High Active Ultra-Low Pt Content Oxygen Reduction Reaction Catalyst Derived from Metal Organic Framework for PEMFC

Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy conversion technology. The sluggish oxygen reduction reaction (ORR) at the fuel cell cathode requires 3 to 5 times more catalysts than the hydrogen oxidation reaction at the anode.(1) Currently, platinum is still the catalyst material of choice. However, the heavy use of scarce Pt in the electrodes represents a major cost barrier to the commercialization of PEMFCs. Tremendous efforts have been dedicated to reduce or remove Pt usage through the development of Pt-transition metal catalysts, as highlighted recently by dealloyed PtNi (2), Mo-doped PtNi(3), and ordered PtCo(4); or through the development of transition metals-nitrogen-carbon based PGM-free catalysts (5, 6). In the fuel cell, catalysts should be highly dispersed over the electrode surface to be easily accessible by the reactants, especially at the high fuel cell current density where a large influx of O2 must be converted. For the Pt alloy catalyst of large crystallites, there won't be enough crystallites available to spread over the electrode surface to encounter the incoming O2 if the total loading of platinum has to be maintained ultralow. Furthermore, unprotected Pt nano-alloys may lose their nanostructure and crystallinity from the dissolution and agglomeration during the ORR process. The PGM-free catalysts, on the other hand, have high catalytic site density with uniform distribution when prepared by homogenous precursors such as metal-organic framework or porous organic polymer, rendering them highly efficient in interacting with O2 flux. Their key drawback, however, is the poor stability operated under PEMFC condition. In this presentation, we will describe a method of preparing highly active yet stable electrocatalysts containing ultralow Pt content using Co or Co/Zn zeolitic imidazolate frameworks as precursors. The new catalysts contain Pt-Co core-shell nanoparticles (NPs) situated over the PGM-free catalytically active support (7). The ORR activities of the new catalysts were first tested in the O2 saturated acidic electrolyte by rotating ring disk electrode (RRDE) before fabricated into the membrane electrode assembly (MEA) and evaluated under fuel cell operating condition. The synergistic catalysis between Pt-Co NPs and PGM-free active substrate led to an unprecedented ORR performance in both RRDE and fuel cell. For example, the fuel cell test demonstrated a mass activity of 1.77 A/mgPt for the new catalysts and > 60% retention of the initial mass activity after 30,000 continuous voltage cycles from 0.6V – 1.0V. The characterizations of the fresh and the post-electrochemical test samples were investigated by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), x-ray absorption near-edge structure (XANES), extended x-ray absorption fine structure (EXAFS), transmission electron microscopy (TEM), Raman spectroscopy and BET surface analysis. We also performed DFT calculation on the ORR thermodynamic barriers over both Pt-Co NPs and PGM-free site as the descriptors for reaction pathway. The results reveal that the synergistic interaction between Pt-Co and PGM active substrate contribute significantly to both activity and durability enhancements. Acknowledgments: This work was supported by U.S. Department of Energy, Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy. LC wishes to thank Argonne National Laboratory for Maria Goeppert Mayer Fellowship. The works performed at Argonne National Laboratory's Center for Nanoscale Materials and Advanced Photo Source, U.S. Department of Energy Office of Science User Facilities, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357. H. A. Gasteiger, N. M. Markovic, Just a Dream-or Future Reality? Science 324, 48-49 (2009).B. Han et al., Record activity and stability of dealloyed bimetallic catalysts for proton exchange membrane fuel cells. Energy & Environmental Science 8, 258-266 (2015).X. Huang et al., High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction. Science 348, 1230-1234 (2015 ).D. Wang et al., Structurally ordered intermetallic platinum-cobalt core-shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts. Nat Mater 12, 81-87 (2013).L. Chong et al., Investigation of Oxygen Reduction Activity of Catalysts Derived from Co and Co/Zn Methyl-Imidazolate Frameworks in Proton Exchange Membrane Fuel Cells. ChemElectroChem 3, 1541-1545 (2016).J. Shui, C. Chen, L. Grabstanowicz, D. Zhao, D. J. Liu, Highly efficient nonprecious metal catalyst prepared with metal-organic framework in a continuous carbon nanofibrous network. Proc Natl Acad Sci U S A 112, 10629-10634 (2015).L. Chong et al., Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks. Science, (2018).