Fuel Cell Catalyst Layers: A Polymer Science Perspective - Chemistry

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Fuel Cell Catalyst Layers: A Polymer Science Perspective Steven Holdcroft Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401445h • Publication Date (Web): 28 Jun 2013 Downloaded from http://pubs.acs.org on July 4, 2013

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Chemistry of Materials

Fuel Cell Catalyst Layers: A Polymer Science Perspective Steven Holdcroft

Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada, V5A 1S6

The polymeric ionomer plays a vital role in PEM fuel cell device technology, not simply as the membrane that transports protons and water from one electrode to another, but as the binder and transport medium responsible for electrochemical activity within the catalyst layer. This perspective examines critical features of the catalyst layer ionomer. It highlights the current understanding of interactions of ionomer in catalyst inks, where the microstructure of the catalyst layer is largely formed, and in the catalyst layer itself. Properties important to the design and function of next generation ionomers and the challenges faced in replacing PFSA ionomers with hydrocarbon-based analogues are closely examined. Keywords: Fuel cells, ionomer, polymer electrolyte membranes, catalyst layers, catalyst inks, membrane electrode assemblies.

Introduction. The proton exchange membrane fuel cell (PEMFC) has received extraordinary attention as a power converter in technology sectors ranging from automotive power, stationary power, to microelectronics. Improvements in fuel cell power density and reduced costs have increased this intensity. At the heart of these advances are developments in materials chemistry, on which fuel cell technology is highly dependent. These and future advancements have a firm basis in nanotechnology, and are commensurate with the development of nanoanalytical tools that facilitate the rationale design of materials with controlled nanostructure. Fuel cell technology stands on the brink of large-scale commercialization yet the technology sector suffers from a perception that current devices are not sufficiently durable, of limited operational flexibility, and not costcompetitive. The automotive sector is especially demanding and further reductions in cost are required in order to compete with incumbent ICE technology. It is estimated that the automotive fuel cell system must reach a cost target of $30/kW, possess 5000 h lifetime and operate with 60% fuel efficiency.1-3 To achieve these goals, the materials chemistry of fuel cells must demonstrably advance. Particular areas of fuel cell materials science have received more attention than others. In the field of Pt catalyst development, improvements in utilization and activity have led to significant reductions in Pt loading; but the increasing cost of Pt threatens to undermine this progress. Significant resources are being dedicated to the sole purpose of reducing Pt loadings even further. Two strate-

gies have emerged: the design of Pt-based catalysts with enhanced intrinsic activity/utilization and the complete replacement of Pt (and other precious metal-based catalysts). For these topics, the reader is directed to recent reviews describing the exceptional advances made towards understanding structure-property relationships of nanoparticle metallic electrocatalysts and non-precious metal-based fuel cell catalysts.4-7 Another area of intense research activity, intimately connected to the catalyst cost-reduction challenge, concerns the catalyst support. Currently, for practical purposes and low cost, carbon materials are used extensively to support nanoparticles of Pt. However, carbon supports do not fully meet durability requirements for automotive applications. Carbon corrosion is identified as a contributor to loss of catalyst activity because carbon-based electrodes are subject to potentials that can cause its oxidation, resulting in increasing electrical resistance and detachment of Pt particles. Significant effort is focused on investigating strategies to replace carbon. For a status report on this subject, the reader is directed to a comprehensive review of the non-carbon catalyst support materials.8 In connection with these developments, new catalyst structures have been proposed that completely mitigate the need for ionomer, relying instead on the ability of the support to form nanostructured thin films (NSTF) onto which catalyst is directly deposited to form electronically conductive and electrochemically active films. The catalyst layers are sufficiently thin that proton transport does not need the aid of an ionomer. NSTF catalyst layers, however, require a more sophisticated fabrication approach than conventional catalyst layers. For a detailed discussion of NSTF

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catalyst layers the reader is directed to work of the 3M group and related reports.9-13 Another subject that continues to receive significant attention is the polymer electrolyte membrane: firstly, in the form of improvements to PFSA ionomer membranes; secondly, in the design and exploration of hydrocarbon ionomer membranes. PFSA ionomer membranes are currently the most stable and reliable commercially-available FC membranes. Despite their relatively long history and wide scale adoption, research into PFSA ionomers continues unabated. The vast majority of reports on PEMFCs utilize long side chain (LSC) PFSA ionomer (e.g., Nafion®) and much has been reported on the structure-properties relationships of these particular ionomers.14 For automotive applications, it is preferable that PEMFCs operate at temperatures above 100 °C and under low ambient humidity but LSC-PFSA ionomer loses proton conductivity above 90 °C and under low RH.1 Attention is thus being paid to short-side-chain (SSC) PFSA ionomers that possess longer sequence lengths between side chains (for a given IEC) and afford a higher degree of crystallinity and higher thermal transition temperatures. By virtue of their enhanced crystallinity, higher IEC PFSAs ionomer membranes can be prepared that do not excessively swell upon exposure to water.15, 16 While PFSA ionomer clearly represents the state-of-the-art PEMFC membrane technology, it is recognized as possessing drawbacks for large-scale, deployment of fuel cells, including high synthetic cost due to its dependence on fluorine-based chemistry, high permeability of reactant gases, and as, briefly alluded to above, its sensitivity to temperature and humidity. Much effort is therefore directed to the design and integration of hydrocarbon-based ionomer membranes. For this, the reader is referred to one of a number of reviews on the topic.17-21 Focus of this Perspective. This perspective concerns neither the catalyst, the support, nor the membrane per se but rather the combination and interplay of the catalyst, support and ionomer that collectively form the catalyst layer. As shown in Figure 1, the catalyst layer is the region where fuel and oxidant are converted to products and where the device converts chemical energy into electrical energy. As illustrated by the electron microscopy images, the catalyst layer, typically cast from an ink, is hierarchical on a multitude of length scales. Several questions are posed in this perspective, including: how does the dispersant affect the aggregation of the ionomer in the catalyst ink and how does this translate to the morphology of the catalyst layer? How does the catalyst and support interact with the ionomer to affect the catalyst layer porosity, electrochemical surface area and conductivity? How is ionomer distributed throughout the catalyst layer? How important are the mass transport properties of the ionomer on fuel cell performance? How can pertinent properties be measured? Do data for the ionomer in the form of macroscopic films reflect the properties of ultra-thin films present in catalyst layers?

20 Reasons for Studying CL ionomers: n Improved Pt utilization m Improved stability of catalysts Controlled gas permeability Lower RH/ higher T operation Improved compatibility with PEM Recycling of catalyst

Figure 1. (Left) Illustration of a PEMFC showing the function of a catalyst layer. (Right) Hierarchical electron micrographs (bottom) SEM of a membrane-electrode-assembly, (top right) SEM of the catalyst layer, (top left) TEM of catalyst aggregates.

