Elucidating the Geometric Active Sites for Oxygen Evolution Reaction on Crystalline Iron-Substituted Cobalt Hydroxide Nanoplates

Abstract

Transition-metal (oxy)hydroxides are among the most active and studied catalysts for the oxygen evolution reaction in alkaline electrolytes. However, the geometric distribution of active sites is still elusive. Here, using the well-defined crystalline iron-substituted cobalt hydroxide as a model catalyst, we reported the scanning electrochemical cell microscopy (SECCM) study of single-crystalline nanoplates, where the oxygen evolution reaction at individual nanoplates was isolated and evaluated independently. With integrated prior- and post-SECCM scanning electron microscopy of the catalyst morphology, correlated structure–activity information of individual electrocatalysts was obtained. Our result reveals that while the active sites are largely located at the edges of the pristine Co(OH)2 nanoplates, the Fe lattice incorporation significantly promotes the basal plane activities. Our approach of correlative imaging provides new insights into the effect of iron incorporation on active site distribution across nano-electrocatalysts.

Recent Developments in Single-Entity Electrochemistry

“… 171 Aside from crystallographic orientations of nanocrystals, other studies in this category have correlated electroactivity to tensile strain 173 and Fe lattice incorporation. 170 These works demonstrate application of electrochemical SPM to reveal fundamental single-entity level structure–activity relationships to guide materials design.…”

“…Another work by Varhade et al investigated the electrocatalytic activity of hexagonal spinel Co 3 O 4 nanoparticles for OER via SECCM and correlative SEM and atomic force microscopy (AFM) to confirm that (110)-exposed edge surface had higher catalytic activity compared to (111)-exposed top planes due to the higher atomic density of Co atoms present on the edge surface . Aside from crystallographic orientations of nanocrystals, other studies in this category have correlated electroactivity to tensile strain and Fe lattice incorporation . These works demonstrate application of electrochemical SPM to reveal fundamental single-entity level structure–activity relationships to guide materials design.…”

“…A trend in SEE with SECCM is the study of single nanocrystals with controlled size, shape, facet, and surface area. ,− Jeong et al employed SECCM to resolve activity for electrochemical CO 2 reduction (eCO 2 RR) at faceted single-crystalline Au nanocrystals . Three differently faceted nanocrystals were designed to have the same surface area.…”

Single Entity Electrocatalysis

“…225 The degree of Fe incorporation in Co(OH) 2 nanoplates increases the basal plane activity towards OER. 226 Not only does elemental ratios affect the electrocatalytic activity, but the effect of size on nanomaterials' electrocatalytic activity, for example, has continued to be debated. For NiFe 2 O 4 nanocrystals, it was found with SECCM that current densities of single particles decreased with increasing particle diameters and that the activity seemed to be sensitive to added Au content only below ∼800 nm.…”

“…Peng et al used SECCM to analyze individual single-crystalline iron-substituted cobalt hydroxide nanoplates, where they were able to improve the inherent OER activity of basal planes by increased Fe incorporation and demonstrate higher intrinsic catalytic activity at edge sites of pristine cobalt hydroxide plates. 226 Sun et al used SECM (with ∼15 nm resolution) to map OER activity at NiO nanosheets. 194 The activities mapped with SECM were correlated to electron tomography and ab initio calculations to reveal that the (100) facet, which is the primary facet of the edge sites, is responsible for the observed enhanced OER activity at edge sites as compared to surface planes.…”