Edson A. Ticianelli

Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells

Electrochemical studies on low catalyst loading gas diffusion electrodes for polymer electrolyte fuel cells are reported. The best performance is obtained with an electrode formed from 20 wt% Pt/C, 0.4 mg Pt cm−2 and 1.1 mg Nafion® cm−2 in the catalyst layer and 15% PTFE in a diffusion layer of 50 µm thickness, for both the cathode and the anode. However, it is also observed that the platinum requirement can be diminished to values close to 0.2 mg Pt cm−2 in the cathode and 0.1 mg pt cm−2 in the anode, without appreciably affecting the good characteristics of the fuel cell response. The experimental fuel cell data were analysed using theoretical models of the electrode structure and of the fuel cell system. It is seen that most of the electrode systems present limiting currents and some also show linear diffusion components arising from diffusion limitations in the gas channels and/or in the thin film of electrolyte covering the catalyst particles.

Oxygen electrocatalysis on thin porous coating rotating platinum electrodes

Abstract This work discusses the electrocatalysis of the oxygen reduction reaction on platinum on carbon thin porous coating rotating disk electrodes in alkaline and acid media. The electrochemical techniques considered are cyclic voltammetry, steady state polarization and impedance spectroscopy. Both the dc and ac polarization results are analyzed and simulated with the thin film/flooded agglomerate model. Cyclic voltammetry allows an inspection of the crystal facet structure of the Pt particles and the determination of effective surface areas which are lower than those determined from X-ray diffraction techniques or transmission electron microscopy. The dc polarization and impedance results show clearly a duplication of the Tafel slope due to structural effects on the porous electrode. Experiments in alkaline solutions show that in this medium there is an important contribution from the carbon to the kinetics of the reaction, which is negligible in acid medium. The effect of Pt particle size on the oxygen reduction electrocatalysis in both electrolytes is correlated with the predominant facets of the platinum crystallites.

The Electro-oxidation of Ethanol on Pt-Ru and Pt-Mo Particles Supported on High-Surface-Area Carbon

The oxidation of ethanol was studied on Pt-Ru and Pt-Mo electrocatalysts, supported on high-surface-area carbon, prepared by reduction with formic acid. The catalysts, containing up to 40 atom % of the second metal, were characterized by X-ray diffraction, energy-dispersive X-ray analyses and X-ray absorption experiments. The oxidation of ethanol was studied by cyclic voltammetry, chronoamperometry, and differential electrochemical mass spectroscopy experiments. Within the conditions used in this work, it was found that the activity for ethanol oxidation increased with the contents of the second metal and with the concentration of ethanol.

The CO Poisoning Mechanism of the Hydrogen Oxidation Reaction in Proton Exchange Membrane Fuel Cells

The CO tolerance mechanism of the hydrogen oxidation reaction was investigated on several highly dispersed carbon-supported nanocrystalline Pt and binary Pt alloys. For this purpose, current/potential behavior was derived from half-cells under actual proton exchange membrane fuel cell operating conditions and correlated with expressions derived from kinetic models. Kinetic analyses have shown that the CO poisoning effect on Pt/C, PtRu/C, and PtSn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded adsorbed CO. For all catalysts, the onset of CO oxidation occurs via the bridge-bonded species, but for PtRu/C and PtSn/C, the reaction starts at smaller potentials. Under this condition, the hydrogen oxidation currents are generated on the vacancies of a carbon monoxide adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. The linearly adsorbed CO is oxidized at higher overpotentials, leading to an increase of the holes on the CO layer and thus of the rate of the hydrogen oxidation process.

Effects of the carbon powder characteristics in the cathode gas diffusion layer on the performance of polymer electrolyte fuel cells

The effects of two different carbon powders (oil-furnace carbon black and acetylene-black) as materials for carbon cloth-based cathode gas diffusion layers on the performance of polymer electrolyte fuel cell (PEFC) electrodes were investigated. The carbon powder characteristics affect the reversible potential of the cell (E8) and both the linear and non-linear polarization components. The best fuel cell performance was obtained at an oxygen pressure of 5 atm with acetylene-black in the cathode gas diffusion layer. # 2002 Elsevier Science B.V. All rights reserved.

