Hydrogenated Graphenes by Birch Reduction: Influence of Electron and Proton Sources on Hydrogenation Efficiency, Magnetism, and Electrochemistry

Abstract

Interest in chemical functionalisation of graphenes today is largely driven by associated changes to its physical and material properties. Functionalisation with hydrogen was employed to obtain hydrogenated graphenes (also termed graphane if fully hydrogenated), which exhibited properties including fluorescence, magnetism and a tuneable band gap. Although the classical Birch reduction has been employed for hydrogenation of graphite oxide, variation exists between the choice of alkali metals and alcohols/water as quenching agents. A systematic study of electron (Li, Na, K, Cs) and proton sources (tBuOH, iPrOH, MeOH, H2O) has been performed to identify optimal conditions. The proton source exerted a great influence on the resulting hydrogenation with water and out-performed alcohols, and the lowest carbon-to-hydrogen ratio was observed with sodium and water with composition of C1.4H1O0.3. Although ferromagnetism at room temperature correlates well with increasing hydrogen concentrations, small contributions from trace iron impurities cannot be completely eliminated. In contrast, hydrogenated graphenes exhibit a significant paramagnetic moment at low temperatures that has no correlation with impurities, and therefore, originates from the carbon system. This is in comparison to graphene, which is strongly diamagnetic, and concentrations of paramagnetic centres in hydrogenated graphenes are one order of magnitude larger than that in graphite. Nonetheless, hydrogenation over a particular level might also excessively disrupt intrinsic sp(2) conjugation, resulting in unintended reduction of electrochemical properties. This was observed with heterogeneous electron-transfer rates and it was postulated that hydrogenated graphenes should generally have high defect densities, but only moderately high hydrogenation, should they be employed as electrode materials.

Near‐Stoichiometric Bulk Graphane from Halogenated Graphenes (X = Cl/Br/I) by the Birch Reduction for High Density Energy Storage

“…Advancing on our previous reports of direct Birch reduction on graphite oxide (GO), we now show that the use of halogenated graphenes as starting material, instead of GO, considerably improves the hydrogen storage up to 7.44 wt%, which is a marked improvement in gravimetric hydrogen content. Since the graphene scaffold is a light‐weight material and its hydrogenated product is stable at both room temperature and on atmospheric exposure, additional equipment (such as tanks, pressure regulators, or insulation, which weigh down the system) are not required, as with conventional compressed hydrogen storage methods; thus strengthening its potential in meeting the U.S. DOE's ultimate system target.…”

Chemistry of Graphene Derivatives: Synthesis, Applications, and Perspectives

“…It has been demonstrated that single‐layer graphene is hydrogenated uniformly, but graphene with more layers and sealed edges is not, which supports the idea that hydrogenation occurs from the edges of graphene by intercalation of alkali metal into the structure, where the defects are located . A comprehensive study of the alkali metal used for reduction in NH 3 and various proton sources yielded graphanes with various hydrogen content, and revealed that the best combination, Na in NH 3 with water as a proton source, yielded graphane with composition of C 1.4 H 1 O 0.3 . Besides graphene, carbon nanotubes can be opened by potassium in NH 3 to yield graphane nanostripes with well‐defined structure .…”

“…[71a] Ac omprehensive study of the alkali metal used for reduction in NH 3 and various proton sourcesy ieldedg raphanes with varioush ydrogen content, and revealed that the best combination, Na in NH 3 with water as ap roton source, yielded graphane with composition of C 1.4 H 1 O 0.3 . [78] Besides graphene,c arbon nanotubes can be openedb yp otassium in NH 3 to yield graphane nanostripesw ith well-defined structure. [79] Also, various other graphite-based nanostructures were used for the synthesis of graphane-based sheetsw ith controlled morphology.…”

Planar Polyolefin Nanostripes: Perhydrogenated Graphene

“…The influence of electron and proton sources on the hydrogenation efficiency during Birch reduction has been thoroughly examined. It was concluded that water as a quenching agent and potassium as a source of electrons represent the optimal combination to achieve the highest degree of hydrogenation . The modified Birch reduction method (Benkeser reaction), utilizing alkyl amine as a solvent (allowing to perform the reaction at room temperature), has been used for graphene hydrogenation .…”

“…It was concluded that water as aq uenching agent and potassiuma sasource of electrons represent the optimal combination to achieve the highest degree of hydrogenation. [18] The modified Birch reduction method (Benkeser reaction), utilizing alkyl amine as as olvent (allowing to perform the reaction at room temperature), has been used for graphene hydrogenation. [19] The last category of graphane preparation combines various synthetic approaches.…”