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Journal Cover Electrocatalysis
  [SJR: 0.817]   [H-I: 17]   [2 followers]  Follow
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1868-2529 - ISSN (Online) 1868-5994
   Published by Springer-Verlag Homepage  [2352 journals]
  • Elementary Reaction Steps in Electrocatalysis: Theory Meets Experiment
    • Authors: Axel Groß; Ludwig A. Kibler
      Pages: 499 - 500
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0389-3
      Issue No: Vol. 8, No. 6 (2017)
  • Local Impact of Pt Nanodeposits on Ionomer Decomposition in Polymer
           Electrolyte Membranes
    • Authors: S. Helmly; M. J. Eslamibidgoli; K. A. Friedrich; M. H. Eikerling
      Pages: 501 - 508
      Abstract: Abstract Based on recent theoretical studies, we designed a multistep experimental protocol to understand the impact of environmental conditions around Pt nanodeposits on membrane chemical degradation. The first experiment probes the local potential at a Pt microelectrode for different rates of permeation of hydrogen and oxygen gases from anode and cathode side. The subsequent degradation experiment utilizes the local conditions taken from the first experiment to analyze local rates of ionomer degradation. The rate of ionomer decomposition is significantly enhanced in the anodic H2-rich membrane region, which can be explained with the markedly increased amount of H2O2 formation at Pt nanodeposits in this region. Graphical Abstract Impact of Pt nanodeposits on chemical membrane degradation
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0353-2
      Issue No: Vol. 8, No. 6 (2017)
  • Electrocatalytic Oxidation of Formate and Formic Acid on Platinum and
           Gold: Study of pH Dependence with Phosphate Buffers
    • Authors: Areeg Abdelrahman; Johannes M. Hermann; Ludwig A. Kibler
      Pages: 509 - 517
      Abstract: Abstract The effect of electrolyte pH on the electrooxidation of formic acid/formate is studied using cyclic voltammetry on polycrystalline rotating disk and single-crystalline Pt and Au electrodes in phosphate-based solutions over a wide range of pH (1–12). A non-linear relationship between oxidation current and pH at constant overpotential is identified for both metals. Surface structure influences the reaction for both Pt and Au electrodes. The results in terms of pH dependence are in agreement with those reported in literature. However, experimental evidence shows that adsorbed phosphates cause dramatic changes in the behaviour of the oxidation of formic acid on Pt and Au electrodes due to site blocking and competitive adsorption. The pH dependence on the catalytic activity for formic acid oxidation on Pt is more complex, due to the poisoning of the electrodes by adsorbed CO in addition to intricate anion adsorption effects. The role of the phosphates in the electrocatalyzed reaction is more than maintaining the pH of the system. Rather, various phosphate anions strongly adsorb on the surface, block reactive surface sites and quantitatively decrease oxidation currents. The blocking effect of the phosphate anions increases with increasing pH value. A more considerable blocking effect is established for Au. In addition, a strong pH dependence on overpotential is identified for Pt. In general, oxidation kinetics of formic acid depends strongly on pH, the nature of the adsorbed phosphate species and the electrode potential. Graphical Abstract ᅟ
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0380-z
      Issue No: Vol. 8, No. 6 (2017)
  • Electrochemical Kinetics: a Surface Science-Supported Picture of Hydrogen
           Electrochemistry on Ru(0001) and Pt/Ru(0001)
    • Authors: M.P. Mercer; H.E. Hoster
      Pages: 518 - 529
