Proton exchange membrane fuel cells (PEMFC) are attractive energy sources for power trains in vehicles because of their low operating temperature that enables fast start-up and high power densities. Cost reduction and durability are the key issues to be solved before PEMFCs can be successfully commercialized. The major portion of fuel cell cost is associated with the catalyst layer which is typically comprised of carbon-supported Pt and ionomer. The degradation of the catalyst layer is one of the major failure modes that can cause voltage degradation and limit the service life of the fuel cell stack during operation. To develop a highly durable and better performing catalyst layer, topics such as the causes for the degradation, modes of failure, different mechanisms and effect of degradation on fuel cell performance must be studied thoroughly. Key degradation modes of catalyst layer are carbon corrosion and Pt dissolution. These two modes change the electrode structure in the membrane electrode assembly (MEA) and result in catalyst layer thinning, CO2 evolution, Pt deposition in the membrane and Pt agglomeration. Alteration of the electrode morphology can lead to voltage degradation. Accelerated stress tests (ASTs) which simulate the conditions and environments to which fuel cells are subject, but which can be completed in a timely manner, are commonly used to investigate the degradation of the various components. One of the current challenges in employing these ASTs is to relate the performance loss under a given set of conditions to the various life-limiting factors and material changes. In this study, various degradation modes of the cathode catalyst layer are isolated to study their relative impact on performance loss ‗Fingerprints‘ of identifiable performance losses due to carbon corrosion are developed for MEAs with 0.4 mg cm−2 cathode platinum loadings. The fingerprint is used to determine the extent of performance loss due to carbon corrosion and Pt dissolution in cases where both mechanisms operate. This method of deconvoluting the contributions to performance loss is validated by comparison to the measured performance losses when the catalyst layer is subjected to an AST in which Pt dissolution is predominant. The limitations of this method iv are discussed in detail. The developed fingerprint suggests that carbon loss leading to CO2 evolution during carbon corrosion ASTs contributes to performance loss of the cell. A mechanistic model for carbon corrosion of the cathode catalyst layer based on one appearing in the literature is developed and validated by comparison of the predicted carbon losses to those measured during various carbon corrosion ASTs. Practical use of the model is verified by comparing the predicted and experimentally observed performance losses. Analysis of the model reveals that the reversible adsorption of water and subsequent oxidation of the carbon site onto which water is adsorbed is the main cause of the current decay during ASTs. Operation of PEM fuel cells at higher cell temperatures and lower relative humidities accelerates Pt dissolution in the catalyst layer during ASTs. In this study, the effects of temperature and relative humidity on MEA degradation are investigated by applying a newly developed AST protocol in which Pt dissolution is predominant and involves the application of a potentiostatic square-wave pulse with a repeating pattern of 3s at 0.6 V followed by 3s at 1.0 V. This protocol is applied at three different temperatures (40°C, 60°C and 80°C) to the same MEA. A diagnostic signature is developed to estimate kinetic losses by making use of the effective platinum surface area (EPSA) obtained from cyclic voltammograms. The analysis indicates that performance degradation occurs mainly due to the loss of Pt in electrical contact with the support and becomes particularly large at 80°C. This Pt dissolution AST protocol is also investigated at three different relative humidities (100%, 50% and 0%). Electrochemical impedance spectroscopy measurements of the MEAs show an increase in both the polarization and ohmic resistances during the course of the AST. Analysis by cyclic voltammetry shows a slight increase in EPSA when the humidity increases from 50% to 100%. The proton resistivity of the ionomer measured by carrying out impedance measurements on MEAs with H2 being fed on the anode side and N2 on the cathode side is found to increase by the time it reaches its end-of-life state when operated under 0 % RH conditions.

