Browsing by Author "Pritzker, Mark"

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Advanced Heteroatom Doped Nanocarbon Materials as Platinum Catalyst Supports for Fuel Cells
(University of Waterloo, 2016-03-17) Hoque, Md Ariful; Chen, Zhongwei; Pritzker, Mark
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.
Electrodeposition of p-Type Cuprous Oxide and its Application in Oxide Solar Cells
(University of Waterloo, 2017-01-06) Yang, Yiyi; Pritzker, Mark; Li, Yuning
Cuprous oxide (Cu2O) is a promising material for the fabrication of oxide solar cells due to its abundance, nontoxicity, stability and ability to be deposited under mild conditions. At the moment, the reported efficiencies for Cu2O-based solar cells are well below its theoretical limit and most of the high performance devices are fabricated using energy-intensive processes. Electrodeposition of Cu2O thin films is a well-known process although a systematic study of the dependence of Cu2O film properties on the deposition conditions has not been reported, especially in the context of photovoltaic applications. The purpose of this study is to optimize the electrodeposited p-type Cu2O film properties for the application in heterojunction solar cells through the manipulation of electrodeposition conditions. A range of Cu2O morphologies are formed by varying the parameters during the constant potential deposition of Cu2O thin films from a copper-lactate electrolyte including applied potential, temperature, pH, copper concentration and lactate concentration. The variation in morphology is mostly attributed to the relative growth rates of <111> and <100> orientations. It is found that the variation in growth orientation is mostly due to the relative rates or reaction and transport. When reaction kinetics is the limiting factor, <100> growth is preferred when the conditions are close to thermodynamic equilibrium. On the other hand, when transport limitation in the system is significant, <111>-oriented growth and cubic grains are favored as the surface concentration of CuLac2 is reduced. In other conditions, mixed orientations tend to be formed. Evidence of adsorption of lactate ions onto the (111) faces is also observed, promoting <100> directed growth at high lactate concentration. In addition, significant evidence of film dissolution during deposition is observed at high electrolyte pH of 13 and above. The performance of Cu2O thin films in photovoltaic devices is evaluated using an FTO/Cu2O/AZO cell structure. It is found that the electrolyte pH has the most prominent effect on the device performance among different electrodeposition parameters. Performance variables including JSC, VOC and FF all improve significantly as the Cu2O deposition pH increases. An optimal cell efficiency of ~1.55% is obtained using Cu2O thin films deposited at a high pH of 13.4, which exceeds the majority of previously reported efficiencies using the same deposition technique. The reason for the improved efficiency is attributed to the dissolution and reformation of Cu2O grains during deposition at high pH through the improvement of charge carrier mobility and band gap in the Cu2O layer. Film orientation of Cu2O also plays a minor role in cell efficiency. Alternative electrodeposition procedures are investigated. Galvanostatic deposition is found to be less suitable than potentiostatic deposition for solar cell fabrication due to the difficulty in controlling Cu metal content in the film. Evidence of dissolution by oxidation is observed under both pulsed and stirred conditions. Different morphologies to the typical polyhedral shapes with higher carrier concentration and low carrier mobility are obtained in both cases. These effects are attributed to the lowering of diffusion limitations during the deposition process either by allowing more time for transport or by increasing transport rate. These processes result in lower device performance compared to potentiostatic deposition under similar conditions. Finally, FTO/Cu2O/ZnO devices are fabricated by depositing ZnO onto electrodeposited Cu2O layer through a seed-layer assisted chemical growth method in zinc nitrate-HMTA solution. Ordered ZnO nanorods are obtained using this technique and their dimensions can be controlled by adjusting bath compositions. Smooth ZnO thin films with no pin-hole defects are obtained although the performance of Cu2O/ZnO devices is not as high as devices with AZO layers.
