Walter Mérida
Relevant Thesis-Based Degree Programs
Affiliations to Research Centres, Institutes & Clusters
Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
In this Ph.D. research study, a non-equilibrium thermodynamic model and a non-equilibrium multilayer thermodynamic model are developed to assess the thermal performance of liquefied natural gas (LNG) storage tanks by reducing the computation time compared with computational fluid dynamics (CFD) methods. The non-equilibrium thermodynamic model incorporates a resistance-capacitance network to evaluate the thermal performance of vertical and horizontal LNG storage tanks. This model is validated against two sets of experimental data, and compared with the equilibrium thermodynamic model. The results indicate that, under stationary conditions, the equilibrium and non-equilibrium thermodynamic models predict the pressure and temperature profiles in the vertical and horizontal LNG storage tanks with good accuracy. However, the non-equilibrium thermodynamic model is the only model that can predict the thermal performance of the storage tanks under dynamic operating conditions such as sudden pressure changes due to the vapor return to a tank or LNG dispensing to heavy-duty trucks. Also, the results highlight that the LNG holding time in horizontal storage tanks is longer than that in vertical storage tanks under dynamic operating conditions.The non-equilibrium multilayer thermodynamic model is developed to study the thermal stratification and rollover in LNG storage tanks. This model divides the liquid and vapor domains in the tanks into multi-layers, considers the boundary layer formation along the tank vertical walls and, uses adaptive mesh to accommodate the changes in the LNG level over time. This model is verified against experimental data available in the literature to predict the thermal stratification and rollover in cryogenic storage tanks. The parametric study indicates that in atmospheric LNG storage tanks, the amount of heat transfer to the tank and the tank aspect ratio (i.e., diameter to height ratio) do not affect the rollover start time. However, they affect the LNG evaporation rate. The rollover start time is directly affected by the ratio between the amount of fresh LNG loaded to the tank (cargo) and the amount of LNG left in the tank (heel) before loading the cargo. This work suggests that the cargo to heel ratio should be monitored regularly to prevent the rollover in LNG storage tanks.
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By powering fuel cell electric vehicles hydrogen can contribute to greenhouse gas emissions reduction in British Columbia (B.C.) The province is well positioned to capitalize on its natural resources and policies towards the development of a hydrogen fueling supply chain (HFSC). However, such development requires significant investment with high risks of negative cash flow for years to decades.A spatially explicit multi-period optimization model was developed to design a minimum-cost HFSC based on a mixed integer linear programming formulation. The model was applied to the light duty passenger vehicle sector in B.C. under three hydrogen demand scenarios. The model considered different capacities for all components of the supply chain, covered the on-site production and capacity expansion options as well as minimum storage requirement for fueling stations. Different combinations of the current and potential environmental mandates and the government economic instruments were integrated in the model explicitly. The model measured the effectiveness of the policies on reducing the cost and greenhouse gas (GHG) emissions of the HFSC for each demand scenario. To this end, the GHG emissions were monetized using the social cost of carbon. The results suggested that hydrogen can be cost competitive with gasoline. However, the cost optimal hydrogen infrastructure relied heavily on steam methane reforming (SMR), with small GHG emissions reduction benefits. Nonetheless, the monetary benefits of well to wheels (WTW) GHG emissions reduction justified the switch from gasoline to SMR-based hydrogen. It was found that central electrolysis can be financially justified by addition of production tax credits or electricity incentives to the current provincial carbon control policies (i.e., carbon tax and low carbon fuel standard).This study assessed the effectiveness of current policies in emissions mitigation from the road freight transport. Moreover, the WTW energy requirement and GHG emissions reduction potential of the all-electric trucking were measured to meet the provincial emissions reduction targets. The results suggested that the B.C. hydroelectricity will fall short of generating sufficient energy to support all-electric trucking. Thus, B.C. has to undertake policies to incentivize electricity generation from diversified renewable energy resources.
