Growth of MAPbX3 (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) me... more Growth of MAPbX3 (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) method, and the crystallization pathway illustrated by the solubility curve of MAPbCl3 in DMSO, compared with inverse temperature crystallization (ITC) method.
Lithium-ion batteries (LIBs) have been widely utilized as power sources for mobile devices. Recen... more Lithium-ion batteries (LIBs) have been widely utilized as power sources for mobile devices. Recently, their use has been expanded to large-scale applications such as electric vehicles (EVs) and energy storage systems (ESSs). These batteries have dominated the energy industry due to their unmatchable properties that include a high energy density, a compact design, and an ability to meet a number of required performance characteristics in comparison to other rechargeable systems. Two vital parameters for LIBs are their stable and safe operation. Since, the cathode serves as a central component of LIBs, the overall cell performance is significantly affected by the chemical and physical properties of the cathode. Cathodes tend to react with the electrolytes and, hence, undergo surface modifications accompanied by degradation. These side-reactions result in an erosion of battery performance and rate capability, thereby causing a reduced battery life and power capacity. Surface coating is the most simple, economical and effective method to protect the cathode surface from degradation and detrimental interfacial reactions with the electrolyte. For the present investigation, a variety of coatings have been used to coat cathode surface. The aim is to explore the effect of these coatings on the physicochemical characteristics as well as electrochemical and thermal properties of spinel cathode using characterization techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and differential thermal analysis (DTA)/ thermogravimetric analysis (TGA).
Water electrolysis has already been implemented for many decades on an industrial scale. It does,... more Water electrolysis has already been implemented for many decades on an industrial scale. It does, however, remain an active area of research and development for the pursuit of materials that have enhanced electrocatalytic performance and prolonged durability. For example, the accumulation of gas bubbles on the surfaces of electrodes during water electrolysis remains a challenge. Persistent bubbles on the surfaces of electrocatalysts can block electrolyte access to the electrode and reduce the electrochemically active surface area. At current densities that are relevant to industrial applications an extensive portion of the electrode can become covered with bubbles. There are a range of solutions that are being utilized in industrial settings, but also additional solutions being sought to optimize the efficiency of these systems. These solutions include applied shear flows across the surfaces of the electrodes, the implementation of ultrasonic induced cavitation, and an increase in electrolyte temperature. These solutions require further expenditures of energy and decrease the overall energy efficiency of the system. Alternative approaches may yield a lower energy demand for maintaining accessibility of the electrolyte to the electrochemically active surface area. One approach is through the design of electrode surface architectures that can assist with the removal of gas bubbles during water electrolysis. This work includes a review of the progress made in preparing structured electrodes for the alkaline based water electrolysis and specifically for the oxygen evolution reaction. These structures can influence the dynamics that occur at the electrode-electrolyte interface, such as the growth, coalescence, and release of oxygen gas bubbles. Understanding the correlations between the structures on the electrodes and their influence on the function of these materials towards the gas evolution are sought to guide the future development of optimal surface structures that are self-cleaning. Designs of electrode surface architectures are sought that can improve the efficiency of electrodes toward this and other gas evolution reactions.
The preparation and screening of nanoparticle (NP) electrocatalysts for improved electrocatalytic... more The preparation and screening of nanoparticle (NP) electrocatalysts for improved electrocatalytic oxygen evolution reactions (OER) will require a better understanding and optimization of the interactions between NPs and their support. First-row transition metals are used extensively as electrocatalysts in electrochemical energy storage and conversion systems. These electrocatalysts undergo transformations in their phase and surface morphology, which are induced by oxidizing potentials in the alkaline medium. A template-assisted approach to prepare electrodes with regular surface morphologies was used to monitor interactions between the NPs and their support both before and after prolonged electrochemical aging. A templateassisted method was used to prepare uniform surface inclusions of nickel ferrite (NiFe 2 O 4 ) NPs on conductive nickel (Ni) supports for evaluation toward the OER. Electron microscopy-based methods were used to assess the resulting transformations of the embedded NPs within the Ni support matrix. Electrochemical aging of these textured electrodes was conducted by cyclic voltammetry (CV) techniques, which resulted in the growth of a 200 nm thick Ni oxy(hydroxide) film on the surfaces of the Ni supports. The growth of the active surface layer led to the encapsulation of the NiFe 2 O 4 NPs as determined by correlative energy dispersive X-ray spectroscopy (EDS) techniques. The NP-modified electrodes exhibited reduced overpotentials and higher sustained current densities for the OER when compared to pure Ni supports. The well-defined morphologies and NP surface inclusions prepared by the template-assisted approach could serve as a platform for investigating additional NP-support interactions for electrocatalytic systems.
