E Embrecht Barendrecht

Influence of inserted anions on the properties of polypyrrole

Selected properties of supported and free-standing polypyrrole films have been studied as a function of the anion, incorporated in the polymer during electrochemical polymerization. The anion was varied in the range: Cl−, ClO−4, benzenesulfonate (BS), toluenesulfonate (TOS), 1,3-benzenedisulfonate (BDS) and 1,3,5-benzenetrisulfonate (BTS). The degree of oxidation of polypyrrole as determined by elemental analysis, varied from ca 0.50, when Cl− or ClO−4 is used, to ca 0.30 for the “organic” anions. Electron micrographs of the films show an increasing smoothness of the films with increasing charge of the ions. The electrical conductivity of free-standing films shows the highest value for the BTS-containing films and decreases in the series: BTS > TOS > BS ⋍ BDS > Cl− ⋍ ClO−4. The reduction and re-oxidation of thin supported films is a slow process for TOS- and BS-containing films. In these cases, the rate of the process is determined by the diffusion of the anions in the film from Q—t12 plots; diffusion coefficients of 4.3 × 10−11 cm2 s−1 for TOS and 4.8 × 10−11 cm2 s−1 for BS can be calculated. For Cl− and ClO−4 containing films, the process is faster and determined by the transition of the polymer from the oxidized to the reduced state (or vice versa). From the linear part of the Q—t12 curves a diffusion coefficient of 3.1 × 10−9 cm2 s−1 for Cl− and 3 × 10−9 cm2 s−1 for ClO−4 can be estimated. Also for films containing BDS and BTS ions, the process seems to be determined by the transition of the polymer, instead of by the diffusion of the ions. The differences in behaviour between the various organic ions are explained in terms of a more or less polar character of the ions. Ellipsometric experiments, carried out in ClO−4 and toluenesulfonate solutions, support the observed differences in behaviour during reduction and re-oxidation of the polymer films.

The reduction of dioxygen at polypyrrole-modified electrodes with incorporated Pt particles

Abstract The electrodeposition of Pt particles in a polypyrrole layer leads to electrodes, showing electrocatalytic activity for the reduction of dioxygen in aqueous media. The electrode preparation, with the current density and time of deposition as main parameters, influences the final product composition, ie the ratio of water to hydrogen peroxide, as dioxygen can be reduced to water or hydrogen peroxide, respectively, via a four- or a two-electron mechanism. By using the rotating ring—disc technique the amount of hydrogen peroxide in the reaction product can be detected. When Pt particles are deposited with a high current density, mainly water is produced. At low current densities of Pt deposition, hydrogen peroxide is the main product. These effects are explained in terms of the availability of the Pt surface for dioxygen.

Oxidation of hydrogen at platinum-polypyrrole electrodes

Polypyrrole, an electronic conducting polymer, is used as a matrix for the dispersion of Pt particles. These particles can be included by two methods, viz. (1) by electrochemical depositin of platinum particles on a polypyrrole covered glassy carbon disc and (2) by incorporation of Pt particles during the polymerization of pyrrole on a glassy carbon disc. As a model reaction the oxidation of hydrogen at these electrodes is studied. The polypyrrole electrodes prepared by method (1) exhibit a good catalytic behaviour. The other type of electrodes however show, despite the higher Pt loading, much less activity. Additionally, electrodes were prepared according to method (1) with poly(N-methylpyrrole) and polyaniline as the conducting polymer. These electrodes show a similar diffusion limited behaviour for the oxidation of hydrogen as polypyrrole-modified electrodes, however the oxidation starts at a much higher potential.

Oxygen reduction on pyrolysed carbon-supported transition metal chelates

Abstract A review is given of the pyrolysis behaviour of carbon-supported transition metal chelates and the developed theories explaining the increased activity and stability towards oxygen reduction. Additional measurements which a new carbon-modified rotating ring-disc electrode show that the central metal ionN 4 unit remains the active site after pyrolysis. The selectivity is increased after pyrolysis since relatively less H 2 O 2 is detected at the ring. The number of available active sites is an important parameter for the resulting activity. During the heat treatment, different processes take place: migration of the chelates over the carbon support, increasing the number of availableactive sites; reaction of the outer fringes of the organic skeleton retaining the metal ionN 4 unit; and degration of the catalyst with loss of activity. The condition of the pyrolysed catalyst and its corresponding activity are determined by the relative rate of these processes at the given pyrolysis temperature.

