is shown in Figure 4i. In a sunny autumn day (October 15th, 2018), the condensed water was observed on the PVC cover after 30 min under natural sun. The prototype could achieve water-collection rates of 0.5- 0.8 kg m-2 h-1 in sunny autumn days (7 days), depending on the average solar intensity. The lower water-collection rate of the prototype than the indoor solar evaporation rate of AMS is mainly ascribed to the lower solar intensities than 1 sun during the field tests. As presented in Figure S18, both the solar evaporation rate and surface temperature were significantly reduced with the decrease of solar intensity. Assuming that the sun shines for 10 h, 5 to 8 kg of fresh water could be generated by using the prototype design and 1 m2 of AMS, which is enough for the drinking water for a family of three people. It should be noted that the cost of such a prototype with 1 m2 of AMS is estimated to be ~$12.5 (Calculation S3). 3． Summary and Conclusions In summary, we have demonstrated the scalable and low-cost fabrication of AMS as well as a facile and smart system for highly-efficient solar steam generation toward water purification. The as-prepared AMS exhibits high porosity, low density (~0.016 g cm-1), ultralow thermal conductivity (~0.038 W m-1 K-1) and a hydrophilic capillary network (water contact angle = 0o). With the assistance of a thermal insulator and water transfer channels, AMS has achieved an ultrafast solar evaporation rate of 1.98 kg m-2 h-1 and a high photothermal efficiency of 92% under 1 sun, superior to most reported materials to-date. Both control experiments and computational studies have demonstrated the function and superiority of the proposed design with thermal insulator and water transfer channels, which displays more significant benefit as solar irradiation becomes more intense. AMS has also 16

revealed excellent anti-fouling performance, strong salt-rejection ability and remarkable cycling stability when used to purify polluted river water, strong acid/alkaline water and seawater. A solar evaporation-condensation prototype made from recycled waste and AMS has been developed for scalable fresh water generation: the prototype with 1 m2 of AMS is estimated to cost ~$12.5, but expected to generate 5- 8 kg of fresh water daily under natural sun. The facile scalability, low cost and high portability of the prototype with AMS enable its significant potential for both domestic and industrial water purification, possibly for sewage treatment and seawater desalination. More efforts should be devoted to improving the evaporation-condensation rate of the prototype with large-scale AMS for fast water generation.

4． Experimental section 4.1 Materials Porous poly-melamine-formaldehyde sponge (MS) in different size were purchased from Tongen Commercial and Trading Co. Ltd (Wuhan, China) at a price of $250 m-3. 4.2 Materials preparation Preparation of calcinated melamine sponges in air (AMS) To fabricate AMS, pristine MS pieces with size of 10 cm× 6 cm× 2 cm were put into a tube furnace and annealed at 400 ℃ for 2 h with a heating rate of 10 ℃/min. The AMS in large size (40 cm × 40 cm × 6 cm) was obtained by annealing MS in a lab-developed furnace (Figure S1) under the same conditions.

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Preparation of various water samples The pure water was deionized water. A solution with 3.5 wt % NaCl was used to model seawater. The river water was collected from Qingshui River (Chengdu, Sichuan Province, P. R. China). The acidic liquid was a 0.1 mol L-1 of HCl solution, and its pH was 1-2. The alkaline liquid was a 0.1 mol L-1 of NaOH solution and the pH was 13-14 (see Figures 4d, 4e), as measured with a pH paper. 4.3 Materials characterizations The thermogravimetric analysis (TGA, Metteler-Toledo TGA/DSC 1/1600) of pristine MS was performed in air at a heating rate of 5 ℃ min-1. The Fourier transform infrared (FTIR) spectra were measured using an infrared spectrometer (Bruker, TENSOR27, USA) using KBr discs. The morphologies of all samples were observed using field emission scanning electron microscopy (FESEM, JEOL-7800). The contact angles were characterized on Kruss Drop Shape Analyzer (Kruss Gmbh DSA255). The Raman spectra measurements were conducted on a confocal micro-Raman spectrometer (Renishawin ViaReflex) in a backscattering configuration. All the samples were excited by 532 nm emission line from a DPSS laser at room temperature. A 50x objective (Leica) was used to focus laser light onto the sample and also collect the scatted light. The thermal conductivity of sponges was measured using Hot Disk TPS 2500S, which is based on the transient hot-wire method. The X-ray diffraction (XRD) analysis of all samples was performed on a Bruker-D8 Advanced X-ray Diffractometer (Cu Kαradiation: λ=1.5415 Å) at a scanning rate of 2 deg/min. The absorbance

