Levich Analysis Research Articles

Glucose oxidation on gold in alkaline solution: A DEMS and microkinetic modeling study

The mechanism and kinetics of the electrochemical oxidation of glucose on gold surfaces in 0.1 M NaOH solution are investigated by rotating disk electrode and differential electrochemical mass spectrometry coupled to microkinetic modeling. For this purpose, the glucose mass-transport parameters (solution viscosity and density, glucose diffusion coefficient) were determined in the relevant conditions. Koutecký–Levich analysis of rotating disk electrode current-potential curves revealed that the glucose oxidation reaction (GOR) follows a one-electron mechanism at 0.6 V vs RHE, while H2 production could be evidenced by DEMS in this potential range. This suggests that the glucose oxidation selectively produces gluconate at 0.6 V vs RHE through an electrochemical oxidative dehydrogenation (EOD) mechanism. A microkinetic model of the current-potential curves was developed, taking into account the hydrogen electrode reactions kinetics, the dissociative adsorption/desorption of glucose, and the glucose electrooxidation on gold surfaces. The dissociative adsorption of glucose was found to be the rate-determining step of the reaction. It is also revealed that the release of anodic H2 through the Tafel step is triggered by the consumption of the adsorbed reaction intermediates. The gluconate electrooxidation into further products has to be considered for potential above 0.7 V vs RHE.

Direct Comparisons of Oxygen Electro-Reduction across Molecular and Extended Oxides

Our work is focused on understanding hydrogen transfer in redox-active metal oxides as candidates for next-generation catalytic architectures, using oxygen electro-reduction as a model reaction. We are specifically studying the interrelationships between hydrogen transfer electrocatalysis across what we term the “molecules-to-materials design continuum” by directly comparing electrochemical properties of molecular tungsten-based polyoxometalates (polyoxotungstates) and extended tungsten oxides.The goal of this study was to critically compare trends of activity and selectivity towards the two-electron oxygen reduction reaction using a series of three catalyst composites derived from molecular polyoxotungstates: (1) the intact molecule adsorbed on a conductive carbon support; (2) the same composite after thermal decomposition of the molecule to bulk WO3; and (3) an intermediate state comprising the partially decomposed molecule.Catalyst composites were synthesized by wet impregnation of the polyoxotungstate (TBA)4W10O32 (TBA = tetrabutylammonium) from acetonitrile on Vulcan carbon, targeting catalyst loadings that yield well-dispersed molecular units. We then used thermal treatments over a range of temperatures associated with progressive thermal decomposition to the intermediate and fully decomposed states. Thermal decomposition reactions and products were further characterized using thermogravimetric analysis and in-situ powder X-ray diffraction. Electrocatalytic measurements were made using standard analytical protocols comprising rotating-disk electrode voltammetry of carbon-supported catalysts deposited as thin films on glassy carbon substrates.On thermal treatment, we observed that the polyoxotungstate loses stabilizing counterions with progressive rearrangement into disordered clusters and ultimately crystalline nanoparticles of orthorhombic WO3. Each of these various forms exhibit broadly similar reactivity towards the oxygen reduction reaction. Electroanalytical results suggest all three tungsten-based composites are poor oxygen reduction catalysts with activities comparable to that of the carbon support. Nonetheless, Levich analysis suggests high selectivity for 2-electron conversion to H2O2 and catalytic onsets that correlate well with distinct voltametric features corresponding to H-activation and, in the case of extended oxides, bulk proton insertion. Ongoing work is focused on extending these studies toward more tractable hydrogen transfer reactions and identifying alternative catalysts with more thermodynamically accessible mechanisms for H activation.More broadly, we hypothesize that these types of apples-to-apples comparisons between molecular and extended catalysts will reveal vital information about the impact of active site composition and electronic structure on H-transfer catalysis.

