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Effects of reduction of graphene oxide on the hydrogen storage capacities of metal graphene nanocomposite
Catalysis Today xxx (xxxx) xxx–xxx
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Effects of reduction of graphene oxide on the hydrogen storage capacities of metal graphene nanocomposite ⁎
A. Ngqalakwezia,b,c, , D. Nkazia, G. Seifertc,d, T. Nthob a
School of Chemical and Metallurgical engineering, University of the Witwatersrand, Johannesburg, South Africa Advanced Materials Division, Mintek, Private Bag x3105, Randburg, 2194, Gauteng, South Africa c Materials for Energy Research Group, University of the Witwatersrand, Johannesburg, South Africa d Theoretical Chemistry, Technische Universität Dresden, 01062, Dresden, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Reduction Hydrogen storage capacity
Light-weight materials such as graphene have attracted a great interest for hydrogen storage applications. In this study, a metal-on graphene nanocomposite was synthesized for hydrogen storage for on-board applications. The graphene used as support was obtained from precursor graphite oxide, which was synthesized using the Improved Tours method and later reduced by L-Ascorbic acid and ammonia. The precursor graphite flakes used in the synthesis of graphite oxide were 100 μm in size. The XRD patterns depicted a low angle shift in GO, 11.3° from graphite 30° proved successful oxidation of graphite. The FTIR and Raman band showed L-Ascorbic reduced the GO better than ammonia through the observed decrease on the functional bands after reduction. The elemental analysis carried out using XRF showed the successful loading of Ca on the graphene matrix and TEM images also confirmed this. The hydrogen storage capacity of the nanocomposite was tested using TGA and TPD equipped with Mass Spectrometer. The Ca/graphene nanocomposite reduced by ammonia exhibited a large hydrogen storage uptake at 4.98 wt. % compared to L-Ascorbic acid.
1. Introduction One of the sustainable development goals globally is to make the environment a cleaner place. As such, the possibility of using hydrogen as an alternative source of energy has garnered a lot of attention due to its zero emission characteristic to the environment, high energy content and its abundant availability [1,2]. However, the major challenge with hydrogen is the storage. The storage of hydrogen is governed by the US Department of Energy (DOE), which sets the standards for on-board systems. To date, no material has fully met the DOE requirements for on-board applications. Various materials have been experimentally tested for solid state storage of hydrogen because gaseous and liquid storage of hydrogen pose cost and energy challenges. One of the material tested for solid storage of hydrogen is graphene based material [3–5] because graphene has intriguing properties such as extra-ordinary mechanical strength, transport performance, increased movement of charge carriers and high surface area [4–7]. Graphene is an allotrope of carbon which is characterized by a monolayer of sp2 carbon atoms. Graphene can be synthesized through graphite intercalation,
chemical vapour deposition, micromechanical cleavage and the reduction of graphite oxide [8–12]. The reduction of graphite oxide has been the common route of producing graphene. The method of reduction of graphene has an effect on the structure of graphene and thereby its hydrogen storage capacity. It has been reported that thermally reduced graphene acquires a significant number of structural defects which act as binding sites for hydrogen [4,15]. However Graphene alone as a physisorbent material does not take up a lot of hydrogen and thereby has to be enhanced through the addition of active metals for practical on-board applications. The incorporation of active nanometals on graphene helps in tuning the electronic structure and surface morphology of graphene [13]. A number of metal/graphene nanocomposites have been synthesized in this regard and a lot of DFT studies have been performed [14]. A Pd/graphene nanocomposite was synthesized with 6.7 wt% at 50 bars [14]. In this work ammonia and L-Ascorbic were used as reductants instead of hydrazine. Hydrazine due to its hazardous nature and cost, remains undesirable as a reductant. Furthermore in this work, different reduction techniques such as chemical reduction and thermal reduction
⁎
Corresponding author at: School of Chemical and Metallurgical engineering, University of the Witwatersrand, Johannesburg, South Africa. E-mail addresses: [email protected] (A. Ngqalakwezi), [email protected], [email protected] (D. Nkazi), [email protected] (T. Ntho). URL: http://[email protected] (D. Nkazi). https://doi.org/10.1016/j.cattod.2019.06.029 Received 14 March 2019; Received in revised form 31 May 2019; Accepted 10 June 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: A. Ngqalakwezi, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.06.029
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are explored. Calcium was also used to improve the catalytic activity of graphene in the uptake of hydrogen. The effect of the reduction techniques on the hydrogen storage capacity of graphene and the metal incorporation thereof were studied. Fig. 2. The thermal reduction process of GO.
2. Experimental section de-ionised water. The NH3 solution was slowly added to the graphite oxide solution. The temperature was raised to 70°C and it was kept there for 2 h. The resultant solution of reduced graphene was then aged for 2 days and freeze dried.
2.1. Synthesis of graphite oxide Precursor graphite oxide was synthesized using the Improved Tour Method which permits a large scale synthesis of hydrophilic oxidized graphene compared to the traditional modified Hummer’s method [15]. In this method, graphite flakes (5 g, Sigma) were weighed and mixed with H2SO4 and H3PO4 in a ratio of 9:1 (360 ml, ACE and 40 ml, Sigma). The solution was stirred at 400 rpm where the temperature was kept below 10°C in an ice bath. Then, KMnO4 (18 g, Sigma) was added slowly to the graphite solution. The mixture was taken off the ice bath and heated to 50°C and stirred for 3 h. The solution was then cooled and H2O2 (3 ml, ACE) was added to quench the reaction. The resultant graphite oxide was dialyzed 6–7 times in 5 l de-ionized water (18.2 MΩ cm purified by a Milli-Q water system). The dialyzed graphite oxide was filtered and dried at 120°C (Fig. 1) [15].
