Alexander Oleinick

Abstract:

Provided that hydrogen can be safely stored and transported at high densities, hydrogen fuel cells would offer highly efficient and completely environmentally friendly solutions for vehicle propulsion. However, neither compressed gas nor liquid hydrogen seems safe enough for everyday use in mobile applications given the risk of accidents and their possible consequences in urban environments related to the high flammability of hydrogen. 

Therefore, approaches consisting in adsorbing hydrogen atoms (i.e., H rather than H₂) appear to offer an excellent solution for its storage in order to propel light and heavy vehicles in urban environments and evidently in between cities. Moreover, compared to solutions based on electric batteries, the ecological and environmental advantage would be obvious.

Among the possible materials for storing hydrogen, graphene (Gr) has been suggested to be one of the most promising materials provided that a significant H/Gr weight ratio (ww%) is achieved. Specifically, the US Department of Energy (DOE) has set a target of 5.5 ww% to be achieved by 2025 as a condition for becoming operational. This goal is however extremely far from what can be achieved presently by direct storage of hydrogen molecules (H₂) even using single graphene monolayers (SLGr) by physical adsorption because this requires extreme temperatures and pressures given the too weak attractive van der Waals forces and the stability of H₂ with respect to its two dissociated atoms (H) and the intrinsic stability of the graphene sp²-carbon resonant network.

In this respect, if possible, a stable chemical adsorption of atomic hydrogen on SLGr appears as the Holy Grail with a theoretical maximum storage capacity of 7.7 ww% for Gr-H atomic adducts and optimal safety conditions and performance. However, generating compact Gr-H adducts from H₂ again requires extreme temperatures and pressures.

On the contrary, hydrogen atoms can easily be produced under mild conditions by electrochemical reduction of common aqueous acidic solutions at the surface of a platinum nanoelectrode placed in straight contact with a SLGr (see adjacent figure) and spread spontaneously on the graphene sheet by reversible adsorption and site to site hopping diffusion at room temperature and atmospheric pressure and without significantly distorting the graphene geometry suggesting a physisorption rather than a covalent bonding [1]. The amount of platinum needed is minimal because it only serves as a catalyst as we were able to demonstrate using dynamic electrochemistry and SERS isotopic Raman spectroscopy. In addition, the process significantly rapid kinetics and its storage capacity (6.6 ww%, viz., more than 85% of the theoretical maximum, being already higher than the target set by DOE [2]) are associated with a substantial stability of the Gr-H layers at room temperature and atmospheric pressure. Furthermore, one would not need to proceed through the intermediacy of hydrogen gas for Gr-H reservoir loading, nor for its unloading to feed fuel cells anodes, removing then two important steps. 

The authors would like to acknowledge support from joint sino-french CNRS IRP NanoBioCatEchem. CA thanks Xiamen University for his Distinguished Visiting Professor position.