Chemical Engineering

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This is the collection for the University of Waterloo's Department of Chemical Engineering.

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Now showing 1 - 20 of 1015

3D bioprinting of liver-mimetic construct with alginate/cellulose nanocrystal hybrid bioink
(Elsevier, 2018-03-01) Wu, Yun; Lin, Zhi Yuan (William); Wenger, Andrew; Tam, Kam C.; Tang, Xiaowu (Shirley)
3D bioprinting is a novel platform for engineering complex, three-dimensional (3D) tissues that mimic real ones. The development of hybrid bioinks is a viable strategy that integrates the desirable properties of the constituents. In this work, we present a hybrid bioink composed of alginate and cellulose nanocrystals (CNCs) and explore its suitability for extrusion-based bioprinting. This bioink possesses excellent shear-thinning property, can be easily extruded through the nozzle, and provides good initial shape fidelity. It has been demonstrated that the viscosities during extrusion were at least two orders of magnitude lower than those at small shear rates, enabling the bioinks to be extruded through the nozzle (100µm inner diameter) readily without clogging. This bioink was then used to print a liver-mimetic honeycomb 3D structure containing fibroblast and hepatoma cells. The structures were crosslinked with CaCl2 and incubated and cultured for 3 days. It was found that the bioprinting process resulted in minimal cell damage making the alginate/CNC hybrid bioink an attractive bioprinting material.
3D N-doped hybrid architectures assembled from 0D T-Nb2O5 embedded in carbon microtubes toward high-rate Li-ion capacitors
(Elsevier, 2019-02) Tolami Hemmati, Sahar; Li, Ge; Wang, Xiaolei; Ding, Yuanli; Pei, Yu; Yu, Aiping; Chen, Zhongwei
Herein, a unique nitrogen-doped T-Nb2O5/tubular carbon hybrid structure in which T-Nb2O5 nanoparticles are homogeneously embedded in an in-situ formed nitrogen-doped microtubular carbon is synthesized, utilizing a facile and innovative synthesis strategy. This structure addresses the poor electron conductivity and rate capability that hinder T-Nb2O5's promise as an anode for Li-ion devices. Such a distinctive structure possesses a robust framework that has ultrasmall active nanocomponents encapsulated in highly conductive carbon scaffold with hollow interior and abundant voids, enabling fast electron/ion transport and electrolyte penetration. Moreover, nitrogen-doping not only ameliorates the electronic conductivity of the heterostructure, but also induces pseudocapacitance mechanism. When evaluated in a half-cell, the as-prepared material delivers a specific capacitance of 370 F g−1 at 0.1 A g−1 within 1–3 V vs. Li/Li+ and excellent cyclability over 1100 cycles. A high energy density of 86.6 W h kg−1 and high power density of 6.09 kW kg−1 are realized. Additionally, a capacitance retention as high as 81% after 3500 cycles is achieved in an Li-ion Capacitor (LIC) with activated carbon as the cathode and nitrogen-doped T-Nb2O5/tubular carbon as the anode.
3D Silicone Whipping Additive Manufacturing (SWAM): Technology, Applications, and Research Needs.
(University of Waterloo, 2022-01-20) Saggu, Gurkamal
Additive manufacturing has become increasingly popular and is developing in many technologies. This thesis focusses on the additive manufacturing with paste technology, more specifically the goal is to further develop a technology for SWAM (Silicone Whipping Additive Manufacturing). SWAM uses a device similar to a traditional filament 3D printer, but it deploys a paste supplied by a pump instead of feeding a thermoplastic filament through a heated nozzle. There are many parameters that are common to both technologies, but the role of several variables is still not described or discussed in the literature. This is relevant because there are limited technologies capable of exploring the advantage of additive manufacturing with soft or elastomeric materials. A successful SWAM technology be used for developing prototypes that require soft materials, like seat cushion for automotive applications and used in tissue for soft robotics. One of the key parameters controlling the properties of a part manufacture by SWAM is the whipping mechanism of the paste. In this thesis, experiments are presented based on the relationship of SWAM parameters and “Line Printing” characteristics. Line Printing is an experimental method developed here to obtain further insight on the mechanism of SWAM and to enable correlation between SWAM parameters and properties of printed parts. The whipping extrusion techniques has successfully elucidated the liquid rope coiling effect. The SWAM parameters were selected as: Print speed of the nozzle, Diameter of the nozzle, Flow rate of the silicone feed, and Deposition height of the falling silicone paste. Whereas the Line Printing characteristics such as Filament deposition height, Filament line width and Filament loops have been determined. Filament loops are further subdivided into Single loops and Multiple loops. As a result, Line Printing may be defined as the novel free forming techniques for reproducing a single segment of the desired feed onto the printed bed using various forms of fabricating methods. It may also be called filament printing, filament fabrication, or line prototyping. It would be an aim to study the behavior of the material deposition as well as its transformed deposition ranging from straight-line printing to multiple or mixed-looped line printing. Hence, novel empirical relationships can be generated based on two boundaries elementary parametric conditions that are print speed and the other is flow rate. A novel parameter named GL ratio was introduced here to describe the transformation of the filament through filament characteristics. It would thoroughly define the feed flow dropping criterion. It is the ratio of the change in the feed flow rate to the change in the print speed of the nozzle while the other parameters are held constant. Five transformations of the line filament have occurred which are named as straight line, zig- zag/wavy loop filament, single loops filament, mix loops filament and multiple loops filament. Out of the 5, there are two unstable states, and the rest are stable states. Instability of filament is relatively high at the transition state because of the changing patterns of the silicone rope coiling effect, where GL is equal to unity. Line printing can be formed into multiple layers thus producing the bulk form which would be a porous structure, thus giving rise to a 3D shape due to controlled deposition of the filament due to the X-Y-Z location of the nozzle. In automotive applications, this technique can be used to create prototypes for an in-seat car cushion as a substitute for polyurethane (PUR) seat cushions. Herein, silicone has been used because of its versatile applications. The in-seat silicone cushions have been produced to perform force-deflection test to identify the mechanical feature. Viscoelastic properties of silicone concluded that novel SWAM can print lower density in-seat cushion along with variable firmness. The printed cushion showed that they can withstand the applied force for a longer time-period without decreasing with time. This feature would reveal that silicone can be a good material for seat cushioning. Hard-skinned robots can be modified to humanoid robots which would be ideal in the education system, especially for the autistic children. Soft tissue mimicking materials have been obtained through the line printing technique of the novel SWAM. Parts can be printed to resemble the firmness of fat and muscle tissues due to the displayed similar properties to that of real tissues through mechanical experiments. This could be applied in robots assisting autistic children for example, so they would learn response to external stimuli through humanoid robots in a more natural interactive environment
Accelerated Durability Testing via Reactants Relative Humidity Cycling on Polymer Electrolyte Membrane Fuel Cells
(University of Waterloo, 2010-09-30T18:19:15Z) Panha, Karachakorn
Cycling of the relative humidity (RH) levels in the reactant streams of polymer electrolyte membrane (PEM) fuel cells has been reported to decay fuel cell performance. This study focuses on the accelerated durability testing to examine different modes of membrane failure via RH cycling. A single PEM fuel cell with an active area of 42.25 cm2 was tested. A Greenlight G50 test station was used to establish baseline cell (Run 1) performance with 840 hours of degradation under high-humidity idle conditions at a constant current density of 10 mA cm-2. Under the same conditions, two other experiments were conducted by varying the RH. For the H2-air RH cycling test (Run 2), anode and cathode inlet gases were provided as dry and humidified gases. Another RH cycling experiment was the H2 RH cycling test (Run 3): the anode inlet gas was cycled whereas keeping the other side constantly at full humidification. These two RH cycling experiments were alternated in dry and 100% humidified conditions every 10 and 40 minutes, respectively. In the experiments, the fuel cells contained a GoreTM 57 catalyst coated membrane (CCM) and 35 BC SGL gas diffusion layers (GDLs). The fuel cell test station had been performed under idle conditions at a constant current density of 10 mA cm-2. Under the idle conditions, operating at very low current density, a low chemical degradation rate and minimal electrical load stress were anticipated. However, the membrane was expected to degrade due to additional stress from the membrane swelling/contraction cycle controlled by the RH. In this work the performance of the 100% RH humidified cell (Run 1) was compared with that of RH cycling cells (Run 2 and Run 3). Chemical and mechanical degradation of the membrane were investigated using in-situ and ex-situ diagnostic methods. The results of each measurement during and after fuel cell operation are consistent. They clearly show that changing in RH lead to an overall PEM fuel cell degradation due to the increase in membrane degradation rate from membrane resistance, fluoride ion release concentration, hydrogen crossover current, membrane thinning, and hot-spot/pin-hole formation.
