Wallace Woon-Fong Leung

Application of a Bilayer TiO2 Nanofiber Photoanode for Optimization of Dye‐Sensitized Solar Cells

Figure 1 . Schematic diagram of fabrication procedure and SEM images of the bilayer TiO 2 nanofi bers photoanode. Since it was introduced in 1991, the lowcost, high-effi ciency, dye-sensitized solar cell (DSSC) has attracted intensive interest as a promising candidate for future green energy. [ 1 ] The respective record high power conversion effi ciency (PCE) of 11% has persisted for more than a decade, [ 2 ] and this needs to be further improved before the technology can be commercialized. The overall sunlight-to-electric-power conversion process of a DSSC can be summarized as the combination of photogeneration, charge-carriers transport, and collection. Considerable efforts have been made to increase the light harvesting, such as increasing the surface area of the semiconductor, [ 1 ] the development of new dyes and dyes mixture with strong and broad absorption spectra, [ 3 ] the introduction of photonic crystal, [ 4,5 ] or inclusion of a lightscattering layer [ 6–8 ] to trap more light in the device. To improve the charge-carrier transport capability of active layer, employment of functional nanostructured photoanodes such as 1D nanostructures (nanotubes, nanowires, and nanofi bers), nanotrees, and nanoforests [ 12–14 ] have been used in DSSC. Using single-walled carbon nanotube-TiO 2 as photoanode, Dang et al. [ 15 ] recently improved the electron collection and thus the power effi ciency up to 10.6%. While these methods improve the effi ciency of DSSC to a certain extent, these processes are complicated and costly because they involve surface modifi cations, synthesis of composite nanostructure, or combining different types of dyes. One promising method to improve the PCE of DSSCs is to use a 1D nanostructure photoanode with high surface area and excellent electron transport properties to harvest and collect effectively photogenerated electrons. Here, an innovative bilayer TiO 2 nanofi ber photoanode was investigated combining both smallerand larger-diameter TiO 2 nanofi bers that are readily fabricated in one step by simple, cost-effective, nozzleless

Electrospun TiO2 Nanorods with Carbon Nanotubes for Efficient Electron Collection in Dye-Sensitized Solar Cells

A high power conversion efficiency of 10.24% can be obtained in a dye-sensitized solar cell by incorporating multiwall carbon nanotubes inside a TiO2 nanorod photoanode. The multiwall carbon nanotubes in the nanorod can effectively collect and transport photogenerated electrons reducing the recombination as well as improving efficiency of the device.

Quantitative evaluation of motor functional recovery process in chronic stroke patients during robot-assisted wrist training

This study was to investigate the motor functional recovery process in chronic stroke during robot-assisted wrist training. Fifteen subjects with chronic upper extremity paresis after stroke attended a 20-session wrist tracking training using an interactive rehabilitation robot. Electromyographic (EMG) parameters, i.e., EMG activation levels of four muscles: biceps brachii (BIC), triceps brachii (TRI, lateral head), flexor carpiradialis (FCR), and extensor carpiradialis (ECR) and their co-contraction indexes (CI) were used to monitor the neuromuscular changes during the training course. The EMG activation levels of the FCR (11.1% of decrease from the initial), BIC (17.1% of decrease from the initial), and ECR (29.4% of decrease from the initial) muscles decreased significantly during the training (P<0.05). Such decrease was associated with decreased Modified Ashworth Scores for both the wrist and elbow joints (P<0.05). Significant decrease (P<0.05) was also found in CIs of muscle pairs, BIC&TRI (21% of decrease from the initial), FCR&BIC (11.3% of decrease from the initial), ECR&BIC (49.3% of decrease from the initial). The decreased CIs related to the BIC muscle were mainly caused by the reduction in the BIC EMG activation level, suggesting a better isolation of the wrist movements from the elbow motions. The decreased CI of ECR& FCR in the later training sessions (P<0.05) was due to the reduced co-contraction phase of the antagonist muscle pair in the tracking tasks. Significant improvements (P<0.05) were also found in motor outcomes related to the shoulder/elbow and wrist/hand scores assessed by the Fugl-Meyer assessment before and after the training. According to the evolution of the EMG parameters along the training course, further motor improvements could be obtained by providing more training sessions, since the decreases of the EMG parameters did not reach a steady state before the end of the training. The results in this study provided an objective and quantitative EMG measure to describe the motor recovery process during poststroke robot-assisted wrist for the further understanding on the neuromuscular mechanism associated with the recovery.

