Ionomer Electrolyte Research Articles

Linking the ionomer film morphology and nanoscale oxygen transport properties in fuel cells

Transport processes are crucial for the performance of electrochemical energy conversion devices and attract wide attentions. This work focuses on the critical oxygen transport process in the ionomer electrolyte film on a Pt electrode, which highly limits the performance of low Pt-loading fuel cells. Reduction of oxygen transport resistance may be achieved by optimizing the ionomer film morphology. Therefore, the relationship between ionomer film morphology and oxygen transport characteristics is explored by altering ionomer side chain lengths in this work. The results show that the swollen structure with larger water agglomerates in the ionomer film with shorter ionomer side chains is detrimental to the formation of oxygen transport paths. However, the multilamellar structure with an alternating alignment of water agglomerates and PFSA ionomer agglomerates in the ionomer film with longer ionomer side chains has a larger water-PFSA interface, which provides more oxygen transport paths and thus reduces the oxygen transport resistance. This work inspires the novel design concept of the ionomer electrolyte film with low local oxygen transport resistance, i.e., enlarging the water-PFSA interface parallel to the oxygen transport direction via altering the ionomer material properties, which is valuable for the development of low Pt-loading fuel cells.

Ionomer film structure under a constant electric potential in proton exchange membrane fuel cells

ABSTRACT Ionomer electrolyte covering Pt/C electrode provides the sites of electrochemical reactions and plays a significant role in the performance of proton exchange membrane fuel cells. The nanoscale ionomer films have been explored by many molecular dynamics (MD) simulations. However, the significant electric field formed in the electrode–electrolyte interface was usually ignored. In this study, MD simulations integrated with the constant electric potential method are carried out to explore the ionomer film structure on the charged Pt electrode. Pt atoms in the electrode have various and changing charges under the constant electric potential, which thus affects the ionomer film structure on the Pt electrode. Specifically, the perfluorosulfonic acid side chains become more stretched and tend to arrange on the location with positively charged Pt atoms. The water molecules rotate toward a preferred orientation and their diffusivity increases under a higher electric potential. In addition, the multilayered structure of ionomer film on the Pt electrode remains unchanged, whereas the water molecules closely on the negatively charged Pt electrode increases. This study provides an approach for the investigation of the electrode–electrolyte interface with a constant electric potential, and the findings can help better understand the complex ionomer film structure on a Pt electrode.

Influence of Proton Activity Gaps between Electrodes on Open-Circuit Potential of H2/H2 and H2/Air Cells

In polymer electrolyte fuel cells, proton activity thermodynamically correlates with the relative humidity (RH) as well as water uptake (λ) of the ionomer electrolyte. Proton activity differences between the two electrodes arise from RH control, and the activity terms of the Nernst equation could become significant. We investigated the effect of proton activity on open-circuit potentials (OCPs) and cell performance by controlling the RH of H2/H2 and H2/air cells, and on the kinetic current (ik ) using a rotating disk electrode (RDE). In the H2/H2 cell, OCPs were caused by the RH difference between electrodes; thermodynamic analysis was performed to rationalize the experimental OCP by correlating the RH and λ. The H2/air cell performance was low owing to the high resistance and low exchange current density at RH < 30%. The RDE measurements showed that ik decreased with increasing concentrations (proton activities) of HClO4, H2SO4, and CF3SO3H (TFMSA).

Macrophase-Separated Organic Ionic Plastic Crystals/PAMPS-Based Ionomer Electrolyte: A New Design Perspective for Flexible and Highly Conductive Solid-State Electrolytes.

A material design approach was taken for the preparation of an organic ionic plastic crystal (OIPC)-polymer electrolyte material that exhibited both good mechanical and transport properties. Previous attempts to form this type of electrolyte material resulted in the solvation of the OIPC by the ionomer and loss of the plastic crystal component. Here, we prepared, in situ, a macrophase-separated OIPC-polymer electrolyte system by adding lithium bis(fluorosulfonyl)imide (LiFSI) to a (PAMPS–N1222) ionomer. It was found that an optimal compositional window of 40–50 mol % LiFSI exists whereby the electrolyte conductivity suddenly increased 4 orders of magnitude while exhibiting elastic and flexible mechanical properties. The phase behavior and transport properties were studied using differential scanning calorimetry and 7Li and 19F solid-state nuclear magnetic resonance spectroscopy. This is the first example of a fabrication principle that lends itself to a wide range of promising OIPC and ionomeric materials. Subsequent studies are required to characterize and understand the morphology and conductive nature of these systems and their application as electrolyte materials.

