Experimental Investigation and modeling of hydrogen storage in graphene nanoplatelets incorporated silicon oxycarbide ceramics

The present work focuses on synthesizing graphene nanoplatelets (GNP) incorporated amorphous silicon oxycarbide ceramic (Si-O-C) for hydrogen adsorption. The changes in the structure of the ceramic upon addition of GNP has been studied using X-ray diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and Fourier Transform Infrared (FT-IR) spectroscopy. A theoretical framework to quantify these changes is proposed in line with existing structural model. Hydrogen adsorption studies have been carried out at 100K and 2 bar using Sievert’s apparatus. Maximum gravimetric storage density (GD) of 0.16 wt.% was observed after adding 0.3 wt.% of GNP, in comparison to 0.35 wt.% for the non-GNP sample. Pores sized 2-5 nm were found to be critical for hydrogen adsorption. The study finds that GNP addition leads to an increase in the size of the silica nanodomains. This increase in nanodomain size results in increase in the pore sizes of the existing mesopores, thereby reducing the overall hydrogen uptake. Further, the addition of GNP beyond 3 wt.% is found to increase the coordination of mixed bonds (Si-O-C) in the interface which results in agglomeration and subsequent loss of porosity in the composite.

Bangalore, India) were incorporated in the mixture before the curing stage. The hydrotalcite helps in synthesizing aligned pores in the ceramic [14]. The R18 sample discussed in reference [14] was chosen for the studies since it has the highest specific surface area and pore volume, which are key to hydrogen adsorption. GNP was added in different weight fractions of 0.3 wt.%, 3wt.%, and 6 wt.%to the polymeric precursor mixture to establish its effect on the pore morphology and structure of the Si-O-C. The precursor mixture was then magnetically stirred for 20min at 600rpm. This was followed by the addition of platinum (0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution in xylene, Pt2%, [O{Si(CH3)2CH=CH2}2 Pt, Sigma Aldrich, USA], 100 ppm by weight of Pt relative to PHMS, as the catalyst for the curing reactions. After Pt addition, again the mixture was stirred for 15 min at 600 rpm. This was followed by pouring these stirred precursors blends in a ceramic crucible for overnight curing at 220˚C [14]. The curing process was followed by pyrolysis till 1000˚C. Pyrolysis was carried out in a tubular furnace at a heating rate of 4˚C/min and in an atmosphere of argon, followed by a dwell time of 1 h.
The methodology followed for the synthesis is shown in Fig.1. The synthesis was followed by the hydrogen adsorption using Sievert's apparatus, seen in ref. [13] and characterization studies on the composite. The mean value of GD obtained have been plotted, with the maximum 6 standard deviation being ±0.005 wt.%. Further, the Van-der Waals equation has been considered for describing the behavior of hydrogen at 100 K in the adsorption chamber [15]. It assumes a slow-motion of the hydrogen molecules from the reference chamber to the adsorption chamber due to low temperatures. The procedure and methodology for calculating and interpreting GD in this manner is detailed in [13].

Structural Characterization:
X-ray diffraction (XRD) was carried out using Rigaku -Ultima IV (Japan) with Cu: Kα (λ = 0.154 nm) to establish the effect of graphene on the pyrolysed samples. Raman spectroscopy (UniRAM, micro-Raman mapping system, laser λ=533nm) was carried on the pyrolysed samples to determine the type of carbon existing after GNP addition. Brunauer, Emmet, and Teller (BET, BESORP-mini II, Japan) analysis were carried out to find the SSA, average pore size, and total pore volume in the pyrolysed samples. The average pore diameter has been calculated as ratio of 4V/A, where V is the volume and A is the surface area of the nano-pores [16]. The mesopore size distribution in the samples was determined from the Barrett-Joyner-Halenda (BJH) curves.
Further, Fourier Transform Infrared Spectroscopy (JASCO Japan, FT-IR 4200, Transmission type) studies were also carried out to study the effect of the GNP addition on to the structure of the Si-O-C.

Elemental Analysis
X-ray photoelectron spectroscopy (XPS) studies were taken up to find the effect of GNP addition on the structure of the Si-O-C through the change in the binding energies (B.E.) of atoms. Since the physisorption of hydrogen is a surface energy phenomenon hence it becomes relevant to associate it with the binding energy of the atoms at the surface and not in the bulk ceramic.
Further, it was also used to find the average elemental composition on the surface of the

