Robust, metallic Pd17Se15 and Pd7Se4 phases from a single source precursor and their use as counter electrodes in dye sensitized solar cells

Thin films of conducting palladium selenide phases (Pd17Se15 and Pd7Se4) are prepared using a single source molecular precursor by thermolysis. Varying the mole ratios of palladium and selenium precursors results in palladium organo-selenolate complexes which on thermolysis at different temperatures yield Pd17Se15 and Pd7Se4 phases that are very stable and adherent to the substrate. The organo-selenolate complexes are characterized using small angle XRD, Se-77 NMR and thermogravimetric analysis (TGA). The palladium selenide films are characterized by various techniques such as XRD, XPS, TEM and SEM. Electrical conductivities of the films are determined using the four probe method. The strong adherence of the films to glass substrates coupled with high corrosion resistant behavior towards strong acid and alkaline environments render them to be very effective as electrocatalysts. The catalytic activity towards the I-3(-)/I- redox couple, which is an important reaction in the regeneration of the dye in a dye-sensitized solar cell, is studied. Between the two phases, the Pd17Se15 film shows superior activity as the counter electrode for dye sensitized solar cells with a photocurrent conversion efficiency of 7.45%.

Rational screening low-cost counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cells have attracted intense research attention owing to their ease of fabrication, cost-effectiveness and high efficiency in converting solar energy. Noble platinum is generally used as catalytic counter electrode for redox mediators in electrolyte solution. Unfortunately, platinum is expensive and non-sustainable for long-term applications. Therefore, researchers are facing with the challenge of developing low-cost and earth-abundant alternatives. So far, rational screening of non-platinum counter electrodes has been hamstrung by the lack of understanding about the electrocatalytic process of redox mediators on various counter electrodes. Here, using first-principle quantum chemical calculations, we studied the electrocatalytic process of redox mediators and predicted electrocatalytic activity of potential semiconductor counter electrodes. On the basis of theoretical predictions, we successfully used rust (alpha-Fe2O3) as a new counter electrode catalyst, which demonstrates promising electrocatalytic activity towards triiodide reduction at a rate comparable to platinum.

PEDOT-coated counter electrodes for dye-sensitized solar cells

Poly(3,4-ethylenedioxythiophene) (PEDOT)-coated counter electrodes for dye-sensitized solar cells have been prepared at ambient temperature and without the use of iron-based oxidants, using an ionic liquid for the synthesis of the PEDOT. These electrodes show comparable electrocatalytic performance with conventional Pt-coated counter electrodes (solar cell efficiencies >7.5%).

Single precursor route to efficient cobalt sulphide counter electrodes for dye sensitized solar cells

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electropolyrnerization of polypyrrole on PFIL-PSS-modified electrodes without added support electrolytes

A novel polyelectrolyte-functionalized ionic liquid (PFIL)/poly(4-styrene sulfonate sodium) (PSS) modified electrode composed of the coaxial and coplanar working, reference and counter electrodes, was used to electropolymerize the polypyrrole. The PFIL/PSS was modified on the integrated electrode (IE) and connected by the working, reference and counter electrodes, resulting in an available charge transfer and lower Ohmic potential drop between the working and counter electrodes. Then polypyrrole (PPy) film was successfully prepared electrochemically without any participation of supporting electrolytes, only in a pyrrole monomer solution. The resulting PPy film in PFIL/PSS matrix exhibited a preferable electroactivity. Subsequently, influence of the modifications on the formation of PPy was further discussed. The results indicated that the synergetic cooperation of PFIL and PSS components accomplished such a successful electropolymerization of PPy.

Studies on Carbon Mediated Paste Screen Printed Sensors for Blood Glucose Sensing Application

Ready-to-use screen printed glucose sensors are fabricated using Prussian Blue (PB) and Cobalt Phthalocyanine (CoPC) mediated carbon inks as working electrodes. The reference and counter electrodes are screen printed using silver/silver chloride and graphitic carbon paste respectively. The screen printed reference electrodes (internal reference electrode (IRE)) are found to be stable for more than 60 minutes when examined with saturated calomel electrode. Optimal operating voltage for PB and CoPC screen printed sensors are determined by hydrodynamic voltammetric technique. Glucose oxidase is immobilized on the working electrodes by cross-linking method. PB mediated glucose sensor exhibits a sensitivity of 5.60 mA cm(-2)/mM for the range, 10 to 1000 mu M. Sensitivity of CoPC mediated glucose sensor is found to be 5.224 mu A cm(-2)/mM and amperometeric response is linear for the range, 100 to 1500 mu M. Interference studies on the fabricated glucose sensors are conducted with species like uric acid and ascorbic acid. PB mediated sensors showed a completely interference-free behavior. The sensing characteristics of PB mediated glucose sensors are also studied in diluted human serum samples and the results are compared with the values obtained through standard clinical method. The co-efficient of variation is found to be less than 5%. (C) 2015 The Electrochemical Society. All rights reserved.