2.2.3. Chemical reduction: L-Ascorbic acid Graphite oxide was mixed with 250 ml of de-ionized water and stirred at 250 rpm. Then, L-Ascorbic acid (5.28 g, Sigma) was mixed with 300 ml of de-ionised water. The L-Ascorbic solution was slowly added to the graphite oxide solution. The temperature was raised to 70°C and it was kept there for 2 h. The resultant solution of reduced graphene was then aged for 2 days and freeze dried. 2.2.4. Calcium loading on graphene Precursor graphene gel was prepared adding and mixing deionised water with the dried reduced graphene. This step could be simultaneously done with the reduction step. To synthesize Ca\graphene, Ca/ Cl2 (1.542 g, ACE) was dissolved in deionized water and was slowly added to graphene gel. The temperature was raised to 70°C and it was stirred for an hour. The resultant solution was sonicated and centrifuged at 4000 rpm. The precipitate was washed extensively with deionized water and centrifuged again. The precipitate was dried in an oven at 180°C and then dried in a furnance for 2 min at 300°C.
2.2. Reduction of graphite oxide 2.2.1. Thermal reduction The graphite oxide was placed in a crucible and then placed in the oven for approximately 24 h. The graphite oxide was reduced at 200°C. The resultant reduced graphene was weighed and then taken for characterization (Fig. 2) [17].
2.3. Characterization 2.2.2. Chemical reduction: Ammonia Graphite oxide was mixed with 300 ml of de-ionized water and stirred at 250 rpm. Then, NH3 (10 ml, ACE) was mixed with 100 ml of
X-ray powder diffraction (p-XRD) using an enhanced resolution Bruker AXS D8 X-ray advanced powder diffractometer equipped with
Fig. 1. Schematic diagram of the synthesis of GO from precursor graphite flakes. 2
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band position is in an in-plane mode which involves the sp2 hybridized carbon and is extremely sensitive to the number of layers in graphene [18,15]. The shift observed in the G-band position of the graphene is due to the increase in the layer thickness which results in the band shifting to a lower energy [17]. The band position can be correlated to the number of layers available in a sample through this relation: [17]
CuKα-radiation (40 mA, 1.78897 Å, 40 kV) with LinxEye detector was utilized to acquire the crystal structure of the catalyst. The XRD spectra was recorded in the 2θ range of 5 to 80◦ diffraction angle with a step size of 0.02. A Perkin Elmer Raman Station 400 benchtop Raman spectroscopy with an excitation wavelength of 785 nm laser (100 mW at the sample) and a lateral resolution of ˜ 1 μm was utilized to characterize the samples. The Raman mapping was executed with an accumulation time of 3 s at each spot and a grip spacing of 0.25 mm. The detector utilized was a temperature controlled Charged Coupled Device (CCD) detector (−50 °C) incorporating a 1024 × 256 pixel sensor. The spectra were processed using Spectrum software supplied by PerkinElmer. Transmission Electron Microscopy (TEM) was carried out using the JEOL Jem 2100 F with an electron accelerating voltage that ranges from 80 kV to 200 kV. The samples were prepared on perforated copper grids coated with carbon FTIR characterization was carried out using a Perkin Elmer Spectrum two (UATR Two). The background spectrum was subtracted from all FTIR recorded spectra. Then a NitonTM XL3t XRF analyser was utilized to determine the elemental analysis of the samples at room temperature. Ammonia temperature programmed desorption (NH3 –TPD) using AutoChemTM II 2920 with automated catalyst characterization system was utilized to determine the desorption kinetics. Thermo-gravimetric analysis (TGA) using the SDT Q600 TA instrument was used to measure the hydrogen uptake.
ωG = 1581.6 +
11 (1 + n1.6)
The shift between the ammonia reduced graphene and the Ascorbic reduced showed no significant difference. The G band is also affected by the temperature and in the reduction experiments the temperature was kept constant at 70°C. On the other hand, the D band in the Raman spectroscopy appears around ˜ 1327-1342 cm−1 wavelength. The D band is regarded as the disorder band because it appears due to the defects that occur during the oxidation and reduction of graphene [16,17]. The D band can be observed at ˜ 1324 cm-1, ˜ 1332 cm-1, ˜ 1322 cm-1 and ˜ 1340 cm-1 for graphite flakes, GO, Ascorbic reduced graphene and ammonia reduced graphene respectively. On the subsequent reduction of graphene, it can be observed that the D band has an increased intensity than the G band because of the degradation of the sp2 hybridized carbons and the formation of the functional groups on the plane resulting in the creation of defects on the plane [16]. The D band has low intensity on the graphite because of the small number of defects on it. The 2D band can be observed on the graphite band at ˜
3. Results and discussion
2714 cm-1. The 2D band appears due to the vibrational process of the two phonon lattice and the increase in number of layers results on the decrease of symmetry and intensity of this band [15]. The ID/ IG intensity ratio was increased from 0.84 graphite oxide to 1.19 in ammonia reduced graphene and 1.21 in ascorbic reduced graphene. This was indicative of the formation sp2 domains and the reduction of the functional groups on the plane
3.1. Structure/texture characterization of the materials 3.1.1. Raman Raman characterization, a vibrational technique that is sensitive to bonding within molecules and geometric structure, was used to identify the structural differences during the reduction and oxidation processes of graphene [16,17]. The vital bands considered are the 2D, D and G bands. Fig. 3 shows the spectra graphite flakes A, graphite oxide B, ammonia reduced graphene C and L-ascorbic reduced graphene D. The Fig. 3 (a) shows the graphite flakes G band at ˜ 1582 cm−1 shifted to ˜ 1598 cm−1 and ˜ 1596 cm−1 in the case of ammonia reduced graphene and L-Ascorbic reduced graphene respectively. The G
3.1.2. XRD The XRD results are presented in the Figure below (4) in a comparative way. While doing the XRD measurements some of the samples were crushed to obtain powder. Graphite depicts a very sharp peak at ˜ 2 θ = 30 ° which corresponds to the packing of atomic layers and
Fig. 3. Raman spectra of the sample A. graphite flakes, B. GO, C. NH3 reduced graphene D. Ascorbic acid reduced graphene. 3
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Fig. 4. XRD patters of the samples where A. graphite flakes, B.GO, C. Ascorbic acid reduced graphene D. NH3 reduced graphene, E. Thermally reduced graphene.
