Invited: Electrocatalyst Layer Degradation of PEM Fuel Cells

Fuel cells have the potential to replace the internal combustion engine in vehicles and provide power in stationary because they are energy-efficient, clean, and fuel-flexible. The durability of MEAs (Membrane Electrode Assemblies) of PEM (Polymer Electrolyte Membrane) fuel cells is a major barrier to the commercialization of these systems for transportation applications.  Power transients, shut-down/start-up, temperature cycling, RH (relative humidity) cycling, are all operating conditions imposed by the rigid requirements for transportation applications.  These operational transients have the effect of inducing a number of chemical and structural degradation mechanisms on electrocatalysts and MEAs, changing their performance.  Past improvements have largely been made possible because of the fundamental understanding of the underlying degradation mechanisms. By investigating component and cell degradation modes; defining the fundamental degradation mechanisms of components and component interactions new materials can be designed to improve durability.

One of the major degradation mechanisms involves the catalyst and catalyst layer, including corrosion of the catalyst support leading to structural changes in the catalyst layer. Cathode catalyst layer thinning is regularly observed with increased losses associated with both catalyst activity and to mass transport limitations which are coupled to loss of porosity in the catalyst layer. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume.  Carbon corrosion is induced by air/hydrogen (or hydrogen/air) fronts during shut-down/start-up which cause high potentials leading to carbon support corrosion.  The degree of the effect is dependent on many factors including the types of carbon utilized as the catalyst support, water content, and the anode catalyst layer. An example of cathode catalyst layer is changing porosity during carbon corrosion accelerated stress testing, where the initial catalyst layer porosity is approximately 40%, decreasing to 33% over 10 hours, and is only about 7% after 40 hours, as measured by TEM. A reduction of the median pore diameter of the carbon particles in the electrode structure is also measured.  Electrode structure degradation has been shown to occur in microscopically localized regions, with support corrosion leading to detachment and agglomeration of catalyst particles, while weakening of the carbon structure allows collapse of electrode pores and can severely limits gas transport.  Post-analysis, after drive cycle operation, shows localized bands of carbon corrosion that are correlated increased Pt particle sizes and closer Pt-Pt interparticle spacings. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume.

Membrane degradation has been noted to affect both the membrane and the electrode structure.  After testing, post-characterization detects electrode regions (channel area) where less Nafion (less fluorine signal) compared to the rest of the electrode surface. Other regions show bands where no electrode is bonded to the membrane surface. Characterization also shows regions (under the lands) where a part of the membrane remained attached to the catalyst layer. In this region, there was a delamination of the membrane from the PTFE reinforcement. Delamination of the membrane from the PTFE reinforcement has also been noted with a large amount of particles (analyzed as Si-O by EDS) present along the two reinforcement interfaces. At the cathode side, the membrane (between the reinforcement and the cathode electrode layer) has virtually been removed.

The material design and fabrication methods of producing the catalyst layer can greatly affect the catalyst layer durability.  Utilization of different electrode materials can stabilize the catalyst layer void volume, thus preventing a portion of the performance loss.