The demands on the alternative energy sector necessitates high quality research and development t... more The demands on the alternative energy sector necessitates high quality research and development to produce economical, reliable and environment friendly energy sources. Proton exchange fuel cells (PEFCs) offer a versatile and dependable alternative to conventional energies both in the transportation and stationary power sectors. Their widespread use is, however, still restricted due to their high cost and the limited availability of platinum (Pt) resources. The high cost of PEFCs is attributed in part to the higher loading of Pt based catalyst at the cathode. The reduction of Pt loadings at the cathode is generally accompanied by significant performance losses due to sluggish oxygen reduction reaction (ORR) kinetics. This issue can be resolved by maximizing the Pt utilization without sacrificing the performance. Hence, on-going research and development work is seeking to better utilize Pt with lower loadings along with optimized performance. Compared to their solid or bulk counterparts, porous structures exhibit a high electrochemical active surface area (A ecsa) and, thus, can enhance the Pt utilization. These porous nanostructures can restructure during fuel cell cycling, and structural collapse can lead to an eventual decrease in Pt utilization.1 Stabilizing agents such as surfactants are usually employed during the Pt nanoparticle (NP) synthesis to prevent NP aggregation. These surfactants can also assist in the formation of the mesoporous structure.2 This study describes the electrochemical deposition of a stable porous Pt structure in the presence of surfactants. A site-directed electrodeposition offers advantages to the wet chemical synthesis of Pt nanocatalysts as it ensures that the Pt is deposited onto specific regions of a support that have a sufficient ionic and electrical conductivity. In this study, we demonstrated the use of anionic, cationic, and non-ionic surfactants to produce porous Pt using electrodeposition and the influence of these surfactants in stabilizing the porous structure during the ORR in acidic medium. The conditions for electrodeposition, such as concentration of surfactants and potential for the nucleation and growth stages were each optimized through a series of experiments. These surfactants included cetyl trimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS), and polyethylene glycol octadecyl ether (Brij 78). We utilized scanning electron microscopy techniques to evaluate the porosity of the deposited NPs and their surface coverage. In these experiments, an initial applied pulse was used to induce nucleation of the Pt followed by the growth of these materials with further electrodeposition at a lower potential. These parameters were evaluated for their influence on the final product, such as the porosity, A ecsa, and the surface coverage these materials. The porosity, composition, and crystallinity of these mesoporous particles were confirmed using transmission electron microscopy (TEM) and selected area electron diffraction techniques. These surfactant systems each resulted in the formation of porous Pt. The porous Pt was also subjected to durability testing by cycling the applied potential over a range of oxidizing potentials. Further analysis of these materials by TEM indicated that the mesoporous structure was maintained after the durability tests with a negligible change in their half-wave potential towards the ORR. The durability testing conducted after the removal of surfactants using Soxhlet extractor confirmed the role of the surfactants in helping to stabilize the porous Pt structure.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, Jun 1, 2016
Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobici... more Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobicity and a high adhesion to water, were prepared by spray coating of progressively smaller hydrophilic silica particles along with hydrophobic nanofibers onto surfaces of interest. Various polymer structures were achieved by tuning the spray coating flow rates during deposition of the polymer nanofibers and silica particles. At a reduced flow rate, polystyrene fibers were formed with diameters of less than 100 nm. Water contact angles (WCAs) of coatings prepared from the hierarchical assemblies of silica particles blended with polystyrene nanofibers were greater than 110°. Coatings prepared from the hierarchical assemblies either with or without incorporation of the polymer nanofibers pinned water droplets to their surfaces even after inverting the substrates, similar to the properties of a rose petal. Hierarchical coatings of silica particles without the polystyrene nanofibers also exhibited a high adhesion to water, pinning at least 30% more water on its surfaces. Conversely, hierarchical coatings containing the polystyrene nanofibers exhibited an increased water mobility across their surfaces. Further water retention experiments were performed to determine the ability of the different coatings to efficiently condense water vapor, as well as their efficiency to remove this condensed liquid from their surfaces. Both types of hierarchical coatings exhibited an excellent ability to retain water at a low humidity, while establishing a self-limiting condition for retaining water at a high humidity. These coatings could be prepared on a relatively large-scale and with a relatively low cost on the surfaces of a variety of materials to enhance their water resistance, water retention and/or ability to condense water vapor.
