Comprehensive understanding of oxygen transport at Gas/ionomer/electrocatalyst triple phase boundary in PEMFCs

Reducing Pt loading of proton exchange membrane fuel cells (PEMFCs) without sacrificing performance remains challenging due to the severe oxygen transport resistance in cathode, especially the local oxygen transport resistance (RLocal) at the gas/ionomer/electrocatalyst triple phase boundary (TPB). Here, the detailed oxygen adsorption mechanism at surface ionomer is quantified for the first time via a self-built characterization system based on quartz crystal microbalance (QCM). The results show that the surface oxygen adsorption amount quasi-logarithmically increases as oxygen partial pressure increases. The ionomer surface is further investigated by molecular dynamics simulations and it is found that there exists not only non-uniformity in morphology, but also separation of hydrophilic and hydrophobic phase, which leads an inhomogeneity of interaction with oxygen. As a consequence, the extended form of Toth model is proved to be more appropriate for the surface oxygen adsorption on ionomer film due to higher correlation coefficients R2, better applicability at low pressure and varying temperature. Ultimately, we successfully predict the oxygen adsorption amount and the corresponding trend of RLocal at practical fuel cell operating condition with the adsorption model disclosed in this work, which is potential to estimate the oxygen adsorption in other type of electrochemical devices.