Highly Durable Electrocatalyst with Low-Loading Pt-Co Dispersed over Graphene Nanofiber for Oxygen Reduction Reaction Under PEMFC Condition

Abstract

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 polymer fiber containing cobalt zeolitic imidazolate framework as precursor. The new catalyst contains Pt-Co nanoparticels situated over the PGM-free catalystically active support (7,8). The ORR activities of the new catalyst was first tested in 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 PtCo NPs and the PGM-free active substrate led to an unprecedented oRR performance in both RRDE and fuel cell. For example, the fuel cell test demonstrated a mess activity of 2.43 A/mgPt for the new catalysts and > 80% retention of the initial mass activity after 60000 continuous voltage cycles from 0.6 – 1.0 V.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 Argonne National Laboratory through Maria Goeppert Mayer Fellowship, US; Overseas Outstanding Youth Fund project, shanghai Jiao Tong university, Shanghai, China, State Key Laboratory of Metal Matrix Composites, shanghai Jiao Tong university, Shanghai, China. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials and Advanced Photo Source, and Hydrogen research center, shanghai Jiao Tong university.Reference: 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. 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