Charge and Hydrogen Storage Capacities of Electrodeposited Graphene Derivatives

Abstract

Graphene’s theoretical capacitance and physical hydrogen storage abilitiesmark it as one of the best candidates for energy storage application. Graphene oxide (GO) is easy to manufacture in bulk synthesis and can be considered as a precursor for graphene synthesis. The application of graphene on the surface of electrodes can be achieved by different techniques as drop-coating and electrodeposition. It has been reported that the electrochemical hydrogen storage is favored in samples with a well-developed porosity and a low content in surface oxygen complexes. Both features indicate that the unsaturated carbon atoms in the carbon materials have an important role for the hydrogen uptake. It has also been suggested that the total capacity of hydrogen storage is proportional to the interlayer distance and not to the carbon specific surface area.The present work presents an investigation of electrodeposited GO coatings (ED), which have subsequently been electroreduced to various degrees of rGO, and relate their charge capacitance and hydrogen storage capacity to the preparation mode, morphological structure, and surface chemistry. These properties were compared to those of drop-cast (DC) coatings of GO.The ED coatings were characterized by more accumulated graphene sheets imperfections as observed by cross section TEM analysis. These coatings, when reduced at -1.6 V vs Hg/HgO showed more efficient removal of phenolic groups than DC ones treated at the same potential (remaining contents of 2.1 and 18.1 %, respectively). They also showed lower charge transfer resistance (5.2 and 28 Ω cm2, respectively), higher capacitance (73.2 and 42.6 F/g, respectively), and higher hydrogen storage capacity (119 and 57 mAh/g, respectively). Moreover, they showed higher stability towards H2 charge/discharge cycles (retained hydrogen capacities of 95 and 40 % after 15 and 6 cycles for ED and DC coatings reduced at -1.5 V, respectively).The removal during the electroreduction process of phenolic groups located at the graphene sheets edges of the ED coatings and their higher pseudo-capacitance can explain their lower charge transfer resistance and their higher pseudo-capacitance, respectively. Their superior stability towards hydrogen charge/discharge cycles is suggested to stem from evolving hydrogen escape routes created during the ED process. The superior performance properties of coatings obtained by ED and subsequently electro-reduced make them promising electrode materials for energy storage.