Antoine Bonnefont

Analysis of diffuse-layer effects on time-dependent interfacial kinetics

Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA 02139(February 1, 2008)We investigate the subtle effects of diffuse charge on interfacial kinetics by solving the governingequations for ion transport (Nernst-Planck) with realistic boundary conditions representing reactionkinetics (Butler-Volmer) and compact-layer capacitance (Stern) in the asymptotic limit ǫ = λ

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.

Spatially resolved ATR-FTIRS study of the formation of macroscopic domains and microislands during CO electrooxidation on Pt.

Electrooxidation of CO in CO-saturated sulfuric acid electrolyte solutions with controlled mass transport is investigated with spatially resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy under galvanostatic and potentiostatic conditions. The reaction conditions are chosen such that steady states with intermediate current densities and intermediate average CO coverages are accessible. We demonstrate that under these conditions the reaction never proceeds uniformly on the electrode surface. Instead, macroscopic domains form spontaneously, composed of areas with high CO coverage and areas essentially free of adsorbed CO molecules. The average coverage within the CO-covered domains depends on the electrolyte concentration and the applied potential and can vary between saturation coverage and a few tenths of a monolayer. However, the absence of a red-shift of the CO vibrational band points to a further substructuring of the domains in densely packed CO microislands. These microislands most likely also form in the boundary layer between the CO-rich and CO-free electrode domains. This hierarchical patterning of the electrode surface is attributed to the interplay of autocatalytic reaction steps, spatial coupling through migration or the galvanostatic control of the experiment, and molecular interactions between molecules co-adsorbed on the electrode surface.

Platinum group metal-free NiMo hydrogen oxidation catalysts: high performance and durability in alkaline exchange membrane fuel cells

We introduce a new platinum group metal-free (PGM-free) hydrogen oxidation electrocatalyst with superior performance in anodes of alkaline exchange membrane fuel cells (AEMFCs). A carbon-supported bimetallic nickel–molybdenum catalyst was synthesized by thermal reduction of transition metal precursors on the surface of a carbon support (KetjenBlack 600J). The mass-weighted activity of 4.5 A gMe−1 determined in a liquid electrolyte 0.1 M NaOH using a rotating disk electrode (RDE) technique is comparable to the value reported for Pd/C with a comparable particle size under similar conditions. This NiMo/KB catalyst was integrated in a membrane electrode assembly (MEA) using an alkaline exchange membrane and ionomer. Single AEMFC tests performed in a H2/O2 configuration resulted in a record power density output of 120 mW cm−2 at 0.5 V, the MEA was found to be durable under the conditions of potential hold of 0.7 V for 115 h. For the first time, operando X-ray computed tomography (CT) experiments were performed demonstrating liquid water formation at the PGM-free anode during cell operation, and in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) and X-ray absorption spectroscopy (APXAS) were used to study the role of molybdenum in hydrogen adsorption.

Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media

The influence of the oxidation of Ni electrodes on the kinetics of the hydrogen oxidation (HOR) and evolution reactions (HER) has been explored by combining an experimental cyclic voltammetry study, microkinetic modeling and X-ray photoelectron spectroscopic analysis. Almost 10 times enhancement of the activity of Ni in the HOR/HER has been observed after its oxidation under the contact with air at ambient conditions and assigned to the presence of NiO species on the surface of metallic Ni. The experimental cyclic voltammetry curves have been analyzed with the help of kinetic model in order to shed light on the mechanism of the HOR/HER for two types of Ni electrodes and its dependence on the presence of NiO on the surface of the electrode. The main features of the experimental current-potential curves can be reproduced with a kinetic model assuming that the free energy of the adsorbed hydrogen intermediate is increased and that the kinetics of the Volmer step is enhanced in the presence of nickel oxide species. The kinetic model provides evidence for the switching from the Heyrovsky–Volmer mechanism on metallic Ni to Tafel–Volmer mechanism on the activated electrode, where surface oxide species co-exist with metal Ni sites.Graphical Abstract

On the effect of Cu on the activity of carbon supported Ni nanoparticles for hydrogen electrode reactions in alkaline medium

