Electrochemistry of anodic fluorine gas evolution at carbon electrodes: Part III characterization of activated carbon anodes following onset of the ‘anode effect’

The exceptional activity of carbon anodes for the fluorine evolution reaction (FER) in a KF·2HF melt at 358 K that can be produced, based on a previously reported procedure for reactivation following onset of the anode effect, has been characterized. The activated carbon anodes that have been treated by applying the procedure exhibit unique, exceptionally active, polarization behaviour. This is manifested as a high exchange current-density, a low Tafel slope in the high current-density range, resistance to further anodic effects, facilitation of the detachment of F₂ gas bubbles and good electrode-life properties. At polarization current-densities of 0.1 and 0.2 A cm^–2, the anode potential is 1.5 and 2.5 V lower, respectively, for the activated that for a normal non-activated carbon electrode. The improvements of the activated carbon anodes are associated with facilitation of the detachment of the F₂ bubble/film. The F₂ bubble/film adherence effect is the main cause for the abnormally high polarization and anode effect for a normal, unactivated carbon anode, as concluded in our previous studies (Part I). It is shown that the facilitation of F₂ bubble/film detachment is due to two physical properties of the activated carbon electrode: a much smaller solid/gas/liquid contact angle and a much smoother surface. As determined by means of ESCA, it is shown that, compared with an unactivated carbon anode, the activated anodes have a smaller extent of surface fluorination corresponding to thinner CF films. This may lead both to a favourable contact angle and a smaller barrier layer for activated electron tunnelling at the activated carbon anodes used for the FER, than at normal non-activated carbon electrodes.     

Electrochemistry of anodic fluorine gas evolution at carbon electrodes: Part II studies on the ‘CF’ film by the current-interruption,AC impedance,ESCA and auger techniques

The CF films that are formed on the surface of carbon anodes used for the fluorine evolution reaction (FER) in KF·2HF melts at 358 K have been studied by bothin situ electrochemical current-interruption and a.c. impedance methods, and byex situ surface spectroscopy [ESCA (XPS) and Auger] techniques. The surface analysis measurements indicate that a thin CF (CF₂) film, 1.7 nm in thickness is formed on the carbon anodesurface. Results from depth profiling analyses of the film indicate that it is not uniform, higher levels of CF and F components being found towards the carbon anode surface. Thein situ electrochemical measurements demonstrate that an abnormally small interfacial capacitance, (1.6–2.7)×10^–7 F cm^–2, arises in the course of the FER at carbon anodes; this was attributed to the presence of a passive dielectric CF film on the carbon electrodes. The determined interfacial capacitance does not change significantly with potential in the potential range studied, which implies that the thickness of the CF film on the fluorine-evolving carbon anodes may be independent of potential.     

Bubble behaviour during oxygen and hydrogen evolution at transparent electrodes in KOH solution

For oxygen and hydrogen evolving transparent nickel electrodes in KOH solutions, parameters characterizing the behaviour of bubbles which are adhered to the electrode surface during gas evolution, have been determined in dependence on current density, i, velocity of solution flow, v, pressure, p, temperature, T, and concentration of KOH. Based on experimental data a new basic bubble parameter, J, has been introduced, which accounts for the bubble behaviour. It has been found that J = a₁i^h₁ and J/(J₀?J) = a₂v^h₂ where J₀ = J at v = 0 ms^?1 and a₁,a₂h₁ and h₂ are empirical constants; some of these depend on nature of gas evolved. Moreover, the parameter J is almost proportional to the KOH concentration, increases in a decreasing rate with increasing pressure and increases linearly with the reciprocal of the absolute temperature.     

Oxidation kinetics of 4-methylanisole in methanol solution at carbon electrodes

A quantitative investigation of the electrochemical oxidation of 4-methylanisole in methanol solution at carbon electrodes has been performed. The oxidation reaction is shown to be complex, resulting in the formation not only of the corresponding diacetal, but also of several intermediate products and side products. Voltammetric measurements and preparative batch syntheses reveal a substantial influence of the choice of both electrode material and supporting electrolyte. The highest selectivity and the most rapid reaction rates are observed at graphite electrodes with potassium fluoride supporting electrolyte, whereas polished surfaces of glassy carbon are far less reactive and result in substantial formation of side products. The observed oxidation kinetics can be represented with a simple empirical model, consisting of three oxidation steps in series yielding respectively an ether, an acetal and an ester. The experimental voltammetric curves have been used to determine the anisole diffusivity in the electrolyte solution and provide fitted values for the kinetic parameters of the three oxidation steps.     

