Electrodeposited Cobalt Nanosheets on Smooth Silver as a Bifunctional Catalyst for OER and ORR: In Situ Structural and Catalytic Characterization.

Developing earth-abundant, cost-effective, and active bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to boosting sustainable energy systems such as electrolyzers and lithium-air batteries. However, the performance of promising cobalt-based materials is impaired by the external effects of binders and carbon additives as well as inhomogeneous electrode fabrication. In this work, binder- and carbon-free flower-like Co-decorated Ag catalytic nanosheets were in situ-synthesized via a simple electrodeposition approach. The morphology, composition, and structure of Co/Ag before and after OER were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Co/Ag thin film electrodes with various Co contents exhibited a bifunctional activity toward ORR and OER due to a synergistic effect. XPS analysis suggested the formation of Co3O4 as the main active species for OER. In particular, Co (83%)/Ag surface revealed a 60 mV lower ORR overpotential than a pure Ag surface and even lower than drop-casted Co3O4 nanoparticles on Ag surface. Only 1.5% peroxide was generated, suggesting a four-electron transfer ORR. In addition, the OER onset potential on Co/Ag is 60 mV less than Co3O4. Tafel slopes of 71 and 75 mV dec-1 were obtained for ORR and OER, respectively. Importantly, the three-dimensional (3D) growth mechanism of a cobalt layer (∼1 nm) on a well-defined atomic smooth Ag surface is unraveled by in situ electrochemical scanning tunneling microscopy (EC-STM). EC-STM suggests that Co prefers to nucleate at the step edges of Ag and grows in a 3D, forming nanoparticles, where the deposition/dissolution process of the Co adlayer on Ag is reversible. This investigation may provide insights into design strategies of efficient oxygen electrocatalysts.

Electrochemical Reduction of O2 in Ca2+ -Containing DMSO: Role of Roughness and Single Crystal Structure.

In this study, the oxygen reduction reaction (ORR) in Ca2+ -containing dimethyl sulfoxide (DMSO) at well-ordered and rough electrode surfaces is compared by using cyclic voltammetry, differential electrochemical mass spectrometry, rotating ring disk electrode, and atomic force microscopy measurements. Slightly soluble CaO2 is the main product during early ORR on gold electrodes; after completion of a monolayer of CaO and/or CaO2 , which is formed in parallel and in competition to the peroxide, only superoxide is formed. When the monolayer is completely closed on smooth annealed Au, no further reduction occurs, whereas on rough Au a defect-rich layer allows for continuous formation of superoxide. CaO2 formed either via two subsequent 1  e - transfer steps or by disproportionation of superoxide may be deposited on top of the CaO/CaO2 adsorbate layer. The slow dissolution of the peroxide particles is demonstrated by AFM. Whereas a smooth CaO/CaO2 -covered electrode shows severe deactivation and a CaO/CaO2 -covered rough electrode allows for diffusion-limited superoxide formation, on single crystals peroxide formation is more pronounced. The reason is most likely the lack of nucleation sites for the blocking CaO/CaO2 layer. RRDE investigations showed sluggish reoxidation kinetics of the dissolved peroxide, which are most likely due to ion pairing with Ca2+ . The apparent transfer coefficient is estimated by using variation of the electrode roughness, confirming the result of the usual Tafel analysis and indicating an equilibrated first 1  e - transfer.

The Oxygen Reduction Reaction in Ca2+ -Containing DMSO: Reaction Mechanism, Electrode Surface Characterization, and Redox Mediation*.

In this study the fundamental understanding of the underlying reactions of a possible Ca-O2 battery using a DMSO-based electrolyte was strengthened. Employing the rotating ring disc electrode, a transition from a mixed process of O2 - and O2 2- formation to an exclusive O2 - formation at gold electrodes is observed. It is shown that in this system Ca-superoxide and Ca-peroxide are formed as soluble species. However, there is a strongly adsorbed layer of products of the oxygen reduction reaction (ORR) s on the electrode surface, which is blocking the electrode. Surprisingly the blockade is only a partial blockade for the formation of peroxide while the formation of superoxide is maintained. During an anodic sweep, the ORR product layer is stripped from the electrode surface. With X-ray photoelectron spectroscopy (XPS) the deposited ORR products were shown to be Ca(O2 )2 , CaO2 , and CaO as well as side-reaction products such as CO3 2- and other oxygen-containing carbon species. It is shown that the strongly attached layer on the electrocatalyst, that was partially blocking the electrode, could be adsorbed CaO. The disproportionation reaction of O2 - in presence of Ca2+ was demonstrated via mass spectrometry. Finally, the ORR mediated by 2,5-di-tert-1,4-benzoquinone (DBBQ) was investigated by differential electrochemical mass spectrometry (DEMS) and XPS. Similar products as without DBBQ are deposited on the electrode surface. The analysis of the DEMS experiments shows that DBBQ- reduces O2 to O2 - and O2 2- , whereas in the presence of DBBQ2- O2 2- is formed. The mechanism of the ORR with and without DBBQ is discussed.

