Mass transport aspects of polymer electrolyte fuel cells under two-phase flow conditions

Die Visualisierung und Quantifizierung von Flüssigwasseransammlungen in Polymerelektrolytmembran-Brennstoffzellen konnte mittels Neutronenradiographie erreicht werden. Dank dieser neuartigen diagnostischen Methode konnte erstmals die Flüssigwasseransammlung in den porösen Gasdiffusionsschichten direkt nachgewiesen und quantifiziert werden. Die Kombination von Neutronenradiographie mit ortsaufgelösten Stromdichtemessungen bzw. lokaler Impedanzspektroskopie erlaubte die Korrelation des inhomogenen Flüssigwasseranfalls mit dem lokalen elektrochemischen Leistungsverhalten. Systematische Untersuchungen an Polymerelektrolyt- und Direkt-Methanol-Brennstoffzellen verdeutlichen sowohl den Einfluss von Betriebsbedingungen als auch die Auswirkung von Materialeigenschaften auf die Ausbildung zweiphasiger Strömungen

Manipulated Electrochemical Surface Reactions Induced By Oscillatory Electric Potentials on Metal Based Electrodes

This research effort investigates the manipulation of surface electrochemical reactions induced by oscillating electric potentials on the surface of metal-based electrodes. Specifically, this research presents experimental data identifying modified electrochemical surface reactions caused by low magnitude electric potential oscillations on multilayered catalytic membranes and on implanted biometallic alloys. The scope of this effort consists of four major components: (1) perform an exhaustive literature review and analysis of the current understanding in applied surface electrochemistry and develop potential theoretical frameworks by which to interpret the experimental results; (2) identify the electrochemical manipulation via electrical oscillation while reacting nitric oxide on a multilayered ceramic membrane in combustion exhaust; (3) demonstrate that low magnitude electric potential oscillation are sufficient to induce corrosion of implanted biometallic alloys; (4) evaluate ambient electromagnetic radiation as a contributing factor to the complex corrosion of implanted ASTM F1537 CoCrMo.Although electrochemistry has been a driving force of many modern technologic advancements and the fundamental relations of electrochemistry have existed for over 100 years, current techniques do not adequately address the possibility for high frequency spatially and temporally varying electromagnetic potential fields and their effects on surface reactivity. Modern electrochemical theory remains focused on quasi-steady state reduction and oxidation reactions. Expanding upon existing theoretical models, such as the Newns-Anderson model for surface adsorbate systems, three feasible mechanisms of action are proposed by which spatially and temporally varying electromagnetic fields may interact to alter surface chemical reactions at the boundary of a metal-based electrode: direct shifting of the d-band center on the metallic surface, a photon-phonon interaction leading to the creation of a phonon-polariton, and/or the evolution of a complex three dimensional field with surface normal. When investigating a multilayered ceramic electrochemical catalytic membrane for automotive emission reduction, it was concluded that the presence of high frequency, low magnitude electric potential oscillations resulted in a manipulation of the predicted chemical pathway during the conversion of NO into diatomic nitrogen and oxygen. The electrical activity altered the initial surface electrochemical reaction toward the less probable formation of N2O, instead of NO2, ultimately resulting in greater NO reduction efficacy, compared to a Pt catalyst. Electrical stimulation (at 200 mVpp, \u3e25MHz) of ASTM F1537 CoCrMo within a simulated synovial fluid resulted in significant corrosion activity. The chemical composition of corrosion products grown via electrical stimulation match that of recovered in vivo corrosion products. The corrosion products contain primarily Cr2O3, CrO3, phosphates, molybdates, CrOH, and CoOH, with varying concentrations of Ca, P, and Co. Furthermore, this work demonstrates that the ambient electromagnetic field in a standard university laboratory can induce sufficient electrical activity to initiate the corrosion of ASTM F1537 CoCrMo in a simulated synovial fluid environment. Samples shielded from electrical activity did not demonstrate corrosion activity, whereas samples subjected to ambient electromagnetic activity showed formation of Cr2O3 and potentially CrO3 with significant concentrations of Ca, P, N, and Na. The work presented throughout this thesis provides foundational experimental data which identifies a novel electrochemical reaction manipulation phenomenon arising from temporally and spatially varying electromagnetic fields. This electrochemical phenomenon is believed to persist across a general electrochemical system and deserves significant future study

Screen Printed Carbon Electrode Based Microfluidic Biosensor for Sweat Cortisol Detection

