Electrochemical Quartz Crystal Microbalance Experiments Research Articles

Bottom-up evolution of perovskite clusters into high-activity rhodium nanoparticles toward alkaline hydrogen evolution

Self-reconstruction has been considered an efficient means to prepare efficient electrocatalysts in various energy transformation process for bond activation and breaking. However, developing nano-sized electrocatalysts through complete in-situ reconstruction with improved activity remains challenging. Herein, we report a bottom-up evolution route of electrochemically reducing Cs3Rh2I9 halide-perovskite clusters on N-doped carbon to prepare ultrafine Rh nanoparticles (~2.2 nm) with large lattice spacings and grain boundaries. Various in-situ and ex-situ characterizations including electrochemical quartz crystal microbalance experiments elucidate the Cs and I extraction and Rh reduction during the electrochemical reduction. These Rh nanoparticles from Cs3Rh2I9 clusters show significantly enhanced mass and area activity toward hydrogen evolution reaction in both alkaline and chlor-alkali electrolyte, superior to liquid-reduced Rh nanoparticles as well as bulk Cs3Rh2I9-derived Rh via top-down electro-reduction transformation. Theoretical calculations demonstrate water activation could be boosted on Cs3Rh2I9 clusters-derived Rh nanoparticles enriched with multiply sites, thus smoothing alkaline hydrogen evolution.

Electropolymerization of thiazole derivatives bearing thiophene and selenophene and the potential application in capacitors

• Three thiazole derivatives bearing thiophene and selenophene were used as monomers for electropolymerization. • Pseudocapacitor performance of deposited polymer layers was investigated on macroelectrode and ultramicroelectrode. • Pseudocapacitor behavior is related to the anion intercalation/deintercalation processes by faradaic reactions of oligomer chains. • The approximate molecular weight of the solvated anion and the number of solvent molecules surrounding a solvated anion were analyzed. Three thiazole derivatives bearing thiophene and selenophene are synthesized and used as monomers for electropolymerization. The electropolymerization process is studied using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques. Deep understanding is obtained regarding the pseudocapacitor performance of deposited polymer layers by subjecting the macroelectrode and ultramicroelectrode to CV and galvanostatic charging–discharging experiments in three kinds of electrolytes. Notably, the highest specific capacitance is observed in the derivative bearing selenopheno[3,4- d ]thiazole and selenophene in a solution of tetrabutylammonium tetrafluoroborate. Furthermore, the electropolymerization rate is influenced by the kind of chalcogenophene and the CV scan rate. A spectroelectrochemistry experiment reveals the optical and electrochromic behavior of the deposited polymer layers. From these results, the pseudocapacitor performance of the deposited polymer layers is related to anion intercalation/deintercalation processes by faradaic reactions of oligomer chains. The EQCM experiments also reveal these processes during electropolymerization and anion intercalation/deintercalation into the deposited polymer layers. Finally, the approximate molecular weight of the solvated anion and the number of solvent molecules surrounding a solvated anion are analyzed using the EQCM data.

Electroless Platinum Deposition Using Co3+/Co2+ Redox Couple as a Reducing Agent.

In the present work, the kinetics of electroless deposition of Pt, using a cobalt ion redox system (Co3+/Co2+) as a reducing agent, has been investigated. The deposition rate of Pt depends on the pH, concentration of reactants, and temperature. The deaeration and bubbling of the plating solution with argon play an essential role. It was found that 0.11 mg cm−2 of Pt films could be deposited on the surface of a roughed glass sheet in one hour without replenishing the solution. Additional data have been obtained on the grounds of electrochemical quartz crystal microbalance experiments. The bubbling (agitation) of the electroless Pt plating solution with argon during the deposition of Pt results in a higher deposition rate and is ca. 3 µg cm−2 min−1. The Pt deposition rate is far less, and is as low as 0.14 µg cm−2 min−1 when the electroless Pt plating solution is not bubbled with argon during the deposition of Pt.