Electrochemical oxidation of glucose in alkaline environment—A comparative study of Ni and Au electrodes

The glucose oxidation reaction (GOR) was studied on Au and Ni electrodes by cyclic voltammetry (CV), rotating disk electrode (RDE) measurements coupled with Koutecky–Levich analysis, Differential Electrochemical Mass Spectrometry (DEMS), in situ Fourier Transform Infrared spectroscopy (FTIRS) and High Performance Liquid Chromatography (HPLC) analysis of the reaction products after electrolysis in potentiostatic mode under continuous flow conditions. This study allowed to identify the reaction products and propose tentative reaction mechanisms. On gold, glucose is adsorbed on metallic sites through its anomeric function (C1) resulting in the formation of gluconate as the main GOR product at potentials close to 0.6 V vs. RHE, with a selectivity towards gluconate close to 100% and a projected faradaic efficiency of ca. 70%. The conversion rate is rather low, close to 20%, due to poisoning of the surface, leading to a strong deactivation. At potentials above 0.7 V vs. RHE, the selectivity towards gluconate decreases and non-selective GOR oxidation proceeds through CC bond cleavage. On nickel, the GOR occurs at high potentials (close to 1.2 V vs. RHE) on Ni(OH)2 and NiOOH sites, and proceeds through C1C2 bond breaking, resulting in arabinose and formate. At higher potentials, the selectivity towards arabinose decreases, formate being the main reaction product.

Size dependent electrocatalytic activities of h-BN for oxygen reduction reaction to water.

Electrocatalytic activities for the oxygen reduction reaction (ORR) of Au electrodes modified by as prepared and size selected (0.45-1.0, 0.22-0.45, and 0.1-0.22 µm) h-BN nanosheet (BNNS), which is an insulator, were examined in O2 saturated 0.5M H2SO4 solution. The overpotential was reduced by all the BNNS modifications, and the smaller the size, the smaller the overpotential for ORR, i.e., the larger the ORR activity, in this size range. The overpotential was reduced by as much as ∼330 mV compared to a bare Au electrode by modifying the Au surface by the BNNS of the smallest size range (0.1-0.22 µm). The overpotential at this electrode was only 80 mV more than that at the Pt electrode. Both the rotation disk electrode experiments with Koutecky-Levich analysis and rotating ring disk electrode measurements showed that more than 80% of oxygen is reduced to water via the four-electron process at this electrode. These results strongly suggest and theoretical density functional theory calculations support that the ORR active sites are located at the edges of BNNS islands adsorbed on Au(111). The decrease in size of BNNS islands results in an effective increase in the number of the catalytically active sites and, hence, in the increase in the catalytic activity of the BNNS/Au(111) system for ORR.

Cu2O-Cu@Titanium Surface with Synergistic Performance for Nitrate-to-Ammonia Electrochemical Reduction.

Transition metals, such as titanium (Ti) and copper (Cu) along with their respective metal oxides (TiO2, Cu2O, and CuO), have been widely studied as electrocatalysts for nitrate electrochemical reduction with important outcomes in the fields of denitrification and ammonia generation. Based on this, this work conducted an evaluation of a composite electrode that integrates materials with different intrinsic activities (i.e., Cu and Cu2O for higher activity for nitrate conversion; Ti for higher faradaic efficiency to ammonia) looking for potential synergistic effects in the direction of ammonia generation. The specific performance of single-metal and composite electrodes has shown a strong dependence on pH and nitrate concentration conditions. Faradaic efficiency to ammonia of 92% and productivities of 0.28 mmolNH3 ·cm-2·h-1 at 0.5 V vs reversible hydrogen electrode (RHE) values are achieved, demonstrating the implicit potential of this approach in comparison to direct N2RR with values in the order of μmolNH3 ·h-1·cm-2. Finally, the electrochemical rate constants (k) for Ti, Cu, and Cu2O-Cu/Ti disk electrodes were determined by the Koutecky-Levich analysis with a rotating disk electrode (RDE) in 3.02 × 10-6, 3.88 × 10-4, and 4.77 × 10-4 cm·s-1 demonstrating an apparent synergistic effect for selective NiRR to ammonia with a Cu2O-Cu/Ti electrode.

