Fuel cells have the potential to provide power for a wide variety of applications ranging from electronic devices to transportation vehicles. Cells operating with H₂ and air as inputs and electric power and water as the only outputs are of particular interest because of their ability to produce power without degrading the environment. Polymer electrolyte membranes (PEMs), with hydrophilic, proton-conducting channels embedded in a structurally sound hydrophobic matrix, play a central role in the operation of polymer electrolyte fuel cells. PEMs are humidified by contact with air (the presence of water in PEMs is essential for proton transport). In addition, PEMs must transport protons to catalyst sites, which are typically crystalline solids such as platinum. The arrangement of the hydrophilic domains in the vicinity of both air and solid substrates is thus crucial. A University of California, Berkeley, and Berkeley Lab group has now provided the first set of data on morphology of PEMs at interfaces by a combination of x-ray scattering and microscopy.

Conceptual diagram of a polymer electrolyte hydrogen fuel cell. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxidant (oxygen or air) is channeled to the cathode on the other side of the cell. At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons. The PEM allows only the positively charged ions to pass through it to the cathode. The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell. Figure courtesy of Wikipedia.

The Berkeley group studied PEMs composed of block copolymers supported by a silicon substrate. Copolymers consist of at least two types of polymer structural units (monomers), which can be arranged in different ways. In block copolymers, a polymer composed of one monomer is linked to another polymer by covalent bonds. In the present case, the blocks were hydrophilic polystyrene sulfonate (PSS) (forming the proton-conducting channels) and hydrophobic polymethylbutylene (PMB) (serving as the matrix through which the channels run). Grazing-incidence small-angle x-ray scattering (GISAXS) data from ALS Beamline 7.3.3 provided information about the orientations of the channels near the air interface and through the interior of 180-nm-thick PEMs before and after exposure to humid air.

In the first sample studied, scattering at incident angles below the critical angle and thus dominated by contributions from the PEM/air surface contained well-defined spots, indicating the presence of hydrophilic channels oriented perpendicular to the surface. This morphology is ideal for water transport. In contrast, scattering at incident angles above the critical angle and thus containing contributions from the entire film, exhibited a scattering ring, indicating the presence of hydrophilic channels parallel to the plane of the film. The scattering ring arises because all orientations of the hydrophilic channels in the plane are equally likely. Transmission electron micrographs from the same sample confirmed the two morphologies determined by GISAXS. The parallel orientation, if it were to exist at the PEM/catalyst interface would lead to poor reaction kinetics, i.e., poor energy-delivery rates.

GISAXS patterns (intensity represented by color as a function of the scattering vectors q_y and q_z) for 180-nm-thick PSS–PMB film on a silicon substrate after exposure to humid air. Data were obtained at two incident angles, α_i = 0.14° (below the critical angle α_c), which measures the channel orientation near the air surface, and α_i = 0.21° (above α_c), which measures the channel orientation through the entire film.

Transmission electron microscopy image of the same sample. Coexisting perpendicular and parallel orientations of cylinders propagate from the air surface (stained with RuO₄) and the silicon substrate, respectively, as shown schematically in the upper right inset. The higher-magnification TEM image shown in the bottom left inset indicates the presence of native SiO₂ layers (bright) on top of the Si substrate and preferential wetting of PSS (dark) on substrate.

The interfacial morphologies depend crucially on molecular structure. The Berkeley group studied a second PSS–PMB copolymer that was identical to the first except that the concentration of sulfonic acid groups in the PSS block was doubled, thereby increasing its hydrophilicity. There was no difference in the bulk morphology of the two samples in the dry state, yet the interfacial properties of the samples were dramatically different. The highly sulfonated sample exhibited parallel hydrophilic cylinders at both air and silicon interfaces. This morphology may hinder water transport from the air because the hydrophilic channels in the PEM are buried beneath a hydrophobic skin.

Ordinarily one might assume that increasing the hydrophilicity of the PEM would lead to better water and proton transport, but these results suggest that this is not true. In the case of the PEM studied here, inappropriate orientation of the proton- and water-transporting channels with increasing sulfonation may lead to poorer performance. While these results demonstrate that one can obtain the orientation of the transporting channels, the relationship between morphology and ion transport is only suggestive and has not yet been determined. Future work will be geared toward determining this relationship.

Research conducted by M.J. Park, S. Kim, A.M. Minor, and N.P. Balsara (University of California, Berkeley, and Berkeley Lab); and Alexander Hexemer (ALS).

Research funding: U.S. Department of Energy, Office of Hydrogen, Fuel Cells and Infrastructure Technologies and Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: M.J. Park, S. Kim, A.M. Minor, A. Hexemer, and N.P. Balsara "Control of domain orientation in block copolymer electrolyte membranes at the interface with humid air," Advanced Materials 21 (2), 203 (2009).

ALSNews Vol. 305