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The confinement-dependent properties of solvents, particularly water, within nanoporous spaces impart unique physical and chemical behavior compared to those of the bulk. This has previously been demonstrated for a U(VI)-based metal organic nanotube that displays ice-like arrays of water molecules within the 1-D pore space and complete selectivity to H

Predicting the redox behavior of magnetite in reducing soils and sediments is challenging because there is neither agreement among measured potentials nor consensus on which Fe(III) | Fe(II) equilibria are most relevant. Here, we measured open-circuit potentials of stoichiometric magnetite equilibrated over a range of solution conditions. Notably, electron transfer mediators were not necessary to reach equilibrium. For conditions where ferrous hydroxide precipitation was limited, Nernstian behavior was observed with an

Low-valent uranium coordination chemistry continues to rely heavily on access to trivalent starting materials, but these reagents are typically prepared from uranium turnings, which are becoming increasingly difficult to acquire. Here we report convenient syntheses of UI3(THF)4 (THF = tetrahydrofuran) and UBr3(THF)4 from UCl4, a more accessible uranium starting material that can be prepared from commercially available uranium oxides. UCl3(THF)2 (1), UBr3(THF)4 (2), and UI3(THF)4 (3) were prepared by single-pot reductions from UCl4 using KH and KC8 and converted to 2 or 3 by halide exchange with the corresponding Me3SiX (where X = Br or I). Reduction of UI4(Et2O)2 (4; Et2O = diethyl ether) and UI4(1,4-dioxane)2 (5) was also shown to cleanly yield 3. Complex 1 was also synthesized separately by the addition of anhydrous HCl to U(BH4)3(THF)2, which was prepared by thermal reduction of U(BH4)4. All three trivalent uranium halide complexes were isolated in high crystalline yields (typically 85-99%) and their formulations were confirmed by single-crystal X-ray diffraction, elemental analysis, and 1H NMR and IR spectroscopy. Elemental analysis conducted on triplicate samples of 1-3 exposed to vacuum for different time intervals revealed significant THF loss for all three complexes in as little as 15 min. Overall, these results offer expedient entry into low-valent uranium chemistry for researchers lacking access to uranium turnings.

The neptunyl (Np(v)O2+/Np(vi)O22+) cation is the dominant form of 237Np in acidic aqueous solutions and the stability of the Np(v) and Np(vi) species is driven by the specific chemical constituents present in the system. Hydrogen bonding with the oxo group may impact the stability of these species, but there is limited understanding of how these intermolecular interactions influence the behavior of both solution and solid-state species. In the current study, we systematically evaluate the interactions between the neptunyl tetrachloride species and hydrogen donors in coordination complexes and in the related aqueous solutions. Both Np(v) compounds (N2C4H12)2[Np(v)O2Cl4]Cl (Np(V)pipz) and (NOC4H10)3[Np(v)O2Cl4] (Np(V)morph) exhibit directional hydrogen bonding to the neptunyl oxo group while Np(vi) compounds (NC5H6)2[Np(vi)O2Cl4] (Np(VI)pyr) and (NOC4H10)4[Np(vi)O2Cl4]·2Cl (Np(VI)morph) assemble via halogen interactions. The Raman spectra of the solid-state phases indicate the activation of vibrational bands when there is asymmetry of the neptunyl bond, while these spectral features are not observed within the related solution phase spectra. Density functional theory calculations of the Np(V)pipz system suggest that activation of the ν3 asymmetric stretch and other combination modes lead to additional complexity within the solid-state spectra. Electrochemical analyses of complexes in the solution phases are consistent with the results of the crystallization experiments as the voltammetric potentials of Np(v)/Np(vi) complexes in the presence of protonated heterocycles differ from the potentials of pure Np(v) and may correlate with the hydrogen bonding interactions.

Electrode kinetics for the hydrogen evolution reaction (HER) on glassy carbon electrodes are inherently slow. Voltammetric responses are marked by large overpotentials η and small exchange current densities j0. HER rates are markedly increased on glassy carbon electrodes modified with composites of Nafion® and siloxane coated micromagnets. Comparison of linear sweep voltammograms of glassy carbon electrodes modified with either Nafion films or composites of magnetized iron oxide microparticles identifies enhanced HER rates where magnetic gradients are established. For magnetized 1 𝜇m γ-Fe2O3 microparticles in Nafion, η is decreased by 0.191 ± 0.019 V at 0.4 mA cm-2 compared to Nafion films. This corresponds to an energetic advantage of -18.4 kJ/mol and a 40-fold increase in j0. For magnetized 5 𝜇m Fe3O4 microparticles in Nafion, η is decreased by 0.28 V at 0.4 mA cm-2, which corresponds to an energetic advantage of -27 kJ/mol and 230-fold increase in exchange current. HER rate on platinum electrodes is unchanged for Pt electrodes modified with Nafion films and with composites of magnetized micromagnets in Nafion. Enhancements are not due to either magnetohydrodynamics or mediation as there is no bulk solvent volume in Nafion to convect and siloxane coating renders the iron oxide microparticles chemically and electrochemically inert. Voltammetry for glassy carbon electrodes modified with Nafion films and with composites of Nafion and demagnetized microparticles are comparable. The chemistry of magnetized and demagnetized composites are the same; the rate enhancements arises from the physical impact of the magnetic gradients in the magnetized composites. The enhanced rate for HER on glassy carbon arises through magnetoelectrocatalysis. Work is undertaken at the University of Iowa. The National Science Foundation (NSF CHE-1309366 and NSF CHE-0809745) and the Army Research Office (W911NF-19-1-0208 (74912-CH-II)) supported these projects.

Electrochemical systems related to energy storage and generation have inherent thermodynamic advantages over thermal energy systems because electrochemical systems have do not require moving parts. Thermodynamic Carnot limitations restrict internal combustion engines (ICEs) to about 40% theoretical energy conversion efficiencies. Thermodynamically, electrochemical energy systems can be 100 % efficient. The high thermodynamic efficiency is however challenged by dynamics of kinetics and transport. Because electrochemical energy systems generally operate at lower temperatures and pressures than ICEs, greater finesse in design of catalysis is needed. Electrochemical energy systems made 40% more efficient would substantially advance the economic and environmental advantages of electrochemical electrochemical energy. Consider mechanisms of physical catalysis where a physical gradient rather than a chemical composition drives chemical change. Magnetoelectrocatalysis is an example of physical catalysis that may provide means to enhance the efficiency of electrochemical energy systems. Magnetoelectrocatalysis exploits magnetic gradients imposed at electrode surfaces to facilitate electron transfer and so electrocatalysis. Here, magnetoelectrocatalysis is shown to increase energy, power, and conversion efficiency of several electrochemical energy systems by 40%. Examples include: Proton exchange membrane (PEM) fuel cells Alkaline batteries (MnO2|Zn) Hydrogen evolution reaction (HER) at glassy carbon MnO2 supercapacitors Magnetic gradient effects on electrochemical efficiency are also observed for several environmentally relevant electrode reactions. CO oxidation on Pt HER on various metal electrodes and photocathodes C1 reactions at rare earth electrocatalysts From these outcomes, it is suggested that magnetoelectrochemical catalysis may provide a path to substantially more efficient electrochemical energy systems, perhaps approaching 40%. Work is undertaken at the University of Iowa. The National Science Foundation (NSF CHE-1309366 and NSF CHE-0809745) and the Army Research Office (W911NF-19-1-0208 (74912-CH-II)) supported these projects.

