Enhanced Cell Performance Research Articles

Rutile TiO2 nanowire array infiltrated with anatase nanoparticles as photoanode for dye-sensitized solar cells: enhanced cell performance via the rutile–anatase heterojunction

The structural and interfacial design of TiO2 photoanodes plays an important role in improving the solar energy conversion performance of dye-sensitized solar cells (DSSCs). Herein, we report that a rutile nanowire (NW) array infiltrated with anatase nanoparticles (NPs) can combine the advantages of the one dimensional electron transportation and light scattering of a NW array and the large dye-loading capacity of NPs, due to the presence of a rutile–anatase heterojunction. The dye-sensitized NW–NP composite film (1.4 μm thick) with a roughness factor of ∼114.7 displays a significantly improved light harvesting ability than a NW array (roughness factor ∼28.2), as manifested by diffuse transmittance and reflection spectra, and even higher light harvesting than a NP film (1.5 μm thick) with a roughness of ∼414.7. Moreover, the dye-sensitized NW–NP composite film shows slower charge recombination kinetics than both the NW array and NP film, as measured by open-circuit photovoltage decay and transient absorption spectroscopy. As a result, the dye-sensitized TiO2 NW–NP composite photoanode exhibits 2.2 times and 1.5 times higher overall efficiency than NW array and NP film photoanodes, respectively, under AM 1.5G simulated solar irradiation, demonstrating the synergistic effect of rutile NW and anatase NP for photoelectrochemical solar energy conversion.

An amphiphilic-like fluoroalkyl modified SiO2 nanoparticle@Nafion proton exchange membrane with excellent fuel cell performance

A new route to enhance the cell performance of a Nafion proton exchange membrane is provided by incorporating fluoroalkyl modified SiO2 nanoparticles. In favor of the achieved amphiphilic-like surface characteristics of SiO2 nanoparticles, the so-formed nanocomposite membrane exhibited great enhancement of single cell performance at 80 °C: ~34% increase in output power relative to the Nafion reference and a superior maximum output power as high as 579.6 mW cm(-2).

Amyloid-like peptide nanofiber templated titania nanostructures as dye sensitized solar cell anodic materials

One-dimensional titania nanostructures can serve as a support for light absorbing molecules and result in an improvement in the short circuit current (Jsc) and open circuit voltage (Voc) as a nanostructured and high-surface-area material in dye-sensitized solar cells. Here, self-assembled amyloid-like peptide nanofibers were exploited as an organic template for the growth of one-dimensional titania nanostructures. Nanostructured titania layers were utilized as anodic materials in dye sensitized solar cells (DSSCs). The photovoltaic performance of the DSSC devices was assessed and an enhancement in the overall cell performance compared to unstructured titania was observed.

Hydrothermally synthesized titania nanotubes as a promising electron transport medium in dye sensitized solar cells exhibiting a record efficiency of 7.6% for 1-D based devices

Hydrothermally synthesized TiO2 nanotubes (TNTs) with a diameter of approximately 10 nm and a length of 250 nm are successfully employed in dye-sensitized solar cells (DSSCs) based on N719 dye and iodide/triiodide electrolyte and exhibiting an efficiency of 7.6% at 1 sun illumination. Randomly oriented TiO2 nanotubes are deposited on FTO glass by the electrophoretic deposition method, and the thickness of the TNT layer and hence the solar cell performance have been shown to depend on the deposition time and the nanotube concentration in the electrolyte solution. The highest efficiency is obtained for the solar cell fabricated with the 6-minute electrophoretically deposited TiO2 film having a film thickness of ∼6 μm. These pristine TNT photoelectrodes exhibit a short-circuit current density (Jsc), an open-circuit voltage (Voc), a fill factor (FF) and an efficiency (η) of 2.4 mA cm−2, 899 mV, 78% and 1.7%, respectively. TiCl4 treatment of pristine TNT photoelectrodes enhances the Jsc, Voc, FF and η to 13.2 mA cm−2, 819 mV, 70.4% and 7.6%, respectively. The TiCl4 treatment is found to be vital for the enhancement of the solar cell performance of hydrothermally synthesised TiO2 nanotube based devices. Enhancement of electron lifetime is noted after treatment of bare TiO2 nanotubes with 0.5 M TiCl4 solution. The electron transport resistance, electron lifetime and charge recombination properties of bare and TiCl4 treated TiO2 nanotubes are investigated by electrochemical impedance measurements. The increased performance of the DSSC fabricated with TiCl4 treated TNTs is due to several factors favouring the enhancement of efficiency. These include the dye loading amount, the decrease in electron transport resistance, the increase in density of states and enhancement of light scattering.

