Search

Graphene and related two-dimensional (2D) materials are being intensively studied for use in membrane separators. The ultrathin nature of 2D materials combined with the opportunities they afford for intentional creation of nanometer-sized pores having controllable size and chemical character could allow for membrane separations with high flux and also high selectivity. Proton transport through graphene is especially interesting because it may occur through the intact pristine graphene rather than through defects / pores. Other cations, for example lithium or sodium, may pass through graphene but that transport is through to occur through defects / pores. This presentation will focus on selective cation transport through graphene layers that are embedded between Nafion membranes. Ion transmission through graphene is measured using a four-electrode cell in which two Luggin capillaries sense the potential difference that develops across the membrane in response to an ion current that is driven by a pair of platinum wire drive electrodes. Proton transmission in this cell configuration occurs more than 100 times faster than transport of any other ion. This difference is thought to reflect the different mechanisms by which protons and other cations pass though graphene. The lecture will discuss defect characterization in CVD graphene films, and will consider the role of defects, which are in fact nanopores, in allowing for selective transport of ions through graphene.

Recent reports from several laboratories indicate that proton transmission through graphene and other 2D materials in contact with proton-conducting electrolytes can occur rapidly with a much lower activation energy than expected from a wide range of computational studies on the proton / graphene interaction energy, and a much higher selectivity relative to transmission of other ions than would be expected based only on ion size. It would be desirable to be able to control transport through the graphene, and many approaches have been tried to rationally control transport rates through graphene via chemical treatments applied to the graphene. One such approach involves use of atomic layer deposition (ALD), in which single layers of inorganic materials, often oxides, are coated in a self-limiting way onto a surface by a series of pulsed reaction steps delivered in series. A common ALD implementation involves coatings of aluminum oxide, or alumina, created by sequential dosing of a surface with trimethyl aluminum and water vapor. We report here on use of the ALD method to coat single-layer CVD graphene supported on copper foil or on Nafion membranes with layers of alumina. Coatings resulting from 50 ALD cycles are found to be thick enough and homogeneous enough to block photoelectron emission from the underlying substrates (copper or Nafion) when the coatings are analyzed by X-ray photoelectron spectroscopy. Electrochemical hydrogen pump experiments were then performed to assess the degree to which ALD treatment of the graphene on Nafion, delivered prior to preparing Nafion | graphene | Nafion sandwich membranes, could change the rate of proton transmission through the graphene that is embedded within the Nafion. As little as two ALD treatment cycles are sufficient to reduce proton transmission currents through graphene by approximately half relative to graphene that is not treated by ALD. Surprisingly, fifty ALD cycles has almost the same effect as two cycles. This finding indicates that that ALD alumina coating is a relatively good proton conductor which is thought to reflect the action of transport pathways on the hydrated surface of the porous ALD alumina layer. The insensitivity of proton transmission to the number of ALD cycles applied suggests that the function of the ALD is to change the transport rates at interfaces, perhaps by creating a new ALD alumina / Nafion interface that protons must traverse in order to pass through the membrane.

Confocal Raman microscopy is being adapted to probe interfaces within bipolar membranes and ionomer-graphene composites of interest for use in fuel cell and related electrochemical devices [1-2]. Although versatile and adaptable to in-situ measurements, confocal Raman microscopy depth-profiling has pitfalls when applied to bulk membranes. Optical aberration and sensitivity to light scattering from sample regions outside the probe beam focus limit depth resolution. With these constraints in mind, initial studies focused on detecting single layer graphene (SLG) as a barrier between two Nafion membranes (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [2]. Using a microscope equipped with oil-immersion optics to provide refractive index matching, the graphene layer was easily detected in confocal Raman microscopy depth-profiles. Spectra were sensitive to defects and stress within the buried graphene layer. Building on the ability to probe membrane-supported SLG, SLG adsorbed to a glass microscope coverslip (SLG/SiO2) was adapted as a platform for confocal aperture alignment and probe volume characterization [1]. Using SLG/SiO2to estimate the limiting axial spatial resolution, the junction separating the AEM/CEM (anion exchange membrane / cation exchange membrane) phases of bipolar polymer membrane (Fumasep FBM) was probed and shown to be consistent with roughness at the boundary on the order of a few micrometers (~3 µm). Continuing efforts are aimed at developing 2-dimensional (x-, y-direction) Raman spatial maps of buried polymer membrane interfaces and model AEM/CEM interfaces for high spatial resolution (< 10 nm) neutron reflectometry depth profiling. 1. Korzeniewski, C.; Kitt, J.P.; Bukola, S.; Creager, S.E.; Minteer, S.D.; Harris, J.M. ”Single layer graphene for estimation of axial spatial resolution in confocal Raman microscopy depth profiling” Anal. Chem.2019, 91, 1049 (DOI:10.1021/acs.analchem.8b04390). 2. Bukola, S.; Liang, Y.; Korzeniewski, C.; Harris, J.M.; Creager, S.E. “Selective proton / deuteron transport through Nafion | graphene | Nafion sandwich structures at very high current density” J. Am. Chem. Soc. 2018, 140, 1743. (DOI: 10.1021/jacs.7b10853)