Metal phosphides from the 3d period exhibit a range of structures and compositions. Many metal-rich phosphides and monophosphides function as heterogeneous electrocatalysts in the hydrogen evolution reaction. This paper describes the direct and tin flux-assisted synthesis of phosphorus-rich metal phosphides with MP2 or MP3 compositions. The facile synthesis of FeP2, CoP3, NiP2, and CuP2 is thermochemically driven by PCl3 formation from reactions of anhydrous metal halides and P4 vapor at 500 °C. Well-crystallized micrometer-sized particles result from these solvent-free reactions. A tin flux leads to more complete reactions at lower temperature for FeP2 and enables synthesis of a monoclinic polymorph of NiP2 rather than the kinetic cubic product formed by direct reaction. These crystalline metal phosphides are investigated as electrocatalyts for hydrogen evolution in acidic and buffered aqueous solutions. All phosphorus-rich products show very good stability in strongly acidic media. The catalytic activit...

Electrocatalysts for hydrogen evolution and oxidation (HER and HOR) reactions, oxygen reduction and evolution (ORR and OER) reactions, and carbon dioxide reduction reactions (CO2RR) are evaluated by Tafel analysis. The Tafel equation specifies the log-linear relationship between current and overpotential 𝛈. Heterogenous electron transfer parameters of exchange current density j o and transfer coefficient 𝛂 are found. Standard heterogenous electron transfer rate k 0 can be found from j o. Conventionally, Tafel analysis is an extension of the Butler-Volmer equation applied at high overpotentials but where mass transport is not significant and the reverse reaction rate is negligible. Applicable at high 𝛈 when electron transfer rates are slow, kinetic parameters are extracted by linear regression. The conventional method is, however, subject to inaccuracies because the linear region is often determined subjectively, without attention to the constraints on overpotential range, no mass transport limitations, and low j o. An algorithm is developed to automate Tafel analysis with the objective to increase measurement accuracy and decrease subjective identification of the linear region. From linear sweep voltammograms (LSVs), j o and α are determined from Tafel slopes in the best fit, linear range. Comparisons of kinetic parameters between conventional and algorithmic Tafel analyses are made for the hydrogen evolution reaction (HER, 2H+ + 2e- ⇌ H2) on various unmodified electrodes and electrodes modified with Nafion® composites. The algorithmic Tafel analysis parameters correlate well with conventional Tafel analyses that respect constraints on mass transport, 𝛈, and j o. Similar agreement is observed between literature and algorithmically fitted kinetic parameters for different electrochemical systems. The algorithm allows for straightforward, rapid Tafel analysis for improved measurement of rate parameters that is independent of user bias in selection of the linear region. Acknowledgments This work was supported by the Army Research Office.

In 1972, Trasatti compiled the exchange current densities j0 and work functions 𝚽 for the hydrogen evolution reaction (HER) on 31 polycrystalline metals at pH 0. Exchange current density measures the HER rate and the work function measures the energy required to remove an electron from the surface of the metal to a point outside the metal. Trasatti showed a plot of log j0 vs 𝚽 linear for the so called d metals and for the sp metals. The slopes are statistically the same for the d and sp metals (6.44 and 6.6 excludes Hg) but the intercepts differ (-35.4 and -38). Here, a thermodynamic specification for the slope of log j0 versus 𝚽 is suggested as 𝛂F/RT, where 𝛂 is the transfer coefficient for heterogeneous electron transfer. Electrochemical potentials for species i in phase j have been used to derive the rates of heterogeneous electron transfer within a transition state context for Butler Volmer kinetics (Bard and Faulkner). The standard chemical potential, activity, and ion charge for species i are 𝝁i 0,j, ai j, and zi is the electrical potential is 𝝓j. Extrapolation of the electrochemical potential to include 𝚽 and derive a rate expression for j0 yield: The electrode potential and the work function group into a common term. The slope of log j0 versus 𝚽 as 𝛂F/RT. An explanation of how Pt with the highest energetic cost to remove an electron 𝚽 yields the highest log j0 is presented. At 25 oC, F/RT = (0.05916 V)-1. Within the electrochemical potential model for log j0 versus 𝚽, the slopes of 6.44 and 6.6 for the d and sp metals at room temperature yield 𝛂 of 0.381 and 0.39. Across 30 metals, 𝛂 is estimated the same. The value of 0.4 is common in measurements of 𝛂 found for the potential dependent term in Butler Volmer kinetics. References Trasatti, S. Work Function, Electronegativity, and Electrochemical Behavior of Metals. III. Electrolytic Hydrogen Evolution in Acid Solution. Electroanalytical Chemistry and Interfacial Electrochemistry 39, 163-184 (1972). A.J. Bard and L. R. Faulkner, Electrochemical Methods, 1980, wiley and Sons, Chapter 2. Acknowledgments This work was supported by the National Science Foundation and the Army Research Office. The University of Iowa Obermann Center for Advanced Studies is acknowledged.

The neptunyl (Np(v)O

Cyclic voltammetry was applied to investigate the permselective properties of electrode-supported ion-exchange polymer films intended for use in future molecular-scale spectroscopic studies of bipolar membranes. The ability of thin ionomer film assemblies to exclude mobile ions charged similarly to the polymer (co-ions) and accumulate ions charged opposite to the polymer (counterions) was scrutinized through use of the diffusible redox probe molecules [Ru(NH3)6]3+ and [IrCl6]2−. With the anion exchange membrane (AEM) phase supported on a carbon disk electrode, bipolar junctions formed by addition of a cation exchange membrane (CEM) overlayer demonstrated high selectivity toward redox ion extraction and exclusion. For junctions formed using a Fumion® AEM phase and a Nafion® overlayer, [IrCl6]2− ions exchanged into Fumion® prior to Nafion® overcoating remained entrapped and the Fumion® excluded [Ru(NH3)6]3+ ions for durability testing periods of more than 20 h under conditions of interest for eventual in situ spectral measurements. Experiments with the Sustainion® anion exchange ionomer uncovered evidence for [IrCl6]2− ion coordination to pendant imidazolium groups on the polymer. A cyclic voltammetric method for estimation of the effective diffusion coefficient and equilibrium extraction constant for redox active probe ions within inert, uniform density electrode-supported thin films was applied to examine charge transport mechanisms.

