Investigation of graphene oxide-hydrogen interaction using in-situ X-ray diffraction studies
Introduction
Enormous energy requirements of today's world are fulfilled by the non-renewable energy sources which are not only limited but also affect our eco-system in a very negative way [1]. Recent research is going on to utilize our non-conventional renewable energy sources and replace conventional fuels with hydrogen, lithium ion batteries, super capacitors etc. Among the green fuels, hydrogen can be a promising candidate to meet the requirements of our growing population. Being a clean fuel, it has calorific value higher than other conventional sources like petroleum [2]. But, hydrogen has low energy density which requires high pressure/and/or low temperature for storage and it's a challenge for practical purposes. Department of energy (DOE), USA has mentioned few targets including 4.5 wt % hydrogen, 0.030 kg hydrogen/L and $ 333/kg stored hydrogen capacity to be achieved by 2020 for onboard application. Different mechanisms have been proposed for storing hydrogen which includes high pressure tank, formation of complex-metal-hydrides and physisorption. The drawbacks of high pressure tanks include low gravimetric and also leakage of hydrogen gas which might cause major accident [3]. The problem with the metal-hydrides lies in the slow rate of hydrogen release which cannot fulfill the need of vehicles [4]. Also, they are too heavy to achieve DOE goals. The physisorption can be considered as a safe option as it involves the weak adsorption of H2 molecules over the surface of host adsorbent [5]. It requires high surface area for maximum loading of the hydrogen gas and host should remain stable even after adsorption [6]. Researchers have shown that the amount of hydrogen loading is directly related to the surface area provided for adsorption. Carbon nanomaterials showed potential for onboard applications like they are light in weight and the process involved in hydrogen releasing is endothermic which cannot lead to explosions. Reports are available on graphene [7], carbon nano tubes [8], [9], [10], [11] where these nanostructures are exploited for hydrogen storage. The experimental results showed that a rippled surface provides more surface area than a planar graphene sheet [4], [12]. In case of carbon nanotubes, large scale synthesis can be a problem as a defined radius is required for storing hydrogen.
Generally, the materials used for hydrogen storage require the understanding of the interaction mechanism of hydrogen with that particular material. Till now, it is not economical to store hydrogen due to the conditions like very low temperature and high pressure [13], [14]. The weak binding strength of hydrogen to these structures poses a challenge to store hydrogen at reasonable temperatures. Several measures have been carried out to increase this binding strength like doping, chemical functionalization etc. But these methods are not economical and also impart impurities to the structures modifying their properties. In-situ XRD measurements have been done to study the effect of hydrogen on Palladium-based alloys [15]. Researchers have shown theoretically that it is possible to bind hydrogen to defective sites of graphene even at room temperature. Also, graphene surface has been modified by different catalysts as shown in literature to maximize its hydrogen intake capacity [16], [17], [18].
One of the carbon nanostructures, Graphene oxide, which is a functionalized form of graphene, has oxygen-containing functional groups present inherently on its edges and planes [19]. Due to its ease of synthesis compared to other nanomaterials, it can be used for storing hydrogen [20]. We, therefore, need to study its interaction with hydrogen.
This work has been carried out using graphene oxide and hydrogen gas to study their interaction at room temperature. We performed in-situ XRD measurements to study the structural changes induced in GO film in different concentration of hydrogen gas at varying pressures. These studies are of utmost importance to open up the possibility of utilizing GO as host sorbent for hydrogen storage for further investigations. In-situ XRD results are supported by FTIR and Raman measurements by which functional groups and consequently the defects induced were analyzed, respectively. Topography of the films was studied using Atomic force microscopy which supports the mechanism we proposed. Elastic recoil detection analysis (ERDA) measurements were done to quantify the amount of hydrogen present in GO films before and after hydrogen environment.
Section snippets
Experimental details
Graphene oxide films were synthesized using procured solution of Graphene oxide from Graphene Supermarket. Different concentrations of solution (5 g/l and 500 mg/l) were mixed to get an optimized concentration. 20 μl solution was drop casted on SiO2 (300 nm)/Si substrate and was dried at room temperature for 24 h. GO films of nearly 600 nm thickness were obtained (as measured by Rutherford backscattering spectrometry).
XRD patterns have been recorded using in-situ XRD facility equipped with Cu Kα
Results and discussion
X-ray diffraction pattern of pristine GO film showed the characteristic peak at ∼12.11° (002) with inter-planar distance of ∼0.73 nm. GO sample was placed in a chamber under vacuum and exposed to different concentrations of hydrogen i.e. 1% H2 (1% H2+ 99% Argon), 10% H2 (10% H2+ 90% Argon) and 100% H2. Pressure of gas was varied from 70 mbar to 900 mbar and diffraction patterns were obtained at different steps as shown in Fig. 1. As it can be observed that there is a shift in the position of
Quantitative analysis of hydrogen using Elastic recoil detection analysis (ERDA)
Rutherford backscattering spectrometry (RBS) is used to determine the thickness of the film. Helium ions of energy 2.8 MeV were bombarded on the sample, tilted at an angle of 15° with respect to the incoming direction of the ions. One detector was placed at 166° with respect to ion beam direction which collected backscattered Helium ions.
Hydrogen concentration in GO pristine film was measured using ERDA and compared with the GO samples exposed to different hydrogen environment. ERDA is one of
Mechanism of interaction of hydrogen with graphene oxide
Graphene oxide, being oxidized form of graphene, contains oxygen-moieties i.e. hydroxyl, epoxy and carboxylic acid. It is shown in literature that hydrogen preferably bind itself to defective sites of carbon rings as they have higher energy as compared to non-defective sites. The oxygen-containing functional groups are considered as defective sites of carbon hexagonal lattice. Intrinsic defects are generally present in GO during the oxidation of graphite. Mechanism for hydrogen interacting with
Conclusion
Graphene oxide films were synthesized and characterized using X-ray diffraction, Raman microscopy and Fourier transform infra-red (FTIR) spectroscopy. These films were exposed to different concentration of hydrogen gas at different pressures varying from 70 mbar to 900 mbar at room temperature. In-situ XRD measurements were performed during the interaction of hydrogen gas with GO and strain induced in films reached up to ∼6.3% in case of lower concentration of hydrogen environment. Hydrogen
Acknowledgement
Chetna Tyagi would like to acknowledge UGC for fellowship. Authors thank Dr. F. Singh and Dr. I. Sulaniya, IUAC for Raman and AFM measurements, respectively. Authors also thank Mr. G.R. Umapathy, IUAC for his help in analyzing ERDA data. Thanks to Mr. Saurabh Sharma, research scholar at IUAC, for his help during in-situ XRD measurements. Thanks to Centre of Basic Sciences, Jamia Milia Islamia, for FTIR measurements. Authors are thankful to DST and BRNS for financial support under Nanomission,
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