As global energy demands increase, the need for clean energy and energy storage is evident. Water... more As global energy demands increase, the need for clean energy and energy storage is evident. Water electrolysis has shown promise as a clean energy storage solution and can be coupled with other clean energy techniques (e.g., wind, solar) to store chemical energy in the form of hydrogen. Alkaline systems are of particular interest as they are more cost effective compared to acidic systems that require precious metal-based catalysts. Alkaline water electrolysis suffers from inefficiencies that include the oxidative half reaction known as the oxygen evolution reaction (OER). Nickel-based electrocatalysts are being developed for the electrochemical transformations of organic species and used in gas evolution reactions, such as the OER. Nickel-based materials are sought after in part for their lower cost relative to precious metal catalysts (e.g., Pt, Ru, Ir), but they lack the higher electrochemical activity achieved by their precious metal counterparts. To develop more active and durable materials and to better understand the mechanisms involved in electrochemical transformations on nickel-based materials, it is essential to understand how these materials evolve and age as a result of electrocatalytic use. In this study, we have preserved the hydrated form of Ni electrocatalysts aged under alkaline conditions relevant to the OER. We prepared a series of electrocatalysts with nano- and micro-scale grains that were polished to a similar nano-scale roughness. First, we electrochemically aged Ni electrocatalysts with nanoscale and microscale grains by potential cycling techniques (i.e., cyclic voltammetry). Following electrochemical aging, the Ni electrocatalysts were preserved by immersion in liquid nitrogen and sublimed in a lyophilizer. After this freeze-drying process, the electrocatalysts were imaged under cryogenic conditions using scanning electron microscopy (SEM) techniques. A comparison was made to aged electrocatalysts where freeze-drying was implemented and those that were allowed to air dry. The surfaces of aged Ni electrocatalysts were all observed to contain an electrochemically active layer with a gel-like form. When the catalysts were air-dried, the layer appeared to have a collapsed, web-like texture. Through the use of transmission electron microscopy analyses, it was determined that these gel-like layers contained predominantly nanocrystalline β-phase nickel hydroxide (β-Ni(OH)2), which likely formed due to relaxation of the OER active beta-phase nickel oxyhydroxide [β-NiOOH] prior to the imaging process. The formation of the gel-like layer covering these electrocatalysts has implications for dynamic processes taking place at their interface with the electrolyte. Processes influenced by the gel-like form of this active layer include the rates of diffusion of electrolyte, the mechanism of O2 bubble nucleation, and the mechanics of bubble release. The results of these studies also have implications for the electrocatalytic activity and stability of other types of electrocatalysts. Further, this work can be extended for the design of new electrocatalysts for a variety of electrocatalytic processes, such as the hydrogen evolution reaction and other gas evolution reactions. Figure 1
An increasing demand for clean and renewable energy provides motivation for additional innovation... more An increasing demand for clean and renewable energy provides motivation for additional innovations to be sought in energy storage. Lithium-ion batteries (LIBs) have seen many advancements in terms of their cathode chemistries, including electrode coatings that are used to increase durability and capacity of cathode materials. These coatings are effective, but create additional interfaces in the battery system, that adds to complexity in characterization. To address many key issues of LIB failures, further insights are necessary that provide details on currently unknown and partially known intermediate states, side reactions, and/or mechanisms of degradation at electrode interfaces. In this work, a method is developed to enable a detailed surface analysis and additional characterization of coated cathode particles harvested from post-mortem LIBs. Procedures are outlined for the safe handling and preparation of post-mortem materials for high-quality single particle electron microscopy analyses, paired with ultramicrotome techniques for high-throughput cross-sectional imaging with elemental and structural analyses of the coatings themselves. Commonly seen in post-mortem results, the materials are kept intact as whole electrodes or large collections of particles while still held within binders and other cathode slurry components from cell fabrication. These samples are often briefly rinsed with electrolyte solvents [e.g., most commonly dimethyl carbonate (DMC)] to rinse away residual lithium salts.1 While important insights have been gained from such studies, details of surface defects are difficult to discern amidst the debris of additives and binders. To address this, we developed a non-destructive procedure for the separation of cathodes particles from their supports, followed by washing to remove binders, additives, and residual electrolyte with relatively safe and accessible solvents. This method enables high-quality single particle imaging by scanning and transmission electron microscopy (SEM and TEM) techniques. Nano-scale features on the particle surfaces can be observed with high-resolution on the post-mortem cathode particles. With this procedure, insights can be gained about the retention of coatings after battery cycling, in addition to observing visible surface defects that result from charge cycling. Typical cross-sectional analyses of interfacial systems involve expensive, time-consuming methods like focused ion beam (FIB) milling that require extensive training, and are low-throughput when FIB lift-out samples are prepared for TEM. An alternative high-throughput technique for the cross-sectional analysis of coated cathode particles by ultramicrotome was previously developed in our group.2 This technique involves embedding coated cathode particles in epoxy, from which ultra-thin slices (<100 nm) that can contain many [approx. 50-200] particles at a time are cut with a diamond knife. The resultant sheets of epoxy are deposited onto TEM grids so the cross-sections can be imaged by TEM. These sections also enable the particle coatings to be characterized separately from the bulk cathode by electron dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). This technique was demonstrated on as-synthesized (pre-electrochemistry) samples but was adapted here for the prepared post-mortem samples to assess degradation of the particles and their coatings. Combining the newly developed techniques for obtaining high-quality images of single whole-particles and particle cross-sections this work provides a safe, relatively inexpensive, and high-throughput methodology for the post-mortem characterization of LIB materials. This data can be correlated to as-synthesized materials and electrochemical cycling data to create a detailed profile of cathode aging and degradation as it relates to LIB failures. We are expanding this method to a variety of standard and coated cathode systems to provide detailed information needed for the next steps towards developing new innovations in electrode designs aimed at achieving more durable LIBs. References: 1) Xiong, R.; Pan, Y.; Shen, W.; Li, H.; Sun, F. Lithium-ion battery aging mechanisms and diagnosis method for automotive applications: Recent advances and perspectives. Renewable and Sustainable Energy Reviews. 2020, 131, 110048 2) Taylor, A. K.; Nesvaderani, F.; Ovens, J. S.; Campbell, S.; Gates, B. D. Enabling a High-Throughput Characterization of Microscale Interfaces within Coated Cathode Particles. ACS Appl. Energy Mater. 2021, 4, 9731−9741
Nonlinear optics provides the functionality of wavelength conversion and switching that allows fo... more Nonlinear optics provides the functionality of wavelength conversion and switching that allows for photonic signal processing and bioimaging. A major obstacle for extreme photonic integration is the requirement to use ultrafast pulsed lasers or to work above the diffraction limit to activate nonlinear optical processes. Here we show that plasmonic enhancement and two dimensional materials provide a promising pathway to extreme photonic integration with nonlinear functionality in nanoscale while using diode lasers.