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This study discusses the development of selective water vapour permeable membranes for the separation of water vapour from air. These membranes are useful as separation media for membrane-based energy recovery ventilation devices. Current generation composite membranes for these devices consist of a polymer film layer which is permeable to water vapour but selective for water vapour over gases. In these composite membranes, this film layer is attached to one surface of a microporous polymeric support substrate. However, as it is demonstrated in this work, the microporous support contributes 30 to 50% of the resistance to water vapour transport in the composite membranes.In an attempt to eliminate this microporous substrate and its associated resistance, a membrane was developed using an electrospun nanofibrous layer as a support structure for the selective coating layer. In these membranes, the electrospun nanofibers are deposited on a non-woven micro-fibrous carrier and then the nanofibers are impregnated with a selectively permeable polymer to make impregnated electrospun nanofibrous membranes (IENM). The nanofibers are found to contribute a resistance to vapour transport in these membranes and their effect on water vapour permeability is quantified through a fiber-filled-film model. An optimization study of the IENMs demonstrated that fiber diameter and fiber volume in the film layer effect the water vapour permeance of these membranes and reducing these variables improved water vapour permeance. IEMNs were demonstrated to have water vapour permeance (>10000 GPU) while still having sufficient selectivity for the application, exceeding the performance of current generation composite membranes.The membranes were demonstrated to be particularly useful in the fabrication of ‘formable membranes’ which could be thermally-formed into exchanger plates for energy recovery ventilator exchangers. Thermal and mechanical properties of the membrane components were reported individually and as complete membranes. A membrane composition was demonstrated to fabricate exchanger plates from formable IENMs.This work contributes to the development of membranes for air-to-air exchangers specifically for energy recovery ventilation. A new class of membrane based on electrospun nanofibers for these devices was successfully demonstrated to have improved performance properties and a novel formable membrane was developed.
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An increasing energy demand and the need to reduce greenhouse gas emissions promotes a need for sustainable energy systems. Polymer-electrolyte-membrane fuel cells, comprising a carbon-platinum catalyst layer (CL), have reached (pre-)commercial viability. However, performance, durability, and cost targets are not entirely met yet. In a larger perspective, this thesis aims to contribute to the development of novel catalyst quality monitoring methods, gradual catalyst loading strategies, and the link between water management and catalyst degradation. The objectives of this thesis are to design tools and develop methods of investigation for catalyst degradation, characterize different accelerated stress test (AST) protocols, indicate stressors of the occurring degradation, deconvolute the performance decay, and to analyze the locally occurring degradation. Carbon corrosion and platinum dissolution are the major degradation mechanisms during the operation. Start-up and shut-down (SU/SD) events where air purges through the hydrogen containing anode, accelerates the progression of these mechanisms, and amplified performance decay results. This study develops an ex-situ tool and a method to link the water content within the CL to the degradation behavior. Higher water content may accelerate CL failure. The comparison of different SU/SD ASTs revealed the degradation behavior and the influence of various mitigation strategies to reduce the occurring voltage decay. The application of an external resistance resulting in the consumption of electrons reduces the degradation. If the voltage is suppressed by applying an external load, the degradation in high current densities is marginal. The deconvolution of the voltage decay at high current densities reveals the ratio of carbon to non-carbon related losses. The non-carbon related losses remain dominant throughout the initial stages of the AST; later the carbon corrosion related losses become significant. The novel multi-channel-characterization-system revealed larger losses towards the cell’s exit during SU/SD and platinum dissolution ASTs. Chemical platinum degradation caused by higher water content towards the exit dominates. Longer residence times of oxygen within the anode promote degradation towards the exit. In conclusion, this study developed novel diagnostics, assessed the ratio of performance decay related to its mechanisms, estimated a dominance of the non-carbon related losses throughout the initial phase, while localizing the degradation.
