Electrochemical impedance spectroscopy (EIS) is a powerful technique that can be applied in-situ to deconvolute the various loss mechanisms in the polymer electrolyte fuel cell (PEFC) that occur at different rates. The frequency response of a PEFC that results from EIS is in essence characterised by energy dissipating and energy storing elements of the cell. It can be represented by an equivalent circuit that is composed of resistors and capacitors respectively. By understanding the arrangement and magnitude of the electrical components in the equivalent electrical circuit, it is possible to generate a deeper understanding of how and where the electrical energy that is generated due to the redox reaction is being dissipated and retained within the real physical system. Although the use of equivalent circuits is often an adequate approach, some electrochemical processes are not adequately described by electrical components. In which case, it is necessary to adopt a more rigorous approach of describing processes through the use of differential equations to describe the physics of the electrochemical system at the frequency domain. Studies in the literature have attempted to construct mathematical models to describe the impedance response of the cathode catalyst layer (CCL) based on conservation equations describing the electrochemical and diffusion processes. However this has resulted in a complicated mathematical analysis which in turn results in complicated solutions. The resulting equations cannot be easily validated against real-world EIS measurements and only analytical results have been reported. In this thesis a mathematical model to describe the impedance response of the CCL has been developed. This model is derived from fundamental electrochemical theory describing the physics of the CCL. The mathematical treatment is simplified by taking into account some considerations based on the EIS theory. The resulting model can be easily applied to real-world EIS measurements of PEFCs and presents parameters commonly known in the electrochemical area. The scientific contribution of this doctoral thesis is mainly divided in two sections: Modelling and Application. The first step of the modelling section develops an equation describing charge conservation in the CCL and together with Ohm s Law equation accounting for ionic conduction, predicts the impedance response of the CCL at low currents. The second step includes the change of oxygen concentration during the oxygen reduction reaction (ORR) into the equation accounting for CCL low current operation. The study of mass transport in the CCL is very complex; the literature has treated it with simplifications and approximations. The finite diffusion distance for oxygen to reach the reaction sites in the CCL forms a complicated network of multi-phase parallel and serial paths and can change in dimension at different operating conditions (flooding, drying). In the mathematical treatment of this doctoral thesis the finite diffusion distance and surface concentration of oxygen in the CCL are considered to be independent of the thickness of the CCL. EIS reflects only bulk measurements based on the total CCL thickness. Even though this results in an over-simplification for the oxygen diffusion in the total CCL, this approach simplifies the mathematical treatment to predict the impedance response of the CCL at high current operation, and as result it can be successfully validated against real-world EIS measurements. In the application section the model is applied with real-world EIS measurements of PEFCs. First the model is applied with EIS measurements presenting inductive effects at high frequencies. The model reveals mechanisms masked at high frequencies of the impedance spectrum by inductance effects. The results demonstrate that the practice of using the real part of the Nyquist plot where the imaginary part is equal to zero to quantify the ohmic resistance in PEFCs can be subject to an erroneous interpretation due to inductive effects at high frequencies. Secondly the model is applied to cathode impedance data obtained through a three-electrode configuration in the measurement system and gives an insight into the mechanisms represented at low frequencies of the impedance complex-plot. The model predicts that the low frequency semicircle in PEFC measurements is attributed to low equilibrium oxygen concentration in the CCL-gas diffusion layer (GDL) interface and low diffusivity of oxygen through the CCL. In addition the model is applied with simultaneous EIS measurements in an Open-Cathode PEFC stack. The factors that limit the performance of the PEFC stack are evaluated with simultaneous EIS measurements and the model. The results show that the change in impedance response of individual cells within the stack is attributed to oxygen limitations, degradation in membrane electrode assemblies (MEAs) and temperature distribution. This EIS knowledge enables an assessment of the state of health in operational fuel cell stacks. In the last section of the application section, the mathematical model translated in the time domain via reverse Laplace Transform predicts the current distribution through the CCL. This provides information to improve the performance of the CCL as well as determine the uptake of product water in the membrane. Finally the conclusions and future work are presented. This doctoral thesis has established a backbone understanding of how the electrochemical and diffusion mechanisms relate to the electrochemical impedance spectra of PEFCs. The goal of a future work is to develop this EIS knowledge into a real-time EIS system for non-intrusive diagnostics of degradation in operational PEFCs. This implies a modification of the model to consider oxygen transport through the CCL thickness as part of a multi-species mixture using mass transport theory including concentrated solution theory to fuel cell engineering.

