Peter G. Pickup

Ionic Conductivity of PEMFC Electrodes Effect of Nafion Loading

The effect of Nafion loading in the cathode catalyst layer of proton exchange membrane fuel cell (PEMFC) electrodes was studied by impedance spectroscopy, cyclic voltammetry, and polarization experiments. Catalyst utilization. determined by cyclic voltammetry, peaked at 76% for a Nafion loading of ca. 30 mass %, and this coincides with the optimum performance obtained in H 2 /O 2 fuel cells. However, the small range of utilizations observed (55-76%) cannot explain the wide range of performances. The impedance results show that the ionic conductivity of the cathode increased greatly with increasing Nafion content, and this is the main factor responsible for the increase in performance up to 30% Nafion. The loss of performance at higher Nafion loadings must have been due to an increasing oxygen transport resistance, because the electronic resistance did not increase significantly. In fact, the highest electronic resistances were observed at low Nafion loadings, indicating that Nafion played a significant role as a binder.

An impedance study of electron transport and electron transfer in composite polypyrrole + polystyrenesulphonate films

At low potentials, the impedance of polypyrrole + polystyrenesulphonate changes from the simple transmission line response observed at high potentials to a more complex response including a high frequency semicircle in the complex plane representation. There is also a shift in the high frequency limiting real impedance as the electronic resistance Re of the polymer becomes comparable with, and then greater than, the solution and film ionic resistances. The ionic conductivity of the polymer composite increases with decreasing potential, and becomes constant at its maximum value in the potential range where the changes in impedance behaviour occur. This greatly simplifies interpretation of the low potential impedance results, and allows unambiguous assignment of circuit elements. The charge transfer resistance Rct, which causes the high frequency semicircles, is due to electron transfer at the polymer|electrode interface. Both Rct and Re decrease exponentially with increasing potential at 60 mV per decade. This observation validates the use, at low potentials, of theoretical models in which conducting polymers are treated as redox polymers despite their failure to follow the Nemst equation at higher potentials.

Electronically conductive anion exchange polymers based on polypyrrole: Preparation, characterization, electrostatic binding of ferrocyanide and electrocatalysis of ascorbic acid oxidation

Abstract Electronically conductive anion exchange polymers have been prepared by the electrochemical polymerization of protonated and quatemized 3-(pyrrol-1-ylmethyl)pyridine. The quaternized polymer has been characterized by elemental analysis, gravimetry, cyclic voltanunetry, scanning electron microscopy and conductivity measurements. Its structure, conductivity and electrochemistry are similar to those of other N-substituted polypyrroles. However, the high concentration of permanent cationic sites (5-6 M ) improve its electrochemical properties, increase its permeability and make it superior to polypyrrole for the binding of anionic electrocatalysts and the preconcentration of ardonic analytes. Voltammetric studies of ferrocyanide and ascorbic acid electrochemistry at poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium] coated electrodes are used to illustrate these points. Ferrocyanide is rapidly ion-exchanged into the polymer with a partition coefficient of 3.2 ×10 4 (vs. 0.01 M NaClO 4 ) to a saturation concentration of 1.4 M . Ascorbic acid can be preconcentrated in the polymer at pH 7 and its oxidation is catalysed both by the polymer itself and by bound ferrocyanide.

Modification of carbon supported catalysts to improve performance in gas diffusion electrodes

The effect of surface oxidation of the carbon support of Pt catalysts with nitric acid has been investigated. Oxidation of the support before deposition of the Pt leads to a decrease in the Pt particle size, as reported by others, and enhanced performance for oxygen reduction in gas diffusion electrodes. Oxidation of the support after Pt deposition produces an even greater enhancement in performance, which we have attributed to enhanced proton conductivity in the catalyst layer. It is speculated that hydrophilic carboxylic acid groups produced by surface oxidation enhance wetting of the catalyst layer.

Conjugated metallopolymers. Redox polymers with interacting metal based redox sites

Hybrid materials which combine the electronic conductivity of conjugated polymers and the redox and optical properties of metal complexes are being developed to take advantage of synergistic electronic interactions. Many systems show a splitting of the metal redox wave analogous to that in dinuclear complexes with through-ligand metal-metal interactions (superexchange). The influence of the conjugated backbone on electron transport between metal centres has been the focus of much work, and the existence of a superexchange pathway has been demonstrated. Several systems have been shown to exhibit catalytic and photocatalytic activities, and a number of applications in sensors have been described.