(1595-1623 cm−1) vibrations and C]O (1719-1754 cm−1) vibrations [19]. The OH- stretch which can be observed in the samples is due to the presence of water in the graphene layers [10]. The decrease in the intensity of the bands can be visibly observed for the samples reduced with Ascorbic acid, while the ammonia reduced graphene spectra shows little change in the intensity of the bands. Therefore this affirms that graphite oxide was successfully reduced with Ascorbic while it was partially reduced with ammonia (Fig. 5).
hexagonal arrangement [3]. This angle shifts lower in GO because of increased interplanar spacing due to the oxidation of the graphite crystalline structure [16]. The GO peak can be observed at ˜ 2 θ = 11.3 ° corresponding to the d001 plane. The broad GO peak confirms the presence of the functional groups in graphite oxide due to the oxidation process [14,3]. It can be noted that the intensity of the oxidation peak at ˜ 2 θ = 11.3 ° decreases in both the thermally reduced graphene and ammonia reduced graphene however it does not disappear. On the other hand a broad peak can be observed on the ascorbic reduced graphene which indicates loss of crystallinity and extreme decrease in the interplanar spacing [16]. The results show that graphite was successfully oxidized and reduced with L-Ascorbic acid (Fig. 4).
3.1.4. XRF X-ray fluorescence, a technique that measures the fluorescent X-ray discharged from a sample due to excitement by X-rays, was utilized to determine the elemental composition of the graphite, reduced graphene and metal incorporated reduced graphene. The graphite mainly consisted of carbon and traces of Ca, Mn and Fe. The reduced graphene depicted new elements such as P and K mainly because KMnO4 and H3PO4 were used during the oxidation of graphite. The XRF of the Ca/ graphene reduced with both NH3 and Ascorbic acid showed an increase of Ca, meaning the Ca was successfully incorporated into the graphene
3.1.3. FTIR results The FTIR spectroscopy is associated with atomic or molecular vibrations in a material. These vibrations are used to fingerprint the functional groups by analysing the frequencies to identify molecules. The FTIR spectra identified several functional groups: C–O (12151252 cm−1) vibrations, OeH (3422 cm−1) stretching vibrations, C]C 4
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Table 3 Elemental comp. of samples reduced with Ascorbic acid and NH3. NH3rGO
AscrGO
Element
%
Element
%
Carbon Mn Si P K Ca
87.411 9.026 0.29 0.713 0.104 0.055
Carbon Mn Si P K Ca
88.277 9.045 0.269 0.731 0.145 0.069
(Tables 1–3). 3.1.5. TEM analysis The surface analysis on the morphological properties of as-prepared graphene, graphene oxide and metal/graphene was examined using Transmission electron microscope as shown in Fig. 6. The samples were prepared by dispersing graphene in a solvent and then pipetting it on the holey carbon coated copper grids. During the sample preparation step the graphite flakes could not be exfoliated and the image captured (Fig. 6 (A)) depicts the graphitic nature of the flakes [20]. GO (Fig. 6 (B)) shows translucent sheets which are entangled on top of each other. GO was observed to be unstable under increased energy beam relative to graphene [21]. Graphene reduced with ammonia, L-Ascorbic acid and thermally reduced graphene can be observed in Fig. 6 c, d and e respectively. Ammonia reduced graphene (Fig. 6 (C)) appears semitransparent and the morphology shows irregular shaped layers with rough surfaces. The tangles and irregularity on the surface are due to the partial reduction of graphene with ammonia. L-Ascorbic reduced graphene (Fig. 6 (D)) shows more transparent layers with folding and wrinkles both on the surface of the structure and edges. Thermally reduced graphene (Fig. 6 (E)) shows flat layers which are stacked. The rough surfaces on the edges and irregularity in the shape can also be observed in the capture image. Fig. 6 (F) ammonia reduced and (G) LAscorbic reduced show observable particles of calcium on the surface which were previously not observed.
Fig. 5. FTIR spectra A. Graphite B. GO, C. Ascorbic acid reduced graphene D. NH3 reduced graphene.
3.1.6. Thermogravimetric analysis Thermogravimetric analysis of Ca/graphene reduced with ammonia and L-ascorbic acid was done on the SDT Q600 TA instrument before and after hydrogen charging. The samples were charged in a 4540 Parr reactor at 10 bars and ambient temperature for 30 min. The temperature was ramped to 500 °C at a ramp rate of 10°C /min under nitrogen gas. Fig. 7 below shows the weight loss of all the samples combined. The weight loss in the Ca/graphene is attributed to the labile oxygen groups and the adsorbed water on the material [23]. The hydrogen capacity of the material was tested through running TGA on non-hydrogen charged Ca/graphene and charged graphene. The Ca/graphene reduced with ammonia had a total weight loss of 50.97 wt% while the non-charged material lost about 45.99%. The calculated hydrogen uptake was up to 4.98 wt% while the Ca/graphene reduced with L-Ascorbic acid was up to 3.99 wt% with weight loss of about 36.31 wt% and 40.30 wt% before and after charging respectively. These results show that the hydrogen uptake of graphene was improved through the incorporation of the Calcium metal onto the graphene matrix. The hydrogen uptake of graphene is reportedly around 2 wt% at 50 bars and 77 K [23,25]. Furthermore these results show the N-doping effect ammonia has on graphene enhances the uptake of hydrogen in the material [29]. According to the results obtained for the structural characterization of the Ca/graphene synthesized and hydrogen storage properties thereof, the uptake of hydrogen may be explained as proposed in by Tranca et.al [14]. The calcium atoms or clusters are immobilized at the carbon nanostructure and can bind hydrogen molecules without
Table 1 Elemental comp. of the precursor graphite flakes. Graphite Flakes Element
%
Carbon Calcium Manganese Iron
96.171 0.516 2.623 0.47
Table 2 Elemental comp. of Ca decorated graphene samples reduced with Asc Acid & NH3. Ca-NH3rGO
Ca-AscrGO
Element
%
Element
%
Carbon Sulphur Calcium Manganese Phosphorus Potassium
75.426 3.809 8.227 5.236 2.066 0.348
Carbon Sulphur Calcium Manganese Phosphorus Potassium
81.589 3.191 7.381 4.234 1.045 0.017
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Fig. 6. TEM morphological analysis of A. Graphite flakes B. GO, C. Ammonia acid reduced graphene D. L-Ascorbic reduced graphene E. Thermally reduced graphene F. Ca/Graphene_Ammonia G. Ca/Graphene_L-Ascorbic acid.
