Benoit Marsan

CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells.

We report an efficient nonplatinized flexible counter electrode for dye-sensitized solar cells. In combination with a solvent-free ionic liquid electrolyte, we have demonstrated a approximately 6.5% cell with an amphiphilic ruthenium polypyridyl photosensitizer showing excellent stability measured under prolonged light soaking at 60 degrees C. Compared to the Pt deposited PEN film, the CoS deposited PEN film shows higher electrocatalytic activity for the reduction of triiodide. This is expected to have an important practical consequence on the production of flexible low-cost and lightweight thin film DSC devices based on the plastic matrix.

Influence of the counter electrode on the photovoltaic performance of dye-sensitized solar cells using a disulfide/thiolate redox electrolyte

Strong scientific interests focus on the investigation of iodine-free redox couples for their application in dye-sensitized solar cells (DSCs). Recently, a disulfide/thiolate-based redox electrolyte has been proposed as a valuable alternative to the conventional I3−/I− system due to its transparent and non-corrosive nature. In the work presented herein, we systematically studied the influence of different counter electrode materials on the photovoltaic performance of DSCs employing this promising organic redox electrolyte. Our investigations focused on understanding the importance of electrocatalytic activity and surface area of the electroactive material on the counter electrode, comparing the conventional platinum to cobalt sulfide (CoS) and poly(3,4-ethylenedioxythiophene) (PEDOT). Electrochemical Impedance Spectroscopy has been used to study in detail the interfacial charge transfer reaction at the counter electrode. By using a high surface area PEDOT-based counter electrode, we finally achieved an unprecedented power conversion efficiency of 7.9% under simulated AM1.5G solar irradiation (100 mW cm−2) which, to the best of our knowledge, represents the highest efficiency that has so far been reported for an organic redox couple.

Cu x Co3 − x O4 Used as Bifunctional Electrocatalyst II. Electrochemical Characterization for the Oxygen Reduction Reaction

Composite film electrodes containing mechanically mixed Co 3 O 4 or CuCo 2 O 4 particles, carbon-black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction reaction (ORR) in O 2 -satwated 1 M KOH solution. The highest current densities were observed with CuCo 2 O 4 and they increased with the oxide content in the film, hence clearly demonstrating the excellent intrinsic electrocatalytic activity of CuCo 2 O 4 for this reaction. The results also showed that CuCo 2 O 4 is a better electrocatalyst than Co 3 O 4 with higher current densities and a greater number of electrons exchanged per O 2 molecule. It was found that the copper-cobalt spinel oxide component favors a total of 4e - in the oxygen reduction process. At the CuCo 2 O 4 -based composite electrode, direct reduction of O 2 into OH - ions (rate constant k 1 ) and the peroxide pathway (formation of HO 2 - ions and their reduction into OH - ions, rate constants k 2 and k 3 ) are proceeding in parallel, with a k 1 /k 2 ratio that increases with the overpotential when the oxide content is greater than 23.5%. At the Co 3 O 4 -based composite electrode, k 1 is very weak with a k 1 /k 2 ratio that decreases rapidly with the overpotential.

The electrodeposition of amorphous molybdenum sulfide

Abstract The electrochemical oxidation of an aqueous solution containing ammonium tetrathiomolybdate at a tin-oxide-coated glass electrode produces a material with MoS 3.8 stoichiometry which consists of amorphous molybdenum trisulfide and elemental sulfur. The nucleation and growth of amorphous molybdenum sulfide have been studied by cyclic voltammetry and potential step experiments. The effect of the ammonium tetrathiomolybdate concentration and solution pH on the current-time transients has been investigated. The analysis of the early part of the transients was carried out using the current-time relationships developed for metal deposition. Our data analysis indicated that film formation occurred by instantaneous nucleation and three-dimensional growth. Subsequently, the formation of bulk film is limited by diffusion or kinetics at low or high MoS 4 2− concentrations respectively.

Preparation and characterization of electrodeposited amorphous molybdenum sulfide

Abstract Amorphous molybdenum sulfide has been electrodeposited onto tin oxide-coated glass electrode from an aqueous ammonium tetrathiomolybdate solution. Cyclic voltammetry, potential step experiments, chemical analysis and scanning electron microscopy (SEM) have been used to characterize the deposited films. Cyclic voltammetry performed in the electrolytic bath, with and without supporting electrolyte, show that the oxidation of MoS 4 2− is completely irreversible. SEM micrographs of the deposited films indicate that the morphologies are thickness independent at high magnification but that cracks are clearly visible at lower magnification for thicker films.

Ionic conductivity studies of polymer electrolytes containing organosulfur species

Abstract The electrolyte composed of a PEO-based polymer complexed with an alkali metal thio/disulfide redox couple is used in an all solid-state electrochemical photovoltaic cell. Total ionic conductivity studies of the electrolytes modified PEO-MT/T 2 (M  Li, Na, K), where T − stands for 5-mercapto-1-methyltetrazole ion and T 2 for its dimere, are reported. As expected, the results show that the conductivity values measured at the same temperature increase with the atomic weight of the alkali cation. A maximum in conductivity is found with system modified PEO-KT/T 2 prepared at a O/K molar ratio of 8/1.

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.

Studies of electrochemical and mass transport properties of a thiolate/disulfide redox couple in polymer electrolytes

Electrochemical and mass transport properties of a thiolate/disulfide couple, formed by 5-mercapto-l-methyltetrazole lithium salt (LiT) and disulfide (T 2 ) dissolved in liquid poly(ethylene glycol) 500 dimethyl ether, PEG(500), DME and soft methoxy-linked poly(ethylene oxide), PMEO, have been studied using a three-electrode microcell and a conventional three-electrode cell. Cyclic voltammetry showed that the electrochemical reversibility of the redox couple is poor in both solvents. The exchange current densities for the reduction and the oxidation reactions in PEG(500) DME-LiClO 4 are 0.24 μA cm -2 and 0.13 μA cm -2 , respectively. The diffusion coefficients of T - and T 2 in the same system are 7.88 x 10 -8 cm 2 s -1 and 6.01 x 10 -8 cm 2 s -1 , respectively, at 23°C, which are about 40 times higher than those measured in PMEO-LiClO 4 . The diffusion coefficients of T - and T 2 in polymer electrolytes based on PMEO showed a VTF behaviour. The diffusion coefficient of T - increases (i) with decreasing LiT concentration and (ii) when T 2 species are present in the electrolyte.

Triplet Energy Transfers in Well-Defined Host-Guest Porphyrin-Carboxylate/Cluster Assemblies.

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)], dppm = (Ph2P)2CH2) via ionic CO2(-)···Pd3(2+) interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd3(2+)] (∼8190 cm(-1)) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680-700 nm arising from a T1-T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1-T1 energy transfer ((3)dye*···[Pd3(2+)] → dye···(3)[Pd3(2+)]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 10(5) s(-1) on the basis of transient absorption measurements at 298 K.

Triplet energy vs. electron transfers in porphyrin- and tetrabenzoporphyrin-carboxylates/Pd3(dppm)3(CO)2+ cluster assemblies; a question of negative charge

Two tetracarboxylatetetrabenzoporphyrinzinc(II), TCPBP (9,18,27,36-tetrakis-meso-(4-carboxyphenyl)tetrabenzoporphyrinatozinc(II)) and TCPEBP (9,18,27,36-tetra-(4-carboxyphenylethynyl)tetrabenzoporphyrinatozinc(II)), and their two corresponding porphyrins, TCPP (tetrakis-meso-(4-carboxyphenyl)porphyrinatozinc(II)) and TCPEP (5,10,15,20-tetra-(4-carboxyphenylethynyl)porphyrinatozinc(II)) as sodium salts in a 1 : 1 mixture of 2MeTHF/MeOH at 77 K exhibit phosphorescence in the near-IR region with maxima ranging from 785 to 1005 nm. The position of these triplet state emissions has been corroborated from DFT (B3LYP) by calculating the total energy difference between the ground, S0, and lowest energy triplet excited state T1 (the calculated position ranges from 797 to 1041 nm). At both temperatures, 77 and 298 K, these dyes make ionic driven host–guest assemblies with the unsaturated redox-active cluster Pd3(dppm)3(CO)2+ ([Pd32+], dppm = Ph2PCH2PPh2 as a PF6− salt). The formation of these assemblies is accompanied by a quenching of the phosphorescence band, without changing the emission lifetime. This phenomenon is consistent with the formation of the non-emissive assemblies dye⋯[Pd32+]x according to dye + [Pd32+] → dye⋯[Pd32+]x (x = 1–4), where only the porphyrin and tetrabenzoporphyrin dyes are phosphorescent. Quenching analysis confirmed that a static quenching mechanism operates. In one case, the quenching rate (kQ) has been evaluated from transient absorption spectroscopy (TAS) where the signal associated with the dyes triplet state exhibits a very short lifetime τ(T1), thus corroborating the efficient quenching. The quenching rate (1/τ(T1)) is much faster (3.0 × 108 s−1) than that expected for the other dye⋯[Pd32+]x assemblies in the literature (∼104 s−1) known for their triplet–triplet energy transfer. Based on the excited state driving force argumentation for oxidative quenching, this fast process is assigned to a predominant photo-induced electron transfer (3dye* + [Pd32+] → dye+˙ + [Pd3+˙]).