Non-woven carbon paper is a porous material composed of carbon composite and is the preferred material for use as the gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cells. This material is both chemically and mechanically stable and provides a free path for diffusion of reactants and removal of products and is electrically conductive for transport of electrons. The transport of species in the GDL has a direct effect on the overall reaction rate in the catalyst layer. Numerical simulation of these transport phenomena is dependent on the transport properties associated with each phenomenon. Most of the available correlations in literature for these properties have been formulated for spherical shell porous media, sand and rock, which are not representative of the structure of the GDL. Hence, the objective of this research work is to investigate the transport properties (diffusion coefficient, thermal conductivity, electrical conductivity, intrinsic and relative permeability and the capillary pressure) of the GDL using experimental and numerical techniques. In this thesis, a three-dimensional reconstruction of the complex, anisotropic structure of the GDL based on stochastic models is used to estimate its transport properties. To establish the validity of the numerical results, an extensive comparison is carried out against published and measured experimental data. It was found that the existing theoretical models result in inaccurate estimation of the transport properties, especially in neglecting the anisotropic nature of the layer. Due to the structure of the carbon paper GDL, it was found that the value of the transport properties in the in-plane direction are much higher than that in the through-plane direction. In the in-plane direction, the fibers are aligned in a more structured manner; hence, the resistance to mass transport is reduced. Based on the numerical results presented in this thesis, correlations of the transport properties are developed. Further, the structure of the carbon paper GDL is investigated using the method of standard porosimetry. The addition of Teflon was found have little effect on the overall pore volume at a pore radius of less than 3 micro meters. A transition region where the pore volume increased with the increase in pore radius was found to occur for a pore radius in the range 3<5.5 micro meters regardless of the PTFE content. Finally, the reduction of the overall pore volume was found to be proportional to the PTFE content. The diffusion coefficient is also measured in this thesis using a Loschmidt cell. The effect of temperature and PTFE loading on the overall diffusibility is examined. It was found that the temperature does not have an effect on the overall diffusibility of the GDL. This implies that the structure of the GDL is the main contributor to the resistance to gas diffusion in the GDL. A comparison between the measured diffusibility and that predicted by the existing available models in literature indicate that these models overpredict the diffusion coefficient of the GDL significantly. Finally, both the in-plane and through-plane thermal conductivity were measured using the method of monotonous heating. This method is a quasi-steady method; hence, it allows the measurement to be carried out for a wide range of temperatures. With this method, the phase transformation due to the presence of PTFE in the samples was investigated. Further, it was found that the through-plane thermal conductivity is much lower than its in-plane counterpart and has a different dependency on the temperature. Detailed investigation of the dependency of the thermal conductivity on the temperature suggests that the thermal expansion in the through-plane direction is positive while it is negative in the in-plane direction. This is an important finding in that it assists in further understanding of the structure of the carbon paper GDL. Finally, the thermal resistance in the through-plane direction due to fiber stacking was investigated and was shown to be dependent on both the temperature and compression pressure.

The polymer electrolyte membrane fuel cell (PEMFC) is an electrochemical device used for the production of power, which is a key for the transition towards green and renewable power delivery devices for mobile, stationary and back-up power applications. PEMFCs consume hydrogen and oxygen to produce power, water and heat. The transient start-up from sub-zero freezing temperature conditions is a problem for the successful, undamaged and unhindered operation. The generation and presence of water in the PEMFC stack in such an environment leads to the formation of ice that hinders the flow of gases, causes morphological changes in the membrane electrode assembly (MEA) leading to reversible and irreversible degradation of stack performance. Start-up performance is highly dependent on start-up operational conditions and procedures. The previous state of the stack will influence the ability to perform upon the next start-up and operation. Water generated during normal operation is vital and improves performance when properly managed. Liquid water present at shut-down can form ice and cause unwanted start-up effects. This phase change may cause damage to the MEA and gas diffusion media due to volume expansion. Removal of high water content at shutdown decreases proton conductivity which can delay start-up times. The United States Department of Energy (DOE) has established a set of criteria that will make fuel cell technology viable when attained. As specified by DOE, an 80 kWe fuel cell will be required by 2015 to reach 50% power in 30 seconds from start-up at an ambient temperature of -20°C. This work investigates freeze start-up in a multi-kilowatt stack approaching both shut-down conditioning and start-up operations to improve performance, moderate fuel cell damage and determine the limits of current stack technology. The investigation involved a Hydrogenics Corporation 5 kW 506 series fuel cell stack. The investigation is completed through conditioning the fuel cell start-up performance at various temperatures ranging from -5°C to below -20°C. The control of system start-up temperature is achieved with an environmental chamber that maintains the desired set point during dwell time and start-up. The supply gases for the experiment are conditioned at ambient stack temperature to create a realistic environment that could be experienced in colder weather climates. Temperature controls aim to maintain steady ambient temperatures during progressive start-up in order to best simulate ambient conditions. The control and operation of the fuel cell is maintained by the use of a fuel cell automated test station (FCATS™). FCATS supplies gas feeds, coolant medium and can control temperature and reactant humidity in reactants according to a prescribed procedure for continuous operation. The iv collection of data occurs by the same system recording cell voltage, temperatures, pressures, flow rates and current densities. A procedural start-up and characterization are conducted in order improve start-of performance and examine reactant flows, coolant activation time, stack conditioning and the effects by freezing temperatures. The resulting degradation is investigated by polarization curves and various ex-situ measurements. In this work, it was found that freeze start-up of a fuel cell stack can be aided and managed by conditioning the stack at shut-down and applying a procedure to successfully start-up and mitigate the damage that freezing can cause.