Nitrogen doped carbonaceous materials as platinum free cathode electrocatalysts for oxygen reduction reaction (ORR)

Comparison of physicochemical properties and electrocatalytic behavior of different N-doped carbonaceous materials as potential catalysts for oxygen reduction reaction (ORR) was attended. Ball-milling of graphite with melamine and solvothermal treatment of graphite oxide, graphene nanoplatelets (GNP) with ammonia were used as preparation methods. Elemental analysis and N2 physisorption measurements revealed the synthesis of N-doped materials with strongly different morphological parameters. Contact angle measurements proved that all three samples had good wettability properties. According to analysis of XRD data and Raman spectra a higher nitrogen concentration corresponded to a smaller size of crystallites of the N-doped carbonaceous material. Surface total N content determined by XPS and bulk N content assessed by elemental analysis were close, indicating homogenous inclusion of N in all samples. Rotating disc electrode tests showed that these N-doped materials weremuch less active in acidic medium than in an alkaline environment. Although the presence of in-plane N species is regarded to be advantageous for the ORR activity, no particular correlation was found in these systems with any type of N species. According to Koutecky–Levich analysis, both the N-containing carbonaceous materials and the reference Pt/C catalyst displayed a typical one-step, four-electron ORR route. Both ball-milled sample with high N-content but with low SSA and solvothermally synthesized N-GNP with high SSA but low N content showed significant ORR activity. It could be concluded that beside the total N content other parameters such as SSA, pore structure, structural defects, wettability were also essential for achieving high ORR activity.

Rapid Measurement of Lactate in the Exhaled Breath Condensate: Biosensor Optimization and In-Human Proof of Concept.

Lactate concentration is of increasing interest as a diagnostic for sepsis, septic shock, and trauma. Compared with the traditional blood sample media, the exhaled breath condensate (EBC) has the advantages of non-invasiveness and higher user acceptance. An amperometric biosensor was developed and its application in EBC lactate detection was investigated in this paper. The sensor was modified with PEDOT:PSS-PB, and two different lactate oxidases (LODs). A rotating disk electrode and Koutecky-Levich analysis were applied for the kinetics analysis and gel optimization. The optimized gel formulation was then tested on disposable screen-printed sensors. The disposable sensors exhibited good performance and presented a high stability for both LOD modifications. Finally, human EBC analysis was conducted from a healthy subject at rest and after 30 min of intense aerobic cycling exercise. The sensor coulometric measurements showed good agreement with fluorometric and triple quadrupole liquid chromatography mass spectrometry reference methods. The EBC lactate concentration increased from 22.5 μM (at rest) to 28.0 μM (after 30 min of cycling) and dropped back to 5.3 μM after 60 min of rest.

Electrocatalytic Behavior of an Amide Functionalized Mn(II) Coordination Polymer on ORR, OER and HER.

The new 3D coordination polymer (CP) [Mn(L)(HCOO)]n (Mn-CP) [L = 4-(pyridin-4-ylcarbamoyl)benzoate] was synthesised via a hydrothermal reaction using the pyridyl amide functionalized benzoic acid HL. It was characterized by elemental, FT-IR spectroscopy, single-crystal and powder X-ray diffraction (PXRD) analyses. Its structural features were disclosed by single-crystal X-ray diffraction analysis, which revealed a 3D structure with the monoclinic space group P21/c. Its performance as an electrocatalyst for oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions was tested in both acidic (0.5 M H2SO4) and alkaline (0.1 M KOH) media. A distinct reduction peak was observed at 0.53 V vs. RHE in 0.1 M KOH, which corresponds to the oxygen reduction, thus clearly demonstrating the material's activity for the ORR. Tafel analysis revealed a Tafel slope of 101 mV dec-1 with mixed kinetics of 2e- and 4e- pathways indicated by the Koutecky-Levich analysis. Conversely, the ORR peak was not present in 0.5 M H2SO4 indicating no activity of Mn-CP for this reaction in acidic media. In addition, Mn-CP demonstrated a noteworthy activity toward OER and HER in acidic media, in contrast to what was observed in 0.1 M KOH.

In Situ X-ray Absorption Spectroscopy of PtNi-Nanowire/Vulcan XC-72R under Oxygen Reduction Reaction in Alkaline Media.

