Area Graphene Papers for Personal Thermal Management

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Aug 5, 2017 - been dedicated to preparing thermal management devices ... the graphene paper provides passive cooling via thermal transmission from.
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Ultrathin, Washable, and Large-Area Graphene Papers for Personal Thermal Management Yang Guo, Chaochao Dun, Junwei Xu, Jiuke Mu, Peiyun Li, Liwen Gu, Chengyi Hou, Corey A. Hewitt, Qinghong Zhang, Yaogang Li,* David L. Carroll,* and Hongzhi Wang* nanocomposites.[11] Similarly, Ko and co-workers demonstrated a transparent heater for wearable electronics applications by constructing a partially embedded Ag NW percolative network on an elastic polydimethylsiloxane substrate.[12] Ag NW-coated textile is also reported by Cui and co-workers to retain body heat due to the conductive network that not only is highly thermal insulating as it reflects human body infrared (IR) radiation, but also allows Joule heating to complement the passive insulation.[6] The same group further processed a nanoporous polyethylene with interconnected pores (50–1000 nm) to develop a promi­ sing mid infrared transparent textile for human body cooling.[13] Chen and co-workers demonstrated a conceptual framework for an infrared-transparent visibleopaque fabric in order to provide personal cooling via thermal radiation from the human body to the ambient environment.[14] These versatile findings create great inspirations to use conductive nanostructures as building blocks for heating and use nanopores as the path for infrared radiating for PTM. However, the limitation of technologies mentioned above is that they are unable to achieve warming and cooling in one textile due to the independence of the functions from the materials. Meanwhile, antioxidation status of metallic nano­ wires deteriorates with the decreased dimension as a result of the high surface-area. To date, macroscopic assembled, free-standing, and chemically stable graphene papers (GPs) have been demonstrated as promising materials in many emerging fields. Noteworthy examples include ultrahigh-power-density supercapacitors,[15]

Freestanding, flexible/foldable, and wearable bifuctional ultrathin graphene paper for heating and cooling is fabricated as an active material in personal thermal management (PTM). The promising electrical conductivity grants the superior Joule heating for extra warmth of 42 °C using a low supply voltage around 3.2 V. Besides, based on its high out-of-plane thermal conductivity, the graphene paper provides passive cooling via thermal transmission from the human body to the environment within 7 s. The cooling effect of graphene paper is superior compared with that of the normal cotton fiber, and this advantage will become more prominent with the increased thickness difference. The present bifunctional graphene paper possesses high durability against bending cycles over 500 times and wash time over 1500 min, suggesting its great potential in wearable PTM.

1. Introduction Wearable devices, with the functions of monitoring real-time physiological[1] and biomechanical signals of the human body[2] and responding to external stimuli,[3,4] are emerging and gradually integrated into personal life. Among which, lightweight, flexible, and comfortable personal thermal management (PTM) becomes more prevalent, bringing with it the potential to wisely adjust body temperature to a thermally safe and comfort state. Therefore, considerable efforts have been dedicated to preparing thermal management devices based on different types of conductive materials including Ag nanowires (NW),[5,6] carbon nanotube,[7] and graphene[8–10] in pioneering work. Recently, Kim and co-workers developed a soft and stretchable heating element for articular thermotherapy using highly conductive Ag NW and thermoplastic elastomer Dr. Y. Guo, Dr. J. Mu, Dr. C. Hou, Prof. Q. Zhang, Prof. Y. Li, Prof. H. Wang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials College of Materials Science and Engineering Donghua University Shanghai 201620, P. R. China E-mail: [email protected]; [email protected] Dr. Y. Guo, Dr. C. Dun, Dr. J. Xu, Dr. C. A. Hewitt, Prof. D. L. Carroll Center for Nanotechnology and Molecular Materials Department of Physics Wake Forest University Winston-Salem, NC 27109, USA E-mail: [email protected]