Electrode Material Dependence, Ion Pairing, and Progressive Increase in Complexity of the α-[S2W18O62]4–/5–/6–/7–/8–/9–/10–Reduction Processes in Acetonitrile Containing [n-Bu4N][PF6] as the Supporting Electrolyte

Large-amplitude Fourier transform alternating current (FTAC) voltammetry has been used to parameterize the electrode kinetics associated with the reduction of α-[S2W18O62]4– in acetonitrile containing [n-Bu4N][PF6] as the supporting electrolyte at glassy carbon (GC), gold (Au), and platinum (Pt) electrodes by experimenter-based heuristic and computer-assisted automated approaches. The electron-transfer kinetics described by the Butler–Volmer relationship are faster at GC than at the metal electrodes. Progressively increasing departures from ideality in the experimental versus simulated data comparisons were found with reduction processes that occur at more negative potentials and with higher electrolyte concentrations. Ion pairing between α-[S2W18O62]4– or its reduced forms and the electrolyte cation may contribute to nonconformance between theory and experiment. Electrochemical quartz crystal microbalance experiments along with other experiments reveal that adsorption of more extensively reduced species may modify the electrode surface and contribute to the asymmetry found in the reduction and oxidation components of the FTAC voltammetric data. Enhanced double-layer effect at negative potentials could also explain why the level of nonideality increases with reduction processes that occur at more negative potentials. The findings in this study are expected to apply to the voltammetric reduction of other negatively charged polyoxometalates.

Electrochemistry at Bimetallic Pd/Au Thin Film Surfaces for Selective Detection of Reactive Oxygen Species and Reactive Nitrogen Species.

In this work, we designed and fabricated Pd/Au bimetallic thin film electrodes with isolated Pd nanoparticles via underpotential deposition of copper on a gold substrate followed by in situ redox replace reaction in a Pd salt solution. The Pd/Au electrode was characterized by AFM and XPS as well as multiple electrochemical techniques including CV and electrochemical quartz crystal microbalance (EQCM) in sulfuric acid and phosphate buffer electrolytes. Results show that the reduction reactions of the analytes (i.e., H2O2 and 3-nitrotyrosine (3-NT)) at the Pd/Au thin film surfaces affect the nature and reactivity of Pd/Au surface electrochemistry including the adsorbed/absorbed hydrogen and/or the premonolayer palladium oxide redox processes at Pd. The EQCM experiment supports the arrangement of small size Pd nanoparticles in the Pd thin film in the presence of gold exhibits unusual properties, acting as a new physicochemical dimension between the electrode and target H2O2 and 3-NT molecules. The Pd/Au thin film was demonstrated as an extremely sensitive and selective probe for detection of common ROS and RNS (i.e., H2O2 and 3-NT). The integration of two different metallic species, Pd and Au, into a surface structure on nanoscale by exploiting their unique surface electrochemistry establishes an innovative analytical method for highly sensitive and selective detection of H2O2 and 3-NT simultaneously. This method has a general scope for detecting a broad range of redox active and nonredox active species simultaneously, which opens up new opportunities to develop new electrocatalytic materials and innovative sensing approaches.

The Influence of the Ligand in the Iridium Mediated Electrocatalyic Water Oxidation.