the ascorbic reduced graphene followed by mass spectrometry was performed in a continuous flow reactor coupled to a quadrupole mass spectrometer. This technique gives information about the desorption profile of the Ca/graphene. The area under peak, which we obtain from the profiles of this analysis is proportional to the amount originally adsorbed. The method was to set for the temperature to ramp to 250 °C linearly. This is to make sure that all strongly bound species were removed and to activate the sample for the adsorption step. The temperature was again ramped up at 10 °C /min to 250 °C while flowing helium. As the temperature was ramped up hydrogen was desorbed and at about 100°C hydrogen was taken up and desorbed around 150°C. The Figure depicts that the NH3 reduced graphene took up more hydrogen than Ascorbic reduced graphene. Fig. 9 shows a graph of the mass spectrometer, which gives the desorption temperatures. The area under the peak of graphs in Fig. 9 give an indication of the total amount adsorbed on the surface of the material.
Fig. 7. TGA weight loss plot against temperature for the Ca/graphene samples reduced with ammonia and Ascorbic acid before and after hydrogen charging.
dissociation via a so called Kubas interaction [25] or also by a unidirectional polarization. This results in binding energies with up to almost 1 eV/H2-molecule [14] considerably larger than with “pristine” graphene (0.07 eV/H2) [26]. The graphical representation of the mechanism is sketched in Fig. 8. The occurrence of Ca particles on the graphene matrix increases the binding energy and thereby facilitates the adsorption of hydrogen molecule due to the polarization of the hydrogen molecule.
4. Conclusion In this work, a Ca/graphene nanocomposite was successfully synthesized. The XRF characterization results proved that the Ca was successfully loaded on the graphene matrix with the Ca percentage moving from 0.516% in graphite flakes to 8.227% in Ca/graphene composite reduced by ammonia. TEM images showed Ca dispersed on the graphene matrix. The hydrogen uptake of the graphene was improved through the incorporation of Calcium from 2 wt% in graphene to 4.98 wt% and 3.99 wt% for Ca/graphene reduced with ammonia and
3.1.7. TPD-MS analysis Temperature programmed desorption of NH3 reduced graphene and 6
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Fig. 8. Ca/graphene hydrogen storage mechanism. Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902. [6] J.H. Jiang, Chemical preparation of graphene-based nanomaterials and their applications in chemical and biological sensors, Small 7 (2011) 2413. [7] S.B. Yang, X.L. Feng, K. Mullen, Sandwhich-like, graphene-based titania nanosheets with high surface area for fast lithium storage, Adv Mater 23 (2011) 3575. [8] Q.J. Xiang, J.G. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (2012) 6575. [9] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large scale pattern growth of graphene films for scretchable transparent electrodes, Nature 457 (2009) 709. [10] S.F. Pei, H.M. Cheng, The reduction of graphene oxide, Carbon 50 (2012) 3210. [11] J. Geng, B.S. Kong, S.B. Yang, H.T. Jung, Preparation of graphene relying on prophrin exfoliation of graphite, Chem Comm 46 (2010) 5091. [12] D.V. Kosynkin, A.L. Higginbotham, A. Sinitskii, J.R. Lomeda, A. Dimiev, B.K. Pricel, J.M. Tour, Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons, Nature 458 (2009) 872. [13] C. Hu, T. Lu, F. Chen, R. Zhang, A brief review of graphene-metal oxide composites synthesis and applications in photocatalysis, J. Chinese Adv. Mater. Soc. 1 (1) (2013) 21–39. [14] Diana C. Tranca, Gotthard Seifert, A first-principles study of metal-decorated graphene nanoribbons for hydrogen storage, De Gruyter 230 (5-7) (2016) 791–808. [15] C. Zhou, A.J. Szpunar, Hydrogen storage perfomance in Pd/Graphene nanocomposites, Applied Materials and Interfaces 8 (2016) 25933–25940. [16] C.D. Marcano, D.V. Kosykin, M.J. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, B.L. Alemany, W. Lu, M.J. Tour, Improved synthesis of graphene oxide, ACS Nano 4 (8) (2010) 4806–4814. [17] F.E.A. Oliveira, B.G. Braga, T.R.C. Tarley, C.A. Pereira, Thermally reduced graphene oxide: synthesis, studies and characterization, J. Mater. Sci. 53 (2018) 12005–12015. [18] M. Wall, The Raman Spectroscopy of Graphene and the Determination of Layer Thickness, Thermo Fisher, USA, 2011. [19] B. Kartick, K.S. Srivastava, I. Srivastava, Green synthesis of graphene, Nanosci. Nanotechnol. 13 (2013) 4320–4324. [20] K. Kanishka, H. De Silva, H.-H. Huang, M. Yoshimura, Progress of reduction of graphene oxide by ascorbic acid, Applied Surafce Science 447 (2018) 338–346. [21] C.M. Willemse, K. Tlhomelang, N. Jahed, P.G. Baker, E.I. Iwuoha, Metallo-graphene nanocomposite electrocatalytic platform for the determination of toxic metal ions, Sensors 11 (2011). [23] K. Kanishka, H. De Silva, Hsin-Hui Huang, Masamichi Yoshimura, Progress of reduction of graphene oxide by ascorbic acid, Appl. Surf. Sci. 447 (2018) 338–346. [25] Gregory J. Kubas, Molecular Hydrogen complexes: coordination of a sigma bond to transition metals, Acc. Chem. Res. 21 (no. 3) (1988) 120–128. [26] Thomas Heine, Lyuben Zhechkov, Gotthard Seifert, Hydrogen storage by physisorption on nanostructured graphite platelets, J. Chem. Soc. Faraday Trans. 6 (5) (2004) 980–984. [29] M. Safardoust-Hojaghan, M. Salavati-Niasari, Degradation of methylene blue as a pollutant with N-doped graphene quantum dot/titanium dioxide nanocomposite, J. Clean. Prod. 148 (2017) 31–36.
