Highly Efficient Solar Steam Generation from Activated Carbon Fiber

May 31, 2019 - Interfacial solar steam generation is a green and promising technique to capture solar energy for brine water desalination, however, it...
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Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4354−4361

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Highly Efficient Solar Steam Generation from Activated Carbon Fiber Cloth with Matching Water Supply and Durable Fouling Resistance Qile Fang,†,‡ Tiantian Li,†,‡ Haibo Lin,†,‡ Rongrong Jiang,† and Fu Liu*,†,‡

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†

Key laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Rd., Ningbo 315201, China ‡ University of Chinese Academy of Sciences, 19 A Yuquan Rd., Shijingshan District, Beijing 100049, China S Supporting Information *

ABSTRACT: Interfacial solar steam generation is a green and promising technique to capture solar energy for brine water desalination; however, it still faces grand challenges of thermal loss and salt fouling to promote the practical application with high performance and durability. In this study, we report that activated carbon fiber cloth (ACFC) with hierarchical microstructures shows superior light-thermal property for solar steam generation. A well-matching water supply path manipulated by cotton fiber nonwoven fabrics (CFNF) can bring about a high evaporation rate of 1.59 kg m−2 h−1 with optimum conversion efficiency of 93.3% under 1 sun. Rate matching between the water supply and vapor evaporation is revealed to be of great importance to full heat exploitation and efficient solar desalination. Moreover, the extra water supply pathway provided by CFNF completely eliminates the salt fouling phenomenon via timely salt dredging, guaranteeing the durability of the self-cleaning solar steam generation system. Thus, the appropriate ACFC+CFNF configuration shows its great potential application in durable and highly efficient solar desalination. KEYWORDS: activated carbon fiber cloth, solar steam generation, desalination, salt fouling, liquid vapor match through bulk water.18,19 In this way, the converted thermal energy is confined at the interface and only heats a small amount of water at the surface, and the surface water is supplied from the bulk water via capillary action or other water paths.20−23 The surface heating holds an advantage over the bulk heating in terms of solar utilization. Interfacial solar steam generation has largely improved the solar-to-steam conversion efficiencies, reaching 60%−85% under 1 sun at ambient conditions without optical concentration,24−27 which is a great progress compared to bulk water evaporation. Local heating is a prerequisite to achieve highly efficient solar steam generation in such an interfacial evaporation system. However, whether the surface local heat is fully utilized is also an important issue worthy of attention. In theory, supplying water transportation at the surface must match the vapor transpiration caused by surface solar heating for optimum vapor production. A scant supply of water to the surface “heating area” makes the evaporation rate lower than

1. INTRODUCTION The global shortage of fresh water is one of the most severe issues that needs to be urgently solved for sustainable development, particularly in developing countries and remote areas. Solar-driven evaporation has elicited ripples of excitement in the research communities of desalination. It efficiently harvests solar energy to produce clean water without consuming any extra energy.1,2 In a solar desalination system, solar absorber and heat management are two crucial elements to enable high solar steam generation,3 and most current work focused on the former to pursue broadband and efficient absorbers, preferably high absorption in the full range of solar spectrum, including carbon-based, plasmonic, and semiconducting absorbers.4−10 In the conventional study, the absorbers are always in direct contact with bulk water, which gives rise to unavoidable thermal loss through bulk water (thermal conductivity ∼0.5 W m−1 K−1).11−14 To overcome this obstacle, interfacial solar heating has been explored recently to minimize the energy loss.3,15−17 The solar energy harvested by a solar absorber is localized at the water−air interface, and a thermal insulation layer is set between the bulk liquid and solar absorber layer to prevent thermal diffusion © 2019 American Chemical Society

Received: March 15, 2019 Accepted: May 31, 2019 Published: May 31, 2019 4354

DOI: 10.1021/acsaem.9b00562 ACS Appl. Energy Mater. 2019, 2, 4354−4361

Article

ACS Applied Energy Materials that of theoretical value, while an excess supply of water will bring about the energy loss, similar to the bulk heating. Thus, the water supply rate must be appropriate to light−thermal conversion for an optimal evaporation, which is an extremely important issue in the heat management of solar steam generation system. However, it is surprising that this key issue was ignored in most previous studies. In addition, the salt fouling on the light absorber surface decays the light absorption and corresponding evaporation due to the accumulated precipitation. Up to now, only a few studies have paid attention to eliminate the salt precipitation and extend the operation time.28,29 Herein we demonstrate that activated carbon fiber cloth (ACFC), a porous member of activated carbon family, shows high-efficiency solar steam generation for brine water desalination, as illustrated in Figure 1a. A well-matching