      Abstract: Abstract In this short review, we compare the kinetics of hydrogen desorption in vacuum to those involved in the electrochemical hydrogen evolution/oxidation reactions (HER/HOR) at two types of atomically smooth model surfaces: bare Ru(0001) and the same surface covered by a 1.1 atomic layer thick Pt film. Low/high H2 (D2) desorption rates at room temperature in vacuum quantitatively correspond to low/high exchange current densities for the HOR/HER in electrochemistry. In view of the “volcano plot” concept, these represent two surfaces that adsorb hydrogen atoms, Had, too strongly and too weakly, respectively. Atomically smooth, vacuum annealed model surfaces are the closest approximation to the idealized slab geometries typically studied by density functional theory (DFT). A predictive volcano plot based on DFT-based adsorption energies for the Had intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of Had intermediates and (ii) local variations in film thickness. The sluggish HER/HOR kinetics of Ru(0001) allows for excellent visibility of cyclic voltammetry (CV) features even in H2-saturated solution. The CV switches between a Had- and a OHad-/Oad-dominated regime, but the presence of H2 in the electrolyte increases the Had-dominated potential window by a factor of two. Whereas in plain electrolyte two electrochemical adsorption processes compete in forming adlayers, it is one electrochemical and one chemical one in the case of H2-saturated electrolyte. We demonstrate and quantitatively explain that dissociative H2 adsorption is more important than H+ discharge for Had formation in the low potential regime on Ru(0001). Graphical Abstract Left: Cyclic voltammograms of Ru(0001) in 0.1 M HClO4, with and without H2 present in solution. Left inset: atomic resolution scanning tunnelling microscope (STM) images of Ru(0001). Centre: volcano plot showing the theoretically predicted hydrogen evolution/oxidation (HER/HOR) current densities on Ru(0001), with variable Had coverage in the adlayer. These values are compared with the exchange current densities for the HER/HOR on Pt(111) and pseudomorphic overlayers of Pt on Ru(0001), where the Pt overlayer thickness is indicated. Temperature programmed desorption (TPD) spectra of D2 on Ru(0001) (upper left) and Pt/Ru(0001) (upper right) are shown along with the respective adsorbate coverage obtained at 300 K. Right: STM image of Pt/Ru(0001). The line indicated how the Pt overlayer thickness varies across the surface, as illustrated at the bottom.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0381-y
      Issue No: Vol. 8, No. 6 (2017)
  • Influence of Step and Island Edges on Local Adsorption Properties:
           Hydrogen Adsorption on Pt Monolayer Island Modified Ru(0001) Electrodes
    • Authors: Sung Sakong; Julia M. Fischer; David Mahlberg; R. Jürgen Behm; Axel Groß
      Pages: 530 - 539
      Abstract: Abstract The influence of steps and island edges on the local electronic structure of a (bi-)metallic single crystalline electrode surface and on the local, site-specific adsorption energy of adsorbed species, the so-called structural effects, was studied by periodic density functional theory based calculations, focusing on longer-range effects. Using hydrogen adsorption energies as a local probe, calculations were performed both for partly Pt monolayer covered planar Ru(0001) surfaces and for a stepped Ru( \(10\bar {19}\) ) surface decorated with a row of Pt atoms. The calculations demonstrate that the steps/island edges affect not only the nearest neighbor adsorption sites but also more distant ones with the extent depending on the particular structure. This longer-range effect is in excellent agreement with recent temperature-programmed desorption and spectroscopy experiments (Hartmann et al. Phys. Chem. Chem. Phys. 14, 10919, 2012). For the interaction of water molecules with partly Pt monolayer covered Ru(0001), similar trends as in the hydrogen adsorption have been found. In addition, hydrogen adsorption energies as a function of coverage have been used to derive the hydrogen coverage as a function of the electrode potential, exhibiting a broad range of stable hydrogen adsorption structures. Graphical Abstract Local adsorption properties of Pt monolayer island modified Ru(0001) electrodes are studied by first-principles calculations
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0354-1
      Issue No: Vol. 8, No. 6 (2017)
  • A Carbon-Free Ag–Co 3 O 4 Composite as a Bifunctional Catalyst for
           Oxygen Reduction and Evolution: Spectroscopic, Microscopic and
           Electrochemical Characterization
    • Authors: Hatem M.A. Amin; Christoph J. Bondue; Santhana Eswara; Ute Kaiser; Helmut Baltruschat
      Pages: 540 - 553
