Poly(aniline), electrochemically deposited on an electrode surface in the presence of poly(acrylic acid), forms a Ðlm which remains protonated, and conducting, at pH 7. The resulting modiÐed electrode is an electrocatalytic surface for NADH oxidation at ]0.05 V vs. SCE in 0.1 M citrateÈphosphate bu †er at pH 7. The amperometric responses of these composite poly(aniline) Ðlms for NADH oxidation were studied in detail and Ðtted to a kinetic model in which the NADH di †uses into the polymer Ðlm and then binds to catalytic sites within the Ðlm where it undergoes reduction to NAD`. The rate determining process depends on the concentration of NADH present and the polymer Ðlm thickness. A comparison of the results presented here for the poly(aniline)Èpoly(acrylate) Ðlms with earlier work on poly(aniline)Èpoly(vinylsulfonate) Ðlms shows that the currents obtained for NADH at these poly(aniline)Èpoly(acrylate) Ðlms are approximately one third of those obtained for the poly(aniline)Èpoly(vinylsulfonate) Ðlms under similar conditions, that the currents saturate at lower NADH concentration and that the response is less stable towards repeated measurements. The poly(aniline)Èpoly(acrylate) Ðlms are, however, less readily inhibited by NAD`and possess the potential advantage that the carboxylate groups can be used as sites for chemical attachment of enzymes or NADH derivatives by using simple coupling reactions.

Proton-exchange-membrane fuel cells (PEMFCs) have received considerable interest as a reliable power source due to their ability to attain high power densities with high energy efficiency. 1 A vital component of the PEMFC is the proton exchange membrane (PEM), which provides the ionic path between the anode and the cathode while separating the two reactant gases. The PEM material most frequently used for this type of application is Nafion ® due to its chemical and mechanical stability and its commercial availability. Since Nafion is relatively costly, much research is being directed toward developing less expensive membrane materials. 2 One such alternative class of membrane derives from studies of radiation-grafted polymerization of styrene monomer into matrices such as fluorinated ethylene propylene (FEP), ethylenetetrafluoroethylene (ETFE), and poly(vinylidene fluoride) (PVDF) with subsequent sulfonation of the polystyrene. 3-6 The grafting and sulfonation processes allow the introduction of ionic conductivity while maintaining the mechanical characteristics of the base polymer. The process uses prefabricated commercial films and thus circumvents difficulties in obtaining thin films of uniform thickness. Parameters such as cross-linking density and membrane thickness can also be controlled. 3,7 The first step in the radiation-grafting technique is irradiation of the base polymer membrane usually with a gamma-source or electron-beam to form radicals on the polymer backbone. This is followed by introduction of the monomer, subsequent copolymerization, and concurrent attachment to the base polymer as a graft chain. Finally, the grafted chains are sulfonated. 3 Scherer's group has studied the radiation grafting approach extensively. They have explored the influence of synthetic conditions on the degree of grafting, structure, and physicochemical properties of FEP-g-polystyrenesulfonic acid (PSSA) films and evaluated their performance, stability, and degradation in PEMFCs. 3,4,7-9 From these studies they have been able to identify important membrane properties, for example: optimum thickness, cross-linking density, and specific resistance required for fuel-cell applications. 3 The performance of PEMFCs is dependent on many factors, but three are prominent and involve the PEM: (i) the ohmic overpotential due to membrane resistance, (ii) the activation overpotential due to slow kinetics of the oxygen reduction reaction (ORR) at the electrode/membrane interface, and (iii) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surface. 10 In order to understand fully the applicability of PEMs in PEMFCs, it is important that the materials be systematically investigated to establish correlations between membrane composition and electrochemical properties. A better understanding of these properties at a fundamental level should lead to further advances in membrane development for applications in PEMFCs.Lehtinen et al. have studied the electrochemical properties of PVDF-g-PSSA membrane...

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Kinetic isotope measurements using [4,4-2H2]NADH and [4-1H, 4-2H]NADH have been used to investigate the mechanism of the electrochemical oxidation of NADH at poly(aniline)-poly(vinyl sulfonate)-modified electrodes. The experiments show a primary kinetic isotope effect for the reaction of 4.2. This is consistent with literature values for the corresponding isotope effect for the oxidation of NADH by two-electron oxidants in homogeneous solution. The result demonstrates that transfer of H from NADH to the modified electrode occurs in the rate-limiting step within the reaction complex.