Figure 2. (a−c) SEM images of the activated carbon fiber cloth (ACFC). (d−f) Elemental composition and maps of ACFC. (g) Water contact angle of ACFC surface. (h) Fire-retardant property of ACFC after flaming for 5 min.

during carbonization and activation process. Although it appears to be hydrophobic based on the element analysis, ACFC displays a superior wettability according to the surface contact angle in Figure 2g, where the water droplet spread instantaneously into the ACFC surface, leading to a contact angle close to zero only after 40 ms. This should be ascribed to the abundant capillary structure in ACFC, and the Laplace pressure drives the rapid liquid movement within the fiber bundles along the carbon fiber direction.30 Therefore, ACFC can automatically draw water from the bulk water to the surface via the capillary interaction. Besides, the abundant nanopores on ACFC (Table S1) can further contribute to its wettability. The thermal stability under sunlight irradiation is important for long time operation. ACFC has a great fireretardant or high-temperature-resistant property due to its high carbonization as shown in Figure 2h, which kept its original morphology after flaming on the alcohol lamp flame for 5 min. 2.2. Light Absorption Behaviors and Light−Thermal Properties of ACFC. The light absorption property of ACFC was evaluated by optical reflection, transmission, and absorption spectra. As shown in Figure 3a, ACFC exhibits a high light absorption >95% within a broadband wavelength from 200 to 2500 nm, which can harvest solar energy to the utmost extent for heat conversion in the solar desalination system. High carbonization leads to closely spaced energy levels of the loosely held highly π electrons that accounts for the broadband light absorption of ACFC.9 In addition, the microstructure of the individual carbon fiber confines the light escape and results in a low reflection below 5% (Figure 3a). As magnified in Figure 3b, regular groove structures can be clearly observed on the carbon fiber surface, similar to a “Roman column” structure in Figure 3c. In such case, the incident light can be guided via zigzag reflections on the wall surface of microgrooves and considerably reduce light reflection to the atmosphere (Figure 3d), giving rise to a high light absorption

Figure 1. (a) Schematics of the solar steam generation devices. (b) Transport path of water and salt on ACFC and CFNF in solar steam generation devices.

water supply path was manipulated via cotton fiber nonwoven fabrics (CFNF), which significantly elevated the evaporation rate from 1.25 to 1.59 kg m−2 h−1 with an optimum conversion efficiency of 93.3% under 1 sun. Rate matching between the water supply and vapor evaporation is first revealed to be of great importance to efficient solar desalination. By balancing this critical factor, one can fully exploit interface heat management. Moreover, the extra CFNF water path beneath the light−thermal material provides a salt dredging capability. The concentrated water was self-cleaned from the ACFC surface through the backward transfer of saline ions in the CFNF water path, maintaining the durable operation, as depicted in Figure 1b.

2. RESULTS AND DISCUSSION 2.1. Structural Properties of ACFC. Activated carbon fiber cloth (ACFC) derived from viscose fiber via hightemperature carbonization and activation is a highly porous free-standing carbon material with favorable mechanical integrity. The scanning electron microscopy (SEM) image in Figure 2a clearly reveals that ACFC is a continuous network woven from fiber bundles, each of which is composed of tens of individual carbon fibers with a width of 10−20 μm (Figure 2b,c and Table S1). As evidenced by the elemental analysis (Figure 2d−f), ACFC is a highly carbonized material with a C content >95 wt % and a small amount of O element, indicating its durable physicochemical stability; besides, the trace P element was also detected on ACFC, which might be introduced by the ammonium phosphate activating agent 4355

DOI: 10.1021/acsaem.9b00562 ACS Appl. Energy Mater. 2019, 2, 4354−4361

Article

ACS Applied Energy Materials

Figure 3. (a) Absorption, reflection, and transmission spectra of ACFC in the wavelength range 200−2500 nm. The absorption spectrum is calculated from reflection and transmission spectra (A = 100% − R − T), and the background area represents the normalized solar spectrum. The inset figure displays the reflection spectra of ACFC in dry and wet states. (b) SEM image of individual carbon fiber in ACFC. (c) “Roman column” model. (d) Light reflection within the groove-structured surface. IR images of the ACFC surface after illumination for 30 min under (e) 1 sun (qi = 1 kW m−2) and (f) 3 sun (qi = 3 kW m−2) without water pumping. IR images of ACFC (g, h) and ACFC+CFNF (i, j) drawing water before and after illumination for 30 min under 1 sun. (k) Temperature elevation of the surface of pure water, ACFC, and ACFC+CFNF drawing water under 1 sun illumination for 30 min.