CO tolerance on PtMo/C electrocatalysts prepared by the formic acid method

Abstract In this work the activity of PtMo/C based materials prepared by the formic acid method was evaluated as electrocatalysts for the hydrogen oxidation reaction in the presence of CO, in polymer electrolyte fuel cells. A very high electrocatalytic activity was observed for an anode formed by PtMo/C (60:40) and supplied with H 2 containing 100 ppm of CO, which presented an overpotential loss of 100 mV at 1 A cm −2 , compared with pure hydrogen. Several electrode configurations based on Mo/C and PtMo/C CO filtering layers, having a Pt-based catalyst layer, were evaluated. In all cases an enhancement of electrocatalytic performance was observed, as compared with the standard Pt/C electrode. It is concluded that the CO tolerance is achieved through an electrochemical surface reaction of adsorbed CO with surface oxides, as proposed by the bifunctional mechanism, acting together with a heterogeneous chemical reaction of CO with water molecules catalyzed by Mo species and resulting in a lowering of the CO concentration in the gas channels of the electrode.

Kinetic study of the hydrogen oxidation reaction on platinum and Nafion ® covered platinum electrodes

In this work an investigation of the hydrogen oxidation reaction at platinum and at platinum/polymer electrolyte interfaces in contact with aqueous solutions composed of mixtures of H2SO4 and K2SO4 (pH range of 0.2–7.5) was carried out. The studies were conducted using cyclic voltammetry, rotating disk electrode and potential step chronoamperometry. The results allowed the determination of the solubility and the diffusion coefficient of the hydrogen gas in the aqueous solutions and the permeability in the polymer electrolyte. Specific theoretical treatments were employed in order to allow the interpretation of the experimental data and to characterize the kinetic and the mechanism of the hydrogen oxidation reaction.

Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells

Polymer electrolyte fuel cell (PEFC) electrodes were prepared by applying different porous gas diffusion half-layers (GDHLs) onto each face of a carbon cloth support, followed by the deposition of a catalyst layer onto one of these half-layers. The performance of PEFCs in H2/air operation using cathodes with GDHLs presenting different characteristics were compared. The best result was obtained using cathodes with GDHLs having polytetrafluorethylene (PTFE) contents of 30 wt % in the gas side and 15 wt % in the catalyst side. This behaviour was explained in terms of a better water management within the cell.

Effect of water transport in a PEFC at low temperatures operating with dry hydrogen

In this work, an experimental study of the polarisation response of a H2/O2 PEFC was carried out at low temperatures as a function of the membrane thickness. Results were analysed using a simplified model describing the transport of water in the membrane. It is seen that the model equations lead to good fittings of experimental current–potential data for polymer electrolyte fuel cells (PEFC) working with dry hydrogen and with Nafion® 115 and 117 membranes. For these membranes, limiting effects due to oxygen diffusion are minimal and the water transport through the membrane is a limiting effect for the cell behaviour at high current densities. With the Nafion® 112 membrane, oxygen diffusion effects dominate the characteristics of the current–potential curve.

Effect of thermal treatment on the performance of CO-tolerant anodes for polymer electrolyte fuel cells

Abstract In this work, carbon-supported Pt:Ru electrocatalysts, mainly for application in polymer electrolyte fuel cells, have been prepared by different methods. The materials were tested in single cells with respect to the hydrogen–oxidation reaction in the presence of CO, and the performances, before and after thermal treatments, compared to that of state-of-the-art, commercial E-TEK catalysts. In most cases, it was found that the performance of the anode improves substantially after the thermal treatment of the material. Of the several methods considered, the preparation of the catalyst via the formation of a sulfite complex, followed by a thermal treatment, gave the best results, comparable to those obtained with the state-of-the-art catalyst.