An understanding of the structure of the catalyst layer at the various hierarchical levels, from the molecular to the macro-scale, is far from complete. Moreover, the experimental variants associated with designing catalyst layers from a new set of materials is dauntingly large, to the extent that researchers often rely on catalyst layer compositions that have been empirically optimized from tried and tested materials such as PFSA ionomer and carbon blacks. By necessity, this perspective draws from the wealth of information available on PFSA ionomer-based catalyst layers but also draws on advances in analytical techniques used to extract as much relevant and reliable information as necessary for catalyst layer development. The properties of PFSA ionomers are therefore highlighted as they provide invaluable insights for the design and integration of alternative ionomers. Role of the Catalyst Layer. The sharp decrease observed in voltage and power density of fuel cell polarization curves in the very low and very high current density regimes are often due to inherent electrochemical kinetics and restricted mass transport within the catalyst layer, respectively. Exchange current densities for the electrochemical oxygen reduction reaction (ORR) are much smaller (~10-10 A cm-2) than for the hydrogen oxidation reaction (HOR) (10-3 A cm-2) and consequently ORR is often the major cause of fuel cell power losses under load. The activity of the catalyst, influenced by its particle size, surface morphology, electronic structure and support structure, is recognized as having an over-riding influence on the electrochemical kinetics of a fuel cell.22-24 The catalyst layer facilitates the transport of reactant gases to catalyst sites and the egress of product water. Under high


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current densities, the significant volume of water produced in the cathode must be removed in order to prevent its flooding.25 Thicknesses of catalyst layers are typically 5-10 microns, which leads to non-uniform reaction rate distributions caused by limitations in gas and proton transport (and ultimately, low utilization of Pt). Significant, advancements in the utilization of Pt in the catalyst layer have been made over the past decades in the form of replacing Pt black with Pt supported on high surface area carbon26 and with impregnation of the catalyst layer with a solid proton conducting polymer, namely, PFSA ionomer.27 Pt loadings have subsequently been reduced from several mg cm-2 to < 0.2 mg cm-2, with concomitant improvements in power density. Incorporating PFSA ionomer directly into the catalyst ink prior to catalyst layer deposition extends the 3-D reaction zone in the catalyst layer and increases catalyst utilization.28 The reader is directed to detailed reviews on the subject for further information.29 The ionomer is typically combined in solution with a supported-catalyst to form a catalyst ink dispersion that can be deposited onto a membrane or gas diffusion layer.30 Contact with and between carbon support particles provides electronic pathways for electrical current flow to and from reaction sites. The ionomer is exposed to incoming gases, water, catalyst and support, and is in contact with the membrane. The ionomer facilitates the transport of protons necessary for ORR and HOR reactions, and plays a crucial role in transporting protons within the catalyst layer by exchanging protons between the catalyst layer and the membrane. It also facilitates access of reactant gases to catalyst sites and the transport of water to and from reaction sites. Therefore, the ionomer should be permeable to gases and water, and induce pore-formation in the catalyst layer during deposition from inks. The ionomer is designed to possess high proton conductivity, negligible electronic conductivity, high gas/water permeability, and to serve as a physical binder for the catalyst/support particles. Moreover, it must be physically compatible with the membrane employed and stable towards electrochemical redox reactions and chemical attack by free radicals. As stated above, low voltage and power densities observed in fuel cell polarization curves in the very low and very high current density regimes are often due to limitations in electrochemical kinetics and poor mass transport within the catalyst layer. Likewise, Ohmic losses observed in the medium current density regime, typically attributed to protonic resistance of the membrane, can be due to poor proton conductivity of the ionomer in the catalyst layer. All these losses can be greatly exacerbated or diminished by the properties and distribution of the ionomer. State-of-the-art membrane-electrode assemblies utilize perfluorosulfonic acid (PFSA) ionomer. The use of PFSA ionomer, however, complicates recycling of catalyst layers for reclamation of Pt. It is also believed that PFSA ionomers promote Pt dissolution during fuel cell operation due to formation of the degradation product, HF. There is therefore intensive research into the development of non-fluorous proton conducting ionomers for the pur-

pose of increasing compatibility of catalyst layers with non-fluorous membranes and/or reducing gas cross-over from anode and cathode and vice versa. Much research emphasis has been placed on the development of hydrocarbon ionomers, such as those based on sulfonated versions of polyimides, polyethersulfones, polyetherketones, polyaryleneethers, polyphenyls, polystyrenics, and polybenzimidazoles. A particularly attractive feature is that properties such as ion exchange capacity, water sorption, proton conduction and gas permeability, can be readily fine-tuned. However, hydrocarbon ionomers differ significantly from PFSA ionomers since, in the majority of instances, sulfonic groups are directly attached to the polymer backbone, which restricts the phase segregation of hydrophilic and hydrophobic domains.31 Also, the, acid group attached to the hydrocarbon polymer is usually much weaker (pKa -2/-1)32 than the corresponding PFSA (pKa -6), meaning that hydrocarbon ionomers often require much higher IECs than PFSA ionomers to achieve similar proton conductivity. This renders the ionomers too brittle when dehydrated and too gelatinous when fully hydrated. This sensitivity to relative humidity causes their conductivity to fall significantly under reduced RH, if optimized for hydrated conditions, or too sorptive under fully hydrated conditions if optimized for low RH conditions. The types and properties of hydrocarbon ionomers as fuel cell membranes have been reviewed extensively and are not reviewed here.2, 31, 33-36 Reports of hydrocarbon ionomer integrated into catalyst layers are much fewer.37 Catalyst Inks - Where the catalyst layer takes shape. Catalyst layers are commonly formed from catalyst ink dispersions, which typically comprise a catalyst deposited onto a support (e.g., Pt/C), ionomer and a dispersing solvent. The significance of the heterogeneous microstructure of the dispersion on the structure and properties of the catalyst layer formed cannot be understated. A critical component of this, though not adequately researched to date, is the nature of the dispersion medium, which governs ink properties such as aggregation size of the catalyst/ionomer particles, viscosity, rate of solidification, and ultimately the physical and mass transport properties of the catalyst layer. The choice of the dispersion medium depends on the method of ink deposition. For instance, screen-printing or roll-to-roll coating usually requires viscous inks with high solids content (> 5 wt%) and high boiling point additives; whereas spray coating requires lower solids content (< 2 wt%) and faster evaporating alcoholic- or water-based solvents. While it is obvious that substrate-supported catalyst, e.g., Pt on C, is insoluble and forms heterogeneous aggregates in the dispersant, it is important to emphasize that considerable evidence exists supporting aggregation of the ionomer in solution as well. The structure, aggregation and morphology of the PSFA ionomers have been comprehensively reviewed.14 A good deal of what we know about PFSA ionomer in solution comes SANS studies of the structure of Nafion® N117 solutions in water and ethanol. Structures including a face-centred-cubic lattice and arrays of parallel and cubic phases of rods have been