the solar absorber. The liquid level of bulk water was lower than the bottom of PS foam, generating an air gap between the PS foam and bulk water to further enhance the thermal insulation effect on account of the lower thermal conductivity of air (∼0.026 W m−1 K−1 at ambient temperature). Then the absorber material of ACFC (cut as band) threaded the two slots and spread smoothly on the PS foam surface, where the braided structure of ACFC made it flexible to hang on the PS support and reach the bulk water for water drawing at two ends. On the strength of fiber bundles structure of ACFC as described above, it can self-pump the bulk water via capillarity to the evaporation surface above PS foam; in spite of this, cotton fiber nonwoven fabric (CFNF, as shown in Figure S1) with high hydroscopicity was set to be an additional water path beneath the ACFC for modulating the water transportation rate from the bulk water. Thus, a typical interfacial solar steam generation device was obtained, and the corresponding water transport path is described in Figure 1b. The salt water was transported to the ACFC surface by both ACFC and hydrophilic CFNF, and vapor evaporation was realized via the interfacial heating. The heating curves of ACFC surface in the desalination system under 1 sun illumination were determined as shown in Figure 3k. The surface temperature of ACFC wetted with drawing water increases from 21 to 32 °C, which is much higher than the elevation of pure water surface without the

of ACFC. It is an effective approach to suppress re-emission losses via spectral and angular selectivity design on the absorber surface,31,32 such as embedded into a reflective cavity with an aperture,33,34 covered with a Bragg filter,35 and a design of asymmetrical nanoscale emitters.36 The “Roman column” structure on ACFC is a such typical surface patterning to enhance light trapping.34 Consequently, ACFC exhibits an outstanding solar−thermal transformation property as verified by the infrared thermal imaging in Figure 3e,f. The ACFC surface heats up to 56 and 113 °C after 30 min illumination under 1 sun (qi = 1 kW m−2) and 3 sun (qi = 3 kW m−2), respectively. The high thermostability of ACFC as displayed in Figure 2h enables it to be structurally stable under high heating temperature by high density of solar illumination, which is of great importance to guarantee the durable light−thermal conversion for long time operation or high light energy input. The decay of light−thermal conversion performance might be encountered on most of polymer-based materials due to the inevitable aging or even degradation. 2.3. Interfacial Solar Steam Generation System. ACFC with hierarchical structures has been demonstrated to be an excellent candidate as a solar absorber; thus, the solar steam conversion and desalination performance were evaluated via a solar steam generation device as designed in Figure 1a. Polystyrene (PS) foam with two slots was plugged at the top of the container, acting as thermal insulation layer and support for 4356

DOI: 10.1021/acsaem.9b00562 ACS Appl. Energy Mater. 2019, 2, 4354−4361

Article

ACS Applied Energy Materials

Figure 4. (a) Mass change of salt water in the solar steam generation device under 1 sun at room temperature of 21 °C and humidity of 40%. (b) Evaporation rate and conversion efficiency of ACFC with different layers of CFNF. (c) Water mass change of ACFC+4 under different optical concentrations (Copt = 1, 2, 3, 4, and 5) at room temperature of 21 °C and humidity of 40%. (d) Calculated evaporation rate and conversion efficiency of ACFC+4 under different optical concentrations. (e) Photograph of a visible steam flow generated under 3 sun illumination. (f) ACFC solar steam production comparison with previous reports, and the detailed data are listed in Table S2.

2.4. Solar Steam Generation Performance. The mass change of the water evaporation was recorded under 1 sun illumination at room temperature of 21 °C and humidity of 40% (Figure 4a), and ACFC reveals a significant promotion for water evaporation with 1.25 kg m−2 h−1 of evaporation rate, which is 3.9 times faster than that of pure water (Figure 4b). Besides the high light−thermal conversion property, a porous weaving structure is another obvious advantage of ACFC contributing to its high water evaporation rate, as the abundant porous structure can facilitate the steam escaping from the light−thermal interface.26,38 In Figure 4a, it is intriguing that the underlying CFNF layer (refers to as ACFC+i, i represents the number of CFNF layers) has a positive impact on the interfacial steam generation, showing a higher mass loss in the mass change−time curves. The evaporation rates increase to 1.45 and 1.59 kg m−2 h−1 when 2 and 4 layers of CFNF mat underneath ACFC, respectively, but slight reduction (1.38 kg m−2 h−1) occurs in the case of six layers of CFNF. It is thus clear that the matching of the water supplying rate and evaporation rate is of great importance for full thermal utilization at the interface to achieve optimal evaporation rate,

solar absorber. We can get a direct observation of the IR images in Figure 3g,h. However, such an equilibrium temperature is lower than that of 56 °C in Figure 3e because partial sensible heat on ACFC is transformed into latent heat for the surface water evaporation, and also a portion of thermal loss might occur in the water path side as both water and ACFC are thermally conductive. On the other hand, the selfpump water or superior wettability property of ACFC renders its lower reflection (