Triple-Phase Pore Network for Oxygen and Proton Transport Model in the Cathode Catalyst Layer of Proton Exchange Membrane Fuel Cells

Abstract

Improvement in the mass transport overpotentials of the oxygen reduction reaction provides a pathway for reducing the high cost associated to platinum group metal loading as well as enhancing the lifetime of electrodes in proton exchange membrane fuel cells (PEMFCs). State-of-the-art PEMFCs typically have Pt loadings of the order of 0.3 mgPtcm-2, which is the main part of the estimated stack cost of $55/kW. Understanding transport phenomena at triple phase boundaries that consist of a (solid) electrocatalyst, ionomer, and (gas-phase) oxygen, where the electrochemical reactions occur has gained importance to improve electrocatalyst accessibility. Mathematical models that address transport of electrons, protons, and oxygen in the cathode is critical for an effective catalyst layer design. Modeling of this multiphysics phenomena is further complicated due to multiscale effects. Pore network model (PNM) is a promising technique to tackle these challenges because of its capability of resolving porous networks locally in greater detail. Moreover, improvements in microscopic imaging techniques (e.g., SEM, TEM, and nano CT) enable the generation of realistic pore networks of the catalyst layers that can be directly integrated into PNM. Most modeling studies to date focus on single networks for oxygen transport in the open pore space, or transport of oxygen and water. A pore network that describes oxygen, proton, and electron transport simultaneously remains uncharted. Here, we construct a triple-phase pore network model using nano CT images of heterogeneous porous catalyst layers. Binary images of solid-open pore space and ionomer-catalyst support are generated, which are then overlapped to create ternary images. Pores and throats are then fit to theseimages to create pore-networks that describe open-pores for oxygen and water transport, ionomer pores for proton transport, and a solid network for electron transport. A schematic of the approach is shown in Figure 1.The study offers a new method that constructs a triple pore network for a porous material in which two levels of open space porosity (micro and macro) exist in connection with solid and ionomer networks. Therefore, arealistic pore network is obtained to be further modeled through the approach presented. Figure 1