Electrochemical water oxidation is the bottleneck of electrolyzers as even the best catalysts, iridium and ruthenium oxides, have to operate at significant overpotentials. Previously, the position of a hydroxyl on a series of hydroxylpicolinate ligands was found to significantly influence the activity of molecular iridium catalysts in sacrificial oxidant driven water oxidation. In this study, these catalysts were tested under electrochemical conditions and benchmarked to several other known molecular iridium catalysts under the exact same conditions. This allowed us to compare these catalysts directly and observe whether structure–activity relationships would prevail under electrochemical conditions. Using both electrochemical quartz crystal microbalance experiments and X-ray photoelectron spectroscopy, we found that all studied iridium complexes form an iridium deposit on the electrode with binding energies ranging from 62.4 to 62.7 eV for the major Ir 4f7/2 species. These do not match the binding energies found for the parent complexes, which have a broader binding energy range from 61.7 to 62.7 eV and show a clear relationship to the electronegativity induced by the ligands. Moreover, all catalysts performed the electrochemical water oxidation in the same order of magnitude as the maximum currents ranged from 0.2 to 0.6 mA cm–2 once more without clear structure–activity relationships. In addition, by employing 1H NMR spectroscopy we found evidence for Cp* breakdown products such as acetate. Electrodeposited iridium oxide from ligand free [Ir(OH)6]2– or a colloidal iridium oxide nanoparticles solution produces currents almost 2 orders of magnitude higher with a maximum current of 11 mA cm–2. Also, this deposited material contains, apart from an Ir 4f7/2 species at 62.4 eV, an Ir species at 63.6 eV, which is not observed for any deposit formed by the molecular complexes. Thus, the electrodeposited material of the complexes cannot be directly linked to bulk iridium oxide. Small IrOx clusters containing few Ir atoms with partially incorporated ligand residues are the most likely option for the catalytically active electrodeposit. Our results emphasize that structure–activity relationships obtained with sacrificial oxidants do not necessarily translate to electrochemical conditions. Furthermore, other factors, such as electrodeposition and catalyst degradation, play a major role in the electrochemically driven water oxidation and should thus be considered when optimizing molecular catalysts.

Redox-State-Dependent Interplay between Pendant Group and Conducting Polymer Backbone in Quinone-Based Conducting Redox Polymers for Lithium Ion Batteries

Conducting redox polymers (CRPs) have attracted increased interest in recent years because of the possibility of combining the favorable electron-transport properties of conducting polymers with the additional functionality provided by the redox active pendant groups (PGs). Herein we present a series of quinone-substituted PEDOT-CRPs where the quinone PGs have been substituted by electron-withdrawing substituents. Introducing electron-withdrawing substituents leads to an increase of the quinone formal potential, making, for example, CF3-substituted CRPs, a promising high-voltage cathode material for lithium ion batteries with a well-defined charge/discharge plateau around 3 V vs Li+/Li0. Interestingly, we find a shift in conductance onset potential concomitant with the quinone formal potential shift, indicating that the polymer backbone conductance is intimately associated with the PG redox chemistry. Through in situ UV–vis, electron paramagnetic resonance (EPR), and electrochemical quartz crystal microbalance experiments as well as by experiments in lithium- and tert-butyl-ammonium-based electrolytes, we show that the conductance delay is caused by the reduced lithiated quinone state, most likely by localizing the polaron charge carrier as indicated by EPR and UV–vis experiments.

Electropolymerization of thiazole derivative bearing thiophene and its application in capacitors and electrochromic devices

A thiazole derivative bearing thiophene as a monomer compound was synthesized and electropolymerized at carbon, gold, platinum, and fluorine doped tin oxide electrodes. From an in-depth analysis of cyclic voltammograms at several scan rates, deposited polymer layers, comprising thiophene functional groups, were shown to clearly exhibit capacitive behavior. This property is related to anion intercalation into these layers during charging and was demonstrated in this study using electrochemical quartz crystal microbalance experiments. It was also shown that in the case of both tetrafluoroborate and hexafluorophosphate anions, the molar ratio of monomers to anions was approximately 1:0.6, which is estimated as the ratio of anion intercalation into thiophene moiety of the polymer layers. This confirms that the capacitance properties of these deposited polymer layers have high stability against electrochemical perturbation. Furthermore, spectroelectrochemistry data obtained during this study indicate that these layers also have switchable electrochromic properties in the visible range.

Electrodeposition of Ag onto Au(111) from Deep Eutectic Solvents

AbstractSilver electrodeposition onto Au(111) has been studied as a function of composition of deep eutectic solvent (DES) type III and IV. The electrochemical investigations were performed by using cyclic voltammetry and in‐situ electrochemical quartz crystal microbalance experiments. Scanning electron microscopy coupled with Auger spectroscopy and atomic force microscopy were used to characterize the deposited metal overlayers. Silver deposition from DES type III shows several analogies to the electrocrystallization from aqueous solutions, such as the presence of underpotential deposition. It is shown that the hydrogen‐bond donor exerts a strong influence on the silver growth mode and on the structure of the metal deposits. In addition, the hydrogen‐bond donor turns out to exert a rather strong influence on the plating process during silver deposition from DES type IV.

