PRISM :: Browsing by Author "Karan, Kunal"

Browsing by Author "Karan, Kunal"

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Open Access
A Confocal Rheology Study of Network Stabilized Bicontinuous Emulsion Gels
(2018-08-24) Malone, Rachel Alexis; Trifkovic, Milana; Karan, Kunal; Bryant, Steven L.
In this thesis, a new bicontinuous soft material was discovered. Bicontinuous intraphase jammed emulsion gels (bipjels) were formed from critical mixtures of water and 2,6-lutidine and were stabilized with commercially produced alumina coated silica nanoparticles. Using a novel confocal rheology platform, the microstructure and rheological properties of the bipjels were simultaneously studied and provided key insights into the morphology and stability of the new materials during their formation, aging, and cooling. Through varying the concentration of the nanoparticles and the initial mixing energy delivered to the bipjel mixtures, the final morphologies could be tuned. A curvature analysis was performed over the aging of the different biPjel samples showing traits of optimal hyperbolic surfaces. The bipjels did not lose their strength upon cooling and liquids remixing which bodes well for their future development as an advanced material. Bipjels represent a new gateway for understanding the role non-interfacially localized particles play in stabilizing non-equilibrium morphologies.
Open Access
An experimental and modeling study of homogeneous gas phase reactions occurring in the modified claus process
(1998) Karan, Kunal; Behie, Leo A.
Open Access
An investigation of mass transfer effects on aeration process for soil remediation
(1994) Karan, Kunal; Mehrotra, Anil Kumar
Embargo
Aqueous Batteries for Large-Scale Energy Storage and Carbon Removal Applications
(2024-08-08) Iyapazham Vaigunda Suba, Prathap; Thangadurai, Venkataraman; Karan, Kunal; Shi, Yujun; Shimizu, George; Roberts, Edward (Ted) PL; Kuss, Christian
Aqueous batteries are safe and environmentally friendly energy storage devices, suitable for intermittent renewable energy sources and carbon removal applications. However, these batteries are hindered by the low operating potential which leads to limitation in energy density. In this thesis, aqueous battery chemistry concept is applied for two different applications such as: a redox-flow battery transformation to an all-gel battery and a seawater-battery for carbon removal application. Flexible, scalable, and low-cost energy storage solutions are required for the widespread use of renewable energy and the mitigation of climate change. In this regard, redox flow batteries are scalable due to their ability to decouple power and energy; however, the commercial applications of these batteries are limited by expensive ion-selective membranes. An auxiliary electrode (AE) mediated membrane-free redox battery concept was demonstrated and different AE materials were screened for their application in vanadium based redox battery. In an AE mediated membrane-free vanadium redox battery, the Coulombic efficiency of the system was limited to 36%. The low Coulombic efficiency observed in AE based battery was mitigated by using a gel-battery design approach in which Bi/BiOCl and V4+/V5+ redox couples were utilized in a gel-based architecture. The Bi/BiOCl conversion reaction based redox couple was demonstrated to work reversibly against traditional vanadium-based redox pair in an aqueous electrolyte. Redox active materials in this cell design are in the gel form, and a traditional membrane or a separator is not required. This proof-of-concept all-gel battery delivered 0.9 V with a volumetric energy density of 22.14 Wh L-1. For carbon removal applications, an aqueous battery with seawater electrolyte was studied. In this battery, iron phosphate (FePO4) electrode was utilized to manipulate ions in different seawater aliquots to generate sodium hydroxide, a base to use in ocean alkalinity enhancement. Electrochemical analysis, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) revealed that Na+ ions can be reversibly intercalated in FePO4. The alkalinity titration was used to determine the increase in alkalinity of seawater. The results indicated that a 16% increase in alkalinity was achieved in 15 ml of synthetic seawater over a period of 3.5 hours (one cycle) using the FePO4 electrode coupled with Pt electrode. A seawater electrolyte based battery with Ni(OH)2/NiOOH redox reaction in place of Pt electrode coupled with FePO4 electrode was studied. The battery delivered a potential of 0.6 V and the galvanostatic cycling experiments revealed that the battery can operate in seawater electrolyte for 50 cycles with 94% Coulombic efficiency.
