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The results obtained from supercapacitors based on carbon electrodes and different types of electrolytes are reported. Two different electrolyte separators were used: a) a porous paper material impregnated with electrolyte solution and, b) a polymer ion exchange membrane. Furthermore electrolyte solutions of 1 M sodium sulphate and 1 M potassium iodide were also used. Four different supercapacitors were realized with these materials formed of carbon based electrodes and: 1)porous separator impregnated with 1 M Na2SO4 (CXPNS) 2)polymer exchange membrane (Na+) and electrodes impregnated with 1 M Na2SO4 solution (CXSNS) 3)solid polymer exchange membrane (Na+) with negative electrode impregnated with 1 M Na2SO4 and positive electrode with 1 M Na2SO4 and 1 M KI (1:1) (CXSNSKI) 4)solid polymer exchange membrane (Na+) with negative electrode impregnated of 1 M Na2SO4 and positive electrode of 1 M KI (CXSNS-KI) The carbon material was a carbon xerogel generously supplied by CSIC-INCAR of Oviedo (Spain); whereas the membrane was a sulfonated PEEK synthesized in our lab, exchanged with Na+. The supercapacitors were characterized by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The objective of the research was to identify a supercapacitor of high specific capacitance and energy, able to operate at 1.6V, having high stability and slow self-discharge rate. The tests evidenced that the solid-state supercapacitor based on Na?SPEEK membrane and KI in positive electrode (CXSNS-KI) exhibited specific capacitance of 208 F?g-1, that was higher compared to that with the porous separator impregnated with 1M Na2SO4 and that with Na?SPEEK electrolyte membrane and electrodes impregnated with sodium sulphate solution (respectively 93 F?g-1 and 83 F?g-1). The solid-state redox supercapacitor showed also low resistance (ESR) and full capacitance retention during 20000 of charge-discharge cycles at 2 Ag-1 and further 500 h at the hold voltage of 1.6 V (floating test). This supercapacitor is considered very interesting for the high specific capacitance and specific energy (208 Fg-1, ? 20 Wh kg-1), high stability and slow self-discharge rate and is very promising for the development of next-generation low cost, high-performance and flexible energy storage devices.

In the last years great efforts were devoted to improve the energy density and the stability of supercapacitors using low cost and environmentally friendly materials. Unfortunately, it was found that supercapacitors exclusively based on capacitive process with carbon-based electrodes and aqueous electrolytes showed limited energy density and stability. In fact, although supercapacitors based on carbon materials with very high specific surface area gave specific capacity exceeding 200 F/g in aqueous electrolyte, the stability became problematic when the potential was kept at 1.5V or higher for long time for the concomitant presence of side irreversible reactions. Recently, many researchers have focused their attention on alternative ways to improve capacitance and energy density consisting in introducing faradaic processes (i.e., with exchange of electrons at the interface electrode /electrolyte) on an electrode of device. Thus one electrode exhibited a higher capacitance and also the capacitance and the energy density of supercapacitor increased. In this work we have studied supercapacitors based on a carbon material coupled with different types of separators and electrolyte solutions to overcome the above limits. Preliminary studies were performed to confirm the limitation of the purely capacitive process to which followed studies that evidenced the positive effect of reversible redox reaction. Four different supercapacitors were realised and studied varying the type of separator (porous paper and sulfonated polyether-ether ketone membrane) and electrolyte solution (sodium sulphate and potassium iodide). A solution of potassium iodide, whether used at the positive electrode, where I-/I3- redox reaction occurred, produced a remarkable additional pseudocapacitance. A specific capacitance of 208 F/g at 1.6 V and stable performance after 20000 of charge/discharge cycles and subsequent 300h of floating condition at 1.6V were obtained from supercapacitor with sPEEK membrane and KI solution as well as a high value of energy density of 20 Wh/kg of capacitor was displayed.

