Steven Holdcroft

Structure‐Morphology‐Property Relationships of Non‐Perfluorinated Proton‐Conducting Membranes

A fundamental understanding of structure-morphology-property relationships of proton exchange membranes (PEMs) is crucial in order to improve the cost, performance, and durability of PEM fuel cells (PEMFCs). In this context, there has been an explosion over the past five years in the volume of research carried out in the area of non-perfluorinated, proton-conducting polymer membranes, with a particular emphasis on exploiting phase behavior associated with block and graft copolymers. This progress report highlights a selection of interesting studies in the area that have appeared since 2005, which illustrate the effects of factors such as acid and water contents and morphology upon proton conduction. It concludes with an outlook on future directions.

Preparation and electrocatalytic properties of conducting films of polypyrrole containing platinum microparticulates

Metallic particles were dispersed in electrically conducting polymer films in order to achieve multi-electron transfer processes in a three-dimensional matrix. Dispersions of Pt in polypyrrole were formed by several electrochemical and chemical methods, and the concentration profiles were determined by Auger Electron Spectroscopy (AES). Films were prepared in which Pt was deposited primarily at the polymer/solution interface, at the metal/polymer interface, or uniformly through the film. The electro-catalytic reduction of oxygen was investigated by rotating disk electrode studies with these films. For the homogeneous film, the catalytic current is limited by the rate of O2 permeation in the film.

Patterning π-conjugated polymers

π-Conjugated polymers show promise as active materials in application areas such as microelectronics, electro-optics, opto-electronics, and photonics. A critical feature in this emerging technology is device fabrication and the reproducible deposition of active material. This review focuses on current trends in the spatial deposition of conjugated polymers.

A stable hydroxide-conducting polymer.

A stable hydroxide-conducting membrane based on benzimidazolium hydroxide and its analogous anion-exchange polymer is reported for the first time. The molecular and polymeric analogues possess unprecedented hydroxide stability in neutral and KOH solutions as the soluble benzimidazolium salt, made possible by steric crowding around the benzimidazolium C2 position, which is usually susceptible to nucleophilic attack by OH(-). The polymers were cast and insolubilized for the purpose of forming membranes by blending with a poly(benzimidazole) followed by hydroxide-activated electrostatic interactions. The resulting membranes possess ionic (OH(-)) conductivities of up to 13.2 mS cm(-1) and represent a new class of anion-exchange polymers and membranes.

Main-chain, statistically sulfonated proton exchange membranes: the relationships of acid concentration and proton mobility to water content and their effect upon proton conductivity

An in-depth analysis has been developed for proton exchange membranes to examine the effect of acid concentration and effective proton mobility upon proton conductivity as well as their relationship to water content. The analysis was carried out on a series of main-chain, statistically sulfonated polymers with varying ion-exchange capacities. These polymer systems consisted of: sulfonated poly(ether ether ketone) (1), poly(ethylenetetrafluoroethylene-graft-polystyrenesulfonic acid) (2), sulfonated polyimide (3) and BAM® membrane (4) with Nafion® (5) as baseline. They represent membranes comprising polyaromatic polymers (1 and 3), one of which is also a rigid-rod polymer (3), vinylic polymers (4) and a vinylic polymer polymerized inside a polymer matrix (2). In order to remove the differences in acid strength for the membranes, proton mobility values at infinite dilution (Xv = 1.0) and 25 °C were calculated and found to be 3.2 (±0.4) × 10−3 cm2 s−1 V−1 (1), 2.9 (±0.4) × 10−3 cm2 s−1 V−1 (2), 1.6 (±0.7) × 10−3 cm2 s−1 V−1 (3) and 2.1 (±0.2) × 10−3 cm2 s−1 V−1 (4). These were then compared with the theoretical value for the mobility of a free proton at infinite dilution. Significant deviations from this value were theorized to be due to possible differences in tortuosity and proximity of acid groups.

