Micro capillaries lotus fabric based aerogel for high efficiency solar steam evaporator and water purification

Micro capillaries lotus fabric based aerogel for high efficiency solar steam evaporator and water purification

INTRODUCTION OF SOLAR STEAM GENERATION

The importance of creating freshwater

Figure 1.1 Projected water stress in 2040[1]

Water stress is considered as one of the major global problems According to the United Nations, a region is considered "water-stressed" when it withdraws 25 percent or more of its renewable freshwater resources[2] In 2018, only 18.4% of the world's total renewable freshwater resources were extracted However, there are already regions experiencing serious problems Northern Africa has critical levels of water stress, whereas Central and Southern Asia have high water stress On the opposite end of the

2 spectrum, 31% of the global population remained in the "no stress" category According to the World Resources Institute's projections for 2040, the problem will only become more widespread According to a report by the Economist Intelligence Unit, urbanization, population growth, climate change, and economic growth are exerting pressure on water systems[3] According to the projections, 44 nations will experience

"extremely high" or "high" water stress by 2040 According to a report by The Economist, a growing number of regions, particularly in East and South-East Asia, are at an increased risk of flooding, which can overwhelm sanitation systems and contaminate potable water sources[3]

In order to safeguard the well-being of communities in affected areas, achieve the UN Sustainable Development Goal 6 on clean water and sanitation, and protect the environment and biodiversity, it is crucial to address water scarcity While human activities have contributed to the water crisis, humans have also developed technologies to improve freshwater acquisition and conservation

Various remedies for water scarcity have been developed, including the construction of dams and reservoirs, the practice of rainwater harvesting, the establishment of aqueducts, the implementation of desalination processes, water reuse systems, and water conservation measures Over the years, numerous scientists and organizations have dedicated their efforts to advancing seawater desalination technology with the aim of enhancing efficiency and reducing energy consumption

By continually improving desalination techniques, it becomes possible to alleviate water scarcity, enhance access to clean water, and mitigate the impact of the water crisis on both humans and the environment These efforts align with the objectives of sustainable development and contribute to a more sustainable future for all.

Desalination of seawater

Figure 1.2 Reverse osmosis is one method of desalination[4]

Desalination is a method which involves removing dissolved salt and minerals from seawater or saline groundwater This method has the benefit of a practically unlimited supply of saltwater There are numerous methods for desalinating seawater, such as boiling, filtration, electrodialysis (using an electric current to remove the ions that make up salts), and reverse osmosis (Figure 1.2) All of these processes are moderately to extremely expensive and require substantial energy input, making the produced water significantly more expensive than that from conventional sources Additionally, the process generates highly saline wastewater that must be disposed of and has a significant impact on the environment Therefore, affordable desalination technologies that utilize renewable energy sources must be developed to address the water shortage and ensure a sustainable future for future generations Research and development of desalination technologies based on renewable energy sources, such as solar, wave, wind, and geothermal energy, are depicted in Figure 1.3

Figure 1.3 The status of the renewable energy operated desalination technologies[5]

Solar Still, PV RO, Solar thermal MED, and Wind RO have reached the application stage among the technologies based on renewable energy Solar evaporation: Solar Still and Solar thermal MED transform solar energy into heat in order to evaporate water Per cubic meter of desalinated water, their operating costs range between 1.3 and 6.5 dollars and 1.0 and 7.3 dollars Consequently, these technologies have the potential to be used as inexpensive desalination technologies in countries with a high number of sunlight hours, such as Vietnam, Bangladesh, etc The performance of solar steam generator technologies is primarily determined by their solar energy conversion efficiency By creating a new photothermal material with increased solar energy conversion efficiency, the efficiency of solar evaporation could be improved Consequently, the objective of my thesis is to produce a new photothermal material with a high solar energy conversion efficiency so as to fully realize the potential of a solar steam generator for desalination In the following section, the working principle of a solar steam generator will be discussed

Solar steam generation (SSG)

Solar irradiation is a promising renewable energy source because the hourly solar flux incident on the earth's surface exceeds the annual global energy demand[6] The earth receives 1361 W/m2 of solar radiation annually, of which 70% is absorbed and the remainder is either reflected or scattered[7] As a tropical nation, Vietnam has a large number of sunshine hours (2000 – 6000 hours/year, or 6 – 7 hours/day) and an average annual solar irradiance of 5 kW/h/m2, creating favorable conditions for the implementation of solar energy-related technologies Efficient solar energy harvesting for steam generation is crucial for a variety of applications, ranging from large-scale power generation, absorption chillers, and desalination systems to compact applications such as drinking water purification, sterilization, and hygiene systems in remote areas where the sun is the only abundant energy source Solar steam generation for seawater desalination has recently attracted a great deal of interest due to its many advantages, such as its use of renewable energy, absence of greenhouse gas emissions, and straightforward fabrication procedure An optimized Solar steam generator could produce 15 to 30 liters of desalinated water per hour given the abundance of sunlight[8]

Figure 1.4 Three components of solar steam generator[9]

Solar steam generation (SSG) is a technology that uses the photothermal effect to produce freshwater from salt water[10] The SSG is utilized in numerous fields, including seawater desalination, waste water purification, thermoelectric systems, etc Three essential components comprise a solar steam generation system: Solar absorber (made of photothermal material), (2) substrates for transporting water to the solar absorber, and (3) water collector for collecting distillate water[11] The SSG system is based on the solar-to-heat conversion process In this process, photothermal material converts light energy into thermal energy by absorbing photons With a sufficient amount of heat, water in container will be evaporated into vapor condensed in the water collector

Several factors influence the performance of a Solar steam generator system: the rate of water supply, the light absorption efficiency of the light absorber, and the thermal conductivity of the photothermal material, which influences heat localization and heat loss minimization Solar steam generator performance relies heavily on the design of photothermal material Since photothermal materials generate heat from solar energy without emitting CO2, it is anticipated that the SSG system will solve the lack of clean water by reducing global warming Numerous research groups have developed photothermal materials for SSG systems in order to maximize the yield of freshwater production In the following section, different types of photothermal materials will be discussed.