SLIM: A Short-Linked, Highly Redox-Stable Trityl Label for High-Sensitivity In-Cell EPR Distance Measurements.

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site-directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in-cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide-functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm, and shows high stability against reduction. Using this label, the guanine-nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.

Online Monitoring of Electrochemical Carbon Corrosion in Alkaline Electrolytes by Differential Electrochemical Mass Spectrometry.

Carbon corrosion at high anodic potentials is a major source of instability, especially in acidic electrolytes and impairs the long-term functionality of electrodes. In-depth investigation of carbon corrosion in alkaline environment by means of differential electrochemical mass spectrometry (DEMS) is prevented by the conversion of CO2 into CO3 2- . We report the adaptation of a DEMS system for online CO2 detection as the product of carbon corrosion in alkaline electrolytes. A new cell design allows for in situ acidification of the electrolyte to release initially dissolved CO3 2- as CO2 in front of the DEMS membrane and its subsequent detection by mass spectrometry. DEMS studies of a carbon-supported nickel boride (Nix B/C) catalyst and Vulcan XC 72 at high anodic potentials suggest protection of carbon in the presence of highly active oxygen evolution electrocatalysts. Most importantly, carbon corrosion is decreased in alkaline solution.

Role of Lattice Oxygen in the Oxygen Evolution Reaction on Co3O4: Isotope Exchange Determined Using a Small-Volume Differential Electrochemical Mass Spectrometry Cell Design.

This work demonstrates the role of lattice oxygen of metal oxide catalysts in the oxygen evolution reaction (OER) as evidenced by isotope labeling together with the differential electrochemical mass spectrometry (DEMS) method. Our recent report assessed this role for Co3O4 using a flow-through DEMS cell, which requires a large volume of electrolyte. Herein, we extend this procedure to different Co3O4 catalyst loadings and particle sizes as well as the mixed Ag + Co3O4 catalyst. We introduce, for the first time, a novel small-volume DEMS cell design capable of using disc electrodes and only <0.5 mL of electrolyte. The reliability of the cell is demonstrated by monitoring gas evolution during OER in real time. This cell shows high sensitivity, high collection efficiency, and very short delay time. DEMS results reveal that only the interfacial part (∼0.2% of the total loading or 25% of surface atoms) of the catalyst is active for OER. Interestingly, the amount of oxygen exchanged on the mixed Ag + Co3O4 catalyst is higher than that on the single Co3O4 catalyst, which illustrates the improved electrocatalytic activity previously reported on this mixed catalyst. Furthermore, the real surface area of the catalysts is estimated using different methods (namely, the ball model, double layer capacitance, isotope exchange, and redox peak methods). The surface areas estimated from the Brunauer-Emmett-Teller (BET) and ball models are comparable but roughly three times higher than that of the redox peak method. Our method represents an alternative approach for probing the mechanism and real surface area of catalysts.

Polyphenols from Rheum Roots Inhibit Growth of Fungal and Oomycete Phytopathogens and Induce Plant Disease Resistance.

A growing world population requires an increase in the quality and quantity of food production. However, field losses due to biotic stresses are currently estimated to be between 10 and 20% worldwide. The risk of resistance and strict pesticide legislation necessitate innovative agronomical practices to adequately protect crops in the future, such as the identification of new substances with novel modes of action. In the present study, liquid chromatography mass spectrometry was used to characterize Rheum rhabarbarum root extracts that were primarily composed of the stilbenes rhaponticin, desoxyrhaponticin, and resveratrol. Minor components were the flavonoids catechin, epicatechin gallate, and procyanidin B1. Specific polyphenolic mixtures inhibited mycelial growth of several phytopathogenic fungi and oomycetes. Foliar spray applications with fractions containing stilbenes and flavonoids inhibited spore germination of powdery mildew in Hordeum vulgare with indications of synergistic interactions. Formulated extracts led to a significant reduction in the incidence of brown rust in Triticum aestivum under field conditions. Arabidopsis thaliana mutant and quantitative reverse-transcription polymerase chain reaction studies suggested that the stilbenes induce salicylic acid-mediated resistance. Thus, the identified substances of Rheum roots represent an excellent source of antifungal agents that can be used in horticulture and agriculture.