Achieving Drag Reduction Through Polymer-Surfactant Interaction
(University of Waterloo, 2013-08-30T14:14:27Z) Mevawalla, Anosh
Drag reduction is a well-observed phenomenon, it was first observed by the British chemist Toms in 1946, yet its mechanism is still unknown to this day. Polymer Drag reduction has found application in reducing pumping costs for oil pipelines (its use in the Trans Alaska Pipeline has resulted in an increase from 1.44 million bbl./day to 2.1356 million bbl./day), increasing the flow rate in firefighting equipment , and in supporting irrigation and drainage systems. Surfactant drag reducers are used industrially in district heating and cooling systems. Though the fields of Surfactant Drag Reduction and Polymer Drag Reduction are each independently well-developed the effect of their interaction on drag reduction is a less explored phenomenon. Through a well chosen pairing of surfactant and polymer, drag reduction can be maximized while minimizing surfactant and polymer concentrations cutting down on cost and environmental impact. The focus of this work was to determine if there was any positive interaction between the polymers Polyethylene Oxide (PEO) and Anionic PolyAcrylAmide (PAM) and the surfactant Amphosol CG (Cocamidopropyl Betaine) as well as any interaction between the polymers themselves. Both polymers are popular drag reducers while Amphosol is a practically nontoxic (LD50=5g/kg) zwitterionic surfactant and is readily biodegradable. In order to determine if any interaction was present and at what concentration was this most notable 4 techniques were used: Surface tension, Conductivity, Relative Viscosity and Shear Viscosity measurement. From this analysis the polymer Saturation point (PSP), Critical aggregation concentration (CAC) and Critical micelle concentration (CMC) were found as well as the concentrations that optimized the viscosity for the pilot plant runs. The bench scale results were used to pick the optimum concentrations for the polymer surfactant solutions. Pressure readings and flowrate measurements were used to plot the Fanning Friction Factor against the Generalized Reynolds Number for the surfactant polymer mixtures and compared to their pure polymer and surfactant counterparts. The Blasius line was found to hold for water measurements taken and is the base to determine percentage drag reduction. The effect of the presence of amphosol on degradation and overall drag reduction were noted. Other factors considered were pipe diameter and the effect of ionic impurities in the solvent.
Acrylonitrile copolymer/graphene skinned cathode for long cycle life rechargeable hybrid aqueous batteries at high-temperature
(Elsevier, 2018-04-01) Zhi, Jian; Bertens, Koen; Yazdi, Alireza Zehtab; Chen, Pu
For aqueous rechargeable lithium battery (ARLB), excellent cycling stability at elevated temperature is highly desirable in its application of electric vehicles (EVs). However, most state-of-art ARLBs show poor durability under high-temperature operation. Herein, we demonstrate a facile coating approach that can construct a thin acrylonitrile copolymer (ANC)/graphene skin on the top-surface of the LiMn2O4 (LMO) cathode in a rechargeable hybrid aqueous lithium battery (ReHAB). Featuring the continuous coverage and the facile electron transport, the ANC/graphene skinned cathode shows a capacity retention of 61% after 300 cycles at 60 °C, two times larger than the battery without the skin. In the cathode, ANC helps to suppress unwanted interfacial side reactions, and graphene renders a robust ion diffusion framework. Quantitative analysis of Mn suggests that the ANC/graphene skin can greatly suppress dissolution of Mn from the LMO into the aqueous electrolyte, while maintaining the charge transfer kinetics. The polymer-based nanocomposite skin on small (1.15 mAh cell) and large (7 mAh cell) cathodes show similar electrochemical improvement, indicting good scale-up potentials.
Active Site Identification and Mathematical Modeling of Polypropylene Made with Ziegler Natta Catalyst
(University of Waterloo, 2008-09-25T15:32:06Z) Alshaiban, Ahmad
Heterogeneous Ziegler-Natta catalysts are responsible for most of the industrial production of polyethylene and polypropylene. A unique feature of these catalysts is the presence of more than one active site type, leading to the production of polyolefins with broad distributions of molecular weight (MWD), chemical composition (CCD) and stereoregularity. These distributions influence strongly the mechanical and rheological properties of polyolefins and are ultimately responsible for their performance and final applications. The inherent complexity of multiple-site-type heterogeneous Ziegler-Natta catalysts, where mass and heat transfer limitations are combined with a rather complex chemistry of site activation in the presence of internal and external donors, plus other phenomena such as comonomer rate enhancement, hydrogen effects, and poisoning, makes the fundamental study of these systems a very challenging proposition. In this research project, new mathematical models for the steady-state and dynamic simulation of propylene polymerization with Ziegler-Natta heterogeneous catalysts have been developed. Two different modeling techniques were compared (population balances/method of moments and Monte Carlo simulation) and a new mechanistic step (site transformation by electron donors) were simulated for the first time. Finally, polypropylene tacticity sequence length distributions were also simulated. The model techniques showed a good agreement in terms of polymer properties such as molecular weights and tacticity distribution. Furthermore, the Monte Carlo simulation technique allowed us to have the full molecular weight and tacticity distributions. As a result, the 13C NMR analytical technique was simulated and predicted.
Additive Manufacturing of High Temperature Strain Gauges
(University of Waterloo, 2018-12-10) Vandenberg, Jeremy
Additive manufacturing (AM) is quickly leading a new revolution in manufacturing. Aerosol ink jet printing (AJP) is a non-contact printing method that allows for printing on irregular substrates. When paired nanoparticulate ink, the method can print electrical traces and sensors. AJP stands to surpass current thin film technologies by flexibly printing on complex geometries. This thesis details the preliminary work towards employing AJP to create sensors operating in harsh environments. Specifically, the development of materials required to enable printed circuits functioning at temperatures exceeding 1000˚C (1850 ˚F). The high temperature corrosion behavior of devices created from nanoparticles is explored from starting with the synthesis of the nanoparticles themselves. Inks suitable for AJP are formed from the nanoparticles. The inks are subsequently printed into strain gauge designs, sintered to bulk, and tested for conductivity. A technique to create core shell nanoparticles is demonstrated in efforts to make the ink materials more resistant to side reactions during the sintering phase. An additional design aspect is introduced in the form of sol gels to solve the corrosion challenges presented. Sol gels were developed to create ceramic thin films to insulate the manufactured sensors, provide an engineered surface, and encapsulation layer for the devices. Sol gel chemistry is a wet chemical approach for forming ceramics that is also found to be compatible with AJP processes. Only a few sensors produced were suitable for electrical characterization. This was due to side reactions in the sintering process as well as insufficient adhesion of the printed traces to the substrate. The resistive path of the sensor was 31 kohms, which was outside of the testing range for strain gauges. The elevated resistance of these samples is due to impurities and defects in the printed patterns. The findings of this thesis are useful for generating the next generation devices for use in harsh environments. The materials established here can be altered by differing processing techniques to eliminate the barriers to achieving integrated strain gauges by additive manufacturing.
Adsorption Kinetics of Alkane-thiol Capped Gold Nanoparticles at Liquid-Liquid Interfaces.