A simple, cost-effective, microfluidic, field-deployable biosensor with screen printed carbon electrode (SPCE) was developed for detection of sweat cortisol with point of care applications. Cortisol detection in artificial sweat is an important screening tool for diagnosis and monitoring of various health conditions like Addison’s disease, stress disorder, and Cushing’s syndrome. A self-assembled monolayer of graphene oxide (GO) is functionalized on SPCE electrode, onto which cortisol antibodies are immobilized for cortisol detection. Microfluidic system ensured precise and controlled flow of reagents and antibodies. Electrochemical measurement is done using cyclic voltammetry, as a function of cortisol concentrations. Cyclic voltammetry measurement gives current magnitude with applied voltage as a function of time. Scanning Electron Microscopy (SEM) imaging shows the change in surface morphology with the addition of antibody, compared to bare electrode functionalized with GO. The images confirm the antibody binding to selfassembled GO nanosurface on the working electrode. Raman imaging also supports the advantages of surface functionalization with antibodies. It shows presence of GO and antibodies on the biosensor surface suggesting GO self-assembly and antibodies immobilization. Atomic Force Microscopy (AFM) imaging shows surface topography of the developed sensor upon immobilization of self-assembled GO. The evenly distributed GO provided more surface area for antibodies immobilization. Cortisol was detected in the linear range of 0.1 ng/ml to 150 ng/ml, where current magnitude decreased with increasing cortisol concentration due to reduction in number of free electrons. The developed microfluidic biosensor for cortisol detection formed the base for sweat cortisol sensor with POC applications, and can also be used in personalized health diagnosis or monitoring

Electrolyte pH conditioning of Regenesys®

Regenesys® is a regenerative fuel cell based on the sulphur and sodium bromide redox couple. On charging the system sulphur (polysulphide) is reduced to the sulphide and bromide oxidised to bromine. During discharge or power delivery the sulphide is oxidised back to sulphur (polysulphide) and the bromine reduced back to the bromide The bromide electrolyte pH decreases during operation. This is symptomatic of the chemical imbalance within the system. HS- is transported across the membrane where oxidisation by the bromine ultimately yields sulphuric acid. Back diffusion of protons will occur if low pH is maintained. The result is the neutralisation of the polysulphide electrolyte (HS- and OH-) and consequently the inhibition of the hydroxyl dependent discharge process that decreases the discharge capacity. [Continues.

Design of Metallic Nanostructures for Wavelength and Angle Selective Light Management.

Sub-wavelength metal nanoparticles demonstrate a resonant coupling to incident optical fields known as the localized surface plasmon resonance, enabling enhanced absorption, scattering, and nano-focusing of light. In this work, plasmonic properties of metal nanoparticles and nanorods are studied and engineered to realize selective management of incident light as a function of wavelength, angle, and polarization, for application to photovoltaics and selectively transmissive / absorptive systems. For photovoltaics (PV) applications, metal nanoparticle scattering is exploited to realize a wavelength selective backscattering layer. Placed behind a thin film PV absorbing layer, an array of silver nanoparticles backscatters light on resonance while off-resonance light is transmitted, allowing engineering of selective transparency vs. absorption and modulation of photocurrent. Further tuning the array by considering anisotropic particle shape (increasing the aspect ratio), the plasmonic resonance becomes a function of both wavelength and incident angle. We propose employing such a nanorod array to realize an angle selective photovoltaic window for building integration: light normal to the window is off resonance, retaining high transmission and window quality visibility, while angled light, including direct sunlight, is resonantly scattered and harvested for conversion to photocurrent. Optical analysis indicates 20 - 30% improvement in direct sunlight absorption and photocurrent is possible without sacrificing window transparency in the viewing direction. Beyond photovoltaics, we consider integrating angle selective metal nanorods with actuating micro-origami structures to control their orientation with respect to incident light. By tuning the plasmonic and angular properties of the system, we propose a novel method to realize balanced 0 - 90+% transmission modulation of the full visible spectrum for application to adjustable smart glass window coatings, potentially significantly improving on current implementations. Large area patterning of deeply sub-wavelength (10's of nm) metal nanorods remains a challenge for traditional nanofabrication techniques. We investigate and describe ways to realize the structures of interest based on the electrochemical synthesis of high aspect ratio self-assembled nanoporous anodized aluminum oxide (AAO) films, including both bottom-up (electroplating) and top-down (reactive ion etching) approaches. Finally, the anisotropic and angle dependent scattering properties of high aspect ratio AAO itself are considered for similar light management applications.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113523/1/brobrts_1.pd