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identified.38 Studies of solution-cast PFSA ionomer films show that prior to annealing, large fractions of poorly crystallized material, small crystallites and amorphous domains exist,39 and that the degree of crystallinity is dependent on the temperature of the solutions cast.40 It is proposed that in solution, rod-like structures are more stable in polar solvents.41, 42 In dilute solutions the radius of the cylindrical particles are reported to be ~20 Å. Examination of the structural evolution of PFSA ionomers from solution to water swollen membranes to dry membranes reveals that for water contents > 50 vol% an inversion of structure from reverse micellar in dry ionomer to a connected network of polymer rod-like particles in solution occurs.43 More recent studies indicate that in swollen Nafion® membranes ionomer chains form elongated bundles with a diameter of ~40 Å and lengths > 1000 Å, surrounded by electrolyte solution.44 Two aggregation processes are identified: a primary aggregation leading to small rod-like bundles that dissociate into single molecular chains under specific conditions, attributed to interaction of the fluorocarbon backbone; and a secondary aggregation leading to larger particles attributed to electrostatic attraction of side chain ion pairs that can be dissociated into primary aggregation particles.45 More details on the self-assembly processes were revealed using ESR to identify spin probes bound to polymer aggregates in aqueous solution. No ESR-detected aggregation was observed in pure EtOH and DMF solutions.46 A fringed rod model explains the mechanism for the transition between the micellar structure in solution and the structure in swollen membranes. It assumes that at high polymer concentrations, a fraction of chains can be incorporated into more than one rod. Thus, as solvent is lost and the ionomer solution becomes concentrated, intercalation of chains from independent rods and micelles occurs, conferring connectivity of the bundles and providing mechanical strength to the solid films. A natural conclusion of the above is that ionomer dispersions comprise of colloidal aggregates of ionomer chains that largely remain upon casting and coalescence further upon thermal annealing. Figure 2 illustrates how the catalyst particles and ionomer independently form heterogeneous structures in solution prior to their mixing and that a substantial portion of the final morphology of the catalyst layer is dictated by the heterogeneous structures of the individual components.

Figure 2. Illustration of pre-aggregation of catalyst/support and ionomer in their respective dispersants and their influence on the heterogeneous structure of catalyst inks.

In order to elucidate the parameters that influence catalyst layer morphology, researchers have exploited molecular dynamics simulations. Employing coarse-grained models, mesoscale simulations are providing insight into how microphase segregation occurs in the ink and how

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the dielectric properties of the dispersant influences the size of carbon/Pt and ionomer aggregates during solidification. Figure 3 illustrates snapshots of the evolution process.47 Rapid clustering of carbon particles is observed, with the ionomer assembling into a distinct interconnected phase in the voids. The structure further evolves into agglomerated carbon particles partially encapsulated by aggregates of ionomer and clusters of water. The simulation supports the hypothesis that ionomer aggregates interact with pre-formed carbon agglomerates, segregating into ionic and hydrophobic regions. The hydrophilic region contains water, hydronium ions, solvent and ionomer side chains to form a 3-D network with the ionomer side chains located at the interface between ionomer domains and water filled pores. Another finding is that ionomer does not penetrate the agglomerates of Pt/C, but forms a distinct proton-conducting phase.

Figure 3. Spontaneous microstructure formation as illustrated by molecular dynamics simulations employing a coarsegrained model. Left to right: Snapshots after increasing time ® intervals. Carbon particles (gray beads), Nafion backbone (red, removed in “c”), side chains (green), hydronium ions inside the pore network (blue). Water beads omitted for clarity. Reprinted from ref. 47. Copyright 2007 American Chemical Society.

Experimental data on the effect of dispersion media on the dispersability of ionomer in inks has been investigated by several groups. PFSA ionomer can be solubilized in organic solvents having a dielectric constant, ε > 10, whereas colloidal solutions are formed for solvents having a dielectric constant of 3 – 10, while PFSA precipitates when ε < 3.48, 49 Catalyst layers prepared using n-butyl acetate (ε = 5.0) yield more efficient electrodes compared to those prepared by the solution method, suggesting that the colloidal method improves continuity of ionomer network and higher porosity of the catalyst layer.50 From complementary work, it is concluded that catalyst layer electrodes improve in performance when prepared from less polar dispersion media, and is ascribed to increased ionic conductivity and improved secondary pore structure.51 Similar conclusions have been found elsewhere,52, 53 affirming that the dispersion medium influences the distribution of ionomer in the catalyst structure. Experimental details of the nano- and meso-structure of catalyst inks are lacking, largely because of the difficulty in dealing with highly dynamic, opaque heterogeneous liquids that settle with time. The accuracy of dynamic


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light scattering (DLS) technique is questionable because the dispersions must be excessively diluted to study which may modify the structure of the original ink. SANS and SAXS, with their higher penetration depth, have been applied to concentrated solutions in order to mitigate this concern. Ultra-small-angle X-ray scattering (USAXS), in conjunction with cryo-TEM, has been used to provide information on the structure, particle size, and interactions of carbon and ionomer, and the influence on catalyst layers.54 Particle size, size distribution and geometry are not uniform in the ink. Moreover, a direct correlation between the structure of the ink and the catalyst layer exists. USAXS analysis confirms that the size of the Pt/C catalyst and PFSA ionomer agglomerates change with ink composition, illustrating the complex and dynamic nature of the interactions. Using TEM analysis, catalyst inks were found to comprise rod-like ionomer particles 3 nm in diameter and 20 nm in length, spherical carbon particles 30-70 nm in diameter, rod-like aggregated carbon black particles 300 nm in diameter, and larger aggregated particles >400 nm in size. Overwhelming evidence exists, therefore, that suggests significant aggregation of the ionomer and of the Pt/C particles occurs in their respective dispersions prior to their mixing, and that preaggregation is captured to a degree and translated into the catalyst layer. Evidence also exists that a non-trivial portion of the ink consists of smaller structures that play a role in how the pre-aggregated solids interact as solvent evaporates. In the following sections consideration is given to how aggregation phenomena translate into the distribution of ionomer in the catalyst layer; how analytical, spectroscopic and microscopy technologies are being directed to facilitate the elucidation of the structure; and how catalyst layer morphology affects the electrochemical and mass transport properties. At this point it is worth noting that elucidation of the rate limiting processes within the catalyst layer requires comprehensive analyses of the components both individually and collectively. A rudimentary ex-situ analysis of catalyst layers involves porosimetric analysis to determine pore size and pore size distributions, SEM analysis to evaluate agglomerate size and distribution, and TEM analysis for ionomer distribution. In situ fuel cell analyses usually involve cyclic voltammetry (CV) to determine the electrochemical surface area of the catalyst, chronoamperometry to determine gas crossover, electrochemical impedance spectroscopy (EIS) to determine protonic conductivity, I-V polarization curve analyses under high oxygen stoichiometry to determine ORR kinetics, and I-V performance and EIS under reduced RH and under load to evaluate mass transport resistance.55 Rate limiting processes can then be determined and evaluated in context of the ex-situ analyses, and only then can a rationale for their improvement be established. Ionomer–Pt/C Interactions. As the impact of the ionomer in the catalyst layer becomes more apparent, its distribution is attracting closer scrutiny. Advancements in electron microscopy have provided valuable insights into how ionomer interacts with the catalyst and

support. Much can be drawn from micrographs that reveal the presence of an ionomer skin a few nm thin around carbon agglomerates, giving the appearance of an idealized 3-phase interface(Figure 4).56, 57

Figure 4. SEM (left) and TEM images (right) showing catalyst layer and ionomer strands that bind Pt/C agglomerates, and an ultra-thin ionomer film, respectively. Reprinted from ref. 56. Copyright 2006 The Electrochemical Society.

