Environmentally sustainable cellulose fibers based aerogel photothermal material for solar driven clean water

Comparison on the specifications and performance of the Cellulose based aerogel in this thesis and other type of photothermal materials... a Cellulose suspension in Tannic acid solution,

|d-d VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN HOANG GIANG

ENVIRONMENTALLY SUSTAINABLE CELLULOSE FIBERS-BASED AEROGEL PHOTOTHERMAL MATERIAL FOR SOLAR-DRIVEN CLEAN WATER

MASTER’S THESIS

VIETNAM NATIONAL UNIVERSITY, HANOI

VIETNAM JAPAN UNIVERSITY

NGUYEN HOANG GIANG

ENVIRONMENTALLY SUSTAINABLE CELLULOSE FIBERS-BASED AEROGEL PHOTOTHERMAL MATERIAL FOR SOLAR-DRIVEN CLEAN WATER

MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD

RESEARCH SUPERVISOR:

Dr PHAM TIEN THANH

Hanoi, 2022

Author of the thesis

Nguyễn Hoàng Giang

ACKNOWLEDGEMENTS

Firstly, I would like to express my sincere gratitude to Dr Pham Tien Thanh, my

supervisors at Vietnam Japan University, for his enthusiasm, support, and patient

guidance throughout the implementation of my research for master thesis

I'd want to take this opportunity to thank Assoc Prof Dr Pham Xuan Nui from Hanoi

University of Mining and Geology for his useful support and recommendation during my

research His suggestions on the purification of cellulose from sugarcane bagasse had been

very useful for my research

I would like to express my gratitude to all lab-mates from both MNT Lab and MEE Lab

for their necessary assistance I really enjoy my research works and the moments we

had together

Last but not least, I really want to thank to MNT’s professors, lecturers, staffs for their

support me during the internship period Without them, I could not have enough

knowledge and experience to research and study in here

I want to express my gratitude to my mother, father, brother, and friends, who have

always been there for me and have always supported and encouraged me throughout my