(University of Waterloo, 2012-03-08T16:01:51Z) Ferdous, Sultana
The pendant drop technique was used to characterize the adsorption behavior of n-dodecane-1-thiol and n-hexane-1-thiol capped gold nanoparticles at the hexane-water interface. The adsorption process was studied by analyzing the dynamic interfacial tension versus nanoparticle concentration, both at early times and at later stages (i.e., immediately after the interface between the fluids is made and once equilibrium has been established). Following free diffusion of nanoparticles from the bulk hexane phase, adsorption leads to ordering and rearrangement of the nanoparticles at the interface and formation of a dense layer. With increasing interfacial coverage, the diffusion-controlled adsorption for the nanoparticles at the interface was found to change to an interaction-controlled assembly and the presence of an adsorption barrier was experimentally verified. At the same bulk concentration, different sizes of n-dodecane-1-thiol nanoparticles showed different absorption behavior at the interface, in agreement with the findings of Kutuzov et al. [1]. The experiments additionally demonstrated the important role played by the capping agent. At the same concentration, gold nanoparticles stabilized by n-hexane-1-thiol exhibited greater surface activity than gold nanoparticles of the same size stabilized by n-dodecane-1-thiol. 1.6 nm, 2.8 nm, and 4.4 nm nanoparticles capped with n-dodecane-1-thiol, and 2.9 nm, and 4.3 nm particles capped with n-hexane-1-thiol were used in this study. The physical size of the gold nanoparticles was determined by TEM image analysis. The pendant drop technique was also used to study the adsorption properties of mixtures of gold nanoparticles at the hexane-water interface; and also investigate the effects of different factors (i.e., temperature, pH or ionic strength) on interfacial tension (IFT). The interfacial properties of mixtures of these nanoparticles, having different sizes and capping agents, were then studied. No interaction was found between the unmixed studied nanoparticles. Using the theory of non-ideal interactions for binary mixtures, the interaction parameters for mixtures of nanoparticles at the interface were determined. The results indicate that nanoparticle concentration of the mixtures has a profound effect on the interfacial nanoparticle composition. A repulsive interaction between nanoparticles of different size and cap was found in the mixtures at the interface layer. The interfacial tension for mixtures was found to be higher than the interfacial tension for non-mixed nanoparticle suspensions. The nanoparticle composition at the interface was found to differ from the composition of nanoparticles in the bulk liquid phase. The activity of unmixed nanoparticles proved to be a poor predictor of the activity of mixtures. It was observed that the most active nanoparticles concentrated at the interface. The effects of temperature, pH and ionic strength concentration on the equilibrium and dynamic IFT of 4.4 nm gold nanoparticles capped with n-dodecane-1-thiol at the hydrocarbon-water interface was studied. The pendant drop technique was also used to study the adsorption properties of these nanoparticles at the hexane-water and nonane-water interface. The addition of NaCl was found to cause a decrease of the equilibrium and dynamic IFT greater than that, which accompanies the adsorption of nanoparticles at the interface in the absence of NaCl. Although IFT values for acidic and neutral conditions were found to be similar, a noticeable decrease in the IFT was found for more basic conditions. Increasing the temperature of the system was found to cause an increase in both dynamic and equilibrium IFT values. The adsorption of functionalized gold nanoparticles at liquid-liquid interfaces is a promising method for self-assembly and the creation of useful nanostructures. These findings contribute to the design of useful supra-colloidal structures by the self-assembly of alkane-thiol capped gold nanoparticles at liquid-liquid interfaces.
Adsorption of an Organic Dye with Cellulose Nanocrystals
(University of Waterloo, 2013-07-23T17:35:54Z) Batmaz, Rasim
In developing countries many industries use dyes to colour their products, such as textiles, rubber, paper, cosmetics, leather, plastics, and food industries. Such a wide range of using dyes in many industries increases the demand of dye, and currently 100,000 dyes are commercially available with a rough estimated production of 10⁶ tones/year. Without proper treatment, dye effluent can be mixed with surface and ground water system and it may finally enter the drinking water system. Therefore, the treatment of dye effluents before discharge to the environment has become an global challenge due to the stability and adverse effects of dyes. Among the present methods, adsorption has been preferred to other conventional techniques due to the simple design and operation, low initial investment, effectiveness and insensitivity to toxic substances. The high surface area and the presence of permanent negative charge on the surface makes cellulose nanocrystal (CNC) an excellent candidate for the adsorption of basic (cationic) dyes. The objective of this project is to evaluate the adsorption properties of CNC for the removal of methylene blue from aqueous solution by changing the parameters, such as adsorbent dosage, initial dye concentration, pH, temperature and salt concentration. It was found that the adsorption is independent of pH, however increase in temperature and ionic strength decreased the removal percentage slightly. The Langmuir and Freundlich isotherms were used to evaluate the feasibility of the adsorption process. The adsorption capacity of CNC was determined using the linearized form of Langmuir model. It possessed a value of 118 mg/g at pH 9 and 25 °C. To enhance the adsorption, CNC was oxidized with TEMPO reagent to convert primary hydroxyl groups to carboxyl groups that provides more negative charge. After the oxidation, the adsorption capacity increased from 118 to 769 mg/g.
Adsorption of EOR Polymers and Surfactants on Carbonate Minerals
(University of Waterloo, 2014-09-24) Shoaib, Mohamad
Polymer and Surfactants are widely used to improve the oil recovery from a reservoir. One of the main issues with injection of these chemicals is their adsorption over reservoir rock surfaces. The first part of this study focuses on understanding the adsorption characteristics of a newly proposed polymer “Schizophyllan” for application in high temperature (120°C) and high salinity (250 g/l) carbonate reservoirs which are typical in Middle East. In the static adsorption experiments, the effect of parameters like mineral type, salinity, background ions, and temperature on adsorption was investigated. We find that adsorption density over minerals decreases with salinity and temperature. The adsorption of the polymer is higher on carbonate rocks compared to silica and kaolin. Dynamic adsorption using the core flow experiments is also studied. The adsorption in the presence of oil is low compared to the adsorption when there was no oil in the core. The change in viscosity of polymer as a result of flowing through the core has also been reported. In the second part we study the adsorption of a switchable surfactant (Neutral to Cationic) which is a promising candidate for CO2 foam for mobility control. The adsorption shows a significant reduction with an increase in salinity and temperature. This is a promising result for further selection of this surfactant for the field applications.
Adsorptive Removal of Refractory Sulphur and Nitrogen Compounds from Transportation Fuels
(University of Waterloo, 2011-09-02T18:22:31Z) Iravani, Amir
The reduction of sulphur in transportation fuel has gained significant importance as the regulatory agencies worldwide react to air quality concerns and the impact of sulphur oxides on the environment. The overall objective of this research was to identify, develop and characterize, based on underlying scientific principles, sorbents that are effective in removal of refractory sulphur compounds from fuel through the process of selective adsorption. It was determined that impregnation of powdered activated carbon with a transition metal (TM) significantly boosted the adsorption performance of the activated carbon. It is hypothesized that the impregnation resulted in the formation of new adsorptive sites that strongly interacted with the lone pairs of electrons on sulphur and nitrogen while having minor impact on the existing oxygen functional groups on the surface of the activated carbon. The percent loading of the TM was determined through wet adsorption study. The best performing sorbent was shown to have maximum adsorption capacities of approximately 1.77 and 0.76 mmol-S/g-sorbent for DBT and 4,6 DMDBT, respectively, with approximately 100% regenerability through solvent wash and thermal treatment. On average, the PTM impregnation showed approximately 137% increase in adsorption capacity of the activated carbon. The sorbent also has good adsorption capacities for organo-nitrogen compounds (i.e., quinoline and carbazole) and a low selectivity towards aromatics, which is desired in adsorptive desulphurization. The surface morphology of the activated carbon, the oxygen functional groups on the surface of the activated carbon, as well as strong (chemisorption) interaction between the TM’s partly vacant and far reaching ‘d’ orbital and lone pair electrons on sulphur and nitrogen are considered to be the main contributing factors to the observed enhancement. It was established in this study that the adsorption isotherms of the impregnated activated carbons best fit Sips isotherm equation, which is a combination of the Langmuir and Freundlich equations. This finding fits well with our initial hypothesis regarding the introduction of new adsorptive sites as a result of TM impregnation and that the sites did not fit well with Langmuir’s monolayer and uniform adsorption mechanism. A kinetic study of the sulphur adsorption using a flow reactor showed a good fit with pseudo second order kinetic model, indicative of an adsorption that is highly dependent on the concentration of available sites on the surface of the sorbent. On average, as expected, the TM impregnated ACC exhibited a higher initial rate of adsorption. The adsorption onto TM sites tends to be more exothermic than adsorption (mainly physisorption) on activated carbon. Therefore, more thermodynamically favoured chemisorption is expected to occur more rapidly than physisorption. It was determined that on average, the initial adsorption rate does not change significantly with temperature while the sulphur adsorption capacity decreases with increase in temperature. It is postulated that the increase in temperature increases surface diffusivity but impedes diffusion flux. The impediment of the diffusion flux will result in reduction in adsorbed quantity. It was also shown that the intra-particle diffusion exists in the adsorption of DBT on TM impregnated activated carbon, however, it is not likely that the overall adsorption is controlled or noticeable impacted by it. As the temperature of the reactor increases the Weber-Morris intra-particle diffusion plot moves away from the origin, and thus intra-particle diffusion becomes less of a controlling mechanism. This further confirms the fact that the boundary layer (i.e., surface diffusion) and potentially adsorptive interactions at the surface are the dominating mechanisms in the sulphur adsorption onto TM impregnated activated carbon. It was determined that the distribution of TM species on the surface of the activated carbon is relatively inhomogeneous, with some areas showing well dispersed TM species while other areas showing large clusters. Different impregnation method that can improve dispersion on the surface may significantly enhance adsorption performance of the sorbent. Furthermore, in this study impregnation of activated carbon using several other transition metals were examined. It was determined that other less expensive transition metals can also improve the adsorption performance of the activated carbon. Further study on less expensive options for impregnating the activated carbon may be beneficial.
