Aerogels are of current interest in the aerospace community due to their light weight and low thermal conductivity, making them suitable for a variety of applications, notably cryotank insulation.

These gels typically exhibit a complex structure; the smallest feature is a “primary” particle of amorphous silica, typically 2-5nm in diameter. The primary particles aggregate to form “secondary” particles, typically an order of magnitude larger, and these, in turn, form pearl-necklace structures whose details depend on the density. The gels appear to exhibit fractal dimensionality, at least over a small range of length scales.

In this work, we investigate the relationship between the structure of the gels, their dimensionality and density, and their thermal conductivity. We model the secondary-particle aggregate structure using a modified Diffusion Limited Cluster Aggregation (DLCA) model. The model produces qualitatively different structures at low and high densities that are consistent with experimental observation. At lower densities, we find evidence for a transition from fractal behavior at small length scales to approximately compact behavior at larger lengths.

We model the thermal conductivity using a variant of the random resistor network approach that has been used to describe, e.g. hopping electrical conduction in doped semiconductors. In our model, each secondary particle is assigned an effective thermal conductance that depends on the particle's size, and on the details of its contacts with neighboring particles; the conductivity of the gel network is obtained using standard numerical techniques. The scaling of the thermal conductivity with density and fractal dimension is discussed.

The performance of solid oxide fuel cells is limited largely by ion transport in the electrolyte. Thin film electrolytes of yttria-stabilized zirconia were deposited by pulsed laser deposition. The electrolyte material was subjected to heavy ion irradiation and heat treatment and the effects on conductivity were measured using electrical impedance spectroscopy. Following irradiation there is a drop in conductivity by a factor of 3-4. After heat treatment at 800°C, the conductivity recovers to the as-deposited value.

Gases evolved on heat-treatment of intact fullerite C60, single-walled (SW) and multi-walled nanotubes (MWNTs) and chemically modified carbon nanomaterials (deuterofullerites C60Dx, hydrogenated SWNTs and fluorinated MWNTs) are studied by mass spectrometry.

Ruthenium-iridium oxide films were prepared in an horizontal hot-wall MOCVD apparatus and their electrocatalytic activity was investigated in the oxygen evolution reaction. Metal-acetylacetonates were used as precursors and oxygen as the reactant gas. Appropriate deposition conditions for obtaining bi-metallic oxide films were found to be deposition temperature (Tdep), 673 K and total pressure (Ptot), 133.32 Pa. X-Ray diffraction (XRD), Rietveld refinement and scanning electron microscopy (SEM) results demonstrated that the MOCVD method is a viable alternative to grow RuxIr1−xO2 films consisted of a single-phase with a very homogeneous distribution of elements. The electrocatalytic activity of the RuxIr1−xO2 films was investigated by cyclic voltammetry and anodic polarization curves in a 0.5 M H2SO4 solution at room temperature. The single-phase Ru0.4Ir0.6O2 films showed to be very promising for the oxygen evolution reaction due to a synergistic effect.

Carbon nanotubes (CNTs) possess remarkable mechanical and physical characteristics due to their unique structure. The high electrical conductivity of CNTs in combination with their structural and chemical properties make this material attractive for improvement of the performance of the electrodes in electrochemical devices, such as polyelectrolyte-membrane fuel cells. The requirements in particular for this application are manifold. Since the electrode is the reaction layer in a fuel cell, the electrolyte, being proton conductive, has to be combined with catalytic activity and electron conductivity, as well as chemical stability against the reactants. Employed as an electrode, CNTs offer feasibilities to enhance the efficiency of fuel cells due to an improved contact between electrolyte-membrane, electrode and catalyst. An other promising capacity is the large specific surface area of CNTs on which catalyst particles can be distributed in small clusters. This provides an increase of the catalyst efficiency on account of a higher reactive surface area which enables a reduction of the amount of catalyst required. In combination with a good electron conductivity, a CNT based electrode is a potential alternative fuel cell electrode. The presented work focuses on a membrane assembly, consisting of CNTs as electrode and sulfonated polyether ether ketone (SPEEK), which is used as polyelectrolyte in direct methanol fuel cells (DMFCs) and provides good proton conductivity. Nanocomposites were produced by a modified drop casting technique, leading to the formation of an asymmetric structure. Due to the process, the compounded membranes provided a single-sided electron conductivity on account of the CNTs. By using different 3D-structured CNT-carpets, varying in thickness, density and setup, the properties of the electrode membrane could be adjusted for its special application. In consideration of the insertion of platinum and ruthenium particles as catalyst in nano-sized clusters on the CNTs, carpets were modified and grown in a CVD-process at the TUHH. The evaluation of the assemblies comprised SEM-pictures in order to analyse the 3D-nano-structure, measurements of the surface conductivity as well as tests in a fuel cell.

The goal of this research is to investigate the activities of a non-noble nano-catalyst (Ni/SiO2) using Si-microreactors for steam reforming of methanol to produce hydrogen for fuel cells. The supported catalyst was synthesized by sol-gel method using Ni (II) salts and Si(C2H5O)4 as starting materials. EDX results indicate that the actual loading of Ni (5-6%) is lower than the intended loading of 12 %. The specific surface area of the silica sol-gel encapsulated Ni nano-catalyst is 452 m2/g with an average pore size of ∼ 3 nm. Steam reforming reactions have been carried out in a microreactor with 50 µm channels in the temperature range of 180-240 °C and atmospheric pressure. Results show 53% conversion of methanol with a selectivity of 74 % to hydrogen at 5 l/min and 200 °C. The magnetic properties of the catalysts were performed using a Vibrating Sample Magnetometer (VSM) to study the activity of the catalysts before and after the steam reforming reactions. The VSM results indicate much higher activity in the microreactor compared to macro-reactor and Ni forms non-ferromagnetic species faster in the microreactor.

Single wall carbon nanotubes (SWCNT) have previously been considered potential catalyst supports in proton exchange membrane fuel cells (PEMFC) [1]. Earlier research and development of SWCNT for PEMFC catalyst supports has been advanced by utilizing differing SWCNT purities and carbon blacks in differing ratios. This study validates the performance of SWCNT and introduces new concepts for SWCNT membrane fabrication.

At the Hawaii Fuel Cell Test Facility a systematic evaluation of the impact of impurities in hydrogen is underway to evaluate the effects on the performance of PEM fuel cells. Initial tests are being conducted using carbon monoxide and hydrocarbon contaminants. The effects of carbon monoxide poisons at atmospheric and pressurized operating conditions have shown a strong dependence on concentration of the impurity over the range 6.7 µmole/mole to 29.3 µmole/mole. Additionally, benzene and toluene were tested at 20 µmole/mole. Although both benzene and toluene showed no evidence of fuel cell degradation, on-line gas analysis of the exit anode stream showed that toluene hydrogenation occurs in the anode resulting in 90% conversion of the toluene to methyl-cyclohexane.

Hydrogen adsorption mechanism into the porous metal-organic frameworks (MOFs) has been studied by density functional theory calculation. The selected functionals for the prediction of interaction energies between hydrogen and potential adsorption sites of MOFs were utilized after the evaluation with the various functionals for interaction energy of H2···C6H6 model system. The adsorption energy of hydrogen molecule into MOFs was investigated with the consideration of the favorable adsorption sites and the orientations. We also calculated the second favorable adsorption sites by geometry optimization using every combination of two first absorbed hydrogen molecules. Based on the calculation of first and second adsorption sites and energies, it has been suggested that the hydrogen adsorption into MOFs follows a cooperative mechanism in which the initial metal sites initiate the propagation of the hydrogen adsorption on the whole frameworks. In addition, the interaction mode between the simple benzene ring with hydrogen is significantly changed when the benzene ring has been incorporated into the framework of MOFs.

For the purpose of developing the corrosion-resistant and low-cost metallic bipolar plates for direct methanol fuel cell (DMFC), Ti mesh, stainless steel and Si(100) were coated with TiN by using the filtered cathodic vacuum arc system (FCVA). These TiN films have received considerable attention because of its high anti-corrosion behavior and low contact-resistance. In order to improve the corrosion protective ability of TiN films and decrease pinholes of coating, growth modifications such as thickness of the coatings and bias applied to substrates have also been carried out. The microstructures and composition of TiN film were identified by the instrumental analyses such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion behavior of TiN coatings was studied in 0.5 M H2SO4 solutions by using potentiodynamic polarization method. The DC bias of −150 V was applied to the substrates to achieve a dense structure of approximately 400 nm coating of TiN, so that good corrosion protection of the Ti mesh and stainless steel substrates can be achieved. The TiN coating on stainless steel exhibited excellent corrosion behavior especially in lower corrosion current than 2×10−7 A/cm2.

A general, efficient and experimentally simple method for the synthesis of covalently crosslinked class II hybrid polymers based on PolyEtherEtherKetone (PEEK) with a high degree of sulfonation (DS = 0.8) was performed. The synthetic strategy foreseen two steps: the first one, direct sulfonation, led to the introduction of sulfonic acid groups and to the formation of sulfone bridges among repeat units. The second step, lithiation and silylation, led to the formation of covalent bonds between the organic backbone and Si(OH)3 moieties. The materials were characterized by 1H NMR Spectroscopy, ATR/FTIR Spectroscopy, Thermogravimetric Analysis (TGA) and Mass Spectrometry. Preliminary proton conduction measurements in dry conditions were performed. The introduction of silicon groups in the system improved thermal stability.

In view of the development of a miniature PEM fuel cell operating at low temperature on hydrogen or direct methanol as anodic feed and air as cathodic feed, we present preliminary results on the performances obtained by employing silicon-based microfabrication techniques and advanced materials which can be easily integrated in silicon technologies, such as porous silicon for the electrode fabrication and PEEK-WC for membranes alternative to Nafion. Powder blasting and reactive ion etching (RIE) have been used for obtaining anode and cathode micro-channels for hydrogen and methanol, in glass and in silicon bipolar plates. Localized micro-fusion processes has been successfully employed for sealing between silicon and membrane.

Composite membranes, prepared by mixing sulfonated polyether ether ketone (SPEEK) and WO3·2H2O in dimethylacetamide, were characterized by Thermogravimetry, Electrochemical Impedance Spectroscopy and Water Uptake measurements to evaluate their possible performance as proton exchange membranes (PEM). The body of results indicated the existence of a coordinative interaction between the water molecules of tungsten oxide and the sulfonic acid (-SO3H) groups of SPEEK. Moreover, EIS data demonstrated that the proton conductivity of the composite membranes is higher than both that of pure SPEEK and pure tungsten oxide, suggesting the presence of ion-rich regions where the proton transfer is favored. The SO3H - H2O interaction not only lead to enhancement of the proton conductivity of the membranes but also to improvement of their heat resistance as well as to decrease their water solubility.

This study is aimed at developing novel hydrides for hydrogen storage through a new synthesis technique utilizing high hydrogen overpressure at elevated temperatures denoted as Molten State Processing, MSP. This synthesis technique holds the potential of fusing different known complex hydrides at elevated temperatures and pressures to form new complexes having different sorption and thermodynamic properties. Complex hydrides produced by this method (e.g. NaMgH3, K3AlH6 and Na2LiAlH6) were identified through structural determination and thermodynamic characterization.

Current fuel cell proton exchange membranes (PEM) rely on a random network of conducting hydrophilic domains to transport protons across the membrane. Despite extensive investigation, details of the structure of the hydrophilic domains in these membranes remain unresolved. In this study a dynamic self-consistent mean field theory has been applied to obtain the morphologies of hydrated Perfluorosulfonic Acid (PFSA) (equivalent weight of 1100) as a model for Nafion® at several water contents. A coarse-grained mesoscale model was developed by dividing the system into three components: backbone, side chain, and water. The interaction parameters for this model were generated using classical molecular dynamics. The simulated morphology shows phase separated micelles filled with water, surrounded by side chains containing sulfonic groups, and embedded in the fluorocarbon matrix. For λ<6 (λ gives the ratio of water molecules to sulfonic groups), the isolated domains obtained from simulation are nearly spherical with a domain size smaller than that fitted to experimental SANS data. For λ>8; the domains deform into elliptical and barbell shapes as they merge. The simulated morphology, hydrophilic domain size and shape are generally consistent with some experimental observations.

The lithium borohydride has been modified by ball milling with metal oxides and metal chlorides as the additives. The modified lithium borohydrides released 9 wt% hydrogen starting from 473K. The dehydrided modified lithium borohydrides absorbed 7-9 wt% hydrogen at 873K and 7 MPa. The modification with additives reduced the dehydriding starting temperature from 673K to 473K and moderated the rehydrogenation conditions from 923K/15 MPa to 873K/7 MPa. XRD and SEM analysis revealed the formation of the intermediate compound that may play a key role in changing the reaction path resulting in the lower dehydriding temperature and reversibility. The additives reduced the dehydriding temperature and improve the reversibility, but it also reduced the hydrogen storage capacity.

We have prepared novel transition metal fluorite materials for a 2-step hydrogen reformer process that generates low CO hydrogen for fuel cell applications. The nanopowder materials, which are converted to an oxygen vacancy rich form in the first step of the process using methane or any hydrocarbon fuel are subsequently brought back to their original state using water vapor while generating pure hydrogen in the process. We have observed large weight losses in TGA measurements below 500°C with our nanomaterials that we do not observe in similar powders with larger grain size that comprise an efficient low temperature first step. We have also observed decreases in lattice parameter in high temperature XRD measurements consistent with formation of a high concentration of oxygen vacancies. The process has produced fuel efficiencies as high as 65% with typical CO content below 30 ppm using methane fuel. Additionally, since we are using nanomaterials, we have enabled low temperature hydrogen generation with good cycling stability at 500°C.