Embargo
Chemical Durability of Polymer Electrolyte Membrane of Fuel Cells
(2024-09-13) Mansoor Basha, Abdul Bashith; Karan, Kunal; Kibria, Md; Roberts, Edward
The commercialization of polymer electrolyte membrane fuel cells (PEMFCs), especially for heavy-duty vehicles with lifetimes exceeding one million miles, is hindered by the limited durability of the polymer electrolyte membrane (PEM). Nafion, a perfluorosulfonic acid (PFSA) membrane, is the benchmark for PEMFCs, offers excellent proton conductivity but undergoes chemical degradation due to reactive free radicals generated by gas crossover. This degradation damages the polymer chains, compromising membrane integrity and reducing overall stack performance. Addressing this challenge is key to advancing PEMFCs for long-term, large-scale applications. This dissertation addresses these durability concerns by investigating alternative membrane architectures aimed at overcoming the barriers to large-scale PEMFC commercialization. Specifically, graphene, with its exceptional gas barrier properties, is applied as a coating on PEMs to enhance chemical stability. Graphene's ability to block atom and molecule transport while allowing proton permeation is exploited in this work. Accelerated stress testing shows a 72% improvement in chemical stability for graphene-coated membranes compared to uncoated ones, attributed to the reduced gas crossover and free radical formation. Single-layer graphene coatings achieved a 20% reduction in gas crossover, while double-layer coatings reduced it by 55%, without significantly impacting membrane ionic resistance. However, challenges related to water management at high current densities were noted, warranting further investigation. Beyond membrane architecture, this dissertation investigates the understanding of potential decay during the OCV hold test. During OCV hold, an approximate 100 mV decay is observed in the first 24 hours. Significant recovery of this OCV loss has been achieved through the conventional “wet recovery process,” which involves lowering the cell potential under over-humidified conditions, confounding the effects of potential reduction and catalyst/ionomer reorganization due to humidification. This study introduces a novel “dry recovery protocol,” using 30% RH and applying 30 minutes of low potential hold in an H2/N2 environment followed by H2/Air. OCV recovery of 82%, 73%, 62%, and 58% was achieved at hold potentials of 0.13, 0.2, 0.6, and 0.8 V, respectively. The small changes in electrochemically active surface area and hydrogen crossover rate observed after 48 hours cannot fully explain the nearly 100 mV OCV drop, suggesting oxide coverage increases as the dominant factor for sharp OCV decay.
Open Access
Coarsening Dynamics in Co-Continuous Polymer Blends and Composites
(2023-04-18) Shah, Rajas Sudhir; Trifkovic, Milana; Bryant, Steven; Karan, Kunal; Natale, Giovanniantonio; Wong, Joanna; Mohraz, Ali
Co-continuous polymer blends find extensive use across multiple industries. However, the morphology of these blends is thermodynamically unstable, which causes coarsening during processing. As a result, it is essential to understand the coarsening mechanism to improve their performance. Unfortunately, the current literature lacks a comprehensive explanation of the microstructural changes that occur during coarsening. To address this issue, the study employs 4D laser scanning confocal microscopy coupled with rotational rheometry to observe the coarsening dynamics in real-time. The aim is to understand the interplay between viscoelasticity and morphology during coarsening. The results show that when the ratio of interfacial tension to viscosity of blends exceeds a critical value, they tend to break down into droplet-matrix structures. The study also focuses on improving the stability and functionality of blends by incorporating solid particles. In literature, particle surfaces are often chemically modified, requiring complex and costly treatments. Therefore, this study examines the stability of co-continuous blends with pristine particles and how particle size and size distribution affect the stability. The study investigates the coarsening dynamics of blends filled with five different silica particles of diameters ranging from 5 nm to 490 nm. The results show that particle size does not play a role in blend stability when particles are thermodynamically driven to their preferred polymer phase. However, a striking effect is achieved when particles are kinetically trapped at the interface. The interparticle interaction governs their extent of agglomeration and, consequently, their ability to stabilize the morphology. The most effective, 140 nm and 250 nm particles are then added into blends under different loading ratios to simulate polydispersity in four different blends. The study demonstrates that polydispersity in particle size does not negatively affect blend stability. The efficacy of particles to suppress coarsening is dependent on their initial localization in the blend, which is determined by polymer characteristics. Overall, the study's findings provide a better understanding of the coarsening mechanism and stabilization of co-continuous morphology in polymer blends using particles. The results contribute to the fundamental knowledge of these materials and can aid in improving their design in various industries.
Open Access
Computational Study on Removal of Naphthenic Acids from Petroleum-based Systems
(2017-12) Wu, Chongchong; Gates, Ian; De Visscher, Alex; Karan, Kunal; Siegler, Hector
Naphthenic acids (NAs) are toxic compounds found in crude oil and products of petroleum processing including produced water. Reactions of hydroxyl radicals with NAs and ionic liquids (ILs) extraction have been reported to be effective in removing NAs from petroleum-based systems. However, the reaction and interaction mechanisms are not fully understood. In this thesis, a density functional theory study was conducted to explore mechanisms and kinetics of reactions between benzoic acid (BA), benzoate (BZ) and hydroxyl radicals as well as the extraction mechanisms of model NAs by ILs. The relationships between physicochemical properties of ILs and their intramolecular and intermolecular interactions were also investigated. The results show that all reaction pathways between BA, BZ and hydroxyl radicals involve the formation of pre-reactive complexes. The reaction rate constants for the addition reactions are highest for BZ in the aqueous phase, followed by BA in the aqueous phase, then by BA in the gas phase. For interactions between ILs and NAs, the main extraction mechanism by 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) for NAs without long alkyl chain is hydrogen bonding, whereas van der Waals interaction and hydrogen bonding are the dominant extraction mechanisms for NAs with long alkyl chain. With the incorporation of biodegradable substitutional groups, hydrogen bonding is still the main extraction mechanism, however, the interaction energy is higher than that of [BMIM][BF4]. More intermolecular hydrogen bonds occur when model NAs are absorbed by [BMIM][BF4] with -COOH or -COOCH3. Additionally, the interaction energy between model NAs and ILs with -COOH or -COOCH3 is higher than that with -OH, -NH2, or -OCH3. Aggregation behaviour and hydrogen bonds in ILs influence their densities and self-diffusion coefficients.