Advanced Carbon Nanomaterials as Non-Precious Metal Catalysts for Fuel Cells
(University of Waterloo, 2017-08-24) Zamani, Pouyan
Polymer electrolyte membrane fuel cells (PEMFCs) are electrochemical devices that efficiently convert hydrogen and oxygen into electricity and water. Their clean point of operation emissions and continuous operation have resulted in PEMFCs being highly touted as integral components of sustainable energy infrastructures, most notably in the transportation sector as a green alternative to the internal combustion engine. The issues associated with hydrogen production and distribution aside, the commercial viability of PEMFCs into the auto- motive sector is hindered by their high cost and inadequate long-term operational stability. The main factor behind both of these problems is the platinum-based electrocatalysts used at the cathode to facilitate the inherently sluggish oxygen reduction reaction (ORR). These expensive precious metal catalysts comprise almost half of the overall PEMFC stack cost and tend to degrade in the cathode environment that is very corrosive due to the acidic and potentiodynamic conditions. The current cost targets for PEMFCs are unattainable unless the extensive reliance on this precious metal is alleviated. The cost reduction can ultimately be accomplished by developing alternative cathode catalysts for the ORR. Research on new platinum catalyst supports or nanostructured platinum alloys to increase ORR activity on a precious metal mass basis have been largely successful. This approach is not ideal, however, due to the volatile pricing and geopolitical instabilities that can likely affect the supply of platinum. For these reasons, the development of entirely non-precious metal catalysts (NPMCs) for the ORR is highly desirable. This is the objective of this thesis, as will be presented in the following sections. ‎Chapter 4 describe the operation of one-dimensional nanofibers are prepared by electrospinning an iron–polyaniline/polyacrylonitrile (Fe–PANI-PAN) metal-polymer blend, followed by subsequent heat treatment. PANI was selected as it has previously been shown to be an ideal nitrogen precursor to produce some of the most active NPMCs to date, owing to its aromatic ring structure with a high content and uniform distribution of nitrogen species that can readily form nitrogen-doped graphitic carbon structures during heat treatments. PAN was also helpful as a low-cost polymer carrier to overcome the poor solubility of PANI in solution and as a secondary source of nitrogen. The addition of 10 wt. % PANI to the electrospinning mixture provides 100 and 70 mV improvements to the ORR onset potential and half-wave potential, respectively, rendering the most active NPMCs prepared by electrospinning to date. The high activity is attributed to the porous structure of the nanofibers, combined with the increased nitrogen content provided by the PANI incorporation. This unique synthetic approach, therefore, provides practical progress towards the development of one-dimensional NPMCs for PEMFC applications. Nitrogen-functionalized graphene substances have proved to be promising electrocatalysts for the ORR due to their high activity and exceptional stability in the alkaline environment. However, they exhibit much lower catalytic activity in acidic electrolytes. Hence, in ‎Chapter 5, a hierarchically porous Co-N functionalized graphene aerogel is provided as an active catalyst for the ORR in an acid medium. In the synthesis procedure, PANI is introduced as a pore-forming substrate to promote the self-assembly of graphene structures into the porous aerogel networks and as a nitrogen precursor to induce in-situ nitrogen-doping. Accordingly, a Co-N decorated graphene aerogel framework with a large surface area (485 m2 g−1) and a plenty of meso/macropores are formed after pyrolysis. Such complex structures provide an excess amount of exposed active sites for the ORR and also ensure secure mass transfer. These advantages render significant catalytic activity with the improved onset and half-wave potentials, low peroxide yield and remarkable stability in acid medium. In the next project (‎Chapter 6), we apply an ammonia treatment to tune the structure and activity of electrocatalysts derived from iron, polyaniline and carbon nanotubes (CNTs). By controlling the NH3 reaction conditions, we were able to tune the chemistry of nitrogen incorporation, including concentration and dopant type. The final catalyst had a robust morphology consisting of highly porous 2-D in-situ formed graphene-like structures that, along with the intermixed 1-D CNTs, were decorated with an abundance of nitrogen and iron species. The catalyst derived under the optimized condition (F-P-C_Ar-NH900) exhibited high catalyst activity, including an E1/2 of 0.80 V vs RHE through RDE testing. Under H2-air conditions that are application-friendly, current densities of 77 mA cm-2 at 0.8 V and 537 mA cm-2 at 0.6 V were achieved. Furthermore, a maximum power density of 335 mW cm-2 at 0.6 V was observed. The number of electrons transferred per reduced oxygen molecule was determined to be 3.90 by RRDE indicating that the catalyst exhibited very good selectivity toward the 4-electorn transfer reaction. These electrochemical evaluations indicate that the chemical modification of Fe-PANI-CNT catalyst by NH3 results in a highly promising Pt-free PEMFC ORR electrocatalyst. In ‎Chapter 7, we report the design of 3-dimensional graphitic meso-porous carbon spheres wrapped with 2-dimensional graphenized sheets. This heterostructure has a large electroactive surface area, abundant pore channels and tuned chemical structures that leads to improved electrocatalytic performance. The nano-channels, acting as nanoscale reactors, provide easily accessible active sites, effective mass transfer and smooth charge transfer across the highly conductive carbon matrix. The obtained catalyst delivers a high maximum power density of 0.82 W cm−2 in a single H2−O2 fuel cell measurement, ranking it as one of the most promising NPMCs in PEMFCs. Moreover, fairly good fuel cell stability was also observed through accelerated degradation testing. This work provides a new avenue for NPMC design that can be a step towards practical commercial PEMFCs. Following the previous studies, an efficient strategy of utilizing dual nitrogen sources for preparing highly active Fe-N-C electrocatalyst with in-situ formed graphene-like structures and tuned micro/meso/macro-porous morphology is reported in ‎Chapter 8. This approach is achieved by simultaneously using PANI as a graphene precursor and introducing phenanthroline (Phen) as a pore-forming agent, followed by several post–treatments. This research was accomplished via introducing Phen into the pores of carbon support by ball-milling, which was then covered with a PANI shell through polymerization of aniline, followed by several subsequent pyrolysis and acid leaching steps leading to the formation of in-situ 3D porous graphene-like morphologies with multiple types of pores. Here, Phen acts as a pore-forming agent that is capable of expanding the external PANI shell during the decomposition. Simultaneously, PANI shell converted to graphene-like structures through graphenization in the presence of iron species during pyrolysis processes. Extensive physical characterization indicates the final catalyst provides rich, porous graphene frameworks decorated with uniformly dispersed active sites. The catalyst exhibits high maximum power densities of 1.06 W cm−2 and 0.38 W cm−2 in H2−O2 and H2−air fuel cell tests, respectively, representing one of the highest reported values to date for NPMCs in PEMFCs. Moreover, good fuel cell durability is also observed through accelerated degradation testing. The unprecedented performance of this electrocatalyst in fuel cell is linked to the highly porous graphene frameworks with a vast distribution of pore sizes that maximizes the number of active sites with enhanced accessibility, facilitates the mass-transport properties, and improves the carbon corrosion resistance. ‎Chapter 9 provides a summary of the conclusions of this body of work, along with strategies that can be engaged to capitalize on the scientific advancements made in this thesis. In summary, this research extends from catalyst synthesis to their actual use in a PEMFC, in order to develop commercially viable NPMCs. Various suggestions for prospect works are recommended in the last part of this chapter to further relate the knowledge to design highly active, durable, and low-cost NPMCs.
