Investigation of low cost carbonaceous materials for application as counter electrode in dye-sensitized solar cells

Influence of Nitrogen and Sulfur Doping of Carbon Xerogels on the Performance and Stability of Counter Electrodes in Dye Sensitized Solar Cells

In this work, carbon xerogels (CXGs) doped with nitrogen or sulfur have been investigated as DSSC counter electrodes. CXGs have been prepared by a sol–gel method from resorcinol and formaldehyde and subsequent carbonization. Nitrogen doping has been carried out by introducing melamine into the synthesis process along with resorcinol and formaldehyde, while sulfur has been incorporated by direct reaction of the carbon material with elemental sulfur. The counter electrodes for DSSCs have been prepared by airbrushing on conductive glass (fluorine-doped tin oxide, FTO), and their electrochemical behavior has been evaluated, observing that the introduction of heteroatoms such as nitrogen or sulfur leads to an improvement in efficiency compared to the undoped material thanks to a decrease in charge transfer resistance.

Polymer Electrolyte Membranes for Water Photo-Electrolysis

Water-fed photo-electrolysis cells equipped with perfluorosulfonic acid (Nafion^® 115) and quaternary ammonium-based (Fumatech^® FAA3) ion exchange membranes as separator for hydrogen and oxygen evolution reactions were investigated. Protonic or anionic ionomer dispersions were deposited on the electrodes to extend the interface with the electrolyte. The photo-anode consisted of a large band-gap Ti-oxide semiconductor. The effect of membrane characteristics on the photo-electrochemical conversion of solar energy was investigated for photo-voltage-driven electrolysis cells. Photo-electrolysis cells were also studied for operation under electrical bias-assisted mode. The pH of the membrane/ionomer had a paramount effect on the photo-electrolytic conversion. The anionic membrane showed enhanced performance compared to the Nafion^®-based cell when just TiO₂ anatase was used as photo-anode. This was associated with better oxygen evolution kinetics in alkaline conditions compared to acidic environment. However, oxygen evolution kinetics in acidic conditions were significantly enhanced by using a Ti sub-oxide as surface promoter in order to facilitate the adsorption of OH species as precursors of oxygen evolution. However, the same surface promoter appeared to inhibit oxygen evolution in an alkaline environment probably as a consequence of the strong adsorption of OH species on the surface under such conditions. These results show that a proper combination of photo-anode and polymer electrolyte membrane is essential to maximize photo-electrolytic conversion.

One-Pot Solvothermal in Situ Growth of 1D Single-Crystalline NiSe on Ni Foil as Efficient and Stable Transparent Conductive Oxide Free Counter Electrodes for Dye-Sensitized Solar Cells

One-dimensional single-crystal nanostructural nickel selenides were successfully in situ grown on metal nickel foils by two simple one-step solvothermal methods, which formed NiSe/Ni counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The nickel foil acted as the nickel source in the reaction process, a supporting substrate, and an electron transport "speedway". Electrochemical testing indicated that the top 1D single-crystal NiSe exhibited prominent electrocatalytic activity for I₃^- reduction. Due to the metallic conductivity of Ni substrate and the outstanding electrocatalytic activity of single-crystal NiSe, the DSSC based on a NiSe/Ni CE exhibited higher fill factor (FF) and larger short-circuit current density (J_sc) than the DSSC based on Pt/FTO CE. The corresponding power conversion efficiency (6.75%) outperformed that of the latter (6.18%). Moreover, the NiSe/Ni CEs also showed excellent electrochemical stability in the I^-/I₃^- redox electrolyte. These findings indicated that single-crystal NiSe in situ grown on Ni substrate was a potential candidate to replace Pt/TCO as a cheap and highly efficient counter electrode of DSSC.

Investigation of Supported Pd-Based Electrocatalysts for the Oxygen Reduction Reaction: Performance, Durability and Methanol Tolerance

Next generation cathode catalysts for direct methanol fuel cells (DMFCs) must have high catalytic activity for the oxygen reduction reaction (ORR), a lower cost than benchmark Pt catalysts, and high stability and high tolerance to permeated methanol. In this study, palladium catalysts supported on titanium suboxides (Pd/Ti_nO_2n–1) were prepared by the sulphite complex route. The aim was to improve methanol tolerance and lower the cost associated with the noble metal while enhancing the stability through the use of titanium-based support; 30% Pd/Ketjenblack (Pd/KB) and 30% Pd/Vulcan (Pd/Vul) were also synthesized for comparison, using the same methodology. The catalysts were ex-situ characterized by physico-chemical analysis and investigated for the ORR to evaluate their activity, stability, and methanol tolerance properties. The Pd/KB catalyst showed the highest activity towards the ORR in perchloric acid solution. All Pd-based catalysts showed suitable tolerance to methanol poisoning, leading to higher ORR activity than a benchmark Pt/C catalyst in the presence of low methanol concentration. Among them, the Pd/Ti_nO_2n–1 catalyst showed a very promising stability compared to carbon-supported Pd samples in an accelerated degradation test of 1000 potential cycles. These results indicate good perspectives for the application of Pd/Ti_nO_2n–1 catalysts in DMFC cathodes.

