The Interplay of Oxygen Reduction Reaction and Iron Dissolution from Fe-N-C Electrocatalysts

Abstract

Fe-N-C catalysts are regularly proposed as promising earth-abundant and cheap catalysts replacing platinum group metal catalysts for fuel cells (FCs). Besides the activity, especially the stability of those materials remains challenging. It was found that the electrochemical activity and durability of Fe-N-C catalysts are superior in alkaline compared to acidic media, [1-3] yet most of their degradation studies are done in acidic media. [4-9] Moreover, although these are systematic works, discrepancies in these results from aqueous model systems (AMS) [5,6] and FC testing [7-9] remain puzzling. For example, the origin of the dissolved Fe species was found to be from poorly active sites in AMS [5] yet from highly active sites in operating FCs. [7,8] Additionally, the harmful effects of reactive oxygen species (ROS) on Fe-N-C catalysts are proven in AMS [6] but not directly correlated to the durability in FCs. [9] To bridge this gap, a gas diffusion electrode (GDE) half-cell coupled with inductively coupled plasma mass spectrometry (ICP-MS) has been developed to study on-line dissolution in realistic catalyst layers. [10] In this work, using a GDE-ICP-MS, we investigate the impacts of oxygen reduction reaction (ORR) on Fe leaching from realistic Fe-N-C alkaline catalyst layers. [11] For the first time, Fe dissolution is measured online at current densities above -100 mA·cm-2. The novel results show that compared to the model Ar-saturated environment, the Fe dissolution is dramatically higher during ORR. Furthermore, between 0.6 and 1.0 VRHE, we unveil an interesting correlation between Fe dissolution and charge transfer events. This subsequently leads to our hypothesis that the instability of the coordinated Fe during Fe3+/Fe2+ redox transitions is responsible for Fe leaching from Fe-N-C catalysts in alkaline media in this potential region. The novel insights into Fe-N-C catalyst degradation in realistic conditions can lead to rational design of this promising platinum group metal free catalyst for efficient, durable, and affordable FCs.