Thermal conductance of single-molecule junctions.

Single-molecule junctions have been extensively used to probe properties as diverse as electrical conduction1-3, light emission4, thermoelectric energy conversion5,6, quantum interference7,8, heat dissipation9,10 and electronic noise11 at atomic and molecular scales. However, a key quantity of current interest-the thermal conductance of single-molecule junctions-has not yet been directly experimentally determined, owing to the challenge of detecting minute heat currents at the picowatt level. Here we show that picowatt-resolution scanning probes previously developed to study the thermal conductance of single-metal-atom junctions12, when used in conjunction with a time-averaging measurement scheme to increase the signal-to-noise ratio, also allow quantification of the much lower thermal conductance of single-molecule junctions. Our experiments on prototypical Au-alkanedithiol-Au junctions containing two to ten carbon atoms confirm that thermal conductance is to a first approximation independent of molecular length, consistent with detailed ab initio simulations. We anticipate that our approach will enable systematic exploration of thermal transport in many other one-dimensional systems, such as short molecules and polymer chains, for which computational predictions of thermal conductance13-16 have remained experimentally inaccessible.

Self-Healable and Stretchable Ionic-Liquid-Based Thermoelectric Composites with High Ionic Seebeck Coefficient.

The advancement of wearable electronics, particularly self-powered wearable electronic devices, necessitates the development of efficient energy conversion technologies with flexible mechanical properties. Recently, ionic thermoelectric (TE) materials have attracted great attention because of their enormous thermopower, which can operate capacitors or supercapacitors by harvesting low-grade heat. This study presents self-healable, stretchable, and flexible ionic TE composites comprising an ionic liquid (IL), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM:OTf); a polymer matrix, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP); and a fluoro-surfactant (FS). The self-healability of the IL-based composites originates from dynamic ion-dipole interactions between the IL, the PVDF-HFP, and the FS. The composites demonstrate excellent ionic TE properties with an ionic Seebeck coefficient (Si ) of ≈38.3 mV K-1 and an ionic figure of merit of ZTi  = 2.34 at 90% relative humidity, which are higher than the values reported for other IL-based TE materials. The IL-based ionic TE composites developed in this study can maintain excellent ionic TE properties under harsh conditions, including severe strain (75%) and multiple cutting-healing cycles.

Improved performance of dye-sensitized solar cells using dual-function TiO(2) nanowire photoelectrode.

A unique, hierarchically structured, aggregated TiO(2) nanowire (A-TiO(2)-nw) is prepared by solvothermal synthesis and used as a dual-functioning photoelectrode in dye-sensitized solar cells (DSSCs). The A-TiO(2)-nw shows improved light scattering compared to conventional TiO(2) nanoparticles (TiO(2)-np) and dramatically enhanced dye adsorption compared to conventional scattering particles (CSP). The A-TiO(2)-nw is used as a scattering layer for bilayer photoelectrodes (TiO(2)-np/A-TiO(2)-nw) in DSSCs to compare the cell performance to that of devices using state-of-the-art photoelectrode architectures (TiO(2)-np/CSP). The DSSCs fabricated using bilayers of TiO(2)-np/A-TiO(2)-nw show improved power conversion efficiency (9.1%) and current density (14.88 mA cm(-2)) compared to those using single-layer TiO(2)-np (7.6% and 11.84 mA cm(-2)) or TiO(2)-np/CSP bilayer structures (8.7% and 13.81 mA cm(-2)). The unique contribution of the A-TiO(2)-nw layers to the device performance is confirmed by studying the incident photon-to-current efficiency. The enhanced external quantum efficiencies at approximately 520 nm and 650 nm clearly reveal the dual functionality of A-TiO(2)-nw. These unique properties of A-TiO(2)-nw may be applied in other devices utilizing light-scattering n-type semiconductor.

FTO-free counter electrodes for dye-sensitized solar cells using carbon nanosheets synthesised from a polymeric carbon source.