Types of photothermal materials

Photothermal materials are materials that can absorb and convert light energy into heat efficiently[12] Photothermal materials can be generally classified into four main subgroups: 1) metallic nanostructures, 2) inorganic semiconductor materials, 3) polymeric materials and 4) carbon-based light absorbing materials [13]

Metal nanostructures are one of the most widely studied materials for photothermal energy conversion due to surface plasmon resonance (SPR) effects[13;14] This effect can significantly amplify light absorption, contributing to a rapid light-to- heat conversion and a heightened photothermal effect.[13] The development of metallic

7 nanostructures materials with high light absorption from Au, Ag, CuS, Cu, Al, and Pd nanoparticles has been the subject of extensive research

The structural stability and recyclability of materials with metallic nanostructures are two of their many benefits Recyclability of Au nanoparticles is 98% [13], and plasmonic Pd nanoparticles exhibit outstanding stability after cycling for 144 hours in solar steam generation Because the nanoparticles can be synthesized using wet chemical methods, metallic nanostructures materials could be produced in large quantities While they show promising results in photothermal conversion, their high cost of raw material prevents mass production, restricting their usefulness

Inorganic semiconductor materials, in contrast to metallic materials, feature a band gap between their valence and conduction bands, allowing for novel optical energy applications The energy bandgap and light-absorbing capacity of nanostructures can be modulated by adjusting the size, structure, and composition of the semiconductors to change the position of the valence band (VB) and/or conduction band (CB)[16]

Numerous studies have been conducted on solar photovoltaic and photocatalytic semiconductor applications However, the observed solar cell efficiency falls far short of the thermodynamic limits of photovoltaic energy conversion This loss of thermodynamic efficiency is caused by the recombination of the free hot carriers, which heats the crystal lattice structure of the semiconductor material This phenomenon is known as local carrier thermochemistry[17]

A number of organic compounds, typically conjugated polymers, have been proposed and developed as solar collectors due to their advantageous chemical action, low cost, light weight, and ease of handling[13] For instance, natural organic materials are also suitable as photothermal materials Compared to metallic nanostructures materials and semiconductor photothermal materials, natural polymeric based photothermal have higher stability and simpler fabrication method[18]

Aerogels are a type of synthetic porous ultralight material derived from a gel in which the liquid component has been replaced with a gas without the gel structure collapsing significantly Recently, there has been a growing interest in using aerogels as substrate materials for the development of polymeric photothermal materials This is due to several favorable properties associated with aerogels, including:

 3D interconnected porous structure: Aerogels possess a unique 3D porous structure with interconnected pores, which provides a large specific surface area This structure enhances the material's water transportation ability, allowing efficient movement of water molecules within the aerogel

 High porosity and low density: Aerogels exhibit high porosity, meaning they have a significant volume of empty space within their structure This results in a low density, making aerogels lightweight materials The high porosity and low density of aerogels contribute to their low thermal conductivity, minimizing heat transfer and reducing energy loss

 Simple fabrication method: The fabrication process of aerogels typically involves a freeze-drying technique, which is relatively simple and widely used

 Numerous hydroxyl groups: Aerogels contain numerous hydroxyl (-OH) groups on their surface These hydroxyl groups have a hydrophilic nature, enabling the formation of hydrogen bonds with water molecules This interaction helps reduce the enthalpy of water evaporation, allowing water to evaporate more easily from the aerogel's surface

The combination of these properties makes aerogels promising substrates for the development of polymeric photothermal materials By utilizing aerogels as a base, researchers can enhance the water transportation ability, improve thermal insulation, and optimize the evaporation performance of photothermal materials

Aerogels are divided into three groups depending on their precursor:

 Organic Aerogel (Made from precursors such as: Polyurethane, resorcinolformaldehyde, Polystyrene, polyimide, cellulose[19]

 Inorganic Aerogel (Made from SiO2 or various types of alkoxysilanes: Al2O3, TiO2, ZrO2 , SiC, v.v.)[19]

 Carbon Aerogel (Made from carbon nano tube, carbon and graphene)[19]

Almost all the types of aerogels are synthesized using traditional sol-gel chemistry A synthetic method for making aerogels usually involves three steps: A synthetic procedure for preparation of aerogels consist mainly three steps: (1) Formation of sol-gels; (2) Aging of the sol-gels; and (3) Drying of the sol-gels to form aerogels

[20] In the sol-gel formation process, All of the precursors are distributed uniformly throughout the reagent mixture after being mixed together in the proper proportions Next, sol-gel is then formed at low temperatures in the reagent mixture, either through reaction[20] or rearrangement of solid phase Finally, the aerogel structure is obtained by freezedrying the gel below the triple point of water to remove the water For example, nanocellulose has been successfully fabricated from a variety of natural ingredients for use in oil spill removal, and aerogel has been successfully fabricated from rice straw

[21], pineapple leaves [22], and sugarcane bagasse [23]

Aerogel-based photothermal materials have shown promising potential in various applications, such as thermochemical reactions, electrostatic lithography, catalysis, light-driven actuation, sensing, photothermal electrodes and especially solar energy conversion[54] However, there are several challenges and limitations associated with current photothermal materials that need to be addressed to move towards practical applications: The primary connection found in natural aerogel materials is the weak hydrogen bond, which means that these materials are not currently suitable for practical applications Additionally, expensive additives like Polypyrrole and Carbon nanotubes (CNF) are used to enhance the material's sunlight absorption, which further raises the production costs Consequently, enhancing the durability of aerogels derived from natural materials and reducing the associated expenses remain significant challenges for their practical utilization.

Justify the selection of research material and method

In Section 1.4, numerous types of photothermal materials are introduced Metallic nanoparticles and semiconductors are characterized by a complicated fabrication procedure and a high fabrication cost In addition to the previously mentioned advantages, aerogel-based photothermal materials offer additional benefits

10 such as low cost, a simple manufacturing method, and the ability to make use of agricultural waste Therefore, this thesis focuses on the fabrication of cellulose-based aerogel photothermal material using natural fiber and PVA The chosen research orientation holds significant potential for applications in solar energy utilization and water treatment in practical application

Lotus fiber is a type of organic fiber derived from the stems of the lotus plant, a widely prevalent aquatic plant in Vietnam's ponds and lakes When lotus plants are harvested by farmers, they are often left with a substantial quantity of stem remnants that no longer possess visual appeal and contribute to agricultural waste However, these discarded lotus stems can be repurposed to create lotus silk, a sustainable and environmentally friendly natural fiber Lotus fiber boasts a multitude of exceptional qualities including its lightweight nature, impressive strength, effective insulation properties, and remarkable lightness[24] At normal temperature and in a dry state, lotus fiber exhibits superior fiber strength compared to both cotton and viscose fibers[25].

