Site-density engineering of single-atomic iron catalysts for high-performance proton exchange membrane fuel cells

The design and development of highly efficient non-precious metal single-atomic ORR catalysts for proton exchange membrane fuel cells (PEMFCs) are highly desirable but challenging. Herein, we report a novel polydopamine (PDA)-metal complex-assisted pyrolysis strategy for producing zeolitic imidazolate framework-derived catalysts with a hierarchically porous carbon support and highly exposed dense-FeN4 sites (Z8@DA-FIP-950-C). The resultant Z8@DA-FIP-950-C catalyst shows remarkably enhanced performance for oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.828 V in 0.1 M HClO4 solution, which is close to commercial 20 wt% Pt/C catalyst. Impressively, the Z8@DA-FIP-950-C exhibits peak power densities of 982 and 454 mW cm-2 in H2/ O2 and H2/air PEMFCs, respectively, which are superior to most of non-precious metal catalysts reported to date. In addition, we construct the quantitative relationship between the active site activity and ORR performance, and prove the dominating role of the FeN4 site density to the observed excellent PEMFC performance. This work demonstrates a facile strategy to prepare the 3D hierarchically porous carbons with a maximized exposure of high-dense FeN4 sites (without acid treatment), providing a useful guidance for the design and development of novel highly-efficient single-atom catalysts for the renewable energy applications.