      Abstract: Abstract A key challenge for rechargeable metal–air batteries is the development of a cost-effective bifunctional catalyst for both oxygen evolution (OER) and reduction (ORR) reactions. Here, we took the advantages of high OER activity of Co3O4 spinel and high ORR activity of Ag to develop a carbon-free oxygen electrode, e.g., for Li–air batteries. The optimized Ag + Co3O4 catalyst was further characterized and exhibited a good bifunctional activity in alkaline media. From rotating ring-disk electrode results, the mixed Ag + Co3O4 catalyst revealed significantly lower (∼320 mV) overpotential for ORR than single Co3O4, and a slightly lower overpotential than pure Ag. A four-electron pathway was also elucidated. The OER activity of the mixed catalyst is 1.5-fold compared to pure Co3O4, although the Co3O4 loading is only 10%, suggesting a large synergistic effect. The potential difference between OER and ORR (i.e., the sum of the overpotentials at 1 mA cm−2) is ca. 0.85 V, which is comparable to noble metal based catalysts. To better understand the origin of this synergism, an XPS analysis was performed, demonstrating that only after oxidation of the mixed catalyst, Co3O4 was reduced to Co(OH)2 at potentials of the ORR, probably due to the presence of Ag+. This redox switching, which was not observed for pure Co3O4, is a probable explanation for the increased catalytic activity. The morphology and the electrochemically active surface area of Ag on the surface were examined by electron microscopy and lead-underpotential deposition, respectively. These results also show that when 88% of the Ag surface is blocked by Co3O4 particles, the residual 12% free Ag sites altogether have a higher activity for ORR than the (100%) pure Ag surface, i.e., the activity per Ag site is increased by more than a factor of 10. The combination of low cost and high performance endows this catalyst as a promising candidate for energy devices, and the present synergistic effect opens a new track for high activity. Graphical Abstract ᅟ
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0364-z
      Issue No: Vol. 8, No. 6 (2017)
  • Oxygen Reduction in Alkaline Media—a Discussion
    • Authors: Anna Ignaczak; Renat Nazmutdinov; Aleksej Goduljan; Leandro Moreira de Campos Pinto; Fernanda Juarez; Paola Quaino; Gustavo Belletti; Elizabeth Santos; Wolfgang Schmickler
      Pages: 554 - 564
      Abstract: Abstract We propose a complete reaction sequence for oxygen reduction in alkaline solutions, in which the first two steps occur in the outer sphere mode. The oxygen-oxygen bond is broken in the third step, which involves adsorption of OH, which is desorbed in the last step. We have investigated the sequence by quantum-chemical methods and determined the energies of activation. Whether the reaction follows a four- or a two-electron mechanism, depends critically on the energy of adsorption of OH. We surmise that our mechanism holds on all electrodes which interact weakly with oxygen, in particular on gold, silver, and graphite. We explain, why Au(100) is a better catalyst than Au(111), why at high overpotentials the reaction on Au(100) reverts to a two-electron mechanism, and why this does not happen on silver. Graphical Abstract We propose a mechanism for oxygen reduction in alkaline solution and support each step by theoretical calculations based on DFT and on our own theory. In particular we explain the central role of OH adsorption in the breaking of the oxygen-oxygen bond
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0365-y
      Issue No: Vol. 8, No. 6 (2017)
  • Fates of Hydrogen During Alumina Growth Below Yttria Nodules in FeCrAl(RE)
           at Low Partial Pressures of Water
    • Authors: Vedad Babic; Christine Geers; Bo Jönsson; Itai Panas
      Pages: 565 - 576
      Abstract: Abstract Oxidation of FeCrAl(Re), when exposed to ∼35 ppm of water as sole supply of oxygen in predominantly nitrogen atmosphere, has two characteristic signatures. One is the internal nitridation owing to chromia nodules acting windows toward nitrogen permeation locally short-circuiting the protective α-Al2O3 scale. The second remarkable feature is the growth of thick, apparently defect-rich alumina scale under yttria-rich nodules. Hence, one part of the present study comprises exploratory DFT calculations to discriminate between the impacts of chromia and yttria viz. nitrogen permeation. The second part concerns boundary conditions for apparent rapid growth of alumina under yttria nodules. Yttria-associated surface energy stabilization of defect-rich alumina in presence of water was argued to involve hydrolysis-driven hydroxylation