Core-shell structured graphene aerogels with multifunctional mechanical, thermal and electromechanical properties

Journal Pre-proof Core-shell structured graphene aerogels with multifunctional mechanical, thermal and electromechanical properties Jannatul Dil Afroze, Md Jaynul Abden, Ziwen Yuan, Chaojun Wang, Wei Li, Yuan Chen, Liyong Tong PII:

S0008-6223(20)30203-7

DOI:

https://doi.org/10.1016/j.carbon.2020.02.057

Reference:

CARBON 15107

To appear in:

Carbon

Received Date: 14 November 2019 Revised Date:

9 February 2020

Accepted Date: 18 February 2020

Please cite this article as: J.D. Afroze, M.J. Abden, Z. Yuan, C. Wang, W. Li, Y. Chen, L. Tong, Coreshell structured graphene aerogels with multifunctional mechanical, thermal and electromechanical properties, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.02.057. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Contributions: JD Afroze and L Tong conceived and designed the experiments after discussion with Y Chen; JD Afroze proposed synthesized all samples, and conducted all SEM, TEM, mechanical, electroand thermomechanical measurements; MJ Abden performed the XRD and Raman measurements and analyzed the results; Z Yuan contributed to the XPS experiments; C Wang conducted the TGA test. L Wei contributed to the discussion of results; JD Afroze analyzed the data and wrote the manuscript; L Tong and Y Chen supervised the project and participated in analysis and discussion of test results as well as write up and revision of the paper.

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Core-shell structured graphene aerogels with multifunctional mechanical, thermal and

2

electromechanical properties

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Jannatul Dil Afrozea, Md Jaynul Abdenb, Ziwen Yuanc, Chaojun Wangc, Li Weic, Yuan Chenc,*,

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Liyong Tonga,*

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a

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2006, Australia

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b

School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW

School of Computing, Engineering and Mathematics, Western Sydney University, NSW 2751,

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Australia

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c

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Australia

School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006,

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Corresponding author email: [email protected] (L. Tong); [email protected]

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(Y. Chen)

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1 2 3

ABSTRACT

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Many engineering applications demand lightweight materials with multifunctional mechanical

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properties. Graphene aerogels (GAs) have emerged as a potential candidate. However, GAs reported

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so far exhibit weak mechanical strength. Here, we report a two-step freezing method with assistance

7

of borate cross-linkers to synthesize a core-shell structured GA. The large temperature gradient can

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control the nucleation and growth of ice crystals, leading to the formation of a densely packed core

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and sparsely packed shell. This unique structure can be turned for high compressive strength (43.43

10

KPa at 50% strain) and elasticity through consecutive distribution of mechanical loads between the

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core and shell. It can fully recover from 70% strain and 100 compression cycles under 50% strain.

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The GA also shows excellent compression sensitivity to electrical resistance, and the first-ever

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reported creep resistance for GAs with negligible residual strain under a static force of 4 kPa up to

14

200 °C in the air. The as-formed core-shell GAs exhibit stable piezoelectric effects, ultralow thermal

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conductivity ( 0.023 W m-1K-1) and superior electrical conductivities (up to 52.99 S/m at 70%

16

strain). The unique architecture and its multifunctional mechanical properties make it promising for a

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range of applications, including flexible sensors, actuators, thermal insulation, and electronics.

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Keywords: graphene aerogel, freezing, compressive strength, piezoresistive sensing.

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1. Introduction

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Lightweight carbon materials with excellent mechanical properties and multifunctionality are

4

desirable for many applications [1, 2]. For example, three-dimensional graphene aerogels (3D GAs)

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are lightweight carbon materials with high electrical conductivity and large surface area [3-5], which

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are promising for applications, such as energy storage, adsorption, catalysis supports, and sensors [6-

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10]. In addition to electrical conductivity and surface area, it is desirable to obtain 3D GAs with high

8

mechanical strength, flexibility, and thermo-mechanical stability. Previous studies demonstrated that

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it is possible to achieve some of these mechanical properties by regulating the geometry and

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chemical compositions of GAs [11]. For example, GAs with aligned pore structures have beneficial

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mechanical properties compared with those with random pore structures [12-14]. However,

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sustaining structural integrity over large strain, little energy dissipation, as well as creep resistance

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ability with stable piezoresistivity remains a big challenge, as these properties are tough to attain

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simultaneously [15-17].