A single polymer electrolyte membrane fuel cell (PEMFC) is comprised of several sub-millimetre thick layers of varying porosity sandwiched together. The thickness of each layer, which typically ranges from 10 to 200μm, is kept small in order to minimize the transport resistance of heat, mass, electrons, and protons, that limit reaction rate. However, the thickness of these materials presents a significant challenge to engineers characterizing the transport properties through them, which is of considerable importance to the development and optimization of fuel cells. The objective of this research is to address the challenges associated with measuring the heat conduction and gas diffusion transport properties of thin porous media used in PEMFCs. An improvement in the accuracy of the guarded heat flow technique for measuring thermal conductivity and the modified Loschmidt Cell technique for measuring gas diffusivity are presented for porous media with a sub-millimetre thickness. The improvement in accuracy is achieved by analyzing parameters in each apparatus that are sensitive to measurement error and have the largest contribution to measurement uncertainty, and then developing ways to minimize the error. The experimental apparatuses are used to investigate the transport properties of the gas diffusion layer (GDL) and the microporous layer (MPL), while the methods would also be useful in the study of the catalyst layer (CL). Gas diffusion through porous media is critical for the high current density operation of a PEMFC, where the electrochemical reaction becomes rate-limited by the diffusive flux of reactants reaching reaction sites. However, geometric models that predict diffusivity of the GDL have been identified as inaccurate in current literature. Experimental results give a better estimate of diffusivity, but published works to date have been limited by high measurement uncertainty. In this thesis, the effective diffusivity of various GDLs are measured using a modified Loschmidt cell and the relative differences between GDLs are explained using scanning electron microscopy and the method of standard porosimetry. The experimental results from this study and others in current literature are used to develop a generalized correlation for predicting diffusivity as a function of porosity in the through-plane direction of a GDL. The thermal conductivity and contact resistance of porous media are important for accurate thermal analysis of a fuel cell, especially at high current densities where the heat flux becomes large. In this thesis, the effective through-plane thermal conductivity and contact resistance of the GDL and MPL are measured. GDL samples with and without a MPL and coated with 30%-wt. PTFE are measured using the guarded steady-state heat flow technique described in the ASTM standard E 1225-04. Thermal contact resistance of the MPL with the iron clamping surface was found to be negligible, owing to the high surface contact area. Thermal conductivity and thickness of the MPL remained constant for compression pressures up to 15bar at 0.30W/m°K and 55μm, respectively. The thermal conductivity of the GDL substrate containing 30%−wt. PTFE varied from 0.30 to 0.56W/m°K as compression was increased from 4 to 15bar. As a result, the GDL contain- ing MPL had a lower effective thermal conductivity at high compression than the GDL without MPL. At low compression, differences were negligible. The constant thickness of the MPL suggests that the porosity, as well as heat and mass transport properties, remain independent of the inhomogeneous compression by the bipolar plate. Despite the low effective thermal conductivity of the MPL, thermal performance of the GDL can be improved by exploiting the excellent surface contact resistance of the MPL while minimizing its thickness.