Nano-size IrOx catalyst of high activity and stability in PEM water electrolyzer with ultra-low iridium loading

The balance of catalyst loading, activity and stability remains a challenge for the anode of proton exchange membrane (PEM) water electrolyzers. Here we report a nano-size IrOx/Nafion® composite catalyst that exhibits both outstanding activity for oxygen evolution reaction (OER) and stability in a PEM water electrolyzer. The IrOx/Nafion® catalyst layer is fabricated using a flame-based cost-effect process, reactive spray deposition technology. The IrOx/Nafion® catalyst shows >10 times improvement in OER mass activity compared to IrOx nanoparticles synthesized using the wet chemistry method. The IrOx/Nafion® catalyst also achieved ∼4,500 h of stable operation in MEA electrolyzer at 1.8 A cm−2 and 80 °C with ultra-low iridium loading of 0.08 mg cm−2. Analysis of the IrOx structure and the electrochemical performance revealed three key factors for balancing high stability and activity: (1) high ratio of Ir (IV) to Ir (III) species and high content of hydroxide on the surface; (2) high anodic charge and surface area due to nano-size IrOx particles that are well-dispersed in the Nafion® ionomer electrolyte; (3) homogeneous anode catalyst layer morphology.

Preliminary Study On The Development Of Sulfonated Graphene Oxide Membranes As Potential Novel Electrolytes For PEM Fuel Cells

In this work, preliminary results about the use of a graphene oxide-based membrane as a possible alternative electrolyte for proton exchange membrane fuel cells (PEMFCs) are reported. Currently, one of the key components of a PEMFC is the ionomer electrolyte, typically the perfluorosulfonic acid membrane known as Nafion, due to its high proton conductivity in standard operating conditions (temperature range: 60-80 °C and relative humidity of 60-100 %), good mechanical properties and its effectiveness as gases separator. However, Nafion limits fuel cell maximum operating temperature, reduces its conductivity upon dehydration and suffers shrinkage and swelling if water content changes significantly [1]. An interesting alternative aiming to enhance FCs efficiency and to overcome drawbacks related to Nafion employment could be represented by graphene oxide (GO). GO is obtained by chemical functionalization and exfoliation of graphite and is attracting attention of the scientific community due to its easy dispersion in water, electronically insulating properties and gas barrier properties. Moreover, GO self-standing membranes have been found impermeable to many gases, whilst allowing permeation of water and minimizing fuel crossover [2]. Some works have reported the use of GO in composite membranes with known proton conductors such as Nafion, S-PEEK (sulfonated poly ether ether ketone), PBI (polybenzimidazole) and PVA (polyvinyl alcohol) [3]. Our research group has developed and characterized a pure GO membrane as a self-standing electrolytic membrane. In a typical experiment, a GO aqueous solution was prepared, then it was vacuum filtered and the cake was dried at 40 °C for 6 hours. A uniform self-assembled membrane was obtained and morphologically characterized by SEM, TGA, XRD and FT-IR analysis. A homogeneous surface and the presence of functional groups, i.e. hydroxyl, epoxy oxygen and carboxyl groups, which should be able in ensuring proper proton conductivity [4], have been achieved. A thickness of around 2 μm was measured. Since thickness is strongly related to initial GO solution concentration, other experiments are going to be done in order to increase it. After proper activation in a sulfuric acid solution, proton conductivity of the prepared membranes was determined by using impedance spectroscopy in the frequency range 1 MHz-1 Hz with an AC amplitude of 500 mV. Preliminary measurements were performed at 25 °C and variable relative humidities (RHs), ranging from anhydrous conditions to 100 %. RH was controlled by using sealed-off glass cell specifically designed to be immersed in a thermostatic bath with saturated aqueous solutions of different salts. A value of 8 mS/cm was found at 25 °C and anhydrous conditions. This is a very promising result since at the same operating conditions Nafion 212 has shown a conductivity of almost one magnitude lower. Conductivity increased upon increasing RH but such effect was less sharp than the one observed for Nafion and needs further tests and detailed investigation. Measurements at different temperatures, as well as in-situ electrical tests, are in progress and will be shown during the conference.