Behavior of nanoconfined water in porous materials has important implications for the development of advanced water purification and storage. In the current study, the kinetics of water sorption from the vapor phase into a metal organic nanotube ((C4N2H6)[(UO2)(C4O4NH5)(C4O4NH6)]·2H2O (UMON)) are investigated with varying relative humidity. The UMON compound contains nanoconfined water molecules arranged in an ice-like array along the length of its one-dimensional pore and exhibits complete specificity to liquid water. Total hydration of the material is observed upon exposure to relative humidity of 60% or higher. Water uptake curves are modeled as diffusion and irreversible condensation in the pore, which leads to a modeled diffusion coefficient of (1.2 ± 0.6) × 10−12 cm2 s−1 for water in UMON nanochannels. This value is much lower than observed for other porous material and is most similar to water diffusivity in low-density amorphous ice. In addition, on exposure to various solvent vapors, the UMON material maintained specificity for water in the gas phase.

Lanthanides are common in catalysis and advanced technologies, but difficult to assess voltammetrically. Voltammetry for seven lanthanide(III) trifluoromethanesulfonate complexes Ln(OTf)3 is enabled at Nafion® modified platinum electrodes in acetonitrile. Voltammetric morphologies for La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, and Yb3+ trifluoromethanesulfonate (triflate) complexes exhibit reduction waves between 0.2 to 0.1 V and −0.7 to −0.8 V vs NHE with coupled chemical processes.

In a thin layer sonoelectrochemistry cell, a simple high frequency (ultrasonic) oscillator placed a short distance from a flat electrode surface focuses ultrasound at the electrode solution interface to impact rates of interfacial processes. Traditionally, sonoelectrochemistry in a bulk fluid uses high energy oscillators such as sonic horns that drive turbulent cavitation. In the thin layer arrangement, no cavitation is visible but rates of interfacial electrochemical events are enhanced. Effective thin layer sonoelectrochemistry requires attention to properties of the solvent and electrode material, frequency and amplitude of the oscillator, and the alignment and distance between the faces of the oscillator and the electrode. When properly configured, the thin layer sonocell enhances energy delivered to the fluid electrode interface through constructive interference. Because there is no cavitation, voltammetric morphologies remain similar to those in quiescent electrolytes. Here, conditions to set up an effective thin layer sonoelectrochemical cell are described.

Traditional sonochemistry (TS) in bulk solution is violent and expensive. Typically, a high power (> 60 W) ultrasonic horn is directed at a planar working electrode. Compression and rarefaction oscillations elicit cavitation bubble formation. Upon bubble collapse, high temperature (> 5000 K) and pressure (> 1000 atm) are produced locally, which enhances rates of chemical reactions. Bubble collapse near the electrode causes solvent jets directed at the surface to effect the surface cleaning and mass transport enhancements attributed to TS. While productive, TS obscures quantitative measurement of electron transfer kinetics and mass transport and requires a large laboratory footprint. An alternative and efficient sonochemistry is needed. Thin Layer Sonochemistry (TLS) exploits natural acoustics in a thin layer electrochemical cell. Low power transducers (e.g., quartz crystal oscillators) are appropriate because TLS achieves resonance in the cell to amplify input sound to the limit set by the solvent. TLS is more efficient and allows finer control of experimental and acoustic parameters. The footprint of TLS is minimal, and so TLS is appropriate for energy production applications (e.g., fuel cells) in portable devices. Experience suggests a subtle relationship between acoustic parameters and resonance. Here, a model is presented for calculation and explanation of resonance in a thin layer sonochemical cell. Background physics are explained and a step-by-step assembly guide is presented. References: Suslick, K. S.; Didenko, Y.; Fang, M. M.; Hyeon, T.; Kolbeck, K. J.; McNamara, W. B. III; Mdleleni, M. M.; Wong, M. "Acoustic Cavitation and Its Chemical Consequences" Phil. Trans. Roy. Soc. London A, 1999, 357, 335-353. Johna Leddy, Chester G. Duda, Jacob Lyon, and William J. Leddy III, US Patent 10,300,453 B2, "Thin Layer Sonoelectrochemistry and Sonoelectrochemistry Devices and Methods," 28 May 2019. D.L. Parr IV and J. Leddy, "Sonoelectrochemistry - A Sketch of Thin Layer Advantages," ECS Trans. (2019) 92(10), 65-73; Open Access, DOI:10.1149/09210.0065ecst

Electrochemical reactions drive many chemical processes using well established protocols and hardware. Most improvements of electrochemical reactions require substantial changes to existing protocols and equipment, However, our previous work has shown that electrochemical systems may be impacted by the presence of magnetic field gradients at the electrode surface. Using ion exchange polymer films with embedded magnetic microparticles, it is possible to improve reaction kinetics. Previous cyclic voltammetric studies of metal tris-bipyridine complexes at electrodes modified with composite films of Nafion and magnetized microparticles exhibit substantially higher currents than at electrodes modified with simple Nafion films. The aim of this study is to show magnetic field gradients can enhance currents for redox probes other than metal tris-bipyridine complexes and to use methods alternative to cyclic voltammetry (CV). Iron (III) perchlorate is the first transition metal species evaluated by impedance to exhibit enhanced electron transfer rates at electrodes modified with magnetized microparticles in Nafion. The enhancement is attributed to improvements in kinetics of heterogeneous exchange rate and self exchange rate. Electrochemical impedance spectroscopy (EIS) and CV studies at magnetically modified electrodes are compared to non-magnetic analogs for the iron (III) perchlorate redox couple. This redox couple shows kinetic rate enhancements for the magnetically modified electrodes over the non-magnetic analogs.

