Graphene–Carbon Composites for Solar and Low‐Voltage Powered Efficient Interfacial Evaporation

Nanocarbon materials have great potential for sustainable energy harvest and energy utilizations, such as solar thermal stream generation, and interfacial evaporation. However, the evaporation rate is far too low for practical applications. The technologies are not ready yet for industries requiring rapid, energy‐efficient, and low‐cost evaporation processes such as distillation and sterilization. A flexible ultrathin graphene–carbon cloth (CC)‐based carbon–carbon composites is prepared by in situ electrochemical reduction of graphene oxide. The carbon–carbon composite materials demonstrate high performance in photothermal evaporation; more importantly, all carbon‐based devices can also be operated as low‐voltage Joule heating elements in wide temperature range up to 389 °C. Even when a very low voltage of 3 V is applied, the graphene–CC heater can reach a very high heating speed up to 112 °C s–1, and a steady‐state temperature up to 292 °C within 10 s only. The low‐voltage heater promises to be the most effective solution for high‐speed interfacial evaporation since its evaporation rate can reach up to 45.87 kg m−2 h−1, enhanced by one order of magnitude compared to the best solar power photothermal seawater desalination devices ever reported.


Introduction
In recent years, nanocarbon materials have shown attractive ability in the fields of photothermal utilization and interface evaporation due to their wide spectral absorption capacity and www.advsustainsys.com form of carbon materials or relying on a single function. The electric energy is a stable energy source, which can be integrated with a wide range of power grid and infrastructures, and electricity can enable new operations with a variety of physical principles and electronics, such as magnetic, microwave, and sensors, toward smart energy harvest and storage systems.
In the conventional evaporation and distillation process, the solution is often heated to boiling point, which is energy intensive but not optimized or efficient due to the related limited fixed size of interface area and a low amount of liquid being treated. The use of micro-nanostructures to achieve interface evaporation can effectively reduce the energy consumption and is affected by the volume of the solution being processed. [11] In addition, the surface properties of the nanomaterial, especially the hydrophobicity and hydrophilicity, have greater impacts on the interface evaporation. Hydrophilic materials can rapidly transfer solution to the hot zone by capillary, but the ideal evaporating layer acts as the zone of local limitation of heat and reaction for interfacial evaporation is necessary to avoid the adsorption of excess water and cause energy loss. [26] In order to achieve higher rate of interface evaporation, in this study, for the first time, we proposed and successfully demonstrated the graphene-carbon composite materials as both high-rate solar evaporation device and a novel, low-voltage-driven electrical heater as the energy source of steam generation and distillation.

Results and Discussion
Flexible reduced GO composited with carbon cloth (rGO-CC), prepared by electrochemical deposition and in situ Joule heat reduction induced by electrical field, has been successfully used as solar and electric heaters for pure water production by efficient evaporation powered by clean energy. The proper-ties of materials, the impact of hydrophilicity/hydrophobicity, the electrothermal performances, and the water evaporation efficiency were studied thoroughly.
In the electrochemical deposition process, as shown in Figure 1a, the GO was dispersed in deionized water and then stirred uniformly to obtain an aqueous solution with the concentration of 0.5 mg mL −1 . A piece of CC and a platinum plate were used as the positive and the negative electrodes, respectively. These two electrodes were kept facing each other maintaining about 2 cm space. Figure 1b shows a schematic diagram of the fabrication procedures. The GO-CC sample was obtained by the electrochemical deposition with the applied voltage of 5 V for 4 h. After that, the sample was dried overnight in a vacuum oven at room temperature. After that, the reduction of GO was achieved by Joule heating with the power generated by applying a 3 V voltage to the GO-CC sample for 5 min under Ar atmosphere. The image of the obtained rGO-CC sample is shown in Figure S1, Supporting Information. A schematic diagram of the efficient water evaporation device using the composite material is shown in Figure 1c. The polystyrene foam is used as the floating and supporting substrate, and 2D flexible porous hydrophilic paper served as the water-guiding media, which is responsible for transferring the bottom salt water to the upper layer. The porous paper is in intimate contact with the top rGO-CC layer by physical methods. A low voltage is applied on the rGO-CC to obtain a high temperature in the open air environment, and the water conducted to the upper portion can be rapidly heated and converted into vapor. This composite material has excellent thermal conductivity, electrical conductivity, and flexibility. As shown in Figure 1d-g, the rGO-CC exhibits good electrothermal performances at low voltages. High temperature, homogeneity, and large-area thermal field can be achieved in both straight and curved configurations.