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3D GAs have been obtained via chemical vapor deposition (CVD) methods on sacrificial

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templates [18, 19] or by 3D printing of graphene-polymer inks [10, 13]. However, most of the 3D

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GAs reported so far have random pores and exhibit weak mechanical strength [20-22]. Further, these

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methods are costly and complicated [11]. The strength of aerogels relies on their cellular

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architecture, density, constituted components [23], and bonding patterns [9].

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Freeze casting water containing graphene hydrogels (GHs) followed by freeze-drying can partly

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regulate pore structures of resulting Gas via different freezing temperatures and directional freezing

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[24, 25]. In particular, a few studies reported that the pore structures of GAs depend on ice crystals

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formed from water trapped between graphene oxide (GO) sheets of GHs. The growth of these ice

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crystals and the aggregation of GO sheets can be influenced by the freezing temperature of GHs [26].

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One-step freezing of GHs, followed by ambient drying, have produced GAs with some ordered pores 3

1

[27, 28]. However, these GAs still showed poor compressibility and would collapse under low

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strains (< 20%) [29-31]. On the other hand, we notice that the excellent strength and stiffness of

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natural plants originate from their hierarchical core-shell structures, and the borate chemistry of

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cellular compounds can strengthen cell walls to provide mechanical supports to plants’ intercellular

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structures [32, 33]. Thus, it may be possible to mimic the structure of natural woods [34, 35] and use

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their corresponding borate chemistry [36] to assist the synthesis of GAs.

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Here, we demonstrated a new method to synthesize core-shell structured GA. A dual temperature

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gradient was created by a two-step freezing method to control the ice crystal growth. Further, borate

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was used as cross-linkers to enhance the interactions among graphene sheets. The morphology and

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physicochemical properties of resulting GAs were compared with GAs prepared by conventional

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one-step freezing methods. Comprehensive studies were carried out to examine mechanical

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properties, temperature-invariant creep resistance behaviors, and electromechanical properties of

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GAs. The potential correction between the microstructure of GAs and their properties was discussed.

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2. Experimental

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2.1 Preparation of graphene aerogels

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GO aqueous dispersion was purchased from Graphenea. Ethylenediamine (C2H8N2, EDA, 99.5%),

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sodium borate decahydrate (Na2B4O7·10H2O, 99%) were used as received from Sigma-Aldrich. As

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illustrated in Fig. 1, a high concentration GO aqueous dispersion (10 mg mL−1, 10 mL) was first

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mixed with a weak reducing agent, i.e., EDA aqueous solution at 10 µL mL−1. Next, about 80 µL 5

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wt.% Na2B4O7 solution was added to the mixture under stirring (maintaining at the concentration of

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8 µL/mL), which served as a cross-linker for GO sheets. The mixture was further sonicated for 30

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min in an ice-water bath. Afterward, 1 mL of the mixture was poured into a sample bottle, which was

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placed inside a 25-mL Teflon-lined autoclave and maintained at 130ºC for 14 h. After the

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hydrothermal assembly, the wet GHs were formed and then dialyzed with ethanol/deionized (DI)

4

1

water mixture (1:100) for 6 h. These GHs were frozen at -20°C for 12 h in a temperature and

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humidity controlled environmental chamber and then dipped into liquid N2 of -196°C for 10 min.

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Afterward, the frozen GHs were dried at the ambient condition for 48 h to obtain the final GAs,

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which are denoted as GA-2. It should be noted that the synthesis of GA-2 involved two separate

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freezing steps at -20°C and -196°C, respectively, which is different from the one-step freezing

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method previously reported [27]. For comparison, we also prepared GAs using a one-step freezing

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method by extending the time of the first freezing step at -20°C for an extra 3 h without the second

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freezing step at -196°C. GAs synthesized by the one-step freezing method are denoted as GA-1.

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Fig. 1. Schematic illustration of the synthesis and structures of GAs by the one-step and two-step

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freezing methods and the photos of GHs and GAs at different synthesis steps.

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2.2 Characterization of physicochemical properties

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The dimension and mass of GAs were measured using a caliper with an accuracy of 0.01 mm and

2

an analytical balance (XP24, Mettler Toledo) with an accuracy of 0.001 mg. Their density was

3

calculated using the measured mass and volume. Their surface morphology and microstructures were

4

examined by a scanning electron microscope (SEM, Sigma HD FEG, Zeiss) and transmission

5

electron microscope (TEM, JEOL JEM-2100). The crystal structures were examined by an X-ray

6

di ractor (XRD, D8 Discover 25, Bruke) under a Cu Kα radiation (λ = 1.54059 Å). Their chemical

7

structures were characterized using a Raman spectrometer (Lab RAM HR Evolution, HORIBA)

8

under a 514 nm laser excitation and an X-ray photoelectron spectrometer (XPS, Thermo Scientific).

