RESEARCH FRONT CSIRO PUBLISHING
Aust. J. Chem. http://dx.doi.org/10.1071/CH16557
Communication
Proton Conductivity of Graphene Oxide on Aging Mohammad Razaul Karim,A,B,D Md. Saidul Islam,A Nurun Nahar Rabin,A Ryo Ohtani,A Masaaki Nakamura,A Michio Koinuma,A and Shinya HayamiA,C,D A
Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. B Department of Chemistry, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh. C Institute of Pulsed Power Science, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. D Corresponding authors. Email:
[email protected];
[email protected]
The aging effect on the proton conductivity of graphene oxide (GO) is investigated. Characterizations of oxygenated functional groups and measurement of the proton conductivity have been performed using freshly prepared GO and the same sample after preserving for three years under ambient conditions. Although GO retains its layered structure, a slight deviation in its powder X-ray diffraction (PXRD) pattern and Raman spectra upon aging implies some changes in the interlayer distance and functional groups. Decomposition of epoxy groups on aging has been recognised by X-ray photoelectron spectroscopy (XPS) analysis. The proton conductivity was found to be reduced by 25 % after three years of aging. Manuscript received: 4 October 2016. Manuscript accepted: 23 November 2016. Published online: 22 December 2016.
Evaluation of the stability of proton conductivity of a proton conductor with aging is necessary for justification of its device application, especially in fuel cells. Herein, we study the effect of aging on the proton conductivity of freshly prepared graphene oxide (GO) and aged graphene oxide (aGO) obtained after preserving the same GO sample for three years at room temperature and pressure. The characterization of GO and aGO reveals some changes in chemical structure as a result of aging. Temperature and relative humidity (RH) dependent conductivities show that the proton conductivities of GO are retained at ,80 % on aging. GO has been evidenced as a new class of proton conductor as it contains oxygenated functional groups embedded in a stable hydrophobic carbon skeleton.[1] Alongside some classical proton conductors, including phosphoric acid (101104 S cm1), nafion (101105 S cm1), carboxylic acids (105106 S cm1), and related hybrids, GO is attracting ongoing interest for its cheap, inert, stable nature and ease of sample fabrication.[2] In fact, carbon-based proton conductors have been investigated worldwide for a long time. In addition to fuel cells, proton conductors are used in impedance based sensors, chemical filters, and biological transport systems.[3] The possibility of GO acting as a proton conductor was first proposed by Raidongia et al. who reported its ion transporting behaviour.[4] Later, GO was used as components in hybrid proton conductors, until we reported pristine GO as a proton conductor itself by measuring its in-plane proton conductivity.[5] Later, it became possible to increase the conductivity of GO by fabricating a multilayer assembly, intercalating sulfate ions at the GO walled channels, or increasing the extent of hydrophilicity of the GO Journal compilation CSIRO 2016
surface by ozonation.[6] We also observed that the conductivity of GO is impaired when its hydrophilic sites are blocked by metal ions or alkyl chains.[7] Beyond these experimental verifications, real applications of GO in fuel cells were reported by Tateishi et al.[8] However, the multiple cycle operation of fuel cells in harsh conditions and with prolonged fixation of the solid electrolytic component inside the devices are possible reasons for the decay in their power output. Therefore, it is necessary to study the stability of GO’s proton conductivity with aging. To the best of our knowledge, such an investigation has not been reported to date. The decomposition of epoxy groups to carboxylic and hydroxy groups is the reported decay process in GO. This process is accompanied by aging or the exposure of GO to UV light and high temperatures.[9] Although this change in functional groups does not significantly affect the total oxygen content of GO, its effect on proton conductivity should be evaluated, as we observed that the epoxy groups and carboxylic groups transport protons through the interior and edges, respectively, of the GO nanosheets.[10] GO was prepared from bulk graphite powder according to a modified Hummers’ method.[6a] The proton conductivity of GO and aGO was measured by a four probe method. Details of experiments are given in the Supplementary Material. Sample characterizations are presented in Fig. 1. Scanning electron microscopy (SEM) images (Fig. 1a, b) show the same layered structure of GO and aGO. In Fig. 1c, the Raman spectra of graphite, GO, and aGO show the presence of two characteristic peaks for carbon materials known as the ‘D’ band (attributed to the breathing mode of the graphitic A1g symmetry) and the ‘G’ band (attributed to the www.publish.csiro.au/journals/ajc
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Fig. 1. SEM images of GO (a) and aGO (b). Raman spectra (c) and PXRD pattern (d). Deconvoluted XPS spectra for GO (e) and aGO (f).