Excessive water formation inside the polymer electrolyte membrane (PEM) fuel cell’s structures leads to the flooding of the cathode gas diffusion layer (GDL) and cathode gas flow channels. This results in a negative impact on water management and the overall cell performance. Liquid water generated in the cathode catalyst layer and the water moved from anode to cathode side due to electro-osmotic drag transport through the GDL to reach the gas flow field channels, where it is removed by air cathode gas stream. Due to high and uniform capillary force distribution effect of the pores through the GDL plane and surface tension between the water droplets and gas flow field channels surfaces, liquid water tends to block/fill the pores of the GDL and stick to the surface of the GDL and gas flow channels. Therefore, it is difficult to remove the trapped water in GDL structure which can lead to flood of the PEM fuel cell. The GDL surfaces are commonly treated uniformly with a hydrophobic material in order to overcome the flooding phenomena inside PEM fuel cell. Despite the importance impact of the surface wettability of both channel and GDL surface characteristics especially for the cathode side on the water management, few experimental studies have been conducted to investigate the effect of the two-phase flow in cathode gas flow channel and their crucial role. The work presented in this thesis covers contributions that provide insight, not only into the investigation of the effects of hydrophobic cathode GDL and cathode gas flow channels, on water removal, two phase flow inside the channel, and on PEM fuel cell performance, but also the superhydrophobic and superhydrophilic GDLs and gas flow channels effects. Further, the effects of a novel GDL designs with sandwich and gradient wettability with driving capillary force through GDL plane have been investigated. Two-phase flow especially in the cathode gas flow field channels of PEM fuel cell has a crucial role on water removal. Hence, in this research, ex-situ investigations of the effects of channels with different surface wettability; superhydrophobic, hydrophobic, slightly hydrophobic, and superhydrophilic on the two-phase flow characteristics have been tested and visualized at room temperature. Pressure drop measurements and two-phase flow visualization have been carried out using high speed camera. The effect of the various coating materials on graphite and GDL surface morphology, roughness, static contact angle (θ), and sliding contact angle (α) have been investigated using scanning electron microscopy (SEM), Profilometry, and sessile drop technique, respectively. It has been observed that the two-phase flow resistance is considerably affected by surface wettability of the channels. Further, the overall cell performance can be improved by superhydrophobic gas flow channels mainly at high current density over slightly hydrophobic and superhydrophilic cases tested. In addition, sandwich wettability GDL has been coated with a silica particle/ Polydimethylsiloxane (PDMS) composite. The porometric characteristics have been studied using, method of standard porosimetry (MSP). It has been found that sandwich wettability GDL has superhydrophobic surfaces with (θ = 162±2°), (α = 5±1°), and the internal pores are hydrophilic, while the mean pore radius is 7.1μm. This shows a low resistance to gas transport. On the other hand, performance testing indicates that (PEM) fuel cell equipped with sandwich wettability GDL results in the best performance compared to those with raw (non-coated) (slightly hydrophobic), PTFE coated (commercial with micro-porous layer (MPL)) (superhydrophobic), and silica coated (superhydrophilic) GDL. The wettability gradient has been introduced through plane of the one side hydrophobic GDL by coating one side of non-coated GDL with 15 wt. % of PTFE solution; however, the other side remains uncoated. The effects of wettability gradient on the water removal rate, droplet dynamics, and PEM fuel cell performance have been covered in this thesis. Water removal rate is determined using a 20 ml syringe barrel, wherein a 13 mm diameter GDL token is fixed on the barrel opening. The droplets penetrating through the GDL are visualized via a high speed camera to study the droplets’ dynamic characteristics. The GDL wettability gradient has a significant impact on water removal rate, droplets’ dynamic characteristics, and consequently enhances the overall PEM fuel cell performance.

abstract: Proton exchange membrane fuel cells (PEMFCs) run on pure hydrogen and oxygen (or air), producing electricity, water, and some heat. This makes PEMFC an attractive option for clean power generation. PEMFCs also operate at low temperature which makes them quick to start up and easy to handle. PEMFCs have several important limitations which must be overcome before commercial viability can be achieved. Active areas of research into making them commercially viable include reducing the cost, size and weight of fuel cells while also increasing their durability and performance. A growing and important part of this research involves the computer modeling of fuel cells. High quality computer modeling and simulation of fuel cells can help speed up the discovery of optimized fuel cell components. Computer modeling can also help improve fundamental understanding of the mechanisms and reactions that take place within the fuel cell. The work presented in this thesis describes a procedure for utilizing computer modeling to create high quality fuel cell simulations using Ansys Fluent 12.1. Methods for creating computer aided design (CAD) models of fuel cells are discussed. Detailed simulation parameters are described and emphasis is placed on establishing convergence criteria which are essential for producing consistent results. A mesh sensitivity study of the catalyst and membrane layers is presented showing the importance of adhering to strictly defined convergence criteria. A study of iteration sensitivity of the simulation at low and high current densities is performed which demonstrates the variance in the rate of convergence and the absolute difference between solution values derived at low numbers of iterations and high numbers of iterations. / Dissertation/Thesis / M.S.Tech Chemistry 2011