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The surface morphologies of CC, GO-CC, and rGO-CC are exhibited in Figure 2 and Figure S2, Supporting Information. The clean fibers of the CC with diameter of about 10 µm can be clearly identified in Figure 2a. The GO surface on CC is relatively smooth and flat with slight thin curls at the edges, which confirms the small number of GO layers (Figure 2b,c). However, in Figure 2d,e, it can be found that the rGO reduced by Joule heat has been tightly coated on the carbon fibers, showing obvious folds at the edges and lots of wrinkles in the middle plate. The rGO coated on CC behaves just like pieces of paper on 1D column arrays and most of them show translucent features at the end of the edges (Figure 2f). In addition, the compositing structures have extra spaces filled by air due to the gap between the 2D graphene and the column-shaped carbon fibers (Figure 2e; Figure S2, Supporting Information). Since the oxygen-containing functional groups of the GO were reduced during Joule heating, the rGO sheets agglomerated and overlapped with each other. Due to the high temperature of the Joule heating process and the rapid heating rate, a lot of cracks formed in the rGO. When the voltage was applied, the temperature increased rapidly, and the thermal energy generated can destroy carbon-oxygen bonds forming CO or CO 2 . [27,28] The rapid release of the gases caused the expansion of the graphene layers in the vertical direction, and the rGO became thicker and denser compared with the GO. The X-ray diffraction (XRD) patterns of the CC, GO-CC, and rGO-CC materials are illustrated in Figure 3a and Figure S3, Supporting Information. It can be seen that compared to the CC, the XRD peak of the GO-CC obtained by electrochemical deposition of GO on CC and the XRD peak of rGO-CC reduced by Joule heating have not changed much, and all of them have the same strong diffraction peaks at around 2θ = 26° corresponding to the (002) reflection of carbon corresponding to the stacking structures of aromatic layers. In the XRD pattern of GO-CC, a weak diffraction peak of graphite oxide (001) can be detected.
The Raman spectrum of these carbon composites is shown in Figure 3b; the D peak at 1344 cm −1 and the G peak at 1588/1575 cm −1 are conspicuous in all samples. The number of defects and the degree of disorder are direct evidences of the degree of graphitization, which can be easily evaluated by the intensity ratio of I D /I G . [25] This ratio is the lowest in CC, indicating that CC has the highest degree of graphitization. As the GO was deposited on the CC with the electrochemical method, then in the GO-CC composite it was reduced by the electrical thermal heating, the ratio gradually increased, indicating that the defects and structure disorder were obviously increased and the thermal reduction induced plenty of defects after partial removal of functional groups. [29] In order to analyze the elemental valence states of the above three materials, X-ray photoelectron spectroscopy (XPS) was used to analyze the samples. The contents of oxygen in CC, GO-CC, and rGO-CC were 3.6, 30.6, and 16 at%, respectively. Simultaneously, the oxygen content of the CC is very low; therefore, the atomic ratio of carbon to oxygen (C/O) is also very large (C/O = 35.9). The prepared GO-CC contains a large amount of oxygen-containing functional groups with a C/O ratio of 2.7. After the heat reduction, the oxygen-containing functional groups were partially removed, and the O 1s peak of the rGO-CC decreased and the C/O ratio increased to 6.57. The fitting peaks of C 1s from XPS spectra show that the presence of CC/CC (284.8 eV), CO (285.6 eV), and OCO (287.8 eV) functional groups is about 87.5%, 9.6%, and 2.9%, respectively (Figure 3d; Figure S4a,b, Supporting Information). The hydrophilicity of the three materials was tested. As shown in Figure 3e-g, the contact angles of the CC, GO-CC, and rGO-CC were 124°, 55°, and 81°, respectively, indicating that the hydrophobicity of CC and GO-CC is the best.