spectrum

of

AMS

was

measured

using

Spectrophotometer, coupled with an Agilent integrating sphere. 18

Cary

5000

UV-Vis-NIR

4.4. Evaluation of solar-driven vaporization in the laboratory Solar evaporation test without thermal insulator (Figure S4b) The solar evaporation tests were performed under the same solar illumination of 1 kW m-2 (1 sun). AMS (with dimensions of 2 cm × 2 cm × 0.3 cm) was inserted in the center of a piece of polystyrene (PS) foam (4.5 cm in diameter, 0.3 cm in thickness) and floated on water in a beaker (4.5 cm in diameter). The whole set-up exposed exactly under simulated sunlight (Surius SS150A, Zolix, Beijing, China). The water weight loss was recorded by an electrical balance every 10 min. The surface temperature of AMS was measured and recorded by an IR camera (FLIR-E64501, Tallinn, Estonia). The solar evaporation rate was calculated based on the weight loss of water over one hour at the steady state. Solar evaporation test with thermal insulator (Figure S4a) A piece of PS foam (4.5 cm in diameter, 0.5 cm in thickness) with two tiny gaps was utilized as the thermal insulator. The hydrophilic air-laid paper (2 cm wide) was threaded through the gaps in PS foam and soaked in bulk water to transport water (Figure 3a). AMS with size of 2 cm× 2 cm× 0.3 cm was wetted and placed on the PS foam. Thus, the PS foam prevents direct contact between AMS and bulk water. All the other set-up and operation were the same as those in the solar evaporation test without thermal insulator. Evaluation of water quality (Figure S13) A multimeter was utilized to measure the ohmic resistance of different water samples. Two electrodes were clamped using a piece of PS foam, and the water sample was contained in a bottle cup. The resistance was recorded from the instrument screen. The pH value was measured using a pH meter. 19

4.5. Macro-scale solar evaporation modeling A computational model was developed to model the solar evaporation process in AMS. The governing equations are presented in Calculation S3. The finite volume method (FVM) was utilized to discretize the partial differential equation and the SIMPLE scheme method was used to model the pressure-velocity coupling. As shown in Figure S8, the solar evaporation systems with/without thermal insulator were both modeled. To simplify the computation, only AMS and bulk water were taken into account. The dimensions of the AMS were set at 2 cm× 2 cm× 0.3 cm. Based on the experimental conditions, the system with thermal insulator was modeled with a piece of AMS and a layer of water with 0.5 mm thickness, while the system without thermal insulator was modeled with AMS and bulk water of 15 mm thick. The solar irradiation on the top surface of AMS was modeled by the method of solar ray tracing. The AMS boundary condition was defined as a pressure outlet with the gage pressure of 0 Pa, representing the top side of the porous AMS. The turbulence parameters of AMS were set based on the specification method of intensity and hydraulic diameter. The side and bottom walls of the computational domain were assumed as thermal isolation, with zero heat flux and stationary walls with no velocity slip. The interface boundary condition was applied at the AMS-water interface, allowing the transfer of heat and mass through the interface [70]. The operating pressure was set to be 101,325 Pa and the initial operating temperature was set as 293 K. The specified operating density of vapor was set to be 0.6 kg m-3 and the gravity constant was chosen as -9.8 m s-2. Face-meshing and boundary-layer meshing were applied to generate the mesh on the surface and on the interface. Elements of “Quad/Tri” type were created. The volume mesh was generated with 20

the element of “Hex/Wedge”. The interval sizes of surface/interface mesh and volume mesh in this work were set to be 0.5 mm. The thermo-physical properties of AMS and the liquid/vapor phases of water are presented in Table S1. All simulations were conducted using ANSYS Fluent 18.0.

