A multi-chromic supercapacitor of high coloration efficiency integrating a MOF-derived V2O5 electrode.

Modern technological trends in smart electronic devices demand more intelligent automation. Simultaneous integration of energy storage and multicolor electrochromism in a single device improves user-device interfacing based on a salient human-readable output. In this work, primarily metal-organic framework (MOF) derived V2O5 was synthesized which, as an electrochromic material, shows high optical modulation of 35% at 485 nm, with very fast switching speeds (2.9/3.4 s for coloring/bleaching). The multiple coloration states of the V2O5 electrode make it worthy for further integration as a smart negative electrode in a multicolored electrochromic asymmetric supercapacitor, where the electrochromic polyaniline electrode serves as the counter electrode. The device demonstrates a high coloration efficiency of 137.2 cm2 C-1 and an areal capacitance of 12.27 mF cm-2 and an energy density of 2.21 × 10-3 mW h cm-2 at a current density of 0.05 mA cm-2. By virtue of its different chromatic states during charging and discharging, smart visual tracking of the state of charge of the supercapacitor can be realized. Such a design of energy storage devices will have promising practical application in futuristic smart multifunctional electronic devices.

Temperature tunable flexible photo absorbers based on near-infrared 1D photonic crystal hybridized W-doped VO2nanostructures.

Vanadium dioxide is a potential candidate for energy efficient smart windows and have crystalline phase transition temperature (Tc) at 68 °C. So far, literatures mainly emphasis on different synthetic strategies of tungsten doped VO2which is a most effective dopant to reduceTcof VO2to near room temperatures. Until now, there is no report shows the incorporation of flexible 1D photonic crystals as spectrally selective, temperature tunable device to control the changes in optical transmission modulations of W-VO2nanostrtcures, especially in the near IR region for smart window application. W-doped VO2with various tungsten contents were synthesized with a facile hydrothermal route. We found that, with 1.1 at% of tungsten doping in intrinsic VO2, the metal to insulator transition temperature is brought down to 37 °C from 68 °C. IR transmission of VO2thin film can be reduced from 70% to 40% around room temperature, after doping. Significant absorption enhancement has been observed for both VO2and W-doped VO2films, deposited over tunable SiO2/Ta2O5based distributed Bragg reflector (DBR) fabricated over flexible PET (poly-ethylene terephthalate) substrates. On depositing VO2over ∼70% reflecting DBR, optical transmission is reduced to ∼15% from 35% while the temperature varies to 380 K from 300 K in IR regime. Number of stacks plays a crucial role for effective IR extinctions. A high quality DBR is fabricated by increasing no. of stacks from 4 to 7, with optical transmission of DBR reduced to nearly 5% in stop band. However, with 1.1 at% of W-VO2over such 95% reflecting flexible DBR, optical transmission vanishes nearly, around room temperature itself in the stop bands of that DBR, which clearly indicates the significant absorption enhancement. W-VO2/DBR hybrid can substantially modulate the solar heat flux and also imbuing DBR over flexible PET substrates offers retrofitting of the existing windows for energy economy. Thus these structures have promising potential applications for optical devices and practical design for smart windows.

Complimentary effects of annealing temperature on optimal tuning of functionalized carbon-V2O5 hybrid nanobelts for targeted dual applications in electrochromic and supercapacitor devices.

Herein, carbon nanosphere-decorated vanadium pentoxide (C@V2O5) hybrid nanobelts were grown via a single step hydrothermal route with improved electronic conductivity as compared to that of pristine oxide. This hybrid nanomaterial exhibits different complimentary ranges of optimum post-growth annealing temperatures, which are suitable for dual applications either in electro-chromic smart windows or in supercapacitors. C@V2O5 nanobelts annealed at 350 °C appear to favor electro-chromic applications. They exhibit maximum dynamic optical transmission modulation as they switch from yellow to dark green, fast switching response, and high visible transmittance. In contrast, C@V2O5nanobelts annealed at 250 °C have been found to be most suitable for supercapacitor applications. They display a high specific capacity and an enhanced diffusion coefficient. Moreover, they exhibit long lifetimes with a capacity retention of ∼94% even after 5000 cycles of operation. Therefore, the obtained results clearly indicate that optimization of the post-growth annealing temperatures is very important and rather complementary in nature in terms of determining the most favorable device functionalities. It enables us to optimally tune these hybrid nanomaterials for targeted, device-specific, energy applications in either electrochromic or supercapacitor technologies simply based on the annealing temperature alone.

Turning on the Protonation-First Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes.

Electrocatalytic reduction of CO2 to CO is reported for the complex, {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), containing four pendant methoxy groups, where [(MeO)2Ph]2bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6'-dimesityl-2,2'-bipyridine ligand in [fac-MnI(mes2bpy)(CO)3(CH3CN)](OTf), which prevents Mn0-Mn0 dimerization, the [(MeO)2Ph]2bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO2 to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)2Ph]2bpy ligand framework on electrocatalytic CO2 reduction and its dependence upon the concentration and pKa of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.

Ambiguous electrocatalytic CO2 reduction behaviour of a nickel bis(aldimino)pyridine pincer complex.

The electrochemical properties of two Ni(NNN)X2 pincer complexes are reported where X = Cl or Br and NNN is N,N'-(2,6-diisopropylphenyl)bis-aldiminopyridine. Cyclic voltammetry under 1 atm of CO2 suggests electrocatalytic CO2 reduction activity, however, bulk electrolysis shows a poor Faradaic efficiency for CO evolution with a high Faradaic yield for H2 evolution.