Graphene nanosheets (GNs) are prepared from natural graphite by a simple ecofriendly microwave-assisted exfoliation technique. The as prepared GNs are decorated with platinum (Pt) nanoparticles by a simple chemical reduction method and used as a low-cost counter electrode (CE) material for dye-sensitized solar cells (DSSCs). Structure and morphology of the prepared GNs and Pt-decorated GNs (Pt-GNs) are evaluated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The electrochemical behavior of GNs and Pt-GNs are compared with std. Pt using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies. These studies indicate that the Pt-GNs-based counter electrode offered superior electrocatalytic activity towards the I-/I3(-) redox mediator with enhanced charge transfer rate and exchange current density at the electrode/electrolyte interface over std. Pt and GNs-based counter electrodes. DSSCs are fabricated with std. Pt, GNs and Pt-GNs to determine the photovoltaic performance under 1 Sun illumination (100 mW cm(-2), AM 1.5). It is found that the cell fabricated with 1 wt% Pt-decorated GNs as counter electrode showed an 11% improvement in photovoltaic cell efficiency compared with the cell assembled with std. platinum and other reported graphene-Pt-based composites as counter electrodes.

MWCNT-based supercapacitors (SC) containing methylene blue (MB) as redox active electrolyte were studied. MWCNTs were employed as model of electrode active material due to their ideal double-layer behavior facilitates the investigation of the energy storage mechanisms involved. MB led to a cell capacitance enhancement equal to 4.5 times the original cell capacitance of MWCNTs in sulphuric acid with a capacitance reduction of only 12% after 6000 charge–discharge cycles. The potential evolution of each electrode during galvanostatic cycling revealed that MB redox reaction develops in both electrodes simultaneously in the voltage range of 0–0.104 V and that this is the main cause of cell capacitance enhancement. Beyond this voltage range, the Faradaic contribution from the MB redox reaction decreases because the anode behaves as a capacitative electrode with a rather reduced charge-capacity due to the small surface area of MWCNTs. By means of a modified assembly composed of a Nafion membrane and MB and sulfuric acid solutions located in the cathode and anode compartments, respectively, it was demonstrated the limiting role of the capacitative electrode in the cell charge-capacity in this type of hybrid devices., The authors acknowledge the financial support of the MICINN (Project MAT2010-20601-C02-01). Silvia Roldán thanks MICINN for a FPI predoctoral research grant.

Three classes (carbides, nitrides and oxides) of nanoscaled early-transition-metal catalysts have been proposed to replace the expensive Pt catalyst as counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Of these catalysts, Cr(3)C(2), CrN, VC(N), VN, TiC, TiC(N), TiN, and V(2)O(3) all showed excellent catalytic activity for the reduction of I(3)(-) to I(-) in the electrolyte. Further, VC embedded in mesoporous carbon (VC-MC) was prepared through in situ synthesis. The I(3)(-)/I(-) DSC based on the VC-MC CE reached a high power conversion efficiency (PCE) of 7.63%, comparable to the photovoltaic performance of the DSC using a Pt CE (7.50%). In addition, the carbide catalysts demonstrated catalytic activity higher than that of Pt for the regeneration of a new organic redox couple of T(2)/T(-). The T(2)/T(-) DSCs using TiC and VC-MC CEs showed PCEs of 4.96 and 5.15%, much higher than that of the DSC using a Pt CE (3.66%). This work expands the list of potential CE catalysts, which can help reduce the cost of DSCs and thereby encourage their fundamental research and commercial application.

Insufficient energy density and poor cyclic stability is still challenge for conductive polymer-based supercapacitor. Herein, high performance electrochemical system has been assembled by combining poly (3,4-ethylenedioxythiophene) (PEDOT):poly (styrene sulfonate) (PSS) redox electrode and potassium iodide redox electrolyte, which provide the maximum specific capacity of 51.3 mAh/g and the retention of specific capacity of 87.6% after 3000 cycles due to the synergic effect through a simultaneous redox reaction both in electrode and electrolyte, as well as the catalytic activity for reduction of triiodide of the PEDOT:PSS.

