Effects of Ionomer Morphology on Oxygen Reduction on Pt

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. Sluggish oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) is a bottleneck for the commercialization of PEFCs as automotive power sources. While replacing platinum as ORR catalyst in polymer electrolyte fuel cells (PEFCs) continues to be heavily pursued, 1,2 the metal, in spite of its high price, still remains the catalyst of choice for commercial applications. Because of that, much effort has been made to optimize cell performance with as little platinum as possible, i.e., to maximize the utilization of platinum. 3 Among various approaches to increase Pt utilization in PEFCs, the most promising are those simultaneously targeting multiple areas, where improvement is desired, e.g., Pt content in catalyst particles, strength of catalyst interaction with the support, particle agglomeration, etc. [4][5][6] However, the accessible surface area of Pt in actual catalyst layers is always smaller than expected from the mass loading and the catalyst particle size distribution due to the agglomeration, electronic isolation, and/or ionic isolation. [7][8][9][10] The ionic isolation may result from Pt particles not in contact with polymer electrolyte or Pt particles only in contact with the non-conducting regions of the polymer electrolyte. In either case, oxygen reduction reaction (ORR) cannot occur on such particles, as there is no mechanism for transporting protons to the reaction site. A few recent studies clearly demonstrate that the interfacial morphology of Nafion must play a central role in the ionic isolation of catalyst particles. Using electrochemical techniques, McGovern et al. 11 demonstrated that the surface of a nano-crystalline platinum catalyst is partially blocked by Nafion. A similar conclusion was reached by Ohma et al. 12 in case of smooth polycrystalline Pt surfaces. In other recent studies, 13,14 it was shown that the morphology of Nafion at smooth interfaces with gas is strongly affected by ionomer hydration and that ion conduction to the surface is very heterogenous on the...

On the use of Nafion® in electrochemical studies of carbon supported oxygen reduction catalysts in aqueous media

“…The preferred orientation of Nafion® molecules in contact with an oxidized Pt is that with its hydrophilic component facing the hydrophilic oxidized surface. In presence of liquid water (electrolyte), Nafion® maintains an open morphology [36] at its interface with bare Pt surfaces, even if its hydrophobic component previously occupied that interface in absence of liquid water [67]. Moreover, as oxygen was not eliminated during the preparation of the Pt catalyst inks, the surface of Pt particles was at last partially oxidized and thus strongly hydrophilic.…”

“…A scan rate of 100 mV s −1 was used in the ECSA, whereas a 10 mV s − 1 scan rate was applied in the kinetic studies. Slow scan rates may enable a potential/Pt surface driven restructuring [67] and possibly also promote Nafion® desorption and formation of small ionomer aggregates. The lack of a similar effect for Pt4.8G may be associated with the stronger interaction of its more graphitic surface with the hydrophobic Nafion® backbone.…”

Enhancing Pt/C Catalysts for the Oxygen Reduction Reaction with Protic Ionic Liquids: The Effect of Anion Structure

“…The interface between the Nafion and the metal has received great attention due to the strong irreversible sulfonate anions adsorption on the Pt. 12,[14][15][16] The degree of sensitivity to the anions highly depends on the Pt facets.12 For example, the facet Pt(111) has been reported to be most sensitive to the anion adsorption. 17,18 To accommodate operation of PEMFC under low humidity conditions, high ionomer to carbon ratios have been adopted to ensure sufficient proton transport, 19 but the excessive sulfonic groups might reduce the catalyst performance.…”

“…The interface between the Nafion and the metal has received great attention due to the strong irreversible sulfonate anions adsorption on the Pt. 12,[14][15][16] The degree of sensitivity to the anions highly depends on the Pt facets. 12 For example, the facet Pt(111) has been reported to be most sensitive to the anion adsorption.…”

Mass-Transport Resistances of Acid and Alkaline Ionomer Layers: A Microelectrode Study Part 1 - Microelectrode Development

“…In the case of catalyst layers, microelectrodes act as a model analogue, allowing for the study of HOR and ORR in a more controlled and defined environment and architecture than in a typical fuel cell (2,4). Many studies have used microelectrodes to study Nafion and these electrochemical reactions, covering a wide range of topics, but most have focused on ORR (1,2,4,5). Some researchers studied the mass-transport characteristics of oxygen gas in Nafion or the platinum/ionomer interface (1,5,6), whereas others focused on the kinetics of the reaction itself on the electrode (4,7,8).…”