Frédéric Maillard

Size effects on reactivity of Pt nanoparticles in CO monolayer oxidation: The role of surface mobility

In the present paper we study the reactivity of model Pt nanoparticles supported on glassy carbon. The particle size effect is rationalized for CO monolayer oxidation exploring electrochemical methods (stripping voltammetry and chronoamperometry) and modelling. Significant size effects are observed in the particle size interval from ca. 1 to 4 nm, including the positive shift of the CO stripping peak with decreasing particle size and a pronounced asymmetry of the current transients at constant potential. The latter go through a maximum at low COads conversion and exhibit tailing, which is the longer the smaller the particle size. Neither mean field nor nucleation & growth models give a coherent explanation of these experimental findings. We, therefore, suggest a basic model employing the active site concept. With a number of reasonable simplifications a full analytical solution is obtained, which allows a straightforward comparison of the theory with the experimental data. A good correspondence between experiment and theory is demonstrated. The model suggests restricted COads mobility at Pt nanoparticles below ca. 2 nm size, with the diffusion coefficient strongly dependent on the particle size, and indicates a transition towards fast diffusion when the particle size exceeds ca. 3 nm. Estimates of relevant kinetic parameters, including diffusion coefficient, reaction constant etc. are obtained and compared to the literature data for extended Pt surfaces.

Influence of particle agglomeration on the catalytic activity of carbon-supported Pt nanoparticles in CO monolayer oxidation

Fuel cell electrocatalysts usually feature high noble metal contents, and these favour particle agglomeration. In this paper a variety of synthetic approaches (wet chemical deposition, electrodeposition and electrodeposition on chemically preformed Pt nuclei) is employed to shed light on the influence of nanoparticle agglomeration on their electrocatalytic properties. Pt loading on model glassy carbon (GC) support is increased systematically from 1.8 to 10.6 μg Pt cm−2 and changes in the catalyst structure are followed by transmission electron microscopy. At low metal loadings (≤5.4 μg Pt cm−2) isolated single crystalline Pt nanoparticles are formed on the support surface by wet chemical deposition from H2PtCl4 precursor. An increase in the metal loading results, first, in a systematic increase of the average diameter of isolated Pt nanoparticles and, second, in coalescence of nanoparticles and formation of particle agglomerates. This behaviour is in line with the previous observations on carbon-supported noble metal fuel cell electrocatalysts. The catalytic activity of Pt/GC electrodes is tested in CO monolayer oxidation. In agreement with the previous studies (F. Maillard, M. Eikerling, O. V. Cherstiouk, S. Schreier, E. Savinova and U. Stimming, Faraday Discuss., 2004, 125, 357), we find that the reaction is strongly size sensitive, exhibiting an increase of the reaction overpotential as the particle size decreases below ca. 3 nm. At larger particle sizes the dependence levels off, the catalytic activity of particles with diameters above 3 nm approaching that of polycrystalline Pt. Meanwhile, Pt agglomerates show remarkably enhanced catalytic activity in comparison to either isolated Pt nanopraticles or polycrystalline Pt foil, catalysing CO monolayer oxidation at ca. 90 mV lower overpotential. Enhanced catalytic activity of Pt agglomerates is ascribed to high concentration of surface defects. CO stripping voltammograms from Pt/GC electrodes, comprising Pt agglomerates along with isolated single crystalline Pt nanoparticles from 2 to 6 nm size, feature double voltammetric peaks, the more negative corresponding to CO oxidation on Pt agglomerates, while the more positive to CO oxidation on isolated Pt nanoparticles. It is shown that CO stripping voltammetry provides a fingerprint of the particle size distribution and the extent of particle agglomeration in carbon-supported Pt catalysts.

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.