Thickness-tunable polyimide nanoencapsulating layers and their influence on cell performance/thermal stability of high-voltage LiCoO2 cathode materials for lithium-ion batteries

We have previously demonstrated polyimide (PI) gel polymer electrolyte (GPE)-based nanoencapsulation as a new surface modification strategy for high-voltage cathode materials. In this study, in an endeavor to attain a more comprehensive understanding of the PI GPE-based surface modification, effects of structural variation of PI encapsulating layers (specifically, focusing on PI coating thickness) on cell performance and thermal stability of high-voltage (4.4 V) LiCoO2 are investigated. Herein, PI coating thickness is tuned between approximately 10 and 40 nm by varying polyamic acid (synthesized from pyromellitic dianhydride/oxydianiline) concentration of coating solutions. As PI coating thickness is increased, discharge C-rate capability of cells is deteriorated due to undesired rise of ionic and electronic resistance of thick PI coating layers. On the other hand, thick PI encapsulating layers are effective in mitigating interfacial exothermic reaction between delithiated LiCoO2 and liquid electrolyte. Notably, among the various PI coating thicknesses, average thickness of 10 nm imparts well-balanced enhancement in cell performance and thermal stability. These results demonstrate that structural fine-tuning (particularly, coating thickness) of PI encapsulating layers, acting as ion-conductive protective conformal thin films, plays a significant role in optimizing their beneficial coating effects on high voltage LiCoO2.

Enhancing Charge Transfer Kinetics by Nanoscale Catalytic Cermet Interlayer

Enhancing the density of catalytic sites is crucial for improving the performance of energy conversion devices. This work demonstrates the kinetic role of 2 nm thin YSZ/Pt cermet layers on enhancing the oxygen reduction kinetics for low temperature solid oxide fuel cells. Cermet layers were deposited between the porous Pt cathode and the dense YSZ electrolyte wafer using atomic layer deposition (ALD). Not only the catalytic role of the cermet layer itself but the mixing effect in the cermet was explored. For cells with unmixed and fully mixed cermet interlayers, the maximum power density was enhanced by a factor of 1.5 and 1.8 at 400 °C, and by 2.3 and 2.7 at 450 °C, respectively, when compared to control cells with no cermet interlayer. The observed enhancement in cell performance is believed to be due to the increased triple phase boundary (TPB) density in the cermet interlayer. We also believe that the sustained kinetics for the fully mixed cermet layer sample stems from better thermal stability of Pt islands separated by the ALD YSZ matrix, which helped to maintain the high-density TPBs even at elevated temperature.

High-efficiency dye-sensitized solar cells using ferrocene-based electrolytes and natural photosensitizers

A new and promising dye-sensitized solar cell (DSSC) bilayer design was developed using an Fe2+/Fe3+ (ferrocene) liquid electrolyte and natural dyes extracted from Hypericum perforatum, Rubia tinctorum L. and Reseda luteola. The photovoltaic parameters controlling the device performance were then investigated. A DSSC based on quercetin dye displayed the most efficient solar to electricity conversion efficiency compared with other dyes with a maximum η value of 2.17%. Maximum overall conversion efficiencies under simulated sunlight that was comparable to natural photosynthesis were increased by 15%. The identification of appropriate additives for improving VOC without causing dye degradation may result in further enhancement of cell performance, making the practical application of such systems more suitable for achieving economically viable solar energy devices.

