A Pore-Scale Model of Oxygen Reduction in Ionomer-Free Catalyst Layers of PEFCs

We present a model for oxygen reduction in water-filled, cylindrical nanopores with platinum walls. At one end, the pores are in contact with a polymer electrolyte membrane. The electrostatic interaction of the protons with the charged pore walls drives proton migration into the ionomer-free channels. We employ the Stern model to relate the surface charge density at the pore walls to the electrode potential. Proton and potential distributions within the pores are governed by the Poisson–Nernst–Planck theory and the oxygen distribution by Fick's law. Assuming a small local current density from oxygen reduction, we found an approximate analytical solution to the transport equations. The metal surface charge density and the corresponding proton conductivity of the pores are tuned by the deviation of the electrode potential from the potential of zero charge of the metal phase, which is the key determinant of the effectiveness of platinum utilization. Other determinants of pore performance are the Helmholtz capacitance, electrokinetic parameters, and pore size and length. Upon upscaling, the model is consistent with polarization data for ionomer-free, ultrathin catalyst layers in polymer electrolyte fuel cells (PEFCs). We discuss the implications of the model for the materials selection and nanostructural design of such catalyst layers.