Open Access
Developing Cold Flow Technology for Pipeline Transportation of Paraffinic 'Waxy' Mixtures
(2019-04-29) Haj-Shafiei, Samira; Mehrotra, Anil K.; Husein, Maen M.; Karan, Kunal
The terms hot flow and cold flow refer to the bulk temperature of a ‘waxy’ crude oil being above and below its wax appearance temperature (WAT), respectively. The deposit thickness has been reported to decrease substantially when the crude oil in a pipeline is in the cold flow regime and the deposit thickness approaches zero when the crude oil temperature is the same as the surrounding temperature because of the absent of temperature driving force. The cold flow regime was characterized by two-phase flow, in which solid wax crystals are suspended in the liquid phase. However, achieving cold flow of a ‘waxy’ crude oil would invariably involve solid deposition on the cooling surface especially in the hot flow regime. This thesis is focused on understanding and developing the cold flow technology for pipeline transportation of paraffinic ‘waxy’ crude oils. Solids deposition in the cold flow regime from a wax–solvent mixture was studied experimentally using a cold finger apparatus to develop stable two-phase solid-in-liquid suspension and to prove the reduction of deposit thickness in the cold flow regime. This study further investigated a novel methodology for accomplishing cold flow condition without any significant deposition in the hot flow regime using both cold-ﬁnger and flow-loop apparatuses. In this approach, the effect of cooling rate was investigated on the temperature difference between the mixture and the coolant as well as the extent of wax deposition. It was found that the temperature difference and the deposit mass increased with the cooling rate. With a constant temperature difference, no deposition was observed above the WAT. The results indicated that the deposition in the hot flow regime could be decreased substantially, or even be avoided when the waxy mixture is cooled at a low cooling rate. In addition, a steady-state heat-transfer model along with the effect of deposit aging was developed for the formation of a deposit-layer from wax–solvent ‘waxy’ mixtures in a pipeline under turbulent ﬂow. The trends in the model predictions compared satisfactorily with those reported from bench-scale experimental studies as well as the predictions from an unsteady-state moving boundary problem formulation.
Open Access
Development of Novel Nanoporous Carbon Scaffolds for PEMFC Applications
(2021-03-26) Atwa, Marwa H; Birss, Viola I; Birss, Viola; Karan, Kunal; Roesler, Roland; Ponnurangam, Sathish; Gasteiger, Hubert
The microstructure of conventional polymer electrolyte fuel cell (PEMFC) catalyst layers (CLs) consists of a mixture of high surface area carbon powder, Pt or Pt alloy nanoparticles (NPs), and an ionomer, which acts as a binder and a proton-conducting electrolyte. These CLs contain complex pathways and a poorly controlled distribution of Pt NPs and ionomer, which can cause kinetic and transport limitations for the oxygen reduction reaction (ORR). Therefore, we have developed a self-supported, robust, and binderless carbon membrane with an ordered, tunable pore structure, namely the nanoporous carbon scaffold (NCS). The NCS films have a high surface area (200-610 m2/g), ultra-low tortuosity (close to 1), high porosity (ca. 90%) and electrical conductivity (? 2 S/cm), and excellent 3-dimensional scalability (1-150+ cm2 demonstrated area, 1-1000 µm thickness). Herein, the effect of the microstructure of various NCS materials, loaded with both catalytic Pt NPs and ionomer, on the kinetics of the oxygen reduction reaction (ORR), which is the rate limiting reaction in PEMFCs, was explored. Two different methods of Pt NP loading (wet impregnation and atomic layer deposition (ALD)) were examined and two types of NCS films with two different structures (monodisperse and bimodal pore size distributions) were prepared and then integrated into a membrane electrode assembly (MEA) as a PEMFC cathode. For the Pt-loaded NCS films with monodisperse pore sizes (uniform pore size distribution with a standard deviation of ± 10-15%) of either 22, 50, or 85 nm, it was found that ALD Pt loading results in a much narrower NP size distribution than wet impregnation, and that the ALD-Pt/NCS85 catalysts out-perform the ALD-Pt/NCS22 materials, likely as Nafion can better penetrate the larger pores to provide proton conductivity. The much smaller neck diameter may be the real problem, where it was shown that larger neck diameters, keeping all other factors constant, can significantly lower the mass transport resistance. The bimodal NCS12 films possess a Craspedia (flower)-like structure, with the spheres (~900 nm) fully saturated with close-packed 12 nm primary pores and connected to neighboring spheres through non-porous carbon fibers, leaving ~ 0.25 µm secondary pores between the spheres. It was found that the 12 nm pores contain the vast majority of the highly dispersed ca. 2-3 nm Pt NPs, and yet contain no Nafion, thus indicating that water must be conducting protons in these pores during the ORR. These Pt NPs also show signs of surface strain, perhaps contributing to their high ORR activity. These factors, including the absence of blocking or poisoning of the Pt NPs by Nafion, have resulted in an extraordinary ORR mass activity (0.58 ± 0.1 A/mgPt), retained after 10k ADT cycles, and a very high limiting current of close to 2.7 A/cm2 in MEA cathode studies.
