Patrick Ozil

Modeling impedance diagrams of active layers in gas diffusion electrodes: diffusion, ohmic drop effects and multistep reactions

Abstract The dc and ac responses of an active layer in a gas diffusion electrode (GDE) have been predicted from theory, considering both diffusion and ionic ohmic drop inside the active layer and a multistep mechanism. This complete approach required numerical resolution. One-step, two-step (Volmer–Heyrovsky) and three-step mechanisms have been investigated. Numerical simulations from original models using dimensionless parameters lead to impedance diagrams involving a nearly perfect semicircle at low frequency and a 45° angle straight line at high frequency, so proving, respectively, the influence of mass and charge transport processes. Finally, a three-step oxygen reduction mechanism gives rise to theoretical impedance diagrams similar to the experimental ones.

Electrocatalysis, diffusion and ohmic drop in PEMFC: Particle size and spatial discrete distribution effects

The main purpose of this work is to investigate the size effect (catalytic and spherical diffusion or migration effects) for oxygen reduction and hydrogen oxidation on a thin active layer. The kinetic current densities are generally corrected from the diffusion in the active layer using the classical homogeneous flooded model. In order to take into account possible effects due to the real discrete distribution of the catalyst phase, a modified model was elaborated. The effects due to the discrete character of the catalyst distribution remain small for cathodic oxygen reduction due to the slow kinetics and can not explain the SA decrease. As a result, the size effect for ORR is therefore only a catalytic one. On the contrary, for anodic hydrogen oxidation a significant diffusion effect appears at the particle level and is not negligible vs the catalytic particle effect. Results point out an increase of the SA and MA for the smallest particles. As a consequence, the size effect for HOR is therefore due to both catalytic and spherical diffusion effects.

Catalyst gradient for cathode active layer of proton exchange membrane fuel cell

The experimental use of catalyst gradients within the active layer of a PEMFC cathode allows studying the influence of the location effect of Pt nanoparticles on cathode performances. The catalyst gradient effect quantitatively and qualitatively depends on porosity: it is stronger for non-porous active layers than for porous ones. The origin of the catalyst gradient effect is not the same according to porosity: oxygen diffusion is a rate limiting step for non-porous active layers, while ionic-ohmic drop is limiting for porous ones. Thus, the catalyst utilisation efficiency increases with the preferential location of Pt nanoparticles close to the gas diffusion layer side in non-porous active layers and close to the proton exchange membrane side in porous active layers. In the latter case, better performances are observed. Therefore, the optimisation of catalyst utilisation is obtained with thin porous active layers and with preferential location of Pt nanoparticles close to the proton exchange membrane side. These experimental results are confirmed by modelling both diffusion and ionic ohmic drop within active layers with catalyst gradients.

Nucleation and Growth of Zinc from Chloride Concentrated Solutions

The electrodeposition of metals is a complex phenomenon influenced by a number of factors that modify the rates of nucleation and growth and determine the properties of the deposits. In this work the authors study the influence of the zinc chloride (ZnCl{sub 2}) concentration on the zinc nucleation process on glassy carbon, in a KCl electrolyte under conditions close to those employed in commercial acid deposition baths for zinc. The electrochemical study was performed using cyclic voltammetry and potentiostatic current-time transients. The charge-transfer coefficient and the formal potential for ZnCl{sub 2} reduction were evaluated from cyclic voltammetry experiments. The nucleation process was analyzed by comparing the transients obtained with the known dimensionless (i/i{sub m}){sup 2} vs. t/t{sub m} response for instantaneous or progressive nucleation. The results show that the nucleation process and the number density of sites are dependent on ZnCl{sub 2} concentration. Scanning electron microscopy analysis of the deposits shows that the deposits are homogeneous and compact although a change in the morphology is observed as a function of ZnCl{sub 2} concentration. Evaluation of the corrosion resistance reveals the influence of the nucleation process on the subsequent corrosion resistance of the zinc deposits.

