M.J. Carmezim

Electrochemical behaviour of thermally treated Cr-oxide films deposited on stainless steel

The semiconductive behaviour of amorphous and crystalline Cr-oxide films deposited on stainless steels was studied by capacitance and photoelectrochemical measurements. The amorphous films presented metallic behaviour. For the crystalline films, bandgaps were determined for direct and indirect transitions. Doping densities evaluated from Mott–Schottky plots were below 1019 cm−3 for the films treated at high temperature, and decreased with the treatment temperature. The crystalline films were described as a thick layer of non-stoichiometric chromium oxide with a bi-polar character.

Electrodeposition and characterization of nickel–copper metallic foams for application as electrodes for supercapacitors

Nickel–copper metallic foams were electrodeposited from an acidic electrolyte, using hydrogen bubble evolution as a dynamic template. Their morphology and chemical composition was studied by scanning electron microscopy and related to the deposition parameters (applied current density and deposition time). For high currents densities (above 1xa0Axa0cm−2) the nickel–copper deposits have a three-dimensional foam-like morphology with randomly distributed nearly-circular pores whose walls present an open dendritic structure. The nickel–copper foams are crystalline and composed of pure nickel and a copper-rich phase containing nickel in solid solution. The electrochemical behaviour of the material was studied by cyclic voltammetry and chronopotentiometry (charge–discharge curves) aiming at its application as a positive electrode for supercapacitors. Cyclic voltammograms showed that the Ni–Cu foams have a pseudocapacitive behaviour. The specific capacitance was calculated from charge–discharge data and the best value (105xa0Fxa0g−1 at 1xa0mAxa0cm−2) was obtained for nickel–copper foams deposited at 1.8xa0Axa0cm−2 for 180xa0s. Cycling stability of these foams was also assessed and they present a 90xa0% capacitance retention after 10,000 cycles at 10xa0mAxa0cm−2.

Electrodeposition: a versatile, efficient, binder-free and room temperature one-step process to produce MnO2 electrochemical capacitor electrodes

The use of room temperature cathodic electrodeposition to produce MnO2 electrochemical capacitor electrodes is demonstrated. By employing a permanganate-based bath, birnessite-type MnO2 electrodes are directly obtained with no further heat treatment. Moreover, a crystalline/amorphous structural transition is observed as the applied current density is increased up to 50 mA cm−2.

Electrolytes in Metal Oxide Supercapacitors

Abstract Energy stored by the supercapacitors and their performance are highly dependent on the nature of the electrolyte and electrode material chosen to build the device. This chapter reviews in detail the electrolytes used in supercapacitors based on binary and ternary metal oxides, their hybrids, as well as symmetric and asymmetric devices. In this context, the most widely used electrolytes such as aqueous, redox additive, solid or gel polymers, and nonaqueous electrolytes are reviewed. Moreover, the effect of different electrolytes on electrochemical properties of metal oxide–based supercapacitors is discussed with their advantages and disadvantages. Charge storage basic mechanisms are a subject tackled through the ultimate state of the art. Additionally, the main challenges in the electrolyte field are addressed as well as the possible strategies that could be used in the future.