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Fuel cells are a promising energy conversion technology compatible with developing renewably sourced primary energy distribution. Proton exchange membrane (PEM) fuel cells are particularly suitable for automotive and portable applications. The present thesis advances novel PEM fuel cell porous transport layer (PTL) characterization and materials research. These layers link macro and nano scales by mediating energy and mass transport between reactant distribution channels and catalyst layers. Contemporary commercial PTLs are limited in selection. Moreover, typical characterization methods ignore essential material anisotropy. Herein, a novel transport layer synthesis concept is introduced. By adapting electrospinning technology, structures with engineered morphology are created. PTLs are produced with fibre diameters from 0.2 to 1.6 µm, and are characterized experimentally ex-situ and in-situ. Electrospun PTLs are shown to deliver 85% of equivalent commercial PTL current densities. Furthermore, the state-of-the-art for electronic resistance measurement of PTLs is improved, with rigorous attention given to the anisotropy of the fibre-based media. Novel method and apparatus provide this information as a function of mechanical strain. PTL in-plane resistivities are a unique contribution, where for commercial materials 4.5x10-⁴ to 1.5x10-⁴ Ω∙m are observed for strains from 0.0 to -0.5 m∙m-¹. Finally, electrospun PTLs are developed to investigate the effect of within-plane anisotropy upon fuel cell performance. Electrospun layers are produced with progressively greater fibre alignment to effect anisotropy. This anisotropy is visualized via microscopy, and quantified using the aforementioned electronic resistivity methods. In-situ results with electrospun PTLs, of anisotropy ratios from 1 to 6, suggest greater performance with average fibre alignment perpendicular to gas distribution channels. The present thesis’ contributions strengthen development of a PTL structure-property-performance relationship. With integration into a cell-level relationship, this can empower rational PEM fuel cell design.
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High frequency data (HFD) of three site studies in different geographic locations were analyzed to reproduce the power spectrum illustrated by Van der Hoven in 1957. His work represents the basis of wind energy standards such as averaging and variability in the frequency domain. The results presented in this thesis unveil discrepancies with Van der Hoven’s approach. A major eddy-energy peak is illustrated at a period of 2 days and a smaller eddy-energy peak contribution at frequencies higher than the region known as the spectrum gap. The variance in the microscale region was calculated by integrating the Power Spectral Density (PSD) over the corresponding range of frequencies. The economic value of this energy variance based on the turbulence kinetic energy of the wind data set is calculated. It is also concluded that, given the results of the study, HFD analysis in the frequency domain uncovers eddy-energy peaks that determine energy fluctuations in the short and long terms. An algorithm was developed to detect delay times in the turbulence kinetic energy (TKE) and the energy dissipation rate ε on a continuous basis (thereby identifying the highest cross-correlation coefficients between them). The Kolmogorov turbulence order is applied to calculate the energy dissipation rate ε through the identification of the inertial subrange. The time scale in the variations of both parameters was successfully calculated and it is close to the time the air takes to circulate between the surface and the top of the atmosphere’s mixed layer. High correlation coefficients are found in the three site studies from 4am to 8am, and from 8pm to 12pm. The cross-correlation function also determines delay time scales in the range of 10-20 minutes and approximately 2 hours. The energy dissipation rate can be calculated to characterize wind variability in a particular site that might affect the performance of a wind turbine. With these results, more information is generated that can be used in the wind turbine’s control system routines to improve its response under wind turbulence variations.
