Advanced Materials Research Vols. 15-17

Abstract: By combining the advantages of a catalytic palladium (Pd thin film) with a high-performance GaAs-based transistor, an interesting hydrogen sensor is fabricated and demonstrated. For the studied device, a 50 Å undoped GaAs cap layer is grown to prevent the Al0.24Ga0.76As Schottky layer from oxidizing and to suppress the Fermi level pinning effect. The sensing mechanism can be described as the dissociation of hydrogen molecules and the polarization of hydrogen atoms. The drain-source variation ΔIDS is caused by the polarization of a dipolar layer resulting in the modulation of the gate potential and carrier density level. Experimentally, a high hydrogen detection sensitivity SJ value of 275.8 mA/mm-ppm H2/air can be obtained under the 14ppm H2/air gas. Even under a very low hydrogen concentration (≤4.3 ppm H2/air) at 303K, the considerable current variation can be observed. Moreover, the fast hydrogen response is found. Therefore, the studied device reveals the promise for high-performance hydrogen sensor applications.

Abstract: Currently, pressurized gas is the leading technology for vehicular on-board hydrogen storage. During refueling, the hydrogen is expanded from the high-pressure fueling station cylinders, into the “empty” vehicle cylinder. The mass of the gas inside a cylinder can be calculated from the knowledge of the pressure and average gas temperature. However, during the fill process, the compression of the gas inside the cylinder leads to a rapid increase in temperature, this phenomenon along with the continuous introduction of cooler gas creates an evolving spatial distribution of gas temperature within the cylinder. In order to determine a correlation between the massaveraged gas temperature and local measurement of gas temperature, this study presents a CFD model of the filling of a hydrogen compressed gas cylinder. The model developed in this study is 2D and axi-symmetric, and solves the governing equations for compressible, unsteady, viscous turbulent flow. The model incorporates real gas effects, convective heat transfer from the gas to the cylinder walls and conduction through the cylinder walls to ambient. The results of the model show a large spatial variation of gas temperature within the cylinder during filling. The modeling results also help to identify the optimum location for the onboard gas temperature sensor such that the local measurement best represents the mass-averaged temperature of the gas within the cylinder. Hence allowing for the calculation of the mass of gas within the cylinder without using an expensive flow meter.

Abstract: A new rapid manufacturing technique for the production of SOFC anodes for direct oxidation of hydrocarbon fuels has been demonstrated. Composite anodes with doped ceria as catalyst and ion conductor and copper as electronic conductor have been fabricated by plasma spraying in air. The process, which can be readily automated and scaled up for mass production, provides a rapid method to produce anodes with mixtures of low and high melting temperature components in several minutes. These anodes previously have required complex multi-step, multi-day processes involving infiltration of sintered pre-forms. This work demonstrates the feasibility of using plasma spray processing to manufacture composite Cu-SDC coatings for application in direct-oxidation SOFC anodes.

Abstract: Extensive interfacial reactions are known to occur between Fe-Co based perovskite cathode materials and the standard solid oxide fuel cell (SOFC) yttria stabilized zirconia (YSZ) electrolyte. Thin films of gadolinia doped ceria (GDC) could be used as a diffusion barrier between the cathode and the electrolyte. The present work investigates spin coating thin diffusion reaction inhibiting films onto SOFC electrolytes. The chemical and structural evolution of ethylene glycol based precursor solution is studied by means of rheology, x-ray diffraction (XRD), high temperature XRD (HT-XRD), Fourier-transformed infrared spectroscopy (FTIR) and differential thermal analysis (DTA). The studies show that cerium formate is formed as an intermediate resin. Thin films, up to 500 nm thick, of gadolinia doped ceria (GDC) are successfully produced by multiple spin coating of polymerized ethylene glycol derived solutions on 200 1m thick YSZ tapes. The GDC and YSZ interfacial surface morphology and film thickness are studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). These films are shown to successfully prevent the creation of non-conducting reaction phases at the cathode-electrolyte interface by blocking interdiffusion.

Abstract: Porous composite cathodes containing (La0.8Sr0.2)0.98MnO3 (LSM) and yttria stabilized zirconia (YSZ) for use in solid oxide fuel cells (SOFCs) have been produced by air plasma spraying. Deposition was carried out using axial powder injection for increased deposition efficiency and composition control. A number of composite cathodes were produced using different combinations of parameter values within the identified range. Successful coatings were then characterized for composition and porosity using EDX and SEM. As a result of these tests, combinations of input parameter values were identified that are best suited to the production of coatings with microstructures appropriate for use in SOFC composite cathodes.

Abstract: In the attempt to develop materials for interconnects of planar-type solid oxide fuel cells (SOFC), the oxidation behavior and scale electrical properties of four, commercially available, ferritic stainless steels were investigated by oxidizing at 800°C for times up to 900 h in the dry and wet air environments. The growth kinetics of oxide scales were significantly deviated from the parabolic growth law, exhibiting the temporal growth exponent being much less than 0.5. Titanium addition in ferrite stainless steels was detrimental to oxidation resistance since its fast out-diffusion onto the scale surface enhanced the scale growth. For the alloys without Ti contents, the presence of water vapor in the oxidizing environment enhanced the scale growth rate, but it resulted in a drastic, microstructural change of oxide scale that consisted of Cr2MnO4 spinel and Cr2O3, which in turn lowered the area specific resistance (ASR) and the apparent activation energy for conduction.

Abstract: The presence of water within the polymeric ionic conductor and catalyst layers is indispensable for proton exchange membrane fuel cell (PEMFC) operation. The water transport within the membrane is always a balance between at least two competing mechanisms (water back-diffusion from the cathode to the anode, and electro-osmotic drag from the anode to the cathode). We present an experimental technique for the analysis of water transport within and through the membrane. Mass spectrometry was used to investigate diffusion and electro-osmotic drag in real time (detection rates < 100 ms). We present data on the net water transport resulting from the analysis at room temperature in Nafion115.

Abstract: The use of redox fuel cells, in which oxygen is replaced by other oxidants such as ferric ions, can have significant advantages. The redox fuel cell can achieve high efficiencies and has other fuel cell advantages. Bioregeneration is one method of creating a closed cathode system with efficient catholyte regeneration. In the work discussed here, a Fe3+/Fe2+ redox simulated bio-electrolyte catholyte is characterized over a range of electrolyte concentrations and fuel cell operating conditions using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS).

Abstract: This investigation considers the development of microchannel reactors with catalystcoated walls for fuel-processing applications. In particular, the focus is to study the possibility of direct etching of microchannels into aluminum and alumina using a solid-state UV laser. Microchannels of a scale between 10μm-200μm across and 10μm-100μm have been ablated into aluminum and alumina bases. It was found that single scans resulted in narrow channels (20μm- 30μm in width) with shapes described by a Gaussian-like distribution. Multiple scans allowed fabrication of channels with a larger width, but of a similar depth. The surface quality was observed to be quite uneven, with roughness on the order of 1μm-2μm.

Abstract: When fine-sized 45ZnO·55P2O5 glass powders were mixed with water, hydration immediately occurred, resulting in the formation of a viscous hydrogel. 31P MAS-NMR spectra showed that the hydrogel contains orthophosphates and long polyphosphates. 45ZnO·55P2O5 glass-derived hydrogel (45ZP gel) showed high conductivities of 9.9 mS/cm at 30 °C. The conductivities were related to the proton amount in the hydrogel. The electric double-layer capacitor (EDC) cell for the 45ZP gel showed no oxidation-reduction processes in the range of 0~1 V and the specific capacities of 2.40 F/g. The possibility as an EDC electrolyte may be related to the conductivities of the hydrogels.