Poisoning mechanism of ammonia on proton transport and ionomer structure in cathode catalyst layer of PEM fuel cells

TL;DR

This study uses molecular dynamics simulations to reveal how ammonia poisons proton transport in PEM fuel cell cathodes by forming ion clusters that block ionomer pathways, with temperature effects influencing cluster stability.

Contribution

It uncovers the detailed molecular mechanisms of ammonia poisoning in PEM fuel cells, including ion cluster formation and temperature effects, providing insights for improving catalyst layer resilience.

Findings

Ammonium replaces hydronium ions at sulfonic acid sites via van der Waals and electrostatic forces.

Ammonia derivatives form ion clusters that absorb hydronium ions, hindering proton transport.

Increasing temperature dissociates ion clusters, restoring proton transport efficiency.

Abstract

Ammonia has strong poisoning effects on cathode catalyst layers of proton exchange membrane (PEM) fuel cells, but the poisoning mechanism is still unclear. In this study, all-atom molecular dynamics simulations are employed to investigate the poisoning mechanisms of ammonia. The results show that ammonium can replace the hydronium ions at the charged sites of sulfonic acid group of the ionomer side chain, and the adsorption of ammonium to sulfonic acid group can be attributed to van der Waals force and electrostatic interaction. Furthermore, other ammonia derivatives, amino and imino ions, can capture hydronium ions to form ion clusters. These ion clusters have strong capability to absorb hydronium ions, and their structures change with ammonia content and temperature. The main mechanism of formation of these clusters is due to the formation of relatively stable hydrogen bonds between ions within the clusters. These mechanisms significantly reduce the efficiency of proton transport, thereby decreasing the catalyst layer's performance in electrochemical reactions. We also discover that the increase in temperature leads to the dissociation of large ion clusters, the blockage in the ionomer layer can be alleviated, and the proton transport efficiency can be restored. The understanding of the poisoning mechanisms obtained in this study is helpful for subsequent research aimed at resolving ammonia poisoning and enhancing the anti-poisoning performance of catalyst layers.