Increasing the catalyst utilization is one way to improve the performance of a fuel cell. In this study, the hydrophobic pillared micro structures (HPMS) is used to increase the oxygen/ catalyst layer interface and thereby raise the performance by about 40%. By using the HPMS of a large size, the performance is improved by around 20%, which is just the same as the increment of the contact surface between oxygen and catalyst layer. By halving the loading of the small HPMS and thus increasing the contact surface between oxygen and the catalyst layer by half of the previous amount, the performance increase is also halved. These experimental results indicate that the main reaction zone is near the surface between oxygen and the catalyst layer. Consequently, there is no obvious decrement in the performance when the Pt loading of cathode is halved, just as the experimental results indicated. As a result, the utilization of catalyst is raised substantially.

Different groups in the forestry sector were likely to have different views regarding the responsibility for an environmentally sustainable forestry and the benefits of certification and this study aimed to examine the attitudes that existed in the forestry sector to the certification schemes FSC and PEFC. In the study a survey and a questionnaire were sent out to different groups. The investigation was made with respect to three groups: forest owners, forest contractors and employees working with the certification schemes on a daily basis at the forest company Södra. The groups agreed that the owner of the forest had the main responsibility for an environmentally sustainable forestry. The study revealed that all three groups highly valued the environmental aspect of certification. Forest owners also identified financial gain as a benefit. The forest contractors felt that the certification gave orderliness to their work but also saw an economic cost from being certified. The forest company employees found certification as an access to a widermarket with certified products. Half of the forest owners estimated their knowledge of the certification schemes to be inadequate while forest contractors and the employees of Södra valued their knowledge as sufficient. The forest owners had less education about the certification schemes than the other groups. / Olika grupper i skogsbranschen riskerar att ha olika syn när det gäller ansvaret för ett miljömässigt hållbart skogsbruk och nyttan med certifiering och den här studien gjordes för att undersöka vilka inställningar som fanns i skogsbranschen till certifieringssystemen FSC och PEFC. Arbetet gjordes med avseende på tre grupper: skogsägare, skogsentreprenörer och anställda som arbetar på skogsföretaget Södra med certifiering i sitt dagliga arbete. I studien skickades en enkät och ett frågeformulär ut. Undersökningen visade att de tre grupperna värderade miljömässigt hållbart skogsbruk med certifiering högt. Som skogsägare fick man en certifieringspremie för det certifierade virke som man sålde. Skogsägarna såg också en ekonomisk vinning med certifiering. Skogsentreprenörerna ansåg att certifiering i högre grad gav ordning och reda i deras arbete men att det fanns en ekonomisk kostnad med att vara certifierad. Skogsföretagsgruppen menade att certifierade produkter gav tillgång till en större marknad för skogsföretaget. Grupperna var överens om att ansvaret för ett miljömässigt hållbart skogsbruk låg på de/den som ägde skogsmarken. Samtidigt uppskattade hälften av skogsägarna att deras kunskaper om certifieringarna var otillräckliga medan skogsentreprenörerna och skogsföretagsgruppen ansåg att deras kunskaper var tillräckliga. Detta kunde bero på att skogsägarna fick mindre utbildning av certifiering än de andra två grupperna. Det här arbetet har visat att grupperna inte har olika syn när det gäller vikten av att ett miljömässigt hållbart skogsbruk bedrivs.

En viktig målsättning i den svenska skogspolitiken är att skogsbruket ska bedrivas på ett hållbart sätt och i Skogsvårdslagen är produktionsmålen och miljömålen jämställda. Skogsägare kan utöver detta åta sig att certifiera sin fastighet, antingen genom FSC eller PEFC. Certifiering är ett marknadsdrivet, frivilligt åtagande och handlar om att ta ett större ansvar för miljö och natur än vad Skogsvårdslagen kräver.  Syftet med denna studie är att jämföra förekomst av naturvärden på certifierade fastigheter och icke-certifierade fastigheter genom fältinventering av utvalda kriterier. Kriterierna valdes utifrån mätbara och synliga indikatorer på naturvärden och utifrån faktorer som speglar variationsrikedom, hänsyn, kontinuitet, flora-och faunaobjekt och återväxt. I denna studie har vi arbetat fram en funktionell inventeringsmall i syfte att kartlägga naturvärden på fastighetsnivå.  En slutsats från inventeringen var att naturens egna förutsättningar är en viktig faktor för förekomsten av naturvärden i högre grad än om fastigheten är certifierad eller inte.

