Investigating Formate, Sulfate, and Halide Anions in Reversible Zinc Electrodeposition Dynamic Windows.

Reversible metal electrodeposition (RME) is an emerging and promising method for designing dynamic windows with electrically controllable transmission, excellent color neutrality, and wide dynamic range. Zn is a viable option for metal-based dynamic windows due to its fast switching kinetics and reversibility despite its very negative deposition voltage. In this manuscript, we study the effect of the supporting electrolyte anions for Zn electrodeposition on transparent tin-doped indium oxide. Through systematic additions or removal of components of the electrolytes, we are able to establish a link between the anions and the effectiveness of Zn RME. This insight allows us to design practical two-electrode 25 cm2 Zn dynamic windows that switch to <1% within 20 s. Lastly, we demonstrate that the accumulation of Zn(OH)2 species on the working electrode degrades the optical contrast of Zn windows during long-term cycling. However, the elimination of these species through acid immersion allows the windows to cycle at least 500 times. Reversible Zn electrodeposition in the presence of a polyethylene glycol additive further improves the cycle life to greater than 1000 cycles. Taken together, these studies highlight important design principles for the construction of robust dynamic windows based on Zn RME.

Hybrid bilayer membranes as platforms for biomimicry and catalysis.

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O2 and CO2 reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Hybrid Dynamic Windows with Color Neutrality and Fast Switching Using Reversible Metal Electrodeposition and Cobalt Hexacyanoferrate Electrochromism.

Dynamic windows, which electronically switch between clear and dark states on demand, save energy in buildings and automobiles by minimizing the lighting, heating, and cooling costs. In this study, we design dynamic windows by combining reversible metal electrodeposition (RME) and electrochromism, two promising approaches for optoelectronics. These hybrid dynamic windows tint on both electrodes. Reversible Bi and Cu electrodeposition is elicited on the working electrode, while an electrochromic cobalt hexacyanoferrate thin film darkens on the counter electrode. By facilitating plane parallel ion diffusion and dual tinting, this architecture enables dynamic windows to switch more rapidly than devices based on traditional RME schemes. We demonstrate that 25 cm2 devices switch with 58% optical contrast between a transparent and a color-neutral dark state in 20 s. Moreover, these hybrid dynamic windows switch 1000 times without significant degradation in optical performance.

Nanostructured Ni-Cu Electrocatalysts for the Oxygen Evolution Reaction.

Ni-based materials are promising electrocatalysts for the oxygen evolution reaction (OER) for water splitting in alkaline media. We report the synthesis and OER electrocatalysis of both Ni-Cu nanoparticles (20-50 nm in diameter) and Ni-Cu nanoclusters (<20 metal atoms). Analysis of mass spectral data from matrix-assisted laser desorption/ionization and electrospray ionization techniques demonstrates that discrete heterobimetallic Ni-Cu nanoclusters capped with glutathione ligands were successfully synthesized. Ni-Cu nanoclusters with a 52:48 mol % Ni:Cu metal composition display an OER onset overpotential of 50 mV and an overpotential of 150 mV at 10 mA cm-2, which makes this catalyst one of the most efficient nonprecious metal OER catalysts. The durability of the nanocluster catalysts on carbon electrodes can be extended by appending them to electrodes modified with TiO2 nanoparticles. Infrared spectroscopy results indicate that the aggregation dynamics of the glutathione ligands change during catalysis. Taken together, these results help explain the reactivity of a novel class of nanostructured Ni-Cu OER catalysts, which are underexplored alternatives to more commonly studied Ni-Fe, Ni-Co, and Ni-Mn materials.

Dual Tinting Dynamic Windows Using Reversible Metal Electrodeposition and Prussian Blue.

Electronically tintable windows based on reversible metal electrodeposition are a promising alternative to electrochromic materials due to the high optical contrast imparted by the opacity of thin metal films. While traditional metal-based dynamic windows utilize a metal counter electrode frame, metal electrodeposition on the working electrode must be paired with a plane-parallel counter electrode that facilitates transverse ion migration to enable uniform metal electrodeposition on a large scale. In this manuscript, we develop dynamic windows that combine reversible Bi electrodeposition on the working electrode with a Prussian blue counter electrode that acts as both a charge storage layer that intercalates K+ ions and an anodically coloring material. Since both electrodes darken during window tinting, 25 cm2 devices switch with an optical contrast of ∼67.5% at 700 nm in only 3 s, making them the fastest dynamic windows using reversible metal electrodeposition reported on this scale.

Membrane-Modified Metal Triazole Complexes for the Electrocatalytic Reduction of Oxygen and Carbon Dioxide.