Advanced Genomic Engineering Strategy based on Recombineering Protocols to “Tailor” Escherichia coli Strains
(University of Waterloo, 2011-05-24T17:33:54Z) Sukhija, Karan
A systematic approach based on bacteriophage Lambda (Lambda Red) and flippase-flippase recognition targets (FLP-FRT) recombinations was proposed for genomic engineering of Escherichia coli. For demonstration purposes, DNA operons containing heterologous genes (i.e. pac encoding E. coli penicillin acylase and palB2 encoding Pseudozyma antarctica lipase B mutant) engineered with regulatory elements, such as strong/inducible promoters (i.e. Ptrc and ParaB), operators, and ribosomal binding sites, were integrated into the E. coli genome at designated locations (i.e. lacZYA, dbpA, and lacI-mhpR loci) either as a gene replacement or gene insertion using various antibiotic selection markers (i.e. kanamycin and chloramphenicol) under various genetic backgrounds (i.e. HB101 and DH5α). The expression of the inserted foreign genes was subject to regulation using appropriate inducers [Isopropyl β-D-1-thiogalactopyranoside (IPTG) and arabinose] at tuneable concentrations. The developed approach has paved an effective way to “tailor” plasmid-free E. coli strains with desired genotypes suitable for various biotechnological applications, such as biomanufacturing and metabolic engineering.
Advanced Heteroatom Doped Nanocarbon Materials as Platinum Catalyst Supports for Fuel Cells
(University of Waterloo, 2016-03-17) Hoque, Md Ariful
The pressing demand for high performance, operationally stable and inexpensive electrocatalyst materials for proton exchange membrane fuel cells (PEMFCs) has spurred significant research and development interest in this field. Until now, fuel cells based on commercially available Pt/C electrocatalysts have not met some of the technical challenges to the widespread commercial adoption of PEMFCs. The main issues associated with the commercial validity of PEMFCs are the high cost and inadequate long term operational stability of Pt/C catalysts typically used to facilitate the inherently sluggish oxygen reduction reaction (ORR). Therefore, the replacement of Pt/C with novel and more effective catalyst materials is critical. These expensive precious metal catalysts make up a large portion of the overall PEMFC stack cost and suffer degradation under harsh potentiodynamic conditions. Therefore, careful electrocatalyst design strategies must be developed to reduce the cost of ORR catalysts with sufficient activity and stability to meet the technical targets set for the use of PEMFCs. In this work, two approaches are applied to develop new electrocatalyst materials for PEMFCs. The first is to design unique sulfur-doped graphene (SG) and sulfur-doped CNT (S-CNT) supports with the objective of replacing the traditional carbon black to enhance stability toward carbon corrosion. The second is to deposit Pt nanoparticles and nanowires onto SG and S-CNT with the objective of exceeding the activity and stability possible with conventional catalysts. These two catalyst technologies are developed with the ultimate objective of integrating the Pt electrodes into membrane electrode assembly (MEA) to provide excellent PEMFC performance. The first study focuses on the use of SG prepared by a thermal shock/quench anneal process as a unique Pt nanoparticle support (Pt/SG). These materials are subjected to a variety of physicochemical characterizations and electrochemical investigation for the ORR. Based on half-cell electrochemical testing in acidic electrolyte, Pt/SG demonstrated increased ORR activity and unprecedented stability over the state-of-the-art commercial Pt/C, maintaining 87% of its electrochemically active surface area following accelerated durability testing. Density functional theory (DFT) calculations highlighted that the interactions between Pt and graphene are enhanced significantly by sulfur doping, leading to a tethering effect that can explain the outstanding electrochemical stability. Furthermore, sulfur dopants resulted in a downshift of the Pt d-band center, explaining the excellent ORR activity and rendering SG as a new and highly promising class of catalyst supports for electrochemical energy technology and PEMFCs. The beneficial impacts of SG support can be utilized by growing more stable nanostructures such as Pt nanowires on SG to further improve the activity and stability of Pt catalysts. Toward this end, we carried out the direct growth of platinum nanowires on SG (PtNW/SG) by a simple, surfactant free solvothermal technique. The growth mechanism, including Pt nanoparticle nucleation on SG, followed by nanoparticle attachment with orientation along the <111> direction is also highlighted. PtNW/SG demonstrated increased Pt mass activity and a specific activity that is 188% higher than state-of-the-art commercial Pt/C catalysts. Most notably, under a harsh potentiodynamic condition (potential cycles: 3000, potential range: 0.05 to 1.5 V vs RHE), PtNW/SG retained 58% of its electrochemically active surface area and 67% of its ORR activity in comparison to Pt/C that retained less than 1% of its surface area and activity and so failed. Given the evidence that SG is a promising support for Pt catalysts, the next logical step is to investigate the influence of sulfur on catalytic materials. Accordingly, we study the effects of sulfur on the electrochemical activity and stability of various SG supported platinum nanowires (PtNW/SGs). To investigate the influence of sulfur, a series of SG materials with varying sulfur contents ranging from 0.35 to 3.95 at% are investigated as Pt nanowire catalyst supports. Based on the physico-chemical characterizations, electrochemical measurements and DFT calculations, the amount of sulfur is shown to significantly affect the electrokinetics of the Pt nanowires. The best ORR kinetics are observed for the Pt nanowires supported on graphene with 1.40 at% sulfur. At higher sulfur contents, further enhancements are not observed, and in fact, leads to a loss of activity. At lower sulfur contents, the beneficial role of sulfur does not have a marked impact on performance so that the characteristics and performance more closely resemble that obtained with undoped graphene supports. Obviously, the beneficial effect of sulfur dopant species can be utilized by doping sulfur into other types of carbon supports such as CNT (S-CNT). Finally, we report on the synthesis, characterization and electrochemical evaluation of S-CNT-supported Pt nanowires (PtNW/S-CNT). PtNW/S-CNT synthesized by a modified solvothermal method demonstrated an increased mass activity and a specific activity 570% higher than state-of-the-art Pt/C. The stability of PtNW/S-CNT is also shown to be very impressive through accelerated degradation testing. Only insignificant changes to the electrochemically active surface area (ECSA, 93% retention) and mass activity (81% retention) of PtNW/S-CNT are observed over the course of cycling, in contrast to sizable losses observed with commercial Pt/C (<1% retention in ECSA and mass activity) under same conditions.
Advanced membrane for water desalination and ion separation applications
(University of Waterloo, 2023-04-25) zarshenas, kiyoumars
Population growth, contamination of fresh water, and climate change are increasing pressure on water supplies, accelerating the need for technological solutions that will improve access to clean water for drinking and sanitation. Membrane technology especially reverse osmosis (RO) and nanofiltration (NF) processes as sustainable routes for water desalination and purification are valuable from an environmental and economic standpoint. At present, RO and NF are the most energy-efficient technologies that provide us with safe and affordable drinking water, but they still need to be improved in terms of cost, affordability, and energy consumption. To achieve these improvements, advances in membrane materials are needed. The most commonly used semi-permeable membrane in RO and NF are polyamide (PA) thin-film composite (TFC) membranes which are fabricated on porous polymeric supports by in-situ polycondensation of two reactive monomers, namely polyamine and polyacyl chloride, at the interface of two mutually immiscible solvents. The main objective of my thesis was to use functional nanomaterials and nanotechnology tools to develop high-performance polyamide thin-film composite (TFC) membranes for water purification and desalination. PA-TFC membranes are flexible and the chemistry and performance of both top-layer and sublayer can be individually manipulated to maximize the overall membrane performance. In the first phase of my doctoral thesis, for the first time, an approach of using an atomic layer deposited (ALD) monomer-affinitive titanium dioxide (TiO2) nanofilm to modify the sublayer of TFC was proposed to form a thin, smooth, and highly cross-linked PA selective top layer. The functional TiO2 nanofilm increases the affinity between modified sublayer and amine monomer provide a more efficient and subtle tuning of the adsorption and diffusion of amine monomer during the interfacial polymerization process. The obtained TFC membrane with optimal ALD TiO2 coverage improved RO performance by obtaining a high permeance of 1.8 L m-2 h-1 bar-1 and high salt rejection rate of 96% in a dead-end process. This work reveals that coating functional nanomaterials by ALD is a practical manipulation technique for the controllable fabrication of promising TFC membranes and the optimization of sublayer materials. In the second phase of this thesis, we offered a facile, green, and cost-efficient approach for coating a stable layer of plant-derived polyphenol tannic acid (TA) on the surface of MXene (Ti3C2Tx) nanosheets. Then, high-performance reverse osmosis polyamide thin film nanocomposite (RO-PA-TFN) membranes were fabricated by incorporation of the modified MXene (Ti3C2Tx-TA) nanosheets in the polyamide selective layer through interfacial polymerization (IP). The strongly negative charge and hydrophilic multifunctional properties of tannic acid not only boosted the chemical compatibility between Ti3C2Tx MXene nanosheets and polyamide matrix to overcome the formation of nonselective voids, but also generated a tight network with selective interfacial pathways for efficient monovalent salt rejection and water permeation. In comparison to the neat thin film composite (TFC) membrane, the optimum TFN (Ti3C2Tx-TA) membrane with a loading of 0.008%wt nanofiller yielded a 1.4-fold enhancement in the water permeability while maintaining at a high NaCl rejection rate of 96% in a dead-end process and enhanced anti-fouling tendency. To the best of our knowledge, this is the first research on tannic acid-modified Ti3C2Tx MXene nanosheets and their utilization in the IP-based TFN membrane. This research offers a facile way for the development of modified MXene nanosheets to be successfully integrated into the polyamide selective layer to improve the performance and fouling resistance of thin film nanocomposite membranes. In the last phase of this thesis, for the first time, a novel IP template, graphene oxide nanoribbons (GONR) was proposed to act perfectly in response to two needs including minimizing the funnel effect and mediating the IP reaction toward desired PA properties. The coated GONR template not only efficiently served the gutter layer role, but also properly regulated the adsorption and transport of amine monomers at the interface of GONR through manipulating electrostatic interaction, capillary rise, and nanoconfinment of IP template by different loadings of GONR. The optimized loading of GONR at 0.02 g.m-2 resulted in a desired hybrid GONR/PA TFC NF membrane with nano-striped crumple structure beyond the PA context, an ultrathin PA nanofilm with a thickness of 15 nm, and a narrow pore size distribution and high crosslinking degree of 80% that simultaneously improve the permeability and selectivity, and successfully passed the upper bound trade-off with permeance of 21.3 L.m-2.h-1.bar-1 and great rejection of 98% for Na2SO4 under 5 bar of pressure. This research provides a new understanding on taking the advantage of a template method thorough an optimized GONR ultrathin network to make a desired selective TFC membrane for more affordable and efficient nanofiltration and ion separation processes.