Parallel Pool Analysis of Transient Spectroscopy Reveals Origins of and Perspectives for ZnO Hybrid Solar Cell Performance Enhancement Using Semiconducting Surfactants.

Recently, the performance of ZnO nanocrystals as an electron acceptor in a solar cell device was significantly increased by a semiconducting surfactant. Here we show, using transient absorption spectroscopy and a parallel pool analysis, that changes in the quantum efficiency of charge generation account for the performance variation among semiconducting-surfactant-coated, surfactant-coated, and uncoated ZnO nanoparticles. We demonstrate that even better surfactant design to suppress fast recombination could still lead to a further doubling of device efficiency.

Improved molten carbonate fuel cell performance via reinforced thin anode

Abstract The effects of anode thickness on electrochemical performance and cell voltage stability of molten carbonate fuel cell (MCFC) were examined using single cell test. It was found that supported thin nickel-aluminum (Ni–Al) anode with small pore size enhanced cell performance by reducing its mass transfer resistance and crossover. The stability of cell voltage was also observed. This was achieved after 0.25 mm thick anode was reinforced with Ni 60 mesh. Unsupported 0.3 mm thick anode yielded poor performance due to deformation and cracks after a long thermal exposure. The performance was improved significantly after all the anodes were reinforced with Ni mesh.

Design and fabrication of stair-step-type electrolyte structure for solid oxide fuel cells

The design and the fabrication of novel stair-step electrolyte based on yttria stabilized zirconia are presented. The novel electrolyte has gradually reduced oxide ion transport paths achieved by the stair-step design. The mechanical and electrochemical performance of the novel electrolyte are investigated and compared to those of standard electrolyte support. Three-point bending tests indicate that the fracture displacement and force measured for the novel electrolyte are 11% and 32% less than those of the standard electrolyte support, respectively. However, the cell based on the novel electrolyte exhibits 40% higher electrochemical performance than the standard electrolyte supported cell at an operation temperature of 700 °C. Impedance analyses revealed that the enhanced cell performance is mainly due to the decrease in the ohmic resistance of the cell achieved by the novel electrolyte design. In addition, the electrode resistances are found to be decreased due to the increased electrochemical reaction zones since the contact area between the novel electrolyte and both electrodes are increased by the novel electrolyte design. Moreover, the cell with novel electrolyte produced 0.47 Wcm−2 peak power at 750 °C while the standard electrolyte supported cell shows almost the same power output at around 800 °C. Thus, novel designed electrolyte also offers some amount of reduction in the operation temperature of solid oxide fuel cells. Copyright © 2012 John Wiley & Sons, Ltd.

Superior Hybrid Cathode Material Containing Lithium-Excess Layered Material and Graphene for Lithium-Ion Batteries

Graphene-wrapped lithium-excess layered hybrid materials (Li(2)MnO(3)·LiMO(2), M = Mn, Ni, Co, hereafter abbreviated as LMNCO) have been synthesized and investigated as cathode materials for lithium-ion batteries. Cyclic voltammetry measurement shows a significant reduction of the reaction overpotential in benefit of the graphene conducting framework. The electrochemical impedance spectroscopy results reveal that the graphene can greatly reduce the cell resistance, especially the charge transfer resistance. Our investigation demonstrates that the graphene conducting framework can efficiently alleviate the polarization of pristine LMNCO material leading to an outstanding enhancement in cell performance and cycling stability. The superior electrochemical properties support the fine hybrid structure design by enwrapping active materials in graphene nanosheets for high-capacity and high-rate cathode materials.