Accuracy in the effective diffusion coefficient of the gas diffusion layer (GDL)/microporous layer (MPL) is important to accurately predict the mass transport limitations for high current density operation of polymer electrolyte membrane (PEM) fuel cells. All the previous studies regarding mass transport limitations were limited to pure GDLs, and experimental analysis of the impact of the MPL on the overall diffusion in the porous GDL is still lacking. The MPL is known to provide beneficial water management properties at high current operating conditions of PEM fuel cells but its small pore sizes become a resistance in the diffusion path for mass transport to the catalyst layer. A modified Loschmidt cell with an oxygen-nitrogen mixture is used in this work to determine the effect of MPL on the effective diffusion coefficients. It is found that Knudsen effects play a dominant role in the diffusion through the MPL where pore diameters are less than 1 μm. Experimental results show that the effective diffusion coefficient of the MPL is only about 21% that of its GDL substrate and Knudsen diffusion accounts for 80% of the effective diffusion coefficient of the GDL with MPL measured in this study. No existing correlations can correlate the effective diffusion coefficient with significant Knudsen contribution.

Durability of carbon-supported platinum nanoparticle is one of the limiting factors for PEM fuel cell commercial applications. In our research work, we applied both experimental and mathematical simulative tools to study the mechanisms of Pt/C catalyst degradation. An accelerated catalyst degradation protocol by cycling the cathode potential in a square-wave profile was applied to study the losses of cell performances, catalyst ORR activity, and Pt active surface areas. Post-mortem analyses of cathode Pt particle size by X-ray diffraction and platinum distributions in CCMs by SEM/EDS were also conducted. Increased cell temperature and relative humidity was found to accelerate the cathode catalyst degradation. High membrane water contents or abundant water/ionic channels within the polymer electrolyte were believed to accelerate Pt ion transport and cathode degradation. After degradation tests, significant amount of Pt loss into the membrane forming a Pt "band" was observed through cathode platinum dissolution and chemical reduction of soluble Pt ions by permeated hydrogen from the anode. Platinum deposition was confirmed at a location where the permeated hydrogen and oxygen had the complete catalytic combustion over the deposited Pt clusters/particles as the catalyst. A cathode degradation model was built including the processes of platinum oxidation, dissolution/replating, diffusion of Pt ions and Pt band formation in electrolyte. A simplified bi-modal particle size distribution was applied with equal numbers of small and large type particles uniformly distributed in the cathode initially. Processes of Pt mass exchange between two types of particles were demonstrated to cause the overall particle growth. This was due to the particle size effect that platinum dissolution from the small type particles and replating of Pt ions onto the large particles was favored. Parametric study found the increased upper cycling potential was the dominated factor to accelerate the catalyst degradation. Also high frequency of potential cycle and low surface oxide coverage accelerated the degradation rate. Pt dissolution and oxidation processes in perchloric acid were preliminary investigated, and both chemical and electrochemical processes of oxidation and dissolution were believed to be involved under closed-circuit fuel cell conditions with oxygen presence at cathode.

abstract: This work aimed to characterize and optimize the variables that influence the Gas Diffusion Layer (GDL) preparation using design of experiment (DOE) approach. In the process of GDL preparation, the quantity of carbon support and Teflon were found to have significant influence on the Proton Exchange Membrane Fuel Cell (PEMFC). Characterization methods like surface roughness, wetting characteristics, microstructure surface morphology, pore size distribution, thermal conductivity of GDLs were examined using laser interferometer, Goniometer, SEM, porosimetry and thermal conductivity analyzer respectively. The GDLs were evaluated in single cell PEMFC under various operating conditions of temperature and relative humidity (RH) using air as oxidant. Electrodes were prepared with different PUREBLACK® and poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining catalytic layer with a Pt-loading (0.4 mg cm-2). In the study, a 73.16 wt.% level of PB and 34 wt.% level of PTFE was the optimal compositions for GDL at 70 °C for 70% RH under air atmosphere. For most electrochemical processes the oxygen reduction is very vita reaction. Pt loading in the electrocatalyst contributes towards the total cost of electrochemical devices. Reducing the Pt loading in electrocatalysts with high efficiency is important for the development of fuel cell technologies. To this end, this thesis work reports the approach to lower down the Pt loading in electrocatalyst based on N-doped carbon nanotubes derived from Zeolitic Imidazolate Frameworks (ZIF-67) for oxygen reduction. This electrocatalyst perform with higher electrocatalytic activity and stability for oxygen reduction in fuel cell testing. The electrochemical properties are mainly due to the synergistic effect from N-doped carbon nanotubes derived from ZIF and Pt loading. The strategy with low Pt loading forecasts in emerging highly active and less expensive electrocatalysts in electrochemical energy devices. This thesis focuses on: (i) methods to obtain greater power density by optimizing content of wet-proofing agent (PTFE) and fine-grained, hydrophobic, microporous layer (MPL); (ii) modeling full factorial analysis of PEMFC for evaluation with experimental results and predicting further improvements in performance; (iii) methods to obtain high levels of performance with low Pt loading electrodes based on N-doped carbon nanotubes derived from ZIF-67 and Pt. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

