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Control of Edge/In-Plane Interactions toward Robust, Highly Proton Conductive Graphene Oxide Membranes Benbing Shi, Hong Wu, Jianliang Shen, Li Cao, Xueyi He, Yu Ma, Yan Li, Jinzhao Li, Mingzhao Xu, Xunli Mao, Ming Qiu, Haobo Geng, Pengfei Yang, and Zhongyi Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04156 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019
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Control of Edge/In-Plane Interactions toward Robust, Highly Proton Conductive Graphene Oxide Membranes Benbing Shi†,‡, Hong Wu*,†,‡, , Jianliang Shen†,‡, Li Cao†,‡, Xueyi He†,‡, Yu Ma†,‡, Yan Li†,‡, Jinzhao Li†,‡, Mingzhao Xu†,‡, Xunli Mao†,‡, Ming Qiu†,‡, Haobo Geng†,‡, Pengfei Yang†,‡, Zhongyi Jiang*,†,‡ †Key
Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China. Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China. ABSTRACT: Graphene oxide (GO) membrane, bearing well-aligned interlayer nanochannels and well-defined physicochemical properties, promises fast proton transport. However, the deficiency of proton donor groups on the basal plane of GO and weak interlamellar interactions between the adjacent nanosheets often cause the low proton conduction capability and poor water stability. Herein, we incorporate the sulfonated graphene quantum dots (SGQD) into graphene oxide (GO) membrane to solve the above dilemma via synergistically control the edge electrostatic interaction and in-plane @=@ interaction of SGQD with GO nanosheets. SGQD with three different kinds of electron-withdrawing groups are employed to modulate the edge electrostatic interactions and improve the water swelling resistant property of GO membranes. Meanwhile, SGQD with abundant proton donor groups assemble on the sp2 domain of GO via in-plane @=@ interaction and confer the GO membranes with low energy-barrier proton transport channels. As a result, the GO membrane achieves enhanced proton conductivity of 324 mS cm-1, maximum power density of 161.6 mW cm-2, and superior water stability when immersed into water for one month. This study demonstrates a strategy for independent manipulation of conductive function and non-conductive function to fabricate high-performance proton exchange membranes.
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KEYWORDS: graphene oxide membrane, sulfonated graphene quantum dots, edge/in-plane interactions, water stability, proton conductivity.
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Recently, two-dimensional (2D) laminar membranes have become the research hotspot owing to the simple preparation process and well-aligned interlayer nanochannels, which dramatically reduce the transport resistance for small molecule/ion.1-7 Graphene oxide (GO), as a representative two-dimensional (2D) material, bearing abundant oxygen containing groups, has been widely utilized as nanoscale building blocks for proton propagation.8,9 GO membrane with regular interlayer channel, along with absorbed water molecules, render continuous hydrogen network for fast proton transport.10 Intrigued by the proton channel in GO membrane, tremendous efforts have been devoted to explore the proton conduction mechanisms and high performance GO membranes.11,12
Proton hops through the formation/cleavage of hydrogen bond network (Grotthuss mechanism) was suggested as the possible way for the movement of protons in GO, and the in-plane C-O-C groups are recognized as the main contributor for proton migration.13,14 In the interlayer of GO membrane, the insufficient proton donor groups limited the proton conductivity (often lower than 10-2 S cm-1).15,16 At the edge of the GO nanosheets, the presence of -CO2H groups endows the GO with negative charge, and electrostatic repulsion interaction along with the strong hydrophilicity induced the instability of GO membrane in water.17,18 The low proton conduction capability of the interlayer channel and weak water stability severely hamper the application efficiency of GO membranes. During the past decades, researchers mainly focused on the enhancement of proton conduction capability of GO membrane, and various methods were explored with the aim to increase the available proton donor groups. These methods could be divided into two categories: (i) molecular intercalation and (ii) chemical modification. Owing to the versatile intercalation chemistry of GO, the organic or inorganic proton donors could be facilely incorporated into the interlayer of GO to promote the proton conductivity of GO-based nanocomposites.19-23 Chemical modification is an alternative method to tether the acid groups onto the GO surface, which 3 ACS Paragon Plus Environment
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avoids the loss of acid groups under the operation condition (hydrated condition).24-26 For improving the water stability of GO membranes, covalent cross-linking by glutaraldehyde,27 thiourea,28 and diamine29 have been widely used. However, the cross-linking usually occurs at the proton conductive region of GO and consumes proton conductive groups (-OH, C-O-C) as well as reduces the water content in the GO interlayer. Currently, there still often exists a trade-off relation between proton conductivity and stability in GO membranes, which is primarily arisen from the undesirable coupling between the enhancement of proton conductivity and the stability.23,30,31 Developing a strategy to simultaneously enhance the proton conductivity and water stability will be of fundamental and technological significance.