Experimental and Modeling Study of Zinc-Cerium Redox Flow Batteries
(University of Waterloo, 2021-04-26) Amini, Kiana; Pritzker, Mark
Redox flow batteries (RFBs) are one of the most promising energy storage technologies that are expected to have an increasing market size in the near future due to their scalability, safety, long-life and system flexibility. The development of RFBs will facilitate the utilization of clean renewable sources of energy, such as sun and wind power, by resolving their intermittency problem. RFBs are also capable of enhancing the stability of the electrical grid by storing the excess electrical power and releasing it during peak hours of electricity demand. Among different types of RFBs, those based on zinc and cerium metals are attractive since they offer a large open-circuit cell voltage and thus have the potential to provide high output energy density. Motivated by these advantages, the present work is focused on the improvement of zinc-cerium RFBs through both experimental and modeling studies. In this work, in situ polarization and electrochemical impedance spectroscopy measurements were combined to accurately identify the sources of voltage loss in each half-cells of the zinc-cerium RFBs during battery operation. This analysis revealed that the kinetic losses at the zinc negative half-cell are responsible for most of the voltage drop at low and intermediate current densities, while limitations to the mass transfer of Ce(IV)/Ce(III) on the positive side are the main reasons for the sharp potential drop at high current densities. At 50% state of the charge, the exchange current density for zinc oxidation was estimated to be 7.4 mA cm-2, while the corresponding value for cerium reduction was found to be 24.2 mA cm-2, confirming that the slower kinetics of the negative half-cell limited the battery performance at low and mid-range current densities. Next, extensive life-cycle analyses were performed to pinpoint the exact cause of the battery capacity fade over multi-cycle experiments. Combination of in situ monitoring of the half-cell electrode potentials and colorimetric titration revealed that the combination of hydrogen evolution side reaction (HER) in the negative half-cell, crossover of protons, and accumulation of Ce(IV) in the positive electrolyte results in the eventual capacity fade of the system. The research contributions outlined above underpin the rest of the work, where a novel electrolyte is designed to increase the kinetic rate of the Zn/Zn(II) redox reaction and enhance the charge and voltage efficiency of the zinc redox reaction and consequently the full-cell battery at low and intermediate current densities. Extensive half-cell potentiometric and galvanostatic electrochemical techniques revealed that the amount of zinc deposited, charge, voltage and energy efficiencies of Zn/Zn(II) always remain significantly higher in an alternative mixed methanesulfonate-chloride negative electrolyte compared to the conventional methanesulfonic acid (MSA) solutions. Given these promising half-cell studies, the electrolyte was further tested in a bench-scale zinc-cerium RFB and the results showed that a zinc-cerium RFB with this new electrolyte has a significantly longer life and notably higher charge and voltage efficiencies compared to conventional zinc-cerium RFBs. Whereas the battery containing pure MSA negative electrolyte was able to operate for 42 h with 97 cycles at a current density of 25 mA cm, it was able to operate for more than 75 h with 166 cycles when mixed MSA-chloride was used on the negative side. Lastly, to simulate the operation of zinc-cerium RFBs, a two-dimensional transient model accounting for three modes of transport (migration, diffusion and convection) coupled with electrode kinetics was developed. The developed model was validated against the above-described extensive set of experimental data. This included full-cell voltages, positive and negative electrode potentials monitored via reference electrodes inserted into the battery set-up and Ce(IV) concentrations in the positive electrolyte measured by colorimetric titration over multi-cycle experiments. This was the most comprehensive comparison carried out between the output of an RFB model and experimental data extracted during operation of a redox flow battery. The validated model was then used to predict the cell voltages and limiting redox reactions during battery operation for different model parameters to provide a direction toward improving the performance of zinc-cerium RFBs.
Fabrication and Characterization of Nanoparticle Microporous Layers on Platinized Titanium Fiber Felt for Electrolyzer Anodes
(University of Waterloo, 2024-09-20) Jamali, Nooruddin; Gostick, Jeff; Pritzker, Mark
This study is concerned with the incorporation of various nanoparticles in the microporous layers (MPL) on titanium fiber felts for use at the anode in proton exchange membrane (PEM) water electrolyzers. The nanoparticle MPLs were coated onto Ti fiber felt using various methods. Three types of nanoparticles were utilized: indium tin oxide (ITO), tin (Sn) and titanium (Ti). The ITO and Sn nanoparticles were applied using an electrospraying technique, with Nafion as a binder (in the case of ITO) to ensure adhesion to the felt substrate and polyvinylpyrrolidone (PVP) as a surfactant to prevent nanoparticle sedimentation. This method resulted in uniformly smooth coatings. In contrast, Ti nanoparticles were deposited via a solvent evaporation method without a binder. This was followed by sintering of the nanoparticle-coated Ti felt at 750°C for 1 hour under an argon atmosphere. The resulting MPLs underwent comprehensive characterization, including surface imaging via scanning electron microscopy (SEM), assessments of permeability and porosity and measurements of electrical conductivity. The final and critical phase of characterization involved testing the samples in a laboratory-scale water electrolyzer. The electrolyzer setup included titanium bipolar plates with a once-through 2.1 x 2.1 cm – flow field leading to the membrane electrode assembly with an active area of 0.9 x 2.0 cm. All cells used to characterize performance consisted of a commercial carbon fiber cathode coated with an MPL (SGL 22BB) and a Hydrion N-115 catalyst-coated membrane. The tests revealed that the performance using sintered MPLs was superior to that of the electrosprayed MPLs and surpassed that of the baseline case (Ti felt with no coating). The sintered Ti coating with the lowest loading operated the best indicating that the rougher and thinner MPL was the best choice. The poor performance of the electrosprayed MPLs is attributed to the higher interparticle resistance due to the presence of non-conducting materials (dispersant and binder) as reflected in the lower conductivity of these MPL.