Ce travail de thèse s'inscrit dans le cadre de la durabilité des PEMFC et s'intéresse plus particulièrement à la dégradation des électrocatalyseurs de type Pt/C qui sont utilisés dans leurs électrodes. L'objectif visé était la détermination des mécanismes responsables de leur dégradation, par combinaison d'expériences d'électrochimie en cellule à trois électrodes, de microscopie électronique en transmission et à balayage, et de spectrométrie de masse (DEMS). Dans un premier temps nous avons mis en évidence que les méthodes de caractérisation employées peuvent influencer les dégradations observées. Ainsi, la morphologie des nanocatalyseurs Pt/C peut-être altérée par l'observation MET elle même, comme d'ailleurs par la séquence de mesures par CO-stripping. / This thesis is part of the durability of PEMFC and is particularly interested in the degradation of electrocatalysts of type Pt / C that are used within their electrodes. The objective was to determine mechanisms responsible for their degradation, by combining experiments in electrochemical cell with three electrodes, transmission electron microscopy and scanning, and mass spectrometry (DEMS). Initially we have shown that the characterization methods used can influence the degradation observed. Thus, the morphology of Pt nanocatalysts / C may be altered by the TEM observation itself, as does the sequence of measurements by CO-stripping.

Neste trabalho, a reação de redução de oxigênio (RRO) foi estudada em catalisadores nano-particulados de Pt e ligas de PtM (M = Co, Cr, Fe e Ni) suportados em carbono, preparados localmente por método de impregnação, para aplicação em células a combustível de eletrólito polimérico (CCEP). A caracterização física destes materiais foi realizada através das técnicas de energia dispersiva de raios x (EDS), difração de raios x (DRX), absorção de raios x (XAS) e microscopia eletrônica de varredura e transmissão. Os testes eletroquímicos dos catalisadores foram realizados com o uso de voltametria cíclica, medidas de polarização em estado estacionário e espectroscopia de impedância eletroquímica. Estes estudos foram conduzidos em meia-célula usando eletrodos de disco/anel rotatórios e tendo ácido sulfúrico (0,5 mol L-1) como eletrólito e em células unitárias CCEP contendo membranas de Nafion® 212 (N212) e Nafion® 112 (N112), alimentadas com H2 no ânodo e O2/ar no cátodo, em diferentes temperaturas e pressões. Finalmente, foram também realizados estudos de durabilidade tanto dos catalisadores como das membranas poliméricas, os quais foram submetidos a procedimentos de degradação acelerada (PDA). Os resultados dos estudos em meia-célula mostraram que os catalisadores bimetálicos (PtM) são menos ativos cataliticamente para a RRO comparados à Pt pura, fatos que não se confirmaram nos testes em células unitárias. Por outro lado, após a aplicação do PDA os catalisadores apresentaram mudanças significativas em suas propriedades estruturais e eletrônicas que levaram à diminuição da atividade frente a RRO. No geral as células a combustível com N212 apresentaram melhor desempenho do que aquelas com N112, quando operadas com ar no cátodo, porém os estudos confirmaram que a degradação da membrana leva à redução do desempenho devido o aumento do cruzamento de gás, principalmente de H2. / In this work, the oxygen reduction reaction (ORR) was studied on nano-particulated Pt and PtM (M = Co, Cr, Fe e Ni) alloy electrocatalysts supported on carbon, prepared by an impregnation method, for utilization on polymer electrolyte fuel cell (PEFC). The physical properties of the materials have been investigated by energy dispersive X-ray analyses (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and scanning and transmission electron microscopy. The electrochemical investigations were carried out using cyclic voltammetry, steady state polarization measurements and electrochemical impedance spectroscopy. Studies were conducted on half-cells with rotating ring-disk electrodes having 0.5 mol L-1 H2SO4 as electrolyte and on PEFC single cells built with Nafion® 212 (N212) and Nafion® 112 (N112) membranes, feed with H2 and O2/air at several temperatures and pressures. Finally, durability studies of either, the catalysts and membranes, were carried out, after they were submitted to accelerated degradation procedures (ADP). The half-cell results indicated a lower activity for the ORR of the bimetallic electrocatalysts, compared to pure Pt, but this was not confirmed by the single cell tests. On the other hand, after the ADP, the catalysts showed significant changes on the morphological and electronic properties, which leaded to a reduction of the activity for the ORR. The single cells with N212 presented higher performance than those with N112, when operating with air supplied cathodes, but the results confirmed that the degradation of the membranes leads to a reduction of the fuel cell performance by increasing the gas crossover, mainly of H2.