Molecular dynamics study of ammonium based co-cation plasticizer effect on lithium ion dynamics in ionomer electrolytes

Polymer lithium single-ion conductors, i.e., ionomers, are promising solid electrolyte materials for battery applications. Adding an organic co-cation into these polymer electrolytes can lower the glass transition temperature and decrease the interactions between the tethered backbone anion and the lithium, leading to improved lithium ionic conductivity. Here we conduct a computational investigation on how three ammonium based co-cations, differentiated in size and structure, affect lithium-ion dynamics in an ionomer electrolyte. Our results show that the lithium-ion hopping has a close relationship with lithium coordination structures and could be enhanced by adding the organic co-cation. Ionic conductivity, however, depends not only on lithium hopping frequency but also on polymer morphology; an interconnected or percolating ionic aggregate structure appears to favour a more continuous lithium-ion hopping process, whereas the larger sized organic co-cations can hinder this transport.

(Invited) The Importance of the Cathode Catalyst Support in the Pursuit of Low-Cost PEM Fuel Cells

Because fuel cells are sized based on their high-power requirement, improvements in the high-power capability (W/cm2) can directly be translated to reduced active area and thus reduced cost. For proton-exchange-membrane (PEM) fuel cells, state-of-the-art PtCo catalysts on several different carbon supports have exhibited impressive and similar kinetic (low-current-density) activity. However, it is now becoming clear that the carbon-support selection can be important in determining their high-power performance. At beginning of life, we have recently observed that active nanoparticles reside predominantly within some porous carbon support particles and are apparently shielded from direct contact with the solid ionomer electrolyte as shown in the second illustration in Figure 1. This raises questions involving O2 and proton transport mechanisms and possible rate limitations within these primary carbon particles. In this work, we will present a mathematical model that includes the cathode-reactant-transport processes within a porous carbon support, allowing estimation of Pt utilization as a function of material parameters and operating conditions. We will also report recent experimental work that we are bringing alongside the model in attempts to validate and develop it further. Finally, we will discuss the implications for optimizing the support and the active-material location within it. References 1) E. Padgett et al., Proceeding of Microscopy & Microanalysis 2015 Volume 21, Issue S3, August 2015, pp. 799-800. This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. Figure 1