Studying the oxygen reduction reaction (ORR) in the alkaline electrolyte has proven to promote better catalytic responses and accessibility to commercialization. Ni-nanowires (NWs) were synthesized via the solvothermal method and modified with Pt using the spontaneous galvanic displacement method to obtain PtNi-NWs. Carbon Vulcan XC-72R (V) was used as the catalyst support, and they were doped with NH3 to obtain PtNi-NWs/V and PtNi-NWs/V–NH3. Their electrocatalytic response for the ORR was tested and PtNi-NWs/V provided the highest specific activity with logarithmic values of 0.707 and 1.01 (mA/cm2Pt) at 0.90 and 0.85 V versus reversible hydrogen electrode (RHE), respectively. PtNi-NWs showed the highest half-wave potential (E1/2 = 0.89 V) at 1600 rpm and 12 μgPt/cm2 in 0.1 M KOH at 25.00 ± 0.01 °C. Additionally, the catalysts followed a four-electron pathway according to the Koutecký–Levich analysis. Moreover, durability experiments demonstrated that the PtNi-NW/V performance loss was like that of commercial Pt/V along 10,000 cycles. Electrochemical ORR in situ X-ray absorption spectroscopy results showed that the Pt L3 edge white line in the PtNi-NW catalysts changed while the electrochemical potential was lowered to negatives values, from 1.0 to 0.3 V versus RHE. The Pt/O region in the in situ Fourier transforms remained the same as the potentials were applied, suggesting an alloy formation between Pt and Ni, and Pt/Pt contracted in the presence of Ni. These results provide a better understanding of PtNi-NWs in alkaline electrolytes, suggesting that they are active catalysts for ORR and can be tuned for fuel cell studies.

Electroreduction of Oxygen on Carbide-Derived Carbon Supported Pd Catalysts

In recent years there have been multiple studies trying to improve the activity and stability of Pd-based catalysts for oxygen reduction reaction (ORR).1 The primary goal is to substitute Pt used for the fuel cell applications. In this work the standard borohydride reduction method was used to deposit Pd on different carbon materials, which were prepared from various metal carbides. These carbide-derived carbon (CDC) materials had varying amounts of micro and mesopores and were further ball-milled to obtain smaller carbon particle sizes. The influence of the support material on the prepared catalysts was studied. From the obtained catalysts it was observed that low micro-porosity lead to a higher order of agglomeration of the deposited Pd particles with electrochemical surface area being 3 times smaller in alkaline solution as compared to highly microporous Pd/CDC2. Obtained Pd/C catalysts were studied using thermogravimetry, scanning electron microscopy, cyclic voltammetry, and CO stripping. The ORR was explored using the rotating disk electrode (RDE) method. From Figure 1 it is evident that the Pd particles have a higher order of agglomeration on Pd/CDC3 and Pd/CDC4, which were the two catalysts with the lowest amount of micropores in the original CDC material. The results of thermogravimetric analysis indicated around 20% of Pd deposition for all studied catalysts except for Pd/CDC3 characterized with smaller mass loss compared with the others, which could be due to fewer sites where Pd can deposit. The results of the thermogravimetric analysis also indicated that the Pd deposition shifted the thermooxidation temperature of the materials to lower values compared to the Pd-free CDC materials. RDE polarization curves were measured in both 0.1 M KOH and 0.1 M HClO4 solution. According to the Koutecký–Levich analysis the number of electrons transferred per O2 molecule for all the catalyst was around 4. There are notable differences in the diffusion-limited current plateaus, which could be due to different coverage of the Pd/C catalysts due to the larger carbon particles present in some of the CDC materials, even though the CDC materials were ball-milled, still rather large (2-6 μm) carbon particles could be observed depending on the CDC material. In alkaline media, the specific activity (SA) values increased with the number of mesopores and decreased with the increase of the number of micropores, where highest SA could be observed for Pd nanoparticles supported on CDC3 and CDC4. Figure 1 . SEM images of a) Pd/CDC1, b) Pd/CDC2, c) Pd/CDC3, d) Pd/CDC4, e) Pd/CDC5, and f) Pd/Vulcan carbon. Reference H. Erikson, A. Sarapuu, J. Solla-Gullon, and K. Tammeveski, J. Electroanal. Chem., 780, 327-336 (2016). Figure 1

Platinum-Free Electrocatalyst for Oxygen Reduction Reaction Derived from the Direct Pyrolysis of Watermelon Peels

