Browsing by Subject "Fuel cells"
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Item Cathode catalysts for low-temperature fuel cells : analysis of surface phenomena(2013-12) Mathew, Preethi; Manthiram, Arumugam; Goodenough, John B.The electrochemical oxygen reduction reaction (ORR) steps on a noble metal catalyst in an acidic aqueous electrolyte depend on the nature of the catalytic surface with which the O₂ molecule interacts. It has been assumed that the O₂ molecules interact directly with a bare noble-metal surface. By studying the nature of chemisorbed species on the surface of a metal catalyst as a function of the voltage on the anodic and cathodic sweeps, it is shown here that the O₂ reacts with a surface covered with oxide species extracted from the aqueous electrolyte and not from the O₂ molecules; the ORR is more active when the surface species are OH rather than O. Moreover, the strength of the chemical bond of the adsorbed species was shown to depend on the relative strengths of the metal-metal versus metal-oxide bonds. The Pt-Pt bonds are stronger than the Pd-Pd bonds, and the relative Pd-O bonds are stronger than the relative Pt-O bonds. As a result, the chemisorbed O species is stable to lower anodic potentials on Pd. CO oxidation to CO₂ occurs at a higher potential on Pd than on Pt, which is why Pd (not Pt) is tolerant to methanol. Experiments with alloys show the following: (1) methanol tolerance decreases with the increase of Pt in the Pd-Pt alloys with Pd₃Pt/C showing an initial tolerance that decreases with cycling; (2) OH is formed on Pt₃Co/C and core-shell Pt-Cu/C, which results in a higher activity and durability for the ORR on these catalysts; (3) a 300°C anneal is needed to stabilize the Pd₃Au/C catalyst that forms an O adsorbate; and (4) OH is formed on Pd₃Co/C and Pd₃CoNi/C. These studies provide a perspective on possible pathways of the ORR on oxide-coated noble-metal alloy catalysts.Item Degradation mechanisms of Pt and Pt alloy nanocatalysts in proton exchange membrane fuel cells(2017-05) Rasouli, SomayeSadat; Ferreira, Paulo J. (Paulo Jorge); Manthiram, Arumugam; Yu, Guihua; Nakashima, Naotoshi; Higashida, Kenji; Kongkanand, AnusornThe goal of this PhD research is to fundamentally understand the degradation mechanisms and durability issues of Pt and Pt-alloy nanocatalysts in the cathode of proton exchange membrane fuel cells (PEMFCs). The primary tool for this research has been state-of-the-art transmission electron microscopy, including aberration-corrected TEM/STEM, in-situ TEM heating, 3D tomography, and Energy Dispersive Spectroscopy (EDS). In order to reveal the degradation mechanisms of nanocatalysts, both indirect and direct TEM methods were used. In the first part of this research, we performed post-mortem transmission electron microscopy (TEM) on the membrane electrode assembly (MEA) of PEMFCs. Using a thorough composition and morphological analysis of the catalysts after fuel cell cycling, we showed that the mechanisms proposed in the literature do not fully explain the degradation of the nanocatalysts. Accordingly, new mechanisms were proposed, namely: 1- Modified Ostwald ripening until adjacent particles make contact with each other and coalesce, 2-preferential deposition of single atoms and atomic clusters between two or more particles and consequently bridging between them. To evaluate these proposed mechanisms mentioned above, the second part of this work focused on determining the behavior of Pt and Pt-alloy nanoparticles during different stages of fuel cell cycling. The first challenge was to find a way to ensure that I was observing the exact same nanoparticles during the various stages of cycling. To accomplish this, we developed an experimental setup which replicates on a TEM grid the effect of voltage cycling on the cathode of an MEA. Using this approach, it was possible to track the behavior of a single nanoparticle at different stages of voltage cycling on the nano-atomic scale. Through these direct observations, we demonstrated that due to carbon corrosion the defects appear at the carbon/nanoparticle interface, which in turn result in particle migration and consequently coalescence. We also revealed the mass transfer mechanisms during the coalescence of nanoparticles. In addition, we revisited the commonly held view on the mechanism of particle dissolution and deposition. Thus, during the later stages of cycling, when the concentration of dissoluble Pt reaches a critical amount, single atoms and atomic clusters appear on the carbon support, which consequently move toward other particles and re-deposit on their surface. This dissolution happens preferentially at the corners and steps of the nanoparticle, while re-deposition occurs on {111} type planes. Contrary to the literature, it turned out that re-deposition is not necessarily an isotropic process as atomic clusters can deposit between two or more particles and bridge them. Furthermore, we investigated the atomic surface evolution and phase segregation of Pt3Co and PtNi nanoparticles under the effect of voltage through advanced spectroscopy technique such as EDS. While it is generally accepted in the literature that larger particles grow at the expense of smaller ones, this study showed that in case of alloys, deposition of Pt occurs on the surface of smaller particles rather than larger ones. This is due to the thicker Pt rich surfaces on the smaller particles, since the Pt rich surface act as nucleation sites for re-precipitation of Pt.Item Development of alternative cathodes for intermediate temperature solid oxide fuel cells(2009-08) Kim, Junghyun; Manthiram, ArumugamItem