Functionalized graphene oxide for efficient capacitive deionization

ENERCHEM

Functionalized graphene oxide for efficient capacitive deionization

ENERCHEM

Pouch-sealing as an effective way to fabricate flexible dye-sensitized solar cells and their integration with supercapacitors

The scientific interest in integrated energy harvesting and storage (HS) devices has increased exponentially in the last decade since they represent an optimal solution to power portable electronic devices and low consuming Internet of Thing (IoT) sensor nodes. The integration of energy storage devices with photovoltaics can allow to avoid problems such as continuous battery replacement and periodic maintenance, reducing overall costs. In this context, dye sensitized solar cells (DSSCs) integrated with a supercapacitor represent the best choice in terms of lifetime, charge-discharge efficiency, and simplicity of connection avoiding electrical signal conditioning between the two devices. DSSCs have many similarities with supercapacitors, with the only aspect that remains uncovered being the sealing of the device. Herein we propose a common vacuum sealing technology for the integration of a supercapacitor and a DSSC made with shared current collectors, to maximize the integration between the two technologies. The HS device showed a maximum overall photon to electrical conversion and storage efficiency (OPECSE) of 6.10% under only 0.1 SUN illumination, thanks to the high photoconversion efficiency showed by the pouch sealed DSSC, equal to 6.62%. The HS device showed a high stability under bending condition and repeated photo-charge/discharge cycling

Modeling of the dye loading time influence on the electrical impedance of a dye-sensitized solar cell

A hemisquaraine dye molecule (CT1) was used as TiO2 sensitizer. The influence of the dye-adsorption time on the electrical impedance of a CT1-based dye-sensitized solar cell (DSC) was analyzed. Differently from what we observed with commercial Ru dye-based DSC, a non-monotonic effect of the impregnation time on the impedance has been found and the dye loading time is much reduced, a desirable outcome in economic grounds. This feature is analyzed in terms of the dye molecules tendency to aggregate close to the TiO2/electrolyte interface. A physical model that fits well the experimental data is proposed, which also takes into account a correction related to the difference between the illuminated area of the cell and the total area available in the electrical measurements

A Facile and Green Synthesis of a MoO2-Reduced Graphene Oxide Aerogel for Energy Storage Devices

A simple, low cost, and "green" method of hydrothermal synthesis, based on the addition of l-ascorbic acid (l-AA) as a reducing agent, is presented in order to obtain reduced graphene oxide (rGO) and hybrid rGO-MoO2 aerogels for the fabrication of supercapacitors. The resulting high degree of chemical reduction of graphene oxide (GO), confirmed by X-Ray Photoelectron Spectroscopy (XPS) analysis, is shown to produce a better electrical double layer (EDL) capacitance, as shown by cyclic voltammetric (CV) measurements. Moreover, a good reduction yield of the carbonaceous 3D-scaffold seems to be achievable even when the precursor of molybdenum oxide is added to the pristine slurry in order to get the hybrid rGO-MoO2 compound. The pseudocapacitance contribution from the resulting embedded MoO2 microstructures, was then studied by means of CV and electrochemical impedance spectroscopy (EIS). The oxidation state of the molybdenum in the MoO2 particles embedded in the rGO aerogel was deeply studied by means of XPS analysis and valuable information on the electrochemical behavior, according to the involved redox reactions, was obtained. Finally, the increased stability of the aerogels prepared with l-AA, after charge-discharge cycling, was demonstrated and confirmed by means of Field Emission Scanning Electron Microscopy (FESEM) characterization

High rejection stacked single-layer graphene membranes for water treatment

Nowadays, the production of pure water from saltwater and wastewater is one of the most challenging issues. Polymeric materials represent, at the moment, the best solution for membranes technology but new materials with improved functionalities are desirable to overcome the typical limitations of polymers. In this work, graphene membranes with superior filtration properties are fabricated by stacking up to three graphene layers on a porous support and exploiting the intrinsic nanopores of graphene to filter diclofenac (drug), and methylene blue (dye). The rejection improves increasing the number of the stacked graphene layers, with the best results obtained with three graphene layers. Mass diffusion properties depend on the size of the probe molecule, consistently with the existence of intrinsic nanometer-sized pores within graphene. From the results of an in depth transmission electron microscopy analysis and molecular dynamics simulations it is inferred that graphene stacking results in a decrease of effective membrane pore sizes to about 13 Å diameter which corresponds to 97% rejection for diclofenac and methylene blue after one hour filtration

