Indirect charge transfer of holes via surface states in ZnO nanowires for photoelectrocatalytic applications

In this work, ZnO nanowires with high aspect ratio were obtained by fast and simple electrochemical anodization. Morphological, structural and photoelectrochemical characteristics of the synthesized ZnO nanowires were evaluated by using different techniques: field emission scanning electron microscopy, atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–VIS spectroscopy, Mott-Schottky analysis and photoelectrochemical impedance spectroscopy. The synthesized ZnO nanowires presented high roughness and high crystallinity. Besides, surface defects were identified in the sample. The value of the donor density (N_D) was in the order of 10¹⁹ cm^?3 in the dark and 10²⁰ cm^?3 under illumination. In addition, the ZnO nanowires presented good photosensibility, with a photocurrent density response 85 times higher than a ZnO compact layer, and lower resistance to charge transfer. The charge transfer processes taking place at the ZnO/electrolyte interface were studied, since these processes strongly influence the photoelectrocatalytic efficiency of the material. According to the results, the charge transfer of holes in the synthesized ZnO nanowires occurs indirectly via surface states. In this regard, surface states may be an important feature for photoelectrocatalytic applications since they could provide lower onset voltages and higher anodic current densities.     

Image-Based Lifelogging: User Emotion Perspective

Lifelog is a digital record of an individual’s daily life. It collects, records, and archives a large amount of unstructured data; therefore, techniques are required to organize and summarize those data for easy retrieval. Lifelogging has been utilized for diverse applications including healthcare, self-tracking, and entertainment, among others. With regard to the image-based lifelogging, even though most users prefer to present photos with facial expressions that allow us to infer their emotions, there have been few studies on lifelogging techniques that focus upon users’ emotions. In this paper, we develop a system that extracts users’ own photos from their smartphones and configures their lifelogs with a focus on their emotions. We design an emotion classifier based on convolutional neural networks (CNN) to predict the users’ emotions. To train the model, we create a new dataset by collecting facial images from the CelebFaces Attributes (CelebA) dataset and labeling their facial emotion expressions, and by integrating parts of the Radboud Faces Database (RaFD). Our dataset consists of 4,715 high-resolution images. We propose Representative Emotional Data Extraction Scheme (REDES) to select representative photos based on inferring users’ emotions from their facial expressions. In addition, we develop a system that allows users to easily configure diaries for a special day and summaize their lifelogs. Our experimental results show that our method is able to effectively incorporate emotions into lifelog, allowing an enriched experience.     

Heat and mass transfer analysis of combined convection in a horizontal rectangle

In this study, the heat and mass transfer of combined free and forced convection in the horizontal rectangle is explored. The governing equations together with the boundary conditions are solved numerically by using the finite volume method. The innovative idea in this study is to appropriately modify the Semi-Implicit Method for Pressure-Linked Equations algorithm and thereby, the numerical solutions of the flow variables such as the temperature and the concentration in addition to the components of velocity and the pressure are computed. The Richardson numbers (Ri) for distinct gases and liquids are calculated for different Rayleigh numbers at low (Re = 50) and high (Re = 5000) Reynolds numbers. The dimensionless parameters, such as the Reynolds number (Re), the Prandtl number (Pr), and the Schmidt number (Sc) are appropriately chosen to calculate the Richardson numbers. Consequently, combined free and forced convection effects are analyzed. Furthermore, the heat and mass transfer aspect for distinct gases and liquids is critically examined using empirical correlations. The accuracy and the validation of these results are ensured owing to the solutions obtained from correlations being advised in this study and those are existing in the literature.     

Investigation on hydrogen-fueled combustion characteristics and thermal performance in a micro heat-recirculation combustor inserted with block

Numerical investigation on the premixed H₂/air combustion in a micro heat-recirculation combustor inserted with/without block is conducted. Effects of block setting, heat-recirculation, and flow rate on combustion characteristics and thermal performance are depicted and analyzed. The results demonstrate that the block enhances the flame stability and preheating effect, which also reduces the heat loss via exhaust gas, while it shortens reactants residence time. The combustor setting with a transverse block gains a better thermal performance than that inserted with a longitudinal block. With the increase of transverse block height, the high-temperature zone is broadened and radiation is improved. However, the block with a height of 10 mm separates the fluid field and weakens the effects of heat recirculation, leading to a lower outer wall temperature. Furthermore, the appropriate block insertion method and height contribute to the significant improvement of heat transfer, radiant efficiency and further optimization of micro power generator.     

Charging strategy through analysis of charging influence factors of ultra-light hydrogen storage with polyethylene terephthalate liner

A lightweight type 4 vessel with a polyethylene terephthalate (PET) liner is analyzed. The derived heat transfer coefficients between the gas and wall are applied, and a parametric study is performed. An optimized charging strategy is also developed. Firstly, when the injected hydrogen temperature decreases, the charging time increases, and the charged gas temperature decreases. Secondly, the higher the ambient temperature, the shorter the charging time, and the higher the charged gas temperature. Thirdly, the larger the mass flow rate, the shorter the charging time, and the higher charged gas temperature. Fourthly, as the initial pressure inside the vessel increases, the charging time shortens, and the charged gas temperature decreases. Fifthly, using the formulated charging strategy, during summer, the charged gas temperature decreases by approximately 9 °C. In winter, the charging time is reduced by approximately 58 s. The results provide important information of temperature control for ensuring vessel safety.     

A numerical study on the performance of PEMFC with wedge-shaped fins in the cathode channel

The gas flow field design has a significant influence on the overall performance of a proton exchange membrane fuel cell (PEMFC). A single-channel PEMFC with wedge-shaped fins in the cathode channel was proposed, and the effects of fin parameters such as volume (0.5 mm³, 1.0 mm³, and 1.5 mm³), number (3, 5, and 9), and porosity of the gas diffusion layer (GDL) (0.2, 0.4, 0.6, and 0.8) on the performance of PEMFC were numerically examined based on the growth rate of power density (GRPD) and polarization curve. It was shown that wedge-shaped fins could effectively improve the PEMFC performance. With an increase in fin volume, the distributions of oxygen mass fraction in the outlet area of the cathode channel were lower, the drainage effect of the PEMFC improved, and GRPD also increased accordingly. Similar results were obtained as the number of fins increased. The GDL porosity had a greater effect than the wedge-shaped fins on the improvement in PEMFC performance, but the influence of GDL porosity weakened and the GRPD of porosity decreased as the porosity increased. This study provides an effective guideline for the optimization of the cathode channel in a PEMFC.     

Qualitative analysis of fluid dynamics and heat transfer characteristics in porous structures towards aqueous phase reforming hydrogen production

Porous structure is commonly used as a catalytic support in the field of fixed-bed reactor for hydrogen production from methanol aqueous phase reforming. The fluid dynamics and heat transfer characteristics of the fluid in a specific porous structure are considered profound influences for catalytic reactions. This study presented a qualitative investigation on the flow and heat transfer characteristics of fluids in variable porous structures. Specifically, the discrete average interpolation method was employed to obtain the value of a continuous structure. From the results, using the area-average temperature equation was 11% (±1.5%) larger and precise than that from the previous work using traditional hydrodynamic equation. The proposed variable porous structure could not only increase the turbulence intensity of the fluid which could further enhance reaction mass transfer but also enhance the convective heat transfer of the fluidic reactants. Some of these findings could inspire the reactor design for improving catalytic efficiency of hydrogen production.     

Performance assessment of gas crossover phenomenon and water transport mechanism in high pressure PEM electrolyzer

To improve proton exchange membrane (PEM) electrolyzes’ performance the voltage loss through them should be avoided. In this work, it is intended to analyze losses including of diffusion loss, ohmic loss due to electrode, bipolar plate (BP), and membrane resistances, and gas crossover associated with the water transferring mechanisms. All of the losses are associated with water transferring mechanisms, which is created due to electro-osmoic drag, pressure differential between the anode and cathode sides, and diffusion. Furthermore, the effect of membrane thickness, cathode pressure, and operating temperature on the hydrogen crossover is examined. In addition, the contribution of ohmic loss due to electrode bipolar plate (BP), and membrane resistances is studied and, the contribution of different losses on the cell performance is discussed. Results show that raising cathode pressure from 1 to 40 bar lead to the increment of anodic hydrogen content from 1.038% to 21% at the specific current density of 10,000 A/m2. Enhancing the thickness of membrane has considerable impact on decrementing anodic hydrogen content, but the mass transfer loss rises from 0.022 to 0.027 V with enhancing membrane thickness from 50 to 300 μm, respectively. Furthermore, the contribution of voltage losses, assigned to each of losses are equal to 85%, 3%, and 12% for activation, diffusion and ohmic losses, respectively. It is found that, from the reported contribution for ohmic loss, the contribution of electrode BP, and membrane resistances are 31% and 69%, respectively.     

Geosynthetic reinforced piled embankment modeling using discrete and continuum approaches

Understanding the load transfer mechanism can support engineers having more economical design of geosynthetic reinforced piled embankments. This study aims to investigate the load transfer mechanisms by two different numerical methods including the Discrete Element Method (DEM) and the Finite Difference Method (FDM). The DEM model adopts (a) discrete particles to simulate the micro-structure of the granular materials and (b) coupled discrete element – finite element method (DEM-FEM) to capture the interaction between granular materials and geotextiles. On the other hand, the FDM model uses an advanced constitutive soil model considering the hardening and softening behaviour of the granular materials. The numerical results show that the geotextiles can only contribute to the vertical loading resistance in cases where the soils between piles are soft enough. In terms of design, an optimum value of the geotextile tensile stiffness can be found considering the load, the soft soil stiffness and the thickness of the embankment. Both the DEM and the FDM show that a high geotextile tensile stiffness is not required since an extra stiffness will slightly contribute to the efficiency of the geosynthetic reinforced piled embankments. Nevertheless, both models are useful to optimize the design of geosynthetic reinforced piled embankments.     

Design and computational analysis of a metal hydride hydrogen storage system with hexagonal honeycomb based heat transfer enhancements-part A

In this study, design and performance analysis is carried out for a 10 kWh metal hydride based hydrogen storage system. The system is equipped with distinctive aluminium hexagonal honeycomb based heat transfer enhancements (HTE) having higher surface area to volume ratio for effective heat transfer combined with low system weight addition. The system performance was studied under different operating conditions. The optimum absorption condition was achieved at 35 bar with water at room temperature as heat transfer fluid where up to 90% absorption was completed in 7200 s. The performance of the reactor was observed to significantly improve upon the addition of the HTE network at a minimal system weight penalty.