Improving Productivity in Bioreactors through Control of Cell Heterogeneity
(University of Waterloo, 2022-12-22) Vitelli, Michael; Budman, Hector; Pritzker, Mark
Whooping cough, also referred to as pertussis, is a highly contagious bacterial respiratory tract disease. At Sanofi Pasteur, the fermentation step in the manufacturing of the vaccine for pertussis involves a sequence of reactors of increasing volume in which the final cell population from one reactor in the train is used to inoculate the following reactor. A main challenge with this operation is that the yield of the vaccine antigens can be highly variable. In particular, pertactin, which is generated in low levels and is highly variable relative to that of the other antigens of the vaccine, has a highly variable production rate thus posing a major bottleneck to the overall productivity. Based on the findings of previous studies by our group (Zavatti, 2019), oxidative stress appears to be related to the variability in productivity of antigens and pertactin in particular. To explain the observed variability of the process, we also hypothesize that the time profiles of dissolved oxygen, pH, temperature and aeration rates during fermentation may not accurately capture the presence of highly stressed cells within the cell population since they only reflect averaged measures of the cell population at any given time. Instead, only cytometric analysis of the heterogeneity of the cell population can provide a correct measure of the level of stress and its impact on productivity. Also, based on the hypothesis that population heterogeneity influences overall productivity, it was further hypothesized that the evolution of the fermentation process depends on the heterogeneity of the inoculum used to start the fermentation process. Then, it is argued that the process can be improved by using an inoculum that is tailored by means of cell sorting. The current work focuses on the impact of oxidative stress and the growth profile of Bordetella pertussis and heterogeneity of intracellular concentrations for four main areas of application: 1. investigation of the possible origins of the oxidative stress in the manufacturing process. 2. development of a metabolic model describing the effect of oxidative stress on the growth of B. pertussis. 3. development of a coupled population balance-oxidative stress model to relate the heterogeneity of intracellular concentrations to B. pertussis growth profiles. 4. development of a protocol to sort B. pertussis on the basis of surface antigen concentration with the purpose of re-culturing. Zavatti (2017) found that a high reactive oxidative species (ROS) at the beginning of the fermentation is associated with a low pertactin yield. One of the purposes of developing a mechanistic model is to determine whether the low growth rate is caused by high ROS levels or the two are merely correlated. This information could help to identify operating conditions that lead to high ROS. The model of Himeoka and Kaneko (2017) was developed to describe the explain the general behaviour during the lag, exponential growth, stationary and death phases without resorting to detailed mechanisms. In this study, we have adapted and extended this model to understand and describe the relation between cell growth, oxidative stress and NADPH under different oxidative conditions during the pertussis vaccine production. In view of the differences between B. pertussis to other bacteria, a main goal of the study is to assess via flask studies and model predictions whether the ROS level is the key determinant of growth under different ROS-inducing conditions rather than other factors such as substrate (glutamate) inhibition. Evaluation of the root-mean-square-error (RMSE) and Akaike information index (AIC) showed that the fit of this new oxidative stress model to the experimental data was considerably better than that of a Contois-based model. The AIC is a particularly useful measure of the trade-off between model dimensionality and predictability in this case since the oxidative stress model involves a greater number of parameters than the Contois model. Also, the model and experimental data verified that high ROS levels at the beginning of flask culture is correlated to low growth profiles but is probably not the cause of the reduced biomass concentration. Population balance models (PBMs) were formulated to describe the evolution in time of the cell population in terms of growth and oxidative stress. Flow cytometry data was used to gain insight into the distribution of important quantities (e.g., cell size, intracellular concentrations of metabolites) over the entire cell population. A coupled population balance-oxidative stress model was developed to predict distributions in cell size and intracellular glutamate, ROS, NADPH and NADP+ concentrations in shake flask cultures of B. pertussis. The major advantage of using a PBM is that it accounts for the distributions and can predict the heterogeneity of the cell population with respect to experimental conditions that are averaged out in bulk models. When comparing the coupled population balance – oxidative stress model to the bulk oxidative stress model, it is apparent that the PBM provides much better predictions of the intracellular ROS concentration. We hypothesize that due to the nonlinear relations between cell growth and oxidative stress, intracellular and cell surface quantities can be better modeled with population balance models. Flow cytometry sorting is an extension of flow cytometry that enables cells to be sorted based on any property measured via flow cytometry. The actual sorting operation occurs downstream from the detectors that measure the light scattering and fluorescence energy. One of the hypotheses of the current work was that productivity can be enhanced by sorting a population of highly producing cells followed by cultivation of the sorted population. To avoid a lengthy a validation of the manufacturing process by mutating B. pertussis cells to find a high pertactin-producing strain, a sort based on epigenetics was analyzed. However, the sorted cell populations were not able to maintain the properties for which they were selected, although the protocol was able to select B. pertussis cells which grew at a faster rate than the control seed provided by Sanofi.