Comprehensive Study of Poly(2,3,5,6-tetrafluoroaniline): From Electrosynthesis to Heterojunctions and Ammonia Sensing.

In this work, we report for the first time on a comprehensive study of poly(2,3,5,6-tetrafluoroaniline) (PTFANI). Contrary to the nonfluorinated polyaniline (PANI) or its analogues bearing one fluorine atom, PTFANI is a poorly conductive material. We present a comprehensive study of the electrosynthesized PTFANI from its monomer in an acidic aqueous medium. PTFANI was fully characterized by a potential-pH diagram, spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) measurements, as well as by a morphological study. Combined with the X-ray photoelectron spectroscopy (XPS) analysis, it allowed us to understand the redox properties of this polymer compared to those of the unsubstituted PANI. At pH < 1.85, no proton transfer occurred during the electrochemical process, but the insertion of anions at the site of the protonated imines was demonstrated through the EQCM and XPS experiments. PTFANI showed a lower ratio of 1 ClO4- per 3 2,3,5,6-tetrafluoroaniline units compared to that of PANI. The behavior at pH > 1.85 was different; no anion upload was observed during the electron transfer, but 1 H+ per electron was involved during the transition between the leucoemeraldine and emeraldine base forms. It should also be noted that the oxidation of the emeraldine into the pernigraniline form was not accessible in PTFANI because of the electron-withdrawing effects of the fluorine atoms. However, we took advantage of the unique behavior of PTFANI to build heterojunctions, by combining with a highly conductive molecular material, namely lutetium bisphthalocyanine, LuPc2. The obtained double-lateral heterojunction exhibited a particularly interesting sensitivity to ammonia, even under humid atmospheres, with a limit of detection of 450 ppb. This work paves the way for the use of PTFANI in other electronic devices and as a sensor not only in the field of air quality monitoring but also in the field of health diagnosis in measuring the human breath.

In-Depth Study of Zn Electrode Passivation in Alkaline Solutions for Zn Batteries

Recently, increasing demand for energy storage options has rekindled work on Zn based alkaline batteries, particularly Zn-Air battery systems. Secondary alkaline Zinc-Air batteries hold several distinct advantages over other large-scale systems for energy storage. Such batteries use materials that are low cost, Zn is abundant in the earth’s crust and aqueous alkaline electrolytes are cheap and easy to work with, have a low toxicity and high chemical stability associated with them. To make Zn-air batteries viable for large scale use, issues associated with Zn electrode passivation need to be addressed that lead to a loss of performance during cycling. During the oxidation of Zn to discharge the battery, a passive layer of ZnO is formed on the electrode. This reduces the discharge current possible. Additionally, passivation is linked to electrode morphology changes between cycles, producing inconsistent performance and a reduction in capacity of the battery. Here we describe the current knowledge about Zn passivation and recent work done to better understand Zn passivation in alkaline solutions. Zn passivation is not well understood. There are multiple mechanism of passivation proposed and studies often have presented different results and conclusions despite using the same experimental conditions. Recently there have been calls in the literature for new in-depth studies that clear up the confusion present in the understanding of Zn passivation and dissolution processes in general[1, 2]. The Electrochemical Quartz Crystal Microbalance (EQCM) is a technique which can observe mass changes on an electrode at the order of 15 nanograms. The change in mass observed can be correlated to the charge passed at a given point during an experiment to give a value of mass change per mole of electron passed. This can then be used to determine the equivalent mass of the species being added to or removed to or from the electrode during an electrochemical reaction. EQCM experiments in KOH solutions of various concentrations and saturated to different levels with Zn have been conducted to observe the process by which the passivation layer forms on the surface and the process by which it is removed. Cyclic voltammogram (CV) experiments show that peaks associated with passivation removal give values of 8 to 16 grams per mole electron which corresponds well with the transition of ZnO or Zn(OH)2 to Zn metal on the surface. The transition of the passive layer to active Zn metal on the surface may contribute to electrode shape change. During the anodic dissolution part of the CV sweep, initially values near 32 grams per mole of electron are observed in the EQCM plots, corresponding to direct removal of Zn in a 2-electron reaction. The mass to charge ratios subsequently transition to values near 49 grams per mole electron which would indicate that ZnO or Zn(OH)2 is being formed and removed from the surface. This suggests that the formation of the passivating Zn species is potential dependent and is not solely precipitation of a film on the electrode surface. Linear sweep voltammetry and chronoamperometry experiments will be discussed as well as the impacts of additives to the solution on the electrochemistry of the electrode. Additionally, rotating disc electrode experiments were conducted using a Zn electrode in KOH solutions of various molarities saturated with Zn to understand the mass transport dependence of the passivation behavior. Changes in the location and height of peaks associated with passivation with rotation speed and sweep rate show a mass transport dependence on when passivation occurs but not on the removal of the passivation layer. Dissolution steps were conducted using Zn plate electrodes with different electrolyte concentrations and saturations of Zn at various potentials to observe major influencing factors in the passivation layer formation. These showed that passivation is heavily dependent on the amount of Zn in the solution as well as the potential applied. We conclude that the formation of the passive layer may depend on the ‘escape’ of the ZnO and Zn(OH)­2 species formed during dissolution from the electrode surface. This will be discussed in the context of the various proposed passivation mechanisms and will lead to some thoughts about improved electrode design. Acknowledgement We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). Bockelmann, M., et al., Electrochemical characterization and mathematical modeling of zinc passivation in alkaline solutions: A review. Electrochimica Acta, 2017Mainar, A., et al., Alkaline aqueous electrolytes for secondary zinc-air batteries: an overview. International Journal of Energy Research, 2016 Figure 1