Thermal effects on the measurement of photocurrent at anodic films on nickel electrodes

The influence of heat on the measurement of photocurrents at nickel electrodes in a 1 M NaOH solution was investigated by using photocurrent, photothermal and thermal current methods under the irradiation of laser beams with wavelengths of 488, 532 and 632 nm, respectively. It was found that the photocurrent appeared in the potential (vs. Hg/HgO electrode) region lower than −0.1 V was caused by the p-type semiconductor property of Ni(OH)₂ anodic film. In the Ni(OH)₂/NiOOH redox potential region the appearance of photocurrent was likely caused by the local temperature variation rather than the semiconductor properties of the anodic layer. Similar phenomena was observed for β-Ni(OH)₂ electrode. During the light irradiation an increase of temperature at the surface of the working electrode may disturb the measurement of photocurrent.     

The anodic evolution of sulphur from sodium polysulphide melts—I. Voltammetric measurements on planar carbon electrodes

The anodic evolution of sulphur from sodium polysulphide melts has been studied at planar vitreous carbon and pyrolytic graphite electrodes. A reaction mechanism is proposed, in which the primary product of the electrode reaction is a partly-discharged polysulphide anion, which undergoes chemical reactions in the diffusion layer to form sulphur. Phase formation is primarily a homogeneous process which does not proceed to a passivating film of sulphur upon the electrode. At high cd nucleation can occur upon the electrode leading to passivation, but the sulphur formed in this way does not wet the electrode, and depassivation can occur spontaneously.     

Electrocatalytic activation of ruthenium electrodes for the Cl2 and O2 evolution reactions by anodic/cathodic cycling

Ruthenium electrodes subjected to an anodic/cathodic potential cycling regime from 0.06 to 1.4E _H develop a changed state of surface oxidation in comparison with that observed in the initial sequence of potentiodynamic sweeps. The cycling effect is analogous to that known at iridium electrodes but here refers to monolayers. The kinetics of Cl₂ and O₂ evolution of these two types of oxidized surfaces were studied by steady-state polarization experiments. Current densities for Cl₂ evolution at the cycled Ru surface oxide areca 30 times greater than those at the original oxidized Ru surface. Oxygen evolution current densities are increased byca 8 times. The effect is a true electrocatalytic one since the real area of the Ru surfaces remains constant withinca 5%. The mechanisms of Cl₂ or O₂ evolution appear to remain unchanged, so the electrocatalytic effects observed are tentatively attributed to a change of potential range over which Ru(III) and Ru(IV) oxidation states arise in the oxide film causing modification of electron transfer rates or adsorption of ions and intermediates.Based on a report to Hooker Chemicals and Plastics Corp., Research Centre, Niagara Falls, NY, July 1976, on a research project supported by Hooker Research Center at the University of Ottawa, 1974–76.Where part of this work (on O₂ evolution electrocatalysis) was carried out by MV.     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.     

Carbon electrodes: Part 2. The anodic oxidation of N-propylamine

The anodic oxidation of a primary aliphatic amine, n-propylamine, has been studied at electrodes of vitreous carbon and pyrolytic graphite. The electrode reaction has been found to differ greatly for the two materials, as evidenced by the voltammetric behaviour at rotating disc electrodes and the results of preparative electrolyses. The observations at the vitreous carbon electrode are in agreement with the mechanism proposed by Mann and Barnes for the oxidation of propylamine at a stationary platinum electrode. A novel mechanism is proposed for the oxidation at a pyrolytic graphite anode.Part 1 M. P. J. Brennan and O. R. Brown,J. Appl. Electrochem.,2 (1972) 43.     

The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H₂SO₄ in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i_o2 follows the relation, log i = A - (E⁰_a - αFη)/2·3 RT - d/d_o. The experimental activation energy E_o = E^o_a = αFη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d_o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH^?_ad or O^2?_ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl^? and Ce⁴⁺ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.     

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO2 film electrodes

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO₂ film electrodes were studied, in connection with their physical and chemical properties. In general, the anodic characteristics of noble metal-doped electrodes were much better than those of base metal-doped electrodes. The oxygen evolution reaction on noble metal-doped electrodes reflected each characteristic of the doped noble metals and the active center for this reaction was considered to be on the noble metal sites. On the other hand, the chlorine evolution reaction on such electrodes was found to be independent of the kind of doped noble metals and the probable mechanism for this reaction was proposed in which the spillover of Cl radical from noble metal to Sn sites was involved.     