9

The thermal properties were measured by a thermogravimetric (TG) analyzer (Q600) up to 950 °C

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with a temperature increasing rate of 10 °C min−1 in air. The thermal conductivity of the hydrogels

11

was determined by a hot-disc thermal constant analyzer (TPS 2500 S, Horiba).

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2.3 Characterization of mechanical, thermo-mechanical and electromechanical properties

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The compression tests of GAs were conducted on a universal mechanical test machine (5570,

15

Instron) equipped with Bluehill software. GA samples with a cylindrical geometry (8 ± 0.1 mm in

16

length and 7 ± 0.1 mm in diameter) were fixed between two flat-surface compression stages within a

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100 N load cell. The sample was then compressed in a perpendicular direction applying different

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strain and then recovered to the original state at a constant crosshead velocity of 2 mm min−1. The

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cycling test was repeated 100 times with an applied strain of 50% to measure the compressive

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behavior and recovery of the GAs. The desired strain was input into the software that commands the

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Instron towards the desired strain level. The command input and processing of commands in Instron

22

is quite simple and straightforward. The applied strain was measured automatically by the instrument

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following the formula; Compressive strain: ɛ =

24

the sample measured manually by the caliper and input in the system, and L2 is calculated by the

25

instrument. The energy loss coe cient (ξ) was determined as the loop area relative to the area under

(

6

)

× 100 %; where L1 is the initial length of

1

the loading curve, and the total loss strain (η) was observed as the di erence between the

2

corresponding strain values when the stress was zero during a loading-unloading process.

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A dynamic mechanical analyzer (DMA Q800, TA Instruments) was used to evaluate the creep

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(thermo-mechanical) performance of GAs. The temperature of GA samples was maintained by a

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forced convection oven. Cylindrical GAs was loaded between two vertically parallel plate

6

compression heads. The GAs was preconditioned under ∼3 % strain to keep full contacts between

7

the compression heads and the samples, which prevented slipping throughout the measurements.

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Before data collection, GAs was equilibrated to the experimental temperature. The creep and

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recovery tests were performed from 25 to 200 ºC with the applied stress of 1 and 4 kPa for 30 min.

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The electrical conductivity and surface resistance were measured using a precision source meter

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(Alginate 2900A, Keysight) and its associated software using the four-probe method under a current

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range up to 100 mA, with a linear probe head (2.0 mm space). The source meter was synchronized

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with the universal mechanical test machine (5570, Instron) to measure the pressure-responsive

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conductivity. The samples were compressed and released at a rate of 2 mm/min, up to a maximum

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strain of 70%. For better electrical contact two surfaces of tested cylindrical GA block were coated

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with a thin layer of silver epoxy paint to connect two thin copper wires, and then the other two ends

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of the copper wires were connected to the source meter [37-39]. The conductivity was calculated

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using the equation: = , where

is the resistivity.

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3. Results and discussion

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3.1 Formation of core-shell structures in graphene aerogels

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Two different types of GAs were synthesized using the one- and two-step freezing methods,

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respectively, as described in Section 2.1. The resulting GA-1 and GA-2 have a cylindrical shape with

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8 ± 0.1 mm in length and 7 ± 0.1 mm in diameter. Their average densities are 17.7 ± 0.5 mg cm-3 for 7

1

GA-1 and 18.0 ± 0.5 mg cm-3 for GA-2. Despite their similar average density, they exhibit different

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microstructures. SEM images in Fig. 2 show longitudinal and cross-sectional views of GAs. GA-2

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exhibits a unique core-shell structure. Fig. 2d shows that the central core area of GA-2 spans

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mm in diameter, which counts for 7% of the cross-sectional area of GA-2. The core area contains

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small micropores with an average pore area of 300 ± 150 µm2 and average pore size of 20 ± 10 µm

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(Fig. 2i). The analysis of SEM images was conducted using the software ImageJ. The analysis results

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in Fig. S1-S3 in the Supplementary data indicate that the core area has a porosity of

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shows that the central core of GA-2 is surrounded by a sparse shell area, which has larger pores with

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an average pore area of 31500 ± 3500 µm2 and average pore size of 230 ± 20 µm. Fig. S2 shows that

1.85

85%. Fig. 2e

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the shell area has a porosity of

95%. In contrast, Fig. 2a-c show that GA-1 has a random porous

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structure in both longitudinal and cross-sectional directions. The pore size of GA-1 varies from 100

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to 220 ± 20 µm, and its pore area is 30000 ± 3400 µm2 with a porosity of

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Table S1 in the Supplementary data for details).