abstract: CubeSats are a newly emerging, low-cost, rapid development platform for space exploration research. They are small spacecraft with a mass and volume of up to 12 kg and 12,000 cm3, respectively. To date, CubeSats have only been flown in Low Earth Orbit (LEO), though a large number are currently being designed to be dropped off by a mother ship on Earth escape trajectories intended for Lunar and Martian flyby missions. Advancements in propulsion technologies now enable these spacecraft to achieve capture orbits around the moon and Mars, providing a wealth of scientific data at low-cost. However, the mass, volume and launch constraints of CubeSats severely limit viable propulsion options. We present an innovative propulsion solution using energy generated by onboard photovoltaic panels to electrolyze water, thus producing combustible hydrogen and oxygen for low-thrust applications. Water has a high storage density allowing for sufficient fuel within volume constraints. Its high enthalpy of formation provides more fuel that translates into increased &#8710;V and vastly reduced risk for the launch vehicle. This innovative technology poses significant challenges including the design and operation of electrolyzers at ultra-cold temperatures, the efficient separation of the resultant hydrogen and oxygen gases from liquid water in a microgravity environment, as well as the effective utilization of thrust to produce desired trajectories. Analysis of the gas combustion and flow through the nozzle using both theoretical equations and finite-volume CFD modeling suggests an expected specific impulse of 360 s. Preliminary results from AGI's Satellite Toolkit (STK) indicate that the &#916;V produced by the system for an 8kg CubeSat with 6kg of propellant in a LEO orbit (370 km altitude) is sufficient for an earth escape trajectory, lunar capture orbit or even a Mars capture orbit. These results suggest a promising pathway for an in-depth study supported by laboratory experiments to characterize the strengths and weaknesses of the proposed concept. / Dissertation/Thesis / Masters Thesis Aerospace Engineering 2015

abstract: As the demand for renewable and alternative energy continues to increase with both large industrial companies and average homeowners, there continues to be a challenge of efficient energy storage. Several main alternative energy producers such as wind turbines, hydroelectric dams, and solar photovoltaic arrays have become more commonly used over the past decade for generating energy. One of the most common issues with these alternative energy producers is the intermittent production and supply of energy due to fluctuations in weather conditions, peak loads, and instantaneous power draw. To counteract these issues, storage units such as battery banks and proton exchange membrane fuel cells are introduced to provide electricity for the unmet energy demands. In this study, a solar photovoltaic array and fuel cell hybrid system has been set up to provide the energy needs for an average Arizona residential household. A bench test setup has revealed that a solar photovoltaic array and the fuel cell hybrid system can produce enough energy to power an Arizona household that on average consumes 37.7 kWh/d. Additionally, a Mathworks MATLAB/Simulink model of the hybrid system has been created to simulate specific scenarios which provide insight into the system’s reaction to various conditions such as varying solar irradiance and temperature variables and poor weather conditions. Finally, the economic impact of the hybrid system was simulated using HOMER Legacy to analyze the cost effectiveness of a 25-year project. / Dissertation/Thesis / Masters Thesis Engineering 2017

A multifunctional structural polymer electrolyte membrane (PEM) fuel cell was designed, developed and manufactured. The structural fuel cell was designed to represent the rear rib section of an aircraft wing. Custom membrane electrode assemblies (MEA s) were manufactured in house. Each MEA had an active area of 25cm2.The platinum loading on each electrode (anode and cathode) was 0.5mg/cm2. Sandwiched between the electrodes was a Nafion 212 electrolyte membrane. Additional components of the structural fuel included metallic bipolar plates and end plates. Initially all the components were manufactured from aluminium in order for the structural fuel cell to closely represent an aircraft wing rib. However due to corrosion problems the bipolar plate had to be manufactured from marine grade 361L stainless steel with a protective coating system. A number of different protective coating systems were tried with wood nickel strike, followed by a 5μm intermediate coat of silver and a 2μm gold top coat being the most successful. Full fuel cell experimental setup was developed which included balance of plant, data acquisition and control unit, and a mechanical loading assembly. Loads were applied to the structural fuel cells tip to achieve a static deflection of ±7mm and dynamic deflections of ±3mm, ±5mm, and ±7mm. Static and dynamic torsion induced 1° to 5° of twist to the structural fuel cell tip. Polarisation curves were produced for each load case. Finite element analysis was used to determine the structural fuel cell displacement, and stress/strain over the range of mechanical loads. The structural fuel cells peak power performance dropped 3.9% from 5.5 watts to 5.3 watts during static bending and 2% from 6.2 watts to 6.1 watts during static torsion. During dynamic bending (2000 cycles) the structural fuel cell peak power performance dropped 11% from 6.7 watts to 6 watts (3mm deflection at 190N), 23% from 6.3 watts to 4.8 watts (5mm deflection at 270N), and 41% from 7.2 watts to 5 watts (7mm deflection at 350N). During dynamic torsion (2000 cycles) the structural fuel cell peak power performance dropped 16% from 6 watts to 5.1 watt (3° of torsional loading), and 30% from 6.4 watts to 4.3 watts (5° of torsional loading). The simulated (finite element modelling) displacement of -6.6mm (At maximum bending load of 364.95N) was within 9% of the actual measured displacement of -7.2mm at 364.95N. Furthermore the majority of the simulated strain values were within 10% of the actual measured strain for the structural fuel cell.