An Electrochemical Quartz Crystal Microbalance Investigation of Manganese Oxide Deposition and Dissolution in Sulfuric Acid Relevant for Zinc Electrowinning

Manganese(II)-ions are present in varying amounts in the electrolyte used for zinc electrowinning, based on the manganese content in the ore. The manganese ions are oxidized at the anode during electrowinning to form manganese dioxide alongside the oxygen evolution reaction. This has both positive and negative consequences for the lead-based anodes used in the industry today. The manganese dioxide prevents corrosion of the lead and hence lowers the amount of lead as an impurity in the produced zinc. Manganese dioxide also increases the selectivity of the oxygen evolution reaction to the chlorine evolution reaction. On the other hand, the manganese dioxide layer grows in thickness over time leading to an increase in cell resistance in addition to a possibility of flaking off and short circuiting the cell. The anodes therefore have to be removed from the cells at regular intervals to physically remove the MnO2 layer, which is a labor intensive task. Lead-based electrodes are not ideal as anode materials for several reasons. They are not very active for oxygen evolution and they introduce lead impurities to the produced metal. Lead is also unwanted for environmental reasons. It is desirable to replace them with electrochemically active dimensionally stable anodes (DSAs). The aim of this study is to investigate manganese oxide deposition and dissolution on noble metals by using conventional electrochemical methods and electrochemical quartz crystal microbalance (EQCM). In this work EQCM was used to provide additional information about the manganese oxide deposition and dissolution from a sulfuric acid solution. The electrode materials studied were platinum and gold. The electrolyte consisted of 0.1 M H2SO4 with 5 mM Mn2+-ions and the experiments were conducted at room temperature. Cyclic voltammograms were recorded for a number of different upper reversal potentials and sweep rates. In addition, sweep-hold and step-hold experiments were conducted. In these experiments the potential was swept or stepped to various potentials at which manganese oxide deposition occurred and held for a given time. These sequences were followed by a negative-going sweep from the hold potential. To complement the EQCM experiments, manganese oxide was deposited on a platinum disc at two different potentials (1.45 V and 1.55 V) and studied by scanning electron microscopy (SEM) as a function of deposition time. The cyclic voltammograms showed an oxidation peak commencing at about 1.42 V and peaking around 1.46 V in the positive-going scan (Fig 1), as observed elsewhere [1]. The oxidation peak coincided with an increase in mass. Clear indications of a nucleation, growth and collision mechanism were observed when the upper reversal potential was chosen from the rising part of the oxidation peak, i.e. below 1.46 V. This peak occurs below the onset of oxygen evolution and is thus related to the onset and continuous formation of manganese oxide. The shape and size of the oxidation peak were similar on gold and platinum. Subsequently, a cathodic peak (reduction of manganese oxide) was observed in the negative-going scan at potentials just positive of 1.3 V, which also appeared similarly on both electrode materials. Interestingly, a second reduction peak at around 1.2 V was observed at both electrodes, but only when the upper limit was sufficiently positive (above 1.5 V). Voltammograms recorded at different sweep rates showed no clear trend in the peak current for the oxidation peak. However, the mass deposited did show a very clear dependence and was inversely proportional to the sweep rate. The SEM images showed that the deposit formed at the higher potential had more dendrites than the deposit formed at the lower potential although almost the same amount of charge was passed in the deposition experiments. The change from one to two reduction peaks when reversing the sweep at potentials above 1.55 V indicates a change in the reduction and deposition mechanism. The lack of sweep-rate dependence of the oxidation peak shows that it is at least not entirely governed by bulk diffusion of Mn2+. To be able to implement DSAs in zinc electrowinning it is important to understand the mechanism of manganese oxide deposition and dissolution. W.H. Kao and V.J. Weibel, Electrochemical oxidation of manganese(II) at a platinum electrode, J. Appl. Electrochem., 22 (1992) 21-27. Figure 1