Table 1.1 The tensile properties of cotton, viscose and lotus fiber in normal temperature state, wet state, and dry state[25]

According to Table 1.1, lotus fiber exhibits significantly higher tenacity compared to viscose and cotton fiber This suggests that incorporating lotus fiber as a reinforcing material in aerogel-based photothermal material can greatly enhance its strength when compared to natural fibers By utilizing a composite structure with lotus fiber and PVA, the resulting material can benefit from the superior mechanical

11 properties of lotus fiber This reinforcement mechanism has the potential to improve the overall strength and durability of the aerogel-based photothermal material

Figure 1.5 SEM image of one spiral lotus fiber bundle with conglutination of several lotus fibers[52]

Figure 1.5 illustrates that lotus fibers are composed of multiple smaller fibers interconnected This unique structure offers improved distribution of lotus fibers within the material When these fibers are separated, it enhances the overall distribution of fibers throughout the aerogel-based photothermal material Consequently, this improved distribution facilitates enhanced water conductivity The material benefits from dual water transport routes: (1) through the porous structure of the aerogel material itself and

(2) via the lotus fibers As a result, the water transport capability of the material can be increased, allowing for efficient movement of water through both pathways

Presently, there is a lack of research groups in Vietnam focused on the development of aerogel based photothermal materials using lotus fibers Therefore, the advantages mentioned earlier make lotus fiber an ideal choice as the primary component for producing photothermal materials in cellulose-based aerogels.

Cellulose based aerogel fabrication procedure

Figure 1.6 Image of lotus fiber[26]

The thesis involves the fabrication of aerogels using lotus fiber as the primary material The weight composition of lotus fiber is as follows:

- Cellulose: polysaccharide of β-glucose, connected by 1,4-glicozit bonding with a molecular weight of 10.000 – 150.000, account for 74% of the dry weight[27]

- Hemicellulose: polysaccharide of glucose and other monomers rather than glucose such as arabinoxylan, glucomannan, and xyloglucan and etc, account for 1.1% of the dry weight[27]

- Lignin: natural polymer made of phenylpropane, act as connection between cellulose and hemicellulose, account for 8.1% of the dry weight[27]

- Other soluble substances(wax, ash, pectin, impurities), account for 16.8 % of the dry weight[27]

The lotus fiber consists of various substances, including hemicellulose and lignin, which are non-crystalline in nature Cellulose, on the other hand, exists in both crystalline and non-crystalline forms To extract cellulose from impurities, a selective hydrolysis process is employed, wherein non-crystalline substances (lignin, hemicellulose, and non-crystalline cellulose) are separated from cellulose due to their higher susceptibility to hydrolyzing agents like NaOH and NaClO Once the cellulose is extracted, it is dispersed in distilled water to form a suspension The suspension is then subjected to freeze-drying, resulting in the formation of cellulose aerogel However, pure cellulose-based aerogel typically exhibits poor mechanical properties and easily

13 disintegrates when exposed to water To overcome this limitation, crosslinkers such as PVA, PVP, and BTMSE are incorporated during the aerogel fabrication process to enhance its mechanical properties[21-23] In the thesis, PVA was chosen as the crosslinker due to its affordability and wide availability.

Purposes of the thesis

The thesis focuses on the production of a PVA composite cellulose-based aerogel using lotus fiber for its potential application in seawater solar desalination To enhance the material's light absorption capacity and mechanical properties, tannic acid and Fe 3+ salts were incorporated into the cellulose-PVA suspension The fabricated material was subjected to characterization using various methods, including SEM (Scanning Electron Microscopy), BET (Brunauer-Emmett-Teller analysis), FT-IR (Fourier Transform Infrared spectroscopy), and XRD (X-ray Diffraction) Furthermore, the evaporation performance of the material was evaluated through experimentation in both controlled laboratory conditions and real-world scenarios

The main objectives of the master's thesis are as follows:

- To produce aerogel by utilizing lotus fiber, PVA, tannic acid, and Fe (III) as the primary components

- To conduct a comprehensive characterization of the material's properties through various methods, including SEM imaging, BET analysis, XRD examination, UV- VIS spectroscopy, FT-IR spectroscopy, and contact angle measurements

- To investigate the thermal conversion properties of the material and explore its potential application in the SSG (solar steam generation) system

- To assess the seawater desalination capabilities of the material under both indoor and outdoor conditions

EXPERIMENTS

Fabrication of photothermal materials

- Tannic acid (C76H52O46, powder), FeCl3 (powder) Ethanol, NaOH, NaClO, PVA were purchased from Xilong Scientific Co., Ltd, China

- Ultra-pure water, which is obtained from Water distill D4000 (VJU), was used to prepare solutions

In the thesis, lotus fiber was chosen as the primary component for synthesizing photothermal material within a carbon-based aerogel The separation of lotus fiber from the lotus stem was performed by artisan Pham Thi Thuan Figure 2.1 illustrates the fabrication procedure for cellulose-based aerogel, which includes the production of both white and black aerogel The fabrication process consists of three steps: 1 Extraction of cellulose, 2 Preparation of cellulose suspension, and 3 Preparation of cellulose-based aerogel from the suspension The cellulose extraction (Step 1) and aerogel preparation (Step 3) were identical for both black and white aerogel However, there was a slight difference in Step 2, the preparation of cellulose suspension, for black aerogel, as it involved the use of tannic acid and Fe 3+ for its fabrication

Figure 2.1 Fabrication process of cellulose – based aerogel (including white and black aerogel)[19]

Cellulose extraction from lotus fiber, which includes cellulose and various impurities like lignin, hemicellulose, wax, protein, and sugar, was performed using a chemical process with the following procedures:

Step 1: Extraction of cellulose (for both black and white aerogel)

1 The lotus fiber is immersed in a solution of Ethanol:H2O (1:1) and kept at 80°C for 3 hours It is then washed using a vacuum filter

2 The product from Step 1.1 is immersed in a solution of NaOH (2M) and kept at 80°C for 3 hours It is washed with a vacuum filter until the pH reaches 7

3 The product from Step 12 is immersed in a solution of NaOH (1%) and NaClO (1%) and kept at 80°C for 2 hours It is washed with a vacuum filter until the pH reaches 7

Step 2.1: Preparation of suspension of white aerogel

1 The extracted cellulose is mixed with distilled water (M1)

2 PVA is dissolved in distilled water to obtain a PVA solution (M2)

3 M1 is slowly added to M2 under vigorous stirring until the substances are evenly distributed in the mixture

4 The mixture is sonicated for 10 minutes at 200 W to obtain the final suspension

Step 2.2: Preparation of suspension of black aerogel

1 The extracted cellulose is mixed with a tannic acid solution for 1 hour (M1) The concentrations of tannic acid used are 3, 5, and 10 mg/ml