Thermal Assisted Oxygen Annealing for High Efficiency Planar CH3NH3PbI3 Perovskite Solar Cells

We report investigations on the influences of post-deposition treatments on the performance of solution-processed methylammonium lead triiodide (CH3NH3PbI3)-based planar solar cells. The prepared films were stored in pure N2 at room temperature or annealed in pure O2 at room temperature, 45°C, 65°C and 85°C for 12 hours prior to the deposition of the metal electrodes. It is found that annealing in O2 leads to substantial increase in the power conversion efficiencies (PCEs) of the devices. Furthermore, strong dependence on the annealing temperature for the PCEs of the devices suggests that a thermally activated process may underlie the observed phenomenon. It is believed that the annealing process may facilitate the diffusion of O2 into the spiro-MeOTAD for inducing p-doping of the hole transport material. Furthermore, the process can result in lowering the localized state density at the grain boundaries as well as the bulk of perovskite. Utilizing thermal assisted O2 annealing, high efficiency devices with good reproducibility were attained. A PCE of 15.4% with an open circuit voltage (VOC) 1.04 V, short circuit current density (JSC) 23 mA/cm2, and fill factor 0.64 had been achieved for our champion device.

Lead Iodide Thin Film Crystallization Control for High-Performance and Stable Solution-Processed Perovskite Solar Cells

PbI2 thin film crystallization control is a prerequisite of high-quality perovskite thin film for sequentially solution-processed perovskite solar cells. An efficient and simple method has been developed by adding HCl to improve perovskite thin film quality, and an efficiency of 15.2% is obtained. This approach improves coverage, uniformity, and stability of pervoskite thin film.

Efficiency enhancement by defect engineering in perovskite photovoltaic cells prepared using evaporated PbI2/CH3NH3I multilayers

We report, for the first time, on the synthesis of perovskite films by thermal annealing of evaporated lead(II) iodide (PbI2)/methylammonium iodide (CH3NH3I) multilayers. Detailed characterization of the resulting films is presented. Our work demonstrates that compact, high quality and uniform perovskite films can be grown using this technique. Optimization of the device structure was achieved by careful design of the layer thickness and the number of PbI2/CH3NH3I pairs used in the formation of the absorber layer. Utilizing additional annealing steps in a controlled atmosphere was shown to result in significant improvement in the device performance. Our experimental data indicate that O2 treatments may result in substantial reduction in the trap density of the device and thereby significant improvement in the lifetimes of the carriers. A high power conversion efficiency (PCE) of 12.5% was recorded for the champion device.

Experimental Investigation on Continuous Filtration of Sub-Micron Aerosol by Filter Composed of Dual-Layers Including a Nanofiber Layer

This study concerns filtration of sub-micron aerosol using dual nanofiber and microfiber layers. The first part compares the filtration performance of nanofiber (300 nm diameter fibers) and microfiber (1.8 μm diameter fibers) filters. Test aerosol is sodium chloride (NaCl) particulates ranging from 41 to 514 nm generated from atomization. Experimental results show that the fractional efficiency and quality factor of nanofiber filter is higher than microfiber filter, but microfiber filter has lower rate of increase in pressure drop under continuous loading. The second part aims to utilize the dust holding capacity of microfiber filter to relieve the rapid clogging and skin formation of nanofiber filter. In this work, three types of filters formed by stacking of microfiber and nanofiber layers are tested to investigate the loading characteristics of inhomogeneous dual-layer fibrous filter under the challenge of sub-micron NaCl aerosol. Experimental results show that the filter with microfiber layer upstream of nanofiber layer has a lower increase rate in pressure drop than the filter with constituent layers in reversed arrangement. The semi-empirical model of polydisperse aerosol loading, as modified from the version of monodisperse aerosol, predicts deposition profile across filter depth that explains the pressure drop results. Serving two purposes, the dual-layer filter with microfiber (upstream) and nanofiber (downstream) is able to utilize the strength of nanofibers in clean stage filtration and microfibers in loaded stage filtration. Moreover, a lower pressure drop increase implies that the filter does not require frequent cleaning by backpulses of compressed air, which might destroy the fragile nanofibers.

Stochastic finite difference lattice Boltzmann method for steady incompressible viscous flows

With the advent of state-of-the-art computers and their rapid availability, the time is ripe for the development of efficient uncertainty quantification (UQ) methods to reduce the complexity of numerical models used to simulate complicated systems with incomplete knowledge and data. The spectral stochastic finite element method (SSFEM) which is one of the widely used UQ methods, regards uncertainty as generating a new dimension and the solution as dependent on this dimension. A convergent expansion along the new dimension is then sought in terms of the polynomial chaos system, and the coefficients in this representation are determined through a Galerkin approach. This approach provides an accurate representation even when only a small number of terms are used in the spectral expansion; consequently, saving in computational resource can be realized compared to the Monte Carlo (MC) scheme. Recent development of a finite difference lattice Boltzmann method (FDLBM) that provides a convenient algorithm for setting the boundary condition allows the flow of Newtonian and non-Newtonian fluids, with and without external body forces to be simulated with ease. Also, the inherent compressibility effect in the conventional lattice Boltzmann method, which might produce significant errors in some incompressible flow simulations, is eliminated. As such, the FDLBM together with an efficient UQ method can be used to treat incompressible flows with built in uncertainty, such as blood flow in stenosed arteries. The objective of this paper is to develop a stochastic numerical solver for steady incompressible viscous flows by combining the FDLBM with a SSFEM. Validation against MC solutions of channel/Couette, driven cavity, and sudden expansion flows are carried out.

Flow and mixing in rotating zigzag microchannel

Abstract The flow and mixing in rotating zigzag microchannel was investigated experimentally and numerically with objective of improving mixing, which is largely due to recirculating crossflow in the cross-sectional plane of the channel and the bend connecting tilted channel segments. Unlike the conventional rotating radial channel, crossflow in the zigzag channel is highly intensified from a combination of: (a) centrifugal acceleration component in the cross-sectional plane due to the inclined channel segments, (b) centrifugal acceleration generating Gortler vortices at “channel bends”, and (c) Coriolis acceleration. When the channel segment in the zigzag channel is inclined towards rotation direction (prograde), all three accelerations are aligned intensifying the crossflow; however, when it is inclined opposite to rotation (retrograde), Coriolis acceleration competes with the other two accelerations producing complex flow. Unlike a stationary zigzag channel, flow in a rotating prograde bend with outlet in the direction of rotation further induces Coriolis acceleration which adds onto the centrifugal acceleration producing enhanced crossflow and mixing, vice versa for a retrograde bend. A numerical model has been developed accurately accounting for the interactions of throughflow, crossflow and material dispersion by diffusion and convection in a rotational platform. An experimental microfluidic platform with rotating zigzag microchannel has also been developed. Experimental results on mixing quality carried out at two rotation speeds compared well with prediction from the numerical model. The overall mixing quality of a rotating zigzag channel is much improved compared with that of a stationary zigzag channel and of a rotating radial channel, due to the intensified crossflow driven by the additional acceleration components. A study on different bend angle on mixing quality in zigzag channel revealed that there is no optimal bend angle to achieve superior mixing enhancement, as a result of the complex flow pattern generated by the three competing accelerations.

A Novel Ionic-Liquid Strain Sensor for Large-Strain Applications

A novel liquid strain sensor was developed by using ldquoroom-temperature ionic liquidrdquo as the piezoresistive gauge material. Polydimethylsiloxane, with microchannels, was used to form gauge structures, and carbon fibers were used as electrodes. The strain performance was examined by electrochemical impedance spectroscopy at room temperature, and curve fitting was applied for explaining the strain response. The results show that a maximum true strain of up to 55% can be measured with good repeatability. A parabolic relationship between the real part of the impedance (Zre) and the true strain (epsiv) is observed, mainly due to the resistance change of the electrolyte. The demonstrated ionic-liquid-based strain sensor is of low cost, is environmentally friendly, and is promising for a wide variety of applications.