Open Access
Effect of Temperature on Asphaltene Deposition Mechanisms in Horizontal Flow
(2021-12-20) Do, Nicson; Yarranton, Harvey W.; Bryant, Steven L.; Karan, Kunal
Asphaltene deposition is a longstanding flow assurance issue that has been extensively investigated at near ambient temperatures where asphaltenes typically precipitate as glassy particles. However, at the higher temperatures sometimes encountered in deep formations, asphaltenes may come out of solution as liquid droplets. Deposition at these conditions has not been rigorously examined. The purpose of this study was to investigate deposition behavior in the liquid droplet regime and compare it with deposition in the glassy particle regime. An apparatus was designed and commissioned to investigate deposition mechanisms over a range of temperatures in horizontal laminar flow using a test fluid of bitumen diluted with n-heptane. Pre-diluted bitumen and additional n-heptane were fed through a static mixer to induce asphaltene precipitation and the subsequent mixture was then displaced through a capillary tube test section. The pressure drop across the test section was monitored for indications of deposition during the flow period, and the capillary tube was removed from the apparatus at the end of each experiment to measure the mass and location of the deposit. Asphaltene deposition was assessed considering the following variables: capillary tube lengths from 3 to 30 cm, solvent contents in the feed from 65 to 90 wt% n-heptane, fluid flow rates of 2 and 4 cm³/min, and temperatures from 50 to 130°C. In the glassy particle regime, a highly porous, orifice-like deposit with a high solvent content formed near the inlet of the test section. The fully developed wet deposit occupied 37% of the tube volume on average. The initial deposition rate increased as the solvent content in the feed increased. Cycles of deposition and erosion were observed during the flow period. The results were consistent with the literature. In the liquid droplet regime, periodically unstable stratified flow was observed. The heavy phase hold up cycled from 70 to 98% of the tube volume. The solvent content of the heavy phase was lower compared to glassy particle deposits and consistent with equilibrium heavy phase compositions reported in the literature. The heavy phase accumulated faster as the solvent content in the feed increased. In one experiment, an orifice-like deposit appeared to form at the start of the experiment indicating that, even in the liquid regime, deposition may occur near a flow disturbance. However, the effect of the orifice was overwhelmed by the accumulation of the flowing heavy phase. The results suggest that the models and treatments developed for deposition in the glassy particle regime may not apply in the liquid regime.
Open Access
Fabrication and Performance of Fuel Cell Catalyst Layer Made with Graphitic and Carbon Black Catalyst Supports
(2023-05-11) Poojary, Sushmit Sadanand; Karan, Kunal; Roberts, Edward; Ponnurangam, Sathish; Thangadurai, Venkataraman
Hydrogen fed polymer electrolyte membrane fuel cells (PEMFCs) are at the forefront of clean energy technologies. Their application in transportation sector is now considered to be primarily in heavy duty vehicles (HDVs). Currently, the technological challenges for PEMFC development for HDV application is to increase the fuel cell stack durability. One of the components considered weak link for PEMFC durability is the carbon support for platinum catalyst used in the cathode and anode catalyst layers. Conventionally used carbon black type catalyst support have poor corrosion resistance as they are prone to both cathodic and anodic corrosion. Graphitization of the carbon black or use of graphene as catalyst support has been reported in the literature. However, the challenges associated with the fabrication and performance of graphene-based catalyst layer need to be addressed. In this thesis, base line data for catalyst layer made with commercially available carbon-black type catalyst support is first established. As a part of this study, the influence of ionomer side chain length or equivalent weight (Aquivion-825 and Nafion-1100) on catalyst layer properties and cell performance was quantified for the commercial catalysts. The observation of higher ORR kinetic activity (A/cm2Pt) for the Aquivion-825 CL than the Nafion-1100 CL is an interesting finding and is hypothesized to different interfacial protonic concentrations between the two CLs at the Pt/ionomer interface. Aquivion-825 CL had a higher local oxygen transport resistance than the Nafion-1100 CL, which is also indicative of changes in the Pt/ionomer interface and is consistent with a stronger contact between Pt and ionomer in the case of more acidic ionomers. Similar dependence on Pt utilisation as a function of relative humidity (RH) is seen for the two CLs (ratio of electrochemically active area at any RH to that at 100% RH). As anticipated, a significant impact of humidity on proton conduction is seen. The CL with the larger equivalent weight ionomer (Nafion-1100) demonstrates lower conduction. A graphene-based catalyst was fabricated, first by using one-step electrochemically exfoliated graphene co-doped with nitrogen and phosphorus, upon which platinum catalyst was subsequently deposited. The in-situ electrochemical characterization showed that the carbon black or Vulcan carbon-based CL outperformed the graphene-based CL. Even though the CV demonstrated that there was electrochemically active Pt present in the CL, the poor performance, absence of a limiting current, and high CL protonic resistance suggest that the reactant gas is unable to reach the active sites due to poor CL porosity, possibly as a result of the stacking of the graphene layers. These results highlight a significant problem in the development of graphene-based catalyst layers for PEM fuel cells.
