Abstract
Developing efficient and inexpensive catalysts for the sluggish oxygen reduction reaction (ORR) constitutes one of the grand challenges in the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Despite recent achievements in designing advanced Pt-based and Pt-free catalysts, current progress primarily involves an empirical approach of trial-and-error combination of precursors and synthesis conditions, which limits further progress. Rational design of catalyst materials requires proper understanding of the mechanistic origin of the ORR and the underlying surface properties under operating conditions that govern catalytic activity. Herein, several different groups of iron-based catalysts synthesized via different methods and/or precursors were systematically studied by combining multiple spectroscopic techniques under ex situ and in situ conditions in an effort to obtain a comprehensive understanding of the synthesis-products correlations, nature of active sites, and the reaction mechanisms. These catalysts include original macrocycles, macrocycle-pyrolyzed catalysts, and Fe−N–C catalysts synthesized from individual Fe, N, and C precursors including polymer-based catalysts, metal organic framework (MOF)-based catalysts, and sacrificial support method (SSM)-based catalysts. The latter group of catalysts is most promising as not only they exhibit exceptional ORR activity and/or durability, but also the final products are controllable. We show that the high activity observed for most pyrolyzed Fe-based catalysts can mainly be attributed to a single active site: non-planar Fe–N4 moiety embedded in distorted carbon matrix characterized by a high potential for the Fe2+/3+ redox transition in acidic electrolyte/environment. The high intrinsic ORR activity, or turnover frequency (TOF), of this site is shown to be accounted for by redox catalysis mechanism that highlights the dominant role of the site-blocking effect. Moreover, a highly active MOF-based catalyst without Fe–N moieties was developed, and the active sites were identified as nitrogen-doped carbon fibers with embedded iron particles that are not directly involved in the oxygen reduction pathway. The high ORR activity and durability of catalysts involving this second site, as demonstrated in fuel cell, are attributed to the high density of active sites and the elimination or reduction of Fenton-type processes. The latter are initiated by hydrogen peroxide but are known to be accelerated by iron ions exposed to the surface, resulting in the formation of damaging free-radicals.
Original language | English |
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Pages (from-to) | 65-82 |
Number of pages | 18 |
Journal | Nano Energy |
Volume | 29 |
DOIs | |
State | Published - Nov 1 2016 |
Externally published | Yes |
Funding
The PANI-derived catalysts were prepared and provided by Gang Wu (Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY) and Piotr Zelenay (Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545). We appreciate financial assistance from the U.S. Department of Energy , EERE ( DE-EE-0000459 ). Use of the National Synchrotron Light Source, Brookhaven National Laboratory (BNL), was supported by the U.S. Department of Energy, Office of Basic Energy Sciences . Synchrotron spectroscopy in this publication was made possible by the Center for Synchrotron Biosciences grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NIBIB). Support from beamline personnel Dr. Erik Farquhar and Mark Chance (X3B) are gratefully acknowledged. 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. Department of Energy 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 .
Keywords
- Active site
- In situ XAS, redox catalysis
- Non-platinum group catalyst
- Oxygen reduction