Development of electrochemical probe microscopy and related techniques

This thesis presents work on the development of a number of scanned electrochemical probe microscopies. Such techniques have widespread applications, from materials science to the life sciences. Advances in flexible instrumentation, coupled with the theoretical description of electrochemical systems, are central themes which allowed for the fruitful investigation of a variety of experimental systems. Theoretical descriptions of scanning ion conductance microscopy (SICM) were developed, particularly to investigate the effect of tip-geometry on imaging resolution. This technique has already found a number of applications in the life sciences, but image resolution has not previously been addressed adequately. Images were recorded showing tip-convolution that were in agreement with theoretical predictions. The scanning microcapillary contact method (SMCM) was developed, as a method of assessing spatial heterogeneities in electrode activity on the submicron length-scale. An electrolyte filled microcapillary containing a reference/auxiliary electrode was approached to a substrate (working) electrode surface, via piezoelectric positioners. Contact of the electrolyte meniscus with the substrate electrode was sensed by a current flowing. Electrochemical measurements were performed before the microcapillary was retracted and another point on the sample was characterised. Spatial heterogeneities in electrode activity were imaged on a sub-micron length-scale and the activity of basal plane highly oriented pyrolytic graphite (HOPG) was demonstrated. Tip position modulation scanning electrochemical microscopy (SECM-TPM), where an ultramicroelectrode (UME) is oscillated perpendicularly to a surface and an amperometric current is recorded, was investigated experimentally and theoretically. A model including convective mass-transport was developed that gave an accurate description of the experimental situation. It was demonstrated that SECM-TPM is a potentially powerful approach for the measurement of the permeability of a sample. SECM experiments were performed investigating the growth of Ag particles at a liquid/liquid interface, which was caused through the electrodissolution of a Ag UME in an aqueous phase, and the reduction of the Ag+ ion by an electron donor in the organic phase. A model was created that allowed for the interpretation of data. Cyclic voltammetry investigations of HOPG covered with a Nafion film containing a redox mediator confirmed the activity of basal plane HOPG, as demonstrated by SMCM measurements. Nafion slowed diffusion sufficiently to allow the spatial-decoupling of surface sites with different activity

Development of innovative materials used in electrochemical devices for the renewable production of hydrogen and electricity