Carbon-based nanomaterials derived from the pyrolysis of heteroatom-containing carbonaceous precursor was shown to exhibit significant electrocatalytic activity towards oxygen reduction reaction (ORR). Biomass from agricultural waste could be seen as possible low-cost precursor for the preparation of carbon-based ORR electrocatalysts. In this study, watermelon peels (WP) were directly pyrolyzed under N2 atmosphere to produce novel carbon-based ORR electrocatalysts. WP were oven-dried and pulverized using a mechanical grinder to produce WP powder. A weighed amount of the resulting powder was then transferred into a ceramic boat and inside a quartz tube furnace. The WP powder was pyrolyzed from room temperature to pyrolysis temperature T at a rate of 5 °C/min under N2 atmosphere. The temperature was held at T for an hour, followed by natural cooling back to room temperature. The pyrolysis temperature was optimized, with T = 800 °C, 900 °C, and 1000 °C. The resulting pyrolysis products were characterized using cyclic voltammetry (CV), rotating disc electrode voltammetry (RDE), and scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDX). CV revealed that WP pyrolyzed at 1000 °C gave the most positive onset potential (Eonset = 0.40 V) and highest current density (j = 10.79 mA/cm2), compared to those pyrolyzed at 800 °C (Eonset = 0.30 V; j = 2.047 mA/cm2) and 900 °C (Eonset = 0.30 V; j = 3.494 mA/cm2). Levich analysis from RDE data showed that the electrocatalysts prepared at 1000 °C proceeded closest to the ideal four-electron route, compared to those prepared at 800 °C and 900 °C. SEM-EDX analysis showed the changes WP underwent in terms of surface morphology and elemental composition, which are then correlated with the apparent electrocatalytic activities of pyrolyzed WP. These findings present the feasibility of using WP as low-cost precursor for ORR electrocatalysts, which could be utilized in fuel cells.

Cobalt-Based Coordination Polymer for Oxygen Reduction Reaction.

Lack of control over the structure and electrically nonconductive properties of coordination polymers (CPs) creates a major hindrance to designing an active electrocatalyst for oxygen reduction reaction (ORR). Here, we report a new semiconductive and low-optical band gap CP structure [{Co3(μ3-OH)(BTB)2(BPE)2}{Co0.5N(C5H5)}], 1, that exhibits high-performance ORR in alkaline medium. The electrical conductivity of compound 1 was measured using impedance spectroscopy and found to be 5 × 10–4 S cm–1. The Ketjenblack EC-600JD carbon used as a support for all the electrochemical methods such as cyclic voltammetry, rotating disk electrode, rotating ring-disk electrode and Koutecký–Levich analysis. The as-synthesized Co-based catalyst has the ability to reduce O2 to H2O by a nearly four-electron process. The crystal structure of 1 shows that the trimeric unit {Co3(μ3-OH)(COO)5N3} and monomeric unit {Co(COO)2(NC5H4)2}2+ are linked with BTB and BPE linkers to form a three-dimensional structure. Theoretical calculations predict that the monomeric center acts as an active catalytic site for ORR. This could be due to the efficient overlap of highest occupied molecular orbital–lowest unoccupied molecular orbital between monomer and O2 molecule. This CP, 1, shows facile 3.6-electron ORR, and it is inexpensive compared with widely used Pt catalysts. Therefore, this CP can be used as a promising cathode material for fuel cells in terms of efficiency and cost effectiveness.

Oxygen Reduction Reaction As the Essential Process for Cathodic Electrodeposition of Metal Oxide Thin Films