Water transport in an operating hydrogen-air fuel cells has gathered the interests of both resear... more Water transport in an operating hydrogen-air fuel cells has gathered the interests of both researchers and developers working on fuel cells. Better water removal can extend the operation regime of fuel cells and, therefore, increase its maximum cell power and overall energy efficiency. However, state-of-art in situ fuel cell diagnostics cannot provide an insight into the water transport phenomenon. Recent research shows that pressure controlling techniques have the potential to address issues with water transport. The purpose of this work is to develop an in-situ diagnostic tool from cathode pressure oscillations for hydrogen-air polymer electrolyte fuel cells named as electrochemical pressure impedance spectroscopy (EPIS). The response between the cell voltage and the pressure oscillations is analogous to the electrochemical impedance spectroscopy (EIS)(See image). The relation between EPIS response and water transport phenomenon is intensively studied in this work. With the help from our industrial partner, Greenlight Innovation Corp., we are able to integrate this diagnostic method into a fuel cell test station. Experimental data are fitted by the mathematical derivation of the response from theoretical cell components from which are extracted crucial parameters and diagnostic information for water transport. Figure 1
ABSTRACTWe demonstrate an alternative route to synthesize functionalized silica nanoparticles thr... more ABSTRACTWe demonstrate an alternative route to synthesize functionalized silica nanoparticles through incorporation of alcohol compounds in the Stöber process. The Stöber process has been widely utilized for the synthesis of silica nanoparticles due to its simplicity and reliability. Silane based compounds have been incorporated in this process in order to tailor surface properties of the silica nanoparticles. These compounds do, however, have limitations in their utility due to side reactions with water and intermolecular polymerization. In this article, we report the incorporation of alcohol based reagents in the Stöber process as an alternative means of synthesis and functionalization of silica nanoparticles. In particular, choline chloride was chosen as an exemplary alcohol to be incorporated in the process for tuning overall surface charge of the silica nanoparticles. These silica nanoparticles with incorporated choline chloride were characterized by atomic force microscopy (AFM), zeta potential measurements, and X-ray photoelectron spectroscopy (XPS) in comparison with silica nanoparticles synthesized from the traditional Stöber process. While the size and shape of the nanoparticles exhibited little difference between the two synthetic routes, the zeta potential of the choline chloride incorporated nanoparticle was ∼10 mV higher than that of the traditional silica nanoparticles. Composition of the choline chloride containing silica nanoparticles was verified by XPS with the observation of strong N1sand C1ssignals. The methods introduced in this article could be expanded to incorporate a range of alcohol containing compounds including choline chloride for the synthesis of silica nanoparticles with a tuned surface chemistry.
Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characteriz... more Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characterize electrochemical processes. It is based on the dynamic analysis of two electrical observables, that is, current and voltage. Electrochemical cells with gaseous reactants or products (e.g., fuel cells, metal/air cells, electrolyzers) offer an additional observable, that is, the gas pressure. The dynamic coupling of current and/or voltage with gas pressure gives rise to a number of additional impedance definitions, for which we use the term electrochemical pressure impedance. It also gives rise to different experimental probing approaches. In this article we present a model-based study of electrochemical pressure impedance spectroscopy (EPIS). Possible quantifications and realizations of EPIS are discussed. The study of generic cell geometries consisting of gas reservoir, diffusion layer(s) and electrochemically active layer(s) reveals distinct spiral-shaped features in the Nyquist plot. Using the example of a sodium/oxygen (Na/O2) cell, the dynamic spatiotemporal behavior of the state variables is quantified and interpreted. Results are compared to first experimental EPIS measurements by Hartmann et al. [J. Phys. Chem. C118, 1461, 2014]. A sensitivity analysis highlights the properties of EPIS with respect to geometric, transport, and kinetic parameters. We demonstrate that EPIS is sensitive to transport parameters that are not well-accessible with standard EIS
We report a size fractionation of titania (TiO2) nanoparticles absorbed from the environment and ... more We report a size fractionation of titania (TiO2) nanoparticles absorbed from the environment and found within wild Dittrichia viscosa plants. The nanoparticles were isolated by extraction and isolation from distinct plant organs, as well as from the corresponding rhizosphere of wild, adult plants. The collected nanoparticles were characterized by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS). More than 1,200 TiO2 nanoparticles were analyzed by these techniques. The results indicated the presence of TiO2 nanoparticles with a wide range of sizes within the inspected plant organs and rhizospheres. Interestingly, a size selective process occurs during the internalization and translocation of these nanoparticles (e.g., foliar and root uptake), which favors the accumulation of mainly TiO2 nanoparticles with diameters <50 nm in the leaves, stems, and roots. In fact, our findings indicate that among the total number of TiO2 nanoparticles analyzed, the fraction of the particles with dimensions <50 nm were 52% of those within the rhizospheres, 88.5% of those within the roots, 90% of those within the stems, and 53% of those within the leaves. This significant difference observed in the size distribution of the TiO2 nanoparticles among the rhizosphere and the plant organs could have impacts on the food chain, and further biologicals effects that are dependent on the size of the TiO2.
