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 Ni-0.8:Fe-0.2 oxide, giving direct evidence that water and hydroxide in the interlayer are able to create stable Ni-IV/Fe-IV 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 Ni-0.8:Fe-0.2 oxide also exhibits an anomalously high active site density and higher activity than Ni0.8Fe0.2OOH LDH.