Rapid Synthesis and Correlative Measurements of Electrocatalytic Nickel/Iron Oxide Nanoparticles

Electrocatalytic core-shell nanoparticles, such as nickel/iron oxides for the oxygen evolution reaction (OER) in alkaline electrolytes, require rapid synthesis and measurement for practical use. To meet this challenge, we investigated a novel process of adding Ni(II) species to Fe nanoparticles immediately after synthesis, which we expected to yield Ni-rich shells around Fe-rich cores. Cyclic voltammetry showed that the overpotential decreased as the molar ratio of Ni to Fe in the synthesis vessel increased from 0.2 mol Ni:1 mol Fe to 1.5 mol Ni:1 mol Fe, consistent with an increase of Ni composition. Unexpectedly, the overpotential increased abruptly at 2.0 mol Ni:1 mol Fe. X-ray photoelectron spectroscopy revealed that this synthesis ratio resulted in less Ni at the nanoparticle surfaces than lower synthesis ratios. These results demonstrate the sensitivity of rapid electrochemical measurements to surface composition, and the limits of Ni(II) adsorption and reduction to rapidly form Ni-rich shells around Fe-rich cores. Cyclic voltammetry also showed that the onset of the methanol oxidation reaction (MOR) correlates with the oxidation of Ni(OH)₂ to NiOOH. Therefore, tuning materials to improve performance as OER catalysts also improves their performance as MOR catalysts.