Galina A. Tsirlina

ELECTROCATALYTIC ACTIVITY PREDICTION FOR HYDROGEN ELECTRODE-REACTION - INTUITION, ART, SCIENCE

Abstract The factors which determine the electrocatalytic activity of metals in the hydrogen reaction are described. The problems are formulated which arise when plotting the volcano relationship between the current density logarithm and the metal—hydrogen bond energy. Experimental data on exchange currents and electron work function W e were selected. It was shown that the scatter in electrochemical data and the absence of reliable results for metals with low W e are the deciding factors for plotting the correlation relationships. It was concluded that, to date, one can only suggest that there is a tendency for the exchange-current to increase with the increase in W e , without giving any quantitative relationships.

On the influence of the metal loading on the structure of carbon-supported PtRu catalysts and their electrocatalytic activities in CO and methanol electrooxidation

PtRu (1:1) catalysts supported on low surface area carbon of the Sibunit family (S(BET) = 72 m(2) g(-1)) with a metal percentage ranging from 5 to 60% are prepared and tested in a CO monolayer and for methanol oxidation in H(2)SO(4) electrolyte. At low metal percentage small (<2 nm) alloy nanoparticles, uniformly distributed on the carbon surface, are formed. As the amount of metal per unit surface area of carbon increases, particles start coalescing and form first quasi two-dimensional, and then three-dimensional metal nanostructures. This results in a strong enhancement of specific catalytic activity in methanol oxidation and a decrease of the overpotential for CO monolayer oxidation. It is suggested that intergrain boundaries connecting crystalline domains in nanostructured PtRu catalysts produced at high metal-on-carbon loadings provide active sites for electrocatalytic processes.

Electrocatalytic oxygen reduction reaction on perovskite oxides: series versus direct pathway.

The mechanism of the oxygen reduction reaction (ORR) on LaCoO(3) and La(0.8)Sr(0.2)MnO(3) perovskite oxides is studied in 1 M NaOH by using the rotating ring disc electrode (RRDE) method. By combining experimental studies with kinetic modeling, it was demonstrated that on perovskite, as well as on perovskite/carbon electrodes, the ORR follows a series pathway through the intermediate formation of hydrogen peroxide. The escape of this intermediate from the electrode strongly depends on: 1) The loading of perovskite; high loadings lead to an overall 4 e(-) oxygen reduction due to efficient hydrogen peroxide re-adsorption on the active sites and its further reduction. 2) The addition of carbon to the catalytic layer, which affects both the utilization of the perovskite surface and the production of hydrogen peroxide. 3) The type of oxide; La(0.8)Sr(0.2)MnO(3) displays higher (compared to LaCoO(3)) activity in the reduction of oxygen to hydrogen peroxide and in the reduction/oxidation of the latter.

Quinones Electrochemistry in Room-Temperature Ionic Liquids

The two-step electrochemical reduction of tetrachloro-1,2-benzoquinone (chloranil), 2-methyl-1,2-benzoquinone (toluquinone), and 9,10-anthraquinone in two room-temperature ionic liquids is addressed by means of voltammetry on a platinum electrode. For the subsequent quinone/radical anion (Q/Q(•-)) and radical anion/dianion (Q(•-)/Q(2-)) redox reactions, the experimental data on formal potentials in 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF(6)]) and literature data for the same reactants in various aprotic molecular solvents are considered in the framework of a common potential sequence (Fc(+)/Fc scale) and compared with solvation energies computed at various levels. For the Q/Q(•-) couple, the agreement appeared to be satisfactory when solvation is described at the polarized continuum model (PCM) level. In contrast, for the Q(•-)/Q(2-) couple, the account for specific solvation at the molecular level is crucial.

Nanoparticles of Pt hydrosol immobilised on Au support: an approach to the study of structural effects in electrocatalysis

The electrochemical properties of platinum hydrosol, obtained by the reduction of H2PtCl6 by sodium citrate (NaH2Cyt), were studied for the first time. Hydrosol particles of mean diameter dc≈2.0 nm, as estimated from UV–vis absorbance spectra, were immobilised on a Au support under anodic polarisation to form stable Ptcol/Au electrodes. High Pt coverages were found from a combination of cyclic voltammetry and scanning tunnelling microscopy data. Immobilised Pt particles (most probably, small uniform aggregates) with dc of about 4.5 nm and a spread of size distribution ca. 1.5 nm demonstrated pronounced adsorption of hydrogen and oxygen similar to pure Pt materials, high electrocatalytic activity with respect to formic acid oxidation as well as a relatively high tolerance towards CO poisoning.