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As fuel cells reach the early stages of commercialization, the need for effective research methods and efficient characterization tools is increasing. Industrial demands allowing for serial production require methods for standardized testing and efficient quality control while developing new materials and production methods. This thesis work presents two methodologies: Design of Experiments (DoE) applied to Polymer Electrolyte Membrane Fuel Cells (PEMFCs) to analyze for sparsity of effects and performance mapping; and a systematical Voltage Loss Breakdown (VLB) method based on experimental polarization featuring anodic contributions. Both concepts were validates and the effects of commercial porous transport layer (PTL) material on the fuel cell’s voltage under galvanostatic conditions were investigated. The results of this work demonstrate the use of DoE to assess the differences and parameter dependencies of different materials in the PTL of PEMFC. Split-plot and general blocked design models were used to analyze the voltage and pressure drop of PEMFC at different current densities of 1.0 A cm^-², 1.4 A cm^-² and 1.6 A cm^-². The empirical models show good fit and prove that these methodologies based on experimental designs can be useful to predict and analyze fuel cell performance within this design space. The use of designed experiments allows a scientifically objective analysis of the data compared to one-factor-at-a-time (OFAT) testing while reducing the overall required test runs. Our results show, that this analysis can capture and model the effects of PTL materials and operating conditions as statistically and physically significant. The VLB method developed in this work systematically analyzes the different dominant loss contributions and shows the relevance of the anode under varying operating conditions. A reference electrode system was designed and validated in order to measure the anodic and cathodic contributions to the cell’s polarization separately. A mathematical approach was developed to break down the polarization curve into the individual contributing losses, distinguishing between anode and cathode and the individual kinetic, ohmic and mass-transport overpotentials. Based on this study it can be concluded, that a micro-porous layer (MPL) leads to reduced mass-transport losses inside the cathode electrode and decreases the ohmic losses.
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The durability and reliability of fuel cell products need to be improved. The lack of early diagnosis and failure-prevention techniques is one of the limiting factors. This thesis presents a non-invasive method for the early diagnosis of flooding, dehydration and low fuel stoichiometry (three common failure modes). The method is based on micro sensing electrodes (SE) that are placed at appropriate locations in a single cell. These electrodes have a characteristic potential response to each of the failure modes, which enables detection prior to overall fuel cell failure. The specific features in the measured responses (or combinations thereof) can be used to discern between different failure modes, and initiate corrective actions.This thesis also reports on the separation of anodic and cathodic potentials in a working fuel cell via reference electrodes maintained at constant conditions. The reference electrodes consisted of four platinized platinum electrode wires and two patches of the same catalyst layer used in the anode. All the reference electrodes were unaffected by the operating conditions of the fuel cell and those with patches provided the most stable potentials. Individual anodic and cathodic overpotentials (activation, ohmic, concentration and mass transport) were obtained in a segmented and un-segmented fuel cell for the first time. An array of reference electrodes and gases with different diffusion coefficients were used to discern the different overpotentials. The results show that the anodic overpotentials cannot be ignored, even if the conditions are changed at the cathode only. The oxygen concentration has an effect on the anode and in particular hydrogen oxidation and proton flux. Under dry conditions the current in-plane gradients are very large and the heat generation profiles are affected, creating an uneven temperature distribution in the catalyst layer due to concurrent effects of the half-cell reactions and the water vaporization.The combination of reference electrodes and multi-component gas analysis, allows the measurement and calculation of kinetic and diffusion parameters that can be used for modeling and to understand the behavior of different layers of a fuel cell.