Open Access
Fabrication of a Highly Transparent Conductive Thin Filmfor Deicing and EMI Shielding Applications
(2020-08-26) Hosseini, Ehsan; Karan, Kunal; Shankar, Karthik; Thangadurai, Venkataraman; Ponnurangam, Sathish; Egberts, Philip
Transparent conducting films (TCFs) market has been continuously evolving and becoming available in a wide variety of applications, including touch screen panels, organic light-emitting diodes (OLEDs), photovoltaic solar cells, heaters and very recently, electromagnetic interference (EMI) shields. A majority of research and development (R&D) on TCFs have been focused on materials that could replace the commonly-used indium tin oxide (ITO) – the substance that is not as favorable as before. Nevertheless, despite its drawbacks such as limited availability, brittleness, toxicity, and high cost, industry has not been persuaded to completely put an end to the ITO’s era. The underlying reasons for this include: (i) some of the proposed alternatives could not even reach the ITO’s unique properties of the optical transparency of > 90% and sheet resistance of < 10 ? sq–1, (ii) some reported materials excel only in one of the ITO’s property, either the transmission or sheet resistance, and (iii) new materials with similar or better properties than ITO, end up creating new challenges such as higher production cost, complex processing with more energy consumption, use of specialized, expensive methods, and the inferior mechanical stability of the end product. On the other hand, for transparent EMI shielding applications, in particular, metals as the most widely-used materials are not desirable anymore due to being corrosive, having low optical transmission and heavyweight. In this thesis, two novel fabrication techniques are introduced for making high performance (i) filler-free and (ii) hybrid TCFs. The ~ 50 nm filler-free highly conducting film is made of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), also known as PEDOT:PSS. To the best of our knowledge, it is the first all-plastic transparent EMI shield developed to date, which is only based on an intrinsically conductive polymer (ICP). The ultra-thin shield offered the total shielding effectiveness of 15 dB in the X-band frequency range (8.2–12.4 GHz) with an appreciable transparency of 97.1%. The shield also has a record thickness-specific shielding figure-of-merit of 300 dB µm–1 – far exceeding the best values for micron-thick metal-based, as well as carbon- and MXene-based composite materials shields. The other fabricated hybrid TCF, not only outperforms both the ITO and other reported TCFs to date but also circumvents the shortcomings of the previously-developed technologies and offers unprecedented new properties. The developed TCF presents the optical transparency of 91%, the sheet resistance as low as 6.4 ? sq–1 and the total EMI shielding of 23 dB in the X-band frequency range. The TCF owes its remarkable features partly to the outstanding qualities of its constituents: PEDOT:PSS, and silver nanowires (AgNWs), the most electrically-conductive material. The next critical part, contributing to the advanced attributes of the TCF, is its layer-by-layer (LBL) structure in which AgNWs are sandwiched in between PEDOT:PSS ultra-thin layers. Last but not least, the sandwich assembly of the TCF in between glass substrates in targeted applications such as deicing vehicles windshields and EMI shielding, makes the thin film electrode protected against harsh environmental conditions in winters, oxidation and scratches – a feature most current TCFs lack of. In addition to the two aforementioned significant applications, being identified and experimented, other functionality features of our fabricated TCF have been under ongoing research in collaboration with other groups, such as in biosensors and Li–S batteries applications, to name a few.
Open Access
Fluorescent polycatecholamine nanostructures as a versatile probe for multiphase systems
(RSC Advances, 2018-09-13) Ozhukil Kollath, Vinayaraj; Derakhshandeh, Maziar; Mudigonda, Thanmayee; Islam, Muhammad Naoshad; Trifkovic, Milana; Karan, Kunal; Mayer, Francis D.
Shape and size controlled nanostructures are critical for nanotechnology and have versatile applications in understanding interfacial phenomena of various multi-phase systems. Facile synthesis of fluorescent nanostructures remains a challenge from conventional precursors. In this study, bio-inspired catecholamines, dopamine (DA), epinephrine (EP) and levodopa (LDA), were used as precursors and fluorescent nanostructures were synthesized via a simple one pot method in a water–alcohol mixture under alkaline conditions. DA and EP formed fluorescent spheres and petal shaped structures respectively over a broad spectrum excitation wavelength, whereas LDA did not form any particular structure. However, the polyepinephrine (PEP) micropetals were formed by weaker interactions as compared to covalently linked polydopamine (PDA) nanospheres, as revealed by NMR studies. Application of these fluorescent structures was illustrated by their adsorption behavior at the oil/water interface using laser scanning confocal microscopy. Interestingly, PDA nanospheres showed complete coverage of the oil/water interface despite its hydrophilic nature, as compared to hydrophobic PEP micropetals which showed a transient coverage of the oil/water interface but mainly self-aggregated in the water phase. The reported unique fluorescent organic structures will play a key role in understanding various multi-phase systems used in aerospace, biomedical, electronics and energy applications.