Square wave voltammetry (SWV) is used to investigate fast electron transfer kinetics across a wide range of redox active species. SWV is an inherently faster, frequency dependent method as compared to most potential sweep methods. Sweep methods commonly implement an applied potential as a staircase of small potential steps. The frequencies of SWV can be related to sweep methods as the product of SWV frequency and the potential increment at each staircase step. Recent work in our group has shown that k0 measurements for tris-bipyridine transition metal complexes can be made using SWV as the wave morphology changes with k0. Heterogeneous electron transfer rates are not reliably measured at the slower voltage perturbations (scan rates) available with cyclic voltammetry (CV). The heterogeneous electron transfer rates k0 for M(bpy)3 z+ complexes, where M = Ru2+, Os2+, Cr3+, Co2+, Co3+, and Fe2+ are sufficiently rapid that k0 is not reliably measured by CV for most of these complexes. Analysis of the SWV data is possible but not as facile as analysis of CV data. To better evaluate k0 measurements, an alternative data analysis algorithm for quasireversible heterogeneous electron transfer is developed based on classical finite difference simulations. The algorithm can be envisioned as converting voltammograms between CV and SWV. Because CV and SWV both contain a potential parameter and a time parameter (mV/s in CV; frequency and step height in SWV), the algorithm uses a “universal scan rate” to convert between all of the other potential and time-dependent parameters. The purpose of the algorithm is to simplify quantitative SWV data analysis and present information in context of the more commonly used CV.

Tafel analysis is widely used to characterize electrochemical kinetics and assess the properties of electrocatalysts for use in fuel cells, electrolyzers, and other applications. Conventional Tafel analysis is an extension of the Butler-Volmer equation at high overpotentials under conditions where mass transport is not significant and the reverse reaction rate is negligible compared to the studied half-reaction. Determination of kinetic parameters from a Tafel plot involves linear regression in regions of large overpotential. This method is limited in part by the subjective determination of linearity, as the kinetic parameters obtained by the regression may vary significantly depending on the chosen linear region. In an effort to increase measurement quality and decrease subjectivity, an algorithm has been developed that generates a Tafel plot from a linear sweep voltammogram (LSV) and determines the exchange current density j 0, charge transfer coefficient α, and Tafel slope of closest fit. Comparisons of kinetic parameters between conventional and algorithmic Tafel analysis are made for the hydrogen evolution reaction (HER, 2H+ + 2e- → H2) on different metal electrodes and Nafion® composite electrodes. The algorithmic Tafel analysis parameters correlate well with conventional methods. Similar agreement is observed between literature and algorithmically fitted kinetic parameters for different electrochemical systems. The developed algorithm allows for straightforward, rapid, and user bias independent Tafel analysis and can be easily used to increase measurement quality.

Technologically, the hydrogen evolution reaction (HER) is the foundation of many electrochemical energy systems. Fundamentally, HER is important as 2H+ + 2e ⇌ H2 defines the standard reference potential E0 H = 0.000 V in thermodynamics. All evolutions in kinetic theory are critically tested against HER. In electrocatalysis, the HER mechanism is historically specified in three steps. The first step, Volmer, couples concerted adsorption and reduction of proton to form electrochemically adsorbed hydrogen. H+ + e ⇄ Hads (Volmer) (1) Once the first proton electroadsorbs, H2 forms by two parallel pathways. The Heyrovsky step proceeds as a second proton is electroadsorbed to the same metal atom and hydrogen gas desorbs from the common metal center. Hads + H+ + e ⇄ H2 (Heyrovsky) (2) In the Tafel step, electrochemically adsorbed hydrogen forms on an adjacent metal center and H2 desorbs. Hads + Hads ⇄ H2 (Tafel) (3) Commonly, HER electrode kinetics are evaluated according to the Volmer-Heyrovsky-Tafel scheme, where rate is measured as exchange current densities j₀. For HER at metal electrodes, log j₀ depends strongly on the electrode metal where log j₀ falls between -3 for Pt, Re, and Pd and -12 for Hg. Although metals are not explicit in the Volmer-Heyrovsky-Tafel scheme, j₀ is evaluated in view of Steps (1) to (3). Here, metal electrode properties are introduced as the standard potential E0 M of the electrode metal. For metal cation reduction to metal M⁰, Mz+ + ze ⇌ M⁰ E0 M log j₀ values are cataloged in a paper by Trasatti (Electroanalytical Chemistry and Interfacial Electrochemistry 1972, 39, 163-184), where he partitioned 31 metal electrodes into d and sp metals. For the transition metals, the d metals, log j₀ is well and linearly correlated with E0 M . For the sp metals, there is little to no correlation of log j₀ with E0 M . For the transition metals, several observations and a model sketch are noted. HER rates as log j₀ increase with more positive values of E0 M . For the initial step of the metal dependent HER process, it is sketched that electron(s) release from M0 to form Mz+ immediately at the electrode surface. For this initial step, formation of Mz+ immediately at the electrode solution interface yields log j₀ linearly dependent on E0 M . Within transition state or activated complex theory, a proposed transition state is formed of electron(s) shared between Mz+ and H+. Formation of the transition state [(1/z)Mz+⋯e⋯H+]‡ precedes the Volmer step, and where applicable, is a rate determining step. Overpotential η pinned by E0 M may characterize the counter intuitive observation that rate (log j₀) increases as E0 M is more positive. This sketch of HER yields a kinetic model where a component of the activation energy is set by F(E0 H -E0 M ). The sketch of an electrode dependent transition state may pose a general approach to characterizing electrocatalysis and interfacial electron transfers. The proposed transition state introduces physical properties of specific electrode materials into the rate expressions a priori.

Polymer electrolyte fuel cells (PEFCs) are often formed with the cation exchange polymer Nafion® sandwiched between two electrodes and run on humidified H2 and O2 gases fed to electrodes held in tight gaskets. The tight gasket opening is only slightly wider than the electrodes. When run on dry H2 and O2, tight gasketed cells fail as water becomes imbalanced across the membrane. Here, the cathode is framed by a narrow perimeter washer so the cathode is centered in a generous Nafion window and surrounded by a thin (≈ 0.05 cm) reservoir volume. Unlike tight gasketed cells, water generated at the cathode crosses the large Nafion window to sustain self humidification at 0.522 V on dry H2 and O2 at 70, 37, and 25 oC and dry H2 and air at 37 oC for 1 atm pressure and moderate flow rates. Built from commercial components, window gasketed PEFCs provided stable, steady state current and power at 0.5 V for >170 h on dry H2 and oxidant. Tight gaskets always fail to yield self humidification whereas window gaskets enable effective self humidification on dry gases. A sketch is provided of how the Nafion window enhances water flux from the cathode. Because humidification hardware can be eliminated, window gasketed PEFCs may be especially useful in portable fuel cell systems. Submitted In Honor of Andrej Wieckowski.

