Hydrogen Oxidation and Oxygen Reduction Reaction at the Platinum-Alkaline Ionomer Interfaces

Over the past years, several highly conductive and stable anion exchange polymer electrolytes have been prepared for the use of membranes or ionomeric binders in alkaline membrane fuel cells (AMFCs).¹ The electrochemical behaviors of ionomeric binder at the catalyst-electrolyte interface are of particular interest since this plays a major role in AMFC performance and durability.

In this study, hydrogen oxidation/evolution reaction (HOR/HER), oxygen reduction reaction (ORR), polymer chain relaxation and cation stability of polymer electrolytes are investigated using polycrystalline platinum microelectrodes. Two radically different anion exchange ionomers (i.e. benzyl trimethyl ammonium functionalized polyphenylene² and phenyl pentamethyl guanidinium functionalized perfluorinated ionomers³, Fig.1) were prepared for the comparative study.

The HOR/HER kinetics at the ammonium functionalized ionomer-Pt interface gradually decreased with time, while the reaction kinetics at the guanidinium functionalized ionomer-Pt interface was stable. Microelectrode experiments using trimethyl ammonium, tetramethyl guanidinium and sodium hydroxide indicated that the time-dependent behavior of the ammonium functionalized ionomer attributed that carbonate/bicarbonate adsorption on platinum surface due to the greater CO₂ affinity of benzyl ammonium cation⁴. As a result, the HOR/HER kinetics of the ammonium functionalized ionomer were substantially lower than those of the guanidinium functionalized ionomer after ~ 50 h equilibrium under 40°C, 100% RH conditions.

The ORR kinetics at the ammonium functionalized ionomer-Pt interface was much resistant to the carbonate/bicarbonate adsorption and only slightly deteriorated kinetics was observed. The ORR kinetics at the guanidinium functionalized ionomer-Pt interface, on the other hand, slightly increased over time. Furthermore, notable increase of oxygen permeability of the Pt electrode with guanidinium functionalized ionomer was observed. Microelectrode experiments using perfluorinated sulfonic acid and sulfonated polyaromatic ionomers disclosed that the change of ORR kinetics and oxygen permeability were related to the long range order ionomer relaxation behavior.

Cation stability of these ionomers was also investigated during potential cycling from 0.2 – 1.0 V. The cation stability of benzyl trimethyl ammonium functionalized ionomer was less than that of phenyl pentamethyl guanidinium functionalized ionomer. Density functional theory calculations on stability of model cations indicates that the barrier energy of nucleophilic substitution (S_N2) of benzyl ammonium is 22.3 kcal/mol which is slightly lower than the S_N2 barrier energy of central carbon of guanidinium (i.e 23.8 kcal/mol).

Acknowledgments
We thank Drs. Dae-Sik Kim (LANL), Cy Fujimoto (SNL), and Michael Hibbs (SNL) for preparing the polymer electrolytes. U.S. Department of Energy Fuel Cell Technologies Program (Technology Development Manager: Dr. Nancy Garland) funded this research. We thank the Korea Institute of Energy Research for supporting Visiting Scholar Program.

References
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