Dr. P. Li Department of Physics Wake Forest University Winston-Salem, NC 27109, USA Dr. L. Gu Fashion Art Design Institute Donghua University Shanghai 200051, P. R. China Dr. L. Gu College of Textiles North Carolina State University Raleigh, NC 27695, USA

DOI: 10.1002/smll.201702645

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multistimuli responsive (light, temperature, and moisture) actuators,[16] programmable instant self-folding walking devices,[4] thermal management for high-power electronics and portable devices,[8,10] electronic skin for sensing human touch,[17] energy conversion and storage devices,[18,19] electrothermal application and deicing,[20] and planar lighting.[21] The superior chemical/mechanic stability, high thermal conductivity, and low electrical resistance of graphene also make it a suitable material for PTM, and yet has been rarely reported. Despite these benefits, GPs have relatively low out-of-plane thermal conductivity when combined in large-area and freestanding sheets, due to the relatively thick structure. An ideal graphene material for PTM is expected to be lightweight, flexible, highly thermally conductive, and scalable. However, a great challenge is the assembly of the macroscopic freestanding GPs with high out-of-plane thermal conductivity and large-area. Here, we report a freestanding, flexible/foldable, and largearea ultrathin GPs with high out-of-plane thermal conductivity and sensitive electrothermal response, and their application to wearable bifunctional PTM devices for heating and cooling. The heating part is achieved by taking advantages of its joule heating, while the cooling part benefits from its high out-ofplane thermal conductivity, which forms the ultrathin and compact lateral structure of the GPs.

2. Results and Discussions The simple fabrication process of ultrathin GPs is illustrated schematically in Figure 1. To achieve freestanding GPs, the graphene oxide (GO) gels are spread out on the prewashed substrate by an automatic coater (step 1). The wet GO papers were dried overnight at room temperature (step 2). The GO papers are then peeled off from the substrate when dried (step 3). The reduction process uses a mild method by vitamin C with a long reflux time (step 4). The challenge for developing high out-ofplane thermal conductivity GPs is to reduce the thickness of the GPs while still remaining freestanding. We achieved the large-area and freestanding GPs by blade-coating and using a commercially available sandpaper substrate. It helps the GO papers peel off completely and readily even when the thickness is as low as 1 µm, due to the fact that the rough surface of sandpaper stores a small amount of air in the gap between the substrate and the GO papers. The slow drying at room temperature avoids breaking the long-range oriented structure of the ultrathin GPs. The mild, slow, and long-time reduction with vitamin C is also beneficial to reduce the formation of defects and ensure thorough reduction from the surface to center of the GPs. Comparing with hydroiodic acid,[22] vitamin C impedes the rapid formation of a hydrophobic surface, which facilitates the diffusion of the reductant molecules into the

Figure 1.  Preparation process of the GPs by blade-coating. Step 1: Spreading the gels out on prewashed (with alcohol) sandpaper to form oriented wet paper. Step 2: Drying the wet paper at room temperature. Step 3: Peeling off the GO paper from substrate. Step 4: Reducing the GO paper to graphene paper using vitamin C.

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Figure 2.  Morphological characterizations of the GPs. a) Large-area and freestanding graphene paper with the size of 25 cm × 15 cm and thickness of 1 µm. b) A handmade craft using graphene paper which involves sharp folding and twisting, suggesting the high-degree of flexibility to be shaped on demand, inset is the SEM of the bending point of the graphene paper. c) Stretching test with weight of 300 g. d–g) The cross-sections of the freestanding, macroscopic assembled properties of GPs with different thicknesses. Surface morphology based on h) SEM and i) AFM measurement after stretching, no voids or breakages were observed.