Charge transfer across the water/nitrobenzene interface by three-electrode system

A droplet of aqueous solution containing a certain molar ratio of redox couple is first attached onto a platinum electrode surface, then the resulting drop electrode is immersed into the organic solution containing very hydrophobic electrolyte. Combined with reference and counter electrodes, a classical three-electrode system has been constructed, Ion transfer (IT) and electron transfer (ET) are investigated systematically using three-electrode voltammetry. Potassium ion transfer and electron transfer between potassium ferricyanide in the aqueous phase and ferrocene in nitrobenzene are observed with potassium ferricyanide/potassium ferrocyanide as the redox couple. Meanwhile, the transfer reactions of lithium, sodium, potassium, proton and ammonium ions are obtained with ferric sulfate/ferrous sulfate as the redox couple. The formal transfer potentials and the standard Gibbs transfer energy of these ions are evaluated and consistent with the results obtained by a four-electrode system and other methods.

Highly Electrocatalytic Activity of RuO2 Nanocrystals for Triiodide Reduction in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are promising alternatives to conventional silicon devices because of their simple fabrication procedure, low cost, and high efficiency. Platinum is generally used as a superior counter electrode (CE) material, but the disadvantages such as high cost and low abundance greatly restrict the large-scale application of DSCs. An efficient and sustainable way to overcome the limited supply of Pt is the development of high-efficiency Pt-free CE materials, which should possess both high electrical conductivity and superior electrocatalytic activity simultaneously. Herein, for the first time, a two-step strategy to synthesize ruthenium dioxide (RuO2) nanocrystals is reported, and it is shown that RuO2 catalysts exhibit promising electrocatalytic activity towards triiodide reduction, which results in comparable energy conversion efficiency to that of conventional Pt CEs. More importantly, by virtue of first-principles calculations, the catalytic mechanism of electrocatalysis for triiodide reduction on various CEs is investigated systematically and it is found that the electrochemical triiodide reduction reaction on RuO2 catalyst surfaces can be enhanced significantly, owing to the ideal combination of good electrocatalytic activity and high electrical conductivity.

Turning Indium Oxide into a Superior Electrocatalyst:Deterministic Heteroatoms

The efficient electrocatalysts for many heterogeneous catalytic processes in energy conversion and storage systems must possess necessary surface active sites. Here we identify, from X-ray photoelectron spectroscopy and density functional theory calculations, that controlling charge density redistribution via the atomic-scale incorporation of heteroatoms is paramount to import surface active sites. We engineer the deterministic nitrogen atoms inserting the bulk material to preferentially expose active sites to turn the inactive material into a sufficient electrocatalyst. The excellent electrocatalytic activity of N-In2O3 nanocrystals leads to higher performance of dye-sensitized solar cells (DSCs) than the DSCs fabricated with Pt. The successful strategy provides the rational design of transforming abundant materials into high-efficient electrocatalysts. More importantly, the exciting discovery of turning the commonly used transparent conductive oxide (TCO) in DSCs into counter electrode material means that except for decreasing the cost, the device structure and processing techniques of DSCs can be simplified in future.

Flow injection amperometric determination of hydrogen peroxide in household commercial products with a ruthenium oxide hexacyanoferrate modified electrode

Hydrogen peroxide was determined in oral antiseptic and bleach samples using a flow-injection system with amperometric detection. A glassy carbon electrode modified by electrochemical deposition of ruthenium oxide hexacyanoferrate was used as working electrode and a homemade Ag/AgCl (saturated KCl) electrode and a platinum wire were used as reference and counter electrodes, respectively. The electrocatalytic reduction process allowed the determination of hydrogen peroxide at 0.0 V. A linear relationship between the cathodic peak current and concentration of hydrogen peroxide was obtained in the range 10-5000 mu mol L(-1) with detection and quantification limits of 1.7 (S/N = 3) and 5.9 (S/N = 10) mu mol L(-1), respectively. The repeatability of the method was evaluated using a 500 mu mol L(-1) hydrogen peroxide solution, the value obtained being 1.6% (n = 14). A sampling rate of 112 samples h(-1) was achieved at optimised conditions. The method was employed for the quantification of hydrogen peroxide in two commercial samples and the results were in agreement with those obtained by using a recommended procedure.

Determination of malic acid in real samples by using enzyme immobilized reactors and amperometric detection

The malate dehydrogenase (MDH) and ascorbate oxidase were immobilized independently, onto silanized controlled porous silica and packed in a tygon tube. The reactors were inserted in the flow system, and the malic acid was determined by measurement of NADH produced by enzymatic reaction. The NADH was reoxidized in a wall jet cell that consisted of spectrographic graphite, Ag/AgCl, KCl(sat), and steel needle as work, reference, and counter electrodes, respectively. The current intensities were measured at 390 mV. The malate calibration curve shows a linear range from 5.0 x 10(-6) to 1.0 x 10(-4) molL(-1), the lifetime was 40 analyses, after that a decrease of 20% on the response is observed. Three different citric juices were analyzed and a good correlation between the proposed method and spectrophotometric commercial kit were obtained.

SYNTHESIS OF GRAPHENE AND ITS APPLICATIONS FOR DYE-SENSITIZED SOLAR CELLS

Graphene, which is a two-dimensional carbon material, exhibits unique properties that promise its potential applications in photovoltaic devices. Dye-sensitized solar cell (DSSC) is a representative of the third generation photovoltaic devices. Therefore, it is important to synthesize graphene with special structures, which possess excellent properties for dye-sensitized solar cells. This dissertation research was focused on (1) the effect of oxygen content on the structure of graphite oxide, (2) the stability of graphene oxide solution, (3) the application of graphene precipitate from graphene oxide solution as counter electrode for DSSCs, (4) the development of a novel synthesis method for the three-dimensional graphene with honeycomb-like structure, and (5) the exploration of honeycomb structured graphene (HSG) as counter electrodes for DSSCs. Graphite oxide is a crucial precursor to synthesize graphene sheets via chemical exfoliation method. The relationship between the oxygen content and the structures of graphite oxides was still not explored. In this research, the oxygen content of graphite oxide is tuned by changing the oxidation time and the effect of oxygen content on the structure of graphite oxide was evaluated. It has been found that the saturated ratio of oxygen to carbon is 0.47. The types of functional groups in graphite oxides, which are epoxy, hydroxyl, and carboxylgroups, are independent of oxygen content. However, the interplanar space and BET surface area of graphite oxide linearly increases with increasing O/C ratio. Graphene oxide (GO) can easily dissolve in water to form a stable homogeneous solution, which can be used to fabricate graphene films and graphene based composites. This work is the first research to evaluate the stability of graphene oxide solution. It has been found that the introduction of strong electrolytes (HCl, LiOH, LiCl) into GO solution can cause GO precipitation. This indicates that the electrostatic repulsion plays a critical role in stabilizing aqueous GO solution. Furthermore, the HCl-induced GO precipitation is a feasible approach to deposit GO sheets on a substrate as a Pt-free counter electrode for a dye-sensitized solar cell (DSSC), which exhibited 1.65% of power conversion efficiency. To explore broad and practical applications, large-scale synthesis with controllable integration of individual graphene sheets is essential. A novel strategy for the synthesis of graphene sheets with three-dimensional (3D) Honeycomb-like structure has been invented in this project based on a simple and novel chemical reaction (Li2O and CO to graphene and Li2CO3). The simultaneous formation of Li2CO3 with graphene not only can isolate graphene sheets from each other to prevent graphite formation during the process, but also determine the locally curved shape of graphene sheets. After removing Li2CO3, 3D graphene sheets with a honeycomb-like structure were obtained. This would be the first approach to synthesize 3D graphene sheets with a controllable shape. Furthermore, it has been demonstrated that the 3D Honeycomb-Structured Graphene (HSG) possesses excellent electrical conductivity and high catalytic activity. As a result, DSSCs with HSG counter electrodes exhibit energy conversion efficiency as high as 7.8%, which is comparable to that of an expensive noble Pt electrode.

Vertical electric field induced bacterial growth inactivation on amorphous carbon electrodes

The objective of the present work is to understand the vertical electric field stimulation of the bacterial cells, when grown on amorphous carbon substrates in vitro. In particular, the antibacterial activity against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli are studied using MTTassay, live/dead assay and inner membrane permeabilization assays. In our experiments, the carbon substrate acts as one electrode and the counter electrode is positioned outside the culture medium, thus suppressing the current, electrokinetic motions and chemical reactions. Guided by similar experiments conducted in our group on neuroblastoma cells, the present experimental results further establish the interdependence of field strength and exposure duration towards bacterial growth inactivation in vitro. Importantly, significant reduction in bacterial viability was recorded at the 2.5 V/cm electric field stimulation conditions, which does not reduce the neural cell viability to any significant extent on an identical substrate. Following electrical stimulation, the bacterial growth is significantly inhibited for S. aureus bacterial strain in an exposure time dependent manner. In summary, our experiments establish the effectiveness of the vertical electric field towards bacterial growth inactivation on amorphous carbon substrates, which is a cell type dependent phenomenon (Gram-positive vs. Gram-negative). (C) 2014 Elsevier Ltd. All rights reserved.