Effect of benzylideneacetone on the electrodeposition mechanism of Zn–Co alloy

The influence of benzylideneacetone (BA) on the mechanism of Zn–Co alloy electrodeposition onto AISI 1018 steel was studied in chloride acidic solutions. Results indicate that BA modifies the exchange current densities of zinc and cobalt such that the alloy is electrodeposited via a normal codeposition mechanism. Analysis of the deposits by Auger spectroscopy and X-ray diffraction shows that BA increases the cobalt concentration in the electrodeposited alloys and gives deposits with a constant concentration profile of both Zn and Co. BA also inhibits the formation of zinc hydroxide in the initial deposition stages, which supports the proposed mechanism of normal codeposition. Finally, it is shown that BA modifies the morphology of the deposits by inducing a reduction in the cluster size, leading to compact, smooth and shiny coatings.

Copper ion removal by Thiobacillus ferrooxidans biomass

Thiobacillus ferrooxidans removed copper ions at pH 1.4 with a specific uptake capacity of 0.7 g Cu 2+ g -1 dry weight. A 0.6 M copper ion tolerant T. ferrooxidans was isolated after successive subcultures on ferrous sulphate oxidation without loss of production, 52 mg protein (mol Fe 2+ oxidised) -l . This adaptation lead to a decrease in copper ion uptake to 0.09 g Cu 2+ g -1 dry weight.

Modelling the mode of operation of PEMFC electrodes at the particle level: influence of ohmic drop within the active layer on electrode performance*

Numerical modelling of charge transfer using the finite element method within the whole active layer of proton exchange membrane fuel cell (PEMFC) electrodes is proposed in order to study the electrocatalyst utilization as characterized by the effectiveness factor. In this way, two modified approaches based on the thin film and agglomerate models are developed for studying ionic ohmic drop effects in the active layer at both the electrolyte layer and electrocatalyst particles scales. The catalyst phase is considered to be a network of spherical nanoparticles instead of the classical representation as a uniform distribution over a surface (thin film model) or in a volume (agglomerate model). Simulations point out unexpected effects at the local level due to the discrete distribution of the catalyst phase as nanoparticles. Finally, the results are applied to the practical case of oxygen reduction and hydrogen oxidation.

Polymer Electrolyte Membrane Fuel Cell Modelling: d.c. and a.c. Solutions

A model of a polymer electrolyte membrane fuel cell (PEMFC) is proposed to predict the polarization curves and corresponding impedance diagrams for a small membrane electrode assembly sandwiched between bipolar plates. This theoretical approach takes into account mass transport for the following species: reactants and products along the flow channel, and water across the membrane. A global mass transport coefficient allows characterization of the mass transport flux through the gas-backing layer of the GDE, and the flooded agglomerate model is used to describe the active layer within the gas diffusion electrode. Applying this model to usual PEMFC shows that diffusion plays a crucial role at cathode for high current densities when using air as reactant. Finally, the influence of working conditions and PEMFC structure is investigated.

Chromium precipitation by the acidophilic bacterium Thiobacillus ferrooxidans

Thiobacillus ferrooxidans tolerates Cr 3+ up to 75 mM during growth on ferrous sulphate without modification of its bacterial activity (53 μg protein mM -1 Fe 2+ oxidised). At pH 1.4 and in the presence of Cr 2 O 3 , a maximal uptake capacity of 509 mg.g -1 dry weight was obtained with biomass harvested in the middle of the exponential growth phase. Cr 6+ uptake by T. ferrooxidans resulted in the precipitation of a chromium-rich compound on the surface bacterial cells, as observed by transmission electron microscopy.

Cementation and corrosion at a RDE: Changes in flow and transfer phenomena induced by surface roughness

Cementation and corrosion were investigated in a batch cell to highlight the improvement of the kinetics observed after a certain lapse of time. Experiments on cementation with the Cd(II)/Zn and Ag(I)/Cu systems, and on corrosion with the Ce(IV)/Zn and Cr(VI)/Zn couples, were carried out on a rotating disc electrode immersed in relatively concentrated solutions. Time variations of the concentration of the reacting species and SEM observations showed that the change in reaction kinetics was due to the roughness of the changing surface of the electrode, depending on the chemical system considered. For corrosion, the average roughness was shown to exceed the thickness of the Nernst diffusion layer, and the rougher surface created allows local flow disruption and mass transfer enhancement. Besides, corrosion by Cr(VI) species results in greater roughness of the metal surface with more significant flow disruption than with Ce(IV) at the same concentration.