of said interface. Subsequent inward growth of the alumina scale was associated with outward diffusion of oxygen vacancies to be accommodated by the remaining proton producing a hydride ion upon surfacing at yttria-decorated alumina interfaces. The latter comprises the cathode process in a quasi-Wagnerian context. Two fates were discussed for this surface ion. One has H−–H+ recombination to form H2 at the interface in conjunction with OH– accommodation upon hydration, while the second allows hydrogen to be incorporated at VO sites in hydroxylated grain boundaries of the growing alumina scale. The latter was taken to explain the experimentally observed rapid oxide growth under yttria-rich nodules. Space charge due to proton reduction was proposed to cause transient inward cationic drag. Graphical Abstract Impacts of chromia and yttria nodules, coexisting in an alumina barrier oxide, viz. nitrogen permeation at low partial pressures of water was addressed. Furthermore, yttria-associated surface energy stabilization of defect-rich alumina in presence of water was argued to involve hydrolysis-driven hydroxylation of the interfaces. Inwards oxide growth is conditioned by dis posal of hydrogen. Two fates were discussed. One has H−–H+ recombination to form H2 at the interfaces, while the second would allow hydrogen to be incorporated in oxygen vacancies in the hydroxylated grain boundaries of the growing alumina scale.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0368-8
      Issue No: Vol. 8, No. 6 (2017)
  • Methanol Oxidation on Pt(111) from First-Principles in Heterogeneous and
    • Authors: Sung Sakong; Axel Groß
      Pages: 577 - 586
      Abstract: Abstract The catalytic oxidation of methanol on Pt(111) has been addressed based on first-principles electronic structure calculations. The chemical environment corresponding to the conditions in heterogeneous and electro-catalysis has been taken into account in a grand-canonical approach. Furthermore, the aqueous electrolyte in electrocatalysis has been described in an implicit solvent model. Thus, we find characteristic differences between the methanol oxidation paths in heterogeneous and electro-catalysis. The presence of the aqueous electrolyte stabilizes reaction intermediates containing hydrophilic groups thus also influencing the selectivity in the methanol oxidation. In addition, adsorbed hydrogen on Pt(111) is shown to render the electro-oxidation of methanol less efficient. Graphical Abstract The difference between methanol oxidation in heterogeneous and electro-catalysis has been studied theoretically from first principles employing a grand-canonical approach.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0370-1
      Issue No: Vol. 8, No. 6 (2017)
  • Hydrogen Evolution Reaction on Nanostructures Electrodes—a Scenario
           on Stepped Silver Surfaces
    • Authors: M. F. Juárez; M. Ávila; A. Ruderman; E. Santos; E. P. M. Leiva; O. A. Oviedo
      Pages: 587 - 593
      Abstract: Abstract We have investigated the scenario for the hydrogen evolution reaction at stepped silver surfaces in acid solutions at high overpotentials using a simple kinetic model. Two independent types of sites, at the steps and at the terraces, were considered. The rate constants for the Volmer and Heyrovsky reactions were estimated. Both reactions occur with rate constant about two orders of magnitude larger at step sites than at terrace sites. Calculations of the activation energy for these reactions using our theory of electrocatalysis give similar results. Graphical Abstract Volmer reaction at terrace and step sites. Solvent interactions of the proton when approaches the surface
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0371-0
      Issue No: Vol. 8, No. 6 (2017)
  • New Platinum Alloy Catalysts for Oxygen Electroreduction Based on Alkaline
           Earth Metals
    • Authors: U. G. Vej-Hansen; M. Escudero-Escribano; A. Velázquez-Palenzuela; P. Malacrida; J. Rossmeisl; I. E. L. Stephens; I. Chorkendorff; J. Schiøtz
      Pages: 594 - 604