Growth of MAPbX3 (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) me... more Growth of MAPbX3 (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) method, and the crystallization pathway illustrated by the solubility curve of MAPbCl3 in DMSO, compared with inverse temperature crystallization (ITC) method.
Lithium-ion batteries (LIBs) have been widely utilized as power sources for mobile devices. Recen... more Lithium-ion batteries (LIBs) have been widely utilized as power sources for mobile devices. Recently, their use has been expanded to large-scale applications such as electric vehicles (EVs) and energy storage systems (ESSs). These batteries have dominated the energy industry due to their unmatchable properties that include a high energy density, a compact design, and an ability to meet a number of required performance characteristics in comparison to other rechargeable systems. Two vital parameters for LIBs are their stable and safe operation. Since, the cathode serves as a central component of LIBs, the overall cell performance is significantly affected by the chemical and physical properties of the cathode. Cathodes tend to react with the electrolytes and, hence, undergo surface modifications accompanied by degradation. These side-reactions result in an erosion of battery performance and rate capability, thereby causing a reduced battery life and power capacity. Surface coating is the most simple, economical and effective method to protect the cathode surface from degradation and detrimental interfacial reactions with the electrolyte. For the present investigation, a variety of coatings have been used to coat cathode surface. The aim is to explore the effect of these coatings on the physicochemical characteristics as well as electrochemical and thermal properties of spinel cathode using characterization techniques like scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and differential thermal analysis (DTA)/ thermogravimetric analysis (TGA).
Water electrolysis has already been implemented for many decades on an industrial scale. It does,... more Water electrolysis has already been implemented for many decades on an industrial scale. It does, however, remain an active area of research and development for the pursuit of materials that have enhanced electrocatalytic performance and prolonged durability. For example, the accumulation of gas bubbles on the surfaces of electrodes during water electrolysis remains a challenge. Persistent bubbles on the surfaces of electrocatalysts can block electrolyte access to the electrode and reduce the electrochemically active surface area. At current densities that are relevant to industrial applications an extensive portion of the electrode can become covered with bubbles. There are a range of solutions that are being utilized in industrial settings, but also additional solutions being sought to optimize the efficiency of these systems. These solutions include applied shear flows across the surfaces of the electrodes, the implementation of ultrasonic induced cavitation, and an increase in electrolyte temperature. These solutions require further expenditures of energy and decrease the overall energy efficiency of the system. Alternative approaches may yield a lower energy demand for maintaining accessibility of the electrolyte to the electrochemically active surface area. One approach is through the design of electrode surface architectures that can assist with the removal of gas bubbles during water electrolysis. This work includes a review of the progress made in preparing structured electrodes for the alkaline based water electrolysis and specifically for the oxygen evolution reaction. These structures can influence the dynamics that occur at the electrode-electrolyte interface, such as the growth, coalescence, and release of oxygen gas bubbles. Understanding the correlations between the structures on the electrodes and their influence on the function of these materials towards the gas evolution are sought to guide the future development of optimal surface structures that are self-cleaning. Designs of electrode surface architectures are sought that can improve the efficiency of electrodes toward this and other gas evolution reactions.
The preparation and screening of nanoparticle (NP) electrocatalysts for improved electrocatalytic... more The preparation and screening of nanoparticle (NP) electrocatalysts for improved electrocatalytic oxygen evolution reactions (OER) will require a better understanding and optimization of the interactions between NPs and their support. First-row transition metals are used extensively as electrocatalysts in electrochemical energy storage and conversion systems. These electrocatalysts undergo transformations in their phase and surface morphology, which are induced by oxidizing potentials in the alkaline medium. A template-assisted approach to prepare electrodes with regular surface morphologies was used to monitor interactions between the NPs and their support both before and after prolonged electrochemical aging. A templateassisted method was used to prepare uniform surface inclusions of nickel ferrite (NiFe 2 O 4 ) NPs on conductive nickel (Ni) supports for evaluation toward the OER. Electron microscopy-based methods were used to assess the resulting transformations of the embedded NPs within the Ni support matrix. Electrochemical aging of these textured electrodes was conducted by cyclic voltammetry (CV) techniques, which resulted in the growth of a 200 nm thick Ni oxy(hydroxide) film on the surfaces of the Ni supports. The growth of the active surface layer led to the encapsulation of the NiFe 2 O 4 NPs as determined by correlative energy dispersive X-ray spectroscopy (EDS) techniques. The NP-modified electrodes exhibited reduced overpotentials and higher sustained current densities for the OER when compared to pure Ni supports. The well-defined morphologies and NP surface inclusions prepared by the template-assisted approach could serve as a platform for investigating additional NP-support interactions for electrocatalytic systems.