Effects of adding varied amounts of copper to carbon-supported nickel particles on their structure, composition and electrocatalytic activity for the hydrogen oxidation and evolution reactions in alkaline medium have been explored. Ni1-xCux/C catalysts were prepared by the incipient wetness impregnation. Comprehensive characterization of the catalysts included X-ray powder diffraction, X-ray photoelectron spectroscopic, transmission electron microscopic and cyclic voltammetric analyses, while atomistic Monte Carlo simulations have been undertaken to obtain further insights into the structure of the bimetallic NiCu nanoparticles. We found that compared to monometallic Ni, NiCu nanoparticles show lower propensity towards oxidation under ambient conditions. Furthermore, we report that adding Cu allows increasing the surface-weighted electrocatalytic activity, and the specific surface area of Ni1-xCux/C electrodes, both contributing to a ca four-fold enhancement of the mass-weighted activity. The nature of the synergistic interactions between Ni and Cu is proposed on the basis of the analysis of experimental data and Monte Carlo structural modelling results.

Cooperative Behaviour of Pt Microelectrodes during CO Bulk Electrooxidation

Cooperative behaviour of an array of microelectrodes: The interplay of bistable reaction kinetics with global coupling results in a spontaneous sequential activation of electrodes when the applied current is increased.

In situ synchrotron far-infrared spectromicroscopy of a copper electrode at grazing incidence angle.

Synchrotron far-infrared spectroscopy in situ was successfully carried out on a copper microelectrode using a grazing-angle objective attached to a Bruker IRscope II microscope. The thin-layer spectroelectrochemical cell was constructed out of Teflon and fitted with a 20 microm-thick Mylar window; the copper electrode was 500 microm in diameter. Measurements were carried out in 0.1 M NaOH solution as a function of applied potential between -1.4 and 0 V versus a Hg/Hg2SO(4) reference electrode. Results demonstrate that with the present technique it is possible to obtain in situ spectra with excellent signal-to-noise ratio for surface oxide films formed electrochemically with less than 1 nL of active solution volume. The surface film on copper at 0 V consisted mainly of CuO with possibly some Cu(OH)2 also present. This interpretation is consistent with previous works and thermodynamic calculations.

Dissipative solitons and backfiring in the electrooxidation of CO on Pt.

Collisions of excitation pulses in dissipative systems lead usually to their annihilation. In this paper, we report electrochemical experiments exhibiting more complex pulse interaction with collision survival and pulse splitting, phenomena that have rarely been observed experimentally and are only poorly understood theoretically. Using spatially resolved in-situ Fourier transform infrared spectroscopy (FTIR) in the attenuated total reflection configuration, we monitored reaction pulses during the electrochemical oxidation of CO on Pt thin film electrodes in a flow cell. The system forms quasi-1d pulses that align parallel to the flow and propagate perpendicular to it. The pulses split once in a while, generating a second solitary wave in the backward moving direction. Upon collision, the waves penetrate each other in a soliton-like manner. These unusual pulse dynamics could be reproduced with a 3-component reaction-diffusion-migration model with two inhibitor species, one of them exhibiting a long-range spatial coupling. The simulations shed light on existence criteria of such dissipative solitons.

1/f 2 noise in bistable electrocatalytic reactions on mesoscale electrodes

The formation of a self-organized spatial domain during current-controlled CO oxidation, a kinetically bistable reaction, is investigated experimentally and by deterministic simulations as a function of the electrode size and of the supporting electrolyte concentration. Decreasing the microelectrode size leads to the suppression of the spatial instability at the electrode and thus stabilizes the S-NDR branch of the reaction. The critical microelectrode size capable of supporting sustained domain formation is shown to be strongly affected by the sulfuric acid concentration, the characteristic time of the positive feedback loop increasing with the sulfate concentration. Furthermore, we demonstrate that for microelectrode diameters close to the instability threshold, small amplitude electrochemical potential fluctuations appear in the system. These potential fluctuations cannot be captured by deterministic mathematical models and are attributed to a strong enhancement of molecular fluctuations or intrinsic noise in the vicinity of the spatial instability. Analysis of the electrochemical noise revealed a 1/f 2 frequency dependence and several common features with neuronal shot noise.