A study of the oxygen electrochemistry of ruthenium and iridium

This thesis is concerned with an investigation of the anodic behaviour of ruthenium and iridium in aqueous solution and particularly of oxygen evolution on these metals. The latter process is of major interest in the large-scale production of hydrogen gas by the electrolysis of water. The presence of low levels of ruthenium trichloride ca. 10-4 mol dm-3 in acid solution give a considerable increase in the rate of oxygen evolution from platinum and gold, but not graphite, anodes. The mechanism of this catalytic effect was investigated using potential step and a.c. impedance technique. Earlier suggestions that the effect is due to catalysis by metal ions in solution were proved to be incorrect and it was shown that ruthenium species were incorporated into the surface oxide film. Changes in the oxidation state of these ruthenium species is probably responsible for the lowering of the oxygen overvoltage. Both the theoretical and practical aspects of the reaction were complicated by the fact that at constant potential the rates of both the catalysed and the uncatalysed oxygen evolution processes exhibit an appreciable, continuous decrease with either time or degree of oxidation of the substrate. The anodic behaviour of iridium in the oxide layer region has been investigated using conventional electrochemical techniques such as cyclic voltammetry. Applying a triangular voltage sweep at 10 Hz, 0.01 to 1.50V increases the amount of electric charge which the surface can store in the oxide region. This activation effect and the mechanism of charge storage is discussed in terms of both an expanded lattice theory for oxide growth on noble metals and a more recent theory of irreversible oxide formation with subsequent stoichiometry changes. The lack of hysteresis between the anodic and cathodic peaks at ca. 0.9 V suggests that the process involved here is proton migration in a relatively thick surface layer, i.e. that the reaction involved is some type of oxide-hydroxide transition. Lack of chloride ion inhibition in the anodic region also supports the irreversible oxide formation theory; however, to account for the hydrogen region of the potential sweep a compromise theory involving partial reduction of the outer regions of iridium oxide film is proposed. The loss of charge storage capacity when the activated iridium surface is anodized for a short time above ca. 1.60 V is attributed to loss by corrosion of the outer active layer from the metal surface. The behaviour of iridium at higher anodic potentials in acid solution was investigated. Current-time curves at constant potential and Tafel plots suggested that a change in the mechanism of the oxygen evolution reaction occurs at ca. 1.8 V. Above this potential, corrosion of the metal occurred, giving rise to an absorbance in the visible spectrum of the electrolyte (λ max = 455 nm). It is suggested that the species involved was Ir(O2)2+. A similar investigation in the case of alkaline electrolyte gave no evidence for a change in mechanism at 1.8 V and corrosion of the iridium was not observed. Oxygen evolution overpotentials were much lower for iridium than for platinum in both acidic and alkaline solutions.

Oxygen reduction reaction catalyzed by epsilon-MnO2: Influence of the crystalline structure on the reaction mechanism

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on carbon supported epsilon-manganese dioxide (epsilon-MnO2/C). The epsilon-MnO2/C catalyst was prepared via thermal decomposition of manganese nitrate and carbon powder (Vulcan XC-72) mixtures. X-ray powder diffraction (XRD) measurements were performed in order to determine the crystalline structure of the resulting composite, while energy dispersive X-ray analysis (EDX) was used to evaluate the chemical composition of the synthesized material. The electrochemical studies were conducted using cyclic voltammetry (CV) and quasi-steady state polarization measurements carried out with an ultra thin layer rotating ring/disk electrode (RRDE) configuration. The electrocatalytic results obtained for 20% (w/w) Pt/C (E-TEK Inc., USA) and alpha-MnO2/C for the ORR, considered as one of the most active manganese oxide based catalyst for the ORR in alkaline media, were included for comparison. The RRDE results revealed that the ORR on the MnO2 catalysts proceeds preferentially through the complete 4e(-) reduction pathway via a 2 plus 2e(-) reduction process involving hydrogen peroxide as an intermediate. A benchmark close to the performance of 20% (w/w) Pt/C (E-TEK Inc., USA) was observed for the epsilon-MnO2/C material in the kinetic control region, superior to the performance of alpha-MnO2/C, but a higher amount of HO2- was obtained when epsilon-MnO2/C was used as catalyst. The higher production of hydrogen peroxide on epsilon-MnO2/C was related to the presence of structural defects, typical of this oxide, while the better catalytic performance in the kinetic control region compared to alpha-MnO2/C was related with the higher electrochemical activity for the proton insertion kinetics, which is a structure sensitive process. (C) 2012 Elsevier Ltd. All rights reserved.