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91% (see Fig. S1-S3 and

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Fig. 2. SEM images of longitudinal and cross-sectional views of (a, b) GA-1 and (d, e) GA-2. (c, f)

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different cross-sectional regions of GA-1 and GA-2. (g, h) oriented hexagon-like cellular structures

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in the core and shell regions of GA-2, respectively. (i) The mean pore areas of five different areas as

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marked in (b) and (e), respectively.

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The average pore size of GA-1, the shell of GA-2, and the core of GA-2 is 220 ± 20 µm, 230 ± 20

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µm and 20 ± 10 µm, respectively. These pore sizes are broadly on par with the mean pore size of

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GAs previously reported [24]. Previous studies reported that the mean pore size of GAs would

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decrease at lower freezing temperatures [24, 26, 40, 41]. For example, GAs with various pore sizes

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were synthesized using a one-step freezing method with freezing temperatures ranging from -10 to -

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170 ºC in a previous study [21]. Our results suggest that the correlation between the mean pore size

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of GAs and their freezing temperature persists despite distinct synthesis parameters used in different

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studies [21].

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It should be noted that the freezing time and temperature used in our two-step method was

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selected based on a parametric study on the influence of freezing time using the one-step freezing

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method. We prepared several GA samples by freezing GHs for 8, 10, 12 to 15 h at -20 °C. Fig. S4 in

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the Supplementary data shows a photo of these GA samples. The 15 h freezing GA sample (GA-1)

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can retain the original shape of the water containing GH after ambient drying (i.e., 7 mm in diameter

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and 8 mm in height). In contrast, the other three GAs with shorter freezing time all show some

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volumetric shrinkage or structural collapses. The 12 h freezing GA sample exhibits a hyperboloid

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shape with the diameter of its upper and lower section at 6 mm and the middle section at around 4

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mm and a height of 6.2 mm. Considering this sample has a smaller shrinkage compared to the other

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two GA samples, we selected 12 h freezing at -20 °C as the first step in our two-step method.

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1

As illustrated in the schematic drawings in Fig. S5 in the Supplementary data, when a GH

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cylinder is placed in a freezing medium, ice crystals would nucleate and grow from the outer surface

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of the GH cylinder toward its inner center in all directions [25]. We propose that the core of GH

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cylinders would not be fully consolidated after the freezing at -20 °C for 12 h. Thus, the partially

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frozen GH can be further consolidated at a lower temperature with a large temperature gradient from

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the surface to the core [21,34,35,36]. When the GH was dipped in -196 ºC liquid N2, large ice

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crystals were fully solidified near to the surface, while smaller ice crystals were formed near to its

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core. Thus, the resulting GA-2 after drying has the core-shell porous structure as shown in Fig. 2d.

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3.2 Physiochemical properties

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The crystal and chemical structures of GAs were examined by XRD, Raman spectroscopy, XPS,

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and TG analysis (TGA), respectively. Fig. 3a shows the XRD spectra of GO sheets, GA-1 and GA-2.

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The diffraction peak of GO at

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show weak and broad features related to single- or few-layer graphene structures. These XRD results

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are similar to those previously reported for 3D GAs [13]. Raman spectra in Fig. 3b have two

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apparent peaks for all three samples. The strong D-band peak at

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while the G-band peak at

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(ID/IG) for GO is 0.95, while this ratio of GA-1 and GA-2 increases to 1.17. This is due to partially

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restored conjugation of sp2 regions of graphene sheets in GAs [28].

12° disappears after the hydrothermal reaction. GA-1 and GA-2

1350 cm-1 indicates defects in GO,

1590 cm-1 is related to graphitized carbon structures. The intensity ratio

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Fig. 3. The characterization of physicochemical properties of GO sheets, GA-1, and GA-2, (a) XRD

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patterns, (b) Raman spectra, (c) XPS spectra, and (d) TGA profiles.