Results and Discussions
FT-IR studies were conducted to confirm the formation of the ceramics at 1000˚C. The FT-IR data was normalized and plotted ( Fig. 2). It shows four distinguishable features for all the four samples. The first is a high-intensity absorption band at around 3500 cm -1 , assigned to the hydroxyl group which represents adsorbed water on the surface. In addition, the bands near 1050 cm -1 and 800 cm -1 are attributed to Si-O-Si and Si-C, respectively [14].  Further, XRD studies were carried out to find the effect of GNP addition on the structure of the ceramic. The characteristic peak corresponding to carbon (2θ = 26˚) can be observed in the samples containing GNP. The intensity of the carbon peak gets higher with an increase in the wt.% of GNP in the ceramic, as seen in Fig. 3. Further, the absence of any sharp peaks around 2θ = 36˚, 72˚ and presence of a broad halo at 2θ = 23˚ (amorphous SiO2) for the GNP free SiOC ceramic sample (SiOC 1000C) confirms its amorphous nature. Moreover, it confirms that GNP is the sole contributor of crystallinity in the ceramics. Additionally, Raman spectroscopy ( Supplementary Fig. 1) was carried out to find the nature of carbon existing in the composite after pyrolysis. The D and G peaks corresponding to the carbon from GNP were observed at 1336 cm -1 and 1556 cm -1 , respectively. The ID/IG ratio was found to be 0.83 (Supplimentary Fig. 1(b)), indicative of the orderness in the carbon of the incorporated GNP [17]. However, the peak intensities for D and G peaks were reduced when compared to pure GNP (ID/IG= 0.43, Supplementary Fig. 1(a)). This is probably due to the strong background fluorescence of the amorphous ceramic samples [8,18].
Having studied the characteristics of the GNP incorporated ceramic, further work focused on finding the elemental composition and the binding energies of the respective atoms on the surface of the specimens, using XPS. The resultant change in the ceramic surface structure upon addition of varying weight percentages of GNP is reflected through the changes in the binding energy values of the atoms, shown in Table 1 and Fig.4. We can observe from Table 1   condensation. This increased multilayer adsorption capability has resulted in higher SSA (Table   2) for these sample. These characterization studies were followed by modeling the adsorption of hydrogen in these amorphous ceramics.
The property used to study the hydrogen storage capacity of the system is the gravimetric storage density (GD) which is defined as follows:

GD =
Mass of hydrogen adsorbed

Mass of hydrogen adsorbed + Mass of adsorbent
Eq. 1 For finding the adsorption storage capacity for the ceramic without GNP, where multilayer adsorption takes place through capillary condensation, the procedure and methodology required for calculating and interpreting GD is detailed in [13]. The role of the free carbon for hydrogen storage has been neglected because it is generally present in disordered form [17].
However, after addition of the GNP, the adsorbent is multicomponent and hence a new approach needs to be defined for establishing the GD of hydrogen in this material. The modeling of the GD requires finding microstructural features of the material and a framework to define the adsorption mechanism. Therefore, in this study, a model used by Saha et. al [20], hereafter GNP, as seen in Table 1. These mesopores are critical to hydrogen adsorption in these ceramics and are constituted by the Si-O bonds [22]. The G.D for these GNP incorporated ceramics is then calculated by incorporating the above considerations into the modified expanded graphite model as detailed below. Where V m ( , T) is the molar volume of adsorbed hydrogen between the graphene layers. The adsorbed hydrogen is at a pressure of and temperature T where is defined as Where is the externally applied charging pressure of the hydrogen gas, and are the moles of hydrogen adsorbed and desorbed recorded from the experiment respectively. The correction factor is established wrt. ref. [13] as CF = (n − 1) * e −( / * ) , where n is the number of graphene layers and is the adsorption energy of hydrogen with carbon in the graphitic domain.
The correction factor is calculated per interlayer spacing of the graphitic domain value. The adsorption energy ( ) depends on the composition of the interface layer (p) and is computed as follows, The value of ( 2 + ℎ ) is obtained from molecular dynamics simulation as detailed in [13].
Therefore, the value of GD is calculated by the equation adapted from the modified expanded graphite model [13] as follows, only considers the ordered GNP which has been added in the ceramic. The presence of the ordered structure of GNP after pyrolysis at 1000˚C has been reported in refrence [17].
This establishment of the equation for gravimetric storage density was followed by the hydrogen adsorption studies on the composite.

Hydrogen storage in GNP incorporated SiOC polymer derived ceramic
Hydrogen storage studies were carried out on the synthesized composites at 100 K and pressure up to 2 bar using Sievert's apparatus. The usage of pressure till 2 bar is an attempt to achieve promising uptake of hydrogen at low pressures, considering its practical usage in future.
Gravimetric Density (GD) of hydrogen storage is then calculated and is then plotted with pressure ranging from 1 bar -2 bar at 3 discrete pressures as shown in Fig. 6 (a) and Table 2.  As seen in Fig.6 (a), the hydrogen storage capacity decreases with addition of GNP. As discussed earlier, the adsorption in GNP free ceramic is largely due to capillary condensation in the mesopores (2-5 nm). However, after addition of GNP, this volume is exponentially reduced as seen in Fig. 6(b) and Table 2 leading to reduction in adsorption capacity. The highest GD of 0.16 wt.% was found to be for the 0.3 wt.% GNP sample. Further, this reduction in the mesopore (2 -5 nm) volume can largely be attributed to changes in the electronegativity of the ceramic after addition of GNP, thereby affecting the interdomain bonding i.e., Si-O, Si-C, and the graphitic part(C) and size of the nanodomains in the ceramic [23,24]. Further from Table 4 we can observe that for the ceramic without GNP addition, the silica nanodomain size is 1.4 nm, which is indicative of mesopores (<5 nm). These mesopores are key to multilayer adsorption in this material [22]. The effect of mesopores sized, 2-5 nm can also be seen through reference [22].
This results in a storage capacity of 0.35 wt.% for the ceramic. However, with the addition of GNP, the silica nanodomain size increases from 1.4 nm to 2.6 nm. The addition of GNP