Electrogeneration of inorganic chloramines on boron-doped diamond anodes during electrochemical oxidation of ammonium chloride, urea and synthetic urine matrix.

Ubiquitous presence of chloride in water effluents may result in the unavoidable electrogeneration of active chlorine species when considering the application of electrochemical advanced oxidation processes as water treatment technologies. However, less attention has been drawn to the subsequent generation of other combined chlorine species such as chloramines. In this work, the electrogeneration of chloramines has been assessed in different water matrices containing NHCl4, urea or synthetic urine. The yield of chloramines has been followed in-situ by differential electrochemistry mass spectroscopy (DEMS) during electrochemical advanced oxidation process with boron-doped diamond (BDD) anodes. Furthermore, the influence of several variables such as chloride concentration, pH or organics concentration on the different distribution of inorganic monochloramine, dichloramine and trichloramine released as products has been considered.

K-O2 electrochemistry: achieving highly reversible peroxide formation.

Since the advent of the lithium-air battery, researchers have focused on understanding the underpinning mechanisms of the oxygen reduction and evolution reaction in aprotic solvents. In this work, the oxygen reaction in the presence of potassium ions in dimethyl sulfoxide was exploited as a model system to refine the present mechanistic picture of oxygen reduction in aprotic environments. In a combined approach utilizing differential electrochemical mass spectrometry in a generator-collector arrangement as well as classical electrochemical techniques, the reversible formation of insoluble peroxide as well as of slightly soluble superoxide is shown. As opposed to other peroxides in other non-aqueous metal-oxygen systems, potassium peroxide can be reoxidized to superoxide with an overpotential of as little as 100 mV. The investigation of the effect of the oxygen partial pressure between 0 and 1 atmosphere demonstrates how the precipitation of superoxide increases the oxidation overpotential of the peroxide and establishes a link between this work and other studies, in which the reversibility of the peroxide formation has not been identified.

Mono and dual hetero-structured M@poly-1,2 diaminoanthraquinone (M = Pt, Pd and Pt-Pd) catalysts for the electrooxidation of small organic fuels in alkaline medium.

Oxidation of some small organic fuels such as methanol (MeOH), ethanol (EtOH) and ethylene glycol (EG) was carried out in an alkaline medium using palladium (Pd)-platinum (Pt) nanoparticles/poly1,2-diaminoanthraquinone/glassy carbon (p1,2-DAAQ/GC) catalyst electrodes. Pd and Pt were incorporated into the p1,2-DAAQ/GC electrode using the cyclic voltammetry (CV) technique. The obtained Pd/p1,2-DAAQ/GC, Pt/p1,2-DAAQ/GC, Pt/Pd/p1,2-DAAQ/GC and Pd/Pt/p1,2-DAAQ/GC nanocatalyst electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and CV methods. Real active surface area (A real) achieved by carbon monoxide (CO) adsorption using differential electrochemical mass spectroscopy (DEMS) technique. The electrochemical activity was evaluated and normalized to A real per metal loading mass. The electrocatalytic oxidation of the small organic fuels at the prepared nanocatalyst electrodes was studied in 1.0 M NaOH solutions by CV and chronoamperometric (CA) techniques. Pt/Pd/p1,2-DAAQ/GC nanocatalyst electrode exhibited enhanced catalytic activity, better durability and higher tolerance to carbon monoxide generated in the oxidation reaction when compared with the other three studied nanocatalysts. The present investigation suggests that the studied nanocatalysts can be successfully applied in direct oxidation of small organic fuels, especially MeOH.

Antimony deposition onto Au(111) and insertion of Mg.

