(Invited) Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale Ni _y Fe_1–Y ox

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 (Ni y Fe1–y Ox, 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 Ni y Fe1–y Ox 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 Ni y Fe1–y Ox 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 (y = 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 Ni y Fe1–y Ox (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 Ni y Fe1–y Ox 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.