Detection of Pt z + Ions and Pt Nanoparticles Inside the Membrane of a Used PEMFC

The physical and chemical state of proton exchange membrane fuel cell (PEMFC) electrocatalysts (Pt/C) were investigated after 529 h of operation under fuel cell relevant conditions (333 K, 0.12 W cm -2 ) and 123 h of rest time under inert atmosphere (N 2 ). Upon aging, pronounced corrosion of the cathode electrocatalyst (carbon-supported platinum nanoparticles: Pt/C) was evidenced by field-emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical techniques. Carbon corrosion was witnessed by the decrease of the cathode thickness (-60%) and by the presence of nonsupported Pt particles inside the electrode. At the cathode, the corrosion of Pt nanoparticles produces Pt z+ (*) ions which diffuse in the ionomer phase or in solution [(*) stands for ionic species present in the ionomer phase or in solution]. These ions are highly mobile inside the membrane electrode assembly (MEA) and may cross over from the cathode to the anode through the PEM. The driving force for that is the electro-osmotic drag (if Pt z+ (*) ions combine with anions and carry a negative charge) and the chemical diffusion (concentration gradient of oxidized platinum species). Consequently, Pt z+ (*) ions were detected by ultraviolet (UV) spectroscopy in the PEM. Due to the high mobility of Pt z+ (*) ions, FEG-SEM and HRTEM analysis of the cross sections of MEAs revealed a pronounced change of Pt distribution after operation. Size distributions of both anode and cathode electrocatalysts evidenced an increase of the mean particle size, tailing toward large particle sizes and particle agglomeration. Nonspherical Pt nanoparticles were detected inside the PEM, the size and shape distribution of which strongly depend on the distance from the cathode. We believe Pt z+ (*) ions are reduced chemically inside the membrane by H 2 crossing over the PEM and both chemically and electrochemically at the anode/cathode.

Membrane and Active Layer Degradation upon PEMFC Steady-State Operation I. Platinum Dissolution and Redistribution within the MEA

We studied proton exchange membrane fuel cell membrane electrode assemblies (MEAs) degradation after fuel-cell operation. Anode and cathode pronounced degradation was monitored by chemical [energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS)], physical [scanning electron microscopy (SEM), transmission electron microscopy], and electrochemical (ultramicroelectrode with cavity) techniques. Aged MEAs underwent severe redistribution of most elements (Pt, C, F), coupled to a dramatic change of Pt particles shape, mean particle size and density over the carbon substrate. Among the various scenarios for Pt redistribution, Pt dissolution into Pt z+ species and transport in the ionomer or the proton exchange membrane play important roles. The Pt z+ dissolution/transport is likely favored by activators/ligands (F- or SO x -containing species) originating from the alteration of the polymers contained in the MEA. From SEM observations, the source of Pt z+ species is the cathode, while EDS and XPS show some SO,- and F-containing species origin from the anode. Local chemical analyses (SEM-EDS and XPS) revealed the excess Pt monitored in aged MEAs is associated with F excess. For instrumental limitation concerns, we could not detect the S element, but SO x -containing species could also act as counter ions during Pt z+ transport within the MEA. Pt corrosion/ redistribution is associated with the decrease of Pt-active area as revealed by CO ad -stripping voltammograms.

Preparation of methanol oxidation electrocatalysts: ruthenium deposition on carbon-supported platinum nanoparticles

Methanol oxidation electrocatalysts were prepared from Ru electrochemical or spontaneous deposition on commercial-grade carbon-supported Pt nanoparticles (Pt-Vulcan XC72, E-TEK). The resulting Ru coverage was estimated by cyclic voltammetry in supporting electrolyte. The maximum electrocatalytic activity for methanol oxidation at room temperature was observed at lower Ru coverage for spontaneous deposition than for electrodeposition; θRu ∼10% vs ∼20%, respectively. On the other hand, higher current densities for methanol oxidation were obtained in the case of electrodeposited Ru. These two results were related to the presence of non-reducible ruthenium oxides in the spontaneous deposit. The present work provides evidence that (i) efficient DMFC electrocatalysts can be achieved by Ru deposition on Pt nanoparticles, and (ii) formation of a PtRu alloy is not a required condition for effective methanol electrooxidation.