Fig. 9. Ca/Graphene TPD-MS desorption profiles.
L-Ascorbic acid respectively. In terms of the objectives of this paper, it was observed that ammonia enhances the hydrogen uptake of graphene while it partially reduces graphite oxide. This was observed in hydrogen uptake analysis that was done on the Ca/graphene nanocomposite reduced by ammonia and L-Ascorbic. The characterization results depicted that L-Ascorbic reduced graphite oxide more than ammonia. The successful oxidation of the graphite also proved that the Improved Tour Method efficiently oxidizes graphite flakes Acknowledgements The Authors are thankful to Mintek and the University of the Witwatersrand for funding the work. References [1] S. Dutta, S.K. Pati, Novel properties of graphene nanoribbons: a review, J. Mater. Chem. 20 (2010) 8207–8223. [2] S. Patchkovskii, S.J. Tse, N.S. Yurchenkov, L. Zhechkov, T. Heine, G. Seifert, Graphene nanostructures as tunable storage media for molecular hydrogen, PNS 102 (30) (2005) 10439–10444. [3] A. Venkatesan, N.R. Patel, S.E. Kannan, Reduced graphene oxide for room temperature hydrogen storafe application, Adv. Mat. Res. (2015) 91–95. [4] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Sci 321 (2008) 385. [5] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau,
7
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Effects of reduction of graphene oxide on the hydrogen storage capacities of metal graphene nanocomposite ⁎
A. Ngqalakwezia,b,c, , D. Nkazia, G. Seifertc,d, T. Nthob a
School of Chemical and Metallurgical engineering, University of the Witwatersrand, Johannesburg, South Africa Advanced Materials Division, Mintek, Private Bag x3105, Randburg, 2194, Gauteng, South Africa c Materials for Energy Research Group, University of the Witwatersrand, Johannesburg, South Africa d Theoretical Chemistry, Technische Universität Dresden, 01062, Dresden, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Reduction Hydrogen storage capacity
Light-weight materials such as graphene have attracted a great interest for hydrogen storage applications. In this study, a metal-on graphene nanocomposite was synthesized for hydrogen storage for on-board applications. The graphene used as support was obtained from precursor graphite oxide, which was synthesized using the Improved Tours method and later reduced by L-Ascorbic acid and ammonia. The precursor graphite flakes used in the synthesis of graphite oxide were 100 μm in size. The XRD patterns depicted a low angle shift in GO, 11.3° from graphite 30° proved successful oxidation of graphite. The FTIR and Raman band showed L-Ascorbic reduced the GO better than ammonia through the observed decrease on the functional bands after reduction. The elemental analysis carried out using XRF showed the successful loading of Ca on the graphene matrix and TEM images also confirmed this. The hydrogen storage capacity of the nanocomposite was tested using TGA and TPD equipped with Mass Spectrometer. The Ca/graphene nanocomposite reduced by ammonia exhibited a large hydrogen storage uptake at 4.98 wt. % compared to L-Ascorbic acid.
1. Introduction One of the sustainable development goals globally is to make the environment a cleaner place. As such, the possibility of using hydrogen as an alternative source of energy has garnered a lot of attention due to its zero emission characteristic to the environment, high energy content and its abundant availability [1,2]. However, the major challenge with hydrogen is the storage. The storage of hydrogen is governed by the US Department of Energy (DOE), which sets the standards for on-board systems. To date, no material has fully met the DOE requirements for on-board applications. Various materials have been experimentally tested for solid state storage of hydrogen because gaseous and liquid storage of hydrogen pose cost and energy challenges. One of the material tested for solid storage of hydrogen is graphene based material [3–5] because graphene has intriguing properties such as extra-ordinary mechanical strength, transport performance, increased movement of charge carriers and high surface area [4–7]. Graphene is an allotrope of carbon which is characterized by a monolayer of sp2 carbon atoms. Graphene can be synthesized through graphite intercalation,
chemical vapour deposition, micromechanical cleavage and the reduction of graphite oxide [8–12]. The reduction of graphite oxide has been the common route of producing graphene. The method of reduction of graphene has an effect on the structure of graphene and thereby its hydrogen storage capacity. It has been reported that thermally reduced graphene acquires a significant number of structural defects which act as binding sites for hydrogen [4,15]. However Graphene alone as a physisorbent material does not take up a lot of hydrogen and thereby has to be enhanced through the addition of active metals for practical on-board applications. The incorporation of active nanometals on graphene helps in tuning the electronic structure and surface morphology of graphene [13]. A number of metal/graphene nanocomposites have been synthesized in this regard and a lot of DFT studies have been performed [14]. A Pd/graphene nanocomposite was synthesized with 6.7 wt% at 50 bars [14]. In this work ammonia and L-Ascorbic were used as reductants instead of hydrazine. Hydrazine due to its hazardous nature and cost, remains undesirable as a reductant. Furthermore in this work, different reduction techniques such as chemical reduction and thermal reduction
⁎
Corresponding author at: School of Chemical and Metallurgical engineering, University of the Witwatersrand, Johannesburg, South Africa. E-mail addresses: [email protected] (A. Ngqalakwezi), [email protected], [email protected] (D. Nkazi), [email protected] (T. Ntho). URL: http://[email protected] (D. Nkazi). https://doi.org/10.1016/j.cattod.2019.06.029 Received 14 March 2019; Received in revised form 31 May 2019; Accepted 10 June 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: A. Ngqalakwezi, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.06.029
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are explored. Calcium was also used to improve the catalytic activity of graphene in the uptake of hydrogen. The effect of the reduction techniques on the hydrogen storage capacity of graphene and the metal incorporation thereof were studied. Fig. 2. The thermal reduction process of GO.