In this manuscript, an electrochemical architecture is designed that controls the kinetics of proton transfer to metal triazole complexes for electrocatalytic O2 and CO2 reduction. Self-assembled monolayers of these catalysts are attached to a glassy carbon electrode and covered with a lipid monolayer containing proton carriers, which acts as a proton-permeable membrane. The O2 reduction voltammograms on carbon are similar to those obtained on membrane-modified Au electrodes, which through the control of proton transfer rates, can be used to improve the selectivity of O2 reduction. The improved voltage stability of the carbon platforms allows for the investigation of a CO2 reduction catalyst inside a membrane. By controlling proton transfer kinetics across the lipid membrane, it is found that the relative rates of H2, CO, and HCOOH production can be modulated. It is envisioned that the use of these membrane-modified carbon electrodes will aid in understanding catalytic reactions involving the transfer of multiple protons and electrons.

Controlling Proton and Electron Transfer Rates to Enhance the Activity of an Oxygen Reduction Electrocatalyst.

An electrochemical approach is developed that allows for the control of both proton and electron transfer rates in the O2 reduction reaction (ORR). A dinuclear Cu ORR catalyst was prepared that can be covalently attached to thiol-based self-assembled monolayers (SAMs) on Au electrodes using azide-alkyne click chemistry. Using this architecture, the electron transfer rate to the catalyst is modulated by changing the length of the SAM, and the proton transfer rate to the catalyst is controlled with an appended lipid membrane modified with proton carriers. By tuning the relative rates of proton and electron transfer, the current density of the lipid-covered catalyst is enhanced without altering its core molecular structure. This electrochemical platform will help identify optimal thermodynamic and kinetic parameters for ORR catalysts and catalysts of other reactions that involve the transfer of both protons and electrons.

Effect of Concentration on the Electrochemistry and Speciation of the Magnesium Aluminum Chloride Complex Electrolyte Solution.

Magnesium batteries offer an opportunity to use naturally abundant Mg and achieve large volumetric capacities reaching over four times that of conventional Li-based intercalation anodes. High volumetric capacity is enabled by the use of a Mg metal anode in which charge is stored via electrodeposition and stripping processes, however, electrolytes that support efficient Mg electrodeposition and stripping are few and are often prepared from highly reactive compounds. One interesting electrolyte solution that supports Mg deposition and stripping without the use of highly reactive reagents is the magnesium aluminum chloride complex (MACC) electrolyte. The MACC exhibits high Coulombic efficiencies and low deposition overpotentials following an electrolytic conditioning protocol that stabilizes species necessary for such behavior. Here, we discuss the effect of the MgCl2 and AlCl3 concentrations on the deposition overpotential, current density, and the conditioning process. Higher concentrations of MACC exhibit enhanced Mg electrodeposition current density and much faster conditioning. An increase in the salt concentrations causes a shift in the complex equilibria involving both cations. The conditioning process is strongly dependent on the concentration suggesting that the electrolyte is activated through a change in speciation of electrolyte complexes and is not simply due to the annihilation of electrolyte impurities. Additionally, the presence of the [Mg2(µ-Cl)3·6THF]+ in the electrolyte solution is again confirmed through careful analysis of experimental Raman spectra coupled with simulation and direct observation of the complex in sonic spray ionization mass spectrometry. Importantly, we suggest that the ∼210 cm-1 mode commonly observed in the Raman spectra of many Mg electrolytes is indicative of the C3v symmetric [Mg2(µ-Cl)3·6THF]+. The 210 cm-1 mode is present in many electrolytes containing MgCl2, so its assignment is of broad interest to the Mg electrolyte community.

Proton transfer dynamics dictate quinone speciation at lipid-modified electrodes.

Proton-coupled electron transfer (PCET) reactions are ubiquitous in biochemistry and alternative energy schemes. Natural enzymes utilize quinones in proton transfer chains and energy conversion processes. Here, we utilize a bio-inspired hybrid bilayer membrane system to control the reaction mechanism of a quinone molecule covalently bound to an electrode surface. In particular, by impeding proton access to the quinone moiety, we change the reaction pathway from a PCET process to a pure electron transfer step. We further alter the reaction pathway to a stepwise PCET process by controlling the proton flux through the use of an alkyl proton carrier incorporated in the lipid membrane. By modulating proton availability, we control the quinone reaction pathway without changing the molecular structure of the redox species. This work provides unique insight into PCET reactions and a novel electrochemical platform for interrogating them.