GPs. As a mild reductant, the vitamin C is also able to avoid damaging human skin by eliminating acidic residue, which can be indicated by the rinse pH values (Figure S1, Supporting Information). Figure 2a shows the freestanding, macroscopic assembled GPs fabricated at room temperature by the facile and scalable blade-coating method. The coverage surface area is 25 cm × 15 cm in centimeter-scale with various designed thicknesses (d = 1, 2, 4, and 8 µm). However, a wide range of geometries can be easily accessed if required by using a larger coating machine, or a continuous production line. Moreover, the as-fabricated graphene paper is as flexible as normal paper/fabrics, which can be sharply folded and twisted into a vast variety of crafts, such as an origami dog in Figure 2b. As shown in Figure 2c, the as prepared ultrathin GPs can hold a weight of 300 g steadily. The detailed mechanical properties of the as-fabricated GPs are characterized by stress–strain tests in Figure S2 (Supporting Information). The thinnest paper (d = 1 µm) exhibits a high tensile fracture strength (σ) up to 112.6 ± 11.6 MPa. With increased thickness, the fracture strength increases slightly, with a roughly constant fracture strain. For example, graphene paper with thickness around 4 µm possesses the highest strength σ = 130.8 ± 9.8 MPa. The present mechanical properties are comparable to previously reported GPs or films with a similar oriented structure based on wet-spun, vacuum filtration, or spray-coating approaches.[8,20] Beyond this value, the fracture strength begins to decrease which might be because of the introduced defects

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with increased thickness. Nevertheless, from the stress–strain tests, it is believed that all free-standing GPs we made are tough enough to be used in PTM thanks to the formation of long-rang ordered nanostructures. As a typical example, cross-sections of the GPs are given in Figure 2d–g, demonstrating the uniform thickness throughout the entire area. Graphene paper with controlled thickness can be conveniently prepared by modulating the concentration of GO gel, all revealing the lamellar nanostructures with regular packed single or few-layer graphene sheets. This is also in agreement with the few wrinkles observed in scanning electron microscopy (SEM) images. The surface morphology of the fabricated graphene paper after stretching with a weight of 300 g is studied through SEM and atomic force microscopy (AFM) (Figure 2h,i), no cracks or voids were observed, suggesting its great toughness. This offers a facile approach to fabricate scalable graphene paper that is suitable to be applied in wearable PTM devices. As revealed in Figure S3 (Supporting Information), the X-ray photoelectron spectra (XPS) C1s spectrum where changes are observed in the peaks corresponding to oxygen containing groups of the GO and GPs. The X-ray diffraction pattern (Figure S4, Supporting Information) of GO is characterized by a strong peak at 2θ = 10.7° while the GPs exhibit a broad peak at 24°, indicating that GO was effectively reduced and there is an existence of π–π stacking between the graphene sheets, which leads to the ordered layer structure. The reliability of mechanical, electrical, and thermal properties of the as-fabricated graphene paper was also tested over

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Figure 3.  Mechanical stability of the GPs. a) Stress of 4 µm graphene paper in a variety of bending cycles. Here, all cycle performances are bent at 180° with bending radius 5 mm. Reliability of graphene paper with different thicknesses over numerous bending cycles. b) Optical images for mechanical test in (a) of a GP in its normal and bending states. c) Electrical conductivity versus bending cycles. d) Thermal conductivity as the function of bending cycles. e) Change of GPs resistance with wash time. The resistance stabilized without obvious decay during the wash test in 1500 min. f) The experimental apparatus for wash test. The inset shows partial enlarged details of the graphene paper holder for wash test.

numerous bending cycles. First, superior mechanical properties are verified using a universal material testing machine (Figure 3a,b). After 200 bending cycles, the bending stress still remains constant within the systematic error of the testing machine, suggesting no fracture occurs during the bending process. This is mainly because of the strong interaction of the π–π attractions among the adjacent nanosheets and the long-range,

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highly ordered nanostructures (Figure 2d–g). Meanwhile, all GPs exhibit high out-of-plane thermal conductivity and excellent electrical conductivity depending on thickness (as summed up in Table 1). Moreover, the electrical conductivity and thermal conductivity display excellent cycle stability, which demonstrates no apparent change in performance upon repeated bending for up to 500 cycles under bending radius 5 mm, as