      Abstract: Abstract The energy efficiency of polymer electrolyte membrane fuel cells is mainly limited by overpotentials related to the oxygen reduction reaction (ORR). In this paper, we present new platinum alloys which are active for the ORR and based on alloying Pt with very abundant elements, such as Ca. Theoretical calculations suggested that Pt5Ca and Pt5Sr should be active for the ORR. Electrochemical measurements show that the activity of sputter-cleaned polycrystalline Pt5Ca and Pt5Sr electrodes is enhanced by a factor of 5–7 relative to polycrystalline Pt. Accelerated stability testing shows that after 10,000 electrochemical cycles, the alloys still retain over half their activity. The stability is thus not quite on par with the similar Pt-lanthanide alloys, possibly due to the somewhat lower heat of formation. Graphical Abstract Left: The structure of Pt5Ca with a compressed Pt overlayer. The compression increases activity for the Oxygen Reduction Reaction (ORR). Right: The measured ORR activity of the best Platinum alloys.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0375-9
      Issue No: Vol. 8, No. 6 (2017)
  • Electrodeposition of Ag Overlayers onto Pt(111): Structural,
           Electrochemical and Electrocatalytic Properties
    • Authors: Ludwig A. Kibler; Khaled A. Soliman; Alan Plumer; Christopher S. Wildi; Eric Bringley; Jonathan E. Mueller; Timo Jacob
      Pages: 605 - 615
      Abstract: Abstract Epitaxially grown Ag overlayers have been fabricated by electrochemical deposition onto Pt(111). The electrochemical behaviour of these Ag overlayers has been studied by cyclic voltammetry, and their adsorption properties are significantly influenced by the underlying Pt(111) substrate and markedly different from those of Ag(111). A characteristic voltammetric peak for OH adsorption on pseudomorphic Ag islands has been observed for alkaline solution. A deposition–dissolution hysteresis in the underpotential deposition region for the Ag bilayer suggests exchange processes between subsurface Ag and Pt atoms. Theoretical DFT calculations confirm the stability of a pseudomorphic Ag monolayer. However, it is shown for two and three Ag layers that the formation of sandwich structures is theoretically favoured, i.e. Ag layers tend to be separated by single Pt layers. While Ag displaces hydrogen adsorbed in the underpotential region, the activity of Ag monolayers for the hydrogen evolution reaction (HER) is very close to that of Pt(111). Also, Tafel slopes for HER on the first pseudomorphic Ag monolayer on Pt(111) and for blank Pt(111) are almost identical, whereas thicker overlayers are more Ag-like. It is shown by theoretical calculations for the case of an Ag monolayer on Pt(111) that hydrogen can be adsorbed on the Pt subsurface layer. Graphical Abstract ᅟ
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0386-6
      Issue No: Vol. 8, No. 6 (2017)
  • Dynamics of the Interaction of Formic Acid with a Polycrystalline Pt Film
           Electrode: a Time-Resolved ATR-FTIR Spectroscopy Study at Low Potentials
           and Temperatures
    • Authors: Z. Jusys; R. J. Behm
      Pages: 616 - 629
      Abstract: Abstract Aiming at more insights into the interaction of formic acid with a Pt electrode, we have studied the dynamics of formic acid interaction with a polycrystalline Pt film electrode in the potential range around the onset of the reaction, from 0.0 to 0.4 V (reversible hydrogen electrode (RHE)), by a combination of electrochemical and in situ IR spectroscopy transient measurements. The measurements were performed under well-defined mass transport conditions; IR spectra were acquired in an attenuated total reflection (ATR) configuration with a time resolution of up to 25 ms (rapid scan mode). To slow down the reaction kinetics and thus stabilize short-living adsorbed intermediates, measurements were performed at ambient and low reaction temperatures (3 °C). Kinetic H/D isotope effects, introduced by using deuterated formic acid, were explored to learn more about the contribution of C–H bond splitting in the rate-determining step in formic acid dehydration (COad formation). Rapid scan ATR-FTIRS measurements show no measurable time delay between the appearance of the bands related to adsorbed bridge-bonded formate species and adsorbed CO at higher potentials (>0.1 V) and no detectable formate signals at low potentials (≤0.1 V), although COad is still formed even at 0.0 V. Adsorption of HCOOH species at low potentials (0.0–0.1 VRHE) is indicated by a band developing at around 1720 cm−1, which is isotope shifted upon deuteration of the C–H bond. Consequences of these and other observations, including the observation of a bell-shaped potential dependence of the initial rate for COad formation in the potential range 0.0–0.4 V or distinct kinetic H/D isotope effects in the rate constants for COad formation, on the mechanistic understanding of the formic acid–Pt interaction are discussed. Graphical Abstract ᅟ
