ABSTRACT
This chapter focuses on the role of multiscale molecular modeling simulations for fundamental understanding of catalyst layer degradation in polymer electrolyte fuel cells (PEFCs). The development of stable and inexpensive materials is the most important technological challenge that nowadays PEFC developers are facing. A profound insight based on the theory and modeling of the pertinent materials will help us figure out how fuel cell components with optimal specifications and higher life-time could be made and how they could be integrated into operating cells. This chapter highlights major challenges and perspectives in the molecular modeling of the degradation of nanomaterials for PEFCs, particularly Pt and carbon in catalyst layers. We exclusively focus on morphology and microstructure changes in catalyst layer of PEFCs
as a result of carbon corrosion and platinum stability on carbon support. Extensive multiscale molecular modeling techniques can unravel the microstructure of catalyst layer as a function of carbon loss and Pt degradation, ionomer and water morphology, water and ionomer coverage, and overall changes in carbon and Pt surface area. Such multiscale simulations establish a relationship between the aging of catalyst layer and the selection of carbon particles, role of Pt and the level of ionomer structural changes during catalyst layer degradation process. 11.1 IntroductionThe performance and durability of Polymer Electrolyte Fuel Cells (PEFCs) are strongly dependent on the complex processes that occur in the membrane electrodes assembly (MEA). Degradation phenomena and preventing aging of MEA materials are among major hurdles limiting near-term commercialization of PEFCs. The durability of MEA for automotive applications should exceed the threshold of 3000 hours for early commercialization.1Among the ingredients in MEA, the catalyst layers (CL), in particular the cathode CL (CCL), are the most critical ones. A highly performing CL requires efficient conduction of electrons and protons, high catalytic activity, stability and utilization of Pt, and fast transport of reactants (oxygen and hydrogen) and water. Due to the low intrinsic rate of the oxygen reduction reaction (ORR), the CCL is the main source of voltage losses in an operating PEFC. Conventional CCLs are random heterogeneous media that consist of a solid phase comprising carbon particles or agglomerates decorated with catalyst nanoparticles (typically Pt-based) as electron conductors and catalyzing reactions, a proton-conducting network of Nafion® ionomer, and a particular water-filled porous network for gaseous transport.2,3 During preparation in an organic solvent, Pt/C particles and ionomer molecules self-organize into agglomerated structures with internal porosities that depend on the composition of the ink.4,5 During fuel cell operating condition, microstructure properties of cathode catalyst layer (CCL) and its ingredients (i.e., Pt and carbon particles, Nafion ionomer), the polymer electrolyte membrane (PEM), and gas diffusion layer undergo several changes. Although Pt nanoparticles are very stable in acidic electrolytes at
low electrode potentials, considerable dissolution of Pt in CCL takes place at high potentials between 0.7 and 1.1 V vs. normal hydrogen electrode (NHE), which is at typical PEFC cathode operating condition.6 Both steady-state and drive-cycle conditions produce extensive catalytic oxidation, dissolution and redistribution of the Pt atoms.7,8 Extensive nanoparticle coarsening and redistribution are also observed during PEFC operation, leading to the reduction of the specific catalytic surface area and concomitantly loss of the electrochemical activity.9 Furthermore, transmission electron microscopy (TEM) observations obtained after PEMFC operation show the formation of Pt single nanocrystals within the PEM.10