Indiana University-Purdue University Indianapolis (IUPUI) / An integrated micro PEM fuel cell system with self-regulated hydrogen generation from ammonia borane is reported to power portable electronics. Hydrogen is generated via catalytic hydrolysis reaction of ammonia borane solution in microchannels with nanoporous platinum catalyst electroplated inside the microchannels. The self-regulation of the ammonia borane solution is achieved by using directional growth and selective venting of hydrogen bubbles in microchannels, which leads to agitation and addition of fresh solution without power consumption. The device is fabricated on combination of polystyrene sheets cut by graphic cutter, a stainless steel layer cut using wire electrical discharge machining and bonding layers with double-sided polyimide tape. Due to the seamless bonding between the hydrogen generator and the micro fuel cell, the dead volume in the gas connection loops can be significantly reduced and the response time of self-regulation is reduced.

The construction sector is responsible for 20% of Greenhouse Gas (GHG) emissions, of whichdiesel-powered construction equipment are large contributors. Currently there are many ongoing Fuel Cell (FC) powered construction equipment projects as it is seen as an attractiveoption to power the futures zero-emission heavy-duty machines. Although an attractivealternative, hydrogen FC has drawbacks such as releasing liquid water and water vapor viathe exhaust as a byproduct which in their working environment can cause a suite of issues. Agap in the literature on the water exhausted is present and therefore this degree project seeksto investigate the amount, and ratio, of liquid water and water vapor released from threetypical construction equipment drive cycles which would allow further investigation onappropriate management. The method used for this degree project was to modify a pre-mademodel in Simulink built with Simscape blocks. The model was modified to represent a FCsystem used in a test-rig by implementing experimental and measured data for design andoperating parameters. Different pressures, temperatures, and cathode inlet RelativeHumidity (RH) were investigated to find their effect on the performance and water in theexhaust. A sensitivity analysis of different unknown parameters was also conducted tounderstand their influence on the results. For the reference case, the results showed that foran articulated hauler, the water in the exhaust was 26% liquid which translates to 8.6 kg for a1-hour drive cycle. The crawler excavator and wheel loader, both had 30-minute drive cyclesand had 1.1 kg liquid water with a liquid water ratio of 7% and 0.7 kg liquid water with aliquid water ratio of 5% in the exhaust respectively. For a full 8-hour workday with twoparallel FCs connected, the articulated hauler liquid water amount is 137.6 kg, the crawlerexcavator 35.2 kg, and the wheel loader 22.4 kg. Overall, it was found the liquid water ratiocould be changed to a large extent with different operating parameters, where thetemperature had the greatest influence. The system and stack efficiencies did not changeconsiderably with different operating parameters, meaning that the total water in the exhaustremained similar for the different respective drive cycles.