Investigation and Enhancement of Zn-Ce Redox Flow Battery Performance Through Experimental and Modeling Studies
(University of Waterloo, 2025-01-20) Yu, Hao; Pritzker, Mark; Gostick, Jeff
The transformation from energy based on fossil fuels to that based on sustainable options such as wind, solar and hydroelectric sources is crucial to reduce air/water pollution and carbon emissions. However, the production of electricity from these sustainable sources is typically intermittent in nature and can perturb the stability of the existing power grid. Redox flow batteries (RFB) have emerged as promising devices for grid-scale energy storage to stabilize power systems and improve their efficiency. Among the different types of RFBs, zinc- and cerium-based RFBs are promising for large-scale applications that require high output power density due to their low cost and high cell voltage. Motivated by its potential for future applications, this work focuses on the performance improvement of Zn-Ce RFBs through both experimental and modeling studies. Many of the findings and general ideas for RFB performance improvement are also applicable to other RFB systems and to commercial scale RFBs in real-life scenarios. In this work, the effect of different positive supporting electrolytes on the performance of a bench-scale Zn-Ce RFB has been studied. The effectiveness of mixed methanesulfonic/sulfuric acid, mixed methanesulfonic/nitric acid and pure methanesulfonic acid has been assessed and compared. The Ce(III)/Ce(IV) reaction exhibits faster kinetics and the battery exhibits higher coulombic efficiency in the mixed 2 mol/L MSA-0.5 mol/L H2SO4 electrolyte compared to that achieved in the commonly used 4 mol/L MSA electrolyte due to lower H+ crossover and higher Ce(IV) solubility. The rate of the fade in coulombic efficiency in the mixed MSA-H2SO4 electrolyte is 0.55% per cycle over 40 charge-discharge cycles, while the fade rate is 1.26% in the case of 4 mol/L MSA. Furthermore, the positive electrode reaction is no longer the limiting half-cell reaction even at the end of long-term battery charge-discharge operation. The effect of ion crossover on the overall Zn-Ce RFB performance has also been investigated through the measurement of the Zn(II), Ce(III), Ce(IV) and H+ concentrations on both sides of a Nafion 117 membrane during charge-discharge cycles. As much as 36% of the initial Zn(II) ions transfer from the negative to the positive electrolyte and 42.5% of the H+ in the positive electrolyte has crossed over to the negative side after 30 charge-discharge cycles. Both of these phenomena contribute to the steady fade in battery performance over the course of operation. Based on these findings, experiments aimed at reducing the concentration gradient driving crossover by intentionally adding different amounts of Zn(II) to the positive electrolyte at the outset of operation have been conducted. This approach has been shown to reduce the crossover of Zn(II) from the negative side to the positive side, improve both the battery coulombic and voltage efficiencies and reduce the decay of battery performance. Since the ion crossover phenomena is very commonly observed, this strategy to improve battery overall performance and reduce ions crossover by minimizing concentration gradient is not only applicable to similar lab-scale RFB research, but also beneficial for real-life RFB applications. Since the positive electrode reaction becomes the limiting half-cell reaction during the course of battery operation, two strategies have been investigated to regenerate the positive electrolyte by converting the accumulated Ce(IV) ions back to Ce(III) ions. The first strategy which utilizes RuO2 as a catalyst for Ce(IV) reduction improves the voltage efficiency from 71.1% to 77.8% over 16 cycles but reduces the coulombic efficiency from 74.1% to 57.8% due to the leakage of RuO2 catalyst through the porous filter into the positive electrolyte. The method utilizing H2O2 to regenerate the positive electrolyte improves the average coulombic efficiency from 63.7% to 68.3% and the average voltage efficiency from 56.8% to 76.1% over 30 cycles. Similar battery performance and life-cycle improvement can also be expected if these electrolyte regeneration methods are applied on a commercial scale. Furthermore, the implementation of these regeneration methods should also reduce the overall operating costs since it will reduce the frequency with which electrolytes have to be replaced. Finally, a transient 2-D model for the Zn-Ce RFB that accounts for the crossover of different electroactive species through the membrane has been developed. All three modes of transport (migration, diffusion and convection) coupled with electrode kinetics of Zn/Zn(II) and Ce(III)/Ce(IV) redox couples as well as HER and OER side reactions are included in the model. This model has been successfully validated against measurements of the evolution of the cell voltage, negative and positive electrode potentials and ion crossover during the course of 5 charge-discharge carried out in our laboratory. The validated model is then used to simulate the battery behaviour when operated under various operating conditions and using positive electrodes with different geometries. The results obtained provide useful information for the future design of Zn- or Ce-based RFBs with the aim of further improving their performance.