Electronically Conducting Proton Exchange Polymers as Catalyst Supports for Proton Exchange Membrane Fuel Cells. Electrocatalysis of Oxygen Reduction, Hydrogen Oxidation, and Methanol Oxidation

A variety of supported catalysts were prepared by the chemical deposition of Pt and Pt‐Ru particles on chemically prepared poly (3,4‐ethylenedioxythiophene)/poly (styrene‐4‐sulfonate) (PEDOT/PSS) and PEDOT/polyvinylsulfate (PVS) composites. The polymer particles were designed to provide a porous, proton‐conducting and electron‐conducting catalyst support for use in fuel cells. These polymer‐supported catalysts were characterized by electron microscopy, impedance spectroscopy, cyclic voltammetry, and conductivity measurements. Their catalytic activities toward hydrogen and methanol oxidation and oxygen reduction were evaluated in proton exchange membrane fuel‐cell‐type gas diffusion electrodes. Activities for oxygen reduction comparable to that obtained with a commercial carbon‐supported catalyst were observed, whereas those for hydrogen and methanol oxidation were significantly inferior, although still high for prototype catalysts.

Coupling of ion and electron transport during impedance measurements on a conducting polymer with similar ionic and electronic conductivities

The ionic and electronic conductivities of poly-[1-methyl-3-(pyrrol-1-ylmethyl)pyridinium perchlorate] have been measured as a function of potential in various solvents by impedance spectroscopy. The ionic conductivity of this polymer shows a strong solvent dependence, ranging from 0.31 mS cm–1 in water to 8.0 µS cm–1 in propylene carbonate. By changing the solvent and the potential, the ionic to electronic conductivity ratio for the polymer has been varied from 1 to 1, allowing a thorough exploration of the validity of a dual resistance transmission line model of the polymer. Overall, the experimental results support the model, and reasonably accurate electronic and ionic conductivities are obtained when data are analysed according to the model. However, an anomalous dependence of ionic conductivity on oxidation state has been observed. There appears to be a direct relationship between the electronic and ionic conductivities of the polymer when they are of similar magnitude, indicating a coupling of ion and electron transport.

Ionic and Electronic Conductivity of Poly‐(3‐methylpyrrole‐4‐carboxylic Acid)

The electronic and ionic conductivities of poly-(3-methylpyrrole-4-carboxylic acid) have been measured in situ by twin electrode voltammetry and impedance spectroscopy, respectively. The electronic conductivity of this polymer, ca. 10 -2 (Ω cm) -1 , is about 1000 times higher than its ionic conductivity, allowing application of a simple transmission line circuit to model ion transport within the polymer. Under almost all conditions investigated the impedance response was similar to that of a transmission line and unambiguous measurements of the polymers ionic conductivity could be obtained

Characterization of polymer supported catalysts by cyclic voltammetry and rotating disk voltammetry

Abstract Polymer supported catalysts prepared by the chemical deposition of Pt on chemically prepared poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) have been studied as thin films in a Nafion matrix on carbon electrodes. Cyclic voltammetry has revealed that the Pt utilization is lower than for a commercial carbon supported catalyst, and this has been attributed to electronic isolation and poisoning of some of the Pt. Rotating disc voltammetry of oxygen reduction is essentially the same (similar n values, Tafel slopes, and real exchange current densities) at electrodes coated with polymer and carbon supported catalysts, with the current at any potential being approximately proportional to the active Pt area determined by cyclic voltammetry.

Characteristics of Polypyrrole/Nafion Composite Membranes in a Direct Methanol Fuel Cell

Commercial perfluorosulphonic acid membranes (Nafion) have been impregnated with polypyrrole by in situ polymerization to decrease the crossover of methanol in direct methanol fuel cells (DMFCs). Modified membranes produced by polymerization of the pyrrole with hydrogen peroxide and iron(III) have been evaluated in a DMFC. Both methods produce membranes that can provide enhanced cell performance, although membranes produced with iron(III) as the oxidizing agent for the polymerization require additional treatments to restore their conductivity and promote bonding to the electrodes. Performance gains result from substantial reductions of the cathode overpotential, while anode overpotentials increase due to the lower conductivities of the modified membranes. Part of the beneficial effect at the cathode appears to be due to lower water crossover from the anode to the cathode.