The energy density of carbon based supercapacitors (CBSCs) was significantly increased by the addition of an inorganic redox species [Ce2(SO4)3] to an aqueous electrolyte (H2SO4). The development of the faradaic processes on the positive electrode not only significantly increased the capacitance but also the operational cell voltage of these devices (up to 1.5 V) due to the high redox potentials at which the Ce3+/Ce4+ reactions occur. Therefore, in asymmetric CBSCs assembled using an activated carbon as negative electrode and MWCNTs as the positive one, the addition of Ce2(SO4)3 moderately increases the energy density of the device (from 1.24 W h kg−1 to 5.08 W h kg−1). When a modified graphite felt is used as positive electrode the energy density of the cell reaches values as high as 13.84 W h kg−1. The resultant systems become asymmetric hybrid devices where energy is stored due to the electrical double layer formation in the negative electrode and the development of the faradaic process in the positive electrode, which acts as a battery-type electrode., The authors thank MINECO (project MAT2010-20601-C02-01) for the financial support provided. Patricia Díaz thanks FPI predoctoral research grant (FPI:BES-2011-045394).

Dye-sensitized solar cells (DSCs) are a low cost and colorful promising alternative to standard silicon photovoltaic cells. Though many of the highest efficiencies have been associated with sensitizers absorbing only in the visible portion of the solar radiation, there is a growing interest for NIR sensitization. This paper reviews the efforts made so far to find sensitizers able to absorb efficiently in the far-red NIR region of solar light. Panchromatic sensitizers as well as dyes absorbing mainly in the 650–920 nm region have been considered.

Unsymmetrical Zn phthalocyanine consisting of six S-aryl groups at alpha-positions and a carboxy anchoring group at beta-position has been designed and synthesized for dye-sensitized solar cells (DSCs) applications. The unsymmetrical phthalocyanine has been characterized by elemental, MALDI-MS, IR, H-1 NMR, UV-Vis, fluorescence (steady-state & lifetime) and electrochemical (including spectroelectrochemical) methods. The Q-band absorption maxima of the unsymmetrical phthalocyanine was red-shifted due to the presence of S-aryl groups, which destabilizes the HOMO level consistent with electrochemical and in situ spectroelectrochemical studies. The redox processes are assigned to the macrocyclic ring-based electron transfer processes, the LUMO of the unsymmetrical phthalocyanines lies above the TiO2 conduction band, and the HOMO is well below the potential of the I/I-3 redox electrolyte. The experimental results are supported by DFT/TD-DFT studies. The new unsymmetrical phthalocyanines was tested in DSCs using I/I-3 redox electrolyte system. (C) 2013 Elsevier B.V. All rights reserved.

Three new sterically demanding unsymmetrical zinc phthalocyanines have been designed and synthesized as sensitizers for dye-sensitized solar cells. All three unsymmetrical phthalocyanines have been completely characterized by elemental analyses, mass spectrometry, FT-IR, H-1 NMR, UV-Visible, and fluorescence (steady-state and life-time) spectroscopies as well as electrochemical methods. Photophysical properties (absorption, emission and redox properties) indicate that the LUMO of unsymmetrical phthalocyanines lies above the TiO2 conduction band and HOMO is below the redox electrolyte. The experimental results are supported by DFT/TD-DFT studies. Electrochemical and in-situ spectroelectrochemical studies suggest that the redox reactions belong to the macrocyclic ring-based electron transfer processes. All three unsymmetrical phthalocyanines were tested in DSSC using I-/I-3(-) redox electrolyte system. (C) 2013 Elsevier Ltd. All rights reserved.

Zinc(II) phthalocyanine dyes that contain triarylamine-terminated bisthiophene and hexylbisthiophene groups have been synthesized by a convergent approach by using carboxytriiodoZnPc as a precursor. Further transformation of the iodo groups by a Pd-catalyzed reaction allowed easy preparation of further extended p-conjugated carboxyZnPcs. These dyes have been used as sensitizers in dye-sensitized solar cells, which exhibit a panchromatic response and moderate overall efficiencies.