Advanced Nanostructure Materials for Hybrid Supercapacitors
(University of Waterloo, 2017-07-05) Abureden, Salah
Hybrid supercapacitors (HSCs) are electrochemical devices that combine the characteristics of batteries and supercapacitors in one asymmetric cell. Lithium ion batteries (LIBs) and supercapacitors (SCs) represent two ends of the power and energy density spectrum. On one end of the Ragone plot spectrum, LIBs utilize faradaic reactions to provide high energy densities (150–250) Whkg-1, however, this relatively slow reaction process limits the power density of LIBs (<1000 W kg-1). The faradic mechanism intercalates/de-intercalates lithium into the active material, which causes changes in the chemical phase and increases the likelihood of material degradation, resulting in a limited cycle life (500-300 cycles). On the other end of the spectrum, SCs are based on electrostatic charge collection, which involve fast, reversible adsorption and desorption of ions on the surface of the active material without phase change or chemical reactions. For this reason, SCs are well known for their high power densities (~10 000 W kg-1) and long cyclability (>100 000 cycles), however SCs suffer from limited energy densities (<10 Whkg-1). Combining half-battery and half supercapacitor in one device to bridge the energy/power density gap, while improving cycle life is a promising solution to meet the evolving energy requirements. However, the rapid fading of the power density, reduced capacitance retention and reduced cyclability at high power rates are the main challenges hindering the development of HSC devices. The power density decays significantly because the faradic material at the battery part can’t adjust its rate of charge-discharge reactions to match the adsorption-desorption rate of SCs. The capacitance retention and cycle life correspond to the stability of the material on the battery component. To overcome these challenges, it is important to develop a new type of nanostructure materials with improved electrochemical capabilities. In this work, we investigate a new class of nanostructure materials with high stability and improved reaction kinetics for the faradic component of the HSC. The development strategy introduces alterations to the intrinsic characteristics of the materials, without changing their chemical phase. The investigated materials included: 1D nickel doped lithium titanate oxide nanofibers, 2D vanadium-modified chalcogens nanosheets anchored graphene nanosheets, and reconciled 2D vanadium disulfide nanosheets with prominent 3D ultra-small nanoparticles attached to graphene nanosheets. The developed materials exhibit outstanding electrochemical performance. In chapter 5, we report how a simple and scalable electrospinning technique was utilized to synthesis 1D nickel doped lithium titanate oxide nanofibers (Ni-LTONF10). The physiochemical characterization confirmed: 1) the successful insertion of nickel into the lattice of the lithium titanate oxide nanofibers (LTONF) without changing its chemical structure and, 2) that the nickel was homogenously distributed throughout the nanofibers to the atomic level, resulting in significantly enhanced ion diffusion and electrical conductivity. This unique coupling of 1D morphology and nickel doping of LTO was investigated as the anode material for lithium ion batteries capable of demonstrating outstanding rate capabilities (up to 50 times higher than theoretical capacity (50 C)). The investigated nanofibers also performs 3 times better than nickel-doped nanoparticles demonstrated in other recent reports and shows outstanding ability to maintain high capacity even at 50 C. Ni-LTONF10 shows 20 times higher capacity compared to un-doped lithium titanate nanofibers at 50 C. Specifically, Ni-LTONF10 displays an initial capacity of 190 mAhg-1 at 0.2 C which is 9% higher than the theoretical capacity of LTO, 150 mAhg-1 at 5 C, 116 mAhg-1 at 20 C and 63 mAhg-1 at 50 C. Additionaly, a hybrid supercapacitor was fabricated using Ni-LTONF10, showing superior energy density at high power density. The device was capable of delivering an energy density of 60 Wh kg -1 at a power density of 1.5 kW kg-1 and also retained a high energy density of 35 Wh kg -1 at 5 kW kg-1. In chapter 6, we discuss the development of vanadium-modified binary chalcogens (NiCo2S4) wrapped with graphene (VNCS), forming tuned 2D sheet-on-sheet nanostructure. This unique material has been synthesized using a facile solvothermal method and is used as an electrode material for supercapacitors capable of demonstrating outstanding improvement in cyclability and capacitance retention at high power rates. The VNCS material shows a superior performance with 430% improvement in capacitance retention at high power rates (50 A g-1) and 140% improvement in capacitance retention after 10,000 cycles at 10 A g-1, when compared to the un-modified material. Specifically, the VNCS showed an initial capacitance of 1340 F g-1 at 2 A g-1 and outstanding capacitance retention at 50 A g-1 (1024 F g-1). Impressively, the capacitance retention after 10,000 cycles at 10 A g-1 exceeds 90%. These exceptional results are considered at the top of reported work in literature as discussed in chapter 6. Moreover, a hybrid supercapacitor (HSC) was fabricated using the VNCS showing superior performance. The HSC delivers an energy density of 45.9 Wh kg -1 at 0.87 kW kg-1 and maintains a superior energy density of 33.6 Wh kg -1 at 9 kW kg-1 indicating the excellent potential of this material in hybrid supercapacitor applications The sheet-on-sheet structure reduced particle aggregation, provided larger surface areas with more electroactive sites for ion diffusion, enhanced the charge-discharge kinetics, which allows for faster electron transport. The morphology and structure characterization techniques confirmed that the vanadium is homogenously distributed throughout the binary chalcogens, resulting in significantly enhanced material stability at high power rates. HRTEM analysis confirmed the role of vanadium in fine-tuning the nano-architecture of the material and showed the dislocation in material structure. In chapter 7, we introduce the use of a safe and simple solvothermal method to synthesize a distinctive, flower bouquet-like, 2D vanadium disulfide (VS2) nanosheet structure with ultra-small prominent 3D VS2 nanoparticles (10-25 nm) on its surface and anchored on the surface of graphene nanosheets (VS2/G). This inimitable material has been tested in supercapacitors and showed superior capacitance, cyclability and capacitance retention at high power rates. The VS2/G showed 130 % higher capacitance at 1 A g-1 compared to other recent reports and remarkably improved capacitance at higher current densities. The material also showed a distinctive ability to maintain capacitance after long cycles at high current densities. Specifically, VS2/G showed 211 F g-1 at 1 A g-1, 135 F g-1 at 20 A g-1 and 97 % capacitance retention after 8000 cycles at 5 A g-1. The VS2/G was tested in a full cell HSC and showed superior energy density of 46.93 Wh kg-1 at a power density of 0.91 kW kg-1 and retained high energy density of 23.11 Wh kg-1 even when the power density was increased ten-fold (9.40 kW k g-1) highlighting the excellent potential of this material to bridge the gap between battery and supercapacitor technologies. The unique morphology of VS2 nanosheets embedded on graphene nanosheets with ultra-small VS2 nanoparticles distributed uniformly at the surface of the nanosheets was confirmed by different characterization techniques including SEM and TEM. The presence of graphene and the harmonized synergy between the 2D sheet-on-sheet morphology with the 3D ultra-small VS2 nanoparticles has a number of advantages. As such, it 1) hinders the agglomeration of the material and provides a large contact area with the electrolyte; and 2) generates strong covalent interactions between the VS2 with the graphene surface. These characteristics lead to an increase in capacitance due to the increase in the number of electroactive sites and improve the charge transfer kinetics, while paving shorter ion diffusion pathways, all resulting in stable and reversible charge transfer processes. Chapter 8 summarizes and concludes the thesis and suggests potential future works for capitalizing on the reported scientific achievements. The introduction of effective changes to the intrinsic characteristics of materials and the development of tuned and novel nanostructure materials, using simple and inexpensive methods, pave the way toward the development of commercial and industrial scale hybrid supercapacitor devices. Future work can investigate the design and development of thick electrodes in an attempt to exploit the high stability of the developed materials and increase the material loading on the electrodes leading to higher energy densities without scarifying the power densities. The increased working voltage and the long cycling life along with the high capacitance retention of the developed materials at high power rates can be used as a base to investigate the design of multi-stack device for industrial scale applications such as electrical vehicles, backup systems, transportation, etc. It is also recommended to further investigate incorporating the developed material in other energy storage and conversion devices.