The formation of a thin ionomer skin is consistent with coarse-grained molecular dynamics simulations. Studies of self-organization processes of catalyst inks consisting of agglomeration of Pt on graphitized carbon black, the formation of ionomer aggregates, the evolution of the porous network, and the creation of interfaces between the distinct phases, predict a thin adhesive ionomer film partially covering carbon agglomerates. In the case of Pt/C agglomerates, Pt facilitates the formation of an ordered array of ions on the ionomer film surface.58 Simulations reveal a preferential orientation of charged ionomer side chains that depends on the surface wetting properties of Pt/C agglomerates. The effect of Pt nanoparticles on the catalyst layer microstructure was to distort the ionomer film, leading the ionomeric side chains to point partly towards the Pt/C surface. Simulations indicate that Pt particles influence the thickness of the ionomer film, with the morphology of ionomer being different from that in the membrane. In contrast, in the presence of highly hydrophobic carbon, such as Vulcan XC-72, the ionomer interacts with the hydrophobic surface and the ionomeric chains point away from the agglomerate. The elucidation of specific interactions between PFSA ionomer and Pt/C agglomerates is paramount to the advancement of ionomer/catalyst systems and remains a challenge. Experimental evidence that does exist is derived from studies of ionomer-Pt interactions, typically involving electrochemically-determined active surface area, ORR activity and CO adsorption. More detailed molecular-scale information can be obtained from in-situ infrared spectroscopy,59, 60 and surface-enhanced Raman spectroscopy (SERS to extract information on the structure of PFSA ionomer adsorbed on electrode surfaces. By comparing vibration assignments of Raman and IR spectra of model compounds, SERS spectra can be interpreted to reveal that PFSA ionomer adsorbs on Pt through sulfonate groups alone,61 which differs from a previous assertion that -CF3 groups also interact with the surface.62 Carbon Supports. The influence of the hydrophobicity/hydrophilicity of the catalyst support on the interaction of ionomer is receiving increasing attention. It is therefore important to discuss the pore structure of car-


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bon blacks and the aggregates they form in order to understand the synergistic effect of the support and the ionomer in controlling catalyst layer electrochemical and mass transport properties. Carbon supports are widely employed because of their low cost, chemical stability, high surface area and affinity for supporting metallic nanoparticles. Carbon black and graphitized carbon are common support materials used in FC technology and range in specific area from < 10 to >2000 m2/g.63 A single carbon black particle consists of a pseudo-spherical arrangement of quasi-graphitic microcrystallites a few layers thick and ~ 20-30 Å in width, resulting in complex, high surface area, porous materials. Carbon blacks have a propensity to aggregate, forming agglomerates of carbon particles exhibiting a bimodal pore size distribution. These include 2-20 nm sized pores within agglomerates of carbon particles and larger >20 nm sized pores that exist between aggregates of agglomerates. These are often referred to as primary and secondary pores, respectively.48, 64 In addition there exists a < 2 nm pore structure that resides within the primary carbon particle, but these are generally not discussed in the context of fuel cell electrochemistry. The hierarchy of pores found for carbon blacks is illustrated in Figure 5.

Figure 5. Representation of micro-, meso, and macroporous structure of carbon blacks due to the primary carbon particle, agglomerated particles, and aggregation of agglomerates. Reprinted from ref. 65. Copyright 2010 American Chemical Society.

Using BET N2 adsorption studies, it is observed that the aggregates of carbon particles found in the powder form are largely preserved in the structure of the catalyst layer.65 By comparing two commonly used carbon supports, Ketjen Black and Vulcan XC-72, which differ significantly in total available surface area, pore volume and fraction of micropores, it is found that the higher surface area of Ketjen Black is largely due to its higher proportion of micropores (< 2 nm), which are largely electrochemically inaccessible. Catalyst layers prepared with various ionomer contents retain key features of the sorption isotherms exhibited by the native carbon powder. Catalyst layers based on ionomer and high surface area Ketjen Black possess larger pore volumes in the secondary pore size range (2-20 nm) than low surface area carbons for identical ionomer loadings. Through diminishment of the

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secondary pore size volume fraction, it is surmised that ionomer is distributed on the surface of agglomerates covering the intra-agglomerate pores (< 20 nm), i.e., on the surface of secondary pores, and that the primary pore space (2-20 nm) and the < 2 nm pores are in accessible to ionomer. These findings are in agreement with proposed agglomeration models and dynamics simulations.64 Taking the assumption that ionomer is evenly distributed over the mesoporous surface area to the extreme; the ionomer thickness can be estimated from the ratio of ionomer volume to mesoporous surface area. For high surface area Ketjen Black and low surface area Vulcan XC72 with 30 wt% ionomer, the ionomer film is calculated to be 1-2 nm and ~ 3 nm thick, respectively.65 The values are consistent with TEM images and course grain model simulations. Moreover, to achieve an ionomer thickness on low surface area carbon similar to that observed for 30 wt% loading on high surface area carbon, the mass fraction of ionomer should be substantially reduced. An alternative viewpoint is that low surface area, mesoporousforming carbons require much less ionomer than high surface area carbons to form a contiguous proton conducting network. This is important because it is essential that ionomer contents be as low as possible to allow sufficient void space in the catalyst layer for unimpeded gaseous mass transport while creating a sufficiently connected percolation ionomer network for proton conduction. Significant blocking of open pore space by ionomer is observed with higher ionomer loadings, and this occurs with lower ionomer loadings when low surface area carbon blacks are employed. The optimum value of 30 wt% ionomer, empirically determined for PFSA ionomers, is clearly not optimal for all carbon supports because of their different mesoporous surface areas and must be adjusted for each catalyst support/ionomer couple. Figure 6 shows a simplified illustration of the delicate balance between water, ionomer, proton conductivity and gas transport for water sorbed into Ketjen Black (high surface area) and Vulcan XC-72 (low surface area) based catalyst layers with 5, 30, and 50 wt% ionomer loading.66 In the low ionomer content case, the ionomer content is too low to provide optimal conductivity, even though low surface area carbon supports allow for a more uniform distribution - absorbed water provides a contiguous link between disconnected patches of ionomer: The low ionomer content, however, facilitates rapid gas access to the reaction sites. As ionomer content catalyst layer is increased, pore space becomes filled, proton conductivity increases, but catalyst sites become blocked to gas. This is exacerbated in low mesoporous surface area catalyst layers.