The evaporation rate of photothermal devices under normal solar irradiation (1 kW m −2 ) is a key indicator and figure of merit for applications. Hydrophobic CC and GO-CC prepared by electrodeposition and rGO-CC obtained by in situ reduction of Joule heat were used to test the effect of photothermal evaporation, as shown in    Under the illumination of 1 kW m −2 , as shown in Figure 4b, the steady-state evaporation rate of rGO-CC reached 2.54 kg m −2 h −1 , which is five times higher than that of natural evaporation (0.51 kg m −2 h −1 ). The evaporation rate of hydrophobic CC is 2.4 kg m −2 h −1 , which is better than GO-CC (2.34 kg m −2 h −1 ). This should be due to the fact that GO-CC contains a large amount of oxygen-containing functional groups and is hydrophilic (Figure 3c-g), which causes it to absorb more aqueous solution and lose much heat. The hydrophobic CC absorbs sunlight and converts it into heat, and the porous cloth that is in close contact with the bottom of CC is heated to evaporation. The macroscopic pores of CC are large and it is favorable for the escape of water vapor; therefore, the evaporation rate is high. The temperature change can also be found during the evaporation recorded by the infrared camera in real time. The average surface temperature of the hydrophilic GO-CC device is the lowest (about 38.5 °C). Due to the strong solar absorption, the rGO-CC with less hydrophilicity has an average surface temperature of 41 °C (Figure 4c). We tested the stability of these devices and performed multiple cycles, each for more than 1 h. The average evaporation rate is shown in Figure 4d. It can be seen that the evaporation rate after long-term operation is relatively stable.
The electrothermal performance of the flexible rGO-CC heating elements was studied carefully as shown in Figure 5. After the power (3 V) was turned on, the temperature of the flexible heaters (CC, GO-CC, and rGO-CC) increased rapidly over time and then reached the steady-state temperature of 269, 150, and 298 °C, respectively, as shown in Figure 5a. When the power was disconnected, the temperature dropped rapidly to room temperature. Among them, GO-CC and rGO-CC reached the steady state within 10 s, while the CC heater can achieve a high temperature around 10 s, but the temperature was still in a slow rising state before the power was turned off. According  www.advsustainsys.com to the formula P = U 2 /R (where P is the power, U is the applied voltage, and R is the resistance), the rGO-CC reached the highest temperature at voltage of 3 V that can be attributed to its lowest resistance. The temperature versus time curves of rGO-CC at different applied voltages ranged from 0.5 to 3 V as shown in Figure 5b, and the temperature reached the steady state of 30.9, 56.7, 107.9, 157.3, 226.4, and 299.6 °C within about 10 s, respectively. As the applied voltages on the same piece of rGO increased, the resistance gradually decreased and the power increased exponentially (Figure 5c). GO is a layered compound with large interlayer distances obtained by deep oxidation of graphite in the liquid phase. There are a large number of polar groups in GO-CC. With the Joule heating, the interlayer water is desorbed and the oxygen-containing group is decomposed. With the rise in heating temperature, the interlayer distance decreases rapidly, and the electrical conductivity gradually increases. However, the reduction by the Joule thermal has not yet reached the graphite-like state due to the low temperature. As the temperature increases, the rGO-CC will still translate into the graphite-like state, and the electrical conductivity also increases. Both the temperature and the heating rate rose linearly when the applied heating power was increased, as shown in Figure 5d. The temperature reached 389 °C and the heating rate achieved as high as 82 °C s -1 , respectively.
The cycle stability at low voltage (3 V) of the rGO-CC is shown in Figure 5e, and the performances are very stable. The temperature increased rapidly and stabilized after the voltage was applied, and then dropped quickly to room temperature when the power was turned off, indicating that the electrical properties of the flexible heater are relatively stable especially during the application of voltage and heat generation. Although the temperature that the heater can reach is directly determined by the input power rather than the resistance, the input voltage of the heater with low resistance is relatively small when the same power is applied, which is very important for practical applications. The low voltage can be achieved with simple equipment and at high level of safety, so the applications will be wider. In addition, the thermal conductivity of the rGO-CC is excellent; therefore, the response time of the temperature is very short. The high steadystate temperature and the fast response time are both very important for heaters, allowing the heaters to extend their applications enormously. The steady-state temperature (389 °C) and fast response time (10 s) of the rGO-CC flexible heater were achieved at a low driving voltage (3.5 V), as shown in Figure 5f. With the help of Joule heating elements CC (hydrophilic/hydrophobic), GO-CC, and rGO-CC, steam and fresh water can be efficiently produced from the saline water with low voltage power sources.  (Figure 6b). The GO-CC and hydrophilic CC showed a similar performance by reaching the evaporation rate of 26.79 and 25.04 kg m −2 h −1 , respectively. Although the dried rGO-CC exhibited higher temperature performance at the same voltage due to the decrease in electrical resistance, it still has similar water evaporation performances to the GO-CC and hydrophilic CC, as it contained a large amount of hydrophilic chemical functional groups. Comparing the evaporation rates at different voltages, the water evaporation rate of the four heaters at 1 and 2 V is shown in Figure 6c. However, GO-CC, rGO-CC, and hydrophilically modified CC containing lots of polar chemical functional groups can absorb much water thus the evaporation rate is greatly limited at 3 V. The infrared images of the rGO-CC device, as shown in Figure 6g-j, indicate the evolution of the energy distribution. Due to the difference of the hydrophilicity, even at the same applied voltage, the average surface temperature and heat distribution of these three devices were different, as shown in Figures S6 and S7, Supporting Information. The hydrophobic CC can maintain direct connection with the thin water layer transferred by the porous paper and keep itself be relatively dry rather than retain much water, so the water molecules at the surface can be heated and evaporate rapidly under high temperature.