Temperature and pressure dependence of O2 reduction at Pt ∣ Nafion® 117 and Pt ∣ BAM® 407 interfaces

Kinetic and mass transport parameters are determined for the oxygen reduction reaction at the interface between a 50 μm radius platinum disk electrode and two solid polymer electrolyte membranes — Nafion® 117 (Du Pont) and a BAM® 407 membrane (Ballard Advanced Materials Corporation). These materials are investigated over a range of temperatures (303–343 K) and oxygen pressures (2–5 atm absolute), at 100% relative humidity, using a solid-state electrochemical cell. Slow-sweep voltammetry yields exchange current density data, while chronoamperometry allows determination of the diffusion coefficient and solubility, for the different conditions specified. This information is used to calculate activation energies for the oxygen reduction process and for diffusion of oxygen in the membranes, as well as the enthalpies of dissolution. Nafion® 117 and the BAM® 407 membrane have similar permeabilities (Dbcb values), however, the diffusion coefficients and solubilities of oxygen are very different — being related to the water content of the respective material. Both membranes exhibit a Henry’s Law dependence for the limiting current of the oxygen reduction reaction with increasing oxygen pressure.

On the Micro-, Meso-, and Macroporous Structures of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

In this work, N(2) adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer-Emmett-Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles (<2 nm), mesopores ascribed to the void space inside agglomerates (2-20 nm), and meso- to macroporous space inside aggregates of agglomerates (>50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume, in contrast to Vulcan XC-72, for which the corresponding fraction of micropores was 15% of the total pore volume. The microstructure of the carbon support was found to be a significant factor in the formation of the microstructure in the three-component CLs, serving as a rigid porous framework for distribution of platinum and the ionomer. It was found that platinum particle deposition on Ketjen Black occurs in, or at the mouth of, the supports micropores, thus affecting its effective microporosity, whereas platinum deposition on Vulcan XC-72 did not significantly affect the supports microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm, which are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells I. Theoretical Modeling

The effect of Nafion loading on the electrode polarization characteristics of a proton exchange membrane fuel cell is studied with a macrohomogeneous model. The composition dependence of performance is rationalized by first relating mass fractions of the different components to their volume fractions and thereafter involving concepts of percolation theory to parameterize effective properties of the cathode catalyst layers. In particular, we explore systematically the effect of Nafion content on the performance. For a uniform layer, the best performance is obtained with a Nafion content of about 35 wt %, representing an optimum balance of proton transport, oxygen diffusion, and electrochemically active surface area. With the help of this modeling tool, we propose a nonuniform Nation catalyst layer and the modeling indicates that such a layer improves performance. Our preliminary experiments (to appear in Part II) confirm this claim. The two cases of nonuniform Nation distribution across the entire thickness include: a three-sublayer structure with equally thick layers, simulating a constant gradient, and a two-sublayer structure with variable thickness of the sublayers. Compared with the optimum Nafion content (35 wt %) in uniform distribution, the three-sublayer structure with higher Nation content on the membrane side exhibits significantly enhanced performance.

Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells II. Experimental Study of the Effect of Nafion Distribution

Gas diffusion electrodes (GDEs) containing a graded distribution of Nafion were prepared and characterized, and their performance as fuel cell cathodes compared to GDEs possessing a uniform distribution of Nafion. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and porosimetry are used to characterize the variations in electrochemical properties, ionic conductivity, and microstructures. The cathodic performance was improved over uniform electrodes at intermediate and high levels of polarization when the Nafion content in the GDE was higher toward the catalyst layer/membrane interface and lower toward the catalyst layer/carbon paper interface since this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Fuel cell performance is much poorer when the gradient of Nafion content is reversed, i.e., highest at the catalyst layer/carbon paper interface since this distribution disfavors proton and gas transport in the regions where they need to be maximized.

Solid Polymer Electrolytes Based on Ionic Graft Polymers: Effect of Graft Chain Length on Nano-Structured, Ionic Networks

The length of graft chains in graft polymers is controlled in order to dictate the formation of a nanochannel network of ions in a non-ionic matrix. Graft polymers were prepared by copolymerization of styrene with poly(sodium styrene sulfonate) (PSSNa) macromonomers. The latter were prepared with controlled molecular weight and narrow polydispersity by stable free radical polymerization. Phase separation of ionic aggregates occurs to a greater extent in films prepared from amphiphilic polymers possessing longer graft chains. Films prepared from polymers containing low ion content comprise of isolated ionic domains and exhibit low ionic conductivity. Increasing the ion content with the membrane, by increasing the number density of ionic graft chains in the polymer, results in ionic domains that coalesce into a network of nanochannels, and a dramatic increase in ion conductivity is observed. The ionic network is developed to a greater extent for films based on longer ionic graft chain polymers; an observation explained on the basis of phase separation.