Redox potential and electrocatalysis of O2 reduction on transition metal chelates

Abstract The redox potentials of iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and cobalt tetraazaannulene (CoTAA), irreversibly adsorbed on pyrolytic graphite, were investigated as a function of the pH by cyclic voltammetry. The reduction of oxygen was studied under the same conditions with the rotating disc electrode technique. At high pH, where the reduction to H2O2 on the cobalt-containing chelates is more reversible, the value of the redox potential is less important. At low pH, where this reduction is irreversible, there is a clear correlation between the redox potential of the central metal ion and the O2 reduction behaviour. In acid media, a 470 mV more positive redox potential of CoTAA compared to that of CoPc, resulted in a half-wave potential, E 1 2 , for the O2 reduction being 400 mV more positive than the E 1 2 of CoPc.

Oxygen reduction at polypyrrole electrodes - I.Theory and evaluation of the RRDE experiments

The rotating ring-disc electrode (rrde) is used to determine the heterogeneous reaction rate constants for the cathodic reduction of molecular oxygen at a polypyrrole electrode in 0.5 M H2SO4. During the rrde experiments, a substantial concentration of hydrogen peroxide is built up in the bulk electrolyte. The theory is adapted to these circumstances and from the results, the various reaction rate constants are calculated. In the modified theory, the bulk peroxide is used to determine the activity of the ring electrode and it is possible to correct the ring current in situations where no diffusion limitation of the peroxide oxidation occurs.

Reduction of nitric oxide at a platinum cathode in an acidic solution

Abstract The reduction of nitric oxide at a platinum electrode in 4 M H 2 SO 4 was investigated by the measurement of potential/current relations and by the determination of the current efficiencies for the hydroxylamine, nitrous oxide, ammonia, hydrazine and hydrogen formation at fixed potentials in the potential range from 0 to −400 mV vs sce . The potential/current curve for the reduction of nitric oxide has two waves, of which the limiting currents depend strongly on the potential and the time of the pretreatment of the electrode and on the direction of the potential change during the measurements. In the potential range of the first wave ( viz ϵ > −30 mV) nitric oxide is reduced only to nitrous oxide. In the whole potential range of the second wave ( viz from −50 to −250 mV) nitric oxide is reduced to hydroxylamine, ammonia and nitrous oxide. Formation of hydrazine has not been detected. From the literature and from the relations of the current efficiencies a possible mechanism is proposed for the reduction of nitric oxide.

The cathodic reduction of oxygen at cobalt phthalocyanine: Influence of electrode preparation on electrocatalysis

Oxygen reduction has been investigated in acid and alkaline solutions on electrodes with the same catalyst (cobalt phthalocyanine or cobalt tetrasulfonato-phthalocyanine), but prepared by different methods. It is concluded that the number of active sites can differ widely from the expected number of active sites as derived from the total amount of catalyst present on the electrode surface. In acid solution, where the reduction is irreversible, the half-wave potential shifts with respect to the number of active sites, according to a theoretical description. For the determination of the “intrinsic catalytic activity” (turnover number), electrodes with an irreversibly absorbed (sub)monolayer are recommended.

The mechanism of oxygen reduction at iron tetrasulfonato-phthalocyanine incorporated in polypyrrole

Abstract The reduction of oxygen at iron tetrasulfonato-phthalocyanine (FeTSPc) incorporated in polypyrrole is studied. Compared with the results of FeTSPc adsorbed on pyrolytic graphite (Cp), a remarkable shift in the reduction onset potential in 0.05 M H 2 SO 4 is observed, but unfortunately thin polypyrrole/FeTSPc films show high instability. Thicker films (equivalent to 10 mC or more) are more stable and lead to an increased four-electron reduction of oxygen. It is proposed that dimeric FeTSPc species are present in polypyrrole and that they are responsible for the marked shift in the reduction onset potential.

Characterization of NiCo2O4 electrodes for O2 evolution. Part I. Electrochemical characterization of freshly prepared NiCo2O4 electrodes

Abstract In order to correlate the electrocatalytic activity and spinel structure, the NiCo2O4 catalyst has been characterized using cyclic voltammetric and rotating ring-disc electrodes. The surface morphology and composition of the freshly prepared NiCo2O4 layer are found to depend on the thermal treatment, particularly on the temperature: the increase in activity with decreasing temperature has been correlated with the increase in surface area and the change in surface composition. The voltammogram of a fresh NiCo2O4 electrode exhibits two anodic oxidation peaks, representing one-electron transfer surface redox reactions. The first anodic peak has been assigned to a Co 2+ 3+ transition, whereas the second oxidation peak can be attributed to a M 3+ 4+ transition (M = Co or Ni). Furthermore, it was shown that the highest oxidation state up to 1.55 V (vs. RHE) is 4+, and that the hydroxyl ion plays an important role in the electrochemical reactions prior to oxygen evolution. Three different models have been suggested for the conjugated electrochemical processes, before oxygen evolution takes place.