Research Article www.acsami.org
Molecular Functionalization of Graphene Oxide for Next-Generation Wearable Electronics Hadis Zarrin, Serubbabel Sy, Jing Fu, Gaopeng Jiang, Keunwoo Kang, Yun-Seok Jun, Aiping Yu, Michael Fowler, and Zhongwei Chen* Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario Canada N2L 3G1
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ABSTRACT: Acquiring reliable and efficient wearable electronics requires the development of flexible electrolyte membranes (EMs) for energy storage systems with high performance and minimum dependency on the operating conditions. Herein, a freestanding graphene oxide (GO) EM is functionalized with 1-hexyl-3-methylimidazolium chloride (HMIM) molecules via both covalent and noncovalent bonds induced by esterification reactions and electrostatic πcation−π stacking, respectively. Compared to the commercial polymeric membrane, the thin HMIM/GO membrane demonstrates not only slightest performance sensitivity to the operating conditions but also a superior hydroxide conductivity of 0.064 ± 0.0021 S cm−1 at 30% RH and room temperature, which was 3.8 times higher than that of the commercial membrane at the same conditions. To study the practical application of the HMIM/GO membranes in wearable electronics, a fully solid-state, thin, flexible zinc−air battery and supercapacitor are made exhibiting high battery performance and capacitance at low humidified and room temperature environment, respectively, favored by the bonded HMIM molecules on the surface of GO nanosheets. The results of this study disclose the strong potential of manipulating the chemical structure of GO to work as a lightweight membrane in wearable energy storage devices, possessing highly stable performance at different operating conditions, especially at low relative humidity and room temperature. KEYWORDS: flexible electronics, graphene oxide, zinc−air batteries, supercapacitors, ion conductivity, ionic liquids
1. INTRODUCTION Entering the golden era of wearable electronics has urged the development of fully solid, lightweight, flexible, and efficient energy storage devices (e.g., batteries and supercapacitors) to be reliable at different environments.1−3 To accomplish this, the compartments of energy storage systems (e.g., electrodes and electrolytes) must be replaced with flexible materials having high sustainability at various operating conditions.3−5 Specifically, the ion-conductive electrolytes in these systems are voluminous and undesirable since their performance is highly dependent on the operating temperature and relative humidity (RH) levels.6,7 Thus, it is essential to replace them with a flexible and lightweight electrolyte membrane (EM) possessing high and stable ionic conductivity at not only fully humidified and elevated temperatures but also low RH and room temperature conditions. Recently, the research has focused on the development of graphene oxide (GO)-based EMs to (i) enable the fabrication of lightweight and flexible electronics and (ii) increase the compatibility of carbon-based electrodes with the electrolyte.8−17 Because of its ultrahigh surface area, yet nanoscale thickness,18,19 GO nanosheets and their derivatives can simply stack and form stable, lightweight, and flexible membranes. However, the rate of ion conduction in GO-based EMs is © 2016 American Chemical Society
highly sensitive to the changes of the operating conditions and extremely dependent on the existence of water in the system (i.e., RH). GO is intrinsically proton-conductive, electrical insulator and highly hydrophilic. Thus, the interlayer spacing widens as RH increases,20 enabling water molecules to be trapped within, and as a result increases the rate of ion transfer. Accordingly, when the humidity decreases, the ion conductivity will drastically decline (e.g., by 1 order of magnitude).15 Since a practical ion-conductive EM requires a minimum ion conductivity of 0.01 S cm−1 at different operating conditions,21 it is vital to efficiently boost that for the GO-based EMs with slightest dependency on the operating conditions (e.g., RH and temperature). The existence of enormous oxygenated groups (e.g., hydroxide, epoxy, and carboxyl groups) on the basal plane and edges of the nanosheets enables chemical functionalization of GO19 and manipulation of its electrical and physicochemical properties for the desired application. Herein, to develop a flexible GO-based EM with high ion conductivity and minimum dependency on the operating conditionsespecially at low RH Received: June 6, 2016 Accepted: August 31, 2016 Published: August 31, 2016 25428
DOI: 10.1021/acsami.6b06769 ACS Appl. Mater. Interfaces 2016, 8, 25428−25437
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Scheme of the experimental procedure used for functionalizing GO nanosheets with HMIM and produced freestanding x-HMIM/GO membranes. (b) Digital photo of freshly vacuum-filtered 10-HMIM/GO membrane with 47 mm diameter. (c) Digital photo showing the flexibility of produced 10-HMIM/GO membrane. (d) Cross-sectional SEM image of 10-HMIM/GO membrane with a thickness of ∼26.7 μm. EDX mapping of (e) carbon, (f) oxygen, and (g) nitrogen elements distributed between the layers of 10-HMIM/GO membrane. GO (40 mg) was dispersed into DDI water and then mixed with HMIM and KOH at room temperature for 48 h. The homogeneous solution was centrifuged several times with DDI water to remove any excess of KOH and unreacted HMIM. Then, the resulted x-HMIM/ GO was redispersed into DDI water and casted to a freestanding membrane via the vacuum filtration method using an Anodisc membrane (47 mm in diameter, 0.2 μm pore size, Whatman), followed by air drying and peeling from the filter. The thicknesses of all membranes were in the range of ∼27−32 μm. 2.2. Physicochemical Characterization. The structural integrity and the distribution degree of elements in the x-HMIM/GO membranes were shown by cross-sectional SEM images and EDX mapping (LEO FESEM1530). XRD (INEL XRG 3000) and Raman spectrometry (Bruker Senterra, 532 nm laser) were utilized to study the changes in the crystallinity of the samples and effects of disorders. The elemental and chemical-bonding analyses of x-HMIM/GO membranes were determined via XPS (Thermo Scientific Al KAlpha X-ray source) and FTIR (Avatar 320) to elucidate the possible functionalization mechanism. To assess the thermal behavior of GO and x-HMIM/GO membranes, the thermogravimetric analysis (TGA) was performed under nitrogen with a TGAQ500 V20.10 instrument in the temperature range from 25 to 800 °C at the heating rate of 10 °C min−1. The water contact angle on the surface of freestanding membranes was measured using CA 2500 XE equipment associated with the software (AST Products, Billerica, MA) to inspect how the hydrophilicity changes with the loading increase of HMIM in GO. 2.3. Zinc−Air Battery Fabrication. To prepare the air electrode in the Zn−air battery setup, gas diffusion layer (GDL) (Ion Power Inc., 25BC) was coated with the catalyst ink prepared by sonicating a mixture of active material and carbon black in a 2:1 mass ratio dispersed in isopropyl alcohol with binder (AS-4 ionomer, Tokuyama Inc.) of 10 μL mg−1 of total mixture. The as-received Co3O4 nanoparticles (