Adsorption of ochratoxin A (OTA) anodic oxidation product on glassy carbon electrodes in highly acidic reaction media: Its thermodynamic and kinetics characterization

We study the thermodynamics and kinetics of the adsorption of a redox couple having quinone nature on glassy carbon electrodes. This couple is produced by the anodic oxidation of mycotoxin ochratoxin A in 10% acetonitrile + 90% 1 M HClO₄ aqueous solution. The quasi-reversible redox couple was studied by both cyclic (CV) and square wave (SWV) voltammetric techniques. The Frumkin adsorption isotherm best described the specific interaction of the redox couple with carbon electrodes. By fitting the experimental data, we obtained values of −28.4 kJ mol⁻¹ and 0.70 ± 0.02 for the Gibbs free energy of adsorption and the interaction parameter, respectively. SWV fully characterized the thermodynamics and kinetics of the adsorbed redox couple, using a combination of the “quasi-reversible maximum” and the “splitting of SW peaks” methods. Average values of 0.609 ± 0.003 V and 0.45 ± 0.06 were obtained for the formal potential and the anodic transfer coefficient, respectively. Moreover, a formal rate constant of 10.7 s⁻¹ was obtained. SWV was also employed to generate calibration curves. The lowest concentration of mycotoxin was 1.24 × 10⁻⁸ M (5 ppb), measured indirectly with a signal to noise ratio of 3:1.     

Structural evolution of multi-walled carbon nanotube/MnO2 composites as supercapacitor electrodes

Multi-walled carbon nanotube (MWCNT)/MnO₂ supercapacitor electrodes containing MnO₂ nanoflakes in the MWCNT network are fabricated through the oxidation of manganese acetate with poly(4-styrenesulfonic acid) (PSS) dispersed MWCNTs. The structural evolution of the electrodes under charge/discharge (reduction/oxidation) cycles and its impact on the electrodes’ electrochemical properties are evaluated. Structural evolution involves the dissolution of MnO₂ upon reduction, the diffusion of the reduced Mn species from the MWCNT network toward the electrolyte solution, and the deposition of MnO₂ on the electrode surface upon oxidation. Electrode structural changes, including the electrode dissolution and the growth of the MnO₂ crystals, are scan rate dependent and have deteriorating effect on the electrode's electrochemical properties including the specific capacitance and cyclic stability.     

Determination of the charge-transfer kinetics of ferrocene at platinum and vitreous carbon electrodes by potential steps chronocoulometry

Potential step chronocoulometric methods were used to measure kinetic parameters for the oxidation of ferrocene at platinum vitreous carbon electrodes in acetonitrile. The formal rate constants observed, k^o_f ～ 0.02 cm s^?1, were independent of electrode material and of the supporting electrolyte chosen (LiClO₄, TEAP or TEABF₄). After correcting for the effects of double layer structure, the experimental activation energy for electron transfer was found to be ΔG≠ (exp) = 0.03 ±0.01 eV molecule^?1 which agreed within 20% with a theoretical value ΔG≠ (theor) = 0.26 eV molecule^?1 calculated for activation by solvent repolarization alone from the Born equation.     

Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes

Carbon materials are often used as catalyst supports, and for catalysts in electrodes of a polymer electrolyte fuel cell, carbon black has been used. Recently, it was found, however, that activated carbon could replace carbon black and besides, significantly improve the activity of the electrode catalyst layer for oxygen reduction. In the present study, to optimize the pore structure of activated carbon for further activity improvement, the influence of the pore structure on the activity was investigated using activated carbon of various specific surface areas and mean pore diameters. A catalyst layer was formed from activated carbon loaded with platinum and a polymer electrolyte. The activity of the layer was measured in an oxygen-saturated perchloric acid solution, supporting the layer on a rotating glassy carbon disk electrode. We found that increases in the specific surface area and mean pore diameter increased the activity and that the latter was more effective than the former mainly due to the enhanced mass-transfer in the pores; the catalyst layer formed from activated carbon with the largest mean pore diameter was the most active. Unless pores excessively develop and lose connections between particles, a large pore diameter is therefore desired for the fuel cell electrodes.     

Electrosynthesis at oxide coated electrodes Part 1 the kinetics of ethanol oxidation at spinel electrodes in aqueous base

The oxidation of ethanol at Ni/Co spinel coated electrodes in aqueous base has been investigated. It is shown that these electrodes are very stable and have a good catalytic activity for the conversion of ethanol to acetic acid (for example 24 mA cm^–2 for the oxidation of 0.1 mol dm^–3 ethanol), far superior to the nickel anodes previously used for the reaction in similar conditions. TheI-E curve for the oxidation of ethanol at a spinel shows a well formed wave prior to oxygen evolution at a potential where the spinel surface itself undergoes oxidation. The limiting current is partially mass transport controlled. The influence of the electrolysis conditions, the composition of the spinel coated electrodes and of parameters in their preparation, on the rate of ethanol oxidation is described.     