1. Introduction Oxygen reduction reaction (ORR) has frequently been employed to electrodeposit metal oxide thin films [1-4]. ORR can often be limited by mass transport of oxygen because of its low solubility in water. It is therefore important to precisely grasp bulk concentration of O2 and its diffusion in a wide temperature range. Optically transparent electrodes such as F-doped SnO2 (FTO) coated glass is often used as the substrate for the purpose of thin film characterization and device fabrication. So it is important to understand the kinetics of such electrodes for the ORR. In this study, we have carried out hydrodynamic electrochemical analysis employing rotating disk Pt and FTO electrodes (RDEs) to quantify each of these limiting factors. 2. Experiment Commercial Pt RDE or a home-made RDE setup employing an FTO (Asahi-DU, 10 Ω/sq.) was used as the working electrode. 0.1 M KCl aqueous solution (pH 5.6) was used as the electrolyte to which a mixture of Ar and O2 is bubbled through. The O2 concentration was separately measured with an Ebara DIO-1350 sensor, based on fluorescence quenching, while the temperature was controlled by a thermostat in a range between 298 and 353 K. 3. Result and Discussion Care must be taken for the bulk concentration of O2 in real electrochemical experimental systems, because the gas phase in a closed cell should be equilibrated with water vapor under a total pressure of 1 atm, and salting out effect by the electrolyte also need to be taken into account. The actual O2 concentrations were measured in O2 saturated water and 0.1 M KCl between 298 and 353 K and compared with the values calculated from the handbook data as shown in Fig. 1. Both the plots are quite nicely overlapping with the respective curves. It is then safe to take C O2 (III) as the bulk O2 concentration for analysis of the electrochemical data measured in 0.1 M KCl. Diffusion coefficient (D) of O2 was precisely determined from Levich analysis of diffusion limited plateau current at Pt RDE. The curves measured at 298 K are chosen an example. The plateau current indicates diffusion limit (j D) of the ORR, and is proportional to ω 0.5 (rad0.5 s-0.5), as expressed by the following Levich equation, j d=0.62nFD 2/ 3 v 1 /6 C * ω 1/2 (1) D was calculated as 1.92×10-5 cm2 s-1 for 298 K from the slope of Levich plot, which nicely compares with the literature value. The same analysis has been carried out for different temperatures. Arrhenius plot between D and 1/T gave a reasonable straight line to yield frequency factor A d and activation energy E a as 8.41×10-3 cm2 s-1 and 15.0 kJ mol-1, respectively. Having known the saturation concentration of O2 and the diffusion coefficient, O2 flux density can be precisely known for any given temperature, partial pressure and w. Blend of Butler-Volmer equation and Levich equation yields Koutecky-Levich (K-L) equation to describe current in the mixed regime, namely, where current is controlled both by charge transfer kinetics and mass transport, j -1=j d -1+j k -1=(0.62nFD 2/3 v -1/6 C *)-1 ω -1/2+(nFCk 0 exp(-a nF(E-E eq)/RT))-1 (2) K-L plot, j -1 vs. ω -0.5, was drawn from the J-V curve to determine a j k value from the intercept with the ordinate for a given overpotential. Extrapolation of the linear fit of logj k vs. overpotential for E–E eq=0 yields the standard charge transfer rate constant k 0. Such analysis has been conducted for the Pt RDE over a wide range of temperature. A reasonably straight Arrhenius plot was obtained to indicate thermal activation of the charge transfer process with a frequency factor (A k) of 9.82×102 cm2 s-1 and an activation energy of 53.2 kJ mol-1. The same analysis has been attempted with the FTO RDE. However, its electrode kinetics was highly unstable. Simply degreased FTO was poor for the ORR and was not able to reach the diffusion limit. Electrochemical activation resulted in a reasonable activity to allow determination of k 0 as 1.82×10-7 cm s-1 at 343 K, which is found to be 2 orders smaller than that of Pt (2.16×10-5 cm s-1) at the same temperature. [1] T. Yoshida, K. Terada, D. Schlettwein, T. Oekermann, T. Sugiura, H. Minoura, Adv. Mater. 2000, 12, 1214. [2] T. Yoshida, T. Oekermann, K. Okabe, D. Schlettwein, K. Funabiki, H. Minoura, Electrochemistry 2002, 70, 470. [3] A. Goux, T. Pauporté, T. Yoshida, D. Lincot, Langmuir 2006, 22, 10 545. [4] T. Yoshida, J. Zhang, D. Komatsu, S. Sawatani, H. Minoura, T. Pauporté, D. Lincot, T. Oekermann, D. Schlettwain, H. Tada, D. Wöhrle, K. Funabiki, M. Matsui, H. Miura and H. Yanagi, Adv. Func. Mater. 2009, 19, 17. Figure 1

Enhanced catalytic activity of nanoshell carbon co-doped with boron and nitrogen in the oxygen reduction reaction