Open Access
Friction and Adhesion of Atomically Thin Films: A Study of Few-layer Graphene
(2017) Gong, Peng; Egberts, Philip; Egberts, Philip; Birss, Viola; Karan, Kunal; Federico, Salvatore
In order to gain better insight on the fundamental mechanisms governing friction and adhesion at the nanoscale, this thesis examines the friction and adhesion properties and mechanisms on the atomically-thin material, graphene. First, through conducting load dependent measurements of friction on mechanical exfoliated graphene samples, the dependence of the friction behaviour on graphene as a function of number of graphene layers, sliding history, environmental humidity, and air exposure time were examined. A mechanism was proposed to fully explain these experimental observations. Secondly, the finite element method (FEM) was applied to investigate the adhesion between a nanoscale tip and graphene covering a silicon substrate. The simulations, contrary to prior experimental results, showed a slight increase in the pull-off force as layer number increased. In addition, it was revealed that the layer-dependent pull-off forces result from the increasing tipgraphene interactions. This work contributes to gaining better insight on the applications to the lubrication mechanisms of graphene.
Open Access
Fundamental Understanding of the Cathodic Catalyst Layer of an Anion Exchange Membrane-Based CO2 Electrolyzer: Catalyst Optimization and Ionomer Characterization
(2023-09-20) Alihosseinzadeh, Amir; Ponnurangam, Sathish; Karan, Kunal; Kusoglu, Ahmet; Mahinpey, Nader; Nassar, Nashaat; Roberts, Edward (Ted)
CO2 electrolysis exhibits significant potential in addressing climate change and facilitating the transformation of captured CO2 into valuable chemicals and fuels. Recent years have witnessed remarkable advancement in the design of CO2 electrolyzers, offering future commercial viability. These design advancements encompass cell configuration, catalytic materials for the cathode and anode, as well as suitable membranes and ionomers. This thesis is focused on the cathodic catalyst layer (CCL) of a gas-fed anion exchange membrane-based CO2 electrolyzer, a pivotal component. The CCL consists of a porous nanocomposite structure comprising electrocatalyst coated with a nanothin ionomer film. The pores facilitate CO2 gas transport, the connected network of catalyst allow electron transport, the percolated distribution of ionomer ensures ion diffusion, the ionomer coverage of catalyst provides the required electrochemical interface and together these features provide for efficient electrochemical reactions. Specifically, the work targeted on optimizing an Ag-based catalyst for CO production and characterizing an imidazolium-based anion exchange ionomer within the CO2 electrolysis environment. While prior research mostly employed highly loaded (1-3 mg.cm-2) commercial Ag nanoparticles, which is spray-coated on the electrode with a maximum mass-specific activity of around 100-150 mA.mgAg-1, this study introduces an electrodeposition technique to synthesize Ag catalysts. This technique not only enhances the electrical contact with the substrate, but also enables control over catalyst size, morphology, and crystallography, which influences catalyst performance and catalyst utilization. Various Ag electrodes with distinct sizes and structures, ranging from polycrystalline to dendritic, were synthesized and assessed for CO2 reduction. An optimized dendritic Ag catalyst, with 0.29 mg.cm-2 Ag loading and maximum (220)/(111) facet ratio, exhibited a high mass-specific activity of 362 mA.mgAg-1, current density of 105 mA.cm-2, and 94% CO selectivity at a 3 V cell potential, maintaining robust performance over extended 100 h CO2 reduction reaction. The catalyst/ionomer interface plays a pivotal role in CO2 reduction, necessitating not only highly electrochemically-active sites to maximize catalyst performance, but also optimized CCL microstructure for ion transport to minimize catalyst utilization. The characteristics of an imidazolium-based anion exchange ionomer (Sustainion, XA-9) were probed under CO2 electrolysis conditions. The influence of relative humidity (0-95%), film thickness (8-61 nm), chemical environment (N2 vs CO2 exposure), and ionomer counterion exchange (Cl- to OH-) on hydration properties, ionic conductivity, and CO2 adsorption/reduction was investigated. Results reveal that swelling, water content (λ), and ionic conductivity of the ionomer films in Cl- form (XA9-Cl), increase with rising relative humidity, with a confinement behavior observed below 26 nm thickness; (22% swelling, 32 wt.% water content, and 13 mS.cm-1 ionic conductivity, at 95% RH on a 26 nm thick film). Although the CO2 exposure did not change the swelling properties of the XA9-Cl ionomer thin film, the water content was slightly increased due to the carbonate/bicarbonate formation in the ionomer. The ionic conductivity of the XA9-Cl thin film responds differently in the N2 vs CO2 chemical environment, displaying a crossover at λ > 6. CO2-exposed ionomer films exhibit higher conductivity at lower λ but lower conductivity at higher λ compared to N2-exposed films. This phenomenon could arise from bicarbonate species enhancing ion conductivity at lower λ, while exhibiting reduced mobility and conductivity compared to OH- ions under high humidity conditions. The transformation of the ionomer to its OH- form improves swelling, water content, and ionic conductivity, reaching levels of 26%, 36 wt.%, and 33 mS.cm-1, respectively, at 95% RH on a 26 nm thin film. Moreover, the OH- form ionomer demonstrates improved CO2 adsorption and reduction on an Au electrode, confirmed by CV analysis. Nevertheless, thicker films manifest postponed CO2 reduction owing to transport limitations in the ionomer film.
