Electroanalytical Assessment of the Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale Ni_yFe_1–yOx

Abstract

Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiFeOx, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective NiFeOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for NiFeOx aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiFeOx materials; however, 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 nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size.

Sustainable Electrocatalytic Architectures Enable Rechargeable Zinc–Air Batteries with Low Voltage Hysteresis

“…Previously, we indexed Ni-rich varieties of Ni crystal habit, was not observed. 30 Here, we observe a small peak at 74.4°2θ for the 275 °C-annealed Ni 2 FeOx, which best matches to cubic NiO (ICDD #01-078-4367) or isostructural FeO (ICDD #01-077-7981). Electron diffraction further confirms that the Ni 2 FeOx forms the cubic rock-salt crystal phase, closely matching that of NiO.…”

“…3,14,17,19,20,36−56 The electrocatalysts, down-selected from our prior three-electrode investigations of ORR and OER performance, are α-MnO 2 for efficient ORR 57 and Ni 2 FeOx primarily for OER, 29 although nickel-ferrite aerogels also express a bifunctional character. 30 We evaluate these electrocatalyst-functionalized air cathodes versus 3D-Zn-sponge anodes in prototype Zn−air cells. We cycle the Zn−air cells at a technologically relevant current density of 10 mA cm −2 to areal energies of 18−36 mW h cm −2 (areal capacities of 15−30 mA h cm −2 ), which map to practical system-level specificenergy values that compete with Li-ion battery systems.…”

“…The use of bifunctional electrocatalysts may simplify cathode preparation, but it is commonly difficult to find materials that catalyze both ORR and OER effectively and stably. ,, The second, more flexible, approach uses two electrocatalysts, one designed for ORR and the other for OER. Manganese oxides are proven, inexpensive ORR electrocatalysts in alkaline electrolytes, particularly the 2 × 2 tunneled cryptomelane-type α-MnO 2 . − For OER in alkaline electrolytes, metal oxides, oxyhydroxides, and layered double hydroxides comprising first-row transition-metals, for example, nickel (Ni), iron (Fe), and cobalt (Co), are among the most electrocatalytically active. − Catalysts for ORR and OER synthesized from Ni, Fe, and Mn are especially desirable because Co is expensive and relies on unsustainable high-risk supply chains; the impetus to develop Co-free cathodes for Li-ion batteries − is therefore equally important for Zn–air batteries.…”

NiFeOx as a Bifunctional Electrocatalyst for Oxygen Reduction (OR) and Evolution (OE) Reaction in Alkaline Media

“…Average pore size distribution for samples at 100-450 °C (Figure 3b) shows pores are in a bi-modal size that have less than 2 nm micropores and mesopores between 10-50 nm, while for the sample heat-treated at 600 °C, most of the pores are in the meso to macropore range (10-60 nm). The pore size distributions for those samples calcined at 100-450 are similar to layered and pillared solids, while the sample at 600 °C is similar to porous Usually, electrocatalytic activity is directly related to the accessible surface area for the oxygen redox reaction [24]. BET surface area, pore volume, and pore size distribution results are presented in Figure 3 and Table 1.…”

“…The TEM result shows that in most of the samples, the particles have well dispersed-nanorod structures which are favourable catalytic active sites for oxygen redox reaction [23]. Usually, electrocatalytic activity is directly related to the accessible surface area for the oxygen redox reaction [24]. BET surface area, pore volume, and pore size distribution results are presented in Figure 3 and Table 1.…”