Abstract
The oxygen reduction reaction (ORR) at the interface between platinum and Nafion 1100 equivalent weight was studied as a function of temperature (20–80 °C), humidity (10–100%), scan rate, the manner in which Nafion film was deposited, and the state of the Pt surface using ultramicroelectrodes employing cyclic voltammetry and chronoamperometry. ORR on smooth electrodes was strongly inhibited under specific conditions dependent on temperature, humidity, and scan rate. From the data presented, we postulate that dynamic changes in the molecular structure of the ionomer at the platinum interface result in differences in ORR voltammetry for films prepared and equilibrated under different conditions. The lack of similar changes for rough, platinized electrodes has been attributed to differences in initial ionomer structure and a higher energy barrier for ionomer restructuring. These model system studies yield insight into the ionomer-catalyst interface of particular interest for polymer electrolyte fuel cells.
Highlights
Background currentsPt black surface.— As the nature of the PtNafion interface was of specific concern, we investigated rough, platinized surfaces to compare with smooth polycrystalline Pt surfaces
Films produced by slow evaporation of the casting solution in the drying chamber apparatus filled with ∼0% relative humidity (RH) ultrahigh purity (UHP) argon were smooth and the thickness of the film over the 100 μm Pt disk was relatively uniform (Fig. 2b)
This finding remains in agreement with hydrophobicity of Nafion surface,[56] which has been frequently invoked as the origin of the so called Schroeder’s paradox,[47,57,58] i.e., the inability of Nafion and some other polymeric materials to achieve the state of full hydration when in contact with 100% RH water vapor, in spite of the water activity in the gas phase being unity
Summary
Background currentsPt black surface.— As the nature of the PtNafion interface was of specific concern, we investigated rough, platinized surfaces to compare with smooth polycrystalline Pt surfaces. As opposed to smooth Pt/Nafion interfaces (Fig. 5), neither hydrogen UPD nor Pt oxide formation/reduction charges on the platinized Pt electrodes were measurably changing with the scan rate between 5 mV s−1 and 500 mV s−1.
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