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Results of XPS confirm hydrothermal reduction of GO sheets and the cross-linking effects of

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Na2B4O7. Fig. 3c shows XPS survey scans of the three samples. C, N, and O peaks appear at 285.3,

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400.5, and 533.2 eV, respectively. GA-1 and GA-2 have N peaks due to the use of EDA in the

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hydrothermal reaction. The deconvolution of their C1s spectra shown in Fig. S6 in the

9

Supplementary data. The XPS peaks at 284.2, 285.4, 286.7, and 288.4 eV in the C1s spectrum of GO

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sheets can be assigned to C-C, C-OH, C-(epoxy/ether) and C=O, respectively. In comparison, weaker

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C=O/COOH peaks in GA-2 indicate that some O-containing functional groups are removed after

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hydrothermal reaction. The sharp peak at 285.0 eV can be assigned to the C-O-B bond, indicating the

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cross-linking role played by Na2B4O7 in GA-2 [28].

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Further, TGA profiles in Fig. 3d show that GA-1 and GA-2 have enhanced thermal stability

15

compared to GO sheets. GO sheets have ~ 10-15 wt.% mass loss at 100°C, which can be ascribed to

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the removal of solvents. The decomposition of O-containing functional groups at 200 °C causes 11

1

another 20 wt.% mass loss. GO sheets have a weight loss of

95.3 wt.% at 600°C. In comparison,

2

GA-2 only has a weight loss of 64.1 wt.% at 600 °C. The improved thermal stability of GA-2

3

suggests that it may be used in applications with high temperatures up to 300 °C.

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3.3 Mechanical properties

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The mechanical behaviors of GA-2 and GA-1 were first investigated using a cyclic compression test

7

at set strains. The measured mechanical properties were compared to reveal the influence of their

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structural features, in particular, the influence of the presence of a dense core in GA-2. The

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compressive stress-strain curves at a set strain were recorded for up to 100 loading and unloading

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cycles. Fig. 4a and 4b depict the typical stress-strain curves measured with a set strain of 50 % for

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GA-2 and GA-1, respectively. The stress-strain curves for both GA-2 and GA-1 are similar except

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for the first loading segment. The strengths at 50 % set stain in the 1st, 2nd, 20th, 50th and 100th

13

loading cycles reach 43.43, 39.52, 31.67, 29.27, and 28.74 KPa for GA-2 as shown in Fig. 4a; and

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they are 31.95, 28.31, 22.33, 17.43, and 15.84 KPa for GA-1, as shown in Fig. 4b. Although there are

15

no obvious differences on density between GA-2 and GA-1 as they fabricated from same GO

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suspension of 10 mg mL-1, the GA-2 is stronger than GA-1 by 36, 39.6, 41.8, 68, and 81.4% for the

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1st, 2nd, 20th, 50th, and 100th loading cycles. The first loading curve exhibit in GA-1 what can be

18

defined as a pseudo-hardening behavior where the slope of the curve increases rapidly up to 35%

19

strain. However, the high energy dissipation and energy loss coefficient (ξ) of GA-1 (Fig. 4c) is a

20

result of micro rupturing during compression at 35% strain and it remains after the cycle [42, 43].

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This damage contributes to the non-recoverable deformation and the decrease in mechanical

22

properties of GA-1. The compressive strength of GA-2 with density of 18 mg/cm3 is much higher in

23

comparison to those in literature reported earlier with similar density, e.g. 18.9 KPa with density of

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17.8 mg/cm3 at 60% strain [38] and 25.6 KPa with density of 16.5 mg/cm3 at 50% strain [14]. The

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well-organized core-shell architectures with borate bridged walls providing necessary strength at the

12

1

expense of the strong framework can maximize mechanical strength compared with other bulk

2

aerogels of analogous bulk density [14, 38, 44, 45]. Without sodium borate, the GA exhibits a plastic

3

deformation with a structural breakdown during the compression cycle (see Video 1 in the

4

supplementary data and Fig. S7a) due to the absence of borate linking/bridging. Borate ions can

5

serve as cross-linkers to covalently bond oxygen containing functional groups to create mechanically

6

robust structures. As shown in Fig. S7c, the compressive strength of GA-2 is increased around two

7

times by using borate as a chemical cross-linker.

8

Fig. 5c depicts the variation of the strength recovery ratio and ξ for GA-2 and GA-1, respectively.

9

GA-2 retains 66.2 % of the original strength at 50 % set strain after 100 loading cycles, while GA-1

10

only retains 49.6 % of the original strength.

