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