Graphene coated nickel foams for hydrogen storage

Hybrid composites where graphene (Gr) and other 2D materials replicate the meso-and micro-structure of 3D porous substrates have shown innovative functionalities in catalysis and energy-related fields. Concerning hydrogen storage, the high surface-to-volume ratio exhibited by both 2D and 3D components of the hybrid material is expected to increase the efficiency of surface chemisorption and bulk absorption of hydrogen in comparison to the flat counterparts. To explore this possibility, we have grown single layer Gr on porous nickel foams and have investigated the interaction with H atoms as a function of the temperature by using X-ray photoelectron spectroscopy and thermal programmed desorption (TPD) at the SuperESCA beamline of Elettra.

The growth of Gr on the Ni foam was obtained by exposing the sample at 773 K to ethylene. Selected C 1s spectra taken at increasing growth time are shown in Fig.1a. Upon exposure to ethylene, the carbide phases (N₁-N₄ components) observed in the pristine sample disappear, while new Gr components (labeled C₀ and C₁) progressively increase in intensity and eventually saturate. The component C₀ is attributed to Gr_S regions grown on the (111) foam grains, where the interaction with the support is as strong as that between Gr and the ordered Ni(111) surface; C₁ is attributed to Gr_W regions that are rotated with respect to (111) grains, or are grown on Ni grains exposing different orientations, and therefore, are interacting weakly with the support.

Figure 1: a) C 1s spectra measured during the Gr growth after 0, 3,10,16 and 44 minutes of exposure to ethylene at 773 K and (bottom) at RT after growth; b) C 1s spectra acquired on the Gr/foam hydrogenated with the same H dose at the indicated temperatures T_H and c) TPD curves measured during sample annealing.

Fig. 1b shows the C 1s spectra measured on the Gr/foam exposed to a flux of H atoms at temperatures T_H between 78 and 298 K. Starting from T_H=98 K, the C 1s line shapes appear broadened on the high binding energy (BE) side, due to the appearance of a component (labeled A) at 285.0 eV, and also on the low BE side, due to another component (labeled B) at 284.1 eV. A and B are ascribed to C atoms directly bonded to H atoms and to their first neighbors, and therefore indicate the occurrence of H chemisorption on Gr. From 198 K, some intensity is transferred from C₀ to C₁, because at this temperature the H atoms start to intercalate below Gr_S, which detaches gradually from the substrate. The intercalation under the nearly free-standing Gr_W remains undetected, because here the penetration of H underneath does not cause any measurable extra-shift of the C₁ component.

Fig. 1c shows H₂ TPD curves measured while heating the Gr/foam hydrogenated at increasing T_H. The desorption of H atoms chemisorbed on Gr originates solely the weak peak G at ~ 650 K.  Hence, all other TPD features correspond to the desorption of H atoms intercalated below Gr and residing at the Ni foam surface or even diffused into the Ni bulk. Hence, differently from Gr_S, where H atoms intercalate only for T_H ~ 198 K, intercalation below Gr_W occurs at much lower temperatures. The TPD curves up to T_H=173 K are dominated by the D peak, due to the desorption of H atoms penetrated in metastable subsurface sites of the Ni foam. The H₂ release at higher temperatures is related to the slower desorption of bulk H atoms and to the release of H atoms chemisorbed on the Ni surface. It turns out that the highest quantity of loaded hydrogen is detected for T_H= 113 K and amounts to ~ 5 times the quantity which saturates the Ni (111) surface with equivalent macroscopic lateral dimension.

The comparison of the results discussed above with those obtained for the bare Ni foam (not shown here) demonstrates that the presence of the Gr cover does not reduce the effectiveness of H loading. Moreover, it contributes to stabilizing in part the loaded hydrogen, although does not appear effective in increasing significantly the stored amount with respect to the bare support. However, the invaluable advantage anyway provided by the presence of Gr is the protection of the foam surface from the adsorption of contaminants, which might limit H chemisorption and consequently subsurface diffusion and loading in the bulk. Although the fundamental aspects of Gr/foam hydrogenation were here investigated in a regime far below the saturation of the bulk absorption, these measurements can be the starting point for further investigations aimed at establishing the ultimate storage capability of these hybrid nanostructured tanks.

This research was conducted by the following research team:

Gaetana Petrone¹, Francesca Zarotti¹, Rosanna Larciprete¹, Roberto Felici², Paolo Lacovig³, Daniel Lizzit³, Ezequiel Tosi³, Silvano Lizzit³

¹ CNR- Instituto dei Sistemi Complessi, Roma, Italy
² CNR-SPIN, Roma, Italy
³ Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy

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