life

Nguyen Hoang Giang

Hanoi, 2022

TABLE OF CONTENTS

LIST OF TABLES i

LIST OF FIGURES ii

LIST OF ABBREVIATIONS iv

CHAPTER 1: INTRODUCTION OF SOLAR STEAM GENERATION 1

1.1 The importance of converting seawater into freshwater 1

1.2 Desalination of seawater 2

1.3 Solar steam generation (SSG) 3

1.4 Types of photothermal materials 5

1.4.1 Metallic nanoparticles 5

1.4.2 Metal Oxides 5

\1.4.3 Biomass based photothermal materials 6

1.4.3 Aerogel based photothermal materials 7

1.5 Justify the selection of research material and method 8

1.6 Cellulose based aerogel fabrication procedure 9

1.7 Purpose of thesis 10

CHAPTER 2: EXPERIMENTS 11

2.1 Fabrication of photothermal materials 11

2.1.1 Chemicals 11

2.1.2 Preparation of natural porous materials 11

2.2 Characterization of photothermal materials 14

2.3 Investigate the photothermal material’s performance 15

2.3.1 Investigation of the material thermal behavior under laboratory condition 15

2.3.2 Investigation of the material’s performance under laboratory condition 16

2.3.3 Investigation of the material’s performance under real condition 17

CHAPTER 3: RESULTS AND DISCUSSIONS 18

3.1 Explanation on the fabrication of aerogels from ground sugarcane bagasse 18

3.1.1 Extraction of cellulose from ground sugarcane bagasse 19

3.1.2 Preparation of white cellulose-based aerogel from extracted cellulose 20

3.1.3 Preparation of black cellulose-based aerogel from extracted cellulose 21

3.2 The surface morphologies of photothermal materials 23

3.2.1 SEM images of the photothermal material 23

3.2.2 Brunauer-Emmett-Teller (BET) analysis results 24

3.2.3 X-ray diffraction and EDS analysis results 25

3.3 Surface structure of the photothermal material 26

3.2.1 EDS analysis results 26

3.2.2 FT-IR spectra 27

3.2.3 Wetting behavior measurement 28

3.3 Mechanical properties of the aerogels 29

3.4 Thermal conductivity of the aerogels 31

3.5 Evaporation performance of the aerogel 31

3.5.1 Thermal behavior and FT-IR of the aerogel 31

3.5.2 Evaporation performance of the aerogel in the experiment condition 32

3.5.3 Evaluation the solar energy evaporation efficiency of the black aerogel 34

3.5.4 Evaluation the structural stability of the black aerogel 36

3.5.5 Evaluation performance of the aerogel in real condition 38

CONCLUSION 40

REFERENCES 43

LIST OF TABLES

Table 2.1 Composition of white and black aerogel samples 14 Table 2.2 List of equipment used for the material characterization 14 Table 3.1 EDS analysis results of White aerogel and Black aerogel 27 Table 3.2 Concentration of substances in the final suspension of the aerogel samples 29 Table 4.1 Comparison on the specifications and performance of the Cellulose based aerogel in this thesis and other type of photothermal materials 41

LIST OF FIGURES

Figure 1.1 Projected water stress in 2040 1

Figure 1.2 a) Severe drought in the Mekong delta, b) Map of saline invasion of the Mekong delta 2

Figure 1.3 Technological status of renewable energy desalination technologies 3

Figure 1.4 Working principle of Solar steam generator 4

Figure 1.5 Tungsten oxides material in a stimulation experiment 6

Figure 1.6 Photothermal material based on lotus seed pods 7

Figure 1.7 Phase diagram of water, illustrate the freeze-drying process 8

Figure 1.8 Image of sugarcane bagasse 9

Figure 2.1 Fabrication process of cellulose – based aerogel (including white and black aerogel) 12

Figure 2.2 a) White cellulose-based aerogel, b) black cellulose-based aerogel 13

Figure 2.4 Setup of the experiment to measure thermal behavior of the aerogels 15

Figure 2.6 a) Working principle of the evaporation device, b) The evaporation device under natural sunlight, c) dimension of the inner sample container, d) dimension of the outer compartment 17

Figure 3.1 (a) Ground sugarcane bagasse, (b) Sugarcane bagasse during the treatment with Ethanol:H2O 1:1 solution, (c) Sugarcane bagasse during the treatment with NaOH solution (d) Sugarcane bagasse during the treatment with NaOH:NaClO solution, (e) Extracted cellulose in a vacuum filter, (f) Cellulose suspension, left overnight 19

Figure 3.2 (a) Cellulose suspension, (b) PVA solution, (c) Mixture of distilled water, cellulose, PVA in the ultrasonic device (d) Final suspension of cellulose in PVA solution, (e) A piece of white aerogel (f) A piece of white aerogel, placed on top of a leaf 20

Figure 3.3 Crosslinking between PVA and cellulose in the white aerogel 20

Figure 3.4 (a) Cellulose suspension in Tannic acid solution, (b) PVA + FeCl3 solution, (c) Mixture of distilled water, cellulose, PVA, Tannic acid, FeCl3 in the ultrasonic device, (d) Final suspension containing cellulose PVA, Tannic acid, FeCl3, (e) A piece of black aerogel, (f) Aggregation of cellulose microfibrils in the suspension when the concentration of tannic acid was 10 mg/ml 21

Figure 3.5 (a) Crosslinking between PVA, tannic acid and cellulose in black aerogel (b) Formation of complexes between tannic acid and Fe3+ 22

Figure 3.6 SEM images of (a) aerogel’s cross section (bar: 200µm), (b) aerogel’s cross section (bar: 100µm), (c) SEM images of aerogel’s cross section (bar: 50 µm), (d) aerogel’s surface (bar: 200µm), (e) aerogel’s surface (bar: 100µm), (f) SEM images of aerogel’s cross section (bar: 50µm) 23

Figure 3.7 BET Isotherm of the aerogels 24

Figure 3.8 (a) XRD spectra of ground sugarcane bagasse and extracted cellulose, (b) XRD spectra of extracted cellulose and white aerogel, (c) XRD spectra of white aerogel

and black aerogel 25

Figure 3.9 a) EDS spectrum of White aerogel (CBA1), (b) EDS spectrum of black aerogel (CBA2) 26

Figure 3.10 FT-IR spectra of Ground sugarcane bagasse, extracted cellulose, White aerogel and Black aerogel in the wavenumber range of 500 – 4000 cm-1 27

