Modeling of Ultrathin Catalyst Layers in Polymer Electrolyte Fuel Cells: Proton Transport and Water Management

Ultrathin catalyst layers (UTCLs) are emerging as a promising alternative to conventional catalyst layers in polymer electrolyte fuel cells. In comparison, UTCLs have dramatically reduced Pt loading and thicknesses and are ionomer–free. We explore two open questions in the theory of UTCLs (1) the proton transport mechanism within the ionomer–free layer and (2) water management in membrane electrode assemblies (MEAs) with UTCLs. To investigate (1), we present a UTCL model, which assumes the protons are drawn into the UTCL via their interaction with the metal surface charge. We consider a continuum model of a water–filled, cylindrical nanopore with charged walls. We derive the relation between metal potential and surface charge density from a Stern double layer model. The model suggests the proton concentration and reaction current density to be highly dependent on the charging properties of the metaljsolution interface, which are parameterized primarily by the potential of zero charge. Therefore, materials for UTCLs should be selected not only for their intrinsic mass activities and durability, but also for their charging properties. A systematic evaluation of the interplay of electrostatic, kinetic, and mass transport phenomena in UTCL demanded an impedance variant of the model. Based on the general set of transient equations, we have derived analytical impedance expressions and equivalent circuit representations in 4 limiting cases. While the UTCL model suggests the charging of the metaljsolution interface to be crucial to performance, theoretical studies on the charging behaviour of platinum are limited. We present a generalised computational hydrogen electrode that enables the ab initio simulation of metaljsolution interfaces as a function of pH and potential. To address (2), we present a water balance model to MEAs with UTCLs. The model relates the current densities, capillary pressure distributions, and fluxes of vapor and liquid water. Analysis of the model suggests that UTCLs require efficient liquid transport paths out of the MEA at low and moderate temperature. We discuss strategies for increasing the current density for the onset of GDL flooding, via enhanced liquid permeabilities, vaporization areas, and gas pressure differentials.