Insights into the Active Electrocatalytic Areas of Layered Double Hydroxide and Amorphous Nickel–Iron Oxide Oxygen Evolution Electrocatalysts

Layered double hydroxide (LDH) and amorphous nickel–iron (oxy)hydroxides (Ni1–xFexOOH) are emerging catalysts for the electrochemical oxygen evolution reaction (OER). It is still unresolved if the layered two-dimensional (2D) structure allows for active catalytic sites to exist below the traditional electrode/electrolyte interface. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) to directly measure active site densities in situ. We determined that Ni0.8Fe0.2OOH LDH showed a 10-fold increase in the active site density compared to rock salt Ni0.8:Fe0.2 oxide, giving direct evidence that water and hydroxide in the interlayer are able to create stable NiIV/FeIV active species at layers below the electrode/electrolyte interface. This result suggests that electrolyte permeability of the 2D LDH structure is a major contributor for its increased catalytic activity. Amorphous Ni0.8:Fe0.2 oxide also exhibits an anomalously high active site density and higher activity than Ni0.8Fe0.2OOH LDH.