Water at the interface and in the bulk of ultrathin ionomer films confined to platinum surfaces affects the electrochemical activity and proton transport in electrochemical devices, yet it is little explored. This is the first study wherein the water distribution and the hygroexpansion of three different sulfonated fluoropolymers, varying in the composition and length of sulfonic group terminating side chains, as confined thin films (∼15 nm) on a Pt surface are examined by neutron reflectometry, ellipsometry, and quartz crystal microbalance. For the shorter side-chain perfluorosulfonated acid (PFSA) ionomers, Nafion EW 1100 and 3M EW 725, one to two monolayers equivalent of interfacial water, similar to adsorbed water on bare Pt, under wet conditions (97% humidity) was observed. In contrast, for the longer chain perfluoroimide acid (PFIA) ionomer, five monolayers equivalent of interfacial water was observed. In the bulk phase of the films, the PFIA ionomer exhibited a multilamellar structure comprising a water-rich layer near the interface region whereas the PFSA ionomer had a more uniform distribution of water. Our study for the first time demonstrates the strong effect of the side chain of sulfonated fluoropolymers on the Pt/ionomer interfacial water.

We cross-correlate the hydration-dependent structure and properties – microscopic and macroscopic – of a thin Nafion ionomer film on an electrochemically pertinent Pt substrate.

We introduce a novel self-standing, nanoporous carbon scaffold (NCS, 25 μm thick), with an ordered inverse opal pore structure (∼85 nm pore) as a microporous layer (MPL) in a polymer electrolyte membrane fuel cell. Unlike previous studies, through chemical functionalization of the pore surfaces, the wettability of the MPL is controllably modified without altering the pore structure. Ex situ environmental scanning electron microscopy experiments revealed water sorption in the hydrophilic NCS under moderate relative humidity (RH) conditions but not in the hydrophobic NCS, wherein water condensation on the surface was noted only at high RH. The influence of structure and wettability of different MPLs on cell performance was gleaned from steadystate cell polarization behavior. For cells operated under dry conditions (≤80% RH), the limiting current for cells with a hydrophilic NCS MPL was the highest while that for cells with a hydrophobic NCS MPL was the lowest regardless of the level of water saturation (RH).

Typically, in polymer electrolyte-based electrochemical devices such as electrolyzers and fuel cells, ionomers in the catalyst layers are present as ultrathin films coating the electrochemically active component. Acidic ionomer thin films have been extensively characterized over the past decade, yet there are few reports on the alkaline ionomer thin films. Here, we present a study on anion-exchange ionomers; specifically, we investigate the water content and conductivity of fluoride, bromide, and carbonate forms of 50 nm thick FAA3 and PPO ionomer thin films at 30 °C and 0−90% RH. A thermodynamic analysis was performed to compute the Gibbs free energy of anionic interaction with water to discuss the impact of anion type on the anionic mobility. Structural analysis using GISAXS was performed on the anion-exchange ionomer thin films. Furthermore, conductivity and water content relationships between FAA3 thin films and membranes and between FAA3 and PPO thin films were compared and discussed in terms of structure and ion clustering.

A proton exchange membrane fuel cell (PEMFC) was segmented to measure local current density, electrochemical surface area, and high frequency resistance (HFR) distribution in the land-channel direction at resolution of 350 μm. An in-house catalyst coated membrane of 3 mm × 3 mm active area was prepared to represent a small area in a larger scale cell with 1 mm land and channel widths. This design was employed to measure current density and HFR distribution at 60 • C with several different operating conditions. Local electrical resistance was also measured separately so that local protonic resistances can be discerned from local HFR. To analyze the effect of the land-channel geometry a method was developed to quantify the sources of current distribution, such as distributions of oxygen concentration at the electrode, oxygen transport resistance, cathode catalyst layer resistance, and membrane water content. Current density distribution is strongly correlated with the distribution of membrane water content and electrode resistance in dry condition, and oxygen concentration distribution in wet condition, while in moderate condition both oxygen concentration and water content in membrane are critical to the local current density distribution. The results imply the limitation of uniform condition assumption used in a differential cell study. Proton exchange membrane fuel cell (PEMFC) is an electrochemical reactor with three geometrical directions: flow channel (length scale 1 to 100 cm), land-channel (length scale 100 to 1000 μm), and through-plane (length scale 10 to 100 μm) directions. Because of the geometry of a PEMFC the distribution of reactants and products in these directions are highly inhomogeneous, as a result making current generation non-uniform. In order to study the local non-uniformities in the flow-channel direction, segmentation of a PEMFC to many differential cells is widely used. Here the differential cell means a fuel cell with small enough active area so that various conditions within the cell can be considered uniform along the flow channel direction. On the other hand, because of its relatively smaller length scale, variations in the land-channel direction are often neglected in most of such studies. However, due to the non-uniform transport length distribution from channel to catalyst layer in the land-channel direction, the differential cells aligned in the flow direction give only the averaged values over each differential cell, and therefore the local information in the land-channel direction cannot be captured.In a wet condition, for example, due to the land-channel geometry liquid water tends to distribute non-uniformly in gas diffusion layer (GDL). Many modeling studies present in literature have predicted liquid water distribution in land-channel direction.1-4 One example of such study is numerical investigation of liquid water saturation distribution in the land-channel direction of a flow field with 1 mm wide land and channel presented by Wang et al. 5 According to their findings, li...