The ratio of evaporation rate to the applied power is also compared with different heaters and different voltages, as shown in Figure 6d. The GO-CC heater was relatively stable, and the ratio increases linearly with voltage increase. The ratio of the hydrophilic modified CC was lower than other heaters. The rGO-CC and hydrophobic CC were relatively constant at 1 and 2 V, while the gap becomes larger at 3 V with the performance of CC 4.68 and rGO-CC 4.17 kg m −2 h −1 W −1 . At low voltage, the water evaporation rate is substantially linear with the input voltage, and the ratio of evaporation rate to input power of these heaters is also close except for hydrophilic CC. However, the water evaporation rate of hydrophobic CC is about 1.8 times higher than that of other heaters under 3 V. It indicates that the hydrophilicity increases the electrical resistance to a certain extent, resulting in a slight decrease of the input power, but more importantly, the hydrophilic heater has a strong ability to capture water molecules, which resulted in more water molecules being heated simultaneously and cannot easily escape. Since the hydrophobic heater maintains its own conductivity and the water molecules are easily heated and escape from the hot zone of the heater, the hydrophobic heaters can achieve faster water evaporation rates.
The energy conversion efficiency (η) is used as a measure to evaluate the electric energy-to-vapor efficiency. The formula is defined as where m is the mass loss rate per unit area calculated from the slop of mass loss curves at the steady state, h LV is the total enthalpy of sensible heat (315 J g −1 , from ≈25 to 100 °C with specific heat of water 4.2 J g −1 K −1 ) and liquid-vapor phase change (2256 J g −1 ). P input is the input power by Joule heating and t is the running time. As the voltage increases, the energy conversion efficiency gradually increases. The energy conversion efficiency of the hydrophobic CC of 3 V is 80.2% (Table S1, Supporting Information). We also tested different samples size (4 × 2 cm), as shown in the Figure S8, Supporting Information. After the sample was enlarged, the electrothermal conversion and the Joule heat evaporation performance slightly decreased, but basically maintained a high evaporation rate. We designed a device (Figure 6e) to achieve rapid evaporation of water with low voltage and to collect fresh water. Since www.advsustainsys.com the temperature was lower than 100 °C ( Figure S7, Supporting Information), only water molecules in the brine were heated to vapor, and most of the ions were still in the brine (Figure 6f). The effect of desalination was carefully examined, and the concentration of the primary ions was eliminated ( Figure S9, Supporting Information). After desalination, the calcium ions, potassium ions, magnesium ions, and sodium ions decreased from 9942, 1168, 8018, and 11210 mg L −1 to 1.2, 0.21, 8.1, and 8.5 mg L −1 , respectively.