Based on the physicochemical structure and proton tranfer pathway of GO, in this study, we proposed a decoupled strategy and utilized pre-designed sulfonated graphene quantum dots (SGQD) to overcome the above-mentioned dilemma of GO membrane by synergistically controlling the edge interaction and the in-plane interaction (as illustrated in Figure 1a). The GO membranes were prepared by vacuum assisted co-assembly in the presence of SGQD. The SGQD possessed sp2 carbon atom, protonated amide groups, and abundant sulfonic acid groups. These features rendered SGQD the capability of generating electrostatic and @=@ interaction to the edge and in-plane of GO nanosheets. SGQD with three different kinds of electron-withdrawing groups were utilized to control the edge electrostatic interaction, which rendered the membrane elevated mechanical and water stability. Arisen from the in-plane @=@ interaction, SGQD spontaneously assembled on the basal plane of GO and rendered abundant proton donor groups, significantly elevated the proton conductivity of GO membrane.
RESULTS AND DISCUSSION The chemical structure of GQD and SGQD are illustrated Figure 1b. The basal plane of GQD and SGQD are composed of the abundant sp2 clusters as that of graphene.32,33 The edges 4 ACS Paragon Plus Environment
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of GQD and SGQD are carboxyl, hydroxyl, amide, and sulfonic acid groups, respectively. Pristine GQD was synthesized by direct pyrolysis of citric acid according to the literature.34 SGQD is obtained by hydrothermal reaction using GQD and sulfonic acid group containing monomer as shown in Figure S1. The resultant SGQD are designated as SGQD-SA, SGQDOA, and SGQD-PA according to the functional groups attached to GQD. The morphologies and microstructures of GQD and SGQD are probed by TEM, as shown in Figure 1c, d. Both of the GQD and SGQD display uniform nanoparticle structures with the average diameters around 5 nm (Figure S2). High-resolution TEM image (HRTEM, inset of Figure 1c) shows the crystallinity of the GQD and the in plane lattice spacing is 0.23 nm, corresponding to the (1120) crystal planes of graphene.35 Upon modification, the SGQD exhibits good monodispersity and the basal plane keeps the pristine crystalline texture without dimensional deformation. The AFM image (Figure S3) reveals the thickness of SGQD is about 1.2 nm, corresponds to the two-layered graphene layers.36 The surface functional groups of the GQD and SGQD are probed by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). As shown in Figure 1e, GQD exhibits typical adsorption bands at around 1633 and 1732 cm-1, assigning to the C=O groups attached to the GQD surface. The C=C stretching at around 1600 cm-1 is widened due to the overlapping from C=O stretching at 1633 cm-1.37 SGQD exhibit two absorption peaks at around 1058 cm-1 and 1456 cm-1 for the vibration bands associated with O=S=O of –SO3H and C-N group.38,39 The high-resolution C1s, N1s, and S2p XPS spectra of the SGQD are shown in Figure S4, which indicate the existence of sp2 C=C bonds, -N-C=O, and -SO3H group on the surface of SGQD. The N and S elemental composition of the obtained SGQD are at the close level about 3.8-4.05 % and 3.86-4.00 %, respectively (Table S1). Figure 1f displays the UV absorption spectra of GQD and GO-GQD solution, the GQD solution shows a peak at 265.3 nm corresponds to the @J@ transition of C=C.40 An absorption band at 357 nm is assigned to the =@ transition attributed to C=O.41 Upon decoration with sulfamic acid (SA), orthanilic acid (OA), and p5 ACS Paragon Plus Environment
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aminobenzene sulfonic acid (PA), the peak for =@ transition changed due to the electronwithdrawing effect of the functional groups. Elevating the electron-withdrawing capability of the functional group, the peak for
=@
transition shift to lower wavelength or even
disappeared. Similar to the other literature reported, both of the obtained GQD and SGQD exhibit bright fluorescence (inset of Figure 1f).42 GO was synthesized using the improved Hummer method.43 The lateral dimensions of GO are about 1-2 O' with some nanoscale wrinkles (Figure 1g). The HRTEM image (Figure 1h) of GO reveals the sp2 and sp3 carbon network (as labeled by yellow mark), and the ratio of the oxidized/non-oxidized C atom is calculated to be 0.84 based on the Raman spectra (inset of Figure 1h). The thickness of the GO nanosheet is about 1.6 nm as probed by AFM (Figure S5), hinting the two-layered structure. The FTIR spectrum (Figure S6) of GO shows various absorption peaks due to the stretching vibration of the oxygen containing groups, such as – CO2H group (1734 cm-1), -C-OH group (1386 cm-1), -C-O-C group (1224 cm-1).43-45 The thermal stability of the GO, GQD, and SGQD were measured by TGA, which exhibit the excellent thermal stability up to 180 oC (Figure S7), sufficient for the practical application in fuel cells or other devices.
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spite of this, felicitously tuning the electron withdrawing capability of the ligand tethered to the amide groups would protonate the N atom by increasing electron density. Here, the molecular electrostatic potential (MEP) and charge distribution of the functional group in SGQD-SA, SGQD-OA, and SGQD-PA were calculated in Figure S8. The charge of the atoms is related to the electron density and electron-rich atom display negative charge, and vice versa. The charge of the N atom in the functional group follows the order of SGQD-SA(0.551)