2 PVA and FeCl3 are dissolved in distilled water to obtain a PVA + Fe 3+ solution The concentrations of FeCl3 and PVA are determined as per Table 2.1 (M2)

3 M1 is slowly added to M2 under vigorous stirring until the substances are evenly distributed in the mixture

4 The mixture is sonicated for 10 minutes at 200 W to obtain the final suspension

Step 3: Preparation of aerogel from suspension (for both white and black aerogel)

1 The suspension is placed in an ultra-low temperature freezer and kept at -70°C for at least 6 hours

2 The frozen suspension is then freeze-dried for 3 days to obtain the final aerogel.

Figure 2.2 a) White cellulose-based aerogel, b) Black cellulose-based aerogel

In the study, one sample of white aerogel, labeled as CC1, and three samples of black aerogel, denoted as CC2, CC3, and CC4, were fabricated The white aerogel was a cellulose-PVA composite aerogel, where PVA served as the crosslinker between cellulose chains, thereby enhancing the mechanical properties of the material On the other hand, the black aerogel was composed of cellulose, PVA, and tannic acid Tannic acid formed complexes with both itself (tannic acid-Tannic acid complexes) and with other components within the aerogel structure The tannic acid could be crosslinked

17 either with cellulose or with PVA, contributing to the overall properties of the black aerogel The composition of all aerogel samples are shown in Table 2.1:

Table 2.1 Composition of white and black aerogel samples

Concentration of substances in the final suspension Cellulose

Characterization of photothermal materials

In order to gain insights into the cellulose-based aerogel, its morphology, surface structure, light absorbance, crystal structure, specific surface area, elemental composition, and mechanical properties were thoroughly characterized These investigations aimed to provide a comprehensive understanding of the aerogel's physical and chemical properties, as well as its potential applications Table 2.1 shows list of equipment that were used for the aerogel’s characterization

Table 2.2 List of equipment used for the material characterization

NO Name of equipment Purpose

1 JSM-IT100, EDS - Take SEM image to study material’s morphology.

2 XRD Mini Flex 600 - Take XRD spectrum to study the crystal structure of samples.

- Taking UV-VIS-IR sample to study the light absorption of the material.

- Taking FT-IR spectra to identify functional groups on the surface of the photothermal materials.

5 NOVA touch 4LX - Study the material’s specific surface area using

6 THB-500 linseis - Study the aerogel’s thermal conductivity.

- Solar stimulator to evaluate the water evaporation ability of SSG system.

8 Shimadzu AG-X - Study the mechanical properties of samples

Investigate the photothermal material’s performance

2.3.1 Investigation of the material thermal behavior under laboratory condition

Figure 2.3 Setup of the experiment to measure thermal behavior of the aerogels[19]

To investigate the thermal behavior of the aerogels, experiments were conducted by subjecting both the white and black aerogels to a solar stimulator under sunlight intensity of 0.96 kW/m² for a duration of 10 minutes Figure 2.3 illustrates the setup and procedure of the experiment Throughout the experiment, the temperatures of both samples were continuously monitored using a FLIR C2 camera, allowing for real-time thermal imaging and temperature measurement This enabled the analysis of how the aerogels responded to solar radiation and provided valuable data on their thermal properties and heat transfer characteristics

2.3.2 Investigation of the material’s performance under laboratory condition

Figure 2.4 a) Solar steam generator containing water supply path, absorber and source of water b) Solar simulator[19]

Figure 2.4 illustrates the experimental setup used to assess the performance of the photothermal material using a solar simulator A mini Solar Steam Generation (SSG) system was constructed using a 100 ml glass beaker filled with seawater Inside the beaker, a piece of polystyrene foam, wrapped in gauze pads, was placed to provide buoyancy and prevent direct heat transfer from the photothermal material to the bulk water The photothermal material itself was positioned on top of the polystyrene foam, floating on the surface of the seawater, as depicted in Figure 2.3 The presence of the polystyrene foam ensured that the photothermal material remained above the water surface, while the gauze pads facilitated water transfer to the surface of the photothermal material This allowed for evaporation of the water upon heating and absorption by the photothermal material For the evaporation experiment, the mini SSG system was placed under the Oriel® Sol1ATM solar simulator and positioned on top of an electrical balance The experiment was conducted for 60 minutes, under sunlight intensity of 0.96 kW/m² conditions Throughout the experiment, the temperature variations of both the material's surface and the bulk water were continuously monitored using a FLIR C2 camera Additionally, the mass changes of the mini SSG system were recorded at 10- minute intervals, providing insights into the evaporation process and the performance of the photothermal material

RESULTS AND DISCUSSIONS

Explanation on the fabrication of aerogels from Lotus fiber

The fabrication process of the cellulose-based aerogel from ground lotus fiber involved three main steps Initially, the raw lotus fiber, which contained hemicellulose, lignin, and other minor impurities, underwent a chemical treatment using substances like NaOH, NaClO, and ethanol This treatment resulted in the extraction of cellulose, predominantly comprising cellulose microfibers, while effectively removing impurities such as lignin and hemicellulose Subsequently, the obtained cellulose was suspended in a solution and subjected to sonication for a duration of 10 minutes This sonication process helped disperse the cellulose microfibers uniformly within the suspension, ensuring a homogeneous mixture Finally, the suspension was freeze-dried to obtain the cellulose-based aerogel To determine the density of the aerogel sample, the weight of the sample was measured using an analytical balance and then divided by its volume This density measurement provided an indication of the aerogel's compactness and porosity The aerogel’s porosity was calculated from the estimated density using the formula:

𝑝 𝑏 ) where 𝛷 is the aerogel’s porosity 𝜌𝑎 is the aerogel density and 𝜌𝑏 is the crystalline

3.1.1 Extraction of cellulose from ground sugarcane bagasse a) b) c) d) e)

Figure 3.1 (a) Raw lotus fiber, (b) Lotus fiber during the treatment with Ethanol:H2O 1:1 solution, (c) Lotus fiber during the treatment with NaOH solution (d) Lotus fiber during the treatment with NaOH:NaClO solution, (e) Extracted cellulose

Figure 3.1 visually depicts the transformation of raw lotus fiber during the cellulose extraction process This process involves the removal of impurities, including hemicellulose, lignin, wax, protein, and a portion of non-crystalline cellulose[53]:

- Ethanol washing: The lotus fiber sample is washed with ethanol to remove impurities that are soluble in polar solvents This helps eliminate substances such as fat, wax, ash, pectin, and amino acids that can be easily dissolved in ethanol

- NaOH treatment: The washed lotus fiber is then subjected to treatment with a NaOH solution NaOH helps hydrolyze non-crystalline substances present in the fiber, such as hemicellulose and lignin

- NaOH/NaClO treatment: In this step, the lotus fiber sample is treated with a mixture of NaOH and NaClO The purpose of this treatment is to remove the remaining lignin, a complex and resistant polymer present in the fiber The

22 combination of NaOH and NaClO helps break down and dissolve the lignin, leaving behind cellulose as the primary component

As the extraction proceeds, these impurities undergo hydrolysis and dissolve into the solution, leading to a noticeable change in the solution's color, as illustrated in Figure 3.1b and Figure 3.1c Once the extraction is complete, the resulting purified cellulose exhibits a bright white color, as depicted in Figure 3.1e This change in appearance reflects the successful removal of impurities, resulting in a purified cellulose product that is visually distinguishable due to its clean and bright appearance

3.1.2 Preparation of white cellulose-based aerogel from extracted cellulose

Figure 3.2 (a) Cellulose suspension, (b) PVA solution, (c) Final suspension of cellulose in PVA solution, (d) A piece of white aerogel

Figure 3.3 Crosslinking between PVA and cellulose in the white aerogel sample[23]

Figure 3.2 illustrates the different stages of the fabrication process for white aerogel Initially, the extracted cellulose was mixed with distilled water, and then this mixture was slowly added to the PVA solution The resulting suspension was subjected to sonication at 200 W, resulting in the formation of the final suspension (as depicted in Figure 3.2c) Subsequently, the final suspension underwent freeze drying, leading to the formation of the white aerogel (as shown in Figure 3.2d) The white aerogel exhibited a porosity of 97,05% and a density of 0.0443g/cm 3 During the aerogel formation process, PVA acted as a crosslinker, effectively linking the cellulose microfibers and contributing to the formation of a stable aerogel structure (as depicted in Figure 3.3)

3.1.3 Preparation of black cellulose-based aerogel from extracted cellulose

Figure 3.4 (a) Cellulose suspension in Tannic acid solution, (b) PVA + FeCl3 solution, (c) Final suspension containing cellulose PVA, Tannic acid, FeCl3, (d) A piece of black aerogel

Figure 3.5 (a) Crosslinking between PVA, tannic acid and cellulose in black aerogel

(b) Formation of complexes between tannic acid and Fe3+[28]

Figure 3.4(a-c) showcases the sequential steps involved in the fabrication process of black aerogel Initially, a mixture of cellulose and tannic acid was slowly added to the PVA + FeCl3 solution, resulting in a color change of the solution to black (as depicted in Figure 3.4c) This color change occurred due to the formation of complexes between tannic acid and Fe3+ The black color persisted in the suspension, indicating the presence of the tannic acid-Fe 3+ complexes Subsequently, the final suspension was freeze-dried, leading to the formation of black aerogel (as depicted in Figure 3.4d) During the aerogel formation process, PVA underwent crosslinking with tannic acid and cellulose through multiple hydrogen bonding interactions between their -OH groups (shown in Figure 3.5a) This crosslinking played a important role in the formation of a more stable aerogel structure Additionally, the complexes between tannic acid and Fe 3+ were formed immediately after mixing the two solutions (as shown in Figure 3.5b) Three types of black aerogel samples, denoted as CC2 (with a tannic acid concentration of 3 mg/ml), CC3 (with a tannic acid concentration of 6 mg/ml), and CC4 (with a tannic acid concentration of 10 mg/ml were successfully fabricated These samples exhibited a porosity of 95.64% and a density of 0.0658 g/cm 3

The surface morphologies of photothermal materials

3.2.1 SEM images of raw lotus fiber and extracted cellulose

Figure 3.6 SEM images of (a) raw lotus fiber (bar: 20àm), (b) extracted cellulose (bar:

Figure 3.6 shows SEM images of raw lotus fiber and cellulose fibers after the extraction process In Figure 3.6a, the raw lotus fiber can be observed, consisting of several smaller fibers interconnected with a diameter of approximately 2-4 μm and

26 around 15-17 fibers linked together This interconnected structure contributes to the mechanical strength of lotus fibers However, after the extraction process, these fibers are separated, as depicted in Figure 3.6b The extraction process leads to better dispersion of these individual fibers within the PVA gel during aerogel fabrication The improved dispersion increases the contact area between the fibers and the PVA, enhancing the cross-linking ability between the two components

3.2.1 SEM images of the photothermal material

Figure 3.7 SEM images of a) white aerogel’s surface; b) cross-section and longitudinal section surface of black aerogel

Figure 3.7 showcases SEM images of surface of both white and black aerogel samples, providing insights into its 3D porous structure The images demonstrate that the aerogel exhibits a network of interconnected pores with varying sizes ranging from 0.2 to 50 àm This porous architecture enables efficient water transportation in the longitudinal direction and diffusion in the lateral direction Notably, Figure 3.7b reveals

27 the presence of enclosed thermal insulating chambers dispersed among the interconnected pore structure These chambers, with sizes ranging from 50 to 100 àm, contribute to a significant reduction in the material's thermal conductivity It's important to note that these thermal insulating chambers are only visible in the cross-section SEM images Furthermore, these images highlights the random distribution of cellulose microfibers throughout the material structure These microfibers, with lengths of 50 to

60 àm and diameters of appoximately 4 àm, enhance the overall structural integrity of the aerogel Based on the SEM images, it can be inferred that the cellulose-based aerogel exhibits excellent water transportation capacity, low thermal conductivity, and good durability[19]

3.2.2 Brunauer-Emmett-Teller (BET) analysis results

By performing the Brunauer-Emmett-Teller analysis, the specific surface area of the aerogel can be quantitatively determined, providing insights into its textural properties and potential for applications that rely on surface interactions, such as catalysis, adsorption, and filtration