The Flip-Flop Diffusion Mechanism across Lipids in a Hybrid Bilayer Membrane.

In this study, we examine the mechanism of flip-flop diffusion of proton carriers across the lipid layer of a hybrid bilayer membrane (HBM). The HBM consists of a lipid monolayer appended on top of a self-assembled monolayer containing a Cu-based O2 reduction catalyst on a Au electrode. The flip-flop diffusion rates of the proton carriers dictate the kinetics of O2 reduction by the electrocatalyst. By varying both the tail lengths of the proton carriers and the lipids, we find the combinations of lengths that maximize the flip-flop diffusion rate. These experimental results combined with biophysical modeling studies allow us to propose a detailed mechanism for transmembrane flip-flop diffusion in HBM systems, which involves the bending of the alkyl tail of the proton carrier as the rate-determining step. Additional studies with an unbendable proton carrier further validate these mechanistic findings.

Proton transfer dynamics control the mechanism of O2 reduction by a non-precious metal electrocatalyst.

Many chemical and biological processes involve the transfer of both protons and electrons. The complex mechanistic details of these proton-coupled electron transfer (PCET) reactions require independent control of both electron and proton transfer. In this report, we make use of lipid-modified electrodes to modulate proton transport to a Cu-based catalyst that facilitates the O2 reduction reaction (ORR), a PCET process important in fuel cells and O2 reduction enzymes. By quantitatively controlling the kinetics of proton transport to the catalyst, we demonstrate that undesired side products such as H2O2 and O2(-) arise from a mismatch between proton and electron transfer rates. Whereas fast proton kinetics induce H2O2 formation and sluggish proton flux produces O2(-), proton transfer rates commensurate with O-O bond breaking rates ensure that only the desired H2O product forms. This fundamental insight aids in the development of a comprehensive framework for understanding the ORR and PCET processes in general.

The Interplay of Al and Mg Speciation in Advanced Mg Battery Electrolyte Solutions.

Mg batteries are an attractive alternative to Li-based energy storage due to the possibility of higher volumetric capacities with the added advantage of using sustainable materials. A promising emerging electrolyte for Mg batteries is the magnesium aluminum chloride complex (MACC) which shows high Mg electrodeposition and stripping efficiencies and relatively high anodic stabilities. As prepared, MACC is inactive with respect to Mg deposition; however, efficient Mg electrodeposition can be achieved following an electrolytic conditioning process. Through the use of Raman spectroscopy, surface enhanced Raman spectroscopy, (27)Al and (35)Cl nuclear magnetic resonance spectroscopy, and pair distribution function analysis, we explore the active vs inactive complexes in the MACC electrolyte and demonstrate the codependence of Al and Mg speciation. These techniques report on significant changes occurring in the bulk speciation of the conditioned electrolyte relative to the as-prepared solution. Analysis shows that the active Mg complex in conditioned MACC is very likely the [Mg2(µ-Cl)3·6THF](+) complex that is observed in the solid state structure. Additionally, conditioning creates free Cl(-) in the electrolyte solution, and we suggest the free Cl(-) adsorbs at the electrode surface to enhance Mg electrodeposition.

Photoresponsive molecular switch for regulating transmembrane proton-transfer kinetics.

To control proton delivery across biological membranes, we synthesized a photoresponsive molecular switch and incorporated it in a lipid layer. This proton gate was reversibly activated with 390 nm light (Z-isomer) and then deactivated by 360 nm irradiation (E-isomer). In a lipid layer this stimuli responsive proton gate allowed the regulation of proton flux with irradiation to a lipid-buried O2 reduction electrocatalyst. Thus, the catalyst was turned on and off with the E-to-Z interconversion. This light-induced membrane proton delivery system may be useful in developing any functional device that performs proton-coupled electron-transfer reactions.

Anion transport through lipids in a hybrid bilayer membrane.

In this report, we use a hybrid bilayer membrane (HBM) as an electrochemical platform to study anion diffusion through a lipid monolayer. We first append lipid on a self-assembled monolayer (SAM) that contains a covalently bound Cu(I)/Cu(II) redox center. We then perform cyclic voltammetry (CV) using different anions in bulk solution and extract thermodynamic and kinetic information about anion transport. We analyze the results using linear combinations of fundamental chemical trends and determine that anion transport quantitatively correlates to polarity and basicity, a relationship we formalize as the lipid permeability parameter. In addition, we discuss how our findings can be interpreted according to the two leading mechanisms describing ion permeability through lipids. Our results demonstrate that anion transport in a HBM is best described by the solubility-diffusion mechanism, not the pore mechanism.

