(Invited) Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale NiyFe1–Yox

Wednesday, 4 October 2017: 12:00
Chesapeake K (Gaylord National Resort and Convention Center)
J. S. Ko (Naval Research Laboratory), C. N. Chervin (Former Staff Scientist at U.S. Naval Research Laboratory), M. Vila (Naval Research Enterprise Internship Program, Undergraduate Research Student), P. A. DeSario, J. F. Parker, J. W. Long, and D. R. Rolison (U.S. Naval Research Laboratory)
The efficient electrocatalysis of reactions involving molecular oxygen determines the performance of many electrochemical energy-storage and -conversion systems. Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiyFe1–yOx, where y=0.1–0.9) prepared as high surface area nanoarchitectures using epoxide-based sol–gel chemistry. Wet gels were processed using either ambient-pressure or supercritical-fluid drying to render ambigel or aerogel nanoarchitectures, respectively. For this series of NiyFe1–yOx materials, we correlate electrocatalytic activity with Ni:Fe stoichiometry and nanoarchitecture. In order to ensure in-series comparisons, calcining at 350°C in air was necessary to crystallize the respective NiyFe1–yOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), biphasic for intermediate stoichiometries (0.5 ≤ y ≤ 0.67), and rock-salt for the most Ni-rich material (= 0.9). For the OER, Ni-rich nanoarchitectures (y > 0.5) exhibit high activity (low overpotential, η), while both the ambigel and aerogel series in the intermediate Ni:Fe stoichiometry range (0.33 ≤ y ≤ 0.67) exhibit a monotonic increase in current density with progressively higher specific surface area. The commensurate OER activity obtained for ambigel and aerogel expressions across a given Ni:Fe stoichiometry highlights the fact that competitive electrocatalytic performance can be achieved without the additional complexity of supercritical-fluid extraction. We also find improved OER performance (η decreases from 390 to 373 mV) for Ni0.67Fe0.33Ox aerogel when heated at 300°C under flowing Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiyFe1–yOx (y < 0.5) materials. As the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a NiyFe1–yOx nanoarchitecture that provides high OER activity with decent ORR activity. We find that assessing catalytic activity requires an appreciation of the multivariate interplay between Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size rather than basing electrocatalyst selection on Ni:Fe stoichiometry alone.