Figure 3 8 BET Isotherm of the aerogels

Figure 3.8 displays the BET Isotherm of the aerogel, which provides valuable information about its specific surface area Analysis of the BET isotherm revealed that the average specific surface area of the aerogel was determined to be 9.35 m²/g This specific surface area value is comparable to that of other types of aerogels The high specific surface area observed in the aerogel is consistent with the findings from the SEM analysis, which indicated the presence of an interconnected three-dimensional porous structure This porous structure ensures that the material possesses excellent water absorption and transportation abilities The interconnected pores create pathways for efficient water absorption and movement within the aerogel, allowing for effective utilization of its surface area Overall, the combination of a high specific surface area and a three-dimensional porous structure contributes to the aerogel's favorable properties, particularly its capacity for water absorption and transportation These characteristics make the aerogel a promising material for applications where efficient water management is required

Figure 3.9 a) XRD spectra of raw lotus fiber and extracted cellulose, b) XRD spectra of white aerogel and black aerogel

Figure 3.9a illustrates a comparison between the X-ray diffraction (XRD) spectra of raw lotus fiber and extracted cellulose In the XRD spectrum of raw lotus fiber, a broad bump is observed in the range of 2θ = 16° to 28°, indicating the presence of cellulose along with impurities such as lignin, hemicellulose, waxes, protein, and water

The low degree of crystallinity in the ground lotus fiber can be attributed to the presence of these impurities as well as non-crystalline cellulose On the other hand, the XRD spectrum of extracted cellulose exhibits a distinct peak at 2θ = 22.16° and a bump around 2θ = 41°, indicating the presence of crystalline cellulose This clear peak at 2θ = 22.16° signifies an increase in crystallinity compared to the raw lotus fiber The difference between the two XRD spectra suggests that the extraction process effectively removes impurities such as lignin, hemicellulose, waxes, and protein, resulting in a higher purity of cellulose Furthermore, the increase in crystallinity observed in the extracted cellulose can be attributed to the hydrolysis of non-crystalline cellulose Non-crystalline cellulose is more susceptible to hydrolysis than crystalline cellulose, leading to its conversion into crystalline cellulose This transformation contributes to the enhanced crystallinity and the appearance of the clear peak at 2θ = 22.16° in the XRD spectrum of the extracted cellulose[29] Overall, the XRD analysis confirms that the extraction process successfully removes impurities and increases the crystallinity of cellulose, resulting in a higher purity and more crystalline form of cellulose in the extracted sample

Figure 3.9b presents a comparison of the X-ray diffraction (XRD) spectra between the white aerogel, consisting of cellulose and PVA, and the black aerogel, containing cellulose, PVA, and the complexes formed between tannic acid and Fe 3+ ions Both XRD spectra exhibit similar patterns with broad peaks observed around 2θ = 22° However, in the XRD spectrum of the black aerogel, the peaks are slightly shifted towards higher 2θ values This shift can be attributed to the presence of amorphous Tannic acid, Fe 3+ ions, and the complexes formed between them It is important to note that the XRD peaks of Fe could not be observed in the spectrum, which is consistent with findings in other publications[3] This is likely because Fe exists in a non- crystalline form, and the amount of tannic acid and Fe 3+ ions used in the fabrication of the aerogel is smaller in comparison to the amounts of cellulose and PVA present Overall, the XRD analysis indicates that both the white and black aerogels possess similar crystal structures characterized by broad peaks at around 2θ = 22° The slight shift in peaks observed in the black aerogel spectrum can be attributed to the presence of amorphous components, specifically tannic acid, Fe 3+ ions, and their complexes.

Chemical composition of the photothermal material

Figure 3.10 FT-IR spectra of white aerogel and 3 black aerogel samples with different tannic mass in the wavenumber range of 500 – 4000 cm -1

Figure 3.10 shows the FT-IR spectra of white aerogel and 3 black aerogel samples with different tannic mass in the wavenumber range of 500 – 4000 cm -1 The FT-IR spectra analysis revealed that all the samples exhibit peaks within the same wavenumber range, indicating the presence of similar functional groups Specifically, the peak at around 3300 cm-1 corresponds to the vibration of -OH groups or -NH groups

In the case of the black aerogel, this peak appeared higher compared to the other samples due to the presence of -OH groups in the polyphenol structures of tannic acid[8] Additionally, peaks at around 2926 cm-1 can be assigned to the C-H stretching vibrations in the aromatic ring of tannic acid Peaks at 1613 cm-1 could be attributed to C=O bonds in carbonyl groups or C=C bonds in aromatic rings, which are commonly found in tannic acid The intensity of the C=O or C=C peak in the FT-IR spectrum of the black aerogel was higher than that of the other spectra, confirming the presence of tannic acid in the black aerogel Furthermore, a peak at around 1034 cm-1 corresponds to the C-O stretching vibrations These peaks collectively indicate the presence of various components in the aerogel, including cellulose, PVA, tannic acid, and remaining

31 lignin and hemicellulose Overall, the FT-IR spectra demonstrate that the material possesses several hydrophilic groups, such as OH and C=O, which contribute to its hydrophilicity

Figure 3.11 Photographic images of the aerogel’s contact angles and water droplet permeation processes: a) white aerogel, b) black aerogel

In Figure 3.14, the wetting behaviors of the aerogel-based photothermal material were evaluated using water contact angle measurements The results showed that the white aerogel sample had a complete absorption of 18 μL of water droplets within 3.854 seconds, indicating its high water absorption capacity (Figure 3.14a) On the other hand, the black aerogel sample demonstrated a rapid permeation of water droplets within 0.492 seconds, indicating its high water permeability (Figure 3.14b) The enhanced water permeation rate of the black aerogel material can be attributed to the presence of tannic acid, which contains numerous hydrophilic -OH groups These hydrophilic groups contribute to the increased hydrophilicity of the photothermal material, allowing water droplets to quickly penetrate and spread within the material.