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0392-8
      Issue No: Vol. 8, No. 6 (2017)
  • Merging Empirical Valence Bond Theory with Quantum Chemistry to Model
           Proton Transfer Processes in Water
    • Authors: Sebastian Dohm; Eckhard Spohr; Martin Korth
      Pages: 630 - 636
      Abstract: Abstract Proton transfer processes in water are of fundamental importance for, among others, electrochemical proton discharge. Empirical valence bond (EVB) approaches were shown in the past to be a versatile tool for modeling complex phenomena such as proton discharge at metal electrodes. By replacing empirical fitting procedures with on-the-fly quantum chemistry (QC) calculations, we arrive at a transferable and systematically tunable description of proton transfer in water with EVB. Graphical Abstract ᅟ
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0396-4
      Issue No: Vol. 8, No. 6 (2017)
  • Water Structure and Mechanisms of Proton Discharge on Platinum Electrodes:
           Empirical Valence Bond Molecular Dynamics Trajectory Studies
    • Authors: Johannes Wiebe; Eckhard Spohr
      Pages: 637 - 646
      Abstract: Abstract We present a comparative analysis of molecular dynamics trajectory studies of the influence of surface charge, ion strength, and ion adsorption on the interfacial water structure and the possible pathways of proton transport and discharge on negatively charged platinum(111) electrodes. The model used is a reactive force field based on a nine-state empirical valence bond model. It incorporates both proton transfer between water molecules and simultaneous electron and proton transfer to the metal (discharge). The interfacial water polarization is the result of the competition between the electrical field influence of the smooth surface charge and the point-like local charges of adsorbed positive or negative ions, which leads to variations of the prevalent proton discharge pathways depending on system composition. Graphical Abstract In trajectory calculations of proton adsorption and discharge from aqueous solutions onto charged platinum electrodes, the presence of electrolytes in the adsorbate layer influences proton discharge mechanisms via changes in water orientation and via exclusion of discharge sites through repulsive cation-proton interactions.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0398-2
      Issue No: Vol. 8, No. 6 (2017)
  • Theoretical Studies on the CO 2 Reduction to CH 3 OH on Cu(211)
    • Authors: Shan Ping Liu; Ming Zhao; Wang Gao; Qing Jiang; Timo Jacob
      Pages: 647 - 656
      Abstract: Abstract CO2 reduction has been pursued for decades as an effective way to produce useful fuels and to mitigate global warming at the same time. Methanol synthesis from CO2 hydrogenation over Cu-based catalysts plays an important role in the chemical and energy industries. However, fundamental questions regarding the reaction mechanism and key reaction intermediates of this process are still unclear. To address these issues, we studied the CO2 hydrogenation process using density functional theory (DFT) combined with van der Waals (vdW) force corrections, finding that the most effective pathway proceeds along the reaction series CO* → CHO* → CH2O* → CH2OH* → CH3OH* with the reactive intermediate CH2O*, which is consistent with experimental findings. Additionally, we find that water molecules play an inhibiting role in the reaction, while H bonds and vdW forces have an essential effect on the reaction mechanisms. These findings shed light on the reaction mechanism of CH3OH formation from CO2 hydrogenation and reveal the essence of H2O in this reaction, providing a useful basis for preceding studies. Graphical Abstract Adsorption configurations of COH on (a) bare Cu(211) and (b) on Cu(211) with a co-adsorbed H2O chain. The corresponding reaction pathways on these two surfaces (c and d) have been calculated using density functional theory combined with van der Waals force corrections. Based on these calculations we obtain the most promising pathway and reveal the drastic effect of water molecules.