Our research group has recently demonstrated the ability to probe within composite membranes formed by sandwiching single layer graphene (SLG) between two Nafion layers (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [1,2]. Using a microscope equipped with an oil-immersion objective, the graphene layer is readily detected in confocal Raman microscopy depth-profiles (z-direction profiles) of the composite membranes. Spectra are sensitive to defects and stress within the buried graphene layer. This presentation will discuss the use of confocal Raman microscopy for structural characterization and spatial profiling in the development of composite polymer electrolyte membranes that contain a graphene layer as a barrier to ion transport. In a second application area, infrared spectroscopy is being adapted to investigate properties of Nafion ionomer particles in dilute dispersions and during transformation to gel and solid phases through the use of time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Since the properties of dispersing fluids can affect the mechanical strength of solution-cast Nafion thin films [3], it is of interest to gain insights into ionomer properties in the dispersion and during phase changes. A well-resolved peak associated with vibrational motions of the Nafion side chain -SO3 -end group is shown to be sensitive to gel-phase development. The persistence of hydration water in the vicinity of the -SO3 -group helps maintain a constant local refractive index that limits optical distortion in the band as the ionomer assembles into the phase-separated framework structure of the solid membrane. ATR FTIR measurements predict trends in evaporative water loss from the dispersion and entry into the gel-phase consistent with expectations of gravimetric measurements and earlier studies [3] of Nafion ionomer dispersion properties. 1. Bukola, S., et al. J. Am. Chem. Soc. 2018, 140, 1743. 2. Bukola, S., et al., ACS Appl. Nano Mater. 2019, 2, 964. 3. Kim, Y.S., et al., Macromolecules 2015, 48, 2161.

High-rate proton transmission through single-layer graphene has been reported by several research groups but the mechanism by which it occurs is still not clear. Low activation energies for proton transmission have been experimentally obtained but are difficult to reconcile with computational modeling studies that predict high activation energies and low transmission rates for protons passing through pristine single-layer graphene at ambient temperatures. It seems likely that a reaction coordinate with an activated complex structure involving proton passage through graphene is involved, though the structural details of such a complex are still not clear. This lecture will review some of our recent experimental results on proton transmission rates through CVD single-layer graphene at variable temperature, with special attention to a kinetics model whereby experimentally-obtained proton-transfer resistances are interpreted as proton-transfer resistances that are linked to heterogeneous proton—transfer rate constants, in a manner that is formally similar to the Butler-Volmer model used to treat electron-transfer rate data in the low overpotential limit. The heterogeneous proton-transfer rate constants are then interpreted using a pre-equilibrium model for protons approaching graphene, coupled to a first-order rate constant for proton transmission through graphene from the pre-equilibrium complex. This first-order proton-transmission rate constant has fundamental significance and may be interpreted in terms of an activation energy and frequency factor for proton transmission. In applying such an analysis to our proton transmission rate data, we obtain a proton-transmission frequency factor that is within a factor of two of the vibrational frequency of the O-H bond in water, suggesting that O-H bond vibration is involved in the rate-determining step of proton transmission.

