E. Passalacqua

Hybrid Nafion–silica membranes doped with heteropolyacids for application in direct methanol fuel cells

Nafion–silica composite membranes doped with phosphotungstic and silicotungstic acids have been investigated for application in direct methanol fuel cells at high temperature (145°C). The phosphotungstic acid-based membrane showed better electrochemical characteristics at high current densities with respect to both silicotungstic acid-modified membrane and silica–Nafion membrane. A maximum power density of 400 mW cm−2 was obtained at 145°C in the presence of oxygen feed, whereas the maximum power density in the presence of air feed was approaching 250 mW cm−2. The results indicate that the addition of inorganic hygroscopic materials to recast Nafion extends the operating range of a direct methanol fuel cell. Operation at high temperatures significantly enhances the kinetics of methanol oxidation.

Sulfonated polysulfone ionomer membranes for fuel cells

Abstract Sulfonated polysulfone (SPSU) membranes with different sulfonation levels have been prepared and evaluated as proton exchange membranes in polymer electrolyte fuel cells (PEFC). The membranes have been characterized by ion-exchange capacity (IEC), thermal analysis, proton conductivity and single cell performance. The introduction of sulfonic groups in the base polymer produces an increase in glass transition temperature ( T g ) from 190 to about 200–220°C and a thermal decomposition of polymer at lower temperature. Proton conductivity of 4.3×10 −2 S·cm −1 at 80°C on a SPSU membrane with IEC=1.25 meq/g was obtained. Membrane/electrodes assemblies (MEAs) prepared with SPSU gave, in single cell tests at 80°C, power densities of 400 and 500 mW·cm −2 in H 2 /air and H 2 /O 2 , respectively.

Influence of the structure in low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the electrodes on a polymer electrolyte fuel cell (PEFC) performance has been investigated. Electrodes have been prepared by varying the composition of diffusion and, or catalyst layer. Improvements have been obtained by introducing a hydrophobic carbon layer between the carbon paper and the catalyst layer for the gas diffusion backing. High performance has been achieved with low Pt-loading electrodes (0.15 mg/cm 2 ) by including the ionomer Nafion in the catalyst ink. Electrodes have been characterized by SEM-EDX analyses and electrochemical tests in a 50 cm 2 single cell.

Analysis of platinum particle size and oxygen reduction in phosphoric acid

Abstract The influence of particle size on the reduction of oxygen on supported platinum (Pt) electrocatalysts (Pt on carbon black) in phosphoric acid was investigated. The results confirm earlier evidence that indicates the mass activity (A g −1 Pt) and specific activity (μA cm −2 Pt) of Pt at 900 mV ( vs rhe ) change with particle size in a manner which corresponds to the change in the relative fraction of Pt surface atoms on the (111) and (100) faces of the Pt particle, assuming a cubo-octahedral geometry. It is further concluded from the measurements in this study, and from some published data, that the interparticle separation distance betweeen Pt particles on the carbon support is not the factor controlling the activity of Pt for oxygen reduction.

The influence of Pt on the electrooxidation behaviour of carbon in phosphoric acid

Abstract The electrooxidation behaviour of Pt/C catalysts for phosphoric acid fuel cells (PAFC) has been investigated by potentiostatic tests in the potential range 0.6 ÷ 1.0 V (rhe). Results indicate that the corrosion rate increases with an increase in Pt content, and is most noticeable at 0.6 V. At higher potentials anodic dissolution of Pt takes place, thus no metal is available to catalyze the corrosion of carbon. It is concluded that a definite influence of the presence of the supported metal on the extent of carbon corrosion under practical conditions does exist, but the understanding of mechanisms involved in the Pt-promoted carbon corrosion requires further studies.

Relationship between physicochemical properties and electrooxidation behaviour of carbon materials

Various carbon blacks, with widely differing physicochemical properties, were characterized and investigated in corrosion experiments. The oxygen concentration, which was obtained from elemental analysis and infrared analysis, showed a strong correlation to the specific corrosion rate at 1 V in 85% H3PO4 at 170°C. The corrosion rate increased with an increase in the surface concentration of oxygen. A hypothesis to explain this corrosion behaviour is proposed.

Analysis of the chemical cross-over in a phosphotungstic acid electrolyte based fuel cell

The mechanism of the chemical cross-over in a low temperature fuel cell based on a phosphotungstic acid electrolyte (PWA) was investigated. It was observed that a parasitic chemical process involving redox reactions of the reagent gases with the electrolyte caused a coulombic loss and a detrimental effect on the polarization curves. The total consumption of reagent gases was determined and compared to the Faradaic consumption. Polarization curves carried out under constant gas utilization (0.66) showed that the decrease of electrochemical activity with respect to the experiments conducted with a stoichiometric-excess of reagent gas feed was quite limited. It was observed that the electrochemical losses were more significant when the cell was operated at low current density or under open circuit conditions, whereas, the electrochemical activity of the PWA fuel cell appeared to be little affected by the chemical cross-over at high current densities.

Influence of physicochemical properties on the performance of Pt/C porous electrodes for oxygen reduction in phosphoric acid

Oxygen reduction on Pt supported on carbon black in PTFE-bonded porous electrodes in 98% H3PO4 at 170°C was investigated. The influence of the Pt surface area and the volume of H3PO4 in the electrode structure (per cent acid occupation, PAO) on the performance of porous electrodes (ie current-potential behavior, potential at a constant current density of 200 mA cm−2, oxygen gain) were evaluated. The results clearly show that the Pt surface area has a major influence on the Tafel slope, electrode polarization, electrode potential decay at constant current density, and oxygen gain. On the other hand, these parameters do not show a clear trend with PAO.

TiO2-based photoelectrodes in photoelectrochemical cells: Performance and mechanism of O2 evolution

Abstract Titanium laminae oxidized in air and CO2 atmosphere have been investigated in the photo-oxidation of water. Factors influencing the photoactivity are the presence of the sub-oxide interface, the overall layer thickness and the non-stoichiometry of the rutile layer. The mechanism of O2 evolution and the presence of some intermediates in the oxidation of water over titanium oxide photoanodes has been related to structural properties, derived from XRD and XPS electrochemical properties (polarization currents, cyclic voltammetry, FBp, zpc) and to the performance in PEC cell of five specimens of lowest and highest photoelectrochemical efficiency.

Mechanism of O2 evolution on TiO2 photoanodes

Abstract The mechanism of O 2 evolution over titanium oxide photoanodes has been studied by relating structural properties, derived from XRD and XPS, and electrochemical properties currents, (cyclic voltammetry and flatband potentials) to the performance in PEC cells of five specimens of lowest and highest photoelectrochemical efficiency.