On the Mechanism of Synergistic ORR in Pt@PGM-Free Catalyst

Abstract

Maximizing the platinum utilization represents a critical research area in the commercial PEM fuel cell development. This is particularly true at the fuel cell cathode where to promote sluggish oxygen reduction reaction (ORR) requires three to four times more Pt in the catalyst than its counterpart in anode. In the electrode, Pt should be highly dispersed to be fully accessible by the reactants. This becomes more important at the high fuel cell current density domain where a large influx of O2 must be converted. By simply dispersing platinum to smaller nanoparticles (NPs) has its limitation. Past studies indicate that the ideal NPs should have dimension in between 4 nm to 6 nm to balance the needs between high Pt exposure and low dissolution rate. At ultralow cathode loading of 0.03 mgPt/cm2 to 0.06 mgPt/cm2 in the PEM fuel cell, such size requirement adds a significant constraint to the available number of NPs for the effective oxygen conversion. At Argonne National Laboratory, we recently developed a new type of ORR catalyst by integrating ultralow loading of Pt (~3 wt.%) over PGM-free catalytically active support derived from the zeolitic imidazolate frameworks. [1] The new catalyst not only demonstrated excellent platinum mass activity and polarization current density in PEM fuel cell test at the cathodic Pt loading < 0.04 mg/cm2, but also showed very good durability during multiple fuel cell voltage cycling in the accelerated stress test (AST). Various characterization tools including XPS, XRD, XAS, TEM have identified the new catalyst containing highly strained Pt-Co core-shell NPs over hig-density and uniformly distributed Co/N/C and Co@graphene PGM-free active support. DFT calculation found two ORR catalytic routes over Pt and PGM-free active site with a crossover path at the formation of hydrogen peroxide, leading to synergistic catalysis. In this presentation, I will discuss the synergistic catalysis from the catalyst design perspective, supported by our recent experimental observation. I will also share our thoughts on the new research direction of this approach. Acknowledgement: This work is supported by U. S. Department of Energy, Fuel Cell Technologies Office through Office of Energy Efficiency and Renewable Energy. The works performed at Argonne National Laboratory’s Center for Nanoscale Materials, an U.S. Department of Energy Office of Science User Facility, is supported by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357. [1] L. Chong, J. Wen, J. Kubal, F. G. Sen, J. Zou, J. Greeley, M. Chan, H. Barkholtz, W. Ding, and D.-J. Liu, “Ultralow-loading Platinum-Cobalt Fuel Cell Catalysts Derived from Imidazolate Frameworks,” Science 362, 1276 (2018).