Oxygen-participated electrochemistry of new lithium-rich layered oxides Li3MRuO5 M = Mn, Fe)

We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R (3) over barm) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn3+ and low spin configuration for Ru4+ where the itinerant electrons occupy a pi*-band. The onset of a net maximum in the chi vs. T plot at 9.5 K and the negative value of the Weiss constant (theta) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn3+ and Ru4+ are partially oxidized to Mn4+ and Ru5+ in the sloping region at low voltage, while in the long plateau, O2- is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O2- ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O2- (plateau) while Fe seems to retain its 3+ state.

Oxygen: problems and solutions in electrochemistry

Different aspects of the electrochemistry of oxygen are examined through four experimental examples: corrosion, passivation via organic thin films, oxygen reduction and water oxidation catalysis are outlined in order to outline the very different ways and circumstances in which oxygen plays a major role in electrochemistry.

Direct electrochemical formation of nanostructured amorphous Co(OH)2 on gold electrodes with enhanced activity for the oxygen evolution reaction.

The oxides of cobalt have recently been shown to be highly effective electrocatalysts for the oxygen evolution reaction (OER) under alkaline conditions. In general species such as Co3O4 and CoOOH have been investigated that often require an elevated temperature step during their synthesis to create crystalline materials. In this work we investigate the rapid and direct electrochemical formation of amorphous nanostructured Co(OH)2 on gold electrodes under room temperture conditions which is a highly active precursor for the OER. During the OER some conversion to crystalline Co3O4 occurs at the surface, but the bulk of the material remains amorphous. It is found that the underlying gold electrode is crucial to the materials enhanced performance and provides higher current density than can be achieved using carbon, palladium or copper support electrodes. This catalyst exhibits excellent activity with a current density of 10 mA cm-2 at an overpotential of 360 mV with a high turnover frequency of 2.1 s-1 in 1 M NaOH. A Tafel slope of 56 mV dec-1 at low overpotentials and a slope of 122 mV dec-1 at high overpotentials is consistent with the dual barrier model for the electrocatalytic evolution of oxygen. Significantly, the catalyst maintains excellent activity for up to 24 hr of continuous operation and this approach offers a facile way to create a highly effective and stable material.

The mobility of oxygen ions in CaF2

The a.c. conductivity of CaF2 samples containing a fine dispersion of CaO particles has been measured in the temperature range 630 to 1100 K. The conductivity of the dispersed solid electrolyte is two orders of magnitude higher than that for pure polycrystalline CaF2 in the middle of the temperature range. Transport measurements on pure single crystals of CaF2 and polycrystalline samples, with and without CaO dispersion, using Fe+FeO and pure Fe as electrodes, clearly indicate that fluorine ions are the only migrating ionic species with a transport number of almost unity, contrary to the suggestion of Chou and Rapp [1, 2]. The enhanced conductivity of the dispersed solid electrolyte probably arises from two effects. A small solubility of oxygen in CaF2 results in an increase in the fluorine vacancy concentration and conductivity. Adsorption of fluorine ions on the surface of the dispersed particles of CaO results in a space charge region around each particle with enhanced conductivity. Measurements on a galvanic cell incorporating CaF2 as the solid electrolyte and oxide electrodes show that the e.m.f. is a function of the activity of CaO at the electrode/electrolyte interface. The response to an oxygen potential gradient is, therefore, through an exchange reaction, which establishes an equivalent fluorine potential at the electrode/electrolyte interface.

Electrocatalysis and redox behavior of Pt(2+) ion in CeO(2) and Ce(0.85)Ti(0.15)O(2): XPS evidence of participation of lattice oxygen for high activity

