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

Abstract

Surface confinement of oxygen inside layers of Nafion self-assembled on carbon-supported oxygen reduction reaction (ORR) catalysts was studied. It is demonstrated that oxygen accumulates in the hydrophobic component of the polymer remaining in contact with the carbon surface. The amount of surface confined oxygen increases with the degree of carbon surface graphitization, which promotes the self-assembly of the polymer. Planar macrocyclic ORR catalysts possessing a delocalized system of π electrons such as Co and Fe porphyrins and phthalocyanines have virtually no effect on the surface confinement of oxygen, in accordance with their structural similarity to graphitic carbon surfaces where they adsorb. Platinum particles in carbon-supported ORR catalysts with high metal contents (20%) disrupt the self-assembly of Nafion and virtually eliminate the oxygen confinement, but the phenomenon is still observed for low Pt loading (4.8%) catalysts.

Application of a non-noble Fe-N-C catalyst for oxygen reduction reaction in an alkaline direct ethanol fuel cell

“…O 2 could have been enough time to adsorb on the catalyst surface (also considering the very high specific surface area of this Fe-N-C catalyst, which is about 1500 m 2 g −1 ), and subsequently being reduced during the voltammetric experiment, with the appearance of a peak similarly to what happens in the CV plots in Figure 1 and Figure 2. Additionally, some O 2 could adsorb on the Nafion self-assemblies locally present on the catalyst surface [43,44], and could have enough time to diffuse toward the ORR active sites and be reduced therein, since the LSV are performed at moderately low scan rate (5 mV s −1 ).…”

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.…”

“…As the sole electrolyte in PEMFCs is Naon, this issue may be related to the Naon sensitivity of the Pt 3 Co catalysts discussed above. In fact, it has previously been noted that surface support modication can alter catalysts compatibility with Naon such as surface graphitization, 54,57 posing a challenge in MEA fabrication or additional support functionalization for materials synthesized in this fashion.…”

Oxygen Absorption in Electrocatalyst Layers Detected by Scanning Electrochemical Microscopy

“…This could lead to a spillover of self‐assembled ionomers onto the active sites of the catalysts, namely Pt or macrocyclic compounds, and build obstacles to the ORR by blocking the pathways [26] . Furthermore, they discussed the novel concept of oxygen confinement within the self‐assembled Nafion [22,27] . They reported that oxygen could be confined within the interface of the hydrophobic part of the self‐assembled ionomer and the graphitic surfaces of the catalyst's carbon support in aqueous media.…”

“…[ 19 , 20 , 21 ] The properties, structure, and behavior of a self‐assembled thin layer of Nafion on various substrates has also been investigated in detail. [ 19 , 22 , 23 , 24 , 25 ] Chlistunoff and Sansiñena[ 22 , 26 , 27 ] published a series of contributions on ORR taking place on carbon‐supported catalysts in which they placed great emphasis on the role of Nafion self‐assembly by means of cyclic voltammetry and rotating ring disc electrode (RRDE) methods in an aqueous media. They demonstrated that Nafion self‐assembles through its hydrophobic component on the graphitic surfaces of the catalyst's carbon support.…”