Effect of the Side-Chain Structure of Perfluoro-Sulfonic Acid Ionomers on the Oxygen Reduction Reaction on the Surface of Pt

Abstract

The effect of side-chain structures in perfluoro-sulfonic acid ionomers on the adsorption of the terminal sulfonate moiety on the surface of Pt is investigated with voltammetry and surface-enhanced infrared absorption spectroscopy (SEIRAS). Analyses with lowmolecular-weight model anions with and without an ether group in the perfluoro-alkyl chain indicate that the anions are adsorbed on Pt through one or two oxygen atom(s) of the terminal sulfonate group and that the oxygen atom of the ether group also interacts with the Pt surface, leading to stronger adsorption of the anions with an ether group. On the basis of the results obtained with the model anions, the adsorption of the terminal sulfonate moieties in perfluorinated sulfonic acid ionomers and its effect on oxygen reduction reaction (ORR) is discussed. It is shown that the ionomers having longer side chains more strongly block ORR due to the flexibility of the side chains.

Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity

“…A recent vibrational spectroscopy study showed that the sulfonic groups of Nafion coordinate with Pt . A more recent study has found evidence of ether groups in the Nafion side chain to interact with Pt surface as well . However, these studies do not provide evidence of whether or why Nafion densification on Pt would occur.…”

“…From this, it was inferred that Au and Pt surfaces have a lower affinity to the sulfonic acid/water phase than to the more hydrophilic SiO 2 surface. However, from spectroelectrochemical measurements, it is known that Pt surfaces have strong affinity for sulfonic group, resulting in poisoning of the Pt catalyst in fuel cells . In another NR study, interfacial structure in hydrated Nafion on Pt, PtO, and glassy carbon substrates was observed .…”

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

“…207,208 Perfluorosulfonic acids (PFSA) such as Nafion used for polymer electrolytes also specifically adsorb on the Pt(111) surface and decrease the ORR activity. [209][210][211] The IRAS and SEIRAS measurements suggested that the sulfonate group in PFSA adsorbs on Pt via one or two oxygen atoms, and the ether group also interacts with the surface. 211,212 The adsorbed species formed during the surface oxidation such as OH ad is a key factor in the ORR activity as well as the specifically-adsorbed electrolyte species.…”

“…[209][210][211] The IRAS and SEIRAS measurements suggested that the sulfonate group in PFSA adsorbs on Pt via one or two oxygen atoms, and the ether group also interacts with the surface. 211,212 The adsorbed species formed during the surface oxidation such as OH ad is a key factor in the ORR activity as well as the specifically-adsorbed electrolyte species. The equation of OH ad formation is as follows:…”

Transport and Electrochemical Interface Properties of Ionomers in Low-Pt Loading Catalyst Layers: Effect of Ionomer Equivalent Weight and Relative Humidity

“…Pt catalysts not in direct contact with ionomer (see Figure 1 a) may not be accessible to protons under low relative humidity (RH) conditions. Recently, it has been identified that the chemical structure of ionomer dictates the chemical environment of the electrochemically active interface including poisoning by sulfonic group and ultimately affecting the oxygen reduction reaction (ORR) activity [ 7 ].…”