Dissolution Behavior of Electrodeposited Ni–W Alloys

Nickel (Ni)–Tungsten (W) alloys were electrodeposited galvanostatically (at–10 mA cm–2) on copper substrate with 3 different W contents under the controlled hydrodynamic conditions and then the anodic dissolution behaviors of the alloys were observed by potentiodynamic polarization and electrochemical quartz crystal microbalance (EQCM) techniques. While the structure of the electrodeposited Ni–W alloy with low W content (15.90% W) was crystalline, that of the alloy with high W content (50.80% W) was nano-crystalline according to X-ray diffraction patterns. The increase in the W content of the electrodeposited Ni–W alloy resulted decrease at pH 3 and increase at pH 7 and 12.5 in the anodic currents of the alloy. The pH dependent dissolutions caused electrodeposited alloy surface to have W—enrichment at pH 3 and Ni—enrichment at pH 7 and 12.5. These observations indicated that the selective dissolution of Ni or W was the main mechanism in the anodic dissolution of the electrodeposited Ni–W alloys. The EQCM experiments conducted at pH 7 supported the presence of the selective dissolution mechanism that the anodic dissolution potential of W was 0.42 V lower than that of Ni in the electrodeposited Ni–W alloys.

Catalytic Activity of an Iron-Based Water Oxidation Catalyst: Substrate Effects of Graphitic Electrodes.

The synthesis, characterization, and electrochemical studies of the dinuclear complex [(MeOH)Fe(Hbbpya)-μ-O-(Hbbpya)Fe(MeOH)](OTf)4 (1) (with Hbbpya = N,N-bis(2,2′-bipyrid-6-yl)amine) are described. With the help of online electrochemical mass spectrometry, the complex is demonstrated to be active as a water oxidation catalyst. Comparing the results obtained for different electrode materials shows a clear substrate influence of the electrode, as the complex shows a significantly lower catalytic overpotential on graphitic working electrodes in comparison to other electrode materials. Cyclic voltammetry experiments provide evidence that the structure of complex 1 undergoes reversible changes under high-potential conditions, regenerating the original structure of complex 1 upon returning to lower potentials. Results from electrochemical quartz crystal microbalance experiments rule out that catalysis proceeds via deposition of catalytically active material on the electrode surface.

Pinpointing the active species of the Cu(DAT) catalyzed oxygen reduction reaction.

