PacSurf2014 Session EH-WeP: Energy Harvesting & Storage Poster Session

In a polymer electrolyte membrane fuel cell (PEMFC), one of the most important factors affecting the performance is ohmic loss arising from the contact resistance at the interface of the gas diffusion layer (GDL) and bipolar plate (BP). As a method to reduce the contact resistance between the bipolar plates and gas diffusion layer, the contact area of the bipolar plates were increased by inducing roughness on the surface. The environment of the highly porous GDL being pressed by an external compaction pressure was simulated, and the contact area between the GDL and bipolar plates was calculated. The calculated contact resistance using the contact area was compared with the experimental contact resistance results of the bipolar plates polished with various grades of abrasive paper. As the average surface roughness increased, the contact resistance values decreased, which is in good agreement with the results of this study. In a single cell test, the efficiency of the cell increases when the rough bipolar plate is used.

ZnO nanowires (NWs) can be grown by chemical approach at low temperature (<100 ℃) on any substrate and any shape substrate. A relatively small force is required to induce the mechanical agitation, so that it can be fabricated sensitive devices.[1] But piezoelectric device based on ZnO NWs, which can not improve the piezoelectric properties of a single material due to a low piezoelectric d constant of ZnO NWs (d≃12pC/N).

In this paper, we demonstrated fully flexible and transparent piezoelectric touch sensor based on ZnO NWs, and composed that touch sensor with BaTiO3 of Perovskite structure for improving piezoelectric properties. In order to maintain the flexibility of sensing spot, the BaTiO3 (10 wt%) are dispersed in flexibility-improved SiO2 capping solution and coated on ZnO NWs surface as a capping layer by spray coating method. Also, By replacing Indium tin oxide (ITO) electrodes with transparent flexible CNT-Ag nanowires electrode, the flexibility of the entire structure was enhanced. ITO is commonly used as transparent electrodes. However due to its high cost and limited supply of indium, the fragility and lack of flexibility of ITO layer, its alternative are being sought. It is expected that conductive films using carbon nanotubes and Ag NWs could be a prospective relacement of ITOs.

The ZnO NWs based sensor generated the output voltage of ~ 50 mV. The sensor with BaTiO3 generates a higher output voltage (~1.2 V) than a ZnO NWs based sensor. We confirmed that the output voltage of sensor with ZnO NWs and BaTiO3 was dramatically increased. We measured the resistance of capping layer and CNT-Ag NWs electrode during the periodic bending. When bent and flexed over 1,000 cycles, the films did not show significant degradation in sheet resistance compared to Ag thin film and ITO film on the same PET substrate. The bending test results conducted to confirm the mechanical stability of capping layer and CNT-Ag NWs as a electrode. The measurement results suggest that the our Piezoelectric touch sensors are suitable for flexible device such as flexible touch sensor, wearable and rollable touch panel.

[1] Zhong Lin Wang, "Piezopotential gated nanowire devices: Piezotronics and piezo phototronics", Nano Today, 5, 2010, pp.540~552

For an environmentally-friendly energy system, hydrogen is expected as a resource to replace fossil fuels. Recently, steam methane reforming (SMR) is mainly used for hydrogen production. However, the promotion and the cost reduction of hydrogen production in SMR is strongly desired for stable supply of hydrogen because SMR requires a large amount of heat. To increase hydrogen production, Maegami et al. proposed the vibrationally-excited method, in which infrared light vibrationally excites a C-H bond of a CH₄ molecule [1] . While hydrogen production is promoted by the vibrational excitation of a CH₄ molecule, the detailed analysis at atomic scale is necessary for higher efficient hydrogen production. Thus, by using the first -principles molecular dynamics (FPMD) simulation method, we examined the effect of vibrational excitation of the C-H bond on chemical reaction dynamics for hydrogen generation from CH₄ and H₂O.

To reveal the chemical reaction dynamics, we simulated collision process of a H₂O molecule with a CH₄ molecule in the vibrationally-excited state by using our development FPMD code “Violet” [2]. The vibrationally-excited state was reproduced by extending a C-H bond . After the collision, a dissociation of C-H bond was observed. Moreover, the H atom of the dissociated C-H bond reacted with a H atom of the H₂O molecule, and H₂ and CH₃OH were generated. Next, to examine the effect of vibrational excitation, we simulated collision processes with the collision angle from -60° to 60° and collision energy from 9 eV to 20 eV in the ground state and the vibrationally-excited state. In the ground state, hydrogen molecules were generated in the range of collision angle from -50° to -10° and collision energy from 17 eV to 20 eV. On the other hand, in the vibrationally-excited state, hydrogen molecules were generated in the range of collision angle from -60° to 0° and collision energy from 14 eV to 20 eV. Therefore, in the vibrationally-excited state, H₂ molecules were generated in a wider range of collision angle and lower collision energy than those in the ground state. This simulation result suggests that the H₂ generation was promoted by vibrational excitation, which is consistent with the experiment [1] . We also examined the later process after the H₂ and CH₃OH were generated. Accordingly, CH₂(OH)₂, HCHO, HCOOH, and CO were observed as intermediate products. Consequently, we indicated the chemical reaction dynamics of H₂ generation from H₂O and vibrationally-excited CH₄ in gas phase.

[1] Y. Maegami, F. Iguchi, and H. Yugami, Appl. Phys. Lett., 97, 231908 (2010).

[2] T. Shimazaki and M. Kubo, Chem. Phys. Lett., 503, 316 (2011).

Semiconductors in the form of thin films are essential components in the development of solar devices. Uniformity in thickness and composition is then crucial for its application; therefore conditions of deposit are the key for its performance. There exist a variety of techniques and chemical methods by which thin films can be obtained. Among them, chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) methods represent very attractive alternatives to molecular beam epitaxy (MBE), chemical vapor deposition (CVD), sputtering, evaporation, etc., because of their relative simplicity. In order to obtain a suitable absorbent layer, the material must have a thickness between 1 and 2 microns for good conversion of sunlight into electricity. In this work lead sulfide (PbS) thin films was prepared by using CBD and SILAR techniques successively. The resultant films were used as a P-type material in a CdS/PbS solar cells and their preparation and performance were compared. In the case of SILAR method, films with the desirable thickness were obtained in an easier way than in the CBD one. Thin films were grown on glass substrate by CBD and SILAR methods at room temperature, controlling the time of deposition in CBD technique and the number of cycles for the SILAR case. The crystalline structure and optical proprieties were characterized by X-ray diffraction, UV-VIS and infrared spectroscopies. The PbS films showed polycrystalline structure and preferred orientation with the cubic cell characteristics. Finally it was observed that the band gap energy decrease as the thickness increases.

Stainless steels are vital construction materials in all areas of industry, combining excellent corrosion resistance with good mechanical properties. For these reasons, stainless steels are used extensively in power stations of all varieties – of particular interest is the use of stainless steel in Pressurised Water Reactor (PWR) type nuclear power plants, such as the Sizewell B power station, Suffolk, UK.

Stainless steel’s corrosion resistance is derived from the formation of a passivation layer at the surface of the material. Under atmospheric conditions this is thought to be a vanishingly thin layer of Chromia (Cr₂O₃), however, under conditions found in the coolant cycles of a PWR, it is thought that the passivation layer grown forms a double layer – the inner layer consisting of corrosion resistant, non-stoichiometric Chromite (FeCr₂O₄), while the outer layer consists of non-stoichiometric Nickel Ferrite (NiFe₂O₄). The thickness of this film is believed to vary with the steel surface finish, and the Environment Degradation Group at the University of Birmingham has recently begun a programme to study the dependence of corrosion rate and passivation layer thickness on surface finish, system chemistry and temperature.

The samples were ground to a 120 and 1200 grit finish using silicon carbide paper to produce samples with significantly difference roughness levels (approximate R_a values of 1050 and 110 nm, respectively), before being inserted into a flowing rig, where they were exposed to deoxygenated water at pH 10 and 300˚C at a pressure of 10 MPa. Sets of samples were removed from the rig every 250 hours, up to 1000 hours total exposure time.

Among the configurations of hybrid solar cells (CSOs ), the bilayer solar cells involving the heterostructure of a p-type soluble conjugated polymer and n-type chalcogenide film is one of the most studied . This bilayer system being one of the simplest c onfigurations still presents a number of factors that affect the performance of the CSOs . Among these factors is the collection of electric charge. To improve the collection of charge, dissociation of excitons in the interface of the semiconductor and the metal is required. Within the promising p-type organic semiconductors for organic solar cells is poly (3- hexylthiophene) ( P3HT) because of its relatively high mobility, solubility and easy processing .The cadmium sulfide ( CdS) is one of the n-type semiconductor widely employed as window layer in solar cells due to its good optical properties. In this work, we present the results of the CSOs with bi layer structure in which different electrical contact configurations were tested . It was used P3HT as p-type organic semiconductor and as n-type semiconductor CdS. The films were deposited by sping coating and chemical bath ( CBD), respectively. The morphological and structural properties of the films were studied by scanning electron microscope (SEM ), x-ray diffraction and Uv -vis spectroscopy . The electrical properties of the films and the CSOs were obtained from the analysis of current curves ( I) versus voltage ( V) in darkness and under illumination.

This work presents a hybrid solar cell made with quantum dots and poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN), which is commonly used as electron transport layer in organic solar cells. Those cells were fabricated by Successsive Ionic Layer Adsorption and Reaction (SILAR) method and spin coating. The configuration used in this work was: TiO₂/PbS/PFN/CdS/ZnS. The presence of PFN produced an increase of 33% in the short circuit current (J_SC) respect to the sample of reference: TiO₂/PbS/CdS/ZnS. Such increase in current caused an increment of the cell efficiency from 2.6% to 3.6%. PTB7 was also used has donor in our hybrid solar cells instead of quantum dots, when the PFN is added in this system, the current increased 60% respect to the cell of reference and the efficiency increased from 1.1% to 3.0%.

Electrochemical capacitors (Ecs) as a class of energy storage devices with high power density and long lifetime are important in applications from commercialized pocket electronics to hybrid vehicles. Although, the low energy density of traditional electrochemical capacitors limits their application. The key to achieve high energy density is to prevent graphene sheets aggregation during processing. It has been found that three dimension (3D) hydrogels with 3D interconnected pores are potential electrode materials for supercapacitors. Recently, 3D MnO₂/graphene nanosheets hydrogel as electrode materials was developed to improve the specific capacitance and increase the operating voltage. However, the Ecs based on these composites usually have poor cycling stabilities due to separation of MnO₂ nanosheets and reduced graphene oxide (rGO) during cycling. So, it is crucial to use polymers by cross-linking. Among the various polymers, poly(vinyl pyrrolidone) (PVP) is a strong candidate due to its good affinity to water. Thus, aqueous and organic electrolytes are able to diffuse into the interlayers of graphene sheets to form electric double layers.

In this study, we report novel synthesis of 3D PVP/MnO₂/rGO nanosheets hydrogel and their application in supercapacitors. PVP chains were used as a cross-linking between MnO₂ and rGO nanosheet; acting as a spacer to prevent the restacking of graphene nanosheets. The first, MnO₂ nanosheets were deposited onto both surfaces of graphene oxide nanosheets in aqueous suspension, and then PVP coating onto MnO₂/GO nanosheets. The composite of 3D PVP/MnO₂/rGO nanosheets hydrogel was prepared by self-assemble with present of Vitamin C. The 3D PVP/MnO₂/rGO nanosheets hydrogel composite was successfully synthesized and confirmed by scanning electron microscope (SEM), transmission electron microscope (TEM) and raman spectroscopy. Then, asymmetric supercapacitors with high energy, power densities and good cycling stabilities were fabricated using the 3D PVP/MnO₂/rGO nanosheets hydrogel as the positive electrode and a pure rGO hydrogel as the negative electrode.