The hydrogen evolution reaction (HER) is critical technologically in electrochemically energy systems. HER is also important fundamentally as 2H⁺ + 2e ⇌ H₂ provides the standard reference potential E0 H = 0.000 V in thermodynamics and HER serves as a primary and critical test case for kinetics theory. For HER electrocatalytic rates, the mechanism is historically broken down into three steps. The reaction sequence begins with adsorption and reduction of proton to form electrochemically adsorbed hydrogen. This is the Volmer step. H⁺ + e ⇄ Hads (Volmer) After the first Hads is formed, H2 can be formed by two pathways. In the Heyrovsky pathway, a second proton undergoes electrochemical hydrogen adsorption at the same metal atom to form hydrogen gas. Hads + H⁺ + e ⇄ H2 (Heyrovsky) In the Tafel pathway, electrochemically adsorbed hydrogens on adjacent metal atoms react to form H2. Hads + Hads ⇄ H2 (Tafel) HER electrode kinetics measured as exchange current densities j₀ are evaluated in accord with the Volmer-Heyrovsky-Tafel scheme. For HER at metal electrodes, log j₀ is known highly dependent on the metal of the electrodes with rates that vary > 10 orders of magnitude. HER data are evaluated according to these three steps, despite that properties of the metal are not explicit in the Volmer-Heyrovsky-Tafel scheme. Here, properties of the metal electrode are introduced through the standard potential E0 M. For metal M0, Mz+ + ze ⇌ M⁰ E0 M Data for log j₀ are taken from a paper by Trasatti (Electroanalytical Chemistry and Interfacial Electrochemistry (1972) 39, 163-184). Trasatti grouped the 31 metal electrodes into d and sp metals. For the d metals (transition metals), log j₀ is well and linearly correlated with E0 M, whereas for the sp metals, little to no correlation is found. Several points are considered. Higher HER rates are observed at transition metal electrodes with more positive values of E0 M. It is sketched that the initial step in the metal dependent HER process is release of electron(s) to form Mz+ immediately at the electrode surface. Where formation of Mz+immediately at the electrode solution interface is the initial step, log j₀ is anticipated linearly dependent on E0 M. The behavior is sketched within transition state or activated complex theory and is based on a transition state formed of electron(s) shared between the metal cation Mz+ and the proton H+. Formation of the transition state [(1/z)Mz+⋯e⋯H+]‡ precedes the Volmer step, and where applicable, is a rate determining step. Implications for overpotential η pinned by E0 M are considered. This may account for the counter intuitive observation that higher log j₀ are found on metals such as Pt where E0 M is more positive. A kinetic model is sketched where a component of the activation energy is set by F(E0 H-E0 M). Finally, the process of proposing an electrode dependent transition state and evaluating the thermodynamic and kinetic consequences may yield a general method for modeling interfacial electron transfer and electrocatalysis. Such a process may introduces electrode specific properties into the rate expressions a priori.

Spreadsheets enhance student acquisition of electrochemical concepts at both undergraduate and graduate levels. The interactive nature of compu-graphical methods allows students more rapid and therefore extensive exploration of electrochemical concepts in thermodynamics, kinetics, and electrochemical methods. Here, several instructional tools are described with focus on the potential axis. Potential Axis - A Spreadsheet for Thermodynamics, Voltammetry, and Electrolysis For reactions A + ne ⇌ B at standard potential EA⁰ and C + me⇌D at EC⁰, calculation of standard cell potential Ecell⁰ = EA⁰ -EC⁰ for net reaction mA + nD ⇌ mB + nC yields thermodynamic information of standard free energy for the reaction ΔGcell⁰ = -nmFEcell⁰. In turn, ΔGcell⁰ identifies spontaneity when ΔGcell⁰ < 0 and equilibrium when ΔGcell⁰ = 0. When more than two reactions are possible or when pH changes for proton dependent reactions or oxygen is present or not, developing chemical intuition about the system and identifying stable species and thermodynamic reaction energies is more complicated. By plotting the reactions on a potential axis, thermodynamic assessment is readily obtained. Under IUPAC conventions with positive potential E to the right, reaction A + ne ⇌ B is plotted as B|A at potential EA⁰ on the potential axis. On addition of a second reaction C + me ⇌ D where EA⁰ < EC⁰, a simplified potential axis is B|A........D|C where potential is positive to the right. When chemical species are on the outside of the E⁰ lines, reaction is spontaneous. For example, if the axis for the A B couple and the C D couple is B|A........D|C , B and C react to form A and D spontaneously. Any other combinations of species are thermodynamically stable (B+A+D and C+D+A). A potential axis is shown below for several copper species at pH = 0. Because Cu and O₂ are outside the E⁰ vertical axes at 0.34 and 1.23 V vs NHE, Cu reacts spontaneously with O₂ based on thermodynamic potentials ECu²⁺|Cu⁰ and EO₂|H₂O⁰. Several other observations about copper electrochemistry are of note. Copper metal Cu is stable in strong acid. Cu⁺ is not observed in water because Cu⁺ is outside the vertical E⁰ lines for Cu⁺|Cu²⁺ and Cu|Cu⁺, so Cu⁺ reacts spontaneously with itself (disproportionation) to form copper metal and cupric cation. Cu is stable in HCl but not oxidizing HNO₃. Nickel metal and Zn metal (Zn|Zn²⁺ not shown at -0.76 V vs NHE) react spontaneously with both HCl and HNO₃. Potential axes are also useful for mapping voltammetry and identifying the sequence of electrolysis steps. Consider a system that contains Cu, Cu²⁺ and Cl⁻ in an aqueous system that is not degassed (contains O₂) at pH = 0. Based on the reactions plotted on the potential axis for copper, the species that are present and can be oxidized will be oxidized in sequence as potential E is swept from negative to positive. Species are oxidized in the sequence Cu then H₂O; because water is the solvent, chloride will not be accessible for oxidation to chlorine. Species that are present and can be reduced, are reduced in ranked in order for E swept from positive to negative as O₂, Cu²⁺, and H⁺. If pH changes, the formal potentials E⁰' for O₂, H⁺, and NO₃⁻ will also shift and the order of reactions can change. Note that as shown, neither concentrations other than standard conditions nor metal ions precipitated as oxides and hydroxides are not included but can be introduced. Several other ideas will be discussed briefly. Compu-graphical finite difference simulations for cyclic voltammetry for use at the undergraduate and graduate levels. (Thanks to Alanah Fitch who first suggested the form for undergraduates.) Useful ideas to convey include: • the concept of flux as number per area per time • electrochemical systems are circuits composed of electron and ion conductors • relationship between measurement rates and kinetic rates • difference between equilibrium (thermodynamics) and steady state (kinetics) • diffusion length ℓ² ≈ Dt (diffusion coefficient and time) and use in characterizing micro- and nano-structures on electrodes ...... Figure 1