Acknowledgements The authors would like to thank the financial support from the National Natural Science Foundation of China (51602038), the Sichuan Science and Technology Support Program (2017HH0101) and the central university funds (A03013023601098, A03018023801074). We greatly appreciate the kind help from Mr. Jingde Li of SCU on the characterizations of AMS.

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(2017) 1-9.

Feng (Frank) Gong is working as an Assistant professor in University of Electronic Science and Technology of China (UESTC). He received his Ph.D. in Mechanical Engineering in 2015 from National University of Singapore, and worked as a postdoctoral fellow at the University of Oklahoma in 2016. His current research interests are focused on photothermal conversion materials, carbon nanocomposite, micro-scale heat transfer and multiscale modeling of energy conversion, transfer and storage.

Hao Li is a third-year undergraduate student at University of Electronic Science and Technology of China. His current research mainly focuses on photothermal conversion materials for high efficiency solar vapor generation.

Wenbin Wang is now pursuing his Bachelor's degree at University of Electronic Science and Technology of China. He is currently focusing on graphene-based functional materials and photothermal conversion materials.

Jigang Huang is a joint Ph.D. candidate in School of Manufacturing Science and Engineering at Sichuan University, Chengdu, China, and Department of Mechanical Engineering at Northwestern University, IL, US. His research interests include multi-scale modeling and experiment of heat transfer and lubrication, additive manufacturing (AM) process development, process modeling and optimization. He obtained his Master and Bachelor degrees from Sichuan University, Chengdu, China.

Dawei (David) Xia is a graduate student in Nanoengineering in University of California San Diego (UCSD), currently under the supervision of Prof. 26

Zheng Chen. He received B.E. degree in Renewable Energy Materials and Devices from University of Electronic Science and Technology of China (UESTC) in 2018. His research interests focus on the synthesis and characterization of nanostructured materials towards energy storage and conversion. Jiaxuan Liao is working as a full professor and a Ph. D. supervisor in University of Electronic Science and Technology of China (UESTC). He received his Ph.D. in Materials Science and Engineering in 2001 from Harbin Institute of Technology, and worked as a postdoctoral fellow at Lanzhou Institute of Chemical Physics of Chinese Academy of Sciences in 2003. His current research interests are focused on energy conversion materials and devices, electronic functional materials and devices, electronic thin films and integrated devices and new energy materials and devices. Mengqiang Wu is working as a full professor and a Ph. D. supervisor in University of Electronic Science and Technology of China (UESTC). He received his M.S. degree in Physical Chemistry from Sichuan University in 1995 and Ph.D. in Microelectronics and Solid State Electronics from the University of Electronic Science and Technology of China in 2002. He worked as a visiting scientist within the University of Cambridge, UK and a postdoctoral fellow at the University of Southampton, UK. His current research interests are focused on advanced materials and devices for energy storage and conversion. Dimitrios Papavassiliou is the C.M. Sliepcevich Professor of Chemical, Biological and Materials Engineering at the University of Oklahoma. He received his M.S. and Ph.D. degrees from the University of Illinois at Urbana-Champaign in Chemical Engineering. His research contributions are in the areas of nanofluids, microfluidics, transport in porous media and turbulent dispersion. Between 2013 and 2016 he served as the Fluid Dynamics program director at the National Science Foundation of the US.

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Highlights  Scalable production of solar evaporator is achieved by calcinating melamine sponge in air (AMS)  AMS yields a solar evaporation rate of 1.98 kg m-2 h-1 with a photothermal efficiency of ~92% under 1 sun  AMS achieves high efficiency, stability and cycling capability on river water and seawater  AMS demonstrates remarkable anti-fouling performance when purifying strong acid/alkaline water

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