Magnesium-based secondary batteries have been regarded as a viable alternative to the immensely popular Li-ion systems owing to their high volumetric capacity. One of the largest challenges is the selection of Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg2+ compared with Li+. In this work, we prepared very thin films of Sb by electrodeposition on a Au(111) substrate. Monolayer and multilayer deposition (up to 20 monolayers) were characterized by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Monolayer deposition results in a characteristic row structure; the monolayer is commensurate in one dimension, but not in the other. The row structure is to some extent maintained after deposition of further layers. After dissolution of the Sb multilayers the substrate is roughened on the atomic scale due to alloy formation, as demonstrated by CV and STM. Further multilayer deposition correspondingly leads to a rough deposit with protrusions of up to 3 nm. The cyclic voltammogram for Mg insertion/de-insertion from MgCl2/AlCl3/tetraglyme (MACC/TG) electrolyte into/from a Sb-modified electrode shows a positive shift (400 mV) of the onset potential of Mg deposition compared to that of a bare Au electrode. From the charge of the Mg deposition, we find that the ratio of Mg to Sb is 1:1, which is somewhat less than expected for the Mg3Sb2 alloy.

Fast and Simultaneous Determination of Gas Diffusivities and Solubilities in Liquids Employing a Thin-Layer Cell Coupled to a Mass Spectrometer, Part II: Proof of Concept and Experimental Results.

A new method for simultaneously determining gas diffusivities and solubilities in liquids was presented and discussed in detail in Part I of this series. In this part of the series, the new measurement cell was employed to determine oxygen solubilities and diffusivities in 20 different dimethyl sulfoxide-based electrolytes. In addition, a comparison to values available in literature was made. From the temperature dependence of the diffusivity between 20 and 40 °C an activation barrier of 19 kJ mol-1 for the diffusion of oxygen in pure dimethyl sulfoxide was found. Moreover, qualitative agreement between Jones-Dole viscosity coefficients and the dependence of the diffusivity on the electrolyte concentration was confirmed. The temperature-dependent solubility measurements revealed an unexpected increase of the oxygen solubility for temperatures above 30 °C. While the oxygen solubility in the case of the alkali-perchlorates decreases with increasing electrolyte concentration, a pronounced salting-in effect for lithium bis(trifluoromethane)sulfonimide was observed.

Fast and Simultaneous Determination of Gas Diffusivities and Solubilities in Liquids Employing a Thin-Layer Cell Coupled to a Mass Spectrometer, Part I: Setup and Methodology.

Transport properties and solubilities of volatile species in liquid solutions are of high interest in different chemical, biological, and physical systems. In this work, a new approach for determining the diffusivity and solubility of gases in liquids simultaneously is presented. The method presented relies on the diffusion of a volatile species through a thin, liquid layer and the subsequent detection of the species using a mass spectrometer. Evaluation of the time development of the resulting transient yields the diffusion coefficient, while the concentration of the species in the liquid layer can be calculated from the steady-state value of the flux into the mass spectrometer. Apart from the geometry of the thin layer and the calibration constant of the mass spectrometer no additional or external data are required. Experimental results of the temperature-dependent solubility and diffusivity of oxygen in dimethyl sulfoxide are presented in our companion paper Part II and serve as a proof of concept.

Surface morphology and adlayer structure of Se on Rh(111).

The deposition of Se from SeO32- solutions was examined in the submonolayer regime by cyclic voltammetry, scanning tunneling microscopy and atomic force microscopy. Up to a coverage of ca. 0.5 (Se atoms to substrate atoms) a smooth adlayer is obtained with a 2 × âˆš3 structure. When the coverage is increased, at around 0.55 V further deposition is paralleled by a roughening starting at the monoatomic steps. The adsorbed Se is stable in the SeO32- free solution. For coverages below 0.25, separate domains for the Se covered regions and Se free regions were observed for potentials above 0.6 V. Since this is the potential of the spike corresponding to adsorption of OH at the clean Rh(111) surface in HClO4, we have to assume that Se and OH adsorb in separate domains. At lower potentials, where OH is desorbed, Se spreads over the complete surface which then appears completely smooth in the STM images. When the coverage is about 0.25 or above, the roughening is also observed in SeO32- free solution, demonstrating that the rough structures are not due to disordered deposition, but really due to a roughening by place exchange.

How many surface atoms in Co3O4 take part in oxygen evolution? Isotope labeling together with differential electrochemical mass spectrometry.