Electrophysiological measurement such as EEG and peripheral nerve conduction studies are commonly used in neurology research and medical diagnoses. Bioelectrochemical structures and reactions at the molecular to micrometer scale propagate charge through space. Electrophysiological measurements made across distances on the scale of centimeters map collective effects of many charge propagation events. In peripheral nerves, charge propagates along nerves in the limbs at ~50 m/s. Voltages measured from the surface of the skin are in the range of 6 to 8 µV for peripheral nerves and as high as 40-60 µV in EEG studies. Charge propagation events that generate measured electrophysiological signals can be viewed with the same electrostatic fundamentals that describe how potentials are established in electrochemical systems. Polarization of a synaptic membrane generates a flux of positive charge and then a charge balancing flux of negative charge as the membrane depolarizes. The flux separates charge that generates a dipole. The dipole propagates down a peripheral nerve as the membrane separates charge sequentially. A sketch of the dipole is based in electrostatics. From an electrostatic perspective, the separation and magnitude of the charges establishes an electrical potential Φ(x,y,z). Consider a single dipole at an instant in time. Evaluation of charge separation yields a map of potential in space as shown in the Figure. The gradient in the electrical potential establishes the electric field. Migration is driven by potential gradients such that the electric field may play a role in dipole propagation. The Figure is an instantaneous snapshot of potential associated with a dipole in space. In electrophysiological measurements, multiple dipoles events yield the measured voltages as nerve impulses that propagate in time along a peripheral nerve tract. A sketch of the correlations between electrophysiological measurements and electrochemical fundamentals is presented. .... Figure 1

Although diffusion is a pervasive concept in chemistry, minimal discussion in chemical education literature provides intuitive explanation of diffusion and mass transport. Explanation of diffusion is often given as a dichotomy: (1) a drunken sailor described by a discrete binomial distribution and (2) a continuous solution of the diffusion (heat) equation. As the dichotomy suggests, there is a persistent dislocation between the drunken sailor and error functions as the two explanations do not align to provide a unified and intuitive explanation of this important and fundamental concept. Here, we present an introduction to diffusion through models and demonstrations to bridge the intuition gap of diffusion in the classroom. Discrete diffusion is explained initially with the classic drunken sailor narrative and also by the Galton board (Figure 1). The Galton board provides a much improved and intuitive explanation of discrete diffusion via hands-on demonstration of the binomial probabilities. Transition into continuous diffusion evolves from an explanation of flux (Figure 2.) and concentration gradients, the drivers of diffusion. Finally, we provide a demonstration suitable for any classroom with diffusion of physical therapy putty. The broadening of a physical therapy putty disk with time is modeled with the diffusion equation. Time dependence of the putty radius is observed and compared with the standard diffusion length metric. Figure 1

In electrochemical energy systems such as fuel cells, important reactions include the oxygen reduction reaction (ORR) and methanol oxidation. Even with platinum catalysts, oxygen reduction kinetics in water at acidic pH are slow. In H₂ proton exchange membrane fuel cells (PEM FCs), ORR is the rate determining kinetic process. In direct reformation fuel cells run on methanol with an oxygen or air cathode, methanol oxidation kinetics are rate limiting. Limitations include slow electron transfer processes coupled to generation of oxidation by-products that adsorb to the electrode and lead to passivation. Sonoelectrochemistry in thin layers of electrolyte enhances electrochemical rates without visible cavitation using low power oscillators [1]. Here, thin layer sonoelectrochemistry coupled with cyclic voltammetry is used to evaluate impacts of sonication in a thin layer on the electrochemical response of oxygen and methanol . For voltammetry at platinum electrodes for saturated oxygen in acidic aqueous electrolyte, the rate of the ORR is enhanced several orders of magnitude as shown by fit of the cyclic voltammetric data. For stoichiometric (50:50) methanol in water with acid electrolyte, oxidation and reduction currents at platinum electrodes are increased almost ten fold as shown in cyclic voltammetric responses. Sonication may increase the rates of electron transfer and remove deposited partial oxidation products. Improved kinetics of oxygen reduction and methanol electrolysis may provide a means to improved electrochemical energy technologies, including hydrogen and direct reformation fuel cells. The energy needed to drive the oscillator represents a low parasitic loss on fuel cell performance. Reference [1] Johna Leddy, Chester G. Duda, Jacob Lyon, and William J. Leddy III, "Thin Layer Sonoelectrochemistry and Sonoelectrochemistry Devices and Methods," published 28 May 2015 as US Patent Application 20150147594 A1.

Of the vitamins identified by the National Institutes of Health, all 13 are electroactive. Vitamins serve as enzymes and coenzymes in metabolic reactions. As catalysts and co-catalysts, vitamins neither provide energy nor serve as structural components of larger molecules. The formal potentials E0' for the vitamins near physiological pH are plotted on a potential axis [1] as shown in Figure 1. The potential axis provides perspective on the energetics of various species. For example, E0' for the four lipid soluble vitamins A, D, E, and K lie outside potential window for water at pH 7. Vitamin C and all the B vitamins are water soluble, with vitamins C and B12 near the middle of the water window. Review of the literature reveals the vitamins are grouped mechanistically as single electron transfer reagents (B3, B7, B2, C, and D), vitamins that can be both oxidized and reduced (B1, B5, B6, B9, and E), and vitamins that undergo two successive and distinct reductions (B12 and K). Mechanistically, vitamin A is most complex. The potential axis and voltammetric mechanisms provide tools to assess and compare various properties of the vitamins. This includes: Electrochemical stability in air, water, and acid Antioxidant characteristics that include interactions with reactive oxygen species (ROS) Electrochemical instability of vitamin pairs Possible cooperative interactions between vitamins in health and medicine Possible mechanisms for vitamin recycling in situ For electrochemical mechanisms that are composed of electron transfer steps (E) and chemical steps (C), the majority of the voltammetric responses are EC mechanisms of various types. For B1, B6, B7, and B9, the reduction of the vitamin is a simple electron transfer, E. Vitamin K voltammetry is EE. Vitamins B1, B9, B5, B6, and E can be reduced and oxidized. Disproportionation of the reduced and oxidized forms can regenerate the initial vitamin species. Vitamin C has a formal potential near the middle of the water window and undergoes ring opening on oxidation in an ECirrev process. With the lowest E0' for oxidation of a vitamin, vitamin C may serve to reduce superoxide, peroxyl, and hydroxyl radicals and thus mitigate effects of ROS. The thermodynamic (E0') and mechanistic (EC) characteristics of the 13 vitamins will be surveyed from the perspective of a potential axis to highlight the similarities and distinctions of these 13 electroactive and metabolically critical species. Reference [1] M. D. Lovander, J. D. Lyon, D. L. Parr IV, J. Wang, B. Parke, and J. Leddy, CRES3T Review—Electrochemical Properties of 13 Vitamins: A Critical Review and Assessment, Journal of The Electrochemical Society, 165 (2) G18-G49 (2018). Figure 1. Potential Axis with 13 Vitamins. From reference [1]. Figure 1

Simulations of cyclic voltammograms for a system with a spatially variant diffusion profile are presented. An expanded version of Fick’s Second Law is used to account for a diffusion coefficient that varies with x, distance normal to the electrode surface. Parameters to define the slope and magnitude of a linearly graded diffusion profile are described. A one dimensional explicit finite difference simulation method is used and parameters are made to be dimensionless. The simulation is vetted by comparison with Nicholson and Shain simulations and determination of the resolution limits. Morphological changes in cyclic voltammograms are observed that indicate an approach to steady state diffusion.