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The significance of the two-step freezing process for enabling compressibility in GA-2 can be

12

further established by analyzing their ξ values. The energy dissipation mechanism throughout the

13

compression process of cellular GA governed by the fracture and friction or sliding between the

14

interconnected graphene cell walls [46, 47]. Depending on the compression cycles, the ξ value of

15

GA-1 ranges from 89.31% (1st cycle) to 69.24 % (after 100 cycles), which is much higher than that

16

of GA-2 (see Fig. 4c). The 1st cycle yields a ξ of 78.18 % for GA-2, and this coefficient remains

17

constant at 55.55 % after 50 cycles. These findings confirm that the mechanically strong small core

18

area of GA-2 would reduce the possibility of inter-sheet friction and permanent damage of the

19

connecting cell walls [46]. Moreover, the less energy dissipation in the cycling process indicates that

20

more elastic energy was stored, facilitating the recovery of GAs from large compression strains.

21

Fig. 4d shows the total strain loss in the loading and unloading cycles for GA-1 and GA-2. The

22

strain loss in loading for both samples exhibits a similar trend. Although there are no apparent

23

differences in strain losses in the 2nd loading cycle between GA-1 and GA-2, GA-2 is much more

24

stable. For example, the strain loss of GA-1 reaches 11.9 % in the 100th cycle, which is 39.3% higher

25

than the strain loss value of GA-2. Similarly, compared with GA-2, the unloading strain loss of GA-1 13

1

increases by 139.6 % (from 2.98 to 7.14 %) and 86.35 % (from 5.13 to 9.56%), respectively in 1st

2

and 2nd cycles. It should be noted that the strain loss of GA-1 increases continuously in a linear

3

fashion up to 100 cycles while GA-2 delivers a stable strain loss after 50 cycles, indicating excellent

4

structural integrity of the GA-2. The highest strain loss obtained in GA-1 is 16.6 % after 100 cycles,

5

which are higher than the highest strain loss value of 11. 53 % for GA-2.

6 7 8

Fig. 4. Compressive properties of GA-2 and GA-1, (a, b) the stress-strain curves at 50 % strain;

9

Inset: experimental snapshots of one compression cycle, (c) strength recovery ratio and energy loss

10

coefficient, and (d) total strain loss of GA-2 at 50% strain for 100 cycles, (e) the compressive stress-

11

strain curves of GA-2 at different set strain, and (f) ultimate strength and strength recovery ratio for

14

1

several graphene porous materials reported in literature; the numbers in parentheses signify the

2

corresponding strain and compressive cycles.

3 4

More interestingly, GA-2 exhibits good elastic behavior (see Video 2 in the supplementary data)

5

and can sustain a higher compressive ε of 70% and able to recover its original level after the release

6

of the stress. Fig. 4e depicts the compressive stress-strain curves of GA-2 at 30%, 50%, and 70% set

7

strains. In comparison with the maximum strengths at 70% strain of GA-1 (Fig. S8) or other reported

8

GAs [44, 48-50], GA-2 possesses much higher compressive strength (60.4 KPa). To further

9

demonstrate the advantages of the unique core-shell structured GA prepared by the two-step freezing

10

process, we compared its mechanical properties with other studies of GAs in the literature [14, 25,

11

31, 38, 45, 48-51]. The strength is plotted as a function of strength recovery ratio as shown in Fig. 4f.

12

Although the strength recovery ratio of GA-2 is modest compared with some of the previous studies,

13

its strength is high, even after 100 cycles with ultimate stress of 28.7 KPa, which is comparable with

14

initial strength values of many previously reported GAs at similar strain [25, 48].

15

The elasticity of GA-2 was further examined by in situ SEM imaging. As shown in Fig. 5, the

16

cellular walls of GA-2 can recover to their original condition after releasing from a 50%

17

compression. Both its shell and core regions have no significant structural changes upon structural

18

evolution during compression. Fig. S9b in the supplementary data shows that the height of the GA-2

19

samples recovers almost to its original one even after 100 compression cycles at 50% strain. The

20

microstructure of GA-2 before (Fig. S9c) and after (Fig. S9d) 100 compressive cycles were also

21

traced by using SEM. Fig. S9d show that its pore walls made of graphene sheets can bend and buckle

22

without cracks and most of the sheets were able to recover their original state indicating that GA-2

23

experiences small structural damage.

