Microwave-assisted synthesis of a nanoamorphous (Ni0.8,Fe0.2) oxide oxygen-evolving electrocatalyst containing only “fast” sites

Abstract

Nickel–iron oxyhydroxides (Ni_1−xFe_xOOH) are non-precious metal electrocatalysts for the oxygen evolution reaction (OER) that have high efficiency in alkaline media. It has been suggested that the layered-double hydroxide (LDH) crystal structure of Ni_0.8Fe_0.2OOH contains two types of catalytic sites, “fast” Fe sites and “slow” Ni sites, which may limit the overall activity because only 20% of the catalytic surface is highly active. Herein, we report a facile microwave-assisted synthesis route of creating a nanoamorphous nickel–iron oxide electrocatalyst that contains only “fast” catalytic sites. Benchmarking experiments on flat electrodes (roughness factors <1.4) showed that the microwave-assisted, nanoamorphous (Ni_0.8,Fe_0.2) oxide had a low OER overpotential of 286 mV at a current density of 10 mA cm⁻². We measured the kinetic rate constant of the active sites directly with the surface interrogation mode of scanning electrochemical microscopy (SI-SECM). We show that the microwave-assisted, nanoamorphous (Ni_0.8,Fe_0.2) oxide has only one type of catalytic site with an OER kinetic rate constant of 1.9 s⁻¹ per site. We compared this to a crystalline Ni_0.8Fe_0.2OOH that was synthesized via electrochemical conditioning of crystalline Ni_0.8:Fe_0.2 oxide, and verified that the Ni_0.8Fe_0.2OOH contained two types of catalytic sites – “fast” sites with an OER rate constant of 1.3 s⁻¹ per site and “slow” sites with an OER rate constant of 0.05 s⁻¹ per site. The percentage of “fast” sites in the crystalline Ni_0.8Fe_0.2OOH was well matched to the total iron atom content, while 100% of the sites were “fast” in the microwave-assisted, nanoamorphous (Ni_0.8,Fe_0.2) oxide.