Quantum chemical modelling of the heterogeneous electron transfer: from qualitative analysis to a polarization curve ☆

State-of-the-art in the field of quantum chemical modelling of the heterogeneous electron transfer processes is reviewed. Novel approaches originating from interplay between quantum chemistry and modern theory of charge transfer are discussed and illustrated by recent results on the calculation of relevant kinetic parameters for various electrochemical systems. Emphasis is made on the modelling of the inner-sphere reorganization and the works of approach, as well as on the consideration of reaction layer as an orientational ensemble of reagents. Recent approaches to the estimation of electronic transmission coefficient are analyzed. A possibility to employ traditional phenomenological theory to the analysis of experimental data is re-examined in the framework of microscopic treatment.

The role of charge distribution in the reactant and product in double layer effects for simple heterogeneous redox reactions

Abstract The role of charge distribution in transition metal complexes is considered for simple heterogeneous electron transfer reactions. On the basis of quantum chemical calculations for the couples [Co(NH 3 ) 6 ] 3+/2+ , [Fe(H 2 O) 6 ] 3+/2+ , and Fe(CN) 6 3−/4− , and a simple model for the double layer, it is shown that the double layer effect for these reactions depends more on the charge on the ligands than on that on the central metal ion. The results of calculations based on the double layer model are discussed with respect to experimental data obtained at mercury and single crystal gold electrodes.

Contemporary understanding of the peroxodisulfate reduction at a mercury electrode

Specific aspects of the kinetics of anion electroreduction at high overvoltages are addressed by various traditional procedures to treat experimental data. The expansion of the Frumkin correction concept is proposed in terms of the reaction volume which increases with increasing negative electrode charge for anionic reactants. A molecular level approach based on quantum chemical calculations of the work terms and the electrode–reactant electronic coupling is employed for calculations of the reaction volume for the case of peroxodisulfate S2O82− and the ion pair Na+·S2O82−, the species which are to be considered as possible reactants in Na2S2O8 solutions with sodium salts as supporting electrolytes. Estimates of the partial rate constants for the anion and ion pair are reported as well. New experimental data are presented for solutions of equal ionic strength and equal degree of association but with different total reactant concentration. These data confirm the possibility of local ion pair formation at low negative charge. In parallel the previously ignored problem of correction for mass transport limitations is discussed for a system with two simultaneously discharging species of various charges. It follows from this reconsideration that previous data cannot be interpreted for sure as the absence of an ion pair contribution to the total current. The problem of separation of contributions from corresponding parallel steps to the total current is discussed, and the fast formation of an ion pair preceding the electron transfer was found to be rather probable.

Platinum Electrodeposits on Glassy Carbon: The Formation Mechanism, Morphology, and Adsorption Properties

The deposition of platinum on glassy carbon (GC) is studied by chronoamperometry. Basic tendencies of the formation of aggregate platinum particles on the oxidized carbon surface are established. These include a primary instantaneous nucleation of platinum under diffusion control and the beginning of a secondary nucleation prior to filling primary active centers. The deposit morphology is examined byex situ methods of scannng electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). A globular structure of platinum, formed by crystallites 3–5 nm in size, is revealed. A comparison of the STM, SEM, and TEM data demonstrates a high information value and accuracy of STM in studies of disperse materials in both nanometer and submicron ranges. Various coulometry techniques intended for the determination of the true surface area of deposited platinum are compared. The most informative techniques are the voltammetry of desorption of copper adatoms and chemisorbed carbon monoxide at, respectively, low and high platinum contents. Differences in the formation kinetics and properties of aggregate particles in Pt/GC and Pt/Pt are found, specifically, smaller Pt/GC crystallites and higher degrees of their concrescence (screening)

Electrochemical characterisation of Pd modified ceramic carbon electrodes: partially flooded versus wetted channel hydrophobic gas electrodes

Inert metal modified composite ceramic carbon electrodes (CCE) were recently introduced and found potential applications as biosensors and gas electrodes. The electrodes comprise graphite powder dispersed in hydrophobically modified silicate xerogels. The interconnected graphite network provides percolative conductivity. The silicate backbone provides rigidity and porosity. The hydrophobic moieties reject water and thus limit the thickness of the electrochemically active portion of the electrodes. Inert metal dispersion is introduced for catalysis. The characterisation of the geometric configuration of the wetted section of the gas electrodes and the inert metal dispersion in porous electrodes poses an interesting challenge since these cannot be resolved by spectroscopic or microscopic techniques or by gas adsorption isotherms. It is demonstrated that electrochemical techniques provide a means to characterise the morphology of the wetted section of Pd-modified gas electrodes. The surface area of the palladium dispersion in CCEs was characterised by cathodic stripping of adsorbed oxygen and underpotential copper deposition (upd), and the active volume of the palladium in the CCE was estimated by electrochemical formation of b-phase palladium hydride. Finally, the parameters that were obtained by the electrochemical characterisation were used in order to fit the potential‐current polarisation curves of oxygen reduction on CCEs of different compositions and preparation protocols.