      PubDate: 2017-11-01
      DOI: 10.1007/s12678-017-0403-9
      Issue No: Vol. 8, No. 6 (2017)
  • Spinel Manganese Ferrites for Oxygen Electrocatalysis: Effect of Mn
           Valency and Occupation Site
    • Authors: Ye Zhou; Yonghua Du; Shibo Xi; Zhichuan J. Xu
      Abstract: Abstract Spinel catalysts have been widely explored for the electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To consolidate the understanding on electrocatalysis by spinel family, intermediate spinels should be deliberately examined because most spinel oxides are of intermediate structure. Here, we report an investigation on the ORR and OER performance of intermediate spinel MnFe2O4. The modulation of cation oxidation state and inversion degree of spinel MnFe2O4 were achieved by a simple annealing process. X-ray absorption spectroscopy analysis reveals that the Mn occupancy in octahedral sites varied from 0.25 ~ 0.41 and Mn cations were oxidized from 2+ to 3+ with increasing temperature treatment. Convinced by the leading role of octahedral-geometric, we further reveal the role of Mn oxidation state through normalizing the activity to active Mn[Oh] site number. Our findings clearly indicate that Mn3+ was more catalytically active than Mn2+ in catalyzing ORR and OER. Graphical Abstract Both Mn occupancy in octahedral site and its oxidation state play dominant roles in determining the catalytic activities of spinel manganese ferrites toward oxygen electrocatalysis.
      PubDate: 2017-10-13
      DOI: 10.1007/s12678-017-0429-z
  • Design and Development of Instrumentations for the Preparation of Platinum
           Single Crystals for Electrochemistry and Electrocatalysis Research Part 3:
           Final Treatment, Electrochemical Measurements, and Recommended Laboratory
    • Authors: Nakkiran Arulmozhi; Derek Esau; Julia van Drunen; Gregory Jerkiewicz
      Abstract: Abstract In this contribution, which is the final paper in a topic-focused series of three papers, we report on the development of experimental methodology that is applied to acquire cyclic voltammetry (CV) profiles for hemispherical Pt(111), Pt(110), and Pt(100) as well as polyoriented monocrystalline Pt electrodes. The paper describes an effective, multi-step procedure for cleaning of electrochemical and annealing glassware. The procedure comprises a four-step process for cleaning glassware and a three-step process for cleaning Viton and Teflon parts to minimize any contamination that can originate from these setup components. The contribution also outlines general laboratory practices that need to be followed in order to maintain clean conditions for conducting electrochemical experiments. The paper discusses a thermal treatment methodology that consists of two steps, namely flame-based annealing and induction-based annealing. The application of this approach produces reproducible, clean, defect-free, and atomically-ordered Pt surfaces. The complete procedure of Pt electrode annealing, cooling in a controlled atmosphere, protecting with a droplet of water, and transferring to an electrochemical cell allows one to conserve a metallic character of the electrode surface (it does not form any oxide during the transfer) while maintaining an atomic order. In addition, the paper describes the design and operation of a multi-component electrochemical workstation, which is employed in measurements on platinum electrochemistry and electrocatalysis. The paper presents and discusses the hanging meniscus