Volvo Construction Equipment is one of the leading construction machinery manufacturers in the world. To stay amongst the leaders, research and development projects for new technologies are crucial. The most important path of development today is the reduction of emissions produced by these heavy duty vehicles. To tackle this challenge, several technologies are already used in industry. One example are hybrid machines that combine a conventional diesel engine with batteries, resulting in reduced engine size and pollutants. Another option are full battery-electric vehicles, which can reduce the on-site emissions to zero. The electrochemical processes within batteries are however comparable slow and result in long recharge times. A new focus of development within the industry are hybrid systems combining fuel cells and batteries. Since hydrogen can be refueled almost as fast as convenient fuel, it solves the issue of long recharge times. Additionally, the reaction is emission free, since there is no combustion process and the only byproduct that is emitted from the fuel cell is chemically clean water. This thesis aims to propose an architecture and packaging solution to replace the diesel engine in a large size wheel loader with a fuel cell power system. This also includes all respective auxiliary systems, i.e. energy storage, cooling and electric systems. Achieving the same performance as a conventional large size wheel loader as well as keeping the spatial envelope the same are the main objectives of this work. To achieve these goals, an extensive study on the most common drive cycles is carried out to understand the power demand of the machine. After the selection of an energy storage system based on a MATLAB simulation script, a cooling system is modelled and scaled to fulfill the operating requirements of the different components. Eventually, all systems are modeled and installed into the wheel loader in CATIA V5. The study showed, that the new system architecture of the vehicle can be fitted into the existing engine bay with a slight extension of the rear frame and hood. Two power optimized batteries are combined with one fuel cell. Hydrogen tanks with a filling volume of 478 [L] can be installed in the machine, covering 50% of the customer population curve without degradation of performance. This includes one refill of the wheel loader during the day. The performance parameters match the conventional machine up to a high degree, concluding that the conversion of a large size wheel loader into a fuel cell powered wheel loader is feasible.

I detta examensarbete utforskas påverkan av partikelstorleken hos Pt-katalysatorer på kolbärare på syrereduktionen i polymerelektrolytmembran (PEM)-bränsleceller. Den elektrokemiskt aktiva ytarean, aktiviteten för syrereduktion och tillhörande degraderingshastigheter för de undersökta katalysatorerna beräknas och jämförs. Experimenten utfördes med hjälp av en metod som kallas ”roterande skivelektrod med tunn film” i en syra-vatten-lösning. Den katalytiska aktiviteten beräknades med Koutecky-Levichs ekvation. Resultaten visar att filmkvaliteten är avgörande för att kunna göra en bedömning av syrereduktionssaktiviteten. En homogen distribution av Pt-nanopartiklar är nödvändig för att få en korrekt och pålitlig katalytisk aktivitet. Vidare utvecklas en metodik för utvärdering av filmkvaliteten baserad på den diffusionsbegränsade strömtätheten och på potentialen vid halva topphöjden. Mätningen av syrereduktionsaktiviteten genomfördes enbart med prov vars filmkvalitet godkändes med avseende på denna metodik och den erhållna aktiviteten jämfördes vid 0.9 V vs. RHE. De undersökta katalysatorerna uppvisar snarlik aktivitet för syrereduktionsreaktionen, vilket överensstämmer med att också de uppmätta elektrokemiskt aktiva ytareorna för de olika katalysatorerna är snarlika. Förlusten av elektrokemiskt aktiv ytarea visar sig vara beroende av partikelstorleken och mindre partiklar uppvisar en högre degraderingshastighet. Rapporten avslutas med en diskussion om avvikelserna från tidigare studier och möjligheter för vidare studier. / In this thesis, the particle size effect of carbon-supported Pt catalysts on the oxygen reduction reaction in polymer electrolyte membrane fuel cells is studied. The electrochemically active surface area and the oxygen reduction reaction activities and the associated degradation rate of the investigated carbon-supported Pt catalysts are computed and compared. The experiments were conducted by a method called thin-film rotating disk electrode in an aqueous acid solution, and the catalytic activity was computed using the Koutecky-Levich equation. The results show that thin-film quality is crucial in the oxygen reduction reaction activity measurement. A homogeneous distribution of Pt nanoparticles is necessary for obtaining correct and reliable catalytic activities. Furthermore, in this thesis, a methodology based on the diffusion-limited current density and the half-wave potential for thin-film quality evaluation is developed. The oxygen reduction reaction activity measurement was only applied to samples with good film quality, and the obtained activity results were compared at 0.9 V vs. RHE. The investigated carbon-supported Pt catalysts display similar oxygen reduction reaction activities, due to the fact that the measured electrochemically active surface area results for the particle sizes are similar. The degradation rate was studied in a platinum dissolution test and the results show a particle size-dependent electrochemically active surface area loss. Smaller particles show a faster and larger degradation rate. At the end of the thesis, deviations in this work from existing work are discussed, and possibilities for future work are presented.