24

15

1 2

Fig. 5. In-situ observations of the tracking of GA-2 at 50 % strain, (a) optical photos of GA-2 in a

3

sample holder under different conditions, and corresponding SEM images with different

4

magnifications for (b) shell and (c) core regions of GA-2.

5 6

The above mechanical property results indicate that GA-2 is much more resilient and flexible than

7

GA-1. As shown in Section 3.1, the two types of GAs have significantly different microstructures.

8

We propose that the observed mechanical performance enhancement of GA-2 can be attributed to its

9

unique microstructure. The cross-sectional SEM image of GA-2 (Fig. 2e) shows that it has a shell

10

with a honeycomb structure surrounding a central core, which is different from the disordered

11

structure of GA-1 (Fig. 2b). Previous studies have demonstrated that GAs with honeycomb

12

microstructures have enhanced mechanical strength and elasticity [28, 46]. Fig. 2g and 2 h show that

13

both core and shell regions of GA-2 have ordered honeycomb structures. The SEM image in Fig.

14

S10a in the supplementary data shows that GO sheets are interlinked with their neighboring sheets,

15

forming three-way junctions. Such an interlinked structure is expected to maximize π–π interactions

16

among GO sheets, which are beneficial in strengthening the mechanical properties of GAs [46]. 16

1

Further, suitable pore sizes and pore walls can also contribute to the mechanical performances and

2

elasticity of GAs [52]. GA-2 has well-ordered pores, which provide smooth channels for load

3

transfer. Fig. S10b in the supplementary data shows that the core region of GA-2 has thin pore walls

4

(comprising of thin layers of stacked graphene sheets) and small pore sizes, which can ensure

5

abundant linkages among adjacent graphene sheets.

6

In particular, the superb elasticity of GA-2 may be ascribed to the elastic nature of GO sheets [53]

7

and wrinkled structures formed by GO sheets [54]. “Wrinkles” formed by GO sheets can provide the

8

elastic resisting force for GAs to recover to their original shape after releasing the applied force [55].

9

As shown in Fig. S10b and S10c, GA-2 contains abundant “wrinkles” formed by GO sheets.

10 11

3.4 Temperature-invariant creep resistance behaviors

12

Materials durability and propriety is essential for potential applications in extreme environmental

13

conditions where time- and temperature-dependent behaviors, such as creep, serve as a possible

14

indicator. Fig. 6 depicts creep profiles of GA-2 and GA-1 as a function of temperature under two

15

different stress (1 and 4 kPa) for 30 min at three different temperatures 25, 100, and 200 °C,

16

respectively. The GAs deforms to the equivalent strain instantly and sustains that strain for the whole

17

time period, demonstrating no resolvable creep. Governing by compressive stress-strain curves, in

18

one set of tests, the constant stress of 1 kPa was applied in the elastic region for 30 min with 5 min

19

for recovery. In the other set of tests, 4 kPa stress was applied in the plateau region with the same

20

experimental conditions. Fig. 6 shows that GA-2 reaches to equivalent strain straightaway at all three

21

temperatures under the two different applied stresses. GA-2 represents <1% creep strain in the elastic

22

region, and

23

respectively. These results suggest that there is no plastic creep because the recovery is good enough

24

within a given recovery time. However, the GA-1 show creeps of 2.2 and 3.4% in the elastic region

25

at 100 and 200 °C and

1% creep strain in the plateau region, avoiding any residual strain at 100 and 200 °C,

4% in plateau region at both temperatures (Fig. 6), over the duration of

17

1

applied stress (1 and 4 KPa). Besides, under constant stress of 4 KPa for 30 min, GA-1 shows

2

residual creep strain of 4.6%, 6.0% and 6.2% at 25, 100 and 200°C, respectively.

3

It is apparent that creep strain slightly increases with rising temperatures in both elastic and

4

plateau regions as a consequence of relaxation of the bridging junctions along with recoverable

5

slippage among the sheets of GAs, which is temperature-sensitive [56]. The creep recovery and

6

resistances are intensely dependent on the microstructures of the materials. The relative changes of

7

creep and recover strains are more apparent for highly oriented honeycomb-like structured GAs. The

8

strong bridging and core-shell microstructure provides a reflective exposition for GA-2’s endurance

9

and recovery improvement under creep conditions. GA-2 also has a low thermal conductivity of

10

0.023 W m-1 K-1, which is beneficial for thermal insulation applications.

11 12

Fig. 6. Creep profiles of GA-2 and GA-1 at temperatures of 25, 100 and 200 °C under a constant σ of

13

(a) 1 kPa, and (b) 4 kPa for 30 min.