Advanced Nanostructured Electrode and Materials Design for Zinc Air Batteries
(University of Waterloo, 2013-04-25T19:26:49Z) Scott, Jordan
Zinc air batteries have great promise as a new age energy storage device due to their environmental benignity, high energy density in terms of both mass and volume, and low cost Zinc air batteries get their high energy density by using oxygen from the air as the active material. This means that all the mass and volume that are normally required for active material in a battery are replaced by a thin gas diffusion electrode which allows for oxygen from the air to diffuse into the cell. Although this seems ideal, there are many technical challenges associated with the cell being open to the atmosphere. Some of these issues include electrolyte and electrode drying out, poor reaction kinetics involving sluggish reaction, the need for bifunctional catalysts to charge and discharge, and durability of the gas diffusion electrode itself. The bifuntional catalysts used in these systems are often platinum or other precious metals since these are commonly known to have the highest performance, however the inherent cost of these materials limits the feasibility of zinc air systems. Thus, there is a need to limit or remove the necessity for platinum carbon catalysts. There are many types of non precious metal catalysts which can be used in place of platinum, however their performance is often not as high, and the durability of these catalysts is also weak. Similar limitations on feasibility are invoked by the poor durability of the gas diffusion electrodes. Carbon corrosion occurs at the harsh caustic conditions present at the gas diffusion electrodes, and this corrosion causes catalyst dissolution. Moreover, many issues with zinc electrode fabrication limit durability and usable anode surface area within these systems. There is a need for a stable, porous, high surface area anode with good structural integrity. These issues are addressed in this work by three studies which each focuses on solving some of the issues pertaining to a crucial component of zinc air batteries, those being the gas diffusion electrode, the zinc electrode, and the bifunctional catalyst necessary for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). The first study addresses the need for improvements to the zinc anode electrode. A new process is proposed for the production of porous zinc electrodes in which the porosity can be easily controlled. This process involves the mixing of atomized zinc powder with a filler compound such as ammonium chloride. The mixture is then pressed into a pellet and heat treated to a temperature which simultaneously sublimes/decomposes the filler compound, and anneals the zinc structure to improve structural integrity. The resultant porous anode showed significantly charge and discharge potentials over the solid plate anode, while allowing for increased control of porosity over other porous electrodes due to the ability to adjust pore size based on the filler compound particle size. The discharge potentials observed from these porous anodes were 20% greater than zinc plate anodes at 100mA, but up to 200% greater at elevated currents of 200mA. Similarly the charging potentials were 53.8% lower at 100mA, and 55.5% lower at 200mA., suggesting greatly improved performance by the porous anode. The second study addresses the need for more durable gas diffusion electrodes. In this study, the bifunctional catalyst was bound directly to a stainless steel current collector via polymer binding in an attempt to remove the possibility of carbon corrosion and catalyst dissolution. The new gas diffusion electrode was successful in eliminating carbon corrosion, wherein, the durability of cells which incorporate this type of electrode was significantly increased. The durability of cell was increased to a point where little to no degradation occurred over 1000 cycles of full cell testing, showing great promise for future use and commercial viability. The final study addresses the need for durable and high performance non precious metal catalysts. The effects of catalyst morphology were studied wherein various morphologies of spinel type cobalt oxide were synthesized and compared. Cobalt oxide nanosheets were successfully synthesized and compared to nanoparticles of comparable size. The cobalt oxide nanosheets showed better charge and discharge potentials as well as durability of the nanoparticles. Impedance analyses showed reduced charge transfer and cell component resistances associated with the nanosheet morphology. Cobalt oxide nanosheets were further compared against platinum carbon. Cobalt oxide nanosheets showed significantly better durability as well as lower charging potentials and higher discharge potentials over 75 cycles. After 75 cycles the platinum carbon had lost 55.7% of its discharge potential wherein cobalt oxide nanosheets lost none of its discharge potential. Three issues pertaining to three major cell components a zinc air were addressed with promising solutions proposed for each. This work provides a basis for advanced zinc electrode fabrication in which further improvements can be incorporated to address other issues pertaining to zinc electrode use. This work set up a basis for electrode design which focuses on non carbon supported catalysts, eliminating the issue of carbon corrosion and associated catalyst dissolution. Finally, the results from the morphology study elucidate the benefits of controlled morphology for bifunctional catalysts, showing how morphology can be adjusted to improve performance by improving cell and charge transfer resistances.