High specific energy is always desirable in battery systems, especially for electric vehicles. A key towards improving specific energy for a particular combination of anode and cathode active material is to maximize the mass fraction of active material, which means minimizing the fraction of inactive material such as the separator, the binder, electrode additives for improving electronic conductivity, and the current collector. A seemingly simple strategy for maximizing the mass fraction of active materials is to make the electrodes as thick as possible. However, ion transport in thick electrodes can be particularly problematic since salt diffusion into and through the electrode pore space is slow and can easily limit the battery charge / discharge rate. An ideal microstructure is one that allows for direct line of sight or path for the electrode active material access to the electrolyte, for rapid ion-transport throughout the entire electrode thickness. Therefore, a need exists for electrode fabrication methods that create microstructures that maximize electrolyte transport, as is needed to achieve high-rate charging/discharging in electrodes having high mass loading. Optimal microstructures in electrodes help to achieve the ideal balance of active material and electrolyte mass to give high gravimetric and volumetric energy density. As to design a microstructure with the goal of Li ion salt transport facilitation, we synthesized ordered porous microstructure electrodes using combination of freeze cast and tape cast techniques. This method enjoys the combination of conventional tape casting, a main technique in manufacturing Li ion electrodes in industrial scale, and freeze casting. This study simulate two dimensional ordered porous electrode cross-sections and presents the electrochemical performance results of Mo-doped LTO in a electrochemical cell consist of a lithium metal anode, polymer separator, and liquid ethylene carbonate and ethyl methyl carbonate electrolyte. The electrochemical performance of ordered porous microstructure is investigated by varying electronic conductivity, tortuosity, particle size, electrode thickness, and C rate. We show that these engineered porous electrodes considerably reduces the electrolytes mass transport limitation and electrochemical response of electrode is independent of tortuosity, making it possible to synthesize ultra thick electrode for high energy and power density applications. Simulation results show that 10 micron thickness of wall and pore freeze tape cast Mo-doped LTO with electronic conductivity of 1 S/m, tortuosity as high as 10, electrode thickness of 458 micron and mass loading of 30 mg/cm^2 can deliver full capacity even at 2 C-rate.

This talk will cover a wide range of recent studies involving proton transmission across CVD graphene layers incorporated into proton-exchange-membrane (PEM) sandwich structures. Many low-temperature electrochemical systems including fuel cells and water electrolysis cells involve proton transmission across proton-exchange membranes integrated with electrodes. A key to optimizing such systems is the accomplishment of selective proton transmission at high rates, while simultaneously inhibiting crossover transmission of other molecular species such as water, hydrogen / oxygen, methanol, and other species. Single-layer CVD graphene has been shown to accomplish this. Proton transmission across graphene in Nafion | graphene | Nafion sandwich structures has been shown to occur with an area-normalized resistance of less than 0.05 ohm cm2 at room temperature, with high selectivity relative to other ions. The graphene layer is nearly defect free and should be an excellent barrier to other species besides protons. The talk will include studies of proton transmission rates using hydrogen pump cells, water electrolysis using PEM cells, and related work. Graphene characterization studies using confocal Raman spectroscopy and X-ray photoelectron spectroscopy, and graphene defect characterization using chemical etching of graphene on copper, will also be covered. Effects of the graphene structure (single vs double layer) and the nature of the 2D material (graphene vs. hexagonal boron nitride) on proton transmission rates, and the rates for other ions besides proton, will also be discussed.

Proton transmission rates across single-layer CVD graphene embedded in sandwich structures with proton-exchange membranes made from the perfluorosulfonic acid (PFSA) ionomer Nafion were measured and found to occur more than 100 times larger than for any other cation. Measurements were made for protons and a series of other cations including Li+, Na+, K+, Rb+, Cs+ and NH4 + using a four-electrode method in which two platinum electrodes drive ionic current through the membrane and two reference electrodes installed in Luggin capillaries sense the transmembrane potential difference induced by the forced ion flow. Proton transmission rates across graphene were between 150 and 350 times larger than for any of the other cations studied. Characterization studies of graphene on Nafion using confocal Raman microscopy and X-ray photoelectron spectroscopy, and defect counting for graphene on copper by chemical etching through graphene on copper will also be reported. Findings are consistent with transport through physical defect structures for all cations except protons. Proton transmission occurs at very high rates and may involve sites that are more widely distributed and that have much higher proton selectivity than the sites responsible for transmission of other ions. Proton transmission rates across graphene were also measured using electrochemical hydrogen pump cells at variable temperature, which provides an activation energy for proton transmission across graphene. Rate constants for proton transmission are interpreted using a charge-transfer resistance model that allows for interpretation of ionic currents in terms of a rate constant for interfacial proton-transfer. Anrhenius analysis of the rate constants gives values for the activation energy and frequency factor for the interfacial proton-transfer reaction.