Figure 3.11 Wetting behavior measurement of the aerogel 28

Figure 3.12 Tensile and compressive strength testing of the aerogels 29

Figure 3.13 Crosslinking between tannic acid, PVA and cellulose 30

Figure 3.14 (a) UV-VIS-IR spectra of the white and black aerogel samples and (b) the maximum temperature of white and black aerogel under 1 sun illumination 31

Figure 3.15 Mass change of the seawater, white aerogel, black aerogel in the solar steam generator under 1 sun illumination 32

Figure 3.16 (a) Surface temperature change of white aerogel and black aerogel in the evaporation experiment, (b) IR images, indicating the temperature change of black aerogel during the evaporation experiment (c) IR images, indicating the temperature change of white aerogel during the evaporation experiment 33

Figure 3.17 a) Temperature variation of black aerogel and the bulk water in the same Solar steam generator b) IR image indicate the material surface temperature and bulk water monitoring areas 34

Figure 3.18 Evaporation rates of the blank seawater, white aerogel and dark aerogel Error! Bookmark not defined Figure 3.19 a) Black aerogel (CBA3)’s rate of evaporation after 50 evaporation cycles, b) Physical appearance of the CBA3 samples after 11 days of exposure to seawater 36 Figure 3.20 a) Self-cleaning mechanism of the aerogel b) Self-cleaning performance of the Black aerogel (CBA3) 37

Figure 3.21 (a) Real water desalination device during the experiment, (b) Parameters: solar flux, temperature, humidity variation on Day 2 from 08:00 – 18:00 (c) Parameters: solar flux, temperature, humidity variation on Day 3 from 08:00 – 18:00, (d) Performance of the desalination device 38

LIST OF ABBREVIATIONS

CBA: Cellulose-based aerogel

EDS: Energy Disperse X-Ray Spectroscopy

FT-IR: Fourier-Transform Infrared Spectroscopy

PVA: Polyvinyl alcohol

RO: Reverse Osmosis

SEM: Scanning Electron Microscope

SSG: Solar steam generation

UV-Vis-nIR: Ultraviolet-Visible-Near Infrared

CHAPTER 1: INTRODUCTION OF SOLAR STEAM GENERATION

1.1 The importance of converting seawater into freshwater

Figure 1.1 Projected water stress in 2040 [3]

Water stress is considered as one of the major global problems It is projected that by 2050, about 5.7 billion people would be affected by the water scarcity and the number would not stop increasing Figure 1.1 shows the projected level of water stress

by countries in 2040 Several world economic power: USA, China and Australia would

be severely affected by water stress, damaging their economy The damaging economy

of USA and China would affect the economy of many other countries that depends on them Human beings are responsible for the own water crisis Economic activities such

as industrial production, natural resources mining, intensive farming that use lots of pesticide and chemical fertilizer have polluted soil, air and water environment, sped up the climate change process, thus reducing the availability of fresh-water for human consumption [25] These activities would not like to halt in the future as nations are fiercely compete with each other for economical supremacy As a result of climate change, the water stress in many countries including Vietnam have become more severe Due to severe drought and the change in flowrate of Mekong River, the Mekong delta

of Vietnam is severely affected by water stress due to saline invasion [21]

well-1.2 Desalination of seawater

Desalination process is classified as Membrane technology and Solar desalination technologies Membrane [25] use pressure (Reverse Osmosis), electrical (Electrodialysis) as the driving force to separate ions from water by overcoming natural osmotic pressures [27] These technology use Membrane desalination technologies are expensive and consume a large amount of energy from non-renewable sources, thus it could not provide viable and sustainable desalination options As a result, affordable desalination technologies that use sustainable source energy must be developed to address the water scarcity and to ensure sustainable living for our offspring Figure 1.3 show status of Research and development of desalination technologies based on renewable energy such as: solar energy, wave, wind, geothermal energy and etc

Figure 1.3 Technological status of renewable energy desalination technologies [2]

Among the renewable energy based technologies, Solar Still, PV-SwRO and Solar CSP/MED, Wind RO have reached the state of application Among these, Solar evaporation: solar still and Solar CSP/MED convert solar energy into heat to evaporate water They have relatively low cost of operation of 1.3 – 6.5 $ and 2.0 – 2.5 $ per cubic meters of water desalinated As a result, these technologies could potentially be applied

as an affordable desalination technologies for countries that have high number of sunlight hours like Vietnam, Bangladesh and etc Solar energy conversion efficiency is the key factor governing the performance of a solar steam generator technologies The efficiency of a solar evaporation could be improved by fabricating new photothermal material with improved solar energy conversion efficiency Therefore, my thesis aim is

to fabricating new photothermal material with high solar energy conversion efficiency

so as to fully realize the potential of solar steam generator for desalination Working principle of a Solar steam generator will be discussed in the next Section [2], [28]