Dinuclear CuII complexes bearing two 3,5-diamino-1,2,4-triazole (DAT) ligands have gained considerable attention as a potential model system for laccase due to their low overpotential for the oxygen reduction reaction (ORR). In this study, the active species for the ORR was investigated. The water soluble dinuclear copper complex (Cu(DAT)) was obtained by mixing a 1 : 1 ratio of Cu(OTf)2 and DAT in water. The electron paramagnetic resonance (EPR) spectrum of Cu(DAT) showed a broad axial signal with a g factor of 2.16 as well as a low intensity Ms = ±2 absorption characteristic of the Cu2(μ-DAT)2 moiety. Monitoring the typical 380 nm peak with UV-Vis spectroscopy revealed that the Cu2(μ-DAT)2 core is extremely sensitive to changes in pH, copper to ligand ratios and the presence of anions. Electrochemical quartz crystal microbalance experiments displayed a large decrease in frequency below 0.5 V versus the reversible hydrogen electrode (RHE) in a Cu(DAT) solution implying the formation of deposition. Rotating ring disk electrode experiments showed that this deposition is an active ORR catalyst which reduces O2 all the way to water at pH 5. The activity increased significantly in the course of time. X-ray photoelectron spectroscopy was utilized to analyze the composition of the deposition. Significant shifts in the Cu 2p3/2 and N 1s spectra were observed with respect to Cu(DAT). After ORR catalysis at pH 5, mostly CuI and/or Cu0 species are present and the deposition corresponds to previously reported electrodepositions of copper. This leads us to conclude that the active species is of a heterogeneous nature and lacks any structural similarity with laccase.

Redox‐Induced Switching of the Viscoelasticity of DNA Layers Observed by using Electrochemical Quartz Crystal Microbalance on the Millisecond Timescale

AbstractThe interactions of single‐stranded DNA (ssDNA) immobilized on gold electrodes with hexammine ruthenium(III) (RuHex) and hexammine cobalt(III) (CoHex) were investigated with chronocoulometric and electrochemical quartz crystal microbalance experiments. The modified surfaces were created by immobilizing ssDNA with tris(dithioserinol) linkers to form a compact, self‐assembled monolayer, followed by passivation with 6‐mercapto‐1‐hexanol (MCH). Upon addition of the redox indicator during a potential jump for RuHex and CoHex, we observed a dramatic increase in the frequency response in comparison to the signal observed with only low‐ionic‐strength hybridization buffer, 10 mM tris(hydroxymethyl)‐aminomethane (Tris)/H2SO4 (pH 7.4). RuHex and CoHex responded quite differently, even on a millisecond time scale. As the expected mass change is less than a factor of 35 to the observed frequency response, we concluded that the applied potential jumps lead to switching of the DNA layer viscoelasticity.

Two New Sandwich-Type Manganese {Mn5}-Substituted Polyoxotungstates: Syntheses, Crystal Structures, Electrochemistry, and Magnetic Properties.

Herein we report two pentanuclear MnII-substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}(AsW9O33)2]9- and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2(SbW9O33)2]8- (bpy = 2,2'-bipyridine), whose structures have been obtained by single-crystal X-ray diffraction (SCXRD), complemented by results obtained from elemental analysis, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy, and thermogravimetric analysis. They consist of two [B-α-XW9O33]9- subunits sandwiching a cyclic assembly of the hexagonal [{Mn(bpy)}2Na(H2O)2(MnCl)2{Mn(H2O)}]9+ and [{Mn(bpy)}2Na(H2O)2(MnCl){Mn(H2O)}2]10+ moieties, respectively, and represent the first pentanuclear MnII-substituted sandwich-type polyoxometalates (POMs). Both compounds have been synthesized by reacting MnCl2·4H2O with trilacunary Na9[XW9O33]·27H2O (X = AsIII and SbIII) POM precursors in the presence of bpy in a 1 M aqueous sodium chloride solution under mild reaction conditions. SCXRD showed that the alternate arrangement of three five-coordinated MnII ions and two six-coordinated MnII ions with an internal Na cation formed a coplanar six-membered ring that was sandwiched between two [B-α-XW9O33]9- (X = AsIII and SbIII) subunits. The results of temperature-dependent direct-current (dc) magnetic susceptibility data indicated ferromagnetic interactions between Mn ions in the cluster. Moreover, alternating-current magnetic susceptibility measurements with a dc-biased magnetic field showed the existence of a ferromagnetic order for both samples. Electrochemistry studies revealed the presence of redox processes assigned to the Mn centers. They are associated with the deposition of material on the working electrode surface, possibly MnxOy, as demonstrated by electrochemical quartz crystal microbalance experiments.

