ORNL, LANL study provides insights into performance of non-precious metal fuel-cell catalysts; atomic-level observations
In order to reduce the cost of next-generation polymer electrolyte fuel cells for vehicles, researchers have been developing alternatives to the prohibitively expensive platinum and platinum-group metal (PGM) catalysts currently used in fuel cell electrodes. New work at Los Alamos (LANL) and Oak Ridge national laboratories (ORNL) is now resolving difficult fuel-cell performance questions, both in determining efficient new materials and understanding how they work at an atomic level. The research is described this week in the journal Science.
Building on previous studies, the Los Alamos-led team has synthesized catalysts comprising low-cost platinum alternatives—iron-nitrogen-carbon catalysts synthesized with two nitrogen precursors that developed hierarchical porosity—that yield performance comparable to the standard PGM fuel cell catalyst used in vehicle applications. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared.
Using sophisticated microscopy at Oak Ridge National Laboratory (ORNL), researchers were able to directly observe the single-atom active sites in the novel material where catalysis takes place, which provided unique insights into the PGM-free material’s efficiency potential.
Nitrogen-carbon catalysts have emerged as a promising alternative to costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen–air fuel cell.
What makes this exploration especially important is that it enhances our understanding of exactly why these alternative catalysts are active. We’ve been advancing the field, but without understanding the sources of activity; without the structural and functional insights, further progress was going to be very difficult.—Piotr Zelenay, leader of the project at Los Alamos National Laboratory
Platinum aids in both the electrocatalytic oxidation of hydrogen fuel at the anode and electrocatalytic reduction of oxygen from air at the cathode, producing usable electricity. Finding a viable, low-cost PGM-free catalyst alternative is becoming more and more possible, but understanding exactly where and how catalysis is occurring in these new materials has been a long-standing challenge. This is true, Zelenay noted, especially in the fuel cell cathode, where a relatively slow oxygen reduction reaction, or ORR, takes place that requires significant loading of platinum.
The performance of the new iron-nitrogen-carbon (Fe-N-C) electrocatalyst approaches that of platinum catalysts, a significant advance, as documented in fuel cell test-stand performance.
Through the use of ORNL’s aberration-corrected scanning transmission electron microscope and electron energy loss spectroscopy, ORNL researchers were able to provide the first direct observation of the often proposed ORR active site, FeN4, at an atomic level.
With both this performance and the atomic visualization of the reaction sites, we are closing the gap to replace platinum with a high-performance catalyst poised to be scaled up for potential application in fuel cells for automotive applications.—Karren More, ORNL microscopy team lead
In addition, the high activity of Fe-N-C catalysts and the FeN4 active-site structure was predicted by computer modeling conducted at Los Alamos, as was the possible reaction pathway.
This work was supported by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Microscopy was performed as part of a user project supported by ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. Computational resources were provided by the Institutional Computing program of Los Alamos National Laboratory.
Hoon T. Chung, David A. Cullen, Drew Higgins, Brian T. Sneed, Edward F. Holby, Karren L. More, Piotr Zelenay (2017) “Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst” Science Vol. 357, Issue 6350, pp. 479-484 doi: 10.1126/science.aan2255