New insight into the effect of Co2+ contamination on local oxygen transport in PEMFCs

Local oxygen transport resistance is increased due to the blocked and intermittent local oxygen pathway caused by Co 2+ contamination in ionomer and a significant performance loss in high current density is observed. • Co 2+ contamination compacts SO 3 - in ionomer and deteriorates the ionomer aggregation. • Ionomer modulus increases and water content reduces as Co 2+ increases. • Co 2+ contamination in ionomer leads to blocked and intermittent local oxygen pathway. • Local oxygen transport resistance increases as degree of Co 2+ contamination increases. Pt alloy electrocatalysts (e.g., Pt 3 Co) for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have attracted a great deal of interest from worldwide researchers in past decades, due to the excellent ORR activity and cost-effective properties. However, the long period operation would yield sever dissolution of the non-precious metal which effects the ionomer microstructure, thus leading to significantly decreased ORR activity of catalyst and aggravated of the oxygen transport resistance in cathode catalyst layers (CCLs), especially the local oxygen transport resistance caused by the ultrathin ionomer film covering on catalyst surface. Herein, in this paper, the effects of Co 2+ contamination on structure of ionomer films and the corresponding local oxygen transport behavior in CCLs are explored via physicochemical characterizations and molecular dynamics (MD) simulations, limiting current method, as well as fuel cell performance measurement. It is found that Co 2+ contamination reduces water content and increases the modulus of the ultrathin ionomer film, while reducing the distance between the sulfonic acid groups and increasing the aggregation size of the ionomer, which is related to the volume of hydrophilic domains. Additionally, the electrochemical measurement results demonstrate that as the degree of Co 2+ contamination increases from 0 to 85%, the local oxygen transport resistance increases from 27.74 s cm -1 to 42.43 s cm -1 , and the peak power density of fuel cell reduces from 510 mW cm −2 to 453 mW cm −2 . The reason behind lies on that contaminated Co 2+ tends to connect with sulfonate, then suppress the ionomer mobility and dislocate the local oxygen transport channels.