1.3 Solar steam generation (SSG)

Solar energy is considered as an abundant source of renewable energy Annually, the earth receives an amount of 1361 W/m2 of sun radiation in which 70% were absorbed while the rest is either reflected or scattered away [2] As a tropical country, Vietnam has a great number of sunshine hours (2000 – 6000 hour/years, equivalent to 6 – 7 hours/day) and an average annual solar irradiation of 5 kW/h/m2, creating favorable

condition for the application of solar energy related technologies Recently, Solar steam generation for seawater desalination attract great attention because of its major strengths such as: renewable energy use, no greenhouse gas emission, simple fabrication process With the abundant amount of sunlight, an optimized Solar steam generator could produce 15 – 30 Litres of desalinated water per hour [14]

Figure 1.4 Working principle of Solar steam generator [29]

Solar steam generation (SSG) is a technology that take advantage photothermal effect to distill water SSG have seen application in seawater desalination, wastewater treatment and etc Solar steam generation system consists of three components: (1) Solar absorber (made of photothermal material), (2) substrates that transport water to the solar absorber and (3) Water collector to collect distilled water

During the SSG system operation, the photothermal material absorbs light energy and converts the energy into thermal energy Meanwhile, water is constantly transported

to the light absorber by the substrate to sustain the evaporation Finally, evaporated water is condensed in the water collector The performance of a SSG system depends

on several factors: rate of water supply, absorption efficiency of the light absorber, thermal conductivity of the photothermal material that affect the heat localization and

heat loss minimization Design of photothermal material take a crucial role in ensuring the good performance of a Solar steam generator Different classification of photothermal material will be discussed in the next Section

1.4 Types of photothermal materials

Photothermal material could be understood as type of material that have good ability to convert light energy to heat Based on the precursor, photothermal materials are classified into groups including: metallic nanoparticles, metal oxides, polymers, semiconductors and carbonaceous based photothermal material [29]

1.4.1 Metallic nanoparticles

Metallic nanoparticles (NPs) take advantage of Localized surface plasmon resonance (LPSR) effect The incident photon is absorbed when its frequency matches the localized surface plasmon resonance frequency of the photothermal material Absorbed energy is converted into heat Numerous researches have been done to develop NPs material with good light absorption ability from Au, Ag, CuS, Cu, Al, and

Pd nanoparticles

Advantages of metallic nanoparticle including structural stability, and recyclability For example, plasmonic Pd nanoparticles shows outstanding stability after cycling for 144 hours in solar steam generation and the recyclability of Au nanoparticles

is 98% [29] Also, metallic nanoparticles could be easily mass produced because the nanoparticles is synthesized by wet chemical method However, in the nanoparticle must

be coated or attached to the surface of substrate such as Bacterial cellulose, which is a complicated fabrication procedure

1.4.2 Metal Oxides

Metal oxides also demonstrate high light absorption ability WO2.9 with a light absorption capacity of 90,6% and a light-to-heat conversion efficiency of 86,9% have been successfully synthesized A water efficiency of 81% have achieved with the material [11]

Figure 1.5 Tungsten oxides material

In other work, another kind of metal oxide material was successfully fabricated

by dispersing polydimethylsiloxane (PDMS)-modified Fe3O4 nanoparticles on the graphene sheet’s surface The material demonstrated high light-to-heat conversion within the surface temperature of about 100˚C [12] Nickel nanoparticles (Ni-NPs) and titanium dioxide nanotubes (TiO2-NTs) with high light absorption ability of 96,83% in the wavelength range of 300 – 2500 nm have also been successfully fabricated The high absorption ability are achieved by light trapping and LPSR properties of the hybrid material However, the material has a few shortcomings such as complicated fabricated procedure and high-power density [13]

