Frédéric Gloaguen

Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition

Abstract The kinetics of oxygen reduction in methanol-containing acid electrolyte was investigated at platinum-based electrodes using the porous rotating disk electrodes (RDE) technique. Utilization of commercial-grade (E-TEK) carbon-supported Pt particles with narrow size distribution provided evidences for a particle size effect on the tolerance of oxygen reduction electrocatalysts to methanol competition. In methanol-containing perchloric acid electrolyte, the mass activity (MA, A g −1 Pt) for oxygen reduction increases continuously with a decrease in particle size from d =4.6 to 2.3 nm, whereas in methanol-free electrolyte MA is roughly independent of the size, when d ≤3.5 nm. Effects of addition of a second metal to Pt were also investigated. Based on particle size considerations Pt:Cr–C appears to be a more active catalyst than Pt–C for oxygen reduction in methanol-containing electrolyte.

Electrocatalytic oxidation of methanol on platinum nanoparticles electrodeposited onto porous carbon substrates

To achieve methanol fuel cell electrodes allowing a high catalyst use, thin layers of various carbon powders and recast Nafion® were electrochemically plated with platinum. The resulting Pt deposits were characterized by hydrogen and carbon monoxide electrosorption, as well as by transmission electron microscopy. Methanol oxidation was then carried out using cyclic voltammetry. The influence of the amount of carbon surface oxides and the effect of Pt specific surface area on the Pt catalytic activity were thus investigated.

Kinetic study of electrochemical reactions at catalyst-recast ionomer interfaces from thin active layer modelling

The objective of the following study was to test proton exchange membrane fuel cell catalysts. A mixture of supported catalyst and recast ionomer (Nafion®) was deposited on a rotating disc electrode (RDE). The resulting thin active layer was immersed in a dilute sulphuric acid solution. The RDE technique allows correction of mass transfer limitation in solution. To calculate the kinetic parameters from the current-potential relation, a mathematical model was written taking into account gas diffusion, ohmic drops and interfacial kinetics within the thin layer. Analytic and/or numerical expressions for the effectiveness factor and for the current-potential relation were obtained. The oxygen reduction reaction at various Pt/C-recast Nafion® interfaces demonstrates the validity of this test procedure.

Platinum electrodeposition on graphite: electrochemical study and STM imaging

The electrodeposition of Pt on a thermally oxidized HOPG surface was performed by single potential perturbation in dilute chloroplatinic acid solutions. The quantitative analysis of the current versus time transient responses, on the time scale of seconds, indicated a low saturation density of nuclei of 2×106 cm−2. The characterization by STM revealed a heterogeneous distribution of deposited Pt on the substrate surface. Most of the deposits were composed of agglomerates of spherical nanoparticles. Local particle densities exceeding 1010 cm−2 were observed, which is several orders of magnitude higher than the saturation coverage of nuclei evaluated from the analysis of the transient at t>0.3 s. From the electrochemical and the structural investigations, it appears that Pt electrodeposition on graphite involves several faradaic steps on the time scale. Firstly, at t 0.3 s, it is very likely that the more rapid growth of a small number (2×106 cm−2) of Pt particles gives a transient response, which is adequately described by the model of Scharifker and Hills for a diffusion controlled growth. As a result, it seems that cluster diffusion contributes significantly to the structural evolution of platinum electrodeposits on graphite.

Oxygen reduction on well-defined platinum nanoparticles inside recast ionomer

Abstract The objective of the study was to investigate the effect of particle size on the catalytic activity for oxygen reduction reaction at platinum/recast ionomer interface. To obtain experimental evidence of this effect, porous electrodes of well-defined geometry and very well calibrated Pt particles on graphite were used. The catalytic powders were prepared by cationic exchange and characterised by TEM and H and CO electrochemical adsorptions. For oxygen reduction, a loss of catalytic activity with the decreased platinum particle size is confirmed. This activity loss is correlated to the stronger adsorption of oxygenated species under inert atmosphere and during oxygen reduction. No effect of the inter-particles distance was found even when the particles are 1.2nm in diameter and about 10nm away; the use of graphite powder also prevents a too strong shielding effect of the catalyst support.

Influence of a pendant amine in the second coordination sphere on proton transfer at a dissymmetrically disubstituted diiron system related to the [2Fe]H subsite of [FeFe]H2ase.