Electronic states of CeO(2), Ce(1 -aEuro parts per thousand x) Pt (x) O(2 -aEuro parts per thousand delta) , and Ce(1 -aEuro parts per thousand x -aEuro parts per thousand y) Ti (y) Pt (x) O(2 -aEuro parts per thousand delta) electrodes have been investigated by X-ray photoelectron spectroscopy as a function of applied potential for oxygen evolution and formic acid and methanol oxidation. Ionically dispersed platinum in Ce(1 -aEuro parts per thousand x) Pt (x) O(2 -aEuro parts per thousand delta) and Ce(1 -aEuro parts per thousand x -aEuro parts per thousand y) Ti (y) Pt (x) O(2 -aEuro parts per thousand delta) is active toward these reactions compared with CeO(2) alone. Higher electrocatalytic activity of Pt(2+) ions in CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) compared with the same amount of Pt(0) in Pt/C is attributed to Pt(2+) ion interaction with CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) to activate the lattice oxygen of the support oxide. Utilization of this activated lattice oxygen has been demonstrated in terms of high oxygen evolution in acid medium with these catalysts. Further, ionic platinum in CeO(2) and Ce(1 -aEuro parts per thousand x) Ti (x) O(2) does not suffer from CO poisoning effect unlike Pt(0) in Pt/C due to participation of activated lattice oxygen which oxidizes the intermediate CO to CO(2). Hence, higher activity is observed toward formic acid and methanol oxidation compared with same amount of Pt metal in Pt/C.

Fundamental Studies of Carbon Oxygen Bond Activation in Nickel Diphosphine Ether Complexes. And, Metallomacrocycles as Ligands: Synthesis and Characterization of Aluminum-Bridged Bisglyoximato Complexes of Iron and Cobalt

In order to develop better catalysts for the cleavage of aryl-X bonds fundamental studies of the mechanism and individual steps of the mechanism have been investigated in detail. As the described studies are difficult at best in catalytic systems, model systems are frequently used. To study aryl-oxygen bond activation, a terphenyl diphosphine scaffold containing an ether moiety in the central arene was designed. The first three chapters of this dissertation focus on the studies of the nickel complexes supported by this diphosphine backbone and the research efforts in regards to aryl-oxygen bond activation.

Chapter 2 outlines the synthesis of a variety of diphosphine terphenyl ether ligand scaffolds. The metallation of these scaffolds with nickel is described. The reactivity of these nickel(0) systems is also outlined. The systems were found to typically undergo a reductive cleavage of the aryl oxygen bond. The mechanism was found to be a subsequent oxidative addition, β-H elimination, reductive elimination and (or) decarbonylation.

Chapter 3 presents kinetic studies of the aryl oxygen bond in the systems outlined in Chapter 2. Using a series of nickel(0) diphosphine terphenyl ether complexes the kinetics of aryl oxygen bond activation was studied. The activation parameters of oxidative addition for the model systems were determined. Little variation was observed in the rate and activation parameters of oxidative addition with varying electronics in the model system. The cause of the lack of variation is due to the ground state and oxidative addition transition state being affected similarly. Attempts were made to extend this study to catalytic systems.

Chapter 4 investigates aryl oxygen bond activation in the presence of additives. It was found that the addition of certain metal alkyls to the nickel(0) model system lead to an increase in the rate of aryl oxygen bond activation. The addition of excess Grignard reagent led to an order of magnitude increase in the rate of aryl oxygen bond activation. Similarly the addition of AlMe3 led to a three order of magnitude rate increase. Addition of AlMe₃ at -80 °C led to the formation of an intermediate which was identified by NOESY correlations as a system in which the AlMe3 is coordinated to the ether moiety of the backbone. The rates and activation parameters of aryl oxygen bond activation in the presence of AlMe3 were investigated.

The last two chapters involve the study of metalla-macrocycles as ligands. Chapter 5 details the synthesis of a variety of glyoxime backbones and diphenol precursors and their metallation with aluminum. The coordination chemistry of iron on the aluminum scaffolds was investigated. Varying the electronics of the aluminum macrocycle was found to affect the observed electrochemistry of the iron center.

Chapter 6 extends the studies of chapter 5 to cobalt complexes. The synthesis of cobalt dialuminum glyoxime metal complexes is described. The electrochemistry of the cobalt complexes was investigated. The electrochemistry was compared to the observed electrochemistry of a zinc analog to identify the redox activity of the ligand. In the presence of acid the cobalt complexes were found to electrochemically reduce protons to dihydrogen. The electronics of the ancillary aluminum ligands were found to affect the potential of proton reduction in the cobalt complexes. These potentials were compared to other diglyoximate complexes.