motion of in-plane bond stretching for C sp2 atoms (E2g) pairs) at 1350 and 1580 cm1, respectively.[11] The G band position shifts from 1577 cm1 in graphite powder to 1589 and 1587 cm1 in GO and aGO. Respective hardening and softening implies a structural change during oxidation of graphite and aging of GO.[12] The peak intensity ratio of the D and G bands (ID/IG value) changes from 0.655 in graphite to 0.932 and 0.876 in GO and aGO, respectively. The initial increase in ID/IG values implies the conversion of sp2 carbon sites to sp3, while the decrease in this value implies the recovery of some sp2 carbon sites after aging.[13] In graphite, the carbon atoms exist in an sp2 hybridized state. On oxidation, some carbon atoms are changed into epoxy sites having an sp3 form of carbon. On aging, some epoxy groups seem to be decomposed. In the powder X-ray diffraction (PXRD) pattern (Fig. 1d), the characteristic peak
for graphite at ,278 (2y) indicates its amorphous structure.[14] In GO, the sharp peak disappears and wider peaks around 2y ¼ 11.08 (GO) and 11.92 (aGO) appear. The calculated ˚ for GO and aGO. For interlayer distances are 7.98 and 7.42 A graphene - GO conversion, the interlayer distance increases but aging seems to reduce some epoxy groups of GO, which causes the shortening of the interlayer distance. The C 1s X-ray photoelectron spectroscopy (XPS) spectra of GO (Fig. 1e) and aGO (Fig. 1f) exhibit two peaks near 285 and 287 eV. After deconvolution, the existence of epoxide (–O–) and hydroxy (–OH) groups at 286.8–287.0 eV with carbonyl (–C=O) and carboxy (–COOH) groups at 287.8–288.0 and 289.0–289.3 eV, respectively, are observed.[15] The C/(C þ O) ratio for GO and aGO are 31.9 and 30 %, respectively. The relative amounts of epoxy, hydroxy, carboxylic, ketonic groups, and C=C sp2
Proton Conductivity of Graphene Oxide after Aging
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contents are determined as 33.2, 4.1, 9.2, 3.5, and 13.6 % in GO and 32.3, 3.8, 7.6, 6.8, and 31.3 % in aGO which indicates some reformation of oxygenated sites. Fig. 2a shows the Nyquist plots for the impedance measurement of GO and aGO at 303 K and 90 % RH. The distorted semicircular curves followed by the second circle imply proton driven conductivity. The proton oriented conductivity was confirmed further from the isotope effect.[6] In Fig. 2b, at room temperature the RH dependent conductivities of GO and aGO increase from 1.9 106 and 1.1 106 S cm1 at 40 % RH to 7.1 105 and 1.6 105 S cm1 at 80 % RH. These data imply some losses of s values of GO due to aging. In Fig. 2c, the temperature dependent conductivities show that at 80 % RH, the s values for GO are nearly 1.25 times higher than for aGO. At 303 K and 80 % RH, the s value for GO and aGO are 7.1 105 and 4.1 105 S cm1, respectively. The conductivities increase with temperature and reach 2.2 104 and 1.6 104 S cm1 at 343 K. Fig. 2d represent the plots for ln(sT) versus T1. The calculated activation energies (Ea) for proton conduction in GO and aGO of 0.303 and 0.312 eV comply with the Grotthuss mechanism of proton transfer. From repeated studies we found that the proton conductivities remain almost constant for almost six months. While comparing the proton conductivity of metal-doped GO with pristine GO in another report, we observed that after 16 months the conductivity suffers some loss. At 303 K the conductivity of 16 months old GO was 1.43 106 and 1.91 105 S cm1 at 40 and 80 % RH.[7b] However, a detailed study on the change in chemical structure and proton conductivity using freshly prepared samples and three
years aging of the sample only was needed. On aging, some reformations of oxygenated functional groups of GO take place, which is confirmed from the XPS analysis. Conversion of some epoxy groups into a ketonic form in aGO seems to result in some net loss in the proton conductivity values. Losses in the O/(C þ O) ratio might also contribute to the conductivity decay. The recovery of C=C carbon bonds in aGO is significant. This typical double bond is electron rich with delocalized p-orbitals, and attributes insulation properties towards proton conduction.[15] Therefore, a loss in s value is evident. Shrinking of the GO interlayer seems to result from the reduction of epoxy sites. The narrow interlayers at aGO hinder hydrodynamics and the hydrogen bond reformation process.[16] We propose that both the chemical and physical changes associated with the aging of GO are responsible for the lowering of proton conductivity values. In conclusion, we have characterized and measured the proton conductivity of freshly prepared GO and three years aged GO. Although the layered structure of GO and aGO are the same, Raman spectroscopy shows some recovery of sp2 carbon sites, while PXRD analysis shows shrinking of the interlayer distances. The XPS analysis shows recovery of graphite’s sp2 carbon sites and some loss in oxygen content and epoxy groups. Both the temperature and RH dependent conductivity values show that the proton conductivity of GO is reduced by 25 % on aging for three years. The activation energy for proton conduction was unchanged while the magnitudes suggest a Grotthuss type mechanism for transferring protons.
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