Membranes, Electrodes, and Membrane-Electrodes Assemblies Analyzed before and after Operation By Atomic Force Microscopy
Authors: Hiesgen Renate , Morawietz Tobias , Handl Michael , Friedrich K. Andreas 
Published: February 2020

In fuel cells, performance and life time rely to a large extend on the nanoscale properties of the materials used as ionomer or in electrodes. As membranes mostly perfluorinated sulfonic acid (PFSA) membranes, e.g. Nafion®  and Aquivion®, are used. Recently, novel materials such as sulfonated multi-block copolymers are considered for application due to their promising conductivity.

The nanostructure of PFSA has been investigated since many decades, however, mostly on pristine samples, equilibrated at different humidity. Extensive studies by SAXS and TEM provided insight into the hydrophobic/hydrophilic phase separation. Typical separation sizes obtained are in the range of 3-7 nm depending on the water uptake. Open questions concerning the conductive structure and the transient behavior of the membranes remain, especially during dynamic fuel cell operation and many efforts are based on understanding b modelling. Aspects of the membranes that have not been considered sufficiently are the changes in the nanostructure induced by current flow and the interfaces to the electrodes. In particular, a hydrophobic surface layer at pristine membranes exists that prevents fast water exchange through the interface, and which persists even after storage in water. Recently, atomic force microscopy (AFM) investigations have also shown that a major difference in conductive nanostructure of the ionomer occurs upon current flow [1].

Fuel cell membranes and electrodes were investigated as pristine material and after operation to identify changes due to aging of the materials and learn about degradation processes on a nanometer-scale. A material-sensitive tapping mode AFM was combined with current measurements to investigate the structure, phase separation, and conductive structure of surfaces and cross sections.  As ionomer materials Nafion® and Aquivion® PFSA that differ in the lengths of their sulfonated side chains, and novel non-fluorinated sulfonated multi-block copolymers were investigated. To avoid the influence of the hydrophobic surface layer, freshly microtome cut cross sections were investigated using a gas tight chamber with humidity controlled ambient atmosphere.

The phase separation of all ionomers was analyzed using the capacitive current distribution. In Aquivion® PFSA samples and in sulfonated multi-block copolymer, larger connected ionic areas were found compared with Nafion. In the capacitive current, the total hydrophilic phase is visible that could be part of a conductive network under operation. In adhesion force mapping, water-rich areas beneath the surface reveal a low adhesion, and further differences in water-rich and water-poor areas arise at cross sections. In membranes, equilibrated at different humidity, circular isolated areas, mostly 30 to 50 nm large, with low adhesion were visible. They were attributed to water-rich regions within the polymer. Due to the small size of single domains, numerous ionic domains must be contained inside such a water droplet.

After forcing a steady-state current through a membrane by application of a voltage (named activation), the water distribution determined by adhesion force mapping differed significantly from that of equilibrium samples. In the direction of applied voltage, micrometer-long 5 nm high and 50-80 nm low-adhesive structures were found that were attributed to a water-rich ionically conductive phase that formed a branched conductive network. This conductive phase did not resemble pores and could be better regarded as water sheets. 

In cross sections of pristine PFSA samples, we found unexpected large-scale ordered structures consistent with multilayer polymer semi-crystallinity and heterogeneous conductivity with embedded water-rich regions, low in adhesion and higher capacitive current (Fig. 1). In comparison of Nafion® and the short side-chain Aquivion® PFSA, the latter ionomer revealed a systematic shift of step heights to smaller sizes, indicating a stacking in layers with a basic unit of 1.7 nm in Nafion and 1.5 nm in Aquivion PFSA.

Fuel cell electrodes are complex porous compounds with nanosized catalyst particles supported on conducting mesoporous carbons and ionic conducting PFSA. Thick PFSA layers around the Pt-decorated carbon may hinder the diffusion of oxygen to the catalyst and decrease performance. With material-sensitive and conductive atomic force microscopy the identification of different components in electrodes in combination with the local current can be achieved. We will present the distribution of ionomer in cross sections of electrodes cut from membrane-electrode assemblies before and after operation.

[1] Renate Hiesgen, Tobias Morawietz, Michael Handl, Martina Corasaniti, K. Andreas Friedrich, Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies,  Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2014.11.122

Figure 1: Atomic force microscope image of a freshly cut cross section of Nafion® 212, equilibrated and measured at 40 % relative humidity: (a) adhesion mapping and (b) capacitive current overlaid on 3-D topography.

Figure 1