2. Experimental section de-ionised water. The NH3 solution was slowly added to the graphite oxide solution. The temperature was raised to 70°C and it was kept there for 2 h. The resultant solution of reduced graphene was then aged for 2 days and freeze dried.
2.1. Synthesis of graphite oxide Precursor graphite oxide was synthesized using the Improved Tour Method which permits a large scale synthesis of hydrophilic oxidized graphene compared to the traditional modified Hummer’s method [15]. In this method, graphite flakes (5 g, Sigma) were weighed and mixed with H2SO4 and H3PO4 in a ratio of 9:1 (360 ml, ACE and 40 ml, Sigma). The solution was stirred at 400 rpm where the temperature was kept below 10°C in an ice bath. Then, KMnO4 (18 g, Sigma) was added slowly to the graphite solution. The mixture was taken off the ice bath and heated to 50°C and stirred for 3 h. The solution was then cooled and H2O2 (3 ml, ACE) was added to quench the reaction. The resultant graphite oxide was dialyzed 6–7 times in 5 l de-ionized water (18.2 MΩ cm purified by a Milli-Q water system). The dialyzed graphite oxide was filtered and dried at 120°C (Fig. 1) [15].
2.2.3. Chemical reduction: L-Ascorbic acid Graphite oxide was mixed with 250 ml of de-ionized water and stirred at 250 rpm. Then, L-Ascorbic acid (5.28 g, Sigma) was mixed with 300 ml of de-ionised water. The L-Ascorbic solution was slowly added to the graphite oxide solution. The temperature was raised to 70°C and it was kept there for 2 h. The resultant solution of reduced graphene was then aged for 2 days and freeze dried. 2.2.4. Calcium loading on graphene Precursor graphene gel was prepared adding and mixing deionised water with the dried reduced graphene. This step could be simultaneously done with the reduction step. To synthesize Ca\graphene, Ca/ Cl2 (1.542 g, ACE) was dissolved in deionized water and was slowly added to graphene gel. The temperature was raised to 70°C and it was stirred for an hour. The resultant solution was sonicated and centrifuged at 4000 rpm. The precipitate was washed extensively with deionized water and centrifuged again. The precipitate was dried in an oven at 180°C and then dried in a furnance for 2 min at 300°C.
2.2. Reduction of graphite oxide 2.2.1. Thermal reduction The graphite oxide was placed in a crucible and then placed in the oven for approximately 24 h. The graphite oxide was reduced at 200°C. The resultant reduced graphene was weighed and then taken for characterization (Fig. 2) [17].
2.3. Characterization 2.2.2. Chemical reduction: Ammonia Graphite oxide was mixed with 300 ml of de-ionized water and stirred at 250 rpm. Then, NH3 (10 ml, ACE) was mixed with 100 ml of
X-ray powder diffraction (p-XRD) using an enhanced resolution Bruker AXS D8 X-ray advanced powder diffractometer equipped with
Fig. 1. Schematic diagram of the synthesis of GO from precursor graphite flakes. 2
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band position is in an in-plane mode which involves the sp2 hybridized carbon and is extremely sensitive to the number of layers in graphene [18,15]. The shift observed in the G-band position of the graphene is due to the increase in the layer thickness which results in the band shifting to a lower energy [17]. The band position can be correlated to the number of layers available in a sample through this relation: [17]
CuKα-radiation (40 mA, 1.78897 Å, 40 kV) with LinxEye detector was utilized to acquire the crystal structure of the catalyst. The XRD spectra was recorded in the 2θ range of 5 to 80◦ diffraction angle with a step size of 0.02. A Perkin Elmer Raman Station 400 benchtop Raman spectroscopy with an excitation wavelength of 785 nm laser (100 mW at the sample) and a lateral resolution of ˜ 1 μm was utilized to characterize the samples. The Raman mapping was executed with an accumulation time of 3 s at each spot and a grip spacing of 0.25 mm. The detector utilized was a temperature controlled Charged Coupled Device (CCD) detector (−50 °C) incorporating a 1024 × 256 pixel sensor. The spectra were processed using Spectrum software supplied by PerkinElmer. Transmission Electron Microscopy (TEM) was carried out using the JEOL Jem 2100 F with an electron accelerating voltage that ranges from 80 kV to 200 kV. The samples were prepared on perforated copper grids coated with carbon FTIR characterization was carried out using a Perkin Elmer Spectrum two (UATR Two). The background spectrum was subtracted from all FTIR recorded spectra. Then a NitonTM XL3t XRF analyser was utilized to determine the elemental analysis of the samples at room temperature. Ammonia temperature programmed desorption (NH3 –TPD) using AutoChemTM II 2920 with automated catalyst characterization system was utilized to determine the desorption kinetics. Thermo-gravimetric analysis (TGA) using the SDT Q600 TA instrument was used to measure the hydrogen uptake.
