Selective Yield of the Distorted Fe-N4-Cx Active Site with Immunity for Anion Poisoning for Oxygen Depolarized Cathode Applications

Abstract

Engendering anion immunity has tremendous implications in electrocatalysis in aqueous acidic electrolytes. This is specially exemplified by oxygen reduction reaction and its connotations with application for energy conversion (phosphoric acid), oxygen depolarized cathodes (ODC) (HCl recovery) and metal air batteries. For ODC cathodes used for chlorine generation, it is noteworthy that an energy saving of ~700 kWh per ton of chlorine gas (up to 30%) can be realized. Need for immunity for anion poisoning is particularly manifest as a result of dependence on costly and rare platinum group metal (PGM) catalysts considering their vulnerability to impurities such as halides. In this abstract we will demonstrate a new class of pyrolyzed M-Nx-C (M=Fe and/or Co) catalysts, which are immune to anion poisoning and are the leading non-platinum group metal (non-PGM) catalysts for ORR owing to their high activity and stability. Unique aspect of these emanates from the use of metal organic framework (MOF) precursors such as Zn based zeolitic imidizolate framework (ZIF, a subclass of metal organic framework) as a sacrificial template1. Herein, we report a facile, low cost and scalable synthetic process of ZIF-8 using a reactive ball-milling approach. A highly active Fe-based ORR catalyst using the as-synthesized ZIF-8 as precursor is demonstrated in hydrochloric acid electrolysers. The ZIF-8 derived catalyst shows a prominent ORR activity with an on-set potential of 0.92 V and a half wave potential of 0.78 V in 0.1 M HClO4 electrolyte in the rotating disk electrode (RDE). The high ORR activity can be partly attributed to the ultra-high surface area as it is closely related to the high density of active sites and the intrinsically improved mass transport properties of the MOF. In addition, combined x-ray adsorption spectroscopy (XAS) and Mössbauer spectroscopy reveal that this catalyst contains high concentration of the distorted Fe-N4-Cx moiety featured with a large Fe out-of-plane displacement and elongated Fe-N bond distance, which is shown to exhibit high intrinsic activity toward ORR owing to the moderate Fe-O binding energy and high Fe2+/3+ redox transition potential.2 Remarkably, the highly active ORR catalyst sustains its activity even in presence of high concentrations of Cl- (Fig. 1), thereby opening up promising avenues for the application of Fe-Nx-C catalysts such as ODCs in hydrochloric acid recovery electrolyzers (Fig. 2). Acknowledgement The authors gratefully acknowledge financial support from the Massachusetts Clean Energy Council via their catalyst award (2013). The authors also gratefully acknowledge the financial support from Denora North America (2014). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. Use of Beamline 2-2 at SSRL was partially supported by the National Synchrotron Light Source II, Brookhaven National Laboratory, under U.S. Department of Energy Contract No. DE-SC0012704. Use of the beamline 9-BM in Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.