1.4.3 Biomass based photothermal materials

Photothermal material could also be fabricated from biomass material such as: pomelo peel [17], lotus seed pod [30], finger citron fruit [23] Compared to metallic nanoparticle and metal oxide based photothermal material, natural based photothermal have higher stability and simpler fabrication method [16], [23]

Figure 1.6 Photothermal material based on lotus seed pods

Figure 1.6 illustrates properties of lotus seedpod, an example of biomass-based photothermal material Biomass-based photothermal material have microporous and vascular bunder structures that could ensure good water transportation ability, low thermal conductivity, and good light absorption ability Moreover, biomass based photothermal material has excellent durability in different conditions because it contain mostly lignin, cellulose and hemicellulose Numerous methods have been implemented

to fabricate biomass based photothermal material such as carbonization in inert gas environment, spin coating to coat a layer of active carbon onto the material surface, fabricate a layer of complexes between polyphenolic compounds and metal ions such as

Cu2+ and Fe3+ Although research on biomass based photothermal material [22, 24, 31] has proved its potential, large scale application of biomass based photothermal material

is still a great challenge

1.4.3 Aerogel based photothermal materials

Aerogel is defined as a synthetic porous material, originated from a gel in which the liquid phase of the gel is replaced by gas More than 90% of aerogel’s volume is air [32] Aerogel has numerous applications including photothermal material fabrication thanks

to its porosity and low thermal conductivity Based on the type of precursor, aerogel could be classified as:

• Organic Aerogel (Made from precursors such as: Polyurethane, resorcinol- formaldehyde, Polystyrene, polyimide, cellulose [33]

• Inorganic Aerogel (Made from SiO2 or various types of alkoxysilanes: Al2O3, TiO2, ZrO2 , SiC, v.v.) [33]

• Carbon Aerogel (Made from carbon nano tube, carbon and graphene)

Figure 1.7 Phase diagram of water, illustrate the freeze-drying process

Generally, the fabrication of aerogel material has two main steps: gelation to create gel structure and drying to replace the liquid phase in gel structure by air In the gelation process, the precursors are mixed together in a desired proportion to create a reagent mixture in which all precursor are evenly distributed Next, gel is formed in the reagent mixture at low temperature either by reaction [33] or rearrangement of solid phase Finally, the aerogel structure is obtained by removing water from the gel by freeze-drying method in which the sample is cooled below the triple point of water, freeze Aerogel has been successfully fabricated from various natural ingredient such as rice straw [18], pineapples leaves [19] or sugarcane bagasse [20] for application in oil spill removal,and form nanocellulose to for application in seawater desalination [17] using simple fabrication methods

1.5 Justify the selection of research material and method

Numerous types of photothermal material have been introduced in Section 1.4 Among these, metallic nanoparticles and metal oxides have complicated fabrication procedure, high fabrication cost Carbon-based photothermal material have simple and cheap fabrication method, but its scalability is limited by the size of carbon-based natural such

as fingered citron, coconut husk, v.v Unlike, these type of photothermal material, aerogel based photothermal material could be fabricated from natural cellulose using a

simple, and scalable fabrication methods Therefore, cellulose-based aerogel is selected

as the research orientation for the thesis Also, cellulose-based aerogel (CBA) has been fabricated from pre-made cellulose nano fibrils (CNF) [33] for application in seawater desalination The use of pre-made (CNF) might increase the fabrication rate We want

to develop a solution for recycling an enormous amount of industrial waste as well as reduce the fabrication cost of the existing CBA fabrication cost Therefore, sugarcane bagasse was selected as the main ingredient for fabrication of photothermal material from CBA

1.6 Cellulose based aerogel fabrication procedure

Figure 1.8 Image of sugarcane bagasse [33]

In the thesis, aerogel was fabricated from sugarcane bagasse, a biodegradable material, released as sugar production waste Sugarcane bagasse accounts for 30% of sugarcane’s fresh weight Composition of sugarcane dry weight are as follow:

- Cellulose: polysaccharide of β-glucose, connected by 1,4-glicozit bonding with a

molecular weight of 10.000 – 150.000, account for 45 – 55% of the dry weight;