Electroactive intermediate states in anodic O2 evolution at Ni---Mo-containing glassy metals

Digital recording of potential relaxation transients, following interruption of current, has been applied to the experimental study of the potential-dependence of the intermediate states, I, involved as the kinetically-significant reaction intermediates in anodic O₂ evolution at three Ni---Mo-based glassy metals. I species are to be regarded as not simply chemisorbed OH or O entities in the elementary steps of the O₂ evolution process but also involve local high oxidation states of the metal ions of the oxide film on which the O₂ evolution process takes place through a mediator mechanism.Analysis of the potential-decay transients shows that two regions of adsorption pseudocapacitance associated with potential-dependence of coverage by I are involved: one arising from the bulk oxide of the film and the second, smaller component, from the surface region of the oxide film where the O₂ evolution process takes place.The potential-decay behaviour, coupled with evaluation of the Tafel polarization relations and pH effects in the kinetics, enables some new aspects of electrocatalysis for anodic O₂ evolution on oxide films to be suggested.     

On the adsorption and kinetics of phase transients of adenosine at the different carbon electrodes modified with a mercury layer

Optical roughness of a pyrolytic graphite electrode with basal (PGEb) and edge orientation (PGEe), a glassy carbon electrode (GCE), and the same electrodes modified with mercury (MFE) was studied by an optical diffractive element (DOE) based sensor. Electrochemical characterisation of these electrodes used was performed by cyclic voltammetry (CV) and capacitance measurements (C-E curves and electrochemical impedance spectroscopy, EIS). The kinetics of phase transients of adenosine adsorbed on PGEb, and GCE modified with mercury layer of different thicknesses (thickness was changed from 0.02 to 2 μm) was studied by chronoamperometry (j-t curves) and capacitance measurements (C-E curves). In acidic (pH 5) solution adenosine forms two different two-dimensional (2D) physisorbed condensed layers on the MFE. The first of these (region I) is located at more positive potential; the centre of this adlayer is situated around −0.4 V. The second 2D physisorbed film (region III) is formed at more negative potentials; the centre of the region III is around −1.3 V. The 2D condensed films (adlayer I and III) of adenosine still exist on the PGEb substrate modified by 0.02 μm thick mercury layer. Adlayers III and I of adenosine exist on the GCE modified with mercury layer down to 0.02 and 0.2 μm, respectively. The kinetics of phase transients of the adenosine films taking place by a potential jump from dilute adsorption region (state II and IV) to the 2D physisorbed film (region I and III) at the PGEb and GCE substrates modified with mercury (Hg-PGEb and Hg-GCE) were studied, respectively. During both of the phase transients of IV→III and II→III of adenosine a polynucleation and growth process on both the Hg-PGEb and Hg-GCE was detected. The phase transients of II→I are characterised by an exponential decay of the current without the current maximum (the adsorption process took place only). It was observed that the phase transients of IV→III are the fastest at the HMDE and gradually slow down on 2 μm Hg-GCE and 2 μm Hg-PGEb, respectively.     

Electrolyte wetting effects in the comparative polarization behaviour of the hydrogen evolution reaction at Ni-Mo-Cd electrodes in KF · 2HF and KOH · 2H2O melts

In the electrolyte process of fluorine production from KF · 2HF melts at 85° C, unusually high polarization arises both at the cell anodes (carbon) and cathodes (mild steel) at moderate current densities. The anomalous polarization behaviour in fluorine evolution at carbon anodes has been extensively studied but much less is known about the origin of hyperpolarization in hydrogen evolution at the cathodes. Here, the results of comparative polarization studies at a Ni-Mo-Cd composite cathode material in a KF · 2HF melt and a corresponding aquo-analogue melt of KOH · 2H₂O are reported. Large differences are observed which are attributed to the different wetting characteristics of these two melts at the interface of the Ni-Mo-Cd cathodes that are microporous. Such electrode material also offers a major improvement of cathode polarization over that at mild steel surfaces. Conclusions on wetting effects are based on: (i) a comparative determination of hydrogen-bubble contact angles at the electrode in the two melts and (ii) an evaluation of the double-layer capacitances of the electrode interfaces with the two melts as determined by the fast potential-relaxation method, at short times after current interruption. The apparent double-layer capacitances at a given electrode are very different in the two melts.     

The influence of F doping on the activity of PbO2 film electrodes in oxygen evolution reaction

The PbO₂ and F⁻-doped PbO₂ (F-PbO₂) film electrodes have been prepared by an electrodeposition process onto platinum substrate. The changes of their electrochemical activity for oxygen evolution (OE) reaction with accelerated electrolysis time have been studied. The results show that the activity of the electrodes decreases with increasing electrolysis time. This can be attributed to the conductivity variations, which are caused by the decrease of the oxygen vacancy concentration in PbO₂ films during OE. The mechanism of the decrease of the oxygen vacancies has been investigated. For F-PbO₂ film electrodes, though its electrochemical activity is somewhat lower than that of undoped one, its activity stability is much higher in comparison with that of PbO₂. The influence of F⁻ doping on the activity of PbO₂ film electrodes has been also discussed.