Quantitative characterization of uniform density, electrochemically inert films on electrodes is achieved by cyclic voltammetry (CV) of a redox probe that partitions from electrolyte into the film. Electrochemically inert films generate no faradaic current in the voltammetric window of the probe. In simulation models, probes pre-equilibrate into films, electrolyze at electrodes, diffuse in film and solution, and extract across film solution interfaces. Film thickness is . Diffusion length δ approximates distance from the electrode where voltammetry perturbs probe concentration; δ ∝ ν−1/2 for scan rate ν. At high ν, δ < and voltammetric morphologies are typical of semi-infinite linear diffusion. As ν slows, δ and CV morphologies can change with relative probe flux in the film and solution. For higher solution flux, voltammograms assume sigmoidal (S-shaped) characteristics; higher film flux generates gaussian (thin layer CV) characteristics. For film and solution diffusion coefficients D f and Ds and κ the equilibrium ratio of probe concentration in film to solution, diagnostics yield κ √ (D f /Ds ) and 2/D f . Because diagnostics apply for all ν, films are fully parameterized by CV alone. Without these diagnostics, full characterization requires a second, steady state voltammetric measurement. Diagnostics are vetted with [Ru(bpy)3]2+ (probe) in inert polymer films of Nafion and of poly(styrenesulfonate). © 2013 The Electrochemical Society. [DOI: 10.1149/2.025306jes] All rights reserved.

Magnetically modified, dye sensitized, n-type semiconductor electrodes were examined as a case study for magnetic field effects on electron transfer reactions in electrochemical power systems. Magnetic field effects arise through kinetics and mass transport; magnetic fields have negligible influence on system thermodynamics (i.e., no effects on open-circuit potential). Enhancements in the photocurrent response of dye sensitized TiO2 semiconductor electrodes are observed.

Electron transfer events are commonly considered in light of the transfer of charge. But, electrons also have properties of spin. Electron spins interact with magnetic fields. In the presence of a localized magnetic field, electron transfer rates of electrochemical reactions change. Correlations between a redox species’ number of unpaired electrons and the change in electron transfer rate have been found. Many important reactions that include the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and carbon dioxide reduction have been shown to be kinetically limited. ORR limits performance of many fuel cell and air batteries. Increased rates of HER facilitate energy generation. The presence of lanthanides at electrode surfaces can mediate the rates of such important reactions. For example, Ytterbium triflate extracted into Nafion on an electrode in acetonitrile increases oxygen reduction currents by 15%. Despite largely similar properties, lanthanide magnetic properties are diverse because the number of unpaired electrons in the d orbitals ranges from 0 to 7. Voltammetry for lanthanides extracted into Nafion films on electrodes undertaken in the presence of some of these species will be presented. Correlations between lanthanides (and their spin and magnetic properties) and the type and magnitude of effects on important reactions will be discussed.

Recent evidence highlights a magnetic effect on electron transfer processes for the hydrogen evolution reaction (HER) [1]. To further evaluate these observations from Trasatti’s data, studies of various diamagnetic electrodes under magnetic modification were undertaken. Here, results for magnetically modified carbon black electrodes are presented. Electrodes are modified by drop casting a suspension of Nafion and carbon black onto a glassy carbon electrode. Electrodes are magnetically modified on inclusion of magnetized microparticles. HER is undertaken voltammetrically at these electrodes and evaluated for impacts on the HER rates under magnetic modification. Magnetically enhanced electron transfer rates has broad impacts on electron transfer theory and electrocatalyst design. Here, implications for design of magnetoelectrocatalysts are presented. References [1] Trasatti, S. Work Function, Electronegativity, and Electrochemical Behavior of Metals. III. Electrolytic Hydrogen Evolution in Acid Solution. Electroanalytical Chemistry and Interfacial Electrochemistry 39, 163-184 (1972).

In 1972, Trasatti carefully compiled the rates of hydrogen evolution reaction (HER) on 31 metal electrodes near pH 0 [1]. The HER is a surface reaction that can be represented as Hsoln⁺ ⇌ Hads⁺ Hads⁺ + e ⇌ Hads ∙ 2 Hads ∙ ⇌ H2,ads H2,ads ⇌ H2,soln Trasatti also defined the electrochemical work function Φ, which is the energy needed to remove an electron from the electrode surface to a redox species immediately at the electrode surface. The rates of the HER reaction are embedded in the standard exchange current density j0 (A/cm2) that measures the standard heterogeneous rate of electron transfer, k0. Trasatti demonstrated that plot of log j0 versus Φ yields two parallel lines as shown, where regression yields log j₀ = (6.4₄ ± 0.2₄) Φ (eV) - (35.₄±1.₁) with R² = 0.98 for the higher rate line and log j₀ = (6.5 ± 0.5₆) Φ (eV) - (38.₅ ± 2.₄) with R² = 0.93 for the lower rate line. The slopes are the same but the intercepts differ by 3. For a given Φ, metals on the upper line have HER rates that are 1000 x higher than metals on the lower line. In this presentation, the magnetic properties of the electrodes are shown to describe the binary segregation of the data into two discrete lines. The magnetic properties of the electrodes are set by paired and unpaired electron spins. The results are evidence of a magnetic effect on electron transfer. References [1] Trasatti, S. Work Function, Electronegativity, and Electrochemical Behavior of Metals. III. Electrolytic Hydrogen Evolution in Acid Solution. Electroanalytical Chemistry and Interfacial Electrochemistry 39, 163-184 (1972). Figure 1

It is common practice to estimate the standard (E0) or formal (E0’) potential from cyclic voltammetric data. A frequent estimate of E0 or E0’ is taken from either the peak potential (Ep), the half wave potential (E1/2), or the average of the cathodic and anodic peak potentials (Epc+Epa)/2. Often, this suffices to identify the standard or formal potential within 100 mV, but confidence in such estimates is not always justified. Mechanisms that include both electron transfer (E) and chemical (C) steps can impact the quality of estimated standard and formal potentials. In some systems, limitations to the estimated standard and formal potentials depend on the rates of heterogeneous electron transfer and homogenous reactions. Here, the quality of estimated standard and formal potentials based on peak potentials, half-wave potentials, and average potentials for the cathodic and anodic sweeps are described for various mechanisms. Cases where the standard and formal potentials are well estimated from measured cyclic voltammetric data are identified and the quality of the estimated standard and formal potentials are characterized. That is, specify whether the estimate is within 100 mV, 50 mV, or 10 mV for a given mechanism. Situations where the estimate fails are identified and alternative means to estimate the standard and formal potential are provided. Several reaction schemes that include EC, EE, CE, and the various ECE mechanisms are characterized.