Studies of the protonation of [Fe2(CO)4(kappa2-PNP)(mu-pdt)] (1; PNP = (Ph2PCH2)2NCH3) by HBF4.Et2O showed that the nature of the reaction product depends on whether the reaction is conducted in acetone or in dichloromethane. In acetone, an N-protonated form, 2, is isolated. Tautomerization of 2 in CH2Cl2 gives rise to a mu-hydride species 3. Variable-temperature NMR experiments have been performed to clarify the processes involved.

Electrodes modified by electrodeposition of CoTAA complexes as selective oxygen cathodes in a direct methanol fuel cell

The oxygen reduction reaction (ORR) at cobalt tetraazaanulene (CoTAA) modified electrodes was investigated. As a first approach, modified electrodes were prepared by electrodeposition of CoTAA on glassy carbon (GC). The modification of the GC surface was monitored by u.v.–vis. differential reflectance spectroscopy (UVDRS). The recorded spectra (i.e., absorbance as a function of wavelength and time) showed that the electrodeposition of CoTAA at 0.8 V vs Ag|AgCl, that is, at a potential where the TAA ligand is oxidized to TAA+•, seems to produce a thin polymer film. Starting from these preliminary results, porous rotating disc electrodes (RDEs) were prepared by electrodeposition of CoTAA (0.8 V vs Ag|AgCl, 1 min) on graphite powder embedded in a recast Nafion® film. The use of a porous RDE allowed comparison of the activity and selectivity of Pt nanoparticles and CoTAA for the ORR under experimental conditions close to those of a fuel cell cathode, that is, at the catalyst|Nafion® interface. The activity towards the ORR of a porous electrode modified by electrodeposition of CoTAA is not affected when methanol is present in the electrolyte phase, whereas a noticeable decrease in the activity of Pt-based oxygen cathodes was observed under the same conditions. Half-cell life tests showed that CoTAA-modified electrodes and Pt-based electrodes have a comparable stability over a period of 90 min.

An evaluation of the macro-homogeneous and agglomerate model for oxygen reduction in PEMFCs

The kinetics of oxygen reduction reaction on platinum/carbon powders in a Nafion film were evaluated with rotating disk electrode and gas diffusion electrode. The effects of the activation, mass transport and ohmic overpotentials were simulated via an “effectiveness factor” approach. The macro-homogeneous model was suitable to simulate the ORR kinetics at the RDE. On the other hand, it was found that the macro-homogeneous model does not simulate the operation of a porous gas diffusion cathode in PEMFC. With this model, the diffusion overpotential in the cathode is considerably overestimated. Conversely, the good agreement between calculated and experimental Tafel plots demonstrates the validity of the agglomerate model, even though the active layers of the PEMFC electrodes were thin and contained no PTFE. These results provided evidence for a two step transport process in the active layer of PEMFC electrodes.

Electrochemical hydrogen production in aqueous micellar solution by a diiron benzenedithiolate complex relevant to [FeFe] hydrogenases

The diiron hydrogenase model Fe(2)(bdt)(CO)(6) (1, bdt = benzenedithiolate) was dispersed in aqueous micellar solution prepared from sodium dodecyl sulfate (SDS). Aqueous solution of 1 showed no sign of decomposition when left in contact with air over a period of several days. Current-potential responses recorded at a dropping mercury electrode over pH 7-3 were consistent with reduction of freely diffusing species. Catalysis of proton reduction was observed at pH < 6 with current densities exceeding 0.5 mA cm(-2) at an acid-to-catalyst ratio of 17. Bulk electrolysis at -0.66 V vs. SHE of solution of 1 at pH 3 confirmed the production of hydrogen with a Faradaic efficiency close to 100%. A mechanism involving initial reduction of 1 and subsequent proton-coupled electron transfer is proposed.

A super-efficient cobalt catalyst for electrochemical hydrogen production from neutral water with 80 mV overpotential

Self-assembled molecular iron and cobalt catalysts (MP4N2, M = Fe, Co) bearing a multihydroxy-functionalized tetraphosphine ligand electrocatalyze H2 generation from neutral water on a mercury electrode at −1.03 and −0.50 V vs. NHE, respectively. Complex CoP4N2 displays extremely low overpotential (Eonset = 80 mV) while maintaining high activity and good stability. Bulk electrolysis of CoP4N2 in a neutral phosphate buffer solution at −1.0 V vs. NHE produced 9.24 × 104 mol H2 per mol cat. over 20 h, with a Faradaic efficiency close to 100% and without apparent deactivation.