Complexation and electrochemical sensing of anions by amide-substituted ferrocenyl ligands

New amide-containing ferrocenyl ligands, L1–5, were prepared and the voltammetric and 1H NMR investigations of anion binding were carried out in organic media. The electrochemical recognition ability of L1–5 towards F−, HSO4−, H2PO4− and ATP2− is based on the synergy between H-bonding to amide protons in anion complexation to reduced, neutral ligands and ion-pairing interactions developed with the oxidized, cationic form of the ligands. The strength of the anion–ligand interactions depends on the number of ferrocene centers and amide groups in the receptor, and on the accessibility of the binding sites. Clear two-wave cyclic voltammetry features allowed the amperometric titration of H2PO4− and ATP2− by ligands L4 and L5 built from a cyclotriveratrylene structural unit, and containing a combination of three ferrocene centers with three (L4) or six (L5) amide groups.

Membrane and Active Layer Degradation Following PEMFC Steady-State Operation II. Influence of Pt z + on Membrane Properties

This paper (Part II) aims to provide an understanding of the degradations originating in the membrane or the binder and which dramatically affect the membrane electrode assembly performance and therefore the lifespan of protein exchange memory fuel cells. In Part I, evidence was provided of the corrosion of the platinum catalysts and the diffusion/migration through the membrane of platinum cations. In this paper we establish that these platinum cations induce physical cross-links that modify both the thermomechanical properties and the conductivities of the membrane. Since physical cross-links are reversible, ex situ experiments were performed to recover the performance of pristine membranes by acidic treatment. Membrane performance was found to be much more affected in the case of membranes aged in the hot zone rather than in the cold zone.

Beyond conventional electrocatalysts: hollow nanoparticles for improved and sustainable oxygen reduction reaction activity

Long-term catalytic performance of electrode materials is a well-established research priority in electrochemical energy conversion and storage systems, such as proton-exchange membrane fuel cells. Despite extensive efforts in research and development, Pt-based nanoparticles remain the only – but an unstable – electrocatalyst able to accelerate efficiently the rate of the oxygen reduction reaction. This paper describes the synthesis and the atomic-scale characterization of hollow Pt-rich/C nanocrystallites, which achieve 4-fold and 5-fold enhancement in specific activity for the oxygen reduction reaction over standard solid Pt/C nanocrystallites of the same size in liquid electrolyte and during real proton-exchange membrane fuel cell (PEMFC) testing, respectively. More importantly, the hollow nanocrystallites can sustain this level of performance during accelerated stress tests, therefore opening new perspectives for the design of improved PEMFC cathode materials.

Synthesis and Properties of Platinum Nanocatalyst Supported on Cellulose-Based Carbon Aerogel for Applications in PEMFCs

Platinum nanoparticles supported on nanostructured cellulose-based carbon aerogels (carbonized aerocellulose, CAC) were evaluated in proton exchange membrane fuel cell (PEMFC). The CAC substrate was synthesized through the dissolution, gelation, regeneration, supercritical CO2 drying and pyrolysis of cellulose. The Pt nanoparticles deposition was performed by impregnation of the CAC with H2PtCl6, followed by Pt z species reduction either under H2 at 300-400C or in basic NaBH4 solution. While H2 reduction leads to uniform Pt nanoparticles well-dispersed over the CAC surface, larger agglomerates form with NaBH4 reduction, as revealed by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The reduction methods influences the quantity of platinum deposited, which may be increased by using multiple impregnation/reduction steps. The specific surface area of Pt and specific/mass activities towards oxygen reduction of the Pt/CAC materials, investigated using the rotating disk electrode setup, are similar to those of commercial Pt/Carbon Black (CB). Finally, PEMFC unit cell testing demonstrates that a Pt/CAC sample synthesized using three successive impregnation/reduction steps, loaded at ca. 14 wt Pt/(Pt C), competes with state-of-the-art Pt/CB electrocatalysts of comparable Pt loading