This study explores the use of the information theory entropy equation in representations of videos for children. The calculated rates of information in the videos are calibrated to the corresponding perceived rates of information as elicited from the twelve 7 to 10 year old girls who were shown video documents. Entropy measures are calculated for several video elements: set time, set incidence, verbal time, verbal incidence, set constraint, nonverbal dependence, and character appearance. As hypothesized, mechanically calculated entropy measure (CEM) was found to be sufficiently similar to perceived entropy measure (PEM) made by children so that they can be used as useful and predictive elements of representations of children’s videos. The relationships between the CEM and the PEM show that CEM could stand for PEM in order to enrich representations for video documents for this age group. Speculations on transferring the CEM to PEM calibration to different age groups and different document types are made, as well as further implications for the field of information science.

Europeiska Unionen presenterade den 8 juni 2020 sin vätgasstrategi i syfte för att minska koldioxidutsläppen. Det unionen vill uppnå med sin vätgasstrategi är att uttnytja konceptet Power-to-X där elektricitet omvandlas till energi. Om elektricitetkällan kommer från förnyelsebar energi kommer grön vätgas produceras. Problemet med vätgas idag är lagring, transport och hanteringstrukturen för ämnet men vätgas kan lagras i flertal applikationer. En av de mest lovande lagringsalternativen är ammoniak som bildas när vätgas med kvävgas reagerar med varandra via ammoniaksyntes. Eftersom vätgasproduktionen idag använder fossila bränslen kommer även dess applikation göra det, men med grön vätgas kommer dess applikation även att bli grön. Idag står ammoniakproduktion för 2 % av fossilbränsleanvändning globalt och frigör mer än 400 miljoner ton CO2 årligen. Dessa utsläpp skulle försvinna om produktionen av ammoniak gjordes med Power-to-X konceptet. Syftet med detta examensarbete är att undersöka förutsättningarna ur ett ekonomiskt, tekniskt och säkerhetsmässigt hållbart Power-to-X koncept i form av en Grön Ammoniakanläggning i Norra Sverige. Det innebär att processer för en ammoniakproduktion skall analyseras ur ett teknisk synvinkel där fokus på funktion mot grön ammoniak är i fokus. Den ekonomiska synvinkeln innebär vad kapitalkostnaden (CAPEX) blir för anläggningen samt driftkostnaden (OPEX) som processen får. Arbetet innehåller först en analys av de processer som krävs för att kunna producera ammoniak. Därefter en analys över möjliga tekniker för dessa processer, hur väl de fungerar mot grön ammoniak och vilka antaganden som är i detta arbete. Anläggningen skulle vara storskalig vilket innebär en produktion på 500 ton NH3 $/dag. Det är även antaget en kontinuerlig eltillförsel samt att elnät redan är tillgänglig. Detta gav att vätgasproduktionen gjordes med en PEM-elektrolys, där kvävgas fås från kryogen destillation och ammoniak produceras med HB-processen. Resultatet visades att anläggningens CAPEX och OPEX blev 2 820 MSEK respektive 1 272 MSEK/år. Den dominerande faktorn för kapitalkostnaden var för vätgasproduktion som utgjorde 60 % av CAPEX. Den höga kostnaden för PEM-elektrolys är dels för att utvecklingen av processen inte är fullbordad, där utvecklingen för tekniken skulle kunna ge en stor kostnadsreducering. Det elbehov som anläggningen kräver är 1,6 TWh och och utgör en påverkan på OPEX är 55,4 %. Den process som kräver mest energi är vätgasproduktionen vilket omfattar 94 % av hela anläggningens totala elbehov. En stor anledning till de dyra driftkostnaderna är elpriset. I detta arbete valdes elpriset till ett medelvärde för SE1 i Sverige under en 10 års period. I ett verkligt scenario hade vätgasproduktionen kunnat optimeras för uppnå billigare drift.