Investigation and Propagation of Defects in the Membrane Electrode Assembly of Polymer Electrolyte Membrane Fuel Cells: Quality Control Analysis
(University of Waterloo, 2019-05-24) Arkhat, Muneendra Prasad; Pritzker, Mark
Polymer electrolyte membrane fuel cells (PEMFC) have the potential to deliver high power density with a lower weight and volume compared to other fuel cells. However, some of the barriers to the successful commercialization of PEMFCs include problems associated with durability, stability and cost. Fuel cell defects that arise and propagate in the membrane electrode assembly (MEA) components during manufacturing and subsequent operation are the biggest factors limiting their durability and stability, leading to shortened lifetimes, reduced performance or cell failure. Defects in the production line must be minimized if PEMFCs are to become reliable electrochemical energy devices on a commercial scale. A conventional PEMFC electrode consists of layers (CL) of nanoscale Pt catalyst particles mixed with an ionomer on a high surface area carbon support deposited on the polymer electrolyte membrane (PEM) and sandwiched between gas diffusion media (GDM). The defects in these components originate from the raw materials used in the catalyst layers, process conditions during catalyst mixing, coating techniques, drying process, thickness variations in the casting substrate and the temperature and humidity of the processing environment. These defects can lead to reduced performance and can increase fuel cell degradation, specifically in the MEA components. Understanding the MEA component defects that affect fuel cell performance and lifetime is integral to the successful development of an on-line quality control strategy. Previous research studies have been conducted on defects in catalyst-coated membranes (CCMs) and gas diffusion layers (GDLs) with various dimensions that have been introduced artificially at specific locations, which does not satisfactorily mimic the situation with real manufacturing defects. Very few studies on real defects have been reported to date with limited work on localized effects on CL defects such as loss of catalyst, the morphology of defect growth or the effect of defect location within the CCM on the resulting cell performance. This has limited our fundamental and comprehensive understanding of the nature of defects in the beginning-of-life (BOL) state and the manner in which they may or may not propagate during PEMFC operation. The focus of this research is to analyze real catalyst layer defects and membrane pinholes on commercial CCMs that are developed during mass production. Specifically, the objectives of this study are to: (i) develop a non-destructive method to identify and quantify defects in CCM electrodes, (ii) implement a defect analysis framework to age CCMs using open-circuit voltage(OCV)- accelerated stress tests (AST), (iii) characterize the electrochemical performance of CCM/MEAs with varying extent of manufacturing defects (catalyst layer thickness, degree of catalyst non-uniformity) and compare this to a baseline, defect-free CCM/MEA using ASTs as well as in-situ and ex-situ methods and (iv) investigate defects on GDL-microporous layer (MPL) using infrared (IR) imaging and surface conductivity measurements. The first set of quality control experiments were performed on CCMs by using optical microscopy to characterize catalyst layer defects. Defects such as micro/macro cracks, catalyst clusters, missing catalyst layer defects (MCLDs), void/empty areas, CL delamination and pinholes in the CCM were characterized in terms of areal dimension (size, shape, and orientation) prior to electrochemical analysis. The OCV-AST protocol was developed to age defected CCMs in a custom-designed test cell and track defect propagation and behavior during aging. The geometric features of the defects were quantified and their growth measured at regular time intervals from beginning-of-life (BOL) to end-of-life (EOL) until the OCV had dropped by 20% from its initial value (as per the DOE-designed protocol). Overall, two types of degradation were observed: surface degradation caused by catalyst erosion and crack degradation caused by membrane mechanical deformation. Furthermore, the catalyst layer defects formed during the decal transfer process exhibited a higher growth rate at middle-of-life (MOL-1) before stabilizing by EOL. The results of the crack propagation analysis during AST showed that the defected area covered under cracks increased from 2.4% of the total CL area at BOL to 10.5% by EOL with a voltage degradation rate of 2.55mV/hr. This type of analysis should provide manufacturers with baseline information that will allow them to select and reject CCMs, increasing the lifetime of fuel cell stacks. Once the CCM defects were analyzed comprehensively, research was carried out on the MEA stack. MEAs containing defected CCMs (incomplete catalyst layer defects-MCLD), pinhole across sealant and artificial pinholes at inlet/middle/outlet were investigated using a cyclic open-circuit voltage (COCV)-AST. Different RH cycling periods from 80% RH to 20% RH with time delays from 5 mins to 30 mins were applied to the cathode to study the propagation of defects and their effect on overall cell performance. In-situ analysis included the measurement of polarization curves, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) to measure electrode degradation. Non-destructive ex-situ analysis using IR thermography was conducted every 100 cycles to monitor the evolution of defects in the MEA. The growth of pinholes was studied on the basis on hydrogen crossover curves. Sealing defects were found to have a major impact on performance loss compared to catalyst layer defects. It was also observed that MCLDs degraded within a short period of time and developed pinholes although the extent of this degradation depended on defect thickness. The MCLD defects were unstable and observed to continually grow due to gradual loss of catalyst particles inside the defected areas that accelerated pinhole formation in CCMs. This effect was clearly reflected in the continuous decay of OCV during the fuel cell operation. Therefore, CCMs leaving the production line with missing and /or thin portions of CL are not recommended for MEA fabrication as they ultimately affect the long-term stability of PEMFC. The last set of quality control experiments was conducted on GDL-MPL defects in samples that were being aged by RH cycling in a custom-design test cell. Thermal image analysis using IR thermography was carried out by passing DC current through the GDL sheet mounted on a porous vacuum stage to identify hot and cold spots reflecting defective areas. The morphological features and surface conductivity of MPL cracks were characterized using optical microscopy and four-point probe conductivity measurements. Interestingly, the nature of defects/cracks propagation in the GDL-MPL was found to affect cell performance in the mass transfer region at high currents. Crack propagation in GDL-MPL increased mass transport losses due to water flooding on the cathode, which was clearly observed in the polarization curves. Finally, the overall effects of catalyst layer defects, membrane pinholes and GDL defects on cell performance were compared. MEA sealant defects (pinholes) had such a negative effect on cell performance that EOL was reached after only ~ 50 hours of COCV operation at 80% - 20% RH cycling. Thus, the detection of such a defect in a CCM should be sufficient cause to reject it for use in a commercial stack. We also observed that CCMs with defects that led to 70% reduced thickness of the CL failed faster than those with the same type of defects that had resulted in 30% reduced thickness of the CL, presumably due to less available catalyst for electrochemical reactions. Clearly, CL defects should be given high priority in quality control inspection strategies devised by CCM electrode manufacturers and PEMFC operators.