Investigation of the activity and stability of Pd-based catalysts towards the oxygen reduction (ORR) and evolution reactions (OER) in iron–air batteries

Palladium on carbon has proved to be a suitable bi-functional catalyst for application in the air electrode of a metal–air battery.

Improved photocurrent of a poly (3,4-ethylenedioxythiophene)-ClO₄⁻/TiO₂ thin film-modified counter electrode for dye-sensitized solar cells

We prepared a poly(3,4-ethylenedioxythiophene) (PEDOT)-ClO₄⁻-supported TiO₂ thin-film electrode as a counter electrode on a transparent conductive oxide glass electrode for a dye-sensitized solar cell (DSSC) using a combination of sol-gel and electropolymerization methods. The photocurrent-voltage characteristics indicate that DSSCs with PEDOT-ClO₄⁻/TiO₂ thin-film counter electrodes had a high photovoltaic conversion efficiency similar to that of PEDOT-ClO₄⁻/TiO₂ particle composite-film electrodes. Furthermore, it was found that the photocurrent was increased by attaching a reflector to the opposite side of the transparent counter electrode.

Flexible carbon nanotube-based composite plates as efficient monolithic counter electrodes for dye solar cells

We demonstrate a general approach to fabricate a novel low-cost, lightweight and flexible nanocomposite foil that can be effectively implemented as a monolithic counter-electrode in dye solar cells. The pivotal aim of this work was to replace not only the platinum catalyzer film, but even the underlying transparent conductive oxide-coated substrate, by means of a monolithic counter electrode based on carbonaceous materials. According to our approach, a proper dispersion of multiwalled carbon nanotubes (MWCNTs) has been added to a dilute polypropylene solution in toluene. The composite solution has been then adequately mixed and subsequently dried by means of a controlled solvent evaporation process; the resulting powder has been modeled by compression molding into thin plates. Four different series of plates have been realized by tuning the carbon nanotubes concentration from 5 wt % to 20 wt %. Finally, a specifically setup reactive ion etching treatment with oxygen plasma has been carried out onto the plate surface to remove the residual polymeric capping layer and allow the embedded CNTs to protrude on top of the surface. A fine-tuning of the morphological features has been made possible by adjusting the plasma etching conditions. For all the treated surfaces, the most meaningful electrochemical parameters have been quantitatively analyzed by means of both electrochemical impedance spectroscopy and cyclic voltammetry measurements. An as high as 13.8 mA/cm(2) photocurrent density, along with a solar conversion efficiency of 6.67%, has been measured for a dye solar cell mounting a counter-electrode based on a 20 wt % CNT nanocomposite.

Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells

When applied on the counter electrode of a dye-sensitized solar cell, functionalized graphene sheets with oxygen-containing sites perform comparably to platinum (conversion efficiencies of 5.0 and 5.5%, respectively, at 100 mW cm(-2) AM1.5G simulated light). To interpret the catalytic activity of functionalized graphene sheets toward the reduction of triiodide, we propose a new electrochemical impedance spectroscopy equivalent circuit that matches the observed spectra features to the appropriate phenomena. Using cyclic voltammetry, we also show that tuning our material by increasing the amount of oxygen-containing functional groups can improve its apparent catalytic activity. Furthermore, we demonstrate that a functionalized graphene sheet based ink can serve as a catalytic, flexible, electrically conductive counter electrode material.

Efficient dye-sensitized solar cells using red turnip and purple wild sicilian prickly pear fruits

Dye-sensitized solar cells (DSSCs) were assembled by using the bougainvillea flowers, red turnip and the purple wild Sicilian prickly pear fruit juice extracts as natural sensitizers of TiO(2) films. The yellow orange indicaxanthin and the red purple betacyanins are the main components in the cocktail of natural dyes obtained from these natural products. The best overall solar energy conversion efficiency of 1.7% was obtained, under AM 1.5 irradiation, with the red turnip extract, that showed a remarkable current density (Jsc = 9.5 mA/cm(2)) and a high IPCE value (65% at lambda = 470 nm). Also the purple extract of the wild Sicilian prickly pear fruit showed interesting performances, with a Jsc of 9.4 mA/cm(2), corresponding to a solar to electrical power conversion of 1.26%.