Development of anode catalysts for direct alcohol fuel cells(2010-08) Lee, Eungje; Manthiram, Arumugam; Goodenough, John B.; Bard, Allen J.; Ferreira, Paulo; Meyers, Jeremy P.Direct alcohol fuel cells (DAFC) are attracting considerable interest to meet a variety of energy needs as they offer higher efficiency with less pollution compared to other conventional energy-conversion devices. However, the sluggish alcohol oxidation reaction kinetics and durability problems of the conventional Pt-Ru anode catalyst hamper the commercialization of the DAFC systems. With an aim to overcome these problems, there have been intensive efforts to alloy Pt-Ru with other metals. Although such strategies have led to some enhancement in activity, the durability problem caused by the instability of Ru could still not be alleviated. In this regard, this dissertation focuses on the development of non-Ru electrocatalysts with high activity and durability for DAFC applications. First, Ru-free, Pt-based bimetallic electrocatalysts for methanol oxidation reaction (MOR) were studied. Particularly, Pt-Sn and Pt-CeO₂ catalysts were synthesized, respectively, by a polyol method and a one-step reverse microemulsion (RME) method. The prepared samples are investigated for phase and morphological evaluations by various material-characterization techniques. Cyclic voltammetry and accelerated durability tests revealed that these alternative catalysts have much higher stability with a catalytic activity for MOR comparable to that of Pt-Ru. In the case of Pt-CeO₂, an improved particle morphology is obtained by the RME synthesis, and the advantage of the RME method is reflected by a higher catalytic activity in comparison to that of Pt-CeO₂ synthesized by the conventional synthesis method. It has been known that Pt-Sn is better than Pt-Ru for ethanol oxidation reaction (EOR), and the direct ethanol fuel cells (DEFC) employing Pt-Sn as the anode catalyst have better durability than the DMFC system employing a Pt-Ru anode catalyst. Therefore, this dissertation then focused on the enhancement of the catalytic activity for EOR by incorporating a third metal M to the Pt-Sn catalyst. Following the synthesis and characterization of the Pt-Sn-M (M = Mo and Pd) alloys, the effect of M on the enhanced catalytic activity of Pt-Sn-M is presented. The activity enhancement of the above catalysts is based on the promoting effect of the second or third elements added to Pt. However, in the final chapter of this dissertation, the activity enhancement of Pt nanoparticle itself through the formation of low energy surfaces is investigated. Carbon-supported Pt nanoparticles are synthesized in mixed water-ethylene glycol solvent, and the positive effect of the mixed solvent on both the morphology and surface structure of the Pt nanoparticles for COad oxidation is discussed.Item Dynamic modeling and analysis of proton exchange membrane fuel cells for control design(2016-05) Headley, Alexander John; Chen, Dongmei, Ph. D.; Wei, Li; Beaman, Joseph J; Ezekoye, Ofodike A; Mullins, Charles BThis dissertation seeks to address a number of issues facing the advancement of Proton Exchange Membrane (PEM) fuel cell technology by improving control-oriented modeling strategies for these systems. Real-time control is a major ongoing challenge for PEM fuel cell technologies, particularly with regards to water and temperature dynamics. This can lead to a number of operational concerns, such as membrane flooding and dehydration, which can seriously diminish the efficiency, reliability, and long term health of the system. To combat these issues, comprehensive models that are capable of capturing the dynamics of the key operating conditions and can be processed in real time are needed. Also, given the inherently distributed nature of the system, such a model would ideally account for the changes in the conditions from cell-to-cell in the stack, which can be very significant. With this goal in mind, the main focus of this dissertation is the development and experimental validation of control-oriented modeling techniques for PEM fuel cell stacks. The first major work in this study was the verification of a relative humidity model in response to varying loads. Through this work, a multiple control volume (CV) approach was developed and experimentally validated to model the distribution of operating conditions more accurately while keeping the computational expense sufficiently low. To optimize the modeling efforts, further analysis of the temperature and vapor distribution was performed starting from first principles. This led to the creation of various techniques to optimally size CVs based on the parameters and operating conditions of the system in question. Finally, it was noted throughout the testing that the performance of the membrane electrolyte assemblies in the test stack declined significantly from their initial state. To compensate for this, a Kalman filter was implemented to quantify the membrane degradation. SEM analysis of membranes from the test stack confirmed the validity of this technique. This work can be used to significantly improve real-time models for PEM fuel cells for model-based control applications.Item Effect of anode properties on the performance of a direct methanol fuel cell(2010-12) Garvin, Joshua Joseph; Meyers, Jeremy P.; Hidrovo, CarlosThis thesis is an investigation of the anode of a direct methanol fuel cell (DMFC) through numerical modeling and simulation. This model attempts to help better understand the two phase flow phenomena in the anode as well as to explain some of the many problems on the