Supercapacitors are the focus of much research at the present time. They offer a potential solution for reversible energy storage in the fields of space, aircrafts or transportation (hybrid vehicles).

An important research line, aiming at increasing both energy and power densities, is devoted to asymmetric transition metal oxides / activated carbon (C) systems. RuO2-based devices exhibit the highest capacitance, more than 700 F/g, but their cost limits the applications to small electronic devices. Less expensive oxides such as cobalt oxides (especially Co3O4), MnO2, V2O5, Fe3O4, NiO, Ni(OH)2, as well as electrically conducting polymers, have been extensively studied in the past decades, or used in commercial devices; they EACH exhibit each drawbacks and advantages with regard to applications. But no system has been investigated as much as the C/MnO2 one, which is particularly interesting because it can work in aqueous media at tensions up to 2 V, and high stability in ageing has been demonstrated. Nevertheless, the performances of the system, especially in terms of power density, are limited by the poor electronic conductivity of MnO2. This problem is usually solved by simply mixing conductive carbon materials (carbon black, CNTs…) with MnO2 or by developing more elaborated grafting or decoration strategies. The combination of oxide and carbonaceous species is widely reported in the literature, whereas combining oxides with different natures is less frequently encountered. We propose in this project to synthesize and develop original materials enhancing, through a synergistic effect, the interesting properties of manganese, cobalt and nickel oxide/hydroxide, the drawbacks of each component being overbalanced by the good complementary properties of the other components. We aim at gathering in one single material (or composite), the good pseudocapacitive behavior of manganese, the good electronic conductivity associated to cobalt oxides, the high capacity of nickel hydroxide, as well as the enhanced conduction properties of carbon.

The present PhD project aims at designing and manufacturing new classes of hybrid composite electrodes based on assemblies of graphene (for enhanced double layer capacitance) and porous transition metals oxides (for additional faradaic capacitance due to multiple reversible redox processes) directly applied on metallic current collectors. The combined advantages of graphene with those of transition metals oxides will enable supercapacitors with high energy density, working in environmentally friendly aqueous electrolytes, which are an acknowledged need.

Expected Project outcomes:

- High energy density electrodes, consisting of graphene combined with transition metallic oxides, with superior specific capacitance (> 700 F/g) and energy density (> 50 kWh/kg) in aqueous electrolytes. - Implementation of flexible fabrication processes capable of producing novel classes of nanostructured porous oxides/graphene composite materials;

- Fostering Hi-Tech applications, combining the traditional electrodeposition of producing metallic oxides with new advanced materials – graphene and graphene oxide;

- Advanced physical, chemical and electrochemical characterization and fundamental knowledge on electrochemical behavior of such electrodes, predicting efficiency, performance and lifetime.

As a conclusion, the originality and novelty of this project is based on the fact that:

- It aims at capitalizing and gathering in one single composite material the best properties individually observed for manganese, cobalt and nickel oxides/hydroxides, with the specific contribution of carbonaceous materials in terms of electronic conductivity.

- It will use a real material engineering strategy through original synthesis methods involving either electrodeposition or exfoliation/restacking processes of slabs with complementary properties.

Fig 1: Schematic representation of the various strategies that we intend to develop for this project:
 various composities will be targeted, resulting from an interstratification of Mn, Co, Ni based oxide /hydroxide and graphene slabs

Project Partners and their Roles

IST-ICEMS Lisbon: development of the new composite electrodes based on co-electrodeposited graphene and transition metallic oxides/hydroxides, electrochemical characterization, physic-chemical study

ICMCB Bordeaux: Synthesis of nanocomposites by exfoliation; electrochemical , chemical, physical and microscopic characterization