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Chemistry of Materials core and sulfonate groups project outwards, are retained upon adsorption on hydrophilic surfaces.69

Figure 7. (Top) Water droplet angle on ultrathin (100 nm) ® ® Nafion films (right) and Nafion Membrane (left). (Bottom) Illustration of the evolution from a hydrophilic to hydrophobic surface with increasing thickness, based on bundles of ionomer chains. Reprinted from ref. 69. Copyright 2011 The Electrochemical Society.

Figure 6. Illustration of increasing coverage of agglomerates with increasing ionomer content, the corresponding blocking of pore space and active sites, and the effect of high vs. low surface area carbons. Active reaction sites having the optimal contact with reactant gas and ionomer are shown in pink. Examples of a reaction site hindered by excessive ionomer and water, i.e., blocked sites, are shown in green. Reprinted from ref. 66. Copyright 2011 American Chemical Society.

Ultra-Thin Ionomer Films. Assuming that ultrathin, nm-thick films of ionomer coat agglomerates of Pt/C particles raises several questions: Do ultrathin thin films possess the same properties as bulk ionomer? Is Pt catalyst inside the agglomerate (not in contact with ionomer) electrochemically active? In relation to the former, it is observed that the proton conductivity of recast Nafion® thin films (10 nm to 210 µm) decreases with decreasing thickness.67 Films 100 nm thick, for example, are an order of magnitude lower in conductivity than bulk material. Under dry conditions, proton conductivity is more sensitive to thickness, which leads to the speculation that the thickness dependence of water absorption in ultrathin films plays a role in determining conductivity. Proton conductivity of ultrathin Nafion® films adsorbed on model substrates examined over a wide range of temperatures and relative humidity reveal that the activation energy is twice that of bulk Nafion® under high RH, with the difference increasing as humidity is decreased.68 It is suggested that the mechanism of proton transport in ultrathin films is different than in the membrane and that the nano-cylindrical structures characteristic of Nafion® membranes may be absent in ultrathin adsorbed films. The water contact angle falls from >100 degrees on membranes to 20 degrees on ultrathin films. It is speculated that the aggregated bundles of PFSA ionomer chains observed in solution, in which nonpolar backbone forms the

Recently it was reported that ultrathin films are hydrated to larger extents than bulk membranes, which is consistent with their higher hydrophilicity, as observed by water contact angle measurements.70 However, these data are representative of thin films on substrates, in the absence of Pt/C. The latter may also play a role in determining water-uptake behaviour of ionomer in catalyst layers for it is also reported that water-vapor uptake by ionomer in the catalyst layer is lower than in the bulk ionomer - with lambda values being 4 as opposed to 10 12 in membranes (at 95% RH).71 In addition, the water uptake kinetics are found to be much faster than expected for a diffusion-controlled process, suggesting that the interfacial character of thin-films cannot be neglected. The question as to whether the Pt nanoparticles inside an agglomerate particle are fuel cell-active if ionomer is restricted to the outside of agglomerates has been addressed theoretically.72 Modeling studies have been undertaken to investigate aspects of current conversion, reactant and current distributions, and catalyst utilization in two extremes of spherical agglomerates: One type consisting of a mixture of Pt/C particles and PFSA ionomer and the other consisting solely of Pt/C agglomerates (and water-filled pores) surrounded by a film of ionomer. The PFSA ionomer-filled agglomerates were found to support a more homogeneous distribution of reaction rates but it was also found that for agglomerates in which ionomer is restricted to the outward surface the proton penetration depth in water-flooded agglomerates could be substantial, providing high catalyst utilization. The notion that ionomer is not required for the transport of protons over small distances is the basis of understanding proton transport in ultrathin catalyst layers such as 3M’s nanostructured thin films (NSTF) which contain no solid polymer electrolyte.6, 13 Here, Pt covers the entire surface of the support and the nanosized hydrophilic pores are typically flooded with water during operation both of which facilitates proton transport due to electrostatic interactions with the surface charge of the pore walls.73


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Further insights into the distribution on ionomer are obtained using electron tomography based on multiple TEM images of a 3D dispersion of ionomer among Pt/C agglomerates. A uniform dispersion of the ionomer and Pt/C agglomerates is observed although it cannot be distinguished whether the ionomer exists solely on the surface of the agglomerates or inside because the electron beam passes through each specimen to yield a 2Dprojection.74 However, given the modeling and experimental studies discussed previously it seems likely that ionomer is largely restricted to the outside of the carbon agglomerate. Non-homogeneous Distribution of Ionomer. Notwithstanding the considerations that ionomer may preferentially adsorb onto preformed agglomerates of Pt/C particles to form ultrathin coatings, there is considerable evidence that ionomer is also dispersed nonhomogeneously throughout the catalyst layer and that much larger domains of ionomer exist that contribute to the binding of catalyst particles.75, 76 A non-homogeneous distribution is expected to influence the transport of dissolved gases, water, and protons. Other techniques able to image and quantify ionomer at the micro-, meso-, and macro scale are serving to accelerate our understanding of ionomer distribution. An emerging technique proving particularly insightful for imaging components on 10 nm - 100 µm length scales is scanning transmission X-ray microscopy (STXM).77 The advantage of STXM over TEM (typically used in combination with dispersive X-ray analysis) is TEM often requires embedding the sample in resin and/or staining with heavy metal ions to enhance contrast, which may alter the morphological structure of the ionomer. STXM does not require this and is also not harmful to the ionomer; whereas e-beam damage of ionomer is a concern in TEM analyses. STXM applied to catalyst layers is illustrated in Figure 8. Here, individual C 1s and F 1s X-ray absorption NEXAFS spectra of the catalyst layer are mapped to distinguish the ionomer from the carbon support.78 The technique has been advanced further to prepare 3D data sets (STXM spectrotomography) for 3D component mapping with spatial resolution of 30 nm.79 The images confirm the non-homogeneity of the catalyst layer and that larger aggregates of ionomer exist.

Figure 8. (Left) Distribution of ionomer (green) and carbon support (red) as determined by scanning transmission xray microscopy (STXM). Left figure reprinted from ref. 78. Copyright 2011 The Electrochemical Society. Right figure reprinted from ref. 79. Copyright 2012 The Electrochemical Society.