14 15

3.5 Electromechanical properties

16

Elastic and strong GAs may find applications in flexible electronics [39, 57]. We further evaluated

17

the electrical conductivity of GA-2 and GA-1 under compression to check suitability for their

18

application in flexible electronics and sensors. Its electrical resistance ratio ((R0 − R)/R0) was

19

analyzed under different applied strains, where R and R0 are the resistances with and without strain.

20

As shown in Fig. 7a and Fig. S11a in the supplementary data, both GA-2 and GA-1 exhibit steady 18

1

responses under a broad range of strains. Its electrical resistance ratio gradually increases from

8 to

2

96% for GA-2 while from 4.9 to 76.1% for GA-1 under the increasing strains from 10 to 70 %. This

3

changing trend may be contributed to the fact that more contacts are established among graphene

4

sheets under higher strains, which leads to more conductive pathways for electron transport. Fig. 7b

5

shows the corresponding conductivity profile of GA-2 and GA-1, respectively. The conductivity of

6

GA-2 increases significantly from 3.00 to 52.99 S/m when the applied strain rises from 10 to 70%,

7

which is much higher than the values of 1.98-8.28 S/m for GA-1 and 1.15-7.48 S/m under the similar

8

strains reported in a recent study [14]. Under 70% strain, the resistance is below 5 Ω, while the

9

highest resistance without compression is about 75 Ω. Fig. 7c and Fig. S11 in the supplementary data

10

show that GA-2 has more steady sensing performance compared to GA-1 and exceptional resilience

11

over 100 loading-unloading cycles under 30 % compression strain

12

excellent reversibility, stability, and resilience of GA-2 can be attributed to the high degree of sheet-

13

to-sheet interactions and its dense core. Overall, its electromechanical properties demonstrate

14

excellent potentials for flexible electronics and sensors.

(see also Fig. S11d). The

15 16

Fig. 7. Electromechanical performance of GA-2. (a) The changes of electrical resistances under

17

di erent applied strains over 10 cycles, (b) the equivalent conductivity-compressive strain profiles of

18

GA-2 in comparison with GA-1 and reference data, and (c) the piezoresistive behavior when GA-2

19

was repeatedly compressed up to 30% of strains for 100 cycles.

20 21

19

1

4. Conclusions

2

In summary, we used a two-step freezing method together with borate cross-linkers to form GAs

3

with a unique core-shell structure. In contrast to conventional one-step freezing methods, this new

4

two-step freezing method can regulate the morphology of ice crystals at different regions in graphene

5

hydrogels. The GA (i.e., GA-2) formed after drying has a dense core with an average pore size of 20

6

± 10 µm, which counts for 7% of the cross-sectional area of GAs, and a porous shell with the pore

7

size of 230 ± 20 µm. GA-2 shows enhanced compressive strength, elasticity, and high conductivity

8

of 3.27 S m−1, which can fully recover from 70% strain and even after 100 cycles under 50% strain.

9

GA-2 also demonstrates a stable piezo-resistive effect with a linear response to applied strains and

10

low thermal conductivity of 0.023 W m−1 K−1. Moreover, GA-2 shows a prominent creep resistance

11

behavior having negligible residual strain under a static force of 4 kPa up to 200°C for 30 min. To the

12

best of our knowledge, this creep resistance behavior has never been reported for GAs. This new GA

13

synthesis method may be extended to design various multifunctional materials with a unique

14

hierarchical core-shell structure. This multifunctional GA may also find multiple applications in

15

flexible electronics, sensors, actuators and so on. The fabricated novel core-shell GA by a two-step

16

freezing strategy exhibits amazing mechanical, electro and thermomechanical properties. The sparse

17

shell and the dense core arrangement augment these properties which can be realized by comparing

18

with the structure and properties of GA-1. Further studies should be conducted to fabricate GAs with

19

different core-shell structures by applying different freezing temperatures and time. Analysis can

20

then be conducted to evaluate the impact of core-shell on the multifunctional performance of GA.

21

Acknowledgments

22

JDA is a recipient of the EITR Scholarship from the Faculty of Engineering at the University of

23

Sydney. The authors wish to express their appreciation to Dr Yixiang Gan and Xu Wang for

24

providing source meter facilities. Y. Chen acknowledges financial supports from the Australian

25

Research Council under the Future Fellowship scheme (FT160100107).

20

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: