With the raise of nanotechnology researching, many special physical and chemical properties were found gradually in nanoscale. Among them, the one-dimension nanostructure owns high specific surface area and excellent electron emission properties. Moreover, the two-dimension arrayed nanostructure has the characteristics of photonic crystal and moth-eye effect. Currently, advanced lithographic methods such as electron beam (E-beam) or deep ultraviolet (DUV) lithography and X-ray lithography are adopted to define periodic nanoscale patterns. But these lithographic equipment are too expensive. Moreover, costly etching methods such as inductively coupled plasma reactive ion etching (ICP-RIE) or electron cyclotron resonance reactive ion etching (ECR-RIE) must be used to form arrayed silicon nanostructure with high aspect ratios. The nanoscale array patterns can be defined on the surface of the silicon wafer by the self-assembly of a polystyrene nanosphere. The photo-assisted electrochemical etching (PAECE) has the advantage of forming nanopore, and the aspect ratio of etched nanopores can be as high as 50:1 which is better than ICP-RIE. Therefore, PAECE is very suitable to fabricate nanostructure. This high-cost drawback makes most of academias and small/medium enterprises hard to invest in nanotechnology. This study combines the self-assembly nanosphere lithography (SANSL) process and photo-assisted electrochemical etching to fabricate a nanostructure array with a high aspect ratio on the surface of a silicon wafer. Experimental results show that the nanosphere array with a nearly perfect arrangement can be obtained in the sample of 1.8 ∗1.8 cm2 by spin coating and vibration coating. Using reactive ion etching (RIE) can transfer the nanosphere array pattern to the silicon nitride layer, and form the etching window of PAECE. The concentration of the HF electrolyte used in PAECE was 2.5 wt%. When PAECE was performed with etching mask can produce deeper and periodic nanopores. The surfactant of SDSS added in the HF electrolyte of PAECE can reduce the contact angle of electrolyte and avoid the phenomenon of hole-reaming. When the voltage of 1 V is used to etch for 12.5 min, the etching depth of the nanopore array structure is about 5.69 £gm and its diameter is about 90 nm, such that the aspect ratio of the pore can reach about 63:1. If the etching voltage was increased, the width of pore will be increased and the depth of pore will be reduced gradually at the same time. When the etching voltage of 2 V is applied to etch for 5 min, the etching height of the nanopillar is about 2 £gm and its diameter is about 100 nm, such that the aspect ratio of the pillar can reach about 20:1. The nanopillar was arranged periodically according to the definition of nanosphere, therefore the arrayed nanopillar can be realized successfully. Dropping the solution which has biological samples into the gap of nanopillar, it will affect the light which goes through the nanostructure and produce specific parameters of polarization. The results showed that when the DI water was dropped into the nanopillar structure, the degree of polarization (DOP) is 0.981, azimuth is 4.86¢X and ellipticity is 2.83¢X. When the solution which has alkaline lysis plasmid of 5 £gg/ml was dropped into the nanopillar structure, the DOP is 0.957, azimuth is 7.7¢X and ellipticity is 3.99¢X. The result shows that the change of polarization parameter has the relations with the concentration of biological samples in solution. Therefore, the measure system can be combined with nanopillar array to develop the photonic crystal biosensor. This study also applies the developed nanopore nanostructure array to fabricate sub-wavelength antireflection structure of solar cell. Experimental results show that the deeper in structure and then the better in antireflective effect. After performing 1 V PAECE for 5 min, the weighted mean reflectance can be reduced to 1.73% under the wavelength range of 280¡V890 nm. Further coating of a silicon nitride layer on the surface of a nanostructure array reduces the weighted mean reflectance even to 0.878 %. Finally, this study also uses various voltage of PAECE to produce nanostructure array with different surface area for the electrode fabrication of fuel cell. Experimental results show that the larger in surface area of sample and then the better in catalysis effect. Two-staged PAECE of 1.5 V and 1.75 V can yield nanopillar with surface area of 14.2 cm2 , which is about 50.2 times higher than a planar electrode. When the surface of such a nanopillar array is coated with platinum of 1000 Å, the reaction current of nanopillar array is 10.2 mA, which is 72.9 times higher than that obtained by only a planar electrode.

The goal of this dissertation was to create a new class of DMFC targeted at high energy density and low loss for small electronic devices. In order for the DMFC to efficiently use all its fuel, with a minimum of balance of plant, a low-loss proton exchange membrane was required. Moderate conductivity and ultra low methanol permeability were needed. Fuel loss is the dominant loss mechanism for low power systems. By replacing the polymer membrane with an inorganic glass membrane, the methanol permeability was reduced, leading to low fuel loss. In order to achieve steady state performance, a compliant, chemically stable electrode structure was investigated. An anode electrode structure to minimize the fuel loss was studied, so as to further increase the fuel cell efficiency. Inorganic proton conducting membranes and electrodes have been made through a sol-gel process. To achieve higher voltage and power, multiple fuel cells can be connected in series in a stack. For the limited volume allowed for the small electronic devices, a noble, compact DMFC stack was designed. Using an ADMFC with a traditional DMFC including PEM, twice higher voltage was achieved by sharing one methanol fuel tank. Since the current ADMFC technology is not as mature as the traditional DMFCs with PEM, the improvement was accomplished to achieve higher performance from ADMFC. The ultimate goal of this study was to develop a DMFC system with high energy density, high energy efficiency, longer-life and lower-cost for low power systems.

Low-temperature fuel cells (LTFC) such as phosphoric acid fuel cells (PAFC) and proton exchange membrane fuel cells (PEMFC) are a promising electrochemical energy system for the conversion of hydrogen to electricity. Many challenges must be overcome before commercialization is possible. This dissertation focuses on the degradation of carbon catalyst supports and PEMFC water management. Kinetic studies are presented on the structure-reactivity relationship including an in-depth study of commercially available and model carbons. A mechanism and numerical model of the electrochemical oxidation of graphene-based carbon is proposed to explain longstanding questions. Three mechanisms are concluded to contribute to the current decay commonly observed during electrochemical oxidation: mass loss, reversible passive oxide formation, and irreversible oxide formation. Water uptake and electro-osmosis are investigated to improve the understanding and aid modeling of water transport in PEMFCs below 0 °C. The implication of an electro-osmotic drag coefficient less than unity is discussed in terms of proton transport mechanisms. Capillary pressure saturation relations are presented for carbon fiber paper which can both be used as gas-diffusion layers in PEMFCs. Boundary and scanning curves for imbibition and drainage are measured to further understanding of the hysteresis observed during PEMFC operation.

Electrodes in a solid oxide fuel cell (SOFC) must possess both adequate porosity and electronic conductivity to perform their functions in the cell. They must be porous to permit rapid mass transport of reactant and product gases and sufficiently conductive to permit efficient electron transfer. However, it is nearly impossible to simultaneously control porosity and conductivity using conventional design and fabrication techniques. In this dissertation, computational design and performance optimization of microarchitectured SOFCs is first investigated in order to achieve higher power density and thus higher efficiency than currently attainable in state-of-the-art SOFCs. This involves a coupled multiphysics simulation of mass transport, electrochemical charge transfer reaction, and current balance as a function of SOFC microarchitecture. Next, the fabrication of microarchitectured SOFCs consistent with the computational designs is addressed based on anode-supported SOFC button cells using the laser ablation technique. Finally, the performance of a fabricated SOFC unit cell is characterized and compared against the performance predicted by the computational model. The results show that the performance of microarchitectured SOFCs was improved against the baseline structure and measured experimental data were well matched to simulation results.

<p>The corrosion of current collector materials in MoltenCarbonate Fuel Cells (MCFC) is investigated. The essential aimsof this investigation were to study the corrosion behaviour ofdifferent materials, in varying cathode and anode MCFCenvironments, and to study the contact corrosion resistancesbetween the MCFC current collector and electrodes. For thesepurposes, pure iron, iron-chromium binary alloys and severalcommercial steels were investigated in molten carbonate meltswithin the pot-cell laboratory set-up. In addition, the contactcorrosion resistances, between an AISI 310 current collectorand two cathodes (NiO and LiCoO₂), were studied in a laboratory fuel cell.Post-tests were done to study the corrosion products formed atthe surfaces.</p><p>In cathode environments, corrosion potential increased overtime as a protective corrosion layer slowly formed. Eventually,the potential reached a stable value close to the cathodeoperating potential. The main cathode reaction, as corrosionpotential increased, changed from water reduction to oxygenreduction. Corrosion rate under the operating cathode conditiondepended on the chromium content; the higher the concentrationof chromium, the lower the corrosion rate. The corrosion ratesof ferritic steels, with high chromium content, and AISI 310were higher at the so-called outlet operating condition incomparison to the standard and so-called inlet conditions. Thecorrosion rate was higher at the beginning of the exposure,which resulted in a relatively fast corrosion layer growth thatslowed as the protective layer was formed. It was shown thatthe corrosion layers, formed on iron-chromium alloys, AISI 310and ferritic high chromium-containing steels, consisted of twolayers. The outer layer was porous and iron rich, while theinner layer was quite compact and rich in chromium and/oraluminiumTherefore, the corrosion behaviour was dependent onthe corrosion layer structure at the metal surface.</p><p>In anode environments, the beneficial behaviour of aluminiumin ferritic alloys, with high aluminium contents, was due tothe formation of aluminium oxide and/or lithium aluminium oxideat the surface. The corrosion rates at the standard and outletconditions were of the same order of magnitude, while thecorrosion rates at the inlet conditions were considerablyhigher. The lower temperatures and higher carbon dioxideconcentrations in the inlet conditions appeared to result in asurface layer deficient in aluminium. A modified theoreticalmodel was developed to evaluate the corrosion current densitiesfrom experimental polarisation curves or linear polarisationresistance measurements in anode environments. The fittingswere found to be very good.</p><p>An experimental method was developed for<i>in-situ</i>measurements of the contributions to the totalohmic losses at the cathode in a laboratory scale MCFC. Thecontact resistance between the cathode and current collectorcontributed quite a large value to the total cathodepolarization. The corrosion layer, formed between the LiCoO₂cathode and AISI 310 current collector, wasiron-rich and more porous, and contained a small amount ofcobalt. This was deemed to consist of a two-phase oxide, whichresulted in a lower conductivity. The corrosion layer, formedbetween the NiO cathode and AISI 310 current collector, wasrich in nickel. The corrosion layers on the AISI 310, incontact with the cathode, had a different composition comparedto samples immersed in carbonate melts.</p><p><b>Key words</b>: molten carbonate fuel cell (MCFC), corrosion,current collector, contact corrosion resistance.</p>

<p>Vehicles powered by fuel cells are from an environmentalaspect superior to the traditional automobile using internalcombustion of gasoline. Power systems which are based upon fuelcell technology require hydrogen for operation. The ideal fuelcell vehicle would operate on pure hydrogen stored on-board.However, storing hydrogen on-board the vehicle is currently notfeasible for technical reasons. The hydrogen can be generatedon-board using a liquid hydrogen carrier such as methanol andgasoline. The objective of the work presented in this thesiswas to develop a catalytic hydrogen generator for automotiveapplications using methanol as the hydrogen carrier.</p><p>The first part of this work gives an introduction to thefield of methanol reforming and the properties of a fuel cellbased power system. Paper I reviews the catalytic materials andprocesses available for producing hydrogen from methanol.</p><p>The second part of this thesis consists of an experimentalinvestigation of the influence of the catalyst composition,materials and process parameters on the activity andselectivity for the production of hydrogen from methanol. InPapers II-IV the influence of the support, carrier andoperational parameters is studied. In Paper V an investigationof the catalytic properties is performed in an attempt tocorrelate material properties with performance of differentcatalysts.</p><p>In the third part of the thesis an investigation isperformed to elucidate whether it is possible to utilizeoxidation of liquid methanol as a heat source for an automotivereformer. In the study which is presented in Paper VI a largeseries of catalytic materials are tested and we were able tominimize the noble metal content making the system more costefficient.</p><p>In the final part of this thesis the reformer prototypedeveloped in the project is evaluated. The reformer which wasconstructed for serving a 5 kWe fuel cell had a highperformance with near 100 % methanol conversion and COconcentrations below 1 vol% in the product stream. The resultsof this part are presented in Paper VII.</p><p><b>Keywords:</b>methanol, fuel cell, vehicle, catalyst,copper, hydrogen, on-board, steam reforming, partial oxidation,combined reforming, oxidative steam reforming, auto-thermalreforming, zinc, zirconium, chromium, aluminium oxide,manganese, characterization, temperature programmed reduction,X-ray diffraction, chemisorption, carbon monoxide, poisoning,reformer.</p>

<p>In this work characterisation methods and fuel cell hardwarewere developed for studying the components of the polymerelectrolyte fuel cell (PEFC). Humidifiers and other componentswere tested in order to develop reproducible and reliableexperimental techniques. A set-up for testing larger cells andstacks was developed.</p><p>A new type of polymer electrolyte membrane fuel cell wasdeveloped for laboratory investigations. Current collectormaterial and gas flow channels can easily be modified in thisconstruction. The electrode potentials can be measured at thegas backing layers, thereby allowing measurement of contactresistances. The use of a reference electrode is alsopossible.</p><p>Contact resistances were studied in situ as a function oftime, clamping pressure, gas pressure and current density.Ex-situ measurements were used to validate the in-situ contactresistance measurements. The validity and error sources of theapplied in-situ measurement methods with reference electrodesand potential probes were studied using both computersimulations and experiments.</p><p>An in-house membrane electrode assembly (MEA) productionline was developed. In-house produced MEAs were utilised inboth membrane degradation and mass transport studies.</p><p>The durability testing of PVDF based membranes membranes wasstudied both by fuel cell experiments and ex-situ testing.Raman spectra were measured for used membranes.</p><p>A current distribution measurement method was developed. Theeffect of inlet humidification and gas composition at thecathode side was studied. In addition, two different flow fieldgeometries were studied. The results of current distributionmeasurements were used to validate a PEFC model.</p><p>Methods for characterising gas diffusion layer (GDL)performance by fuel cell testing and ex-situ measurements weredeveloped. The performance of GDL materials was tested withvarying cell compression and cathode humidity. Porosity, poresize distribution and contact angle were determined. Electricalcontact resistance, thermal impedance and gas permeabilitieswere measured at different compression levels.</p><p>Development work on a stack with stainless steel net wascarried out as well as characterisation studies of differentstack components. Thermal impedances and flow fieldpermeability were measured.</p><p>Mass transport limitations in the cathodes were studied byvarying the electrode thickness, partial pressure and humidityof oxygen.</p><p><b>Keywords:</b>polymer electrolyte membrane fuel cell (PEFC),contact resistance, clamping pressure, stainless steel,membrane degradation, current distribution, gas diffusionlayer, stack, thermal impedance, permeability.</p>

<p>This thesis deals with modeling of two types of fuel cells:the polymer electrolyte fuel cell (PEFC) and the directmethanol fuel cell (DMFC), for which we address four majorissues: a) mass transport limitations; b) water management(PEFC); c) gas management (DMFC); d) thermal management.</p><p>Four models have been derived and studied for the PEFC,focusing on the cathode. The first exploits the slenderness ofthe cathode for a two-dimensional geometry, leading to areduced model, where several nondimensional parameters capturethe behavior of the cathode. The model was extended to threedimensions, where four di.erent flow distributors were studiedfor the cathode. A quantitative comparison shows that theinterdigitated channels can sustain the highest currentdensities. These two models, comprising isothermal gasphaseflow, limit the studies to (a). Returning to a two-dimensionalgeometry of the PEFC, the liquid phase was introduced via aseparate flow model approach for the cathode. In addition toconservation of mass, momentum and species, the model wasextended to consider simultaneous charge and heat transfer forthe whole cell. Di.erent thermal, flow fields, and hydrodynamicconditions were studied, addressing (a), (b) and (d). A scaleanalysis allowed for predictions of the cell performance priorto any computations. Good agreement between experiments with asegmented cell and the model was obtained.</p><p>A liquid-phase model, comprising conservation of mass,momentum and species, was derived and analyzed for the anode ofthe DMFC. The impact of hydrodynamic, electrochemical andgeometrical features on the fuel cell performance were studied,mainly focusing on (a). The slenderness of the anode allows theuse of a narrow-gap approximation, leading to a reduced model,with benefits such as reduced computational cost andunderstanding of the physical trends prior to any numericalcomputations. Adding the gas-phase via a multiphase mixtureapproach, the gas management (c) could also be studied.Experiments with a cell, equipped with a transparent end plate,allowed for visualization of the flow in the anode, as well asvalidation of the two-phase model. Good agreement betweenexperiments and the model was achieved.</p><p><b>Keywords:</b>Fuel cell; DMFC; PEFC; one-phase; two-phase;model; visual cell; segmented cell; scale analysis; asymptoticanalysis.</p>