ωG = 1581.6 +
11 (1 + n1.6)
The shift between the ammonia reduced graphene and the Ascorbic reduced showed no significant difference. The G band is also affected by the temperature and in the reduction experiments the temperature was kept constant at 70°C. On the other hand, the D band in the Raman spectroscopy appears around ˜ 1327-1342 cm−1 wavelength. The D band is regarded as the disorder band because it appears due to the defects that occur during the oxidation and reduction of graphene [16,17]. The D band can be observed at ˜ 1324 cm-1, ˜ 1332 cm-1, ˜ 1322 cm-1 and ˜ 1340 cm-1 for graphite flakes, GO, Ascorbic reduced graphene and ammonia reduced graphene respectively. On the subsequent reduction of graphene, it can be observed that the D band has an increased intensity than the G band because of the degradation of the sp2 hybridized carbons and the formation of the functional groups on the plane resulting in the creation of defects on the plane [16]. The D band has low intensity on the graphite because of the small number of defects on it. The 2D band can be observed on the graphite band at ˜
3. Results and discussion
2714 cm-1. The 2D band appears due to the vibrational process of the two phonon lattice and the increase in number of layers results on the decrease of symmetry and intensity of this band [15]. The ID/ IG intensity ratio was increased from 0.84 graphite oxide to 1.19 in ammonia reduced graphene and 1.21 in ascorbic reduced graphene. This was indicative of the formation sp2 domains and the reduction of the functional groups on the plane
3.1. Structure/texture characterization of the materials 3.1.1. Raman Raman characterization, a vibrational technique that is sensitive to bonding within molecules and geometric structure, was used to identify the structural differences during the reduction and oxidation processes of graphene [16,17]. The vital bands considered are the 2D, D and G bands. Fig. 3 shows the spectra graphite flakes A, graphite oxide B, ammonia reduced graphene C and L-ascorbic reduced graphene D. The Fig. 3 (a) shows the graphite flakes G band at ˜ 1582 cm−1 shifted to ˜ 1598 cm−1 and ˜ 1596 cm−1 in the case of ammonia reduced graphene and L-Ascorbic reduced graphene respectively. The G
3.1.2. XRD The XRD results are presented in the Figure below (4) in a comparative way. While doing the XRD measurements some of the samples were crushed to obtain powder. Graphite depicts a very sharp peak at ˜ 2 θ = 30 ° which corresponds to the packing of atomic layers and
Fig. 3. Raman spectra of the sample A. graphite flakes, B. GO, C. NH3 reduced graphene D. Ascorbic acid reduced graphene. 3
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Fig. 4. XRD patters of the samples where A. graphite flakes, B.GO, C. Ascorbic acid reduced graphene D. NH3 reduced graphene, E. Thermally reduced graphene.
(1595-1623 cm−1) vibrations and C]O (1719-1754 cm−1) vibrations [19]. The OH- stretch which can be observed in the samples is due to the presence of water in the graphene layers [10]. The decrease in the intensity of the bands can be visibly observed for the samples reduced with Ascorbic acid, while the ammonia reduced graphene spectra shows little change in the intensity of the bands. Therefore this affirms that graphite oxide was successfully reduced with Ascorbic while it was partially reduced with ammonia (Fig. 5).
hexagonal arrangement [3]. This angle shifts lower in GO because of increased interplanar spacing due to the oxidation of the graphite crystalline structure [16]. The GO peak can be observed at ˜ 2 θ = 11.3 ° corresponding to the d001 plane. The broad GO peak confirms the presence of the functional groups in graphite oxide due to the oxidation process [14,3]. It can be noted that the intensity of the oxidation peak at ˜ 2 θ = 11.3 ° decreases in both the thermally reduced graphene and ammonia reduced graphene however it does not disappear. On the other hand a broad peak can be observed on the ascorbic reduced graphene which indicates loss of crystallinity and extreme decrease in the interplanar spacing [16]. The results show that graphite was successfully oxidized and reduced with L-Ascorbic acid (Fig. 4).
3.1.4. XRF X-ray fluorescence, a technique that measures the fluorescent X-ray discharged from a sample due to excitement by X-rays, was utilized to determine the elemental composition of the graphite, reduced graphene and metal incorporated reduced graphene. The graphite mainly consisted of carbon and traces of Ca, Mn and Fe. The reduced graphene depicted new elements such as P and K mainly because KMnO4 and H3PO4 were used during the oxidation of graphite. The XRF of the Ca/ graphene reduced with both NH3 and Ascorbic acid showed an increase of Ca, meaning the Ca was successfully incorporated into the graphene
3.1.3. FTIR results The FTIR spectroscopy is associated with atomic or molecular vibrations in a material. These vibrations are used to fingerprint the functional groups by analysing the frequencies to identify molecules. The FTIR spectra identified several functional groups: C–O (12151252 cm−1) vibrations, OeH (3422 cm−1) stretching vibrations, C]C 4
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Table 3 Elemental comp. of samples reduced with Ascorbic acid and NH3. NH3rGO
AscrGO
Element
%
Element
%
Carbon Mn Si P K Ca
87.411 9.026 0.29 0.713 0.104 0.055
Carbon Mn Si P K Ca
88.277 9.045 0.269 0.731 0.145 0.069
(Tables 1–3). 3.1.5. TEM analysis The surface analysis on the morphological properties of as-prepared graphene, graphene oxide and metal/graphene was examined using Transmission electron microscope as shown in Fig. 6. The samples were prepared by dispersing graphene in a solvent and then pipetting it on the holey carbon coated copper grids. During the sample preparation step the graphite flakes could not be exfoliated and the image captured (Fig. 6 (A)) depicts the graphitic nature of the flakes [20]. GO (Fig. 6 (B)) shows translucent sheets which are entangled on top of each other. GO was observed to be unstable under increased energy beam relative to graphene [21]. Graphene reduced with ammonia, L-Ascorbic acid and thermally reduced graphene can be observed in Fig. 6 c, d and e respectively. Ammonia reduced graphene (Fig. 6 (C)) appears semitransparent and the morphology shows irregular shaped layers with rough surfaces. The tangles and irregularity on the surface are due to the partial reduction of graphene with ammonia. L-Ascorbic reduced graphene (Fig. 6 (D)) shows more transparent layers with folding and wrinkles both on the surface of the structure and edges. Thermally reduced graphene (Fig. 6 (E)) shows flat layers which are stacked. The rough surfaces on the edges and irregularity in the shape can also be observed in the capture image. Fig. 6 (F) ammonia reduced and (G) LAscorbic reduced show observable particles of calcium on the surface which were previously not observed.
Fig. 5. FTIR spectra A. Graphite B. GO, C. Ascorbic acid reduced graphene D. NH3 reduced graphene.