Long Term Effects of Airborne Contaminants in PEMFC Cathode

Air pollution is a challenge for the commercialization of proton exchange membrane fuel cells (PEMFCs) [1]. Recently, twenty-one species were selected from a list of more than 200 airborne pollutants (US Environmental Protection Agency database) for single cell accelerated tests [2]. Results showed that most contaminants differently affected cell performance. Seven of these twenty-one species were down selected for detailed studies based on quantitative criteria [2]. The seven contaminants are acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene and propene [2]. All seven contaminants can adsorb on the Pt surface, reduce the effective electrochemical active area for the oxygen reduction reaction (ORR), and even impact the ORR pathway and increase the formation of hydrogen peroxide (H2O2) [3] that promotes ionomer decomposition [4]. Acetonitrile also decreases the proton conductivity of the membrane. The larger H2O2 production rate determined with a rotating ring/disc electrode maintained at 30 °C is not expected to be representative of a PEMFC. Therefore, it is currently unclear if long term operation of a PEMFC exposed to a contaminant will lead to faster catalyst layer ionomer and membrane degradation.A long term test with acetonitrile injected in the cathode was conducted. Acetonitrile was selected because it has several impacts on cell performance and promotes the formation of H2O2. Electrochemical impedance spectroscopy (EIS) was intermittently used to obtain information on cell processes with minimal disturbance to the test. Cyclic voltammetry (CV) was applied before and after the long term test to measure the decrease in electrochemically active Pt sites. Transmission electron microscopy (TEM) was employed to analyze Pt particles agglomeration and Pt deposition in the electrolyte membrane. Inductively coupled plasma mass spectroscopy (ICP-MS) was utilized to detect Pt dissolution in exhaust water. Ion chromatography (IC) was exploited to measure the F– emission rate in the exhaust water and identify other ions including NH4 +. Scanning electron microscopy (SEM) was used to investigate thickness changes of catalyst layers and membrane.Figure 1 shows the cell voltage and high frequency resistance responses during the long term acetonitrile contamination experiment. Initially, the cell voltage at 0.4 A cm–2 was 0.667 V and the high frequency resistance (HFR) was ~0.1 Ω cm2. Within the 300 h pre-poisoning period, the cell voltage decreased with a degradation rate of 1.18 x 10–4 V h–1 and the HFR stabilized at 0.088 Ω cm2 at 220 h. During the first 20 h of acetonitrile exposure, the cell voltage dropped by 0.13 V and the HFR increased by 0.021 Ω cm2. Interestingly, the cell performance was stable during the following ~600 h even though the HFR continued its slow increase. The cell voltage eventually decreased by ~0.02 V during the last 400 h of exposure to acetonitrile. For the recovery phase, the cell voltage and HFR were respectively restored to 0.551 V and 0.109 Ω cm2 during the first 20 h. After that period, an accidental interruption occurred leading to a 4 min OCV event that subsequently increased the voltage and decreased the HFR. After that event, the cell voltage resumed its decay with a degradation rate of 1.40 x 10–4 V h–1. At around 425 h of the recovery period, a second interruption occurred resulting in a cell voltage jump of ~0.02 V which was followed by degradation with a similar rate after the first interruption. The long term exposure to acetonitrile appears to mitigate the regular degradation of cell performance; despite the initial performance loss due to acetonitrile adsorption on Pt and the continuing HFR increase attributed to the effect of acetonitrile reaction products on ionomer electrolyte. The OCV operation may also accelerate the cell performance recovery.The presence of long term contaminant effects was ascertained by focusing on Pt catalysts and ionomer electrolyte degradation. The Pt catalysts’ degradation was probed by comparing EIS, CV, ICP-MS and TEM results with control samples. The ionomer degradation was explored by comparing EIS, IC and SEM results with control samples.ACKNOWLEDGMENTSThis work is supported by the Department of Energy (DE-EE0000467). Authors are also grateful to the Hawaiian Electric Company for their ongoing support to the operations of the Hawaii Sustainable Energy Research Facility.REFERENCES1. J. St-Pierre, in Polymer Electrolyte Fuel Cell Durability, F. N. Büchi, M. Inaba, and T. J. Schmidt, Editors, p. 289, Springer (2009).2. J. St-Pierre, M. S. Angelo, and Y. Zhai, J. Electrochem. Soc., 161, F280 (2014).3. J. Ge, J. St-Pierre, and Y. Zhai, Electrochim. Acta, 134, 272 (2014).4. A. Bosnjakovic and S. Schlick, J. Phys. Chem. B, 108, 4332 (2004). Figure 1

Scaling behavior and local structure of ion aggregates in single-ion conductors.

Single-ion conductors are attractive electrolyte materials because of their inherent safety and ease of processing. Most ions in a sodium-neutralized PEO sulfonated-isophthalate ionomer electrolyte exist as one dimensional chains, restricted in dimensionality by the steric hindrance of the attached polymer. Because the ions are slow to reconfigure, atomistic MD simulations of this material are unable to adequately sample equilibrium ion structures. We apply a novel coarse-graining scheme using a generalized-YBG procedure in which the polymer backbone is completely removed and implicitly represented by the effective potentials of the remaining ions. The ion-only coarse-grained simulation allows for substantial sampling of equilibrium aggregate configurations. We extend the wormlike micelle theory to model ion chain equilibrium. Our aggregates are random walks which become more positively charged with increasing size. Defects occur on the string-like structure in the form of “dust” and “knots,” which form due to cation coordination with open sites along the string. The presence of these defects suggest that cation hopping along open third-coordination sites could be an important mechanism of charge transport using ion aggregates.