Flexible and Floating Photovoltaics

We present our results on dye sensitized solar cells (DSSCs) for flexible or floating photovoltaic devices. In these works, we used polymer electrolyte membranes and metal grids as electrodes substrates in order to preserve the flexibility of the entire structure. These cells aim to be competitive in the near future solar market due to the extremely low cost and easiness of processing. Moreover, they evidently improve their conversion efficiency under low illumination conditions. This feature makes flexible DSSCs extremely interesting to be exploited in particular applications. In addition, we also introduce a smart integration of a DSSC with an electrical double layer capacitor (EDLC) which employs graphene nanoplatelets as active material in a completely flexible architecture. To the best of our knowledge this harvesting-storage (HS) device has the best overall photon-to-electrical conversion and storage efficiency ever attained to date for a flexible DSSC-based non-wired integrated HS device. Noteworthy, this value increases lowering the radiation intensity, thus showing optimal performances in real operation or indoor conditions

Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting

Titanium dioxide (TiO2) and zinc oxide (ZnO) nanostructures have been widely used as photo-catalysts due to their low-cost, high surface area, robustness, abundance and non-toxicity. In this work, four TiO2 and ZnO - based nanostructures, i.e. TiO2 nanoparticles (TiO2 NPs), TiO2 nanotubes (TiO2 NTs), ZnO nanowires (ZnO NWs) and ZnO@TiO2 core-shell structures, specifically prepared with a fixed thickness of about 1.5 μm, are compared for the solar-driven water splitting reaction, under AM1.5G simulated sunlight. A complete characterization of these photo-electrodes in their structural and photo-electrochemical properties was carried out. Both TiO2 NPs and NTs showed photo-current saturation reaching 0.02 and 0.12 mA/cm2, respectively, for potential values of about 0.3 and 0.6 V vs. RHE. In contrast, the ZnO NWs and the ZnO@TiO2 core-shell samples evidence a linear increase of the photocurrent with the applied potential, reaching 0.45 and 0.63 mA/cm2 at 1.7 V vs. RHE, respectively. However, under concentrated light conditions, the TiO2 NTs demonstrate a higher increase of the performance with respect to the ZnO@TiO2 core-shells. Such material dependent behaviours are discussed in relation with the different charge transport mechanisms and interfacial reaction kinetics, which were investigated through electrochemical impedance spectroscopy. The role of key parameters such as electronic properties, specific surface area and photo-catalytic activity on the performance of these materials are discussed. Moreover, proper optimization strategies are analyzed in view of increasing the efficiency of the best performing TiO2 and ZnO - based nanostructures, toward their practical application in a solar water splitting device

New Transparent Laser-Drilled Fluorine-doped Tin Oxide covered Quartz Electrodes for Photo-Electrochemical Water Splitting

A new-designed transparent, conductive and porous electrode was developed for application in a compact laboratory-scale proton exchange membrane (PEM) photo-electrolyzer. The electrode is made of a thin transparent quartz sheet covered with fluorine-doped tin oxide (FTO), in which an array of holes is laser-drilled to allow water and gas permeation. The electrical, morphological, optical and electrochemical characterization of the drilled electrodes is presented in comparison with a non-drilled one. The drilled electrode exhibits, in the visible region, a good transmittance (average value of 62%), a noticeable reflectance due to the light scattering effect of the hole-drilled internal region, and a higher effective surface area than the non-drilled electrode. The proof-of-concept of the applicability of the drilled electrode was achieved by using it as a support for a traditional photocatalyst (i.e. commercial TiO2 nanoparticles). The latter, coupled with a polymeric electrolyte membrane (i.e.Nafion 117) and a Pt counter electrode, forms a transparent membrane electrode assembly (MEA), with a good conductivity, wettability and porosity. Electrochemical impedance spectroscopy (EIS) was used as a very powerful tool to gain information on the real active surface of the new drilled electrode and the main electrochemical parameters driving the charge transfer reactions on it. This new electrode architecture is demonstrated to be an ideal support for testing new anodic and cathodic photoactive materials working in tandem configuration for solar fuels production by water photo-electrolysis