anode side of a DMFC and show how changing some of the anode side properties could alleviate these problems. This type of modeling is important for designing and optimizing the DMFC for specific applications like portable electronics. Understanding the losses within the DMFC like removable of carbon dioxide, conversion losses, and methanol crossover from the anode to the cathode will help the DMFC become more commercially viable. The model is based on two phase flow in porous media combined with equilibrium between phases in a porous media with contributions from a capillary pressure difference. The effect of the physical parameters of the fuel cell like the thickness, permeability, and contact angle as well as the operating conditions like the temperature and methanol feed concentration, have on the performance of the DMFC during operation will be investigated. This will show how to remove the gas phase from the anode while enabling methanol to reach the catalyst layer and minimizing methanol crossover.Item Fabrication of PEM fuel cell bipolar plate by indirect selective laser sintering(2006) Chen, Ssuwei; Bourell, David LeeA new manufacturing technique utilizing Selective Laser Sintering (SLS) has been developed for fabrication of proton exchange membrane fuel cell (PEMFC) bipolar plates. The layer-based nature of SLS offers several advantages for bipolar plate development and manufacturing. This additive process provides the ability to manufacture complex geometries that are otherwise difficult to obtain using conventional manufacturing techniques. SLS fabrication of bipolar plates will significantly benefit bipolar plate design research because prototypes can be produced and tested in a much shorter period of time and at a lower cost. In addition, fuel cell performance with plates having single serpentine, triple serpentine and interdigitated flow field designs were simulated using commercial Computational Fluid Dynamics (CFD) software package, FLUENT, which has a special PEMFC toolbox implemented for performance simulation. The PEMFC model in FLUENT is a multi-phase mixture model that is capable of predicting local current density distribution, temperature distribution and species vii concentration, etc. To verify the simulation results and demonstrate the value of the SLS technique, real plates with abovementioned flow field configurations were constructed based on the established SLS fabrication routes. Experiments were then conducted using a newly assembled PEMFC test unit with a bubble humidifier installed. Improved design and prompt experimental validation of fuel cell bipolar plates became possible through the combination of CFD simulation and the established indirect SLS process. Other novel bipolar plate designs were also proposed.Item Laser sintering for high electrical conduction applications(2012-05) Murugesan Chakravarthy, Kumaran; Bourell, David Lee; Manthiram, Arumugam; Meyers, Jeremy P.; Beaman, Joseph J.; Juenger, Maria G.Applications involving high electrical conduction require complex components that are difficult to be manufactured by conventional processes. Laser sintering (LS) is an additive manufacturing technique that overcomes these drawbacks by offering design flexibility. This study focuses upon optimizing the process of laser sintering to manufacture functional prototypes of components used in high electrical conduction applications. Specifically, components for two systems – high current sliding electrical contacts and fuel cells – were designed, manufactured and tested. C-asperity rails were made by LS and tested in a high current sliding electrical setup. Corrugated flow field plates were created by LS and their performance in a direct methanol fuel cell (DMFC) was tested. This is the first experimental attempt at using laser sintering for manufacturing such complex components for use in high electrical conduction applications. The second part of this study involves optimization the laser sintering process. Towards this, efforts were made to improve the green strength of parts made by LS. Particle size of graphite/ phenolic resin and addition of nylon/11 and wax were tested for their effect upon green strength. Of these, significant improvement of green strength was observed by altering the particle size of the graphite/ phenolic resin system. New methods of improving green strength by employing fast cure phenolic resins with carbon fiber additions were successfully demonstrated. This study also identified a binder system and process parameters for indirect LS of stainless steel –for bipolar plate compression/ injection mold tooling. All the experimental results of this study lead us to believe that laser sintering can be developed as a robust and efficient process for the manufacture of specialized components used in advanced electrical conduction systems.Item Perovskites oxides for metal-air batteries and pseudocapacitor applications(2019-06-12) Alexander, Caleb Tyler; Stevenson, Keith J.; Johnston, Keith P., 1955-; Hwang, Gyeong; Milliron, DeliaWind and solar energy’s rapid development has created a significant need for low-cost energy storage to enable renewables at grid level. To meet these challenges, metal-air batteries and fuel cells are being considered for base-load energy storage while high power applications like frequency regulation and uninterruptable power supplies (UPS) can be addressed using high energy pseudocapacitors. The major bottleneck to metal-air battery and fuel cell commercialization is the sluggish oxygen reactions at the positive electrode that are industrially catalyzed using expensive precious metal catalysts