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- In-situ localized pH, pNa and dissolved O2 measurements during charge-discharge of mixed Ni–Co hydroxide electrodesPublication . Adán-Más, Alberto; Taryba, Maryna; Moura E Silva, Teresa; Guerlou-Demourgues, Liliane; Montemor, M.F.This work reports, for the first time, the use of ion-selective localized electrochemical techniques to elucidate the charge-discharge mechanism of nickel-cobalt hydroxide electrodes for electrochemical energy storage. The charge-discharge mechanism of electrodeposited nickel-cobalt hydroxide electrodes was studied in Na2SO4 0.05 M by localized in situ measurements of pH, pNa and dissolved O2 during cyclic voltammetry. Local pH and pNa distributions were recorded using micro-potentiometric sensors with liquid membrane, while dissolved O2 was monitored using a fiber-optic microsensor. These original results highlight how localized potentiometry can provide new insights to better understand the charge mechanism of metal (hydr)oxide electrodes by directly measuring the concentrations/activities of relevant species at the electrode-electrolyte interface during charge-discharge.
- Nickel-cobalt oxide modified with reduced graphene oxide: Performance and degradation for energy storage applicationsPublication . Adan-Mas, Alberto; Moura E Silva, Teresa; Guerlou-Demourgues, Liliane; Bourgeois, L.; MONTEMOR, FATIMANickel-cobalt oxide is synthesized in combination with electrochemically reduced graphene oxide (Er-GO) by one-step electrodeposition on stainless steel followed by thermal treatment. The presence of reduced graphene oxide leads to enhanced electrochemical response, with a capacity increase from 113 mA h g(-1) to 180 mA h g(-1), and to increased faradaic efficiency and rate capability. Compared to Ni-Co oxide, the addition of reduced graphene oxide increases capacity retention from 58% to 83% after 5000 cycles. The material fade during cycling is studied by means of electrochemical impedance spectroscopy, electron diffraction spectroscopy and scanning electron microscopy. As a result, different degradation mechanisms are identified as source of the capacity decay, such as microstructural cracking, phase transformation and parasitic reactions.
- Enhancement of the Ni-Co hydroxide response as energy storage material by electrochemically reduced graphene oxidePublication . Adán-Más, Alberto; Duarte, Raquel G.; Morais Silva, Teresa; Guerlou-Demourgues, Liliane; MONTEMOR, MARIANickel-Cobalt double hydroxide materials are combined with Electrochemically Reduced Graphene Oxide (Er-GO) on top of Stainless Steel collectors by means of a one-step pulsed electrodeposition technique. Results show that, when Er-GO is integrated into the Nickel-Cobalt hydroxide matrix, there is an improvement of the electrochemical performance of the material, which shows increased capacity, stability and rate capability. As seen by Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), Grazing-Incidence X-Ray Diffraction (GIXD) and Raman Spectroscopy, there is an excellent integration between the materials that leads to the enhanced electrochemical response. Capacity values up to 96 mA h/g and a 62% capacity retention after 5000 cycles were achieved. Moreover, scan rates up to 2000 mV/s without loss in electrochemical response were possible proving its good rate capabilities. A detailed study of the cycling degradation phenomena was also done by means of electrochemical impedance spectroscopy (EIS). Thus, this novel electrodeposited material serves as an excellent material for energy storage applications.
- Application of the Mott-Schottky model to select potentials for EIS studies on electrodes for electrochemical charge storagePublication . Adan-Mas, Alberto; Moura E Silva, Teresa; Guerlou-Demourgues, Liliane; MONTEMOR, MARIAElectrochemical Impedance Spectroscopy (EIS) is a powerful technique to understand the electrode-electrolyte interaction and to evaluate degradation, resistive behaviour and electrochemical activity of energy storage materials used in batteries, pseudocapacitors and supercapacitors among others. However, it can sometimes be misused or under-interpreted. To effectively acquire EIS results, the voltages imposed to the working electrode at which EIS spectra are obtained, shall be critically selected. This work follows a previous study on the EIS response of Nickel-Cobalt hydroxide, and highlights how the Mott-Schottky model can be used as a complementary tool to explain EIS results obtained at different potentials. The Mott-Schottky model is used to understand further the fundamental processes occurring at the electrode-electrolyte interface of nickel-cobalt hydroxide in alkali media and to explain the changes in conductivity of the material that ultimately determine the electrode electrochemical activity. The applicability of the model to assist in the potential selection for EIS studies on other important charge storage materials such as MnOx and MoOx is discussed too.