Modeling and Characterization of Lithium Iron Phosphate Battery Electrodes
(University of Waterloo, 2016-09-29) Farkhondeh Borazjani, Mohammad; Fowler, Michael; Pritzker, Mark
A detailed understanding of lithiation/delithiation dynamics of battery active materials is crucial both for optimizing the existing technologies and for developing new materials. Among all, LiFePO4 (LFP) has been subject to intensive fundamental research due to its intriguing phase-transformation dynamics which, unexpectedly, yields an outstanding rate capability and a long cycle life for electrodes made of this insertion material. In this thesis mathematical models are used as cheap and simple tools to investigate the electrochemical performance of LFP electrodes. The thesis begins with the investigation of the solid-state transport (bulk effects) and electronic conductivity (surface effects) of LFP by means of variable solid-state diffusivity (VSSD) and resistive-reactant (RR) models, respectively. Both models are effectively validated against experimental galvanostatic discharge data over a wide range of applied currents. However, a very small solid-state diffusion coefficient (~〖10〗^(-19) "m" ^2 "s" ^(-1)) is required for both models to fit the experimental data. VSSD model features a particle-size distribution (PSD) which is estimated via model-experiment comparison. The fitted PSD, which is a geometric property and essentially invariable, requires to be different at different rates for the model to match experimental data; it is shifted towards smaller particles in order to accurately predict the electrode performance during galvanostatic discharge at higher applied currents. A contact-resistance distribution (CRD) replaces the PSD in the RR model. The fitted CRD turns out to be extremely broad spanning from ~1" to " ~〖10〗^2 "Ω" "m" ^2. Next, following recent observations of ultra fast lithiation/delithiation of LFP, a simple mesoscopic model is developed which, in contrast to the first part of this research, completely disregards solid-state diffusion limitations. Instead, the model accounts for the inherent inhomogeneity of physico-chemical properties and bi-stable nature of phase-change insertion materials such as LFP with no consideration of any geometric detail of the active material. The entire active material domain is discretized into meso-scale units featuring basic thermodynamic (non-monotonic equilibrium potential as a function of composition) and kinetic (insertion/de-insertion resistance) properties. With only these two factors incorporated, the model is able to simultaneously explain a number of unique features associated with lithium iron phosphate electrochemical performance including the quasi-static potential hysteresis, high rate capability, cycle-path dependence, mismatch in electrode polarization during GITT when compared with continuous cycling at the same current, bell-shaped current response in PITT and the most recently observed memory effect. Detailed analysis of the electrode dynamics suggests that a necessary condition for the memory effect to appear in an LFP electrode is the existence of a non-zero residual capacity at the onset of memory-release charging which may originate either from a non-zero initial SOC or from an imbalanced writing cycle. A memory effect should therefore not be observed in an electrode that has been preconditioned at extremely low currents (i.e., zero initial SOC) and has undergone an extremely slow memory-writing cycle (i.e., approaching a balanced cycle). In the next step, the mesoscopic model developed at the unit level is incorporated into porous-electrode theory and validated by comparing the simulation results with experimental data from continuous and intermittent galvanostatic discharge of a commercial LiFePO_4 electrode at various operating conditions. A bimodal lognormal resistance distribution is assumed to account for disparity of insertion dynamics among elementary units. Good agreement between the model and experimental data confirms the fidelity of the model. Investigation of three different GITT experiments suggests that the slow evolution of electrode polarization during each current pulse and the subsequent relaxation period is contributed by the inter-unit rather than intra-unit Li transport in LiFePO_4 electrodes. As such, GITT experiments once formulated for the determination of diffusion coefficient of inserted species in solid-solution systems may also be used to estimate the single-unit equilibrium potential (i.e., thermodynamic properties) as well as the dynamic properties (e.g., resistance distribution) of phase-change insertion materials. Further analysis of the GITT experiments suggest that, depending on the overall depth-of-discharge of the electrode and the incremental depth-of-discharge of each GITT pulse, the solid-solution capacity available in the Li-rich end-member may be able to accommodate Li insertion entirely without the need for active (closed-circuit) phase transformation. Instead, redistribution of Li among units during relaxation equilibrates the solid-solution composition by transforming a few Li-poor units to Li-rich ones. Despite rigorous research in the literature, this thesis presents the first attempt to quantitatively explain the above-mentioned irregularities simultaneously using a single unifying model and pinpoint the dominant contributing factors under various operating conditions. A realistic account of porous-electrode effects in the experimental validation of the mesoscopic model requires accurate estimation of the electrolyte transport properties. In addition to the modeling of phase-change electrodes, this thesis work presents a novel four-electrode-cell method to determine transport properties and the thermodynamic factor of concentrated binary electrolytes. The cell consists of two reference electrodes (i.e., potential sensors) in addition to the working and counter electrodes. The sensors measure the closed-circuit as well as open-circuit potential in response to an input current across the working and counter electrodes. The new method requires the application of only a single galvanostatic polarization pulse and appropriate concentration-cell experiments. By fitting a suitable model to the data obtained from these experiments, the three independent transport properties of a concentrated binary electrolyte, namely, ionic conductivity, diffusion coefficient and transference number as well as the thermodynamic factor can be determined. In particular, the measurement of the closed-circuit potential using this cell provides a simpler and essentially more accurate means to estimate the transference number than the conventional semi-infinite diffusion method.