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A more accurate representation of the distribution of ionomer in the catalyst layer is one that incorporates both the concept of an ultrathin ionomer film surrounding agglomerates of Pt/C and the presence of large domains distributed inhomogenously throughout the secondary pore structure. The distribution of ionomer does not appear to be homogeneous on any length scale, with the result that individual Pt catalyst particles may be in contact with a thin film of ionomer, large agglomerates of ionomer, or no ionomer at all. This heterogeneous nature has been discussed in an attempt to develop an evaluation method for the effectiveness of Pt under actual fuel cell operating conditions where Pt/C is considered to be in contact with a range of ionomer film thicknesses, from thick to the non-existent, as illustrated in Figure 9.80 Where Pt/C is in contact with a thick film of ionomer, proton conduction at the interface is facile but reactant gas diffusion is hindered. At the other extreme, where the catalyst is not in contact with ionomer, proton transport is impeded, but gas transport to the catalyst is rapid. A thin coating of ionomer provides the optimal transport of proton and gaseous reactant. Using catalyst layers prepared from different volumetric ratios of ionomer to Pt/C, different categories of exposure of Pt/C to ionomer were created and the oxygen reduction reaction kinetics observed can be explained on the basis of the different interfacial transport scenarios. For low ionomer:Pt/C ratios, electrochemically-derived Tafel slopes were greater than ideal due to hindered ionic conduction in very thin ionomer films. For high ionomer:Pt/C ratios, where ionic conductance is high, current density is limited by O2 mass transport.

Figure. 9. Schematic depiction of variation of ionomer film thickness within a catalyst layer; Reprinted from ref. 80. Copyright 2010 Elsevier.

Properties of the Ionomer. The intrinsic properties of the ionomer clearly play a significant part in determining the kinetic and mass transport parameters of the catalyst layer. A few critical aspects of the ionomer are considered below, namely, proton conductivity, ORR kinetics/mass transport and water sorption/transport. Moreover, the relative importance of these parameters in the context of fuel cell operation is discussed as they provide a basis for future ionomer design.81 (i) Proton Conductivity. Much has been reported on the proton conductivity of ionomers, particularly when the ionomer is in the form of a membrane. Only a brief


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recap is required here: Proton conductivity is intimately linked to both acid and water content but also to the strength of the acid and the internal morphology of the membrane. Understanding the interplay of these parameters is essential to understanding proton conductivity and for developing new materials. The transport of protons in aqueous solutions occurs via both the vehicular transport of hydrated protons and structural diffusion. In the case of ionomers, the counter-ions are bound to the polymer chain and thus proton mobility is also dictated by their organization. The effective mean-free path for conduction is critical, as “dead-ends” and high tortuosity lower proton mobility.31, 82 The degree of dissociation of the acid and distance between acidic groups also plays a role. Proton conductivity is related through the NernstEinstein relationship to the product of the activity of protons and proton mobility. Both proton activity and mobility are dependent upon the acid strength and water content. The strength of the acid strength dictates how easily the proton is dissociated from the counter-ion. pKa values of model compound, e.g., triflic acid, is much more negative than p-toluenesulfonic acid,83 meaning that fewer water molecules are required to dissociate a proton from PFSA ionomer than from sulfonated polyaromatics.84 The acid content of the ionomer, as expressed by the ion-exchange capacity (IEC), strongly influences conductivity and is usually the first parameter varied in the study of new ionomers. This is expected given conductivity is dependent upon proton concentration. However, to understand the relationship between conductivity and IEC it is necessary to examine how water uptake is affected by IEC. Generally, for low ionomer water contents the effective proton mobility is low because of incomplete dissociation of the acid and poorly connected hydrophilic channels. For intermediate water contents, typically λ ~ 10 to ~20, increasing the water content has a beneficial effect on conductivity because it hydrates protons and provides improved pathways for conduction. But for larger water contents, excessive swelling dilutes the proton concentration and, despite the proton mobility being high, conductivity is reduced. The other important factor controlling the conductivity of ionomers is the microstructure/morphology, as these determine both the water content and the organization of hydrophilic channels as discussed previously. (ii) Oxygen Reduction Reaction (ORR) Kinetics. The ionomer is expected to strongly influence the kinetics of ORR and HOR on Pt. While kinetic parameters can be extracted from fuel cell polarization curves many factors contribute to the activation polarization losses and only generalized correlations can be made. To provide deeper insight, ex-situ measurements with carefully controlled interfaces and conditions are necessary. Currently, knowledge on the influence of the ionomer on ORR is extrapolated from ex-situ electrochemical techniques using ionomer membranes. ORR kinetic parameters such as exchange current density, Tafel slope and transfer coefficient can be extracted from electroanalytical methods applied to microelectrode/ionomer membrane interfaces

or rotating disk electrode experiments. Typically, dual Tafel slopes are observed for ORR at Pt/ionomer interfaces due to potential-dependent, oxide-covered and oxidefree Pt surfaces.85 PFSA ionomer membranes, such as Nafion® provide faster ORR kinetics than sulfonated polyaromatics, such as poly(aryleneethersulfone),86 and poly(etheretherketone).87 Rotating disk electrodes coated with thin catalyst layers can also be used to evaluate interfaces, as was reported for Pt/carbon black incorporating sulfonated polyimides, but conditions under which these films are studied are limited by electrode flooding.88 The common observation is that non-PFSA ionomers yield inherently slower ORR kinetics, which transposes to larger activation polarization losses in fuel cell cathodes. The origin of this is not yet understood but adsorption processes and the morphology of the ionomer on the Pt/C surface are expected to play dominating roles. (iii) Oxygen Mass Transport. The ionomer as a membrane should be as impermeable to oxygen (and hydrogen) as possible in order to minimize gas crossover, as this lowers the operating voltage of a fuel cell and is believed to contribute to fuel cell degradation.89, 90 It is reasonable to speculate that ionomer in the catalyst layer should be highly permeable to oxygen (and hydrogen) in order to promote transport to the reaction sites. However, whether or not oxygen permeability through the ionomer in the catalyst layer is important for fuel cell operation is not yet clear. Oxygen diffusion is believed faster in the hydrophilic domains of ionomer, while O2 solubility is higher in the hydrophobic regions.91, 92 O2 permeability through Nafion using different experimental setups has been previously reviewed.93, 94 Measurements of gas permeability vary widely but the trend is that PFSA ionomer membranes are more permeable than hydrocarbon ionomers because of the exceptionally high oxygen solubility in the fluorous domains.95 Electroanalytical methods allow for deconvolution of O2 permeability into solubility and diffusion coefficient.96-98 For measurements on various ionomer membranes as a function of IEC, temperature, gas pressure and humidity, sulfonated hydrocarbon membranes exhibit the same dependence to gas pressure and temperature as Nafion: i.e., oxygen solubility increases with pressure while the diffusion coefficient remains relatively unchanged; the diffusion coefficient increases with increasing temperature while O2 solubility decreases; and because the diffusion coefficient increases faster than the drop in solubility, the membrane permeability increases with temperature. Although O2 permeability of the sulfonated hydrocarbon membranes increases with increasing IEC, it is still found to be lower compared to Nafion due to the considerably lower oxygen solubility. This has been confirmed using in situ fuel cell measurements.99 O2 diffusion through ionomer appears limited by the percolation path, which is not only defined by the architecture and morphology of the ionomers but also by the amount of water absorbed by the membrane. It is therefore difficult to compare diffusion behavior among ionomers of different structure. However, some trends stand out:100, 101 Oxygen diffusion in BAM® membranes, for ex-