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Water transport across Nafion membranes was investigated under activitygradients at atmospheric pressure. The activity gradients across the membrane werecontrolled by exposing one side of the membrane to dry gas under laminar flow, whilemaintaining liquid or vapour equilibrium with water on the other side of the membrane.The measurements were made under steady state and transient conditions.The main objective was to identify the rate limiting mechanism among the threemajor water transport processes in Nafion: diffusion in the bulk, and sorption across bothinterfaces. The proposed hypothesis was to represent the overall water transport acrossthe membrane as the sum of resistances across the membrane bulk and interfaces.The experimental implementation required new hardware and techniques. A dualchamber cell with temperature, humidity, and pressure control was designed with a gatedvalve to control the initial starting time for transient measurements. Unlike previouslyreported work, this design enabled the individual control of membrane thickness,temperature, pressure, relative humidity, and dry gas flow rate in each chamber.The ability to control these variables made the experimental results amenable totheoretical simulations. Two phenomenological models were proposed to separate thecontributions of bulk transport and sorption processes. The Varying Diffusion Coefficientmodel (VDC) was a preliminary effort to generate mass transport coefficients. TheVaporization-Exchange Model (VEM) provided an approximation for the steady stateand transient water transport data through the definition of a novel boundary conditionthat describes the kinetics at the interface, while diffusion is described by Fick’ s law.The VEM yielded interfacial water transport rates:kv= 0.75 cms⁻¹ for liquid-, andkv= 0.63 cms⁻¹ for vapour-equilibrated membranes. Such results contribute to fill the gapfor the membrane interfacial kinetics in the fuel cell literature. The analysis with theVEM revealed that interfacial water transport became rate limiting at membranethickness below Ca. 100 μm.Analysis of transient data with VEM generated bulk diffusivity coefficients: D2-7x 10⁻¹° m²s⁻¹, for liquid equilibrated membranes at 30-70°C, which agreed with literaturedata.A case study is presented for Nafion-Si0₂ composite membranes to study theeffect of the membrane water content, by addition of silicon dioxide to Nafionmembranes. The status of water in the membrane was characterized with long-establishedtechniques, such as vapour sorption, scanning calorimetry, and water uptakemeasurements. Information from these measurements was coupled with measurementsfrom the dual chamber cell. Experimental results indicated a threshold at 16 wt% ofsilicon dioxide above which the water transport properties of the composite differedsignificantly from additive-free Nafion. Analysis with the VEM suggested that theaddition of the composite produced structural changes to the polymer matrix.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
This thesis proposes a deep learning framework for estimating the power use of electric vehicle drivetrains, using an artificial recurrent neural network architecture (long short-term memory), and techniques for data curation and analysis. Power samples of battery-electric buses operating in Victoria, B.C. were deployed to train, validate, and test the model. Microtrips were extracted from the dataset by taking out proportions of time spent at standstill. Anomalous trends of the dataset components were detected, marked as outliers, and excluded from the model input using Symbolic Aggregate approXimation (SAX) transformation filtering algorithm. A systematic optimization process is adopted to achieve the best hyperparameter configuration for this estimation task. The resulting fine-tuned deep LSTM model is then created, evaluated, and tested. More effort has been devoted to enhancing the efficacy of the model through investigating alternative model configurations and formulations. Among the several configurations tested, the best power consumption modeling approach yielded an error of about 5% (the RMSE of 0.05747) and a squared correlation coefficient of beyond 90% (R² of 0.92502). [This thesis was revised on October 25, 2021.]
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Abundant materials, cheap synthesis methods and high efficiencies are required for photoelectrodes materials to enable commercial water splitting through photoelectrochemical processes. In this work, the synthesis and improvement through an electrochemical modification of the n-type semiconductor BiVO4 for oxygen evolution is presented. The modification consisted on a dynamic polarization in a three-electrode cell. The pristine and modified BiVO4 were evaluated through electrochemical and photoelectrochemical methods. The modification was found to increase the photocurrent by 80% compared to pristine BiVO₄ at 1.23 V vs RHE and increase in the incident photon-electron conversion efficiency (IPCE) by up to 20% at 400 nm. A photocathode for hydrogen evolution was synthetized and tested along with the BiVO4 photoanode in a two electrode photoelectrochemical cell. Water splitting was obtained without external bias and an improvement of 33% in the solar to hydrogen (STH) efficiency was obtained with the modified material. Based on results gathered through electrochemical, photoelectrochemical and physical-chemical characterization, the modification causes the insertion of hydrogen into the lattice and distorts the crystal structure. Moreover, the amount of V4+ in the photoanode increases and there is OH adsorption in the surface. These results provide evidence for improved overall performance.