The use of carbon cathode catalysts in polymer electrolyte fuel cells instead of the current platinum catalysts is attracting increasing attention. We claim that two factors are important for enhancing the activity of carbon cathode catalysts in the oxygen reduction reaction (ORR): the formation of a nanoshell structure and co-doping with boron and nitrogen. Herein, we investigate the preparation and characterization of active ORR carbon catalysts that combine the above factors. Boron and nitrogen (BN)-doped nanoshell-containing carbon (BN-NSCC) was prepared by carbonizing a mixture of poly(furfuryl alcohol), cobalt phthalocyanine, melamine, and a trifluoroborane–methanol complex at 1000 °C. Transmission electron microscopy and X-ray photoelectron spectroscopy revealed the formation of nanoshell structures with distorted graphitic layers and the introduction of boron and nitrogen atoms, respectively. The ORR activity was evaluated in oxygen-saturated 0.5 mol dm−3 H2SO4 using Koutecky–Levich analysis. The BN-NSCC showed an eight to ten times higher ORR activity than undoped NSCC, with an increased number of electrons participating in the reaction. Tafel analysis revealed a change in the rate-determining step caused by BN-doping. Thus, the combination of a nanoshell structure and co-doping with boron and nitrogen was found to improve the ORR activity of carbon catalysts.

An activity scale of cathode materials for the electrochemical cyclisation of allyl 2-bromobenzyl ether

The electrocatalytic activity of 10 cathode materials (Ag, Au, Cu, Glassy carbon (GC), Ni, Mg, Mn, Pb, Pt and Ti) was examined for the intramolecular cyclisation reaction of allyl 2-bromobenzyl ether to 2-methyl benzopyran. The influence of the cathode materials on the electrocatalytic activity was tested by means of cyclic voltammetry (CV) in an electrolyte solution of allyl 2-bromobenzyl ether (1mM) + tetrabutylammonium perchlorate (0.1M) (Bu4NClO4) in methanol (MeOH). On Au, GC, Ni, Mg, Mn, Pb and Pt one single reduction wave was measured while Ag and Cu exhibited two cathodic reduction waves. Ti did not show a voltammetric response. The best electrocatalytic activity was obtained on Ag which showed a shift of the reduction peak potential of 771mV with respect to GC, the most inert material. In addition to the screening, rotating disk experiments (RDE) in combination with Koutecky Levich analysis and product formation during electrosynthesis were performed. Koutecky Levich analysis of the RDE at peak potential showed that one-electron was transferred at Au, GC, Ni, Mg, Mn, Pb and Pt. On Ag and Cu the number of transferred electrons determined with Koutecky-Levich depends on the potential and can be tuned to specific one- or two-electron transfer. Electrosynthesis showed that on Au, GC, Ni, Mg, Mn, Pb and Pt exclusively 5-methyl benzopyran was formed. On Ag and Cu product formation was depended on the potential and 2-methyl benzopyran and allyl benzyl ether (not cyclized product) were both formed.

An extended Landau–Levich model for the dragging of a thin liquid film with a propagating surface acoustic wave

In this paper we revisit the Landau and Levich analysis of a coating flow in the case where the flow in the thin liquid film is supported by a Rayleigh surface acoustic wave (SAW), propagating in the solid substrate. Our theoretical analysis reveals that the geometry of the film evolves under the action of the propagating SAW in a manner that is similar to the evolution of films that are being deposited using the dip coating technique. We show that in a steady state the thin-film evolution equation reduces to a generalized Landau–Levich equation with the dragging velocity, imposed by the SAW, depending on the local film thickness. We demonstrate that the generalized Landau–Levich equation has a branch of stable steady state solutions and a branch of unstable solutions. The branches meet at a saddle-node bifurcation point corresponding to the threshold value of the SAW intensity. Below the threshold value no steady states were found and our numerical computations suggest a gradual thinning of the liquid film from its initial geometry.