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Table 1.  Mechanical fracture strength and strain, electrical conductivity, and out-of-plane and in-plane thermal conductivities of the GPs with different thicknesses at room temperature (298 K). Thickness [µm]

Strength [MPa]

σ [S cm−1]

ρ [g cm−3]

Cp [J g−1 K−1]

κ [W m−1 K−1]

κ [W m−1 K−1]

Out-of-plane

In-plane

1 ± 0.03

112

87

1.2023

0.5202

2100 ± 300

2600 ± 300

2 ± 0.04

117

94

1.1484

0.5723

1800 ± 200

2100 ± 200

4 ± 0.07

131

112

1.1241

0.5816

900 ± 150

1300 ± 150

6 ± 0.11

116

108

1.0742

0.5793

570 ± 50

890 ± 50

8 ± 0.16

101

105

1.0657

0.5802

490 ± 50

730 ± 50

can be seen in Figure 3c,d. The detailed mechanical fracture strength and strain of various GPs with different thicknesses were shown in Figure S4 (Supporting Information). Comparing with the wearable electronics based on a transfer printing technology,[23] the mechanical deformability and wearability of the present materials might not be that competitive. However, the heater and cooler performances can be better. Actually, the GPs are highly durable against washing tests. Similar to previous reports,[6] we also use the relative electrical resistance to quantify the durability of the present GPs. As can be seen (Figure 3e), the measured resistance remains constant (≈21 Ω) after the first 5 washing cycles, with washing time longer than 1500 min, which is better than the previous reports (Figure S5, Supporting Information). The detailed experimental apparatus for wash testing was shown in Figure 3f. These unique features including their superior mechanical strength, high thermal conductivity, and excellent electrical conductivity pave the way for the direct applications of utilizing graphene paper in PTM.

Active warming up is one of the most important features of PTM. Figure 4a shows the generated temperature profiles with variations of the supplied voltages. The temperature was measured by a thermal couple in close contact with the samples with an error of 1%. Specially, for graphene paper based cloth, a slight voltage supply nearly 2.5 V can already meet the normal requirement of the human body. This is due to the extremely high electrical conductivity of graphene compared with carbon nanotubes based cloth that needs 12 V to reach a satisfied temperature.[6] To ensure safety of the human body, 2.5–3 V is more than enough, however, higher temperature can also be achieved. As can be seen from the joule heating function in Figure 4b, when the supply voltage reaches 8 V, the temperature difference in two ends of the graphene paper becomes about 180 °C, which might be useful in some limited conditions. All the heating procedures were accomplished within the initial 10 s and remained stable before power off. With the increased applied voltage for higher temperature (≈360 °C), the

Figure 4.  Heating property of the GPs. a) Voltage-dependence of temperature in GPs. b) Temperature difference of the heater compared to room temperature based on supplied voltage, which shows an exponential relationship. c) Schematic illustrating heat transfer model of the heating process. d) Infrared images of graphene paper applied a voltage of 3.2 V.

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PTM device become unstable and the performance dropped gradually due to the limited stability of electrodes and the oxidation of graphene paper. A 1D steady-state heat transfer model was adopted to analyze the heating process, as shown in Figure 4c.[13] The detailed mechanism was given in the Supporting Information. Such a fast electrothermal response was confirmed by an infrared thermal camera under a typical supply voltage of 3.2 V (Figure 4d). As can be seen, it only takes 8 s for the graphene paper to rise from room temperature to 38 °C. In addition to the satisfied temperature response, a homogeneously distributed heat flow was observed due to the high thermal conductivity of graphene.[20] In heating process, the hot graphene is the primary heat source which can transfer heat to surrounding of human body by radiation and conduction, in which both thermal radiation and conduction are beneficial. Moreover, it can act as a great bodyguard against human body radiative heat loss after Joule heating under cold temperature conditions (such as