Advanced non-precious metal catalyst for oxygen reduction reaction in polymer electrolyte membrane fuel cells
(University of Waterloo, 2017-09-20) Jiang, Gaopeng
To address the global energy and environmental challenges, the polymer electrolyte membrane fuel cell (PEMFC) is proposed and developed as one of the most promising power source candidates for various applications including electric vehicles, stationary power stations and portable devices due to its high efficiency and low emissions. However, the intrinsically sluggish reaction at the cathode, namely the oxygen reduction reaction (ORR), hinders the large-scale commercialization of the PEMFC as expensive and scarce platinum-based catalysts are used to accelerate this reaction. In order to reduce the cost of PEMFC, non-precious metal catalyst (NPMC) towards ORR has been developed and already brought itself from a pure scientific curiosity to a practically viable option for some commercial applications. In this work, two classes of low-cost NPMCs are investigated. One class is composed of high temperature treated transition metal-nitrogen-carbon M-N-C (M=Fe, Co) complex catalyst, especially iron-nitrogen-carbon complex (Fe-N-C) catalyst. These materials can demonstrate decent ORR activity and durability and provide high power output at moderate operating voltages. The other class with an even lower cost is the metal-free catalyst, which omits the metal content from M-N-C catalysts completely. This type of catalyst demonstrates excellent durability, especially in the presence of species that can cause contamination (e.g. carbon monoxide) or species that can cross-over (e.g. methanol). These two classes of NPMCs are developed and delivered with the ultimate objective of achieving a significant cost reduction in PEMFC while maintaining excellent PEMFC performance and durability. Herein, the research in this thesis starts with novel N, S-co-doped Fe-N-C catalysts to meet the objective of obtaining a highly economical and efficient NPMC. The catalyst is fabricated via pyrolyzing the composite of in-situ polymerized novel N, S-co-containing precursor, polyrhodanine (PRh) onto the acid-treated carbon black via the initiation of FeCl3. The N, S-co-doped Fe-N-C catalyst is obtained after two heat-treatment steps with one acid-leaching step in between. The catalyst demonstrates excellent ORR activity, bearing a half-wave potential of 0.77 V vs RHE in the acidic electrolyte. It also shows an excellent H2-air PEMFC performance, ranking the obtained peak power density (386 mW cm-2 at 0.46 V) among the best reported NMPC catalyst in H2-air PEMFC in the world. The N, S-co-doped Fe-N-C catalyst tends to catalyze the oxygen reduction via four electron pathway according to its number of transferred electrons (>3.94) and low peroxide yield (< 2.8 %). In addition, it demonstrates decent durability, showing only 32 mV downshift after 5000 potential sweep cycles in the ADT tests. The role of sulfur in this N, S-co-doped Fe-N-C is examined with the assistance of different characterizations and the comparison with an S-free nitrogen precursor (polypyrrole, PPy). Sulfur can benefit the ORR activity and durability of FePRh-HT2 catalyst with regard to the morphology, active sites density as well as the molecular structure of active sites. In the presence of sulfur, FeS nanoparticles are formed during pyrolysis and they function as the macropore and mesopore agents to enhance the BET surface area as well as the mass diffusion after acid leaching. The formation of FeS during the pyrolysis inhibits the formation of Fe3C and facilitates the formation of the Fe-Nx active sites, resulting in higher active sites density of FePRh-HT2 as opposed to FePPy-HT2. The sulfur-doped Fe-N4 active site structure demonstrates a better affinity to O2 molecule comparing to Fe-N4 active site structure in the DFT calculation, corroborating the better ORR stability of the FePRh-HT2 catalyst. Therefore, the multi-functionality of sulfur from the precursor PRh endows this N, S-co-doped NPMC with high ORR activity and durability This research also presents the feasibility of highly porous free-standing low-cost metal-free catalysts that are successfully prepared via the solution casting methodology followed by annealing the porous PBI-Py membrane at high temperature. The obtained catalyst film is truly metal-free and has adjustable 3D nano-network structure. The surface area of final catalyst can reach as high as 902 m2 g-1 by tuning the amount of pore agent in the casting solution, The optimal catalyst in this series is 95%PBI-Py-1000 which demonstrates comparable activity (E1/2 = 0.82 V vs. RHE) to commercial Pt/C catalyst in alkaline electrolyte and superior methanol tolerance to that of commercial Pt/C catalyst in RDE tests. The results suggest not only PBI-Py to be a decent nitrogen precursor for metal-free catalysts, but also the solution casting methodology as a feasible methodology for NPMC fabrication. To further reduce the cost of ORR catalysts and introduce another level of environmental-benignity, biomass precursor, cellulose nanocrystals (CNC) are applied to develop N-doped mesoporous carbon nanorods as the metal-free catalysts. With a smooth coating of MF on the surface of CNC, the MFCNC derived fibrous-structure of mesoporous nitrogen-doped carbon nanorods, N-CNR were obtained via one-step pyrolysis. The decent catalytic activity towards ORR in the alkaline electrolyte shapes N-CNR as the excellent metal-free catalysts with ultralow cost. In summary, this thesis focuses on the development of NPMC, including M-N-C catalysts and metal-free catalysts, with high activity and durability towards ORR as well as achieving a cost reduction for the catalyst. Several recommendations for future work based on this work are also included at the end, offering what we believe are meaningful future research directions for the development of NPMC for ORR.
Advanced Silicon Anode Architectures for High Energy Density Lithium-ion Batteries
(University of Waterloo, 2018-09-07) Batmaz, Rasim
The pressing environmental issues and the continuous increase in energy demand have drawn tremendous attention to the development of advanced electrochemical energy storage systems (EESS). Lithium-ion batteries are currently the most developed EESS; however, they are insufficient to meet the requirements of energy intensive applications such as electric vehicles. This stems from the intrinsic limitations of commercial cathode and anode materials. Therefore, their electrochemical properties should be either improved by applying new fabrication techniques or replaced with new generation materials to increase their energy density, power density and stability. Silicon is a promising candidate as a new generation anode material due to its enormous theoretical lithium storage capacity. However, silicon faces some technological hurdles such as poor cycle stability and rate capability. This stems from its intrinsic low electrical conductivity, severe volume change upon reaction with lithium, leading to loss of electrode integrity and formation of an unstable solid electrolyte interphase. In an attempt to address these problems, the proposed research projects have been embodied in this thesis. The main focus of these projects includes: enhancing electronic conductivity, forming stable electrolyte interphase and preventing electrode structural failure. To achieve these goals, novel silicon-carbon composite materials in which the silicon particles were hosted by nano-architectured carbon scaffolds were prepared to improve the electronic conductivity and help form a stable electrolyte interphase. Furthermore, to prevent electrode structure failure, the electrodes prepared from these silicon-carbon composite materials were subjected to thermal treatment to alter the electrode architecture by tuning the chemical structure of the binder. In Chapter 4, an advanced silicon electrode was developed by using commercially available silicon nanoparticles (SiNPs) as the anode material and sulfur-doped graphene (SG) as a carbon support. The electrode slurry was prepared by mixing these components with polyacrylonitrile (PAN) binder and then applied to the current collector. After the electrodes were dried, a thermal treatment was applied to reconstruct the architecture of the electrodes. In this new electrode architecture, PAN polymer is turned into an aromatic structure (cPAN) with 6-membered rings hosting the nitrogen atoms in pyridinic position. Thus, after the treatment, SiNPs are surrounded by the 3D conductive hierarchical architecture of SG sheets and the aromatic structure of cPAN. It was found that the silicon atoms on the nanoparticle surfaces anchor to and covalently interact with the sulfur and nitrogen atoms of this carbonaceous nanoarchitecture. This prevents the agglomeration of silicon particles, maintains the electrode integrity and stabilizes the solid electrolyte interphase leading to a superior reversible capacity of over 1000 mAh g-1 for 2275 cycles at 2.0 A g-1. The excellent performance combined with the simple, scalable and non-hazardous approach render the process as a very promising candidate for lithium-ion battery technology. This lays the basis for the project in Chapter 5. Although a high-performance anode was obtained by utilizing commercially available silicon nanoparticles, the high-cost of nanoparticles hinders the commercialization. To address this challenge, we have fabricated a stable silicon-based anode using low-cost silicon micron particles (SiMPs) by developing a two-step top-down approach. Wet-milling of SiMPs within an electrode precursor slurry (sulfur-doped graphene (SG), polyacrylonitrile (PAN) in dimethylformamide) allows for nanostructuring of the silicon by a straightforward and scalable process. After casting the electrode precursor slurry on the current collector, the electrodes are annealed to achieve an ideally tuned SG-SiMPs-cPAN electrode structure. In this structure, the polymer binder (PAN) is converted into a 3D aromatic network of cPAN that wraps the silicon particles and forms micron-sized channels throughout the electrode structure. These micro-channels act as a mechanical buffer for the anisotropic volume changes of silicon particles during battery charging/discharging, thereby preventing electrode pulverization. This electrode structure delivers excellent capacity (3081 mAh g-1 at 0.1 A g-1) in addition to good rate capabilities and cycle life (1423 mAh g-1 at 2.0 A g-1 for 500 cycles). Furthermore, the efficiency of this technique makes it possible to expand its application to other anode materials that require mechanical robustness and electrical conductivity with the goal of preparing next generation lithium-ion batteries. With a practical goal of fabricating low cost, scalable and facile silicon electrodes, we have removed sulfur-doped graphene from the electrode recipe of Chapter 5 and eliminated ball-milling. In this study, metallurgical-sized silicon is used as the anode material. The electrodes deliver an areal capacity of 3.0 mAh cm-2 at 0.1 A g-1 and more than 1.5 mAh cm-2 at 2.0 A g-1 for high loading electrodes. For moderate loadings, 1030 mAh g-1 (0.5 mAh cm-2) is achieved after 250 cycles at 2.0 A g-1. This excellent performance is attributed to the post-annealing of electrodes in which the in-situ binder graphenization of PAN takes place, leading a 3D robust electrode architecture. The mSiPs are hosted within this architecture which serves as an electron pathway with its π-conjugated aromatic structure and provides channels on the electrode surface to guarantee electrolyte penetration for good ionic conductivity. The partial graphenization of PAN can help to maintain its elastic properties required to accommodate large volume expansion of mSiPs and maintain the electrode integrity. This may lead to the formation of stable SEI that enables good cycling and rate performance. Furthermore, our approach is compatible with industrial slurry fabrication technique and open to be adopted to other electrode materials.

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