PtCeOx/C as a novel methanol-tolerant electrocatalyst of oxygen reduction for direct methanol fuel cells

A prominent methanol-tolerant characteristic of the PtCeOx/C electrocatalyst was found during oxygen reduction reaction process. The carbon-supported platinum modified with cerium oxide (PtCeOx/C) as cathode electrocatalyst for direct methanol fuel cells was prepared via a simple and effective route. The synthesized electrocatalysts were characterized by X-ray diffraction and transmission electron microscopy. It was found that the cerium oxide within PtCeOx/C present in an amorphous form on the carbon support surface and the PtCeOx/C possesses almost similar disordered morphological structure and slightly smaller particle size compared with the unmodified Pt/C catalyst.

Direct electrochemistry of glucose oxidase and biosensing for glucose based on carbon nanotubes@SnO2-Au composite

Multiwalled carbon nanotubes@SnO2-Au (MWCNTs@SnO2-Au) composite was synthesized by a chemical route. The structure and composition of the MWCNTs@SnO2-Au composite were confirmed by means of transmission electron microscopy, X-ray photoelectron and Raman spectroscopy. Due to the good electrocatalytic property of MWCNTs@SnO2-Au composite, a glucose biosensor was constructed by absorbing glucose oxidase (GOD) on the hybrid material. A direct electron transfer process is observed at the MWCNTs@SnO2-Au/GOD-modified glassy carbon electrode. The glucose biosensor has a linear range from 4.0 to 24.0 mM, which is suitable for glucose determination by real samples. It should be worthwhile noting that, from 4.0 to 12.0 mM, the cathodic peak currents of the biosensor decrease linearly with increasing the glucose concentrations in human blood. Meanwhile, the resulting biosensor can also prevent the effects of interfering species.

The real role of carbon in Pt/C catalysts for oxygen reduction reaction

A new electrocatalysis of carbon materials for oxygen reduction reaction (ORR) on Pt/C catalysts was discovered. It was found that there exist two kinds of electroactive sites on these supports of carbon materials, which can effectively electrocatalyze the reduction of peroxide intermediated from oxygen reduction on Pt, as this provides continuous driving force to move the equilibrium toward the production of peroxide from ORR.

Electrocatalytic reduction of oxygen at multi-walled carbon nanotubes and cobalt porphyrin modified glassy carbon electrode

The multi-walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc-NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O-2 reduction. The reduction peak potential of O-2 is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co-exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MVVNTs/CoTMPyP)(n) prepared by layer-by-layer method were investigated, and the results showed that the peak current of O-2 reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.

Electrochemical impedance spectroscopy analysis for oxygen reduction reaction in 3.5% NaCl solution

The electrochemical impedance spectroscopy (EIS) at different potentials has been used to study the oxygen reduction reaction (ORR) in 3.5% NaCl solution on glassy carbon (GC) electrode in this work. Results show that ORR consists of three two-electron reaction steps and both superoxide ion (O-2(-)) and hydrogen peroxide (H2O2), which are produced by ORR, obstruct the diffusion of oxygen to the surface of the electrode and make the EIS results change into a transmissive finite diffusion process with the real part contraction and a reflective finite diffusion process from a semi-infinite diffusion process. The values of electron transfer resistance (R-t) and diffusion resistance (R-d) were calculated from EIS. O-2(-) influenced strongly on the Rt values and induced a maximum at -0.45 V.

The electrochemical reduction reaction of dissolved oxygen on Q235 carbon steel in alkaline solution containing chloride ions

Cyclic voltammetry, electrochemical impedance spectroscopy, and rotating disk electrode voltammetry have been used to study the effect of chloride ions on the dissolved oxygen reduction reaction (ORR) on Q235 carbon steel electrode in a 0.02 M calcium hydroxide (Ca(OH)(2)) solutions imitating the liquid phase in concrete pores. The results indicate that the cathodic process on Q235 carbon steel electrode in oxygen-saturated 0.02 M Ca(OH)(2) with different concentrations of chloride ions contain three reactions except hydrogen evolution: dissolved oxygen reduction, the reduction of Fe(III) to Fe(II), and then the reduction of Fe(II) to Fe. The peak potential of ORR shifts to the positive direction as the chloride ion concentration increases. The oxygen molecule adsorption can be inhibited by the chloride ion adsorption, and the rate of ORR decreases as the concentration of chloride ions increases. The mechanism of ORR is changed from 2e(-) and 4e(-) reactions, occurring simultaneously, to quietly 4e(-) reaction with the increasing chloride ion concentration.