Skogsägare har möjlighet att certifiera sin skogsfastighet i enlighet med FSC och/eller PEFC. Flera av de skogsägare som certifierar sin fastighet känner dock inte till PEFC:s krav. Studiens syfte var undersöka synen på ansvar och behovet av kunskap och information om PEFC. Endast 66 % av skogsägarna ansåg att det var de själva som stod ytterst ansvarig för sin skogsfastighet och lika stor andel ansåg att informationen de fått om PEFC varit tillräcklig. Drygt hälften av skogsägarna ansåg sig ha tillräcklig kunskap om PEFC. Studien visade att flera olika faktorer spelar in på skogsägarnas syn på ansvar och behovet av kunskap och information.

Syftet med studien var att beskriva och analysera hur Sydveds certifieringsprocess upplevs av företagets virkesleverantörer. Studien genomfördes som en enkätundersökning via post och e-post. De frågeställningar som utgjorde stommen av arbetet handlade huvudsakligen om upplevelsen kring certifieringsprocessen, virkesleverantörernas kunskap om certifiering, motiv till certifiering och hur dessa påverkar upplevelsen av certifieringsprocessen. Fokus låg även på huruvida den geografiska spridningen hade någon betydelse för upplevelsen liksom om virkesleverantörerna upplevde ett behov av en förändring i certifieringsprocessen. Sammanfattningsvis visade resultaten främst på en förbättringspotential gällande återkoppling och kunskap. Det som Sydved rekommenderas att utveckla är olika delar av certifieringsprocessen där tyngdpunkten bör ligga på att få ett enhetligt sätt att jobba på inom organisationen. Hela certifieringsprocessen, från introduktion till revision och återkoppling bör ha tydliga riktlinjer, då kedjan inte är starkare än sin svagaste länk.

A transient model of the cathode catalyst layer of a proton exchange membrane fuel cell is presented. The catalyst layer structure can be described as a superposition of the polymer membrane, the backing layer, and some additional platinum particles. The model, which incorporates some of the features of the pseudo-homogeneous models currently present in the literature, considers the kinetics of the electrochemical reaction taking place at the platinum surface, the proton transport through the polymer agglomerates, and the oxygen and water transport within the pores as well as the membrane material of the catalyst layer. Due to the lower porosity of this region and the higher liquid water content, the catalyst layer can be current limiting in the fuel cell. Furthermore, since the cost of the catalyst material is critical, it is important to have a model predicting the effective utilization of this catalyst layer as well as one, which gives insights into how it might be improved. Equations are presented for the mass conservation of reactants and products, the electrical and ionic currents, and the conservation of energy. A discussion of a number of the closure relations such as the Butler-Volmer equation employed is included as is a discussion of the initial and boundary conditions applied. The mathematical model is solved using a finite elements approach developed at the I.U.S.T.I. / Master of Science

A growing number of Swedish forest owners are choosing to certify their forestry. However, there appears to be a lack of field surveys which describe differences between certified and non-certified forests concerning the degree of completed environmental considerations. In order to know to what extent certified forests contribute towards improved environmental consideration, we need studies on how large the difference is in relation to non-certified forests. Differences between certified and non-certified forest clearings were quantified through an inventory of the number of retention trees, dead trees and snags. The result showed that the retained number of the investigated parameters varied greatly between individual clearings, regardless of certification status. However, the results suggested that the certified clearings had an average higher incidence ofthe investigated parameters. In comparison to statistics of southern Sweden's average, the results also suggested that the certified clearings (from the county “Blekinge”) most of the time had a higher incidence of the investigated parameters, while the non-certified clearings most of the time had a lower incidence of the investigated parameters. The environmental considerations on neither the certified nor the non-certified clearings were found to be near the highest amount that could be required by the Swedish forestry act. This observation is interesting, especially since the certified clearings would be expected to maintain a consistently high level in relation to the law requirement.