like Pt and IrO₂. Here, the aim is to replace precious metal-catalysts with low-cost LaNiO₃ perovskites and N-doped CNTs in alkaline conditions and study their synergistic interactions and composite stability. The work is continued by studying the anion-intercalation pseudocapacitance in a perovskite oxide library with composition La [subscript 1-x] Sr [subscript x] BO [subscript 3-δ] (B = Mn, Fe, Co; 0 ≤ x ≤ 1) and found that increasing oxygen vacancy content universally increases the pseudocapacitance while the B-site element controlled the redox potential. The most pseudocapacitive materials were then used to make the first all perovskite asymmetric pseudocapacitors with a maximum energy density of 31 Wh kg⁻¹. This work was followed by using the principles learned to further extent the redox voltage potential difference using LaNi [subscript 1-x] Fe [subscript x] O [subscript 3-δ] and brownmillerite-SrFeO [subscript 2.5] to make an asymmetric pseudocapacitor. Doing this, the redox discharge potential was pushed all the way to 1.1 V which is the highest asymmetric pseudocapacitor discharge peak potential reported to date.Item Simulation, analysis, and mass-transport optimization in PEMFCs(2011-12) Olapade, Peter Ojo; Meyers, Jeremy P.In this dissertation, we present two major lines of numerical investigation based on a control-volume approach to solve coupled, nonlinear differential equations. The first model is developed to provide better understanding of the water management in PEMFC operating at less than 100ºC, under transient conditions. The model provides explanations for the observed differences between hydration and dehydration time constants during load change. When there is liquid water at the cathode catalyst layer, the time constant of the water content in the membrane is closely tied to that of liquid water saturation in the cathode catalyst layer, as the vapor is already saturated. The water content in the membrane will not reach steady state as long as the liquid water flow in the cathode catalyst layer is not at steady state. The second model is to optimize the morphological properties of HT-PEMFCs components so as to keep water generated as close as possible to the membrane to help reduce ionic resistance and thereby increase cell performance. Humidification of the feed gas at room temperature is shown to have minimal effects on the ionic resistance of the membrane used in the HT-PEMFC. Feed gases must be humidified at higher temperature to have effects on the ionic resistance. However, humidification at such higher temperatures will require complex system design and additional power consumption. It is, therefore, important to keep the water generated by the electrochemical reaction as close as possible to the membrane to hydration the membrane so as to reduce the ionic resistance and thereby increase cell performance. The use of cathode MPL helps keep the water generated close to the membrane and decreasing the MPL porosity and pore size will increase the effectiveness of the MPL in keep the water generated close to the membrane. The optimum value of the MPL porosity depends on the operating conditions of the cell. Similarly, decreasing the GDL porosity helps keep water close to the membrane and the optimum value of the GDL porosity depends on the operating conditions of the cell.Item Synthesis and characterization of carbon nanotube supported nanoparticles for catalysis(2007-12) Vijayaraghavan, Ganesh, 1978-; Stevenson, Keith J.This dissertation describes the synthesis and characterization of nitrogen doped carbon nanotube (NCNT) supported nanoparticles for catalysis, specifically, the cathodic oxygen reduction reaction (ORR) in fuel cells. Strategies for synthesis of mono- and bimetallic nanoparticle catalysts through dendrimer based templating techniques and with the aid of metal organic chemical vapor deposition (MOCVD) precursors and efficient assembly protocols of the catalysts with the NCNTs are discussed in detail. Physicochemical properties of the NCNTs and NCNT supported catalysts were characterized using a host of tools including scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis, BET surface area and pore size analysis and electrochemical techniques including cyclic voltammetry, chronocoulometry, chronoamperometry and rotating disk electrode voltammetry. Chapter 1 serves as a general introduction and provides a brief overview of challenges associated with the synthesis, characterization and utilization of graphitic carbons and graphitic carbon supported catalysts in heterogeneous catalysis. Chapter 2 provides an overview of the synthesis and characterization of systematically doped iron and nickel catalyzed NCNTs in an effort to understand the effect of nitrogen doping on ORR. Chapter 3 describes the use of NCNTs as supports for dendrimer templated nanoparticle catalysts for ORR. A facile synthetic strategy for the immersion based loading of catalysts onto NCNTs by spontaneous adsorption to achieve specific catalyst loadings is explored. Chapter 4 details the loading of monodisperse Pt, Pd and PtPd catalysts on the as synthesized NCNTs using MOCVD precursors. The MOCVD route offers promise for direct dispersion and activation of ORR catalysts on NCNT supports and eliminates a host of problems associated with traditional solvent based catalyst preparation schemes. Chapter 5 details future directions on a few topics of interest including efficient electrodeposition strategies for preparing NCNT supported catalysts, studies on PtCu catalysts for ORR and finally prospects of using NCNT supported catalysts in fuel cell applications.