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Hydrogen can play a vital role in a net-zero-emission future because it can be used as fuel in many applications as well as a large scale energy-storage medium. The generation of hydrogen by water electrolysis powered by renewable energy is among the solutions to provide low-carbon hydrogen. Even though the technical feasibility of water electrolysis is demonstrated, the economic analysis and energy system integration on a large scale is not fully covered yet. In this thesis, a techno-economic analysis was performed for large-scale hydrogen production plants (4,000–40,000 kgH₂/day or approximately 10–100 MW). Two electricity pricing schemes in 8 different geographical locations were considered including five Canadian provinces with flat rates and real-time pricing for the wholesale markets in Germany, California, and Ontario. The flat-rate pricing yielded a range for the levelized cost of hydrogen produced via water electrolysis (e.g., $4.21–$4.71/kgH₂ in Québec). For the wholesale electricity markets, an operational strategy was developed that aims to identify if a posted price is high or low based on historical electricity spot prices. The electricity cost can be reduced by 4%–9% in Germany and by 15%–31% in Ontario and California at a capacity factor of 0.9 by implementing this operational strategy. Electrolytic hydrogen production in Ontario combined with underground storage was found to be the cheapest in the three wholesale electricity markets, resulting in a levelized cost of hydrogen of $2.93–$3.22/kgH₂ for alkaline electrolysis and $2.66–$3.54/kgH₂ for proton exchange membrane electrolysis. Compared to steam methane reforming at $2.5–$2.8/kgH₂ (without carbon capture), the electrolytic hydrogen cost is 6%–27% higher. However, this cost becomes comparable to that from steam methane reforming once carbon capture and storage are included in the analysis. Our results suggest that maximizing the use of the electrolytic systems via high capacity factors is economically favorable, especially under integration with wholesale electricity markets.
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Natural gas has gained attraction as an alternative fuel for heavy-duty vehicles due to its lower prices and carbon dioxide (CO₂) emissions as compared to diesel. However, methane is the main component of natural gas, and it is a potent greenhouse gas (GHG). The global warming potentials (GWPs) of methane are 86 and 34 times as high as those of CO₂ in 20- and 100-year horizons, respectively. The potential of natural gas to reduce GHG emissions from heavy-duty vehicles can be undermined if enough methane is emitted along the natural gas supply chain.A high-volume sampling (HVS) system was developed to accurately quantify methane emissions from the pump-to-tank (PTT) segment or the natural gas refueling infrastructure. This segment is the least documented portion in the life cycle analysis of natural gas. The accuracy of the HVS system was validated by comparing the measurements with known flow rates of injected methane and CO₂. The results showed that the HVS system was capable of measuring steady-state leaks and transient emissions with a maximum uncertainty of 6.6%.The utility of the HVS system was demonstrated to measure methane emissions from a pilot and fully-operational time-fill compressed natural gas (CNG) refueling station. A data set of component-level emission rates from compressors, component and nozzle leaks, and nozzle venting events was generated. The results showed that compressors were a significant source of emissions in the pilot station, contributing 88.6% to the annual emissions. Prior to regularly scheduled maintenance, compressor emissions and nozzle leaks in the fully-operational station contributed 32.9% and 66.6% to the annual emissions, respectively. The PTT methane emissions from the pilot and fully-operational stations were 1.4±0.8% and 0.7±0.7% of the total throughputs, respectively. Using these data, practical solutions were recommended and implemented to reduce the PTT methane emissions by 80±20% and 98±2% in the pilot and fully-operational stations, respectively. The HVS methodology presented in this study can be applied to accurately quantify methane emissions from a wide range of natural gas refueling infrastructure including fast-fill CNG and liquefied natural gas (LNG) refueling stations, and LNG bunkering facilities.
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The effects of pH, pellet loading, and available surface area on CaCO₃ pellet growth were measured in highly alkaline liquids. These experiments included three scales. (i) Data from the bench scale reactor were used to predict CaCO₃ pellet diameter in larger scale reactors. (ii) Beaker scale experiments revealed that high pellet loading is more critical to CaCO₃ pellet growth than available surface area. At equal amounts of available surface areas, different retentions were found for different pellet sizes. Elevated temperatures proved detrimental to growth and produced smaller fines. An inversion of mole fractions of CaCO₃ morphologies occurred at the equivalence point of pH 12.3. (iii) Lab-scale fluidized bed reactor, designed and constructed for this thesis, showed that CaCO₃ pellet size is key to calcium retention within a fluidized bed reactor.