Open Access
Heteroatom Doped Carbon Electrode Materials for Redox Flow Battery Application
(2021-08-24) Singh, Ashutosh K; Roberts, Edward PL; Karan, Kunal; Ponnurangam, Sathish; Welch, Gregory; Agar, Ertan
The development and investigation of electrode materials for high electrocatalytic activity and surface area for redox reactions in all vanadium redox flow batteries (VRFBs) is studied in this dissertation. Electrochemical energy storage and especially redox flow batteries (RFBs) are favoured choice in many applications because of their flexibility, efficiency, and scalability. One of the challenges faced by the most developed RFB technology, the all-vanadium redox flow battery (VRFB), is the sluggish kinetics of the VO2+/VO2+ and V2+/V3+ redox couple at the typically used carbon electrodes. The objective of this project is to evaluate factors that are affecting the kinetics of the interfacial charge-transfer process, and to investigate the physiochemical treatment processes to functionalise the electrodes with heteroatoms such as nitrogen, to promote better performance. At the first part of this PhD work, the impact of extended charge-discharge cycling on carbon paper electrodes pre-treated using conventional heat treatment approach was investigated. Electrode degradation along with 70% decrease in charge – discharge capacity was observed after 100 charge – discharge cycles of a single cell vanadium redox flow battery operating at a current density of 80 mA cm?2 at room temperature (23?C). Electrochemical investigation reveals an increase in the activation overpotential at both the positive and negative electrodes originating from significant changes in the composition of oxygen functional groups at the electrode surface after degradation. In order to improve the electrocatalytic activity of carbon paper electrodes, a physiochemical process for nitrogen doping was developed. N-doped carbon paper have been shown to have superior electrocatalytic activity towards several redox reactions such as the [Fe(CN)6]3-/4- redox couple. The effect of pyrolytic pretreatments under different conditions on the performance of carbon paper were also studied to elucidate their electrocatalytic activity from a material physics perspective, using Raman spectroscopy. Although heating of carbon paper in air at around 500°C (a widely used method for activating carbon paper electrodes) increases the surface area by about 16-times compared to untreated and nitrogen-doped carbon paper, the latter exhibits superior electrocatalytic property for VO2+/VO2+, [Fe(CN)6]3-/4-, and the oxygen reduction reaction. The use of graphene material was explored with an aim of increasing the active surface area and electrocatalytic activity of the modified carbon paper electrodes. A novel scalable and high yield method for the graphene production using an electrochemical exfoliation pathway was developed. The electrochemical method involves two steps: (1) intercalation of aqueous ionic OH? species into the graphite at a constant current density of 30 mA cm?2, followed by (2) oxidation of these ionic species in (NH4)2SO4 (0.1M), and under a relatively high anodic voltage (10 V). The resultant graphene flakes were found to have high electrical conductivity of 44230 S m?1 compared to graphene produced using conventional hummers method, with a high graphene yield of about 93%, along with and a low oxidation level with a C:O ratio of about 15 (determined by XPS). The graphene modified carbon paper prepared by dip coating carbon paper electrodes in a graphene – water suspension, were evaluated for application in an all-vanadium redox flow battery and was found to improve the voltage efficiency by around 10% at a current density of 80 mA cm?2.
Open Access
In-situ Variable Temperature Ellipsometry of Nafion Thin Films
(2016) Zhang, Chi; Karan, Kunal
ABSTRACT This work presents the study of thermal behaviour in thin Nafion films. The experiments have been carried out with variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), and goniometer. The results from this investigation focused on thin films between 10 and 700 nm thick on different substrates. The thermal behaviour of thin films was studied by analyzing the thermal expansion coefficients (α) and glass transition temperature (Tg). Moreover, the influence of different substrates on the thin films’ thermal behaviour was investigated. It was found that Nafion thin films did not show clear glass transition behaviour between 25 and 200°C. The α values of thin films showed clear deviation from that of Nafion membrane. Moreover, the thin films on hydrophilic substrates (Au and Pt). showed decrease in α compared to membrane, while intermediate substrates (SiO2, graphene) showed increase. There is a strong correlation between water contact angle and α values.