Highly conductive carbon nanosheets (CNSs) are fabricated using a polymeric carbon source and subsequently applied as the counter electrodes (CNS-CEs) for dye-sensitized solar cells (DSSCs). The CNSs have a similar structure to multilayered graphene, and their high electrical conductivity and electrocatalytic activity enable them to have a dual-function as both CEs and charge supporting electrodes. CNSs form a unique CE material that functions successfully while being metal- and fluorine doped tin oxide (FTO)-free and allowing DSSCs to achieve ∼5% power conversion efficiency. The chemical structure, electrical properties, electrocatalytic activity, and work function of the CNS-CEs prepared under various conditions of carbonization are investigated, and their effects on the performance of the corresponding DSSCs are discussed. Carbonization temperature is shown to have influenced the size of graphitic domains and the presence of heteroatoms and functional groups in CNS-CEs. The change in the graphitic domain size has a marginal influence on the work function of the CNS-CEs and the overpotential for the reduction of the redox couples (I(-)/I3(-)). However, the electrical conductivity of CNS-CEs and the charge transfer resistance at CE/electrolyte interfaces in the DSSCs are considerably influenced by the carbonization condition. Our study shows that CNSs serve as efficient, FTO-free CE materials for DSSCs, and they are appropriate materials with which the effects of the chemical/physical properties of graphene-based materials on the electrode performance of various electrochemical devices may be studied.

Self-healable polymer complex with a giant ionic thermoelectric effect.

In this study, we develop a stretchable/self-healable polymer, PEDOT:PAAMPSA:PA, with remarkably high ionic thermoelectric (iTE) properties: an ionic figure-of-merit of 12.3 at 70% relative humidity (RH). The iTE properties of PEDOT:PAAMPSA:PA are optimized by controlling the ion carrier concentration, ion diffusion coefficient, and Eastman entropy, and high stretchability and self-healing ability are achieved based on the dynamic interactions between the components. Moreover, the iTE properties are retained under repeated mechanical stress (30 cycles of self-healing and 50 cycles of stretching). An ionic thermoelectric capacitor (ITEC) device using PEDOT:PAAMPSA:PA achieves a maximum power output and energy density of 4.59 µW‧m-2 and 1.95 mJ‧m-2, respectively, at a load resistance of 10 KΩ, and a 9-pair ITEC module produces a voltage output of 0.37 V‧K-1 with a maximum power output of 0.21 µW‧m-2 and energy density of 0.35 mJ‧m-2 at 80% RH, demonstrating the potential for a self-powering source.

Aqueous dispersible graphene/Pt nanohybrids by green chemistry: application as cathodes for dye-sensitized solar cells.

Aqueous dispersible nanohybrids (NHBs) of graphene nanosheets (GNSs) and Pt nanoparticles (Pt-NPs) were synthesized through the one-pot reduction of their precursors using an environmentally benign chemical, vitamin C. The concurrent reduction of the precursors, which includes graphene oxide (GO) to GNS and H2PtCl6 to Pt(0), was facile and efficient to yield GNS/Pt-NHBs in which face-centered cubic (fcc) crystalline Pt-NPs with average diameters of ~5 nm were robustly attached on the surface of the GNSs. The conversion yield during Pt reduction was fairly high (∼90%) and the Pt content within the NHBs was easily controllable. The resulting stable aqueous colloidal dispersion of GNS/Pt-NHBs was successfully fabricated as thin films without using any binder by the electro-spray method at room temperature, and the fabricated samples were used as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The electrocatalytic activity of the NHBs for I(-)/I3(-) redox couples in conventional DSSCs was investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis. Doping of GNSs with small amounts of Pt-NPs (<10 wt %) could dramatically enhance the redox kinetics. The enhanced electrocatalytic activity of the GNS/Pt-NHBs was reflected in the performance of the DSSCs. The power conversion efficiency of optimized DSSCs using the NHB-CEs was 8.91% (VOC: 830 mV, JSC: 15.56 mAcm(-2), and FF: 69%), which is comparable to that of devices using the state-of-the-art Pt-based CEs (8.85%).