Mechanical properties of the photothermal materials

The compressive strength of the white aerogel sample (CC1) and three black aerogel samples (CC2, CC3, and CC4) was evaluated using the SHIMADZU AG-X equipment at the School of Engineering, Osaka University

Figure 3.12 The process of measuring the compressive strength of photothermal materials

The process of measuring the compressive strength of photothermal materials is depicted in Figure 3.11 The samples used for measuring mechanical properties have a height of 12mm and a diameter of 40mm During the measurements, the sample was compressed to 90% of its initial height to determine the compressive force and the ability to recover from deformation The measurement results are presented in Figure 3.6 and Table 3.1

Figure 3.13 Compressive stress–strain curves of the aerogels with different chemical compositon

Table 3.1 Young modulus of photothermal materials

Table 3.2 Comparision of Young Modulus between lotus fiber aerogel and other natural-based aerogel

Material Young Modulus (kPa) Reference

Lotus fiber/PVA aerogel 104.7 This work

Pineapple Leaf /Cotton/PVA aerogel 36.13 22

Wood pulp cellulose nano fiber aerogel 101.3 31

Based on the results presented in Figure 3.12, it is evident that the compressive strength of the white and black samples is 0.7 MPa and 1.4 MPa, respectively These values are significantly higher than those typically observed for other cellulose-based aerogels, which are usually below 100 kPa[21-23] This suggests that the lotus fiber- based materials exhibit superior strengthening capabilities compared to other natural cellulose sources Furthermore, it can be observed that the black samples possess approximately twice the compressive strength of the white samples, indicating that the addition of Fe 3+ and tannic acid contributes to enhancing the mechanical properties of the material

In Table 3.1, Young's modulus is estimated from the unloading curve However, due to the significant compression of the CC2 black sample, the oversampling results for this sample may become inaccurate Nevertheless, it is evident that the Young's modulus of the black samples is approximately twice that of the white samples This further confirms that the addition of tannic acid and FeCl3 to the initial solution enhances the strength of the material

Figure 3.14 Crosslinking between tannic acid, PVA and cellulose

The increase in compressive strength of the aerogel with the tannic acid content can be attributed to the crosslinking between tannic acid and either PVA or cellulose through multiple hydrogen bonding between their -OH groups, as shown in Figure 3.13 This crosslinking mechanism enhances the compressive strength of the aerogel, similar to the effect of 1,2-Bis-(trimethylsilyl) ethane (BTMSE) in improving the mechanical properties of cellulose-based aerogels[31] While increasing the content of cellulose and the crosslinker can enhance the mechanical properties, it would also reduce the material's porosity, increase its density, and compromise its thermal conductivity, which is undesirable for a photothermal material Therefore, it is important to optimize the crosslinking process to improve the material's mechanical properties without sacrificing its performance One approach to improving the mechanical properties is by optimizing the cellulose extraction procedure to obtain cellulose nanofibrils instead of cellulose microfibrils Reducing the size of cellulose fibers increases the crosslinking area between cellulose and crosslinkers, thereby enhancing the degree of crosslinking and mechanical strength However, controlling the crosslinking process becomes more challenging as PVA and tannic acid may interact with each other instead of crosslinking with cellulose Further research is needed to explore methods for controlling and

36 optimizing the crosslinking process in order to achieve the desired mechanical properties while maintaining the material's porosity and thermal conductivity

Based on the comparison of Young's modulus values in Table 3.2, it indicates that aerogels made from lotus fiber have a significantly higher Young's modulus compared to other natural cellulose materials This suggests that incorporating lotus fibers into materials can enhance their strength and stiffness The higher Young's modulus of lotus fiber-based aerogels implies that they have improved structural integrity and resistance to deformation, which can be advantageous in various applications that require strong and rigid materials

Figure 3.15 Compressive loading-unloading stress-strain curves of aerogel

Furthermore, a compressive loading-unloading test was conducted on the photothermal material based on aerogels The result is shown on Figure 3.15 It was observed that the material exhibited excellent resilience as it endured 20 cycles of loading and unloading without rupturing Even after the 20th cycle, the material's stress- strain curve still retained a significant hysteresis loop, indicating its impressive elasticity.

Evaporation performance of the aerogel

3.5.1 Thermal behavior and FT-IR of the aerogel

Figure 3.16 a) UV-VIS-IR spectra of the white and black aerogel samples and b) the maximum temperature of white and black aerogel under sun intensity 0.96kW/m2

Figure 3.17 IR images, indicating the temperature changes before and after under sun simulator for 10 minutes: a) white sample; b) black sample

The UV-VIS-IR spectra were measured for the aerogel samples to evaluate their light absorption capabilities, which play a crucial role in determining the material's solar energy conversion efficiency Figure 3.14a displays the absorption spectra of the black and white aerogel samples The white aerogel exhibited an average absorption of approximately 20% in the wavelength range of 300 - 1300 nm, and around 40% in the

38 range of 1300 - 2500 nm In contrast, the black aerogel sample displayed significantly higher average absorptions, with values of approximately 95% in the wavelength range of 300 - 800 nm, and around 70% in the range of 800 - 2500 nm

The increased absorption observed in the black aerogel can be attributed to several factors Firstly, the formation of complexes between tannic acid and Fe 3+ contributes to enhanced light absorption Secondly, the ligand to metal charge transfer phenomenon occurring in these complexes further enhances absorption Lastly, the roughness and porosity of the aerogel's surface structure allow for multiple reflections of sunlight, leading to increased light capture and absorption Hence, the UV-VIS-IR spectra analysis confirms that the aerogel exhibits a high light absorption ability, particularly in the case of the black aerogel sample This characteristic is crucial for efficient solar energy conversion in photothermal applications

Figure 3.14b demonstrates the temperature rise of the white and black aerogel samples when subjected to simulated illumination with a radiation power density of 0.96 kWm -2 The change in samples surface’s temperature before and after the experiment can be observed in Figure 3.17 The surface temperature of the white aerogel reached a stabilized value of approximately 40 °C, while the black aerogel sample achieved a stabilized temperature of around 61 °C This temperature increase indicates the excellent photothermal conversion properties of the black aerogel Based on the characterization results mentioned above, it can be concluded that the black aerogel is well-suited for applications in solar steam generators Its high light absorption ability, as confirmed by the UV-VIS-IR spectra, enables efficient conversion of light energy into thermal energy The significant temperature rise observed in the black aerogel sample further supports its suitability for such applications, as it demonstrates the material's ability to generate and retain heat effectively

Therefore, the black aerogel's excellent light absorption ability and photothermal conversion properties make it a promising candidate for use in solar steam generators, where the efficient conversion of sunlight into thermal energy is essential

3.5.2 Evaporation performance of the aerogel in the experiment condition

The evaporation performance of the aerogels was investigated using the Oriel® Sol1ATM solar stimulator in the experimental setup described in Section 2.3.2 of the thesis The experiment involved a white aerogel sample and two black samples, one with a height of 18mm and another with a height of 25mm The duration of the experiment was 1 hour, during which the mass changes of the samples were monitored using an electrical balance

Figure 3.18 Mass change of the seawater, white aerogel, and black aerogel in the solar steam generator under light intensity 0.96kW/m2

