(513fi) Modeling Transport through Porous Carbon Catalysts in Fuel Cells

Low Pt-loaded catalyst layers (CLs) in polymer-electrolyte fuel cells (PEFCs) suffer significant loses at high-current densities, primarily due to (i) reactant gas transport resistance through ionomer thin films, and (ii) interfacial resistance at ionomer-Pt interface arising from ionomer-Pt interactions. Enhancing local transport of reactants (protons and reactant gas) to the Pt reaction site is required to mitigate these losses and reduce fuel-cell cost. Carbon supports form the primary porous structure of CLs and can be broadly classified into porous and non-porous types. Non-porous carbon supports with Pt particles on external surface result in strong ionomer-Pt interactions causing catalyst poisoning. Porous carbons on the contrary, have Pt particles majorly deposited in interior carbon pores where ionomer cannot penetrate, thus minimizing catalyst poisoning. However, the reactant species must diffuse further into the carbon particles to react, increasing transport resistance.

We study water uptake and transport of reactant species in the ionomer-free interior pores of porous carbon. Probability distribution functions are used to construct pore-size distributions and fit with experimental data. Water uptake is modeled using an adsorption Lennard-Jones interaction potential and capillary condensation, obtaining good agreement with literature reports. Pores capable of proton conduction can either be (i) flooded with condensed water, or (ii) wetted with adsorbed water (monolayer or more) on pore walls. Proton transport in flooded pores obeys Poisson-Boltzmann and Nernst-Planck equations while in pores with adsorbed water layers proton transport occurs via surface diffusion. Transport of reactant gas is modeled using Fick’s law.

Flooded pores are significantly limited by gas transport at high-current densities, whereas wetted-pore performance is highly sensitive to proton transport. From comparison to measurement of active electrochemical surface areas (ECSA), we demonstrate that a significant fraction of Pt particles contributing to the electrochemically active surface area exhibit proton conduction through adsorbed water layers (monolayer or more).