3.1.6. Thermogravimetric analysis Thermogravimetric analysis of Ca/graphene reduced with ammonia and L-ascorbic acid was done on the SDT Q600 TA instrument before and after hydrogen charging. The samples were charged in a 4540 Parr reactor at 10 bars and ambient temperature for 30 min. The temperature was ramped to 500 °C at a ramp rate of 10°C /min under nitrogen gas. Fig. 7 below shows the weight loss of all the samples combined. The weight loss in the Ca/graphene is attributed to the labile oxygen groups and the adsorbed water on the material [23]. The hydrogen capacity of the material was tested through running TGA on non-hydrogen charged Ca/graphene and charged graphene. The Ca/graphene reduced with ammonia had a total weight loss of 50.97 wt% while the non-charged material lost about 45.99%. The calculated hydrogen uptake was up to 4.98 wt% while the Ca/graphene reduced with L-Ascorbic acid was up to 3.99 wt% with weight loss of about 36.31 wt% and 40.30 wt% before and after charging respectively. These results show that the hydrogen uptake of graphene was improved through the incorporation of the Calcium metal onto the graphene matrix. The hydrogen uptake of graphene is reportedly around 2 wt% at 50 bars and 77 K [23,25]. Furthermore these results show the N-doping effect ammonia has on graphene enhances the uptake of hydrogen in the material [29]. According to the results obtained for the structural characterization of the Ca/graphene synthesized and hydrogen storage properties thereof, the uptake of hydrogen may be explained as proposed in by Tranca et.al [14]. The calcium atoms or clusters are immobilized at the carbon nanostructure and can bind hydrogen molecules without
Table 1 Elemental comp. of the precursor graphite flakes. Graphite Flakes Element
%
Carbon Calcium Manganese Iron
96.171 0.516 2.623 0.47
Table 2 Elemental comp. of Ca decorated graphene samples reduced with Asc Acid & NH3. Ca-NH3rGO
Ca-AscrGO
Element
%
Element
%
Carbon Sulphur Calcium Manganese Phosphorus Potassium
75.426 3.809 8.227 5.236 2.066 0.348
Carbon Sulphur Calcium Manganese Phosphorus Potassium
81.589 3.191 7.381 4.234 1.045 0.017
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Fig. 6. TEM morphological analysis of A. Graphite flakes B. GO, C. Ammonia acid reduced graphene D. L-Ascorbic reduced graphene E. Thermally reduced graphene F. Ca/Graphene_Ammonia G. Ca/Graphene_L-Ascorbic acid.
the ascorbic reduced graphene followed by mass spectrometry was performed in a continuous flow reactor coupled to a quadrupole mass spectrometer. This technique gives information about the desorption profile of the Ca/graphene. The area under peak, which we obtain from the profiles of this analysis is proportional to the amount originally adsorbed. The method was to set for the temperature to ramp to 250 °C linearly. This is to make sure that all strongly bound species were removed and to activate the sample for the adsorption step. The temperature was again ramped up at 10 °C /min to 250 °C while flowing helium. As the temperature was ramped up hydrogen was desorbed and at about 100°C hydrogen was taken up and desorbed around 150°C. The Figure depicts that the NH3 reduced graphene took up more hydrogen than Ascorbic reduced graphene. Fig. 9 shows a graph of the mass spectrometer, which gives the desorption temperatures. The area under the peak of graphs in Fig. 9 give an indication of the total amount adsorbed on the surface of the material.
Fig. 7. TGA weight loss plot against temperature for the Ca/graphene samples reduced with ammonia and Ascorbic acid before and after hydrogen charging.
dissociation via a so called Kubas interaction [25] or also by a unidirectional polarization. This results in binding energies with up to almost 1 eV/H2-molecule [14] considerably larger than with “pristine” graphene (0.07 eV/H2) [26]. The graphical representation of the mechanism is sketched in Fig. 8. The occurrence of Ca particles on the graphene matrix increases the binding energy and thereby facilitates the adsorption of hydrogen molecule due to the polarization of the hydrogen molecule.
4. Conclusion In this work, a Ca/graphene nanocomposite was successfully synthesized. The XRF characterization results proved that the Ca was successfully loaded on the graphene matrix with the Ca percentage moving from 0.516% in graphite flakes to 8.227% in Ca/graphene composite reduced by ammonia. TEM images showed Ca dispersed on the graphene matrix. The hydrogen uptake of the graphene was improved through the incorporation of Calcium from 2 wt% in graphene to 4.98 wt% and 3.99 wt% for Ca/graphene reduced with ammonia and
3.1.7. TPD-MS analysis Temperature programmed desorption of NH3 reduced graphene and 6
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Fig. 9. Ca/Graphene TPD-MS desorption profiles.
L-Ascorbic acid respectively. In terms of the objectives of this paper, it was observed that ammonia enhances the hydrogen uptake of graphene while it partially reduces graphite oxide. This was observed in hydrogen uptake analysis that was done on the Ca/graphene nanocomposite reduced by ammonia and L-Ascorbic. The characterization results depicted that L-Ascorbic reduced graphite oxide more than ammonia. The successful oxidation of the graphite also proved that the Improved Tour Method efficiently oxidizes graphite flakes Acknowledgements The Authors are thankful to Mintek and the University of the Witwatersrand for funding the work. References [1] S. Dutta, S.K. Pati, Novel properties of graphene nanoribbons: a review, J. Mater. Chem. 20 (2010) 8207–8223. [2] S. Patchkovskii, S.J. Tse, N.S. Yurchenkov, L. Zhechkov, T. Heine, G. Seifert, Graphene nanostructures as tunable storage media for molecular hydrogen, PNS 102 (30) (2005) 10439–10444. [3] A. Venkatesan, N.R. Patel, S.E. Kannan, Reduced graphene oxide for room temperature hydrogen storafe application, Adv. Mat. Res. (2015) 91–95. [4] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Sci 321 (2008) 385. [5] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau,
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