Rechargeable batteries are important components of many systems and devices and a high capacity battery that can be rapidly charged remains a top research and development priority. Important performance factors like capacity, energy density, and power density strongly depend on chemical nature and the microstructure of the porous electrodes. Highly tortuous porous electrodes and long lithium ion pathway due to pore blockage by inactive material are detrimental to the energy and power density. Our goal is to investigate the potential of fabricating low tortuosity, controlled porosity Li-ion battery electrodes by taking advantage of the hierarchical ordered porous microstructure, which can be obtained using freeze tape casting technique. For this research, Lithium titanate (Li4Ti5O12) powders with and without molybdenum doping (LTO and MoLTO respectively) were synthesized by a solid-state method and used to fabricate electrodes on Cu foil using a normal tape-cast method and a novel freeze-tape-cast. Modest molybdenum doping produces a significant electronic conductivity increase (e.g. 1 mS cm-1 for MoLTO vs 10-7 mS.cm-1 for LTO) that is thought to reflect a partial Ti4+ reduction to Ti3+ with charge compensation by the Mo6+ dopant, producing a stable mixed-valent Ti4+/3+ state. Freeze-tape-cast electrodes were fabricated by a variant of the normal tape-cast method that includes a rapid freezing step in which the solvent in the Cu-foil-supported slurry is rapidly frozen on a cold finger then subsequently sublimed to create unidirectional columnar macropores in the electrode. The effect of Mo doping, freezing rate, electrode thickness on electrochemical behavior of electrodes were investigated. The resulting freeze tape cast electrodes exhibit high porosity and low tortuosity which enhances electrolyte accessibility throughout the full electrode thickness. Freeze-tape-cast electrodes subjected to galvanostatic charge-discharge testing as cathodes in cells vs. a lithium metal anode exhibit higher specific capacity and lower capacity loss at high discharge rates compared with normal-tape-cast electrodes of the same mass loading, despite the fact that the freeze-tape-cast electrodes are nearly twice as thick as the normal tape cast electrodes. We show that these engineered porous electrodes of MoLTO have improved energy and power density compared with randomly oriented uniform porosity electrodes – feature of the current generation of battery electrodes.

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton and deuteron transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures in proton and deuteron form respectively. Ion currents up to 1 A / cm2 were obtained for proton transfer across graphene, and were diminished by a factor of 14 for otherwise identical cells pumping deuterium ions. Proton / deuterium pumping rates were nearly identical to each other for cells having no graphene. Arrhenius analysis of variable temperature studies of deuterium pumping across Nafion | graphene | Nafion membranes yielded an activation energy of approximately 0.5 +/- 0.1 eV for deuterium transfer across graphene. Figure 1

Hydrogen pump cells fabricated from Nafion membranes and Pt-on-carbon electrodes were used to measure rates of proton transfer across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures. Single-layer graphene prepared by chemical vapor deposition onto copper foil was sandwiched between Nafion membranes by a series of hot-pressing steps coupled with chemical etching to remove the copper. Proton current densities in excess of 1 A / cm2 were observed for proton transfer across the graphene at cell bias voltages less than 200 mV. Confocal Raman spectroscopy measurements of the Nafion / graphene sandwich structures are consistent with single-layer graphene having very few structural defects embedded between Nafion membranes. Ion-transfer rates for lithium, sodium, potassium, and other cations were also measured and found to be much lower than rates for proton transfer, in most cases by many orders of magnitude. Two-layer and few-layer (3-5 layer) graphene sandwich structures were also studied and found to give proton currents much lower than single-layer graphene. Proton currents are described in terms of an area-normalized resistance following correction for all other contributions to series resistance in the cells. Resistances are then analyzed in terms of a rate constant for proton self-exchange across graphene. Figure 1