- Hemicellulose: polysaccharide of glucose and other monomers rather than glucose

such as arabinoxylan, glucomannan, and xyloglucan and etc, account for 20 – 25%

of the dry weight

- Lignin: natural polymer made of phenylpropane, act as connection between

cellulose and hemicellulose, account for 18 – 23% of the dry weight

- Other soluble substances: (wax, ash, protein), account for 5 – 3 % of the dry

weight

Among these substances, hemicellulose and lignin are non-crystalline while cellulose exists in both crystalline and non-crystalline form Cellulose is usually

extracted from the impurities by selective hydrolysis in which non-crystalline substances of sugarcane bagasse (lignin, hemicellulose and non-crystalline cellulose) are separated from cellulose because they are more susceptible to hydrolyzing agents such as NaOH, NaClO and etc [33] Subsequently, the extracted cellulose was dispersed

in distilled water to create a suspension Finally, the suspension was freeze-dried to obtain cellulose aerogel Generally, pure cellulose-based aerogel has very poor mechanical properties It tends to disintegrate upon contact with water As a result, crosslinkers, such as PVA, PVP, BTMSE are added during the aerogel’s fabrication to improve the aerogel’s mechanical properties [18 – 20] In the thesis, PVA was used as the crosslinker because of its low-cost, high-availability

1.7 Purposes of the thesis

This thesis reports the fabrication of PVA composite cellulose-based aerogel from sugarcane bagasse for application in seawater solar desalination Tannic acid and

Fe3+ salts were mix into the suspension of cellulose in PVA solution to improve the material’s light absorption capacity Then, the material was characterized by methods such as SEM, BET, FT-IR, XRD The evaporation performance of the material was tested in both experiment and real conditions

The master thesis aims at:

- Fabricate Aerogel from sugarcane bagasse, PVA, tannic acid, Fe (III)

- Characterize the material’s properties using methods such as SEM image, BET, XRD, UV-VIS, FT-IR, contact angle etc

- Investigate the material’s thermal conversion properties and its application in the SSG system

- Investigate the seawater desalination capability of the material under indoor and outdoor conditions

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CHAPTER 2: EXPERIMENTS

This chapter presents a fabrication process of photothermal materials using sugarcane bagasse, PVA, Tannic acid and FeCl3 From the fabricated material, Solar steam generation systems are constructed for seawater desalination

2.1 Fabrication of photothermal materials

2.1.2 Preparation of natural porous materials

In my thesis, sugarcane bagasse was selected as the main ingredient for the synthesis of photothermal material from carbon-based aerogel Sugarcane bagasse was collected from several sugarcane juice stalls The collected sugarcane bagasse was cleaned with water before being dried at 50°C for about 72 hours Then, the dried sugarcane bagasse was grounded before being sieved to obtain particle size of less than

1 mm Figure 2.1 shows the fabrication procedure of cellulose-based aerogel (including white and black aerogel) The fabrication process of both black and white aerogel consists of 3 Step: 1 Extraction of cellulose; 2 Preparation of cellulose suspension; 3 Preparation of cellulose-based aerogel from the suspension The Cellulose extraction (Step 1) and Aerogel preparation (Step 3) of the fabrication procedure for both black and white aerogel were the same while the Step 2 Preparation of cellulose suspension

of black aerogel was slightly different from that of white aerogel as Tannic acid and

Fe3+ were used for the fabrication of black aerogel

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

1 Immerse the ground bagasse to a solution of Ethanol: H2O 1:1, keep at 80°C for 3 hours Wash with a vacuum filter

2 Immerse the product of Step 1 in a solution of NaOH 2M, keep at 80°C for 3 hours Wash with a vacuum filter until pH = 7

3 Immerse the product of Step 2 in a solution of NaOH 1% and NaClO 1%, keep at 80°C for 2 hours Wash with a vacuum filter until pH = 7

Step 2.1: Preparation of suspension of white aerogel

1 Mix the extracted cellulose with distilled water (M1)

2 Dissolve PVA in distilled water to obtain PVA solution, (M2)

3 Slowly added M1 to M2 under vigorous stirring until all substances were evenly distributed in the mixtures

4 Sonicate for 10 min at 200 W to obtain the final suspension

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Step 2.2: Preparation of suspension of black aerogel

5 Mix the extracted cellulose with tannic acid solution for 1 hour (M1) The concentrations of Tannic acid were selected as: 3, 5 and 10 mg/ml

6 Dissolve PVA and FeCl3 in distilled water to obtain PVA+ Fe3+ solution The concentration of FeCl3 and PVA are shown in Table 2.1 (M2)