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In this work, electrochemical impedance spectroscopy (EIS) measurements were carried out on an increasingly complex distribution of cylindrical pores with well-defined geometries. The desired pores were made via drilling on graphite surfaces. The pore structure complexity was gradually increased starting with the evaluation of single pore impedances followed by drilling multiple pores with uniform pore dimensions and finally generating pore size distributions (PSD) having distributed pore radii or depth or a combination of both. The impedance response was interpreted using ZView fitting and a graphical approach and was found to be well described by the existing theory. The impedance response of the resulting porous electrodes was characterised with varying geometrical and electrochemical parameters - pore depth, pore radii, pore density, and electrolyte conductivity. The obtained results suggest a possibility to use measurements on drilled cylindrical pores to interpret more complex PSD as measurements of PSD were successfully modelled by adding the impedance response of single pores.
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Smart surfaces with dynamic and reversible wettability offer much versatility to meet various application needs. However, application of the smart materials remains challenging, mostly due to complex preparation procedure, a small change in the contact angle, and slow wettability switching. A thorough understanding of wetting phenomena is needed to achieve a breakthrough in the design of superwettability systems.This thesis describes an electrochemical technique to evaluate the surface wettability of rough surfaces based on the wetted area under the droplet. The proportionality between the double layer capacitance and the ion accessible solid-liquid interfacial area is exploited to determine the actual wetted area. While the contact angle fails to describe the wetting mode, the electrochemical approach is capable of discerning the effective wetted area. The electrochemical wettability metric is also used to understand an anomalous wetting behaviour at which the contact angle and interfacial area of an intrinsically hydrophilic substrate show a concurrent increase with roughness. These observations contradict predictions from the Wenzel and Cassie- Baxter models. Based on a coupled optical and electrochemical analysis, the limitations of contact angle are highlighted.An in-situ control over the wettability of electrodeposited copper structure through electrochemical modulation of oxidation state is demonstrated for the first time. Precise control over the rate and extent of the wetting transition is achieved by tuning the magnitude and duration of the applied voltage. Moreover, air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min restores the initial superhydrophobicity. Microstructural and electrochemical analysis show that the active wetting control is based on the Faradaic phase transformation of the surface-bound CuxO phase shielding the Cu core. Based on the wetting switching functionality of the Cu-CuxO core-shell structures, a smart oil-water membrane is designed. The as-deposited or air-dried Cu mesh exhibits superhydrophobicity and superoleophilicity, thus is effective for heavy oil-water separation. On the other hand, the application of a small reduction voltage (
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With fossil fuels increasingly being exhausted and environment pollutions getting worse, it is necessary for our generation to look for sustainable, renewable and environmentally safe alternative energy sources. Solar energy is so far the most available resource, with around 120,000 TW of solar energy striking the surface of the earth. However, sunlight is just available in the daytime, can change within hours or seasons, and it is spread over low-density collection areas. An efficient way of energy storage is required for the utilization of solar energy. So far, one of the most practical ways to store a significant amount of energy is through a chemical energy carrier. Hydrogen fuel is one of the prime candidates as a future energy carrier which is environmentally friendly during its production, delivery, and consumption. Hydrogen production by photoelectrochemical water splitting using a semiconductor catalyst could be one of the most promising ways to harvest solar energy. In this thesis, an in-situ potentiodynamic approach (cyclic voltammetry) was used to modify the photoelectrocatalytic properties of nanostructured electrodes in different media. The effect of the morphology was studied by comparing TiO₂ nanotube and nanorod. Also, the influence of the modification on WO₃, ZnO materials was evaluated. The photogenerated charge carrier separation was studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and Mott-Schottky plots. The morphologies of the samples were tested by scanning electron microscope (SEM). X-ray diffraction (XRD) was used to analyze crystallinity. X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemical composition. The experimental data proved that electronic properties could be changed by the self-doping process, hence, improving the optical absorption properties and increase charge transfer rates. In this way, the photoelectrocatalytic activity of semiconductors was enhanced. The observed behaviors from electrochemical measurements suggested that morphology has a vital role in the capacitive properties. A semiconductor tailored via band structure modification indicates that the electrochemical treatment can be a systematic and straightforward technique for developing novel photoelectrocatalysts with enhanced performances under visible light.