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ample, is much more dependent on water content than EFTE-g-PSSA, and DAIS membranes. Nafion® N117 has a higher diffusion coefficient even though it has a much lower water content. Under low water content, Nafion and BAM® ionomer exhibit much higher oxygen solubility than DAIS and ETFE-g-PSSA membranes because of their high fluorous content. For high water contents, oxygen solubility is less affected by the chemical structure of membranes as oxygen solubility appears to approach a constant value. Oxygen permeability through Nafion® N117 is greater than for other ionomers with similar water content, even though the oxygen diffusion coefficient is relatively low. For ETFE-g-PSSA, oxygen permeability is surprisingly low given their high total water contents, but this is related to the relatively low λ values and low “free water” content. While oxygen mass transport parameters differ from one ionomer to another, and are highly sensitive to temperature and RH, the question remains whether these factor into rate-determining processes of fuel cell kinetics. Gas transport resistances in and through the ionomer in the catalyst have been studied by measuring limiting current densities of membrane-electrode-assemblies (MEAs).102 By varying the pressure of the reactant gases, the total gas transport resistances were deconvoluted into a dominant Knudsen diffusion resistance in secondary pores of the catalyst layer and a second resistance due to mass transport resistance in the ionomer. The latter increases in significance upon reducing the Pt loading commensurate with solubilized gas having to diffuse through longer path lengths to reach the catalyst. For investigating the impact of lowering the Pt loading on transport properties and I–V performance, a simple 1-D model for the cathode catalyst layer has been developed, as shown in Figure 10, that illustrates the synergy between oxygen transport through pores/ionomer and proton conduction.103 The analysis indicates that, as Pt content is lowered, the proton transport resistance and Knudsen (pore) diffusion resistance to gaseous oxygen are reduced because the thickness of the catalyst layer is correspondingly reduced, and the losses observed in polarization curves are attributed to transport resistance of oxygen in and through the ionomer. This suggests that oxygen transport in the ionomer influences the fuel cell power density and plays an increasing role as the Pt content is reduced.

Figure 10. Steady-state 1-D transport model of cathode catalyst layer. Reprinted from ref. 103. Copyright 2011 Elsevier.

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With a view towards optimal gas transport and water transport and proton conduction, a three-phase boundary, cathode catalyst layer model incorporating the transport of electrons, protons, and oxygen has been investigated. It is calculated that oxygen transport resistance through the ionomer coating on Pt/C agglomerates is a dominant factor controlling rate determining processes of the cathode catalyst layer activity under normal operating fuel cell conditions.104 It is also suggested that limitations in the transport of oxygen through the ionomer can be offset by reducing the thickness of the ionomer film and increasing the catalyst layer thickness. However, increasing the porosity of the catalyst layer, reducing the ionomer thickness and using thicker layers, decreases the effective proton conductance of the catalyst layer, and results in inadequate proton conductivity and non-uniform overpotentials in the catalyst layer leading to performance losses. Under this scenario, increasing the permeability of oxygen in the ionomer would serve an effective strategy for maintaining satisfactory I-V polarization performance under reduced Pt loading. In complementary work, using ionomer-coated Pt electrodes, the resistance to oxygen dissolution from the gas phase into Nafion is found to account for a large part of the overall oxygen transport losses.105, 106 An agglomerate model was able to fit the experimental results. It is suggested that the rate of oxygen dissolution from the gas phase into the ionomer, rather than inside the agglomerate, is the cause of this oxygen transport loss, and that thin ionomer films are much more resistive to oxygen dissolution than the bulk ionomer In a different approach, fuel cell limiting current densities, in conjunction with electrochemical impedance spectroscopy (EIS) was employed to identify predominant reactant gas mass transport phenomena under reduced RH.107 The limitation of gas diffusion through the pores of cathode catalyst layers was not sufficient to explain the observed decreasing (IR-corrected) limiting current density with increasing RH. EIS revealed a low frequency impedance arc that expands with increasing current density but contracts with increasing RH, and is attributed to oxygen mass transport resistance in the catalyst layer ionomer. Under low RH operation, oxygen transport in the ionomer thin film within catalyst layer becomes an increasingly important factor that contributes to fuel cell potential losses. (iv) Water Transport. Water transport through ionomer membranes is critical for PEMFC operation.108, 109 The net water flux is a combination of electro-osmotic drag and diffusion. The former is due to water(s) of hydration transported with protons from anode to cathode; the latter, a result of the diffusion of water which in turn is caused by a water gradient that develops upon accumulation of water at the cathode and dehydration at the anode. As these processes affect the water content of the membrane, they also affect proton conductivity of the membrane. In the context of the ionomer in catalyst layers, either as ultrathin thin films or through larger domains, the current understanding of these phenomena is


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limited. However, from studying water transport through ionomer membranes trends can be extracted; but it should be noted there are many different methods to measure water transport110 and there are inconsistencies in reported data. Generally, increasing the temperature increases the rate of water transport, whereas reducing the water content (via lowering RH) reduces it. Water self-diffusion coefficients increase with increasing water content.111, 112 The nature of the water present within an ionomer plays a strong role in influencing rates of water transport. For λ > 6, water is said to exist as “free water”, “loosely-bound water” and “non-freezable water”.113 The absence of free water, as found for ionomers with very low λ, water transport is relatively poor. Water diffusion increases with increasing free water content. At first read, it may seem that water transport through ionomer in the catalyst layer may be of negligible importance given the small dimensions of the ionomeric domains, with respect to the thickness of a PEM. However, fuel cell reactions within the catalyst layer are dependent on the transport of water at interfaces. Interfacial transport resistance across membrane-vapour interfaces has been found to contribute significantly to water transport resistance, and can dominate rates of water flux for ionomer films < 50 μm thick.114 Current-sensing AFM reveals that the fractional area of ion-conducting domains at the surface decreases with decreasing humidity, as illustrated in Figure 11, which correlates well with an observed increase in resistance of interfacial water transport.115 These studies lead to the speculation that water transport through ultrathin ionomer films such as those believed to exist in the catalyst layer may be more resistive than previously thought.

the internal morphology of the films controls water uptake (and presumably water transport), and how substrate/ionomer interactions influence ionomer domain orientation in ultrathin films.116 For instance, films cast on hydrophobic surfaces possess a higher concentration of ionomer domains oriented parallel to the substrate, which consequently reduces water uptake. 2D confinement and substrate adhesion constrains in-plane swelling of the film. The observation of anisotropy in ultrathin films may explain interfacial water resistance at PFSA ionomer membranes. However, ex-situ measurements of liquid and vapour water permeability through PFSA ionomer membranes and membranes coated with catalyst layers indicate that the catalyst layer exerts no influence on the interfacial water sorption/desorption kinetics of the membrane interfaces and that the bottleneck for water transport across catalyst-coated membranes appears to be the related to the membrane.117 Since it has previously been shown that significant interfacial water transport processes exist, these contradictory observations lead to the questions regarding the location, nature, and function of the membrane/ionomer interface in catalyst layer, and how it influences interfacial water transport? While the TEM image and cartoon illustration118 shown below (Figure 12) provide some insight into the catalyst layer/membrane interface, the molecular-level description is far from complete.

