Structure of the Ionomer Film in Catalyst Layers of Proton Exchange Membrane Fuel Cells

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• 作者：Qianping He ; Nethika S. Suraweera ; David C. Joy ; David J. Keffer
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：December 5, 2013
• 年：2013
• 卷：117
• 期：48
• 页码：25305-25316
• 全文大小：610K
• 年卷期：v.117,no.48(December 5, 2013)
• ISSN：1932-7455

文摘

The nanoscale structure of the ionomer film located in the catalyst layer of polymer exchange membrane fuel cells (PEMFCs) is of vital importance to proton transport and catalyst utilization. Classical molecular dynamic simulations are conducted to explore the molecular-level structure as well as the structure鈥損roperty relationships in the ionomer film. Twenty-four systems are simulated to investigate the effect of (i) hydration, (ii) ionomer film thickness, (iii) oxidation of the carbon support surface, and (iv) the presence of catalyst nanoparticles on film adhesion and morphology. The ionomer does not form a continuous film on the carbon surface; rather, the ionomer forms irregular patches through which proton transport from the catalyst to the membrane must occur. These ionomer films are not able to retain water to the same extent as bulk ionomer membranes. However, thicker films retain proportionally more water than thinner films, allowing for a larger and better connected aqueous domain required for proton transport. Oxidation of the carbon support surface through either epoxidation or hydroxylation strongly impacts the water distribution throughout the film and thus the film adhesion. Hydroxylation enhances adhesion of the film relative to a pristine surface. Epoxidation can result in partial delamination of the film, an effect that is more pronounced for thinner films. The presence of Pt or PtO nanoparticles impacts the distribution of water and the ionomer. An aqueous layer forms around the nanoparticles and provides pathways for protons into the film. These insights provide a molecular-level basis for the experimental observations such as the inhomogeneous distribution of the Nafion film on the carbon support, the existence of an optimal content of recast ionomer in the catalyst layer, and the impact of surface oxidation on the restructuring of polymer chains and thus on PEMFC performance. This work also implies that oxidation during operation can result in ionomer film delamination, which reduces the binding energy of the catalysts, a possible precursor to catalyst detachment.