Synthesis and electrochemical properties of a perfluorinated ionomer with lithium sulfonyl dicyanomethide functional groups

A novel type of perfluorinated ionomer with lithium sulfonyl dicyanomethide functional groups was prepared for Li-ion batteries with high rate performance. The immobile sulfonyl dicyanomethide anion on the backbone of the polymer chain results in the unity transference number of Li+ and high ionic conductivity of the ionomer electrolyte, which could prevent problems caused by a concentration gradient in the electrolyte. 19F NMR and FT-IR confirmed that the polymer was synthesized successfully. With the appropriate solvent, the ionomer electrolyte's ionic conductivities were observed to be as high as 10−4 S cm−1 at room temperature. The ionomer electrolyte was electrochemically stable up to 4.5 V, and it still had a stable interface with the Li electrode after a long storage time. The ionomer electrolyte has a promising future in practical application for lithium-ion batteries.

Electrochemical performance of lithium/sulfur batteries using perfluorinated ionomer electrolyte with lithium sulfonyl dicyanomethide functional groups as functional separator

Lithium/sulfur (Li/S) batteries using the novel perfluorinated ionomer electrolyte with lithium sulfonyl dicyanomethide functional groups as both separator and electrolyte are demonstrated. Comparison was made with the Li/S batteries with the conventional liquid electrolyte, poly(vinylidenefluoride-hexafluoropropylene) [P(VDF-HFP)] gel polymer electrolyte, the lithiated Nafion ionomer electrolyte, and the novel perfluorinated ionomer electrolyte. The battery with novel perfluorinated ionomer electrolyte shows stable capacity retention compared with the batteries with the conventional liquid electrolyte and the gel electrolyte, and good rate performance compared with the battery with the lithiated Nafion ionomer electrolyte.

Application of lithiated Nafion ionomer film as functional separator for lithium sulfur cells

Lithiated Nafion ionomer film is applied as functional separator for Li–S cells. The ionomer electrolyte is composed of the lithiated Nafion ionomer film and liquid electrolyte. The Li-ion conductivity of the ionomer electrolyte is 2.1 × 10−5 S cm−1 at room temperature. The Li-ion transference number and the elemental sulfur permeability of the ionomer electrolyte shows the polysulfide anions can transport through the ionomer film hardly. The CV result reveals the ionomer electrolyte is electrochemically stable and workable for lithium and sulfur electrodes. The initial discharge capacities of the Li–S cells with ionomer film and with liquid electrolyte are nearly the same. However, the discharge capacity of the Li–S cell with the ionomer film at the 50th cycle is found to be 815 mAh g−1, and the coulombic efficiencies are above 97% along with cycles, showing an improvement in cycle performance as well as in coulombic efficiency, compared with the Li–S cell with liquid electrolyte of which the discharge capacity is 552 mAh g−1 at the 50th cycles, and the coulombic efficiencies are lower than 97% along with cycles.

Hollow nitrogen-containing core/shell fibrous carbon nanomaterials as support to platinum nanocatalysts and their TEM tomography study

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.

Investigation into PEFC Characteristics Using Four-Layer Electrode Catalyst

Water management of the catalyst layer in a polymer electrolyte membrane fuel cell was improved by focusing on the three-phase boundary. To fabricate the cathode catalyst layer, carbon-support with catalyst and fluorocarbon resin were mixed and heat-treated at the melting temperature of fluorocarbon resin. The transmission electron microscopy images of this electrode catalyst showed that the surfaces of the catalyst particle and of the carbon-support were partly coated by thin film of melted fluorocarbon resin. Ionomer electrolyte material was added to this electrode catalyst, and an electrode catalyst with a four-layer structure was fabricated. The durability at 50{degree sign}C of a single cell using the four-layer electrode catalyst as the cathode was examined. The deterioration rate was lower than that using the conventional three-layer electrode catalyst. In addition, a load change test at 50{degree sign}C, and a durability test at room temperature gave good results.