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Proton exchange membrane (PEM) fuel cells are devices that produce zero emission electricity, offering one pathway towards a sustainable energy future. They are in the early stages of commercialization in automotive, backup power and mobile power applications, with cost and lifetime of the cells still representing major barriers. Platinum (Pt) degradation in a PEM fuel cell leads to reduced performance and lifetime, while raw material costs represent a large portion of the overall cost. One of the Pt degradation modes leads to Pt dissolving from the cathode and precipitating in the membrane, forming a “Pt band”. The Pt band is disconnected and unused Pt. If this Pt could be moved, it might be able to be returned to the cathode and be made useful again. This Pt can also have effects on membrane degradation.Pt bands were created in two unique locations using accelerated stress tests (AST) of 10,000 square wave potential cycles from 0.6-1.0V. The locations of the bands were accurately predicted using an existing model and are dependent on the concentrations of oxygen and hydrogen in the membrane. It was hypothesized that increasing the oxygen concentration around the Pt particles in the membrane, would lead to dissolution and movement of the Pt. A new more quantified analysis of the Pt band using SEM imaging is implemented to measure the Pt movement and more fully characterize the Pt band. After two experiments, one trying to move the Pt for 28h and another for 100h, no Pt movement was observed. Another experiment created 2 Pt bands in the same membrane and characterized them for the first time. The second band formed in this experiment did not influence the first band, nor was it influenced by the first band during its formation. The new characterization techniques demonstrated that the distributions of Pt mass and particles were different for the bands formed in the two different locations, while the dual band displayed a superposition of these distributions. The differences distributions have never been quantified before and could have different effects on membrane degradation, which should be the focus of future work.
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The interface between the catalyst layer (CL) and the micro porous layer (MPL) in proton exchange membrane fuel cells (PEMFCs) has been studied in ex-situ experiments. The interfacial morphology, specifically the area, origin and dimensions of interfacial gaps in between compressed CLs and MPLs were investigated with high-resolution X-ray micro computed tomography. In a separate experiment, the electric contact resistance (CR) was evaluated using a custom four-point-probe setup for CLs with different compositions as a function of compression pressure and relative humidity (RH). The interfacial gap area (fraction of the interface separated by gaps) was higher for gas diffusion layers (GDL, with MPL) – catalyst coated membrane (CCM) assemblies with large differences in the surface roughness of the CL and MPL. The interfacial gap area decreased with increasing compression and with increased similarity in roughness. Relatively large continuous gaps were found in proximity of specific cracks in the MPL. These are hypothesized to form due to the presence of large pores on the surface of the GDL, in which the MPL sags and cracks. Relatively small gaps form by means of the regular surface roughness features throughout the CL-MPL interface. Smaller pores on the GDL surface serving as substrate for the MPL could reduce the number of MPL crack-induced gaps. Moreover, adjusting the CL and MPL surface roughness parameters to achieve similar orders of roughness can result in fewer enclosed gaps, and therefore, enhance the mating characteristics. The electric CR followed a similar trend for all the CL compositions, featuring a non-linear decrease in resistance with the increase in the compression pressure. Moreover, the CR was also found to increase with the ionomer content in the CL and with the increase in RH. Physical characterization of the CL surfaces revealed that this increase in the ionomer content enhances the surface roughness features and the surface coverage by the ionomer, both of which affecting the electrical CR towards the MPL. With increasing RH, the CR values doubled for all CL compositions as a result of humidity induced ionomer swelling with the uptake of water.
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