Understanding the charge/discharge mechanisms and passivation reactions in Na-O 2 batteries

Sodium-oxygen batteries are becoming of increasing interest in the research community as they are able to overcome some of the difficulties associated with lithium-oxygen batteries. The interpretation of the processes governing the discharge and charge of these batteries, however, has been under debate since their early development. In this work we combine different electrochemical methods to build up a model of the discharge product formation and decomposition. We initially analyze the formation and decomposition of the discharge products by means of electrochemical impedance spectroscopy. After that, and for the first time, oxygen electrode processes in Na-O2 cells are analyzed by means of electrochemical quartz crystal microbalance experiments. Based on the combination of these two techniques it is possible to evidence the stabilization of the discharge products in the electrolyte prior to their precipitation. The deposition of passivating products that cannot be stripped off during charge is also demonstrated. Cyclic voltammetry experiments at different potential limits further confirm these passivation reactions. In conclusion, this work provides an accurate picture of the mechanism of the Na-O2 cell reactions by combining different electrochemical techniques.

N‐halamine coatings formed via the electroreduction of in situ generated diazonium cations: toward antimicrobial surfaces

Nowadays, antimicrobial N‐halamine polymers and coatings are extensively studied thanks to their qualities such as effectiveness toward a broad spectrum of microorganisms and regenerability. An N‐halamine organic polymer contains nitrogen–halogen covalent bonds. Its antimicrobial properties are due to the oxidizing power of the halide atoms, which are at the oxidation state + I in chloramine (&gt;N‐Cl) or bromamine (&gt;N‐Br) groups. The aim of the present work was to generate a new N‐halamine coating in a two‐step route. First, we have synthesized by electroreduction of in situ generated aminophenyl diazonium cations polyaminophenyl films because these films contain numerous amino functions. Electrochemical quartz crystal microbalance (EQCM) allowed us to determine the mass of the film formed by cyclic voltammetry and to estimate its thickness; the highest one is about 1 µm. Then, the second step of the process was the formation of haloamine functions into the polyaminophenyl film by substitution of H atoms of the amine functions by Br or Cl atoms. The films were observed by SEM, and their composition was determined by Energy‐dispersive X‐ray spectroscopy (EDS). Finally, the EQCM experiments allowed us to estimate that the N‐halamine films that we have prepared contain one Cl atom or one Br atom for 4 or 4.5 phenyl groups, respectively. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Contrasted electrochemical behaviour of Cu(I) and Cu(II) ions in 1-butyl-3-methylimidazolium dicyanamide

The electrochemical behaviours of Cu(I) and Cu(II) ions in 1-butyl-3-methylimidazolium dicyanamide ([BMIm][DCA]) have been examined and compared. A contrasted behaviour is evidenced for Cu(I) and Cu(II) when the ions are in contact with copper deposit. Therefore more specifically, the comproportionation reaction occurring between Cu(II) and Cu(0) has been investigated in this work by means of different electrochemical techniques such as chronopotentiometry, chronoamperometry and electrochemical quartz crystal microbalance (EQCM). Chronopotentiometry and EQCM experiments have conclusively shown the existence of a comproportionation reaction, Cu(II)+Cu(0) →2Cu(I). The reaction presents a pseudo-zero order with respect to copper metal in the presence of an excess of Cu(II) ions. The rate of the overall process has been proven to be of first order with respect to the Cu(II) bulk concentration since we highlighted that the overall reaction at the electrode is controlled by diffusion.