On the use of Nafion® in electrochemical studies of carbon supported oxygen reduction catalysts in aqueous media

Abstract

The paper presents the results of a voltammetric and rotating ring disk electrode (RRDE) study of oxygen reduction reaction (ORR) catalyzed by Fe and Co porphyrins and phthalocyanines adsorbed on a high surface area carbon (Vulcan XC72) and by carbon supported Pt catalysts in presence of various quantities of Nafion®. The results demonstrate that the hydrophobic backbone of Nafion® self assembles on nanoparticles of common carbon supports of ORR catalysts. The phenomenon is promoted by the attractive interactions of the backbone with the graphitic surfaces of carbon particles. It leads to a significant ORR inhibition as a result of the spillover of the hydrophobic Nafion® component onto the catalyst particles. The extent of the inhibition depends on the type and the amount of the catalyst on the carbon surface and the degree of carbon surface graphitization. The activity of the transition metal macrocycles is suppressed by up to two orders of magnitude, whereas that of low Pt loading (4.8%) catalysts by less than one order of magnitude. The results demonstrate a great risk of incorrect catalyst activity determinations when using even very small Nafion® quantities as the catalyst ink dispersing agent.

Nafion Induced Surface Confinement of Oxygen in Carbon-Supported Oxygen Reduction Catalysts

“…On the other hand, the data presented in the previous sections ( Figure 8) suggest that oxygen accumulates predominantly, if not exclusively, at the interfaces between graphitic surfaces and the hydrophobic component of self-assembled Nafion. 16 Highly ordered pyrolytic graphite (HOPG) with its almost atomically flat graphitic surface and demonstrated ability to promote self-assembly of Nafion 16,39 seemed to offer a unique opportunity to determine the quantitative relationship between the concentration of surface confined oxygen and the real graphitic surface area from ORR measurements. The expected contributions from surface confined oxygen in total oxygen reduction currents (charges) are much smaller for HOPG than those observed for the high surface area carbons.…”

“…The inks of Pt catalysts were prepared by mixing the solids with IPA and Nafion followed by sonication. The amount of 5% Nafion solution in the catalyst inks will be denoted 16 by R and corresponds to the ratio of volume of the 5% Nafion solution expressed in microliters to the mass of the carbon support expressed in milligrams. The value of R varied from 0 to 2.7 μL per 1 mg of the carbon support, i.e., R = 1 corresponds to 1 μL of the Nafion solution used for 1 mg of the carbon support and is equivalent to 46.8 μg of pure Nafion per 1 mg of the support.…”

“…As in our previous study, 16 devoted to the selfassembly of Nafion on high surface area carbons, the experiments were performed for a constant total loading of 0.1 mg of the carbon support, which corresponded to ∼0.4 mg carbon cm disk −2…”

“…However, the first CoPC reduction was found to correspond to around 100% generation of hydrogen peroxide (see the number of electrons Inset: logarithms of the kinetic ORR currents determined for the same catalysts from RRDE experiments at 400 rpm. 16 Nafion contents (R) in the catalyst inks and the respective numbers of electrons (n) transferred in the first ORR wave (determined from the respective ring currents at +1.2 V) are listed in the figure.…”

Oxidatively induced exposure of active surface area during microwave assisted formation of Pt3Co nanoparticles for oxygen reduction reaction

“…As a result, our measured activity of the Pt/C reference was also moderately enhanced compared to earlier observations of Pt/C from the same manufacturer but with higher Naon content, 44 consistent with evaluations in literature where Naon concentration are shown to have a signicant inuence. [52][53][54][55][56] In this fashion, the ability of the synthesized NPs to facilitate the oxygen reduction reaction was evaluated by LSV in O 2 saturated 0.1 M HClO 4 electrolyte, as shown in Fig. 5.…”