Les systèmes pile à combustible (SPAC) sont prometteurs pour la production d'énergie électrique. Les applications potentielles concernent les secteurs portable, stationnaire ou transport. La pile à combustible (PAC) est alimentée en hydrogène ; l'hydrogène peut être produit à partir d'un procédé d'électrolyse en utilisant des énergies renouvelables ou à partir d'hydrocarbures transformés en un gaz riche en hydrogène. Un SPAC contient, parmi d'autres éléments, deux composants clés : l'unité de production de combustible et l'empilement de cellules (stack). L'utilisation d'un SPAC alimenté en hydrocarbure comme unité de puissance auxiliaire dans le transport peut être un point de départ intéressant pour cette technologie dans la mesure où elle utilise l'infrastructure de distribution existante.<br /><br />Les SPAC alimentés en hydrocarbure sont des systèmes complexes, multi physiques qui combinent différent domaines énergétiques c'est à dire électro-chimique, électrique, pneumatique et thermique. Un tel système fonctionne seulement dans des conditions opérationnelles bien définies. La mise en oeuvre d'un SPAC demande donc un système de contrôle bien adapté. L'approche classique du développement d'une commande suppose souvent de définir la fonction de transfert du système complet ce qui est difficile, voire impossible, compte tenu de la complexité du système considéré. En conséquence, la structure de commande des systèmes complexes multi-domaines se base souvent sur des observations empiriques ou sur l'expertise. Il s'agit ici de trouver une approche systématique qui permet de déduire la structure de la commande à partir de la description du système. Une telle approche vise à simplifier le développement de la structure de commande pour des systèmes complexes multi domaines et à assurer que la structure de la commande est adaptée aux besoins du système.<br /><br />Dans ce travail, un modèle complet d'un SPAC basse température alimenté en diesel commercial est présenté. Il est adapté au développement de la structure de commande. Le SPAC étudié est destiné à fournir d'une part une puissance électrique de 25kW, d'autre part de la chaleur valorisée sous cette forme ou utilisée pour la climatisation.<br />Dans le chapitre 2, différentes méthodologies de modélisation sont présentées. Leur degré de pertinence est évalué pour modéliser des systèmes complexes multi domaines et pour être utilisé pour le développement de la structure de la commande. La Représentation Énergétique Macroscopique (REM) est identifiée comme la méthodologie la mieux adaptée.<br />Dans le chapitre 3 un modèle de dispositif de transformation du combustible est présenté et implanté dans Matlab/Simulink(TM). Pour obtenir un mélange riche en hydrogène, l'hydrocarbure doit être fractionné. Par la suite, le mélange de gaz doit être purifié afin d'éviter la contamination de la pile à combustible par des dérivés soufrés et/ou par du monoxyde de carbone.<br />Dans le chapitre 4, un modèle de stack est présenté. Il prend en compte les débits gazeux dans les différentes couches, l'humidification de la membrane et la tension de pile.<br />Il existe deux technologies de piles à combustibles basse température. Le modèle est développé pour la technologie la plus connue (Polymer Electrolyte Fuel Cell - PEFC), mais la technologie émergente (High Temperature Proton Exchange Membrane Fuel Cell - HTPEMFC) montre des avantages concernant le volume du système et l'utilisation de la chaleur. Ainsi, les modèles du fuel processeur et du stack pile à combustible ont été adaptés à la technologie émergente. La simplicité de l'adaptation souligne l'avantage de l'utilisation d'une approche modulaire pour la modélisation.<br />Le modèle est validé à partir de valeurs mesurées, de valeurs de la littérature et de valeurs fournies par le constructeur du système.<br />Pour confirmer que le modèle peut être utilisé pour le développement de la structure de commande, celle concernant la température et les débits de gaz pour le SPAC est développée au chapitre 5. La structure de commande peut être obtenue par une inversion bloc par bloc du modèle. L'approche donne une structure de commande mais le choix des régulateurs et leur paramétrage est du ressort du développeur. L'application de la commande montre qu'en utilisant la REM, il est possible de dériver la structure de la commande a partir du modèle d'un système complexe multi domaine, sans avoir besoin de la fonction de transfert du système entier.<br /><br />Le travail est accompli dans le cadre du projet national française GAPPAC composante du programme PAN-H de l'Agence National de la Recherche (ANR). Il regroupe des partenaires industriels (N-GHY, Airbus et Nexter) et <br />universitaires (LMFA, Armines, IFFI, INRETS LTN et l'institut FCLAB).

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. 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. 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. Keywords:Fuel cell; DMFC; PEFC; one-phase; two-phase;model; visual cell; segmented cell; scale analysis; asymptoticanalysis.