Understanding the mechanism underlying the oxygen evolution reaction (OER) on oxides is crucial for the development of many energy storage systems. Here, the mechanism of OER on a Co3O4 spinel catalyst is investigated in alkaline media using 18O-labeling combined with differential electrochemical mass spectrometry (DEMS). This work unravels the role of surface oxygen of the oxide in the OER. It is shown that in H218O-containing electrolyte the amount of 18O16O evolved increases from cycle to cycle together with a concomitant decrease of the amount of 16O2 with each cycle before reaching a steady-state value. 18O16O is also evolved from a H216O solution on a Co3O4 electrode pre-treated in H218O-containing solution, indicating the formation of the 18O-labeled oxide in the previous step. Therefore, the oxide layer takes part in OER via an oxygen exchange mechanism. The total number of oxygen atoms of the oxide participating in OER is 0.1 to 0.2% of the total oxide loading, corresponding to about 10-30% of the surface atoms; these represent the catalytically active sites. Moreover, the real surface area of the catalyst is estimated using different methods (namely the ball model, double layer capacitance method, redox peak method, isotope exchange), and compared to the BET data. The surface areas calculated from the BET data, ball model and redox peak method are similar for small particles, which indicates their smooth surface; however they are smaller than that estimated from double-layer capacitance. For larger particles, the much larger surface area estimated from the redox peak in comparison to that expected from the ball model seems to be due to their roughness. Thus, this work highlights the importance of probing the mechanism when investigating the OER activity of a catalyst.

Determining Solubility and Diffusivity by Using a Flow Cell Coupled to a Mass Spectrometer.

One of the main challenges in metal-air batteries is the selection of a suitable electrolyte that is characterized by high oxygen solubility, low viscosity, a liquid state and low vapor pressure across a wide temperature range, and stability across a wide potential window. Herein, a new method based on a thin layer flow through cell coupled to a mass spectrometer through a porous Teflon membrane is described that allows the determination of the solubility of volatile species and their diffusion coefficients in aqueous and nonaqueous solutions. The method makes use of the fact that at low flow rates the rate of species entering the vacuum system, and thus the ion current, is proportional to the concentration times the flow rate (c⋅u) and independent of the diffusion coefficient. The limit at high flow rates is proportional to D2/3·c·u1/3 . Oxygen concentrations and diffusion coefficients in aqueous electrolytes that contain Li(+) and K(+) and organic solvents that contain Li(+) , K(+) , and Mg(2+) , such as propylene carbonate, dimethyl sulfoxide tetraglyme, and N-methyl-2-pyrrolidone, have been determined by using different flow rates in the range of 0.1 to 80 µL s(-1) . This method appears to be quite reliable, as can be seen by a comparison of the results obtained herein with available literature data. The solubility and diffusion coefficient values of O2 decrease as the concentration of salt in the electrolyte was increased due to a "salting out" effect.

Stick-slip behaviour on Au(111) with adsorption of copper and sulfate.

Several transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick-slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × âˆš7 superstructure on the copper monolayer. Stick-slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick-slip resolution persists at all normal forces investigated.

Electrocatalytic oxidation and adsorption rate of methanol at Pt stepped single-crystal electrodes and effect of Ru step decoration: a DEMS study.

The adsorption and oxidation of methanol at Pt(331) and Ru-step-decorated Pt(331) electrodes are studied recording currents and ion currents by online differential electrochemical mass spectrometry. The CO(2) current efficiencies and the degree of surface poisoning with CO(ad) formed during methanol oxidation are independent of the flow rate, confirming the parallel pathway mechanism. The CO(2) current efficiencies decrease with increasing methanol concentration and increase with increasing potential, whereas those of methyl formate show a reverse trend. At potentials higher than 0.6 V, neither the CO(2) current efficiencies nor the methanol oxidation currents increase with increasing Ru coverage. Instead, methanol oxidation is inhibited due to blocking of the most active platinum step sites. At potentials lower than 0.6 V, however, not only the onset of methanol oxidation shifts negatively, by about 0.1 V, but also the methanol oxidation current and the CO(2) current efficiencies increase. Crucial for the use in fuel cells is the complete oxidation to CO(2), which can be achieved if the reactants first adsorb at the electrode surface along the reaction path with adsorbed CO as an intermediate. Therefore, we directly determine the methanol adsorption rates at Pt(331) as well as at Ru-step-decorated Pt(331), Pt(332), Pt(100), and Pt(11,1,1) electrodes. The methanol adsorption rate is doubled by a double step density in the case of the Pt(331) and Pt(332) electrodes, higher at higher Ru coverages, and increases by a factor of three upon increasing the potential by 0.1 V (corresponding to a Tafel slope of approximately 200 mV dec(-1)). At Pt(331) electrodes with partial step decoration, stripping of adsorbed CO (from CO gas) reveals two adsorbate states, which are also discernable when the adsorbate formed from methanol dehydrogenation is stripped.