Enhanced transport properties in polymer electrolyte composite membranes with graphene oxide sheets

Polymer electrolyte membranes have been widely investigated for high performance fuel cells. Here, we report the synthesis of ionic conductive Nafion/graphene oxide (GO) composite membranes for application in direct methanol fuel cells. GOs interact with both the non-polar backbone and the polar ionic clusters of Nafion because of their amphiphilic characteristics attributable to hydrophobic conjugation and hydrophilic functional groups. Accordingly, GO sheets serve to modify the microstructures of two domains of Nafion. In particular, the transport properties of Nafion are favorably manipulated by the incorporation of GO. This modulated the ionic channels of Nafion and decrease methanol crossover while preserving ionic conductivity. Furthermore, strong interfacial interactions due to the insertion of GO nanofillers into the Nafion matrix improve the thermal and mechanical properties of the material. In particular, we exploit Nafion/GO composite membrane as electrolyte material for direct methanol fuel cell (DMFC) in order to resolve current issue of methanol crossover. This composite membrane-based DMFC compared to the Nafion 112-based DMFC remarkably enhanced cell performance, especially in severe operating conditions.

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach. The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles. To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 μm thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm(2), which can be increased up to 17-18 mA/cm(2) (~25% higher) after application of a simple diffuse back reflector made of a white paint. To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23°C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF(2;), and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF(2;), and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm(2), up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.

High throughput generated micro-aggregates of chondrocytes stimulate cartilage formation in vitro and in vivo

Cell-based cartilage repair strategies such as matrix-induced autologous chondrocyte implantation (MACI) could be improved by enhancing cell performance. We hypothesised that micro-aggregates of chondrocytes generated in high-throughput prior to implantation in a defect could stimulate cartilaginous matrix deposition and remodelling. To address this issue, we designed a micro-mould to enable controlled high-throughput formation of micro-aggregates. Morphology, stability, gene expression profiles and chondrogenic potential of micro-aggregates of human and bovine chondrocytes were evaluated and compared to single-cells cultured in micro-wells and in 3D after encapsulation in Dextran-Tyramine (Dex-TA) hydrogels in vitro and in vivo. We successfully formed micro-aggregates of human and bovine chondrocytes with highly controlled size, stability and viability within 24 hours. Micro-aggregates of 100 cells presented a superior balance in Collagen type I and Collagen type II gene expression over single cells and micro-aggregates of 50 and 200 cells. Matrix metalloproteinases 1, 9 and 13 mRNA levels were decreased in micro-aggregates compared to single-cells. Histological and biochemical analysis demonstrated enhanced matrix deposition in constructs seeded with micro-aggregates cultured in vitro and in vivo, compared to single-cell seeded constructs. Whole genome microarray analysis and single gene expression profiles using human chondrocytes confirmed increased expression of cartilage-related genes when chondrocytes were cultured in micro-aggregates. In conclusion, we succeeded in controlled high-throughput formation of micro-aggregates of chondrocytes. Compared to single cell-seeded constructs, seeding of constructs with micro-aggregates greatly improved neo-cartilage formation. Therefore, micro-aggregation prior to chondrocyte implantation in current MACI procedures, may effectively accelerate hyaline cartilage formation.

Enhancement of hybrid solar cell performance by polythieno [3,4-b]thiophenebenzodithiophene and microplasma-induced surface engineering of silicon nanocrystals

We have developed a room temperature fabrication technique for hybrid bulk heterojunction solar cells based on silicon nanocrystals (Si-ncs) embedded in polythieno[3,4-b]thiophenebenzodithiophene (PTB7) and that exhibit a high open-circuit voltage exceeding 1 V. This type of device outperforms the open-circuit voltage of analogous devices based on Si-ncs within poly(hexylthiophene) (P3HT). We also demonstrate that three dimensional surface engineering of Si-ncs by microplasma processing in ethanol can be used to enhance the electronic interactions with PTB7, without using any surfactant, and increasing the power conversion efficiency.

