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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

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.

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.

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.