Proton switch for modulating oxygen reduction by a copper electrocatalyst embedded in a hybrid bilayer membrane.

Molecular switches gate many fundamental processes in natural and artificial systems. Here, we report the development of an electrochemical platform in which a proton carrier switches the activity of a catalyst. By incorporating an alkyl phosphate in the lipid layer of a hybrid bilayer membrane, we regulate proton transport to a Cu-based molecular oxygen reduction reaction catalyst. To construct this hybrid bilayer membrane system, we prepare an example of a synthetic Cu oxygen reduction reaction catalyst that forms a self-assembled monolayer on Au surfaces. We then embed this Cu catalyst inside a hybrid bilayer membrane by depositing a monolayer of lipid on the self-assembled monolayer. We envisage that this electrochemical system can give a unique mechanistic insight not only into the oxygen reduction reaction, but into proton-coupled electron transfer in general.

Inhibiting platelet-stimulated blood coagulation by inhibition of mitochondrial respiration.

Platelets are important mediators of blood coagulation that lack nuclei, but contain mitochondria. Although the presence of mitochondria in platelets has long been recognized, platelet mitochondrial function remains largely unaddressed. On the basis of a small amount of literature that suggests platelet mitochondria are functional, we hypothesized that the inhibition of platelet mitochondria disrupts platelet function and platelet-activated blood coagulation. To test this hypothesis, members of the tetrazole, thiazole, and 1,2,3-triazole families of small molecule heterocycles were screened for the ability to inhibit isolated mitochondrial respiration and coagulation of whole blood. The families of heterocycles screened were chosen on the basis of the ability of the heterocycle family to inhibit a biomimetic model of cytochrome c oxidase (CcO). The strength of mitochondrial inhibition correlates with each compound's ability to deter platelet stimulation and platelet-activated blood clotting. These results suggest that for this class of molecules, inhibition of blood coagulation may be occurring through a mechanism involving mitochondrial inhibition.

Hybrid bilayer membrane: a platform to study the role of proton flux on the efficiency of oxygen reduction by a molecular electrocatalyst.

In this report, we present a novel platform to study proton-coupled electron transfer (PCET) by controlling the proton flux using an electrode-supported hybrid bilayer membrane (HBM). Oxygen reduction by an iron porphyrin was used as a model PCET reaction. The proton flux was controlled by incorporating an aliphatic proton carrier, decanoic acid, into the lipid layer of the HBM. Using this system, we observed a different catalytic behavior than obtained by simply changing the pH of the solution in the absence of an HBM.

Ferrocene embedded in an electrode-supported hybrid lipid bilayer membrane: a model system for electrocatalysis in a biomimetic environment.

An electrode-supported system in which ferrocene molecules are embedded in a hybrid bilayer membrane (HBM) has been prepared and characterized. The redox properties of the ferrocene molecules were studied by varying the lipid and alkanethiol building blocks of the HBM. The midpoint potential and electron transfer rate of the embedded ferrocene were found to be dependent on the hydrophobic nature of the electrolyte and the distance at which the ferrocene was positioned in the HBM relative to the electrode and the solution. Additionally, the ability of the lipid-embedded ferrocenium ions to oxidize solution phase ascorbic acid was evaluated and found to be dependent on the nature of the counterion.

Inhibition of electrocatalytic O(2) reduction of functional CcO models by competitive, non-competitive, and mixed inhibitors.

Electrocatalytic reduction of O(2) by functional cytochrome C Oxidase (CcO) models is studied in the presence of several known inhibitors like CO, N(3)(-), CN(-), and NO(2)(-). These models successfully reproduce the inhibitions observed in CcO at similar concentrations reported for these inhibitors. Importantly, the data show very different electrochemical responses depending on the nature of the inhibitor, that is, competitive, non-competitive and mixed. Chemical models have been provided for these observed differences in the electrochemical behavior. Using the benchmark electrochemical behaviors for known inhibitors, the inhibition by NO(2)(-) is investigated. Electrochemical data suggests that NO(2)(-) acts as a competitive inhibitor at high concentrations. Spectroscopic data suggests that NO released during oxidation of the reduced catalyst in presence of excess NO(2)(-) is the source of the competitive inhibition by NO(2)(-). Presence of the distal Cu(B) lowers the inhibitory effect of CN(-) and NO(2)(-). While for CN(-) it weakens its binding affinity to the reduced complex by approximately 4.5 times, for NO(2)(-), it allows regeneration of the active catalyst from a catalytically inactive, air stable ferrous nitrosyl complex via a proposed superoxide mediated pathway.