Modification of Hydrphilic and Hydrophobic Surfaces Using an Ionic-Complementary Peptide
(Public Library of Science (PLOS), 2007) Yang, Hong; Fung, Shan-Yu; Pritzker, Mark; Chen, P.
Ionic-complementary peptides are novel nano-biomaterials with a variety of biomedical applications including potential biosurface engineering. This study presents evidence that a model ionic-complementary peptide EAK16-II is capable of assembling/coating on hydrophilic mica as well as hydrophobic highly ordered pyrolytic graphite (HOPG) surfaces with different nano-patterns. EAK16-II forms randomly oriented nanofibers or nanofiber networks on mica, while ordered nanofibers parallel or oriented 60° or 120° to each other on HOPG, reflecting the crystallographic symmetry of graphite (0001). The density of coated nanofibers on both surfaces can be controlled by adjusting the peptide concentration and the contact time of the peptide solution with the surface. The coated EAK16-II nanofibers alter the wettability of the two surfaces differently: the water contact angle of bare mica surface is measured to be <10°, while it increases to 20.3±2.9° upon 2 h modification of the surface using a 29 µM EAK16 II solution. In contrast, the water contact angle decreases significantly from 71.2±11.1° to 39.4±4.3° after the HOPG surface is coated with a 29 µM peptide solution for 2 h. The stability of the EAK16-II nanofibers on both surfaces is further evaluated by immersing the surface into acidic and basic solutions and analyzing the changes in the nanofiber surface coverage. The EAK16-II nanofibers on mica remain stable in acidic solution but not in alkaline solution, while they are stable on the HOPG surface regardless of the solution pH. This work demonstrates the possibility of using self-assembling peptides for surface modification applications.
Multiphysics Modelling of an Alkaline All-Iron, All-Soluble Aqueous Redox Flow Battery
(University of Waterloo, 2021-05-26) Dhillon, Arjun; Pritzker, Mark
The development of redox flow battery (RFB) technologies has attracted considerable attention in recent years. Redox flow batteries are electrochemical energy storage devices that operate as flowing systems. Unlike what is possible in conventional batteries, the ability to size the electrolyte storage tanks and electrodes separately enables the battery energy and power capacities to be decoupled and these important properties to be designed and scaled independently. Such systems are particularly attractive for large-scale grid energy storage, especially in conjunction with intermittent energy generation from renewable sources. As RFBs move from research and development to commercial adoption, the use of mathematical models becomes increasingly important for design and analysis of these systems and is indispensable for ensuring their success. Most RFB modelling to date has focused on the all-vanadium RFB, although novel RFBs are continuously investigated and developed. One such novel RFB is the all-iron all-soluble aqueous RFB that is the focus of the present work. This RFB makes use of iron-cyanide (Fe(II)-CN/Fe(III)-CN) and iron-triethanolamine (Fe(II)-TEOA/Fe(III)-TEOA) redox couples in alkaline aqueous solutions. Both redox couples have fast kinetics and the use of high-pH conditions mitigates the loss of current efficiency due to the hydrogen evolution side reaction. A model has been developed in the present work for the novel all-iron all-soluble aqueous redox flow battery presented by Gong et al. It is the first model to be developed for this RFB. The transient two-dimensional model considers transport of all redox species in the two electrode compartments using porous electrode theory. The side reaction involving the oxidation of TEOA following its permeation across the ion exchange membrane to the positive side is investigated and incorporated into the model. The hydrogen evolution reaction is also incorporated in the model. Parameter values are obtained from literature where available; the remainder of these values are obtained from fitting of the voltage-time curves for charge and discharge to published experimental data. A simulation of a sequence of repeated charge-discharge cycles is conducted and compared with experimental data. The RFB capacity and current efficiency are stable over this duration, which is consistent with experimental observations in the original study. The model has been shown to fit the available experimental data well and describe the behaviour of the RFB. The electrode potentials and reactant species concentrations are found to remain fairly uniform, indicating facile mass transport within the electrode. Recommendations are also made on future experimental and modelling work that can be conducted for this system.