Efficient plasmonic dye-sensitized solar cells with fluorescent Au-encapsulated C-dots.

A simple strategy to improve the efficiency of a ZnO-nanorod-based dye-sensitized solar cell (DSSC) by use of Au-encapsulated carbon dots (Au@C-dots) in the photoanode is presented. The localized surface plasmonic resonance of Au in the 500-550 nm range coupled with the ability of C-dots to undergo charge separation increase the energy-harvesting efficiency of the DSSC with ZnO/N719/Au@C-dots photoanodes. Charge transfer from N719 dye to Au@C-dots is confirmed by fluorescence and lifetime enhancements of Au@C-dots. Forster resonance energy transfer (FRET) from the gap states of ZnO nanorods to N719 dye is also ratified and the energy transfer rate is 4.4×10(8) s(-1) and the Forster radius is 1.89 nm. The overall power conversion efficiency of the plasmonic and FRET-enabled DSSC with ZnO/N719/Au@C-dots as the photoanode, I2/I(-) as the electrolyte and multiwalled carbon nanotubes as the counter electrode is 4.1%, greater by 29% compared to a traditional ZnO/N719 cell.

Energy relay from an unconventional yellow dye to CdS/CdSe quantum dots for enhanced solar cell performance.

A new design for a quasi-solid-state Forster resonance energy transfer (FRET) enabled solar cell with unattached Lucifer yellow (LY) dye molecules as donors and CdS/CdSe quantum dots (QDs) tethered to titania (TiO2 ) as acceptors is presented. The Forster radius is experimentally determined to be 5.29 nm. Sequential energy transfer from the LY dye to the QDs and electron transfer from the QDs to TiO2 is followed by fluorescence quenching and electron lifetime studies. Cells with a donor-acceptor architecture (TiO2 /CdS/CdSe/ZnS-LY/S(2-)-multi-walled carbon nanotubes) show a maximum incident photon-to-current conversion efficiency of 53 % at 530 nm. This is the highest efficiency among Ru-dye free FRET-enabled quantum dot solar cells (QDSCs), and is much higher than the donor or acceptor-only cells. The FRET-enhanced solar cell performance over the majority of the visible spectrum paves the way to harnessing the untapped potential of the LY dye as an energy relay fluorophore for the entire gamut of dye sensitized, organic, or hybrid solar cells.

Photodegradable iron(III) cross-linked alginate gels.

Biocompatible photoresponsive materials are of interest for targeted drug delivery, tissue engineering, 2D and 3D protein patterning, and other biomedical applications. We prepared light degradable hydrogels using a natural alginate polysaccharide cross-linked with iron(III) cations. The "hard" iron(III) cations used to cross-link the alginate hydrogel were found to undergo facile photoreduction to "soft" iron(II) cations in the presence of millimolar concentrations of sodium lactate. The "soft" iron(II) cations have a decreased ability to cross-link the alginate which results in dissolution of the hydrogel and the formation of a homogeneous solution. The photodegradation is done using long wave UV or visible light at neutral pH. The very mild conditions required for the photodegradation and the high rate at which it occurs suggest applications for iron(III) cross-linked alginate hydrogels as light-controlled biocompatible scaffolds.

Nanoscale connectivity in a TiO2/CdSe quantum dots/functionalized graphene oxide nanosheets/Au nanoparticles composite for enhanced photoelectrochemical solar cell performance.

Electron transfer dynamics in a photoactive coating made of CdSe quantum dots (QDs) and Au nanoparticles (NPs) tethered to a framework of ionic liquid functionalized graphene oxide (FGO) nanosheets and mesoporous titania (TiO(2)) was studied. High resolution transmission electron microscopy analyses on TiO(2)/CdSe/FGO/Au not only revealed the linker mediated binding of CdSe QDs with TiO(2) but also, surprisingly, revealed a nanoscale connectivity between CdSe QDs, Au NPs and TiO(2) with FGO nanosheets, achieved by a simple solution processing method. Time resolved fluorescence decay experiments coupled with the systematic quenching of CdSe emission by Au NPs or FGO nanosheets or by a combination of the latter two provide concrete evidences favoring the most likely pathway of ultrafast decay of excited CdSe in the composite to be a relay mechanism. A balance between energetics and kinetics of the system is realized by alignment of conduction band edges, whereby, CdSe QDs inject photogenerated electrons into the conduction band of TiO(2), from where, electrons are promptly transferred to FGO nanosheets and then through Au NPs to the current collector. Conductive-atomic force microscopy also provided a direct correlation between the local nanostructure and the enhanced ability of composite to conduct electrons. Point contact I-V measurements and average photoconductivity results demonstrated the current distribution as well as the population of conducting domains to be uniform across the TiO(2)/CdSe/FGO/Au composite, thus validating the higher photocurrent generation. A six-fold enhancement in photocurrent and a 100 mV increment in photovoltage combined with an incident photon to current conversion efficiency of 27%, achieved in the composite, compared to the inferior performance of the TiO(2)/CdSe/Au composite imply that FGO nanosheets and Au NPs work in tandem to promote charge separation and furnish less impeded pathways for electron transfer and transport. Such a hierarchical rapid electron transfer model can be adapted to other nanostructures as well, as they can favorably impact photoelectrochemical performance.