Study of a Renewable Biomass Powered SOFC

The objective of this work is to explore a solution to utilize biomass carbon by a tubular solid oxide fuel cell integrated with an "in-situ" steam gasification reactor. Non-catalyzed and catalyzed coconut shell carbons were employed as solid fuels, which were directly converted to gaseous fuels to feed the SOFC operated at 800°. Single cells showed variable electrochemical performances according to the catalytic additives. Sample without catalysts exhibited low power density, while those with catalysts such as Fe2O3 and K2CO3 showed higher performances. The enhanced cell performance with catalysts was considered to be the result of increased conversation rate of carbon and consequently high fuel concentration supplied to SOFC.

Solid Oxide Fuel Cell Stack Module Development for Fuel Flexible Power Generation

FuelCell Energy is developing solid oxide fuel cell (SOFC) systems and technology for very efficient, economically viable, coal-to-electricity power plants utilizing synthesis gas (syngas) from a coal gasifier for the Department of Energy Solid State Energy Conversion Alliance (SECA) program. SOFC technology development includes cell performance enhancement, stack building block scale-up, stack tower design and testing, and development of a 250 kW module accommodating multiple stack towers.

Thin catalyst layer deposited on proton-exchange membrane by self-breathing method

Catalyst layers at membrane–electrode interfaces play an important role in the performance of proton-exchange membrane fuel cells. Increasing the effective interface area and reducing the interface charge transfer resistance are key factors in enhancing cell performance. This study reports on a self-breathing process that has been developed to coat a thin catalyst layer on a membrane surface prior to application of Pt/C slurry. To further improve the breathing efficiency, oxygen plasma treatment was applied. The morphology and surface bonding of the processed membranes were characterized by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The interface charge transfer resistances of the membrane electrode assemblies were measured by electrochemical impedance spectroscopy. The results show that the charge transfer resistance of the cell with the self-breathing membrane is one order lower than that of a pristine cell.

Efficient Phthalimide Copolymer‐Based Bulk Heterojunction Solar Cells: How the Processing Additive Influences Nanoscale Morphology and Photovoltaic Properties

AbstractThe power conversion efficiency of poly(N‐(2‐ethylhexyl)‐3,6‐bis(4‐dodecyloxythiophen‐2‐yl)phthalimide) (PhBTEH)/fullerene bulk heterojunction solar cells improves from 0.43 to 4.1% by using a processing additive. The underlying mechanism for the almost 10‐fold enhancement in solar cell performance is found to be inhibition of fullerene intercalation into the polymer side chains and regulation of the relative crystallization/aggregation rates of the polymer and fullerene. An optimal interconnected two‐phase morphology with 15–20 nm domains is obtained when a processing additive is used compared with 100–300 nm domains without the additive. The results demonstrate that a processing additive provides an effective means of controlling both the fullerene intercalation in polymer/fullerene blends and the domain sizes of their phase‐separated nanoscale morphology.

Nanocomposites of TiO2 and double-walled carbon nanotubes for improved dye-sensitized solar cells

It is demonstrated that an incorporation of double-walled carbon nanotubes (DWCNTs) into a TiO2 photo-anode layer results in a significant improvement in the overall energy conversion performance in the dye-sensitized solar cell (DSSC). Comparing to the standard TiO2 anode, the carbon nanotube-containing TiO2 anode with 0.2 wt. % DWCNTs has boosted up the photocurrent density (Jsc) by 43%. The DSSC power conversion efficiency was also improved from ∼3.9% in the case of carbon nanotube-free TiO2 anode to as high as 6.4% with the addition of DWCNTs upon optimized anode annealing. The observed enhancement in the solar cell performance in the presence of the carbon nanotubes is attributed primarily to the noticeable reduction in microcracking and associated robust electrical conduction. Some contribution of the electrical conducting nature of the filler material (DWCNTs) to the improved DSSC properties may be possible; however, it is viewed as a minor effect, considering the small amount of the nanotubes used.