Figure 3.15 demonstrates the mass change of the solar steam generator during the evaporation experiment using a solar simulator The evaporation rates of the white aerogel and black aerogel samples are presented, along with a comparison to the evaporation rate of bulk water under the same conditions The evaporation rate of the white aerogel is reported as 1.25 kg/m²ãh, while the highest evaporation rate among the black aerogel samples is recorded as 2.51 kg/m²ãh This performance surpasses that of most other photothermal materials mentioned in Table 3.3 Notably, all the rates of evaporation surpass the rate of evaporation of bulk water, which is measured as 0.42 kg/m²ãh under the identical experimental conditions The enhanced evaporation rates observed in the photothermal materials are attributed to two main factors Firstly, the heat localization effect stemming from the low thermal conductivity of the photothermal material causes the light energy to be retained as heat in the vicinity of the material's

40 surface This concentrated heat enables the evaporation of a smaller amount of water on the surface of material, rather than dispersing and heating the bulk water, ultimately leading to an accelerated rate of evaporation Secondly, the increased light absorption ability of the black aerogel samples contributes to the increased evaporation rate[32]

By absorbing a greater amount of light energy, these samples generate more heat, further intensifying the localized heating effect and promoting faster evaporation

Figure 3.19 Temperature variation during evaporation experiment between: a) Surface of white and black aerogel samples b) black sample surface, bulk water and black sample body

Figure 3.20 IR images, indicating the temperature change during the evaporation experiment of: a) white aerogel’s surface, b) black aerogel’s surface and c) bulk water and black aerogel’body

In Figure 3.16a, the graph illustrates the surface temperature of the black aerogel and white aerogel samples in the solar steam generator The black aerogel shows a surface temperature stabilization at around 39°C, whereas the white aerogel stabilizes at

42 approximately 30°C This discrepancy indicates that the black aerogel possesses a higher light absorption ability compared to the white aerogel Furthermore, Figure 3.16b demonstrates the comparison between the surface and body temperature of the photothermal material and the bulk water in the solar steam generator The graph confirms the phenomenon of heat localization, where the surface temperature of the photothermal material consistently exceeds that of the bulk water The heat localization effect arises due to the lower thermal conductivity of the photothermal material As a result, when exposed to light energy, the photothermal material tends to retain and concentrate heat near its surface, leading to a higher surface temperature In contrast, the bulk water, with its higher thermal conductivity, disperses the heat more evenly, resulting in a lower surface temperature Therefore, the data presented in Figure 3.16 support the conclusion that the black aerogel exhibits superior light absorption ability and a higher surface temperature, thus confirming the role of heat localization in the enhanced evaporation performance of the photothermal material in the solar steam generator

Table 3.3 Comparison on the specifications and performance of the Cellulose based aerogel in this thesis and other type of photothermal materials

Lotus Fiber cellulose based aerogel

20 days Low cost This work

Cotton Fabric cellulose based aerogel

Low price, simple fabrication, medium scalability

Pt/Au/TiO2 wood carbon

Ferric ion- polyphenol- graphene aerogel

Activated carbon treated wood (AC wood)

Carbon nanotube coated sunflower stalks

ZIF67@MXene/rGO decorated rock wool

Marine biomass based composite aerogel

Geololymer mesoporous bulk carbons driven from biomass

3.5.3 Evaluation the solar energy evaporation efficiency of the black aerogel in dark condition

Figure 3.21 Evaporation rates of the seawater, white aerogel and black aerogel in dark condition

To determine the efficiency of the material in utilizing solar energy for evaporation, an experiment was conducted to measure the evaporation enthalpy of seawater on the surface of a photothermal aerogel The experiment utilized the same solar steam generator and experimental conditions as described in Section 2.3.3 Four beakers of equal size, with a diameter of 42 mm and a volume of 100 ml, were prepared Seawater, a white aerogel sample, and two black aerogel samples were placed in these beakers The white aerogel sample had a diameter and thickness of 42 mm and 18 mm, respectively, while one of the black aerogel samples had a diameter of 42 mm and a thickness of 18 mm, and the other had a thickness of 25 mm The dark evaporation experiment was conducted three times, with each run lasting for 60 minutes at a temperature of 30°C and a pressure of 1 atm In the dark condition, the photothermal material absorbed heat from the surroundings to facilitate water evaporation, eliminating the influence of light intensity on the evaporation rate During dark evaporation, the rate of evaporation is determined by the water evaporation enthalpy, which represents the energy required to convert water into vapor

Figure 3.17 presents the evaporation rates observed during the experiment The seawater, white aerogel, and black aerogel samples with heights of 18 mm and 25 mm exhibited evaporation rates of 0.078 kgm -2 h -1 , 0.15 kgm -2 h -1 , 0.17 kgm -2 h -1 , and 0.24

47 kgm -2 h -1 , respectively The evaporation rates of the white and black aerogel samples with the same height were similar, but the black aerogel sample with a greater height showed a higher evaporation rate This can be attributed to the increased contact area between the aerogel and the external environment, leading to enhanced evaporation Notably, all the aerogel samples demonstrated higher evaporation rates compared to blank seawater As the material's light absorption capacity does not affect the evaporation rate under dark conditions, the rate of dark evaporation solely relies on the water evaporation enthalpy within the material

The water evaporation enthalpy of the photothermal material in the solar steam generator was found to be lower than that of bulk water This is attributed to the presence of numerous hydrophilic -OH groups derived from cellulose, PVA, and polyphenol in tannic acid on the material's surface These hydrophilic groups facilitate water evaporation in the form of single molecules or clusters containing a few to ten molecules Evaporating as clusters allows water to undergo evaporation with a minimal enthalpy change The 3D interconnected pore structure of the aerogel, combined with the abundant -OH groups on its surface, enables hydrogen bonding with water molecules, facilitating the evaporation of water molecules in groups of a few to ten[18] Therefore, the evaporation enthalpy of the aerogel material is reduced

The evaporation enthalpy of the black aerogel material with 25mm height was calculated by the formula[18]:

𝛥𝐻0 𝑚0 = 𝛥𝐻1 𝑚1 where: 𝛥𝐻0 is the evaporation enthalpy of the bulk water = 2450 J/g; 𝑚0 is the dark evaporation rate of bulk water = 0.078 kgm -2 h -2 ; 𝑚1 is the dark evaporation rate of black sample = 0.24 kgm -2 h -2 ; and 𝛥𝐻1 is the evaporation enthalpy of the black sample Using the formula, we found that 𝛥𝐻1 = 796.25 J/g

The black aerogel's solar energy conversion efficiency can be calculated from its evaporation enthalpy using the formula[18]: