Tris Cobalt Research Articles

Cobalt(II) tris(2-pyridylmethyl)amine complexes [Co(TPA)X]+ bearing coordinating anion (X = Cl−, Br−, I− and NCS−): synthesis and application for carbon dioxide reduction

Four cobalt(II) complexes supported by the tripodal ligand tris(2-pyridylmethyl)amine (TPA) and ancillary coordinated anion (X), [Co(TPA)X]+ (X=Cl−, Br−, I− and NCS−) (1–4), have been prepared. Magnetic susceptibility measurement and electronic spectroscopy indicate that they are five-coordinated high spin cobalt(II) complexes. The structures of 2–4 have been determined by X-ray crystallography. Reduction of CO2 to CO has been observed upon irradiation of complexes 1–4 in the presence of fac-Ir(ppy)3 (ppy=C-deprotonated 2-phenylpyridine anion) and under CO2 atmosphere using blue LEDs. All of these Co complexes were found to be efficient and stable CO2 reduction catalyst with TON(CO)>1000 and CO selectivity>90% under 65h of irradiation. Among them, [Co(TPA)Br]+ showed the best performance in CO2 reduction with TON(CO)>1400 after 65h. The coordinated anion X played an important role in the photo-catalysis. Density functional theory (DFT) calculation has also been performed to examine the mechanism and role of coordinated anion X in visible-light induced CO2 reduction.

The alluaudite-type crystal structures of Na2(Fe/Co)2Co(VO4)3 and Ag2(Fe/Co)2Co(VO4)3.

Single crystals of the title compounds, disodium di(cobalt/iron) cobalt tris-(orthovanadate), Na2(Fe/Co)2Co(VO4)3, and disilver di(cobalt/iron) cobalt tris-(orthovanadate), Ag2(Fe/Co)2Co(VO4)3, were grown from a melt consisting of stoichiometric mixtures of three metallic cation precursors and vanadium pentoxide. The difficulty to distinguish between cobalt and iron by using X-ray diffraction alone forced us to explore several models, assuming an oxidation state of +II for Co and +III for Fe and a partial cationic disorder in the Wyckoff site 8f containing a mixture of Co and Fe with a statistical distribution for the Na compound and an occupancy ratio of 0.4875:0.5125 (Co:Fe) for the Ag compound. The alluaudite-type structure is made up from [10-1] chains of [(Co,Fe)2O10] double octa-hedra linked by highly distorted [CoO6] octa-hedra via a common edge. The chains are linked through VO4 tetra-hedra, forming polyhedral sheets perpendicular to [010]. The stacking of the sheets defines two types of channels parallel to [001] where the Na(+) cations (both with full occupancy) or Ag(+) cations (one with occupancy 0.97) are located.

Reversible magnetogenic cobalt complexes

A cobalt tris(2-pyridylmethyl)amine complex cycles between stable paramagnetic Co(ii) and diamagnetic Co(iii) forms with corresponding changes in the MRI contrast.

Kinetics of Regeneration and Recombination Reactions in Dye-Sensitized Solar Cells Employing Cobalt Redox Shuttles

The key to achieving high-efficiency dye-sensitized solar cells (DSSCs) is the realization of a redox shuttle which exhibits quantitative dye regeneration with a minimal driving force. Since the electron diffusion length, Ln, is controlled by recombination to the redox shuttle, an optimal redox shuttle must balance the kinetics of these two key electron-transfer reactions. In this work the dye regeneration efficiency, ηreg, and the electron diffusion length were determined for DSSCs employing cobalt tris(bipyridine), [Co(bpy)3]3+/2+, and cobalt bis(trithiacyclononane), [Co(ttcn)2]3+/2+, redox shuttles from optical and incident photon to current efficiency (IPCE) measurements of the cells under front side and back side illumination directions. The regeneration of the D35cpdt dye was found to be quantitative with [Co(ttcn)2]3+/2+; however, dye regeneration with the current champion redox shuttle [Co(bpy)3]3+/2+ is suboptimal despite a larger driving force of the reaction. The electron diffusion length was found to be shorter for DSSCs with the [Co(ttcn)2]3+/2+ redox shuttle compared to [Co(bpy)3]3+/2+, however, due to faster recombination. The self-exchange rate constants of the two redox shuttles were determined from cross-exchange measurements and were found to differ by over 4 orders of magnitude. Application of Marcus theory allowed the difference in self-exchange rate constants to quantitatively account for the differences in regeneration efficiency and electron diffusion length of the two redox shuttles. Atomic layer deposition (ALD) was used to add a single layer of alumina on the TiO2 film prior to immersing it in the sensitizer solution. This treatment resulted in improved performance for DSSCs employing both redox shuttles; however, the improvement was shown to arise from different causes. The alumina layer reduces recombination to the redox shuttle and thereby increases Ln for [Co(ttcn)2]3+/2+. The alumina layer was also shown to improve the dye regeneration efficiency for the [Co(bpy)3]3+/2+ redox shuttle through reduction of recombination to the oxidized dye. These findings clearly demonstrate the fine balance between the regeneration and recombination reactions when outer-sphere redox shuttles are employed in DSSCs. Isolation of the efficiency-limiting reactions, however, allows for strategies to overcome these barriers to be identified and implemented.

Evaluation of Electrochemical Methods for Determination of the Seebeck Coefficient of Redox Electrolytes

Recent advances in thermoelectrochemical cells, which are being developed for harvesting low grade waste heat, have shown the promise of cobalt bipyridyl salts as the active redox couple. The Seebeck coefficient, Se, of a redox couple determines the open circuit voltage achievable, for a given temperature gradient, across the thermoelectrochemical cell. Thus, the accurate determination of this thermodynamic parameter is key to the development and study of new redox electrolytes. Further, techniques for accurate determination of Se using only one half of the redox couple reduces the synthetic requirements. Here, we compare three different experimental techniques for measuring Se of a cobalt tris(bipyridyl) redox couple in ionic liquid electrolytes. The use of temperature dependent cyclic voltammetry (CV) in isothermal and non-isothermal cells was investigated in depth, and the Se values compared to those from thermo-electromotive force measurements. Within experimental error, the Se values derived from CV methods were found to be in accordance with those obtained from electromotive force (emf) measurements. The applicability of cyclic voltammetry techniques for determining Se when employing only one part of the redox couple was demonstrated.

Carbazole Dye-Sensitized Solar Cells Studied from Femtoseconds to Seconds-Effect of Additives in Cobalt- and Iodide-Based Electrolytes.

Comprehensive studies of all charge-separation processes in efficient carbazole dye-sensitized solar cells are correlated with their photovoltaic parameters. An important role of partial, fast electron recombination from the semiconductor nanoparticles to the oxidized dye is revealed; this takes place on the picosecond and sub-nanosecond timescales. The charge-transfer dynamics in cobalt tris(bipyridyl) based electrolytes and iodide-based electrolyte is observed to depend on potential-determining additives in a similar way. Upon addition of 0.5 M 4-tert-butylpiridine to both types of electrolytes, the stability of the cells is greatly improved; the cell photovoltage increases by 150-200 mV, the electron injection rate decreases about five times (from 5 to 1 ps(-1) ), and fast recombination slows down about two to three times. Dye regeneration proceeds at a rate of about 1 μs(-1) in all electrolytes. Electron recombination from titania to cobalt electrolytes is much faster than that to iodide ones.

Are High-Potential Cobalt Tris(bipyridyl) Complexes Sufficiently Stable to Be Efficient Mediators in Dye-Sensitized Solar Cells? Synthesis, Characterization, and Stability Tests

We report the synthesis of three new, high-oxidation-potential cobalt complexes. The ligand lability of cobalt complexes, especially those of higher redox potential, has been scantly addressed in the current literature on the use of these complexes in dye-sensitized solar cells (DSSCs). In characterizing our high-potential complexes, multiple pieces of evidence reveal a trend of decreasing complex stability with increasing redox potential. These include the appearance of multiple waves in the cyclic voltammetry, a color change upon addition of 4-tert-butylpyridine indicating the formation of a new species, direct NMR evidence of free dissociated ligand in acetonitrile solution, and potential-independent DSSC recombination currents paralleling the stability trends as determined by NMR. We take advantage of a simple quantitative NMR experiment to determine the approximate ligand-binding equilibrium (stability) constants of our complexes in acetonitrile at room temperature. With the above information, we propose alternative, clathrochelating ligand structures for the future study of high potential cobalt mediators for application in DSSCs.

Structure cristalline de type alluaudite K0.4Na3.6Co(MoO4)3.

A new triple molybdate, potassium sodium cobalt tris-(molybdate), K0.4Na3.6Co(MoO4)3, was synthesized using solid-state reactions. The Co(2+) and one Na(+) cation are located at the same general site, each with occupancy 0.5. Another site (site symmetry 2) is occupied by Na(+) and K(+) cations, with occupancies of 0.597 (7) and 0.402 (6), respectively. The other two Na(+) cations and one of the two Mo atoms lie on special positions (site symmetries -1, 2 and 2, respectively). The structure is characterized by M 2O10 (M = Co/Na) dimers, which are linked by MoO4 tetra-hedra, forming infinite layers. The latter are connected firstly by insertion of one type of MoO4 tetra-hedra and secondly by sharing corners with the other type of MoO4 tetra-hedra. This results in an open three-dimensional framework with the cavities occupied by the Na(+) and K(+) cations. The structure is isotypic with Na3In2As3O12 and Na3In2P3O12. A comparison is made with structures such as K2Co2(MoO4)3 and β-NaFe2(MoO4)3 and their differences are discussed.

Enhanced Photovoltaic Properties and Long-Term Stability in Plasmonic Dye-Sensitized Solar Cells via Noncorrosive Redox Mediator

We demonstrate the localized surface plasmon resonance (LSPR) effect, which can enhance the photovoltaic properties of dye-sensitized solar cells (DSSCs), and the long-term stability of size-controlled plasmonic structures using a noncorrosive redox mediator. Gold nanoparticles (Au NPs) were synthesized with a phase transfer method based on ligand exchange. This synthetic method is advantageous because the uniformly sized Au NPs, can be mass produced and easily applied to DSSC photoanodes. The plasmonic DSSCs showed an 11% improvement of power conversion efficiency due to the incorporation of 0.07 wt % Au NPs, compared to the reference DSSCs without Au NPs. The improved efficiency was primarily due to the enhanced photocurrent generation by LSPR effect. With the cobalt redox mediator, the long-term stability of the plasmonic structures also significantly increased. The plasmonic DSSCs with cobalt(II/III) tris(2,2'-bipyridine) ([Co(bpy)3](2+/3+)) redox mediator maintained the LSPR effect with stable photovoltaic performance for 1000 h. This is, to our knowledge, the first demonstration of the long-term stability of plasmonic nanostructures in plasmonic DSSCs based on liquid electrolytes. As a result, the enhanced long-term stability of plasmonic NPs via a noncorrosive redox mediator will increase the feasibility of plasmonic DSSCs.

Highly efficient one-dimensional ZnO nanowire-based dye-sensitized solar cell using a metal-free, D-π-A-type, carbazole derivative with more than 5% power conversion.

Hydrothermally grown one-dimensional ZnO nanowire (1D ZnO NW) and a newly synthesized metal-free, D-π-A type, carbazole dye (SK1) sensitizer-based photovoltaic device with a power conversion efficiency (PCE) of more than 5% have been demonstrated by employing the cobalt tris(2,2'-bipyridyl) redox shuttle. A short-circuit current density (Jsc) of ∼12.0 mA/cm(2), an open-circuit voltage (Voc) of ∼719 mV, and a fill factor (FF) of ∼65% have been afforded by the 1D ZnO NW-based dye-sensitized solar cell (DSSC) incorporating [Co(bpy)3](3+/2+) complex as the one-electron redox mediator. In contrast, the identical DSSC with traditional I3(-)/I(-) electrolyte has shown a Jsc ≈ 12.2 mA/cm(2), a Voc ≈ 629 mV, and a FF ≈ 62%, yielding a PCE of ∼4.7%. The persuasive role of the inherent superior electron transport property of 1D ZnO NWs in enhancing the device efficiency is evidenced from the impoverished performance of the DSSCs with photoanodes fabricated using ZnO nanoparticles (NPs). The DSSCs having ZnO NP-based photoanodes have achieved the PCEs of ∼3.6% and ∼3.2% using cobalt- and iodine-based redox electrolytes, respectively. The electronic interactions between the SK1 sensitizer and ZnO (NWs and NPs) to induce the photogenerated charge transfer from SK1 to the conduction band (CB) of ZnO are evidenced from the significant quenching of photoluminescence and exciton lifetime decay of SK1, when it is anchored onto the ZnO architectures. The energetics of the SK1 dye molecule are estimated by combining the spectroscopic and electrochemical techniques. The electronic distributions of SK1 dye molecule in its HOMO and LUMO energy levels are interpreted using density functional theory (DFT)-based calculations. The electron donor-π linker-acceptor (D-π-A) configuration of SK1 dye provides an intramolecular charge transfer within the molecule, prompting the electron migration from the carbazole donor to cyanoacrylic acceptor moiety via the oligo-phenylenevinylene linker group. The D-π-A-mediated electron movement witnesses the favorable photoexcited electron transfer from the LUMO of SK1 dye to the CB of ZnO through the carboxyl anchoring group.

Cobalt dopant with deep redox potential for organometal halide hybrid solar cells.

In this work, we report a new cobalt(III) complex, tris[2-(1H-pyrazol-1-yl)pyrimidine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (MY11), with deep redox potential (1.27 V vs NHE) as dopant for 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). This dopant possesses, to the best of our knowledge, the deepest redox potential among all cobalt-based dopants used in solar cell applications, allowing it to dope a wide range of hole-conductors. We demonstrate the tuning of redox potential of the Co dopant by incorporating pyrimidine moiety in the ligand. We characterize the optical and electrochemical properties of the newly synthesized dopant and show impressive spiro-to-spiro(+) conversion. Lastly, we fabricate high efficiency perovskite-based solar cells using MY11 as dopant for molecular hole-conductor, spiro-OMeTAD, to reveal the impact of this dopant in photovoltaic performance. An overall power conversion efficiency of 12% is achieved using MY11 as p-type dopant to spiro-OMeTAD.

Polythermal solubility of chromium(III) and cobalt(III) tris(acetylacetonates) in water-tert-butanol mixtures

Solubility data for chromium(III) and cobalt(III) tris(acetylacetonates) in water and water-tert-butanol mixtures over a wide range of temperatures and alcohol concentrations were obtained, and thermodynamic parameters of solution were calculated. The dependences of solubility on the concentration of tert-butanol have an inversion point corresponding to a change in the hydration mechanism, when the solvent structure changes from water to aqueous alcohol.

Dependence of the electrocatalytic performance of platinized counter electrodes on the redox mediator employed in dye-sensitized solar cells

The electrocatalytic properties of platinized counter electrodes (Pt CEs) prepared by various coating methods were investigated with respect to the redox mediator, including the widely used iodide/tri-iodide (I - / I3 ) and the more recently introduced cobalt(II/III)tris(2,2 0 - bipyridine) (Co(bpy)3?/3? ), for application in dye-sensi- tized solar cells (DSCs). The coating methods controlled Pt loading and the surface morphology of the Pt CEs. For a high-performance DSC with a fill factor (0.7, the charge- transfer resistance at the Pt CE/electrolyte interface should be \4.5 X cm 2 for both redox mediators. The I - /I3 -med- iated DSCs were insensitive to Pt loading as low as 0.001 mg cm -2 , while the Co(bpy)3?/3? -mediated DSCs required relatively large Pt loadings of ( 0.005 mg cm -2 . Our results indicated that care should be taken in the preparation of Pt CEs with high transparency and low loading to obtain high-performance DSCs employing cobalt-ligand redox electrolyte.

Shielding effects of additives in a cobalt(ii/iii) redox electrolyte: toward higher open-circuit photovoltages in dye-sensitized solar cells

We report the molecular interactions between cobalt tris(bipyridyl) [Co(bpy)3]2+/3+ and additives of 4-tert-butylpyridine (TBP) in a redox electrolyte and the effects on the performance of dye-sensitized solar cells (DSCs). The interactions and consequent effects led us to the concept of a TBP-shielded [Co(bpy)3]3+ complex, where the [Co(bpy)3]3+ ion was surrounded by TBP molecules via weak ion–dipole attractions. The TBP molecules were able to bi-functionally shield both [Co(bpy)3]3+ ions and the TiO2 surface from undesirable charge recombination, achieving a greater enhancement of open-circuit voltage (VOC) of about 200 mV than 140 mV in the iodide (I−/I3−) system where TBP passivated the TiO2 surface only. The VOC–JSC trade-off problem was thereby mitigated. Our discovery of the molecular interactions between additives and redox species would stimulate the engineering of new additives or redox mediators to improve the performance of DSCs.

The effect of dye coverage on the performance of dye-sensitized solar cells with a cobalt-based electrolyte

The effect of dye coverage of the mesoporous TiO2 electrode on the performance of dye-sensitized solar cells based on the cobalt tris(bipyridine) electrolyte and the D35 dye was studied in detail. The dye coverage was controlled by using a dye bath with different dye concentrations and containing an inert salt, LiClO4, which was found to promote equilibrium conditions in the dye adsorption process. The amount of adsorbed D35 dye on mesoporous TiO2 was reasonably fit using the Langmuir adsorption isotherm, with a binding constant of 55 000 M(-1). Upon increasing the dye coverage on the TiO2 electrode, the electron lifetime in the dye-sensitized solar cell increased remarkably, demonstrating the blocking behavior of the D35 dye at the TiO2-electrolyte interface. Consequently, the solar cell efficiency increased dramatically with the D35 dye coverage.

ASSESSMENT OF THE "MICRO-GREENNESS" OF SYNTHESES USING THE GREEN STAR

This article reports a study to increase the overall greenness of chemical syntheses for first-year university laboratories. The separate evaluation of the micro-greenness of the three stages of synthesis (reaction, isolation and purification) using the Green Star (GS) was implemented and their respective contribution to overall greenness was investigated for two examples: syntheses of cobalt (III) tris(acetylacetonate) and potassium nitrilosulfonate. Results showed that the post-reaction (work-up) steps are the most problematic for overall greenness. Greenness optimization can be achieved by combining the greenest procedures for each step obtained from different protocols available in the literature.

PEDOT counter electrodes for dye-sensitized solar cells prepared by aqueous micellar electrodeposition

Electropolymerization of 3,4-ethylenedioxythiophene (EDOT) was performed in an aqueous micellar solution onto conducting glass and conducting flexible plastic substrates using a simple, scalable process. The background electrolyte in the process consisted merely of a micellar aqueous sodium dodecyl sulfate (SDS) solution. Electrodeposition of poly(3,4-ethylenedioxythiophene) (PEDOT) was conducted at constant current, resulting in homogeneous films, even on large sized conducting glass and plastic substrates (9cm×9cm). The use of water as electrolyte, application on large substrates and applicability on flexible plastic substrates demonstrates the feasibility of this method for upscaling and use in industrial fabrication of DSCs. DSCs were assembled using three different PEDOT thicknesses on conducting glass as counter electrodes and a comparison was made with thermally platinized conducting glass counter electrodes. In cobalt tris(bipyridine)-based electrolyte, the catalytic performance of the PEDOT counter electrodes was significantly higher than that of platinized counter electrodes. DSCs with PEDOT counter electrodes gave higher efficiencies due to higher fill factors and a lower charge transfer resistance. The low charge transfer resistance and good catalytic performance of the PEDOT counter electrodes can be related to its mesoporous morphology resembling crumpled sheets of paper.

Nanostructured TiO2/CH3NH3PbI3 heterojunction solar cells employing spiro-OMeTAD/Co-complex as hole-transporting material

For using 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) as a hole conductor in solar cells, it is necessary to improve its charge-transport properties through electrochemical doping. With the aim of fabricating efficient mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cells, we used tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tris(bis(trifluoromethylsulfonyl) imide)] (FK209) as a p-dopant for spiro-OMeTAD. The mixture of spiro-OMeTAD, FK209, lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and 4-tert-butylpyridine (TBP) exhibited significantly higher performance than mixtures of pristine spiro-OMeTAD, spiro-OMeTAD, and FK209, and spiro-OMeTAD, Li-TFSI, and TBP. Such a synergistic effect between the Co-complex and Li-TFSI in conjunction with spiro-OMeTAD effectively improved the power conversion efficiency (PCE) of the fabricated solar cells. As a result, we achieved PCE of 10.4%, measured under standard solar conditions (AM 1.5G, 100 mW cm−2).

High Seebeck coefficient redox ionic liquid electrolytes for thermal energy harvesting

Manipulation of the cobalt(II/III) tris(bipyridyl) redox couple through anion exchange has improved its solubility in ionic liquids and 3-methoxypropionitrile (MPN). This has allowed the preparation of electrolytes with high Seebeck coefficients, Se = 1.5–2.2 mV K−1, and thereby excellent prospects for thermal harvesting. The unique physical properties of ionic liquids offer ideal characteristics for their use as electrolytes in thermoelectrochemical cells, particularly for applications involving thermal energy available at temperatures in the 100–200 °C range. The power generation characteristics of thermoelectrochemical cells using a series of ionic liquids and MPN with the CoII/III(bpy)3(NTf2)2/3 couple are described. Power densities reached >0.5 W m−2 in unoptimized devices, operating with a 130 °C hot side. The high Seebeck coefficient appears to have its origins in the high-to-low spin transition upon electron transfer in this cobalt complex.

Imidazolium functionalized cobalt tris(bipyridyl) complex redox shuttles for high efficiency ionic liquid electrolyte dye-sensitized solar cells

CoII/III redox couples and their derivatives have been extensively studied and applied as redox mediators for high efficiency organic solvent DSSCs. However, CoII/III complex redox mediator-based ionic liquid electrolyte DSSCs have not been studied so far due to the poor solubility of most CoII/III complexes in ionic liquid electrolytes. Herein, an imidazolium functionalized cobalt tris(bipyridyl) complex redox mediator ([Co((MeIm-Bpy)PF6)3]2+/3+) (((MeIm-Bpy)PF6)3 = 3,3′-(2,2′-bipyridine-4,4′-diyl-bis(methylene))bis(1-methyl-1H-imidazol-3-ium)hexafluorohosphate) with high redox potential and good solubility in ionic liquid electrolyte has been synthesized and applied in ionic liquid electrolyte DSSCs. Using the dye N719 as the photosensitizer, overall power conversion efficiencies of 7.37% and 8.29% are achieved in binary ionic liquid-based electrolyte under 1.5 solar spectrum illumination at 100 and 50 mW cm−2, respectively. Both values are considerably higher than those of the I−/I3− redox couple-based ionic liquid electrolyte.