Ionomer Adsorption Study during Electrochemical Reaction of Pt Electrode Surfaces Using Electrochemical Quartz Crystal Microbalance
Authors: Takei Kaede , Noguchi Takatoshi , Fujita Satoshi , Chikama Katsumi , Kondo Toshihiro 
Published: December 2024

Introduction:

In polymer electrolyte fuel cells (PEFCs), the platinum (Pt) catalyst is covered by the solid polymer electrolyte ionomer. The ionomer used as an electrolyte at both the anode and cathode serves the function of transporting protons and dissolved oxygen to the surface of the Pt catalyst. Therefore, it is considered that the adsorption/desorption behavior of the ionomer during PEFC operation affects the catalytic activity of Pt. While perfluorosulfonic acid (PFSA) based ionomers are now widely used, previous studies have shown that the sulfonic acid groups on the side chains strongly adsorb to the Pt surface at thr cathode and suppress the oxygen reduction reaction (ORR) [1]. In this study, the relationship between the adsorption/desorption of several PFSA-based ionomers, such as NafionⓇ and AquivionⓇ, and the redox behavior of Pt electrodes was investigated by in situ monitoring with an electrochemical quartz crystal microbalance (EQCM), which is a method that can measure the potential-dependent, minute mass of 10-9 g (ng) changes on the electrode surface.

Experiment:

Electrochemical measurements were performed with a three-electrode cell, which was composed of Pt-QCM electrode as a working electrode, Pt wire as a control electrode, and Ag/AgCl as a reference electrode. Prior to the EQCM experiment, electrochemical cleaning of the electrode surface was carried out by using 50 mM H2SO4 solution. We used the aqueous dispersion solutions containing NafionⓇ and AquivionⓇ, and then the cyclic voltammograms (CVs) at scan rate of 50 mV s-1 were acquired at room temperature.

Results and Discussion:

In both aqueous dispersion solutions, we observed the mass changes during Pt oxide formation/removal and hydrogen adsorption/desorption reactions. NafionⓇ exhibits specific adsorption/desorption behavior during the Pt oxide formation/removal reaction process. At the potential approximately 0.5 V in the negative-going scan, cathodic current due to the removal of the surface Pt oxide flowed accompanied by mass increased and then decreased on Pt surface. This result suggested that OH adsorbed on the surface of the platinum oxide is reductively removed from the surface accompanied with NafionⓇ adsorption, and after that the surface Pt oxide is reductively removed accompanied with NafionⓇ desorption. On the other hand, AquivionⓇ showed adsorption/desorption behavior dependent on the reaction process of hydrogen adsorption/desorption on the Pt surface. At the potential range of 0 V or more negative, when hydrogen absorbed Pt surface in negative-going scan, the mass was greatly decreased and when hydrogen removed from surface in the positive-going scan, the mass increased again. Comparing the side chain structures of NafionⓇ and AquivionⓇ, the former has long side chain and one more ether group. It is considered that the difference in the length of these side chains corresponds to the difference in the adsorption/desorption activity on the Pt surface.

References:

[1] K. Kodama, et al., ACS Catal. 8, 694 (2018).