Enhancing Activity and Stability of PGM-Free Oxygen Reduction Reaction Electrocatalysts Synthesized and Characterized Using High-Throughput Methodology

The polymer electrolyte fuel cell (PEMFC) is a promising and clean technology that can provide energy sufficient to power both light-duty and heavy-duty vehicles with full-size cars with long driving ranges. However, the widespread cost-competitive implementation of this technology is hindered by sluggish kinetics of the oxygen reduction reaction (ORR) and the need for costly platinum-based materials to catalyze the ORR. An extensive effort has been underway for more than 50 years to replace the platinum ORR catalyst with a platinum group metal-free (PGM-free) catalyst using earth-abundant materials. Although progress has been made over the past decade in increasing both the ORR activity and durability of PGM-free catalysts ,^1-3 further improvements in these materials, especially in hydrogen-air performance and long-term performance durability are needed for them to be viable for vehicle power. The highest ORR activities for PGM-free materials have been obtained from catalysts derived from iron salts and metal-organic frameworks or carbon-nitrogen-containing polymers,^3-6 though the ORR turnover frequencies and volumetric active site densities of these materials are still approximately one order of magnitude lower than that those of state-of-the-art Pt alloy nanoparticle catalyst.¹ For the general class of pyrolyzed iron-nitrogen-carbon PGM-free materials, it has been determined that variables such as the iron precursor, carbon and nitrogen sources, their relative concentrations, as well as the temperature and atmosphere of pyrolysis are important in determining the activity and stability of the resulting catalysts.^4,7 Changing the synthesis variable and testing their effect on the resulting catalyst properties is a very time-consuming process and only a limited portion of the composition and temperature space have been explored for this broad class of materials. In 2011, Dodelet et al.⁴ developed a new synthetic method for Fe-N-C catalysts using physical mixing, through ball milling, of a zeolitic imidazolate framework (ZIF-8), 1,10-phenanthroline, and iron precursors. High-energy ball milling followed by controlled thermal annealing is an easy and scalable synthetic method in which type of precursors and concentrations can be easily varied.⁸

An automation platform, a multi-port ball-mill, and parallel fixed bed reactors in Argonne's High-throughput Research Laboratory have been utilized to rapidly synthesize unique PGM-free catalysts. A large number of variables have been explored, such as synthesis method (e.g., ball-milling, chemical vapor deposition), nitrogen content, ZIF-8 content, PGM-free metal content, iron precursor, and capping agent to stabilize active sites. These variables have been shown to be important factors in determining the activity and stability of the resulting catalysts by RDE. A multi-channel flow double electrode (m-CFDE) cell was designed and constructed for the simultaneous screening the ORR activity of multiple materials using an aqueous hydrodynamic technique. In addition, a 25-electrode array fuel cell served to test and optimize the electrode composition. The ORR activity is correlated with catalyst Fe speciation, as determined using Fe K-edge X-ray absorption spectroscopy (XAFS) and also with catalyst BET and electrochemically-determined surface areas. This large data set obtained was fed into machine learning to generate new sets of synthesis parameters and conditions to further improve catalyst activity.

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This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat). This research used resources of the Advanced Photon Source, a DOE Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC-02-06CH11357. Argonne is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under the same contract.