7 Slowly added M1 to M2 under vigorous stirring until all substances were evenly distributed in the mixtures

8 Sonicate for 10 min at 200 W to obtain the final suspension

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

1 Put the suspension in an ultra-low temperature freezer, keep at -70°C for at least 6 hours

2 Freeze-dry the frozen suspension for 3 days to obtain the final aerogel

Figure 2.2 a) White cellulose-based aerogel, b) black cellulose-based aerogel One sample of white aerogel, denoted as CBA1 and two samples of black aerogel (denoted as CBA2 and CBA3) were fabricated The fabricated white aerogel was cellulose-PVA composite aerogel in which PVA acts as the crosslinker between cellulose chains, improving the mechanical properties The fabricated black aerogel consists of cellulose, PVA and tannic acid and the complexes between Tannic acid and Tannic acid Tannic acid was either crosslinked with cellulose or PVA Information on the composition of all aerogel samples are shown in Table 2.1:

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Table 2.1 Composition of white and black aerogel samples

Sample ID

Concentration of substances in the final suspension

CBA1 (white) 20 mg/ml 20 mg/ml 0 0

CBA2 (black) 20 mg/ml 20 mg/ml 3 mg/ml 6.0 mg/ml

CBA3 (black) 20 mg/ml 20 mg/ml 5 mg/ml 6.0 mg/ml

2.2 Characterization of photothermal materials

The cellulose-based aerogel was characterized to understand its morphology, surface structure, light absorbance, crystal structure, specific surface area, elemental composition, mechanical properties 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

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

4 Nicolet iS50 FTIR - 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

BET analysis method

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

7 Oriel® Sol1ATM - Solar stimulator to evaluate the water evaporation

ability of SSG system

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Figure 2.3 Some instruments used in this research (a) JSM-IT100 InTouchScopeTM Scanning Electron Microscope (b) Oriel® Sol1ATM Solar Simulators (c) FLIR C2

camera

2.3 Investigate the photothermal material’s performance

2.3.1 Investigation of the material thermal behavior under laboratory condition

Figure 2.4 Setup of the experiment to measure thermal behavior of the aerogels

In order to study the thermal behavior of the aerogels, both white and black aerogel were exposed to the solar stimulator under 1 sun illumination for 10 minutes (Figure 2.4) During the experiment, the temperatures of both samples were constantly monitored using FLIR C2 camera

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2.3.2 Investigation of the material’s performance under laboratory condition

Figure 2.5 a) Solar steam generator containing water supply path, absorber and source

of water b) Solar simulator

Figure 2.5 illustrate the experiment setup to assess the photothermal material’s performance using solar simulator A mini SSG system has been made from a 100 ml glass beaker containing seawater, a piece of polystyrene foam, wrapped in gauze pads and a piece of photothermal material The piece of photothermal material was placed on top of the polystyrene which in turn float on the seawater (as shown in Figure 2.4(a)) The polystyrene piece main roles are to keep the photothermal material above the water surface and to prevent heat transfer from the material to the bulk water The cotton gauze helps to transfer water to the photothermal material’s surface where the water is evaporated under heat, absorbed by the photothermal material For the evaporation experiment, the mini SSG system was put under the Oriel® Sol1ATM solar simulator, and on top of an electrical balance The evaporation experiment was carried out for 60 minutes under 1 Sun illumination During the experiment, the temperature variation of the material’s surface and the bulk water were constantly monitored using a FLIR C2 camera The mass changes of the mini SSG system during the experiment were recorded for every 10 minutes

The device for assessing the material’s performance under real sun condition is shown

in Figure 2.6 The inner sample container (Figure 2.6(c)) is an open top glass box with

an open top area of 100 cm2 The outer compartment has dimensions of 12 cm in width,

24 cm in length and 25 cm in height (Figure 2.6(d)) The experiment setup is illustrated

in Figure 2.6(a) The inside sample container contain seawater, polystyrene foam and photothermal material, similar to the mini SSG mentioned in Section 2.3.2 The device was placed under natural sunlight from 8:00 am to 6:00 pm for 3 consecutive days Under the effect of sunlight, the generated steam condensed in the outer compartment’s top plate before being collected on the outer compartment’s bottom The volume of water evaporated & collected, outdoor temperature, humidity and solar intensity were recorded at regular interval