Lanthanides have sufficiently similar chemical and physical properties that lanthanides are difficult to separate. Among the common properties are standard potentials that fall within 100 mV for Ln3|0. The standard potentials are sufficiently extreme that lanthanide electrochemistry is commonly undertaken in ionic liquids and molten salts. Recently, it was demonstrated that cyclic voltammetry of lanthanide triflates is accessible in acetonitrile at Nafion modified electrodes. Because at least four different oxidation states of the lanthanides are accessible, the voltammetric response is complex, as demonstrated by fitting the mechanism. Interest in benchtop electrochemistry of lanthanides arose from investigations of magnetoelectrocatalysis, where magnetic properties and spin impact rates of electron transfer at electrodes and electrocatalysts. Despite largely similar properties, lanthanide magnetic properties are diverse because the number of unpaired electrons in the d orbitals ranges from 0 to 7. The presence of lanthanides at the electrode surface mediates rates of important reactions that include the oxygen reduction reaction (ORR). Ytterbium triflate extracted into Nafion on electrode in acetonitrile increase oxygen reduction currents by 15%. On introduction of electrochemically inert hematite microparticles into Nafion, oxygen reduction currents are enhanced 35%. Other measures of catalysis that include decreased overpotential are also observed. Reference Johna Leddy and Krysti L. Knoche, Lanthanide Electrochemistry, published 12 November 2015 as US Patent Application, 20150322581-A1

Nafion (DuPont) is a microstructured ion exchange polymer with high water content. Water volume fraction estimates for as-received and proton exchanged Nafion films range from 28 to 43% by volume. The ionic conductivity and anion exclusion of Nafion are often ascribed to the highly cationic, water-filled microdomains of the polymer. A question arises: if Nafion is frozen, will the microdomains be disrupted with subsequent loss of structural integrity and its associated anion exclusion, selectivity, and conductivity? Here, a preliminary assessment of the impact of freezing recast Nafion 1100 films is reported.

Magnetic fields at electrodes increase flux through dynamics. Magnetically modified electrodes sustain a permanent magnetic field because magnetic materials are either attached to the electrode surface or incorporated into the electronic conductor of the electrode. Thus far, magnetically modified electrodes have been produced by coating electrode surfaces with composites of Nafion and magnetic microparticles. Glassy carbon and platinum electrodes as well as fuel cell electrodes modified with Nafion and magnetic microparticles exhibit enhanced flux as compared to similar nonmagnetic composites. Here, magnet incorporated carbon electrodes (MICEs) are presented. Two examples are described: magnet incorporated carbon paste electrodes (MICE-Ps) and magnet incorporated carbon epoxy electrodes (MICE-Es). Voltammetrically driven fluxes for MICEs are higher than those for the corresponding nonmagnetic controls (carbon paste (CPEs) and carbon epoxy (CEEs) electrodes).

A complete derivation is presented for concentration profiles and transition times in a thin layer cell under constant current conditions. Expressions for the concentration across a thin layer cell and the transition time are presented under conditions where the diffusion layer is extremely small (short times) and extremely large (long times) compared to the thickness of the thin layer cell. Excellent agreement with those previously reported in the literature is found. Application to thin layer spectroelectrochemical cells is demonstrated.

In typical applications of electrochemistry, Fick's second law for diffusion is established with spatially invariant diffusion coefficients. However, when the diffusion coefficient is spatially dependent, D(x), Fick's second law in one dimension, as shown below, introduces opportunities to control transport. When the diffusion coefficient varies spatially and the solution is quiescent, steady state flux of the diffusant is possible. Films of nonuniform density and viscosity on electrodes provide a matrix to establish domains with spatially varying diffusion coefficients. To explore impacts of D(x) on probe transport through films under voltammetric perturbation, several inquiries were undertaken. The impact of spatially varying diffusion coefficients is simulated by explicit finite difference for a redox probe moving through a graded film on an electrode. Voltammetric morphologies vary with the structucture of the gradient. Sigmoidal voltammograms result as cyclic voltammetric scan rate slows. Graded diffusion coefficients are established on electrodes by modification of density gradient polymers such as Ficoll^{®}. Sigmoidal voltammograms with limiting currents independent of scan rate are observed at slow scan rates, consistent with steady state transport in a domain where the diffusion coefficient is spatially varying. Ion exchange polymer Nafion cocast with a graded density polymer similarly impacts transport of a probe through the graded Nafion film and sigmoidal voltammograms are observed. The interplay between Fick's second law with spatially varying diffusion coefficients; simulation of voltammetry with D(x); and implementation of graded diffusion coefficients on electrode modification by Ficoll and by Ficoll-Nafion composites will be presented. Figure 1

Nature designs eloquent systems to effect electrochemical changes that pose daunting objectives for address by simple electrodes. In a sequence of electrochemical steps in a highly structured, bio-matrix, biological systems provide energy and kinetic control that yields important products from oxidized reactants. Examples are CO2 and water to form O2 and sugars and dinitrogen to form ammonia. By capturing these biological matrices on electrodes, the biomachines can generate products important in energy and materials. Minteer and coworkers have demonstrated many systems of this type, including capture of the entire Krebs cycle on an electrode where the various components of the cycle are captured in a modified Nafion membrane. By analogy, we have captured algae on electrodes to generate ammonia. Cyanobacteria generates ammonia from dinitrogen with nitrogenase enzyme and from fixed nitrogen of nitrate and nitrite with nitrate and nitrite reductase. Thermodynamically, the conversion of dinitrogen to ammonia is not energetically taxing, but the energetic cost to break the nitrogen nitrogen triple bond is high. In review of the kinetics for ammonia generation from N2, NO3 -, and NO2 - reactants, the thermodynamics were reviewed. From a potential axis and speciation diagram, some reactions are identified with sufficient energy to break the nitrogen triple bond. These reactions and other mechanistic steps in ammonia generation are presented and the role of local pH is considered. The protocol is anticipated applicable to identifying energy generating reactions important in other bioelectrochemical systems.