A major challenge in effectively treating infections is to provide timely diagnosis of a bacterial or viral agent. Current cell culture methods require > 24 hours to identify the cause of infection. The Toll-Like Receptor (TLR) family of proteins from the human immune system can rapidly identify classes of pathogens and has been shown to work well in an impedance-based biosensor, where the protein is attached to a Au electrode via a self-assembled monolayer (SAM). While the sensitivity of these sensors has been good, they contain a high resistance (>1 kΩ) SAM, generating relatively small signals and requiring longer data collection times, which is ill-suited to implementation outside of a laboratory. Here, we describe a refined approach to increasing the signal magnitude and decreasing the measurement time of a TLR-4 biosensor by inserting a redox-active ferrocenyl-terminated alkane thiol into a mixed SAM containing hydroxyl- and carboxyl-terminated alkane thiols. The introduction of ferrocene into the SAM provides an avenue for the electrochemical mediation of soluble redox couples. This dramatically increases the measured current across the SAM when compared to SAMs that do not contain such a redox-active moiety, with confirmation of the intended surface modification of the Au electrodes obtained using a variety of advanced surface techniques. It is shown that these TLR-4 biosensors, utilizing the ferrocene-based SAMs, exhibit a ≤ 1 kΩ interfacial resistance that can be measured in less than one minute. These biosensors respond selectively to their intended target, Gram-negative bacteria, while remaining insensitive to Gram-positive bacteria or viral particles. These sensors were further interrogated with an inexpensive (< $100) open-source hand-held potentiostat that could be easily employed in a non-laboratory setting, including in remote or third-world applications. Figure 1

Battery electrode microstructure strongly affects the mixed ionic / electronic conduction in electrodes that is necessary to achieve rapid charging / discharging, especially in very thick electrodes. Thick electrodes having low porosity will not provide access of the full electrode structure to electrolyte ions, which will limit the effective capacity at high rates. One strategy for combating this limitation is to incorporate macroscopic pores into the electrode. Electrolyte-filled macropores provide an electrolyte reservoir that can provide ions needed for rapid charging / discharging of a thick electrode, due to the shorter overall distance that ions have to travel during charge / discharge. This talk will present recent work on the use of freeze tape casting with water-based electrode slurries to create thick electrodes having large macroscopic through-pores that run perpendicular to the electrode plane. Electrodes having similar mass loading can be charged / discharged more rapidly when these macropores are present, compared with electrodes that do not have macropores. Implications of the work for fabrication of very thick electrodes capable of rapid charging / discharging will be discussed. Figure 1

Electron-transfer mediation reactions via surface-confined redox molecules in monolayer assemblies on electrodes have been studied for decades, but treatments that consider the electrode reaction in terms of microscopic rates for the relevant reactions are not so common. The critical reactions are (1) electron transfer between the immobilized redox molecule and the electrode, and (2) electron exchange between the immobilized redox molecule on the electrode and a second redox molecule in solution. Analytical solutions are available for limiting cases, e.g. no concentration polarization or convective mass transfer via electrode rotation, but not for the general case of cyclic voltammetry in quiescent solution, with full accounting of concentration polarization during the scan. This talk will present a spreadsheet-based digital simulation of this specific situation. A key element needed for the simulation is the surface flux condition, which is solved analytically as a kinetics problem considering reactions 1 and 2 above with a steady-state assumption. Simulated voltammograms will be presented and compared with experimental data for ferrocene-containing monolayers mediating ferrocyanide oxidation in solution. The simulation model allows for prediction of trends with respect to systematic changes in ferrocene surface coverage, ferrocyanide concentration, relative redox potentials for ferrocene and ferrocyanide redox, ferrocene oxidation / reduction rate constant, potential scan rate, and other parameters. Application in electrochemical biosensing at monolayers that also contain bioaffinity ligands will also be considered. Figure 1

not Available.

not Available.

not Available.

not Available.