Open Access
Influence of Polymer Structure, Cation Type, and Substrate on Ionomer Thin Film Hydration Properties
(2023-01-18) Eskandari, Hamideh; Karan, Kunal; Benneker, Anne Maria; Ponnurangam, Sathish; Roberts, Edward (Ted); Thangadurai, Venkataraman; Konika Dishari, Shudipto
Low carbon energy systems are undeniable solutions for addressing environmental and climate change issues facing the world. Hydrogen-fueled polymer electrolyte fuel cell (PEFC) are at the forefront of clean energy technology solutions since they are producing no particulate emissions and only water as a by-product at the point source (cars) and can improve air quality. Critical to their widespread adoption is the need for a reduction in the PEFC stack cost which is mainly attributed to the expensive Platinum (Pt) catalyst and development of new materials in catalyst layer. This work aims at investigating the effect of material (such as ionic polymer or ionomer type, substrate and contamination) and operational conditions (such as temperature and relative humidity, RH) on hydration behavior (water uptake and proton conduction) of ionomer thin film in catalyst layer, and then developing a correlation describing how those parameters effect hydration properties (water uptake and proton conductivity) of the ionomer thin film. The results of this study will enable the engineers to better optimize the performance of their produced PEFCs. In this work, we reported the water content and proton conductivity properties of thin-film ionomers (30 nm) at 80 °C over a wide range of relative humidity (0−90%) for seven different ionomers differing in the side-chain structure, including the number of protogenic groups, with the equivalent weight ranging from 620 to 1100 g/mol of sulfonic acid. The results show that the acid content or equivalent weight of the ionomer is the strongest determinant of both the swelling and the proton conductivity of ionomer films at a given relative humidity. The proton conductivity of low-equivalent-weight ionomers was higher than that of higher-equivalent-weight ionomers. Significantly higher values of both water content and proton conductivity are observed at 80 °C compared to those at 30 °C, implying that room temperature data are not reliable for estimating ionomer properties in the fuel cell catalyst layer. We also studied the impact of exchange of protons with cobalt ions on the humidity dependent (0−90% RH) hydration and conductivity of ∼30 nm thin ionomer films at a fuel cell-relevant temperature (80 °C). A significant suppression (up to 2 orders of magnitude at low RH) in ionic conductivity was observed for all ionomers upon exchange of protons with cobalt ions, evidently because the water content of the ionomer films decreases upon Co2+ exchange. The most interesting finding of the study is that a large variation in conductivity between the H+ form and Co2+ form of ionomer films at a given RH is significantly minimized when conductivity is correlated with the water content. Then we focused on how carbon and Pt substrate impact the 10 nm ionomer swelling rate under different RHs. It was found that films on Pt substrate, have higher swelling rate than those on carbon and SiO2 substrates. These results prove the evidence of better water network and higher proton conductivity in ionomers on Pt substrate, indicating that the interactions between ionomers and substrate affects internal structure of ionomers as well as the film surface especially in ultra thin films (< 10 nm). Moreover, water sorption studies on ionomer thin films shows that water absorption is slower than water desorption. It could indicate that the rate of water absorption controlled by the rate of interfacial transport and swelling while the desorption rate mainly controls by interfacial mass transport.
Open Access
An Investigation of the Evaporation Dynamics of Water Droplets and Na-Cl Water Droplets Suspended in Air by Acoustic Levitation
(2020-09-23) Bunio, Lyndon; Gates, Ian Donald; Benneker, Anne M.; Hejazi, Seyed Hossein; Karan, Kunal
Evaporation of liquid droplets is ubiquitous in nature and has been used across several industrial processes. Many evaporation studies of supported droplets have been done through time, starting with investigations of sessile droplets on solid surfaces, to droplets hanging on thin filaments, to droplets supported on superhydrophobic materials. An emerging method to study free droplets is by using acoustic levitation, a tool that has allowed for investigations of evaporation and crystallization of true free droplets. In the first part of the study, the evaporation of pure water is first investigated and compared to a novel theory for droplet evaporation which includes both diffusive transport away from the system and thermal conduction into it. The results demonstrate that the major control on the evaporation rate is diffusion. In the second part of the study, the evaporation and crystallization dynamics of NaCl water solution droplets is examined with different concentrations. Qualitative results describe the crystallization process and the ‘cup’ shape produced for NaCl concentration ranging from 225-300 g/L. Higher concentration (325 g/L) yield a crystal sphere around the droplet. Quantitative findings compared to theory shows that the NaCl solutions require an enhanced diffusion coefficient to better match the experimental data.
Open Access
Ir-core Pt-shell Nanoparticles for the Electro-oxidation of Ethanol
(2018-12-06) Slaby, Jachym; Birss, Viola; Shi, Yujun J.; Karan, Kunal; Heyne, Belinda J. M.
This work was conducted in order to establish the behavior of Ir@Pt core@shell nanoparticles (NPs) during the catalysis of the ethanol oxidation reaction (EOR). Ir@Pt catalysts with < 1 monolayer of the Pt shell were synthesized using the well-established polyol method, which involves the use of a capping ligand. The NPs were tested for the EOR at both room and elevated (70 ̊C) temperatures, as well as acetaldehyde oxidation at room temperature. Ir was found to be more active towards the EOR at lower potentials and Pt at higher potentials. The presence of the capping ligand made it difficult to establish any cooperativity between the two metals to catalyze the EOR. Thus, similar Ir@Pt NPs were also made without a capping ligand. Unexpectedly, little to no enhancement of the EOR catalytic activity was found for these core@shell NPs. The findings were compared to similar studies of methanol oxidation.