High Performance Fuel Cell Catalysts Synthesized By Fe Metalation of Nitrogen Doped Carbons Derived from Metal Organic Framework ZIF-8

Abstract

Hydrogen fuel cells are potentially a sustainable alternative to internal combustion engines. However, the critical raw materials, platinum group metals (PGMs), necessary to achieve adequate power density in automotive fuel stacks are cost prohibitive. Therefore, it is imperative to replace PGMs with low cost and earth abundant metals such as Fe, Co and Mn. This represents a considerable challenge in the design of PGM-free catalysts for the oxygen reduction reaction (ORR) in acidic electrolyte since current state-of the-art PGM-free catalysts fall short of matching the high activity of PGM catalysts in this environment1. Addressing the challenge of PGM-free ORR catalysis, the most promising class of materials is metal-nitrogen-carbon (MeNC) catalysts. These materials comprise a conductive and highly porous carbon doped with nitrogen moieties which can be regarded as solid-state ligands coordinating the metal centers. The two rational means of improving catalyst activity is through increasing the catalyst site density (SD) and/or turnover frequency (TOF). However, constraints exist which complicate efforts to increase the SD of MeNC catalysts, in particular the Fe-catalyzed graphitization of the carbon support leading to i) low surface area and ii) loss of pyridinic and other metal-coordinating nitrogen moieties at elevated temperature. A threshold Fe content therefore exists (depending on the synthesis approach and pyrolysis conditions) above which Fe nanoparticles nucleate during pyrolysis, diminishing the number of metal-based active sites in the final MeNC material either directly (a fraction of Fe atoms being directed to Fe nanoparticles), or indirectly, as mentioned above.2 One approach to increasing the catalyst SD is by the metalation of commercially available carbons which typically involves mixing of the carbon with Fe and N precursors such as iron acetate and 1,10-phenanthroline, followed by pyrolysis in inert atmosphere.3, 4 Transmetalation has also been shown as an effective method for replacing MgNC or ZnNC sites formed in during a first synthesis step, with FeNC sites using a solvothermal technique2. The advantage of these methods is the decoupling of the carbon support synthesis and Fe metalation, allowing for the aforementioned constraints on SD to be fully or partially overcome. We will report on a facile solid-state method for Fe metalation of nitrogen-doped carbon (NC) synthesized from a zinc-containing metal-organic-framework (ZIF-8). Decoupling the synthesis of NC from the FeNC catalyst mitigates the Fe-catalyzed graphitization leading to increased carbon surface area, pore volume, and surface composition of pyridinic nitrogen. Fe metalization, involving solid-state mixing of anhydrous FeCl2 with the NC followed by a low temperature pyrolysis, results in a catalyst consisting exclusively of coordinated FeNC active sites without altering the morphology or nitrogen speciation of the NC. These improvements of the catalyst structure and composition enable the catalysts to achieve half-wave potentials (E1/2) greater than 0.8 V vs. RHE and near 4 e- selectivity with H2O2 yields as low as 1 %. A broad set of characterization techniques, including: Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and newly developed methods to probe the SD and TOF of FeNC catalysts, nitrite stripping and CO chemisorption, were used allowing for an in-depth understanding of the catalytic materials to guide further catalyst optimization leading to better performance of PGM-free FeNC catalysts for ORR. 5, 6. Acknowledgement: The research leading to these results has received funding from FCH2 JU under grant agreement 779366, CRESCENDO.