Conclusion
To summarize, we have developed a new versatile, flexible device that combines both photothermal and low-voltage heating elements. The solar absorption rate can reach up to 93-97%, and the evaporation rate of rGO-CC can reach up to 2.54 kg m −2 h −1 under normal illumination of 1 kW m −2 . These heating elements achieved higher temperature at lower voltage inputs. Among them, the rGO-CC heater reached 309 °C at 3 V (389 °C at 3.5 V), and the temperature response time was only 10 s, indicating good electrical and thermal conductivity. These porous heaters also have good effect on water evaporation. Although the conductivity and heat transferability of the dried rGO-CC is higher, the hydrophilicity causes more immersion of water, which in turn changes the electrical conductivity of the whole device. The hydrophilic heaters can absorb and retain much more water, thus they have reduced evaporation rate as there are much more water molecules need to be heated simultaneously. The hydrophilic binding force of water molecules is large, which hinders the escape of water molecules. Hydrophilic materials have a stronger binding force on water molecules and also hinder the rate of water evaporation to some extent, while the hydrophobic CC heater can produce more heat at the same voltage. Therefore, in terms of electric heating, to achieve interfacial evaporation, the water evaporation rate of hydrophilic carbon composite rGO-CC and the hydrophobic carbon cloth can reach first-rank interfacial evaporation rate up to 27 and 45.87 kg m −2 h −1 , respectively. The evaporation devices with flexible porous conductive materials utilizing low-voltage Adv. Sustainable Syst. 2020, 4, 1900122 www.advsustainsys.com electricity can achieve rapid water evaporation which is one order of magnitude higher than the best solar powered seawater desalination devices ever reported. [26] Our new carboncarbon materials and devices promise a bright future as an emergency solution for the lack of clean drinking water sources in dry areas, the production of purified fresh water in extreme conditions, and the most reliable sources of water supply for the space explorations.

Experimental Section
Materials: Sodium chloride, potassium chloride, magnesium sulfate, calcium chloride, and graphite were provided by Sigma-Aldrich. The electrical resistivity of the deionized water was 18.2 MΩ·cm. The CC samples were purchased from Fuel Cell Earth.
Fabrication of GO-CC, rGO-CC, and Hydrophilic CC Materials: Typically, the GO were prepared by the modified Hummers method. [30] The GO-CC samples were prepared by electrophoretic deposition in 50 mL GO suspension with the concentration of 0.5 mg mL −1 at an applied voltage of 5 V for 4 h. The CC (2 × 3 cm) was used as the positive electrode. A piece of Pt and a saturated calomel electrode were used as the cathode and reference electrode, respectively. The distance between the two electrodes was 2 cm. Since the GO sheets have negative charge, they moved to the positive electrode under the influence of the voltage, and deposited onto the surface of CC. Then the GO-CC samples were dried overnight in an oven at 60 °C. The rGO-CC can be obtained by in situ heating under Ar atmosphere with voltage of 3 V applied on the dried GO-CC for 30 min. The hydrophobic CC was immersed in nitric acid for 1 h and then washed with deionized water, and the hydrophilic CC was obtained after drying.
Characterizations: The microstructures of these composites were characterized by a super-resolution field-emission scanning electron microscopy (JEOL JSM-7800F Prime). The surface properties of the composites were carried out on the X-ray photoelectron spectrometer (AXIS Ultra DLD, Kratos) and the confocal micro-Raman spectrometer (inVia Qontor). The properties of optical absorption and reflection of these composites were carried out on a Fourier transform infrared spectrometer (Nicolet 6700). The collection of the IR photos during the experiments was done using an IR camera (MAG32, Magnity Electronics, China). The concentration of ions was tested by inductively coupled plasma optical (iCAP6300, Thermo Scientific).
Evaporation Tests: A simulated salt water solution (Ca 2+ 9942 mg L −1 , K + 1168 mg L −1 , Mg 2+ 8018 mg L −1 , and Na + 11210 mg L −1 ) was prepared in a 100 mL plastic cup. One piece of porous paper covered a piece of polystyrene foam tightly. The heater (CC, GO-CC, or rGO-CC) was bound to the top surface of the flat paper. The whole heater device can float on the water. The gap between the device and the cup was carefully closed to avoid interference. The cup together with the heater device was placed on an electronic balance, and the data of the real-time weight loss were obtained through a computer connected with the balance.
Photothermal Power Source: The simulated solar light with diameter of 10 cm was supplied by a solar simulator (Oriel 94043 AAA, Newport). The light intensity was measured and calibrated using an optical power meter (S310C, Thorlabs, Inc.).
Electrothermal Power Source: The electrical power was supplied by a DC power supply (KORAD, KA3005D/P). When using Joule heat to supply energy to realize the interfacial evaporation, both ends of the heater were adhered to the titanium foil with silver conductive glue and connected to the copper wires.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.