Investigation into polymer electrolyte membrane fuel cell characteristics using four-layer electrode catalyst

The use of auxiliary power and the cost of using main power decrease when the reactive gas is applied at low humidity for the operation of polymer electrolyte membrane fuel cells. The electrolyte membrane and the three-phase boundary of the electrode need to be improved to maintain this low humidity. We carried out a study of the electrode catalyst at low humidity in order to improve the cathode catalyst. Carbon support with a catalyst and fluorocarbon resin were mixed and then were treated at the melting temperature of fluorocarbon resin to fabricate the cathode-electrode catalyst layer. The transmission electron microscopy images of this electrode catalyst revealed that the surfaces of the catalyst particle and the carbon support were partly coated with a thin film of melted fluorocarbon resin. Ionomer electrolyte material was added to this electrode catalyst, and an electrode catalyst with a four-layer structure was fabricated. The durability of this four-layer electrode catalyst being operated at low humidity (42% relative humidity on the anode and the cathode) was evaluated by using a cell that contained the catalyst. The results demonstrated that the rate of deterioration was smaller than that of a conventional three-layer electrode catalyst. In addition, the load change test to assess the durability of the cell (current range between 75 and 600 mA cm−2) produced good results.

Investigation of PEFC Characteristics on Four-Layer Electrode Catalyst at 50.DEG.C. or Lower

Water management of the catalyst layer in a polymer electrolyte membrane fuel cell was improved by focusing on the three-phase boundary. To fabricate the cathode catalyst layer, carbon-support with catalyst and fluorine resin were mixed and heat-treated at the melting temperature of fluorine resin. The transmission electron microscopy images of this electrode catalyst showed that the surfaces of the catalyst particle and of the carbon-support were partly coated by thin film of melted fluorine resin. Ionomer electrolyte material was added to this electrode catalyst, and an electrode catalyst with a four-layer structure was fabricated. The durability at 50°C of a single cell using the four-layer electrode catalyst as the cathode was examined. The deterioration rate was lower than that using the conventional three-layer electrode catalyst. In addition, a load change test at 50°C, an OCV test at 50°C, and a durability test at room temperature gave good results.

Gas crossover and membrane degradation in polymer electrolyte fuel cells

Hydrogen gas crossover measurements and durability tests of a single cell under open-circuit conditions were carried out to investigate membrane degradation in polymer electrolyte fuel cells (PEFCs). The limiting current density for hydrogen crossover was approximately 0.8 mA cm −2 at 80 °C under atmospheric pressure, and gas crossover increased with an increase in cell temperature, humidity and hydrogen gas pressure. Under open-circuit conditions, the perfluorinated ionomer electrolyte membrane deteriorated significantly although no net electrochemical reactions took place at the cathode and anode. The mechanism for membrane degradation was discussed in terms of heat generation and hydrogen peroxide formation upon gas crossover and the resulting catalytic combustion, and it was concluded that the latter is the primary reason, in which hydrogen peroxide is most probably formed by gas crossover of oxygen and the resulting catalytic combustion at the anode side. In addition, it was inferred that reactive oxygen radicals (HO and HO 2 ) were formed in the presence of minor impurities such as Fe 2+ and Cu 2+ ions, which could accelerate the membrane degradation.

Study on ion conduction behavior of the plasticized polymer electrolytes based on poly(methylmethacrylate-co-alkali metal methacrylate) by FT-IR spectroscopy

The ion conduction behavior of the plasticized polymer electrolyte based on poly(methylmethacrylate-co-sodium methacrylate) was studied with in-situ FT-IR technique under a dc electric field. This study includes a detailed investigation on the various environments of the plasticizer EC in the ionomer electrolyte. The transport of charge carrier in the ionomer electrolyte was strongly dependent on the ion content of the ionomer and the ionic aggregates were formed in the ionomer electrolyte with around 7 mol% content of the ionomer. From the in-situ FT-IR experiments the transference number could be also calculated.

A Microelectrode Study of Oxygen Reduction at the Platinum/Recast‐Nafion Film Interface

We describe a study of the oxygen reduction kinetics at the Pt/recast Nafion® interface employing a minicell based on a filmed microelectrode. The experimental conditions are identical to those in polymer‐electrolyte fuel cells in that the recast ionomer electrolyte is exposed only to water vapor. We find that the interfacial rate of oxygen reduction near 0.9 V is similar for the Pt/recast ionomer interface and for Pt immersed in dilute aqueous acid solutions. A significant loss of oxygen reduction activity occurs when the recast ionomer electrolyte loses water. We discuss a possible evaluation of catalyst utilization in polymer electrolyte fuel cells.