Figure 12. TEM image and simplified illustration of the membrane/ catalyst layer interface. Left figure reprinted from ref. 117. Copyright 2010 The Electrochemical Society. Right figure reprinted from ref. 118. Copyright 2010 Makoto Adachi.

Figure 11. Current-sensing atomic force microscopy and im® ages on Nafion NE212: Low-current (light green) and high current (blue). RH (left to right): 30, 51, 65, 77, 84%. Reprinted from ref. 115. Copyright 2011 American Chemical Society.

In situ grazing-incidence small-angle X-ray scattering (GISAXS) on 100 nm thick PFSA ionomer films demonstrates the wetting characteristics of thin films and how

Concluding Remarks. The above considerations on the nature of the ionomer in the catalyst ink, its interaction with Pt and C, the ambiguous distribution of ionomers in catalyst layers and the influence of specific properties of the ionomer provide a basis for understanding the function of current ionomers and aids the rationale design of non-PFSA (hydrocarbon) ionomers. As with PFSA ionomer, hydrocarbon ionomers must possess high proton conductivity, low electronic conductivity, sufficient permeability to gases, mechanical binding properties and reasonable chemical stability. The proton conductivity of hydrocarbon ionomers is often lower than PFSA ionomers for a given IEC. This can be overcome by


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increasing the ionomer content in the catalyst layers but this densifies the catalyst layer with the consequence of harmfully reducing mesoporous void space. Increasing the ion exchange capacity of the ionomer in order to increase proton conductivity has the tendency to render the catalyst layer too hydrophillic, swelling excessively in contact with liquid water. The dispersion medium used to prepare the catalyst ink also influences the properties of the catalyst layer because it controls ionomer-ionomer, ionomer-Pt/C, Pt/C-Pt/C interactions. These interactions warrant closer scrutiny at the molecular level in order to correlate and control the degree of aggregation within catalyst inks. Concerning oxygen reduction reaction kinetics, there is sufficient evidence to indicate that exchange current densities are inherently lower in the presence of hydrocarbon ionomer.37 This directly affects the activation polarization of fuel cells which is dictated by inherent rates of electrochemical electron transfer and the active surface area of the catalyst. Oxygen diffusion coefficients are shown to be largely dependent on the water content of the ionomer, and in particular on the amount of “free” water present. Diffusion coefficients for hydrocarbon ionomers can be on par with PFSA ionomers but oxygen solubility is several times lower due to the lower fluorine content or its complete absence. As a consequence, oxygen permeability is also generally lower. In the context of water transport, the rates of transport for hydrocarbon ionomers are typically lower than for Nafion membranes, even when their water contents are larger.31, 113 This is due to the absence of highly organised hydrophilic channels that are characteristic of PFSA ionomer. Recent studies also reveal that the interfacial water transport resistance of certain archetypical hydrocarbon ionomers is larger than Nafion, despite the surface of the hydrocarbon ionomer being more hydrophilic.119 This leads to the speculation that the connectivity of surface ionic domains with the internal structure of the ionomer may play a dominant role in interfacial transport properties - a topic that deserves more scrutiny. Given that the majority of intrinsic properties of hydrocarbon ionomers pertinent to the performance of catalyst layers are inferior to PFSA ionomers, it is not surprising that fuel cell polarisation curves of fuel cells based on model hydrocarbon ionomers are also inferior.37, 120-123 Future advancements in the development of hydrocarbon ionomers specifically designed for catalyst layers must take note of why PFSA ionomers afford superior function and take into account the current deficiencies of hydrocarbon ionomers. To this end, it must be recognized that in addition to maintaining high proton conductivity, water transport and oxygen permeability, the electrochemical activity of the Pt/hydrocarbon ionomer interface must be enhanced to the level demonstrated by Pt/PFSA ionomer interfaces. The vast majority of sulfonated hydrocarbons considered as candidates for the catalyst layer are soluble only in high boiling point, amphiphilic solvents, and are generally not dispersible in alcoholic aqueous solutions such as those used to prepare Nafion solutions. As a conse-

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quence, solutions of hydrocarbon ionomers are generally “truer” polymer solutions than PFSA ionomer dispersions, to the extent that the rod-like aggregate structures characteristic of PFSA ionomers may be largely absent in solutions of hydrocarbon polyelectrolytes. Solubilized, single polymer chains wrap and bind C/Pt aggregates in a manner that may block the catalyst surface rather than activate it. The combination of single chain interactions and the presence of aromatic and heterocyclic motifs that adsorbed onto the catalyst appear detrimental to the electrochemical activity of the Pt/ionomer interface. A stream of thought is that hydrocarbon ionomer in the catalyst layer should mimic that observed by PFSA ionomers, i.e., bundles of ionomer chains which coat the surface of preaggregated Pt/C particles. Such structures in PFSA ionomers are formed at the early stage of catalyst formation, when ionomer is dispersed in solution prior to mixing with dispersions of Pt/C. Here lies the opportunity to control aggregation phenomenon of the hydrocarbon ionomer. Unfortunately, the vast majority of ionomers that would be of interest for use in the catalyst layer are soluble in only in high boiling, aprotic solvents. Dispersions of ionomer in alcoholic solutions are rare, or at least not highly considered in the literature. It can be argued that the quality of the dispersion of ionomer in solution is as important to the function of a catalyst layer as the ionomer itself. In summary, next generation ionomers should possess mass transport properties that are less sensitive to changes in temperature and humidity; support high oxygen/hydrogen permeability; be dispersible in water/alcohol mixtures; and be less susceptible to chemical degradation. This will require fundamental research that addresses a lack of understanding of molecular level interactions of ionomer with catalyst and support materials; aggregation phenomenon; the influence of solidification of catalyst inks on the morphology of catalyst layers; the rate of transport processes (proton, water, gases) through thin films of ionomer; and imaging and quantification of the distribution of ionomer in catalyst layers. Solutions to these challenges require innovative research that is multiand cross-disciplinary. Acknowledgements. The author thanks his current and former students and research associates for their valued contributions to the development of the field, and Dr. Timothy J. Peckham for proof-reading this manuscript and providing insightful comments. References.

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Fuel Cell Catalyst Layers: A Polymer Science Perspective - Chemistry

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