The demands on the alternative energy sector necessitates high quality research and development t... more The demands on the alternative energy sector necessitates high quality research and development to produce economical, reliable and environment friendly energy sources. Proton exchange fuel cells (PEFCs) offer a versatile and dependable alternative to conventional energies both in the transportation and stationary power sectors. Their widespread use is, however, still restricted due to their high cost and the limited availability of platinum (Pt) resources. The high cost of PEFCs is attributed in part to the higher loading of Pt based catalyst at the cathode. The reduction of Pt loadings at the cathode is generally accompanied by significant performance losses due to sluggish oxygen reduction reaction (ORR) kinetics. This issue can be resolved by maximizing the Pt utilization without sacrificing the performance. Hence, on-going research and development work is seeking to better utilize Pt with lower loadings along with optimized performance. Compared to their solid or bulk counterparts, porous structures exhibit a high electrochemical active surface area (A ecsa) and, thus, can enhance the Pt utilization. These porous nanostructures can restructure during fuel cell cycling, and structural collapse can lead to an eventual decrease in Pt utilization.1 Stabilizing agents such as surfactants are usually employed during the Pt nanoparticle (NP) synthesis to prevent NP aggregation. These surfactants can also assist in the formation of the mesoporous structure.2 This study describes the electrochemical deposition of a stable porous Pt structure in the presence of surfactants. A site-directed electrodeposition offers advantages to the wet chemical synthesis of Pt nanocatalysts as it ensures that the Pt is deposited onto specific regions of a support that have a sufficient ionic and electrical conductivity. In this study, we demonstrated the use of anionic, cationic, and non-ionic surfactants to produce porous Pt using electrodeposition and the influence of these surfactants in stabilizing the porous structure during the ORR in acidic medium. The conditions for electrodeposition, such as concentration of surfactants and potential for the nucleation and growth stages were each optimized through a series of experiments. These surfactants included cetyl trimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS), and polyethylene glycol octadecyl ether (Brij 78). We utilized scanning electron microscopy techniques to evaluate the porosity of the deposited NPs and their surface coverage. In these experiments, an initial applied pulse was used to induce nucleation of the Pt followed by the growth of these materials with further electrodeposition at a lower potential. These parameters were evaluated for their influence on the final product, such as the porosity, A ecsa, and the surface coverage these materials. The porosity, composition, and crystallinity of these mesoporous particles were confirmed using transmission electron microscopy (TEM) and selected area electron diffraction techniques. These surfactant systems each resulted in the formation of porous Pt. The porous Pt was also subjected to durability testing by cycling the applied potential over a range of oxidizing potentials. Further analysis of these materials by TEM indicated that the mesoporous structure was maintained after the durability tests with a negligible change in their half-wave potential towards the ORR. The durability testing conducted after the removal of surfactants using Soxhlet extractor confirmed the role of the surfactants in helping to stabilize the porous Pt structure.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, Jun 1, 2016
Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobici... more Surfaces with rose petal like properties, simultaneously exhibiting a high degree of hydrophobicity and a high adhesion to water, were prepared by spray coating of progressively smaller hydrophilic silica particles along with hydrophobic nanofibers onto surfaces of interest. Various polymer structures were achieved by tuning the spray coating flow rates during deposition of the polymer nanofibers and silica particles. At a reduced flow rate, polystyrene fibers were formed with diameters of less than 100 nm. Water contact angles (WCAs) of coatings prepared from the hierarchical assemblies of silica particles blended with polystyrene nanofibers were greater than 110°. Coatings prepared from the hierarchical assemblies either with or without incorporation of the polymer nanofibers pinned water droplets to their surfaces even after inverting the substrates, similar to the properties of a rose petal. Hierarchical coatings of silica particles without the polystyrene nanofibers also exhibited a high adhesion to water, pinning at least 30% more water on its surfaces. Conversely, hierarchical coatings containing the polystyrene nanofibers exhibited an increased water mobility across their surfaces. Further water retention experiments were performed to determine the ability of the different coatings to efficiently condense water vapor, as well as their efficiency to remove this condensed liquid from their surfaces. Both types of hierarchical coatings exhibited an excellent ability to retain water at a low humidity, while establishing a self-limiting condition for retaining water at a high humidity. These coatings could be prepared on a relatively large-scale and with a relatively low cost on the surfaces of a variety of materials to enhance their water resistance, water retention and/or ability to condense water vapor.
As global energy demands increase, the need for clean energy and energy storage is evident. Water... more As global energy demands increase, the need for clean energy and energy storage is evident. Water electrolysis has shown promise as a clean energy storage solution and can be coupled with other clean energy techniques (e.g., wind, solar) to store chemical energy in the form of hydrogen. Alkaline systems are of particular interest as they are more cost effective compared to acidic systems that require precious metal-based catalysts. Alkaline water electrolysis suffers from inefficiencies that include the oxidative half reaction known as the oxygen evolution reaction (OER). Nickel-based electrocatalysts are being developed for the electrochemical transformations of organic species and used in gas evolution reactions, such as the OER. Nickel-based materials are sought after in part for their lower cost relative to precious metal catalysts (e.g., Pt, Ru, Ir), but they lack the higher electrochemical activity achieved by their precious metal counterparts. To develop more active and durable materials and to better understand the mechanisms involved in electrochemical transformations on nickel-based materials, it is essential to understand how these materials evolve and age as a result of electrocatalytic use. In this study, we have preserved the hydrated form of Ni electrocatalysts aged under alkaline conditions relevant to the OER. We prepared a series of