configuration that is used to maintain an electrolytic contact between a monocrystalline Pt electrode and an aqueous electrolyte solution. Finally, the contribution presents CV profiles for hemispherical Pt(111), Pt(110), and Pt(100) as well as polyoriented, monocrystalline Pt electrodes, which are characteristics of high-quality electrodes and clean experimental conditions. These results validate the instrumentation described in the series of papers and the laboratory practices developed in the course of our research. Graphical Abstract ᅟ
      PubDate: 2017-10-12
      DOI: 10.1007/s12678-017-0426-2
  • Fe 3 C Nanorods Encapsulated in N-Doped Carbon Nanotubes as Active
           Electrocatalysts for Hydrogen Evolution Reaction
    • Authors: Lulu Zhang; Yongting Chen; Pingping Zhao; Wei Luo; Shengli Chen; Minhua Shao
      Abstract: Abstract One of the main challenges to generate hydrogen is to develop cheap and highly active electrocatalysts toward hydrogen evolution reaction (HER). Herein, Fe3C nanorods encapsulated in N-doped carbon nanotubes (Fe3C@NCNT) were synthesized by a simple method involving the pyrolysis of a Fe-based metal organic framework (MOF) and melamine. The hybrid composite showed an excellent catalytic activity toward HER in an acidic electrolyte. The overpotential at the current density of 10 mA cm−2 was only 154 mV in a 0.5 M H2SO4 solution, which was lower than most of the non-precious metal and metal-free catalysts. Density functional theory (DFT) calculation results suggested that the synergistic effect between Fe3C nanorods and N dopants could effectively tune the H adsorption energy on carbon resulting in an enhanced HER activity. Graphical Abstract
      PubDate: 2017-10-06
      DOI: 10.1007/s12678-017-0425-3
  • The Sensitive and Selective Enzyme-Free Electrochemical H 2 O 2 Sensor
           Based on rGO/MnFe 2 O 4 Nanocomposite
    • Authors: Gnana Pragasam Jenita Rani; Jayachandran Saravanan; Sunirmal Sheet; Michael Anjello Jothi Rajan; Yang Soo Lee; Alagusundaram Balasubramani; Georgepeter Gnana kumar
      Abstract: Abstract The reduced graphene oxide/manganese ferrite (rGO/MnFe2O4) composite was prepared via a simple and straight forward solvothermal technique. The well-defined and mono-dispersed MnFe2O4 spherical particles with the average particle size of 370 nm, composing of 10-nm-sized primary nanoparticles were densely anchored over MnFe2O4 composite cons support. The cubic spinel structure of MnFe2O4 nanoparticles was unfurled through the diffraction patterns, and its compilation with rGO to form composite has sustained the structure, morphology, and size of MnFe2O4. The as-prepared rGO/MnFe2O4 composite exhibited superior electrocatalytic activity toward the enzyme-free H2O2 detection under neutral conditions. The as-fabricated rGO/MnFe2O4/glassy carbon electrode (GCE) exhibited an excellent enzyme-free H2O2 detection with a wide linear range of 1 μM–22 mM, a lower limit of detection of 0.35 μm (S/N = 3), and a considerable sensitivity of 1180 μA mM−1 cm−2. Furthermore, rGO/MnFe2O4/GCE demonstrated the high selectivity toward H2O2 without the influence of other electroactive species and exhibited the prompt recovery in the range of 97.84–102.23% toward real sample analysis, which ensured the viable applications of rGO/MnFe2O4 composite in electrochemical enzyme-free H2O2 sensors. Graphical Abstract ᅟ
      PubDate: 2017-10-06
      DOI: 10.1007/s12678-017-0418-2
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