SEI Characterization Using Ferrocene/Ferrocenium As Redox Shuttle

The characterisation of the solid electrolyte interphase (SEI) formed at the negative electrode in lithium-ion batteries is proposed by Tang et al. 1 Its principle is to introduce a redox shuttle in the electrolyte after the passive film (SEI) is formed at the electrode surface and to study how the electrochemistry of the shuttle is modified by the presence of the film. It is then possible to derive some film properties such those related to the transport of the redox molecule in the film and those related to the kinetics of the shuttle oxidation/reduction. So far, the majority of the work on shuttle experiments was done on a glassy carbon electrode using ferrocene/ferrocenium as redox shuttle. In this communication, we will describe experiments dedicated to form SEIs at glassy carbon electrodes and provide details on shuttle characterization experiments. We will show that the electrochemistry of the ferrocene/ferrocenium couple is greatly dependent on the formation charge involved in the SEI formation step. The SEI both slows down the kinetics of ferrocenium reduction (low overpotential) and also has an effect on the value of the limiting current density (high overpotential). For thin SEIs, there is a marked influence of the current density with the electrode rotation rate. Koutecky-Levich analysis can be performed on these data in order to derive the so-called kinetic current density. For electrodes on which “thick” SEIs are grown, the current density for ferrocenium reduction is much smaller and much less sensitive to the electrode rotation rate, which indicates that there is little ferrocenium diffusion limitations in the electrolyte at this low current density value. Performing Koutecky Levich analysis on these data is not possible, but the current density value measured at the RDE somehow can be considered as a direct reading of the kinetic current density. Acknowlegdment: This work was supported by the European Union’s 7th Framework programme for research, technological development and demonstration under the SIRBATT project, which has received agreement No. 608502.

Gold nanoparticle decoration of insulating boron nitride nanosheet on inert gold electrode toward an efficient electrocatalyst for the reduction of oxygen to water

Overpotential for oxygen reduction reaction (ORR) at Au electrode is reported to be reduced by 0.27V by the modification with boron nitride nanosheet (BNNS) but oxygen is reduced only to H2O2 by 2-electron process at Au electrode. Here we demonstrate that the decoration of BNNS with gold nanoparticles (AuNP) not only reduces the overpotential for ORR further by ca. 50mV, but also opens a 4-electron reduction route to water. Both rotating disk electrode experiments with Koutecky–Levich analysis and rotating ring disk electrode measurements show that more than 50% of oxygen is reduced to water via 4-electron process at Au–BNNS/Au electrode while less than 20 and 10% of oxygen are reduced to water at the BNNS/Au and bare Au electrodes, respectively. Theoretical analysis of free energy profiles for ORR at the BN monolayer with and without Au8 cluster placed on Au(111) shows significant stabilization of adsorbed oxygen atom by the Au8 cluster, opening a 4-electron reduction pathway.

Levich Analysis and the Apparent Potential Dependency of the Levich B Factor

ABSTRACTLevich analysis is a frequently used method for extraction of diffusional properties of electrochemical active species. The analysis is based on plotting current versus the square root of the rotation rate of a rotating disc electrode. From the slope of this plot, the Levich B factor and the diffusional parameters may be obtained. In this article, we focus on issues related to the extraction of the B factor from experimental measurements. We show that the plot does not fully linearize the data: a prerequisite for the slope extraction. Furthermore, analyzing data at various potentials results in varying values of the B factor, contrary to predictions by theory. Thus, as an alternative, we introduce a ω½-normalized Levich plot. This plot linearized the data and the resulting B factor was potential-independent. The difference between the two methods is ascribed to the consideration that the electrolyte–electrode interface current may be limited. We conclude that the potential-dependent B factor based on the standard Levich analysis is an artifact and the resulting values are systematically underestimated in comparison to results from the ω½-normalized Levich analysis.

Kinetics of hydrogen evolution reaction on nickel modified by spontaneous Ru deposition: A rotating disk electrode and impedance spectroscopy approach

The kinetics of hydrogen evolution reaction (HER) was studied on nickel catalysts prepared using a ruthenium spontaneous deposition method. Rotating disk electrode (RDE) and electrochemical impedance spectroscopy (EIS) were used for electrochemical characterization, and the results of the analysis were compared with pure nickel catalysts. Three different deposition times were used to obtain different ruthenium loads and to analyze their impact on the electrocatalytic activity. Kinetic and thermodynamic parameters of the reaction were obtained for freshly synthesized catalysts, and also for catalysts aged using a short chronoamperometric procedure. The presence of ruthenium was found to shift the reaction onset potential up to about 160 mV to lower overpotentials comparing with pure nickel, which not increases after ageing, showing a substantial improvement in catalytic activity. Results of Tafel and Koutecky–Levich (K–L) analysis have shown that the HER rate determining step is the Volmer reaction with Langmuir type adsorption for both, Ni/Ru fresh and aged catalysts.