electrocatalysts with nano- and micro-scale grains that were polished to a similar nano-scale roughness. First, we electrochemically aged Ni electrocatalysts with nanoscale and microscale grains by potential cycling techniques (i.e., cyclic voltammetry). Following electrochemical aging, the Ni electrocatalysts were preserved by immersion in liquid nitrogen and sublimed in a lyophilizer. After this freeze-drying process, the electrocatalysts were imaged under cryogenic conditions using scanning electron microscopy (SEM) techniques. A comparison was made to aged electrocatalysts where freeze-drying was implemented and those that were allowed to air dry. The surfaces of aged Ni electrocatalysts were all observed to contain an electrochemically active layer with a gel-like form. When the catalysts were air-dried, the layer appeared to have a collapsed, web-like texture. Through the use of transmission electron microscopy analyses, it was determined that these gel-like layers contained predominantly nanocrystalline β-phase nickel hydroxide (β-Ni(OH)2), which likely formed due to relaxation of the OER active beta-phase nickel oxyhydroxide [β-NiOOH] prior to the imaging process. The formation of the gel-like layer covering these electrocatalysts has implications for dynamic processes taking place at their interface with the electrolyte. Processes influenced by the gel-like form of this active layer include the rates of diffusion of electrolyte, the mechanism of O2 bubble nucleation, and the mechanics of bubble release. The results of these studies also have implications for the electrocatalytic activity and stability of other types of electrocatalysts. Further, this work can be extended for the design of new electrocatalysts for a variety of electrocatalytic processes, such as the hydrogen evolution reaction and other gas evolution reactions. Figure 1
An increasing demand for clean and renewable energy provides motivation for additional innovation... more An increasing demand for clean and renewable energy provides motivation for additional innovations to be sought in energy storage. Lithium-ion batteries (LIBs) have seen many advancements in terms of their cathode chemistries, including electrode coatings that are used to increase durability and capacity of cathode materials. These coatings are effective, but create additional interfaces in the battery system, that adds to complexity in characterization. To address many key issues of LIB failures, further insights are necessary that provide details on currently unknown and partially known intermediate states, side reactions, and/or mechanisms of degradation at electrode interfaces. In this work, a method is developed to enable a detailed surface analysis and additional characterization of coated cathode particles harvested from post-mortem LIBs. Procedures are outlined for the safe handling and preparation of post-mortem materials for high-quality single particle electron microscopy analyses, paired with ultramicrotome techniques for high-throughput cross-sectional imaging with elemental and structural analyses of the coatings themselves. Commonly seen in post-mortem results, the materials are kept intact as whole electrodes or large collections of particles while still held within binders and other cathode slurry components from cell fabrication. These samples are often briefly rinsed with electrolyte solvents [e.g., most commonly dimethyl carbonate (DMC)] to rinse away residual lithium salts.1 While important insights have been gained from such studies, details of surface defects are difficult to discern amidst the debris of additives and binders. To address this, we developed a non-destructive procedure for the separation of cathodes particles from their supports, followed by washing to remove binders, additives, and residual electrolyte with relatively safe and accessible solvents. This method enables high-quality single particle imaging by scanning and transmission electron microscopy (SEM and TEM) techniques. Nano-scale features on the particle surfaces can be observed with high-resolution on the post-mortem cathode particles. With this procedure, insights can be gained about the retention of coatings after battery cycling, in addition to observing visible surface defects that result from charge cycling. Typical cross-sectional analyses of interfacial systems involve expensive, time-consuming methods like focused ion beam (FIB) milling that require extensive training, and are low-throughput when FIB lift-out samples are prepared for TEM. An alternative high-throughput technique for the cross-sectional analysis of coated cathode particles by ultramicrotome was previously developed in our group.2 This technique involves embedding coated cathode particles in epoxy, from which ultra-thin slices (&lt;100 nm) that can contain many [approx. 50-200] particles at a time are cut with a diamond knife. The resultant sheets of epoxy are deposited onto TEM grids so the cross-sections can be imaged by TEM. These sections also enable the particle coatings to be characterized separately from the bulk cathode by electron dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). This technique was demonstrated on as-synthesized (pre-electrochemistry) samples but was adapted here for the prepared post-mortem samples to assess degradation of the particles and their coatings. Combining the newly developed techniques for obtaining high-quality images of single whole-particles and particle cross-sections this work provides a safe, relatively inexpensive, and high-throughput methodology for the post-mortem characterization of LIB materials. This data can be correlated to as-synthesized materials and electrochemical cycling data to create a detailed profile of cathode aging and degradation as it relates to LIB failures. We are expanding this method to a variety of standard and coated cathode systems to provide detailed information needed for the next steps towards developing new innovations in electrode designs aimed at achieving more durable LIBs. References: 1) Xiong, R.; Pan, Y.; Shen, W.; Li, H.; Sun, F. Lithium-ion battery aging mechanisms and diagnosis method for automotive applications: Recent advances and perspectives. Renewable and Sustainable Energy Reviews. 2020, 131, 110048 2) Taylor, A. K.; Nesvaderani, F.; Ovens, J. S.; Campbell, S.; Gates, B. D. Enabling a High-Throughput Characterization of Microscale Interfaces within Coated Cathode Particles. ACS Appl. Energy Mater. 2021, 4, 9731−9741
Nonlinear optics provides the functionality of wavelength conversion and switching that allows fo... more Nonlinear optics provides the functionality of wavelength conversion and switching that allows for photonic signal processing and bioimaging. A major obstacle for extreme photonic integration is the requirement to use ultrafast pulsed lasers or to work above the diffraction limit to activate nonlinear optical processes. Here we show that plasmonic enhancement and two dimensional materials provide a promising pathway to extreme photonic integration with nonlinear functionality in nanoscale while using diode lasers.