A Topology-Based Method for the Compartmentalization of Multiphysics Flows
(University of Waterloo, 2020-09-28) Donnelly, Thomas Richard; Abukhdeir, Nasser Mohieddin; Pritzker, Mark
Continuum simulations of multiphysics processes are costly due to the coupling of transport phenomena. Compartment modelling decouples hydrodynamics from other transport phenomena, offering a low-cost simulation alternative for applications such as design screening. While different methods will produce compartment models with different accuracy levels, no rigorous compartmentalization approaches currently exist. In this work, a compartmentalization algorithm is proposed that identifies distinct flow modes from an analysis of the topology of a fluid velocity field. This topological analysis is based on an analogy between modes of fluid flow and deformation modes that have been identified for the molecular alignment of liquid crystals. A velocity alignment vector is defined and used to compute the deformation modes splay, twist, and bend for a velocity field. This topologically-informed compartmentalization algorithm is developed through its application to a test case of steady single-phase laminar flow through a cylindrical vessel with a step increase in cross-sectional area. This case is observed to exhibit unidirectional, recirculatory, and diverging flow based on a continuum simulation. Local alignment deformation, defined as the sum of splay, twist, and bend for a velocity field, is computed and thresholded to segment the domain into compartments dominated by each of the distinct flow modes. These compartments are incorporated into a compartment model, which is validated against the continuum simulation through a comparison of their residence time distributions (RTDs). The compartment model RTD is shown to deviate from the continuum simulation in terms of having lower mean residence time, higher variance, and higher skewness. The deviation between the compartment model and continuum simulation is attributed to the approximation of unidirectional compartments as well-mixed. The compartment model is modified to approximate each unidirectional compartment as a series of ideal continuous stirred-tank reactors (CSTRs), which improves the variance and skewness of the RTD but does not increase the mean residence time. As a final modification to the compartment model considered, an adjustment to the thresholding used for the compartment model is shown to have an insignificant effect on the RTD. While additional work is required to improve the accuracy of the compartment modelling approach proposed, compartment models based on velocity topology offer a promising approach for multiphysics simulation.
WO3/SnO2-Modified Graphite Felts as Novel Carbon-Based Positive Electrodes for Zinc-Cerium Redox-Flow Battery (RFB)
(University of Waterloo, 2021-05-21) Bin Gah, Omar Khaled Omar; Pritzker, Mark
Redox flow batteries (RFBs) are promising large-scale energy storage devices that are potentially cost-effective, easily scaled up in size, safe and versatile. RFBs use the chemical reactions of redox species which are dissolved in liquid electrolytes to store electrical energy when demand is low and deliver it during periods of high demand for electricity. Among various RFBs, zinc-cerium redox flow batteries (Zn-Ce RFBs) have received attention as an attractive energy storage system due to their higher open-circuit voltage (OCV) than any other aqueous-based system that has been proposed. However, the performance of Zn-Ce RFB must be enhanced if it is to be successfully commercialized. The electrode structure plays a significant role in the performance of RFBs. Ideal electrodes should perform efficiently under the severe acidic and oxidizing conditions with satisfactory durability. Taking into account the highly acidic and oxidizing conditions on the cerium half-cell, an electrode employed in that side should maintain outstanding electrochemical behavior toward the Ce(III)/Ce(IV) reaction. Precious metals such as platinum and gold have shown promising performance. However, due to their high cost, the need for cheaper, but effective, electrode materials have become very crucial. Economical carbon-based materials have been shown to provide good kinetic activity for the Ce(III)/Ce(IV) reaction. Particularly, polyacrylonitrile (PAN)-based graphite felt (GF) exhibits excellent chemical and electrochemical catalytic activity due to its excellent stability and conductivity, high specific surface area and enhanced three-dimensional structure. This study concentrated on examining the utilization of modified graphite felts (GFs) as a positive electrode for the Zn-Ce RFB system. Solvothermal method was used to introduce metal oxides electrocatalysts on the GF surface. Tungsten trioxide (WO3) and tin dioxide (SnO2) served as promising metal oxides to enhance hydrophilicity and electrochemical activity of GF. A binary mixture of WO3 and SnO2 was used as a novel candidate to improve the electrochemical activity of pristine GFs for the Ce(III)/Ce(IV) reaction. Half-cell electrochemical characterization was performed using cyclic voltammetry (CV) at room temperature in a 0.05 M Ce(III) MSA + 1.0 M MSA electrolyte. The ratio of the cathodic peak current density to the anodic peak current density (Ipc/Ipa), peak potential separation (ΔEp) and coulombic charge obtained from the CVs were used to assess the activity and reversibility of the Ce(III)/Ce(IV) reaction on the various GF electrode samples. The durability of the modified GF electrodes was evaluated by subjecting them to multiple CV cycles over the voltage range from 0.56 to 1.8 V vs. Ag|AgCl at a scan rate of 10 mV s-1. Modified GFs with the novel binary metal oxide containing both WO3 and SnO2 exhibited superior stability and electrocatalytic performance toward the Ce(III)/Ce(IV) reaction compared to unmodified GF and those modified with the individual oxides alone. Among all GF samples, the binary WO3-SnO2 modified GF showed the highest values of anodic charge (Qa), cathodic charge (Qc) and Qc/Qa. Qc/Qa of as-received GF was found to improve from 17.8% to 50.1% when modified with 0.05M W and 0.05M Sn. The microstructure and composition of the modified GFs were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Overall, findings obtained from these analyses confirmed the successful decoration of W and Sn on the GFs.