<p>Fuel cells are promising candidates for converting chemical energy into electrical energy. The Molten Carbonate Fuel Cell (MCFC) is a high temperature fuel cell that produces electrical energy from a variety of fuels containing hydrogen, hydrocarbons and carbon monoxide. Since the waste heat has a high temperature it can also be used leading to a high overall efficiency.</p><p>Material degradation and the cost of the components are the problems for the commercialisation of MCFC. Although there are companies around the world starting to commercialise MCFC some further cost reduction is needed before MCFC can be fully introduced at the market.</p><p>In this work, alternative materials for three different components of MCFC have been investigated. The alternative materials should have a lower cost compared to the state-of-the-art materials but also meet the life-time goal of MCFC, which is around 5 years. The nickel dissolution of the cathode is a problem and a cathode with lower solubility is needed. The dissolution of nickel for three alternative cathode materials was investigated, where one of the materials had a lower solubility than the state-of-the-art nickel oxide. This material was also tested in a cell and the electrochemical performance was found to be comparable with nickel oxide and is an interesting candidate.</p><p>An inexpensive anode current collector material is also desired. For the anode current collector, the contact resistance should be low and it should have good corrosion properties. The two alternative materials tested had low contact resistance, but some chromium enrichment was seen at the grain boundaries. This can lead to a decreased mechanical stability of the material. In the wet-seal area, the stainless steel used as bipolar/separator plate should be coated. An alternative process to coat the stainless steel, that is less expensive, was evaluated. This process can be a suitable process, but today, when the coating process is done manually there seems to be a problem with the adherence.</p><p>This work has been a part of the IRMATECH project, which was financed by the European Commission, where the partners have been universities, research institutes and companies around Europe.</p>

Fibrous porous media are widely used in various industries such as biomedical engineering, textiles, paper, and alternative energy. Often these porous materials are formed into composite materials, using subsequent manufacturing steps, to improve their properties. There is a strong correlation between system performance and the transport and mechanical properties of the porous media, in raw or composite form. However, these properties depend on the final pore structure of the material. Thus, the ability to manufacture fibrous porous media, in raw or composite forms, with an engineered structure with predictable properties is highly desirable for the optimization of the overall performance of a relevant system. To date, the characterization of the porous media has been primarily based on reverse design methods i.e., extracting the data from existing materials with image processing techniques. The objective of this research is to develop a methodology to enable the virtual generation of complex composite porous media with tailored properties, from the implementation of a fibrous medium in the design space to the simulated coating of this media representative of the manufacturing space. To meet this objective a modified periodic surface model is proposed, which is utilized to parametrically generate a fibrous domain. The suggested modeling approach allows for a high-degree of control over the fiber profile, matrix properties, and fiber-binder composition. Using the domain generated with the suggested geometrical modeling approach, numerical simulations are executed to simulate transport properties such as permeability, diffusivity and tortuosity, as well as, to directly coat the microstructure, thereby forming a complex composite material. To understand the interplay between the xxiii fiber matrix and the transport properties, the morphology of the virtual microstructure is characterized based on the pore size, chord length and shortest path length distributions inside the porous domain. In order to ensure the desired properties of the microstructure, the fluid penetration, at the micro scale, is analyzed during the direct coating process. This work presents a framework for feasible and effective generation of complex porous media in the virtual space, which can be directly manufactured.