Water transport in an operating hydrogen-air fuel cells has gathered the interests of both resear... more Water transport in an operating hydrogen-air fuel cells has gathered the interests of both researchers and developers working on fuel cells. Better water removal can extend the operation regime of fuel cells and, therefore, increase its maximum cell power and overall energy efficiency. However, state-of-art in situ fuel cell diagnostics cannot provide an insight into the water transport phenomenon. Recent research shows that pressure controlling techniques have the potential to address issues with water transport. The purpose of this work is to develop an in-situ diagnostic tool from cathode pressure oscillations for hydrogen-air polymer electrolyte fuel cells named as electrochemical pressure impedance spectroscopy (EPIS). The response between the cell voltage and the pressure oscillations is analogous to the electrochemical impedance spectroscopy (EIS)(See image). The relation between EPIS response and water transport phenomenon is intensively studied in this work. With the help from our industrial partner, Greenlight Innovation Corp., we are able to integrate this diagnostic method into a fuel cell test station. Experimental data are fitted by the mathematical derivation of the response from theoretical cell components from which are extracted crucial parameters and diagnostic information for water transport. Figure 1
ABSTRACTWe demonstrate an alternative route to synthesize functionalized silica nanoparticles thr... more ABSTRACTWe demonstrate an alternative route to synthesize functionalized silica nanoparticles through incorporation of alcohol compounds in the Stöber process. The Stöber process has been widely utilized for the synthesis of silica nanoparticles due to its simplicity and reliability. Silane based compounds have been incorporated in this process in order to tailor surface properties of the silica nanoparticles. These compounds do, however, have limitations in their utility due to side reactions with water and intermolecular polymerization. In this article, we report the incorporation of alcohol based reagents in the Stöber process as an alternative means of synthesis and functionalization of silica nanoparticles. In particular, choline chloride was chosen as an exemplary alcohol to be incorporated in the process for tuning overall surface charge of the silica nanoparticles. These silica nanoparticles with incorporated choline chloride were characterized by atomic force microscopy (AFM), zeta potential measurements, and X-ray photoelectron spectroscopy (XPS) in comparison with silica nanoparticles synthesized from the traditional Stöber process. While the size and shape of the nanoparticles exhibited little difference between the two synthetic routes, the zeta potential of the choline chloride incorporated nanoparticle was ∼10 mV higher than that of the traditional silica nanoparticles. Composition of the choline chloride containing silica nanoparticles was verified by XPS with the observation of strong N1sand C1ssignals. The methods introduced in this article could be expanded to incorporate a range of alcohol containing compounds including choline chloride for the synthesis of silica nanoparticles with a tuned surface chemistry.
Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characteriz... more Electrochemical impedance spectroscopy (EIS) is a widely-used diagnostic technique to characterize electrochemical processes. It is based on the dynamic analysis of two electrical observables, that is, current and voltage. Electrochemical cells with gaseous reactants or products (e.g., fuel cells, metal/air cells, electrolyzers) offer an additional observable, that is, the gas pressure. The dynamic coupling of current and/or voltage with gas pressure gives rise to a number of additional impedance definitions, for which we use the term electrochemical pressure impedance. It also gives rise to different experimental probing approaches. In this article we present a model-based study of electrochemical pressure impedance spectroscopy (EPIS). Possible quantifications and realizations of EPIS are discussed. The study of generic cell geometries consisting of gas reservoir, diffusion layer(s) and electrochemically active layer(s) reveals distinct spiral-shaped features in the Nyquist plot. Using the example of a sodium/oxygen (Na/O2) cell, the dynamic spatiotemporal behavior of the state variables is quantified and interpreted. Results are compared to first experimental EPIS measurements by Hartmann et al. [J. Phys. Chem. C118, 1461, 2014]. A sensitivity analysis highlights the properties of EPIS with respect to geometric, transport, and kinetic parameters. We demonstrate that EPIS is sensitive to transport parameters that are not well-accessible with standard EIS
We report a size fractionation of titania (TiO2) nanoparticles absorbed from the environment and ... more We report a size fractionation of titania (TiO2) nanoparticles absorbed from the environment and found within wild Dittrichia viscosa plants. The nanoparticles were isolated by extraction and isolation from distinct plant organs, as well as from the corresponding rhizosphere of wild, adult plants. The collected nanoparticles were characterized by scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS). More than 1,200 TiO2 nanoparticles were analyzed by these techniques. The results indicated the presence of TiO2 nanoparticles with a wide range of sizes within the inspected plant organs and rhizospheres. Interestingly, a size selective process occurs during the internalization and translocation of these nanoparticles (e.g., foliar and root uptake), which favors the accumulation of mainly TiO2 nanoparticles with diameters <50 nm in the leaves, stems, and roots. In fact, our findings indicate that among the total number of TiO2 nanoparticles analyzed, the fraction of the particles with dimensions <50 nm were 52% of those within the rhizospheres, 88.5% of those within the roots, 90% of those within the stems, and 53% of those within the leaves. This significant difference observed in the size distribution of the TiO2 nanoparticles among the rhizosphere and the plant organs could have impacts on the food chain, and further biologicals effects that are dependent on the size of the TiO2.
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Papers by Byron Gates