Does an Ionomer Penetrate a Carbon Mesopore? Free-Energy Analysis Using Molecular Dynamics Simulations

Abstract

Mesoporous carbon support enhances the catalytic activity and gas diffusivity of the cathode catalyst layers of polymer-electrolyte fuel cells. The key role of the mesopores is believed to be their suppression of catalyst poisoning by the ionomers and reduction of the oxygen diffusivity near the catalysts. However, because of the difficulty to observe the ionomer distributions directly, it is unclear whether the ionomer penetrates the mesopores. Here, using molecular dynamics simulations of ionomer penetration into a seamless carbon mesopore, we show that the penetrability depends upon the pore size and the type of solvent. Free-energy profiles obtained from the simulations indicate that an ionomer in a water solvent easily invades a mesopore with a diameter >2 nm, while in a mixed water−ethanol solvent, it cannot do so. Structural differences between the two solvents suggest that three factors affect penetrability. Our results suggest that the ionomer distribution can be controlled by optimizing the solvent composition in the catalyst ink.

Comprehensive Molecular Dynamics Study of Oxygen Diffusion in Carbon Mesopores: Insights for Designing Fuel-Cell Catalyst Supports

“…The five-walled CNTs were generated using d and h obtained from n a and n b by the cylindrical projection of a graphene sheet of length πd and height h according to a previous study. 47 Finally, a periodic boundary of length h was applied in the direction of the tube axis, and the desired periodic infinite CNT system was obtained. The thickness of the five-layered graphite is approximately 1.7 nm.…”

“…Then, for input pore diameter d 0 and cylinder length h 0 , we can estimate n a and n b as n a = ceil true( π ( normald 0 + 2 normald CC inter n layer ) 3 d CC intra true) ⁣ and goodbreak0em1em⁣ n b = ceil ( h 0 3 normald cc intra ) where ceil( x ) is a rounded-up value of x ; d CC inter = 0.335 nm is the interlayer distance between CNT walls; n layer = 0,1,2,3,4 are the indexes of the CNT layers. The five-walled CNTs were generated using d and h obtained from n a and n b by the cylindrical projection of a graphene sheet of length π d and height h according to a previous study . Finally, a periodic boundary of length h was applied in the direction of the tube axis, and the desired periodic infinite CNT system was obtained.…”

“…To the best of our knowledge, the effect of ionomers on pore diffusion has not been investigated, though a recent study revealed that ionomers can penetrate pores with diameters of a few nanometers and that the penetration length can be adjusted by controlling the ink solvent. 47 Therefore, in this study, oxygen diffusion in the ionomer-filled pores was analyzed by performing MD simulations to understand the diffusion characteristics in the pores based on molecular theory and to obtain knowledge for the design of mesoporous carbon supports for fuel-cell catalyst layers.…”

“…The diffusion characteristics in such pores are known to be different from the characteristics of bulk diffusion and Knudsen diffusion because of the restriction of molecular mobility. − For example, gas diffusion in carbon nanotubes (CNTs) with diameters on the order of a few nanometers occurs faster than Knudsen diffusion; , conversely, gas diffusion through water-filled pores of diameter 1 nm is slower than Knudsen diffusion. , As shown in these examples, molecular-scale diffusion analysis is necessary to understand diffusion in nanometer-scale pores. To the best of our knowledge, the effect of ionomers on pore diffusion has not been investigated, though a recent study revealed that ionomers can penetrate pores with diameters of a few nanometers and that the penetration length can be adjusted by controlling the ink solvent . Therefore, in this study, oxygen diffusion in the ionomer-filled pores was analyzed by performing MD simulations to understand the diffusion characteristics in the pores based on molecular theory and to obtain knowledge for the design of mesoporous carbon supports for fuel-cell catalyst layers.…”

Translating insights from experimental analyses with single-crystal electrodes to practically-applicable material development strategies for controlling the Pt/ionomer interface in polymer electrolyte fuel cells

“…Transmission electron microscopy observations indicated that a large portion of the Pt nanoparticles are located in the pores of the porous carbon particles [53]. Electrochemical measurements [37] and MD simulations [54] indicated that these Pt particles are not covered by ionomers because the ionomer molecules do not easily penetrate the pores. In addition, protons can access the Pt nanoparticles in the pore through liquid water without an ionomer [55].…”