One of the most important challenges for our society is providing powerful devices for renewable energy production. Many technologies based on renewable energy sources have been developed, which represent a clean energy sources that have a much lower environmental impact than conventional energy technologies. Nowadays, many researches focus their attention on the development of renewable energy from solar, water, organic matter and biomass, which represent abundant and renewable energy sources. This research is mainly focused on the development of promising electrode materials and their potential application on emerging technologies such as artificial photosynthesis and microbial fuel cell (MFC). According to desired proprieties of functional materials, this research was focused on two main materials: (1) TiO2 for the development of electrodes for the water splitting reaction due to its demonstrated application potential as photocatalyst material and (2) carbon-based materials for the development of electrodes for MFC. In the first part of the investigation, different TiO2 nanostructures have been studied including: synthesis, characterization and test of TiO2-based materials with the aim of improving the limiting factors of the photocatalytic reaction: charge recombination and separation/migration processes. The photo-catalytic properties of different TiO2 nanostructures were evaluated including: TiO2 nanoparticles (NPs) film, TiO2 nanotubes (NTs) and ZnO@TiO2 core-shell structures. Photo-electrochemical activity measurements and electrochemical impedance spectroscopy analysis showed an improvement in charge collection efficiency of 1D-nanostructures, related to a more efficient electron transport in the materials. The efficient application of both the TiO2 NTs and the ZnO@TiO2 core-shell photoanodes opens important perspectives, not only in the water splitting application field, but also for other photo-catalytic applications (e.g. photovoltaic cells, degradation of organic substances), due to their chemical stability, easiness of preparation and improved transport properties. Additionally, in order to improve the photo-catalytic activity of TiO2 NPs, PANI/TiO2 composite film was synthesized. PANI/TiO2 composite film was successfully applied as anode material for the PEC water splitting reaction showing a significant increase in the photocatalytic activity of TiO2 NPs composite film essentially attributed to the efficient separation of the generated electron and hole pairs. To date, no cost-effective materials system satisfies all of the technical requirements for practical hydrogen production under zero-bias conditions. For this propose, to promote the sustainability of the process, the bias require to conduct PEC water splitting reaction could be powered by MFC systems in which many efforts have been done to improve power and electricity generation as is explained below. In this work, different strategies were also applied in order to improve the performance of anode materials for MFCs. The investigation of commercial carbon-based materials demonstrated that these materials, normally used for other ends are suitable electrodes for MFC and their use could reduce MFC costs and improve the energy sustainability of the process. In addition, to enhance power generation in MFC by using low-cost and commercial carbon-based materials, nitric acid activation (C-HNO3) and PANI deposition (C-PANI) were performed on commercial carbon felt (C-FELT) in order to increase the performance of MFC. Electrochemical determinations performed in batch-mode MFC reveled a strong reduction of the activation losses contribution and an important decrease of the internal resistance of the cell using C-HNO3 and C-PANI of about 2.3 and 4.4 times, respectively, with respect to C-FELT. Additionally, with the aim of solvent different MFC operational problems such as: biofouling, low surface area and large-scale MFC, an innovative three-dimensional material effectively developed and used as anode electrode. The conductive carbon-coated Berl saddles (C-SADDLES) were successfully used as anode electrode in batch-mode MFC. Electrochemical results suggested that C-SADDLES offer a low-cost solution to satisfy either electrical or bioreactor requirements, increasing the reliability of the MFC processes, and seems to be a valid candidate for scaled-up systems and for continuous mode application of MFC technology. In addition, the electrochemical performance and continuous energy production of the most promising materials obtained during this work were evaluated under continuous operation MFC in a long-term evaluation test. Remarkable results were obtained for continuous MFCs systems operated with three different anode materials: C-FELT, C-PANI and C-SADDLES. From polarization curves, the maximum power generation was obtained using C-SADDLES (102 mW•m-2) with respect to C-FELT (93 mW•m-2) and C-PANI (65 mW•m-2) after three months of operation. The highest amount of electrical energy was produced by C-PANI (1803 J) with respect to C-FELT (1664 J) and C-SADDLES (1674 J). However, it is worth to note that PANI activity was reduced during time by the operating conditions inside the anode chamber. In order to demonstrate the wide application potential MFC, this work reports on merging heterogeneous contributions and combining the advantages from three separate fields in a system which enables the ultra-low-power monitoring of a microbial fuel cell voltage status and enables pressure monitoring features of the internal conditions of a cell. The solution is conceived to provide an efficient energy source, harvesting wastewater, integrating energy management and health monitoring capabilities to sensor nodes which are not connected to the energy grid. Finally, this work presented a general concept of the integration of both devices into a hybrid device by interfacing PEC and MFC devices (denoted as PEC-MFC), which is proposed to generate electricity and hydrogen using as external bias the potential produce by microbial fuel cel

Low Temperature Plasma Etching Control through Ion Energy Angular Distribution and 3-Dimensional Profile Simulation.

Plasma etching has become a major part of semiconductor processing because it enables the production of smaller electronics with increased computational power. Plasma etching produces highly anisotropic features, which are needed to maintain feature size critical dimensions (CD) through directional ion wafer bombardment. As the semiconductor industry moves towards smaller feature sizes and higher aspect ratios, a better understanding of ion dynamics and control of the plasma etch processes becomes increasingly necessary. In prior technology nodes, 2-dimensional (2-d) feature profile models served very well to help optimize features and connect reactor scale properties to feature scale CDs. As CDs continue to shrink, the current technology nodes must utilize 3-dimensional (3-d) structures, whose optimization is considerably more difficult and not well represented by 2-d profile simulators. This dissertation investigated the plasma physics and plasma surface interactions in plasma etching chambers using a hybrid plasma equipment model to predict plasma properties and a Monte Carlo feature profile model to predict feature evolution. Algorithms for capturing ion sheath dynamics, controlling dual frequency powers on the same substrate and describing 3-d plasma surface kinetics have been developed and integrated into the models. With the addition of these new algorithms, three challenging areas have been investigated: ion multi-frequency sheath dynamics, control of ion energy angular distributions and 3-d plasma etching. The ion kinetics are found to be controlled through several critical parameters, such as shifting phases, tuning frequencies, and adjusting rf voltage ratios. The 3-d profile model addresses the complex feature pattern layout and aids in the physical understanding of ion 3-d bombardment on surfaces. With this improved capability, correlations of the variability of plasma tool performance with variability of feature dimensions are investigated.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113432/1/yitingz_1.pd